FUNDAMENTAL STUDIES OF COPPER CORROSION IN INTERCONNECT FABRICATION PROCESS AND SPECTROSCOPIC. INVESTIGATION OF LOW-k STRUCTURES

Transcription

1 FUNDAMENTAL STUDIES OF COPPER CORROSION IN INTERCONNECT FABRICATION PROCESS AND SPECTROSCOPIC INVESTIGATION OF LOW-k STRUCTURES Arindom Goswami, M.S., M.Sc. Requirements for the Degree of DOCTOR OF PHILOSOPHY UNIVERSITY OF NORTH TEXAS December 2015 APPROVED: Oliver M. R. Chyan, Major Professor Michael G. Richmond, Committee Member Teresa Golden, Committee Member William E. Acree, Committee Member & Chair of the Department of Chemistry Costas Tsatsoulis, Dean of the Toulouse Graduate School

2 Goswami, Arindom. Fundamental Studies of Copper Corrosion in Interconnect Fabrication Process and Spectroscopic Investigation of Low-k Structures. Doctor of Philosophy (Chemistry -Analytical Chemistry), December 2015, 87 pages, 2 tables, 40 figures, chapter references. In the first part of this dissertation, copper bimetallic corrosion and its inhibition in cleaning processes involved in interconnect fabrication is explored. In microelectronics fabrication, post chemical mechanical polishing (CMP) cleaning is required to remove organic contaminants and particles left on copper interconnects after the CMP process. Use of cleaning solutions, however, causes serious reliability issues due to corrosion and recession of the interconnects. In this study, different azole compounds are explored and pyrazole is found out to be a potentially superior Cu corrosion inhibitor, compared to the most widely used benzotriazole (BTA), for tetramethyl ammonium hydroxide (TMAH)-based post CMP cleaning solutions at ph 14. Micropattern corrosion screening results and electrochemical impedance spectroscopy (EIS) revealed that 1 mm Pyrazole in 8 wt% TMAH solution inhibits Cu corrosion more effectively than 10 mm benzotriazole (BTA) under same conditions. Moreover, water contact angle measurement results also showed that Pyrazole-treated Cu surfaces are relatively hydrophilic compared to those treated with BTA/TMAH. X-ray photoelectron spectroscopy (XPS) analysis supports Cu-Pyrazole complex formation on the Cu surface. Overall Cu corrosion rate in TMAH-based highly alkaline post CMP cleaning solution is shown to be considerably reduced to less than 1Å/min by addition of 1 mm Pyrazole. In the second part, a novel technique built in-house called multiple internal Reflection Infrared Spectroscopy (MIR-IR) was explored as a characterization tool for characterization of different low-k structures.in leading edge integrated circuit manufacturing, reduction of RC time

3 delay by incorporation of porous ultra low-k interlayer dielectrics into Cu interconnect nanostructure continues to pose major integration challenges. The main challenge is that porous structure renders interlayer dielectrics mechanically weak, chemically unstable and more susceptible to the RIE plasma etching damages. Besides the challenge of handling weak porous ultra low-k materials, a lack of sensitive metrology to guide systematic development of plasma etching, restoration and cleaning processes is the major stumbling block. We explored Multiple Internal Reflection Infrared Spectroscopy and associated IR techniques as a sensitive (sub-5 nm) characterization tool to investigate chemical bonding modification across fluorocarbon etch residues and low-k dielectric interface after plasma etching, ashing, UV curing and post-etch cleaning. The new insights on chemical bonding transformation mapping can effectively guide the development of clean-friendly plasma etch for creating ultra low-k dielectric nanostructures with minimal dielectric damages.

4 Copyright 2015 By Arindom Goswami ii

5 ACKNOWLEDGEMENTS Six years ago, I arrived in the USA from India to pursue higher studies. After six years, this journey is on the verge of completion with the grace of God. I take this opportunity to express my sincere gratitude to all those who have encouraged and believed in me to complete this dissertation. First and foremost, I would like to thank my mother Rajalakhmi and sister Lakhirupa for supporting me in this journey. I shall forever remain grateful to my PhD advisor Dr. Chyan for taking me under his wings and making me a good student as well as a better person. I would like to thank my committee members Dr. Acree, Dr. Richmond and Dr. Golden for their comments and critiques of my dissertation in a short span of time which greatly helped me improve my dissertation and make it better. I would also like to thank all my friends and team members for their constant support: Nick Ross, Sirish Rimal, Tamal Mukherjee, Pofu Lin, Dr. Seare Berhe, and past graduates Dr. Simon Koskey, Dr. Jafar Abdelghani. Sirish Rimal and Tamal Mukherjee deserve special mention as they contributed heavily in writing chapter 3, both with their scientific input as well as with MIR-IR and UV irradiation experiments. I would also like to thank CART, especially Saul for being helpful. The financial support from the chemistry department, SRC, INTEL, TEL, ATMI, LAM research, and Freescale semiconductor is deeply appreciated. Lastly and most importantly, I would like to say a big thank you to my late father Dr. Aboni Chandra Goswami for having been such a wonderful father and for touching each of our lives and making it beautiful. I miss you every day. I wish you were here to see this moment. This dissertation is dedicated to you. iii

6 CONTENTS ACKNOWLEDGEMENTS LIST OF FIGURES AND TABLES iii viii CHAPTERS Chapter 1 Introduction and instrumentation Introduction Copper Technology Copper as a replacement for Aluminum Low Resistivity High Electromigration Resistance Challenges to implement Copper Need for planarization: CMP process for Copper Chemical Mechanical Planarization (CMP) Introduction Copper CMP Pourbaix Diagram Experimental Procedure and Instruments Potentiodynamic polarization plots Impedance Measurements X-Ray Photoelectron Spectroscopy (XPS) 22 iv

7 Introduction and Fundamentals Instrumentation Contact angle measurements Physical vapor deposition Micropattern corrosion screening MIR-IR metrology References 34 Chapter 2 Study of Pyrazole as Copper corrosion inhibitor in model alkaline post chemical mechanical polishing cleaning solution Introduction Literature survey of inhibitor performance The structure of copper-azole complexes Benzotriazole Pyrazole Experimental procedure Results and Discussion Effect of substrate on Cu corrosion Cu micropattern corrosion and inhibition Electrochemical Analysis Tafel plots Electrochemical Impedance Spectroscopy (EIS) Water Contact Angle Measurements 56 v

8 2.4.5 Surface Analysis XPS Analysis Proposed Mechanism of Cu Corrosion Summary References 61 Chapter 3 Infrared Spectroscopic Characterization on Low-k Dielectric Nanostructure to Optimize Cu Interconnect Fabrication Introduction Experimental Procedure Characterization of Dielectric Trench Pattern Functional Group Specific Chemical Reaction UV Treatment + Wet Clean Results and Discussions Chemical Bonding Mapping of Low-k Trench Structure after Fluorocarbon Plasma Etch MIR-IR Evaluation of Plasma Damage to Trench Low-k from Different Oxidative Strip Processes Characterization of Chemical Bonding Structure of Model Fluorocarbon Polymer Wet Clean of Post-Plasma Etch Residues UV-Assisted Wet Clean of Fluorocarbon Polymer Mechanism of UV-Induced Structural Disintegration of Fluorocarbon vi

9 Polymer Conclusion References 86 vii

10 LIST OF FIGURES AND TABLES Chapter 1 Figure 1.1 The effect of feature size on the gate and interconnect delay 3 Figure 1.2 Number of metal layers and k-values of ILD layers versus year of production 7 Figure 1.3 A cross-sectional SEM image of a representative multilevel interconnect network 7 Figure 1.4 (a) Dual damascene process. 9 Figure 1.5 A cross-sectional SEM image of a representative multilevel interconnect network with (right) and without (left) CMP process 9 Figure 1.6 Schematic of a CMP tool 11 Figure 1.7 Potential-pH equilibrium diagram for copper-water system at 25 C 13 Figure 1.8 Basic regions in a Pourbaix diagram 15 Figure 1.9 A typical Potentiodynamic polarization plot of Cu electrode shows E corr, I corr, cathodic curve, and anodic curve. 20 Figure 1.10 Nyquist plot and equivalent circuit 21 Figure 1.11 Schematic representation of the XPS instrumentation. 23 Figure 1.12 Schematic diagram of the XPS process, showing photo-electron effect. 23 Figure 1.13 (a) XPS chamber in (b) PHI 5000VersaProbe Scanning XPS 28 Figure 1.14 A sessile liquid drop on a solid surface in equilibrium with the vapor Phase 29 Figure 1.15 Dual magnetron gun sputtering system 31 Figure 1.16: Micropattern Corrosion Screening Technique 32 viii

11 Figure 1.17: Principle of MIR-IR metrology. 33 Figure 1.18: Schematic representation of MIR-IR setup. 34 Chapter 2 Figure 2.1 Structure of benzotriazole (BTA) and Pyrazole 43 Figure 2.2 Micropattern corrosion screening structure 48 Figure 2.3 Time lapsed images of Cu microdots deposited on Ru, Ta and glass in 8 wt.% TMAH solution 50 Figure 2.4 Tafel plots of Ru, Cu and Ta measured in TMAH ph 14 solution 51 Figure 2.5 Inhibitor concentration dependent etch rate of Cu in 8 wt.% TMAH 53 Figure 2.6 Time lapsed images of 50nM Cu/Ru immersed in 8 wt.% TMAH with additional 1mM Pyrazole and 10mM BTA 53 Figure 2.7 Tafel plots of Cu in 8wt.% TMAH and with Pyrazole and BTA 54 Figure 2.8 EIS data (a) Nyquist plot of Cu in TMAH (black), TMAH +BTA (red) and TMAH+Pyrazole (blue) and inset equivalent circuit used to fit data. 56 Figure 2.9 Variation in DI water contact angle of Cu in TMAH and TMAH+ inhibitor 57 Figure 2.10 XPS Cu 2p spectra of; (a) bare Cu, (b) BTA modified Cu and (c) Pyrazole modified Cu and Cu LMM spectra of; (d) bare Cu, (e) BTA modified Cu and (f) Pyrazole modified Cu 59 Figure 2.11 Figure 2.11 XPS N1s spectra of: (a) bare Cu, (b) BTA modified Cu and (c) Pyrazole modified Cu 60 Chapter 3 Figure 3.1 Characterization tools to assist low-k dielectric nanostructure fabrication. 67 ix

12 Figure 3.2 IR spectra of 300 nm CDO film obtained from MIR-IR and T-IR; mabs = miliabsorbance unit. 70 Figure 3.3 Chemical bonding mapping of low-k trench nano-structure formed after fluorocarbon plasma etch process. 71 Figure 3.4 (a) MIR-IR spectra of low-k trench structures after different oxidative plasma strip processes (strip 1 4) to optimize O radical content and (b) respective plot of OH increase and Si-CH 3 decrease from MIR-IR and T-IR respectively. 73 Figure 3.5 (a) FT-IR spectra of 28 nm and 6 nm MFP films deposited by CHF 3 /C 4 F 8 /Ar plasma chemistry only and added feedstock plasma gases (O 2 and NF 3 ) to CHF 3 /C 4 F 8 /Ar plasma recipe; (b) corresponding cross-sectional SEM images (MFP film in green color). 75 Figure 3.6 Representative model chemical bonding structure of functionalized fluorocarbon polymer derived via reductive defluorination of CF x groups, DNPH hydrazine formation of carbonyl and bromination of olefin unsaturations. Differential spectra (i) were obtained by subtracting as-deposited MFP film spectra (ii) from the corresponding derivatized MFP film spectra (iii). 76 Figure 3.7 Time dependent proprietary cleaning solvent treatment for post-etch residue removal on patterned low-k test wafer. 79 Figure 3.8 FT-IR spectra of as-deposited (no UV) 28 nm MFP film, 180 sec UV-treated and 300 sec UV-treated MFP film with subsequent wet clean. 81 Figure 3.9 XPS analysis of (a) C 1s and (b) F 1s of UV- treated and subsequent wet cleans on MFP, (c) SEM images of the corresponding UV and wet cleans on MFP film. 82 Figure 3.10 IR spectroscopic evidence of progressive hydroxyl formation during UV-air x

13 treatment that turn the fluoropolymer film more hydrophilic for improved wet removal. 83 Figure 3.11 Proposed mechanism of UV-assisted fluorocarbon chain dissociation involving excited singlet oxygen via photosensitization of carbonyl groups. 84 Tables Table 1.1 Properties of Low Resistivity Metals 5 Table 1.2 Effects of potential variation in the positive (noble) and negative (active) directions 16 xi

14 CHAPTER Introduction INTODUCTION AND INSTRUMENTATION Advanced and sophisticated electronic gadgets such as high speed computers with quadcore processors and terabit hard disk drives, smartphones, video games with vivid graphics just to name a few, have penetrated into every aspects of our daily life. These wonders of technology with enhanced performance were invented due to the speedy growth of the semiconductor chip manufacturing and technology. Microprocessor chips are the most complex manufactured product known till date. For example, approximately 400 of 45nm transistors could fit on the surface of a single human red blood cell [1]. The number of devices on a chip for silicon integrated circuits follows an extraordinary growth as predicted by Gordon Moore [2], allowing billions of transistors to be integrated on a single chip. This increase in device density has resulted in constant improvement in cost and performance. Prior devices with feature sizes of 1 m and higher have utilized aluminum-based materials (resistivity cm) for the interconnect fabrication effectively. However, it is possible to amplify the device speed by utilizing lower resistance metals as interconnects, such as copper (resistivity cm). The use of copper permits the reduction of resistancecapacitance (RC) delay. For over three decades, silicon dioxide (SiO2) has been the dielectric material of choice for the semiconductor chip manufacturing. But in recent times, due to the ever-increasing demand for high processing speed in semiconductor and scaled-down chip components, SiO2 is no longer acceptable. Thus, low-k materials have come into prominence and started to replace 1

15 SiO 2 as dielectric materials. By lowering the dielectric constant RC delay (R = the resistance of the metal lines; C = the line capacitance), and metal cross-talk between wires can be reduced. In recent years, a variety of low-k materials with a dielectric constant (k) of less than 3 have been investigated. Ultra-low-k materials are polymeric compounds that can either be spin-coated from a solution or plasma deposited by a chemical vapor deposition (CVD) process. In the year 2000, IBM declared the fabrication of Cu interconnect structures for the 130-nm node with SiLK, a polyphenylene based material with a dielectric constant (k) of about 2.7. Carbon-doped oxide (CDO), also called organosilicate glass (OSG), is a group of materials deposited by CVD technique by incorporating hydrocarbon groups into oxide films (SiOCH films) and introducing porogens into the film to create pores. Currently, CDO materials are commercially accessible under the trade names Black Diamond and Coral. They are the materials of choice for contemporary copper dual damascene based interconnect structures [3]. In a characteristic damascene process, a hardmask is used for deposition and patterning of the low-k material. A barrier layer (Ta) deposition is carried forward using physical vapor deposition (PVD) technique to facilitate adhesion and to operate as a diffusion barrier between metal and dielectric material, before inlaying the interconnect metal (Cu). Next, copper is electroplated to a thickness of μm above the dielectric over the whole wafer surface. The surplus copper deposit left during this process is subsequently polished away using chemicalmechanical polishing (CMP process) down to the barrier layer, which is then polished away, exposing the dielectric layer. 2

16 1.2 Copper technology Copper as a replacement for Aluminum In the year 1997, copper has replaced aluminum as the electrical interconnection material on silicon integrated circuits. Low resistivity and high resistance to electromigration make copper an attractive alternative to aluminum. As device dimensions shrink below 0.25 pm, the continued use of aluminum interconnects became questionable. Silicon chip manufacturers have started using copper metallization for sub-0.25 μm generation devices. Figure 1.1: The effect of feature size on the gate and interconnect delay [4] Desired properties of metals for ULSI: [5] Low resistivity (< 4 µω-cm) Easy to be etch Should be stable in oxidizing environment (oxidizable) Excellent adhesion to underlying substrate 3

17 Mechanical and electrical stability Surface smoothness Less contamination of processing equipment Anisotropically etchable with high selectivity with respect to substrate and mask material Depositable over vertical walls with conformal coverage of steps Low film stresses Low resistivity The total circuit signal delay is a combination of the intrinsic device delay and the interconnect delay. Figure 2.1 shows a comparison of the intrinsic gate delay of a metaloxide-semiconductor transistor and the interconnect delay. As feature sizes on a silicon chip decrease (< 1 pm), the circuit signal delay is dominated by the interconnect delay rather than the intrinsic device delay. The delay in signal propagation through an interconnect is given by the product of resistance (R) and capacitance (C). The RC time constant is given by equation (2.1) (1.1) where = interconnect resistivity, t m = interconnect thickness, L = interconnect length, = interlayer dielectric (ILD) permittivity, and = interlayer dielectric thickness. Consequently, the interconnect delay can be reduced by using metals with lower resistivity and/or ILDs with lower dielectric constant. The resistivity of pure aluminum is 2.7 µω cm whereas that of pure copper is considerably lower at 1.7 µω cm [9] Note that in practice, aluminum is alloyed with copper and silicon to increase its electromigration resistance and decrease its reactivity. These alloying additions increase the resistivity of the aluminum interconnect to -3.5 µω cm, making copper even more attractive. 4

18 Table 1.1. Properties of Low Resistivity Metals [6] Ag Al Al alloy Au Cu ~ Resistivity (µωcm) Electromigration Poor Poor Fair-Poor Very Good Good Resistance Corrosion Poor Good Good Excellent Poor Resistance Adhesion to Poor Good Good Poor Poor SiO 2 Si Deep Levels Yes No No Yes Yes RIE Etch No Yes Yes No No As we can see from the table, out of all the metals with low resistivity than aluminum, copper appears to be most attractive. Copper has a resistivity slightly greater than Ag and approximately 50 % lower than the aluminum. Thus, RC delay gets significantly reduced due to incorporation of copper High electromigration resistance The interconnect materials in integrated circuits are subjected to elevated temperatures (~100 C due to Joule heating) and high current densities ( A/cm 2 ) during operation. [7] Such conditions facilitate electromigration to occur within the conductor lines, leading to considerable mass transport in metals. As a result of electromigration, interconnect failure can occur due to formation of pores or discontinuities in the metal. 5

19 Atomic diffusion can be elevated at high operating temperatures of the interconnect. The atomic mobility in the conductor is further enhanced due to the influence of applied electric field on the ionized atoms. Due to the high current densities experienced by the interconnect during operation, the momentum transfer induced by electron dictates and directs to overall mass transfer in the route of electron flow. The effective charge (Z*) on an atom is the sum total of the true charge on an ionized atom and a total number of collisions per unit time between an atom and electrons. [8] (1.2) In equation (1.2), Z = true charge, n e = density of electrons, l e = mean free path of electrons, and = collision cross-section of atoms with electrons. Z* for aluminum is in the range ( ) while for copper the range is ( ). [9] Atomic densities as well as resistivities of aluminum and copper differ by a factor of less than 2. Hence, diffusivity is the major factor that determines the electromigration resistance of these two metals. Copper has a significantly higher melting point than aluminum (1084 C vs. 660 C). Therefore, the bulk diffusivity of copper at 100 C (interconnect operating temperature) is ten orders of magnitude smaller than that of aluminum. Hence, copper can handle higher current densities (up to 5 X 10 6 A/cm 2 ) compared to aluminum ( 2 X 10 5 A/cm 2 ). 6

20 Figure 1.2: Number of metal layers and k-values of ILD layers versus year of production [10] Figure 1.3: A cross-sectional SEM image of a representative multilevel interconnect network [11] Challenges to implement copper During processing (< 450 C) [9] and device operation (~100 C), copper is subjected to higher temperatures. The diffusion of copper increases at higher temperatures due to the exponential 7

21 dependence of the diffusion coefficient, D, on temperature given by (1.3) where D 0 = pre-exponential diffusivity, Q = activation energy, R= gas constant, and T = absolute temperature (K). As a result, copper can diffuse into the isolating dielectric, e.g. silicon dioxide (SiO 2 ). Copper diffusion into SiO 2 is speeded up more by the presence of an electric field for the duration of device operation. The occurrence of copper in SiO 2 results in current leakage across the interlayer dielectric [9]. Therefore, the use of copper as an interconnect require the use of a barrier layer to avert diffusion into the underlying dielectric and substrate at higher processing and operating temperatures and under applied electric field. Besides a diffusion barrier, an adhesion layer is also required as copper does not bond well to SiO 2. Another drawback of copper technology is the reactivity of copper. Copper continues to oxidize for as long as it is exposed to the ambient environment, unlike aluminum which forms a self-limiting oxide. The leading hurdle in incorporating copper into prevalent silicon integrated circuit technology is patterning the interconnects. As copper does not form volatile species below 100 C, common dry etching techniques used on aluminum (such as, reactive ion etching) cannot be employed to pattern copper. Instead, an inlaid metal process like the damascene approach can be used. In the damascene approach, trenches are etched into the dielectric; a diffusion barrier/adhesion layer is deposited, followed by copper deposition to fill the precut trenches. Excess copper is polished away by chemical mechanical planarization (Figure 1.4). This process creates a planar surface with copper surrounded by the diffusion barrier/adhesion layer on all sides but the polished surface. This inlaid metal structure created with the help of CMP has been proven to be a practical manufacturing process for copper interconnections. Damascene architecture together with CMP has been effective in creating 8

22 integrated circuit metallization schemes. Figure 1.4: Dual damascene process. 1.3 Need for planarization: CMP Process for Copper Performance enhancement at the chip level was attained due to multilevel metallization. Yet, it brought in massive challenges in fabrication at the wafer level. Precision and effectiveness of pattern transfer onto photoresist in photolithography step suffered due to the patchy and rough topography which developed as the metallization levels increased. Figure 1.5: A cross-sectional SEM image of a representative multilevel interconnect network with (right) and without (left) CMP process. [12] 9

23 Efficient strategy was vital to rise above this challenge by providing a planar surface which ultimately leads to the introduction and growth of CMP into the manufacturing process. CMP is extensively in use to generate planarized surface due to the following advantages [13]: 1. Higher yields in photolithography 2. Increase in the stacking height of metallization layers up to Elimination of step coverage concerns 4. Can remove surface defects, nanotopographies. CMP is widely used as it can produce global or long-range planarization. Global planarization is necessary in semiconductor manufacturing to planarize the topography of the surface over broad ranges of an image field. 1.4 Chemical Mechanical Planarization (CMP) Introduction At present, CMP is the solitary method that can provide local and global planarity on the surface of wafer. CMP is an ensemble of smoothing and planarizing processes aided by chemical and mechanical forces. The term CMP also refers to chemical mechanical polishing that causes planarization of surfaces. However, polishing and planarization are not synonyms and two different processes. Polishing refers to smoothing of surface not necessarily planar [14]. Though the application of CMP to metal and dielectric thin films is relatively new, it has been under practice for a long time for glass as well as silicon processing [15-16]. A schematic of a CMP tool is shown in Figure 1.6 [17]. The CMP process consists of polishing wafer surfaces against a polymeric polishing pad attached to a table, called platen. Wafer carrier 10

24 holds the wafer against the pad and the wafer is pressed against the surface of the pad covered with polishing slurry. The carrier and the platen rotate in the same direction while the wafer is pressed against the pad. Slurry is a chemically reactive aqueous solution containing abrasive particles, which is fed between the wafer and the pad. Abrasive particles in the slurry cause mechanical abrasion while the chemical activity of the slurry strips away features on the wafer. The process is customized to offer superior material removal rate from elevated areas on surfaces compared to low areas, resulting in planarization. Chemistry alone will not accomplish planarization because most chemical actions are isotropic. Mechanical abrasion alone is not useful as well because of the associated damage of the sample surfaces. Figure 1.6. Schematic of a CMP tool [17]. There are three key components in a CMP process needed for describing polishing [18] I. The surface to be polished. II. The pad that transfer the mechanical force to the surface being polished. 11

25 III. The slurry that provides both chemical and mechanical effects. A fundamental understanding of the chemical-mechanical action of the slurry and other CMP consumables is still emerging. The process is quite complicated, involving a large number of variables. CMP offers several advantages such as: a) global planarization can be achieved which is necessary in building multilevel structures. b) wide range of wafer surfaces can be planarized. c) useful for planarizing multiple materials during the same polish step. d) provides an alternate path of patterning metal, eliminating the need to plasma etch. e) improves metal step coverage due to reduction in topography. f) provide increased IC reliability, speed, yield of sub-0.5 pm devices. Although CMP has many advantages, it suffers from certain challenges as well. These include limitations in end point detection (EPD) during simultaneous polishing of two different materials, dishing of metal lines and erosion of ILD during Cu damascene polishing (occurs due to lack of accurate EPD systems), post-cmp defects like residual abrasive particles, corrosion, scratches, stress cracking and wafer and die scale non-uniformity. Extensive research and process development is continuing to overcome these challenges [19] Copper CMP Chemical-mechanical planarization (CMP) has developed into a crucial step for damascene process [20], making incorporation of copper as interconnect metal viable. Isotropic etching is to be avoided to attain high planarization during the copper damascene process. Thus, the buried areas must be protected against etching. In this respect, two approaches are broadly discussed in 12

26 literature [21]. The first approach is to apply passivation chemistry, whereby the metal surface is covered by a protective layer as a result of reaction between the film and the chemicals. The material is then removed by alternating cycles of chemical passivation and mechanical abrasion. The second approach is to apply dissolution chemistry with inhibiting agents. The CMP of protruding regions is controlled by direct dissolution and mechanical abrasion, whereas the recessed regions are protected by a protective layer formed by the inhibitors. Copper is a relatively noble metal. Figure 1.7 shows the Pourbaix diagram [22] for the copper-water system. Equilibrium diagram shows that copper can be corroded by acidic or strongly alkaline solutions in the presence of oxidizers, while a protective layer of copper oxide is formed in neutral and alkaline solutions. Figure 1.7. Potential-pH equilibrium diagram for copper-water system at 25 C [22]. 13

27 Cu CMP is typically a two-step process. First step involves bulk removal of copper by using a high copper to barrier (Ta/TaN) selective slurry, while in the second step both copper and barrier layers are polished by non-selective slurry. Planarization is accomplished by a symbiotic combination of chemical surface reactions together with mechanical action using abrasive slurry and the pad. To sustain the chemical reactions, the CMP slurries typically consists of: a) a ph-adjusted aqueous background, b) an oxidizer, c) a complexing agent, and d) a corrosion inhibitor [23]. A persistent shrinkage in device dimensions beyond manufacturing at 0.13 pm (130 nm) requires use of Cu metallization as well as low-k dielectrics. Cu/low-k integration poses two major challenging issues: peeling and delamination, besides dishing, erosion and scratches. Delamination and peeling occurs due to materials adhesion and inherent mechanical strength, and hardness as well as high internal stress of metal stacks and shear force during CMP [24]. 1.5 Pourbaix diagram Pourbaix diagrams are graphical illustration of the stability regions for dissolved as well as solid/undissolved species in the aqueous solutions. There are four regions in the diagram corresponding to a) oxidizing (acidic), oxidizing (alkaline), reducing (acidic) and reducing (alkaline) environments (figure 1.8). 14

28 Figure 1.8: Basic regions in a Pourbaix diagram [25] Primarily, these potential-ph diagrams are derived from thermodynamic information. There are 3 types of reactions (lines) described on the diagram: a) Pure electrochemical (charge transfer) reactions (depends only on potential E, but independent of ph) Horizontal to the X-axis. b) Electrochemical reactions involving H + ions (depends on both E and ph) Slanted with definite slopes. c) Pure chemical reactions (mostly acid-base reactions) (depends only on ph, but independent of E) Vertical to the X-axis. Heavy solid lines are used in the Pourbaix diagram to specify equilibrium between two solid species. Solid lines indicate stability between a solid and a dissolved species, and dashed lines indicate stability between two dissolved species. 15

29 Table 1.2: Effects of potential variation in the positive (noble) and negative (active) directions [25]: Potential increases in the positive (noble) direction Loss of electrons (oxidation) is favored. Metal dissolution is favored. The system becomes more oxidizing. Potential decreases in the negative (active) direction Gain of electrons (reduction) is favored. Metal deposition is favored. The system becomes more reducing. Ratio of increases. Ratio of decreases. For studying CMP, heterogeneous diagrams are considered, which show the regions of stability for solids (metal and/or metal oxides) in contact with other solids or dissolved species. Homogeneous diagrams can only show the stability regions for dissolved species. The Pourbaix diagram for the Cu-H 2 O system is shown in Figure 1.7 above. From the Pourbaix diagram: For half-cell reactions under consideration, Pourbaix diagrams are constructed by plotting the Nernst equations (designated by lines). Considering the general reaction aa + bb + ch 2 O + mh + +ne - = 0 (3.16) Placing of the species in the diagram is established by two principles. [26] First, for charge transfer (n 1) reactions, the coefficients a and n are considered to be positive. Species A is the more highly oxidized of the two species and, thus, appears above species B on the diagram. Using the Cu-H 2 O diagram as an example, the reaction for the formation of Cu 2 O 16

30 2Cu + H 2 O Cu 2 O + 2H + + 2e - (1.4) can be rewritten as Cu 2 O + 2H + + 2e - - 2Cu H 2 O = 0 (1.5) and Cu 2 O is placed above Cu on the diagram. Second, for pure chemical reactions (n = 0), the coefficients a and m are taken to be positive. Species A is more basic than species B and appears to the right of species B on the diagram. For example, the stability between Cu 2+ and CuO Cu 2+ + H 2 O CuO + 2H + (1.6) is rewritten as CuO + 2H + - Cu 2+ - H 2 O = 0 (1.7) and CuO is placed to the right of Cu 2+ on the Cu-H 2 0 diagram. Additionally, the stability regions for dissolved species are computed for a given activity of the dissolved species. As a result, the solid and/or dashed lines delineating the stability region of a dissolved species are not absolute and move with changing activity. By convention, an activity of 10-6 is assumed, unless the activity is otherwise known. The two sloping and parallel dashed lines on the diagram specify the region of water stability. Water is a stable species between the lines. Stability limits for water is represented by the oxygen (universal oxidizing agent) and hydrogen (universal reducing agent) reactions. Above the top line, oxygen is evolved due to water decomposition (1.8) in acid or neutral solutions or in basic solutions (1.8a). 2H 2 O O 2 + 4H + + 4e - (1.8) O 2 + 2H 2 O + 4e - 4OH - (1.8a) On the contrary, hydrogen gas is produced below the bottom line, in acid Solutions (1.9) or in neutral or basic solutions (1.9a). 17

31 2H + + 2e - H 2 (1.9) 2H 2 O + 2e - H 2 + 2OH - (1.9a) The lines are computed from the Nernst equations by varying ph and assuming 1 atm of pressure for O 2 and H 2. These metal-h 2 O diagrams are valid only in the absence of complexing agents or insoluble compounds. The addition of one or more chemical species to the system may introduce several new equilibria. For example, the copper-h 2 O potential-ph diagram must be modified to include copper-ammine complexes which form in the presence of ammonia. In addition, localized increases in temperature, pressure, and stress during CMP can modify the potential-ph diagram. For instance, increased temperature increases the stability regions of Cu + and the hydrolysis products of Cu + and Cu 2+ [27]. All of these regions increase at the expense of Cu 2+ and solid copper oxides. 1.6 Experimental Procedure and instruments Potentiodynamic polarization plots Potentiodynamic polarization plots are governed by Tafel equation which relates the rate of an electrochemical reaction to overpotential (η). Potentiodynamic polarization plot is a graphical representation of the logarithm of the current density (i) versus the overpotential (η). A polarized electrode often provides a correlation between current and potential in a region which is given by: η = ±B log (I/I 0 ) (1.10) where η is applied overpotential with respect to the open circuit potential, I is the measured current density, B and I 0 are constants, I 0 is defined as the equilibrium current density, and B is defined as the Tafel Slope. 18

32 To perform a potentiodynamic polarization scan, several components are required. 1. An electrochemical cell which contains a) the metal to be investigated, b) the chemical environment necessary to perform the polarization scan. The electrochemical cell usually contains three electrodes- the working electrode, the counter electrode, and the reference electrode. The working electrode is the sample under investigation (for the dissertation work, small Cu wafer is used as the working electrode). The reference electrode provides a stable reference against which the applied potential can be computed accurately (Ag/AgCl is used). The counter electrode is used to provide the applied current, and are composed of a highly corrosion resistant material (Platinum (Pt) is used for our purpose). 2. The sample surface must be prepared to ensure well-defined initial condition, which does not vary significantly from run to run. 3. An equipment capable of running the measurements and data acquisition. The Potentiodynamic polarization plot is created by running the polarization at about -200 mv from open circuit potential (OCP) and increasing till the potential is +200 mv from OCP. Figure 1-9 (a) shows how the corrosion current (I corr ) can be projected by intersecting extrapolated lines of cathodic and anodic current curves. The corrosion potential (E corr ) is designated by extrapolation of the indicating tip of the Potentiodynamic polarization plot curve to the axis of potential V. By characterizing the slope of two cathodic and anodic branches, the tendency of redox reaction can be found as shown in Figure (b). The Potentiodynamic polarization plot was used to measure the corrosion potential and corrosion current of Cu interconnects, diffusion barrier materials like Ta or Ru in post chemical mechanical planarization (post-cmp) cleaning solution. 19

33 Log of Current A Cathodic Current I corr Corrosion Current Anodic Current Corrosion Potential E corr Potential V Figure 1-9 (a): A typical Potentiodynamic polarization plot of Cu electrode shows E corr, I corr, cathodic curve, and anodic curve Impedance Measurements Electrochemical impedance spectroscopy (EIS) is a method of evaluating the electrical behavior of electrodes as well as electrolyte materials. EIS is useful in evaluating a corrosion process based on measurement of the system s current or voltage response on applying a small amplitude sinusoidal excitation signal (either a known voltage or current) to the electrodes. In EIS, a sinusoidal voltage is applied at varying frequency (1 mhz 100 KHz) to an electrode system under consideration. The corrosion process typically forces the measured current to be out of phase (denoted by the phase angle) with the input voltage [28]. Dividing the input voltage by the output current furnishes the impedance. The response is evaluated in terms of the resultant current amplitude and phase. EIS data is 20

34 usually represented in Nyquist or Bode plots. In Nyquist plot, the real part of EIS data is plotted on the x-axis and the imaginary part on the y-axis. The y-axis in Nyquist plot is negative and each point on the plot represents impedance at a particular frequency. On the other hand, in the case of Bode plot, the logarithm of frequency data is plotted on the x-axis and both the absolute values of impedance ( Z = Z 0 ) and phase shift on the y-axis. Bode plot shows frequency information explicitly, unlike the Nyquist plot. The impedance spectrum reflects redox reactions and migrations across the electrochemical cell. These are determined by the electrical and chemical properties of the corrosive medium and electrode material [29]. Following are some areas of corrosion where EIS has been applied successfully: Rate determination Inhibitor performance Passive layer characteristic Coating performance EIS spectrum is generally analyzed by fitting it to an equivalent electrical circuit to describe the electrochemical system. Most of the circuit elements used in the model are common electrical elements such as resistors, capacitors, and inductors. Figure 1.10 shows a Nyquist plot and a corresponding equivalent circuit. 21

35 Figure 1.10 Nyquist plot and equivalent circuit From the Nyquist plot data, the solution resistance (Rs) can be evaluated by reading the real axis value at high frequency (next to the origin). The real value on the low frequency region is the sum of solution resistance and charge transfer resistance (Rct). The diameter of the semicircle is the polarization resistance and this can be used to find out the corrosion rate of a metal X-Ray photoelectron Spectroscopy (XPS) Introduction and Fundamentals X-ray photoelectron spectroscopy or electron spectroscopy for chemical analysis (ESCA) is derived from the photoelectric effect. This is a highly surface sensitive technique useful in the determination of elemental composition, chemical state analysis, determination of empirical formula, and determination of the electronic state of a material. Some of the advantages of the XPS are: 1. Can detect elements from Li up to U 22

36 2. Surface sensitive ( Å sampling depth) 3. Sensitive to variations in chemical environment 4. Quantitative without use of standards 5. Controllable charging problems with insulators XPS is a non-destructive surface analysis procedure (except when doing a depth profile), and it is owing to the fact that only the ejection of electrons is necessary for analysis. The atomic nuclei being examined remain unaffected during electron spectroscopic measurements, unlike other elemental analysis techniques. XPS involves ejection of an electron (photoemission) from a core level by an x-ray photon of energy hν (Refer to Figure. 1.12). Fig 1.11 Schematic representation of the XPS instrumentation [30] 23

37 Incident x-ray Photo-emitted electron Conduction Band Valence Band Free electron Level Fermi Level 2p 2s 1s L 2, L 3 L 1 K Figure 1.12: Schematic diagram of the XPS process, showing photo-electron effect. As the photoelectrons analyzed eject only from the upper atomic layers of the sample surface being studied ( 100 Å), XPS is a very useful tool for studying interfacial phenomena at the solid-solid and solid-gas boundaries [31]. Next, electron spectrometer is used to analyze the emitted photoelectron energy. The electron kinetic energy (E K ) is an experimental quantity measured by the spectrometer, but is not an intrinsic material property as it is dependent on the incident x-ray photon energy. The electron binding energy (E B ) is a parameter which characterizes the electron, both in terms of its parent element and its atomic energy level. The relationship between these parameters is: (Eq. 1.11) where = work function of the instrument, which is defined as the energy required by the electron to overcome in order to escape the surface. Binding energy (E B ) represents the strength of interaction between electron (n, l, m, s) 24

38 and nuclear charge. In gaseous medium, E B ~ Ionization potential (IP) (in solids, has to be added). E B follows energy of levels as: E B (1s) > E B (2s) > E B (2p) > E B (3s), and so on. E B of orbital increases with the atomic number (z): E B (Na 1s) (z=11) < E B (Mg 1s) (z=12) < E B (Al 1s) (z=13). The reason E B increases with the increase in z, is due to the increase in number of nucleons attracting the electrons. E B of orbital is not effected by isotopes (e.g. E B of ( 7 Li 1s) = E B of ( 6 Li 1s)). Photoelectrons generate x-ray photoelectron spectral peaks, which are termed according to the orbital (l= denoted as s, p, d, f ) and spin (s = ± 1/2) quantum numbers. The total momentum of the photoelectrons (J = l ± s) is incorporated in the naming of a measured x-ray photoelectron spectral peak (e.g. Cu2p 3/2, where l + s= 1+ ½ = 3/2). The XPS tool is equipped to measure all the kinetic energy of all the collected electrons, which also includes the contributions of photoelectron and Auger electron lines. Auger lines result due to ejection of valence level electrons whose kinetic energy is independent of the photon energy. Requirements of XPS leading to Photoelectric Effect: Kinetic energy (E K ) of photoelectrons increases as the binding energy (E B ) decreases (from equation 1.11). Photoemission is directly proportional to the intensity of photons. (more incident photon will result in more emission of photoelectrons). Needs a monochromatic (x-ray) incident beam. A range of kinetic energies can be produced if the valence band is broad. As each element has their unique set of core levels, kinetic energies can be used to fingerprint elements. XPS provides both quantitative and qualitative elemental analysis from a collected spectrum. 25

39 This is accomplished by quantifying the peak intensities which designates an approximate number of the atoms of a specific element present. The atomic concentration of an element A, C A, can be calculated by [32] (Eq. 1.12) where I A = area of the most intense peak of element A, S A = sensitivity factor, and (In/Sn) = ratio for all the elements (n) present on the surface. For well-calibrated XPS systems, the precision of all the quantitative measurements is usually in the range of ±5% Instrumentation Modern XPS instruments mainly contain a sample mount system, x-ray source (monochromator), electrostatic charged-particle energy analyzer, an electron detector enclosed in an ultra high vacuum (UHV) system, and an optional argon ion gun for sputtering or for performing a depth profile analysis. Material of choice for a soft x-ray source should fulfill the following criteria: 1. The line width should not limit the required energy resolution, 2. X-ray energy must be sufficient for the photo-ejection of core electrons for analysis, 3. Suitable x-ray wavelength required to obtain a strong photo-electron signal for analysis. X-ray anodes usually used in XPS are either magnesium (Mg) or aluminum (Al) with energies and ev [33] which translates to wavelengths of and nm respectively. An aluminum source usually produces monochromatic aluminum k-alpha x-rays by focusing and diffracting a non-monochromatic x-ray beam off of a thin disc of natural, quartz crystal. A 2 μm thick Al foil has to be inserted between the anode and the sample to protect the sample from scattered electrons from the anode, heating and contamination effects [34]. 26

40 An XPS spectrometer is employed to measure an electron energy spectrum with the help of an electron energy analyzer. It has the capability to focus and direct the ejected electrons from the sample surface due to x-ray excitation. A handful of electrons with the desired energy range is filtered, collected and detected at the detector. The most common analyzers used for XPS analysis are the double-pass cylindrical mirror analyzer (DPCMA) and the concentric hemispherical energy analyzer (CHA). CHA is the one more extensively used between the two and it is made up of two metal hemispheres, one hemisphere is of concave while the other is of convex shape. They have coincident centers of curvature making the hemispheres concentric. An electric field is generated in between these two spheres as a consequence of placing varying voltages on each sphere. The electrons injected into the gap gets attracted to the bottom (positive applied voltage) and repelled from the top (negative applied voltage). That's why, only selected electrons having the exact energy (called pass energy) is able to reach the detector. Detectors that are commonly used in XPS are channeltrons, microchannel plates, and restrictive anode plates. These detectors amplify each individual electron signals striking the detector making them detectable. The detector is positioned at the rear of the analyzer slit, and it consists of several single detectors linked into a computer, to display the plotted spectrum. An ultra high vacuum (UHV) system (~10-9 torr) is required to detect the low energy electrons emitted from the surface without any elastic collision. For UHV, the mean free path is very high, of the order km). The XPS measurements were carried forward using a PHI 5000Versa Probe Scanning XPS as shown in Figure 1-13 below. A standard Al-K α X-ray source was used at 280 watts and electrostatic analysis in constant pass energy mode of 114.7eV (for survey scans) and 23.5 ev (for high-resolution scans). This instrument can provide a highly focused monochromatic x-ray 27

41 beam (10-100µm) that can be precisely focused on the area of interest. An optional ion gun (100V to 5kV differentially pumped Ar ion gun with regulated leak valve with monolayer resolution) for specimen cleaning and sputter depth profiling is also accessible for use. The Ar ion gun is also used to neutralize the charging insulating materials during the X-ray irradiation. In this dissertation, the XPS was utilized to characterize metallic copper, Cu(I) oxide and Cu(II) oxide while exposing to different chemical environments. (a) (b) Figure 1-13 (a) XPS chamber in (b) PHI 5000VersaProbe Scanning XPS Contact angle measurements Contact angle (θ) is defined as the angle between the surface of the liquid and the outline of the contact surface, when an interface exists between a liquid and a solid. The contact angle of a small, sessile drop, is a function of the surface free energy, and is characterized by the Young- Dupré equation [35] (eq. 1.13) where θ is the contact angle, is the interfacial free energy per unit area, and LV, SV, and SL 28

42 refer to liquid-vapor, solid-vapor, and solid-liquid interfaces, respectively. Figure 1.14 presents an schematic illustration of the parameters used in the Young-Dupré equation. Figure 1.14 A sessile liquid drop on a solid surface in equilibrium with the vapor phase The angle of the liquid drop on the solid surface results due to the balance between the cohesive forces in the liquid and the adhesive forces between the solid and the liquid. As the interaction increases, the liquid spreads until the angle becomes near 0 o. Hydrophobicity and hydrophilicity of a surface can be determined using water to measure contact angle. In this dissertation contact angle measurements were utilized in the study of copper corrosion inhibitors to determine the hydrophobicity of the surface before and after treatment with inhibitors Physical vapor deposition (sputtering) Sputtering is a physical vapor deposition (PVD) process used for material deposition onto a substrate. Sputtering utilizes a plasma, (typically noble gas ions like Ar + ) to knock material off from a target a few atoms at a time. A general definition of plasma is a set of free charged particles moving in random paths, which is on average electrically neutral. In sputtering plasma electrons are more mobile than the large gas ions (Ar + ) and are preferentially heated. More 29

43 electrons are lost to the chamber walls due to their high energy and the plasma becomes positively charged ideally at a constant potential (also known as the plasma potential). The Ar + ions are then accelerated towards the anode target. After colliding with the target the Ar + ions dislodges target atoms, which moves towards the substrate and finally settle on top of it. The electrons released along with the Argon ionization process are also accelerated towards the anode substrate, which collides with additional Argon atoms, creating more ions and free electrons in the process, continuing the cycle. The atom ejection from the target occurs due to energetic particles [36] and only occurs when the kinetic energy of the approaching particles is much greater than the conventional thermal energies. The following principle pertains to dual magnetron sputtering system shown in figure 1.15 that was utilized in all thin film deposition in this dissertation. In case of a magnetron sputtering system, there is a strong magnetic field near the target area. This magnetic field causes spiraling of travelling electrons along magnetic flux lines near the target rather than being attracted toward the substrate. The advantage of this is that, the plasma is confined to an area near the target, without causing damages to the thin film being formed. Also, electrons travel for a longer distance, increasing the probability of further ionizing Argon atoms. This tends to generate a stable plasma with high density of ions. More ions mean more ejected atoms from the target, therefore increasing the efficiency of the sputtering process. The faster ejection rate, and hence deposition rate, minimizes impurities to form in the thin film, and the increased distance between the plasma and substrate minimizes damage caused by stray electron and Argon ions. The desktop pro dual magnetron sputtering system (Denton Vacuum LLC.) (figure 1.15) is capable of depositing thin film of multi-layers or alloys. This sputtering system is equipped with a direct current (DC) power and a radio frequency (RF) power magnetron guns. To sputter 30

44 conducting targets, a DC power supply is generally used. For insulating or semiconductor targets, an RF power supply is required with an automatic or manual impedance matching network between the power supply and the sputtering gun. The high radio frequency corresponds to the alternating currents, which allows sputtering of insulating material even in air ambient. Hence, the magnetic field can be generated on insulating target and sputter-deposit insulating materials like Si by using RF power source. DC RF Turbo pump Figure 1-15: Dual magnetron gun sputtering system Micropattern Corrosion Screening Technique Micropattern corrosion screening technique is a method that is used to study bimetallic corrosion as a result of two different metals being in contact. It employs the use of microdots of ~130 microns diameter and varied thickness depending on experimental need that are deposited on various substrates through a contact mask using standard magnetron sputtering machine. The microdots deposited form a micropattern on the substrate of choice to form a bimetallic contact that can be studied. The samples are then immersed in a corrosive solution and in situ investigation of corrosion behavior is done by visual inspection using a 31

45 metallurgical microscope. Figure 1.16 illustrates micropattern corrosion screening structure. In this dissertation, this novel rapid corrosion screening technique was demonstrated to be useful in the study of bimetallic corrosion as well as corrosion prevention by inhibitors in CMP relevant chemical environments. Figure 1.16: Micropattern Corrosion Screening Technique MIR-IR metrology Multiple internal reflection infrared spectroscopy (or simply MIR-IR) technique is based on internal reflection spectroscopy, pioneered by Fahrenfort [37] and Harrick [38]. Total internal reflection occurs at the interface of two media when radiation propagates from the optically denser medium (refractive index n 1 ) to the optically rarer medium (refractive index n 2, n 1 > n 2 ), and the radiation angle of incidence (θ) is greater than the critical angle (θ c ). The critical angle is a function of refractive indices of two media and is given by: (eq. 1.14) 32

46 Figure 1.17: Principle of MIR-IR metrology. With every reflection, an evanescent wave is propagating into the adjacent optically rarer medium. This evanescent wave can be defined as a standing electric wave normal to the interface of the two media, and it arises due to the superposition of the electric fields of the incident and reflected waves. The amplitude of this evanescent wave (E) decays exponentially with distance from the interface, as given by [38]: where E 0 is the amplitude of the electric field at the interface (z = 0), z is the distance (eq. 1.15) from the interface, and d p is the penetration depth. The penetration depth (d p ) is the distance where the amplitude of the electric field is 1/e of E 0, which is a function of refractive indices n 1 and n 2, the incidence angle θ, and the wavelength of the radiation λ [38]: (eq. 1.16) 33

47 The principle of MIR-IR is shown schematically in figure below: Figure 1.18: Schematic representation of MIR-IR setup [39]. Optically denser medium has refractive index n 1 whereas optically rarer medium has refractive index n 2. Also the radiation angle of incidence is designated by symbol θ and the critical angle is given by θ C. If the optically rarer medium absorbs IR radiation, attenuated total reflection (ATR) results at characteristic wavelengths, which corresponds to the vibrational resonant frequency. 1.7 References G.Moore, Cramming More Components Onto Integrated Circuits Electronics, 38, 8 (1985). 34

48 3. Benjamin D. Hatton, Kai Landskron, William J. Hunks, Mark R. Bennett, Donna Shukaris, Douglas D. Perovic, Geoffrey A. Ozin, Materials chemistry for lowk materials Materialstoday, 9, 3 (2006) 4. S.-P. Jeng, R. H. Havemann, and M.-C. Chang, Process Integration & Manufacturability Issues for High Performance Multilevel Interconnect, M aterials Research Society Symposium Proceedings: Advanced Metallization for Devices and Circuits-Science, Technology, and Manufacturability (Pittsburgh, PA: Materials Research Society), 337 (1994) S. Wolf, and R.N. Tauber, Silicon Processing for the VLSI era, Vol. 1 Process Technology, Lattice Press CA (2001) Steigerwald, S.P. Muraraka, and R.J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials, John Wiley & Sons (1997). 7. S. P. Murarka, Metallization Theory and Practice fo r VLSI and ULSI, Boston. Butterworth-Heinemann, 1993, P. Shewmon, Diffusion in Solids, second edition, Warrendale, PA: The Minerals, Metals, and Materials Society, 1989, S. P. Murarka and S. W. Hymes, Copper Metallization for ULSI and Beyond, Critical Reviews in Solid State and M aterials Sciences, 20 (1995) Lee, J. A.; Moinpour, M.; Liou, H. C. ; Abell, T. Proceedings of Materials Research Soceity, San Francisco, CA, 2003, p. F Li, Yuzhuo Microelectronics applications of chemical mechanical planarization, John Wiley and Sons,

49 13. Zantye, Parshuram B.; Kumar, Ashok; Sikder, A. K. Materials Science and Engineering R 45, 2004, R.J. Guttmann, J.M. Steigerwald, L. You, D.T. Price, J. Nierynck, D.J. Duquette,and S.P. Murarka, Thin Solid Films, 270, 596 (1995). 15. M. A. Fury, Solid-State Technol., 41, 81 (1997). 16. E. Mendel, Solid-State Technol., 10, 27 (1967). 17. R.G. Kelly, J.R. Scully, D.W. Shoesmith, R.G. Buchheit, Electrochemical Techniques in Corrosion Science and Engineering, Marcel Dekker, New York, Steigerwald, J.M., S.P. Murarka, and R.J. Gutmann, Chemical Mechanical Planarization of Microelectronic Materials. 1997: John Wiley and Sons, Inc. 19. V.R.K. Gorantla, Ph.D. Thesis, Clarkson University (2004). 20. R.K. Singh, A. Zutshi, R. Surana, M. Naik, and T. Pan, MRS Bull., Oct (2002) R. Carpio, J. Farkas, and R. Jairath, Thin Solid Films, 266 (1995) M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutis, Chapter IV, NACE, Houston TX (1994). 23. S.V. Babu, K.C. Cadien, and H. Yano, Eds., Chemical Mechanical Polishing-Advances and Future Challenges, Mat. Res. Soc. Symp. Proc. 671 (2001). 24. S. Balakumar, T. Haque, R. Kumar, A.S. Kumar, and M. Rahman, Mat. Res. Soc. Symp. Proc. 867 (2005) M. Pourbaix, Thermodynamics of Dilute Aqueous Solutions, London: Edward Arnold, 1949,

50 27. B. Beverskog and I. Puigdomenech, Revised Pourbaix Diagrams for Copper at 25 to 300 C, Journal of the Electrochemical Society, 144 (1997) L. R. Faulkner, J. Chem. Ed,. 60, 262, (1983). 29. D. D. MacDonald, Electrochim. Acta, 51, 1376 (2006) C. C. Chusuei, D. W. Goodman, Encyclopedia of Physical Science and Technology, 3rd Edition, Vol. 17, Academic Press (2002). 32. S. Nasrazadani, I. Fritsch, C. S. Henry, Analytical Techniques for Materials Characterization. In Advanced Electronic Packaging, edited by W.D. Brown, New York, C. R. Brundle, X-Ray Photoelectron Spectroscopy In Encyclopedia of Materials Characterization, edited by Brundle, C. R., Evans, C. A. Jr, Wilson, S., Massachusetts, Butterworth-Heinemann, H. Bubert, J. C. Rivière, Photoelectron Spectroscopy In Surface and Thin Film Analysis: A Compendium of Principles, Instrumentation, and applications, edited by Bubert, H., Jenett, H., Federal Republic of Germany, Wiley-VCH, T. S. Chow (1998). "Wetting of rough surfaces". Journal of Physics: Condensed Matter 10 (27): L R. Behrisch, Sputtering by Particle bombardment. Springer, Berlin, (1981). 37. J. Fahrenfort. Attenuated total reflection. A new principle for the production of useful infrared reflection spectra of organic compounds, Spectrochim. Acta 1961, 17, N. J. Harrick. Internal Reflection Spectroscopy, John Wiley & Sons, Inc.: New York,

51 39. B. Mizaikoff. Waveguide-enhanced mid-infrared chem/bio sensors, Chem. Soc. Rev., 2013, 42,

52 CHAPTER 2 STUDY OF PYRAZOLE AS COPPER CORROSION INHIBITOR IN MODEL ALKALINE POST CHEMICAL MECHANICAL POLISHING CLEANING SOLUTION 2.1 Introduction At the turn of the millennium, copper has replaced aluminum as the interconnect material in Ultra-large-scale integration (ULSI) devices due to its high electromigration resistance and low electrical resistance [1-2]. Despite substantial reduction in resistance-capacitance (RC) delay, several challenges were encountered in the integration of copper as wiring metal. One of the major challenges is, conventional technique like etching process cannot be applied for copper patterning due to lack of copper compound with high enough vapor pressure at relatively low temperature [3]. A more pragmatic approach to the Cu integration process involves deposition of a diffusion barrier (Ru, Ta, TaN) and Cu metal into interlayer dielectrics (ILD), known as Damascene process, followed by removal of overburden metals by chemical mechanical polishing (CMP) technique [4-5]. Damascene technology is defined as the processes of forming trenches filled with interconnect (copper) and then planarizing it in such a way that allows copper to remain only in the trenches. CMP essentially involves polishing of wafers against a pad under pressure using slurry consisting of abrasive particles and various chemicals. The overburden material is removed from the wafer surface due to the synergistic effects of the 39

53 chemical reactions (between the material being polished and chemicals in the slurry) and the mechanical action due to abrasives. The introduction of copper in semiconductor devices has brought attention to the phenomena of thin film corrosion that must be minimized for optimal device performance, reliability, and longevity. Since CMP and post CMP cleaning processes are inherently wet processes, they expose Cu interconnect and barrier to the corrosive chemicals relevant to these processes. This results in bimetallic corrosion that can be detrimental to the yield and reliability of integrated circuits devices. Furthermore, CMP process leaves Cu residues, organic residues, abrasive particles and other contaminants on the surface that can degrade the electrical properties of ILDs, lower the conductivity of Cu and lead to poor adhesion of the subsequent layers [6]. To avoid reliability issues, an effective post CMP cleaning step is therefore required to remove the residues and contaminants. Both acidic and alkaline cleaning solutions have been developed in recent years that effectively remove Cu residues and contaminants. Recent study has shown alkaline post CMP cleaning solution to be more effective to achieve a good cleaning performance and low surface roughness [7]. One of the advantages of alkaline cleaning chemistry is the selective dissolution of CuO at high ph, leaving Cu 2 O passivating layer on Cu surface. Also at high ph conditions, the negative zeta potential (NZP) aids in keeping the removed particles in the solution and does not allow them to reattach to the wafer surface or brush PVA [8]. Alkaline post CMP cleaning solution used for this study is called Tetra methyl ammonium hydroxide (TMAH), a very strong base (with a pk b ~ 0). Unlike ammonia, it does not form complexes with copper. TMAH solutions can also be obtained in very high purity with minimal ionic contamination. TMAH has been used in proprietary chemicals for effective inhibition and prevention of re-adhesion of particles that are removed during the cleaning 40

54 process [9]. Although the TMAH based solution is efficient in removing the residues and contaminants, it has been documented to be corrosive towards Cu and could therefore cause serious reliability issues [9]. This is a significant drawback as the Cu/diffusion barrier contacts are inevitably exposed to the corrosive chemicals used during the cleaning process. Ruthenium (Ru) has been explored as alternative barrier/liner materials to replace tantalum/tantalum nitride (Ta/TaN) bilayer stack because of the scaling difficulty [10-12]. However, Cu/Ru bimetallic corrosion could be of concern since Ru belongs to the platinum group metals that is more noble than Cu. Previously, we reported a novel bimetallic corrosion testing technique, which is an effective methodology for evaluating bimetallic corrosion of Cu interconnects when exposed to CMP and post CMP cleaning conditions and showed that Cu/Ru exhibit enhanced corrosion compared to Cu/Ta [13]. Micropattern corrosion testing technique has the advantage of allowing actual monitoring of corrosion of Cu when in contact with a barrier metal unlike other methods hence the bimetallic effect can be evaluated. Reliable Cu metallization process is achieved when metal loss is minimized during post CMP cleaning and is achieved by addition of a corrosion inhibitor into the cleaning solution. Significant progress had been made in the study of copper corrosion inhibitors and nitrogen heterocycles have been found to be effective metal corrosion inhibitors. This is because of their chelating action and formation of physical barrier on the surface of the metal that prevent corrosion [14]. Among them, benzotriazole (BTA) has been extensively investigated and used as Cu corrosion inhibitor in both acidic and alkaline post CMP cleaning solutions [15-18]. Neutral BTA exists in two tautomeric forms in equilibrium 1 H-benzotriazole and 2 H-benzotriazole, where the former is the predominant species (99.9%) in both solution and gas phases. Due to the presence of multiple electronegative nitrogen atoms in the ring, BTA has appreciable NH 41

55 acidicity (pk a ~8.2) and at high ph conditions, mostly exists in its highly resonance stabilized conjugate base form, benzotriazolyl anion. BTA has been widely used as metal corrosion inhibitor due to its strong metal-chelating capability and presence of hydrophobic benzene ring (water solubility is wt%). BTA physisorption at the metal interface, formation of BTAmetal complexation monolayer and subsequent molecular self assembly via strong π-π interaction between BTA aromatic rings combine to result a hydrophobic protective film, ([Cu + BTA - ] n ), on the metal surface rendering it inaccessible for any attack from corroding chemicals [19-21]. Inhibition efficiency of BTA for Cu corrosion is a function of the temperature, concentration of BTA, immersion time, oxidation states on Cu surface and the ph. BTA adsorption is much faster on Cu 2 O layer than on CuO or pristine Cu surface and therefore thickness of Cu 2 O underlayer determines thickness of the Cu-BTA protective layer. The adsorption of BTA on an oxide-free Cu surface, which is the case in very high ph conditions, is suggested to be improbable or minimal [22]. According to Pourbaix diagram, at higher ph, Cu surface comprises less oxide layer and mostly exists in Cu 0 state and at ph approaching 14, Cu directly transforms into CuO 2-2 ion [23]. Moreover, due to its poor aqueous solubility and inherent hydrophobicity, BTA contributes to increased organic residue defects on water surface and therefore not efficient when used as Cu corrosion inhibitor in aqueous alkaline post CMP cleaning solution. Therefore, BTA needs to be replaced by an efficient corrosion inhibitor that is effective in highly alkaline (> ph 12) post CMP cleaning formulations and the treated surface is more hydrophilic in nature. One of the potential substitutes is Pyrazole which is readily water soluble unlike BTA. It is a weaker NH acid (pk a ~14.21) than BTA partly due to dimer formation via intermolecular hydrogen bonding. The conjugate base, azolyl anion, is stabilized by two equally contributing 42

56 resonance forms [24]. The inhibition effect of Pyrazole and Pyrazole derivatives on the corrosion of Cu has been studied in hydrochloric acid using impedance spectroscopy and polarization methods [25-26]. It was found that inhibitors adsorbed on Cu surface to form Pyrazole-Cu (II) complex protective layer without changing Cu dissolution mechanism. Research of Pyrazole as Cu corrosion inhibitor has largely been done in acidic solutions and very little in alkaline solutions [27] using electrochemical methods. Figure 1 shows molecular structure of BTA. In this study, we report the effectiveness of an Pyrazole compound in the inhibition of Cu corrosion in 8 wt.% TMAH (ph 14) solution and compare it to BTA under the same conditions. The investigation was carried out in-situ using the micropattern corrosion screening technique. Corrosion potentials and currents measured by electrochemical technique, and XPS analyses complemented well with micropattern corrosion screening results to demonstrate efficiency of Pyrazole in Cu corrosion inhibition in TMAH based alkaline post CMP cleaning solution Figure 2.1 Structure of (a) benzotriazole (BTA) and (b) pyrazole 2.2 Literature survey of inhibitor performance The structure of copper-azole complexes Azoles are organic compounds containing nitrogen atoms with free electron pairs that are 43

57 potential sites for bonding with copper and that enable inhibiting action. Various studies of the structure of the Cu(I) [1] and Cu(II) [29 34] azole complexes concluded that the bonding occurs through the N atoms. Other heteroatoms like sulphur, if present along with nitrogen, can also participate in bonding [35,36], and that the coordination number can be as high as four. As these azole ligands are planar, they impose a planar structure on the resultant coordination complexes. However, in some cases complex could not exist in a coplanar form due to the steric hindrance that can be avoided by plane rotation of some of the ligands [28,30 34]. On the other hand, presence of any significant amount of single bond double bond resonance in the Cu N bond, tends to stabilize the completely planar configuration [32]. Steric hindrance can give rise to a negative interaction among the ligands that can introduce porosity to the complexes [28]. However, the crystal structure may further be stabilized by intramolecular and intermolecular hydrogen bonds [30]. Another characteristic of azole compounds is observed and that is their ability to act as polydentate ligands [30-32] Benzotriazole The most commonly used copper corrosion inhibitor of azole type is Benzotriazole (BTA). BTA consists of benzene and triazole ring. BTA can exist in three forms depending on ph value of the solution. In strongly acidic media it has protonated form BTAH + 2, in weakly acidic, neutral and weakly alkaline media its form is BTA, while in strongly alkaline media it is BTA - [37]. According to the E ph diagrams for systems containing Cu and BTA [38], it is possible to predict whether the inhibiting action can be expected. The same is confirmed by the results of Hope et al. [39] who observed that the adsorption of BTA onto the copper surface was reversible at ph levels below 3, and depends upon both solution ph and the applied electrode potential. At 44

58 lower ph the coordination between Cu and BTA as well as the chemisorption of BTAH + 2 onto the electrode surface occur, whereas at higher ph levels the surface layer [Cu(I) BTA] was observed on the electrode. Dugdale and Cotton [40] observed that benzotriazole, unlike the inhibitors that act only through adsorption, can form chemical bond to the metal or metal oxide surface. Thus, once the metal surface has been pre-treated, the effect is long-lasting and does not require renewal. Electrochemical measurement has indicated that corrosion inhibition by benzotriazole does not result solely from the blocking action of the copper benzotriazole complex. Generally about BTA action it can be said that it is anodic copper corrosion inhibitor which action mechanism includes chemisorption, rearly physisorption on the copper surface that follows Langmuire isotherm followed by the formation of complex Cu(I)BTA. Coordination between BTA molecule and Cu electrode surface occurs via nitrogen atom of triazole ring. BTA molecules can be oriented parallel or vertical regarding surface. The orientation of inhibitor molecule is important because of the possibility of formation of stronger bond if the orientation is parallel due to interaction of p electrons of the ring with vacant d orbitals of copper. [41] According to Tromans and Sun [37], BTA can react with the clean copper surface to form the protective film. The study of Xue et al. [42] conducted on the clean copper surface proved that adsorbed BTA anchored directly to the metal. The metallic copper reacts with BTA at a faster rate providing much better corrosion protection than the oxidized surface. It is proposed that the surface moiety is [Cu(I) BTA - ] and the isolated product is [Cu(II) (BTA - ) 2 ]. Hence, BTA can react with copper metal at zero oxidation state under mild conditions to form (benzotriazolato)copper(1+) according to the following mechanism. In neutral solutions, BTA functions as a ligand by means of the unshared pair of electrons on the pyridine -type nitrogen 45

59 (N2). The first step of the reaction is the formation of a [Cu(0) BTA] complex by ligation of pyridine nitrogen of BTA with metallic copper. In [Cu(0) BTA] complex, both the copper atom and the imino group are activated. On the other hand, as the solution is exposed to air, copper metal can adsorb oxygen from solution, to form adsorbed oxygen species, eg, undissociated dioxygen, O 2. It has been reported that the coadsorbed oxygen shows more basic and oxidizing properties than the oxygen in air or in solution. So, under mild conditions, copper metal is prone to donate electrons and BTA can be deprotonated, resulting in the formation of the complex (benzotriazolato)copper(1+) and water. Two of the nitrogen atoms in the benzotriazolium anion are indistinguishable and are equivalent for coordination. Each cuprous cation coordinates with two nitrogen ligands. It is reasonable to propose the structure of (benzotriazolato)copper(l+) to be that of an infinite polymer with the benzotriazolium anions acting as bridging ligands. The third nitrogen atom in the anion can also ligand to copper surface, forming a polymeric protecting layer Pyrazole The inhibiting effect of pyrazole (Pz) on Cu anodic dissolution in 0.1 M HCl was investigated by Geler and co-workers [43]. Based upon the experimental results, the inhibition effect of Pz can be related to an adsorption process which takes place through the lone-pair of electrons of the azole nitrogen and/or the six delocalized p-electrons of the pyrazole ring. Furthermore the presence of pyrazole changes the mechanism of copper dissolution in chloride media, by decreasing the rate of cuprous species formed and increasing that of the cupric species. The action of pyrazole can be physically interpreted as blockage of the metal surface by copper 46

60 pyrazole species produced during the anodic sweep, however, they are poorly adhered to the electrode surface. It is suggested that the use of Pz derivatives with higher electron density would favour the formation of strongly bound complexes. The same is confirmed in the study conducted by Vera et al. [44], where the ability of the tetradentate 1,5-bis(4-dithiocarboxylate-1- dodecyl-5-hydroxy-3-methylpyrazolyl)pentane, BDTCPP to inhibit copper corrosion in 3.5% NaCl solution is investigated. The inhibitor prevents copper corrosion by physisorption on the metal surface, followed by chemisorption of a protective Cu(II)-complex. BDTCPP acts as a tetradentate ligand to form a 1:1 [Cu(II) BDTCPP] square planar complex. The layer covers the substrate completely, making the surface more smooth and homogeneous, without bare copper exposed to the electrolyte. The high inhibition efficiency can be attributed to the good adherence of the complex on the copper surface. These properties can be related to the structural characteristics of the BDTCPP ligand: planar with high p/lone-pair electron density conjugate heterocyclic 4-dithiocarboxylate-5-hydroxy pyrazolyl moiety, with nitrogen and sulphur atoms having high affinity for copper atoms. 2.3 Experimental Procedure Corrosion measurement in this study was done by micropattern corrosion testing technique described elsewhere [13]. As demonstrated in figure 2, copper microdots (ca. 50 nm thick, 130 µm in diameter pattern transferred via contact mask on to Ru, Ta and glass substrates) as well as Ru and Ta substrates on Si were deposited using magnetron sputtering (Desktop Pro, Denton Vacuum). The actual corrosion process of micropattern array is recorded using an optical microscope (Nikon, Eclipse ME600). The corrosion rate (Å/min) can be estimated by the time of complete disappearance of Cu dots after immersion in the probing chemical solution. Bimetallic 47

61 corrosion screening technique has the advantage of allowing the direct observation of corrosion of a metal in direct contact with another metal hence the effect of bimetallic contact can be determined instantly under different conditions. It is also a fast corrosion screening technique as Cu microdots were designed to be thin enough to permit screening many corrosion inhibitors within a short period of time. Figure 2.2 Micropattern corrosion screening structure Pyrazole (98% Sigma-Aldrich), BTA (97% Sigma-Aldrich), and TMAH (25 wt.% Sigma- Aldrich) were used as received. Model aqueous cleaning solution containing 8 wt.% TMAH (ph 14) in water, as a representative of mainstream proprietary alkaline cleaning solution was prepared using pre-purified water (>18.2 MΩ, Millipore integral 3) [6,45]. All electrochemical measurements were done using CHI 760D (CH Instruments) potentiostat. Cu shot, 5mm in diameter was used as the working electrode. The metal electrode was polished down to 0.5 micron mirror polishing and sonicated in de-ionized water. A three-electrode system with Pt as the counter electrode and Ag/AgCl as reference electrode was used in an electrochemical cell. Electrochemical impedance spectroscopy (EIS) measurements were completed by superimposing an ac signal with amplitude of 5 mv peak to peak and frequency range from 100 khz to 50 mhz. 48

62 The EIS results were analyzed using ZSimpWin software. XPS samples were prepared by sputter depositing Cu on Ru substrate (1cm 2 ) and immersing them in 8 wt.% TMAH solution containing 1mM Pyrazole and 10mM BTA for 20 minutes. The samples were rinsed with de-ionized water and blow-dried with dry nitrogen purge. XPS analyses were conducted ex-situ using a PHI 5000 VersaProbe, a multi-technique surface analyses instrument equipped with Al K α ( ev) radiation and dual-gun charge compensation system for analysis of all sample types. 2.4 Results and Discussion Effect of Substrate on Cu Corrosion The nature of bimetallic contact was studied to determine the effect on the rate of Cu corrosion. Figure 3a shows the time-lapsed images of Cu micropatterns on three different substrates; Ru, Ta and glass submerged in the alkaline TMAH (8 wt.%, ph=14). Ta is currently used as part of the diffusion barrier for Cu interconnects in integrated circuit devices [46-47] while Ru is a new promising candidate for liner metal because Cu can be directly plated onto Ru without Cu seed [48-49]. Glass was chosen as a non-conductive dielectric substrate. As shown in Figure 3, Cu microdots on Ta substrate required over 4X the amount of time as compared to Cu microdots on Ru substrate to corrode completely in TMAH ph=14 solution. Based on the micropattern screening, the Cu corrosion trend follows Cu/Ru (27 min) > Cu/Glass (56 min) > Cu/Ta (120 min). Tantalum has a strong tendency to be oxidized and form tantalum oxide (Ta 2 O 5 ) especially when exposed to aqueous medium, Eq. 1. 2Ta + 5H 2 O = Ta 2 O H e (1) 49

63 Ta exhibits a thermodynamically favorable oxidation reaction that donates electrons through the Cu/Ta bimetallic contact. This results in cathodic protection of Cu microdots. On the other hand, Ru is nobler than Cu therefore Cu oxidation is facilitated through Cu/Ru bimetallic couple. Cu microdots on glass was used to represent Cu only corrosion case and as expected it exhibited a corrosion rate that is in between Cu/Ru and Cu/Ta [13]. Figure 2.3 Time lapsed images of Cu microdots deposited on Ru, Ta and glass in 8 wt.% TMAH solution To confirm the trend of galvanic corrosion, Tafel plots (figure 2.4) were recorded for Cu, Ru and Ta in TMAH ph 14 solution. The corrosion potentials mostly followed a general trend of E corr, Ru > Ecorr, Cu > E corr, Ta which correlated well with the expected metal nobility trend. 50

64 Figure 2.4 Tafel plots of Ru, Cu and Ta measured in TMAH ph 14 solution Cu Micropattern Corrosion and Inhibition Chemical composition of post CMP cleaning solution has a direct effect on bimetallic Cu corrosion process. The continued shrinking of Cu interconnects features in order to satisfy Moore's law has led to increased need to minimize Cu corrosion during post CMP cleaning to an industrially accepted rate of <1Å /min. The Pourbaix potential-ph diagram indicates that alkaline solution condition could facilitate Cu to corrode more readily. Cu bimetallic contact with Ru leads to accelerated corrosion which was confirmed using micropattern corrosion screening technique. The nobility of Ru in TMAH solution and with addition of Pyrazole and BTA was determined using Tafel plots. The i corr and E corr were found to have negligible difference for the three plots therefore confirming that Ru in TMAH is not affected by the presence of corrosion inhibitors.the proposed mechanism of Cu/Ru corrosion is shown below. 51

65 Ru cathode Slow O 2 2H 2 O 4e 4OH ( ORR) Oxygen reduction reaction Cu anode 2Cu 2H O Cu O 2H 2e Cu Cu O 2H OH 2 2Cu 2 H O 2e Slow Cu( OH) 2 2 Figure 2.5 shows corrosion rate of Cu microdots on Ru in 8 wt.% TMAH with respect to increasing concentrations of Pyrazole and BTA. Figure 2.6 displays the time lapsed images of Cu microdots after immersion in the same TMAH solution and with 10mM BTA and 1mM Pyrazole. The time required to completely erode 50nm Cu dots can be used to gauge the relative rate of corrosion. The corrosion rate can therefore be estimated to be inversely proportional to corrosion time. The relative Cu corrosion rate of Cu/Ru micropattern in 8 wt.% TMAH (27Å /min) only partially diminished after addition of up to 10mM BTA (8Å /min). Comparatively, Cu corrosion rate of Cu/Ru micropattern was significantly reduced (<1Å /min) after addition of 1mM Pyrazole in 8 wt.% TMAH solution. Further increase in Pyrazole concentration did not result any substantial corrosion rate decrease. From the results, we conclude that 1mM Pyrazole is needed to achieve the industry acceptable Cu corrosion rate of <1Å /min. Furthermore, it is desirable to keep the inhibitor concentration very low because the excess inhibitor on Cu surface becomes the source of organic contamination that can affect the conductivity and interlayer adhesion of the subsequent layers. This demonstrates that Pyrazole is a better candidate for Cu corrosion inhibition in highly alkaline TMAH solution as compared to the industrial standard, BTA. All subsequent experiments in this paper were carried out using 1mM Pyrazole and 10 mm BTA. 52

66 Figure 2.5 Inhibitor concentration dependent etch rate of Cu in 8 wt.% TMAH Figure 2.6 Time lapsed images of 50nM Cu/Ru immersed in 8 wt.% TMAH with additional 1mM Pyrazole and 10mM BTA 53

67 2.4.3 Electrochemical Analysis Tafel Plots The trend of Cu corrosion was also monitored using Tafel plots. The rate of corrosion can be theoretically calculated from corrosion current which is obtained from extrapolation of anodic and cathodic curves. As shown in Figure 2.7, the corrosion current follow the trend of i corr, no inhibitor> i corr, 10mM BTA > i corr. 1mM Pyrazole which correlated well with the results from micropattern corrosion screening while the E corr remain relatively the same. The disadvantage of Tafel plot method is that the corrosion rate is obtained from fresh electrode/solution and the study is limited to short term study only. Furthermore, corrosion study by Tafel plots is based on a single metal electrode and cannot be directly applied to bimetallic corrosion which are both addressed by micropattern screening technique. Figure 2.7 Tafel plots of Cu in 8wt.% TMAH and with Pyrazole and BTA 54

68 Electrochemical Impedance Spectroscopy (EIS) EIS is an effective technique that is used in the analysis of various steps involved in an electrochemical reaction by measuring the response of impedance system to a small ac potential in a wide frequency range [50]. It provides a method for measuring the resistance against the transfer of ionic species to the metal surface and has been used to evaluate the barrier properties of corrosion inhibitors [51-52]. Corrosion of Cu in 8 wt.% TMAH solution in the presence of BTA and Pyrazole inhibitors was investigated by EIS to substantiate the aforementioned effectiveness of Pyrazole as Cu corrosion inhibitor. The results of the successive impedance scans are shown in figure 2.8 in the form of Nyquist plots. All the impedance data were fitted using an equivalent circuit shown in figure 2.8 (inset) where R s represents the solution resistance, R dl is the charge transfer resistance, Q dl is double layer constant phase element (CPE), R f is layer resistance and Q f (CPE) represent the protective properties of the film on Cu. This equivalent circuit has been used to fit impedance data in studies of corrosion inhibition by organic coatings [51, 53-54]. As seen in the Nyquist plot (figure 2.8), addition of BTA and Pyrazole increases the charge transfer resistance of the Cu electrode in TMAH solution. This is arrived at by evaluating the diameter of the semicircular Nyquist plots which clearly increases with the addition of BTA and Pyrazole to TMAH. The increase in charge transfer resistance could be due to the adsorption of inhibitor molecules on the Cu surface which modifies the surface by decreasing the electrical capacity because of displacement of water molecules and other ions originally adsorbed on the surface. It is particularly important to note that the charge transfer resistance with addition of Pyrazole is ~ 10X higher than that of the TMAH solution contains BTA. This confirms the 55

69 formation of a denser protective layer on the surface of Cu by pyrazole compared to the one formed by benzotriazole which explains why Pyrazole exhibited effective corrosion inhibition shown by Micropattern corrosion screening. Figure 2.8 EIS data (a) Nyquist plot of Cu in TMAH (black), TMAH +BTA (red) and TMAH+Pyrazole (blue) and inset equivalent circuit used to fit data Water Contact Angle Measurement Surface tension or wettability of Cu surface is an important aspect in post CMP cleaning. It has direct impact on the cleaning results as a hydrophilic surface enables relatively easy flushing away contaminants and minimizes watermarks [55]. Cu samples were submerged in TMAH solution and TMAH with 1mM Pyrazole and different concentrations of BTA for 20 minutes followed by DI water rinse and air-dry and subsequent contact angle measurement on the treated surfaces. As shown in Figure 2.9, significant differences of wettability of Cu were 56

70 observed among the test solutions. The TMAH solution containing BTA produced hydrophobic surface and hydrophobicity increased with concentration. Surface hydrophobicity of BTA/TMAH-treated Cu surface is commonly attributed to the specific orientation of BTA molecules relative to the Cu surface where hydrophobic benzene ring is facing away from Cu surface and therefore forming a protective hydrophobic barrier [56]. The TMAH solution containing Pyrazole produced a relatively hydrophilic Cu surface after immersion compared to the ones treated with BTA/TMAH solution. This is likely due to relatively smaller size of Pyrazole molecule owing to the absence of large hyrdrophobic benzene moiety. As a result, a thin hydrophilic Cu-Pyrazole complex is adsorbed Cu surface. Figure 2.9 Variation in DI water contact angle of Cu in TMAH and TMAH+ inhibitor 57

71 2.4.5 Surface Analysis XPS Analysis XPS analysis was done on three substrates; bare Cu, Cu immersed for 20 minutes in TMAH with 10mM BTA and TMAH with 1mM Pyrazole. Figure 2.10 present the spectra of Cu 2p and auger spectra (Cu LMM ). From the results, it is clear the chemical surface states are different for the three substrates. Bare Cu contains CuO and CuOH which is indicated by Cu 2p 3/2 at ev and Cu LMM at ev (Figure 2.10a and b).the intense shake up satellites around ev indicates the presence of Cu 2+ which is evoked by the availability of unfilled d-orbitals (d 9 ) [57-59]. The peak at ev is attributed to either Cu 2 O or metallic Cu. The difference in energy between Cu 2 O and Cu is about 0.1eV [60] and it was difficult to distinguish the two with the resolution of our instrument. Cu LMM spectra on the other hand has a difference of about 2.6 ev between Cu 2 O (570.5 ev) and Cu (567.8 ev). This is as a result of the relaxation energy difference between the materials [61]. In bare Cu analysis, Cu 2 O is not present because of the absence of the characteristic Cu LMM peak at ev. The Cu 2p 3/2 component of Cu immersed in TMAH solution with 10mM BTA (Figure 2.10c) shows a sharp peak at ev which is attributed to Cu 2 O or metallic Cu. This is confirmed by the Cu LMM peak at ev (Figure 2.10d) which is typical of Cu 2 O. This indicates that a protective film is present on Cu surface, but protection is not effective based on micropattern corrosion screening results. Cu substrate immersed in TMAH solution containing 1mM Pyrazole showed a sharp peak on Cu 2p spectra at ev (Figure 2.10e). The actual identification was confirmed by Cu LMM spectra with 2 peaks, the main peak at ev (Figure 2.10f) that is typical metallic Cu and another peak at ev that is Cu 2 O [62-63]. This reveals the absence of Cu 2+ on the substrate treated with Pyrazole and could be as a result of formation 58

72 of Cu(1)-Pyrazole complex on Cu surface. The presence of the Cu 2 O on the surface after treatment with Pyrazole may suggest the adsorption process involves oxidation of Cu atoms to Cu + followed by chemisorption of the inhibitors. Figure 2.10 XPS Cu 2p spectra of; (a) bare Cu, (b) BTA modified Cu and (c) Pyrazole modified Cu and Cu LMM spectra of; (d) bare Cu, (e) BTA modified Cu and (f) Pyrazole modified Cu Figure 2.11 is the N1s spectra of three Cu substrates. As expected, bare Cu (Figure 2.11a) depicts absence of N1s peak. The substrate containing BTA in TMAH (Figure 2.11b) has a single and significantly narrower peak located at ev. The narrower N1s peak indicates that the charge is evenly distributed by the conjugated π structure delocalized over the two N atoms and both N1 and N3 are equivalent [64-65]. This could signify absence of Cu-N bonding and 59

73 explanation of ineffective corrosion inhibition as depicted by micropattern corrosion screening. The sample treated with Pyrazole (Figure 2.11c) has two peaks; one at ev and the other one at ev. This shows that the two N atoms are not equivalent which may indicate that one of the N atoms in Pyrazole molecule is bonded to Cu(1) or Cu as bonding changes the electron environment of one N atom compared to the second one [64]. This therefore demonstrates that the effectiveness of Pyrazole in inhibition of Cu corrosion is through formation of Cu-Pyrazole complex to form the inhibition film. Figure 2.11 XPS N1s spectra of: (a) bare Cu, (b) BTA modified Cu and (c) Pyrazole modified Cu 2.5 Proposed Mechanism of Cu Corrosion Inhibition Based on the data obtained we propose a mechanism of Cu corrosion inhibition in TMAH 60

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79 Chapter 3 INFRARED SPECTROSCOPIC CHARACTERIZATION ON LOW-K DIELECTRIC NANOSTRUCTURE TO OPTIMIZE Cu INTERCONNECT FABRICATION 3.1 Introduction In advanced copper interconnect design, integration of porous ultra low-k (ULK) interlayer dielectrics (ILD) into Cu interconnect nanostructure will continue to pose unprecedented technological challenges in back-end-of-line (BEOL) fabrication beyond 10 nm. Porous dielectrics with carbon-doped silicon oxide (CDO) structural framework can reduce resistancecapacitance (RC) delay. But, replacing stronger Si-O bonds with more reactive Si-C bonds makes CDO easily subjected to collateral plasma-induced damages during reactive ion etching (RIE) pattern transfer and subsequent photoresist ashing. Furthermore, more stringent critical dimension (CD) requirements at sub-10 nm node demands low-k dielectric nanostructure to be fabricated ideally with no damage or minimum (< 1 nm) dielectric damages. Recent development of atomic layer etching process is aimed to achieve ultimate CD control by precisely controlling the etch variability down to atomic level [1]. Given these technological challenges, it is imperative to have a sensitive metrology that can guide every processing steps during the systematic development of plasma etching, restoration and cleaning processes of low-k dielectric nanostructure. Current characterization tools for low-k nanostructure, illustrated in Figure 3.1, provide useful information on cross-sectional profile, 66

80 elemental composition and crystal structure. But, these methods do not yield the chemical bonding information needed to accurately evaluate the ULK ILD integrity, chemical structure of etch residues and how that affects the subsequent cleanability. Figure 3.1. Characterization tools to assist low-k dielectric nanostructure fabrication. From the chemical bonding prospective, the success on fabricating next generation Cu interconnect is really hinged on the ability to control chemical bond breaking and bond formation in highly selective time sequences within restricted nanometer dimension. In this work, we developed a new metrology approach using Multiple Internal Reflection Infrared Spectroscopy (MIR-IR) and associated IR techniques to probe intricate chemical bonding transformation taking place across fluorocarbon etch residues and low-k dielectric interface after plasma etching, ashing, UV curing and post-etch cleaning. New insights obtained on the chemical bonding modification across ULK ILD interfaces, combined with commonly used characterization tools, can facilitate continuous development of plasma etching and post-etch cleaning processes to achieve CD control and minimize dielectric damages for sub-10 nm node. 67

81 3.2 Experimental Procedure Characterization of Dielectric Trench Pattern Trench pattern structures on CDO dielectrics were generated by conventional fluorocarbon based RIE, followed by conformal model fluorocarbon polymer (MFP) deposition using CHF 3 /C 4 F 8 /Ar plasma. Wafers were used directly to prepare attenuated total reflectance (ATR) coupons as reported previously [2]. Both transmission (T-IR) and MIR-IR spectra were measured using Nicolet is50 FT-IR spectrometer, constantly purged under dry air (< 0.1 ppm CO 2 ) to avoid any added contamination. All spectra were collected at 2 cm -1 resolution and are the average of 200 individual spectra. XPS analyses were conducted using a PHI 5000 VersaProbe equipped with Al K α ( ev) radiation. Cross-sectional images were obtained with a Hitachi S-5500 FE-SEM Functional Group Specific Chemical Reaction All chemicals were reagent grade or higher purity, purchased from Sigma-Aldrich and used without further purification. A 2,4-dinitrophenylhydrazine (DNPH) solution was used for C=O identification, where coupons were immersed in a solution heated at 70 C for 16 hours. For C=C, wafer coupons were immersed in bromine solution at 50 C for 1 hour. A Na/naphthalenide etchant (Creative Engineers, Inc.) was used for reductive defluorination reaction. All wafer coupons were rinsed thoroughly with ample amount of ultrapure water and isopropyl alcohol followed by N 2 blow dry before characterization UV Treatment + Wet Clean 68

82 A broadband UV source (160 nm-1100 nm) using specific filters to cut off emissions below 230 nm was used to expose MFP films in atmospheric conditions. Following UV treatment, the samples were subsequently cleaned at 60 C for 4 minutes in a proprietary cleaning solvent typically used in high volume manufacturing interconnect processing for post-etch polymer removal. A second TMAH (tetramethylammonium hydroxide) based proprietary cleaning solvent (ph > 13) was also used for post-etch residues cleaning from test wafer by immersion at 50 C with sonication. 3.3 Results and Discussion Chemical Bonding Mapping of Low-k Trench Structure after Fluorocarbon Plasma Etch Figure 3.2 shows the FT-IR spectra of ~300 nm blanket CDO dielectrics on Si (100) substrate collected using two different FT-IR measurement configurations, MIR-IR and T-IR. MIR-IR showed higher spectral sensitivity with superior resolution of characterizing a thin film compared to conventional T-IR. The dominant IR peak absorption at 2970 cm -1 assigned to asymmetric stretching (a-ch 3 ) of the C-doping in CDO was found to increase by 20 folds in MIR-IR (178 mabs vs. 9 mabs in T-IR). Previous studies in our lab have shown that MIR-IR spectroscopy can increase detection capability by up to 100 fold in determining these critical chemical bonding information from various substrates [3-4]. In addition, other intrinsic important bonding features like the Si-OH, O-SiH x, SiH x, C=O and C=C within CDO film can be observed in MIR-IR spectrum with high clarity, Figure 3.2. The increased sensitivity in MIR-IR is made possible by using a higher refractive index silicon wafer substrate itself as an internal reflection element waveguide allowing for multiple total internal reflections (~80 reflections from a 60 mm Si crystal). Our previous studies have successfully demonstrated MIR-IR as a suitable metrology to 69

83 characterize chemical bonding of hydrogen termination, trace organic adsorption and plasmadeposited polymer thin film deposition on silicon wafer surface with sub-monolayer sensitivity [5-7]. Figure 3.2. IR spectra of 300 nm CDO film obtained from MIR-IR and T-IR; mabs = miliabsorbance unit. A successful pattern transfer by RIE etching, a critical step for fabricating a low-k trench structure, requires both excellent CD control and minimized dielectric damages. While cross section SEM can provide CD measurement, MIR-IR and T-IR with their inherent capability of characterizing 3-dimensional patterned nanostructures are best suited to assess chemical bonding transformation took place after each processing step. As shown in Figure 3.3, obtained IR spectra 70

84 provide detailed chemical bonding signatures across porous low-k trench nano-structure after RIE etch process. Figure 3.3. Chemical bonding mapping of low-k trench nano-structure formed after fluorocarbon plasma etch process. The IR spectra provide chipmaker a useful chemical bonding mapping to use as benchmarking criteria, more discussion at later section, for evaluating 1) low-k dielectric integrity 2) etch residues removal and 3) processing induced dielectric damages. Specifically, Figure 3.3 shows sharp and intense IR peak absorptions at 2970 cm -1 (a-ch 3 stretching) and 1275 cm -1 (Si-CH 3 bending) that are attributed to the critical CH 3 groups (C-doping) in the SiOCH matrix, incorporated to reduce the overall k value of the dielectric material. The broad and dominant absorption band in the region cm -1, assigned to Si-O-Si backbone can be 71

85 deconvoluted (not shown) in three major peaks centered at ~1160 cm -1 (Si-O cage structure), ~1060 cm -1 (Si-O network) and ~1025 cm -1 (suboxide) represents the bulk constituents of the dielectrics [8-9]. Other minor contribution of the low-k film can also be detected on the spectrum (CH 2 at 2930 cm -1, O-SiH x at 2250 cm -1 and SiH x at ~2165 cm -1 ). As for post-etch etch residues the broad vibrational band in the region cm -1 carrying multiple bonding signatures can be accounted for chemical bonding features of thin post-etch fluorocarbon residues (<2 nm) across dielectric surface (Figure 3.3 inset). The chemical identity of thin polymeric etch residues was later deduced by functional group specific chemical reactions, described in details in the following sections. In addition, the reactive species in the plasma such as the radicals as well as the energetic ions impingement can easily break the weaker Si-CH 3 bonds causing to form undesirable surface Si-OH groups, indicative of plasma induced damage to low-k dielectrics. Figure 3.3 show that the broad absorption band centered at ~3440 cm -1 is a contribution from Si- OH bond formation, a useful gauge for water sensitivity of damaged low-k, after etch process MIR-IR Evaluation of Plasma Damage to Trench Low-k from Different Oxidative Strip Processes Plasma processing used in BEOL fabrication poses serious issues to fragile porous low-k and can hinder successful Cu/Low-k integration. Moreover, oxidative plasma stripping of resist stack is known to cause severe damage by abstracting the critical methyl groups (Si-CH 3 ) from the dielectric material. This results in formation of new polar Si-OH bonds that readily adsorbs water molecules (k ~80), raising the overall dielectric constant of the material. In this study, four low-k trench structures, treated with different oxidative strip processes (strip 1 4) targeted to optimize processing gases and chamber pressure to deliver reduced O radical content, were analyzed by 72

86 MIR-IR and T-IR. Since surface anchored Si-OH groups are highly prone to water sensitivity, measuring the change in Si-OH peak intensity can provide a direct assessment of plasma induced damage. Figure 3.4. (a) MIR-IR spectra of low-k trench structures after different oxidative plasma strip processes (strip 1 4) to optimize O radical content and (b) respective plot of OH increase and Si-CH 3 decrease from MIR-IR and T-IR respectively. As shown in Figure 3.4a, MIR-IR show that strip 1 process imparts less damage, quantified from lower Si-OH peak intensity, while strip 4 shows the highest plasma induced damage to low-k dielectrics, also separately verified by MIS structure for increased k value measurement. The trend of Si-OH IR peak absorption from all strip processes corroborates well with the trend of methyl (Si-CH 3 ) loss in the dielectric film from the same. Respective plots of IR peak heights from MIR-IR and T-IR spectra, Figure 3.4b shows that lower Si-OH peak intensity corresponds to retaining maximum C-doping in the dielectric trench structure. Our data demonstrate that MIR-IR metrology is fully capable of differentiating the extent of plasma damage from four 73

87 different stripping processes. Recently, our work has also shown that the enhanced sensitivity of MIR-IR metrology is well suited for dielectric damage quantification on deep sidewalls trench structure to identify the increase in Si-OH and C=O and the decrease in Si-CH 3 correlated well with increased plasma induced damage [10] Characterization of Chemical Bonding Structure of Model Fluorocarbon Polymer To achieve accurate profile control and minimal dielectric damage in anisotropic etching of organosilicate-based dielectrics, fluoropolymer thin films are deliberately deposited on the contour of patterned trench nanostructures [11]. A selective and complete removal of these fluorocarbon polymers to ensure no interference with subsequent silicidation and contact formation is made extremely difficult due to its chemical inertness. In order to develop an efficient clean step with understanding of detailed chemical bonding information, we have deposited thin MFP film, mimicking post-plasma-etch residues, on low-k patterned trench structures for FT-IR characterization. Figure 3.5 compares infrared spectra of MFP film deposited by standard fluorocarbon-based plasma (CHF 3 /C 4 F 8 /Ar) only and with additives (NF 3 and O 2 ). Spectral identification corresponding to different film thicknesses (28 nm and 6 nm) was possible based on differential absorption intensities owing to precise background cancellation method. The strongest absorption band in top spectrum (28 nm MFP film), observed in the region cm -1, contains multiple vibrational signatures. Symmetric and asymmetric stretching vibrations of CF 2 group (1180 and 1230 cm -1 respectively) as a broad band indicated to a complex non-linear structure of MFP film compared to simple linear polytetrafluoroethylene chain. The broad absorption band in the range cm -1 was accredited to olefinic unsaturation with various 74

88 degrees of fluorination and carbonyl group. Presence of branching ( pendant ) CF 3, terminal CF 3, cross-linking CF fractions were also observed at ~1345 and 930 cm -1 respectively [12]. Figure 3.5. (a) FT-IR spectra of 28 nm and 6 nm MFP films deposited by CHF 3 /C 4 F 8 /Ar plasma chemistry only and added feedstock plasma gases (O 2 and NF 3 ) to CHF 3 /C 4 F 8 /Ar plasma recipe; (b) corresponding cross-sectional SEM images (MFP film in green color). In interest of modifying fluoropolymer structure, effect of plasma chemistry with added gases (NF 3, O 2 ) was also studied. According to fluorocarbon-based RIE chemistry, CF x (x = 1-3) and F fragments are generated from parent fluorocarbon plasma where F is the efficient etching agent 75

89 and CF x radicals conflate to deposit fluoropolymer residues. Feedstock additives like O 2 and NF 3 affect the radical concentrations and hence grow fluorocarbon films of variable thickness and functionalities [13]. Accordingly, overall reduction in absorption band intensities were observed in Figure 3.5 when additives were added to the original fluorocarbon plasma recipe optimized for 28 nm fluoropolymer film deposition. Reduction of pendant CF 3 bending vibration band (930 cm -1 ) in both O 2 - and NF 3 -added plasma conditions supports CF 3 radical extinction effect by the additives. Figure 3.6. Representative model chemical bonding structure of functionalized fluorocarbon polymer derived via reductive defluorination of CF x groups, DNPH hydrazine formation of carbonyl and bromination of olefin unsaturations. Differential spectra (i) were obtained by subtracting as-deposited MFP film spectra (ii) from the corresponding derivatized MFP film spectra (iii). 76

90 To decipher overlapping vibrational bands, specific chemical derivatization reactions were utilized to verify and confirm functional groups such as carbonyl, olefin and CF x units embedded in fluoropolymer chain. Differential infrared spectrum from each derivatization reaction clearly showed associated bonding transformations. A schematic representation of the MFP residue based on the assigned vibrational modes is proposed in Figure 3.6 with functional group specific chemical derivatization reactions and corresponding FT-IR spectra. The model bonding structure manifests that the MFP residue consists of lower molecular weight (oligomeric) fluorocarbon configuration with substantial cross-linking and branching. Sodium/naphthalenide solution is capable of breaking strong C-F bond via energetic radialanion mechanism. Progressive reductive defluorination process of 28 nm MFP film is seen by gradual decrease of CF 2 asymmetric stretching vibration band at 1180 and 1230 cm -1, Figure 3.6. Observation of increasing absorption bands at 1393 and 1611 cm -1 were accounted to the alkene bonds formed after defluorination reaction. Brady s test, a classical chemical analysis for carbonyl functionality based on the condensation-dehydration reaction between an aldehyde or ketone and 2, 4-dinitrophenylhydrazine (DNPH) to form hydrazone adduct, was used to test the presence of carbonyl. The decreasing absorbance band at 1710 cm -1 belongs to C=O stretching absorption. Emergence of strong 1619 cm -1 peak is assigned as C=N stretching frequency cm -1 and 1505 cm -1 bands are observed as NO 2 symmetric and asymmetric stretching vibrations respectively. Band at 1596 cm -1 bands was noted as amine bending vibration. Br 2 addition to alkene bond results usual vicinal di-bromo products along with a di-hydroxy product in presence of water/moisture. Increase in C-O bonds (~1100 cm -1 ), OH stretching ( cm -1 ), OHand Br- substituted C-F (~1400 cm -1 ) and reduction of C=C bonds centered around 1670 cm -1 confirmed olefin fractions in MFP structure. In line with the proposed molecular structure model, 77

91 these active unsaturated sites (olefin, carbonyl) are potentially more vulnerable to chemical attacks and therefore can be strategically targeted for improved post-etch residue removal. In the following sections, we explore various wet cleaning methods for the unreactive fluoropolymers from trench patterned structures Wet Clean of Post-Plasma Etch Residues Removal of post-etch residues from deep trench sidewalls and bottom has posed serious challenges especially with smaller technology node structures (higher aspect ratio). More complex and stringent requirements are expected to achieve efficient residues removal without degrading the properties of underlying porous low-k stack [14]. Figure 3.7 shows the time dependent differential IR spectra of patterned low-k test wafer treated with TMAH-based proprietary cleaning solution, targeted especially for etch residues removal. The increasing negative absorbance of CF x stretching band at 1236 cm -1 with treatment time is evidence of progressive polymer removal by wet cleans. In addition, concomitant damage to the underlying porous low-k structure during the wet process was observed in the corresponding IR spectra as characteristic vibrational bands of Si-O-Si array and -CH 3 doping in a low-k microstructure (1024 cm -1, 1178 cm -1 and 1276 cm -1, attributed to siloxane network, siloxane cage and Si-CH 3 bending vibrations respectively). Such low-k structural degradation associated with cleaning process is highly undesirable to obviate device reliability issues. The FT-IR method, therefore, provided a crucial guiding tool to monitor both the fluoropolymer removal efficiency and the damage associated by wet treatment, suggesting the need for a highly selective cleaning formulation that only targets the deposited etch residues polymer without dielectric impairment. 78

92 Figure 3.7. Time dependent proprietary cleaning solvent treatment for post-etch residue removal on patterned low-k test wafer UV-Assisted Wet Clean of Fluorocarbon Polymer In search of a less aggressive and more efficient fluoropolymer removal method, we considered introducing/modifying functionalities within the stable fluorocarbon chain and study its effect on removal efficiency in a subsequent wet clean step. We investigated UV-interactions of model fluorocarbon polymer in atmospheric conditions and its effect on subsequent wet removal efficiency. First part of the investigation includes characterization, structural modification and material loss of 28 nm MFP film upon UV/air irradiation. Top transmission IR 79

93 spectrum in Figure 3.8 representing as-deposited MFP film demonstrates major vibrational features (symmetric and asymmetric CF 2, terminal CF 3 stretching, CF bending, as internal CF 3 vibration, C=O stretching and fluorinated C=C stretching) as identified earlier in Figure 3.5. Progressive UV irradiations in atmospheric condition increasingly reduce CF x band intensity and width. Significant decrease of both the main CF 2 band and C=C/C=O broad band after 300 sec of irradiation indicated major material removal by UV/air exposure. Blue shifted and increased IR band at ~1752 cm -1 during early was assigned to fluorinated C=O groups that were formed due to UV-induced bonding transformations during initial stages of UV/air treatment [15]. Post-UV exposure, the wafers were subsequently wet cleaned which removed remaining structurally modified and more hydrophilic fluorocarbon film as evident from further reduction of characteristic CF 2 stretching band. Finally, a 300 sec UV/air irradiation followed by wet clean was demonstrated to remove the deposited 28 nm fluoropolymer film completely. Separate control experiments (not shown) showed that without UV treatment and O 2 rich experiments, no fluoropolymer removal was achieved by the wet clean formulation alone [16]. UV/air treatment dissect hydrophobic fluorocarbon chain and incorporate hydrophilic O-containing groups that are potentially vulnerable to subsequent solvent/cleaning reagents for better dissolution. 80

94 Figure 3.8. FT-IR spectra of as-deposited (no UV) 28 nm MFP film, 180 sec UV-treated and 300 sec UV-treated MFP film with subsequent wet clean. To support FT-IR data, XPS and SEM analyses were conducted ex-situ as shown in Figure 3.9. The C 1s and F 1s XPS spectra exhibited all the functional groups associated to the as- 81

95 deposited MFP film. F 1s XPS spectra (Figure 3.9b) confirmed only partial removal of fluorine after 300 sec UV/air treatment. Figure 3.9. XPS analysis of (a) C 1s and (b) F 1s of UV- treated and subsequent wet cleans on MFP, (c) SEM images of the corresponding UV and wet cleans on MFP film. However, 300 sec UV/air + wet clean step caused complete fluorine removal. Partial removal of CF x components of MFP after 300 sec UV irradiation was also affirmed by corresponding C 1s XPS spectra (Figure 3.9a). After wet clean, these CF x bands disappeared, and a narrow band at 283 ev remained, which corresponds to Si-CH 3 originating from a post-clean CDO low-k trench structure. SEM images (Figure 3.9c) also indicated progressive material loss with UV irradiation 82

96 time. Part of the improved wet clean efficiency post-uv exposure is contributed to incorporation of hydrophilic groups in the fluorocarbon remnant after UV/air irradiation, as illustrated in Figure Carbonyl centers in model fluorocarbon polymers are more electrophilic due to the adjacent electronegative F atoms. Guthrie and co-workers showed that replacing all six α- hydrogens of acetone by fluorine atoms makes acetone a billion times more reactive to water nucleophile [17]. Resulting hydroxyl groups in as-deposited film after UV/air irradiation can be observed by increased band centered at ~3200 cm -1. Formation of alcohol groups (C OH) render UV-modified MFP more hydrophilic and therefore easier to wet-clean via aqueous/semi-aqueous formulations. Figure IR spectroscopic evidence of progressive hydroxyl formation during UV-air treatment that turn the fluoropolymer film more hydrophilic for improved wet removal. 83

97 3.3.6 Mechanism of UV-Induced Structural Disintegration of Fluorocarbon Polymer To account for the UV-induced bonding transformations occurred in MFP structure, a possible mechanistic route involving photo-interactions between the MFP and incident UV light under atmospheric conditions is proposed. As depicted in Figure 3.11, MFP film first absorbs incident UV light (λ 230 nm) via its carbonyl chromophores to form an excited state, which then transfers its excess energy to ambient triplet O 2 molecules to generate singlet oxygen ( 1 O 2 ). Energetic 1 O 2 then reacts with π bonds in fluoro-substituted olefins embedded in MFP to form 1,2-dioxetane ring intermediate. Finally the strained four-membered ring dissociates into carbonyl products, detected by arisen 1752 cm -1 vibrational band (Figure 3.8). Figure Proposed mechanism of UV-assisted fluorocarbon chain dissociation involving excited singlet oxygen via photosensitization of carbonyl groups. Specific suggestions in the proposed mechanistic pathway are inferred from organic photochemistry principles, literature examples and control experiments and are summarized as following: (i) fragmentation via direct scission of strong C-F bond ( kj/mol) is ruled out due to the low energy UV ( 230 nm), (ii) although both olefins and carbonyls are good 84