Direct Analysis of Photoresist Using Inductively Coupled Plasma Mass Spectrometry (ICP-MS) Application Semiconductor Author Junichi Takahashi Koichi Yono Agilent Technologies, Inc. 9-1, Takakura-Cho, Hachioji-Shi, Tokyo, 192-0033, Japan Abstract A simple method for analyzing photoresists using inductively coupled plasma mass spectrometry is discussed. It is possible to freely aspirate a 10-fold diluted photoresist sample (30% resin) into an inductively coupled plasma mass spectrometer which has been specially designed to handle high matrix samples. The Agilent 7500s with ShieldTorch System features an advanced RF generator which produces a highly robust and high temperature "cool" plasma. The unique combination of the ShieldTorch and cool plasma conditions give effective interference removal of carbon and argon based interferences on 24 Mg ( 12 C 2 ) and 52 Cr ( 40 Ar 12 C) and 39 K ( 38 Ar 1 H), 40 Ca ( 40 Ar) and 56 Fe ( 40 Ar 16 O), which would otherwise limit standard quadrupole inductively coupled plasma mass spectrometry operation for this application. Introduction Manufacturing integrated circuits (ICs) is a complex process involving numerous steps over many weeks. Without constant testing, metal contaminants can be unknowingly introduced at any step of the manufacturing process and particularly during the critical lithography stage. Once a layer of material, such as an oxide layer, is grown or deposited onto the silicon wafer surface, a lightsensitive liquid photoresist layer is then applied. After it has cured, the photoresist prevents etching or plating of the area it covers. There are several different classifications of resists. This application note considers the analysis of the p-type, or positive resist, which becomes soluble when exposed to ultraviolet light. When processing a wafer with a positive resist, a mask having the required template is aligned between an ultraviolet light source and the wafer. Ultraviolet light shines through the clear portions of the mask thereby exposing the template onto the photosensitive resist. The exposed resist becomes soluble to the developer, for example, tetra-methyl ammonium hydroxide (TMAH, 2.38%), and is removed from the wafer surface. The mask pattern is then etched onto the wafer using either a wet or dry etching process, the remaining undeveloped/hardened photoresist is removed and the process is repeated. Metal impurities present in the photoresist can cause a distortion of the electrical properties and reliability of the final semiconductor devices so acceptable limits of impurities are constantly being reduced. Typical acceptable levels of metallic impurities in the photoresist are in the range 10 30 ppb per element with typical elements of interest including: Na, Mg, K, Ca, Cr, Mn, Fe, Ni, Cu and Zn. Consequently, monitoring these elements in photoresist at ultratrace levels is critical and is routinely carried out by photoresist suppliers and some integrated circuit manufacturers.
In the past, photoresist was prepared by acid digestion, followed by analysis using graphite furnace atomic absorption spectrometry (GFAAS). As well as being time-consuming and potentially dangerous, acid digestion leads to the loss of volatiles, for example, boron and arsenic. Other limitations of this approach include the potential introduction of contaminants from the apparatus, acid mixture and other reagents, and the poor sample throughput capabilities of GFAAS. In contrast, photoresist can be analyzed directly for multiple elements using inductively coupled plasma mass spectrometry (ICP-MS), following a simple 1:10 dilution in a suitable solvent. The solvents used will depend upon the chemical composition of the resist, with more common solvents including N-methyl-2-pyrrolidone (NMP), propylene glycol monomethyl ether (PGME) and ethyl lactate. The purity of the solvent is a potential limitation to this approach since detection limits in the photoresist are limited by the level of impurities in the diluent. Also, since some photoresist products are 30% resin, the heavy resist matrix could clog the nebulizer, torch, interface, and drain or could suppress analyte signals. Finally, carbon based spectral interferences, such as C 2 and ArC, can limit the measurement of 24 Mg and 52 Cr respectively. Instrumentation The difficulties associated with analyzing heavy sample matrices, such as instrument clogging and signal drift, can be avoided through a combination of instrument design and optimization of the operating parameters. The fundamental design principle of the Agilent 7500 Series ICP-MS was to effectively decompose the sample matrix and efficiently ionize the analytes of interest. This was achieved primarily through the use of a high temperature plasma sustained by a 27.12 MHz RF generator, which was found to transfer energy throughout the plasma more efficiently than plasmas operated at higher frequencies [1, 2]. In addition, the use of a low flow nebulizer prevents overloading of the plasma and ensures efficient use of the plasma energy in decomposing the sample. A further design consideration to optimize matrix tolerance is the accurate control of the spray chamber temperature. Agilent Technologies uses a Peltier device to cool the spray chamber to below 5 C. Efficient cooling controls the volatility of organic solvents; removes water vapor from aqueous samples; and reduces the solvent load on the plasma which leaves more energy for matrix decomposition and analyte ionization [3]. Finally, the Agilent 7500 Series incorporates a highly efficient ion lens system specifically designed for use with low flow nebulizers to ensure exceptional sensitivity through efficient analyte ion transmission even at very low flow rates. This application uses an Agilent 7500s ICP-MS equipped with the ShieldTorch System and organic solvent introduction kit (Part number G1833-65038). The kit consists of a quartz narrow-bore injector torch (1.5 mm id) with a tapered tip, and spray chamber drain fitting for organic solvents. The Agilent concentric nebulizer used in this study (Part number G1820-65138) achieves the low sample uptake rates required for the analysis of high matrix samples. The specially engineered tapered tip torch, exclusive to Agilent Technologies, (Part number G1833-65424) was designed specifically for the analysis of photoresists and other organic based samples. Removal of Carbon The high carbon content of photoresist, which typically consists of a carrier solvent, photoactive compound and polymers, can lead to deposition of carbon on the sampling cone, eventually leading to clogging of the cone orifice and a reduction in sensitivity. To prevent this, a small percentage of oxygen is added directly into the argon carrier gas, which transports the sample aerosol droplets into the plasma. A 20% oxygen in argon mix is added either via the spray chamber or using a T-connector before the torch. Platinum-tipped interface cones instead of the standard nickel cones are recommended because, with added oxygen, the plasma environment becomes much more reactive. Removal of Spectral Interferences Under standard operating conditions various plasma and matrix based interfering polyatomic species are generated, that may overlap analyte ions of interest (Table 1). For quadrupole ICP-MS the well-established and most effective method of reducing polyatomic interferences in high purity matrices is the use of the ShieldTorch System and cool plasma conditions. 2
Table 1. Potential Interferences on Preferred Analyte Isotopes Argon-based interference Element overlapping species 39 38 K Ar 1 H 40 40 Ca Ar 56 40 Fe Ar 16 O Organic matrix-(carbon)-based interference Element overlapping species 24 12 Mg C 2 52 40 Cr Ar 12 C To operate the ICP-MS in cool plasma mode, the plasma forward power is reduced and the carrier gas flow rate and sampling depth are adjusted, so that the ions are sampled from a region of the plasma where the ionization is carefully controlled. Consequently, ionization of the elements of interest can be maintained, but the potential interfering polyatomic ions can be attenuated, as they are ionized in a different region of the plasma. This method is most effective if the plasma potential is minimized, which can only be achieved effectively by grounding the plasma using a metal shield plate (Agilent ShieldTorch System). Without such a plate, there can be only partial grounding of the plasma. As a result, systems not equipped with a ShieldTorch do not effectively reduce plasma and matrix interferences at high forward power levels. Therefore, systems using other non-shield devices must operate cool plasmas at very low, fragile, powers (around 600W). At such low forward power, there is insufficient plasma energy to decompose the matrix of samples such as organic solvents, so sample digestion or desolvation may be required. In contrast, when the Agilent ShieldTorch System is used, the cool plasma technique is extremely efficient at removing carbon and plasma-based polyatomic interferences, matching the performance of even high resolution ICP-MS. With the Agilent self-aligning shield mount system, cool plasma conditions (950W) can be readily applied to the analysis of heavy photoresist solutions. In routine operation, automatic switching between one set of cool plasma conditions and one set of normal plasma conditions is employed, to cover all the required elements in a single acquisition. Switching between operating modes is automated using Agilent s Multi-Tune software, which means that each autosampler vial is sampled only once, thereby minimizing any potential for sample contamination. Analytical results are then collated and presented in a single report. Normal/Cool plasma switching has proven to be rapid and stable over extended periods of operation, resulting in the excellent signal to background, signal stability and detection capability required for this analysis. Experimental Standard Analytical Method for Photoresist Analysis A simple analytical method was used to analyze the photoresist samples. See Table 2 for the operating parameters used. The optimal gas flow rate was fixed at 20% (0.2 L/min) of oxygen in argon mix and the torch sampling depth was fixed at 17.5 mm. Typically the torch sampling depth for optimal cool plasma performance is shorter than that used under standard operating conditions. Although there is some sensitivity loss by fixing a constant sampling depth, it is sufficient for this application given the ppb level contaminants in the resist and the limitation of trace metal contamination in the diluent solvent. Table 2. ICP-MS Sample Introduction and Plasma Conditions for Normal and Cool Plasma Analysis. Note that the only difference in operating conditions between normal and cool plasma is the plasma power and the make-up gas flow rate, thereby significantly simplifying instrument optimization. Parameter Normal plasma Cool plasma RF Power 1450W 950W Fixed sampling depth 17.5 mm 17.5 mm Nebulizer gas 1.0 L/min 1.0 L/min Make-up gas 0 L/min 0.4 L/min Optional gas flow 20% 20% Spray chamber temp 2 C 2 C Integration time/mass 0.99 sec 0.99 sec Torch Quartz, 1.5 mm tapered injector Nebulizer Concentric nebulizer (self-aspiration) Cool plasma conditions were optimized using a standard tuning solution containing 1 ppb Co in 1% nitric acid. The signal response in counts per second (cps) for 59 Co were maximized and the signal response for the Ar 2 dimer at mass 80 and the C 2 dimer at mass 24 were minimized (carbon does not ionize in cool plasma). Normal plasma conditions were then tuned by maximizing the counts for a 1 ppb tuning solution containing 7 Li, 89 Y and 205 Tl in 1% HNO 3. After tuning using aqueous based standards, a solution of PGME was aspirated for approximately 10 minutes in order to prepare the ICP-MS for organic sample introduction. Without this cleaning 3
stage, the photoresist sample would precipitate on contact with water. Sample Preparation Sample preparation consisted of simply diluting the photoresist samples 1:10 with propylene glycol monomethyl ether (PGME). Method of Quantitation Analysis was performed using the method of standard additions (MSA) by spiking the diluted photoresist sample with 1, 2, and 3 ppb multi-element standard. A MSA calibration curve developed for one given sample can then be applied to all samples of the same matrix type without the need for further additions, so analysis time is not compromised. The concentrations of the trace elements in the unspiked photoresist sample were calculated from the standard addition calibration curve. Concentrations obtained were corrected to account for the 10 dilution factor. Data Acquisition The photoresist sample was freely aspirated into the ICP-MS using standard 0.3 mm capillary tubing from the Integrated Autosampler (I-AS), a clean autosampler designed specifically to avoid contamination at the sample introduction stage (Part number G3160A). A rinse step (using PGME) between samples is recommended to prevent signal drift resulting from photoresist deposition. Results Table 3 shows the detection limits (DL) obtained during this study, for the 1:10 diluted photoresist sample. DLs were calculated using three times the standard deviation (n = 3) of the raw counts of the photoresist divided by the slope of the MSA curve. Using the slope of the MSA curve, rather than counts obtained from a standard, takes into Table 3. Detection Limits in Photoresist (ppb) Element (mass) Plasma mode Detection limit (ppb) *Photoresist (A) (ppb) PGME (B) (ppb) A - B (ppb) Li (7) Cool 0.0004 0.066 0.060 < 0.0004 Be (9) Normal 0.002 0.018 0.025 < 0.002 B (10) Normal 0.08 0.92 0.87 < 0.08 Na (23) Cool 0.01 0.49 0.11 0.38 Mg (24) Cool 0.005 0.179 0.086 0.093 Al (27) Cool 0.06 0.19 0.20 < 0.06 K (39) Cool 0.009 0.34 0.10 0.23 Ca (40) Cool 0.02 0.37 0.25 0.12 Ti (47) Normal 0.2 1.8 2.1 < 0.2 V (51) Normal 0.04 0.35 0.23 0.12 Cr (52) Cool 0.06 2.0 2.1 < 0.06 Mn (55) Cool 0.006 0.066 0.080 < 0.006 Fe (56) Cool 0.3 1.7 1.4 < 0.3 Ni (58) Cool 0.06 0.15 0.15 < 0.06 Co (59) Cool 0.0008 0.075 0.081 < 0.0008 Ni (60) Cool 0.03 0.16 0.14 < 0.03 Cu (63) Cool 0.01 0.13 0.13 < 0.01 Zn (68) Cool 0.08 0.75 0.52 0.23 Mo (95) Normal 0.01 0.06 0.06 < 0.01 Ag (107) Normal 0.005 0.028 0.033 < 0.005 Cd (111) Normal 0.01 0.06 0.06 < 0.01 Ba (138) Normal 0.004 0.033 0.043 < 0.004 Ta (181) Normal 0.007 0.044 0.118 < 0.007 W (182) Normal 0.007 0.036 0.050 < 0.007 Pb (208) Cool 0.004 0.092 0.077 0.015 *Photoresist sample was diluted 1:10 with PGME 4
account any matrix suppression. The reported detection limits are fundamentally limited by the metal impurities in the solvent blank (B) for example, in PGME. Table 4 illustrates 1 ppb spike recoveries in 1:10 diluted photoresist using both normal and cool plasma conditions. An external calibration curve was generated using standards at 0, 1, 2 and 3 ppb added to the PGME. Indium (10 ppb) and rhodium (10 ppb) were added as internal standards for cool and normal plasma elements respectively. The spike recoveries are good, particularly for difficult elements such as Mg, Cr, Ca, K and Fe, which suffer carbon and argon based interferences under normal plasma conditions. Table 4. Spike Recoveries (%) in Photoresist Using Cool (Indium as Internal Standard) and Normal (Rhodium as Internal Standard) Plasma Conditions Element (mass) Plasma mode Concentration (ppb) 1 ppb Spike % Recovery Li (7) Cool 0.003 0.940 94 Be (9) Normal 0.002 0.967 97 B (10) Normal 0.021 0.93 91 Na (23) Cool 0.352 1.49 114 Mg (24) Cool 0.088 1.1 107 Al (27) Cool 0.000 1.1 112 K (39) Cool 0.233 1.40 117 Ca (40) Cool 0.115 1.30 119 Ti (47) Normal 0.000 0.91 91 V (51) Normal 0.106 1.11 100 Cr (52) Cool 0.084 1.3 119 Mn (55) Cool 0.000 1.1 112 Fe (56) Cool 0.272 1.39 112 Ni (58) Cool 0.026 1.2 116 Co (59) Cool 0.000 1.1 114 Ni (60) Cool 0.034 1.2 115 Cu (63) Cool 0.009 1.1 111 Zn (68) Cool 0.173 1.25 107 Mo (95) Normal 0.011 0.99 98 Ag (107) Normal 0.000 0.99 99 Cd (111) Normal 0.000 1.02 102 Ba (138) Normal 0.000 0.98 98 Ta (181) Normal 0.000 1.03 103 W (182) Normal 0.000 1.03 103 Pb (208) Cool 0.000 1.00 100 *Photoresist sample was diluted 1:10 with PGME 5
A 2-hour stability study was performed by adding a 1 ppb standard into 1:10 diluted photoresist (3% resin) sample and analyzing the spiked sample over a 2-hour period. Instrument stability over this period was excellent with %RSD values typically less than 3% for cool plasma and normal plasma elements, despite the complex matrix. Stability plots of representative elements appear in Figures 1 and 2. 3.5 Concentration (ppb) 3.0 2.5 2.0 1.5 1.0 0.5 0 0 20 40 60 80 100 120 Time (min) 7 Li (3.0%) 24 Mg (0.9%) 25 Mg (1.0%) 27 Al (0.7%) 39 K (0.9%) 40 Ca (2.3%) 52 Cr (1.0%) 53 Cr (0.9%) 55 Mn (0.6%) 56 Fe (3.0%) 58 Ni (1.3%) 59 Co (1.0%) 60 Ni (1.3%) 63 Cu (1.3%) 65 Cu (1.9%) 68 Zn (6.7%) 208 Pb (1.7%) Figure 1. Two-hour stability plot for cool plasma elements in 3% photoresist sample. 6
2.5 Concentration (ppb) 2.0 1.5 1.0 10 B (2.3%) 47 Ti (1.6%) 51 V (0.9%) 66 Zn (1.4%) 95 Mo (0.8%) 111 Cd (1.0%) 181 Ta (0.7%) 182 W (0.7%) 0.5 0 0 20 40 60 80 100 120 Time (min) Figure 2. Two-hour stability plot for normal plasma elements in 3% photoresist sample. Conclusions Reliable photoresist analysis can be carried out following simple dilution in a suitable solvent so long as the ICP-MS meets several key design considerations. First, the sample introduction system needs to be optimized to handle high sample matrices over extended periods of time. This includes the use of low flow nebulizers, cooling of the spray chamber and use of a torch injector designed to minimize sample deposition. Second, a highly efficient plasma RF generator is required in order to produce a robust and stable plasma for complete matrix decomposition and sample ionization. A flexible gas control mechanism is also desirable to accommodate the need for oxygen addition when analyzing organic solvents. Finally, an effective plasma grounding mechanism, such as the Agilent ShieldTorch System, is required for elimination of polyatomic interferences through the use of high power cool plasma operation. The Agilent 7500s ICP-MS with ShieldTorch System meets all of the design criteria outlined above. The results outlined in this application note demonstrate that the 7500s higher power (950W) cool plasma effectively breaks down the heavy resist matrix (sample analyzed as 2% 3% resins) and eliminates C-based interferences on Mg and Cr, as well as Ar-based interferences on K, Ca and Fe to reproducibly measure the key analytes at the levels required by the industry. References 1. M.H. Abdalla, R. Diemiastzonek, J. Jarosz, J. M. Mermet and J. Robin, Anal. Chim. Acta 33B, 55 (1978). 2. P.E. Walters, W.H. Gunter and P.B. Zeeman, Spectrochim Acta 41B, 133 (1983). 3. R.C. Hutton and A.N. Eaton, J. Anal. At. Spectrom., 2, 595 (1987). For More Information For more information on our products and services, visit our Web site at www.agilent.com/chem. For more information about semiconductor measurement capabilities, go to www.agilent.com/chem/semicon. 7
www.agilent.com/chem Agilent shall not be liable for errors contained herein or for incidental or consequential damages in connection with the furnishing, performance, or use of this material. Information, descriptions, and specifications in this publication are subject to change without notice. Agilent Technologies, Inc. 2002 Printed in the USA July 26, 2002 5988-7100EN