Applied Surface Science

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1 Applied Surface Science 255 (2009) Contents lists available at ScienceDirect Applied Surface Science journal homepage: Surface properties of silicon oxide films deposited using low-pressure dielectric barrier discharge Yejun Yin a,b, Dongping Liu a, *, Dongming Li b, Jiandong Gu a,b, Zhiqing Feng a, Jinhai Niu a, Guenther Benstetter c, Sam Zhang d a School of Science, Dalian Nationalities University, Dalian , China b School of Mechanical Engineering, Dalian Jiaotong University, Dalian , China c Electrical Engineering Department, University of Applied Sciences Deggendorf, Edlmairstr 6&8, Deggendorf 94469, Germany d School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore , Singapore ARTICLE INFO ABSTRACT Article history: Received 11 February 2009 Received in revised form 17 April 2009 Accepted 17 April 2009 Available online 24 April 2009 Keywords: Conductive atomic force microscopy SiO 2 films XPS The deposition of SiO X films from low-pressure dielectric barrier discharge plasmas has been investigated using tetraethoxysilane (TEOS)/O 2 as the feed gas. Films were analyzed using X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), AFM-based nanoindentation/ nanowear techniques, and conductive AFM. Film deposition rates and hydrocarbon incorporation in the SiO X film decrease with addition of O 2. High-quality SiO X films with extremely low surface roughness are deposited at high oxidant concertrations. Addition of oxidant to the feed gas leads to a change in the SiO X film structure from precursor-like to a dense SiO X structure. The SiO X films deposited with TEOS/O 2 plasmas were found to have soft surface layers, nm thick, which contribute to an improvement of their field emission properties. The effect of gas phase compositions on the surface properties of the conductive surface layer was discussed. ß 2009 Elsevier B.V. All rights reserved. 1. Introduction Si-based materials including Si oxides are of great interests in manufacturing semiconductor devices [1 3], primarily as a gate oxide, as insulating layers between interconnect levels, or as a passivation layer. Plasma enhanced chemical vapor deposition (PECVD) is well suited for room temperature deposition of thin films. A large variety of plasma activation processes (i.e., r.f., DC, microwave plasmas, etc.) have been used to maximise the deposition rate and to improve the film properties [4]. In particular, the deposition of dense thin films is desirable for most applications and many efforts have been made to develop deposition procedures and novel deposition techniques. In our previous reports, low-pressure dielectric barrier discharge (DBD) plasmas have been successfully used to deposit diamond-like carbon films [5,6] and fluorocarbon films [7]. DBDs are non-equilibrium gas discharges which can be operated at atmospheric pressure. The filament discharges produced at atmospheric pressure lead to inhomogeneous deposition of films [8,9]. Usually, the glow-like low-pressure DBD plasmas were used to deposit homogeneous films at room temperature. DBDs are * Corresponding author. addresses: dliu010119@hotmail.com, Dongping.liu@dlnu.edu.cn (D. Liu). characterized by the presence of one or two layers of insulator between the electrodes. The gas spacing between the electrodes can be varied in the range of 0.1 mm to several mm. Low-pressure DBDs consist of uniform (along the whole electrode) glow-like single breakdowns with half-widths of several microseconds, which contributes to the formation of thin films with smooth surfaces. The DBD-induced deposition technique has many advantages, including the simplicity of the experimental set-up, large area deposition of thin films, and a low consumption of feed gas and electric power. The present paper reports on the deposition of SiO X films by low-pressure dielectric barrier discharge plasmas. Films were characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Changes in the chemical bonding structure, deposition rates, and surface roughness were presented. In particular, conductive AFM (CAFM), combined with AFM-based wear tests was used to reveal the soft surface layer of deposited films and analyze their conductivity. The effects of O 2 addition on the film structures are also addressed. 2. Experimental The low-pressure DBD chamber has been described previously [7]. Briefly, this chamber mainly consists of a high-voltage (H.V.) electrode, 2 mm thick quartz tube, and a grounded electrode. The /$ see front matter ß 2009 Elsevier B.V. All rights reserved. doi: /j.apsusc

2 Y. Yin et al. / Applied Surface Science 255 (2009) H.V. Electrode is attached to the bottom of the quartz tube in the air. The pressure of tetraethoxysilane (TEOS) (>95%) was controlled with a needle valve. High purity O 2 (>99.9%) was also introduced into the reactor through a mass flow controller. The pressure in the reactor was monitored with a MKS Baratron capacitance manometer, which is insensitive to differing gas compositions. TEOS and O 2 precursors flow through 4 mm gas spacing between the quartz dielectric and the grounded electrode, on the top of which the silicon (Si) substrate is placed. The O 2 flow rates are in the range of 0 15 sccm, depending on the O 2 pressure. The flow rate of TEOS vapor is controlled by a needle valve. The pressure of TEOS in the reactor was maintained at 80 5 Pa and the ratios of precursor to O 2 were calculated from their partial pressure. For all deposition, the peak-to-peak voltage and the frequency of the power supply source were maintained at 24 kv and 1 khz, respectively. Substrates were removed from the reactor for analysis. XPS analyses were performed on the Kratos AXIS Ultra spectrometer with a monochromatized Al Ka X-ray source ( ev) operated at a reduced power of 150 W (15 kv and 10 ma). The base pressure in the analysis chamber was Pa. The core-level spectra were obtained at a photoelectron take-off angle of 908 measured with respect to the sample surface and were recorded in 0.1 ev step with the pass energy of 40 ev. We used binding energy (BE) (284.6 ev) as the charge correction reference [10 12]. Film thicknesses were obtained by masking a portion of the substrates during deposition and then measuring the resulting step height by profilometry (Surfcorder ET4000m). Deposition rates were calculated by dividing the film thickness by the deposition time. The topography of deposited films was analyzed using an atomic force microscopy (DI 3100), operated in the tapping mode. The silicon tips with a tip radius typically in a range of 5 10 nm, were used to conduct these measurements. Surface root-mean-square (RMS) roughness values were derived from the AFM images. AFM-based nanoindentation tests were performed using a diamond-tipped cantilever. A pyramidal diamond tip with a tip radius lower than 25 nm was used to generate the indentation impressions. The nanohardness was defined as the ratio of the indentation force to the projected area of the residual indentation [13,14]. The nanoindentation tests show a hardness of the silicon (1 0 0) around 11 GPa 15% with the indentation forces ranging from 40 to 100 mn. Diamond-tipped cantilever was also used for nanowear tests using the contact-mode AFM. Wear tests were carried out by scanning a square area pattern at a given loading force. The electrical conduction measurements were carried out with conductive AFM (CAFM) equipped with a conductive tip and a pa preamplifier [15,16]. A constant voltage was applied between the conductive tip and the substrate. The tip bias was kept positive to obtain a relatively stable electron emission. The CAFM tests were performed at the region of wear marks. Electron emission images were obtained simultaneously with the topography. Table 1 Atomic ratios for films deposited from TEOS/O 2 DBD plasmas. TEOS/O 2 C/O C/Si O/Si 1: : : : : from 100% TEOS, the distribution of Si 2p peak is 1.5% Si( O) 1, 5.8% Si( O) 2, 4.5% Si( O) 3, and 88.2% Si( O) 4. Higher oxidant concentration in the organosilane plasmas result in a decrease in hydrocarbon content, thereby producing more stoichiometric SiO X films, compared to the films deposited with r.f. plasmas [18]. Indeed, for films deposited from a 44% TEOS plasma, the distribution is simply 0.4% Si( O) 1, 0.6% Si( O) 2, 3.0% Si( O) 3, and 96% Si( O) 4. Fig. 2 shows the deposition rates of the SiO X films as a function of oxidant addition. With increasing amount of oxidant, the film deposition rate initially increases to a maximum and then declines sharply. Similar behavior has been observed previously for SiO X films deposited from alkoxysilane/o 2 [19], tetramethylcyclotetrasiloxane/o 2 [18], and dimethyldimethoxysilane/o 2 [18] r.f. plasmas. Fig. 3(a) and (b) shows the 3D AFM images for the films deposited from 100% TEOS and 44% TEOS plasmas. These films are homogeneous with very smooth surfaces. The surface RMS roughness values for films deposited with TEOS/O 2 plasmas are 3. Results Table 1 shows the XPS ratios of C/O, C/Si, and O/Si for films deposited at different TEOS:O 2 ratios. The C/O, C/Si, and O/Si ratios in the films appears insensitive to the addition of an oxidant. The C/ O ratios ( ) and C/Si ratios ( ) indicate that the significant amounts of carbon can exist at the surface of the deposited films, and the O/Si ratio >2.0 results from Si C moieties or SiOH species at the film surfaces. Fig. 1 shows respective highresolution XPS spectra and fitting of the Si 2p peak for films deposited from TEOS/O 2 plasmas. The number of O atoms bonded to Si in SiO X films can vary from one to four, denoted by Si( O) 1, Si( O) 2, Si( O) 3, and Si( O) 4, respectively [17]. For films deposited Fig. 1. Curve fit of the high-resolution XPS Si 2p spectra for films deposited from plasmas of (A) TEOS:O 2 = 1:0 and (B) TEOS:O 2 = 1:1.27.

3 7710 Y. Yin et al. / Applied Surface Science 255 (2009) Fig. 2. Dependence of deposition rate on oxidant addition for films deposited from TEOS/O 2 DBD plasmas. Fig. 3. 3D AFM images for the films deposited from plasmas of (a) TEOS:O 2 = 1:0 and (b) TEOS:O 2 = 1:1.27. listed in Table 2. These films exhibit atomic surface roughness (RMS values: nm) and RMS values are independent on the reactive gas compositions. Fig. 4 shows representative 2D AFM images of indents formed with the same loading force of about 80 mn. The triangle impressions result from a pyramidal diamond tip with its cantilever spring constant of 181 N/m. The area of the residual Table 2 RMS roughness, nanohardness, and surface layer thickness for films deposited from TEOS/O 2 DBD plasmas. TEOS/O 2 RMS values (nm) Nanohardness (GPa) Surface layer thickness (nm) 1: : : : : Fig. 4. 2D AFM images of indents for films deposited from plasmas of (a) TEOS:O 2 = 1:0, (b) TEOS:O 2 = 1:0.34, and (c) TEOS:O 2 = 1:1.27. The images have the same scan area of mm 2. These indents were generated with a pyramidal diamond tip at the loading force of about 80 mn. indentation is greatly reduced with increasing O 2 fraction. The nanohardness values increase from 0.64 to 6.6 GPa when TEOS fraction is decreased from 100% to 44% (Table 2). These measurements indicates that polymer-like SiO X films are produced

4 Y. Yin et al. / Applied Surface Science 255 (2009) Fig. 6. FTIR transmission spectra of films deposited from plasmas of (A) TEOS:O 2 = 1:0, (B) TEOS:O 2 = 1:0.34, (C) TEOS:O 2 = 1:0.48, (D) TEOS:O 2 = 1:0.82, and (E) TEOS:O 2 = 1:1.27. Fig. 5. (a) Topography on 1.5 mm 1.5 mm area obtained using tapping mode AFM, and (b) current image recorded at the same area using a Pt/Ir coated tip at the tip bias of 4.7 V. The wear marks were generated using a diamond tip at the loading force of 20 mn on plasma deposited 31 nm SiO X on a p-type silicon substrate. with 100% TEOS plasma while the oxidant addition to reactive gases may result in deposition of the SiO X films with a dense structure. The nanowear tests were performed at a given loading force for an increasing number of wear cycles. The thickness of soft surface layers was estimated from the curves of the wear depths versus the number of wear cycles [16]. The thickness of soft surface layers versus the oxidant addition is listed in Table 2. The SiO X film deposited with a 100% TEOS plasma was not covered with a soft surface layer. However, O 2 addition to the reactive gases leads to the deposition of SiO X films with soft surface layers. Their thickness increases from 0.5 to 1.5 nm when the TEOS fraction is varied from 74.4% to 44%. Fig. 5 presents the results of CAFM tests performed on the 32 nm SiO X film deposited with a 55% TEOS plasma. The CAFM tests were performed using a Pt/Ir coated tip at the tip bias of 4.7 V at the region of the wear mark, where the 1.5 nm soft surface layer was removed. The wear mark 2.0 nm in depth was generated using a diamond tip at the loading force of 20 mn. The emission current image (Fig. 5(b)) of the wear mark exhibits the consistence with its topography image (Fig. 5(a)). Compared to the region where the soft surface layer exists, the wear mark is relatively insulating. This suggests the soft surface layer is conductive, which contributes to an improvement of film field emission properties. The addition of oxidants to TEOS plasmas can reduce the hydrocarbon content and improve SiO 2 film quality. Fig. 6 shows a series of transmission FTIR spectra for the films deposited from plasmas of 100% TEOS (TEOS:O 2 = 1:0), 74.4% TEOS (TEOS:O 2 = 1:0.34), 67% TEOS (TEOS:O 2 = 1:0.48), 55% TEOS (TEOS:O 2 = 1:0.82), and 44% TEOS (TEOS:O 2 = 1:1.27). For the film deposited from 100% TEOS, Fig. 6(A), the absorbance bands at 2950 cm 1 is assigned to C H stretches from CH x (x = 1-3) groups [2,3,12,20]. The most intense absorbance band at 1084 cm 1 is due to Si O Si and Si O C stretches [21,22]. The broad absorbance band at 3500 cm 1 is assigned to SiO H [2,3,18]. The weak absorbances at 960, 885, and 800 cm 1 are attributed to Si CH 3 rocking modes (960 cm 1 for Si CH 3, 885 and 800 cm 1 for Si (CH 3 ) 2 ) [17,18]. The absorbance at 2350 cm 1 is attributed to CO 2 stretches. Fig. 6(B) shows a transmission FTIR spectrum for the film deposited from 74.4% TEOS plasma. The addition of O 2 decreases the intensities of the absorbance bands for hydrocarbons (2950 cm 1 ) and Si CH 3 (960, 885, and 800 cm 1 ), relative to the Si O Si and Si O C absorbance band at 1084 cm 1. This trend is more apparent when the amount of oxygen is increased to 56% [Fig. 6(C) (E)]. When TEOS:O 2 = 1:1.27, all the absorbance bands for hydrocarbons and silicon hydride are eliminated, Fig. 1(B). The Si O Si absorbance band has narrowed and shifted to 1119 cm 1. The reduction of carbon and hydrogen demonstrates a more stoichiometric, high-quality SiO 2 film is produced. 4. Discussion The low-pressure DBD plasmas consist of pulsed glow-like breakdowns [5,6]. The breakdown number per voltage period mainly depends on discharge pressure, the distance of the discharge gas spacing, and the applied voltage. Compared to filament discharges at atmospheric pressure, the low-pressure glow-like discharges result in the SiO X films with very smooth surfaces. The SiO X deposition processes are divided into several steps [23,24]. First, the gas phase precursor is fragmented in the plasma discharge. These fragments can then adsorb on the substrate surface, diffuse, and desorb. A rearrangement or recombination reaction can be produced while the adsorbed atoms diffuse on the surface. High density of activated sites plays a crucial role in depositing uniform films and controlling film deposition rates. The ions in the plasma sheath can be energetic and have enough energy to break the chemical bonds at the film surface. The energetic ions can create activated sites during their interactions with film surfaces. The gas phase species, mostly O atoms, react with the adsorbed precursor fragments, remove the hydrocarbon components and form the SiO X network.

5 7712 Y. Yin et al. / Applied Surface Science 255 (2009) The gas phase fragments in 100% TEOS plasmas are precursorlike, which results in the deposition of precursor-like films. The CH x (x = 1 3), SiO H, and Si CH 3 absorbance peaks in the FTIR spectrum are clearly observed for the SiO X film deposited with 100% TEOS plasmas. The deposited films are polymer-like with the nanohardness of 0.64 GPa. The films are less precursorlike with the addition of an oxidant. The addition of oxidant can strongly affect both the gas phase composition and the film deposition, and improve the quality of the film through chemical reactions in the gas phase and oxidation at the film surface. Reactions between O atoms and adsorbed prescursor fragments increase with oxidant addition, and thus reduce hydrocarbon incoroporation into the deposited SiO X films. Film deposition rates initially increase with addition of oxidant, and then decrease rapidly. The deposition rate is dependent on the concentration of precursor fragments in the gas phase, the adsorption/desorption rate of these speciese on the surface, and the oxidation reactions on the surface. The initial increases observed in the deposition rate are attributed to the increase in gas phase precursor fragmentation, which increases the adsorption rate and thus the deposition rate. The subsequent decrease in the deposition rate with more addition of oxidant is due to a decrease in the gas phase concentration of precursor fragments and an increase in surface oxidation reactions. The ions in the plasma sheath at film surfaces may produce energetic impacts. The ion energy may be transferred to the violent motion of the atoms in the area of the impact, and form thermal spikes [25]. The intense local heating from the thermal spikes can provide the energy required to release atoms from a film surface. The incoming radicals may adsorb the activated sites at the film surface, which contributes to the film growth. The surface layer may be formed during the interactions between the activated species from the gas phase and the film surface. For XPS measurements, the electrons ejected from C 1s orbitals have kinetic energies around 1000 ev and their escape depth is less than 2nm[26]. Since the soft surface layer for the SiO X films deposited with TEOS/O 2 plasmas is typically nm thick, film compositions measured by XPS are mainly from the film surface layer. When O 2 is added, the hydrogen-containing products such as H 2 O may preferably be produced in the gas phase or during the interactions between the gas phase plasmas and the depositing surface. Decreasing the hydrogen content in the deposited films may lead to the change in the surface layer from one polymer-like structure to graphite-like one. The carbon-rich surface layer for films deposited with O 2 addition can obviously contribute to an improvement of the film field emission properties. The uniform and dense ultrathin (<2 nm) SiO 2 layer may be obtained by rapid thermal oxidation in dry oxygen ambient of a silicon substrate [27]. The dense SiO X films deposited O 2 addition are clearly covered with about 1.5 nm thick soft surface layer, which indicates the DBD deposition technique is not an ideal one for depositing ultrathin silicon oxide films. 5. Conclusion SiO X films have been deposited from low-pressure TEOS/O 2 DBD plasmas. The addition of O 2 to the feed gas is essential to deposition of high-quality SiO 2 films. The films were characterized with XPS, AFM, AFM-based nanoindentation, and CAFM. Addition of oxidant to the feed gas leads to a change in the SiO X film structure from polymer-like to a dense SiO X structure. Higher oxidant concentration in the organosilane plasmas results in a decrease in hydrocarbon contents in the deposited SiO X film, thereby produces more stoichiometric SiO X films. The films deposited with TEOS/O 2 plasmas were covered with the soft surface layer. The thin surface layer with a high carbon concentration is obviously conductive, compared to the film bulk. The conductive surface layer can be produced during the interactions between the O 2 -rich plasmas and the growing film surface. Overall, this study has demonstrated the viability of TEOSbased DBD system for production of high-quality SiO X films. Acknowledgment This work was supported by the National Science Foundation of China (NSF ). References [1] D.A. DeCrosta, J.J. Hackenberg, J.H. Linn, J. Electrochem. Soc. 143 (1996) [2] S.C. Deshmukh, E.S. Aydil, Appl. Phys. Lett. 65 (1994) [3] S.C. Deshmukh, E.S. Aydil, J. Vac. Sci. Technol. A 13 (1995) [4] A. 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