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Conversion of a plasma enhanced chemical vapor deposited silicon–carbon–nitride thin film at ultra-low temperature by oxygen plasma
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Thin Solid Films 516 (2008) 885 – 890 www.elsevier.com/locate/tsf
Conversion of a plasma enhanced chemical vapor deposited silicon–carbon–nitride thin film at ultra-low temperature by oxygen plasma Steven M. Smith ⁎, Tim Tighe, Diana Convey, Jaime Quintero, Yi Wei Motorola Inc., Embedded Systems Research Lab EL317, 2100 E. Elliot Rd., Arizona 85284, USA Received 10 July 2006; received in revised form 5 April 2007; accepted 19 April 2007 Available online 3 May 2007
Abstract In this work we present an ultra-low temperature method for the oxidation of an amorphous silicon–carbide–nitride (SiCN) material. The SiCN is deposited on silicon substrates by plasma enhanced chemical vapor deposition using CH4, SiH4, and N2 chemistry. The physical and chemical properties are characterized for the as-deposited SiCN and post-oxidized films are discussed. The SiCN film is exposed to oxygen plasma, where it undergoes a chemical transformation into a binary SiO2 material system. A 1.7 nm/min oxidation rate is typical for this process and compares favorably to oxidation methods utilizing much higher temperatures. The substrate temperature remains extremely low throughout the oxidation process, Ts b 200 °C. Changes in film stress, optical constants, film thickness, surface roughness, and film density are measured. Chemical analysis by X-ray photoelectron spectroscopy is reported for both the as-deposited and oxidized film and confirms the resultant film to be the chemical equivalent of thermally grown SiO2. We discuss applications specifically targeted to the conversion of SiCN to SiO2. © 2007 Elsevier B.V. All rights reserved. Keywords: SiCN; PECVD; Oxidation; SiO2
1. Introduction Ternary silicon carbide-nitride materials have traditionally been sought due to their unique physical and chemical properties. This class of material has been categorized as being extremely hard, possessing low chemical reactivity, and is resistant to oxidation. Researchers have investigated SiCN materials produced by a variety of deposition techniques. These techniques include sputter deposition [1–3], plasma CVD [4–6], and laser ablation [7]. Additionally, numerous permutations based on these techniques have been reported. Variations in material properties are directly affected by the deposition technique, the type of target material or chemical precursor(s) used, and the process parameter space being investigated. For these reasons it often becomes difficult to make direct comparisons of ternary materials produced by varying means. ⁎ Corresponding author. Tel.: +1 480 413 7798; fax: +1 480 413 5453. E-mail address: [email protected] (S.M. Smith). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.153
While numerous publications have referenced oxidation resistance as a basic material property for SiCN materials, few researchers have published oxidation data specific to their referenced film. Of the limited published oxidation rates for SiCN, the method cited is predominantly high temperature air or oxygen exposure. For ion-enhanced sputter-deposited SiCN films, He et al. have shown oxidation rates in flowing O2 at 1000 °C of 2.1 nm/min [8]. When comparing 950 °C wetoxidation of PECVD SiC to SiCN films with varying N content (5.0 at.% and 9.3 at.%), Tsui et al. found the oxidation rate decreased for increasing N-content, 1.33 and 0.73 nm/min respectively. In the same work, a PECVD SiC film (N = 0.0 at.%) was oxidized at a significantly reduced temperature (550 °C) with a reported oxidation rate of 3.0 nm/min [9]. Ternary materials can be altered dramatically by changing the chemical make-up of the film. For example, silicon oxynitride (SiON) materials can be modified to be extremely nitride-like (SiN) or very oxide-like (SiO), where each SiN/SiO variation produces a film having different chemical, physical,
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and optical properties. Likewise, similar material transitions hold true for ternary SiCN as well. As stated previously, the choice of deposition techniques can play an important role in defining the final material property. In this work we use PECVD for its robust process latitudes, as it is capable of depositing virtually any material over virtually any surface or substrate. The use of PECVD in industry is driven by the need to reduce thermal budgets, which are much lower than conventional thermally driven chemical vapor deposition (CVD) processes used to produce very high quality materials. As such, it is not uncommon to produce equivalent films at temperatures 2–5 times lower than those produced by thermal CVD techniques. Additionally, PECVD techniques provide added benefits beyond simple temperature reduction such as significantly higher deposition rates and/or enhanced process flexibility to tailor properties like stress, density, surface roughness, and refractive index. 2. Experimental details 2.1. Deposition and oxidation equipment A ternary Si–C–N film is deposited using SiH4, N2, and CH4 source gases onto oxidized and un-oxidized n-type (100) Sisubstrates using a commercially available PECVD reactor. The system is load-lock designed and configured with an earthground resistively heated aluminum heaterblock (substrate electrode), a closed loop pressure control system, and a mass flow controlled gas distribution network. The reactor can be powered by single or dual frequency (13.56 MHz and variable 100–450 kHz) power supplies that provide power directly to the gas-distribution showerheads, which reside directly above the substrate. Deposition conditions used to produce the SiCN film were derived from earlier work to develop diamond-like carbon films [10]. The deposition temperature is b350 °C and plasma power is in kilowatts. A plasma ash system is used to perform the plasma oxidation of the as-deposited SiCN film. The system is commonly found in integrated circuit (I.C.) manufacturing facilities and is used to remove photo-resist, a material used for pattern transfer steps in I.C. integration schemes. A generic photo-resist strip recipe is selected for this work. A perforated aluminum cylinder with a diameter of 200 mm is placed inside the chamber with the SiCN coated substrates placed in the center of the cylinder on a quartz boat. This set-up effectively simulates a remote O2 plasma system by minimizing high energy charge effects. The O2 plasma process initiates at room temperature and slowly reaches a steady-state temperature of less than 200 °C within 20 min, a direct effect of the plasma energy. Exposure time is the only variable investigated and is necessary to determine the oxidation rate of the SiCN material. 2.2. Analysis tools Stress data were taken using an FSM-128 mapping tool from Frontier Semiconductor Incorporated. The FSM-128 employs a scanning laser to measure the change in curvature of the Si-
substrate. The film induced curvature is used to derive values for film stress using the Stoney equation [11]. Twelve laser scans per sample were taken at 30° offsets and were used to provide a 360° detailed stress map of the wafer surface. Stress values cited in this work are the average stress calculated from the 12 individual laser scans. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics PHI Model 5400 XPS System equipped with a non-monochromated Mg Ka (1253.6 keV) anode running at 12 kV and 200 W. Depth profiles were collected using a Physical Electronics Ion Gun Model 04-300 operated at 3 kV, 25 mA and a 6 mm2 raster. Ultra-high purity argon was used as the sputtering gas. The sputter rate for both the SiCN and SiO2 films under these conditions are near equivalent and found to be ∼2 nm/min. The base pressure of the system during the depth profiles was 1 × 10− 5 Pa. The data was collected using pass energy of 44.75 eV and an energy step of 0.125 eV. The spectrometer was calibrated prior to the experiments using both the position and separation of the Au 4f7/2 (84.0 eV) and the Cu 2p3/2 (932.67) peaks. The background was considered as Shirley type and curve fitting was carried out using a non-linear least-squares fitting method employing a Gaussian–Lorentzian function. Quantification relied upon relative sensitivity factors supplied by the instrument manufacturer and by available in-house standards [12]. The atomic concentrations were found to have an error of less than +/−2% for both the SiCN and SiO2 depth profile. All film thickness and optical constant data were obtained using spectroscopic ellipsometry by means of a Woollam Variable Angle Spectroscopic Ellipsometer (WVASE). Three distinct angles of incidence were employed to collect optical spectra from 400 nm–800 nm wavelengths. A Cauchy layer with Urbach absorption was used for both the as-deposited and oxidized SiCN film. Thermally grown SiO2 is referenced for comparison to the plasma oxidized film. Models for each of these layers were obtained from the WVASE material files. Refractive index (n) and extinction coefficient (k) are reported at 633 nm wavelength for reference to single wavelength (HeNe) ellipsometry. Surface morphology specific to film roughness is determined by atomic force microscopy (AFM). Images were obtained using a Veeco Dimension 3000 AFM system. Statistics gathered from 5 μm × 5 μm images were captured in air using Si-etched tips in tapping mode. Root Mean Square (RMS) of the instrument noise is 0.11 nm. RMS and Z height information is used to describe the film's roughness and general morphology for both as deposited films and after the O2 plasma conversion process. 3. Results and discussion 3.1. Chemical analysis — XPS A silicon wafer is first oxidized by placing in a quartz furnace and flowing high purity oxygen at 1000 °C to effectively grow 200 nm of SiO2. The thermally grown oxide is referenced as the calibration standard for SiO2 in this work. Next, the sample is deposited with 100 nm of PECVD SiCN and is then cleaved, where only one half is subjected to a 60 minute plasma oxidation using the plasma ash tool. Afterwards, both halves are analyzed
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Fig. 1. XPS depth profile for the as-deposited SiCN film (A) and the plasma oxidized SiCN film (B) on the oxidized silicon substrate.
using XPS depth profiling, with the results displayed in Fig. 1. As expected, a small amount of surface oxidation is observed at the surface of the SiCN film and similarly a mono-layer or less of adventitious carbon was observed on the plasma oxidized SiCN surface. The XPS data clearly shows the as-deposited SiCN composition is homogeneous throughout the bulk of the film and is found to be 66% C, 28% Si, and 6% N. The transitional interface between the SiCN film and the thermally grown SiO2 layer is observed after 40 min of depth profiling. The interface region is fully interrogated after 60 min of profiling, where the XPS scans shows complete loss of C and N while the increasing O and Si have leveled off, indicating full transition into the SiO2 layer. Likewise, the XPS chemical profile for the sample receiving the plasma oxidation aligns well to the chemical profile for the thermally grown SiO2, where both profiles produce equivalent compositions of 67% O and 33% Si. Note that for the plasma oxidized sample, the XPS profile time is extended by 25% to insure that chemical analysis on each side of the oxidized SiCN/SiO2 interface has occurred. However, we found no evidence of an interface, thus providing complementary evidence that the plasma oxidized SiCN film is the chemical equivalent of thermally grown SiO2. Equally interesting is the lack of C and N, indicating complete removal during the oxygen plasma reaction.
Fig. 2. Evolution of the SiO2 and SiCN thicknesses as obtained by VASE following the conversion of an SiCN film by O2 plasma as a function of O2 plasma exposure time.
3.2. Oxidation rate and film thickness A 105 nm SiCN film is deposited directly onto a 100 mm silicon wafer and is subsequently oxidized via O2 plasma. VASE analysis is used to measure the thickness of the SiCN and the oxidized equivalent film at varying intervals throughout the plasma oxidation process. The results displayed in Fig. 2 reveals that the total film thickness remains virtually unchanged. Interestingly, the oxidation rate is linear and calculated to be 1.7 nm/min, which indicates that the process is reaction-rate limited and not bounded by diffusion-rate laws. The plasma oxidation rate for our SiCN film compares favorably to the 2.1 nm/min shown by Tsui et al. for their PECVD SiCN material. However, those rates reflect a much higher oxidation temperature (950 °C) [9]. In Fig. 2, film thickness is plotted as a function of oxygen plasma exposure time, where VASE is used to measure the oxidized and non-oxidized layers of the SiCN film. A linear rate response is observed from 0 to 60 min, at which point the SiCN film is completely oxidized and is confirmed via VASE and XPS depth profile analysis techniques. Further oxygen plasma exposure did not alter the SiO2 thickness, indicating that the oxidation reaction is very efficient as it moves through the bulk film. This observation is confirmed by XPS analysis where no carbon or nitrogen is evident in the oxidized film. Temperature of the O2 plasma chamber specific to the chosen recipe shows a rapid increase prior to reaching a steady-state temperature of 180 °C. The temperature measurement data is
Fig. 3. Substrate temperature as a function of O2 plasma exposure time.
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3.3. Stress analysis
Fig. 4. Stress change progression for a 100 nm thick SiCN film as a function of O2 plasma exposure time.
displayed graphically in Fig. 3. Within 20 min from O2 plasma ignition the chamber temperature becomes stabilized, where only a 3 °C increase over the next 40 min of plasma exposure is observed. Note in Fig. 2 we show the SiCN oxidation rate to be linear throughout the 60 minute plasma conversion. We therefore conclude that temperature is not the primary factor driving the oxidation process. This leaves plasma ion-to-radical density as the most likely mechanism for the control of oxidation rates.
A nominal 100 nm thick SiCN film is deposited onto a bare silicon substrate. Film stress is then measured vs. cumulative oxygen plasma exposure times for t = 0, 10, 30, 60, 75, and 90 min and is graphically displayed in Fig. 4. The film stress measured at t0 provides the as-deposited SiCN stress, which is shown to be highly compressive at 750 MPa. At t10 the film stress immediately begins to shift in the tensile direction as the oxygen plasma converts the single layer SiCN film into a SiO2/ SiCN bi-layer. Continued plasma exposure drives stress further in the tensile direction until the SiO2/SiCN bi-layer is converted entirely into a single layer SiO2 film. The film is shown by XPS analysis to be completely converted at t60, where the residual film stress is stabilized at 100 MPa (tensile) and remains unchanged during further plasma exposure. The fact that the postoxidized film stress remains constant is another indicator of the SiO2 film stability. 3.4. Film density analysis Density is determined by the weight-gain method using a Mettler AE-163 micro-balance with accuracy to 10 μg. Five 150 mm silicon substrates were weighed before and after SiCN deposition. The film thickness is determined by VASE analysis.
Fig. 5. AFM images, with scale bars inserted, show surface roughness for (A) as-deposited SiCN — (RMS 0.20 nm), (B) plasma oxidized SiCN — (RMS 0.18 nm), and (C) bare silicon substrate — (RMS 0.17 nm).
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3.6. Optical data Variable-angle spectroscopic ellipsometry is used to measure a 100 nm as-deposited and post-oxidized SiCN film as well as a thermally grown SiO2 film to determine the index of refraction (n) and extinction coefficient (k) for wavelengths ranging from 300 to 800 nm as displayed in Fig. 6. The as-deposited SiCN film is somewhat absorbing, especially at lower wavelengths as indicated by the increasing k values. As expected for the case of the plasma oxidized sample, the film is non-absorbing (k = 0) over the measured wavelength range. For reference we cite specific values for n and k at 632.8 nm, the wavelength of a HeNe laser typically used in single wavelength ellipsometry. When comparing the non-oxidized vs. oxidized film, n is 2.07 and 1.46 whereas k is 0.2 and 0.0 respectively. When comparing optical constants for plasma oxidized SiCN to those of thermally grown SiO2 (the standard), we find that they are nearly identical and within measurement error of the VASE system, as both materials exhibit n and k to be 1.46 and 0.00 respectively. 3.7. Discussions Fig. 6. Optical constants (n and k) of as-deposited SiCN, post-plasma converted SiCN, and the thermally grown SiO2.
The calculated substrate area and the measured film thickness provide the material volume from which a simple density calculation is performed for each of the five SiCN samples. The asmeasured SiCN film density ranged from 1.50–1.55 g/cm3 with a median value of 1.52 g/cm3. The measured density is a reflection of the Si:C ratio specific to this film. For the O2 plasma treated SiCN film, the median density increases to 2.0 g/cm3. The resultant density change is not unexpected and indeed fits quite well to the density of low temperature (b350 °C) PECVD SiOx films and is typically found to be in the range of 2.0–2.2 g/cm3 as determined by in-house deposited thin films. For comparison, the density of the reference thermally grown SiO2 using a dry O2 process has a density of 2.20 g/cm3 [13]. 3.5. Surface roughness A silicon wafer is deposited with 60 nm of SiCN and is then cleaved, with one half receiving an oxygen plasma process. Then, each half is measured for surface roughness by AFM. The RMS roughness for the as-deposited SiCN is very low (0.20 nm) and is characteristic of plasma deposited carbon-rich films [14–17]. Likewise, the plasma oxidized sample shows an equivalent roughness value of 0.18 nm. This trend holds true for Z-range values within a 5.0 μm2 sampling region as well, where the asdeposited SiCN and the plasma oxidized film show a 3.5 nm and 2.4 nm Z-range respectively. Both materials are considered to be very smooth for PECVD thin films and appear to emulate the starting surface roughness of polished silicon substrates, (0.17 nm RMS and 2.35 nm Z-range). The series of AFM images corresponding to the aforementioned samples are shown in Fig. 5.
In Sections 3.1–3.6 we have used systematic chemical and physical analyses to support an ultra-low temperature method for converting a PECVD SiCN film to SiO2 by exposure to oxygen plasma. However, to some extent this evidence would appear to refute one of the three basic tenants generally ascribed to SiCN materials, that they have high resistance to oxidation. Hence, it is relevant to briefly address the two remaining properties most cited for SiCN materials, those being hardness and chemical reactivity. While we do not have exacting data for hardness of the SiCN film referenced in this work, we do offer supporting data based on micro-hardness values obtained for a PECVD SiC film, (SiCN with N = 0). For that film, micro-hardness is measured using a Hysitron Tribo-Indentor with a three-sided pyramidal Berkovich diamond tip and is found to be 19.7 GPa. Many researchers have shown the inclusion of N up to 30–35% during the formation of SiCN leads to enhanced film hardness [8,18]. Therefore it is our belief that the SiCN material reported in this work would have a minimum hardness equivalent to the SiC film (19.7 GPa). To address chemical reactivity, SiCN wet etch rate data is collected for two different chemical etch solutions, a 10:1 buffered oxide etch at ambient conditions and a 50/50 KOH/H2O solution at 95 °C. For these tests, the calculated SiCN etch rate is less than 0.15 nm/min. In addition, the SiCN has been found to be a very effective etch-stop material during chemical–mechanical polishing and has also shown high selectivity to SiO2 during plasma reactive ion-etch processes. Recall in Section 3.2, where it is concluded that temperature has no measurable effect on SiCN oxidation rates. Therefore it becomes more plausible that plasma power and process pressure which is used to control ion and radical density as well as gas residence time are the dominant factors driving the plasma oxidation reaction. To test this theory, we compared the original O2 plasma system (system-1) to a different remote O2 plasma system (system-2) which has the ability to control and maintain steady-state temperature throughout the oxidation process.
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Matching recipes were generated for each system with the only variable being temperature. While system-2 maintained a constant chamber temperature of 250 °C, the temperature inside system-1 was left uncontrolled and thereby emulates the temperature profile displayed in Fig. 3. Subsequent analysis via XPS and VASE showed equal amounts of SiCN were converted into SiO2 during a 30 minute O2 plasma exposure. This finding further supports the premise that temperature is not the driving force in the conversion process. Likewise, when changing process pressure by a factor of four, the oxidation process became virtually non-existent, as less than 2.0 nm of the surface showed indication of SiO2 conversion. A thin film having the capability to alter is chemical and physical make-up in post process will have numerous applications in micro-electro-mechanical-systems (MEMS), microelectronics, and optical devices. As such, we have used the plasma deposited SiCN material in numerous applications. It has been used as a hard-stop layer during CMP of plasma oxides over arrays of memory cells; as a convertible KOH etch-stop layer in the formation of SiOF membranes on silicon substrates for 157 nm lithography applications; as a convertible RIE etch-stop layer to produce multi-wavelength Fabry–Perot filters; and as a convertible hard-coating to mitigate integration issues associated with the fabrication of nano-imprint lithography templates. In each application and after the SiCN has served its intended purpose, it is then converted into SiO2 to ensure device performance remains unaffected. For MEMS devices it may aid in the ability to tune film stress or produce smooth low friction surfaces. Likewise, potential uses in optical devices such as waveguides that can be easily produced by selectively masking the oxidation process to produce channels of high and low index materials while maintaining a smooth surface and interface. 4. Summary In this work we have shown an ultra-low temperature method for the oxidation of a ternary PECVD SiCN material. The resultant material is indistinguishable from thermally grown SiO2 by XPS or VASE analysis. The material transformation occurs virtually without change to film thickness or surface roughness. The oxidation process is not temperature driven as reaction rates remain constant for both increasing and steady-state temperature regimes. Furthermore, the plasma
oxidation process is shown to drive residual film stress from a compressive to tensile state and it decreases the optical constants n and k while simultaneously increasing film density. The conversion of this SiCN film by oxygen plasma at very low temperatures to produce SiO2 has many potential applications in MEMS, microelectronics, and optical devices. Acknowledgements The authors would like to extend many thanks to Bill Dauksher for his useful discussion regarding oxygen plasma processing, to David Standfast for the use of the O2 plasma toolsets and performing the operational set-up for the plasma oxidation process, and to Papu Maniar and his team who support and operate Motorola's Physical Technologies Laboratory. References [1] K.B. Sundaram, J. Alizdeh, Thin Solid Films 370 (2000) 151. [2] S. Ting, Y. Fang, W. Hsieh, Y. Tsair, C. Chang, C. Lin, M. Hseih, H. Chiang, J. Ho, IEEE Electron Device Lett. 23/3 (2002) 142. [3] X. Peng, L. Song, J. Meng, Y. Zhang, X. Hu, Appl. Surf. Sci. 173 (2001) 331. [4] I. Martin, M. Vetter, A. Orpella, C. Voz, J. Puigdollers, R. Alcubilla, Appl. Phys. Lett. 81 (2002) 4461. [5] P. Jedrzejowski, J. Cizek, A. Amassian, J. Klemberg-Sapieha, J. Vleck, L. Martinu, Thin Solid Films 447–448 (2004) 201. [6] D. Zhang, Y. Gao, J. Wei, Z. Mo, Thin Solid Films 337–378 (2000) 607. [7] N. Park, S. Kim, G. Sung, Electron. Telecom. Res. Inst. J. 26/3 (2004) 257. [8] X. He, T. Taylor, R. Lillard, K. Walter, M. Nastasi, J. Phys., Condens. Matter 12 (2000) L591. [9] B. Tsui, K. Fang, C. Wu, Y. Li, IEEE Trans. Semicond. Manuf. 18 (2005) 716. [10] S. Smith, S. Voight, H. Tompkins, A. Hooper, A. Talin, J. Vella, Thin Solid Films 398–399 (2001) 163. [11] G. Stoney, Proc. R. Soc. Lond., Ser. A 82 (1909) 172. [12] J. Moulder, W. Stickle, P. Sobol, K. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perking-Elmer Corporation, Physical Electronics Division, 1992, p. 252. [13] K. Petersen, IEEE Electron Device Trans. 25 /10 (1978) 1241. [14] Y. Kim, S. Yoon, S. Suh, J. Lee, G. Jang, D. Yoon, J. Ceram. Proc. Res. 6 (2005) 205. [15] L. Valentini, J. Kenny, G. Mariotto, P. Tosi, G. Carlotti, G. Socino, L. Lozzi, S. Santucci, J. Vac. Sci. Technol., A, Vac. Surf. Films 19/4 (2001) 1611. [16] G. Yu, B. Tay, Z. Sun, L. Pan, Appl. Surf. Sci. 219 (2003) 228. [17] G. Capote, R. Prioli, P. Jardim, A. Zanatta, L. Jacobsohn, F. Freire, J. NonCryst. Solids 338–340 (2004) 503. [18] Z. Cao, Thin Solid Films 401 (2001) 94.
Thin Solid Films 516 (2008) 885 – 890 www.elsevier.com/locate/tsf
Conversion of a plasma enhanced chemical vapor deposited silicon–carbon–nitride thin film at ultra-low temperature by oxygen plasma Steven M. Smith ⁎, Tim Tighe, Diana Convey, Jaime Quintero, Yi Wei Motorola Inc., Embedded Systems Research Lab EL317, 2100 E. Elliot Rd., Arizona 85284, USA Received 10 July 2006; received in revised form 5 April 2007; accepted 19 April 2007 Available online 3 May 2007
Abstract In this work we present an ultra-low temperature method for the oxidation of an amorphous silicon–carbide–nitride (SiCN) material. The SiCN is deposited on silicon substrates by plasma enhanced chemical vapor deposition using CH4, SiH4, and N2 chemistry. The physical and chemical properties are characterized for the as-deposited SiCN and post-oxidized films are discussed. The SiCN film is exposed to oxygen plasma, where it undergoes a chemical transformation into a binary SiO2 material system. A 1.7 nm/min oxidation rate is typical for this process and compares favorably to oxidation methods utilizing much higher temperatures. The substrate temperature remains extremely low throughout the oxidation process, Ts b 200 °C. Changes in film stress, optical constants, film thickness, surface roughness, and film density are measured. Chemical analysis by X-ray photoelectron spectroscopy is reported for both the as-deposited and oxidized film and confirms the resultant film to be the chemical equivalent of thermally grown SiO2. We discuss applications specifically targeted to the conversion of SiCN to SiO2. © 2007 Elsevier B.V. All rights reserved. Keywords: SiCN; PECVD; Oxidation; SiO2
1. Introduction Ternary silicon carbide-nitride materials have traditionally been sought due to their unique physical and chemical properties. This class of material has been categorized as being extremely hard, possessing low chemical reactivity, and is resistant to oxidation. Researchers have investigated SiCN materials produced by a variety of deposition techniques. These techniques include sputter deposition [1–3], plasma CVD [4–6], and laser ablation [7]. Additionally, numerous permutations based on these techniques have been reported. Variations in material properties are directly affected by the deposition technique, the type of target material or chemical precursor(s) used, and the process parameter space being investigated. For these reasons it often becomes difficult to make direct comparisons of ternary materials produced by varying means. ⁎ Corresponding author. Tel.: +1 480 413 7798; fax: +1 480 413 5453. E-mail address: [email protected] (S.M. Smith). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.04.153
While numerous publications have referenced oxidation resistance as a basic material property for SiCN materials, few researchers have published oxidation data specific to their referenced film. Of the limited published oxidation rates for SiCN, the method cited is predominantly high temperature air or oxygen exposure. For ion-enhanced sputter-deposited SiCN films, He et al. have shown oxidation rates in flowing O2 at 1000 °C of 2.1 nm/min [8]. When comparing 950 °C wetoxidation of PECVD SiC to SiCN films with varying N content (5.0 at.% and 9.3 at.%), Tsui et al. found the oxidation rate decreased for increasing N-content, 1.33 and 0.73 nm/min respectively. In the same work, a PECVD SiC film (N = 0.0 at.%) was oxidized at a significantly reduced temperature (550 °C) with a reported oxidation rate of 3.0 nm/min [9]. Ternary materials can be altered dramatically by changing the chemical make-up of the film. For example, silicon oxynitride (SiON) materials can be modified to be extremely nitride-like (SiN) or very oxide-like (SiO), where each SiN/SiO variation produces a film having different chemical, physical,
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and optical properties. Likewise, similar material transitions hold true for ternary SiCN as well. As stated previously, the choice of deposition techniques can play an important role in defining the final material property. In this work we use PECVD for its robust process latitudes, as it is capable of depositing virtually any material over virtually any surface or substrate. The use of PECVD in industry is driven by the need to reduce thermal budgets, which are much lower than conventional thermally driven chemical vapor deposition (CVD) processes used to produce very high quality materials. As such, it is not uncommon to produce equivalent films at temperatures 2–5 times lower than those produced by thermal CVD techniques. Additionally, PECVD techniques provide added benefits beyond simple temperature reduction such as significantly higher deposition rates and/or enhanced process flexibility to tailor properties like stress, density, surface roughness, and refractive index. 2. Experimental details 2.1. Deposition and oxidation equipment A ternary Si–C–N film is deposited using SiH4, N2, and CH4 source gases onto oxidized and un-oxidized n-type (100) Sisubstrates using a commercially available PECVD reactor. The system is load-lock designed and configured with an earthground resistively heated aluminum heaterblock (substrate electrode), a closed loop pressure control system, and a mass flow controlled gas distribution network. The reactor can be powered by single or dual frequency (13.56 MHz and variable 100–450 kHz) power supplies that provide power directly to the gas-distribution showerheads, which reside directly above the substrate. Deposition conditions used to produce the SiCN film were derived from earlier work to develop diamond-like carbon films [10]. The deposition temperature is b350 °C and plasma power is in kilowatts. A plasma ash system is used to perform the plasma oxidation of the as-deposited SiCN film. The system is commonly found in integrated circuit (I.C.) manufacturing facilities and is used to remove photo-resist, a material used for pattern transfer steps in I.C. integration schemes. A generic photo-resist strip recipe is selected for this work. A perforated aluminum cylinder with a diameter of 200 mm is placed inside the chamber with the SiCN coated substrates placed in the center of the cylinder on a quartz boat. This set-up effectively simulates a remote O2 plasma system by minimizing high energy charge effects. The O2 plasma process initiates at room temperature and slowly reaches a steady-state temperature of less than 200 °C within 20 min, a direct effect of the plasma energy. Exposure time is the only variable investigated and is necessary to determine the oxidation rate of the SiCN material. 2.2. Analysis tools Stress data were taken using an FSM-128 mapping tool from Frontier Semiconductor Incorporated. The FSM-128 employs a scanning laser to measure the change in curvature of the Si-
substrate. The film induced curvature is used to derive values for film stress using the Stoney equation [11]. Twelve laser scans per sample were taken at 30° offsets and were used to provide a 360° detailed stress map of the wafer surface. Stress values cited in this work are the average stress calculated from the 12 individual laser scans. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a Physical Electronics PHI Model 5400 XPS System equipped with a non-monochromated Mg Ka (1253.6 keV) anode running at 12 kV and 200 W. Depth profiles were collected using a Physical Electronics Ion Gun Model 04-300 operated at 3 kV, 25 mA and a 6 mm2 raster. Ultra-high purity argon was used as the sputtering gas. The sputter rate for both the SiCN and SiO2 films under these conditions are near equivalent and found to be ∼2 nm/min. The base pressure of the system during the depth profiles was 1 × 10− 5 Pa. The data was collected using pass energy of 44.75 eV and an energy step of 0.125 eV. The spectrometer was calibrated prior to the experiments using both the position and separation of the Au 4f7/2 (84.0 eV) and the Cu 2p3/2 (932.67) peaks. The background was considered as Shirley type and curve fitting was carried out using a non-linear least-squares fitting method employing a Gaussian–Lorentzian function. Quantification relied upon relative sensitivity factors supplied by the instrument manufacturer and by available in-house standards [12]. The atomic concentrations were found to have an error of less than +/−2% for both the SiCN and SiO2 depth profile. All film thickness and optical constant data were obtained using spectroscopic ellipsometry by means of a Woollam Variable Angle Spectroscopic Ellipsometer (WVASE). Three distinct angles of incidence were employed to collect optical spectra from 400 nm–800 nm wavelengths. A Cauchy layer with Urbach absorption was used for both the as-deposited and oxidized SiCN film. Thermally grown SiO2 is referenced for comparison to the plasma oxidized film. Models for each of these layers were obtained from the WVASE material files. Refractive index (n) and extinction coefficient (k) are reported at 633 nm wavelength for reference to single wavelength (HeNe) ellipsometry. Surface morphology specific to film roughness is determined by atomic force microscopy (AFM). Images were obtained using a Veeco Dimension 3000 AFM system. Statistics gathered from 5 μm × 5 μm images were captured in air using Si-etched tips in tapping mode. Root Mean Square (RMS) of the instrument noise is 0.11 nm. RMS and Z height information is used to describe the film's roughness and general morphology for both as deposited films and after the O2 plasma conversion process. 3. Results and discussion 3.1. Chemical analysis — XPS A silicon wafer is first oxidized by placing in a quartz furnace and flowing high purity oxygen at 1000 °C to effectively grow 200 nm of SiO2. The thermally grown oxide is referenced as the calibration standard for SiO2 in this work. Next, the sample is deposited with 100 nm of PECVD SiCN and is then cleaved, where only one half is subjected to a 60 minute plasma oxidation using the plasma ash tool. Afterwards, both halves are analyzed
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Fig. 1. XPS depth profile for the as-deposited SiCN film (A) and the plasma oxidized SiCN film (B) on the oxidized silicon substrate.
using XPS depth profiling, with the results displayed in Fig. 1. As expected, a small amount of surface oxidation is observed at the surface of the SiCN film and similarly a mono-layer or less of adventitious carbon was observed on the plasma oxidized SiCN surface. The XPS data clearly shows the as-deposited SiCN composition is homogeneous throughout the bulk of the film and is found to be 66% C, 28% Si, and 6% N. The transitional interface between the SiCN film and the thermally grown SiO2 layer is observed after 40 min of depth profiling. The interface region is fully interrogated after 60 min of profiling, where the XPS scans shows complete loss of C and N while the increasing O and Si have leveled off, indicating full transition into the SiO2 layer. Likewise, the XPS chemical profile for the sample receiving the plasma oxidation aligns well to the chemical profile for the thermally grown SiO2, where both profiles produce equivalent compositions of 67% O and 33% Si. Note that for the plasma oxidized sample, the XPS profile time is extended by 25% to insure that chemical analysis on each side of the oxidized SiCN/SiO2 interface has occurred. However, we found no evidence of an interface, thus providing complementary evidence that the plasma oxidized SiCN film is the chemical equivalent of thermally grown SiO2. Equally interesting is the lack of C and N, indicating complete removal during the oxygen plasma reaction.
Fig. 2. Evolution of the SiO2 and SiCN thicknesses as obtained by VASE following the conversion of an SiCN film by O2 plasma as a function of O2 plasma exposure time.
3.2. Oxidation rate and film thickness A 105 nm SiCN film is deposited directly onto a 100 mm silicon wafer and is subsequently oxidized via O2 plasma. VASE analysis is used to measure the thickness of the SiCN and the oxidized equivalent film at varying intervals throughout the plasma oxidation process. The results displayed in Fig. 2 reveals that the total film thickness remains virtually unchanged. Interestingly, the oxidation rate is linear and calculated to be 1.7 nm/min, which indicates that the process is reaction-rate limited and not bounded by diffusion-rate laws. The plasma oxidation rate for our SiCN film compares favorably to the 2.1 nm/min shown by Tsui et al. for their PECVD SiCN material. However, those rates reflect a much higher oxidation temperature (950 °C) [9]. In Fig. 2, film thickness is plotted as a function of oxygen plasma exposure time, where VASE is used to measure the oxidized and non-oxidized layers of the SiCN film. A linear rate response is observed from 0 to 60 min, at which point the SiCN film is completely oxidized and is confirmed via VASE and XPS depth profile analysis techniques. Further oxygen plasma exposure did not alter the SiO2 thickness, indicating that the oxidation reaction is very efficient as it moves through the bulk film. This observation is confirmed by XPS analysis where no carbon or nitrogen is evident in the oxidized film. Temperature of the O2 plasma chamber specific to the chosen recipe shows a rapid increase prior to reaching a steady-state temperature of 180 °C. The temperature measurement data is
Fig. 3. Substrate temperature as a function of O2 plasma exposure time.
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3.3. Stress analysis
Fig. 4. Stress change progression for a 100 nm thick SiCN film as a function of O2 plasma exposure time.
displayed graphically in Fig. 3. Within 20 min from O2 plasma ignition the chamber temperature becomes stabilized, where only a 3 °C increase over the next 40 min of plasma exposure is observed. Note in Fig. 2 we show the SiCN oxidation rate to be linear throughout the 60 minute plasma conversion. We therefore conclude that temperature is not the primary factor driving the oxidation process. This leaves plasma ion-to-radical density as the most likely mechanism for the control of oxidation rates.
A nominal 100 nm thick SiCN film is deposited onto a bare silicon substrate. Film stress is then measured vs. cumulative oxygen plasma exposure times for t = 0, 10, 30, 60, 75, and 90 min and is graphically displayed in Fig. 4. The film stress measured at t0 provides the as-deposited SiCN stress, which is shown to be highly compressive at 750 MPa. At t10 the film stress immediately begins to shift in the tensile direction as the oxygen plasma converts the single layer SiCN film into a SiO2/ SiCN bi-layer. Continued plasma exposure drives stress further in the tensile direction until the SiO2/SiCN bi-layer is converted entirely into a single layer SiO2 film. The film is shown by XPS analysis to be completely converted at t60, where the residual film stress is stabilized at 100 MPa (tensile) and remains unchanged during further plasma exposure. The fact that the postoxidized film stress remains constant is another indicator of the SiO2 film stability. 3.4. Film density analysis Density is determined by the weight-gain method using a Mettler AE-163 micro-balance with accuracy to 10 μg. Five 150 mm silicon substrates were weighed before and after SiCN deposition. The film thickness is determined by VASE analysis.
Fig. 5. AFM images, with scale bars inserted, show surface roughness for (A) as-deposited SiCN — (RMS 0.20 nm), (B) plasma oxidized SiCN — (RMS 0.18 nm), and (C) bare silicon substrate — (RMS 0.17 nm).
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3.6. Optical data Variable-angle spectroscopic ellipsometry is used to measure a 100 nm as-deposited and post-oxidized SiCN film as well as a thermally grown SiO2 film to determine the index of refraction (n) and extinction coefficient (k) for wavelengths ranging from 300 to 800 nm as displayed in Fig. 6. The as-deposited SiCN film is somewhat absorbing, especially at lower wavelengths as indicated by the increasing k values. As expected for the case of the plasma oxidized sample, the film is non-absorbing (k = 0) over the measured wavelength range. For reference we cite specific values for n and k at 632.8 nm, the wavelength of a HeNe laser typically used in single wavelength ellipsometry. When comparing the non-oxidized vs. oxidized film, n is 2.07 and 1.46 whereas k is 0.2 and 0.0 respectively. When comparing optical constants for plasma oxidized SiCN to those of thermally grown SiO2 (the standard), we find that they are nearly identical and within measurement error of the VASE system, as both materials exhibit n and k to be 1.46 and 0.00 respectively. 3.7. Discussions Fig. 6. Optical constants (n and k) of as-deposited SiCN, post-plasma converted SiCN, and the thermally grown SiO2.
The calculated substrate area and the measured film thickness provide the material volume from which a simple density calculation is performed for each of the five SiCN samples. The asmeasured SiCN film density ranged from 1.50–1.55 g/cm3 with a median value of 1.52 g/cm3. The measured density is a reflection of the Si:C ratio specific to this film. For the O2 plasma treated SiCN film, the median density increases to 2.0 g/cm3. The resultant density change is not unexpected and indeed fits quite well to the density of low temperature (b350 °C) PECVD SiOx films and is typically found to be in the range of 2.0–2.2 g/cm3 as determined by in-house deposited thin films. For comparison, the density of the reference thermally grown SiO2 using a dry O2 process has a density of 2.20 g/cm3 [13]. 3.5. Surface roughness A silicon wafer is deposited with 60 nm of SiCN and is then cleaved, with one half receiving an oxygen plasma process. Then, each half is measured for surface roughness by AFM. The RMS roughness for the as-deposited SiCN is very low (0.20 nm) and is characteristic of plasma deposited carbon-rich films [14–17]. Likewise, the plasma oxidized sample shows an equivalent roughness value of 0.18 nm. This trend holds true for Z-range values within a 5.0 μm2 sampling region as well, where the asdeposited SiCN and the plasma oxidized film show a 3.5 nm and 2.4 nm Z-range respectively. Both materials are considered to be very smooth for PECVD thin films and appear to emulate the starting surface roughness of polished silicon substrates, (0.17 nm RMS and 2.35 nm Z-range). The series of AFM images corresponding to the aforementioned samples are shown in Fig. 5.
In Sections 3.1–3.6 we have used systematic chemical and physical analyses to support an ultra-low temperature method for converting a PECVD SiCN film to SiO2 by exposure to oxygen plasma. However, to some extent this evidence would appear to refute one of the three basic tenants generally ascribed to SiCN materials, that they have high resistance to oxidation. Hence, it is relevant to briefly address the two remaining properties most cited for SiCN materials, those being hardness and chemical reactivity. While we do not have exacting data for hardness of the SiCN film referenced in this work, we do offer supporting data based on micro-hardness values obtained for a PECVD SiC film, (SiCN with N = 0). For that film, micro-hardness is measured using a Hysitron Tribo-Indentor with a three-sided pyramidal Berkovich diamond tip and is found to be 19.7 GPa. Many researchers have shown the inclusion of N up to 30–35% during the formation of SiCN leads to enhanced film hardness [8,18]. Therefore it is our belief that the SiCN material reported in this work would have a minimum hardness equivalent to the SiC film (19.7 GPa). To address chemical reactivity, SiCN wet etch rate data is collected for two different chemical etch solutions, a 10:1 buffered oxide etch at ambient conditions and a 50/50 KOH/H2O solution at 95 °C. For these tests, the calculated SiCN etch rate is less than 0.15 nm/min. In addition, the SiCN has been found to be a very effective etch-stop material during chemical–mechanical polishing and has also shown high selectivity to SiO2 during plasma reactive ion-etch processes. Recall in Section 3.2, where it is concluded that temperature has no measurable effect on SiCN oxidation rates. Therefore it becomes more plausible that plasma power and process pressure which is used to control ion and radical density as well as gas residence time are the dominant factors driving the plasma oxidation reaction. To test this theory, we compared the original O2 plasma system (system-1) to a different remote O2 plasma system (system-2) which has the ability to control and maintain steady-state temperature throughout the oxidation process.
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Matching recipes were generated for each system with the only variable being temperature. While system-2 maintained a constant chamber temperature of 250 °C, the temperature inside system-1 was left uncontrolled and thereby emulates the temperature profile displayed in Fig. 3. Subsequent analysis via XPS and VASE showed equal amounts of SiCN were converted into SiO2 during a 30 minute O2 plasma exposure. This finding further supports the premise that temperature is not the driving force in the conversion process. Likewise, when changing process pressure by a factor of four, the oxidation process became virtually non-existent, as less than 2.0 nm of the surface showed indication of SiO2 conversion. A thin film having the capability to alter is chemical and physical make-up in post process will have numerous applications in micro-electro-mechanical-systems (MEMS), microelectronics, and optical devices. As such, we have used the plasma deposited SiCN material in numerous applications. It has been used as a hard-stop layer during CMP of plasma oxides over arrays of memory cells; as a convertible KOH etch-stop layer in the formation of SiOF membranes on silicon substrates for 157 nm lithography applications; as a convertible RIE etch-stop layer to produce multi-wavelength Fabry–Perot filters; and as a convertible hard-coating to mitigate integration issues associated with the fabrication of nano-imprint lithography templates. In each application and after the SiCN has served its intended purpose, it is then converted into SiO2 to ensure device performance remains unaffected. For MEMS devices it may aid in the ability to tune film stress or produce smooth low friction surfaces. Likewise, potential uses in optical devices such as waveguides that can be easily produced by selectively masking the oxidation process to produce channels of high and low index materials while maintaining a smooth surface and interface. 4. Summary In this work we have shown an ultra-low temperature method for the oxidation of a ternary PECVD SiCN material. The resultant material is indistinguishable from thermally grown SiO2 by XPS or VASE analysis. The material transformation occurs virtually without change to film thickness or surface roughness. The oxidation process is not temperature driven as reaction rates remain constant for both increasing and steady-state temperature regimes. Furthermore, the plasma
oxidation process is shown to drive residual film stress from a compressive to tensile state and it decreases the optical constants n and k while simultaneously increasing film density. The conversion of this SiCN film by oxygen plasma at very low temperatures to produce SiO2 has many potential applications in MEMS, microelectronics, and optical devices. Acknowledgements The authors would like to extend many thanks to Bill Dauksher for his useful discussion regarding oxygen plasma processing, to David Standfast for the use of the O2 plasma toolsets and performing the operational set-up for the plasma oxidation process, and to Papu Maniar and his team who support and operate Motorola's Physical Technologies Laboratory. References [1] K.B. Sundaram, J. Alizdeh, Thin Solid Films 370 (2000) 151. [2] S. Ting, Y. Fang, W. Hsieh, Y. Tsair, C. Chang, C. Lin, M. Hseih, H. Chiang, J. Ho, IEEE Electron Device Lett. 23/3 (2002) 142. [3] X. Peng, L. Song, J. Meng, Y. Zhang, X. Hu, Appl. Surf. Sci. 173 (2001) 331. [4] I. Martin, M. Vetter, A. Orpella, C. Voz, J. Puigdollers, R. Alcubilla, Appl. Phys. Lett. 81 (2002) 4461. [5] P. Jedrzejowski, J. Cizek, A. Amassian, J. Klemberg-Sapieha, J. Vleck, L. Martinu, Thin Solid Films 447–448 (2004) 201. [6] D. Zhang, Y. Gao, J. Wei, Z. Mo, Thin Solid Films 337–378 (2000) 607. [7] N. Park, S. Kim, G. Sung, Electron. Telecom. Res. Inst. J. 26/3 (2004) 257. [8] X. He, T. Taylor, R. Lillard, K. Walter, M. Nastasi, J. Phys., Condens. Matter 12 (2000) L591. [9] B. Tsui, K. Fang, C. Wu, Y. Li, IEEE Trans. Semicond. Manuf. 18 (2005) 716. [10] S. Smith, S. Voight, H. Tompkins, A. Hooper, A. Talin, J. Vella, Thin Solid Films 398–399 (2001) 163. [11] G. Stoney, Proc. R. Soc. Lond., Ser. A 82 (1909) 172. [12] J. Moulder, W. Stickle, P. Sobol, K. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Perking-Elmer Corporation, Physical Electronics Division, 1992, p. 252. [13] K. Petersen, IEEE Electron Device Trans. 25 /10 (1978) 1241. [14] Y. Kim, S. Yoon, S. Suh, J. Lee, G. Jang, D. Yoon, J. Ceram. Proc. Res. 6 (2005) 205. [15] L. Valentini, J. Kenny, G. Mariotto, P. Tosi, G. Carlotti, G. Socino, L. Lozzi, S. Santucci, J. Vac. Sci. Technol., A, Vac. Surf. Films 19/4 (2001) 1611. [16] G. Yu, B. Tay, Z. Sun, L. Pan, Appl. Surf. Sci. 219 (2003) 228. [17] G. Capote, R. Prioli, P. Jardim, A. Zanatta, L. Jacobsohn, F. Freire, J. NonCryst. Solids 338–340 (2004) 503. [18] Z. Cao, Thin Solid Films 401 (2001) 94.