The effect of diluent gas and rapid thermal annealing on the properties of plasma-deposited silicon nitride films

Thin Solid Films, 209 (1992) 215-222

215

The effect of diluent gas and rapid thermal annealing on the properties of plasma-deposited silicon nitride films C h u l W o o N a m a n d S e o n g Ihl W o o * Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, PO Box 150, Cheongryang, Seoul (South Korea)

Yong Tae Kim and Suk-Ki Min Semiconductor Materials Laboratory, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul (South Korea) (Received June 17, 1991; accepted October 7, 1991)

Abstract The effects of diluent gases on the deposition rate, content of hydrogen bonds, etch rate, refractive index and fixed charge density of plasma-enhanced chemical vapour deposition silicon nitride films have been studied for various reactant gas compositions. Post-rapid thermal annealing (RTA) of as-grown films was also performed in the temperature range 500-1100 °C. From the results of optical emission spectroscopy it is suggested that metastable N 2 molecules can contribute to silicon nitride film formation as an extra nitrogen source. Metastable N 2 molecules may enhance the dissociation of other feed gases. Lower refractive index, higher deposition rate and higher N-H bond density were obtained for silicon nitride films grown in N2 diluent as compared with films grown in H2 diluent. As the RTA temperature was increased, the content of hydrogen bonds was reduced and the film density increased. Hence the etch rate of silicon nitride films in buffered HF solution decreased with increasing RTA temperature. A minimum value of fixed charge density was obtained at the RTA temperature of 900 °C for all the films investigated.

1. Introduction Plasma-enhanced chemical vapour deposition (PECVD) silicon nitride has been widely used as an interlevel dielectric material [ 1, 2] and in encapsulation layers for GaAs and InP devices [3, 4] in microelectronics applications owing to its low deposition temperature (300-400 °C) and the excellent passivation properties of silicon nitride layers against the diffusion of moisture and alkaline ions [5]. There are many previous studies indicating that the physical and electrical properties of the deposited films are dependent on the type of reaction gas, flow rate, r.f. frequency, power density and substrate temperature [2,3,5,6]. However, certain problems have inhibited its universal application. One of the problems is the high levels of atomic hydrogen formed by reacting Sill 4 and NH3 (and/or H2, N2) in the glow discharge process [2, 5, 6]. These H atoms in plasma nitride films are generally combined with Si and N atoms. The Si-H and N-H bonds decrease the thermal stability, increase the film etch rate and change the electrical properties of the films [2, 7] depending on the types of hydrogen bond. Furthermore, when a thermal annealing process is involved during subsequent microelectronic device

*Author to whom correspondence should be addressed.

0040-6090/92/$5.00

fabrication steps, hydrogen bonds such as Si-H and N - H are broken to form H2 gas which diffuses out from the film. This H 2 evolution results in a significant change in the properties of silicon nitride films. Hence post-heat treatment of PECVD silicon nitride films is required. In this study we report the dependence of deposition rate, refractive index and N-H bond density on the variation in feed gas composition of the SiHn-NH3-H2 and SiHa-NH3-N 2 systems. The possible roles of N 2 diluent gas in the deposition of silicon nitride films are discussed. The effect of post-rapid thermal annealing (RTA) on the hydrogen bond evolution, etch rate and fixed charge density of plasma nitride films is also investigated.

2. Experimental details Silicon nitride films were deposited in a parallel plate, capacitively coupled plasma reactor. The r.f.-excited electrode was the upper electrode which was 32.0 cm in diameter. The radio frequency was 13.56 MHz. Silicon substrates were placed on the lower electrode which was grounded and 28.0 cm in diameter. The distance between the two electrodes was 4.0 cm. The reactor was initially evacuated to 1.3 x 10-a Pa by a diffusion pump and a mechanical pump. The substrate was borondoped (100)-oriented silicon wafer with a resistivity of

© 1992 - - Elsevier Sequoia. All rights reserved

216

C. W. Nam et al. ] Properties of plasma-deposited silicon nitride films

5 - 6 laf~ cm. Two kinds of reactant gas system were used, namely SiH4-NH3-H2 and SiH4-NH3-N2. The composition of the feed gas was varied in such a manner that the Sill 4 flow rate was fixed at 3 standard cm 3 min (sccm) while the NH3 flow rate varied from 3 to 30 sccm. The remainder of the gas was diluent gas, either N2 or H2. The total flow rate was fixed at 60 sccm. The total pressure was maintained at 40 Pa (0.3 Torr) by using a throttle valve. The r.f. power and substrate temperature were fixed at 40 W and 300 °C respectively. The film thickness was measured by an ellipsometer and nanospectrometer (AFT-100). Fourier transform infrared ( F T I R ) spectroscopy measurements (Bomem MB-101) were used to determine the variation in hydrogen bond configurations with feed gas composition and R T A temperature. In addition, the etch rate was measured to determine the correlation between the etch rate in buffered H F solution (48 wt.% HF) and the hydrogen concentration in the film. In order to evaluate the fixed charge density from the flat-band voltage of the C - V curve, high frequency C - V measurements ( 1 MHz) of a capacitor consisting of aluminium dots and a plasma nitride film 1000 A thick prepared at various gas compositions and R T A temperatures were carried out. In order to monitor the plasma deposition process, optical emission spectroscopy (OES) was used. The emission light from the glow discharge of different feed gas mixtures was focused with a quartz lens and introduced into a m o n o c h r o m a t o r (SPEX, 0.25 m) [8]. The experimental apparatus for OES consisted of a photomultiplier tube ( R C A 31034 A) and a conventional photocounting system comprising a discriminator, a multichannel scaler and a personal computer. Emission spectra between 200 and 800 nm with a resolution of 0.1 nm were obtained.

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3.1. Diluent gas effect on the film properties Figure 1 shows the dependence of the deposition rate and refractive index of silicon nitride films on the feed gas composition with different diluent gases. When H2 is used as the diluent gas, the deposition rate increases as the NH3:SiH4 ratio increases, while the refractive index decreases. However, when N2 is used as the diluent gas, the deposition rate decreases as the NH3:SiH 4 ratio increases, while the refractive index is somewhat insensitive to the NH3:SiH4 ratio and is smaller than in the case of H2 diluent gas. When NH3:SiH 4 = 10, the deposition rate and refractive index obtained in the case of N2 diluent gas are similar to those obtained in the case of H2 diluent gas. The IR absorption spectra exhibit peaks at 870, 2170 and 3350 c m - ~which can be assigned to the Si-N lattice vibration mode and the Si-H and N - H stretching modes

0

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NH3/SiH 4 Ratio Fig. 2. (a) IR intensity of N-H stretching band and (b) etch rate of silicon nitride films prepared with gas mixtures of Sill4 NH3-N2(*) and SiH4-NH3-H2(O).

respectively. The absorption band due to Si-H can hardly be detected or is relatively small compared with that due to N - H . Figure 2(a) shows the absorbance of the N - H stretching band at 3350 cm t of films prepared at various gas compositions in the range NH3:SiH4 = 1-10 in H2 or N2 diluent gases. The number of N - H bonds increases with increasing NH3:SiH4 ratio. N - H bonds are more sensitive to the N H 3 : S i I I 4 ratio in the case of N2 diluent gas than in the case of H2 diluent gas. The content of N H bonds in the films grown with N 2 diluent gas is about twice that of N - H

217

C. W. Nam et al./ Properties of plasma-deposited silicon nitride films 25930

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Fig, 3. Typical optical emission spectra observed from the plasma of SiH4-NH3-H2: (a) SiH4:NH3:H2= 3:3:54; (b) SiH4:NH3:H2 = 3:30:27.

bonds in the case of H2 diluent gas at the same value of the NH3:SiH 4 ratio. It is well known that when the hydrogen concentration is high, the etch rate of nitride films in buffered H F solution is also increased [9]. Figure 2(b) shows the dependence of the etch rate on the NH3 :Sill4 ratio and diluent gas. This clearly indicates that the etch rate is increased owing to the increase in content of N - H bonds.

5

3.2. Optical emission spectroscopy

Typical results of OES for the SiH4-NH3-H2 gas system are shown in Fig. 3. Several emission lines are identified [10], such as NH* (336 nm), Sill* (414 nm), Si* (288 nm), H2* (602 nm) and H* (656 nm), which are dominant and sensitive to the gas mixture ratio. The peak at 750 nm is the emission line from excited argon which is used as a reference peak. The flow rate of argon is fixed at 0.5 seem, which is less than 1% of the total flow rate of the feed gas. The emission intensities of excited argon atoms are almost identical in both feed gas compositions, as shown in Figs. 3(a) and 3(b). This result indicates that the excitation efficiency does not change with different gas mixtures. Therefore it is possible that the concentrations of dissociated species such as Sill or N H are proportional to the ratio of the emission intensities of their respective peaks to the emission intensity o f the argon peak providing the validity of the actinometry [11]. Figure 4 shows the variation in relative intensities of NH*, Sill* and H* with NH3:SiH 4 ratio. The intensity of argon at 750 nm is taken as a reference peak. With increasing N H 3 flow rate the relative intensity o f NH* increases while that of

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NH3 Flow Rate (sccm) Fig. 4. Relative intensities of NH* (©), Sill* (×), H*(,) and H2*( + ). Referencepeak is 750 nm of argon. The flow rate of argon (0.5 seem) is less than 1% of total flow rate.

both Sill* and H* is rather constant, indicating that only the addition of N H 3 contributes to the increment in the concentration of N H radicals. In the emission spectrum of the SiH4-NH3-N 2 gas system shown in Fig. 5 we could hardly observe the NH* and Sill* peaks which were dominant in H2 diluent gas. No emission from atomic nitrogen was detectable, partly because of the high dissociation energy of N2 (9.76 eV). Unfortunately, all we can observe are many emission lines originating from various excited states of N2. The argon actinometry is not applicable because the argon peak is not detected. The normalized intensity of the N 2 peaks at 336 and 357 nm for various NH3 :Sill4 ratios is shown in Fig. 6. The intensity of N2 at 336nm

218

C. W. Nam et al. / Properties of plasma-deposited silicon nitridefilms

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Fig. 6. Intensities of N2* at 336 nm (©) and 357 nm (*), normalized to the intensity of N 2 at 336 nm, obtained with a feed gas composition SiH4:NH3:N2 = 3:3:54.

obtained with the feed gas composition SiH4:NH~:N2 = 3:3:54 is taken as 1.0 and other peaks are normalized to the intensity of this peak. As can be seen in this figure, the intensities of the Nz* peaks at 336 and 357 nm decrease with the addition of N H 3.

3.3. Fixed charge density In order to determine the correlation between the fixed charge density and the N - H - t y p e hydrogen bond density, the fixed charge density of an aluminum/silicon nitride/silicon structure was measured. Plasma silicon nitride films 1000/~ thick were deposited with varying NH3:SiH 4 ratios in H 2 and N2 diluent gases. Figure 7 shows that the fixed charge density decreases with

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Fig. 7, Dependence of fixed charge density on feed gas composition. increasing NH3:SiH4 ratio, which is consistent with the work reported by Hezel and Schorner [12].

3.4. Rapid thermal annealing o f SiN films Post-RTA of as-grown films in the temperature range 500-1100 °C was performed to investigate the change in film properties. C - V curves obtained for various R T A temperatures are shown in Fig. 8. The feed gas composition of the as-grown films was SiH4:NH3:H2 = 3:3:54. A large hysteresis of about 6 V is observed with as-grown films. With increasing R T A temperature the hysteresis decreases and the fiat-band voltage is shifted in the direction of positive voltage up to the R T A temperature of 900 °C. However, the hysteresis again increases and the fiat-band voltage is shifted in the direction of negative voltage as the R T A temperature is increased above 900 °C. Figure 9 shows the

C. W. Nam et al. / Properties of plasma-deposited silicon nitride films 1.0

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1100°C

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Fig. 10. IR absorption spectra of silicon nitride films at various R T A temperatures. Deposition conditions of as-grown films: r.f. power 40 W, feed gas composition SiH4:NH3:N 2 = 3:15:42.

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towards accumulation to avoid possible effects of charge injection from silicon into silicon nitride on the fixed charge density. In the case of films grown with a feed gas composition SiHa:NH3:H2 = 3:3:54 it is seen that the fixed charge density is reduced from 9.5 x 10 ~2 to 2.8 x 10 ]2 cm -2 up to an annealing temperature of 900 °C. The intensities of the N - H and Si-H bonds decrease as the RTA temperature is increased to l l00°C. The Si-H stretching band at 2170cm -] and the N - H bending band at 1170 cm- ] are almost completely eliminated at 1100 °C as shown in Fig. 10. An increase in RTA temperature leads to the densification of silicon nitride films, resulting in an increase in film density and a decrease in the etch rate in buffered H F solution as shown in Fig. 11. Figure 12 shows that shrinkage of the film thickness accompanies the RTA process.

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Fig. 9. Effect of R T A temperature on fixed charge density of silicon nitride films: x , SiH4:NH3:H 2 = 3:3:54; + , SiH4:NH3:N 2 = 3:3:54; • , SiH4:NHa:H2 = 3:15:42; ©, SiH4:NH3:N2 = 3:15:42.

dependence of fixed charge density on RTA temperature. The duration of the RTA process was 10 s and the fixed charge density was determined from the initial C - V curves which were recorded starting from inversion

4.1. Role of diluent gas Many previous works reported that the contents of hydrogen bonds (Si-H and N - H ) were dependent upon several deposition variables, i.e. deposition temperature, r.f. power and gas composition. It is now well known that high temperatures or high r.f. powers lead to films containing fewer hydrogen bonds owing to facilitated hydrogen desorption during the deposition process [2]. However, plasma silicon nitride thin films deposited at different gas compositions of NH3:SiH4 ratios with diluent gases ( H E o r N 2 ) do not show a simple correlation between the deposition rate, refractive index, etch

C. W. Nam et al. / Properties of plasma-deposited silicon nitride films

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rate, fixed charge density and the types of hydrogen bonds. It is well known that PECVD silicon nitride films have lower refractive index as the atomic ratio of silicon to nitrogen (Si:N) in the films decreases [13, 14]. Therefore it can be proposed that films deposited at higher NH3:SiH 4 ratio have lower Si:N ratio, resulting in the lower refractive index as shown in Fig. 1. In the case of N 2 diluent gas the refractive index is lower than that of silicon nitride films grown in H2 diluent, indicating that the Si:N ratio in the nitride layers is lower than that of films grown in H2 diluent. This result suggests that the N2 diluent gas itself participates in the deposition process as a nitrogen source. Claassen et al. [14] measured the refractive index of silicon nitride films as a function of NH3 partial pressure with different diluent gases such as argon, H2 and N2. The refractive index with argon diluent was highest, while the lowest value was obtained with N2 diluent.

However, the possible roles of the diluent gases have not been presented. They also reported that the hydrogen content in the nitride films was constant (about 23%) regardless of the diluent gas, which is in disagreement with our results. It is presumed that most of the hydrogen desorption is influenced mainly by the substrate temperature or by the radical concentration in the gas phase. Therefore the higher N H bond content obtained with N2 diluent as shown in Fig. 2(a) indicates that at the same NH3:SiH 4 ratio the concentration of N H , radicals in the plasma is higher when N 2 is used as the diluent gas instead of HR. However, it is not clear that whether Ne is dissociated into N atoms to recombine with H atoms to form N - H or whether N2 diluent gas enhances the dissociation rate of other feed gases s u c h a s N H 3 and S i l l 4. Optical emission spectroscopy shows that in H2 diluent gas the intensity of excited hydrogen (Ha*) is relatively weak compared with other peaks such as Sill or N H (Fig. 3), while the intensity of excited nitrogen (N2*) is strong and dominant in N2 diluent gas. Additionally, when H2 is used as the diluent gas, the emission intensity of excited hydrogen (H2*) is rather constant regardless of the N H 3 flow rate, as shown in Fig. 4. However, the intensities of the N2* peaks decreased with the addition of N H 3 as shown in Fig. 6, which is in good correlation with the deposition rate of nitride films grown in N2 diluent as shown in Fig. 1. Therefore it is reasonable that at least these reactive N2 molecules which are in an electronically excited state would participate in the formation of silicon nitride films as a source of nitrogen, leading to the higher deposition rate and lower refractive index of silicon nitride films grown in N2 diluent gas. The sharp decrease in deposition rate between NH3:SiH 4 = l and 3 in Fig. 1 might be due to the sharp decrease in the amount of energized metastable N2*. The dominant emission lines in the plasma of N 2 diluent gas are observed at wavelengths of 336 and 357 nm as shown in Fig. 5. These emission lines are known to be due to the electronic transition of N2, C31-Ig "-1" B3Mg [10]. However, Jeunehomme [15] reported that the lifetimes of the B 3 H state were relatively short, 4 - 8 gs. Therefore it is readily expected that excited N 2 molecules in t h e B31-Ig state are rapidly relaxed to the lower energy s t a t e A3Yu +. In the case of the A352u + state, which is the metastable state of N2, the lifetime is about 2 s [ 16]. Also, the threshold energy of the A3Yu + state is 6.14eV [17]. This energy is high enough for N H 3 to be decomposed to N H + H 2 (3.9eV) or N H R + H (4.5eV) provided that these metastable N2 molecules transfer their energy through inelastic collisions, i.e. the Penning reaction [ 18]. Consequently, the concentration of reactive NHx radicals in the plasma would be increased by this Penning reaction,

C. IV. Nam et al. / Properties of plasma-deposited silicon nitride films

resulting in the higher concentration of N-H bonds in the films grown in N2 diluent as shown in Fig. 2(a). 4.2. Fixed charge density According to the results of Hezel and coworkers [7, 12], the interface state density of PECVD silicon nitride is attributable to unsaturated silicon dangling bonds, which may be enhanced by ion- and electronbombardment-induced hydrogen desorption during the plasma deposition process. They showed that both fixed charge density and interface state density decreased with increasing deposition or annealing temperature up to 450°C and that there exists some relationship between fixed charge density and interface state density. Silicon dangling bonds are easily generated when the flow rate of Sill 4 is higher than that of NH3, because the dissociation energy of Sill e from SiH4 (2.2 eV) is lower than that of NH from NH 3 (3.9eV). In contrast, when the flow rate of NH 3 is higher than that of SiH4, the concentration of NH radicals from NH3 becomes high, so that the adsorption of N-H bonds on dangling silicon sites is increased. Claassen et al. [6] proposed a simplified reaction scheme for the plasma deposition of silicon nitride from SiH4-NH3-N 2. In the presence of the large amounts of nitrogen and NH radicals, silicon dangling bonds are easily saturated with N-H bonds. This leads to a reduction in incomplete bonding during the deposition process and hence the fixed charge density is also decreased with the increase in N-H bonds. 4.3. Rapid thermal annealing of SiN films Hezel and coworkers [7, 19] reported that the fixed charge density in atmospheric pressure CVD silicon nitride (APCVD) films decreased with increasing deposition and annealing temperature up to 900 °C and that it was correlated with the decomposition of N-H or Si-H hydrogen bonds. The fixed charge density of APCVD silicon nitride films decreased with decreasing hydrogen bonds. For PECVD silicon nitride films, Stein et al. [20] reported similar results: the decrease in N-H bonds is paralleled by a decrease in the fixed charge density after annealing at temperatures up to 500 °C. However, the reason for the reduction in fixed charge density with increasing annealing temperature has not been presented. Considering the results noted in Figs 10-12, the structure of silicon nitride films is rearranged as the Si-H and N-H bonds are evolved out from the films, suggesting that a cross-linking occurs between nitrogen and silicon after hydrogen elimination. As indirect evidence of the cross-linking between silicon and nitrogen, Brown and Khaliq [21] reported that the absorption edge was shifted towards higher energies after the RTA process, indicating that S i N bond distortion is relieved by the

221

combination of hydrogen loss and that reordering of the film structure has occurred. Yokoyama et al. [22] observed the electron spin resonance signal from defects in PECVD silicon nitride films. They measured the spin density originating from silicon dangling bonds as a function of annealing temperature. As a result of annealing, the spin density could be reduced by about one order of magnitude up to an annealing temperature of 600 °C. They attributed this result to the partial decomposition of N-H bonds. H and N atoms might migrate in the silicon nitride network to the site of silicon dangling bonds and relax the network. Therefore, as a result of the cross-linking reaction, the films become more dense and less porous; defects such as silicon dangling bonds, nitrogen vacancies and silicon vacancies are reduced. Then the fixed charge density which is influenced by incomplete silicon dangling bonds is reduced. However, when the annealing temperature is above 900 °C, many defects in silicon nitride can be generated by bond breakage. Thermal defects caused by the high temperature annealing process may be increased. Therefore the fixed charge density rather increases above an annealing temperature of 900 °C as shown in Fig. 9.

5. Conclusions

The deposition rate decreased and the refractive index of silicon nitride films did not change much as the NH3:SiH 4 ratio increased in the case of N2 diluent gas, while the deposition rate increased and the refractive index decreased in the case of H 2 diluent gas. The intensity of the N-H stretching band increased more in the presence of N2 diluent gas than in the presence of H2 diluent gas. The results of optical emission spectroscopy suggest that the concentration of NHx radicals can be increased by inelastic collisions of metastable N 2 molecules. It is also plausible that N2 gas itself supplies nitrogen sources. When these N-H bonds are increasing during the plasma silicon nitride deposition process, the silicon dangling bonds generated are passivated by adsorption of N-H bonds. Therefore the fixed charge density was reduced with increasing N-H bond density. With post-rapid thermal annealing the content of hydrogen bonds was reduced and the film density increased. The etch rate of silicon nitride films in buffered HF solution decreased with increasing RTA temperature. The fixed charge density was reduced with increasing RTA temperature below 900 °C, which could be explained by the densification of films and cross-linking between silicon and nitrogen after hydrogen elimination, leading to a reduction in porous defects and silicon dangling bonds in as-grown films.

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C. W. Nam et ai. / Properties of plasma-deposited silicon nitride films

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10 R. W. B. Pearse and A. G. Gaydon, Identification of Molecular Spectra, Chapman and Hall, London, 4th edn., 1976. 11 J. W. Coburn and M. Chen, J. Appl. Phys., 51 (1980) 3134. 12 R. Hezel and R. Schorner, J. Appl. Phys., 52 (1981) 3076. 13 W. R. Knoll, Thin Solid Films, 168 (1989) 123. 14 W. A. P. Claassen, W. G. J. N. Valkenburg, F. H. P. M. Habraken and Y. Tamminga, J. Electrochem. Soc., 130 (1983) 2419. 15 M. Jeunehomme, J. Chem. Phys., 45 (1969) 1805. 16 D. E. Shemansky, J. Chem. Phys., 51 (1969) 689. 17 R. 1. Hall, J. Mazeau, J. Reinhardt and C. Schermann, J. Phys. B: At. Mol. Phys., 3 (1970)991. 18 V. S. Nguyen, in McD. Robinson and G. W. Cullen (eds.), Proc. 9th Int. Conf. on CVD, Vol. 84 6, Electrochemical Society, Pennington, NJ, 1984, p. 213. 19 R. Hezel and E. Hearn, J. Electrochem. Soc., 125(1978) 1848. 20 J. Stein, V. A. Wells and R. E. Hampty, J. Electrochem. Soc., 126 (1979) 1750. 21 W. D. Brown and M. A. Khaliq, Thin Solid Films, 186 (1990) 73. 22 S. Yokoyama, M. Hirose and Y. Osaka, Jpn. J. Appl. Phys., 20 (1981) L35.