Post-deposition high temperature processing of silicon nitride

Thin Solid Films, 101 (1983) 291 298

291

PREPARATION AND CHARACTERIZATION

POST-DEPOSITION H I G H T E M P E R A T U R E PROCESSING OF SILICON NITRIDE H. J. STEIN, P. S. PEERCY AND R. J. SOKEL*

Sandia National Laboratories, Albuquerque. N M 87185 (U.S.A. ) (Received February 25, 1982 ; accepted November 17, 1982)

Hydrogen, which is normally incorporated in silicon nitride in the form of S i - - H and N - - H bonds, is lost by trap-limited diffusion during high temperature post-deposition processing. Hydrogen loss occurs from films prepared by both atmospheric pressure chemical vapor deposition and low pressure chemical vapor deposition, and such loss appears to be unimpeded by polycrystalline silicon gate layers. We have found that partial reloading of hydrogen-depleted films to reform N - - H and S i - - H bonds can be achieved by annealing in H 2 gas at 900 °C. It is inferred, therefore, that silicon nitride does not reconstruct when hydrogen is lost during high temperature processing. Changes in the equilibrium positive charge parallel changes in the hydrogen concentration in silicon nitride, consistent with a previously reported parallel behavior between positive charge and S i - - H bonds.

1. INTRODUCTION Silicon nitride (SIN) films used for non-volatile memory elements in semiconductor memories are typically deposited by chemical vapor deposition (CVD) techniques at temperatures from 700 to 800 °C. These temperatures may be exceeded during post-nitride-deposition processing, either during drive-in diffusion for doping of polycrystalline silicon (poly-Si) gates or during thermal oxidation. Since SiN is an active charge storage element in metal(or poly-Si)/nitride/oxide/Si (MNOS) structures, there is considerable interest in determining the effects of such high temperature post-deposition processing on the composition and properties of SiN films. We report the effects of post-deposition high temperature annealing on the chemical composition and equilibrium charge for CVD SiN films. Annealing was performed in both N 2 and H2 atmospheres to permit quantitative examination of hydrogen loss and the effect on equilibrium charge in SiN. In addition, the effects of specific processing steps required to form poly-Si/nitride/oxide/Si stacks were examined. The use of high temperature H2 anneals to restore the hydrogen concentration and equilibrium charge toward as-deposited values was also studied. * Present address: INMOS, PO Box 16000,Colorado Springs, CO 80935, U.S.A. 0040-6090/83/0000-0000/$03.00

© ElsevierSequoia/Printedin The Netherlands

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H . J . STEIN, P. S. PEERCY, R, J. SOKEL

A previous study ~ has shown that high temperature processing of M N O S structures increases charge transport in SiN films, and previous workers 2"3 have shown that post-deposition H 2 annealing at temperatures between 800 and 900 °C can have beneficial effects for M N O S device performance. H 2 passivation of SiSiO2 or SiO2-SiN interface states was inferred to explain these beneficial effects. We find that annealing at about 1000 °C or subjecting SiN films deposited at 750 °C to a post-deposition processing step at 1000 °C causes extensive hydrogen loss from SiN films, but partial reintroduction of hydrogen can be achieved by annealing in H2 at 900 °C. 2. EXPERIMENTALDETAILS SiN was deposited onto the native oxides of 3-6 ~2 cm n-type silicon substrates. Prior to deposition the substrates were cleaned using a hydrogen peroxide-sulfuric acid mixture and etched hydrophobic in hydrofluoric acid. After the hydrofluoric acid etch, the substrates were rinsed in deionized water and spin dried. Atmospheric pressure depositions (APCVDs) Were performed in an Applied Materials AMV 1200 reactor with the substrates at 750 °C as measured with an optical pyrometer. The gas flows were 2 standard 1 min 1, 0.01 standard I rain- ~ and 45 standard 1 rain 1 for N H s , S i l l 4 and N 2 carrier gas respectively, to give a 200:1 NH3:SiH4 ratio. Low pressure (260 mTorr) depositions (LPCVDs) of SiN were made using 180 standard cm a min 1 of N H 3 and 20 standard cm 3 min-~ of dichlorosilane in a hot-wall Applied Materials model 5100 system. The deposition rates were approximately 150/~ rain- 1 for A P C V D and about 20/~ min - 1 for LPCVD. The ellipsometrically determined refractive indices were 1.9 and 2.0 for the A P C V D and L P C V D films respectively. Ion backscattering confirmed the presence of a few atomic per cent of oxygen in the A P C V D films. No oxygen was detected in the L P C V D films. Poly-Si layers were deposited onto A P C V D SiN at 750 °C by A P C V D from 80 standard cm 3 m i n - ~ of 5~o Sill4 in N 2 with 43 standard 1m i n - t o f N 2 carrier gas, and at 640 °C by L P C V D (100 standard c m 3 min -1 Sill4 and 500 standard cm 3 rain -~ N 2 ) onto L P C V D SiN. Drive-in diffusions and other non-H2 annealing steps were performed in a tube furnace with a flowing N 2 gas atmosphere. The AMV 1200 reactor used for nitride deposition and a second system in which the samples were radiantly heated by hot Inconel canister walls were used for the H 2 annealing. A thermocouple pressed against a dummy sample was used to measure the temperature in the radiantly heated system. All analysis measurements were made at approximately 300 K. An up-graded Perkin-Elmer 221 spectrophotometer with a CaF prism interchange and a multiple internal reflectance (MIR) attachment were used for MIR measurements 4 of IR absorption by N - - H and S i - - H stretch modes. Depositions for the MIR measurements were made onto polished silicon plates 20 mm × 52 mm x l mm with 45 ° entrance and exit faces. Nuclear reaction analysis of hydrogen profiles in SiN using fluorine or nitrogen has been described previously ~. Films analyzed by nuclear reaction techniques were deposited onto silicon wafer substrates. Measurements of the capacitance C at 1 MHz v e r s u s the bias voltage I/applied to the SiN were performed on both SiN/MIR substrates and SiN/wafer substrates. These C - V measurements were made with a mercury probe contact to the SiN and a large-area metal contact to the silicon back surface.

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3. EXPERIMENTAL RESULTS AND DISCUSSION C - V results for SIN(1500/~)/native oxide/Si structures after various processing steps are shown in Fig. 1. Results are shown for a structure with a 750 °C A P C V D SiN film after a poly-Si deposition at 750 °C and after poly-Si deposition plus a subsequent 30 rain anneal at 900 °C in H 2. The measurements were made after etch removal of the poly-Si layer. Since a negative position of the C - V characteristic on the voltage axis indicates equilibrium positive charge in the dielectric structures, these data indicate that high temperature processing reduces the equilibrium positive charge. IR absorption spectra for the S i - - H and N - - H stretch modes in SiN after different processing steps are shown in Fig. 2; the C - V characteristics shown in Fig. 1 were measured on the same samples. Poly-Si deposition causes a small decrease in S i - - H and N - - H absorption, and annealing in H 2 at 900°C causes a further decrease in the chemically bound hydrogen. It can be seen that the loss of equilibrium positive charge occurs simultaneously with the loss of chemically bound hydrogen. i

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H . J . STEIN, P. S. PEERCY, R. J. SOKEL

While IR absorption indicates the loss of S i - - H and N - - H bonds from the SiN films after high temperature processing, other techniques are required for quantitative measurements of the total hydrogen in the films and to calibrate the IR absorption measurements. We therefore used nuclear reaction analysis techniques to measure the depth distribution and total concentration of hydrogen in the SiN films after various processing steps. Results from these measurements are shown in Fig. 3 for the SiN films prepared in the same deposition as those in Figs, 1 and 2. In contrast with the C - V and IR data, the nuclear reaction analysis data were obtained without removing the 6000 ,~ poly-Si layer. Hydrogen concentrations are plotted against depth through the structures in Fig. 3 (upper scale) where the depth scales were calculated from the stopping powers for silicon and SiN assuming densities of 2.3 g cm 3 and 2.95 g cm 3 respectively. Except for the as-deposited film, the data plotted in Fig. 3(a) are for the same processing steps represented in Figs. 1 and 2. These data show that the hydrogen is distributed throughout the depth of the SiN and its concentration is reduced by annealing in H 2 a t 900 °C. Within the detection limit of approximately 0.05 at./o,°r no hydrogen is trapped in the poly-Si film and neither the poly-Si nor the silicon substrate appear to prevent the loss of hydrogen from the SiN. DEPTH ( ~ g / c r n 2) 0

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(a) (b) Fig. 3. Depth profilesfor hydrogen concentrations through poly-Si/SiN/nativeoxide/Si structures after different high temperature processing steps: e, after poly-Si deposition (750'C); A after poly-Si deposition plus H 2 annealing(900~C): I , after steam oxidation (1000 C); [53,after steam oxidation plus H2 annealing(900°C). Hydrogen profiling results for structures which had undergone a 30 min wet oxidation step at t000 °C prior to the poly-Si deposition are shown in Fig. 3(b). The 1000°C oxidation step caused extensive loss of hydrogen from the SiN film. Subsequent annealing in H 2 at 900°C reintroduced an appreciable amount of hydrogen into the SiN. We also note that the hydrogen concentration in the poly-Si remains at or below the detection limit for all the processing steps. The data in Figs. 1-3 are for A P C V D SiN films which were considerably thicker than the 400-500,~, films typically used for M N O S memory applications. The

POST-DEPOSITION HIGH TEMPERATURE PROCESSING OF

SiN

295

energetics of hydrogen loss are determined by N - - H and Si--H bond dissociation 6, 7, and hydrogen leaves the film by trap-limited diffusion 5. Therefore, for a direct comparison of processing-induced hydrogen loss from SiN films in nitride/oxide/Si structures with that encountered for conventional MNOS devices, the measurement should be made on films prepared by representative deposition conditions and of comparable thickness. To make such a direct comparison, similar studies were performed on thinner films prepared by both APCVD and LPCVD. Unannealed fractions for Si--H and N - - H concentrations in approximately 400 ~ APCVD and approximately 500/~ LPCVD SiN films for 20 min isochronal anneals in N 2 between 650 and 950 °C are plotted in Fig. 4. Both APCVD and LPCVD films show small initial instabilities between N - - H and Si--H at the lower annealing temperatures and loss of N - - H is observed at a slightly higher annealing temperature than that for loss of Si--H. This difference may be a consequence of a slightly higher bond dissociation energy 8 for N - - H than for Si--H. We also observe that the temperature for one-half fractional anneal is approximately 30 °C lower for the APCVD sample than for the LPCVD sample. Since the hydrogen loss occurs by trap-limited diffusion, this difference is attributed to the difference in sample thickness. These measurements on thin films demonstrate that the fractional hydrogen loss from 400-500,~ SiN films during high temperature processing is greater than that indicated by the results in Figs. 1 and 2 for 1500/~ films. The broken line segments at the high temperature end of the data in Fig. 4 show reintroduction of hydrogen into the films by annealing in H z at 900°C after a previous anneal in N2 at 950°C. Significant reintroduction of hydrogen was observed. While the fractional reintroduction is greater for the LPCVD sample than for the APCVD sample, the difference in the quantity of hydrogen reintroduced is not as great as this fractional difference since the initial hydrogen concentration in APCVD SiN is approximately twice that for LPCVD SiN deposited at the same temperature. Furthermore, the anneals were performed in different systems and more work is required to determine whether there is a material dependence for the reintroduction of hydrogen. Temperatures below 900 °C are relatively ineffective for

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296

H . J . STEIN, P. S. PEERCY, R. J. SOKEL

reintroducing hydrogen into SiN for the conditions of these experiments. We infer from the reintroduction of hydrogen into the film during annealing in gaseous H 2 that SiN does not reconstruct during high temperature processing; rather, the bonding sites for hydrogen remain in the film. This is in agreement with previous work v on SiN films deposited at 900°C and subsequently annealed in different gaseous ambients at 1000 and 1100 °C. C - V data for 750 °C LPCVD SiN films after annealing at 950 °C in N 2 and at 900°C in H 2 are shown in Fig. 5. Figure 5(b) shows the effect.of an H2 anneal on an as-deposited approximately 500/~ film whereas Fig. 5(a) shows the effects of annealing in H2 on a film which had been previously annealed at 950 °C in N 2. In addition to the simultaneous loss of equilibrium positive charge and hydrogen on direct annealing in either N 2 or H 2, as previously shown by the data in Figs. 1 and 2 for thicker APCVD films, these results demonstrate an increase in positive charge when hydrogen is reintroduced into a hydrogen-depleted annealed film. This result is in accord with previous work 9 which showed that the equilibrium positive charge increases with S i - - H concentration. Changes in positive charge are therefore shown to accompany changes in the hydrogen concentration in SiN during high temperature processing. The data of Fig. 5 were obtained from isochronal annealing of the same bare surface SiN samples throughout the entire temperature range. The effects of specific processing steps on hydrogen concentration for approximately 500~, 750°C LPCVD SiN films have also been examined. The steps were (1) poly-Si deposition at 640 °C followed by 30 min at 800 °C for drive-in diffusion, (2) poly-Si plus drive-in diffusion and annealing in H2 at 900 °C for 30 min, (3) wet oxidation for 30 min at 1000 °C followed by poly-Si deposition and drive-in diffusion, and (4) wet oxidation, polysilicon deposition and drive-in diffusion followed by an anneal in H z at 900 °C for 30 rain. The effects of these processing steps on the total hydrogen concentration as measured by nuclear reaction analysis and on the S i - - H and N - - H bond concentrations are summarized in Fig. 6. The poly-Si layer was removed from all the samples for these measurements and the S i - - H and N - - H concentrations were

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POST-DEPOSITION HIGH TEMPERATURE PROCESSING OF

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297

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scaled to the total hydrogen concentration using relative absorption cross sections determined previously 5. These results confirm the extensive loss of hydrogen from 500,& SiN films during post-deposition processing at temperatures above that for SiN deposition. Furthermore, the approximately 6000 ~ poly-Si layer does not significantly alter hydrogen loss and reintroduction. Other studies 1°'11 have indicated that hydrogen, particularly atomic hydrogen, transports readily through crystalline silicon and poly-Si at temperatures as low as 350°C and it is quite probable that atomic hydrogen is the controlling species for the loss and reintroduction of hydrogen in SiN. 4.

SUMMARIZING REMARKS AND CONCLUSIONS

Processing steps which require temperatures greater than the temperature used for the deposition of SiN films results in a loss of hydrogen from both silicon and nitrogen bonding sites throughout thin SiN films. The loss is essentially unaffected by a 6000/~ poly-Si layer. The hydrogen loss occurs for both LPCVD and APCVD nitride films in accord with that expected for trap-limited diffusion. The active traps limiting the diffusion of hydrogen are ascribed to incompletely bonded silicon and nitrogen. After substantial hydrogen loss caused by high temperature processing, hydrogen can be partially reloaded into SiN by annealing in H2 at 900 °C. From this reloading we conclude that CVD SiN films deposited at 750 °C do not reconstruct during processing at 1000 °C and that silicon ahd nitrogen sites for bonding hydrogen remain in the films. The equilibrium positive charge decreases and increases in parallel with the concentration of hydrogen. We have presented hydrogen loss and reintroduction results for SiN formed at a single deposition temperature and one reactant gas ratio each for LPCVD and APCVD. Optimization of processing and H 2 annealing sequences and temperatures will depend on the SiN deposition parameters and on the desired characteristics for MNOS devices. For example, a negative flat-band voltage, and hence hydrogen incorporated as Si--H, is desirable for p-channel but may be undesirable for nchannel MNOS devices. Additional studies are needed to separate Hz annealing

298

H.J. STEIN, P. S. PEERCY, R. J. SOKEL

effects on the nitride film itself from those at the semiconductor-dielectric interface for a more complete understanding of equilibrium charge and of injection, trapping and transport of charge in MNOS structures. ACKNOWLEDGMENTS

The authors wish to acknowledge the efforts of Mary Mitchell and Ron Jones for film depositions used in this investigation. This work was Performed at Sandia National Laboratories supported by the U.S. Department of Energy under Contract DE-AC04-76DP00789, REFERENCES

1 J . A . Topich, Electrochemical Society Meet., Vol. 77-1, Electrochemical Society, Princeton, NJ, 1977, Extended Abstracts, p. 323. 2 Y. Yatsuda, S. Minami, R. Kondo, T. Hagawara and Y. Itoh, Proc. llth Conf. on Solid State Devices, Tokyo. 1979, in Jpn. J. Appl. Phys., Suppl. 1, 19 (1980)219. 3 G. Schols, H. Maes, G. Declerck and R. Van Overstraeten, Ret,. Phys. Appl., 13 (1978) 825. 4 N.J. Harrick, Internal Reflection Spectroscopy, Wiley-lnterscience, New York, 1967. 5 P.S. Peercy, H.J. Stein, B.L. DoyleandS. T. Picraux, J. Electron. Mater.,8(1979) ll. 6 H.J. Stein and H. A. R. Wegener, J. Electrochem. Soc., 124 (1977) 908. 7 T.P. Smirnova, V. I. Belyi and L. V. Chramova, Thin Solid Films, 74 (1980) 287 293. 8 A . Q . Gaydon, Dissociation Energies and Spectra of Diatomic Molecules, Chapman and Hall, London, 3rd edn., 1968. 9 H.J. Stein, P. S. Peercy and D. S. Ginley, in G. Lucovsky, S. T. Pantelides and F. L. Galeener (eds.), The Physics of MOS Insulators, Pergamon, Oxford, 1980, p. 147. 10 P.S. Peercy, H. J. Stein and D. S. Ginley, Appl. Phys. Lett., 36 (1980) 678. 11 N.M. Johnson, D. K. Biegelsen and M. D. Moyer, Appl. Phys. Lett., 40 (1982) 882.