Thin Solid Films, 164 (1988) 309 312
309 PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION SILICON NITRIDE FOR MICROELECTRONIC APPLICATIONS* MANJU GUPTA, V. K. RATHI, S. P. SINGH AND O. P. AGNIHOTRI
Semiconductor Engineering Laboratory, Department of Physics 1.I.T. Hauz Khas, New Delhi 110 016 (India) K. S. CHARI
Department of Electronics, Loknayak Bhavan, Khan Market, New Delhi 110 003 (India)
We have deposited low-stress (less than 5 x 107 d y n c m - 2 ) , low-pin-holedensity, low-leakage films of silicon nitride by plasma enhanced chemical vapour deposition (PECVD). The absorption band gap for nitrogen saturated films is 5.15 eV. The refractive index depends on the silane:ammonia ratio, and by adjusting this ratio, we can prepare stoichiometric films with a refractive index of 1.9. A leakage current of less than 1 0 - 9 A c m -2 and a fixed charge density of 2.1 × 10a 1 c m - 2 are obtained for these layers.
There has been considerable interest in low-temperature deposition of silicon nitride for large-scale integrated circuits, especially for passivation and the dielectric for multilevel interconnects. Low temperature is necessary to prevent diffusion of shallow junctions and interdiffusion of metals in multilevel interconnects. Plasma enhanced chemical vapour deposition (PECVD) has the advantage of low processing temperature (less than 400°C) L2. Electrical, optical and mechanical properties of P E C V D silicon nitride are dependent upon deposition system parameters such as power, temperature and reactant gas ratios and their flows. Studies of the pin hole density and stress in the films are important when these films are used for passivation and multilevel interconnects. An integrated circuit consists of active and passive devices including various charge sensitive devices. Hence the interface evaluation is essential to predict the change in the performance of the underlying devices. In this paper, results are given on the stress of the films along with their transport and interface characteristics. 2. EXPERIMENTALDETAILS A plasma deposition system was used for the deposition of silicon nitride films. Reactant gas flows of 10-200 standard cm 3 min - 1 of Sill4 (2% in argon) and 10 and 20 standard cm a m i n - 1 of N H 3 were used. The substrate temperature was varied * Paper presented at the 7th International Conferenceon Thin Films, New Delhi, India, December7-11, 1987. 0040-6090/88/$3.50
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M. GUPTA e t al.
between 100 and 350 °C. Films of different thickness were deposited on bare n/n +silicon (resistivity n-type 1-10 f~ cm) or aluminium coated wafers. The thicknesses and the refractive indices were simultaneously obtained by using a Gaertner ellipsometer L 117. Prior to deposition, silicon wafers were cleaned by the RCA method. Aluminium dots of 0.6 mm diameter, or large area contacts with crosssection 5 mm × 5 mm, were vacuum evaporated to study the current voltage or high frequency (1 MHz) capacitance voltage characteristics. C - V characteristics were measured by using an MSI C - V profiler interfaced with an H P 85 B computer. The pin hole density was measured electrochemically 3 and the stress by Newton's method 4. 3.
Figure 1 shows the refractive index dependence of the silane:ammonia ratio for a fixed r.f. power of 0.13 W c m - 2 and a substrate temperature of 200 °C. It is evident that as we make the silicon-rich films, the refractive index increases. Both the refractive index and the film thicknesses were found to be within + 5% of their average values. Usually the silicon-rich films were found to be absorbing in the visible region whereas the films saturated with nitrogen were rather highly transparent. The value of the direct band gap was determined from the transmission spectra of the films and was found to be 5.15 eV, which agrees well with the value reported for P E C V D Si3N 4. Pin hole densities in the range 15-40cm -2 were observed on films of 1000/~ thickness. In the past, conduction in silicon nitride was attributed mainly to electron transport 5. However, recent studies on M N O S memories have suggested that such conduction is due mainly to holes 6. In our investigation, the current transport was studied by using standard metal-insulator (nitride)-metal (MIM) structures. The current-voltage characteristics were measured in the steady state after a 2 min interval of given gate bias voltage. Figure 2 shows the lg I - E ~ characteristics of a film of 1000 A thick deposited at 0.13 W c m - 2 r.f. power, 200 °C substrate temperature and silane:ammonia ratio of 0.1. The low field conduction appears to be due to -10
I . ~ ~ . ° 'J ~ ° J
-12 /~ / ~
z. E1/2 (102 ~/~ cm )
Fig. 1. Indexof refraction as a functionofsilane:ammoniaratio (substrate temperature, about 200 °C; r.f. power, about 0.13 W cm- 2). Fig. 2. Lg I vs. E i plot measured at 300 K (A1dot diameter, 1mm).
PECVD S i 3 N 4 FOR MICROELECTRONICS
electrode-limited transport 7. The slope of the linear portion of the curve gives a dynamic dielectric constant of 2.2. This value agrees with the value reported by Sinha and Smith 8 and indicates that the charge transport is controlled by the P o o l e Frenkel mechanism. The leakage current density was found to be independent of the contact areas indicating the absence of any significant leakage paths in the grown films. The stress in these films reduces as we increase the silane:ammonia ratio while keeping the temperature of deposition at 200 °C and the r.f. power at 0.13 W c m - 2 The stress is compressive in nature. We believe that this reduction may be due to better matching of silicon-rich films to silicon substrates. Since the films deposited with high silane:ammonia ratio showed high leakage currents, a ratio (SiHa:NH 3 ~ 0.1) for which low leakage currents were observed, was chosen to examine the effects of deposition temperature on the stress in the films. Figure 3 shows the stress vs. temperature behaviour of these films. It is evident from this Fig. 3 that the stress decreases as we increase the deposition temperature. The minimum detectable limit of the measurement system is 5 × 1 0 7 dyn c m - z. For films deposited at 350 °C with a silane:ammonia ratio of 0.1 and r.f. power of 0.13 W c m - z , stress is less than the minimum detectable limit.
5 c -~
3 -~ 0.6
0.2 100 Substrate
I T e m p e r a t u r e (°c)
0 BIAS (VOLTS)
Fig. 3. Stress vs. substrate temperature for constant flow rates. Fig. 4. Normalized capacitance vs. voltage of an AI/Si3N,/n-Si/n*-Si/AI structure (frequency, 1 MHz).
Figure 4 shows the normalized high frequency C - V characteristics of an A1/Si3N4/n-Si/n+-Si/A1 structure with nitride thickness of 1000,&. For a doping density of 2.36 × 101 s c m - 3 of underlying silicon substrate, a fixed charge density of 2.1 × 10~1 cm -2 was obtained on the as-deposited SiaN 4 films with the following deposition parameters: the substrate temperature, 200 °C; SiH4:NH 3 ratio, about 0.1; r.f. power density, about 0.13 W c m - 2 ; chamber pressure, 3 Torr during deposition. REFERENCES 1 R . C . G . Swan, R. R. Mehta and T. P. Gange, J. Electrochem. Sac., S o l i d S t a t e Sci. and Technol., 114 (1967) 713.
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A.K. Sinha, H. J. Levinstein, T. E. Smith, G. Quinane and S. E, Haszko, J. Electrochem. Soc., 125 (1978) 603. 3 C. Blaauw, J. Electrochem. Soc., 137 (5) (1984) 1114. 4 P.N. Kember and S. C. Lindall, Semiconductor International, August 1985. 5 S.M. Sze, J. Appl, Phys., 38 (1967) 2951. 6 P.C. Arnett and Z. A. Weinberg, IEEE Trans. Electron Devices, ED-25 (1978) 1014. 7 J.G. Simmons, in I,. I. Maissel and R. Glang (eds.), Handbook of Thin Film Technology, McGrawHill, New York, 1970, pp. 14-33. 8 A.K. Sinha and T. E. Smith, J. Appl. Phys., 49 (5) (1978) 2256.