Measurement of electron transport coefficients in tetramethylsilane vapour

This paper reports four electron transport coefficients in tetramethylsilane (TMS, Si(CH3)4) vapour which is utilized in plasma-enhanced chemical vapour deposition (PECVD) of thin films such as SiC and SiN. The coefficients, namely the density-normalized effective ionization coefficient (αeff/N), the mean-arrival-time drift velocity (Wm) and the product of the longitudinal electron diffusion coefficient and the gas density (NDL) have been experimentally determined using a double-shutter drift tube method based on an arrival-time spectra theory over a reduced electric field (E/N) range of 30-5000 Td (1 Td = 10-21 V m2). The result shows that the coefficient αeff/N increases rapidly at an E/N value of about 200 Td and then reaches a value of 9.84 x 10-16 cm2 at an E/N value of 5000 Td. It is revealed that the values of the coefficient αeff/N are practically equal to those in SiH4 gas which is a representative gas used in PECVD. The reliability of the αeff/N values is checked by a steady-state Townsend experiment. The drift velocity (Wm) monotonically increases with E/N except at 50 Td < E/N < 200 Td, where a slight negative differential conductivity is observed. The longitudinal diffusion coefficient (NDL) has a minimum value at an E/N value of about 100 Td in the present measurement. Its variation profile against E/N is similar to that in SiH4 although its profile shifts to higher E/N. The ratio of the longitudinal diffusion coefficient to the mobility (DL/μ) is deduced from the Wm and the NDL values, which monotonically increases from 0.1 to about 10 eV with E/N in the present E/N range