Measurement of electron transport coefficients in tetramethylsilane vapour

This paper reports four electron transport coefficients in tetramethylsilane (TMS, Si(CH 3 ) 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 (α eff / N ), the mean-arrival-time drift velocity ( W m ) and the product of the longitudinal electron diffusion coefficient and the gas density ( N D L ) 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 m 2 ). The result shows that the coefficient α eff / N increases rapidly at an E / N value of about 200 Td and then reaches a value of 9.84 × 10 −16 cm 2 at an E / N value of 5000 Td. It is revealed that the values of the coefficient α eff / N are practically equal to those in SiH 4 gas which is a representative gas used in PECVD. The reliability of the α eff / N values is checked by a steady-state Townsend experiment. The drift velocity ( W m ) 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 ( N D L ) 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 SiH 4 although its profile shifts to higher E / N . The ratio of the longitudinal diffusion coefficient to the mobility ( D L /μ) is deduced from the W m and the N D L values, which monotonically increases from 0.1 to about 10 eV with E / N in the present E / N range.