Powder-free plasma chemical vapor deposition of hydrogenated amorphous silicon with high rf power density using modulated rf discharge

Abstract

Deposition of hydrogenated amorphous silicon films from SiH4/He gas mixtures was performed by using a square wave amplitude modulated rf discharge. The modulation was used for controlling radical densities in plasmas which led to a high rate deposition of good quality films. The fairly high deposition rate of 6 Å/s was obtained for a low concentration of 5% SiH4 and a high rf peak power 200 W (0.8 W/cm3) without any appreciable amount of powder particles in the reaction chamber. The optical gap of the films was 1.8–1.95 eV. Emission intensities of HeI 388.9 nm and SiH 413.5 nm linearly increased with rf peak power and were well correlated with the deposition rate.

From molecules to particles in silane plasmas

“…Particle formation in rf silane plasmas is strongly reduced by square-wave power modulation at kHz frequencies [ 10,15,25]. In previous work [7,9,15] with plasma conditions suitable for high quality amorphous silicon deposition, we demonstrated that powder does not form when anions are observed to escape (modulation frequency < 3 kHz), and that powder does form when the anions do not have time to be evacuated in the afterglow (modulation frequency > 3 kHz).…”

“…Other pathways may predominate in high power (-0.8 W cm-3), high pressure (-1 mbar) silane plasmas in which particle formation occurs in a fraction of a second [25,26]. Mandich and Reents [18] disqualify positive ions since bottlenecks occur in cation nucleation (although exceptions exist [ 19,271) and furthermore they are rapidly lost by evacuation across the plasma sheaths.…”

“…If neutrals were the pathway species in low pressure experiments in which particles form slowly in a continuous plasma, the neutral dwell time would have to be at least a second. But kHz modulation is observed to eliminate powder formation, imposing a neutral precursor lifetime of less than a millisecond (NB it is not sufficient that the reactive monomer species disappear rapidly [25] because if the pathway species remains for further growth in subsequent plasma periods, the powder would still continue to grow albeit more slowly due to a lower monomer density). This is a paradoxical situation for neutrals which can only be resolved by taking negative species to be the polymerisation pathway -their dwell time in continuous plasmas is indefinitely long due to sheath trapping, and their lifetime is short in modulated plasmas due to expulsion by reversed sheath fields.…”

Atomic-scale characterization of hydrogenated amorphous-silicon films and devices. Annual subcontract report, 14 February 1995--14 April 1996

“…Watanabe et al 1990, Bouchoule et al 1991, Fukuzawa et al 1994, Howling et al 1994, and Schmidt et al 1993). In addition, the amount and location of particulates is highly dependent on the details of each individual deposition chamber.…”

“…It may be possible to prevent these from growing to the 2-20 nm size range, for example by modulating the discharge and allowing the R nm particles to leave periodically (e.g. Watanabe et al 1990). However, many of these small particles will deposit into the film, probably with undesirable consequences.…”