Pulse-modulated plasma-enhanced chemical vapor deposition of SiO2 coatings from octamethylcyclotetrasiloxane

Silicon dioxide coatings can be grown at low substrate temperatures in a pulse-modulated microwave electron cyclotron resonance (ECR) oxygen plasma with octamethylcyclotetrasiloxane (OMCTS) as the organosilicon precursor. The 2.45 GHz microwave power was pulse-modulated with repetition frequencies of 20 Hz–20 kHz, duty ratios (pulse on-time/pulse period) from 25 to 100%, and peak microwave power from 800 to 2400 W. The coatings are SiO2-like with Si:O ratios of approximately 3:4 and carbon percentages of 20–25%. Pulsed plasma deposition significantly lowers the deposition substrate temperature as the peak power and duty ratio decrease. With 1600 W input continuous microwave power, substrate temperatures were 140–150 °C after 10 min of deposition, while with a 50% pulse duty cycle and 1600 W peak power the temperature decreased to 90 °C. The coating hardness decreased with pulsed operation compared to continuous operation, unless the average microwave power levels were made equivalent. Deposition growth rates depended only weakly on the pulse repetition frequency and duty ratio, but increased strongly as the pulse peak power was raised. Pulsed deposition at 800 W peak power and 50% duty ratio gave a growth rate of 0.5–0.6 μm/min for all frequencies between 20 Hz and 20 kHz, increasing to 0.8–0.9 μm/min at 1600 W peak power.