Highly selective etching of poly-Si by time modulation bias

Highly selective etching of poly-Si is possible by time modulation (TM) bias. Radio frequency (RF) bias applied to a substrate is pulse modulated. The energy of ions impinging on the wafer is varied by the peak-to-peak voltage of RF bias. The average flux of accelerated ions is controlled by the duty ratio of the pulse. The selectivity of poly-Si to SiO2 is about 1.5-fold that of continuous wave bias at a poly-Si etch rate of 100 nm/min. The SiO2 etch rate is suppressed and the selectivity increased, this may be caused by the selective deposition of reaction products including oxygen on the SiO2 surface during off periods of RF bias.