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 SiO 2 is about 1.5-fold that of continuous wave bias at a poly-Si etch rate of 100 nm/min. The SiO 2 etch rate is suppressed and the selectivity increased, this may be caused by the selective deposition of reaction products including oxygen on the SiO 2 surface during off periods of RF bias.