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Highly Anisotropic Etching of Polysilicon by Time-Modulation Bias

The improvement of selectivity for polysilicon (poly-Si) etching without lowering the selectivity to SiO2 is possible through the implementation of time-modulation (TM) bias. RF bias (800 kHz) applied to a substrate in an electron cyclotron resonant (ECR) etcher is pulse modulated with a repetition frequency of 1 kHz. The amplitude of RF voltage controls the energy of the ions accelerated toward the wafer. The flux of accelerated ions is controlled by the duty ratio of the pulse. The taper angle of the sidewall of line patterns is 89° for TM bias and 81° for conventional continuous-wave bias. The etch rate of the poly-Si is ∼300 nm/min and has a selectivity to SiO2 of 17. In addition, microtrenching at the bottom of the space is suppressed by the implementation of TM biasing. Modeling of the etch profile based on the competition of deposition and etching on the sidewall surface can explain the effects of TM bias.