Pulse-time-modulated electron cyclotron resonance plasma etching with low radio-frequency substrate bias

The etching characteristics of polycrystalline silicon (poly-Si) by short-time modulated electron cyclotron resonance (ECR) determined by using a Cl2 etchant were investigated. It was found that the poly-Si etching rate was improved by increasing the pulse interval of the microwaves to more than 50 μs for a 50% duty ratio and applying a 600 kHz rf bias to the substrate. A large amount of negative chlorine ions are generated in the afterglow which occurs for more than 50 μs in high-density, low-pressure plasmas. The low rf bias alternately accelerates both the negative and positive ions to the substrate surface. As a result, the pulse-time modulated ECR plasma can produce high rate and charge-free etching with a 600 kHz rf bias.