The characteristics of silicon etching using radio-frequency (rf) substrate biased ultrahigh-frequency (UHF) plasma determined by using a Cl-2 etchant were investigated. The silicon etching rate and the etching profile were improved by decreasing the substrate bias frequency to less than 600 kHz. It is suggested that a large number of negative chlorine ions is generated in the high-density, low-pressure UHF plasma because of the extremely low electron temperature. The low-frequency substrate bias accelerates the negative and positive ions alternately to the substrate surface. As a result, the low-frequency biased UHF plasma can be used to achieve high-rate, highly anisotropic, and microloading-free silicon etching with a 600 kHz rf substrate bias.