"Notching," which is a kind of local side etching caused by charging of pattern structures, is a serious obstacle to achieving tight critical dimension (CD) control in fabrication beyond quarter micron devices. Although sidewall protection with increased polymer deposition on the sidewall can reduce notching, it tends to enhance the so-called proximity effect, which is the variation of etched profiles observed when the pattern spacing is varied. Therefore, notching has to be suppressed without extra sidewall protection to achieve tight CD control. To solve this problem, we studied the effects of rf bias frequency for both continuous mode and pulse modulated mode electron cyclotron resonance plasma sources with divergent magnetic fields, and found out that lowering the bias frequency can reduce the notching by itself. We found that the notch depth is markedly decreased by lowering the bias frequency from 13.56 MHz to 400 kHz under most conditions. In continuous mode plasma, however, this improvement becomes minimal when the pressure is decreased to reduce the proximity effect. On the other hand, using a pulsed plasma source having a 100 mu s cycle and 25%-50% duty, we succeeded in suppressing notching even in cases of 1 mT and lower. Consequently, we have achieved a vertical etched profile with minimal proximity effect.