Characteristics of the plasma and the poly-Si etching in a pulse modulated helicon wave plasma were studied. The self bias voltage induced on the wafer by r.f, bias of 300 kHz was directly measured with a Pt wafer by high voltage probe. Even though the r.f. bias frequency of 300 kHz was less than the ion plasma frequency, it was observed that the self bias voltage was induced, which contradicts the conventional thoughts. In the off period of the source power in the pulse mode, the weak plasma was sustained only by the r.f. bias power. This weak plasma in the pulse mode induced higher self bias voltage compared with the continuous mode because the self bias voltage decreased as the plasma density increased. The pulse modulated helicon wave plasma with low frequency r.f, bins effectively eliminated the local side etching but enhanced the resist erosion due to higher self bias voltage. The Si etch rate and the selectivity of Si to photo-resist were enhanced by adjusting the duty ratio.