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 wafe
r by r.f, bias of 300 kHz was directly measured with a Pt wafer by high vol
tage probe. Even though the r.f. bias frequency of 300 kHz was less than th
e ion plasma frequency, it was observed that the self bias voltage was indu
ced, 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 de
creased as the plasma density increased. The pulse modulated helicon wave p
lasma with low frequency r.f, bins effectively eliminated the local side et
ching 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 adj
usting the duty ratio. (C) 1999 Elsevier Science S.A. All rights reserved.