Spatial behavior of energy relaxation of electrons in capacitively coupleddischarges: Comparison between Ar and SiH4

The electron energy relaxation in space has been investigated for electropo sitive Ar and electronegative SiH4 discharges by a 1D Particle-in-cell/Mont e Carlo code. The ionization rate has been studied since this rate is stron gly influenced by the energy relaxation mechanism. The ionization rate in t he two kinds of discharges at a low pressure (30 mTorr), a low power (8 W), and a low frequency (13.56 MHz) is regarded as the reference case. The eff ects of pressure, power, and frequency on the ionization rate have been obs erved and compared between the two types of discharges. With the pressure i ncreasing from 30 to 400 mTorr, in the case of the argon discharge the ioni zation peak moves from the plasma bulk (nonlocal behavior of the electron e nergy distribution function) towards the momentary cathodic presheath (loca l behavior). In addition to a similar variation of the ionization rate, in the silane discharge an ionization peak occurs near the momentary anodic pr esheath, and at the high pressure the ionization in the plasma bulk is stil l considerable. The power can only influence the ionization rate quantitati vely. The effect of frequency on the ionization rate is similar for both ki nds of discharges. With increasing frequency the ionization rate tends to a somewhat local character, i.e., the ionization appears closer to the momen tary cathode. This is attributed to the fact that at the high frequency a b ulk electric field in the bulk is found clearly out of phase with the sheat h fields, and the energetic electrons are pushed towards the cathode earlie r in space. Meanwhile, in the silane discharge the distinctive ionization, which is strongly present at the anodic presheath and in the plasma bulk at low frequency, almost disappears. (C) 2000 American Institute of Physics. [S0021-8979(00)02608-6].