Electron energy distribution function in capacitively coupled RF discharges: difference between electropositive Ar and electronegative SiH4 discharges

The electron energy distribution functions (EEDFs) for electropositive Ar and electronegative SiH4 discharges have been studied by means of a one-dimensional particle-in-cell/Monte Carlo (1D PIC/MC) model and have been compared under the same discharge conditions (frequency, power and pressure). The shape of the EEDF at the plasma centre in the hr discharges varies strongly with discharge conditions: at 13.56 MHz and 30 mTorr it is bi-Maxwellian, at 65 MHz and 30 mTorr it is Maxwellian and at 13.56 MHz and 400 mTorr it is Druyvesteyn like. The EEDF in SiH4 discharges, on the other hand, has more or less the same shape under the different conditions, with a strongly populated high-energy tail. This difference is attributed to the significantly different effect of the bulk field on the tail: for the Ar case the field is weak (several tens of volts per metre) and for the SiH4 case the field is strong (several hundred volts per metre). As a result of the strong bulk field in phase with the cathode sheath field, the non-local behaviour of the EEDF in the SiH4 discharge partly results in an EEDF with the most significant tail appearing at a position closer to the anode pre-sheath, in addition the EEDF with a strongly populated tail occurs in the middle of the plasma as in the Ar case. Consequently, electronegative discharges are more non-local than electropositive discharges under the same working conditions.