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 a nd electronegative SiH4 discharges have been studied by means of a one-dime nsional particle-in-cell/Monte Carlo (1D PIC/MC) model and have been compar ed under the same discharge conditions (frequency, power and pressure). The shape of the EEDF at the plasma centre in the hr discharges varies strongl y 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 mo re or less the same shape under the different conditions, with a strongly p opulated high-energy tail. This difference is attributed to the significant ly different effect of the bulk field on the tail: for the Ar case the fiel d 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 bul k 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 sign ificant tail appearing at a position closer to the anode pre-sheath, in add ition the EEDF with a strongly populated tail occurs in the middle of the p lasma as in the Ar case. Consequently, electronegative discharges are more non-local than electropositive discharges under the same working conditions .