Ion energy distributions in SF6 plasmas at a radio-frequency powered electrode

An energy-resolving quadrupole mass spectrometer (E-QMS) was assembled underneath the powered electrode of a diode reactive ion etcher. The plasma ions reach the E-QMS through an orifice in the powered electrode with a diameter of 100 mu m. The ion energy distributions (IEDs) of ionic species from SF6 plasmas in the pressure range of 0.1-1.5 Pa for dc bias potentials between 50 and 300 V and a rf of 13.56 MHz were investigated. The IEDs always show a saddle shaped peak at an energy corresponding to a total potential drop across the sheath given by U-Sh=U-dc+U-p , where U-dc is the dc bias potential and Up is the time averaged plasma potential. In the energy range from 0 eV to eU(dc) there are multiple peaks in the IEDs of SFx+ (x=1.5), whereas the F+-F2+, and S+ IEDs show only a single peak in this range. These peaks are the result of ion generating collisions in the sheath. On pressure variation the IEDs do not change significantly. We also measure IEDs of negative ions. The IEDs of these ions show a broad distribution with an intensity maximum appearing at the half of U-p and a with corresponding to max[U-p(t)], where U-p(t) is the time varying plasma potential. These correlations suggest that these ions originate from the plasma bulk. (C) 1996 American Institute of Physics.