DIAGNOSTIC ON N-2 PLASMA WITH AN ENERGY-RESOLVED QUADRUPOLE MASS-SPECTROMETER AT THE POWERED ELECTRODE IN A REACTIVE ION ETCHING SYSTEM - ION ENERGY-DISTRIBUTION OF N-2(+) AND N+

An energy-resolved quadrupole mass spectrometer (E-QMS) was assembled underneath the powered electrode of a diode reactive ion etcher. The p lasma ions reach the QMS through an orifice in the powered electrode w ith a diameter of 100 mu m. The ion energy distributions (IEDs) of N-2 (+) and N+ from N-2 plasmas in the pressure range 0.1-2 Pa for d.c. bi as potentials between 50 V and 350 V and a frequency of 13.56 MHz were investigated. The IEDs of N-2(+) and N+ always show a saddle-shaped p eak at an energy corresponding to the total potential drop across the sheath given by U-DC + U-PI, where U-DC is the d.c. bias potential and U-PI is the time averaged plasma potential. In the energy range from OeV to eU(DC) there are multiple peaks in the IEDs of N-2(+) owing to symmetric resonant charge transfer collisions, whereas the N+ shows on ly a broad distribution. Thus we conclude that the collision processes of N+ are predominantely elastic. With increasing pressure, the inten sity of the peaks in the IEDs of N-2(+) below eU(DC) exceeds the inten sity of the saddle-shaped peaks at U-DC + U-PI owing to a higher proba bility for charge transfer collisions in the sheath. The intensity of the broad distribution in the IEDs of NC also increases with increasin g pressure, but never exceeds the intensity of the saddle-shaped peak.