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 plasma ions reach the QMS through an orifice in the powered electrode with 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. bias 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 peak 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 only a broad distribution. Thus we conclude that the collision processes of N+ are predominantely elastic. With increasing pressure, the intensity of the peaks in the IEDs of N-2(+) below eU(DC) exceeds the intensity of the saddle-shaped peaks at U-DC + U-PI owing to a higher probability for charge transfer collisions in the sheath. The intensity of the broad distribution in the IEDs of NC also increases with increasing pressure, but never exceeds the intensity of the saddle-shaped peak.