A technique for obtaining time- and energy-resolved mass spectroscopicmeasurements on pulsed plasmas

Abstract

We present a simple technique for obtaining the time-resolved ion energydistribution function (IEDF) at a boundary in pulsed plasmas using acommercial quadrupole mass energy analyser. In this technique, ions areextracted from the plasma at selected parts of the pulse cycle, through thesynchronized electrical biasing of a grid assembly attached to the barrel of theinstrument, forming an electrostatic shutter. This sampling method has theadvantage over the normal technique of electronically gating the detectedion signal to achieve time resolution, since the IEDFs can be obtainedeven when the ion flight time through the instrument (typically 100 µs) is greater than the pulse period or the characteristic time oftransients in the plasma under investigation. The arrangement allowsus therefore to diagnose plasmas pulsed at high frequencies (≥10 kHz). Presently, atime resolution of 4 µs can be obtained, limited only by the driving electronics design. Thetechnique has been tested on a DC magnetron discharge operated inargon. The plasma was pulsed at a low frequency of 2 kHz, but with adischarge voltage waveform containing fast transients (on the µstime scale or faster). The results show clearly the evolution of the IEDFs duringthe pulse, responding to these fast transients with a significant number of ionscreated at plasma potentials above 140 V. These time-evolved IEDFs cannot beobtained using the conventional, manufacturer’s time-resolved method for thisinstrument. However, the new technique does introduce a small distortion in themeasured IEDFs at energies above 120 eV, which is always observed,irrespective of the position of the shutter window during the pulse. This isdue to the transient nature of the discriminating grid bias used in theelectrostatic shuttering. Its effects and possible elimination are discussed.