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

We present a simple technique for obtaining the time-resolved ion energy distribution function (IEDF) at a boundary in pulsed plasmas using a commercial quadrupole mass energy analyser. In this technique, ions are extracted from the plasma at selected parts of the pulse cycle, through the synchronized electrical biasing of a grid assembly attached to the barrel of the instrument, forming an electrostatic shutter. This sampling method has the advantage over the normal technique of electronically gating the detected ion signal to achieve time resolution, since the IEDFs can be obtained even when the ion flight time through the instrument (typically 100 mus) is greater than the pulse period or the characteristic time of transients in the plasma under investigation. The arrangement allows us therefore to diagnose plasmas pulsed at high frequencies (> 10 kHz). Presently, a time resolution of 4 mus can be obtained, limited only by the driving electronics design. The technique has been tested on a DC magnetron discharge operated in argon. The plasma was pulsed at a low frequency of 2 kHz, but with a discharge voltage waveform containing fast transients (on the mus time scale or faster). The results show clearly the evolution of the IEDFs during the pulse, responding to these fast transients with a significant number of ions created at plasma potentials above 140 V. These time-evolved IEDFs cannot be obtained using the conventional, manufacturer's time-resolved method for this instrument. However, the new technique does introduce a small distortion in the measured IEDFs at energies above 120 eV, which is always observed, irrespective of the position of the shutter window during the pulse. This is due to the transient nature of the discriminating grid bias used in the electrostatic shuttering. Its effects and possible elimination are discussed.