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

Abstract

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 µs) 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 µs 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 µs 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.

Two-Dimensional Double Probe Study of the Temporal Evolution of the Charge Carrier Density in a Pulsed Magnetron

“…The electrons may, therefore, have enough energy to ionise in the region close to the substrate, but—as observed—not close to the target. As the value is close to the ionisation limit, it seems likely that changes in the electric field distribution may significantly change the formation of maximum A and this is probably the reason why A is extremely sensitive to changes in the chamber state 16. It may also be the reason why other measurements with a slightly different geometry have not shown the first maximum (A) in the electron density 6,7,11.…”

“…However, these processes become more and more important especially at high pulse frequencies as they begin to dominate the pulse. There have been studies of these transient phenomena by different time‐resolved methods such as Langmuir probes,6–11 optical emission spectroscopy (OES),12–15 and mass spectrometry 16,17. Most of these characterisations have been done along a line‐of‐sight across the discharge,12–14 at a particular position in it,7,8,16,17 or along the axis of symmetry 7,9,18.…”

Ion Energy Distributions in Magnetron Sputtering of Zinc Aluminium Oxide

“…However, there are additional peaks at higher energies between 20 and 50 eV which were also observed by other authors. [14,[16][17][18] Some very weak signals are observed up to 110 eV. These peaks are clearly dependent on the pulse parameters.…”

Quadrupole mass spectrometry of reactive plasmas

“…Time-resolved measurements showed that the low-energy peak originates in the on phase of the pulse (A), high-energy ions are accelerated by a large positive bias generated by a reverse voltage overshoot phase (C). The midrange ions are created during the off phase (B) when the plasma potential rises a few volts above the positive target potential, figure 16, tagged A, B and C in correlation with the stages of a pulse [113,114]. The conclusion was that the plasma potential changes occur on the electron plasma frequency time scale [107,[113][114][115][116].…”