Time Resolved Measurements of Mass and Energy Spectra of Plasma

Valuable information can be obtained when mass spectra and energy distributions are directly measured for mass-identified positive and negative ions moving towards target surfaces in plasma reactors. In most cases, these measurements are time-averaged distributions, including when pulsed power is applied to the plasma.

Time Resolved Analysis

It would be beneficial to have time-resolved data, especially during initiation and decay of pulsed plasmas. To perform such analyses, a Multi-Channel Scalar (MCS) device is incorporated into the ion detector system of a Hiden EQP instrument (Figure 1).

MCS may be employed as a detector for time resolved analysis in beam chopper inlets, ion flight time measurements, and plasma ignition/modulation/extinction analyses.

Typical operating configuration for the Hiden EQP

Figure 1. Typical operating configuration for the Hiden EQP

Hiden's internal MCS facility sequentially directs the mass resolved ion beam detector’s output into one of a number of counting channels (bins) in order to enable high speed time resolved analysis of the ions of interest by determining the number of ions reaching the detector during the channel time.

It may be applied to follow high-speed changes and acquire time resolved data from external events without using any external equipment. Since, it can be synchronously operated with MASsoft's scan generators, it may be applied to collect a data surface wherein one of the axes is time. One example is an energy-time surface for ions in modulated plasma.

Example Data

The data acquired for plasmas in a parallel-plate reactor run using argon as the test gas with power supply from 25kHz AC supply or from an RF supply at 13.6MHz repetitively gated utilizing a square-wave envelope from a signal generator operating at around 500Hz frequency are presented here as examples to demonstrate the new capabilities of the Hiden EQP instrument.

For Ar+ ions at 50mTorr pressure in a 20kHz plasma, a typical variation with time of the energy distribution is presented in Figure 2. The energy scans were performed at intervals of 200nsec. The ion energy distributions for the gated RF plasmas utilizing argon as the test gas are considerably more structured.

Typical time resolved data of Ar+ in a 20kHz 50mTorr plasma

Figure 2. Typical time resolved data of Ar+ in a 20kHz 50mTorr plasma

Figure 3 shows a characteristic family of IED scans for Ar+ ions for a 25W plasma at a pressure of 35mTorr, demonstrating especially the rapidly decaying high energy ions arriving at the sampling orifice of the EQP upon establishment of the plasma.

Typical time resolved data of Ar+ ions in an RF 13.6 MHz 35mTorr plasma

Figure 3. Typical time resolved data of Ar+ ions in an RF 13.6 MHz 35mTorr plasma

Conclusion

In this experiment, the Multi-Channel Scalar device was coupled to the ion detector system of a Hiden EQP instrument. The time resolution used was 50ns. The results revealed initiation and decay of features in the ion energy spectra, which were otherwise impossible to be acquired with time averaged data.

This information has been sourced, reviewed and adapted from materials provided by Hiden Analytical.

For more information on this source, please visit Hiden Analytical.

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