Automated control of an actively compensated Langmuir probe system using simulated annealing

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Abstract

A simulated annealing (SA) method has been developed to deduce 14 Fourier terms in a radio frequency waveform for active compensation of a Langmuir probe system. The active compensation system uses seven harmonics to generate a required waveform. Therefore, 14 heavily interacting continuous parameters need to be tuned before measurements can be taken. Because of the magnitude of the resulting search space, it is virtually impossible to test all possible solutions within an acceptable time. An automated control system employing SA has been developed for online tuning of the waveform. This control system has been shown to find better solutions in less time than skilled human operators. The results are also more reproducible and hence more reliable.

Section snippets

Problem description

This article concerns the use of simulated annealing (SA) to control a diagnostic measurement system for a range of important industrial processes based on plasma technology. The plasmas in question are produced by the passage of electricity through a gas at low pressure, resulting in a partially ionised medium comprising electrons, ions, radicals and neutrals, according to which gases are involved.

The semiconductor industry has been a major exploiter of plasma technology. It uses plasma

Automated control system

Previous work reported by Dyson et al. [3] involved the use of three harmonics for waveform synthesis. In this application, an additive synthesiser (harmonic box) with seven harmonics has been used to generate the appropriate waveform for a Langmuir probe system attached to a gaseous electronics conference (GEC) reference reactor [4]. Fig. 3 shows the schematics of the control system for waveform tuning. The 14 input parameters (seven amplitudes and seven phases) are heavily interacting due to

The implementation of the control system

The system was set up at the laboratories at the Oxford Research Unit of the Open University. Fig. 5 shows the Langmuir probe system attached to the GEC reference reactor used during the development.

System performance

Argon gas is used at a pressure of 100 mTorr. The plasma is usually produced using a power output from the RF generator of 50 W. Fig. 9 shows a typical run of the algorithm. The dotted line represents the best solution found so far, the thick line represents the fitness on each iteration. It can be observed that the algorithm initially explores the search space, while later on (after approximately 400 iterations) it exploits the most promising region. Hence, the behaviour of the algorithm at the

Acknowledgements

This work was funded by the Engineering and Physical Sciences Research Council (EPSRC) under grant reference GR/M71039/01. The authors wish to express their thanks to Fraser G Robertson, Faculty of Technology, the Open University, for assistance with the RF equipment, and to Pierre Barroy, Oxford Research Unit, the Open University, for his assistance in setting-up the experiment.

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