Design and use of a gridded probe in a low-pressure rf argon discharge

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Abstract
Low-pressure rf discharges can be investigated experimentally using many diagnostic techniques. In many cases the simple Langmuir probe can be used to make reliable measurements of basic plasma parameters. In the work reported here, this technique has been extended by employing a gridded Langmuir probe in a parallel-plate rf discharge for the first time. Design considerations and construction of the probe are discussed. It was found that optimum performance could be achieved using a construction based on two independent grids with a nonzero potential difference between them. The probe was used to measure plasma parameters in a low-pressure argon discharge excited at 13.56 MHz between two planar parallel electrodes. Results obtained for different probe orientations along the discharge axis show the electron energy distribution to be approximately Maxwellian in the plasma bulk. However, there is evidence for anisotropy in the electron energy distribution in the region of the plasma nearest to the driven electrode. In fact the high-energy tail observed can be attributed to secondary electron emission from the electrode surface.

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