Plasma processing has a wide and rapidly growing range of applications today. The technology started more or less in the electronics industries for coating and surface modification. Now plasma finds its application in treatment of fabrics, the food industries and a variety of medical applications are in sight as well.
In order to optimize a plasma process and/or to maintain reproducible conditions an elementary understanding and thus diagnostics is required.
Combining a differentially pumped mass spectrometer with an electrostatic energy analyzer the mass and the energy distributions of positive and negative ions and neutrals out of a plasma can be measured.
This method is straight forward. By appropriate ionisation of the neutrals so called radicals may be determined too. On the other side the immersion of any probe into a plasma may disturb the initial situation. In order to extract ions into the instrument an electrical field has to be applied to it. The interaction between this electrical field and the plasma can modify the energy of the ions (mostly for light ions) and their direction too.
Every mass spectrometer which uses electron impact ionisation is quite sensitive to magnetic fields. If an SEM or a C-SEM is used for amplification these devices are influenced by magnetic fields too. Therefore in many experiments magnetic shielding of the instruments is required.
During the detection of neutrals especially high energetic ions may still enter the instrument and the question is how to discriminate between the two species.
Looking closer at mass spectrometric plasma diagnostics the method is still straight forward. However, a careful design of the instruments and an appropriate planning of the experiments are required to avoid artefacts.
Therefore the principle of operation of such mass spectrometers is explained.
Examples of magnetic shielding are given. Design rules are described in general. A method to minimize the influence of the extracting field on the initially energy distribution of the ions is discussed.
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