Apparatus for monitoring and/or controlling plasma processes

Apparatus for monitoring a plasma process in which a plasma is formed to occupy a specified region between two electrodes spaced a predetermined distance from each other, said apparatus comprising, in combination:

• mass spectrometer means including a mass analyzer having an ion inlet and an ion outlet and an ion detector disposed in operative association with said ion outlet;
• output means connected to said detector for providing an output signal representative of the mass spectrum of ions observed by said mass spectrometer means;
• ion-optical means oriented approximately at right angles to a line interconnecting the two electrodes, having an inlet opening located in the vicinity of the specified region and disposed for extracting ions from the plasma and focussing the ions thus extracted onto said ion inlet of said analyzer;
• and
• a housing carrying said ion-optical means and having an open rear end facing said spectrometer means and a front end directed toward the specified region and provided with said inlet opening, wherein said housing is cylindrical and has a diameter which is less than one-fourth the distance between said electrodes.