Abstract
During sputtering the plasma potential of the glow discharge may be influenced by unprotected electrodes situated in the vicinity of the plasma region. A positive electrode potential 
is known to shift the plasma potential by a value of 
in the positive potential direction. A negative electrode potential has no such influence on the plasma potential. A consequence of this behavior of the plasma potential is that all grounded parts of the vacuum system will establish a substantial 
negative potential with respect to the plasma if a positive electrode is introduced. This effect may cause excess contamination of the sputtered films since sputtering during this condition takes place both from the grounded vessel surfaces and from the sputtering target. Also, grounded substrates will then be subjected to unintended bombardment by energetic ions. This is especially important to consider in planar magnetron sputtering, where no energetic bombardment of substrates is supposed to take place. Introducing a negatively biased Langmuir probe into the plasma makes the probe current very sensitive to plasma conductance without affecting plasma potential conditions during sputtering. It is thus possible to monitor the sputter‐cleaning of targets. During cleaning of an Al target the secondary emission factor of electrons caused by ion impact changes considerably. Thus the conductance of the plasma changes. This is detected by the Langmuir probe.