Use of real-time ion-mass spectrometry and discharge voltage trending for analysis of a-Si DC plasma CVD processes

DC discharge voltage monitoring is a convenient non-intrusive tool for in-line monitoring of plasma-deposited amorphous silicon deposition processes. We discuss the observed variations of the discharge voltage due to (i) changing hydrogen dilution of silane, (ii) etch back of silicon by a hydrogen plasma, (iii) admixing of germane and methane and correlate the results with in situ ion-mass spectroscopy. We suggest that negative ion formation by electron attachment to silane and/or higher-silane species causes the changes of discharge voltage. Time dependences following discharge initiation are discussed in terms of the relative concentrations of silane versus higher-silane species.