Problem: CVD of Diamond like carbon

Hello, I just started to use a rf-discharge (13.56) parallel plate reactor for deposition of diamond like carbon (ratio of planes
2(grounded):1(powered)). I know from a large pile of papers that at constant pressure (2Pa) and given power a constant bias should develop over the powered electrode (where the sample is situated). Now my problem: the bias is decreasing with deposition time. Does anyone have experience with this kind of PECVD and has an idea how this can be possible? thanks
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