The total current or flux of ions striking the substrate is an important parameter that must be tightly controlled during plasma processing. Several methods have recently been proposed for monitoring the ion current in situ. These methods rely on passive, noninvasive measurements of the radio frequency (rf) current and voltage signals that are generated by plasma-processing equipment. The rf measurements are then interpreted by electrical models of the plasma discharge. Here, a rigorous and comprehensive test of such methods was performed for high-density discharges in argon at 1.33 Pa (10 mTorr) in an inductively coupled plasma reactor, at inductive source powers of 60-350 W, rf bias powers up to 150 W, and rf bias frequencies of 0.1-10 MHz. Model-based methods were tested by comparison to direct, independent measurements of the ion current at the substrate electrode made using lower frequency (10 kHz) rf bias and modulated rf bias. Errors in two model-based methods are identified and explained by effects that are present in the high-density plasmas but are not included in the models. A third method, based on a new, more accurate numerical sheath model, gives values of the ion current in agreement with the independent measurements.
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