Measuring the ion current in high-density plasmas using radio-frequency current and voltage measurements

The total current or flux of ions striking the substrate is an important pa rameter that must be tightly controlled during plasma processing. Several m ethods have recently been proposed for monitoring the ion current in situ. These methods rely on passive, noninvasive measurements of the radio freque ncy (rf) current and voltage signals that are generated by plasma-processin g equipment. The rf measurements are then interpreted by electrical models of the plasma discharge. Here, a rigorous and comprehensive test of such me thods was performed for high-density discharges in argon at 1.33 Pa (10 mTo rr) 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 MH z. Model-based methods were tested by comparison to direct, independent mea surements of the ion current at the substrate electrode made using lower fr equency (10 kHz) rf bias and modulated rf bias. Errors in two model-based m ethods 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 cur rent in agreement with the independent measurements.