Real-Time, Noninvasive Monitoring of Ion Energy and Ion Current at a Wafer Surface During Plasma Etching
Mark A. Sobolewski
A noninvasive, non-perturbing technique for real-time monitoring of ion energy distributions and total ion current at a wafer surface during plasma processing has been used to monitor rapid changes in CF4/Ar etching plasmas in an inductively coupled, rf-biased plasma reactor. To mimic the effects of process recipe steps or reactor malfunctions, perturbations were made in the inductive source power, gas flow, and pressure, and the resulting effects on total ion current, sheath voltage and ion energy were monitored. During etching of a thermal silicon dioxide film, smaller changes, which are caused by the etch process itself, were also observed. Sheath voltages determined by the noninvasive technique were in good agreement with simultaneous measurements made using a capacitive probe. In addition to providing a demonstration of the speed and accuracy of the technique, the results also provide useful information about the relative importance of different types of equipment malfunctions and suggest methods for minimizing their effects. In particular, operating at constant bias voltage, instead of constant bias power, gave more stable ion energies. The physical mechanisms that cause the observed changes in ion energy are discussed, and a comparison to other process monitoring methods is presented. No other noninvasive, non-perturbing method yields ion current or ion energies as accurately as the technique presented here.