Paper PS-ThP2
The Effect of Radio-Frequency Bias on Electron Density in an Inductively Coupled Plasma Reactor, Measured by a Wave Cutoff Probe

Thursday, October 18, 2007, 5:30 pm, Room 4C

Inductively coupled plasma reactors allow greater, more independent control of ion energy and ion flux than is possible in traditional capacitively coupled plasma reactors. Nevertheless, even in inductively coupled reactors, it is unlikely that perfectly independent control can be achieved. The application of radio-frequency (rf) substrate bias, which is intended to only affect ion energies, may also produce changes in the plasma electron and ion densities and the total ion flux. Such changes are generally believed to be small, but it is not clear how small. Modeling studies of inductively coupled plasmas usually do not consider these bias effects. Experimental measurements of the effect of bias on electron density have been made, using Langmuir probes or microwave interferometry, but the data reported so far are quite limited and often appear to be contradictory. The accuracy of some results may be in doubt, since measured changes are comparable in size to systematic errors present in the measurement techniques. To provide a better characterization and understanding of the effect of bias power on electron density, we performed a detailed study in Ar, CF₄, and Ar/CF₄ plasmas. We measured the electron density with a wave cutoff probe, which avoids problems with deposition and rf compensation that may affect the accuracy of Langmuir probes. The effect of rf bias on electron density was measured as a function of source power, position, pressure, bias frequency, bias amplitude, and time. At selected experimental conditions, results from the cutoff probe were compared to Langmuir probe measurements, and both showed the same effects. Two types of bias-induced changes in electron density were observed. One was a gas composition effect caused by etch or sputter products liberated from the wafer surface. The other was an electron heating effect caused by absorption of bias power by plasma electrons. Simple models of each effect were derived and shown to yield quantitative predictions in agreement with the observations.