Microwave surface-wave plasma sources for wafer etching or deposition are promising alternatives to capacitively- or inductively-coupled sources. Unlike the latter, the source and wafer are decoupled, such that the wafer may be independently biased without affecting the source. Furthermore, microwave surface-wave sources are known to produce relatively dense, quiescent, low-temperature plasmas near the wafer surface, thereby minimizing wafer damage. Our device consists of an RLSA (radial line slot antenna) which transmits 2.45 GHz microwaves into a large quartz resonator disk which then couples to the plasma. We compare 2-D PIC (particle-in-cell) simulations from the VORPAL code¹ with Langmuir probe measurements² of the EEDF (electron energy distribution function) of the plasma. The simulations, a continuation of earlier work,³ include ionization using a Monte-Carlo model with an energy-dependent cross section. Secondary emission from the quartz surface is modeled with energy and incident-angle dependent yield and produces a specific energy spectrum of outgoing particles. Fitting of the probe I-V curves employs a novel method of assuming from the outset two Maxwellian distributions plus a drifting Maxwellian to model a beam component. This method, unlike fitting the curves to polynomials or such, aids in interpretation of the results. We find that the EEDF near the resonator disk is typically dominated by the beam component, transitions to two Maxwellians away from the disk, then thermalizes to a single cold Maxwellian near the wafer surface. Simulations and data for various plasma densities and gas pressures will be presented.

¹C. Nieter and J. R. Cary, J. Comp. Phys. 196, 448 (2004).

²J. P. Zhao et al., poster at this conference

³R. V. Bravenec et al., poster at Gaseous Electronics Conference, Dallas, Oct., 2008.

(Research funded by Tokyo Electron Technology Development Institute. The authors also acknowledge the contributions of C. Roark, D. Smithe, and P. Stolz of Tech-X Corp.)