Ion energy distribution function measurements by laser-induced fluorescence in a dual radio frequency sheath

Abstract

Ion dynamics are investigated in a dual frequency radio frequency sheath as a function of radius above a 30 cm diameter biased silicon wafer in an industrial inductively coupled (440 kHz, 500 W) plasma etch tool. Ion velocity distribution (IVD) function measurements in the argon plasma are taken using laser induced fluorescence. Planar sheets of laser light enter the chamber both parallel and perpendicular to the surface of the wafer in order to measure both parallel and perpendicular IVDs at thousands of spatial positions. A fast (30 ns exposure) charge coupled device camera measures the resulting fluorescence with a spatial resolution of 0.4 mm. The dual-frequency bias on the wafer is comprised of a 2 MHz low frequency (LF) bias and a 19 MHz high frequency bias. The laser is phase locked to the LF bias and IVD measurements are taken at several different LF phases. Ion energy distribution (IED) function measurements and calculated moments are compared for several cases. IEDs were measured at two disparate phases of the phase-locked LF bias. IEDs were found to be multipeaked and were well-approximated by a sum of Maxwellian distributions. The calculated fluxes in the dual frequency case were found to be substantially more radially uniform than the single frequency bias case. For industrial applications, this radially uniform ion flux is evidently a trade off with the undesirable multipeaked structure in the IEDs.