Rf probe technology for the next generation of technologicalplasmas

Abstract

We describe radio frequency (rf) analysis of technological plasmasat the 13.56 MHz fundamental drive frequency and integer narrow-band harmonicsup to n = 9. In particular, we demonstrate the use of harmonic amplitudeinformation as a process end-point diagnostic. Using very high frequency (vhf)techniques, we construct non-invasive ex situ remote-coupled probes: adiplexer, an equal-ratio-arm bridge, and a dual directional coupler used as asingle directional device. These probes bolt into the plasma-tool 50 Ωtransmission-line between the rf generator and matching network, and hence donot require modification of the plasma tool. The 50 Ω probe environmentproduces repeatable measurements of the chamber capacitance and narrow-bandharmonic amplitude with an end-point detection sensitivity corresponding to a2 dB change in the harmonic amplitude with the removal of 1 cm2 of photoresist.The methodology and design of an instrument for the measurement of theplasma-tool frequency response, and the plasma harmonic amplitudeand phase response are examined. The instrument allows the monitoring of theplasma phase delay, plasma-tool short- and long-term ageing, and processend-point prediction.