Paper EW-WeL4
Ion Energy and Ion Flux Measurements at an RF Biased Surface

Wednesday, October 17, 2007, 1:20 pm, Room Exhibit Hall

Impedans is introducing the first floating Retarding Field Energy Analyzer which will allow measurement of the Ion Energy Distribution Function on an RF biased substrate. RFEAs are used in research laboratories to measure ion energy distribution functions. Their use in practise is limited by the need to ground the analyser, whereas most interesting applications are in RF biased substrates. The IEDF is important in understanding the role of ions in processes such as etch. Deviations in ion energy can lead to charging effects on a substrate or wafer and variations in flux can effect etch uniformity. A transition from a high frequency (single peak distribution) to a low frequency (bimodal distribution) sheath is often observed in RF plasma sources. The energy difference between the two peaks of the bimodal distribution is related to the RF modulation of the plasma potential and to the ratio between the ion transit time in the sheath and the RF period. A change in ion mass can result in changes in bi-modal structure and impact the average energy of ions arriving at the substrate or wafer. Due to the complexity of modelling the ion energy profiles, measurements are often necessary to validate the behaviour of a new source design in etch applications. The Impedans RFEA is unique in that it uses the technology developed to float RF Langmuir probes in order to isolate the RFEA from electrical ground. The analyser is built into a dummy wafer and placed on the chuck or wafer holder. The RFEA sensor does not disturb the RF bias and the measured Ion Flux and Ion Energy are similar to that seen by a wafer. By placing multiple sensors on a dummy wafer a spatial scan of the ion energy distribution and flux can be achieved. This is invaluable data to understand the plasma process and its effects on the surface of interest. The Impedans RFEA is designed to operate in hostile environments and can withstand temperatures of several hundred degrees centigrade.