Plasma density measurements in agressive etching gas environment

Plasma density measurement in fluorocarbon and other aggressive etching gases cannot be easily performed by standard Langmuir probe techniques because of the building of non-conducting films on the probe surface. A new technique free of this problem is the surface-wave (SW) probe, a coaxial-cable-fed pin antenna protected by a dielectric tube around it and connected to a microwave network analyzer. Such a probe features characteristic absorption frequencies manifesting themselves as sharp minima of the reflected microwave power. The absorption peaks have been shown to occur when the electron density of the plasma surrounding the probe is close to the surface-wave resonance plasma density n_c(1+epsiv_d), where epsiv_d is the dielectric constant of the tube protecting the probe and n_c = f²/F_p is the cut-off plasma density related to the microwave frequency/via the proportionality constant F_p ap 81 m^-3Hz^-2. Thus the order of the plasma density can be directly estimated from the absorption peak frequency f as n_e ap (1+epsiv_d)(f²/F_p), with further corrections roughly accounting for the antenna length and surface-wave dispersion in quasi- static approximation. In this contribution we tested the underlying theory by going beyond the approximate quasi-static estimates. The probe absorption spectrum was computed by means of a finite-difference electromagnetic field solver. The absorption frequencies computed were found to differ up to 30% from the ones obtained in quasi-static approximation as in Kokura et al. (1999). This difference can usually be tolerated, although it has to be taken into account when higher accuracy is sought. The SW probe operates even after deposition of some dielectric films or some etching of the protecting tube, but the effect on the measurement accuracy has been not studied up to now. Finite difference computat- - ions showed a 1-2% increase of the absorption frequency when the thickness of the protecting quartz tube of 4 mm inner diameter is reduced to 0.6 mm from an initial thickness of 1 mm. This result was further confirmed by using a probe exposed for 5.5 hours to HF plasma which resulted in tube thickness reduction by 0.4 mm and absorption frequency shift in agreement with the theory. Thus we conclude that SW probe measurements are not sensitive to erosion of the protective dielectric up to half the initial thickness. The authors acknowledge valuable help from T Ohkawara and T Yamashita.