Langmuir probe measurements of weakly collisional electronegative RF discharge plasmas

We report on Langmuir probe measurements of low pressure (0.1-10 Pa) O-2 pl asmas in an RF discharge at 13.56 MHz, The probe is actively compensated us ing a three harmonic box and the collisionless theory of Amemiya et al (199 9) for electronegative plasmas is used to fit to the resulting I-V curves. However, it has been shown for example by Shih and Levi (1971) and Sudit an d Woods (1994) that in electropositive plasmas such as He and N at these lo w pressures the ion saturation current can be reduced to as much as one-hal f the expected value due to a small number of collisions in the probe's pre sheath, This effect has been treated theoretically by Shih and Levi (1971) for both spherical and cylindrical probes. In this paper we extend this the ory to cover the case of electronegative plasmas. Here collisions have a gr eater effect as the resulting ohmic drop has to be compared to the smaller collisionless presheath voltage drop in units of (k(B)T(e)/e) of -1/2 gamma , for a negative ion temperature ratio gamma = T-e/T-n. Using the correctio n factors from this theory we are able to obtain the negative ion fraction alpha in our weakly collisional plasmas. The results using a commercial pro cessing rig indicate a maximum in the negative ion density at around 2.5 Pa consistent with measurements based on laser photodetachment by Stoffels et al (1995).