Spatially resolved electron density distribution in an RF excited parallel plate plasma reactor by 1 mm microwave interferometry

The electron distribution in the argon plasma of a capacitively coupled parallel plate reactor (13.56 MHz) is measured with spatial resolution. The line-of-sight integrated electron density is obtained with a quasi-optical heterodyne interferometer operating at the wavelength lambda = 1 mm. The electron density distribution is measured in a sequence of planes between the electrodes whereby the radial distribution is determined in each plane by Abel inversion. This allows us to generate an ( r, z ) mapping of the electron density. The lateral resolution is 6.5 mm in the focal region; the phase resolution amounts to 7 × 10 -5 of a period so that a line density of 2 × 10 14 m -2 represents the lower limit of applicability. The spatial distributions are compared with electron density measurements at similar discharges by other groups and with probe measurements in the midplane of our discharge.