160 GHz Gaussian beam microwave interferometry in low-density rf plasmas

160 GHz Gaussian beam microwave interferometry is realized for electron density analysis in low pressure rf plasmas. Measurement of electron densities lower than 1016 m−3 with corresponding phase shift less than 0.3° demands high stability of the interferometer frequency and minimum disturbance due to external interfering voltages and mechanical vibrations of the optical components. The interferometer consists of a frequency stabilized (phase lock loop) heterodyne system operating at a frequency of fMWI = 160.28 GHz and wavelength of λMWI = 1.87 mm, respectively. A quasi-optical setup is used, considering specially designed horn antennas and elliptical mirrors as well as components which have to comply with the aperture limit in relation to the Gaussian microwave beam and its optimal coupling and focusing into the plasma center. A spatial and temporal resolution of about 10 mm (beam waist 5 mm) and 0.2 µs is achieved, respectively. In cc-rf plasma the lowest measurable phase shift is in the order of 0.01°, which corresponds to a line-integrated electron density of about 5 × 1013 m−2 or an electron density of 5 × 1014 m−3 averaged over the electrode diameter. Results are presented and discussed concerning line-integrated electron density in an asymmetric argon cc-rf plasma in dependence on rf power and total pressure.