Electrostatic probe analysis of microwave plasmas used for polymer etching

Abstract

Microwave plasma etching [S. Dzioba, G. Esten and H. M. Nagub, J. Electrochem Soc. 129, 2537 (1982); K. Suzuki, S. Okudaira, S. Nishimatsu, K. Usami, and I. Kanomata, J. Electrochem. Soc. 129, 2764 (1982)] is a novel technique which promises to have wide application in the semiconductor industry for the removal of silicon and polymeric materials. Its advantages include relatively high concentrations of both ions and chemically excited species and the ability to etch isotropically. This allows high etch rates to be achieved with a minimum of ion bombardment. Langmuir probe studies have been performed on microwave (915 MHz) plasmas in O2 to determine the spatial variation of the plasma parameters (ion density, electron temperature, and space potential), as well as their dependence on gas pressure, microwave power, and the addition of CF4. Probes have also been used in microwave plasmas with an rf bias applied to an internal electrode (combination plasmas). Ion densities on the order of 2×1010 cm−3 were typical in the spatial afterglow of microwave plasmas with an O2 pressure of 100 mTorr, and values as high as 7×1010 cm−3 were observed at lower pressures. Average electron temperatures between 1 and 2 eV were prevalent in the microwave plasmas while rf biasing the substrate electrode leads to a value of 7 eV near the etching surface.