Optical and electrical diagnostics of fluorocarbon plasma etching processes

This article reviews recent work concerning the role of CF and CF 2 radicals in etching and polymerization processes occurring in capacitively coupled radio-frequency plasmas in fluorocarbon gases used for the selective etching of SiO 2 layers in microelectronic device fabrication. Laser-induced fluorescence (LIF) was used to determine time-resolved axial concentration profiles of these species in continuous and pulse-modulated CF 4 and C 2 F 6 plasmas. Calibration techniques, including broad-band UV absorption spectroscopy, were developed to put the LIF measurements on an absolute scale. A novel technique was used to determine the ion flux to the reactor walls in these polymerizing environments. The mass distribution of the ions arriving at the reactor walls was determined using a quadrupole mass spectrometer. It was found that CF x radicals are produced predominantly by the reflection of neutralized and dissociated CF x + ions at the powered electrode surface. When the fluorine atom concentration is high, the CF x radicals are destroyed effectively by recombination catalysed by the reactor walls. When the fluorine atom concentration is lowered, the CF 2 concentration rises markedly, and it participates in gas-phase oligomerization processes, forming large C x F y molecules and, after ionization, large C x F y + ions. These species appear to be the true polymer precursors. This mechanism explains the well known correlation between high CF 2 concentrations, polymer deposition and SiO 2 over Si etch selectivity.