Optical and electrical diagnostics of fluorocarbon plasma etching processes

This article reviews recent work concerning the role of CF and CF2 radicals in etching and polymerization processes occurring in capacitively coupled radio-frequency plasmas in fluorocarbon gases used for the selective etchin g of SiO2 layers in microelectronic device fabrication. Laser-induced fluor escence (LIF) was used to determine time-resolved axial concentration profi les of these species in continuous and pulse-modulated CF4 and C2F6 plasmas . Calibration techniques, including broad-band UV absorption spectroscopy, were developed to put the LIF measurements on an absolute scale. A novel te chnique was used to determine the ion flux to the reactor walls in these po lymerizing environments. The mass distribution of the ions arriving at the reactor walls was determined using a quadrupole mass spectrometer. It was found that CFx radicals are produced predominantly by the reflection of neutralized and dissociated CFx+ ions at the powered electrode surface. When the fluorine atom concentration is high, the CFx radicals are destroy ed effectively by recombination catalysed by the reactor walls. When the fl uorine atom concentration is lowered, the CF2 concentration rises markedly, and it participates in gas-phase oligomerization processes, forming large CxFy molecules and, after ionization, large CxFy+ ions. These species appea r to be the true polymer precursors. This mechanism explains the well known correlation between high CF2 concentrations, polymer deposition and SiO2 o ver Si etch selectivity.