In situ x-ray photoelectron spectroscopy (XPS) and thermal desorption measurements indicate that etching of polyimide (PI) in a microwave oxygen plasma results in increased carbonyl (C=O) bonds on the etched surfaces, which can be removed by heating. Addition of CF4 to oxygen plasma enhances the etch rate, but also gives rise to fluorine coverages which appear to scale with the atomic F concentration in the gas mixture. XPS study of selected fluorinated model compounds shows that changes in the C is spectra of PI surfaces, etched in plasmas with different CF4 concentrations can be interpreted either as formation of Teflon-like passivation layers or as simply fluorinated benzenes. Exposure to x-ray irradiation at a constant flux or isothermal heating in vacuum caused the fluorine coverage to desorb in two stages, suggestive of two different fiuorine-containing desorbing species. Experimental evidence obtained by multiple internal reflectance and transmission Fourier transform infrared spectroscopy indicates that for UP to 50% CF4 additions, etching is accompanied by an increasing degree of fluorination of aro-atic carbon. This is in contrast to the general belief that formation ofa Teflon-like layer is responsible for inhibition of etch rate at higher CF4 concentrations in the gas mixture. A selfconsistent model is proposed to explain these observations. © 1987, American Vacuum Society. All rights reserved.