Control of etch selectivity

Abstract

Selective etching of separate materials in a manufacture of a device, such as a layer of silicon dioxide on a substrate of silicon in a semiconductor device, is accomplished in a reaction chamber having an RF electromagnetic field which interacts with plural etchants in gaseous phase to produce ions for etching the materials. The ratio of the concentration of etchants affect the relative rates of etching the respective materials. By pulsing the rf excitation waveform, intervals of deenergization of the field are produced repetitively wherein, during any one of these intervals, there is a decay in the concentration of each ionized etchant. Rates of decay and the resulting lifetimes differ for each of the etchants. Thereby, by adjustment of the duration of the deenergization interval, the average concentration of one etchant relative to the average concentration of a second etchant can be varied to attain selective etching of the materials. Continuous monitoring of etchant concentrations, as by ultraviolet absorption spectroscopy, permits automatic control of the modulation to attain a desired etch selectivity in real time.