Silicon loss and transient etch rate in selective reactive ion etching of oxide overlayers

Silicon dioxide to silicon etch selectivity in reactive ion etching (RIE) employing fluorocarbon gases is due to the formation of a fluorocarbon film on the substrate. This implies that etch selectivity is not achieved instantly but that the substrate will exhibit a transient etch rate and hence material loss during the film formation period. The amount of silicon lost during overetching for CF4/H2 RIE was determined by Rutherford backscattering spectrometry employing a buried marker. For standard etching conditions, about 7 nm of silicon is lost within the first minute of overetching as compared to 1.5 nm expected from steady-state etch rate data. The Si etch rate varies strongly as a function of time and decreases by a factor of greater than 10 within the first 30 s of overetching to approach a steady-state value.