Precision Reactive Sputter Etching and Its Applications

A precision reactive sputter etching process was developed for accurately etching single crystal silicon. This allows one to modify existing device structures and to create new devices in a repeatable way. The process relies on the in situ monitoring of an optical interference signal from a polysilicon film deposited over an SiO₂ layer. By this method the etch depth can be controlled to within ±10 nm. This technique has been used to study the device characteristics of thin base and thin emitter transistors without any noticeable device degradation arising from ion impact damage. This is achieved by using Cl₂/Ar gas. Two wafers etched at a time in four runs showed a standard deviation of less than 14% in current gain. For the thin emitter study, the current gain decreased significantly for emitters thinned below 200 nm because of enhanced hole diffusion. We believe that this technique will become increasingly important in the future for studying the device characteristics and for making new device structures.