The application of end point detection in the fabrication of optical and microwave devices

Reactive ion etching is becoming increasingly widespread in the fabrication of III-V semiconductor devices. Particular applications include optical waveguides, heterojunction bipolar transistors (HBTs) and FET/HEMT type devices. Etch depth control is a critical issue in the fabrication of many devices. Optical waveguides require accurate etch depths for optical mode control whereas in the fabrication of HBTs there is a requirement to access a thin base layer. It is possible to fabricate devices using a "dead reckoning" method-however, this has proved not to be very accurate and a form of end point detection is required. A number of end point detection techniques are available and include optical emission spectroscopy, optical reflectometry and mass spectroscopy. This paper will compare all three methods and demonstrate weaknesses and strong points for each. Particular attention will be given to optical reflectometry and mass spectroscopy in the fabrication of HBTs.