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

Abstract

Reactive ion etching is becoming increasingly widespread in the fabrication of III-V semiconductor devices. Particular applications include optical waveguides, heterojunction bipolar uansistors(HBTs) and ET/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 requued. A number of end point detection techniques are available and include optical emission spectroscopy, optical reflectometxy 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 Wrication of HBTs. Introduction.One of the most critical areas in III-V device fabrication is accurate and controllable mesa formation and buried layer contact. While wet etching of material is suitable for large or coarse devices it lacks the lateral and vertical controllability required for modem microwave and optical devices.For these devices a "reactive ion process" or "dry etch" process is utilised due its high unifohty and anisotropic etch nature.[ 11 Although this technique offers a number of advantages it is still far from ideal due to the run to run etch variation and the lack of etch depth control. In order to overcome these difficulties one technique that is gaining wider use is that of "end point detection". [2] This report investigates two of these end point detection techniques, mass spectroscopy and optical interferometry. As a vehicle to assess these techniques the heterojunction bipolar transistor has been used since it requires accurate etches to a thin base layer in order to reduce the parasitic resistance [3]. Introduction to Techniques. Opiical Reflectometry. The technique of optical reflectomeuy uses the interference of the reflected laser light source from different layers within the substrate. The intensity of the reflected beam is dependant on the Fresnel coefticient of the materials. The amplitude of the intexference pattern obtained is dependant on the difference in refractive indices between the etching substrate and the layer below it. The period between each of the fringes being given by ?J2n where h is the wavelength of light from the laser diode source and n is the refractive index of the material being etched. [4]Wafers were etched in a Plasma Technoiogy RlESOClp system. This system is a parallel plate reactive ion etcher which uses silicon tetrachloride as the etch gas. A Mickeldore Technology Ltd. Dynarmc Optical Reflectivity Instrumentation System (DORIS) was mounted centrally over a window in the top electrode. The DORIS system uses a 670nm laser diode mounted in a perspex block. This block also contains a photodiode detector which is offset ...