Ion Beam Etching System for Mercury Cadmium Telluride and III-V Compound Semiconductors
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ION BEAM ETCHING SYSTEM FOR MERCURY CADMIUM TELLURIDE AND rn-V COMPOUND SEMICONDUCTORS GEOFFREY K. REEVES*, PATRICK. W. LEECH+, and PATRICK BOND* Royal Melbourne Institute of Technology, Melbourne, Vic., Australia. +Telecom Australia Research Labs., Clayton, Vic., Australia. ABSTRACT This paper describes a laboratory built ion beam etching system and its performance when used for etching Hgl_xCdxTe, GaAs and InP. The etching system provides a means for forming device mesas on a wide range of semiconductors without having to resort to wet chemical etches. The system uses a Kaufmann ion source, a rotating platform and two flow controllers to allow the variation of gas ratios and flows. INTRODUCTION Increasing interest in Mercury Cadmium Telluride (MCT) as an optoelectronic device material has resulted in the development of essential fabrication procedures such as contact formation[l] and wet chemical etching[2]. However the fabrication of semiconductor devices requires etching techniques that provide good dimensional control with low surface contamination levels. These etching characteristics are desirable for obtaining the optimum performance from semiconductor devices, and thus dry etching techniques have become well established for device fabrication[3]. When etching is undertaken for line or mesa formation, then etch characteristics such as etchrate uniformity, anisotropy and linewidth fidelity are important. However when further device processing is to be undertaken on the etched surface (e.g. Schottky or ohmic contacts), then the physical and electrical nature of the etched surface becomes critical. In order to improve the quality of optoelectronic devices using MCT, a study of some of the etching characteristics of this material (in conjuction with InP and GaAs) was undertaken on a small laboratory-built ion beam etching system. ETCHING SYSTEM The laboratory-built etching system allows control over gas flow rate, ion beam current density, beam voltage, accelerator voltage and incident beam etching angle. While the uniformity of etching rate is limited to a relatively small area (2cm x 2cm), this is not a major limitation as most of the sample areas used for our device fabrication are of this order or less. Etching action is essentially ion beam milling with the option of chemical assistance due to the presence of reactive gases bled into the chamber. The etching system with Kaufman ion source is shown in figure 1 with the stainless steel chamber removed. The 5cm diameter ion source(A) is mounted on an adjustable quadrant arm(B) and is situated approximately 12cm above the sample holder(C). The sample holder is water cooled and can rotate at 60 revolutions per minute. A Faraday cup(D) for measuring beam current density is located on top of a shutter(E) which can cover the sample holder. A reactive gas can be introduced separately via a circular gas feed ring(F), providing additional control of the etching conditions. The chamber is cryopumped down to approximately 1 x 10-6 torr prior to etch commencement. The p
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