Nanostructure Fabrication: Dry Etching Damage
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NANOSTRUCTURE FABRICATION: DRY ETCHING DAMAGE G.F.DOUGHTY, R.CHEUNG, M.A.FOAD, M.RAHMAN, N.P.JOHNSON, P.D.WANG, AND C.D.W.WILKINSON Department of Electronics & Electrical Engineering, University of Glasgow, Glasgow G12 8Q0, Scotland UK.
N.I.CAMERON,
ABSTRACT Although the directed energy and chemical reactivity of dry etching permits the fabrication of nanostructures with precise geometries, it also causes unwanted electrical and optical changes to the surface, changes generally referred to as "damage". This paper discusses the extent and the impact of dry-etching damage on rI-V and I[-VI compound semiconductors as assessed by a very wide range of techniques: the performance of devices such as MESFETs, and measurements of other properties - surface uniformly and precisely, Schottky junction characteristics, cut-off of epitaxial wire conductance, integrated photoluminescence, X-ray reflectivity, DLTS, TEM imaging and Raman scattering. We distinguish an important difference between the nature of damage on sidewalls and on surfaces normal to the directed ions, and report on progress towards establishing a model of the nature of dry etching damage. This model is applied to indicate what kinds of processes are likely to give etching with low damage.
INTRODUCTION Fabrication of nanostructures usually demands the geometrical precision that only dry etching can provide. Dry processing can etch fine structure geometry faithfully. Wet etching cannot routinely achieve uniform and precise close packing of vertical walled features. However, the directed energy and chemical reactivity of dry etching tends to make other changes to the surface that are less beneficial. These unwanted changes, as detected by electrical, optical or surface science examinations, are generally referred to as "damage". Dry etching processes For any particular application, a dry etching process is required to make a suitable compromise between the following factors: 1. etching should be selective between the mask and substrate, and also between the various layers of a device. For nanostructures the process must be compatible with electron-beam lithography masks. Selectivity depends on the proportion and type of chemically-active species used in the process. 2. Anisotropy - wall profiles must be vertical, or of reproducible slope. .3. Surfaces and walls must be smooth, which depends on mask quality, substrate
Mat. Res. Soc. Symp. Proc. Vol. 236. @1992 Materials Research Society
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material, and process parameters (particularly any that result in deposition as well as etching). 4. The etching must produce little damage, with little effect on the electrical or optical performance of a device. This depends greatly on the type, energy and chemistry of the ions involved. 5. The process must be slow enough to control etch depth accurately. We have been investigating the use of several processes for etching rn-V and IIVI compound semiconductors. Among these are: Ion-Beam Etching (IBE), using an Oxford OAR 2255 unit, with an IonTech 25mm diameter Kaufman
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