Photochemical Dry Etching of Semiconductors and its Relationship to Semiconductor Electronic Properties
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PHOTOCHEMICAL DRY ETCHING OF SEMICONDUCTORS AND ITS RELATIONSHIP TO SEMICONDUCTOR ELECTRONIC PROPERTIES CAROL I. H. ASHBY Sandia National Laboratories, 87185
Division 1126,
P.
0. Box 5800, Albuquerque,
NM
ABSTRACT and selectivity of photochemical dry etching The general utility processes which require direct participation of photogenerated carriers is The largely determined by the electronic properties of the semiconductor. laser which is used to produce the carriers responsible for etching can also provide in situ measurement of some of the important electronic properties which influence the etching process. INTRODUCTION There are three fundamental mechanisms whereby irradiation of the surface of a semiconductor can produce spatially localized dry etching [1]. The first of these simply involves heating the surface to increase the rate of a thermally activated chemical reaction. The second involves absorption of the laser photons by a reactant precursor molecule, either in the gas phase or while adsorbed on the surface, and subsequent dissociation to The third involves the photogeneration of produce a more reactive etchant. electron-hole pairs in the surface region of the solid and the subsequent When a laserparticipation of these carriers in the etching reaction. induced chemical reaction requires such electronic excitation of the semiconductor, those factors which control photocarrier generation or This subsequent carrier behavior can control the chemical reaction as well. is exemplified by the photochemical dry etching of GaAs [2] and Ga(As,P) [3] by low concentrations of Cl atoms under conditions which produce negligible etching in
the absence of photogenerated
electon-hole pairs.
Dependence on
photocarrier generation has also been reported for dry etching of Si [4,5] Reaction rates for this third type of process are proportional to the free carrier flux at the surface. Consequently, those factors which determine the magnitude of that flux, such as photon absorption depth, impurity concentrations, surface band bending, carrier lifetimes, and The recombination processes, necessarily control the photochemical etching. understanding of these photochemical etching processes can be aided by optical diagnostics such as photoluminescence and photoreflectance spectroscopies. These are nondestructive in situ diagnositic techniques which permit measurement of electronic properties of direct-gap semiconductors under a variety of reaction conditions and may provide a means for rapid determination of appropriate reaction conditions to achieve a high degree of etching selectivity. In this paper, the discussion is limited to the physics and chemistry of this third type of process.
ELECTRONIC PROPERTIES
In all cases, the chief concern is the free carrier density at the surface, since this quantity ultimately determines the maximum rate at which etching can occur. The final free carrier density at the surface is determined by the balance between those processes which create free carriers, those which destroy them, an
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