Condensed Chlorine Etching of GaAs Induced by Excimer Laser Radiation
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CONDENSED CHLORINE ETCHING OF GaAs INDUCED BY EXCIMER LASER RADIATION M.B. Freiler, Ming Chang Shih, G. Haase*, R. Scarmozzino, and R.M. Osgood, Jr. MicroelectronicsSciences Laboratories,Columbia University, New York, NY
* Currently at University of Wisconsin, Madison ABSTRACT We report excimer laser induced etching of GaAs samples covered with a layer of condensed Cl2 . The experiments were performed at low temperatures (120-150 K) and in a chlorine ambient (PC12=1-40 mTorr). Spatially well-resolved, anisotropic etching has been demonstrated with an observed etch rate of 0.25 A/pulse (0.1 p•rmin for typical parameters). The etch rate is characterized as a function of the various system parameters (pressure, temperature, laser rep-rate, etc.), and a model is proposed to describe the etching mechanism. INTRODUCTION There are several ways that anisotropic chemical etching can be achieved. Previous work studying ion beam etching of Si at low temperatures [1,21 showed that anisotropic etching is achievable at low temperatures because the spontaneous etch rate, which tends to be isotropic, would be reduced. Also, anisotropy can be achieved by forming a passivating layer on the sidewalls [3]. Furthermore, the creation of the etchant species in an adlayer increases anisotropy by reducing the influence of etching by gas phase atoms, which impinge the surface at many different angles, causing an undercut. In previous work reported by Liberman, et al. [4], a layer of physisorbed chlorine was formed on GaAs (110) samples cooled to 85 K prior to irradiation by an excimer laser at low fluences. Products leaving the surface were measured by temperature-programmed-desorption (TPD) and time-of-flight (TOF) measurements, and the condition of the sample surface was probed by Auger and LEED. TPD experiments after excimer laser irradiation showed the formation of reaction products not observed without irradiation. In particular, AsCI3 and Ga were observed to desorb at 175 and 210 K, respectively. TOF measurements showed that C1 and Cl, were ejected from the surface by the laser beam during irradiation, indicating that the laser is capable of causing desorption of adsorbed species. It was also observed that the amount of AsCI 3 desorbed was much greater when 193 nm radiation is used than when 351 nm was used. This is consistent with measurements of the optical absorption of condensed Cl, [5], which showed a large peak at about 200 nm. We speculate that this is due to charge transfer between neighboring chlorine molecules, resulting in dissociation [4]. The above results suggest a process for anisotropic etching of GaAs. By lowering the substrate temperature in a chlorine ambient, a layer of C12 is physisorbed on the GaAs surface. Excimer laser radiation at 193 nm dissociates the chlorine molecules to form reactive CI radicals, whiich are confined to the illuminated zone and react with the GaAs surface. The physisorbed Cl2, as well as the etching products (AsC13 and Ga), are then desorbed by the laser, resulting in etching. Good spatia
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