Excimer-Laser-Assisted Etching of Gallium Arsenide: The Effect of Substrate Temperature
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EXCIMER-LASER-ASSISTED ETCHING OF GALLIUM ARSENIDE: THE EFFECT OF SUBSTRATE TEMPERATURE
MICHAEL R. BERMAN McDonnell Douglas Research Laboratories, P. 0. Box 516, St. Louis, MO 63166
ABSTRACT The excimer-laser-assisted etching of GaAs in Cl 2 at 308 nm has been studied as a function of substrate temperature in the range of 23 to 1500 C. Rectangular channels were etched into (100)-oriented, undoped GaAs wafers. The fluence from the xenon chloride excimer laser at the GaAs surface was 175 mJ/cm 2 . At temperatures below 75 0 C, the etch rate was independent of substrate temperature and the etch rate increased with Cl 2 pressure from 0.4 to 2.0 Torr. At substrate temperatures above 750 C, the etch rate increased with increasing substrate temperatures and was independent of Cl 2 pressure. Analysis of the temperature dependence of the total etch rate above 75 ° C according to an Arrhenius expression gave an activation energy of 11.8 kcal/mol, which is consistent with the enthalpy of vaporization of GaCl 3. The onset of the temperature dependent region coincides with the melting point of GaCl 3 .
INTRODUCTION There has been considerable interest in the etching of gallium arsenide by chlorine [1-14]. Excimer-laser-assisted etching is promising as an alternative to currently used techniques for anisotropic etching of gallium arsenide [ 12-14]. Excimer-laser-assisted etching can be used for projection or mask etching, and it avoids ion-induced damage to the GaAs lattice observed following irradiation by ion beams [ 15]. Such ion-induced damage can negatively affect the properties of optoelectronic devices created using ion etching techniques. The etching of GaAs by Cl 2 is in several respects controlled by the removal of the products of the etching reaction from the surface. At room temperature, a layer of solid GaCl 3 forms on the surface which inhibits further etching [3]. Deposition of etch products onto the surface also affects the etch rates [14]. In this study, we look at the effect of substrate temperature on the excimer-laser-assisted etching of GaAs by chlorine. The effect of substrate temperature on the GaCl 3 surface layer greatly affects the etching mechanism and the etch rates that are observed.
EXPERIMENTAL Excimer-laser-assisted etching of GaAs was performed in a stainless steel reaction cell into which chlorine gas was added. Approximately I x 1-cm square samples of (100)-oriented, undoped GaAs (Sumitomo) were mounted on a heatable, nickel-plated brass holder. Temperatures were monitored with a thermocouple located 1 mm below the surface of the Mat. Res. Soc. Symp. Proc. Vol. 201. 1 1991 Materials Research Society
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sample holder. Temperatures were maintained to ± I K during the experimnnts. The reaction cell was evacuated by a liquid-nitrogen trapped oil diffusion pump to -10- to 10 Torr prior to each experiment. A portion of the beam of a 308-nm XeCl excimer laser (Lambda Physik EMG 201E) was selected by a 1x 10 mm rectangular aperture and imaged onto the GaAs surface at normal incidence with a dem
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