Advanced Device Fabrication With Angled Chlorine Ion Beam Assisted Etching

Abstract

Angled ion beam assisted etching (IBAE) has been used in conjunction with a variety of lithographic techniques to produce structures in GaAs and GaAlAs with controlled side-wall geometries. In the IBAE process an argon ion beam and a jet of chlorine gas are simultaneously incident on the sample. The etching occurs due to a chemical process involving chlorine, but is highly anisotropic because of the argon ion beam. In fact, the slope of the etched wall is determined by the angle at which the sample is tilted with respect to the ion beam. A number of different side-wall contours have been generated by using fixed tilt angles and computer-controlled dynamic tilting. We are currently utilizing this technology to fabricate vertical field effect transistors (FETs), resonant tunneling transistors, surface emmitting laser arrays and quantum-wire structures. This article describes the angled IBAE technique and its use to fabricate novel devices and structures. The basic chlorine IBAE and angled chlorine IBAE processes and equipment have been described elsewhere. 1-3 A schematic drawingof the etching geometry and computer-controlled sample stage used for angled etching is shown in Fig. 1. The tilt angle of the wafer is defined as the angle formed by the normal of the wafer surface to the axis of the argon ion beam, shown as 6 in Fig. 1. In angled chlorine IBAE the tilt angle is also the angle that the etched sidewall makes with the normal of the wafer surface. Figure 2 shows a scanning electron microscope (SEM) micrograph of etched walls in (100) GaAs with the edge alignment along the (01 1) cleavage plane. The first micrograph shows a sidewall etched at four different tilt angles for four different time intervals. The tilt angle schedule was 30° for 20 min, 40° for 10 min, 50° for 5 min, and 60° for 2.5 min. The second micrograph shows a curved sidewall obtained by computer-controlled etching using 800 discrete tilt angles. Etching was initiated with the ion beam 35O from the normal to the sample, and the angular motion of the sample holder was accelerated during the run. As the angle between the ion beam and the sample normal increases, the top edge of the mask shadows areas with more vertical sidewalls so that virtually any concave shape can be generated. For this work we adjusted the system operating parameters to give a normal-incident etch rate of 40 to 50 nm min-l in GaAs. We operated the system with a 500-eV argon ion beam at a current density of 0.02 rnA cm-2 which gave an argon ion beam pressure of 0.1 mTorr at the sample surface. The chlorine beam pressure at the sample surface was 2.8 mTorr. With these parameters the normal-incidence etch rates for A Ga As with x from 0.08 to 0.80 were 40 nm mine' to within 10%. No roughness was observed atbakA1GaAs heterointerfaces. The masking materials were baked AZ- 1470 photoresist, pyrolytically deposited phosphosilicate glass and evaporated nickel. The respective etch rates for these materials were 4.7 nm min-l, 1.2 nm min-l, and 0.4 nm min-l. The technique was used to fabricate a monolithic two-dimensional GaAdAlGaAs laser diode array with light emission normal to the surface. This was accomplished by fabricating an array of edge-emitting quantum-well double-hetemstructure lasers with deflecting adjacent to the