Dry processing of high resolution and high aspect ratio structures in GaAs-AlxGa_1−xAs for integrated optics

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Abstract
The properties of ion-beam milling, rf-sputter etching, and plasma etching for the fabrication of high resolution integrated optical structures in GaAs and AlxGa1−xAs were investigated. The ion-beam milling rates for a 500-eV Ar beam were measured as a function of the angle of incidence for A2-1350 photoresist, GaAs, Al, Ti, and Ta in vacuum. For Ti and Ta, the ion-beam milling rates were measured for various O2 partial pressures in the target chamber. For the fabrication of high-resolution patterns, a technique was developed in which a very thin resist pattern is transferred by plasma etching into a layer of Ta prior to ion-beam milling. Both ion-beam milling and rf-sputter etching through the Ta metal mask were used for the fabrication of gratings in GaAs. From measurements made in this study, it was found that rf-sputter etching in an Ar atmosphere results in a resolution comparable to ion-beam milling at normal incidence, but the material removal efficiency is somewhat lower. Third-order Bragg diffraction gratings (period = 0.36–0.37 μm) for GaAs-AlxGa1−xAs heterostructure lasers and ridged stripes 10 μm wide and 1.5–2.0 μm deep have been prepared by these techniques. High-aspect ratio or skewed-profile gratings were obtained by selection of the ion-beam milling or rf-sputter etching conditions. As previously reported, these structures have been overgrown by MBE AlxGa1−xAs layers to form distributed feedback lasers.