Ion-assisted processing of GaAs for optical devices

Gas etching techniques, such as reactive ion etching, ion beam milling and chemically assisted ion beam etching, have been used to produce a range of optical and electronic components in GaAs. Large structures, typically 10-100μm deep, such as spherical microlenses, mesas, columns and fibre-location-channels, may be fabricated in GaAs by a fast reactive ion etching (RIE) process using Freon-12 (CCl2F2). Three-dimensional ir microlenses of spherical geometry have been fabricated in single crystal (III) and (100) GaAs. Microlens structures of this form were transferred from a hemispherical photoresist mask cap into the GaAs substrate by ion beam milling and by CCl2F2 assisted ion beam etching. Etching rates for crystalline GaAs by RIE with CCl2F2 were fast; at a power density of 6 W cm-2 and gas pressure of 18 Pa (∼150 mtorr) an etch rate of 3.5 μm min-1 was recorded. However, the selectivity between photoresist mask and GaAs using RIE was more difficult to control. By inert argon ion beam milling at a beam energy of 500 eV and current density 0.55 mA cm-2, etch rates for crystalline GaAs of circa 700 A min-1 were obtained. Improving surface finish is also important if highly optically efficient lenses are to be produced with reduced losses (i.e. scattering, aberrations). Low damage etching of GaAs has been demonstrated with ≤100 eV ion beams, resulting in improved surface finish. (author)