Crystallographic dependence of the lateral undercut wet etch rate of Al_0.5In_0.5P in diluted HCl for III-V sacrificial release

The authors investigated the use of InAlP as a sacrificial layer lattice-matched to GaAs when diluted hydrochloric acid is used for sacrificial etching. They show that InAlP can be used to fabricate submicrometer air gaps in micro-opto-electro-mechanical systems and that a selectivity toward GaAs larger than 500 is achieved. This selectivity enables fabrication control of the nanometer-size structures required in photonic crystal and high-index contrast subwavelength grating structures. The crystallographic dependence of the lateral etch rate in InAlP is shown to be symmetric around the 〈110〉 directions where an etch rate of 0.5 μm/min is obtained at 22 °C in HCl:2H2O. Since the etch rate in the 〈100〉 directions exceeds by ten times that of the 〈110〉 directions, InAlP may be used in sacrificial release of high-aspect ratio structures. Free-hanging structures with length to air-gap aspect ratios above 600 are demonstrated by use of critical point drying following the sacrificial etch.