Hetero-Micromachining of Epitaxial III/V Compound Semiconductors

Due to their special properties (direct band gap, piezoelectric effect) III/V compound semiconductors are of high potential for the realization of monolithic micro-opto-electro-mechanical systems (MOEMS). Hetero-micromachining (HMM) is a novel technique for the fabrication of miniaturized sensors and actuators which is based on III/V compound semiconductor layers epitaxially grown on (001) silicon. Using this concept MOEMS may be realized in combination with well-established silicon microelectronics. In this contribution hetero-micromachining of indium phosphide, gallium arsenide and stacks of different III/V compound semiconductor layers is described. Mechanical structures (cantilevers, membranes) were realized exploiting the etching selectivity of these materials against silicon in KOH solution. Both etching and fracture properties of InP cantilevers are dependent on the concentration of silicon impurities in the layer. For GaAs a fracture limit in excess of 1.5 GPa was found which is close to figures known for standard silicon wafer material. Micromirrors of various designs were fabricated by hetero-micromachining of InP. Actuation is performed using the bimorph effect between the InP and a metallization deposited on top of the mirror suspensions. Depending on the design of the suspensions mirror deflections up to 0.07° per milliwatt of electrical input power could be achieved at excitation frequencies ranging from the quasistatic case to several kilohertz. For the higher frequency range a piezoelectric actuator was designed based on highly resistive III/V-semiconductors. Using iron doping a resistivity of 7.106Ωcm at 2 V could be obtained with InP which can be further improved by reducing the unintentional incorporation of silicon impurities during heteroepitaxy.