Structural characterization of free-standing gallium arsenide coiled membranes produced by micromachining

Abstract

High-resolution X-ray diffraction techniques have been employed successfully to evaluate crystalline quality and long-term stability of coiled membranes. The process of fabrication involves photolithography, implantation by 2 MeV N^{2+} ions inn-type GaAs substrates, followed by selective etching. A five-crystal X-ray diffractometer was employed in (+, −, +) setting with an Mo Kα1exploring beam for high-resolution X-ray diffractometry and topography experiments. The exploring-beam width was reduced to illuminate different segments of the coiled membrane. Diffraction curves recorded from the bulk crystal surrounding the sensor had a half width of 26 arcseconds, whereas the half widths from sensor segments were in the range ∼58 to ∼166 arcseconds. Different segments (particularly vertical ones) were identified from the observed angular separations between different diffraction peaks as well as from the shape of the diffraction peaks. It was found that different segments of the sensor were tilted with respect to one another and the tilt angles were in the range 15–212 arcseconds. High-resolution X-ray diffraction topographs recorded from (5\bar 11) and (400) diffracting planes revealed that the sides of the trough below the membrane created by etching are not vertical, but tapered. Also, there is a thin strip of crystal freely hanging over the tapered regions as a result of underetching. The surface of the cavity is uneven. The structural perfection of different membrane segments could also be ascertained from the contrast in topographs.