SURFACE FREE-ENERGY MODEL OF SILICON ANISOTROPIC ETCHING

Silicon micromachining is used extensively for the microfabrication of integrated circuits, microsensors and microactuator devices. We address both calculations of the surface free energy of the crystal planes of silicon and the results of etching single-crystal silicon spheres in KOH and CsOH under controlled conditions. The silicon spheres were prepared by the cup-grinding method. They were etched in a constant temperature bath at 50 and 75-degrees-C and inspected under the SEM. An optical reflectance technique was used to determine the angle of inclination of the etch facets. The order of etch rates for the crystal planes was {311}, {522} > {100} > {111}. The fastest etching planes in KOH were {320} and in CsOH were {110}. Also, with KOH etching, the high index plane {432} was observed. The surface free energy of the crystal planes was calculated based upon the number of bonds on the surface. A minimum in surface free energy occurred for all the low index planes i.e., {100}, {110}, and {111}. There was also a minimum near the {522} planes in agreement with the experimental observations that these planes show anisotropic etching behavior. The {110} and {522} planes had a significant number of in-plane bonds. When the in-plane bond density was added to the surface bond density a good fit with the observed hierarchy of etch rates was obtained for the slower etching planes: {311}, {522} > {100} > {111}.