Microelectronic and micromechanical structures formed in semiconductor substrates are the focus of much current development effort. The ability to form vertical walls is a fundamental requirement of such efforts. Vertical wall structures have been formed in (111) silicon however, to date no one has been able to systematically form vertical wall structures in the more common and less expensive (100) silicon. Prior art methods have only been able to isolate the (111) plane in (100) silicon to create walls at an angle of cos.sup.-1 (1/.sqroot.3) degress to the substrate surface (in (100) silicon, the (111) plane lies at an angle of cos.sup.-1 (1/.sqroot.3), or approximately 54.7356.degree. to the outer surface of the silicon). No one has been able to create wall structures in (100) silicon at angles of greater than 54 degress, let alone the vertical structures needed for many microelectronic and micromechanical devices.
Etching of crystals, including semiconductor crystals, by various etchants has been studied over the past forty years. Herring was among the first to apply Wulf's thermodynamic equilibrium constructions to crystal thermal etching rather than applying kinetics. In addition, Herring derived stability criteria for surfaces according to their surface-free energy. Batterman developed stability theories by etching pits in germanium and noting which planes became dominant over time and noting where etching hillocks appeared on germanium spheres. A refinement of Batterman's principals, accomplished by Irving, included the stability of crystalline corners in germanium. Frank et al. were able to calculate etching trajectories from Frank's kinematic theory of crystal growth. From Frank's theory, transient as well as equilibrium etching shapes of crystals can be predicted. Jaccodine was the first to apply Wulf's equilibrium constructions to chemical wet etching of crystals. Weirauch studied the correlation between the slow etching planes on crystalline silicon spheres and equilibrium surfaces that developed from a mass slot opening on a flat silicon surface. Kendall et al. developed a wagon wheel method for measuring the etch rates in important directions without the use of etched spheres. Kaminsky developed an exact processing procedure for producing geometrically precise silicon structures. Kendall et al. presented a hydration etching model which predicted the etchant concentration in which the etch rate peaked for various etchants. Seidel developed a model which explains the anisotropic etching of silicon by alkaline hydroxides as a result of surface state differences. Ipyam's experiments with cesium hydroxide showed that the dependance of the etch rate of silicon is proportional to the fourth power of the water concentration.
None of these examinations resulted in any etching methods that will allow for the formation of vertical wall structures in (100) silicon. Accordingly, a need has arisen for a method of anisotropic etching of (100) silicon to facilitate the formation of vertical wall structures.
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