Screening Multidentate Ligands for Control of Anisotropic Etching

A micromachined test structure was used to measure the etch rate of silicon in potassium hydroxide (KOH) as a function of surface orientation, which quantifies etchant anisotropy. By adding multidentate ligands, containing hydroxyl groups, to etchant solutions, the effects of these ligands on etch rate were determined. Bidentate ligands 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol each suppressed etching of the (110) surface to differing degrees, with 1,2-ethanediol having almost no effect, 1,3-propanediol having a moderate effect, and 1,4-butanediol having the greatest effect. The etch rates of (001) and (111) were unaffected. Summary of Research: Screening multidentate ligands for control of anisotropic etching: The design and fabrication of complex, threedimensional structures often relies on anisotropic etchants. Because anisotropic etchants preferentially etch silicon in specific directions, they allow investigators to precisely carve out three-dimensional structures. For example, many alkaline etchants, such as KOH and TMAH, etch (110)-oriented surfaces much more rapidly than they etch (001)and (111)-oriented surfaces. When combined with photolithography, such etchants are useful in the fabrication of membranes, flow channels, and suspended structures. Unfortunately, an etchant’s inherent anisotropy and chemistry can limit the range of possible structures and processing steps available in device fabrication. In addition, some etchants introduce undesirable morphological effects. For example, many etchants etch (001)-oriented surfaces rapidly, but etch (111)-oriented surfaces slowly. As a (001) surface is etched, it can spontaneously form macroscopic, pyramidal hillocks as slow-etching (111) facets gradually develop. In other cases, an etched surface may be macroscopically smooth, but contain nanoscale roughness that degrades device performance. One way to enable greater flexibility in anisotropic etching is to modify etchants using ligands: compounds with functional groups that can bind to a crystal surface. Such ligands should alter the reactivity of surfaces to an etchant, and thus change the etchant’s anisotropy. What types of compounds have this effect? Can they be combined to tune the relative reaction rates of various crystal faces? Micromachined structure enables rapid assay of facespecific etch rates: Identifying ligands with the appropriate reactivity requires screening of candidate compounds, which involves the systematic measurement of face-specific etch rates. Producing silicon of varying orientations, however, is expensive, and accurate etch rate measurements are timeconsuming. To facilitate face-specific etch rate measurements, we fabricated the circular array of high-aspect-ratio, triangular wedges illustrated in Figure 1. Micromachined from a single crystal of silicon, the wedges’ sidewalls present an array of surfaces, including the (001), (110), and (111) surfaces. When the structure is immersed in an etchant, all of the surfaces are etched simultaneously. The etch rate of each face is revealed by the wedge’s size and shape, as shown in Figure 2(b). Etching a wedge’s sidewalls causes an apparent retraction of the tip, where the sidewalls intersect. Because the included angle of the wedge is only 1o, this retraction is 115 times the side wall’s etch depth. The resulting amplification allows rapid quantification of etch rates by microscopy.