Influence of Etching Potential on Convex Corner Anisotropic Etching in TMAH Solution

Anisotropic etching with tetramethylammonium hydroxide (TMAH) water solutions is a simple and CMOS-compatible way to obtain geometrical patterns in single-crystal silicon wafers. The fabrication of trenches and other features is although limited by the need to compensate convex corners which tend to be etched very fast. Such compensation produces footings at the bottom edge of the etched walls, yielding a complex and scarcely predictable geometry which might affect the performance of devices in applications such as fluidics. The etch rates for different crystal planes are affected not only by the TMAH concentration and etching temperature but also by the etching reaction potential. In this work, shallow TMAH etching of single-crystal silicon wafers in 25-wt% TMAH at 90°C is examined up to a depth of 30 m, at potentials ranging from -1 to -2 V, using etching masks to obtain compensated convex corners. Identification of the sidewalls as {311} planes is performed by angle measurement on SEM and optical images. The etch ratio of the (100) crystal plane versus both (311) and (111) planes is measured at the various potentials. Morphological differences between cathodic and anodic potentials with respect to the open-circuit potential (OCP) are examined: Cathodic etching (between OCP and -2 V) yields footing patterns and a pronounced undercut, while anodic etching (between -1 V and OCP) produces smooth sidewalls with no footing, an increased (100)/(311) etch ratio, and a decreased (100)/(111) etch ratio.