The evolution from convex corner undercut towards microneedle formation: theory and experimental verification

In this paper, we present the mechanism of silicon convex corner undercutting of etch masks in pure aqueous KOH solution (29% KOH, 79 degrees C) during microneedle fabrication. The process was documented by images and measurements in an optical microscope. These measurements have been used to determine the Miller indices of the crystal planes comprising the microneedle as well as for mathematical interpretations of etch rates, etch times and mask design. Under these etch conditions, the final structures have convex corner undercutting planes with Miller indices in the {h 12} family, in agreement with literature results. However, we show that the index h decreases during the etch process, from more than 14 (nearly vertical walls) to a stable value of 3. The final microneedle is formed by {312} planes and a small base of {121} planes. Stable crystal plane formation permits the fabrication of microneedles with different heights with one etch depth using different mask sizes. We demonstrate that the etch bath conditions play an important role in the formation of the needle shape.