Effect of Transport and Reaction on the Shape Evolution of Cavities during Wet Chemical Etching

The effect of fluid flow, transport, and reaction on the shape evolution of two‐dimensional cavities during wet chemical etching was studied. Finite element methods were employed to solve for the fluid velocity profiles and for the etchant concentration distribution in cavities of arbitrary shape. A moving boundary scheme was developed to track the shape evolution of the etching cavity. In the case of pure diffusion and under mass‐transfer control, a mask with finite thickness resulted in significantly better etch factor (etch anisotropy) as compared to an infinitely thin mask, albeit the etch rate was essentially unaffected. With fluid flow past the cavity, the etch rate increased fourfold and the etch factor increased by 40% as compared to pure diffusion, under the conditions examined. In addition, the etch rate, etch factor, and cavity wall profiles showed a strong dependence on etch time as the cavity aspect ratio (depth/width) increased with time during etching.