Forced and Natural Convection Effects on the Shape Evolution of Cavities during Wet Chemical Etching

The effect of forced and natural convection on the shape evolution of deep cavities during wet chemical etching was investigated. Etching was assumed to be limited by removal of the dissolution products away from the vicinity of the active surface. Finite element methods were employed to solve for the fluid velocity and product concentration distributions in cavities of irregular geometries resulting from etching. Forced convection was found very ineffective for rinsing deep cavities. The etching rate decreased sharply with time as the cavity became deeper during etching. At the same time, the etching rate distribution along the active surface became nearly uniform, degrading etch anisotropy. In contrast, natural convection was effective for rinsing the dissolution products out of the cavity. Both the etching rate and the etch factor remained at relatively high values throughout etching, even at later times when the cavity became deeper. The cavity wall profiles and the corresponding flow and concentration fields showed some interesting features, especially for the case of natural convection. The results have important implications for deep anisotropic etching and other related processes.