Dual-doped TMAH silicon etchant for microelectromechanical structures and systems applications

Tetra-methyl ammonium-hydroxide (TMAH), is an anisotropic silicon etchant that is gaining considerable use in silicon micromachining due to its excellent silicon etch rate, etch selectivity to masking layers, degree of anisotropy, and relatively low toxicity. However, a shortcoming of TMAH is that it aggressively etches exposed aluminum layers. A dual-doped low concentration TMAH solution is presented, which incorporates a silicate additive for aluminum passivation and an oxidizer additive for improving etch rate and surface quality. Using etch and under-etch experiments, the dual-doped 5 wt % TMAH solution is shown to have characteristics comparable to those of high concentration TMAH solutions with the added benefit of improved etch rate, smoother etched surfaces, and selectivity to silicon over aluminum. Such an etchant can find effective use in batch fabrication and integration of microelectromechanical systems within the framework of standard foundry processes.