Characterization of TMAHW silicon etchant using ammonium persulfate as an oxidizing agent

Tetra-methyl ammonium-hydroxide-with-water solution, or TMAHW, is an anisotropic silicon etchant that is gaining considerable use in silicon micromachining as an alternative to the more commonly used KOH (potassium hydroxide) and EDP/EPW (ethylenediamine-pyrocatechol-water) etchants. This is mainly due to its excellent compatibility with CMOS processing, etch selectivity, anisotropy, and relatively low toxicity. TMAHW exhibits a high etch rate for lower concentrations of solutions; however, this rate falls dramatically due to the inevitable formation of hillocks on the etched surface. For higher TMAHW concentrations, hillock formation can be suppressed at the expense of lower etch rates. In this paper, an improved TMAHW solution, which incorporates an oxidizing agent additive producing higher etch rates and a better surface quality, will be examined and characterized. The influence of temperature and concentration of these TMAHW solutions is studied in order to optimize this anisotropic silicon etching procedure for its use as an efficient process step in the fabrication of microelectromechanical systems (MEMS).