**2. Characterization of the etch rate anisotropy**

Different etchants can give different etching properties. As one of the most important properties, the etch rate anisotropy clearly manifests the etching behavior in a concentrated solution. In this chapter, the characterization of the etch rate anisotropy is studied by using hemispherical silicon specimens, with and without surfactant in TMAH solutions. Especially, surfactant-modified etching process is analyzed in detail because of its benefit in MEMS applications.
