Handbook of Silicon Based MEMS Materials and Technologies

6.3. Etch Stop Layers

There are two epi layer structures that are used as etch stops in MEMS applications. The most common method is the use of a very heavily B doped epi layer, as described in Section 6.3.1. A second method using pseudomorphic SiGe is described in Section 6.3.2. The choice of etch stop is dictated by the liquid phase chemistry to be used in the etch process.

6.3.1. Heavily Boron Doped Epitaxial Etch Stop Layers

Extremely heavily B doped epitaxy, referred to as p⁺⁺ epi, is often used as an etch stop layer when KOH is used in a liquid phase etching solution [6]. An etch selectivity of ∼1000:1 is observed for Si doped with B at < 10¹⁹ cm⁻³ compared to a doping concentration of > 10²⁰ cm⁻³, with the higher etch rate observed for ...