ABSTRACT
Bulk micromachining means that three-dimensional features are etched into the bulk of crystalline and noncrystalline materials. In contrast, surface micromachined features are built up, layer by layer, on the surface of a substrate (e.g., a single-crystal silicon wafer). Dry etching defines the surface features in the x, y plane, and wet etching releases them from the plane by undercutting. In surface micromachining, shapes in the x, y plane are unrestricted by the crystallography of the substrate. For illustration, in Figure 5.1, we compare an absolute pressure sensor based on poly-Si and made by surface micromachining with one made by bulk micromachining in single-crystal Si. Not reflected in this figure is the fact that the surface micromachined devices typically end up quite a bit smaller than their bulk micromachined counterparts.
