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Micromachining is often divided into two categories: bulk and surface micromachining. 'Bulk' micromachining generally refers to processes involving wet chemical etching of structures formed out of the silicon substrate and so is limited to fairly large, crude structures. 'Surface' micromachining allows intricate patterning of thin films of polysilicon and other materials to from essentially 2D layered parts (since the thickness of the parts is limited by the thickness of the deposited films). In addition to these two types of micromachining there is in fact a third type of micromachining as well, namely, 'mold' micromachining, in which the part is formed by filling a mold which was defined by photolithographic means. Historically micromachining molds have been formed in some sort of photopolymer, be it with x-ray lithography (LIGA) or more conventional UV lithography, with the aim of producing piece parts. Recently, however, several groups including ours at Sandia have independently come up with the idea of forming the mold for mechanical parts by etching into the silicon substrate itself. In Sandia's mold process, the mold is recessed into the substrate using a deep silicon trench etch, lines with a sacrificial or etch-stop layer, and then filled with any of a number of mechanical materials. The completed structures are not ejected from the mold to be used as piece parts, rather the mold is dissolved from around selected movable segments of the parts, leaving the parts anchored to the substrate. Since the mold is recessed into the substrate, the whole micromechanical structure can be formed, planarized, and integrated with standard silicon microelectronic circuits before the release etch. In addition, unlike surface-micromachined parts, the thickness of the molded parts is limited by the depth of the trench etch (typically 10- 50 micrometers ) rather than the thickness of deposited polysilicon (typically 2 micrometers ). The capability of fabricating thicker (and therefore much stiffer and more massive) parts is critical for motion-sensing structures involving large gimballed platforms, proof masses, etc. At the same time, the planarized mold technology enables the subsequent fabrication of features (for example flexible springs and flexures), much finer than those possible with bulk processes.
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