Silicon Surface Micromachining

Key element of surface micromachining is the deposition of polycrystalline silicon on top of a sacrificial layer. Freestanding bridges, cantilevers, vibrating beams, resonating structures are formed from the polysilicon anchored on a silicon substrate by removing the sacrificial oxide. Typically, the polycrystalline silicon layers are 2µm thick.

The Bosch process features a polycrystalline silicon thickness of 10µm for the freestanding structures. The large thickness is achieved using an epitaxial reactor for the deposition of the polysilicon, hence called epipoly. A proprietary trench technique allows the formation of vertical sidewalls with large aspect ratio (see Fig. 2). In addition to the functional poly layer there is a second thin poly-Si underneath which serves as interconnect, shield or counter electrode. The second poly-Si layer is isolated from the epipoly by the sacrificial material and from the substrate by an oxide film.
Special features of the Bosch process include:

• Large maximum feature size (>1mm because of low stress and strain gradient)
• Narrow gaps (~2µm)
• Small effects of mounting induced stress
• Design flexibility (two interconnect layers, horizontal and vertical capacitive elements)
• Large working capacitance (>1pF, enables off-chip evaluation)
• 2-chip solution (off-chip evaluation for maximum flexibility)

Benefits of microsystem technology?

• Cost reduction (batch fabrication, fewer components)
• Reliability (fewer components, fewer interconnects)
• Size and weight (miniaturization, no maintenance on the system)
• Integration of systems as well as functions (self test, accuracy, control)

The Bosch Foundry offers this process in so-called multi-project-wafer runs. This service is embedded in a high volume production environment where more than 100 million accelerometers have been produced so far.