Page 121 - An Introduction to Microelectromechanical Systems Engineering

100 MEM Structures and Systems in Industrial and Automotive Applications

Silicon
Glass

Metal contact to
top wafer

Air damping vias Silicon
Inertial mass

Metal contact to
middle wafer

Metal electrode Contact to
substrate

Metal contact to
lower wafer

Figure 4.16 Illustration of a bulk micromachined capacitive accelerometer. The inertial mass in
the middle wafer forms the moveable electrode of a variable differential capacitive circuit. (After:
accelerometer product catalog of VTI Technologies of Vantaa, Finland.)

pattern information is encoded in each of the three masking layers. Timed etching
simply translates the encoded information into a variable topography in the silicon
substrate. The end result is a thin support hinge member with a much thicker inertial
mass. The recesses on either side of the mass form the thin gaps for the two-plate
sense capacitors.

Silicon

1. Etch recess cavities in silicon 2. Deposit and pattern three masking
layers; anisotropic etch silicon

Mass

Hinge

3. Remove first masking layer; 4. Remove second masking layer;
anisotropic etch silicon anisotropic etch silicon
Figure 4.17 Process steps to fabricate the middle wafer containing the hinge and the inertial
mass of a bulk micromachined capacitive accelerometer similar to the device from VTI
Technologies. (After: [20].)