Page 10 - Spring 2009

Fig. 10. Hohm and Sessler 1983 patent.
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oxide (ZnO). The objective of this work was to create a microsensor that could possibly be used to monitor film thick- ness in the IC wafer fabrication area as a process control tool.
In July 1983 Dietmar Hohm and Gerhard Sessler applied for a German patent titled “Silicon-based capacitive trans- ducers incorporating silicon dioxide electret,”21 (Fig. 10). This first version of a microphone used a silicon dioxide charged electret backplate and is described in detail in Hohm and
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The prototype was comprised of a 10 mm x 10 mm backplate produced from p-type silicon with a 2 micron thick silicon dioxide top layer. The diaphragm was a 13 micron thick Mylar membrane with an aluminum coating sep- arated from the backplate by a 30 micron thick polymer ring. A 1 mm diameter hole in the backplate provided a path to a back volume formed with the microphone housing. The long
Multhaupt (1984).
term charge stability of the SiO2 electrode was evaluated over 20 months and found to have no measurable decay. In 1989
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In 1989 Hohm and Hess24 presented an externally biased silicon microphone comprised of a diaphragm formed on one silicon wafer and the backplate formed on a second wafer and the system bonded together to form a microphone (Fig. 11). The backplate was made from a sili- con wafer with a SiO2 electrode layer which also included a spacer layer to form the electrode gap. Two rectangular slits were formed in the backplate to reduce the stiffness of the air layer between the diaphragm and back plate. The diaphragm was made from a silicon nitride (Si3N4) layer produced on a separate silicon wafer. Both diaphragm wafer and backplate wafer were glued together to form the final transducer which measured 1.7 mm x 2 mm. Experimental
described prototypes of a silicon electret micro- phone produced using a silicon backplate wafer with a SiO2 coating as the electret and using a second wafer with etched through holes to form a spacer for the diaphragm.
Murphy et al.
Fig 11. Cross-section of Silicon Microphone.
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