Y. Lu,
In response to your question regarding piezoresistive bulk vs surface
micromachined pressure sensors here is my reading on this issue. I believe that
bulk micromachined devices are still the preferred solution in terms of
manufacturing, performance and applications.
Fundamentally, the difficulty with surface micromachining is the fabrication of
rugged differential and gage devices. This is due to the small near micron gaps
created in the silicon processing. There are also issues with the lower
sensitivity of polysilicon devices (lower gage factor), overall accuracy and
the difficulty of designing low pressure devices with this technology.
On the positive side it is easier to build extremely small, low cost
polysilicon absolute pressure sensor devices that have good high pressure and
overpressure capabilities (due to the built-in high pressure backstop during
fabrication). There are a number of product applications where these
characteristics could be commercialized.
Regards,
Andy Mirza
MOTOROLA Inc.
Sensor Products Division
Phoenix, Arizona