Medical Applications of Micro-Electro-Mechanical Systems (MEMS) Technology - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 54

MEMS Process Sequence development and Process Integration
The making of MEMS devices involves bringing together a multiplicity of
processing steps into what is called a process sequence [22]. The number of pro-
cessing steps involved in fabricating a MEMS device can vary from less than
ten for a simple process sequence to a few hundred for a more complicated
one. A common thread in MEMS device implementation is that most MEMS
process sequences are customized to the device that is being fabricated. That
is, the process sequence for any one MEMS device is likely to be vastly differ-
ent from the process sequence used to implement any other MEMS device.
This is quite a different situation from what is found in microelectronics,
where there are typically only three components made, namely transistors,
resistors, and capacitors, and a small number of process sequences com-
monly used for fabrication, including CMOS, Bipolar, and BiCMOS. In con-
trast, there are a vastly larger number of MEMS devices, each having its own
unique and custom process sequence. This means that the development of
the process sequence for any MEMS device is typically very challenging,
and most successful MEMS development projects engage MEMS fabrication
experts early and extensively. Process integration is defined as understand-
ing, characterizing, and optimizing to the greatest extent possible the inter-
relationship of the individual processing steps in a process sequence. Given
the customization of MEMS process sequences, it should not be surprising
that process integration is of critical importance in any MEMS development
effort. Specifically, skilled MEMS fabrication technologists having relevant
practical experience are needed to successfully develop a process sequence for
a MEMS device. Moreover, the skills of these technologists must be directly
relevant to MEMS device process development, that is, device development
skills learned in other related fields such as microelectronics are usually not
sufficient and do not directly transfer well to MEMS process sequence devel-
opment. The learning of MEMS process development and process integra-
tion requires many years of education and practical experience and cannot
be adequately covered in a small section of a chapter. Readers are referred to
[22] for more information about this subject.
MEMS Device Technologies
Microsensor Technology
Many types of sensors exist that use micromachined devices, including pres-
sure; acoustic; temperature (including infrared focal plane arrays); inertial
(including acceleration and rate rotation sensors); magnetic field (Hall, mag-
netoresistive, and magnetotransistors); force (including tactile); strain; opti-
cal; radiation; and chemical and biological [9, 12, 37, 38]. As discussed above,