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290 MEMS and Microstructures in Aerospace Applications

13.2 WAFER HANDLING

Particular attention and concern must be given to the handling and processing of
MEMS devices to optimize yield and reduce the possibility of inducing latent
defects into the product line. Handling considerations from the microelectronic
world are not totally adequate and need to be supplemented in the MEMS device
arena. Although MEMS wafer fabrication has been developed around the micro-
circuit IC type manufacturing process and equipment, the devices are different.
MEMS fabrication does not identically track the CMOS IC world either. In general,
the IC design rules have been the basis for the MEMS fabrication tools. These rules
and practices, however, have their limitations.
MEMS designs often call for a double-sided wafer processing as opposed to the
one-sided processing world of microelectronic manufacturing. Second-side process-
ing for microcircuits is normally only a backside grind or other thinning techniques
such as etching. In addition, for MEMS, there may be significant variation in the
thickness of the wafer as seen in such components as accelerometers, and of course
different thickness for different technology types. MEMS devices are heteroge-
neous as opposed to the homogeneity of ICs. Material variations in the MEMS
device arena cover a broader range than in the microcircuit work where there is a
more limited choice of substrate material (germanium, silicon, gallium arsenide,
silicon on insulator, etc.) with the dominant material being silicon. Moreover,
traditional substrate material from the microcircuit world such as glass and quartz
may be used with MEMS.
At the MEMS wafer level, concern and attention must be paid to prevent
damage to structures from tooling, considering both the front and back sides of
the wafer. For example, with processing that involves both sides of the wafer, care
must be taken to minimize particles and other contaminants at the back of the wafer.
Equipment may need to be modified to work with both sides of the wafer. Wafer
thickness and equipment versatility must also be considered. Since different tech-
nologies drive different thicknesses, awareness that the clean room will have
various thicknesses in process at the same time adds to the complexity of tooling.
It would not be unrealistic to find thicknesses of device types varying by as much as
several hundred microns. Foundries are more vulnerable to the concerns of varying
MEMS thickness in manufacturing than a custom MEMS house. Foundries attempt
some design guidelines in this area to minimize the complexity of their processing
job. Traditional processing tools often must be adapted in order to accommodate
these differences.
Inspection criteria do not exist for anomalies induced by back and front
side handling, and therefore extra precaution at the design stage is necessary.
For example, it is essential to assure that there is an adequate, dedicated ring
area around the perimeter of the wafer for handling. These areas may
typically have a width of 7 mm. Tool marks and contact with chucks should be
constrained to these areas, and the area of interest should be clean. However, marks
or anomalies that will be removed or cleaned up at a later step should not be cause
for rejection.

© 2006 by Taylor & Francis Group, LLC