Page 43 - MEMS Mechanical Sensors

32 Materials and Fabrication Techniques

mechanical properties. Other adhesives include PMMA, polyamides, silicone rub-
bers, and negative photoresist [38]. Waxes can also be used as temporary adhesives
during processing. Generally such bonds can be achieved at temperatures under
150°C and are relatively soft, providing some degree of stress relief for the wafers.
They are, however, unsuitable for hermetic seals, can degrade over long periods of
time, and can possess poor thermal stability [39].

2.3.5.5 Vacuum Bonding
A bonding stage may be carried out during a fabrication process to trap a vacuum in
a cavity, which may, for example, contain a micromachined feature such as a reso-
nator that requires a sufficiently high vacuum to operate. In this case the bonding
process has to be carried out in a vacuum, with the component parts being brought
together under vacuum. If anodic bonding is used, gas generation during the anodic
bonding process and gas desorption from the inner surface of the sealed cavity neces-
sitate the use of a getter within the cavity that is able to withstand the bonding tem-
–5
perature [40]. Vacuums of 4 ×10 torr have been achieved using this approach.

2.3.5.6 Bond Aligning

Some devices require accurate alignment between the two components being
bonded together. Equipment is commercially available to enable wafers to be
aligned and bonded to each other with an accuracy of a few micrometers. Glass-to-
silicon bonding alignment is straightforward because of the transparency of the
glass. For silicon-to-silicon bonding the aligners use infrared systems similar to those
used in double-sided alignment in lithography. Equipment is available with various
options so that anodic bonding, eutectic bonding, or silicon fusion bonding can be
done in various environments including vacuum.

2.3.6 Thick-Film Screen Printing
Screen printing is one of the oldest forms of graphic art reproduction and involves
the deposition of an ink (or paste) onto a base material (or substrate) through the use
of a finely woven screen having an etched pattern of the desired geometry. The term
“thick-film” can often be misinterpreted, so it is worth noting that it does not neces-
sarily relate to the actual thickness of the film itself. The typical range of thicknesses
for thick-film layers, however, is between 0.1 and 100 µm. The process is commonly
used for the production of graphics and text onto items such as T-shirts, mugs, and
pencils, and it is very similar to that used for microelectronic thick-films. The degree
of sophistication for the latter is, however, significantly higher in order to provide
high-quality, reproducible films for use in electronic systems.
The technology for manufacturing thick-film hybrid microelectronic circuits
was introduced in the 1950s. Such circuits typically comprised semiconductor
devices, monolithic integrated circuits, discrete passive components, and the thick-
films themselves. In the early days of the technology, the thick-films were mainly
resistors, conductors, or dielectric layers. Evidence of thick-film hybrids can still be
found in many of today’s commercial devices such as televisions, calculators, and
telephones.