Page 44 - MEMS Mechanical Sensors

2.3 Fabrication Techniques 33

A typical mask, or screen, is made of a finely woven mesh of stainless steel,
nylon, or polyester, which is mounted under tension on a metal frame and coated
with a UV-sensitive emulsion. The desired pattern is exposed onto the screen photo-
graphically, leaving open areas through which a paste can be deposited. The pastes
comprise a finely divided powder (typically 5-µm average particle size), a glass frit,
and an organic carrier that gives the ink the appropriate viscosity for screen-
printing. Typically, thick-film pastes are resistive, conductive, or dielectric in nature
and are deposited onto substrates such as alumina or insulated stainless steels. Sili-
con has also been used as a base material to make devices such as micropumps, like
the one described by Koch et al. [41].
The screen is held in position at a distance of around 0.5 mm away from and
parallel to the substrate. The paste is poured onto the upper surface of the screen. A
squeegee then traverses across the screen under pressure, forcing the ink through the
open areas and onto the substrate leaving the required pattern. A wide variety of
commercial screen printers, specifically developed for thick-film processing, are
available for this task.
After screen-printing, the deposited films are dried in either a box oven or, more
typically, in an infrared belt drier. This is usually achieved at a temperature of
around 150°C. This stage of the processing removes the organic carriers that were
present in the paste and produces a rigid film that can be handled or even over-
printed with further layers. The final step is to fire (or sinter) the films to form a
solid composite material. The glass frit melts during the annealing phase and bonds
the film to the substrate and also binds the active particles together. This phase of
processing is undertaken in a belt furnace at temperatures up to 900°C. The furnace
operator has control over the peak temperature, throughput speed, and dwell time.
After firing, the film is firmly attached to the substrate and additional screen printed
layers can be added if needed, and the print, dry, fire cycle is then repeated. In addi-
tion to fabricating circuits, thick-film technology has been widely used as a means of
making a variety of sensors [42].

2.3.7 Electroplating
Electroplating is used in many MEMS processes to obtain thick layers of a metal or
alloy. Processes for depositing various different metals and alloys have been adapted
for electroplating onto silicon. Those most commonly used are for depositing Ni,
NiFe, Au, and Cu. A plating base, such as Ti, Ti/Pt, or Cr/Au has to be deposited
onto the silicon. The uniformity, morphology, and composition of the deposited
layer depend primarily on the design and operating parameters of the electroplating
bath. To produce a patterned electroplated layer on silicon, a resist pattern (referred
to as a mold) has to be applied. This resist pattern also influences the growth uni-
formity, morphology, and composition of the layer. Therefore, it is common in elec-
troplating MEMS structures that some effort has to be put in on optimization to
achieve a successful outcome. Even by MEMS standards, the deposited layers are
unusually thick, sometimes of the order of 500 µm. There are photoresists (dis-
cussed earlier in this chapter) available with which it is possible to form molds of
this thickness for electroplating. The process of electroplating using an optical pho-
toresist as a mold is sometimes referred to as UV-LIGA in contrast with the X-ray
LIGA process described in the next section. The highest aspect ratios that can be