How Do We Make Small Things? - Introduction to BioMEMS - page 11

1.4 Micromachining
he photoresist pattern may later be transferred onto the underlying substrate to form nonpla-
nar structures by a variety of processes including chemical etches and deposition of materials;
although the concept is almost as old as photolithography, it is now known by the term coined
for it in the 1980s—“ micromachining .” he term micromachining also includes other strategies
such as laser patterning. Historically, the micromachining ield started in the 1980s as a spin-of
of microelectronics, using recycled facilities (as many chemicals used for etching are incompat-
ible with electronics) and the same silicon wafers that had been conveniently developed for inte-
grated circuit processing (photoresist spreads well on smooth, circular wafers). Glass and quartz
wafers were introduced soon for applications requiring electro-osmotic low, and provided a
transparent substrate. Hence, silicon and glass have always been considered the “traditional
substrates” among the micromachining community.
Some micromachined devices are already ubiquitous and have had a huge impact in society,
such as the accelerometers in your car's airbag deployers or the nozzle in the printer head of your
oice's inkjet printer. he airbag deployer is based on a thin slab of silicon that hangs over an
etched cavity; because of its minuscule size, the slab reacts extremely fast. he inkjet nozzle is also
an etched cavity covered with a thin membrane that has a microfabricated hole; a small heater
inside the cavity vaporizes the ink, which forms a bubble that ejects a tiny droplet out of the hole,
onto a nearby surface such as a sheet of paper. Because of the size of the cavity, the ink cools down
in less than a millisecond and is ready again for the formation of another bubble (i.e., it can print
fast), and owing to the size of the hole, the printer has a resolution superior to that of the eye.
Micromachining techniques are not simple for a nonexpert and require a lot of ine-tuning of
materials processing and expensive facilities. However, several devices can be made at once on the
same wafer, and the price of processing several wafers is almost the same as processing one; so,
the cost per unit can be quite low, ensuring fast commercial dissemination for everyday objects.

1.4.1 Etching: Wet versus Dry, Isotropic versus Anisotropic
he etching chemistry must be chosen such that it attacks the substrate but not the photo-
resist, or with as much selectivity as possible ( Figure 1.9 ). Chemical etches can be carried
out using solutions (a “ wet etch ”) or using a reactive-gas plasma (a “ dry etch ,” also termed
“ reactive ion etch ”). We distinguish between a chemical etch that proceeds in every direc-
tion homogeneously (“ isotropic etch ”) and one that proceeds in a preferential direction
(“ anisotropic etch ”). Virtually every imaginable combination of chemical etches, substrates,
and masking materials has been tried in micromachining. When micromachining is per-
formed by the addition or subtraction of shallow layers on the surface of the wafer, we term
it “ surface micromachining ”; on the other hand, when the processing involves deep etches
through a good portion of the thickness of the wafer, we speak of “ bulk micromachin-
ing .” Nearly all commercial MEMS devices (e.g., accelerometers) are fabricated by surface

Photoresist
pattern

Selective etch Photoresist

removal

FIGURE 1.9 Etching.using.a.photoresist.mask.