Page 22 -

1.3 Miniaturized Systems with Microoptics and Micromechanics 11
mirror facets for LDs. (c) A wet-etchingwindow is made with a resist,
and the microcantilever is undercut by selective etchingto leave it freely
suspended.
These processes are compatible with laser fabrication, so an MC structure
can be fabricated at the same time as an LD structure. Furthermore, because
a single-crystal epitaxial layer has little residual stress, precise microstructures
can be obtained without significant deformation.
Combined use of the above micromachiningprocesses will be useful in the
future. However, processingof electronics and MEMS must be compatible and
should be held down to low costs. In many actual microsystems, microassem-
bly, bonding, and packing techniques will also play important roles. Moreover,
to apply the merit of the mask process to the MEMS with an arrayed struc-
ture, it is imperative to increase the yield rate.

1.3Miniaturized Systems with Microoptics
and Micromechanics

1.3.1 Important Aspects for Miniaturization
We see that the miniaturization techniques described earlier will provide many
new optical MEMS that will environmentally friendly due to their smallness,
reliable due to the integration process, and low in cost owing to mass pro-
duction. However, new problems arise as a result of the miniaturization. Un-
derstandingthe scalinglaws and the important aspects of miniaturization
will help readers in choosingthe appropriate actuator mechanism and power
source.
Feynman presented the concept of sacrificed etchingto fabricate a silicon
micromotor 20 years ago [1.32]. At the same time, he pointed out the necessity
of friction-less and contact sticking-free structure for the MEMS because of
the relative increase of the surface effect in such microdevices.
Figure 1.12 shows the general characteristics of scaling laws. As the object
3
2
size [L] decreases, the ratio of surface area [L ] to volume [L ] increases.
Weight depends on volume, while drag force depends on surface area, which
renders surface forces highly important in microstructures. Faster evaporation
associated with larger surface-to-volume ratios has important consequences in
analytical equipment such as µ-TAS.
2
3
Response time is proportional to [mass/frictional force], i.e., [L /L ]=[L],
which leads to fast response. The Reynolds number is proportional to [inertia
2
2
4
force/viscous dragforce], i.e., [L /L ]=[L ], which leads to laminar flow.
2
3
5
2
Movingenergy is proportional to [mass × velocity ], i.e., [L × L ]=[L ],
which leads to low energy consumption.
Almost all micromotors and microactuators have been built based on elec-
2
trostatic actuation, nevertheless, electrostatic force is proportional to [L ],
4
but electromagnetic force is proportional to [L ]. This is because the plate for