Page 199 - Microsensors, MEMS and Smart Devices - Gardner Varadhan and Awadelkarim

MICROSTEREOLITHOGRAPHY 179

simple mechanical float mechanism through to an optical proximity sensor system, which
monitors the current resin height and allows this to be fed back into a computer-based
closed-loop control system.
A number of other steps are required in the coating process, such as the lowering
of the part in the vat full of resin, the wiping away of any excess materials, and the
smoothing of the remaining materials to provide the desired thickness of liquid resin
above the previously solidified layer.
The thickness of the layer typically ranges from 100 to 500 um. The recoater must be
controlled very precisely to achieve thinner layers, because a recoating error of 25 um
becomes very significant when building with thickness below 100 um (Jacobs 1996).
When a smooth, uniform, and accurate coating of the liquid resin has been achieved
over the previously solidified polymer layer, the process is not finished until sufficient
resin remains within the vat to compensate for any shrinkage that can occur during
curing.
An imaging system for SL includes a light source (laser or lamp), beam delivery,
and focusing elements (Figure 7.7). The laser (or lamp) chosen for the system must be
appropriate for the resin to be used. Wavelength, output beam shape, and power available
are all important characteristics (see equations in Section 7.1.1). Beam delivery elements
are employed to fold the path of the laser beam and therefore make the SL system as
compact as possible.
A typical SL system employs two orthogonally mounted, servo-controlled,
galvanometer-driven mirrors to direct the laser beams onto the surface of the vat. The
beam is passed though a focusing objective and then hits the resin surface. The beam
exposure is controlled by a shutter according to an on-off command generated by the
build file. A mechanical shutter requires typically about 1 ms to actuate, and so has now
been replaced by an acoustic optical modulator that has a much lower actuation time of
about 1 us, thus allowing a much faster fabrication process. The time needed to write a
layer is always a critical parameter of any beam writer because it relates both to throughput
and cost.
Applications of SL vary considerably from the quick-cast tooling through to struc-
tural analysis. Consequently, SL can speed up product development and improve product
quality through superior design and prototyping.

7.2 MICROSTEREOLITHOGRAPHY

The principle of MSL is basically the same as that of SL (Section 7.1), except that the
resolution of the process is lower. In MSL, a UV laser beam is focused down to a 1 to
2 um-diameter spot that solidifies a resin layer of 1 to 10 um in thickness, whereas in
conventional SL, the laser beam spot size and layer thickness are both on the order of
100 to 1000 um. Submicron control of both the x-y-z translation stages and the UV beam
spot enables the precise fabrication of complex 3-D microstructures.
MSL is also called microphotoforming and was first introduced to fabricate high
aspect ratio and complex 3-D microstructure in 1993 (Ikuta and Hirowatari 1993). In
contrast to conventional subtractive micromachining, MSL is an additive process, and
therefore, it enables the fabrication of high aspect ratio microstructures with novel smart
materials. The MSL process is, in principle, compatible with silicon microtechnology