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1.2 Fabrication Methods 7
LIGA-Based
flexures

Movable
filter elements
light path Plunger
Perspective Electromagnetic
(substrate drive
cutaway)

Stationary
structure
Light path
Light path
Transmission window
C00219-02

Side view
(cutaway) Transmission window
Fig. 1.7. LIGA-based tunable IR filter showing vertical parallel plate filter structure
and linear magnetic drive actuator [1.25]. Courtesy of J. Allen Cox, Honeywell, USA

Figure 1.7 shows an LIGA-based tunable infrared (IR) filter [1.25]. Grat-
ings with free-standing nickel walls as high as 50 µm with periods on the order
of 10 µm were fabricated by LIGA. The linear actuator utilizes a permalloy
electromagnet with an air gap because of the large power (0.1 mN) necessary
to adjust the spacing of the grating. Furthermore, simple 3-D microstructures
will be fabricated by the LIGA process [1.26].

Photoforming

Complicated 3-D microstructures have been fabricated by stackingpreshaped
layers made by solidifyinga thin resin layer with UV light [1.27,1.28]. There
are two solidification methods: a free surface and a fixed surface solidification.
In the case of the free surface, solidification occurs at the resin/air interface,
leadingto perturbation on the surface. On the other hand, in the case of the
fixed surface, solidification occurs at the stable window/resin interface, leading
to smoother structures. Photoformingexhibits the followingcharacteristics:

1. complicated microstructures can be fabricated
2. laser beam can be deflected easily by scanningmirrors
3. no need for mask process
4. low throughput due to direct laser beam writing.