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1.3 Miniaturized Systems with Microoptics and Micromechanics 13
Free-space Micro-optical Bench and Sensors

Vertical micromirrors can be fabricated by anisotropic etchingon (100) silicon
just like the V-groove described in Sect. 1.2.1. The (111) planes are perpen-
dicular to the Si surface and atomically smooth. Therefore, high-aspect-ratio
mirrors can be formed. Figure 1.13 shows an on-chip Mach-Zehnder interfer-
ometer produced by Uenishi [1.36]. Micromirrors are reported several µms
thick and 200 µmhigh.
Free-space micro-optical elements held by 3-D alignment structures on
a silicon substrate have been demonstrated usinga surface-micromachining
technique in which the optical elements are first fabricated by a planar process
and then the optical elements are folded, into 3-D structures, as shown in
Fig. 1.14 [1.37]. Figure 1.15 shows the schematic of the out-of-plane micro-
Fresnel lens fabricated on a hinged polysilicon plate (a), and the assembly
process for the 3-D micro-Fresnel lens (b) [1.38]. A Fresnel lens stands in
front of an edge-emitting LD to collimate its light beam.
To achieve on-chip alignment of hybrid-integrated components such as an
LD and a micro-optical element, a micro-XYZ stage consisting of a pair of

Laser incidence
Micromirror (Si plate)
Laser beam

1 mm
Fig. 1.13. An on-chip Mach-Zehnder interferometer produced by anisotropic etch-
ing on (100)silicon [1.36]. Courtesy of Y. Uenishi, NTT, Japan
(a) (b) Staple holding
Staple holding
Optical
Sacrificed layer element
Optical element
Sacrificed layer

Si substrate Si substrate
Fig. 1.14. Free-space micro optical elements held by 3-D alignment structures on
a silicon substrate, fabricated using a surface-micromachining technique. Optical
elements were first fabricated by planar process and then folded into 3-D structures
[1.37]