Page 182 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 161

through mutual inductance coupling (the mirror does not respond to this high fre-
quency). This coupling is a strong function of the position and orientation of the
coils relative to the primary coil. These sense voltages then become a direct measure
of the angular position of the mirror and are used in a closed-loop electronic circuit
to spatially lock the mirror.
The details of the fabrication process are not available, but, once again, one can
design a fabrication sequence that can produce a similar device. The starting material
is a SOI substrate polished on both sides. The first fabrication steps cover the forma-
tion of the drive coils and corresponding interconnects on the front side of the SOI
wafer. A gold seed layer, typically 50 to 100 nm thick, is sputtered on both sides of
the wafer, then followed by standard lithography on the front side to delineate the
coil layout. The thin gold layer on the back side will ultimately serve as the reflecting
surface of the mirror. Electroplating 5–20 microns of gold on the front side forms the
coils and bond pads. The next step is the delineation of the torsional hinges, also on
the front side of the wafer. This is completed using standard lithography, followed
by standard RIE. It may be necessary to delineate the suspension hinges just prior to
the electroplating if the thickness of the gold is more than 5 µm in order to avoid the
deposition of resist over the thick topographical features of the gold coils. The fabri-
cation is completed by etching from the back side of the wafer the contour of the mir-
ror and using the embedded silicon oxide layer as an etch stop. Either DRIE or wet
anisotropic etching (e.g., KOH or TMAH) can be used. The very last step is the
removal of the exposed silicon oxide layer using hydrofluoric acid.
It is evident from this process that the thickness of the suspension is determined
by the thickness of the top SOI layer, typically a few micrometers thick. As a result,
the mechanical properties of the suspension are very predictable and well con-
trolled. Similarly, the thickness of the mirror is determined by the thickness of the
handle layer (thick bottom layer) of the SOI wafer and is uniform—the measured
surface flatness over the 3-mm diameter mirror is less than 15 nm RMS with local
roughness of approximately 2 nm. The gold layer on the back side of the wafer pro-
vides a very high reflectivity in the near infrared spectrum.

Achromatic Variable Optical Attenuation

A variable optical attenuator (VOA) is a dynamic optical component used in fiber-
optical telecommunications to adjust the intensity of light inside the fiber. A VOA
typically maintains the power below 20 mW, which corresponds to the onset of
nonlinear effects such as four-wave mixing, Brillouin scattering, and Raman scatter-
ing [40, 41]. Key characteristics of a VOA are spectral range (typically between
1,528 to 1,620 nm), insertion loss (a measure of light lost within the component
exclusive of the required attenuation, typically less than 1 dB), polarization-
dependent loss (a measure of the difference in loss between the two orthogonal
polarizations, typically less than 0.5 dB), wavelength dependence of attenuation
(typically less than 0.3 dB over the spectral range), and finally size (a volume less
3
than 1 cm is highly desirable). All loss parameters are measured in dB.
Numerous implementations using MEMS technology have emerged in the past
few years. The following example is a product by Lightconnect, Inc., of Newark,
California, that utilizes a principle of operation and a structure that are identical to
the GLV discussed earlier in this chapter [42]. The basic concept is to use diffraction