Page 162 - An Introduction to Microelectromechanical Systems Engineering

Fiber-Optic Communication Devices 141

bright and dark states in excess of 200. The fill ratio—the percentage area available
to reflect light—is approximately 70%, with a potential for further improvement by
reducing the unused space between ribbons—the pitch, and not the spacing, deter-
mines the diffraction angle.
A key advantage of the GLV over other display technologies is its fast speed.
The small size and weight of the ribbon, combined with the short stroke, provide a
switching speed of about 20 ns, about one thousand times faster than the DMD. At
these speeds, the address and support electronics become simple. There is no longer
a need for fast memory buffers, such as those required for conventional active
matrix liquid crystal displays, to compensate for the mismatch in speeds between
the electronics and the display elements. Moreover, there is little power required to
actuate the very small ribbons.
The very fast switching has also allowed Silicon Light Machines to explore a
simpler scheme, whereby the projected image of a single row of pixels is rapidly
scanned through the optics to build a two-dimensional picture. Projection at video
rate for a high-resolution display requiring 1,000 horizontal lines implies a data
scan rate of 60,000 lines per second. Incorporating 256 shades of gray increases the
bit refresh rate to 15.4 MHz, which corresponds to a pixel switching every 65
ns—well within the capability of the GLV. This new scheme allows simplifying the
GLV to a single row of pixels instead of a two-dimensional array and hence reduces
associated manufacturing costs.
The fabrication involves the surface micromachining of the ribbons and their
release by etching a sacrificial layer. The process begins with the deposition of an
insulating 500-nm thick silicon dioxide layer over a silicon wafer, followed by the
sputter deposition or CVD of tungsten. The tungsten is patterned using standard
lithography and etched in SF -based plasma to define the electrodes for electrostatic
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actuation. The sacrificial layer is then deposited. The details of this layer are not
publicly available, but many possibilities exist, including organic polymers. This
layer is very thin, measuring approximately 130 nm, one quarter the wavelength of
green light. Silicon nitride and aluminum are deposited next, followed by patterning
in the shape of narrow ribbons. The release step is last. Oxygen plasma is useful for
the removal of organic sacrificial layers, such as photoresist. It is also possible to
consider using sputtered amorphous silicon as a sacrificial layer. Its selective
removal, however, may require an exotic etch step involving xenon difluoride
(XeF ). This etchant sublimes at room temperature from its solid form and reacts
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spontaneously with silicon to form volatile SiF . Its advantage over SF or CF is that
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it does not require a plasma, and it does not etch silicon nitride, silicon oxide, or alu-
minum. But xenon difluoride is a hazardous chemical, reacting with water moisture
to form hydrofluoric acid. It is not used in the integrated circuit industry.

Fiber-Optic Communication Devices

The rise and fall of scores of start-up companies during the bubble years
(1997–2001) of the fiber-optic telecommunication industry left a legacy of technical
innovations and novel designs, especially as relating to MEMS. In the economic
downturn, many companies closed their doors, and it could be years before their
intellectual property is applied to other fields. A few companies have survived and