Enabling Manufacturing Technologies - Implications of Emerging Micro-and Nanotechnologies - page 156

TABLE 4-2 High Complexity of the Digital Mirror Device

Label DMD Level Function

A Electrode Bias Electrode

B Electrode Address Electrode

C Hinge/Beam Beam and Hinge Posts

D Hinge/Beam Hinges

E Mirror Mirror and Mirror Support Post

A monolithic mirror approach using an array of aluminum alloy mirrors was
developed to overcome the particle problems of the polymer mirrors. The alumi-
num mirrors worked much better in regard to dust but rapidly failed in operation
because of the stress inherent in the mirror layer after plasma etching. Also, the
TI printer division interested in the printer application fell on hard times, so the
continuation of the program was always in doubt.
New DARPA money was supplied, administered by the Air Force. DARPA
was then interested in large projection displays. During the 1990s development
problems continued. (For instance, in operation, the mirrors would stick perma-
nently to the underlying films; the mirrors did not optimally fill the optical
aperture, leading to a checkerboard display image; there was high optical inser-
tion loss from a mismatch in the pixel size and the size of the arc lamp; a very
high light flux on the DMD was required; arc lamp longevity was insufficient;
and the cost of engineering of a small sophisticated light handling system, includ-
ing an optically flat window on the package top, was high.) By the late 1980s the
DMD was being touted as a MEMS success story, but these manufacturability
issues continued to threaten the viability of commercialization.
During the 1990s, TI, using its own funds, gradually worked out solutions to
the DMD manufacturability problems. Mirror reliability was solved with a hid-
den mirror hinge made out of an amorphous material. Since there were no grain
boundaries in the hinge, there were no sources of the cracks that had led to high
failure rates in the metal hinges. Stiction was fixed by fabricating springs and a
fluorocarbon coating on the back side of the mirrors. The springs produced a
restoring force when the mirror touched the base layer. The hidden-hinge design
allowed for a more optically efficient pixel, filling the aperture. Obtaining a
robust, very bright light source turned out to be an enormous problem, and the
light source is still a major cost component in the DMD system.
One of the veteran engineers on the project at Texas Instruments, Michael
Mignardi, recently gave an IEEE seminar detailing the scale of TI's effort to
successfully produce the DMD. 69 Even after feasibility had been demonstrated in
the late 1980s, it took another 10 years to reach profitability, with the effort being
spread roughly equally among device improvement, packaging, and testing. Low
outgasing adhesives were used to prevent the buildup of a sticky layer inside the
package, which would result in stiction problems. A zeolite getter was included