Page 160 - An Introduction to Microelectromechanical Systems Engineering

Imaging and Displays 139

silicon along edge scribe lines to a depth that allows breaking the individual dice
apart at a later stage. An oxygen-plasma etch step removes both sacrificial layers
and releases the micromirrors. A special passivation step deposits a thin, antistiction
layer to prevent any adhesion between the yoke and the landing pads. Finally, a sin-
gulation process breaks apart and separates the individual dice. The packaging of
the DMD is discussed in Chapter 8.
Reliability is the sine qua non of the commercial success of DMD technology.
The designs described here are the result of extensive efforts at Texas Instruments
aimed at understanding the long-term operation of the pixels as well as their fail-
ure modes. The DMD micromirrors are sufficiently robust to withstand normal
environmental and handling conditions, including 1,500G mechanical shocks,
because the weight of the micromirrors is insignificant. The major failure and
malfunction mechanisms are surface contamination and hinge memory. The latter
is the result of metal creep in the hinge material and causes the mirror to exhibit a
residual tilt in the absence of actuation voltages. The reliability of the DMD is
further discussed in Chapter 8.

Grating Light Valve™ Display
The Grating Light Valve™ (GLV) is a novel display concept invented initially at
Stanford University. Silicon Light Machines of Sunnyvale, California, a division of
Cypress Semiconductor Corp. of San Jose, California, is developing a commercial
product based on the licensed technology [3]. The fundamental light-switching con-
cept relies on closely spaced parallel rows of reflective ribbons suspended over a sub-
strate (see Figure 5.5). The separation gap between the ribbons and the substrate is

Diffracted light
Incident light
Reflected light Incident light

Ribbons

Aluminum (50 nm)
Si N (100 nm)
4
3
Air gap (130 nm)
Tungsten (100 nm)
Oxide (500 nm)
Silicon Silicon
Unactuated - reflective state Actuated - diffractive state
Figure 5.5 Illustration of the operating principle of a single pixel in the GLV. Electrostatic
pull down of alternate ribbons changes the optical properties of the surface from reflective
to diffractive. (After: [3].)