Page 156 - An Introduction to Microelectromechanical Systems Engineering

Imaging and Displays 135

does not specify the type of material, but one could use an organic polyimide film or
photoresist that can sustain the subsequent thermal cycles of the fabrication
process. Standard lithography and etching methods are applied to define contacts
through the sacrificial layer to the underlying metal. These contacts also serve to
form anchor points for the suspended plate. A 0.5-µm thick silicon nitride layer is
deposited at low temperature and patterned using standard lithography in the shape
of the suspended plate. The next deposition step is critical because it defines the thin
temperature-sensitive resistor. Two families of materials exhibit suitable sensing
properties:

• Vanadium oxides (VO ,V O , and V O );
2 2 3 2 5
• Lanthanum manganese oxides (La A MnO ; A = Ca, Sr, Ba, or Pb).
1-x x 3
Sputtered vanadium oxides have a convenient sheet resistance (~25 kΩ per
square at 25ºC), acceptable 1/f noise, high absorption of infrared radiation, and a
large TCR of about –0.2% per degree Celsius. Lanthanum manganese oxides yield
even larger TCRs in the range of –0.3% per degree Celsius with low 1/f noise. The
combination of low noise and high TCR is critical to increasing sensitivity. After the
deposition and patterning of the resistive element, another silicon nitride layer is
applied for encapsulation of the sensitive components. Removal of the sacrificial
layer by plasma etching releases the silicon nitride plate. An oxygen plasma is effec-
tive at isotropically removing organic materials, including polyimide and photore-
sist. Finally, the parts are diced, then packaged under vacuum (<10 Pa residual
pressure) to reduce heat loss by conduction.
The readout electronics (see Figure 5.1) activate a column of pixels by applying
a voltage to their corresponding address column; they then measure the current
from each transistor as a constant pulse voltage sequentially scans the address rows
for the pixels in the column. The estimated change in temperature for an incident
radiation power of 10 −8 W is only 0.1ºC. The corresponding resistance change is a
measurable –10Ω for a 50-kΩ resistor. The thermal capacity of a pixel is 10 −9 J/K,
determined by the very small thermal mass of the suspended plate. Consequently,
the thermal response time, defined by the ratio of thermal capacity to thermal con-
ductance, is less than 10 ms, sufficiently fast for most imaging applications. The
signal-to-noise ratio is limited by thermal noise and 1/f noise to about 49 dB. Special
circuits perform a calibration step that subtracts from the active image the signal of
a blank scene. The latter signal incorporates the effects of nonuniform pixel resis-
tance across the array. An intermittent shutter provides the blank scene signal,
therefore allowing continuous calibration.

Projection Display with the Digital Micromirror Device TM
The Digital Micromirror Device™ (DMD) is a trademark of Texas Instruments of
Dallas, Texas, which developed and commercialized this new concept in projection
display technology referred to as Digital Light Processing™ (DLP). U.S. Pat-
ent 4,615,595 (October 7, 1986) describes the early structure of the DMD.
The technology has since undergone continuous evolution and improvements.
Texas Instruments first introduced its new product family of DLP-based projection
systems in 1996.