Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-10-14
2001-10-09
Dang, Hung Xuan (Department: 2873)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S336100, C250S338100, C250S352000
Reexamination Certificate
active
06300632
ABSTRACT:
TECHNICAL FIELD
This invention relates to an uncooled infrared focal plane imagers and microelectromechanical infrared detectors for use therein.
BACKGROUND ART
Bolometers are thermally isolated detectors that are heated by infrared radiation absorbed on their surface. The temperature rise is converted to an electrical signal using a thermistor element. Infrared bolometer detectors are widely used for thermal targeting, night vision systems, and motion sensing. The most sensitive bolometers available are cryogenically cooled superconductive devices. Often their cooling system is heavy and bulky; hence these devices are not adequate for lightweight and portable systems. In addition, the cryogenic cooler is the most expensive component in the photon detector IR camera, and has finite lifetime only around 2,000 hours.
Recent work by Honeywell and Texas Instruments has demonstrated the feasibility of uncooled bolometric devices. Honeywell's device used a micromachined array of microbridge-type bolometric pixels 50×50 &mgr;m
2
each. The temperature detector was a VO
x
resistor patterned on the microbridge which has a sharp transition in its resistance near room temperature. Similar technology has been applied into different developed commercial available infrared imagers. The Texas Instruments device uses a pyroelectric pixel of barium strontium titanate (BST) that is patterned directly on the substrate. Both devices have a backplane of interfacing electronics for multiplexing of the pixel signals. Recently, the research work has been concentrating on the monolithic fabrication of infrared detector using micromachining techniques which trade the fill factor with potential high yield and lower production cost.
In all of these technologies, the infrared detectors have highly process dependent responsivities and offsets requiring elaborate calibration procedures and the extensive use of corrective electronics.
Most conventional bolometers operate in the open loop scheme shown in FIG.
1
. Upon the absorption of incident radiation, the temperature of a thermally isolated plate or absorber
10
increases changing the electrical characteristic of a thermistor
12
. The thermally isolated plate
10
defines a thermally isolated area. The electrical signal of the thermistor
12
is typically amplified yielding voltage V
o
representing the power of the incoming radiation. One of the major problems with this mode of operation is that the measured signal is influenced by many material properties, offsets and drifts which make the response difficult to predict.
For example in the simple open loop detector circuit shown in
FIG. 2
, the bolometer R
d
with temperature coefficient of resistance &agr; is connected through biasing resistor R
1
to the voltage source V. For incident power &Dgr;&PHgr;, the bolometer temperature increase &Dgr;T
d
obeys:
C
ⅆ
Δ
T
d
ⅆ
t
+
G
0
Δ
T
d
=
W
h
+
Δ
Φ
(
1
)
where:
C=the heat capacitance of the bolometer element;
G
0
&Dgr;T
d
=the conductive and radiative heat flow for the element; and
W
h
=the self-generated thermal power in the bolometer.
The responsivity of this detector circuit is:
R
=
Δ
υ
ΔΦ
=
(
R
1
R
1
+
R
d
)
I
ϵ
R
d
α
G
e
(
1
+
ω
2
τ
2
)
1
/
2
(
2
)
where
G
e
=
G
-
α
G
0
(
Δ
T
d
)
(
R
1
-
R
d
R
1
+
R
d
)
,
τ
=
C
/
G
e
(
3
)
and
G
e
: effective thermal conductance;
G: thermal conductance for a small temperature change;
&egr;: the emissivity of the bolometer; and
&ohgr;: sinusoidal radiation input frequency.
The responsivity thus depends on the material properties like &agr;, and R
d
that vary from run-to-run hence a calibration procedure is needed. In a focal plane imaging array, this must be done for each pixel in the array.
DISCLOSURE OF INVENTION
An object of the present invention is to provide an uncooled infrared focal plane imager and microelectromechanical infrared detector for use therein wherein both the imager and the detector are heat-balancing.
Another object of the present invention is to provide an uncooled infrared focal plane imager and microelectromechanical infrared detector for use therein wherein both the imager and the detector utilize an active pixel heat-balancing technique based on electrothermal feedback principles.
Yet another object of the present invention is to provide an uncooled infrared focal plane imager and microelectromechanical infrared detector for use therein which are formed utilizing a commercial CMOS process plus a single electrochemical etch stop releasing step.
Yet still another object of the present invention is to provide an uncooled infrared focal plane imager and microelectromechanical infrared detector for use therein which employ heat balance between incoming infrared radiation and active device power dissipation.
In carrying out the above objects and other objects of the present invention, an uncooled infrared focal plane imager is provided. The imager includes a plurality of microelectromechanical infrared detectors disposed in a plane. Each of the infrared detectors includes a substrate and a sensor positioned in an area thermally isolated from the substrate for sensing temperature and to provide a corresponding temperature signal representing incident power of infrared energy. Each of the infrared detectors also includes a heater for controllably heating the sensor based on the temperature signal. A change in incident power of the infrared energy is substantially balanced by a corresponding change in heat generated by the heater to heat the sensor. The imager also includes a signal processor for processing the temperature signals from the infrared detectors to obtain an infrared image.
Preferably, the heater is a circuit element such as a MOSFET and the sensor is a thermistor element such as a MOSFET.
Also, preferably, the substrate is a semiconductor substrate.
Still, preferably, each of the infrared detectors includes a comparator which compares the temperature signal with a reference temperature signal and provides an output signal based on the comparison wherein the heat generated by the heater is based on the output signal.
Preferably, the sensor and heater are PMOS devices and wherein the sensor and the heater comprise stages of a cascade amplifier.
Still further in carrying out the above objects and other objects of the present invention, a microelectromechanical infrared detector constructed in accordance with the above is provided.
The uncooled infrared focal plane imager includes two-dimensional arrays of infrared detectors thermally isolated from their surroundings. The detectors respond to incoming infrared radiation through temperature changes. Materials used for these detectors are chosen based on material properties, such as electrical resistance, pyroelectric polarization, or dielectric constant, each of which are dependent on temperature variations. Uncooled infrared imagers are fundamentally different from the traditional cryogenically cooled, mechanically scanned, linearly arrayed photon detector-based infrared imagers. The uncooled focal plane infrared imaging systems do not have complex moving components and do not require complicated cooling mechanisms and therefore are lightweight, small, have low power requirements, require simplified control logistics, and are more reliable.
The present invention provides a new type of uncooled heat balancing focal plane infrared imager based microelectromechanical system (MEMS) technology suitable for many applications such as night vision for military systems, collision avoidance and vision enhancement for automotive safety systems, and wafer temperature radiometry for semiconductor process control. Compared to current uncooled infrared imager technology, this invention provides a high performance uncooled infrare
Liu Chien-Chang
Mastrangelo Carlos H.
Brooks & Kushman P.C.
Dang Hung Xuan
The Regents of the University of Michigan
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