Radiant energy – Infrared-to-visible imaging – Including detector array
Reexamination Certificate
1999-10-04
2002-08-27
Hannaher, Constantine (Department: 2878)
Radiant energy
Infrared-to-visible imaging
Including detector array
C250S338100
Reexamination Certificate
active
06441372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an infrared focal plane array detector and a method of producing the same, and more particularly to an infrared focal plane array detector which is less likely to be influenced by an offset signal and a temperature drift and a method of producing the same.
2
. Description of the Related Art
A bolometer detector has a resistance value which varies in response to a temperature variation, and is used widely for detection of infrared rays with the characteristic made most of. A mechanism of operation of a conventional infrared focal plane array detector which employs a bolometer detector and is an example of an application is described with reference to FIG.
1
.
A plurality of unit cells
400
each including one bolometer detector
401
are arranged in a two-dimensional array in FPA (FOCAL PLANE ARRAY)
417
such that they may form columns and rows. Bolometer detector
401
in each unit cell
400
is connected to ground terminal
403
through unit cell selecting transistor or switch
402
. The gate electrodes of unit cell selecting switches
402
are controlled by vertical shift register
410
through horizontal lines
406
and are selected such that all of the unit cell selecting switches
402
connected to one horizontal lines
406
may be put into a closed state. Consequently, on each vertical line
404
, bolometer detector
401
on one of the unit cells
400
is selectively connected to ground terminal
403
.
Vertical lines
404
are connected to an on-chip amplifier
407
through vertical line selection switches
405
and over to output line
421
. Vertical line selection switches
405
are controlled by horizontal shift register
409
such that one of vertical lines
404
may be connected to an on-chip amplifier
407
at a given time.
A light receiver of each bolometer detector
401
is in a high thermally insulating state with respect to asubstrate, and consequently, thermal energy by infrared radiation is temporarily stored into bolometer detector
401
. As a result, the temperature of bolometer detector
401
rises, and a resistance variation corresponding to the temperature variation appears with bolometer detector
401
. Temperature information of an image pickup object can be obtained by externally reading out the resistance variation through output terminal
408
.
For the readout circuit, for example, integration circuit
440
is used. In integration circuit
440
, electric current flowing through integration transistor
441
while a fixed voltage is applied to bolometer detector
401
for a fixed time is integrated by means of integrating capacitor
442
. Actually, integrating capacitor
442
connected in series to bolometer detector
401
is charged up to a predetermined voltage in advance, and then, after integrating capacitor
442
is energized with the fixed voltage for a fixed time, the remaining voltage of integrating capacitor
442
is read out. Integration transistor
441
acts to keep the voltage of output terminal
408
fixed even if the terminal voltage of integrating capacitor
442
varies during the integration operation. Accordingly, the remaining voltage of integrating capacitor
442
after the integration operation comes to an end depends upon the resistance value of bolometer detector
401
within the integration period and includes information regarding the amount of heat radiation received from the image pickup object. Information of the infrared radiation amount can be read out electrically in this manner.
The infrared focal plane array detector which employs a bolometer element described above, however, has a problem in that offset components of the output signal are so high that a sufficient gain cannot be obtained. A principal part of the offset components of the signal arises from a large variation of the bolometer resistance by a temperature rise caused by joule heating of the bolometer detector within the integration period. Although the offset components occupy the greater part of the output signal, it does not include temperature information of the object at all. Therefore, the dynamic range of an amplifier cannot be used effectively, and the signal gain cannot be raised. If the integration time is increased, then the temperature resolution is augmented as much. However, since this increases also the offset amount simultaneously, a sufficient integration time cannot be assured.
The infrared focal plane array detector which employs a bolometer described above has another significant problem in temperature drift. Although each bolometer detector is thermally isolated from the substrate, actually a little heat exchange with the substrate is present through signal readout wires. Therefore, if the ambient temperature varies and the substrate temperature varies, then the element temperature of the bolometer detector varies with a long time constant. This makes a cause of a temperature drift. In order to prevent this, FPN (FIXED PATTERN NOISE) correction must be performed frequently during operation, and this significantly deteriorates the operability of an infrared camera which employs the infrared detector.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a new infrared focal plane array detector which eliminates the drawbacks of the prior art described above and particularly removes an offset and eliminates an influence of a temperature drift thereby to augment the accuracy in detection of infrared rays and a method of producing the infrared focal plane array detector.
An-infrared focal plane array detector of the present invention comprises first bolometer detectors and at least one set of second bolometer detectors and a plurality of third bolometer detectors being provided on the same substrate. First bolometer detectors have, between the first bolometer detectors and a substrate, a thermal isolation structure for thermally isolating the first bolometer detectors and the substrate from each other and disposed in an infrared detection area on the substrate for receiving infrared rays incoming thereto. Second bolometer detectors have a thermal isolation structure between the second bolometer detectors and the substrate for thermally isolating the second bolometer detectors from the substrate and are so structured as to receive no infrared rays incoming to the infrared focal plane array detector. Third bolometer detectors are directly formed on the substrate. The bolometer detectors forming a circuit for removing an offset in output signals therefrom and reducing a temperature drift thereof.
Preferably, an arbitrary one of the first bolometers disposed in the infrared detection area and one of the third bolometer detectors disposed on the same column as the first bolometer detector are connected to each other at a first junction, and one of the second bolometer detectors disposed on the same row as the first bolometer detector and another one of the third bolometer detectors disposed on the same column as the second bolometer detector and different from the third bolometer detector are connected to each other at a second junction, and further, a bridge circuit is formed from the four bolometer detectors and the first junction and the second junction form midpoints for detecting a difference in voltage of the bridge circuit. Alternatively, the infrared focal plane array detector may be constructed such that an arbitrary one of the first bolometer detectors disposed in the infrared detection area is connected to one of the second bolometer detectors disposed on the same column as the first bolometer detector, and one of the third bolometer detectors disposed on the same row as the first bolometer detector is connected to another one of the third bolometer detectors disposed on the same column as the third bolometer detector, and further, a bridge circuit is formed from the four bolometer detectors and a junction between the first bolometer detector and the second bolometer detector and another junction between the two third bolometer det
Hannaher Constantine
NEC Corporation
Sughrue & Mion, PLLC
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