Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1998-11-17
2004-07-13
Luu, Thanh X. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C348S241000
Reexamination Certificate
active
06762398
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging device for converting an electromagnetic radiation such as visible light, infrared rays, ultraviolet rays, X-rays, etc. into an electric signal, and more particularly to an infrared imaging device for converting infrared rays into an electric signal.
2. Description of the Related Art
Infrared imaging devices are classified into a quantum type which detects incident infrared rays with a photodiode or the like and a thermal type which converts an increase in temperature of a structural body due to incident infrared rays into an electric signal with a thermoelectric transducer. Both types of infrared imaging devices are used to measure a temperature distribution over the surface of a subject to be imaged, for example.
One conventional infrared imaging device is disclosed in Japanese patent application No. 098009/96, for example, which is an earlier invention by the inventor of the present invention.
FIGS. 1 and 2
of the accompanying drawings are a cross-sectional view and a circuit diagram, respectively, of the disclosed conventional infrared imaging device. The conventional infrared imaging device is a thermal-type infrared imaging device. As shown in
FIG. 1
, the infrared imaging device has a semiconductor substrate
20
, a scanning circuit
21
on the surface of the semiconductor substrate
20
, and a photodetector on the scanning circuit
21
for converting incident infrared rays into an electric signal. The scanning circuit
21
and the photodetector comprise an integrated matrix of pixels for generating a signal representing a two-dimensional infrared image. The photodetector comprises an infrared absorbing layer
29
for absorbing infrared rays, a diaphragm (silicon oxide film)
28
for preventing heat from being dissipated away, and a thermoelectric transducer
27
for converting heat into an electric signal.
The diaphragm
28
has its lower layer removed by etching, so that it is of a floating film-like structure. The thermoelectric transducer
27
comprises a bolometer whose electric resistance varies depending on the temperature, the bolometer being made of titanium. An infrared ray applied to each of the pixels is absorbed by the infrared absorbing layer
29
at each pixel, increasing the temperature of the diaphragm
28
at each pixel. The increase in the temperature is converted into an electric signal by the titanium bolometer. Electric signals generated by the respective pixels are successively read by the scanning circuit
21
.
The infrared imaging device also has a silicon oxide film
22
, cavities
23
, ground lines
24
, signal lines
25
, and vertical selection lines
30
.
As shown in
FIG. 2
, the scanning circuit
21
of the infrared imaging device has source followers
907
,
912
, load transistors
913
, horizontal switches
909
,
916
, horizontal signal lines
911
, NPN transistors
902
, PNP transistors
904
, integrating capacitors
905
, ramp waveform generators
915
, pixel switches
920
, horizontal signal lines
918
, titanium bolometers
901
,
903
, level converters
921
,
922
,
923
,
924
,
925
,
926
,
927
,
927
for being supplied respectively with horizontal data
929
, a horizontal clock
930
, an S/H pulse
931
, a reset pulse
932
, horizontal data
933
, a horizontal clock
934
, vertical data
935
, and a vertical clock
936
, a horizontal shift register
910
for outputting horizontal pulses I
1
-I
5
, a horizontal shift register
917
for outputting horizontal selection signals H
1
-H
128
, and a vertical shift register
919
for outputting vertical selection signals V
1
-V
128
.
In
FIG. 2
, each of the titanium bolometers
901
is disposed on the corresponding diaphragm
28
, and is sensitive to incident infrared rays. When a voltage V
b1
is applied to the base of an NPN transistor
902
, a voltage (V
b1
−V
BE
) is applied to the titanium bolometer
901
where V
BE
represents a base-to-emitter voltage of the NPN transistor
902
. If the titanium bolometer
901
has a resistance R
b1
, then a current I
c1
=(V
b1
−V
BE
)/R
b1
flows through the collector of the NPN transistor
902
.
The titanium bolometers
903
are disposed on the semiconductor substrate
20
, and hence are not sensitive to incident infrared rays. This is because the titanium bolometers
903
are used as a reference with respect to the titanium bolometers
901
. When a voltage V
b2
is applied to the base of an NPN transistor
904
, a current I
c2
=(V
b2
−V
BE
)/R
b2
flows through the collector of the NPN transistor
904
where R
b2
represents the resistance of the titanium bolometer
903
.
When no incident infrared ray is applied, the currents I
c1
, I
c2
are in equilibrium with each other, and almost no current flows in the integrating capacitor
905
. When an incident infrared ray is applied, the temperature of the thermally isolated diaphragm
28
rises, changing the resistance of the titanium bolometer
901
on the diaphragm
28
. The change in the resistance of the titanium bolometer
901
changes the current I
c1
. Since the resistance of the titanium bolometer
903
on the semiconductor substrate
20
does not change, the current I
c2
does not change. Because of the changing current I
c1
, there is developed a current difference &Dgr;I=(I
c2
−I
c1
) which is stored in the integrating capacitor
905
. The current difference &Dgr;I comprises a signal component and a bias component which cannot be removed, with a larger bias component being removed.
Another conventional imaging device is an amplification-type solid-state imaging device as disclosed in Japanese laid-open patent publication No. 289381/89, for example. The amplification-type solid-state imaging device disclosed has a photodiode and a current mirror that are combined with each other for reducing the effects of the threshold voltage VT and parasitic capacitance of an amplifying element.
Japanese laid-open patent publication No. 78218/94 reveals an imaging device in which the difference between an output signal produced by a pixel when a reset time is long and an output signal produced by the pixel when the reset time is short is determined to remove fixed pattern noise (FPN).
In an imaging device disclosed in Japanese laid-open patent publication No. 242330/96, the difference between an output signal produced by a pixel immediately before the signal is reset and an output signal produced by the pixel immediately after the signal reset is determined to correct signal variations of a reading circuit.
The imaging device shown in Japanese laid-open patent publication No. 098009/96 is capable of cutting off a larger bias component and extracting a signal component, but cannot increase the amplification for signals if there are large variations between the pixels.
In imaging devices composed of a plurality of pixels, there are usually variations between the pixels. These variations between the pixels may be caused by variation between detectors such as bolometers or variations in threshold voltages and parasitic capacitances of amplifying elements. In a bolometer-type infrared imaging device, for example, bolometer resistances vary from several percentages to several tens of percentages due to variations of the thickness of bolometer films, variations of specific resistances, and variations of patterned dimensions.
Such pixel variations may pose a serious problem in reading signals. For example, when a subject having a temperature difference of 1° C. is imaged, the temperature of the bolometer temperature changes by about 1 m° C., and the resistance of the bolometer changes by about 0.001% if the temperature coefficient of resistance of the bolometer is 1%/° C. In order to read such a small resistance change, it should preferably be amplified by an amplifying circuit. If there are large resistance variations between the pixels, however, the dynamic range of the amplifying circuit is limited by the large resistance variations, and the amplification fac
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