U.S. patent application number 13/728009 was filed with the patent office on 2013-09-26 for uncooled infrared imaging device.
The applicant listed for this patent is Masaki Atsuta, Hideyuki Funaki, Hiroto Honda, Koichi Ishii, Honam Kwon, Keita Sasaki, Kazuhiro Suzuki. Invention is credited to Masaki Atsuta, Hideyuki Funaki, Hiroto Honda, Koichi Ishii, Honam Kwon, Keita Sasaki, Kazuhiro Suzuki.
Application Number | 20130248714 13/728009 |
Document ID | / |
Family ID | 47605356 |
Filed Date | 2013-09-26 |
United States Patent
Application |
20130248714 |
Kind Code |
A1 |
Honda; Hiroto ; et
al. |
September 26, 2013 |
UNCOOLED INFRARED IMAGING DEVICE
Abstract
An uncooled infrared imaging device according to an embodiment
includes: reference pixels formed on a semiconductor substrate and
arranged in at least one row; and infrared detection pixels
arranged in the remaining rows and detecting incident infrared
rays. Each of the reference pixels includes a first cell located
above a first concave portion. The first cell includes a first
thermoelectric conversion unit having a first infrared absorption
film; and a first thermoelectric conversion element. Each of the
infrared detection pixels includes a second cell located above a
second concave portion, and having a larger area than the first
cell. The second cell includes: a second thermoelectric converting
unit located above the second concave portion; and first and second
supporting structure units supporting the second thermoelectric
converting unit above the second concave portion. The second
thermoelectric converting unit includes: a second infrared
absorption film; and a second thermoelectric conversion
element.
Inventors: |
Honda; Hiroto;
(Yokohama-shi, JP) ; Suzuki; Kazuhiro; (Tokyo,
JP) ; Funaki; Hideyuki; (Tokyo, JP) ; Atsuta;
Masaki; (Yokosuka-Shi, JP) ; Sasaki; Keita;
(Yokohama-shi, JP) ; Ishii; Koichi; (Kawasaki-Shi,
JP) ; Kwon; Honam; (Kawasaki-Shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Honda; Hiroto
Suzuki; Kazuhiro
Funaki; Hideyuki
Atsuta; Masaki
Sasaki; Keita
Ishii; Koichi
Kwon; Honam |
Yokohama-shi
Tokyo
Tokyo
Yokosuka-Shi
Yokohama-shi
Kawasaki-Shi
Kawasaki-Shi |
|
JP
JP
JP
JP
JP
JP
JP |
|
|
Family ID: |
47605356 |
Appl. No.: |
13/728009 |
Filed: |
December 27, 2012 |
Current U.S.
Class: |
250/338.4 |
Current CPC
Class: |
G01J 5/12 20130101; G01J
2005/0077 20130101; H04N 5/33 20130101; G01J 5/023 20130101; H01L
27/14649 20130101; G01J 5/22 20130101; G01J 5/20 20130101 |
Class at
Publication: |
250/338.4 |
International
Class: |
H01L 27/146 20060101
H01L027/146; G01J 5/20 20060101 G01J005/20 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 23, 2012 |
JP |
2012-068158 |
Claims
1. An uncooled infrared imaging device comprising: a semiconductor
substrate; an imaging region formed on the semiconductor substrate,
a plurality of pixels being arranged in a matrix form in the
imaging region, the plurality of pixels including a plurality of
reference pixels arranged in at least one row, and a plurality of
infrared detection pixels arranged in the remaining rows, each of
the infrared detection pixels being configured to detect an
incident infrared ray, each of the reference pixels including a
first cell located above a first concave portion formed in a
surface portion of the semiconductor substrate, the first cell
including a first thermoelectric converting unit including a first
infrared absorption film and a first thermoelectric conversion
element, the first infrared absorption film being configured to
absorb the incident infrared ray and convert the incident infrared
ray into heat, the first thermoelectric conversion element being
configured to convert the heat obtained by the first infrared
absorption film into an electrical signal, each of the infrared
detection pixels including a second cell located above a second
concave portion formed in a surface portion of the semiconductor
substrate, the second cell including a second thermoelectric
converting unit located above the second concave portion and first
and second supporting structure units supporting the second
thermoelectric converting unit above the second concave portion,
the second thermoelectric converting unit including a second
infrared absorption film and a second thermoelectric conversion
element, the second infrared absorption film being configured to
absorb the incident infrared ray and convert the incident infrared
ray into heat, the second thermoelectric conversion element being
configured to convert the heat obtained by the second infrared
absorption film into an electrical signal, the first cell having a
smaller area than the second cell when viewed from an incident
direction of the infrared ray; a plurality of row select lines
formed in the imaging region, the plurality of row select lines
corresponding to the rows of the plurality of pixels, each of the
row select lines being connected to one end of the thermoelectric
conversion element of each pixel of the corresponding one of the
rows, each of the row select lines selecting the pixels of the
corresponding one of the rows; and a plurality of signal lines
formed in the imaging region, the plurality of signal lines
corresponding to columns of the plurality of pixels, each of the
signal lines being connected to the other end of the thermoelectric
conversion element of each pixel of the corresponding one of the
columns, each of the signal lines being used for reading the
electrical signal from each pixel of the corresponding one of the
columns.
2. The device according to claim 1, wherein, when
G.sub.th.sub.--.sub.IMG represents a heat conductance of each of
the infrared detection pixels, C.sub.th.sub.--.sub.IMG represents a
heat capacity of each of the second cells, G.sub.th.sub.--.sub.TB
represents a heat conductance of each of the reference pixels,
C.sub.th.sub.--.sub.TB represents a heat capacity of each of the
first cells, and tsel represents a duration of flowing of current
to the infrared detection pixel and the reference pixels, the
following equation is established, 1 G th_IMG { 1 - exp ( - tsel C
th_IMG / G th_IMG ) } = 1 G th_TB { 1 - exp ( - tsel C th_TB / G
th_TB ) } ##EQU00005## where one of the left-hand value and the
right-hand value of the equation is 0.9 to 1.1 times larger than
the other one of the left-hand value and the right-hand value.
3. The device according to claim 1, further comprising a plurality
of differential clamp circuits corresponding to the respective
signal lines, each of the differential clamp circuits including: a
coupling capacitor having one terminal connected to the
corresponding one of the signal lines; a differential amplifier
having a positive-side input terminal connected to the other
terminal of the coupling capacitor; a feedback capacitor located
between the positive-side input terminal of the differential
amplifier and an output terminal of the differential amplifier; and
a feedback switch connected in parallel to the feedback
capacitor.
4. The device according to claim 3, further comprising a plurality
of read transistors corresponding to the respective signal lines,
each of the read transistors having one of a source and a drain
connected to the output terminal of the differential amplifier,
each of the read transistors having the other one of the source and
the drain connected to a read line, a gate of each of the read
transistors receiving a select signal from a column select
circuit.
5. The device according to claim 1, further comprising a
differential amplifier configured to amplify a difference between
signal voltages obtained from the infrared detection pixels and the
reference pixels.
6. The device according to claim 1, wherein the first and second
thermoelectric conversion elements are series-connected diodes.
7. The device according to claim 1, wherein the first and second
thermoelectric conversion elements are series-connected resistors.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority from prior Japanese Patent Application No. 2012-68158
filed on Mar. 23, 2012 in Japan, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] An embodiment described herein relates generally to an
uncooled infrared imaging device.
BACKGROUND
[0003] Infrared rays are radiated from a heat source even in the
dark, and characteristically have higher permeability than visible
light in smoke or fog. Accordingly, infrared imaging can be
performed during day and night. Also, temperature information about
an object can be obtained through infrared imaging. In view of
this, infrared imaging can be applied to a wide variety of fields
such as the defense field and the fields of security cameras and
fire detecting cameras.
[0004] In recent years, "uncooled infrared imaging devices" that do
not require cooling mechanisms have been actively developed. An
uncooled or thermal infrared imaging device converts an incident
infrared ray of approximately 10 .mu.m in wavelength into heat with
an absorption mechanism, and further converts the temperature
change at the heat sensitive unit caused by the small amount of
heat into an electrical signal with a thermoelectric conversion
means. The uncooled infrared imaging device reads the electrical
signal, to obtain infrared image information.
[0005] For example, there have been infrared sensors each using a
silicon pn junction that converts a temperature change into a
voltage change by applying a constant forward current.
Characteristically, such infrared sensors can be mass-produced
through a silicon LSI manufacturing process using a SOI (Silicon On
Insulator) substrate as a semiconductor substrate. Also, the
rectification properties of the silicon pn junctions serving as the
thermoelectric conversion means are utilized to realize the row
select function, and accordingly, the pixels can be designed to
have very simple structures.
[0006] One of the indicators of the performance of an infrared
sensor is NETD (Noise Equivalent Temperature Difference), which
indicates the temperature resolution of the infrared sensor. It is
critical to reduce the NETD, or reduce the infrared sensor
temperature difference equivalent to noise. To do so, it is
necessary to increase signal sensitivity and reduce noise.
[0007] Thermoelectric conversion elements are sensitive to
temperature components other than temperature rises caused by
incident infrared rays, or to the temperature of the semiconductor
substrate and the self-heating temperature at the time of flowing
of current. To correct those "offset temperatures", reference
pixels are provided.
[0008] Like an infrared detection pixel, a conventional reference
pixel reflects the influence of the temperature of the
semiconductor substrate, but has a different self-heating
temperature from that of an infrared detection pixel. The
difference in self-heating temperature is much larger than a signal
generated from an incident infrared ray, and therefore, it is
necessary to correct the difference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a circuit diagram of an uncooled infrared imaging
device according to an embodiment;
[0010] FIGS. 2(a) and 2(b) are a plan view and a cross-sectional
view of an infrared detection pixel according to the embodiment,
respectively;
[0011] FIGS. 3(a) and 3(b) are a plan view and a cross-sectional
view of a reference pixel according to the embodiment,
respectively; and
[0012] FIGS. 4(a) and 4(b) are graphs showing the self-heating
amounts in an infrared detection pixel and a reference pixel,
respectively.
DETAILED DESCRIPTION
[0013] An uncooled infrared imaging device according to an
embodiment includes: a semiconductor substrate; an imaging region
formed on the semiconductor substrate, a plurality of pixels being
arranged in a matrix form in the imaging region, the plurality of
pixels including a plurality of reference pixels arranged in at
least one row, and a plurality of infrared detection pixels
arranged in the remaining rows, each of the infrared detection
pixels being configured to detect an incident infrared ray, each of
the reference pixels including a first cell located above a first
concave portion formed in a surface portion of the semiconductor
substrate, the first cell including a first thermoelectric
converting unit including a first infrared absorption film and a
first thermoelectric conversion element, the first infrared
absorption film being configured to absorb the incident infrared
ray and convert the incident infrared ray into heat, the first
thermoelectric conversion element being configured to convert the
heat obtained by the first infrared absorption film into an
electrical signal, each of the infrared detection pixels including
a second cell located above a second concave portion formed in a
surface portion of the semiconductor substrate, the second cell
including a second thermoelectric converting unit located above the
second concave portion and first and second supporting structure
units supporting the second thermoelectric converting unit above
the second concave portion, the second thermoelectric converting
unit including a second infrared absorption film and a second
thermoelectric conversion element, the second infrared absorption
film being configured to absorb the incident infrared ray and
convert the incident infrared ray into heat, the second
thermoelectric conversion element being configured to convert the
heat obtained by the second infrared absorption film into an
electrical signal, the first cell having a smaller area than the
second cell when viewed from an incident direction of the infrared
ray; a plurality of row select lines formed in the imaging region,
the plurality of row select lines corresponding to the rows of the
plurality of pixels, each of the row select lines being connected
to one end of the thermoelectric conversion element of each pixel
of the corresponding one of the rows, each of the row select lines
selecting the pixels of the corresponding one of the rows; and a
plurality of signal lines formed in the imaging region, the
plurality of signal lines corresponding to columns of the plurality
of pixels, each of the signal lines being connected to the other
end of the thermoelectric conversion element of each pixel of the
corresponding one of the columns, each of the signal lines being
used for reading the electrical signal from each pixel of the
corresponding one of the columns.
[0014] The following is a description of an embodiment, with
reference to the accompanying drawings.
[0015] FIG. 1 shows an uncooled infrared imaging device
(hereinafter also referred to as the infrared sensor) according to
an embodiment. The infrared sensor of this embodiment includes an
imaging region formed on a semiconductor substrate. In this imaging
region, there are two reference pixels 11.sub.1 and 11.sub.2 that
are arranged in one row and two columns, and infrared detection
pixels 12.sub.ij (i, j=1, 2) arranged in two rows and two columns,
for example. Although an imaging region normally accommodates more
pixels than the above, the imaging region in this embodiment
accommodates pixels, or reference pixels and infrared detection
pixels, which are arranged in three rows and two columns, for
descriptive purposes. Each of the pixels includes a pn junction
(diodes) serving as a thermoelectric conversion element, which will
be described later in detail.
[0016] The first terminal (the anode-side terminal of the pn
junction) of each of the reference pixels 11.sub.1 and 11.sub.2 is
connected to a row select line 45.sub.0. The second terminals (the
cathode-side terminals of the pn junctions) of the reference pixels
11.sub.1 and 11.sub.2 are connected to vertical signal lines
(hereinafter also referred to simply as the signal lines) 44.sub.1
and 44.sub.2, respectively. The respective first terminals (the
anode-side terminals of the pn junctions) of the infrared detection
pixels 12.sub.11 and 12.sub.12 of the first row are connected to a
row select line 45.sub.1. The second terminals (the cathode-side
terminals of the pn junctions) of the infrared detection pixels
12.sub.11 and 12.sub.12 are connected to the vertical signal lines
44.sub.1 and 44.sub.2, respectively. The respective first terminals
(the anode-side terminals of the pn junctions) of the infrared
detection pixels 12.sub.21 and 12.sub.22 of the second row are
connected to a row select line 45.sub.2. The second terminals (the
cathode-side terminals of the pn junctions) of the infrared
detection pixels 12.sub.21 and 12.sub.22 are connected to the
vertical signal lines 44.sub.1 and 44.sub.2, respectively.
[0017] Each of the row select lines 45.sub.0, 45.sub.1, and
45.sub.2 is connected to a row select circuit 5. The rows are
sequentially selected one by one by the row select circuit 5, and a
bias voltage Vd is applied to each selected row. One terminal of
each signal line 44.sub.i (i=1, 2) is connected to a power source
Vss via a load transistor 41.sub.i, and the other terminal is
connected to the positive-side input terminal of a differential
amplifier 20.sub.i via a coupling capacitor 22.sub.i. The load
transistors 41.sub.1 and 41.sub.2 operate in a saturation region,
and supply a constant current to the pixels of a selected row in
accordance with a gate voltage GL1 applied to the gates thereof.
That is, the load transistors 41 function as constant current
sources. The source voltage of each of the load transistors
41.sub.1 and 41.sub.2 is represented by Vd0.
[0018] A voltage V2 is applied to the negative-side input terminal
of each differential amplifier 20.sub.i(i=1, 2), and the output
terminal of each differential amplifier 20.sub.i is connected to a
read line 33 via a read transistor 32.sub.i. Upon receipt of a
select signal from a column select circuit 6, each read transistor
32.sub.i(i=1, 2) is switched on.
[0019] A feedback switch 23.sub.i and a feedback capacitor 24.sub.i
are connected in parallel between the positive-side input terminal
and the output terminal of each differential amplifier 20.sub.i
(i=1, 2). A coupling capacitor 22.sub.i (i=1, 2), a differential
amplifier 20.sub.i, a feedback switch 23.sub.i, and a feedback
capacitor 24.sub.i constitute a differential clamp circuit. As will
be described later, this differential clamp circuit holds signals
that are output from the reference pixel connected to the
corresponding signal line, and amplifies a differential signal when
an infrared detection pixel connected to the corresponding signal
line is selected.
[0020] When the row select circuit 5 applies the bias voltage Vd to
a selected row, a series voltage Vd-Vd0 is applied to the pn
junctions of the reference pixels or the infrared detection pixels
connected to the selected row. For example, if the row select line
45.sub.0 is selected, the series voltage Vd-Vd0 is applied to the
pn junctions of the reference pixels 11.sub.1 and 11.sub.2. If the
row select line 45.sub.1 is selected, the series voltage Vd-Vd0 is
applied to the pn junctions of the infrared detection pixels
12.sub.11 and 12.sub.12. If the row select line 45.sub.2 is
selected, the series voltage Vd-Vd0 is applied to the pn junctions
of the infrared detection pixels 12.sub.21 and 12.sub.22. All the
pn junctions of the pixels of the unselected rows are
inversely-biased. Therefore, the unselected row select lines are
separated from the signal lines 44.sub.1 and 44.sub.2. That is, it
can be said that the pn junctions each have a pixel select
function.
[0021] The potential of each of the vertical signal lines 44.sub.1
and 44.sub.2 when any infrared ray is not received is defined as
Vs1. Upon receipt of an infrared ray, the pixel temperature of each
infrared detection pixel 12.sub.ij (i, j=1, 2) becomes higher.
Accordingly, the potential of each of the vertical signal lines
44.sub.1 and 44.sub.2 becomes higher than Vs1. For example, when
the object temperature varies by 1 K (Kelvin), the temperature of
an infrared detection pixel 12.sub.ij (i, j=1, 2) varies by
approximately 5 mK. With the thermoelectric conversion efficiency
being 10 mV/K, the potential of each of the vertical signal lines
44.sub.1 and 44.sub.2 increases by approximately 50 .mu.V, which is
much smaller than the increase of the bias voltage Vd.
[0022] Upon receipt of an infrared ray, the potential of each of
the vertical signal line 44.sub.1 and 44.sub.2 becomes
Vd-(Vf0-Vsig-Vsh). Here, Vf0 represents the forward voltage of a pn
junction when no infrared ray is received, and Vsig represents the
voltage signal based on the temperature rise in the later described
thermoelectric converting unit of an infrared detection pixel. Vsh
represents the change in voltage due to the Joule heat generated
when a current is applied to the pn junction. The self-heating
amount is expressed by the following equation (1):
T cell ( t ) = I f V f G th { 1 - exp ( - t C th / G th ) } ( 1 )
##EQU00001##
[0023] Here, I.sub.f represents the amount of current determined by
the operating points of the load transistors 41.sub.1 and 41.sub.2,
V.sub.f represents the forward voltage of the pn junction, t
represents the time elapsed from the start of flowing the current,
and C.sub.th and G.sub.th represent the heat capacity and the heat
conductance of the infrared detection pixel 12.sub.ij (i, j=1, 2),
respectively. Those items will be described later. In a case where
t is sufficiently smaller than the value expressed by
C.sub.th/G.sub.th, the equation (1) is approximated by the
following equation (2), and the temperature increases in proportion
to time.
T cell ( t .fwdarw. 0 ) = I f V f C th t ( 2 ) ##EQU00002##
[0024] FIG. 2(a) is a plan view of an infrared detection pixel
12.sub.ij (i, j=1, 2). FIG. 2(b) is a cross-sectional view of the
infrared detection pixel 12.sub.ij (i, j=1, 2), taken along the
section line A-A of FIG. 2(a). The infrared detection pixel
12.sub.ij (i, j=1, 2) includes a cell 160 formed on a SOI
substrate. The SOI substrate includes a supporting substrate 181, a
buried insulating layer (a BOX layer) 182, and a SOI
(Silicon-On-Insulator) layer formed with silicon single crystals.
The SOI substrate has a concave portion 185 formed in a surface
portion thereof. The concave portion 185 is formed by removing part
of the supporting substrate 181.
[0025] The cell 160 includes a thermoelectric converting unit 161,
and supporting structure units 167a and 167b that support the
thermoelectric converting unit 161 above the concave portion 185.
The thermoelectric converting unit 161 includes diodes (two diodes
in FIG. 2(a)) 162 connected in series, an interconnect 163
connecting those diodes 162, and an infrared absorption film 164
formed to cover the diodes 162 and the interconnect 163.
[0026] The supporting structure unit 167a includes: a connective
interconnect 168a that has one end connected to the corresponding
vertical signal line and has the other end connected to one end of
the series circuit formed with the series-connected diodes 162; and
an insulating film 169a coating the connective interconnect 168a.
The other supporting structure unit 167b includes: a connective
interconnect 168b that has one end connected to the corresponding
row select line and has the other end connected to the other end of
the series circuit formed with the series-connected diodes 162; and
an insulating film 169b coating the connective interconnect
168b.
[0027] The infrared absorption film 164 generates heat upon receipt
of an incident infrared ray. The diodes 162 convert the heat
generated by the infrared absorption film 164 into an electrical
signal. The supporting structure units 167a and 167b have elongate
shapes so as to surround the thermoelectric converting unit 161.
With this arrangement, the thermoelectric converting unit 161 is
supported on the SOI substrate, while being substantially
heat-insulated from the SOI substrate.
[0028] Having such a structure, each infrared detection pixel
12.sub.ij (i, j=1, 2) stores the heat generated from an incident
infrared ray, and can output the voltage based on the heat to the
signal line 44.sub.1 or 44.sub.2. The bias voltage Vd from the row
select line 45.sub.1 or 45.sub.2 is transferred to the
thermoelectric converting unit 161 via the interconnect 168b. The
signal that has passed the thermoelectric converting unit 161 is
transferred to the vertical signal line 44.sub.1 or 44.sub.2 via
the interconnect 168a.
[0029] FIG. 3(a) is a plan view of a reference pixel 11.sub.i (i=1,
2). FIG. 3(b) is a cross-sectional view of the reference pixel
11.sub.i (i=1, 2), taken along the section line A-A of FIG. 3(a).
The reference pixel 11.sub.i (i=1, 2) includes a cell 160a formed
on a SOI substrate. The SOI substrate includes a supporting
substrate 181, a buried insulating layer (a BOX layer) 182, and a
SOI (Silicon-On-Insulator) layer formed with silicon single
crystals. The SOI substrate has a concave portion 185a formed in a
surface portion thereof. The concave portion 185a is formed by
removing part of the supporting substrate 181.
[0030] The cell 160a includes a thermoelectric converting unit 161a
formed on the buried insulating layer 182. The thermoelectric
converting unit 161a includes diodes (two diodes in FIG. 3(a)) 162a
connected in series, an interconnect 163a connecting those diodes
162a, and an infrared absorption film 164a formed to cover the
diodes 162a and the interconnect 163a. One end of the series
circuit formed with the series-connected diodes 162a is connected
to a vertical signal line via an interconnect 165a, and the other
end of the series circuit is connected to a row select line via an
interconnect 165b. The interconnects 165a and 165b are formed in
regions where the concave portion 185a is not formed. The concave
portion 185a is formed below the thermoelectric converting unit
161a. However, the thermoelectric converting unit 161a is connected
to the portion of the buried insulating layer 182 formed outside
the region where the concave portion 185a is formed and to the
portion of the insulating film 164 formed on the portion of the
buried insulating layer 182, via the buried insulating layer 182
and the insulating film 164 formed on the buried insulating layer
182. Accordingly, unlike the thermoelectric converting unit 161 of
each infrared detection pixel, the thermoelectric converting unit
161a does not need to be supported above the concave portion 185a
by elongate supporting structure units. That is, unlike the
thermoelectric converting unit 161 of each infrared detection
pixel, the thermoelectric converting unit 161a is not
heat-insulated by elongate supporting structure units. Therefore,
the heat generated from an infrared ray is smaller by several
digits than that generated by an infrared detection pixel, and can
be ignored. That is, in each reference pixel, the heat conductance
G.sub.th, which is an indicator of heat insulating properties, is
much higher than that of each infrared detection pixel, and heat
easily escapes from each reference pixel.
[0031] The unit of the heat conductance G.sub.th is W/K, indicating
how many watts of energy is transferred in a case where a heat
conductor exists between two heat baths between which the
temperature difference is 1 K. The heat conductance G.sub.th is
expressed as G.sub.th=.kappa.S/L(W/K), where .kappa.(W/(Km))
represents the heat conductivity, S (m.sup.2) represents the
cross-sectional area, and L (m) represents the length of the
material responsible for heat conduction, or of the connective
interconnects 168a and 168b and the insulating films 169a and 169b
constituting the supporting structure units 167a and 167b. That is,
a structure with a larger cross-sectional area and a shorter length
has higher heat conductance.
[0032] Meanwhile, the heat capacity C.sub.th is the indicator of
how many joules of energy is required to increase the temperature
of an object by 1K, and the unit thereof is J/K. The heat capacity
C.sub.th is expressed as C.sub.th=cdV, where c (J/kg) represents
the specific heat of the material, V (m.sup.3) represents the
volume of the material, and d (kg/m.sup.3) represents the density
of the material.
[0033] The heat conductance G.sub.th.sub.--.sub.IMG and the heat
capacity C.sub.th.sub.--.sub.IMG of each infrared detection pixel
12.sub.ij (i, j=1, 2) are expressed as:
G.sub.th.sub.--.sub.IMG=.kappa..sub.2S/L
C.sub.th.sub.--.sub.IMG=c.sub.2d.sub.2LcWcHc (3)
[0034] Here, S represents the cross-sectional area of the
supporting structure units 167a and 167b, L represents the length
of the supporting structure units 167a and 167b, Lc and We
represent the length and the width of the thermoelectric converting
unit 161, respectively, and Hc represents the height of the
thermoelectric converting unit 161 inclusive of the thickness of
the buried insulating film 182. Meanwhile, .kappa..sub.2, c.sub.2,
and d.sub.2 represent the heat conductivity, the specific heat, and
the density of each of the supporting structure units 167a and
167b, respectively.
[0035] The heat conductance G.sub.th.sub.--.sub.TB and the heat
capacity C.sub.th.sub.--.sub.TB of each reference pixel 11.sub.i
(i=1, 2) are expressed as:
G.sub.th.sub.--.sub.TB=.kappa..sub.1S.sub.TB/L.sub.TB
C.sub.th.sub.--TB=c.sub.1d.sub.1LcWc.sub.TBHc (4)
[0036] Here, S.sub.TB and L.sub.TB represent the cross-sectional
area and the length of the connecting portion 166 between the
thermoelectric converting unit 161a and the interconnects 165a and
165b, respectively, Lc and Wc.sub.TB represent the length and the
width of the thermoelectric converting unit 161a, respectively, and
Hc represents the height of the thermoelectric converting unit 161a
inclusive of the thickness of the buried insulating film 182.
Meanwhile, .kappa..sub.1, c.sub.1, and d.sub.1 represent the heat
conductivity, the specific heat, and the density of the connecting
portion 166 between the thermoelectric converting unit 161a and the
interconnects 165a and 165b, respectively.
[0037] In each reference pixel 11.sub.i (i=1, 2), the plane pattern
of the concave portion 185a is designed so that the width Wc.sub.TB
of the thermoelectric converting unit 161a is smaller than the
width Wc of the thermoelectric converting unit 161 of each infrared
detection pixel 12.sub.ij (i, j=1, 2).
[0038] Therefore, the heat conductance of each reference pixel is
lower than the heat conductance of each infrared detection pixel,
and the heat capacity of each reference pixel is smaller than the
heat capacity of each infrared detection pixel. That is, the
following inequalities are satisfied:
G.sub.th.sub.--.sub.TB<G.sub.th.sub.--.sub.IMG
C.sub.th.sub.--.sub.TB<C.sub.th.sub.--.sub.IMG
[0039] FIGS. 4(a) and 4(b) show graphs of the self-heating amounts
in each infrared detection pixel 12.sub.ij (i, j=1, 2) and each
reference pixel 11.sub.i (i=1, 2). FIG. 4(a) illustrates the
self-heating in a case where the heat capacity
C.sub.th.sub.--.sub.TB of each reference pixel 11.sub.i (i=1, 2) is
equal to the heat capacity C.sub.th.sub.--.sub.IMG of each infrared
detection pixel 12.sub.ij (i, j=1, 2), or where the areas of the
cells 160 and 160a are the same. Once flowing of current is
started, the temperature of each pixel increases according to the
equation (1). As for each infrared detection pixel 12.sub.ij (i,
j=1, 2), the heat conductance G.sub.th.sub.--.sub.IMG is very low,
and the ratio C.sub.th.sub.--.sub.IMG/G.sub.th.sub.--.sub.IMG is
high. Accordingly, the thermal time constant is high, and the
temperature increases according to the equation (2) at the time of
short-time flowing of current. As for each reference pixel 11.sub.i
(i=1, 2), the heat conductance G.sub.th.sub.--.sub.TB is higher
than the heat conductance of each infrared detection pixel.
Therefore, as indicated by the equation (1), the temperature
increase rate becomes lower with time. In this case, the
self-heating temperature rises of both pixels differ from each
other when the flowing of current ends. This difference is on the
order of approximately 100 mK in a case where a current of 1 .mu.A
is applied, and is larger than a signal with respect to an object
temperature change by about two digits.
[0040] However, in a case where the heat capacity
C.sub.th.sub.--.sub.TB of each reference pixel 11.sub.i (i=1, 2) is
made smaller than the heat capacity C.sub.th.sub.--.sub.IMG of each
infrared detection pixel 12.sub.ij (i, j=1, 2), or where the area
of the cell 160a is made smaller than the area of the cell 160 as
in this embodiment, the self-heating temperatures can be made the
same when the flowing of current ends, as shown in FIG. 4(b). At
this point, the design parameters of each pixel should satisfy the
following equation:
I f V f G th_IMG { 1 - exp ( - tsel C th_IMG / G th_IMG ) } = I f V
f G th_TB { 1 - exp ( - tsel C th_TB / G th_TB ) } ##EQU00003##
[0041] Since each infrared detection pixel 12.sub.ij (i, j=1, 2)
and each reference pixel 11.sub.i (i=1, 2) are operated under the
same current and voltage conditions, I.sub.f and V.sub.f are equal.
Therefore, the following equation should be satisfied:
1 G th_IMG { 1 - exp ( - tsel C th_IMG / G th_IMG ) } = 1 G th_TB {
1 - exp ( - tsel C th_TB / G th_TB ) } ( 5 ) ##EQU00004##
[0042] Here, G.sub.th.sub.--.sub.IMG and G.sub.th.sub.--.sub.TB
represent the heat conductance of each infrared detection pixel and
the heat conductance of each reference pixel, respectively,
C.sub.th.sub.--.sub.IMG and C.sub.th.sub.--.sub.TB represent the
heat capacity of the cell of each infrared detection pixel and the
heat capacity of the cell of each reference pixel, respectively,
tsel represents the duration of flowing of current to each pixel,
and the equations (3) and (4) are satisfied. Since the heat
capacities C.sub.th.sub.--.sub.IMG and C.sub.th.sub.--.sub.TB are
proportional to the volumes of the respective cells, a desired heat
capacitance C.sub.th ratio can be achieved by adjusting the area
ratio, as long as the thicknesses are uniform. Even in a case where
the structure is designed to satisfy the equation (5), there might
be deviations from the equation (5) due to low precision in the
manufacturing process or the like. In that case, the same effects
as above can be achieved, as long as one of the left-hand value and
the right-hand value of the equation (5) is 0.9 to 1.1 times larger
than the other one of those values.
[0043] Next, an infrared signal read method is described.
[0044] In this embodiment, one reference pixel is connected to one
signal line of a column of infrared detection pixels, and the
reference pixels exist in a different row from those of the
infrared detection pixels, as shown in FIG. 1. Since the bias
voltage is applied to the row of the reference pixels at a
different time from the time when the bias voltage is applied to
the rows of the infrared detection pixels, signals cannot be
compared at once. Therefore, differential clamp circuits that hold
signals output from the reference pixels and amplify only
differential signals when an infrared detection pixel is selected
are used. Each of those differential clamp circuits includes a
coupling capacitor 22.sub.i (i=1,2), a differential amplifier
20.sub.i (i=1, 2), a feedback switch 23.sub.i (i=1, 2), and a
feedback capacitor 24.sub.i (i=1, 2).
[0045] First, during a first period, the bias voltage Vd is applied
to the row select line 45.sub.0 to which the reference pixels
11.sub.1 and 11.sub.2 are connected. As a result of this, the
potential of each of the vertical signal lines 44.sub.1 and
44.sub.2 becomes Vd-Vf0. This voltage value is 1.0 V, for
example.
[0046] If the feedback switches 23.sub.1 and 23.sub.2 are switched
on (short-circuited) at this point, a voltage follower connection
is formed, and the input voltages V11, V12 and the output voltages
Vo11, Vo12 of the differential amplifiers 20.sub.1 and 20.sub.2
become equal to a DC voltage V2. Here, the DC voltage V2 is a
constant voltage applied to the differential amplifiers of all the
columns, and may be 1.5 V, for example. At this point, the voltage
of each of the coupling capacitors 22.sub.1 and 22.sub.2 on the
side of the vertical signal lines 44.sub.1 and 44.sub.2 is 1.0 V,
and the voltage of each of the coupling capacitors 22.sub.1 and
22.sub.2 on the side of the differential amplifiers 20.sub.1 and
20.sub.2 is 1.5 V.
[0047] When the feedback switches 23.sub.1 and 23.sub.2 are
switched off (opened), the above described potential relationship
is maintained, but the input voltages V11, V12 and the output
voltages Vo11, Vo12 of the differential amplifiers 20.sub.1 and
20.sub.2 are cut off from each other. A current is flowed to the
reference pixels 11.sub.1 and 11.sub.2 in the period from the
switching on of the feedback switches 23.sub.1 and 23.sub.2 until
the switching off of the feedback switches 23.sub.1 and 23.sub.2.
Here, it is essential that the duration of flowing of current
becomes equal to the time tsel in the equation (5).
[0048] During a second period, the bias voltage Vd is applied to
the row select line 45.sub.1 to which the infrared detection pixels
12.sub.11 and 12.sub.12 are connected. As a result of this, the
potential of each of the vertical signal lines 44.sub.1 and
44.sub.2 becomes Vd-(Vf0-Vsig). This voltage value is 1.001 V, for
example. The difference between the potentials at both ends of each
of the coupling capacitors 22.sub.1 and 22.sub.2 is kept. As the
potential on the side of the vertical signal lines 44.sub.1 and
44.sub.2 is higher than that in the first period by 0.001 V, the
potential of V11 (V12) increases to 1.501 V. If the gains of the
differential amplifiers 20.sub.1 and 20.sub.2 are high enough at
this point, the circuits formed with the coupling capacitors
22.sub.1 and 22.sub.2, the differential amplifiers 20.sub.1 and
20.sub.2, and the feedback capacitors 24.sub.1 and 24.sub.2 become
integration circuits. Where Cc represents the capacity value of
each of the coupling capacitors 22.sub.1 and 22.sub.2, and Cfb
represents the capacity value of each of the feedback capacitors
24.sub.1 and 24.sub.2, the following relationship is established:
Vo11=Cc/Cfb.times.Vsig. Where Cc is 5 pF, and Cfg=0.5 pF, for
example, the relationship is expressed as Vo11=10.times.0.001=0.01
(V). As a result, the infrared signal Vsig is amplified
tenfold.
[0049] As described above, this embodiment can provide an uncooled
infrared imaging device that is capable of reducing the influence
of the difference in self-heating temperature between the infrared
detection pixels and the reference pixels.
[0050] Although series-connected diodes are used as the
thermoelectric conversion elements of the infrared detection pixels
and the reference pixels in the above described embodiment, the
same effects as above can be achieved by using series-connected
resistors.
[0051] While certain embodiments have been described, these
embodiments have been presented by way of example only, and are not
intended to limit the scope of the inventions. Indeed, the novel
methods and systems described herein can be embodied in a variety
of other forms; furthermore, various omissions, substitutions and
changes in the form of the methods and systems described herein can
be made without departing from the spirit of the inventions. The
accompanying claims and their equivalents are intended to cover
such forms or modifications as would fall within the scope and
spirit of the inventions.
* * * * *