Thermal infrared detector

Details Thermal infrared detector Thermal infrared detector

2002-11-21

2004-03-23

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Infrared responsive

Reexamination Certificate

active

06710344

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a thermal infrared detector having a thermal isolation structure for minimum temperature resolution.
2. Description of the Related Art
Thermal infrared detectors are available in different types including a bolometer type, a pyroelectric type, and a thermocouple type. These thermal infrared detectors have a thermal isolation structure, i.e., a so-called diaphragm structure, for increased detection sensitivity. In the thermal isolation structure, a diaphragm has an outer edge supported in spaced relation to a substrate by a plurality of beams.
Several thermal isolation structures of conventional thermal infrared detectors will be described below with reference to accompanying drawings.
FIG. 1
of the accompanying drawings schematically shows in plan a pixel of a 320×240 pixel thermistor bolometer-type infrared detector developed by Santa Barbara Research Center (see W. Radford et al., SPIE, Vol. 2746, 1996, page 82). The pixels of the illustrated infrared detector are spaced at a pitch of 48 &mgr;m. Each of the pixels comprises a substrate
101
on which a readout circuit is formed, and a diaphragm (photo-sensitive area)
102
supported on and spaced from the substrate
101
by two beams
103
. The entire chip of the infrared detector, which includes a matrix of those pixels, is encased in an evacuated package. The diaphragm
102
comprises a thin film of thermistor bolometer material and a protective film surrounding the thin film of thermistor bolometer material. Each of the beams
103
comprises an interconnection which electrically connects the readout circuit to the thin film of thermistor bolometer material via a contact
105
, and a protective film surrounding the interconnection.
When an infrared radiation is applied to the diaphragm
102
, the applied infrared radiation is absorbed by the diaphragm
102
, producing heat which increases the temperature of the diaphragm
102
. The rise in the temperature of the diaphragm
102
changes the resistance of the thin film of thermistor bolometer material, and the changed resistance is converted by the readout circuit in the chip into an electric signal, which is converted into an image representative of the detected infrared radiation.
In order to increase the sensitivity of the infrared detector, it is important that the heat absorbed by the diaphragm
102
and transferred to the substrate
101
be minimized, i.e., the thermal conductance of the diaphragm
102
be reduced. In many thermal infrared detectors, the chip is encased in the evacuated package and the beams
103
be thinned to reduce the thermal conductance of the diaphragm
102
.
FIG. 2
of the accompanying drawings schematically shows in plan a pixel of a 320×240 pixel thermistor bolometer-type infrared detector developed by Loral Infrared & Imaging Systems (see C. Marshall et al., SPIE, Vol. 2746, 1996, page 23). The pixels of the illustrated infrared detector are spaced at a pitch of 46.25 &mgr;m. Each of the pixels comprises a substrate
111
, and a diaphragm
112
supported on and spaced from the substrate
111
by two beams
113
, and contacts
115
. The infrared detector shown in
FIG. 2
detects an infrared radiation according to the same principle as the infrared detector shown in FIG.
1
. The infrared detector shown in
FIG. 2
differs from the infrared detector shown in
FIG. 1
in that the beams
113
are bent around an outer peripheral edge of the diaphragm
112
and the beams
113
are longer than the beams
103
shown in FIG.
1
. The diaphragm
112
thus constructed has a small thermal conductance for increased detection sensitivity.
FIGS. 3
a
through
3
e
of the accompanying drawings schematically show in plan a pixel of a thermistor bolometer-type infrared detector developed by National Optics Institute (see H. Jerominek et al., SPIE, Vol. 2746, 1996, page 60). The infrared detector shown in
FIGS. 3
a
through
3
e
has a diaphragm
122
supported on a substrate
121
by beams
123
, and detects an infrared radiation according to the same principle as the infrared detector shown in FIG.
1
.
FIGS. 3
a
through
3
e
show various different thermal isolation structures. Specifically, the thermal isolation structures shown in
FIGS. 3
a
,
3
b
differ from each other with respect to the number of bends of beams
123
. The thermal isolation structure shown in
FIG. 3
b
has a better thermal isolation capability than the thermal isolation structure shown in
FIG. 3
a
, but has a smaller filling factor of the diaphragm
122
, i.e., a smaller occupation ratio of the diaphragm
122
with respect to the pixel. The thermal isolation structure shown in
FIG. 3
c
is designed to increase the thermal isolation capability by changing the manner in which the beams
123
are bent. Each of the thermal isolation structures shown in
FIGS. 3
d
,
3
e
has four beams
123
to support the diaphragm
122
.
FIG. 4
a
of the accompanying drawings schematically show in plan a pixel of a 16×16 pyroelectric infrared detector developed by Toyota Central R&D Labs., Inc., and
FIG. 4
b
is a cross-sectional view taken along line
4
b
—
4
b
of
FIG. 4
b
(see Fujitsuka et al., Journal of Japan Society of Infrared Science and Technology).
In
FIGS. 4
a
and
4
b
, the pixel has sides each 75 &mgr;m long, and includes a cavity
205
defined in an n-type silicon substrate
200
by the bulk micro-machining technology. A diaphragm
201
is supported over the cavity
205
by four beams
202
each having a width of 4 &mgr;m and a length of 59 &mgr;m. The diaphragm
201
comprises a silicon oxide film
203
and an electrode
204
of Ti/TiN. One of the four beams
202
has an interconnection
204
′ of Ti/TiN that is electrically connected to the electrode
204
. Although not shown, a thin film of PVDF (Polyvinylidene Fluoride) as a pyroelectric material is deposited on the electrode
204
of the diaphragm
201
, and a metal film serving as an infrared absorption film and also as an upper electrode is deposited on the thin film of PVDF.
An infrared radiation applied to the diaphragm
201
is absorbed by the diaphragm
201
, producing heat that changes the polarized state of the pyroelectric material and generates a surface charge on the diaphragm
201
. The surface charge is converted by a readout circuit (not shown) on the n-type silicon substrate
200
into an electric signal, which is converted into an image representative of the detected infrared radiation.
While various thermal isolation structures have heretofore been attempted, they have not been optimized to minimize the temperature resolution. Specifically, the temperature resolution can generally be reduced by increasing the length of the beams to provide a better thermal isolation between the diaphragm and the substrate. However, if the pixels are large, then excessively increasing the thermal isolation results in a worse temperature resolution because it increases a thermal time constant that prevents the diaphragm from keeping up with a change in the temperature of the subject being detected.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a thermal infrared detector having beams whose lengths are optimum for minimizing the temperature resolution depending on the size of pixels.
To achieve the above object, a thermal infrared detector according to the present invention has a substrate having a readout circuit and a plurality of pixels patterned on the substrate at a pitch p in the range of 15≦p≦50 (&mgr;m). Each of the pixels has a diaphragm including a thin film of bolometer and spaced from the substrate, two beams by which the diaphragm is supported on the substrate, and interconnections formed respectively on the beams and connecting the readout circuit and the thin film of bolometer to each other. The length of each of the beams is determined by a beam length index which is produced by dividing the length of each of the beams by one-quarter of the peripheral len

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