Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2002-08-01
2004-07-20
Wells, Nikita (Department: 2881)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338100, C250S338400, C250S332000, C250S352000
Reexamination Certificate
active
06765210
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a bolometer-type infrared sensor having a thermal isolation structure. More particularly, the invention relates to a bolometer-type infrared sensor using a resistor with a hysteresis in its thermal characteristic of resistance, and a driving method of the sensor.
2. Description of the Related Art
Conventionally, bolometer-type infrared sensors have typically used bolometer materials without hysteresis in its thermal characteristic of resistance. In recent years, Kawano created an improved bolometer-type infrared sensor having a large temperature coefficient of resistance and a hysteresis in its thermal characteristic of resistance, which is disclosed by the Japanese Non-Examined Patent Publication No. 2000-55737 published in February 2000. This sensor is explained below with reference to
FIGS. 1A and 1B
, and
FIGS. 2
to
4
.
FIGS. 1A and 1B
show the structure of one pixel of the prior-art infrared sensor array, which is termed an infrared sensor below.
As shown in
FIG. 1B
, the sensor has a diaphragm
110
for sensing infrared rays. The diaphragm
110
comprises a thin bolometer film
105
, a dielectric supporting film
103
, a dielectric protecting film
106
, and an infrared absorbing film
107
. The supporting film
103
, which is located on the inner side of the bolometer film
105
, supports the film
105
. The film
106
, which is located to cover the bolometer film
105
on the outer side thereof, is used to protect the film
105
. The infrared absorbing film
107
is used to absorb infrared rays irradiated to the diaphragm
110
.
The diaphragm
110
further comprises electrodes
104
and
104
′ at each end (the lower and upper ends in
FIGS. 1A and 1B
) of the bolometer film
105
. The electrode
104
is connected to a wiring line
114
. The electrode
104
′ is connected to a wiring line
114
′. on operation, a pulsed bias voltage is applied across the electrodes
104
and
104
′ by way of the wiring lines
114
and
114
′. Due to infrared rays
111
applied, the temperature of the bolometer film
105
changes and thus, the bolometer film
105
generates electrical resistance change. As a result, by reading out the electrical resistance change of the film
105
, irradiation of the infrared rays
111
is detected through the change of voltage or current caused by the pulsed bias voltage.
The diaphragm
110
in held on two banks
116
and
116
′ of a substrate
102
by way of two beams
112
and
112
′, thereby forming a suspended structure. This suspended structure is to constitute a thermal isolation structure of the diaphragm
110
(i.e., the bolometer film
105
) from the substrate
102
.
A reflector film
101
is formed on the surface of the substrate
102
sandwiched by the banks
116
and
116
′. A cavity or space
109
is formed between the diaphragm
110
and the reflector film
101
. The distance between the film
101
and the diaphragm
110
is well adjusted in such a way that almost all the infrared rays
111
are absorbed by the infrared absorbing film
107
. Due to absorption of the rays
111
, the temperature of the diaphragm
110
rises and thus, the electrical resistance of the bolometer film
105
changes.
The banks
116
and
116
′ constitute the sidewalls of the cavity
109
. The diaphragm
110
is thermally isolated from the banks
116
and
116
′ by a slit
108
.
The reference numerals
113
and
113
′ denote the roots of the beams
112
and
112
′, respectively. The reference numerals
115
and
115
′ denote the contacts with the wiring lines
114
and
114
′, respectively.
FIG. 2
shows the relationship between the specific resistance &sgr; and the temperature T of the bolometer film
105
used in the prior-art infrared array sensor of
FIGS. 1A and 1B
. A pulsed bias voltage or current is periodically applied to the bolometer film
105
, thereby repeating the temperature cycle shown in FIG.
3
. In
FIG. 3
, t
f
is the frame time and t
ro
is the read-out time. The pulsed bias voltage or current is applied during the read-out time t
ro
. The application timing of the pulsed bias voltage or current is not shown in FIG.
3
. The temperature of the bolometer film
105
is gradually risen or dropped to draw the temperature cycle of FIG.
2
. In this temperature cycle, the maximum variation range of temperature is &Dgr;Tc, which is greater than the hysteresis range &Dgr;Tt of temperature (i.e., &Dgr;Tc >&Dgr;Tt) The maximum variation range &Dgr;Tc is set by adjusting the value of the pulse width t
ro
or voltage in such a way as to be greater than (&Dgr;Tt+&Dgr;Tmax), where &Dgr;Tmax is the maximum temperature change of the temperature sensing section of the bolometer film
105
caused by the possible change of the infrared rays
111
.
Here, when the quantity of the irradiated infrared rays
111
is equal to the reference value, the state of the bolometer film
105
is situated at the point A (temperature: T
obj
) on the temperature falling curve
150
in FIG.
2
. Then, the state of the bolometer film
105
is gradually changed to go along the given temperature cycle. First, the pulsed bias voltage is applied to the film
105
to start raising its temperature. Then, the temperature of the film
105
rises without changing its physicochemical structure and as a result, the specific resistance curve (A→B) intersects with the temperature rising curve
151
at the point B (temperature: T
B
). Since &Dgr;T is greater than &Dgr;Tt, the temperature of the film
105
rises furthermore. When the temperature of the film
105
becomes higher than the temperature. T
B
, the temperature of the film
105
rises with changing its physicochemical structure and as a result, the state of the film
105
reaches the point C (temperature: T
c
=T
obj
+&Dgr;Tc).
Subsequently, when the application of the pulsed bias voltage is stopped and the temperature of the film
105
begins to drop, the temperature of the film
105
drops without changing its physicochemical structure and as a result, the specific resistance curve (C→D) intersects with the temperature falling curve
150
at the point D (temperature: T
D
) Thereafter, the temperature of the film
105
drops with changing its physicochemical structure from the temperature T
D
to the starting temperature T
obj
.
If the quantity of the infrared rays
111
from the object is decreased, the temperature of the bolometer film
105
drops by &Dgr;T
obj
with the temperature cycle in question. Therefore, the temperature cycle curves
150
and
151
are laterally shifted to the lower side (to the left side in
FIG. 2
) by &Dgr;T
obj
and as a result, the point A is shifted to the point A′. The point A′ in
FIG. 2
denotes the starting point of the next temperature cycle.
In this way, by detecting the temperature shift &Dgr;T
obj
, the quantity change of the infrared rays
111
can be known while keeping the temperature coefficient of resistance (TCR) high.
In
FIG. 2
, the starting point of the temperature cycle is placed on the point A, which is located on the temperature falling curve
150
. However, the same result as described above is obtainable if the starting point is placed on a point that is not located on the hysteresis curve
150
and
151
.
FIG. 4
shows the relationship between the specific resistance
6
and the temperature T of the bolometer film
105
, where the starting point is placed on the point C that is shifted to the higher temperature side from the temperature rising curve
151
. The temperature of the bolometer film
105
is dropped and risen to draw the temperature cycle of
FIG. 4
In this temperature cycle, the maximum variation range of temperature is T
1
to T
2
, which is located within the hysteresis range of T
D
to T
u
. The pulsed bias condition (i.e., the voltage value and the pulse width) is set in such a way that &Dgr;Tc is greater than &Dgr;Tt (i.e., &Dgr;Tc >&D
Kalivoda Christopher M.
Wells Nikita
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