Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
1999-08-13
2002-08-20
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S338400, C250S338100, C250S332000
Reexamination Certificate
active
06437331
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a bolometer type infrared sensor, and more particularly to a bolometer type infrared sensor in which a bolometer material having hysteresis in resistance—temperature characteristic is used.
2. Description of the Related Art
A bolometer type infrared sensor is a sensor in which infrared rays are irradiated to a bolometer material that is separated in heat from a substrate and in which the infrared rays are detected based on a resistance change which is caused by a temperature change.
As the bolometer material of such a bolometer type infrared sensor, it is preferable for the bolometer material to have a high temperature coefficient of resistance (TCR) %/K which is a resistance change percentage for every degree of temperature change. Materials having such a characteristic are reported in metal, metallic oxide, and a semiconductor. For example, in Japanese Laid Open Patent Application (JP-A-Heisei 5-206526) and U.S. Pat. Nos. 5,288,649 and 5,367,169, the technique is disclosed in which amorphous silicon (a-Si) doped into an n-type or a p-type semiconductor is used as the bolometer material. Also, vanadium oxide or a bolometer material in which vanadium oxide is used as a base bolometer material are often used for the bolometer. Those characteristics are reported in reference (Solar Energy Materials 14, 205 (1986)) and the reference (Physical Review B 22, 2626 (1980)).
The bolometer material of an a-Si system has a relatively high value of TCR of 3%/K but has a higher resistivity than 1000 &OHgr;cm. When the resistivity is high, a large Johnson noise is generated when the resistance value of the diaphragm is read out. As a result, substantial sensitivity to the infrared rays is not increased so much. On the contrary, when the resistivity is very small, the influence of the wiring resistance appears and the high sensitivity is not attained. Therefore, the resistivity is desirable in a range of 0.01 to 1 &OHgr;cm.
On the other hand, the resistivity is as relatively low as 0.1 &OHgr;cm in the bolometer material composed of vanadium oxide or containing it as the base bolometer material and a sensor having a TCR value of about 2%/K has been obtained. However, to aim at the sensor having higher sensitivity, it is necessary for the sensor to have a larger TCR value. To attain such a larger TCR value by use of vanadium oxide, a way of using the phase transition of vanadium oxide could be considered. The resistance change of equal to or more than 2 digits is generally observed before and after the phase transition of vanadium oxide. Also, by doping various metal elements in vanadium oxide, the transition temperature can be controlled to a suitable temperature. For these reasons, a high sensitive infrared ray detection characteristic can be expected if vanadium oxide is used as the material of the bolometer.
However, it is known that the resistivity of vanadium oxide has a hysteresis to the cycle of the temperature change. Conventionally, the bolometer material having a hysteresis in the resistivity temperature characteristic could not be used for the infrared ray sensing unit for the following reasons.
FIG. 5
is a diagram illustrating a hysteresis curve of a bolometer material having a hysteresis in the resistivity temperature characteristic. In
FIG. 5
, it is supposed that a stable phase on a lower temperature side is a first phase and a stable phase on the higher temperature side is a second phase. In the following description, a point in the figure, i.e., a set of a bolometer temperature and a bolometer resistance corresponding to the bolometer temperature is shown as a state of the bolometer
A resistivity &rgr; of the bolometer material in
FIG. 5
changes as follows with the temperature change. A logarithm of the resistivity is indicated in FIG.
5
.
First, a bolometer temperature is increased from the state a of the first phase. At this time, the resistivity changes gently until the bolometer temperature reaches a temperature corresponding to a critical state b. When the bolometer temperature is increased beyond the critical state b, the phase transition begins so that the resistivity decreases rapidly. Then, when the bolometer temperature reaches a temperature corresponding to a state c, the rapid change of the resistivity happens no longer, even if the bolometer temperature is increased. The resistivity changes gently again.
That is, the bolometer material undergoes the phase transition from the first phase to the second phase while the bolometer state changes from the state b to the states c. The phase transition is completed at the critical state c so that the bolometer state changes to the second phase which is the stable phase on the higher temperature side. The curve bc is referred to as a temperature increasing curve in the following description.
On the other hand, the bolometer temperature of the bolometer material is decreased from the state f in the second phase. At this time, even if the bolometer temperature reaches the temperature corresponding to the state c, the phase transition does not happen. Therefore, the bolometer material maintains a second phase. In this temperature region, the resistivity of the bolometer material changes gently. When the bolometer temperature is further decreased so that the bolometer temperature reaches a temperature corresponding to the state d, the phase transition begins so that the resistivity increases rapidly. The rapid increase of the resistivity continues to the temperature corresponding to state e. Then, when the bolometer temperature is further decreased to pass through the state e, the resistivity of the bolometer material changes gently in this temperature region.
The curve de is referred to as a temperature decreasing curve in the following description. The bolometer material performs the phase transition from the second phase to the first phase in the temperature region corresponding to the temperature decreasing curve.
The point to which an attention should be paid when the bolometer material is used is that the phase transitions shown by the temperature increasing curve and the temperature decreasing curve are a non-reversible process.
Now, it is supposed that the state of the bolometer material is changed from the state a of the first phase via the critical state b to one point p
1
on the temperature increasing curve, and then the temperature is decreased. In this case, the state of the bolometer material does not change along the temperature increasing curve from state p
1
to the critical state b in an opposite direction. The state of the bolometer material changes from state p
1
to the lower temperature side in approximately parallel to curve a-b. That is, the state of the bolometer material changes from the point p
1
to the left side in FIG.
5
.
Then, while the state of the bolometer material reaches the state shown by the point p in
FIG. 5
via the above process, the resistivity gently changes along the curve p-p
1
in
FIG. 5
, if the temperature of the bolometer material is increased again. Thereafter, when the state of the bolometer material reaches the point p
1
on the temperature increasing curve, the resistivity begins to change rapidly along the temperature increasing curve.
The similar phenomenon occurs in the bolometer material when the temperature is decreased from the state f in the second phase.
Now, it is supposed that the state of the bolometer material changes from the state f in the second phase to a state p
2
on the temperature decreasing curve via the critical state d and then the temperature is increased again. In this case, the state of the bolometer material does not change from the state p
2
to the critical state d on the temperature decreasing curve in the opposite direction. The state of the bolometer material changes from the state p
2
to a point on the higher temperature side in approximately parallel to curve d-f. That is, the state of the bolometer changes from the point p
2
to the right side
Hannaher Constantine
Israel Andrew
NEC Corporation
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