Radiant energy – Invisible radiant energy responsive electric signalling – Infrared responsive
Reexamination Certificate
2000-06-14
2001-04-03
Hannaher, Constantine (Department: 2878)
Radiant energy
Invisible radiant energy responsive electric signalling
Infrared responsive
C250S332000, C250S338400
Reexamination Certificate
active
06211520
ABSTRACT:
TECHNICAL FIELD
The present invention relates particularly to a signal reading circuit of an infrared sensor for sensing infrared rays. Also, it relates to an infrared sensor array having these sensors arranged in a planar array.
BACKGROUND OF THE INVENTION
In an infrared sensor, a sensing element (sensing section) absorbs infrared rays emitted from an object to raise the temperature of itself, and temperature information of the object is obtained by detecting the change in characteristics of the sensing element according to the temperature rise.
For describing an example of a conventional bolometer-type infrared sensor, a schematic perspective view of a sensing element section constituting the sensor is shown in FIG.
10
. The bolometer-type infrared sensor is a kind of a heat-type infrared sensor that senses the temperature information of an object by converting the resistance change caused by the temperature rise of a resistor constituting the sensor, into electric current change, voltage change, or the like. In the Figure, the numeral
101
represents a resistor made of a material having a large rate of resistance change with temperature. A heat insulating structure
104
(void section) is formed on a silicon substrate
103
by micromachining to effectively raise the temperature of the resistor
101
and to maintain the raised temperature of the sensing element. The numeral
102
represents an electrode wiring for taking signals out from the sensing element to the outside.
As a heat-type infrared sensor, those utilizing the change in forward voltage of a semiconductor junction diode are also proposed.
FIG. 11
shows a circuit diagram for describing an infrared sensor of junction diode type. The numeral
105
represents junction diodes (here, four diodes are connected), and a constant electric current flows through these diodes
105
from a constant electric current source
106
. When infrared rays (IR) from an object are radiated onto the diodes
105
, the forward voltage of the diodes change. Accordingly, temperature information of the object can be obtained by sensing the amount of change thereof by a signal output line
107
. Since the amount of change in the forward voltage is smaller than that of a bolometer type that uses a material having a large resistance-changing rate, the junction-type sensors are inferior in sensitivity. However, the junction type sensor has an advantage in that it can be fabricated together with the signal reading circuit by using silicon IC processing. Also, the junction type sensor has an advantage that it has little instability of characteristics and little non-uniformity within a wafer surface; the bolometer type tends to have these problems. Also, the problem of low sensitivity of the junction type can be improved by connecting a plurality of diodes.
An infrared sensor array obtained by two-dimensionally arranging a plurality of the above-described heat-type infrared sensors is utilized as a solid image sensor for a dark field observation camera and others. The efficiency of an infrared sensor array is represented by a noise equivalent temperature difference (hereafter referred to as NETD). This is a ratio of the noise in signals between the sensors to the temperature sensitivity. For an improvement of NETD, the control of the noise is important as well as the enlargement of the temperature sensitivity.
The noise is generated in the sensor (or in the sensing element that constitute the sensor) and the reading circuit. By limiting an unnecessary signal band, the noise can be effectively controlled. A representative method of limiting the signal band is an integration circuit.
FIG. 12
represents an example of the method in which a gate modulation circuit (a kind of integration circuits) is used for a quantum-type infrared sensor. In the Figure, the numeral
111
represents a quantum-type sensing element. When the infrared light enters, the carriers generated in the sensing element
111
are transmitted through a load resistance
112
to change the voltage of a node
113
. An electric capacitor
114
has been charged to a certain voltage in advance by a reset switch
115
. The change at the node
113
gives rise to change in the electric current of a MOSFET
117
, thereby altering the amount of electric current that is discharged from the electric capacitor
114
. Accordingly, the electric current value of the signal output line
116
after a predetermined discharging time depends on the amount of carriers generated in the sensing element
111
, i.e. the amount of the incident light. The amount of change in the electric current value of the signal output line
116
is determined by the amount of voltage change in the node
113
, the magnitude of the electric capacitor
114
, the period, of discharging time, and the mutual conductance of the MOSFET
117
. The period of time of electric discharge determines the signal band. The longer the discharge period, the more the band is limited to reduce the noise.
As described above, in a quantum-type sensor, the noise can be suppressed by using a of gate modulation circuit. However, if such a technique is applied to a heat-type sensor, as the output variation greatly varies when the temperature of the sensor changes, a problem rises that a small temperature change caused by the incident infrared rays, which is an aim of the sensing, cannot be read out.
The present invention has been made in order to solve the above-mentioned problems, and to provide a heat-type infrared sensor in which the noise controlling technique used in the quantum-type sensor can be applied. In other words, it is an object of the present invention to provide an infrared sensor wherein the output variations caused by the temperature change in the sensor can be suppressed by sensing the temperature of the whole sensor. It is another object of the present invention to provide an infrared sensor array in which such infrared sensors are arranged in a one-dimensional or two-dimensional array.
DISCLOSURE OF THE INVENTION
According to the first aspect of the present invention, an infrared sensor comprises: a first heat-type infrared sensing element section formed via a dielectric layer on a silicon substrate and having a void formed thereunder, an output voltage thereof depending on an amount of incident infrared rays; a MOSFET of which gate voltage is the output voltage of said first heat-type infrared sensing element section; a second temperature sensing element section formed via a dielectric film on said silicon substrate, an output voltage thereof being utilized as a source voltage of said MOSFET; and an electric capacitor section connected to said MOSFET. The second temperature sensing element section has no heat insulating structure, and its output is related to a temperature variation of the sensor itself. Accordingly, by using the output signal of the second temperature sensing element, it is possible to correct the output of the first heat-type infrared sensing element section which is a true sensing section of infrared rays. Thus, a high-efficient sensor with controlled noise can be obtained.
Also, in the aforementioned infrared sensor, the first heat-type infrared sensing element section preferably comprises a first diode group including a plurality of junction diodes connected with each other, and the second temperature sensing element section preferably comprises a second diode group including a plurality of junction diodes connected with each other. This makes it possible to fabricate the sensor together with a readout circuit of the sensor by means of a silicon IC process.
The number of the diodes in the first diode group is preferably different from the number of the diodes in the second diode group, so that a MOSFET of enhancement type can be used, thereby improving the degree of freedom in designing.
Alternatively, in the aforementioned infrared sensor of the first aspect of the invention, the first heat-type infrared sensing element section may comprise a first resistor, and the second temperature sensing element
Ishikawa Tomohiro
Ueno Masashi
Hannaher Constantine
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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