JP4071122B2 - Thermal infrared solid-state image sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thermal infrared solid-state imaging device that detects a temperature change caused by incident infrared rays using a two-dimensionally arranged semiconductor sensor, and more particularly, a thermal type that outputs an electrical signal from a semiconductor sensor after integration processing by a signal processing circuit. The present invention relates to an infrared solid-state imaging device.
[0002]
[Prior art]
In a general thermal infrared solid-state imaging device, pixels having a heat insulating structure are two-dimensionally arranged, and an infrared image is captured by utilizing the fact that the temperature of the pixel changes due to incident infrared rays. In the case of an uncooled thermal infrared solid-state image sensor, the temperature sensor that constitutes the pixel uses a semiconductor element such as a diode or transistor in addition to a bolometer such as polysilicon, amorphous silicon, silicon carbide, or vanadium oxide. It has been known. A semiconductor element such as a diode is advantageous in making the characteristics of each pixel uniform because variations in electrical characteristics and temperature dependence are very small between solids.
[0003]
In a thermal infrared solid-state imaging device using a diode as a pixel, the pixels are two-dimensionally arranged, connected by a drive line for each row, and connected by a signal line for each column. Each drive line is selected in turn by the vertical scanning circuit and the switch, and the pixel is energized from the power source through the selected drive line. The output of the pixel is transmitted to the integration circuit via the signal line, integrated and amplified by the integration circuit, and sequentially output to the output terminal by the horizontal scanning circuit and the switch (for example, see Non-Patent Document 1).
[0004]
[Non-Patent Document 1]
Ishikawa et al., “Low cost 320 × 240 uncooled IRFPA using conventional silicon IC process”, Part of the SPIE Conference on infrared Technology and Applications XXV, April 1999, Vol. 3698, pages 556 to 564 Page [0005]
[Problems to be solved]
However, the conventional thermal infrared solid-state imaging device has the following problems.
First, since the response of the pixel includes a response due to a change in element temperature in addition to the response of infrared light, if the element temperature control is not performed precisely, a phenomenon that the image signal drifts with the change in the element temperature appears and a stable image There is a problem that cannot be obtained. In other words, it is ideal to completely insulate the pixel and detect only a temperature change due to infrared rays, but in reality, the pixel temperature also changes to some extent with the temperature change of the substrate. For this reason, when the environmental temperature of the infrared imaging device changes during the detection operation, the voltage read from the pixel also changes due to the influence. The change in the output signal due to the change in the environmental temperature is indistinguishable from the change due to the incident infrared ray, so that there is a problem that the measurement accuracy of the infrared ray is lowered and stable image acquisition cannot be performed.
[0006]
In addition, since the final output signal of the element is directly affected by the voltage fluctuation of the power source that drives the pixel, the output signal tends to be unstable. For example, when the pixel is driven with a constant current and the change in the voltage across the pixel is read as a signal to the integration circuit, the voltage read out also changes as the voltage of the power source driving the pixel changes. The magnitude of the output signal will also vary.
[0007]
On the other hand, in order to prevent such temperature drift and power supply drift, various measures such as changing the circuit configuration of the power supply, improving the pixel structure, and adding a correction circuit to the signal readout circuit can be considered. However, when the circuit configuration and the pixel structure become complicated, it is difficult to reduce the size and increase the integration of the thermal infrared solid-state imaging device, and the manufacturing cost also increases.
[0008]
The present invention has been made in view of the above-mentioned problems, and has a simple circuit configuration, has little output fluctuation due to power supply voltage fluctuation and element temperature fluctuation, and is a thermal infrared solid-state imaging device capable of stable image acquisition. It is also intended to provide.
[0009]
[Means for Solving the Problems]
A pixel area having one diode or a plurality of diodes connected in series and having a heat insulation structure and an infrared absorption structure is arranged in a two-dimensional manner, and an anode of the pixel is commonly connected for each row. A plurality of drive lines and cathodes of the pixels are commonly connected for each column, and a plurality of signal lines including a first constant current means at the end of each column and the drive lines are sequentially selected and selected. A vertical scanning circuit for connecting a pixel power supply to a drive line, and a terminal of the signal line provided with the first constant current means is connected to a gate connected to a gate and a drain of the MOS transistor and periodically The gate modulation integration circuit formed from the capacitor to be reset, the sample hold circuit connected to the output of the gate modulation integration circuit, and the output of the sample hold circuit are sequentially selected and removed. A thermal-type infrared solid-state imaging device and a horizontal driving circuit for outputting a,
The MOS transistor in the gate modulation integration circuit has a source connected in common between columns by a bias line, and a reference signal output capable of outputting a reference signal that changes according to the temperature of the entire element to the bias line The circuit is connected,
A low-pass filter is provided between the reference signal output circuit and the bias line .
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
Embodiment 1 FIG.
FIG. 1 is a circuit diagram showing a circuit configuration of a thermal infrared solid-state imaging device according to Embodiment 1 of the present invention. Pixels 1 each having a plurality of diodes connected in series and having an infrared absorption structure and a heat insulation structure are two-dimensionally arranged to form a pixel area. A drive line 3 is commonly connected to each row of the pixels 1. A signal line 12 is commonly connected to each column of the pixels 1, and a constant current source 2 (= first constant current unit) is connected to the terminal of each signal line 12. A voltage is sequentially applied to the drive lines 3 of each row by the vertical scanning circuit 4 and the switch 5, and the pixels 1 are sequentially driven for each row. While the pixels 1 for one row are being driven, the pixels in each column are sequentially selected by the horizontal driving circuit 8 and the switch 9, and the output of the pixel 1 is passed through the integrating circuit 7 provided for each column. Are sequentially read out to the output terminal 10.
[0011]
Energization of the pixel 1 is performed by the power source 6 through the selected drive line 3. Since the constant current source 2 is provided at the end of the signal line 12 connected to the cathode side of the pixel 1, the pixel 1 is driven with constant current. The voltage across the constant current power source 2 is a value obtained by subtracting the voltage across the pixel 1 from the voltage across the power source 6 and the voltage drop in the wiring. Accordingly, the voltage at both ends of the constant current source 2 is integrated and amplified by the integrating circuit 7 and read out as an output to detect a change in the voltage at both ends of the pixel 1, that is, a change in the amount of infrared light incident on the pixel 1. it can.
[0012]
FIG. 2 shows a circuit configuration of the integrating circuit 7. The MOS transistor 13 is an integrating transistor and has a gate serving as an input terminal. The integration capacitor 14 is connected to the transistor 15 and is periodically reset by the transistor 15. A bias voltage that determines the operating point of the integration transistor 13 is applied from the bias line 11. A current corresponding to the voltage across the constant current source 2 input to the gate flows through the integration transistor 13, and the integration capacitor 14 is charged by the current for an integration time determined by the transistor 15. That is, the MOS transistor 13 and the integration capacitor 14 constitute a gate modulation integration circuit. The output of this integration circuit is sampled by a sample hold (S / H) circuit 16 and output through a buffer amplifier 17.
[0013]
However, when the voltage across the constant current source 2 is simply integrated and amplified by the integration circuit 7 shown in FIG. 2, a phenomenon in which the image signal drifts with changes in the element temperature and fluctuations in the power supply voltage appears, and stable image acquisition is possible. Disappear. This is because the change in the voltage across the pixel 1 includes not only the change due to the change in the amount of incident infrared rays but also the change due to the temperature change of the entire element and the change due to the voltage fluctuation of the power source 6. This is because the drift component is also integrated and amplified simultaneously.
[0014]
Therefore, in the present embodiment, the sources of the integrating transistors 13 constituting the gate modulation integrating circuit 7 are commonly connected by the bias line 11, and the reference signal output circuit 18 is connected to the bias line 11 via the bias power supply circuit 21. is doing.
[0015]
The reference signal output circuit 18 includes a reference pixel 19 and a constant current source 20 (= second constant current means) connected in series on the cathode side thereof, and outputs a reference signal that changes in accordance with the temperature of the entire element. It is configured to be able to. The reference pixel 19 has substantially the same structure as the pixel 1 except that it does not have at least one of a heat insulating structure and an infrared absorption structure, and can detect only a change in element temperature. The constant current source 20 is configured such that a current substantially equal to that of the constant current source 2 flows. With this reference signal output circuit, it is possible to output a reference signal for calibrating the drift due to temperature variation of the entire element. That is, the voltage fluctuation that appears at the connection point between the pixel and the constant current source 2 due to the temperature fluctuation of the entire element is substantially the same as the voltage fluctuation at the connection point between the reference pixel 19 and the constant current source 20.
[0016]
The bias power supply circuit 21 includes a level shift circuit, a low-pass filter, and a buffer amplifier. The bias power supply circuit 21 removes the noise component of the reference pixel 19 from the output of the reference signal output circuit 18 and appropriately biases the integration transistor 13. The level of the output voltage of the reference signal output circuit 18 is adjusted so as to be a voltage.
[0017]
In this way, the bias line 11 connected to the source of the integration transistor 13 has a voltage level that serves as a reference signal for calibrating the drift due to the temperature variation of the entire element and at which the operating point of the integration transistor 13 is appropriate. The bias that has it is input. Accordingly, the drift of the voltage across the pixel due to the element temperature variation is input to both the gate and the source of the integration transistor 13, and the drift due to the variation of the element temperature is removed from the current flowing through the integration transistor 13. Can do.
[0018]
In the present embodiment, the reference signal output circuit 18 is driven by the power source 6 that drives the pixel 1. That is, the anode of the reference pixel 19 in the reference signal output circuit 18 is connected to the power source 6 that drives the pixel 1. For this reason, the input to the bias line 11 also serves as a reference signal for calibrating the drift due to fluctuations in the power supply voltage. Accordingly, the drift due to the voltage fluctuation of the power supply 6 is also input to both the gate and the source of the integration transistor 13, and the drift due to the fluctuation of the power supply voltage can be removed from the current flowing through the integration transistor 13. .
[0019]
As described above, according to the thermal infrared solid-state imaging device according to the present embodiment, the bias line 11 of the integration circuit 7 is commonly connected, and the reference signal output circuit 18 and the bias power supply circuit 21 are provided. With the circuit configuration, it is possible to remove a drift component due to variations in element temperature and power supply voltage, and to perform stable image acquisition.
[0020]
Hereinafter, each component of the thermal infrared imaging device according to the present embodiment will be described in detail.
[Pixel 1]
3A and 3B are a cross-sectional view and a perspective view schematically showing an example of the pixel structure. A PN junction diode 902 serving as a temperature sensor is supported on a hollow portion 1103 provided on a silicon substrate 1102 by two long support legs 1101, and an electrode wiring 1104 of the diode 902 is embedded in the support legs 1101. Yes. The hollow portion 1103 increases the thermal resistance between the diode 902 and the silicon substrate 1102 to form a heat insulating structure.
[0021]
Since the diode 902 needs to be a layer independent of the substrate 1102, it is formed on the SOI layer using the SOI substrate, and the buried oxide film under the SOI layer is part of the structure supporting the hollow structure. It has become. In addition, the infrared absorption structure 1106 that is in thermal contact with the diode portion has a structure that protrudes above the support leg 1101 so that infrared rays incident from above can be efficiently absorbed. In FIG. 3B, in order to make the structure of the lower part easy to understand, the infrared absorption structure in the front part of the drawing is omitted.
[0022]
When infrared rays are incident on the pixel 1, the whole pixel 1 including the infrared absorption structure 1106 is absorbed, and heat generated by the absorption is transmitted to the PN junction diode 902. Since the current-voltage characteristics of the PN junction diode 902 change depending on the temperature, an output corresponding to the amount of infrared absorption can be obtained by driving the PN junction diode at a constant current and monitoring the voltage at both ends. The thermal infrared solid-state imaging device has a structure in which a large number of pixels 1 are two-dimensionally arranged and accessed in order.
[0023]
In such an element, characteristic uniformity between pixels is important. The forward voltage of the diode and its temperature dependence have very little variation between solids, and using a diode as a temperature sensor for a thermal infrared imaging device is particularly effective for achieving uniform characteristics. A plurality of PN junction diodes are preferably connected in series in order to increase sensitivity. The sensitivity may be increased by connecting an appropriate temperature-sensitive resistor such as a bolometer film to the PN junction diode.
[0024]
[Reference signal output circuit]
As shown in FIG. 1, the reference signal output circuit 18 has a configuration in which a reference pixel 19 provided outside a pixel area and a constant current source 20 for supplying a constant current thereto are connected in series. The difference between both ends is output. The reference pixel 19 is preferably formed on the same substrate as the pixel area outside the pixel area where infrared rays are incident. The reference pixel 19 has a structure that excludes one or both of the infrared absorption structure and the heat insulation structure so that only the temperature change of the element due to the environment or the like can be detected regardless of the incident infrared ray. In order to form a pixel that does not have a heat insulating structure, for example, in the detector structure shown in FIGS. 3A and 3B, the hollow portion 1103 is not formed in the substrate 1102 or the length of the support leg 1101 is formed. Is sufficiently shorter than that of the pixel 1. In order to form a pixel that does not have an infrared absorption structure, for example, the formation of the infrared absorption structure 1106 may be omitted in the pixel 1 in FIGS. The current of the constant current source 20 is preferably substantially equal to that of the constant current source 2 for the pixel 1. The reference signal output circuit 18 can output a reference signal for calibrating the temperature drift of the element that does not depend on infrared absorption.
[0025]
[Bias power supply circuit]
The output of the reference signal output circuit 18 is input to the bias line 11 via the bias power supply circuit 21. FIG. 4 shows a configuration example of the bias power supply circuit 21. In the example of FIG. 4, the bias power supply circuit 21 includes a level shift circuit 27, a first low-pass filter 28a, a buffer amplifier 29, and a second low-pass filter 28b. The output of the reference signal output circuit 18 is converted to an appropriate voltage level by the level shift circuit 27 by the buffer power supply circuit 21, and then noise is removed by the low-pass filter 28a, and the current drive capability is increased by the buffer 29. The noise is again removed by the low-pass filter 28 b and then applied to the bias line 11 .
[0026]
The level shift circuit 27 is for converting the output voltage of the reference signal output circuit 18 to an appropriate voltage level as the bias voltage of the integration transistor 13. An example of the level shift circuit 27 is shown in FIG. FIG. 5 shows a so-called source follower amplifier. In FIG. 5, the gate of the driver transistor 32 is an input and the source is an output. The gate-source voltage of the driver transistor 32, that is, the level shift voltage between the input and output can be determined by the current value of the current source 33.
[0027]
The two low-pass filters 28a and 28b are for cutting noise of the reference pixel 19 and the constant current source 20 and extracting only the temperature drift component. That is, the low-pass filter plays a role of preventing an increase in noise due to the provision of the reference signal output circuit 18 . In general, in an infrared detector aiming at high S / N, the noise of the power supply system is sufficiently reduced by the power supply circuit, and the noise from the detection unit becomes the main noise component of the apparatus. Accordingly, when the outputs of the pixel 1 and the reference pixel 19 having the same configuration are input to the integration circuit 7, the noise is uncorrelated, and the noise becomes √2 times the conventional value. On the other hand, changes in the output of the detection unit due to changes in the environmental temperature and changes in the power supply voltage due to fluctuations in the power supply circuit characteristics due to changes in the environmental temperature are generally gradual over the order of seconds. Therefore, the band of the bias voltage output circuit for monitoring it may be sufficiently narrower than the band necessary for the signal line for detecting infrared rays. Therefore, if low-pass filters 28a and 28b are inserted between the output of the reference signal output circuit 18 and the input terminal of the integrating transistor 13 so as to pass only voltage fluctuations due to environmental temperature, power supply circuit drift, etc., the reference is made. An increase in noise due to the signal output circuit 18 can be suppressed.
[0028]
Note that a typical value of the noise bandwidth for the pixel of such an infrared solid-state imaging device is several KHz, and therefore, the cut-off frequency may be set to 1/100 or less. From the viewpoint of power supply voltage fluctuations and temperature fluctuations, the fluctuation period is fast and on the order of seconds, so a band of several Hz is sufficient. In this embodiment, the low-pass filter is inserted before and after the buffer 29, but only one of them may be used.
[0029]
An example of the circuit configuration of the low-pass filters 28a and 28b is shown in FIGS. 6 (a) and 6 (b). The configuration shown below can be used for any low-pass filter before or after the buffer amplifier 29. The low-pass filter shown in FIG. 6A uses a passive element and includes a resistor or reactance 30 and a capacitor 31. As the low-pass filter 28b inserted on the rear side of the buffer amplifier 29, a reactance having no DC voltage drop is desirable. On the other hand, as the low-pass filter 28a provided on the front side of the buffer amplifier 29, it is desirable to use a resistor that can easily obtain characteristics as a filter. The resistor 30 may be replaced with the internal resistance of the power supply circuit 6 or the internal resistance of the buffer amplifier 29.
[0030]
The low-pass filter in FIG. 6B is an integrating circuit using an operational amplifier 29 which is an active element, and since this circuit configuration is also common as a low-pass filter, detailed description thereof is omitted.
[0031]
The low-pass filters 28a and 28b in the present invention are not limited to those illustrated in FIGS. 6A and 6B, and other filters (for example, switched capacitor circuits) may be used. In addition, the low-pass filter may be provided only in one of the front and rear sides of the buffer amplifier 29. In this case, it is preferable to leave the filter (= filter 28a) on the front side of the buffer amplifier 29. This is because a large current flows behind the buffer amplifier 29, and a voltage drop at the filter causes a variation in the bias voltage.
[0032]
Embodiment 2. FIG.
In the present embodiment, a thermal infrared solid-state imaging device is configured in the same manner as in the first embodiment except that the configuration shown in FIG. 7 is used as the reference signal output circuit 18. In FIG. 7, a plurality of reference pixels 19 are arranged in parallel, and a constant current source 22 in which a current corresponding to that number is connected in series. That is, when N reference pixels 19 are arranged in parallel, a current N times that of the constant current source 2 is caused to flow by the constant current source 22. Since the temperature of the thermal infrared solid-state imaging device may have a distribution within an element region having a finite size, if a plurality of reference pixels 19 are appropriately distributed and installed within the element region, the temperature is reflected on the average temperature. Output can be taken out.
[0033]
Embodiment 3 FIG.
The same object as that of the second embodiment can also be realized by using the configuration shown in FIG. 8 as the reference signal output circuit 18. In FIG. 8, the reference pixel 19 and the constant current source 20 are connected in series as one set, and the combined structure is distributed and installed in the element region, and each output is averaged by the averaging circuit 23. To do. The constant current source 20 preferably flows substantially the same current as the constant current source 2. The averaging circuit 23 can be realized by a combination of an adding circuit and a dividing circuit, which are well-known circuits using an operational amplifier.
[0034]
Embodiment 4 FIG.
In the present embodiment, a thermal infrared solid-state imaging device is configured in the same manner as in the first to third embodiments except that the configuration shown in FIG. 9 or 10 is used as the reference signal output circuit 18. The reference signal output circuit 18 of FIG. 9 is an example in which a thermistor 24 whose resistance changes with temperature is driven by a constant current source 25 instead of the reference pixel 19. In this configuration, the resistance of the thermistor 24, the current of the constant current source 25, or the voltage value of the level shift circuit 27 is set so that the output voltage of the buffer amplifier 29 in the bias power supply circuit 21 is the same as in the first embodiment. Is preferably set.
[0035]
Further, the reference signal output circuit 18 of FIG. 10 is an example in which the thermistor 24 is driven at a constant current by using the load resistor 26 instead of the constant current source 25 in the configuration of FIG. Also in this example, the resistance of the thermistor 24, the current of the constant current source 25, or the voltage value of the level shift circuit 27 is set so that the output voltage of the buffer amplifier 29 in the bias power supply circuit 21 is the same as in the first embodiment. Is preferably set.
[0036]
Embodiment 5. FIG.
FIG. 11 shows another embodiment of a thermal infrared solid-state imaging device according to the present invention. The present embodiment is the same as the first embodiment except that load resistors 34 and 35 are used instead of the constant current sources 2 and 20. If the resistances of the load resistors 34 and 35 are made substantially the same, the same effect as in the first embodiment can be obtained.
[0037]
【The invention's effect】
According to the thermal infrared solid-state imaging device of the present invention, the sources of the integration transistors are connected in common by a bias line, and a reference signal that changes according to the temperature of the entire device is input to the bias line. The element temperature drift component can be removed, and stable image acquisition is possible.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing a thermal infrared solid-state imaging device according to Embodiment 1 of the present invention.
FIG. 2 is a circuit diagram showing an integration circuit of the thermal infrared solid-state imaging device according to Embodiment 1 of the present invention.
FIGS. 3A and 3B are a cross-sectional view and a perspective view showing a pixel structure of a thermal infrared solid-state imaging device according to Embodiment 1 of the present invention.
FIG. 4 is a circuit diagram showing an example of a bias power supply circuit connected to a reference signal output circuit.
FIG. 5 is a circuit diagram illustrating an example of a level shift circuit.
6A and 6B are circuit diagrams showing examples of low-pass filters. FIG.
FIG. 7 is a circuit diagram showing a reference signal output circuit used in the second embodiment.
FIG. 8 is a circuit diagram showing a reference signal output circuit used in the third embodiment.
FIG. 9 is a circuit diagram showing a reference signal output circuit used in the fourth embodiment.
FIG. 10 is a circuit diagram showing another example of a reference signal output circuit used in the fourth embodiment.
FIG. 11 is a circuit diagram showing a thermal infrared solid-state imaging device according to a fifth embodiment of the present invention.
[Explanation of symbols]
1 pixel, 2 and 20 constant current source, 3 drive line, 4 vertical scanning circuit, 6 power supply, 7 integration circuit, 8 horizontal scanning circuit, 9 switch, 10 output terminal, 11 bias line, 12 signal line, 19 reference pixel, 21 Bias power supply circuit.

Claims (7)

A pixel area having one diode or a plurality of diodes connected in series and having a heat insulation structure and an infrared absorption structure is arranged in a two-dimensional manner, and an anode of the pixel is commonly connected for each row. A plurality of drive lines and cathodes of the pixels are commonly connected for each column, and a plurality of signal lines including a first constant current means at the end of each column and the drive lines are sequentially selected and selected. A vertical scanning circuit for connecting a pixel power supply to a drive line, and a terminal of the signal line provided with the first constant current means is connected to a gate connected to a gate and a drain of the MOS transistor and periodically The gate modulation integration circuit formed from the capacitor to be reset, the sample hold circuit connected to the output of the gate modulation integration circuit, and the output of the sample hold circuit are sequentially selected and removed. A thermal-type infrared solid-state imaging device and a horizontal driving circuit for outputting a,
The MOS transistor in the gate modulation integration circuit has a source connected in common between columns by a bias line, and a reference signal output capable of outputting a reference signal that changes according to the temperature of the entire element to the bias line The circuit is connected,
A thermal infrared solid-state imaging device comprising a low-pass filter between the reference signal output circuit and the bias line .   The thermal infrared solid-state imaging device according to claim 1, wherein the reference signal output circuit is driven by the pixel power source. The reference signal output circuit includes a temperature sensing element whose current-voltage characteristics or resistance changes according to temperature, and a second constant current means connected to the cathode side of the temperature sensing element, The thermal infrared solid-state imaging device according to claim 1 or 2 , wherein a voltage between both ends of the constant current means is output as a reference signal. The reference signal output circuit includes N (N is a natural number) the temperature sensing elements and one second constant current converting means commonly connected to the cathode side of the N temperature sensing elements. 4. The thermal infrared solid-state image pickup device according to claim 3 , wherein the second constant current converting unit passes a current N times that of the first constant current converting unit. The reference signal output circuit includes N (N is a natural number) the temperature sensing elements, N second constant current converting means connected to the individual cathode sides of the temperature sensing elements, and the N number of the current sensing elements. 4. A thermal infrared solid-state image pickup device according to claim 3 , further comprising an averaging circuit that averages the voltages at both ends of the second constant current means and outputs the averaged voltage as a reference signal. 6. The reference pixel according to claim 3 , wherein the temperature-sensitive element is a reference pixel having substantially the same structure as the pixel except that it does not have at least one of a heat insulating structure and an infrared absorption structure. Thermal infrared solid-state image sensor. 6. The thermal infrared solid-state imaging device according to claim 3 , wherein the temperature sensitive device is a thermistor.

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