JP4300305B2 - Thermal infrared image sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a thermal infrared imaging device having means for performance correction.
[0002]
[Prior art]
The thermal infrared imaging device has a structure in which a large number of pixels (for example, about several tens of thousands to several hundred thousand pixels) are arranged on a plane, but the sensitivity of each pixel lacks uniformity. For this reason, if an image is captured without correcting the sensitivity of each pixel, the captured image may differ from the actual subject. Therefore, when using the thermal infrared imaging device, it is necessary to correct the sensitivity of each pixel of the device in order to improve the image quality of the image. Referring to the plan view of the thermal infrared imaging device shown in FIG. 3, the thermal infrared imaging device has an array of m columns × n rows, with the thermal infrared sensor 1 having the heat absorption layer 2 as a pixel and a large number of pixels. Are arranged in a shape. A switching circuit unit 3 is formed in each pixel, and a signal of each pixel is read out via the x address line 4 and the y address line 5 (external circuit is not shown).
[0003]
In a conventional thermal infrared imaging device, as shown in FIG. 5, the output voltage of each pixel (explained on the assumption that the state change of the pixel is converted into a voltage signal) is usually unbalanced for each pixel. . For this reason, it is necessary to provide an external signal processing unit such as an amplifier circuit, an A / D conversion unit, and a signal correction storage unit to correct an imbalance between pixels every time an image is taken. To correct this imbalance between pixels, when using a thermal infrared imaging device as an infrared imaging camera, place an object with a relatively uniform temperature in front of the infrared imaging camera, and the incident infrared energy at that time. There is a method in which the output voltage of each pixel is stored in a storage unit on the assumption that is uniform, and the sensitivity of each pixel is corrected according to the output value at that time. In general, a metal plate or black plate close to room temperature is installed in front of an infrared imaging camera, and sensitivity correction is performed according to the output voltage of each pixel when the image is captured.
[0004]
[Problems to be solved by the invention]
However, in the correction method as described above, it is necessary to prepare and correct an object with a temperature that is within the field of view of the infrared imaging camera every time the infrared imaging camera is used. For example, in order to obtain a practically accurate image, it is necessary to correct at intervals of about 10 minutes even during use. Further, when an object having a uniform temperature is placed in front of the camera, depending on the environmental temperature, it is difficult to maintain a uniform temperature over the entire area depending on the heat capacity of the object. Especially in the case of recent infrared cameras, a nett with a noise equivalent temperature difference (0.1TD) of around 0.1 ° C. has appeared. It becomes difficult. The difficulty also increases when using an object having a temperature higher than the ambient temperature.
[0005]
The present invention has been made to solve the above-described problems of the prior art, and provides a thermal infrared imaging device capable of correcting sensitivity between pixels easily, quickly, and with high accuracy. Objective.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the present invention is configured as described in the claims. That is, in the first aspect of the present invention, means for contacting and separating the diaphragm having the infrared detector mounted thereon from the support substrate, heating means for heating the support substrate, and temperature of the support substrate or the diaphragm are detected. Temperature detection means, storage means for storing the temperature when the diaphragm and the support substrate are in contact with each other, and output signals of the pixels at that time, and output of each pixel based on the stored value of the storage means And a means for correcting the signal. In addition, said temperature shows the temperature of an object itself (a unit may be degree C or K), and does not mean a temperature difference. The storage means and the means for correcting the output signal of each pixel may be provided as an external circuit. However, if there is an empty area on the element, it may be formed in that portion.
[0007]
By configuring as described above, all the pixels can be unified at the same predetermined temperature as the temperature of the support substrate by bringing the diaphragm into contact with the support substrate heated to a predetermined temperature by the heating means. The sensitivity of each pixel can be corrected by detecting the output signal of the pixel. And since there is no need to prepare an object with a uniform temperature as in the prior art, the procedure required for sensitivity correction is extremely easy and quick, and the temperature of each pixel can be made exactly the same temperature. It can be performed with high accuracy. In addition, since the thermal infrared imaging device itself is equipped with a heating means and a temperature detection means, the temperature can be set by the element itself, and the temperature can be set and detected with high accuracy, thereby further improving the accuracy. I can do it.
[0008]
In the second aspect of the invention, since the diaphragm and the support substrate are electrostatically brought into contact with each other, preparation for imaging can be performed very quickly and easily.
[0009]
In the invention described in claim 3, sensitivity correction can be performed precisely, and outputs at three or more different temperatures are detected even when the temperature characteristic of each pixel is not linear. Thus, accurate sensitivity correction is possible even with non-linear characteristics.
[0010]
【The invention's effect】
According to the present invention, the temperature of each pixel constituting the infrared imaging element can be easily, quickly and accurately set to a predetermined temperature that is the same as the substrate temperature. Therefore, the performance of each pixel can be easily corrected according to the output signal at that time. Further, correction is possible even when the temperature characteristics of the infrared sensor serving as each pixel are not linear. Therefore, there is an effect that sensitivity correction of each pixel of the thermal infrared imaging device can be performed easily, quickly and with high accuracy without preparing an object having a uniform temperature as in the prior art.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 and FIG. 2 are diagrams showing an embodiment of one infrared sensor (pixel) constituting the thermal infrared imaging device of the present invention, FIG. 1 is a plan view, and FIG. 2 is a sectional view. FIG. 3 is a plan view showing a schematic configuration of a thermal infrared imaging element formed using the above sensor.
[0012]
First, a pixel structure will be described with reference to FIGS. Here, a bolometer type infrared sensor capable of detecting temperature as a sensor (pixel) will be described. The bolometer type infrared sensor utilizes the characteristic that the electric resistance of the resistor changes according to the temperature change. 1 and 2, reference numeral 11 denotes a semiconductor substrate (for example, a silicon substrate) that serves as a support substrate. A diaphragm 12 is supported by the beam portion 13 and faces the semiconductor substrate 11 with a minute gap 20 (for example, about several μm) therebetween. A resistor 16, an interlayer insulating film 17, an interlayer insulating layer 19, a heat absorption layer 25, and an electrode 30 are formed on the diaphragm 12, and the diaphragm portion 14 is configured as a whole. The diaphragm portion 14 is thermally separated from the semiconductor substrate 11 by the air gap 20. A wiring 15 for connecting the resistor 16 to the outside and a wiring 31 for connecting the electrode 30 to the outside are formed on the beam portion 13, and the interlayer insulating film 17 covers the wiring 31. A temperature detecting means 40 and a heating means 41 are provided on the semiconductor substrate 11. A required number of such pixels are formed in an array on the semiconductor substrate 11 (see FIG. 3). As can be seen from FIG. 2, the resistor 16 is formed on the electrode 30, and the heat absorption layer 25 is further formed thereon. In FIG. 1, only the heat absorption layer 25 is actually visible. However, for the convenience of explanation, the resistor 16 is displayed as shown in the figure, and the electrode 30 and the heat absorption layer 25 are formed on substantially the same plane. It is shown with a solid line.
[0013]
The semiconductor substrate 11 may be formed with a signal processing circuit such as an x, y address decoder or a preamplifier. A temperature detecting means 40 (for example, a bolometer type or pn junction type temperature sensor) and a heating means 41 (for example, a metal film resistor) are formed using the empty area of the semiconductor substrate 11. Since the heating means 41 heats the entire thermal infrared imaging device so as to have a uniform temperature, it is preferably arranged over the entire area. The temperature detecting means 40 may be provided for each pixel. However, if the temperature of the entire semiconductor substrate 11 is uniform, only one temperature detecting means 40 may be provided. In the figure, as an example, a pixel in which the temperature detecting means 40 exists is shown.
[0014]
In addition, a circuit for applying a voltage between the electrode 30 and the semiconductor substrate 11 and a memory for detecting the temperature and the output signal of each pixel at that time (details will be described later) are required. is doing. These circuits may be formed in a portion where the element has a vacant area or may be an external circuit.
[0015]
2 shows an example in which the heating means 41 is provided in the semiconductor substrate 11, it may be formed on the interlayer insulating layer 17 formed on the semiconductor substrate 11 or on the lower portion of the semiconductor substrate 11. Further, the temperature detecting means 40 may be provided anywhere within the element.
[0016]
FIG. 3 is a plan view of a thermal infrared imaging device using the infrared sensor 1 shown in FIGS. 1 and 2. The infrared sensor 1 (pixel) has a heat absorption layer 2 (corresponding to 25 in FIGS. 1 and 2), and a large number of pixels are arranged in an array of m columns × n rows. A switching circuit unit 3 is formed in each pixel, and a signal is read out through the x address line 4 and the y address line 5. An external circuit and a circuit for correcting the output voltage are not shown.
[0017]
The operation will be described below.
In the bolometer-type infrared sensor as described above, incident infrared rays are absorbed by the heat absorption layer 25, and a change in the resistance value of the resistor 16 that occurs in response to a temperature change caused thereby is detected (for example, detected as a voltage change). A signal corresponding to the incident infrared ray is obtained.
[0018]
In the present embodiment, the temperature of the semiconductor substrate 11 is kept at a predetermined temperature by supplying a current to the heating means 41 while heating the temperature detection means 40 while detecting the temperature, and in this state, each pixel is maintained. The thermal separation diaphragm 14 in which the infrared detection part is formed is brought into contact with the semiconductor substrate 11 and thermally short-circuited, and the infrared detection part (the heat absorption layer 25, the resistor 16 formed in the diaphragm part 14 is formed. ) Is the same as the substrate temperature. Note that the thickness of the diaphragm portion 14 is extremely thin, for example, about several μm, and its heat capacity is very small. Thus, if all the pixels are set to the same temperature, the same effect as when an object having a constant temperature is placed in front of the infrared camera can be obtained. Note that the temperature control of the semiconductor substrate 11 as described above can be achieved by controlling the heating means 41 so that the output of the temperature detection means 40 is fed back to coincide with a predetermined temperature. Alternatively, a method of heating the heating means 41 appropriately and detecting the temperature at that time by the temperature detection means 40 is also possible.
As described above, the diaphragm unit 14 is brought into contact with the semiconductor substrate 11 to set the temperature to a predetermined value, and then the diaphragm unit 14 is moved away from the semiconductor substrate 11 to complete the imaging preparation state.
[0019]
Next, the function of bringing the diaphragm portion 14 into contact with or separating from the semiconductor substrate 11 as described above will be described. In the present embodiment, the diaphragm portion 14 and the semiconductor substrate 11 are brought into contact with each other with an electrostatic force. That is, an electric field is applied between the electrode 30 formed on the diaphragm portion 14 and the semiconductor substrate 11 (voltage application circuit is not shown). For example, when the semiconductor substrate 11 side is grounded and a positive voltage is applied to the electrode side, the diaphragm portion 14 is attracted by the electrostatic force, the beam portion 13 is bent, and the diaphragm portion 14 and the semiconductor substrate 11 are in close contact with each other. At this time, the temperature on the semiconductor substrate 11 side is conducted to the diaphragm portion 14 side, and the diaphragm portion 14 having a small heat capacity rapidly becomes the same temperature as the semiconductor substrate 11. The temperature at this time is measured by the temperature detecting means 40. In addition, since the size of the diaphragm portion 14 is, for example, about several tens to several hundreds of μm on one side and the gap 20 is, for example, about 1 to several μm, it can be easily adhered by applying a low voltage.
[0020]
If the applied voltage is removed, the diaphragm portion 14 is separated from the semiconductor substrate 11 by the elastic force of the beam portion 13. It should be noted that a metal layer may be provided on the semiconductor substrate 11 side or a layer of a material having low adhesion to the silicon nitride film may be formed so that the diaphragm portion 14 can be easily separated from the semiconductor substrate 11. Moreover, in order to ease the adhesiveness between these layers and the diaphragm portion 14, grooves may be formed in these layers.
[0021]
Although the above description is an example in which the semiconductor substrate 11 can be used as an electrode with low resistance (high impurity concentration), the semiconductor substrate 11 has a high resistance value (low impurity concentration) or an insulating material. When the support substrate is used, a second electrode is provided at a position facing the electrode 30 on the support substrate (position facing the diaphragm portion 14 and the gap 20), and the electrode 30 and the second electrode A voltage may be applied between the two.
[0022]
Next, a configuration (means for correcting the output signal of each pixel) that performs performance correction (sensitivity calibration) of each pixel using the above temperature control will be described.
FIG. 4 is a diagram showing the concept of output signals at temperatures T1 and T2 for the (m−1, n−1) th pixel and the (m, n) th pixel. The temperature referred to here indicates the temperature of the object itself (the unit may be ° C. or K) and does not mean a temperature difference. Assume that the entire thermal infrared imaging device as shown in FIG. 3 is set to the temperature T1 by the method using the heating means 41 and the diaphragm portion 14 of each pixel is brought into contact with the substrate. At this time, both the (m−1, n−1) th pixel and the (m, n) th pixel have the same temperature T1. The output signal V1 of the (m−1, n−1) th pixel and the output signal V1 ′ of the (m, n) th pixel at this temperature T1 are obtained. Note that the temperature change in the pixel itself is a change in the resistance value of the resistor 16, but here it is treated as being converted into a voltage and output. Next, the entire thermal infrared imaging device is set to a temperature T2 (for example, T2> T1), the diaphragm portion 14 of each pixel is brought into contact with the substrate, and the (m−1, n−1) th pixel of the temperature T2 An output signal V2 and an output signal V2 ′ of the (m, n) th pixel are obtained.
[0023]
If the difference (T2−T1) between the temperatures T1 and T2 is ΔT, the output signal difference (V2−V1) of each pixel is ΔV1, and (V2′−V1 ′) is ΔV1 ′,
In the (m−1, n−1) th pixel, ΔV1 / ΔT = (V2−V1) / (T2−T1).
In the (m, n) th pixel, ΔV1 ′ / ΔT = (V2′−V1 ′) / (T2−T1)
Becomes an output signal change with respect to a temperature change, and this value can be treated as a proportional constant indicating the temperature change of the output signal for each element. The same process is performed for all the pixels, and the data of all the pixels is stored in the storage unit, and the performance of each pixel is corrected when the infrared camera is used.
[0024]
Hereinafter, a case where an average value is used will be described as an example of performance correction (calibration) of each pixel. The proportionality constant (corresponding to the above ΔV1 ′ / ΔT) of the (m, n) th pixel obtained as described above is A (m, n), and the average value of the proportionality constants of all the pixels is A. Consider sensitivity correction for the (m, n) th pixel.
The difference between the output voltage V1 of the (m, n) th pixel at the temperature T1 and the average value V of the output voltages of all the pixels is (V−V1). When the output voltage of the (m, n) th pixel at a certain temperature Tx is Vx, the correction value is [Vx + (V−V1)] × A / A (m, n).
It becomes. Similar correction is performed for other pixels. By performing such correction, it becomes possible to correct the performance of all the pixels based on the average value.
[0025]
Next, a case where performance correction is performed based on a certain pixel will be described. For example, the reference pixel is the (1,1) th pixel, and the proportionality constant of this (1,1) pixel is A0. The difference between the output voltage V1 of the (m, n) th pixel and the output voltage V0 of the (1, 1) pixel at the temperature T1 is (V0−V1). When the output voltage of the (m, n) th pixel at a certain temperature Tx is Vx, the correction value is [Vx + (V0−V1)] × A0 / A (m, n).
It becomes. Similar correction is performed for other pixels. By performing such correction, it becomes possible to correct the performance of all the pixels based on the average value.
As described above, the difference in sensitivity of each pixel can be eliminated.
[0026]
As for the performance correction of each pixel, the proportional constant is obtained from the data of two points, and the method for correcting the data by this is described. However, when the relationship between the output signal of the pixel and the temperature is not linear. If the set temperature is set to three or more points, the output signal at each temperature is measured, the temperature characteristic curve of each pixel is obtained, and correction corresponding thereto is performed, so that high-precision performance correction can be performed.
Further, it is desirable to set the temperature T1, T2, etc. when performing performance correction to a value close to the temperature of the subject to be imaged, for example.
[0027]
The correction as described above has no influence from the environment because the semiconductor substrate 11 is not heated even if the infrared ray is incident through the infrared camera and irradiated to the infrared sensor. Furthermore, since heating and temperature detection are performed using an electrical method, a uniform temperature object for performance correction is not necessary. Therefore, preparation of the infrared camera can be performed quickly.
[0028]
Next, a method for manufacturing the element shown in FIGS. 1 to 3 will be described.
First, an oxide film (for example, a PSG film) serving as a sacrificial layer is formed on the semiconductor substrate 11 in a region serving as the gap 20. Here, a method for forming a sacrificial layer of PSG will be described. A PSG oxide film having a thickness of about 1 μm is formed on the semiconductor substrate 11 by a CVD method or the like. Since the PSG oxide film is a sacrificial etching layer, polyimide or the like may be used. A lattice-like groove (width of about 1 μm) is formed in this PSG oxide film by etching. The same PSG oxide film is formed thereon by the CVD method, and is removed by etching leaving a part. In this way, a sacrificial layer is formed in the portion that becomes the gap 20 (below the diaphragm portion 14 and the beam portion 13). The oxide film serving as the sacrificial layer may be a single layer having a thickness of about 2 μm. However, when the method of forming the PSG oxide film again after forming the grooves as described above, a small tunnel (cavity or “ Since the etching solution permeates through the tunnel when the sacrificial layer is etched, the etching rate can be increased.
[0029]
Next, a film that becomes a diaphragm, for example, a silicon nitride film is formed on the sacrificial layer by LPCVD. This is a structural material for the beam portion 13 and the diaphragm 12. An electrode 30 is formed on the diaphragm 12. Any electrode 30 may be used as long as it is a metal film, but an A1 film is preferable in view of the CMOS process. An interlayer insulating film 17 such as silicon oxide is formed on the electrode 30, and a resistor 16 whose electric resistance changes according to a temperature change is formed and patterned as necessary. After the interlayer insulating film 19 is formed thereon, an etching hole 18 is formed. Next, the heat absorption layer 25 is formed. An amorphous Si layer may be formed between the heat absorption layer 25 and the interlayer insulating layer 19 in order to enhance the adhesion of the heat absorption layer 25. In this case, a gold black film is appropriate as the heat absorption layer 25. Next, an etching solution is injected from the etching hole 18 and the sacrificial layer of the PSG oxide film is removed by etching, thereby forming a void 20 and separately forming the diaphragm portion 14.
[0030]
In the above description, the case where the bolometer type infrared sensor is used as a pixel has been described. However, a pn junction type infrared sensor can be similarly configured.
[Brief description of the drawings]
FIG. 1 is a plan view showing an embodiment of an infrared sensor (pixel) used in a thermal infrared imaging device of the present invention.
FIG. 2 is a cross-sectional view showing an embodiment of an infrared sensor (pixel) used in the thermal infrared imaging device of the present invention.
FIG. 3 is a plan view showing an array configuration of a thermal infrared imaging device of the present invention.
FIG. 4 is a characteristic diagram showing states of output signals of two pixels.
FIG. 5 is a characteristic diagram showing variations in output signals of pixels in a thermal infrared imaging device.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Infrared sensor 2 ... Heat absorption layer 3 ... Switching circuit part 4 ... x address line 5 ... y address line 11 ... Semiconductor substrate 12 ... Diaphragm 13 ... Beam part 14 ... Diaphragm part 15 ... Wiring 16 ... Resistor 17 ... Interlayer insulation Layer 18 ... Etching hole 19 ... Interlayer insulating layer 20 ... Air gap 25 ... Heat absorption layer 30 ... Electrode 31 ... Wiring 40 ... Temperature detection means 41 ... Heating means

Claims (4)

In a thermal infrared imaging device in which a diaphragm having a thermal separation structure is formed with a gap from a support substrate, and an infrared detection unit is formed on the diaphragm, a pixel is used, and an array is formed by a plurality of pixels. ,
Means for contacting and separating the diaphragm from the support substrate;
Heating means for heating the support substrate;
Temperature detecting means for detecting the temperature of the support substrate or the diaphragm;
Storage means for storing a temperature when the diaphragm and the support substrate are in contact with each other, and an output signal of the pixel at that time;
Means for correcting the output signal of each pixel based on the stored value of the storage means;
A thermal infrared imaging device comprising: The means for bringing the diaphragm into contact with and separating from the support substrate is configured to apply a voltage between an electrode formed on the diaphragm and the electrode provided on the support substrate or the support substrate, thereby causing the diaphragm and the The thermal infrared imaging device according to claim 1, wherein the thermal infrared imaging device is in contact with a support substrate. The means for correcting the output signal of each pixel corrects the output signal of each pixel according to at least two different temperatures and the value of the output signal at each temperature. Item 3. The thermal infrared imaging device according to Item 2. The thermal infrared imaging device according to any one of claims 1 to 3, wherein the infrared detector is a bolometer type or pn junction type infrared sensor.

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