JPH05240702A - Semiconductor photodetector - Google Patents

Abstract

PURPOSE:To provide a semiconductor photodetector having such a structure wherein heat radiated from a thermoelectric cooling element can be very efficiently transferred to a heat radiating plate regardless of the size of the thermoelectric cooling element. CONSTITUTION:Grooves 32 are formed in the surface of a heat radiating plate 24 and an adhesive 30 of good heat conductivity is applied to the surface of the heat radiating plate 24. The heat radiating surface of a thermoelectric cooler makes contact with the adhesive 30 with a predetermined pushing force, thereby forcing extra adhesive 30 into the grooves 32 of the heat radiating plate 24 and reducing the film thickness of the adhesive to a predetermined amount. An image sensor chip is placed on the heat absorbing face 21 of the surface of this thermoelectric cooling element. A cap hermetically seals the thermoelectric cooling element and the image sensor chip and is in contact with the heat radiating plate 24, thereby absorbing heat that the thermoelectric cooling element radiates.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor photodetector for detecting a semiconductor photodetector while cooling the semiconductor photodetector using an electronic cooling element.

[0002]

2. Description of the Related Art When a semiconductor light receiving element such as a photodiode is used to detect ultra weak light, it is necessary to cool the photodiode to reduce dark current.

FIG. 4 shows an apparatus for cooling an image sensor assembled in a ceramic package with an electronic cooling element. The photodiode formed in the image sensor is cooled by the electronic cooling element, and the dark current is reduced. In addition, the same figure (a) is a left side view and the same figure (b).
Is a partially cutaway sectional view, and FIG. 7C is a right side view.

An image sensor is formed in the ceramic package 11, and an image picked up by the image sensor is read by the drive read circuit 10. The back surface of this ceramic package 11 is an aluminum block 1.
It is pressed against 2. The aluminum block 12 has good thermal conductivity and is used to improve temperature uniformity, and the electronic cooling element 13 needs to cool both the aluminum block 12 and the ceramic package 11. For this reason, the cooling capacity is insufficient if only one electronic cooling element 13 is used.
4 is used, and the cooling structure has a two-stage structure. Since the cooling structure is a double structure, a large amount of heat is radiated from the heating surface of the electronic cooling element, and it is difficult to sufficiently dissipate the heat by so-called air cooling. May be destroyed. For this reason, a structure is adopted in which cooling water is caused to flow into the heat dissipation block that closely adheres the heat generation surface of the electronic cooler. This cooling water is taken into the device through the outflow / outlet port 16 and travels along the pipe 15 to dissipate the heat dissipation block. Sent to. Further, the cooling surfaces of the ceramic package 11, the aluminum block 12, and the electronic cooling elements 13 and 14 are naturally lower than room temperature. Therefore, when these are exposed, dew condensation may occur, the window of the image sensor may become cloudy, and the electronic cooler may be destroyed. For this reason,
All systems must be put in a shield case to be shielded from the outside air, and the shield case must be filled with dry air. This dry air is taken in through the outflow port 17.

However, in the photodetector thus constructed, it is necessary to cool not only the ceramic package 11 but also other components associated with it.
The heat capacity of the object to be cooled increases. Therefore, in order to cool the entire ceramic package 11, it is necessary to improve the cooling capacity by arranging a plurality of electronic coolers in series as described above. For this reason, the amount of heat released from the electronic cooler increases, and the means for dissipating this is inevitably a water-cooled type. In addition, dry air is also required to prevent condensation. As a result, in the above device, preparation and handling of equipment have been very troublesome.

FIG. 5 shows the structure of a photo-detecting device which solves such a problem.

In this device, the image sensor chip 19 is directly attached to the cooling surface 21 of the electronic cooling element without the ceramic package as in the above device. The temperature of this cooling surface 21 is monitored by the thermistor 20. The electronic cooling element has a cooling function performed by the Peltier element 22 and absorbs heat from the cooling surface 21.
Further, the heat generation surface 23 radiates a heat quantity that is the sum of the heat quantity absorbed by the cooling surface 21 and the power consumption of the Peltier element 22. Therefore, in order to efficiently radiate the heat from the heat generating surface 23, the heat generating surface 23 is strongly pressed against the heat dissipation plate 24 having good heat conductivity. Further, an auxiliary heat dissipation plate 25 is attached to the heat dissipation plate 24 so that the surface area of the heat dissipation plate 24 is increased and the heat dissipation effect is enhanced. The image sensor chip 19, the thermistor 20, and the electronic cooling element are sealed by a cap 28, and are placed in a dry inert gas atmosphere or a vacuum state. The cap 28 is provided with a window member 29 made of, for example, artificial sapphire, and light is taken in from the window member 29. The output of the image sensor 19 and the output of the thermistor 20 are
It is transmitted to the base lead 26 via the gold wire 27 and taken out to the outside.

In such a photo-detecting device, the image sensor chip 19 and the cooling surface 21 of the electronic cooler are at a low temperature, but the inside of the device is kept in a dry inert gas atmosphere or a vacuum state. There is no dew condensation on the inner surface of the window member 29 and the inner surface of the window member 29 is not clouded. Further, on the heat generating surface 23 of the electronic cooling element, the heat quantity of the sum of the heat quantity absorbed by the cooling surface 21 and the power consumption of the percha element 22 is generated as described above, and the generated heat quantity is the heat generating surface 23.
Is connected to the heat radiating plate 24 with a small thermal resistance, so that it is sufficiently transmitted to the heat radiating plate 24, and further to the cab 28 attached to the heat radiating plate 24. For this reason, the portion of the outer shell of the device that comes into contact with the outside air is always higher than room temperature, and therefore dew condensation occurs on the outer surface, and
The surface of the window material 29 will not fog. Also,
By directly attaching the image sensor chip 19 to the electronic cooling element, the cooling efficiency is improved, and the image cooling chip 19 can be sufficiently cooled even if the electronic cooling element has a one-stage configuration. Further, since only one electronic cooling element is required, less heat is generated on the heat generating side of the electronic cooling device, and therefore, electronic cooling by so-called air cooling becomes possible and it is not necessary to use cooling water or dry air.

[0009]

Usually, as shown in FIG. 6 (a), the heat-generating surface 23 of the electronic cooler and the heat-dissipating plate 24 are first connected by an adhesive or a low-temperature molten metal having good heat conductivity. An appropriate amount of 30 is dropped on the surface of the heat radiating plate 24, and then an electronic cooler is placed on this adhesive 30 and restrained from above with an appropriate force. However, at this time, the area of the electronic cooler becomes large as follows, and if the area to be bonded becomes large, the bonding work becomes difficult.

In the image sensor chip 19 arranged on the cooling surface 21 of the electronic cooling element, one photodiode is provided.
They are arranged in one dimension or two dimensions. The dark current generated in this photodiode is reduced to about 1/2 by cooling at 7 ° C.
If there is temperature variation in the surface of the image sensor chip 19, the dark output uniformity (in-plane uniformity in which dark current occurs) immediately deteriorates. In particular, at the edge portion of the electronic cooling element, the Peltier element becomes discontinuous (disappears) there, so that the cooling capacity at this edge portion decreases, and naturally the temperature of the peripheral portion of the cooling surface 21 is lower than that of the central portion. Will go up. Therefore, in order to improve the uniformity of the temperature in the plane of the image sensor chip 19 and to prevent the dark output uniformity from deteriorating, the size of the electronic cooler is made sufficiently larger than the size of the image sensor chip 19, and the electronic cooler is It is necessary to prevent the image sensor chip 19 from being located at the end of the. For this reason, when the area of the image sensor chip 19 to be cooled is large, the area of the electronic cooler also becomes considerably large accordingly.

As described above, the heat generation surface 23 of the electronic cooler generates a sum of heat absorbed by the cooling surface 21 and the power consumption of the electronic cooler itself, so that the heat generation surface 23 heats the heat sink 24. Since the resistance is low, the temperature rise on the heating surface 23 must be suppressed as much as possible. Therefore, when the area of the electronic cooler becomes large as described above, it is necessary to further reduce the thermal resistance with the heat dissipation plate 24. However, in the conventional photodetector, this heat is generated as follows. It was difficult to reduce the resistance.

That is, as a problem of the bonding process, when the amount of the adhesive or the low temperature molten metal 30 is too large, the suppressing force is strengthened from the above to increase the adhesive or the low temperature molten metal 3.
It is necessary to reduce the film thickness of 0 to reduce the thermal resistance. For this reason, the adhesive or the low temperature molten metal 30 is strongly suppressed and sticks out from under the electronic cooler as shown in FIG. 6B, causing a problem in the subsequent process. For example, if an adhesive or low-temperature molten metal is applied to the welding surface between the heat dissipation plate 24 and the cap 28, airtightness will be impaired.

In order to prevent such troubles from occurring, if the restraining force is weakened, it is formed between the heat generating surface 23 and the heat radiating plate 24 of the electronic cooler, as shown in FIG. 6 (c). The film thickness of the adhesive or the low temperature molten metal 30 becomes thicker. For this reason, the thermal resistance from the heat generating surface 23 to the heat dissipation plate 24 becomes high, and heat is not efficiently dissipated.

On the other hand, when the amount of the adhesive or the low temperature molten metal 30 is small, as shown in FIG. 6D, the thermal resistance between the heat generating surface 23 and the heat radiating plate 24 of the electronic cooler is very high. Large air layers are likely to occur. For this reason, not only the heat is not efficiently dissipated, but finally the electronic cooler may be thermally damaged.

[0015]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and is directed to a heat dissipation plate having grooves formed on the surface thereof and a heat conductive material applied to the surface of the heat dissipation plate. An electronic cooling element in which a good bonding agent and the heat-releasing surface are in contact with this bonding agent with a predetermined pressing force to push the excess bonding agent into the groove of the heat dissipation plate to make the bonding agent have a predetermined thickness When,
The semiconductor light receiving element mounted on the heat absorbing surface of the electronic cooling element, the electronic cooling element and the semiconductor light receiving element are hermetically sealed, and the heat radiated by the electronic cooling element is absorbed by contacting the heat radiating plate. The semiconductor photodetection device is formed by including a lid.

[0016]

When the electronic cooling element is brought into contact with the heat radiating plate with a predetermined pressing force, the excess bonding agent interposed between the electronic cooling element and the heat radiating plate is pushed into the groove of the heat radiating plate, and the adhesive around the electronic cooling element. No excess cement is distributed, and
No air layer is formed in the bonding agent. Further, by pressing the electronic cooling element with a predetermined pressing force, the film thickness of the bonding agent is formed to a predetermined thickness, and the thermal resistance from the electronic cooling element to the heat dissipation plate is sufficiently reduced. Therefore, even if the coating amount of the bonding agent fluctuates to some extent, the electronic cooling element is always bonded to the heat sink with a small thermal resistance.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a diagram showing the structure of the main part of a semiconductor photodetector according to an embodiment of the present invention, and FIG. 1A shows the structure of a heat sink. The upper surface of the radiator plate 24 in the drawing is a portion connected to the heat generating surface of the electronic cooler, and is a heat absorbing surface that receives the amount of heat emitted from the electronic cooler.
Grooves 32 are cut on the heat absorbing surface at appropriate intervals up to the end of the heat radiating plate 24 as shown. Therefore, to be precise, the heat dissipation plate 24 is joined to the heat generating surface of the electronic cooler in the rectangular or island-shaped portion excluding the groove 32.

FIG. 2B is a diagram showing a process of connecting the heat sink 24 to the electronic cooler, showing a case where an adhesive or low temperature molten metal 30 as a bonding agent is applied on the heat sink 24. ing. At this time, the amount of the adhesive or the low temperature molten metal 30 may be an appropriate amount, and the allowable range of variation of the coating amount is larger than that in the conventional structure.

FIG. 3C is a view showing the heat generating surface 23 of the electronic cooler adhered to the heat radiating plate 24. The excess adhesive or the low temperature molten metal 30 is pushed into the concave portion of the groove 32, so that it does not overflow from under the heat generating surface 23 of the electronic cooler. Moreover, the adhesive or the low temperature molten metal 3 formed between the heat generating surface 23 of the electronic cooler and the heat dissipation plate 24.
The film thickness of 0 becomes very thin. Further, the adhesive or the low temperature molten metal 30 is evenly distributed between the heat generating surface 23 and the heat dissipation plate 24, and an air layer is not formed unlike the conventional case.
Therefore, since the film thickness is very thin and no air layer is present, the thermal resistance from the heat generating surface 23 of the electronic cooler to the heat radiating plate 24 becomes very small. In addition, the adhesive or the low temperature molten metal 3 is applied by keeping the pressing force to the electronic cooler constant.
0 is always formed to have a predetermined thinness. For this reason, it becomes possible to realize an adhesive process having a small thermal resistance in each manufacturing process.

On the electronic cooler adhered to the heat sink 24, as shown in FIG. 5, the image sensor chip 19 is provided.
Is placed. A chip thermistor 20 is attached to the image sensor chip 19, and the temperature of the surface of the image sensor chip 19 is monitored. The image sensor chip 19 and the thermistor 20 are connected to the outside via the base lead 26 connected to the gold wire 27. Further, the image sensor chip 19, the electronic cooling element and the thermistor 20 are hermetically sealed by the cap 28, and are placed in a dry inert gas atmosphere or a vacuum state. Further, an auxiliary heat dissipation plate 25 is attached to the heat dissipation plate 24.

In the semiconductor photodetector according to this embodiment having the above-mentioned structure, since the electronic cooling element and the heat dissipation plate 24 are bonded with extremely small thermal resistance as described above, the entire image sensor chip 19 is Cools efficiently. Therefore, the dark current generated in the photodiode formed in the image sensor chip 19 can be made extremely small, and the S / N ratio of light detection can be improved as follows.

The image sensor chip 19 is an image pickup device for converting the light input position into time series information, and inside thereof, photodiodes are arranged one-dimensionally or two-dimensionally. The image sensor is composed of various functional units for photoelectric conversion, storage and scanning. A switch means or a transfer means is used for scanning, and the signal charge is carried to a video line which is a common signal line by this scanning. In addition, since each photodiode of the image sensor is geometrically fixed, the image distortion is essentially small, and it is small and lightweight, and has excellent environmental resistance against vibration and shock. It has features such as. Such an image sensor is considered to have a wide range of applications as a sensor having a visual function, and as typical application fields, there are many possible applications such as a video camera, object recognition, a multi-channel detector for a spectrophotometer, distance photometry, and color identification. ..

Further, the image sensor, as described above,
It is roughly classified into a one-dimensional array and a two-dimensional array according to the arrangement of the light receiving parts, and further divided into an address system and a signal transfer system according to the scanning function. FIG. 2 is a circuit diagram formed inside a MOS image sensor chip which is an address type one-dimensional image pickup device array. In an address system represented by a MOS image sensor, a continuous pallet made in a shift register 1 composed of a MOSFET is added to an address switch 2, so that the electric charge discharged in a photodiode 3 is a common signal line. Charged from line 4.

FIG. 3 shows a pixel structure and a circuit structure of a MOS type image pickup device using a method of detecting the charging current by using a resistor. The N-channel MOSFET using the P-type silicon substrate 5 constitutes the address switch 6.
Further, the PN junction portion of the source region 7 constitutes a photodiode, which performs a photoelectric conversion function and also functions as a charge storage portion.

When a positive address pallet is applied to the gate electrode 8, an N channel is formed on the surface of the silicon substrate 5 below the gate electrode 8. Therefore, the potential of the source region 7 forming the photodiode is the potential V of the drain region 9
Charge is supplied from the drain region 9 to the source region 7 through this N channel until it becomes equal to cc. Since the potential of the P-type silicon substrate 5 is 0 V and the potential of the N-type source 7 is Vcc, the photodiode is in a reset state in which a reverse bias of Vcc volt is applied. On the other hand, when the address pulse is not applied to the gate electrode 8, no channel is formed in the silicon substrate 5 below the gate electrode 8. Therefore, the potential of the source region 7 forming the photodiode becomes a floating state,
The accumulation operation is started. That is, when light is incident in this state and carriers are excited in the silicon substrate 5, the charges (holes) accumulated in the photodiode are discharged by the excited carriers (electrons), and as a result, the photodiode potential is increased. Will fall. At this time, when completely discharged,
The potential of the photodiode becomes the minimum value of 0V, and the reverse bias is eliminated.

What is clear here is that the reverse bias of the photodiode in the reset state is Vcc volt, and when it is completely discharged, it becomes 0 volt. Therefore, the maximum value Qmax of the amount of charge that can be discharged by the photodiode eventually becomes the photodiode capacitance. Let Cpd be shown in the following equation.

Qmax = Cpd × Vcc (1) Next, when the address pallet is applied to the gate electrode 8 again, the charging current corresponding to the discharge charge flows into the photodiode again. This charging current amount is detected by an external circuit.

The total amount of discharged charges is proportional to the product of the amount of light incident on the photodiode and the time interval for turning on the gate of the address switch 6. Such an operation mode is called a charge storage mode and is useful for detecting feeble light. The discharge charge amount Qs is given by the following equation, where Ip is the photocurrent of the photodiode, Id is the dark current of the photodiode, and t is the accumulation time (the time interval when the gate is turned on).

Qs = (Ip + Id) × t (2) Now, when the incident light is very weak, the photocurrent Ip in the above equation becomes small, but if the accumulation time t is increased by that much, the total The output charge amount Qs of the signal becomes large, and the signal can be taken out without lowering the S / N ratio.
However, since the maximum discharge charge amount of the photodiode is determined by the equation (1), the dark current component I in the equation (2) is
If d is large, the accumulation time t cannot be made long, and therefore the S / N ratio may not be improved as planned.

However, in the photodiode of the image sensor, the dark current Id can be dramatically lowered by lowering the temperature thereof. When trying to detect ultra weak light, a method of cooling the image sensor to lengthen the accumulation time is often used.

Therefore, by constructing the image sensor with the structure according to the present embodiment, the image sensor chip 19 is cooled very efficiently as described above.
The dark current Id can be sufficiently reduced. Moreover,
The dark current Id can be sufficiently reduced because the image sensor chip 19 can be efficiently cooled even when the electronic cooler has a large area. Therefore, according to the present embodiment, it is possible to perform photodetection with a good S / N ratio regardless of the size of the image pickup element area.

[0032]

As described above, according to the present invention, since the electronic cooling element is brought into contact with the heat dissipation plate with a predetermined pressing force, the excess bonding agent interposed between the electronic cooling element and the heat dissipation plate dissipates heat. The excess bonding agent is not distributed around the electronic cooling element by being pushed into the groove of the plate, and an air layer is not formed in the bonding agent. Further, by pressing the electronic cooling element with a predetermined pressing force, the film thickness of the bonding agent is formed to a predetermined thickness, and the thermal resistance from the electronic cooling element to the heat dissipation plate is sufficiently reduced. Therefore, even if the coating amount of the bonding agent fluctuates to some extent, the electronic cooling element is always bonded to the heat sink with a small thermal resistance.

Therefore, the heat radiated from the electronic cooling element is always transferred to the heat sink very efficiently regardless of the size of the electronic cooling element. Therefore, a semiconductor photodetection device capable of performing photodetection with a good S / N ratio is provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a main part of a semiconductor photodetector according to an embodiment of the present invention.

FIG. 2 is an address-based one-dimensional image sensor array M.
It is a circuit block diagram of an OS image sensor.

FIG. 3 is a diagram showing a pixel structure of the MOS type image sensor shown in FIG.

FIG. 4 is a diagram showing a first conventional semiconductor photodetection device for cooling a MOS image sensor with an electronic cooling element.

5 is a diagram showing a second conventional semiconductor photodetector device which solves the problems of the device shown in FIG. 4;

FIG. 6 is a diagram for explaining a problem that the conventional second device shown in FIG. 5 has.

[Explanation of symbols]

19 ... Image sensor chip, 20 ... Chip thermistor, 21 ... Cooling surface of electronic cooler, 22 ... Peltier element,
23 ... Heat-generating surface of electronic cooler, 24 ... Heat sink, 25 ... Auxiliary heat sink, 26 ... Base lead, 27 ... Gold wire, 28
... Cap, 29 ... Window material, 30 ... Adhesive or low temperature molten metal (bonding agent), 32 ... Groove.

Claims (2)

[Claims] 1. A heat radiating plate having a groove formed on the surface thereof, a bonding agent having good thermal conductivity applied to the surface of the heat radiating plate, and a heat radiating surface contacting the bonding agent with a predetermined pressing force. An electronic cooling element joined by pushing an excessive bonding agent into the groove of the heat sink to make the thickness of the bonding agent a predetermined thickness, and a semiconductor light receiving device mounted on the heat absorption surface of the surface of the electronic cooling element. A semiconductor photodetection device comprising: an element; and a lid that hermetically seals the electronic cooling element and the semiconductor light receiving element and is in contact with the heat dissipation plate to absorb heat radiated by the electronic cooling element. 2. The semiconductor photodetector according to claim 1, wherein the bonding agent is made of a low temperature molten metal.