JP3121424B2 - Semiconductor photodetector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

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

[0002]

2. Description of the Related Art When detecting a very weak light using a semiconductor light receiving element such as a photodiode, it is necessary to cool the photodiode and reduce the dark current.

FIG. 4 shows an apparatus for cooling an image sensor assembled in a ceramic package by an electronic cooling element. The photodiode formed in the image sensor is cooled by the thermoelectric cooler, and the dark current is reduced. FIG. 3A is a left side view, and FIG.
Is a partially cutaway sectional view, and FIG. 3C is a right side view.

[0006] An image sensor is formed in the ceramic package 11, and an image picked up by the image sensor is read out by the drive reading circuit 10. The back of the ceramic package 11 is
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. Therefore, if only one stage of the electronic cooling element 13 is used, the cooling capacity is insufficient.
4 is used, and the cooling structure has a two-stage configuration. Since the cooling structure has a double structure, a large amount of heat is released from the heat generating 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 employed in which cooling water flows through a heat-dissipating block that closely contacts the heat-generating surface of the electronic cooler. This cooling water is taken into the apparatus through an outflow / inlet port 16 and travels along a pipe 15 to dissipate the heat-dissipating block. Sent to Also, 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, fogging of the window of the image sensor may occur, and furthermore, the electronic cooler may be destroyed. For this reason,
All systems must be placed in a shielded case to block out from outside air, and the inside of the shielded case must be filled with dry air. This dry air is taken in via the outlet 17.

[0005] However, in the photodetector configured as described above, it is necessary to cool not only the ceramic package 11 but also other equipment attached thereto.
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 radiation of the electronic cooler increases, and the means for dissipating the heat must be of a water-cooling type. In addition, dry air for preventing dew condensation is required. As a result, in the above-mentioned apparatus, preparation of equipment and handling were very troublesome.

FIG. 5 shows the structure of a photodetector which has solved such a problem.

In this device, the image sensor chip 19 is directly mounted on the cooling surface 21 of the electronic cooling element without using a ceramic package as in the above device. The temperature of the 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 emits a heat amount that is the sum of the heat amount 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 a heat radiating plate 24 having good heat conductivity. Further, an auxiliary heat radiating plate 25 is attached to the heat radiating plate 24, so that the surface area of the heat radiating plate 24 is increased and the heat radiating effect is enhanced. The image sensor chip 19, the thermistor 20, and the thermoelectric cooler 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 material 29 made of, for example, artificial sapphire, and light is taken in from the window material 29. Note that 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.

In such a light detecting device, the temperature of the image sensor chip 19 and the cooling surface 21 of the electronic cooler become low, but the inside of the device is kept in a dry inert gas atmosphere or a vacuum state. Does not occur, and the inner surface of the window material 29 does not fog. Further, the heat amount of the heat absorbed by the cooling surface 21 and the power consumption of the Pelture element 22 are generated on the heat generating surface 23 of the electronic cooling element as described above.
Is connected to the heat radiating plate 24 with low thermal resistance, so that the heat 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 always rises above room temperature, and therefore, dew condensation occurs on the outer surface,
Fogging does not occur on the surface of the window material 29. Also,
By attaching the image sensor chip 19 directly to the electronic cooling element, the cooling efficiency is enhanced, and the electronic cooling element can sufficiently cool the image sensor chip 19 even with a single-stage configuration. Furthermore, since only one electronic cooling element is required, the heat generated on the heat generation side of the electronic cooler is small. Therefore, electronic cooling by so-called air cooling becomes possible, and there is no need to use cooling water or dry air.

[0009]

Normally, as shown in FIG. 6 (a), the connection between the heat generating surface 23 of the electronic cooler and the heat radiating plate 24 is first made of an adhesive having good thermal conductivity or a low-temperature molten metal. 30 is dropped on the surface of the heat sink 24 in an appropriate amount, and then an electronic cooler is placed on the adhesive 30 and pressed down from above with an appropriate force. However, at this time, the area of the electronic cooler increases as follows, and the bonding operation becomes difficult when the area to be bonded increases.

In the image sensor chip 19 disposed on the cooling surface 21 of the electronic cooling element, one photodiode is provided.
They are arranged in two or three dimensions. The dark current generated in this photodiode is reduced to about 1/2 by cooling at 7 ° C., and therefore,
If there is a temperature variation in the plane of the image sensor chip 19, the dark output uniformity (in-plane uniformity in which a dark current occurs) immediately deteriorates. In particular, at the end of the electronic cooling element, the Peltier element becomes discontinuous (disappears) there, and the cooling capacity at this end is reduced. It goes up. Therefore, in order to improve the temperature uniformity in the plane of the image sensor chip 19 and not to deteriorate the dark output uniformity, the size of the electronic cooler is made sufficiently larger than the size of the image sensor chip 19, and It is necessary to prevent the image sensor chip 19 from being located at the end of the image sensor. For this reason, when the area of the image sensor chip 19 to be cooled is large, the area of the electronic cooler becomes considerably large accordingly.

As described above, the heat generation surface 23 of the electronic cooler generates a heat amount equal to the sum of the heat absorbed by the cooling surface 21 and the power consumption of the electronic cooler itself. The resistance value of the heat generating surface 23 must be suppressed as much as possible with a low resistance connection. For this reason, when the area of the electronic cooler is increased as described above, it is necessary to further reduce the thermal resistance with the heat radiating plate 24. It was difficult to reduce the resistance.

That is, as a problem of the bonding process, when the amount of the adhesive and the low-temperature molten metal 30 is too large, the force for suppressing the adhesive and the low-temperature molten metal
The thermal resistance must be reduced by reducing the film thickness of 0. For this reason, the adhesive or the low-temperature molten metal 30 is strongly suppressed and protrudes from under the electronic cooler as shown in FIG. 6B, which causes a problem in the subsequent process. For example, if an adhesive or a low-temperature molten metal reaches the welding surface between the heat radiating plate 24 and the cap 28, the airtightness is impaired.

When the suppressing force is weakened in order to prevent such a problem from occurring, as shown in FIG. 6 (c), the force is formed between the heat generating surface 23 of the electronic cooler and the heat radiating plate 24. The thickness of the adhesive or the low-temperature molten metal 30 increases. For this reason, the thermal resistance from the heat generating surface 23 to the heat radiating plate 24 is increased, and the 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 low. Large air layers are likely to form. For this reason, not only is heat dissipation not performed efficiently, but also there is a possibility that the electronic cooler will eventually be thermally damaged.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in order to solve such a problem, and has a heat radiating plate having a groove formed on a surface thereof and a heat conductive material applied to the surface of the heat radiating plate. An electronic cooling element in which a good bonding agent and a heat emitting surface are brought into contact with the bonding agent with a predetermined pressing force, and an excess bonding agent is pushed into the grooves of the heat sink to reduce the thickness of the bonding agent to a predetermined thickness. When,
The semiconductor light receiving element mounted on the heat absorbing surface of the electronic cooling element, hermetically seals the electronic cooling element and the semiconductor light receiving element, and absorbs heat radiated by the electronic cooling element by contacting the heat sink. And a lid for forming the semiconductor photodetector.

[0016]

When the electronic cooling element comes into contact with the heat radiating plate with a predetermined pressing force, an 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 surrounding area of the electronic cooling element is circulated. No extra cement is distributed, and
No air layer is formed in the bonding agent. Further, when the electronic cooling element is pressed with a predetermined pressing force, the thickness of the bonding agent is formed to a predetermined thickness, and the thermal resistance from the electronic cooling element to the heat radiating plate is sufficiently reduced. Therefore, even if the application amount of the bonding agent slightly fluctuates, the thermoelectric cooling element is always bonded to the heat sink with low thermal resistance.

[0017]

FIG. 1 is a view showing a configuration of a main part of a semiconductor photodetector according to an embodiment of the present invention. FIG. 1A shows a structure of a heat sink. The upper surface of the radiator plate 24 in the figure is a portion connected to the heat generating surface of the electronic cooler, and serves as a heat absorbing surface for receiving the amount of heat released from the electronic cooler.
Grooves 32 are cut in the heat absorbing surface up to the end of the heat radiating plate 24 at appropriate intervals as shown. Therefore, to be precise, the heat radiating plate 24 is joined to the heat generating surface of the electronic cooler in a rectangular or island-shaped portion excluding the groove 32 portion.

FIG. 2B is a view showing a process of connecting the heat radiating plate 24 to the electronic cooler, and shows a state in which an adhesive or a low-temperature molten metal 30 as a bonding agent is applied on the heat radiating plate 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 in which the applied amount fluctuates is larger than in the conventional structure.

FIG. 2C is a view showing a state where the heat generating surface 23 of the electronic cooler is bonded to a heat radiating plate 24. The excess adhesive or the low-temperature molten metal 30 is pushed into the concave portion of the groove 32 and does not overflow from under the heat generating surface 23 of the electronic cooler. In addition, the adhesive or the low-temperature molten metal 3 formed between the heat generating surface 23 of the electronic cooler and the heat radiating plate 24
The film thickness of 0 becomes very thin. Further, the adhesive or the low-temperature molten metal 30 is uniformly distributed between the heat generating surface 23 and the heat radiating plate 24, so that an air layer is not formed unlike the conventional case.
Therefore, since the film thickness is very thin and the air layer does not intervene, the thermal resistance from the heat generating surface 23 of the electronic cooler to the heat radiating plate 24 becomes very small. Also, by keeping the pressing force to the electronic cooler constant, the adhesive or the low-temperature molten metal
0 is always formed to a predetermined thickness. For this reason, it is possible to realize a bonding process in which the thermal resistance is always low for each manufacturing process.

As shown in FIG. 5, the image sensor chip 19 is mounted on the electronic cooler bonded to the heat radiating plate 24.
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 a base lead 26 connected to a gold wire 27. The image sensor chip 19, the thermoelectric cooler, and the thermistor 20 are hermetically sealed by a cap 28, and are placed in a dry inert gas atmosphere or a vacuum. An auxiliary heat radiating plate 25 is attached to the heat radiating plate 24.

In the semiconductor photodetector according to the present embodiment having the above-described structure, the electronic cooling element and the heat radiating plate 24 are bonded to each other with extremely low thermal resistance as described above. Cooled efficiently. Therefore, the dark current generated in the photodiode formed in the image sensor chip 19 can be extremely reduced, and the S / N ratio of the light detection can be improved as follows.

The image sensor chip 19 is an image pickup device for converting a light input position into time-series information, in which photodiodes are arranged one-dimensionally or two-dimensionally. The image sensor includes various functional units for photoelectric conversion, accumulation, and scanning. For scanning, switch means or transfer means is used, and by this scanning, signal charges are carried to a video line which is a common signal line. In addition, since each photodiode of the image sensor is geometrically fixed, the image distortion is essentially small, and it is small and lightweight, has excellent environmental resistance such as vibration and shock, and has a low voltage drive and low power consumption. It has such features as: Such an image sensor is considered to have a wide range of applications as a sensor having a visual function, and a number of typical application fields such as a video camera, object recognition, a multi-channel detector for a spectrophotometer, distance metering, and color identification are conceivable. .

Further, as described above, the image sensor
It is roughly classified into a one-dimensional array and a two-dimensional array according to the arrangement of the light receiving units, and further roughly classified into an address system and a signal transfer system by a scanning function. FIG. 2 is a circuit diagram configured inside a MOS image sensor chip which is an address type one-dimensional image sensor array. In an addressing method represented by a MOS image sensor, a continuous palace formed in a shift register 1 composed of a MOSFET is applied to an address switch 2, so that the electric charge discharged in the photodiode 3 is a video signal which 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 using a resistor. An N-channel MOSFET using the P-type silicon substrate 5 constitutes an address switch 6.
Further, the PN junction of the source region 7 constitutes a photodiode, and performs a photoelectric conversion function and also functions as a charge storage unit.

When a positive address palace is applied to gate electrode 8, an N channel is formed on the surface of silicon substrate 5 under gate electrode 8. Therefore, the potential of the source region 7 constituting the photodiode is changed to the potential V of the drain region 9.
Charge is supplied from the drain region 9 to the source region 7 via the N channel until the charge 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 to which a reverse bias of Vcc volts is applied. On the other hand, when the address pulse is not applied to the gate electrode 8, no channel is formed on the silicon substrate 5 below the gate electrode 8. Therefore, the potential of the source region 7 constituting the photodiode is in a floating state,
The accumulation operation is started. That is, when light is incident in this state to excite carriers 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 reduced. Drops. At this time, when completely discharged,
The potential of the photodiode becomes the minimum value of 0 V, and the reverse bias disappears.

It is clear that the reverse bias of the photodiode in the reset state is Vcc volts and becomes 0 volt when completely discharged, so that the maximum value Qmax of the amount of charge that can be discharged by the photodiode is determined by the photodiode capacity. Assuming that Cpd, the following equation is used.

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

The total discharge charge 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 accumulation mode, and is useful for detecting weak light. The discharge charge amount Qs is represented 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 (time interval for turning on the gate).

Qs = (Ip + Id) × t (2) When the incident light is very weak, the photocurrent Ip becomes small in the above equation, but if the accumulation time t is prolonged, the total , The output charge amount Qs becomes large, and a signal can be extracted without causing a decrease in 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
If d is large, the accumulation time t cannot be long, and therefore, the S / N ratio may not be able to be improved as expected.

However, in the photodiode of the image sensor, it is possible to dramatically lower the dark current Id by lowering the temperature. When detecting an extremely weak light, a method of cooling the image sensor to increase the accumulation time is often used.

Accordingly, by configuring 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,
This 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. For this reason, according to the present embodiment, light detection can be performed with a good S / N ratio regardless of the size of the imaging element area.

[0032]

As described above, according to the present invention, when the electronic cooling element comes into contact with the heat radiating plate with a predetermined pressing force, the extra bonding agent interposed between the electronic cooling element and the heat radiating plate is radiated. No excess bonding agent is distributed around the thermoelectric cooling element by being pushed into the groove of the plate, and no air layer is formed in the bonding agent. Further, when the electronic cooling element is pressed with a predetermined pressing force, the thickness of the bonding agent is formed to a predetermined thickness, and the thermal resistance from the electronic cooling element to the heat radiating plate is sufficiently reduced. Therefore, even if the application amount of the bonding agent slightly fluctuates, the thermoelectric cooling element is always bonded to the heat sink with low thermal resistance.

Therefore, the heat released from the thermoelectric cooler is always transmitted to the radiator plate very efficiently regardless of the size of the thermoelectric cooler. Accordingly, a semiconductor photodetector capable of performing photodetection with a high S / N ratio is provided.

[Brief description of the 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 shows an address type one-dimensional image sensor array M
FIG. 2 is a circuit configuration diagram of an OS image sensor.

FIG. 3 is a diagram showing a pixel structure of the MOS type imaging device shown in FIG. 2;

FIG. 4 is a diagram showing a first conventional semiconductor photodetector that cools a MOS image sensor with an electronic cooling element.

FIG. 5 is a diagram showing a second conventional semiconductor photodetector that has solved the problems of the device shown in FIG. 4;

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

[Explanation of symbols]

19: Image sensor chip, 20: Chip thermistor, 21: Cooling surface of electronic cooler, 22: Peltier element,
23: Heating surface of the 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)

(57) [Claims] 1. A heat sink having a groove formed on a surface thereof, a bonding agent having good thermal conductivity applied to a surface of the heat sink, and a heat emitting surface contacting the bonding agent with a predetermined pressing force. An extra cooling agent is pushed into the groove of the heat radiating plate to reduce the thickness of the joining agent to a predetermined thickness and joined, and a semiconductor light receiving device mounted on the heat absorbing surface of the surface of the electronic cooling device A semiconductor photodetector, comprising: an element; and a lid that hermetically seals the electronic cooling element and the semiconductor light receiving element and that is in contact with the heat sink 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.