JP2017117835A - Photoelectric conversion element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a photoelectric conversion element capable of supporting a wide light quantity range.SOLUTION: A photoelectric conversion element 1A comprises: a plurality of pixels 10 which are formed on a common semiconductor substrate 30 and each of which includes an avalanche photodiode (APD) operating on common bias voltage; a first wiring 21 electrically connected to two or more first pixels 11 included in the plurality of pixels 10 and for collectively taking out output current from the two or more first pixels 11; and a second wiring 22 electrically connected to two or more second pixels 12 included in the plurality of pixels 10 and for collectively taking out output current from the two or more second pixels 12. A resistance value of a quenching resistor 24 of the second pixel 12 is larger than a resistance value of a quenching resistor 23 of the first pixel 11.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a photoelectric conversion element.

  Patent Document 1 discloses a radiation image reading apparatus. This radiation image reading apparatus includes a photoelectric conversion element for reading image information obtained by scanning excitation light on a photostimulable phosphor layer on which a radiation image is recorded. The photoelectric conversion element has a photodiode and a silicon photomultiplier tube. Then, the photoelectric conversion element for reading image information is switched between the photodiode and the silicon photomultiplier tube in accordance with the amount of stimulated light read by the photoelectric conversion element.

JP 2008-287165 A

  2. Description of the Related Art In recent years, a photoelectric conversion element for detecting weak light is known in which pixels made of avalanche photodiodes (hereinafter referred to as APDs) are two-dimensionally arranged. In such a photoelectric conversion element, a common bias voltage is supplied to each pixel, and the output current from each pixel is collected in a lump to perform photon counting, thereby reducing the amount of light incident on the photodiode array. It can measure with high accuracy. An example of such a photoelectric conversion device is MPPC (registered trademark) manufactured by Hamamatsu Photonics.

  However, as the application range of such a photoelectric conversion element expands, it is desired to be able to cope with a wide light amount range from a weak light amount to a relatively large light amount. This invention is made | formed in view of such a subject, and it aims at providing the photoelectric conversion element which can respond to a wide light quantity range.

  In order to solve the above-described problems, a photoelectric conversion element according to the present invention is formed on a common semiconductor substrate and includes a plurality of pixels each including an avalanche photodiode and a plurality of pixels formed on the semiconductor substrate and included in the plurality of pixels. A first wiring that is electrically connected to the first pixel via a quenching resistor and that collectively outputs output currents from the two or more first pixels; and a plurality of wirings formed on the semiconductor substrate. Two or more second pixels included in the pixel are electrically connected to each other through a quenching resistor, and a second wiring that collectively outputs output currents from the two or more second pixels is provided. The resistance value of the quenching resistor of the second pixel is larger than the resistance value of the quenching resistor of the first pixel.

  In this photoelectric conversion element, the resistance value of the quenching resistor of the second pixel is larger than the resistance value of the quenching resistor of the first pixel. In the two or more first pixels having a relatively small quenching resistance value, a relatively large current can be output even when the amount of incident light is weak. In other words, since the current can be output with a high gain with respect to the incident light amount, the lower limit of the detectable incident light amount can be reduced. On the other hand, two or more second pixels having relatively large quenching resistance values can output a relatively small current even when the amount of incident light is relatively large. In other words, since the current can be output with a low gain with respect to the incident light amount, the upper limit of the incident light amount at which the output is saturated can be further increased. Therefore, according to the above-described photoelectric conversion element, an output current is selectively extracted from the first wiring or the second wiring in accordance with the incident light amount, thereby supporting a wide light amount range from a weak light amount to a relatively large light amount. be able to.

  In the above photoelectric conversion element, the light receiving area of the first pixel and the light receiving area of the second pixel may be substantially equal to each other. Thereby, said effect can be easily acquired only by changing the resistance value of quenching resistance with respect to the conventional photoelectric conversion element.

  In the above photoelectric conversion element, the quenching resistance of the second pixel may be longer than the quenching resistance of the first pixel. Alternatively, the width in the direction intersecting with the extending direction of the quenching resistance of the second pixel may be narrower than the width in the direction intersecting with the extending direction of the quenching resistance of the first pixel. For example, by adopting at least one of these configurations, the resistance value of the quenching resistance of the second pixel can be easily made larger than the resistance value of the quenching resistance of the first pixel.

  In the above photoelectric conversion element, the bias voltage applied to the second pixel may be smaller than the bias voltage applied to the first pixel. Thereby, the light quantity range which can respond can be expanded further.

  ADVANTAGE OF THE INVENTION According to this invention, the photoelectric conversion element which can respond to a wide light quantity range can be provided.

It is a top view of the photoelectric conversion element concerning a 1st embodiment of the present invention. It is a top view which expands and shows a part of light-receiving surface. It is a figure which shows roughly the cross-sectional structure of a photoelectric conversion element. It is a figure which shows schematically the structural example of an external circuit. It is a figure which shows the example of the shape of quenching resistance as a 1st modification. It is a top view of the photoelectric conversion element which concerns on a 2nd modification. It is a top view which expands and shows a part of light-receiving surface. It is a figure which shows roughly the cross-sectional structure of a photoelectric conversion element. It is a figure which shows roughly the circuit structure which concerns on a 3rd modification.

  Hereinafter, embodiments of a photoelectric conversion element according to the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant description is omitted.

  FIG. 1 is a plan view of a photoelectric conversion element 1A according to an embodiment of the present invention. The photoelectric conversion element 1 </ b> A includes a semiconductor substrate 30, and the main surface of the semiconductor substrate 30 is a light receiving unit 3 </ b> A that receives light. In the light receiving unit 3A, a plurality of pixels 10 are arranged two-dimensionally. The plurality of pixels 10 each include an APD that operates with a common bias voltage.

  The plurality of pixels 10 include two or more first pixels 11 and two or more second pixels 12. The light receiving area (effective sensitive area) of the first pixel 11 and the light receiving area of the second pixel 12 are substantially equal to each other. In one example, the pitch (center distance) between adjacent first pixels 11 is 50 μm, and the pitch between adjacent second pixels 12 is also 50 μm. In the present embodiment, a first pixel column 11A in which the first pixels 11 are arranged in the column direction, and a second pixel column 12A in which the second pixels 12 are arranged in the column direction, Are arranged alternately in the row direction.

  The photoelectric conversion element 1A further includes a first wiring 21 and a second wiring 22 for signal reading. The first wiring 21 is electrically connected to two or more first pixels 11, and takes out output currents from these first pixels 11 at a time. The second wiring 22 is electrically connected to two or more second pixels 12, and takes out output currents from these second pixels 12 in a lump.

  FIG. 2 is an enlarged plan view showing a part of the light receiving unit 3A. As shown in FIG. 2, the APD of the first pixel 11 and the first wiring 21 are electrically connected via a quenching resistor 23. In other words, one end of the quenching resistor 23 is electrically connected to the APD of the first pixel 11, and the other end is electrically connected to the first wiring 21. Similarly, the APD of the second pixel 12 and the second wiring 22 are electrically connected via a quenching resistor 24. In other words, one end of the quenching resistor 24 is electrically connected to the APD of the second pixel 12, and the other end is electrically connected to the second wiring 22.

  The resistance value of the quenching resistor 24 of the second pixel 12 is larger than the resistance value of the quenching resistor 23 of the first pixel 11. In one example, the resistance value of the quenching resistor 24 is 500 kΩ, and the resistance value of the quenching resistor 23 is 250 kΩ. In another example, the resistance value of the quenching resistor 24 is 750 kΩ, and the resistance value of the quenching resistor 23 is 250 kΩ. In yet another example, the resistance value of the quenching resistor 24 is 1 MΩ, and the resistance value of the quenching resistor 23 is 250 kΩ. The resistance value of the quenching resistor 23 may be a value that does not latch (that is, can be quenched).

  Such a difference in resistance value of the quenching resistor is preferably realized, for example, by making the cross-sectional areas of the quenching resistors different from each other or making the lengths of the quenching resistors different from each other. In the example shown in FIG. 2, the quenching resistor 23 is linearly arranged and shortened to reduce the resistance value of the quenching resistor 23, and the quenching resistor 24 is increased to increase the resistance value of the quenching resistor 24. The resistor 24 is arranged in a spiral shape to make it longer. The quenching resistors 23 and 24 are made of, for example, a light transmissive (translucent) conductive material.

  FIG. 3 is a diagram schematically showing a cross-sectional configuration of the photoelectric conversion element 1A. The photoelectric conversion element 1 </ b> A includes a semiconductor substrate 30. The plurality of pixels 10 described above are formed on this common semiconductor substrate 30. Specifically, the semiconductor substrate 30 has a main surface 30a and a back surface 30b, and a lower surface electrode (cathode) 31 is provided on the entire surface of the back surface 30b. The semiconductor substrate 30 is formed by laminating a region 30c including the back surface 30b and made of n-type Si, and a region 30d including the main surface 30a and made of p-type Si. Inside the region 30d including the main surface 30a, a p-type semiconductor region 32a constituting the first pixel 11 and a p-type semiconductor region 32b constituting the second pixel 12 are arranged at intervals. Is formed. The p-type semiconductor regions 32a and 32b are made of, for example, p-type Si. The APD of the first pixel 11 is configured by forming a pn junction between the region 30d and the region 30c immediately below the p-type semiconductor region 32a. Similarly, the APD of the second pixel 12 is configured by forming a pn junction between the region 30d and the region 30c immediately below the p-type semiconductor region 32b.

A first insulating film 33 is provided on the entire surface of the main surface 30a. The first insulating film 33 can be preferably composed of an insulating silicon compound such as SiO ₂ or SiN. A contact electrode (anode) 34 a is provided on the p-type semiconductor region 32 a and on the first insulating film 33. The contact electrode 34 a is in contact with the p-type semiconductor region 32 a through the opening formed in the first insulating film 33. Similarly, a contact electrode (anode) 34 b is provided on the p-type semiconductor region 32 b and on the first insulating film 33. The contact electrode 34 b is in contact with the p-type semiconductor region 32 b through the opening formed in the first insulating film 33.

  The first wiring 21 and the second wiring 22 are made of metal and are formed on the semiconductor substrate 30. In the present embodiment, the first wiring 21 and the second wiring 22 are the first insulating film located on the region of the semiconductor substrate 30 where neither the p-type semiconductor region 32a nor the p-type semiconductor region 32b is formed. 33 is provided.

The first wiring 21 and the second wiring 22, the first insulating film 33, and the contact electrodes 34 a and 34 b are covered with a second insulating film 35. The second insulating film 35 covers the entire surface of the semiconductor substrate 30 and can be suitably configured by an inorganic insulator such as SiO ₂ or SiN. The quenching resistors 23 and 24 described above are provided on the second insulating film 35. One end and the other end of the quenching resistor 23 are electrically connected to the contact electrode 34 a and the first wiring 21 through an opening formed in the second insulating film 35. One end and the other end of the quenching resistor 24 are electrically connected to each of the contact electrode 34 b and the second wiring 22 through an opening formed in the second insulating film 35.

  Here, a configuration example of an external circuit for reading a signal from the photoelectric conversion element 1A of the present embodiment will be described. FIG. 4 is a diagram schematically showing a configuration example of the external circuit. As shown in FIG. 4, a common bias voltage HV is applied to the cathode of each APD of the plurality of pixels 10, that is, the lower surface electrode 31 (see FIG. 3).

  The anode of the APD of the first pixel 11 is connected to one end of a resistor 41 provided outside the photoelectric conversion element 1 </ b> A via the quenching resistor 23 and the first wiring 21. The other end of the resistor 41 is connected to a reference potential (GND) line 51. One end of the resistor 41 is connected to the photon counting circuit 60, and the voltage drop across the resistor 41 is input to the photon counting circuit 60.

  The photon counting circuit 60 includes a comparator 61 and a counter 62. The comparator 61 compares the reference voltage generated by the D / A converter 63 with the voltage drop across the resistor 41. When the voltage drop across the resistor 41 exceeds the reference voltage (that is, when a current pulse exceeding the threshold is output from the first pixel 11), a signal is sent to the counter 62. The counter 62 counts the number of times the signal is sent from the comparator 61. The count value corresponds to the amount of incident light on all the first pixels 11.

  The anode of the APD of the second pixel 12 is connected to one end of a resistor 42 provided outside the photoelectric conversion element 1 </ b> A via the quenching resistor 24 and the second wiring 22. The other end of the resistor 42 is connected to a reference potential (GND) line 51. One end of the resistor 42 is connected to the A / D converter 72 via the amplifier circuit 70 and the peak hold circuit 71. The voltage drop across the resistor 42 is amplified by the amplifier circuit 70 and then held by the peak hold circuit 71. Then, the held voltage is input to the A / D converter 72. The A / D converter 72 converts the input voltage signal (analog signal) into a digital signal. The digital value corresponds to the amount of incident light on all the second pixels 12.

  The count value output from the counter 62 and the digital value output from the A / D converter 72 are sent to the signal processing unit 80. The signal processing unit 80 employs one having a significant value among the count value and the digital value, and specifies the amount of incident light based on the value.

  The effects obtained by the photoelectric conversion element 1A of the present embodiment described above will be described. In the photoelectric conversion element 1 </ b> A of the present embodiment, the resistance value of the quenching resistor 24 of the second pixel 12 is larger than the resistance value of the quenching resistor 23 of the first pixel 11. The two or more first pixels 11 having a relatively low resistance value of the quenching resistor 23 can output a relatively large current even when the amount of incident light is weak. In other words, since the current can be output with a high gain with respect to the incident light amount, the lower limit of the detectable incident light amount can be reduced. On the other hand, two or more second pixels 12 having a relatively large resistance value of the quenching resistor 24 can output a relatively small current even when the amount of incident light is relatively large. In other words, since the current can be output with a low gain with respect to the incident light amount, the upper limit of the incident light amount at which the output is saturated can be further increased. Therefore, according to the photoelectric conversion element 1A of the present embodiment, the output current is selectively extracted from the first wiring 21 or the second wiring 22 in accordance with the amount of incident light, thereby widening from a weak light amount to a relatively large light amount. It can correspond to the light intensity range.

  Further, according to the present embodiment, it is possible to cope with both a case where the amount of incident light is weak and a case where the amount of incident light is relatively large with a single device that shares the light detection principle of each pixel. As a result, it is expected that the operating voltage is shared, the cost is reduced by configuring the same on the same substrate, and the characteristics are made uniform. Further, according to the present embodiment, it is possible to arrange a large number of pixels 10 in a two-dimensional manner, and a large area light receiving surface can be easily realized. In addition, the degree of freedom of arrangement of the plurality of pixels 10 is high, and it is easy to change the light receiving unit 3A to a shape suitable for a use and an optical system such as a square, a rectangle, a circle, and a polygon.

  Further, as in the present embodiment, the light receiving area of the first pixel 11 and the light receiving area of the second pixel 12 may be substantially equal to each other. Thereby, said effect can be easily acquired only by changing the resistance value of quenching resistance 23 and 24 with respect to the conventional photoelectric conversion element.

  Moreover, the quenching resistor 24 may be longer than the quenching resistor 23 as in the present embodiment. By adopting such a configuration, the resistance value of the quenching resistor 24 can be easily made larger than the resistance value of the quenching resistor 23.

(First modification)
FIG. 5A and FIG. 5B are cross-sectional views of quenching resistors 24 and 23 according to a modification, and intersect with the extending direction of the quenching resistors 24 and 23 (typically vertical). N) shows a cross section. In this example, the width W 1 of the quenching resistor 24 is narrower than the width W 2 of the quenching resistor 23. In the embodiment described above, the resistance values are made different by making the lengths of the quenching resistors 23 and 24 different, but also by making the widths of the quenching resistors 23 and 24 different as in this modification. The resistance value can be easily varied.

(Second modification)
FIG. 6 is a plan view of a photoelectric conversion element 1B according to a second modification of the present invention. The difference between the photoelectric conversion element 1B and the above embodiment is the arrangement of the first pixels 11 and the second pixels 12 on the light receiving surface. In the light receiving unit 3B of the present modification, a plurality of first regions A1 each including K ₁ (K ₁ is an integer equal to or greater than 2 and K ₁ = 16 is illustrated in the figure), respectively. , K ₂ (K ₂ is an integer greater than or equal to ₂ ; in the figure, K ₂ = 16 is exemplified) a plurality of second regions A 2 each including a second pixel 12 are mixed in the light receiving unit 3 B. They are arranged in a two-dimensional form (matrix form). In the example shown in FIG. 6, the first area A1 and the second area A2 are arranged in a checkered pattern.

FIG. 7A and FIG. 7B are enlarged plan views showing the first region A1 and the second region A2, respectively. FIG. 8 is a diagram schematically showing a cross-sectional configuration of the photoelectric conversion element 1B. In one example, in the first area A1, the first pixels 11 are two-dimensionally arranged in M ₁ rows and N ₁ columns (M ₁ and N ₁ are integers of 1 or more, where M ₁ × N ₁ = K ₁ ). ing. The first wirings 21 are arranged every two rows, and the first pixels 11 located on both sides of each first wiring 21 are connected to the first wirings 21 via quenching resistors 23. Electrically connected. Similarly, in the second area A2, the second pixels 12 are two-dimensionally arranged in M ₂ rows and N ₂ columns (M ₂ and N ₂ are integers of 1 or more, where M ₂ × N ₂ = K ₂ ). ing. The second wirings 22 are arranged every two rows, and the second pixels 12 located on both sides of each second wiring 22 are connected to the second wirings 22 via quenching resistors 24. Electrically connected.

  The arrangement of the first and second pixels in the present invention is not limited to the above-described embodiment, and various forms such as the photoelectric conversion element 1B of the present modification are possible. Further, the same effects as those of the photoelectric conversion element 1A of the above-described embodiment can be suitably achieved regardless of the pixel arrangement.

(Third Modification)
FIG. 9 is a diagram showing a circuit configuration according to a third modification of the present invention. In the above embodiment (see FIG. 4), the common bias voltage HV is applied to the plurality of pixels 10. However, in this modification, the bias voltage applied to the second pixel 12 is the first pixel 11. Smaller than the bias voltage applied to. Specifically, a common bias potential HV is applied to the cathodes of the respective APDs. The anode of the APD of the first pixel 11 is connected via the quenching resistor 23, the first wiring 21, and the resistor 41. Are connected to the first reference potential (GND) line 52. On the other hand, the anode of the APD of the second pixel 12 is connected to the second reference potential (GND) line 53 through the quenching resistor 24, the second wiring 22, and the resistor 42. The potential of the second reference potential (GND) line 53 is set higher than the potential of the first reference potential (GND) line 52. As a result, the bias voltage applied to the second pixel 12 is substantially smaller than the bias voltage applied to the first pixel 11.

  According to this modification, the first pixel 11 can increase the sensitivity to the incident light amount and output a larger current with respect to the weak incident light amount. In other words, since the current can be output with a higher gain with respect to the incident light amount, the lower limit of the detectable incident light amount can be further reduced. On the other hand, in the second pixel 12, the sensitivity to the incident light amount can be lowered, and the output current can be further reduced with respect to the large incident light amount. In other words, since the current can be output with a lower gain with respect to the incident light amount, the upper limit of the incident light amount at which the output is saturated can be further increased. Therefore, according to the present modification, it is possible to further expand the compatible light quantity range as compared with the above embodiment.

  Note that the APD cathode of the first pixel 11 and the APD cathode of the second pixel 12 are electrically separated, and the bias voltage applied to the APD cathode of the first pixel 11 is set to the second voltage. The bias voltage applied to the cathode of the APD of the pixel 12 may be larger. Even if it is such a structure, the said effect of this modification can be acquired suitably.

  In the above embodiment, the resistance value of the resistor 41 that converts the output current from the first pixel 11 into a voltage signal is greater than the resistance value of the resistor 42 that converts the output current from the second pixel 12 into a voltage signal. May be larger. Thereby, even when the amount of incident light is weak, the output current from the first pixel 11 can be converted into a voltage signal with a relatively large amplification factor. In other words, since a voltage signal can be generated with a high gain with respect to the incident light amount, photon counting can be performed with high accuracy. On the other hand, even when the amount of incident light is relatively large, the output current from the second pixel 12 can be converted into a voltage signal with a relatively small amplification factor. In other words, since the voltage signal can be generated with a low gain with respect to the incident light amount, the upper limit of the incident light amount at which the output is saturated can be further increased. Therefore, it is possible to further expand the light quantity range that can be handled.

  The photoelectric conversion element according to the present invention is not limited to the above-described embodiment, and various other modifications are possible. For example, the above-described embodiments and modifications may be combined with each other as necessary. Moreover, in the said embodiment and the 1st modification, although exemplifying changing these length and width as a method of making resistance value of quenching resistance 23 and 24 mutually differ, quenching resistance 23, The resistance values of 24 may be different from each other. For example, these resistance values can be suitably varied by making the concentrations of the resistance components added to the quenching resistors 23 and 24 different from each other. In the above embodiment, Si is exemplified as the constituent material of the semiconductor substrate 30 and the p-type semiconductor regions 32a and 32b. However, in the present invention, various semiconductor materials can be adopted for the semiconductor substrate and each p-type semiconductor region.

  DESCRIPTION OF SYMBOLS 1A, 1B ... Photoelectric conversion element, 3A, 3B ... Light-receiving part, 10 ... Pixel, 11 ... 1st pixel, 11A ... 1st pixel row, 12 ... 2nd pixel, 12A ... 2nd pixel row, 21 ... 1st wiring, 22 ... 2nd wiring, 23, 24 ... Quenching resistance, 30 ... Semiconductor substrate, 31 ... Bottom electrode, 32a, 32b ... p-type semiconductor region, 33 ... 1st insulating film, 34a, 34b ... contact electrode, 35 ... second insulating film, 41, 42 ... resistor, 60 ... photon counting circuit, 61 ... comparator, 62 ... counter, 63 ... D / A converter, 70 ... amplification circuit, 71 ... peak hold circuit 72 ... A / D converter, 80 ... signal processing unit, A1 ... first region, A2 ... second region.

Claims (5)

A plurality of pixels formed on a common semiconductor substrate and each including an avalanche photodiode;
Two or more first pixels included in the plurality of pixels formed on the semiconductor substrate are electrically connected via a quenching resistor, and output currents from the two or more first pixels are collectively collected. First wiring to be taken out,
Two or more second pixels included in the plurality of pixels formed on the semiconductor substrate are electrically connected via a quenching resistor, and output currents from the two or more second pixels are collectively collected. And a second wiring to be taken out,
The photoelectric conversion element in which a resistance value of the quenching resistor of the second pixel is larger than a resistance value of the quenching resistor of the first pixel.   The photoelectric conversion element according to claim 1, wherein a light receiving area of the first pixel and a light receiving area of the second pixel are substantially equal to each other.   3. The photoelectric conversion element according to claim 1, wherein the quenching resistance of the second pixel is longer than the quenching resistance of the first pixel.   The width of the second pixel in the direction intersecting with the extending direction of the quenching resistor is narrower than the width of the first pixel in the direction intersecting with the extending direction of the quenching resistor. The photoelectric conversion element according to any one of 3.   The photoelectric conversion element according to claim 1, wherein a bias voltage applied to the second pixel is smaller than a bias voltage applied to the first pixel.

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