Patent Publication Number: US-9419159-B2

Title: Semiconductor light-detecting element

Description:
This is a continuation application of copending application Ser. No. 13/147,871, having a §371 date of Aug. 4, 2011, which is a national stage filing based on PCT International Application No. PCT/JP2010/052210, filed on Feb. 15, 2010. The copending application Ser. No. 13/147,871 is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a semiconductor light-detecting element. 
     BACKGROUND ART 
     A photodiode using compound semiconductors is known as a photodiode with a high spectral sensitivity characteristic in the near-infrared wavelength band (e.g., cf. Patent Literature 1). The photodiode described in Patent Literature 1 is provided with a first light receiving layer comprised of one of InGaAsN, InGaAsNSb, and InGaAsNP, and a second light receiving layer having an absorption edge at a longer wavelength than that of the first light receiving layer and comprised of a quantum well structure. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent Application Laid-open No. 2008-153311 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, such photodiodes using the compound semiconductors are still expensive and their manufacturing steps are also complicated. For this reason, there are desires for practical application of a silicon photodiode being inexpensive and easy to manufacture and having sufficient spectral sensitivity in the near-infrared wavelength band. The conventional silicon photodiodes generally had the spectral sensitivity characteristic with the limit of about 1100 nm on the long wavelength side, but the spectral sensitivity characteristic in the wavelength band of not less than 1000 nm was not enough. 
     It is an object of the present invention to provide a semiconductor light-detecting element using silicon and having a sufficient spectral sensitivity characteristic in the near-infrared wavelength band. 
     Solution to Problem 
     A semiconductor light-detecting element according to the present invention is one comprising: a silicon substrate having a pn junction comprised of a semiconductor region of a first conductivity type and a semiconductor region of a second conductivity type, wherein on the silicon substrate, an accumulation layer of the first conductivity type is formed on a principal surface side of the silicon substrate and an irregular asperity is formed in at least a region opposed to the pn junction in the principal surface, and wherein the region opposed to the pn junction in the principal surface of the silicon substrate is optically exposed. 
     In the semiconductor light-detecting element of the present invention the irregular asperity is formed in at least the region opposed to the pn junction in the principal surface of the silicon substrate. For this reason, light incident into the semiconductor light-detecting element is reflected, scattered, or diffused by the region to travel through a long distance in the silicon substrate. This causes the light incident into the semiconductor light-detecting element to be mostly absorbed in the silicon substrate, without passing through the semiconductor light-detecting element (silicon substrate). In the foregoing semiconductor light-detecting element, therefore, the travel distance of the light incident into the semiconductor light-detecting element becomes long and the distance of absorption of light also becomes long, so as to improve the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the semiconductor light-detecting element according to the present invention, the accumulation layer of the first conductivity type is formed on the principal surface side of the silicon substrate. For this reason, unnecessary carriers generated independent of light on the principal surface side recombine there, so as to reduce dark current. The first conductivity type accumulation layer prevents carriers generated by light near the principal surface of the silicon substrate from being trapped in the principal surface. For this reason, the carriers generated by light efficiently migrate to the pn junction, so as to improve the photodetection sensitivity of the semiconductor light-detecting element. 
     A photodiode according to the present invention is one comprising: a silicon substrate comprised of a semiconductor of a first conductivity type, having a first principal surface and a second principal surface opposed to each other, and having a semiconductor region of a second conductivity type formed on the first principal surface side, wherein on the silicon substrate, an accumulation layer of the first conductivity type having a higher impurity concentration than the silicon substrate is formed on the second principal surface side and an irregular asperity is formed in at least a region opposed to the semiconductor region of the second conductivity type in the second principal surface, and wherein the region opposed to the semiconductor region of the second conductivity type in the second principal surface of the silicon substrate is optically exposed. 
     In the photodiode according to the present invention, as described above, the travel distance of light incident into the photodiode becomes long and the distance of absorption of light also becomes long, so as to improve the spectral sensitivity characteristic in the near-infrared wavelength band. The accumulation layer of the first conductivity type formed on the second principal surface side of the silicon substrate reduces the dark current and improves the photodetection sensitivity of the photodiode. 
     Preferably, a portion in the silicon substrate corresponding to the semiconductor region of the second conductivity type is thinned from the second principal surface side while leaving a surrounding region around the thinned portion. In this case, the photodiode can be obtained with respective light incident surfaces on the first principal surface and second principal surface sides of the silicon substrate. 
     Preferably, a thickness of the accumulation layer of the first conductivity type is larger than a height difference of the irregular asperity. In this case, as described above, it is feasible to ensure the operational effect by the accumulation layer. 
     Advantageous Effects of Invention 
     The present invention successfully provides the semiconductor light-detecting element using silicon and having the sufficient spectral sensitivity characteristic in the near-infrared wavelength band. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a drawing for explaining a manufacturing method of a photodiode according to the first embodiment. 
         FIG. 2  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 3  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 4  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 5  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 6  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 7  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 8  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 9  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 10  is a drawing for explaining the manufacturing method of the photodiode according to the first embodiment. 
         FIG. 11  is a drawing showing a configuration of the photodiode according to the first embodiment. 
         FIG. 12  is a diagram showing changes of spectral sensitivity versus wavelength in Example 1 and Comparative Example 1. 
         FIG. 13  is a diagram showing changes of temperature coefficient versus wavelength in Example 1 and Comparative Example 1. 
         FIG. 14  is a drawing for explaining a manufacturing method of a photodiode according to the second embodiment. 
         FIG. 15  is a drawing for explaining the manufacturing method of the photodiode according to the second embodiment. 
         FIG. 16  is a drawing for explaining the manufacturing method of the photodiode according to the second embodiment. 
         FIG. 17  is a drawing for explaining a manufacturing method of a photodiode according to the third embodiment. 
         FIG. 18  is a drawing for explaining the manufacturing method of the photodiode according to the third embodiment. 
         FIG. 19  is a drawing for explaining the manufacturing method of the photodiode according to the third embodiment. 
         FIG. 20  is a drawing for explaining the manufacturing method of the photodiode according to the third embodiment. 
         FIG. 21  is a drawing for explaining the manufacturing method of the photodiode according to the third embodiment. 
         FIG. 22  is a drawing for explaining a manufacturing method of a photodiode according to the fourth embodiment. 
         FIG. 23  is a drawing for explaining the manufacturing method of the photodiode according to the fourth embodiment. 
         FIG. 24  is a drawing for explaining the manufacturing method of the photodiode according to the fourth embodiment. 
         FIG. 25  is a drawing for explaining a configuration of a photodiode array according to the fifth embodiment. 
         FIG. 26  is a perspective view showing a semiconductor light-detecting element according to the sixth embodiment. 
         FIG. 27  is a drawing for explaining a cross-sectional configuration of the semiconductor light-detecting element according to the sixth embodiment. 
         FIG. 28  is a drawing for explaining a configuration of a photodiode according to the seventh embodiment. 
         FIG. 29  is a drawing for explaining a configuration of a photodiode array according to the eighth embodiment. 
         FIG. 30  is a plan view schematically showing a photodiode array according to the ninth embodiment. 
         FIG. 31  is a drawing showing a cross-sectional configuration along the line XXXI-XXXI in  FIG. 30 . 
         FIG. 32  is a drawing schematically explaining a connection relation of each photodetecting channel with a signal conductor wire and a resistor. 
         FIG. 33  is a plan view schematically showing a MOS image sensor according to the tenth embodiment. 
         FIG. 34  is a drawing showing a cross-sectional configuration along the line XXXIV-XXXIV in  FIG. 33 . 
         FIG. 35  is a plan view showing a pixel in the MOS image sensor according to the tenth embodiment. 
         FIG. 36  is a drawing showing a cross-sectional configuration along the line XXXVI-XXXVI in  FIG. 35 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. In the description, the same elements or elements with the same functionality will be denoted by the same reference signs, without redundant description. 
     (First Embodiment) 
     A method for manufacturing a photodiode according to the first embodiment will be described with reference to  FIGS. 1 to 10 .  FIGS. 1 to 10  are drawings for explaining the manufacturing method of the photodiode according to the first embodiment. 
     The first step is to prepare an n −  type semiconductor substrate  1  comprised of silicon (Si) crystal and having a first principal surface  1   a  and a second principal surface  1   b  opposed to each other (cf.  FIG. 1 ). The n −  type semiconductor substrate  1  has the thickness of about 300 μm and the resistivity of about 1 kΩ·cm. In the present embodiment, a “high impurity concentration” refers to, for example, an impurity concentration of not less than about 1×10 17  cm −3  and is denoted by sign “+” attached to conductivity type. A “low impurity concentration” refers to, for example, an impurity concentration of not more than about 1×10 15  cm −3  and is denoted by sign “−” attached to conductivity type. Examples of n-type impurities include antimony (Sb), arsenic (As), and so on, and examples of p-type impurities include boron (B) and others. 
     Next, a p +  type semiconductor region  3  and an n +  type semiconductor region  5  are formed on the first principal surface  1   a  side of the n −  type semiconductor substrate  1  (cf.  FIG. 2 ). The p +  type semiconductor region  3  is formed by diffusing a p-type impurity in a high concentration from the first principal surface  1   a  side in the n −  type semiconductor substrate  1 , using a mask opening in a central region. The n +  type semiconductor region  5  is formed by diffusing an n-type impurity in a higher concentration than in the n −  type semiconductor substrate  1 , from the first principal surface  1   a  side in the n −  type semiconductor substrate  1  so as to surround the p +  type semiconductor region  3 , using another mask opening in a peripheral region. The p +  type semiconductor region  3  has the thickness of, for example, about 0.55 μm and the sheet resistance of, for example, 44 Ω/sq. The n +  type semiconductor region  5  has the thickness of, for example, about 1.5 μm and the sheet resistance of, for example, 12 Ω/sq. 
     Next, an insulating layer  7  is formed on the first principal surface  1   a  side of the n −  type semiconductor substrate  1  (cf.  FIG. 3 ). The insulating layer  7  is comprised of SiO 2  and is formed by thermal oxidation of the n −  type semiconductor substrate  1 . The insulating layer  7  has the thickness of, for example, about 0.1 μm. Thereafter, a contact hole H 1  is formed in the insulating layer  7  on the p +  type semiconductor region  3  and a contact hole H 2  is formed in the insulating layer  7  on the n +  type semiconductor region  5 . An antireflective (AR) layer comprised of SiN may be formed instead of the insulating layer  7 . 
     Next, a passivation layer  9  is formed on the second principal surface  1   b  of the n −  type semiconductor substrate  1  and on the insulating layer  7  (cf.  FIG. 4 ). The passivation layer  9  is comprised of SiN and is formed, for example, by the plasma CVD process. The passivation layer  9  has the thickness of, for example, 0.1 μm. Then the n −  type semiconductor substrate  1  is polished from the second principal surface  1   b  side before the thickness of the n −  type semiconductor substrate  1  reaches a desired thickness (cf.  FIG. 5 ). This process removes the passivation layer  9  from on the second principal surface  1   b  of the n −  type semiconductor substrate  1 , thereby exposing the n −  type semiconductor substrate  1 . A surface exposed by polishing is also referred to herein as the second principal surface  1   b . The desired thickness is, for example, 270 μm. 
     Next, the second principal surface  1   b  of the n −  type semiconductor substrate  1  is subjected to irradiation with a pulsed laser beam PL, thereby forming an irregular asperity  10  (cf.  FIG. 6 ). In this step, as shown in  FIG. 7 , the n −  type semiconductor substrate  1  is placed in a chamber C, and the n −  type semiconductor substrate  1  is irradiated with the pulsed laser beam PL from a pulse laser generating device PLD located outside the chamber C. The chamber C has a gas inlet port G IN  and a gas outlet port G OUT , and an inert gas (e.g., nitrogen gas, argon gas, or the like) is introduced through the gas inlet port G IN  and discharged through the gas outlet port G OUT , thereby forming an inert gas flow G f  in the chamber C. Dust and other materials made during the irradiation with the pulsed laser beam PL are discharged as trapped into the inert gas flow G f , to the outside of the chamber C, thereby preventing processing debris, dust, and other materials from attaching to the n −  type semiconductor substrate  1 . 
     In the present embodiment, the pulse laser generating device PLD to be used is a picosecond to femtosecond pulse laser generating device and a picosecond to femtosecond pulsed laser beam is applied across the entire area of the second principal surface  1   b . The second principal surface  1   b  is roughened by the picosecond to femtosecond pulsed laser beam, whereby the irregular asperity  10  is formed throughout the entire area of the second principal surface  1   b , as shown in  FIG. 8 . The irregular asperity  10  has facets intersecting with a direction perpendicular to the first principal surface  1   a . The height difference of asperity  10  is, for example, about 0.5 to 10 μm and the spacing of projections in the asperity  10  is about 0.5 to 10 μm. The picosecond to femtosecond pulsed laser beam has the pulse duration of, for example, about 50 fs-2 ps, the intensity of, for example, about 4 to 16 GW, and the pulse energy of, for example, about 200 to 800 μJ/pulse. More generally, the peak intensity is 3×10 11  to 2.5×10 13  (W/cm 2 ) and the fluence is about 0.1 to 1.3 (J/cm 2 ).  FIG. 8  is an SEM image resulting from observation of the irregular asperity  10  formed in the second principal surface  1   b.    
     Next, an accumulation layer  11  is formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1  (cf.  FIG. 9 ). In this step, the accumulation layer  11  is formed by ion implantation or diffusion of an n-type impurity from the second principal surface  1   b  side in the n −  type semiconductor substrate  1  so that an impurity concentration thereof becomes higher than that of the n −  type semiconductor substrate  1 . The accumulation layer  11  has the thickness of, for example, about 1 μm. 
     Next, the n −  type semiconductor substrate  1  is subjected to a thermal treatment (annealing). In this step, the n −  type semiconductor substrate  1  is heated, for example, in the temperature range of about 800 to 1000° C. under an ambiance of N 2  gas for about 0.5 to 1 hour. 
     Next, the passivation layer  9  formed on the insulating layer  7  is removed and thereafter electrodes  13 ,  15  are formed (cf.  FIG. 10 ). The electrode  13  is formed in the contact hole H 1  and the electrode  15  in the contact hole H 2 . The electrodes  13 ,  15  each are comprised of aluminum (Al) or the like and have the thickness of, for example, about 1 μm. This completes the photodiode PD 1 . 
     The photodiode PD 1  is provided with the n −  type semiconductor substrate  1 , as shown in  FIG. 10 . The p +  type semiconductor region  3  and the n +  type semiconductor region  5  are formed on the first principal surface  1   a  side of the n −  type semiconductor substrate  1  and a pn junction is formed between the n −  type semiconductor substrate  1  and the p +  type semiconductor region  3 . The electrode  13  is in electrical contact with and connection to the p +  type semiconductor region  3  through the contact hole H 1 . The electrode  15  is in electrical contact with and connection to the n +  type semiconductor region  5  through the contact hole H 2 . 
     The irregular asperity  10  is formed in the second principal surface  1   b  of the n −  type semiconductor substrate  1 . The accumulation layer  11  is formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1  and the second principal surface  1   b  is optically exposed. That the second principal surface  1   b  is optically exposed encompasses not only the case where the second principal surface  1   b  is in contact with ambient gas such as air, but also the case where an optically transparent film is formed on the second principal surface  1   b.    
     In the photodiode PD 1 , the irregular asperity  10  is formed in the second principal surface  1   b . For this reason, light L incident into the photodiode PD 1  is reflected, scattered, or diffused by the asperity  10 , as shown in  FIG. 11 , to travel through a long distance in the n −  type semiconductor substrate  1 . 
     Normally, Si has the refractive index n=3.5 and air the refractive index n=1.0. In a photodiode, when light is incident from a direction normal to a light incident surface, light remaining unabsorbed in the photodiode (silicon substrate) is separated into a light component reflected on the back surface to the light incident surface and a light component passing through the photodiode. The light passing through the photodiode does not contribute to the sensitivity of the photodiode. The light component reflected on the back surface to the light incident surface, if absorbed in the photodiode, becomes a photocurrent. A light component still remaining unabsorbed is reflected or transmitted by the light incident surface as the light component having reached the back surface to the light incident surface was. 
     In the photodiode PD 1 , where light L is incident from the direction normal to the light incident surface (first principal surface  1   a ), when the light reaches the irregular asperity  10  formed in the second principal surface  1   b , light components arriving thereat at angles of not less than 16.6° to a direction of emergence from the asperity  10  are totally reflected by the asperity  10 . Since the asperity  10  is formed irregularly, it has various angles to the emergence direction and the totally reflected light components diffuse into various directions. For this reason, the totally reflected light components include light components absorbed inside the n −  type semiconductor substrate  1  and light components arriving at the first principal surface  1   a  and side faces. 
     Since the light components arriving at the first principal surface  1   a  and side faces travel in various directions because of the diffusion on the asperity  10 , the light components arriving at the first principal surface  1   a  and the side faces are extremely highly likely to be totally reflected on the first principal surface  1   a  and the side faces. The light components totally reflected on the first principal surface  1   a  and the side faces are repeatedly totally reflected on different faces to further increase their travel distance. The light L incident into the photodiode PD 1  is absorbed in the n −  type semiconductor substrate  1  during travel through the long distance inside the n −  type semiconductor substrate  1  to be detected as a photocurrent. 
     The light L incident into the photodiode PD 1  mostly travels, without being transmitted by the photodiode PD 1 , through the long travel distance to be absorbed in the n −  type semiconductor substrate  1 . Therefore, the photodiode PD 1  is improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     If a regular asperity is formed in the second principal surface  1   b , the light components arriving at the first principal surface  1   a  and the side faces are diffused by the asperity but travel in uniform directions, and, therefore, the light components arriving at the first principal surface  1   a  and the side faces are less likely to be totally reflected on the first principal surface  1   a  and the side faces. This results in increase in light passing through the first principal surface  1   a  and the side faces, and through the second principal surface  1   b , and thus the travel distance of the light incident into the photodiode must be short. Accordingly, it becomes difficult to improve the spectral sensitivity characteristic in the near-infrared wavelength band. 
     An experiment was conducted in order to check the effect of improvement in the spectral sensitivity characteristic in the near-infrared wavelength band by the first embodiment. 
     We fabricated a photodiode with the above-described configuration (referred to as Example 1) and a photodiode without the irregular asperity in the second principal surface of the n −  type semiconductor substrate (referred to as Comparative Example 1), and investigated their spectral sensitivity characteristics. Example 1 and Comparative Example 1 have the same configuration, except for the formation of the irregular asperity by irradiation with the pulsed laser beam. The size of the n −  type semiconductor substrate  1  was set to 6.5 mm×6.5 mm. The size of the p +  type semiconductor region  3 , or a photosensitive region was set to 5.8 mm×5.8 mm. A bias voltage VR applied to the photodiodes was set to 0 V. 
     The results are shown in  FIG. 12 . In  FIG. 12 , the spectral sensitivity characteristic of Example 1 is represented by T 1  and the spectral sensitivity characteristic of Comparative Example 1 by characteristic T 2 . In  FIG. 12 , the vertical axis represents the spectral sensitivity (mA/W) and the horizontal axis the wavelength of light (nm). A characteristic indicated by a chain line represents a spectral sensitivity characteristic where the quantum efficiency (QE) is 100%, and a characteristic indicated by a dashed line, a spectral sensitivity characteristic where the quantum efficiency is 50%. 
     As seen from  FIG. 12 , for example at 1064 nm, the spectral sensitivity in Comparative Example 1 is 0.2 A/W (QE=25%) whereas the spectral sensitivity in Example 1 is 0.6 A/W (QE=72%); thus the spectral sensitivity in the near-infrared wavelength band is drastically improved. 
     We also checked temperature characteristics of spectral sensitivity in Example 1 and Comparative Example 1. We investigated the spectral sensitivity characteristics with increase in ambient temperature from 25° C. to 60° C. and calculated a rate (temperature coefficient) of spectral sensitivity at 60° C. to spectral sensitivity at 25° C. The results are shown in  FIG. 13 . In  FIG. 13 , the characteristic of temperature coefficient of Example 1 is represented by T 3  and that of Comparative Example 1 by characteristic T 4 . In  FIG. 13 , the vertical axis represents the temperature coefficient (%/° C.) and the horizontal axis the wavelength of light (nm). 
     As seen from  FIG. 13 , for example at 1064 nm, the temperature coefficient in Comparative Example 1 is 0.7%/° C., whereas the temperature coefficient in Example 1 is 0.2%/° C., demonstrating lower temperature dependence. In general, an increase in temperature leads to an increase in spectral sensitivity because of increase in absorption coefficient and decrease in bandgap energy. In Example 1, since the spectral sensitivity is sufficiently high even at room temperature, the change of spectral sensitivity due to temperature rise is smaller than in Comparative Example 1. 
     In the photodiode PD 1 , the accumulation layer  11  is formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1 . This induces recombination of unnecessary carriers generated independent of light on the second principal surface  1   b  side, which can reduce dark current. The accumulation layer  11  prevents carriers generated by light near the second principal surface  1   b , from being trapped in the second principal surface  1   b . For this reason, the carriers generated by light efficiently migrate to the pn junction, which can further improve the photodetection sensitivity of the photodiode PD 1 . 
     In the first embodiment, after the formation of the accumulation layer  11 , the n −  type semiconductor substrate  1  is subjected to the thermal treatment. This treatment restores the crystallinity of the n −  type semiconductor substrate  1 , which can prevent such a problem as increase of dark current. 
     In the first embodiment, after the thermal treatment of the n −  type semiconductor substrate  1 , the electrodes  13 ,  15  are formed. This prevents the electrodes  13 ,  15  from melting during the thermal treatment, even in the case where the electrodes  13 ,  15  are made of a metal with a relatively low melting point. Therefore, the electrodes  13 ,  15  can be appropriately formed without being affected by the thermal treatment. 
     In the first embodiment, the irregular asperity  10  is formed by the irradiation with the picosecond to femtosecond pulsed laser beam. This permits the irregular asperity  10  to be appropriately and readily formed. 
     (Second Embodiment) 
     A method for manufacturing a photodiode according to the second embodiment will be described with reference to  FIGS. 14 to 16 .  FIGS. 14 to 16  are drawings for explaining the manufacturing method of the photodiode according to the second embodiment. 
     The manufacturing method of the second embodiment, up to the polishing of the n −  type semiconductor substrate  1  from the second principal surface  1   b  side, is the same as the manufacturing method of the first embodiment, and the description of the previous steps before it is omitted herein. After the n −  type semiconductor substrate  1  is polished from the second principal surface  1   b  side to obtain the n −  type semiconductor substrate  1  in the desired thickness, the accumulation layer  11  is formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1  (cf.  FIG. 14 ). The formation of the accumulation layer  11  is carried out in the same manner as in the first embodiment. The accumulation layer  11  has the thickness of, for example, about 1 μm. 
     Next, the second principal surface  1   b  of the n −  type semiconductor substrate  1  is irradiated with the pulsed laser beam PL to form the irregular asperity  10  (cf.  FIG. 15 ). The formation of the irregular asperity  10  is carried out in the same manner as in the first embodiment. 
     Next, as in the first embodiment, the n −  type semiconductor substrate  1  is subjected to a thermal treatment. Thereafter, the passivation layer  9  formed on the insulating layer  7  is removed and then the electrodes  13 ,  15  are formed (cf.  FIG. 16 ). This completes the photodiode PD 2 . 
     In the second embodiment, as in the first embodiment, the travel distance of light incident into the photodiode PD 2  also becomes long and the distance of absorption of light also becomes long. This allows the photodiode PD 2  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the second embodiment, the thickness of the accumulation layer  11  is larger than the height difference of the irregular asperity  10 . For this reason, even if the irregular asperity  10  is formed by the irradiation with the pulsed laser beam after the formation of the accumulation layer  11 , the accumulation layer  11  remains with certainty. Therefore, it is feasible to ensure the operational effect by the accumulation layer  11 . 
     (Third Embodiment) 
     A method for manufacturing a photodiode according to the third embodiment will be described with reference to  FIGS. 17 to 21 .  FIGS. 17 to 21  are drawings for explaining the manufacturing method of the photodiode according to the third embodiment. 
     The manufacturing method of the third embodiment, up to the formation of the passivation layer  9 , is the same as the manufacturing method of the first embodiment, and the description of the previous steps before it is omitted herein. After the formation of the passivation layer  9 , a portion corresponding to the p +  type semiconductor region  3  in the n −  type semiconductor substrate  1  is thinned from the second principal surface  1   b  side while leaving a surrounding region around the thinned portion (cf.  FIG. 17 ). The thinning of the n −  type semiconductor substrate  1  is carried out, for example, by anisotropic etching based on alkali etching using a potassium hydroxide solution, TMAH (tetramethylammonium hydroxide solution), or the like. The thinned portion of the n −  type semiconductor substrate  1  has the thickness of, for example, about 100 μm, and the surrounding region around it has the thickness of, for example, about 300 μm. 
     Next, the n −  type semiconductor substrate  1  is polished from the second principal surface  1   b  side before the thickness of the surrounding region of the n −  type semiconductor substrate  1  reaches a desired thickness (cf.  FIG. 18 ). The desired thickness herein is, for example, 270 μm. 
     Next, the second principal surface  1   b  of the n −  type semiconductor substrate  1  is irradiated with the pulsed laser beam PL to form the irregular asperity  10  (cf.  FIG. 19 ). The formation of the irregular asperity  10  is carried out in the same manner as in the first embodiment. 
     Next, the accumulation layer  11  is formed on the second principal surface  1   b  side of the thinned portion of the n −  type semiconductor substrate  1  (cf.  FIG. 20 ). The formation of the accumulation layer  11  is carried out in the same manner as in the first embodiment. The accumulation layer  11  has the thickness of, for example, about 3 μm. 
     Next, as in the first embodiment, the n −  type semiconductor substrate  1  is subjected to a thermal treatment and thereafter, the passivation layer  9  formed on the insulating layer  7  is removed, followed by formation of the electrodes  13 ,  15  (cf.  FIG. 21 ). This completes the photodiode PD 3 . 
     In the third embodiment, as in the first and second embodiments, the travel distance of light incident into the photodiode PD 3  also becomes long and the distance of absorption of light also becomes long. This allows the photodiode PD 3  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the third embodiment, prior to the formation of the irregular asperity  10 , the portion corresponding to the p +  type semiconductor region  3  in the n −  type semiconductor substrate  1  is thinned from the second principal surface  1   b  side while leaving the surrounding region around the thinned portion. This permits the photodiode PD 3  to be formed with respective light incident surfaces on the first principal surface  1   a  and the second principal surface  1   b  sides of the n −  type semiconductor substrate  1 . 
     (Fourth Embodiment) 
     A method for manufacturing a photodiode according to the fourth embodiment will be described with reference to  FIGS. 22 to 24 .  FIGS. 22 to 24  are drawings for explaining the manufacturing method of the photodiode according to the fourth embodiment. 
     The manufacturing method of the fourth embodiment, up to the thinning of the n −  type semiconductor substrate  1 , is the same as the manufacturing method of the third embodiment, and the description of the previous steps before it is omitted herein. After the n −  type semiconductor substrate  1  is polished from the second principal surface  1   b  side to obtain the n −  type semiconductor substrate  1  in the desired thickness, the accumulation layer  11  is formed on the second principal surface  1   b  side of the thinned portion of the n −  type semiconductor substrate  1  (cf.  FIG. 22 ). The formation of the accumulation layer  11  is carried out in the same manner as in the first embodiment. The accumulation layer  11  has the thickness of, for example, about 3 μm. 
     Next, the second principal surface  1   b  of the n −  type semiconductor substrate  1  is irradiated with the pulsed laser beam PL to form the irregular asperity  10  (cf.  FIG. 23 ). The formation of the irregular asperity  10  is carried out in the same manner as in the first embodiment. 
     Next, the n −  type semiconductor substrate  1  is subjected to a thermal treatment as in the first embodiment. Then the passivation layer  9  formed on the insulating layer  7  is removed and thereafter, the electrodes  13 ,  15  are formed (cf.  FIG. 24 ). This completes the photodiode PD 4 . 
     In the fourth embodiment, as in the first to third embodiments, the travel distance of light incident into the photodiode PD 4  also becomes long and the distance of absorption of light also becomes long. This allows the photodiode PD 4  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the fourth embodiment, prior to the formation of the accumulation layer  11 , the portion corresponding to the p +  type semiconductor region  3  in the n −  type semiconductor substrate  1  is thinned from the second principal surface  1   b  side while leaving the surrounding region around the thinned portion. This permits the photodiode PD 4  to be formed with respective light incident surfaces on the first principal surface  1   a  and the second principal surface  1   b  sides of the n −  type semiconductor substrate  1 . 
     (Fifth Embodiment) 
     A photodiode array PDA 1  according to the fifth embodiment will be described with reference to  FIG. 25 .  FIG. 25  is a drawing for explaining a configuration of the photodiode array according to the fifth embodiment. 
     The photodiode array PDA 1  is provided with an n −  type semiconductor substrate  1 . A plurality of p +  type semiconductor regions  3  are formed on the first principal surface  1   a  side of the n −  type semiconductor substrate  1  and a pn junction is created between the n −  type semiconductor substrate  1  and each p +  type semiconductor region  3 . The irregular asperity  10  is formed in the second principal surface  1   b  of the n −  type semiconductor substrate  1 . The accumulation layer  11  is formed on the second principal surface  1   b  side and the second principal surface  1   b  is optically exposed. In the photodiode array PDA 1 , since the irregular asperity  10  is formed in the second principal surface  1   b , light incident into the photodiode array PDA 1  is reflected, scattered, or diffused by the asperity  10  to travel through a long distance in the n −  type semiconductor substrate  1 . 
     In the fifth embodiment, as in the first to fourth embodiments, the travel distance of light incident into the photodiode array PDA 1  becomes long and the distance of absorption of light also becomes long. This allows the photodiode array PDA 1  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the photodiode array PDA 1 , the accumulation layer  11  is formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1 . This induces recombination of unnecessary carriers generated independent of light on the second principal surface  1   b  side, so as to reduce the dark current. The accumulation layer  11  prevents carriers generated by light near the second principal surface  1   b , from being trapped in the second principal surface  1   b . For this reason, the carriers generated by light can efficiently migrate to the pn junctions so as to further improve the photodetection sensitivity of the photodiode array PDA 1 . 
     In the photodiode array PDA 1 , the irregular asperity  10  is also formed in regions opposed to the regions between the p +  type semiconductor regions  3  (pn junctions) in the second principal surface  1   b . For this reason, light incident into the regions between the p +  type semiconductor regions  3  is reflected, scattered, or diffused by the irregular asperity  10  formed in the regions opposed to the regions between the p +  type semiconductor regions  3  in the second principal surface  1   b , to be certainly absorbed in the n −  type semiconductor substrate  1 . Therefore, the photodiode array PDA 1  is improved in the photodetection sensitivity, without reduction in detection sensitivity in the regions between the p +  type semiconductor regions  3 . 
     (Sixth Embodiment) 
     A solid-state imaging device SI 1  according to the sixth embodiment will be described with reference to  FIGS. 26 and 27 .  FIG. 26  is a perspective view showing the solid-state imaging device according to the sixth embodiment.  FIG. 27  is a drawing for explaining a cross-sectional configuration of the solid-state imaging device according to the sixth embodiment. 
     The solid-state imaging device SI 1 , as shown in  FIG. 26 , is a back-thinned type solid-state imaging device and BT-CCD (charged coupled device) obtained by thinning the back side of a semiconductor substrate SS by etching with a KOH aqueous solution or the like. A recess TD is formed in a central region of the etched semiconductor substrate SS and a thick frame portion exists around the recess TD. Side faces of the recess TD are inclined at obtuse angles relative to a bottom face BF. The thinned central region of the semiconductor substrate SS is a photosensitive region (imaging region) and light L is incident along the negative direction of Z-axis into this photosensitive region. The bottom face BF of the recess TD of the semiconductor substrate SS constitutes a light incident surface. The frame portion may be removed by etching, whereby the solid-state imaging device is modified into a back-thinned type solid-state imaging device the entire region of which is thinned. 
     The solid-state imaging device SI 1  is provided with a p-type semiconductor substrate  21  as the foregoing semiconductor substrate SS. The p-type semiconductor substrate  21  is comprised of silicon (Si) crystal and has a first principal surface  21   a  and a second principal surface  21   b  opposed to each other. The p-type semiconductor substrate  21  has the thickness set to not more than the pixel pitch P. In the present embodiment, the pixel pitch P is about 10 to 48 μm and the thickness of the p-type semiconductor substrate  21  is about 10 to 30 μm. The present embodiment shows an example of two-phase clock driving and under each transfer electrode, there are regions of different impurity concentrations (not shown) in order to ensure unidirectional transfer of charge. 
     An n-type semiconductor layer  23  as a charge transfer part is formed on the first principal surface  21   a  side of the p-type semiconductor substrate  21 , and therefore a pn junction is formed between the p-type semiconductor substrate  21  and the n-type semiconductor layer  23 . A plurality of charge transfer electrodes  25  as a transfer electrode part are provided through an insulating layer  27  on the first principal surface  21   a  of the p-type semiconductor substrate  21 . On the first principal surface  21   a  side of the p-type semiconductor substrate  21 , isolation regions for electrically separating the n-type semiconductor layer  23  into regions for respective vertical CCDs are also formed though not shown. The n-type semiconductor layer  23  has the thickness of about 0.5 μm. 
     The irregular asperity  10  is formed throughout the entire photosensitive region  29  in the second principal surface  21   b  of the p-type semiconductor substrate  21 . An accumulation layer  31  is formed on the second principal surface  21   b  side of the p-type semiconductor substrate  21  and the second principal surface  21   b  is optically exposed. That the second principal surface  21   b  is optically exposed embraces not only the case where the second principal surface  21   b  is in contact with ambient gas such as air, but also the case where an optically transparent film is formed on the second principal surface  21   b . When the solid-state imaging device SI 1  is a back-thinned type solid-state imaging device wherein the entire region is thinned, the irregular asperity  10  may be formed throughout the entire area of the second principal surface  21   b  of the p-type semiconductor substrate  21 . When the solid-state imaging device SI 1  is a back-thinned type solid-state imaging device wherein only the region near the photosensitive region  29  is thinned, the irregular asperity  10  may be formed throughout the entire area of the second principal surface  21   b  including the peripheral frame portion not thinned in the p-type semiconductor substrate  21 , and the inclined faces connected to the frame portion. 
     A method for manufacturing the solid-state imaging device SI 1  of the present embodiment will be described below. 
     First, the p-type semiconductor substrate  21  is prepared and the n-type semiconductor layer  23  is formed on the first principal surface  21   a  side of the p-type semiconductor substrate  21 . The n-type semiconductor layer  23  is formed by diffusion of an n-type impurity from the first principal surface  21   a  side in the p-type semiconductor substrate  21 . 
     Next, the accumulation layer  31  is formed on the second principal surface  21   b  side of the p-type semiconductor substrate  21 . The accumulation layer  31  is formed by ion implantation or diffusion of a p-type impurity from the second principal surface  21   b  side in the p-type semiconductor substrate  21  so that an impurity concentration thereof becomes higher than that of the p-type semiconductor substrate  21 , as in the aforementioned embodiment. The accumulation layer  31  has the thickness of, for example, about 0.5 μm. The accumulation layer  31  may be formed before the formation of the irregular asperity  10  or may be formed after the formation of the irregular asperity  10 . 
     Next, the p-type semiconductor substrate  21  is thinned as described above. In the case where the accumulation layer  31  is formed after the formation of the irregular asperity  10 , the p-type semiconductor substrate  21  is thinned after the formation of the irregular asperity  10  and thereafter, the accumulation layer  31  is formed. 
     Next, the p-type semiconductor substrate  21  is subjected to a thermal treatment to activate the accumulation layer  31 . The thermal treatment is carried out, for example, in the temperature range of about 800 to 1000° C. under an ambiance of N 2  gas for about 0.5 to 1.0 hour. At this time, the crystallinity of the p-type semiconductor substrate  21  is also restored. 
     Next, the irregular asperity  10  is formed on the second principal surface  21   b  side of the p-type semiconductor substrate  21 . The irregular asperity  10  is formed by irradiating the second principal surface  21   b  of the p-type semiconductor substrate  21  with the pulsed laser beam, as in the aforementioned embodiment. 
     Next, the p-type semiconductor substrate  21  is subjected to a thermal treatment. The thermal treatment is carried out, for example, in the temperature range of about 800 to 1000° C. under an ambiance of N 2  gas for about 0.5 to 1.0 hour. The thermal treatment brings about recovery of crystal defects and recrystallization in the p-type semiconductor substrate  21 , which can prevent such a problem as increase in dark current. The thermal treatment after the formation of the accumulation layer  31  may be omitted, while only the thermal treatment after the formation of the irregular asperity  10  is carried out. 
     Next, the insulating layer  27  and charge transfer electrodes  25  are formed. Since steps of forming the insulating layer  27  and charge transfer electrodes  25  are known, the description thereof is omitted herein. The charge transfer electrodes  25  are comprised, for example, of polysilicon or metal. The insulating layer  27  is comprised, for example, of SiO 2 . A protecting film may be further formed so as to cover the insulating layer  27  and the charge transfer electrodes  25 . The protecting film is comprised, for example, of BPSG (Boron Phosphor Silicate Glass). This completes the solid-state imaging device SI 1 . 
     In the solid-state imaging device SI 1 , when light is incident to the light incident surface (second principal surface  21   b ), the incident light is scattered by the asperity  10  to travel in various directions in the p-type semiconductor substrate  21  because the irregular asperity  10  is formed in the second principal surface  21   b . Since light components arriving at the first principal surface  21   a  and others travel in various directions because of the diffusion on the asperity  10 , the light components arriving at the first principal surface  21   a  and others are extremely highly likely to be totally reflected on the first principal surface  21   a . The light components totally reflected on the first principal surface  21   a  and others are repeatedly totally reflected on different faces or, reflected, scattered, or diffused on the second principal surface  21   b , whereby their travel distance becomes longer. In this manner, the light incident into the solid-state imaging device SI 1  is reflected, scattered, or diffused by the asperity  10  to travel through a long distance in the p-type semiconductor substrate  21 . While the light incident into the solid-state imaging device SI 1  travels through the long distance inside the p-type semiconductor substrate  21 , the light is absorbed in the p-type semiconductor substrate  21  and carriers generated by the light turn to charges at respective pixels of the n-type semiconductor layer  23  to be transferred and detected. Therefore, the solid-state imaging device SI 1  is improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     The solid-state imaging device SI 1  has a risk of reduction in resolution due to occurrence of crosstalk between pixels resulting from reflection, scattering, or diffusion by the asperity  10 . However, since the thickness of the p-type semiconductor substrate  21  is set to not more than the pixel pitch P, the occurrence of crosstalk between pixels can be suppressed in the solid-state imaging device SI 1 . 
     In the solid-state imaging device SI 1 , the accumulation layer  31  is formed on the second principal surface  21   b  side of the p-type semiconductor substrate  21 . This induces recombination of unnecessary carriers generated independent of light on the second principal surface  21   b  side, which can reduce the dark current. The accumulation layer  31  prevents carriers generated by light near the second principal surface  21   b , from being trapped in the second principal surface  21   b . For this reason, the carriers generated by light efficiently migrate to the pn junction, which can further improve the photodetection sensitivity of the solid-state imaging device SI 1 . 
     In the sixth embodiment, after the formation of the accumulation layer  31 , the p-type semiconductor substrate  21  is subjected to the thermal treatment. This restores the crystallinity of the p-type semiconductor substrate  21 , which can prevent such a problem as increase in dark current. 
     In the sixth embodiment, after the thermal treatment of the p-type semiconductor substrate  21 , the charge transfer electrodes  25  are formed. This process prevents the charge transfer electrodes  25  from melting during the thermal treatment even in the case where the charge transfer electrodes  25  are made of a material with a relatively low melting point. Therefore, the charge transfer electrodes  25  can be appropriately formed without being affected by the thermal treatment. 
     In the sixth embodiment, the irregular asperity  10  is formed by the irradiation with the picosecond to femtosecond pulsed laser beam. This permits the irregular asperity  10  to be appropriately and readily formed. 
     Incidentally, in the case of the semiconductor light-detecting element like the solid-state imaging device, it is possible to realize the solid-state imaging device with the spectral sensitivity characteristic in the near-infrared wavelength band, by setting the semiconductor substrate of silicon thick (e.g., about 200 μm). However, in the case of the semiconductor substrate having the increased thickness, it is necessary to apply a large bias voltage of about several ten V to completely deplete the semiconductor substrate, in order to achieve good resolution. The reason for it is to prevent the following phenomenon: if the semiconductor substrate is not completely depleted and a neutral region still remains in part of the semiconductor substrate, carriers generated in the neutral region will diffuse so as to degrade the resolution. 
     As the semiconductor substrate becomes thicker, the dark current also increases. For this reason, it is also necessary to cool the semiconductor substrate (e.g., at −70 to −100° C.), thereby to suppress the increase in dark current. 
     In the solid-state imaging device SI 1  of the sixth embodiment, however, since the irregular asperity  10  is formed in the second principal surface  21   b  as described above, the travel distance of the light incident into the solid-state imaging device SI 1  becomes longer. For this reason, it is feasible to realize the solid-state imaging device with the sufficient spectral sensitivity characteristic in the near-infrared wavelength band, without need for increasing the thickness of the semiconductor substrate (p-type semiconductor substrate  21 ), particularly, the portion corresponding to the photosensitive region  29 . Therefore, when compared to the solid-state imaging device with the spectral sensitivity characteristic in the near-infrared wavelength band based on the increase in the thickness of the semiconductor substrate, the aforementioned solid-state imaging device SI 1  can achieve good resolution with application of an extremely lower bias voltage or without application of any bias voltage. The cooling of the semiconductor substrate also becomes unnecessary, depending upon the intended use. 
     When the semiconductor substrate, particularly the portion corresponding to the photosensitive region, is thinned, there is a risk of occurrence of an etalon phenomenon. The etalon phenomenon is a phenomenon of interference between detection target light incident from the back surface and light resulting from reflection of the incident detection target light on the front surface, and affects the detection characteristic in the near-infrared wavelength band. In the solid-state imaging device SI 1 , however, since light beams reflected on the asperity  10  have dispersed phase differences relative to the phase of the incident light because of the formation of the irregular asperity  10  in the second principal surface  21   b , these light beams cancel each other, so as to suppress the etalon phenomenon. 
     In the sixth embodiment, the p-type semiconductor substrate  21  is thinned from the second principal surface  21   b  side. This allows the semiconductor light-detecting element to be formed with respective light incident surfaces on the first principal surface  21   a  and second principal surface  21   b  sides of the p-type semiconductor substrate  21 . Namely, the solid-state imaging device SI 1  can also be used as a front-illuminated type solid-state imaging device, as well as a back-thinned type solid-state imaging device. 
     In the case where the irregular asperity  10  is formed by the irradiation with the pulsed laser beam after the formation of the accumulation layer  31 , the thickness of the accumulation layer  31  is preferably set larger than the height difference of the irregular asperity  10 . In this case, the accumulation layer  31  remains with certainty even after the irregular asperity  10  is formed by the irradiation with the pulsed laser beam. Therefore, it is feasible to ensure the operational effect by the accumulation layer  31 . 
     (Seventh Embodiment) 
     A photodiode PD 5  according to the seventh embodiment will be described with reference to  FIG. 28 .  FIG. 28  is a drawing for explaining a configuration of the photodiode of the seventh embodiment. 
     The photodiode PD 5  is an avalanche photodiode for detecting low-energy light the wavelength region of which is in the visible to near infrared region. The photodiode PD 5  is provided with a p −  type semiconductor substrate  40 . The p −  type semiconductor substrate  40  is comprised of silicon (Si) crystal and has a first principal surface  40   a  and a second principal surface  40   b  opposed to each other. The p −  type semiconductor substrate  40  includes a photosensitive region  41 . 
     The photosensitive region  41  is disposed in a central region of the first principal surface  40   a  on a plan view. The photosensitive region  41  has the thickness inward from the first principal surface  40   a . The photosensitive region  41  is composed of an n +  type impurity region  43 , a p +  type impurity region  45 , and a region that is depleted with application of a bias voltage in the p −  type semiconductor substrate  40 . The n +  type impurity region  43  has the thickness inside the p −  type semiconductor substrate  40  from the first principal surface  40   a . The n +  type impurity region  43  has an n +  type guard ring  43   a . The n +  type guard ring  43   a  is provided at the peripheral edge of the n +  type impurity region  43 . The p +  type impurity region  45  has the thickness further inside the p −  type semiconductor substrate  40  from the n +  type impurity region  43 . The p −  type semiconductor substrate  40  has a p +  type diffusion blocking region  47 . The p +  type diffusion blocking region  47  is disposed at the peripheral edge of the first principal surface  40   a  on the plan view and has the thickness inward from the first principal surface  40   a . The p +  type diffusion blocking region  47  is provided so as to surround the photosensitive region  41 . 
     The p −  type semiconductor substrate  40  is a silicon substrate doped with a p-type impurity, e.g., such as boron (B). The p +  type impurity region  45  is a region doped with a p-type impurity in a higher concentration than the p −  type semiconductor substrate  40 . The p +  type diffusion blocking region  47  is a region doped with a p-type impurity in a higher concentration than the p +  type impurity region  45 . The n +  type impurity region  43  is a region doped with an n-type impurity, e.g., such as antimony (Sb). The n +  type impurity region  43  (including the n +  type guard ring  43   a ) and the p +  type impurity region  45  constitute a pn junction in the p −  type semiconductor substrate  40 . 
     The photodiode PD 5  has a passivation film  49  deposited on the first principal surface  40   a . The photodiode PD 5  has an electrode  51  and an electrode  53  disposed on the passivation film  49 . In the passivation film  49 , a contact hole H 11  is provided on the n +  type impurity region  43  and a contact hole H 12  is provided on the p +  type diffusion blocking region  47 . The electrode  51  is in electrical contact with and connection to the n +  type impurity region  43  through the contact hole H 11 . The electrode  53  is in electrical contact with and connection to the p +  type diffusion blocking region  47  through the contact hole H 12 . A material of the passivation film  49  is, for example, silicon oxide or the like. 
     The photodiode PD 5  has a recess  55  formed on the second principal surface  40   b  side. The recess  55  is formed by thinning the p −  type semiconductor substrate  40  from the second principal surface  40   b  side and a thick frame portion exists around the recess  55 . The side face of the recess  55  is inclined at an obtuse angle relative to the bottom face of the recess  55 . The recess  55  is formed so as to overlap the photosensitive region  41  on the plan view. The thickness between the bottom face of the recess  55  and the first principal surface  40   a  is relatively small, e.g., about 100-200 μm, and is preferably about 150 μm. Since the thickness between the first principal surface  40   a  and the bottom face of the recess  55  is relatively small as described above, the response speed becomes higher and the bias voltage applied to the photodiode PD 5  is reduced. 
     The irregular asperity  10  is formed throughout the entire second principal surface  40   b  of the p −  type semiconductor substrate  40 . An accumulation layer  57  is formed on the second principal surface  40   b  side of the p −  type semiconductor substrate  40 . In the accumulation layer  57 , a region corresponding to the bottom face of the recess  55 , i.e., the region opposed to the photosensitive region  41  forming the avalanche photodiode is optically exposed. That the second principal surface  40   b  is optically exposed embraces, not only the case where the second principal surface  40   b  is in contact with ambient gas such as air, but also the case where an optically transparent film is formed on the second principal surface  40   b . The irregular asperity  10  may be formed only in the bottom face of the recess  55 , i.e., only in the region opposed to the photosensitive region  51  functioning as the avalanche photodiode. 
     The photodiode PD 5  has an electrode  59 . The electrode  59  is provided on the accumulation layer  57  and is in electrical contact with and connection to the accumulation layer  57 . The region where the electrode  59  is formed in the accumulation layer  57  is not optically exposed. 
     In the photodiode PD 5  having the above configuration, when a reverse bias voltage (breakdown voltage) is applied to the electrode  51  and the electrode  59 , carriers according to the quantity of light incident into the photosensitive region  41  are generated in the photosensitive region  41 . The carriers generated near the p +  type diffusion blocking region  47  flow into the p +  type diffusion blocking region  47 . For this reason, the p +  type diffusion blocking region  47  reduces a tail in an output signal from the electrode  51 . 
     The following will describe a method for manufacturing the photodiode PD 5  of the seventh embodiment. 
     First, the p −  type semiconductor substrate  40  is prepared. The thickness of the p −  type semiconductor substrate  40  is about 300 μm. 
     Next, the p +  type impurity region  45  and p +  type diffusion blocking region  47  are formed on the first principal surface  40   a  side of the p −  type semiconductor substrate  40 . The p +  type impurity region  45  is formed by ion implantation of a p-type impurity in a high concentration from the first principal surface  40   a  side in the p −  type semiconductor substrate  40 , using a mask opening in a central region. The p +  type diffusion blocking region  47  is formed by diffusing a p-type impurity in a high concentration from the first principal surface  40   a  side in the p −  type semiconductor substrate  40 , using another mask opening in a peripheral region. 
     Next, the n +  type guard ring  43   a  and the n +  type impurity region  43  are formed on the first principal surface  40   a  side of the p −  type semiconductor substrate  40 . The n +  type guard ring  43   a  is formed by diffusing an n-type impurity in a high concentration from the first principal surface  40   a  side in the p −  type semiconductor substrate  40 , using a mask opening in a ring shape. The n +  type impurity region  43  is formed by ion implantation of an n-type impurity in a high concentration from the first principal surface  40   a  side in the p −  type semiconductor substrate  40 , using another mask opening in a central region. 
     Next, the surface of the second principal surface  40   b  of the p −  type semiconductor substrate  40  is planarized by polishing. Thereafter, a portion corresponding to the p +  type impurity region  45  in the p −  type semiconductor substrate  40  is thinned from the second principal surface  40   b  side while leaving a surrounding region around the thinned portion. The thinning of the p −  type semiconductor substrate  40  is carried out by anisotropic etching, e.g., alkali etching using a KOH aqueous solution, TMAH, or the like. The thickness of the thinned portion of the p type semiconductor substrate  40  is, for example, about 150 μm and the thickness of the surrounding region is, for example, about 200 μm. 
     Next, the accumulation layer  57  is formed on the second principal surface  40   b  side of the p −  type semiconductor substrate  40 . Here, the accumulation layer  57  is formed by ion implantation of a p-type impurity in a higher impurity concentration than in the p −  type semiconductor substrate  40 , from the second principal surface  40   b  side in the p −  type semiconductor substrate  40 . The thickness of the accumulation layer  57  is, for example, about 1.5 μm. 
     Next, the p −  type semiconductor substrate  40  is subjected to a thermal treatment (annealing) to activate the accumulation layer  57 . Here, the p −  type semiconductor substrate  40  is heated in the temperature range of about 800 to 1000° C. in an ambiance such as N 2  gas for about 0.5 to 1.0 hour. 
     Next, the second principal surface  40   b  of the p −  type semiconductor substrate  40  is irradiated with a pulsed laser beam PL to form the irregular asperity  10 . The irregular asperity  10  is formed by irradiating the second principal surface  40   b  of the p type semiconductor substrate  40  with the pulsed laser beam, as in the aforementioned embodiments. 
     Next, the p −  type semiconductor substrate  40  is subjected to a thermal treatment (annealing). Here, the p −  type semiconductor substrate  40  is heated in the temperature range of about 800 to 1000° C. in an ambiance such as N 2  gas for about 0.5-1.0 hour. The thermal treatment achieves recovery of crystal defects and recrystallization in the p-type semiconductor substrate  40 , so as to prevent the problem such as the increase of dark current. 
     Next, the passivation film  49  is formed on the first principal surface  40   a  side of the p −  type semiconductor substrate  40 . Then the contact holes H 11 , H 12  are formed in the passivation film  49  and the electrodes  51 ,  53  are formed. The electrode  51  is formed in the contact hole H 11  and the electrode  53  in the contact hole H 12 . Furthermore, the electrode  59  is formed on the accumulation layer  57  in the surrounding region around the thinned portion of the p −  type semiconductor substrate  40 . The electrodes  51 ,  53  each are comprised of aluminum (Al) or the like and the electrode  59  is comprised of gold (Au) or the like. This completes the photodiode PD 5 . 
     In the photodiode PD 5 , since the irregular asperity  10  is formed in the second principal surface  40   b , the light incident into the photodiode PD 5  is reflected, scattered, or diffused by the asperity  10  to travel through a long distance in the p −  type semiconductor substrate  40 . 
     In the photodiode PD 5 , where the light is incident from the direction normal to a light incident surface (first principal surface  40   a ), when the light arrives at the irregular asperity  10  formed in the second principal surface  40   b , light components arriving thereat at angles of not less than 16.6° relative to the direction of emergence from the asperity  10  are totally reflected by the asperity  10 . Since the asperity  10  is formed irregularly, it has various angles relative to the emergence direction and diffuses the totally reflected light components into various directions. For this reason, the totally reflected light components include light components absorbed inside the p −  type semiconductor substrate  40  and light components reaching the first principal surface  40   a  and side faces. 
     Since the light components reaching the first principal surface  40   a  and the side faces travel in various directions because of the diffusion on the asperity  10 , the light components reaching the first principal surface  40   a  and the side faces are extremely highly likely to be totally reflected by the first principal surface  40   a  and the side faces. The light components totally reflected by the first principal surface  40   a  and the side faces are repeatedly totally reflected on different faces, whereby the travel distance thereof further increases. The light incident into the photodiode PD 5  is absorbed by the p −  type semiconductor substrate  40  during the travel through the long distance inside the p −  type semiconductor substrate  40  to be detected as a photocurrent. 
     In this manner, the light L incident into the photodiode PD 5  mostly travels, without being transmitted by the photodiode PD 5 , through the long travel distance to be absorbed in the p −  type semiconductor substrate  40 . Therefore, the photodiode PD 5  is improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the photodiode PD 5 , the accumulation layer  57  is formed on the second principal surface  40   b  side of the p −  type semiconductor substrate  40 . This induces recombination of unnecessary carriers generated on the second principal surface  40   b  side, so as to reduce the dark current. The accumulation layer  57  prevents carriers generated near the second principal surface  40   b , from being trapped in the second principal surface  40   b . For this reason, the generated carriers efficiently migrate to the pn junction, so as to further improve the photodetection sensitivity of the photodiode PD 5 . 
     In the seventh embodiment, the p −  type semiconductor substrate  40  is subjected to the thermal treatment, after formation of the accumulation layer  57 . This restores the crystallinity of the p −  type semiconductor substrate  40 , so as to prevent the problem such as the increase of dark current. 
     The accumulation layer  57  may be formed after formation of the irregular asperity  10 . In the case where the irregular asperity  10  is formed by irradiation with the pulsed laser light, after the formation of the accumulation layer  57 , the thickness of the accumulation layer  57  is preferably set to be larger than the height difference of the irregular asperity  10 . In this case, the accumulation layer  57  is certainly left even after the irregular asperity  10  is formed by irradiation with the pulsed laser light. The operational effect by the accumulation layer  57  can be ensured accordingly. 
     In the seventh embodiment, the electrodes  51 ,  53 ,  59  are formed after the thermal treatment of the p −  type semiconductor substrate  40 . This prevents the electrodes  51 ,  53 ,  59  from melting during the thermal treatment, even if the electrodes  51 ,  53 ,  59  are made of materials with a relatively low melting point. Therefore, the electrodes  51 ,  53 ,  59  can be appropriately formed without being affected by the thermal treatment. 
     In the seventh embodiment, the irregular asperity  10  is formed by irradiation with the picosecond to femtosecond pulsed laser beam. This allows the irregular asperity  10  to be appropriately and readily formed. 
     In the seventh embodiment, the p −  type semiconductor substrate  40  is thinned from the second principal surface  40   b  side. This allows us to obtain the photodiode with the respective light incident surfaces on the first principal surface  40   a  side and the second principal surface  40   b  side of the p −  type semiconductor substrate  40 . Namely, the photodiode PD 5  can be used, not only as a front-illuminated type photodiode, but also as a back-thinned type photodiode. 
     Incidentally, concerning the avalanche photodiode, it is possible to realize the semiconductor light-detecting element with a practically sufficient spectral sensitivity characteristic in the near-infrared wavelength band, by setting the semiconductor substrate of silicon thick (e.g., about several hundred μm to 2 mm). However, the avalanche photodiode necessitates the bias voltage for depletion and the bias voltage for avalanche multiplication and, therefore, in the case where the thickness of the semiconductor substrate is set large, it becomes necessary to apply the extremely high bias voltage. The thick semiconductor substrate also increases the dark current. 
     In the photodiode PD 5  of the seventh embodiment, however, since the irregular asperity  10  is formed in the second principal surface  40   b  as described above, the travel distance of the light incident into the photodiode PD 5  is lengthened. For this reason, it is feasible to realize the photodiode with the practically sufficient spectral sensitivity characteristic in the near-infrared wavelength band, without need for increasing the thickness of the semiconductor substrate (p −  type semiconductor substrate  40 ), particularly, the portion corresponding to the photosensitive region  41 . Therefore, the foregoing photodiode PD 5  can achieve the good spectral sensitivity characteristic with application of a lower bias voltage than the photodiode with the spectral sensitivity characteristic in the near-infrared wavelength band based on the increase in the thickness of the semiconductor substrate. In addition, the increase of dark current is suppressed, so as to improve the detection accuracy of the photodiode PD 5 . Furthermore, since the thickness of the p −  type semiconductor substrate  40  is small, the response speed of the photodiode PD 5  improves. 
     In the photodiode PD 5  of the seventh embodiment, the entire region on the second principal surface  40   b  side may be thinned. 
     (Eighth Embodiment) 
     A photodiode array PDA 2  according to the eighth embodiment will be described with reference to  FIG. 29 .  FIG. 29  is a drawing for explaining a configuration of the photodiode array according to the eighth embodiment. 
     The photodiode array PDA 2  is provided with a p −  type semiconductor substrate  40 , and a plurality of photosensitive regions  41  functioning as avalanche photodiodes are arranged on the p −  type semiconductor substrate  40 . 
     The irregular asperity  10  is formed throughout the entire second principal surface  40   b  of the p −  type semiconductor substrate  40 . Namely, the photodiode array PDA 2  has the irregular asperity  10  formed not only in regions opposed to the photosensitive regions  41  functioning as avalanche photodiodes, but also in regions opposed to the regions between the photosensitive regions  41 . 
     In the eighth embodiment, as in the seventh embodiment, the travel distance of light incident into the photodiode array PDA 2  also becomes long and the distance of absorption of light also becomes long. This allows the photodiode array PDA 2  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     The photodiode array PDA 2  of the eighth embodiment, as in the seventh embodiment, can achieve the good spectral sensitivity characteristic with application of a lower bias voltage than a photodiode array with a spectral sensitivity characteristic in the near-infrared wavelength band based on the increase in the thickness of the semiconductor substrate. In addition, the increase of dark current is suppressed, so as to improve the detection accuracy of the photodiode array PDA 2 . Furthermore, since the thickness of the p −  type semiconductor substrate  40  is small, the response speed of the photodiode array PDA 2  improves. 
     In the photodiode array PDA 2 , the irregular asperity  10  is also formed in the regions opposed to the regions between the photosensitive regions  41  in the second principal surface  40   b  of the p −  type semiconductor substrate  40 . For this reason, light L incident between the photosensitive regions  41  is reflected, scattered, or diffused by the irregular asperity  10  formed in the regions opposed to the regions between the photosensitive regions  41  in the second principal surface  40   b  to be absorbed by any one of the photosensitive regions  41 . In the photodiode array PDA, therefore, the detection sensitivity is not lowered between the photosensitive regions  41 , so as to improve the photodetection sensitivity. 
     The photodiode array PDA 2  can also be used as a device for detecting YAG laser light as the photodiode PD 5  of the seventh embodiment. 
     In the photodiode array PDA 2  the entire region on the second principal surface  40   b  side may be thinned. The photodiode array PDA 2  can be used as a photodiode array of either of the front-illuminated type and the back-thinned type. 
     (Ninth Embodiment) 
     A configuration of a photodiode array PDA 3  according to the ninth embodiment will be described with reference to  FIGS. 30 and 31 .  FIG. 30  is a plan view schematically showing the photodiode array PDA 3  of the ninth embodiment.  FIG. 31  is a drawing showing a cross-sectional configuration along the line XXXI-XXXI of the photodiode array PDA 3  shown in  FIG. 30 . 
     The photodiode array PDA 3  is composed of a plurality of semiconductor layers and an insulating layer layered on a substrate  62 . As shown in  FIG. 30 , the photodiode array PDA 3  is a multichannel avalanche photodiode for photon counting in which a plurality of photodetecting channels CH for detection target light to be injected thereinto are formed in a matrix pattern (4×4 in the present embodiment). There are signal conductor wires  63 , resistors  64 , and an electrode pad  65  provided on the top side of the photodiode array PDA 3 . The substrate  62  has, for example, a square shape about 1 mm on each side. Each photodetecting channel CH has, for example, a square shape. 
     The signal conductor wire  63  consists of a readout portion  63   a , connecting portions  63   b , and channel peripheral portions  63   c . The readout portion  63   a  transfers a signal output from each photodetecting channel CH. The connecting portion  63   b  connects each resistor  64  and the readout portion  63   a . Each channel peripheral portion  63   c  is routed so as to surround the periphery of the photodetecting channel CH. The readout portion  63   a  is connected to each of the photodetecting channels CH arranged in two adjacent columns with the readout portion  63   a  in between, and is connected at one end thereof to the electrode pad  65 . Since in the present embodiment the photodiodes are arranged in the 4×4 matrix pattern, there are two readout portions  63   a  as wiring on the photodiode array PDA 3  and these are connected both to the electrode pad  65 . The signal conductor wires  63  are comprised, for example, of aluminum (Al). 
     The resistor  64  is provided for each photodetecting channel CH through one end portion  64   a  and the channel peripheral portion  63   c  and is connected through the other end portion  64   b  and the connecting portion  63   b  to the readout portion  63   a . A plurality of resistors  64  (eight in the present embodiment) to be connected to an identical readout portion  63   a  are connected to the readout portion  63   a . The resistors  64  are comprised, for example, of polysilicon (Poly-Si). 
     Next, the cross-sectional configuration of the photodiode array PDA 3  will be described with reference to  FIG. 31 . As shown in  FIG. 31 , the photodiode array PDA 3  is provided with a substrate  62  having a semiconductor layer with the conductivity type of n-type (first conductivity type), a p −  type semiconductor layer  73  with the conductivity type of p-type (second conductivity type) formed on the substrate  62 , p +  type semiconductor regions  74  with the conductivity type of p-type formed on the p −  type semiconductor layer  73 , a protecting film  76 , a separating portion  80  with the conductivity type of n-type (first conductivity type) formed in the p −  type semiconductor layer  73 , and the aforementioned signal conductor wires  63  and resistors  64  formed on the protecting film  76 . The detection target light is injected from the top side or the bottom side in  FIG. 31 . 
     The substrate  62  has a substrate member SM, an insulating film  61  formed on the substrate member SM, and an n +  type semiconductor layer  72  formed on the insulating film  61 . The substrate member SM is comprised of Si (silicon). The insulating film  61  is comprised, for example, of SiO 2  (silicon oxide). The n +  type semiconductor layer  72  is a semiconductor layer with the conductivity type of n-type comprised of Si and having a high impurity concentration. The thickness of the n +  type semiconductor layer  72  is, for example, 1 μm-12 μm. 
     The p −  type semiconductor layer  73  is an epitaxial semiconductor layer with the conductivity type of p-type having a low impurity concentration. The p −  type semiconductor layer  73  forms pn junctions at the interface to the substrate  62 . The p −  type semiconductor layer  73  has a plurality of multiplication regions AM for avalanche multiplication of carriers generated with incidence of the detection target light, corresponding to the respective photodetecting channels CH. The thickness of the p −  type semiconductor layer  73  is, for example, 3 μm-5 μm. The p −  type semiconductor layer  73  is comprised of Si. Therefore, the n +  type semiconductor layer  72  and the p −  type semiconductor layer  73  constitute a silicon substrate. 
     The p +  type semiconductor regions  74  are formed on the p −  type semiconductor layer  73 , corresponding to the multiplication regions AM of the respective photodetecting channels CH. Namely, each multiplication region AM is a region near the interface to the substrate  62  in the p −  type semiconductor layer  73  located below the p +  type semiconductor region  74  in the lamination direction of semiconductor layers (which will be referred to hereinafter simply as the lamination direction). The p +  type semiconductor regions  74  are comprised of Si. 
     The separating portion  80  is formed between the plurality of photodetecting channels CH to separate the photodetecting channels CH. Namely, the separating portion  80  is formed so as to form the multiplication regions AM in the p −  type semiconductor layer  73  in one-to-one correspondence to the respective photodetecting channels CH. The separating portion  80  is formed in a two-dimensional lattice pattern on the substrate  62  so as to completely surround the periphery of each multiplication region AM. The separating portion  80  is formed so as to penetrate from the top side to the bottom side of the p −  type semiconductor layer  73  in the lamination direction. The separating portion  80  is a semiconductor layer with the conductivity type of n-type an impurity of which is comprised, for example, of P and an impurity concentration of which is high. If the separating portion  80  is formed by diffusion, a long thermal treatment time will be needed, and it is, therefore, considered that the impurity of the n +  type semiconductor layer  72  diffuses into the epitaxial semiconductor layer so as to cause a rise of interfaces of the pn junctions. In order to prevent this rise, the separating portion  80  may be formed in such a manner that a trench is formed by etching near centers of regions corresponding to the separating portion  80  and thereafter the diffusion of the impurity is performed. A light blocking portion may be formed in the trench groove by filling the trench groove with a material to absorb or reflect the light in the wavelength band to be absorbed by the photodetecting channels. In this case, it is feasible to prevent crosstalk caused by influence of emission by avalanche multiplication on neighboring photodetecting channels. 
     The p −  type semiconductor layer  73 , p +  type semiconductor regions  74 , and separating portion  80  form a flat surface on the top side of the photodiode array PDA 3  and the protecting film  76  is formed thereon. The protecting film  76  is made of an insulating layer comprised, for example, of SiO 2 . 
     The signal conductor wires  63  and resistors  64  are formed on the protecting film  76 . The readout portions  63   a  of the signal conductor wires  63  and the resistors  64  are formed above the separating portion  80 . 
     The signal conductor wires  63  function as anodes and the photodiode array may be provided with a transparent electrode layer (e.g., a layer comprised of ITO (Indium Tin Oxide)) over the entire surface on the bottom side (the side without the insulating film  61 ) of the substrate  62 , as a cathode though not shown. Alternatively, as a cathode, an electrode portion may be formed so as to be drawn out to the front side. 
     Now, a connection relation of each photodetecting channel CH to the signal conductor wire  63  and resistor  64  will be described with reference to  FIG. 32 .  FIG. 32  is a drawing for schematically explaining the connection relation of each photodetecting channel CH to the signal conductor wire  63  and resistor  64 . As shown in  FIG. 32 , the p +  type semiconductor region  74  of each photodetecting channel CH is directly connected to the signal conductor wire  63  (channel peripheral portion  63   c ). This establishes electrical connection between the signal conductor wire  63  (channel peripheral portion  63   c ) and the p −  type semiconductor layer  73 . The p −  type semiconductor layer  73  and one end  64   a  of the resistor  64  are connected through the signal conductor wire  63  (channel peripheral portion  63   c ) and the other end  64   b  of each resistor  64  is connected through the connecting portion  63   b  to the readout portion  63   a.    
     In the substrate  62 , the region where the plurality of photodetecting channels CH are formed is thinned from the substrate member SM side, so as to remove a portion corresponding to the region where the plurality of photodetecting channels CH are formed in the substrate member SM. The substrate member SM exists as a frame portion around the thinned region. By removing the frame portion as well, the substrate  62  may have a configuration wherein the entire region is thinned, i.e., the whole substrate member SM is removed. The removal of the substrate member SM can be implemented by etching (e.g., dry etching or the like), polishing, and so on. In the case where the substrate member SM is removed by dry etching, the insulating film  61  also functions as an etching stop layer. The insulating film  61  exposed after the removal of the substrate member S is removed as described later. 
     In the surface of the n +  type semiconductor layer  72 , the irregular asperity  10  is formed throughout the entire region where the plurality of photodetecting channels CH are formed. The region where the irregular asperity  10  is formed in the surface of the n +  type semiconductor layer  72  is optically exposed. That the surface of the n +  type semiconductor layer  72  is optically exposed embraces, not only the case where the surface of the n +  type semiconductor layer  72  is in contact with ambient gas such as air, but also the case where an optically transparent film is formed on the surface of the n +  type semiconductor layer  72 . The irregular asperity  10  may also be formed only in the regions opposed to the respective photodetecting channels CH. 
     The irregular asperity  10  is formed by irradiating the insulating film  61  exposed after the removal of the substrate member SM, with a pulsed laser beam, in the same manner as in the aforementioned embodiments. Namely, when the exposed insulating film  61  is irradiated with the pulsed laser beam, the insulating film  61  is removed and the surface of the n +  type semiconductor layer  72  is roughened by the pulsed laser beam, thereby forming the irregular asperity  10 . 
     After the formation of the irregular asperity  10  by irradiation with the pulsed laser beam, the substrate  62  is preferably subjected to a thermal treatment (annealing). For example, the substrate  62  is heated in the temperature range of about 800 to 1000° C. in an ambiance of N 2  gas or the like for about 0.5 to 1.0 hour. The foregoing thermal treatment achieves recovery of crystal defects and recrystallization in the n +  type semiconductor layer  72  so as to prevent the problem such as the increase of dark current. 
     When the photodiode array PDA 3  configured as described above is used for photon counting, it is operated under an operation condition called a Geiger mode. In the Geiger mode operation, a reverse voltage (e.g., 50 V or more) higher than the breakdown voltage is applied to each photodetecting channel CH. When the detection target light is incident from the top side into each photodetecting channel CH in this state, the target light is absorbed in each photodetecting channel CH to generate a carrier. The generated carrier migrates as accelerated according to an electric field in each photodetecting channel CH to be multiplied in each multiplication region AM. Then the multiplied carriers are taken out through the resistor  64  and through the signal conductor wire  63  to the outside to be detected based on a wave height value of an output signal thereof. Since every channel detecting a photon provides the same quantity of output, the total output from all the channels is detected, thereby counting how many photodetecting channels CH in the photodiode array PDA 3  provided the output. Therefore, the photodiode array PDA 3  accomplishes the photon counting by the single irradiation operation with the detection target light. 
     Incidentally, since the irregular asperity  10  is formed in the surface of the n +  type semiconductor layer  72  in the photodiode array PDA 3 , the light incident into the photodiode array PDA 3  is reflected, scattered, or diffused by the asperity  10  to travel through a long distance in the photodiode array PDA 3 . 
     For example, in the case where the photodiode array PDA 3  is used as a front-illuminated type photodiode array and where the light is incident from the protecting film  76  side into the photodiode array PDA 3 , when the light reaches the irregular asperity  10  formed in the surface of the n +  type semiconductor layer  72 , light components arriving thereat at angles of not less than 16.6° to the direction of emergence from the asperity  10  are totally reflected by the asperity  10 . Since the asperity  10  is formed irregularly, it has various angles relative to the emergence direction and the totally reflected light components diffuse into various directions. For this reason, the totally reflected light components include light components absorbed in each photodetecting channel CH and light components reaching the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72 . 
     The light components reaching the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  travel in various directions because of the diffusion on the asperity  10 . For this reason, the light components reaching the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  are extremely highly likely to be totally reflected on the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72 . The light components totally reflected on the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  are repeatedly totally reflected on different faces, to further increase their travel distance. While the light incident into the photodiode array PDA 3  travels through the long distance inside the photodiode array PDA 3 , it is absorbed in each photodetecting channel CH to be detected as a photocurrent. 
     In the case where the photodiode array PDA 3  is used as a back-thinned type photodiode array and where the light is incident from the front surface side of the n +  type semiconductor layer  72  into the photodiode array PDA 3 , the incident light is scattered by the asperity  10  and travels in various directions in the photodiode array PDA 3 . Since the light components reaching the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  travel in various directions because of the diffusion on the asperity  10 , the light components reaching the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  are extremely highly likely to be totally reflected on each surface. The light components totally reflected on the surface on the protecting film  76  side and the side faces of the n +  type semiconductor layer  72  are repeatedly totally reflected on different faces, or reflected, scattered, or diffused on the asperity  10 , to further increase their travel distance. The light incident into the photodiode array PDA 3  is reflected, scattered, or diffused by the asperity  10  to travel through the long distance in the photodiode array PDA 3  and to be absorbed in each photodetecting channel CH to be detected as a photocurrent. 
     The light L incident into the photodiode array PDA 3  mostly travels through the long travel distance to be absorbed in each photodetecting channel CH, without passing through the photodiode array PDA 3 . Therefore, the photodiode array PDA 3  is improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the ninth embodiment, the irregular asperity  10  is formed in the surface of the n +  type semiconductor layer  72 . For this reason, it induces recombination of unnecessary carriers generated independent of light on the surface side where the irregular asperity  10  is formed, so as to reduce the dark current. The n +  type semiconductor layer  72  functions as an accumulation layer to prevent carriers generated by light near the surface of the n +  type semiconductor layer  72 , from being trapped in the surface. For this reason, the carriers generated by light efficiently migrate to the multiplication regions AM, so as to improve the photodetection sensitivity of the photodiode array PDA 3 . 
     In the ninth embodiment, the irregular asperity  10  is also formed in the surface corresponding to the regions between the plurality of photodetecting channels CH in the n +  type semiconductor layer  72  and the surface is optically exposed. For this reason, light incident into the regions between the plurality of photodetecting channels CH is also reflected, scattered, or diffused by the irregular asperity  10  to be absorbed in any one of the photodetecting channels CH. Therefore, the detection sensitivity is not lowered in the regions between the photodetecting channels CH, whereby the photodetection sensitivity of the photodiode array PDA 3  is further improved. 
     In the ninth embodiment, the thickness of the n +  type semiconductor layer  72  is larger than the height difference of the irregular asperity  10 . For this reason, it is feasible to certainly ensure the operational effect as an accumulation layer by the n +  type semiconductor layer  72 . 
     In the photodiode array PDA 3 , the pn junctions are composed of the n +  type semiconductor layer  72  of the substrate  62  and the p −  type semiconductor layer  73  being the epitaxial semiconductor layer formed on the n +  type semiconductor layer  72  of the substrate  62 . The multiplication regions AM are formed in the p −  type semiconductor layer  73  where the pn junctions are substantialized, and the correspondence of each multiplication region AM to each photodetecting channel CH is realized by the separating portion  80  formed between the photodetecting channels CH. A pn junction surface is composed of an interface between the n +  type semiconductor layer  72  and the p −  type semiconductor layer  73  and an interface between the separating portion  80  and the p −  type semiconductor layer  73 . For this reason, the high-concentration impurity regions are convex and there is no region having a high electric field. Therefore, the photodiode array PDA 3  has no ends (edges) of pn junctions where edge breakdown occurs in the Geiger mode operation. For this reason, the photodiode array PDA 3  does not have to be provided with a guard ring for the pn junction of each photodetecting channel CH. This enables the photodiode array PDA 3  to have a drastically high aperture rate. 
     By achieving the high aperture rate, it also becomes feasible to increase the detection efficiency of the photodiode array PDA 3 . 
     Since the photodetecting channels CH are separated by the separating portion  80 , it becomes feasible to well suppress crosstalk. 
     Even if in the Geiger mode operation a large voltage difference is made between a photodetecting channel with incidence of a photon and a channel without incidence, the channels can be separated well because the separating portion  80  is formed between the photodetecting channels CH. 
     Since in the photodiode array PDA 3  the readout portions  63   a  of the signal conductor wires  63  are formed above the separating portion  80 , the signal conductor wires  63  are prevented from crossing above the multiplication regions AM, i.e., above the photodetection surface. For this reason, the aperture rate is more increased. Furthermore, it is considered to be also effective to suppression of dark current. In the photodiode array PDA 3 , the aperture rate is still more increased because the resistors  64  are also formed above the separating portion  80 . 
     The inventor of the present application discovered from wavelength dependence of after pulse that in the case where the n-type semiconductor substrate was used and the p-type epitaxial semiconductor layer was formed thereon, some of holes generated in the n-type semiconductor substrate went late into the multiplication region to produce an after pulse. In view of this problem, the photodiode array PDA 3  is constructed by removing the substrate member SM from the region where the plurality of photodetecting channels CH are formed, so as to suppress the after pulse. 
     In the ninth embodiment, the number of photodetecting channels formed in the photodiode array does not have to be limited to the number (4×4) in the above embodiment. The number of separating portion  80  formed between the light-detecting channels is not limited to the number described in the above embodiment and modification examples, either, but may be three or more. The signal conductor wires  63  do not always have to be formed above the separating portions  80 . The resistors  64  do not always have to be formed above the separating portions  80 , either. The layers and others are not limited only to those exemplified in the above embodiment. 
     (Tenth Embodiment) 
     A MOS image sensor MI 1  according to the tenth embodiment will be described with reference to  FIG. 33 .  FIG. 33  is a plan view schematically showing the MOS image sensor according to the tenth embodiment.  FIG. 34  is a drawing showing a cross-sectional configuration along the line XXXIV-XXXIV of the MOS image sensor shown in  FIG. 33 . 
     The MOS image sensor MI 1  is provided with a semiconductor substrate  90  of the first conductivity type comprised of silicon. There are a light receiving portion  91 , a vertical shift register  92  for selection of rows, and a horizontal shift register  93  for selection of columns, formed in the semiconductor substrate  90 . The light receiving portion  91 , as shown in  FIG. 34 ( a ) , is arranged on the first principal surface  90   a  side of the semiconductor substrate  90 . A plurality of pixels (not shown) are two-dimensionally arranged in the light receiving portion  91 . The vertical shift register  92  is arranged beside the light receiving portion  91  (on the lift side in  FIG. 33 ). The horizontal shift register  93  is also arranged beside the light receiving portion  91  (on the lower side in  FIG. 33 ) 
     The irregular asperity  10  is formed, as shown in  FIG. 34 ( a ) , in a region corresponding to the light receiving portion  91  in a second principal surface  90   b  of the semiconductor substrate  90 . The accumulation layer  11  is formed on the second principal surface  90   b  side of the semiconductor substrate  90  and the second principal surface  90   b  is optically exposed. The irregular asperity  10  may be formed throughout the entire area of the second principal surface  90   b  of the semiconductor substrate  90 , as shown in  FIG. 34 ( b ) . 
     The below will describe a configuration of each pixel PX arranged in the light receiving portion  91  of the MOS image sensor MI 1 , with reference to  FIGS. 35 and 36 .  FIG. 35  is a plan view showing an enlarged pixel in the MOS image sensor according to the tenth embodiment.  FIG. 36  is a drawing showing a cross-sectional configuration along the line XXXVI-XXXVI in  FIG. 36 . 
     Each pixel PX, as shown in  FIG. 35 , is composed of a light receiving region  101  and an attendant circuit  102 . In a case where the MOS image sensor MI 1  is a PPS (Passive Pixel Sensor), the attendant circuit  102  is composed of a readout FET. In a case where the MOS image sensor MI 1  is an APS (Active Pixel Sensor), the attendant circuit  102  is composed of an amplifying circuit including four transistors and others. 
     The light receiving region  101 , as shown in  FIG. 36 ( a ) , is a photodiode comprised of a pn junction composed of the semiconductor substrate  90  and a second conductivity type semiconductor region  111 . The attendant circuit  102  is arranged beside the second conductivity type semiconductor region  111  (on the left side in  FIG. 36 ( a ) ). The irregular asperity  10  is formed throughout the entire area of the pixel PX in the second principal surface  90   b  of the semiconductor substrate  90 . The irregular asperity  10  may be formed only in a region corresponding to the light receiving region  101  (second conductivity type semiconductor region  111 ) in the second principal surface  90   b  of the semiconductor substrate  90 , as shown in  FIG. 36 ( b ) . 
     In the tenth embodiment, as in the other embodiments, the travel distance of light incident into the MOS image sensor MI 1  also becomes long and the distance of absorption of light also becomes long. This allows the MOS image sensor MI 1  also to be improved in the spectral sensitivity characteristic in the near-infrared wavelength band. 
     In the tenth embodiment, the semiconductor substrate  90  may be thinned after completion of the processing process on the first principal surface  90   a  side of the semiconductor substrate  90 . In this case, the MOS image sensor is obtained with respective light incident surfaces on the first principal surface  90   a  and second principal surface  90   b  sides of the semiconductor substrate  90 . 
     It is noted that the mode shown in the tenth embodiment is not applied only to the MOS image sensor. The mode shown in the tenth embodiment is also applicable to a CMOS image sensor, photo IC, CMOS photo IC, or the like to detect light in the near-infrared wavelength band. 
     The above described the preferred embodiments of the present invention, but it should be noted that the present invention is not always limited to the above-described embodiments but can be modified in various ways without departing from the scope and spirit of the invention. 
     In the first to fifth embodiments the irregular asperity  10  is formed by irradiating the entire area of the second principal surface  1   b  with the pulsed laser beam, but the present invention is not limited to it. For example, the irregular asperity  10  may also be formed by irradiating only the region opposed to the p +  type semiconductor region  3  in the second principal surface  1   b  of the n −  type semiconductor substrate  1  with the pulsed laser beam. 
     In the first to fifth embodiments the electrode  15  is in electrical contact with and connection to the n +  type semiconductor region  5  formed on the first principal surface  1   a  side of the n −  type semiconductor substrate  1 , but it is not limited only to this example. For example, the electrode  15  may be in electrical contact with and connection to the accumulation layer  11  formed on the second principal surface  1   b  side of the n −  type semiconductor substrate  1 . In this case, the electrode  15  is preferably formed outside the region opposed to the p +  type semiconductor region  3  in the second principal surface  1   b  of the n −  type semiconductor substrate  1 . The reason for it is as follows: if the electrode  15  is formed in the region opposed to the p +  type semiconductor region  3  in the second principal surface  1   b  of the n −  type semiconductor substrate  1 , the irregular asperity  10  formed in the second principal surface  1   b  is blocked by the electrode  15 , causing an event of reduction in the spectral sensitivity in the near-infrared wavelength band. 
     The conductivity types of p-type and n-type may be interchanged so as to be reverse to those described above in the photodiodes PD 1 -PD 5 , photodiode arrays PDA 1 - 3 , solid-state imaging device SI 1 , and MOS image sensor MI 1  according to the embodiments of the present invention. 
     Incidentally, there is a conventional technology of “a manufacturing method of a silicon-based detector with a surface layer doped with sulfur and microstructured with a laser” as disclosed in Japanese Translation of PCT International Application Laid-open No. 2008-515196. In the foregoing Laid-open No. 2008-515196, each of positions on a surface of a silicon substrate is irradiated with at least one femtosecond laser pulse and, at the same time, the surface is exposed to a substance containing sulfur, to form a plurality of sulfur-containing portions in the surface layer of the substrate. In this manner, an impurity level is formed in the bandgap of silicon in the Laid-open No. 2008-515196, so as to improve the infrared sensitivity. Therefore, the photodetector disclosed in the Laid-open No. 2008-515196 is different from the semiconductor light-detecting elements of the embodiments of the present invention with the improved spectral sensitivity characteristic in the near-infrared region because of the lengthened travel distance of light incident into the semiconductor light-detecting element and the lengthened distance of absorption of light as well. Furthermore, the photodetector disclosed in the Laid-open No. 2008-515196 detects light by the photoelectric effect, which is different from the semiconductor light-detecting elements of the embodiments of the present invention. 
     Conventionally, there were only semiconductor light receiving elements using compound semiconductors, as semiconductor light receiving elements with practical spectral sensitivity in the wavelength range of not less than 1000 nm. However, the present invention has realized the semiconductor light receiving elements capable of detecting the near-infrared light in the wavelength range of not less than 1000 nm using silicon inexpensive in raw materials and processing cost and easy to process, thereby providing a great industrial advantage. 
     INDUSTRIAL APPLICABILITY 
     The present invention is applicable to semiconductor light-detecting elements and light-detecting apparatus. 
     LIST OF REFERENCE SIGNS 
       1  n −  type semiconductor substrate;  1   a  first principal surface;  1   b  second principal surface;  3  p +  type semiconductor region;  5  n +  semiconductor region:  10  irregular asperity;  11  accumulation layer;  13 , electrodes; PL pulsed laser beam; PD 1 -PD 5  photodiodes; PDA 1 - 3  photodiode arrays; SI 1  solid-state imaging device; MI 1  MOS image sensor.