Photodiode array and radiation detector having depressions of predetermined depth formed in regions corresponding to the regions where the photodiodes are formed in the semiconductor substrate

A photodiode array 1 is provided with an n-type silicon substrate 3. A plurality of photodiodes 4 are formed in array on the opposites surface side to an incident surface of light L to be detected, in the n-type silicon substrate 3. A depression 6 with a predetermined depth more depressed than a region not corresponding to regions where the photodiodes 4 are formed is formed in regions corresponding to the regions where the photodiodes 4 are formed, on the incident surface side of the light L to be detected, in the n-type silicon substrate 3.

TECHNICAL FIELD

The present invention relates to a photodiode array and production method thereof, and to a radiation detector.

BACKGROUND ART

The photodiode arrays of this type conventionally known include back-illuminated photodiode arrays of a type in which light is incident through the opposite surface (back surface) to a surface where bump electrodes or the like are formed (e.g., reference is made to Patent Document 1). The photodiode array disclosed in this Patent Document 1 has photodiodes140of pn junctions made by forming p-layers134of prismatic shape in an n-type silicon substrate133, as shown inFIGS. 26 and 27. A scintillator131is bonded through a negative electrode film136to the back surface (the upper side in the drawing) opposite to the surface where the photodiodes140are formed (the lower side in the drawing). When light resulting from wavelength conversion in the scintillator131is incident into the photodiodes140, each of the photodiodes140generates an electric current according to the incident light. The electric current generated in each photodiode140is outputted through positive electrode135formed on the front surface side, solder ball139, and solder pad138provided on printed circuit board137.

DISCLOSURE OF THE INVENTION

Incidentally, for mounting of the aforementioned photodiode array, e.g., a photodiode array for CT, a flat collet or a pyramid collet can be used as a collet for holding a chip under suction. The flat collet is normally used for flip chip bonding. The CT photodiode array has a large chip area (e.g., the square shape of 20 mm on each side). Where the pyramid collet161used in an ordinary mounter is used as shown inFIG. 25B, chip162will warp due to clearance163between chip162and pyramid collet161. For this reason, in the case where the pyramid collet161is used, the warpage could cause positional misalignment and degrade the mounting accuracy of chip162. The flip chip bonding process requires application of heat and pressure, but the efficiency of thermal conduction is poor in use of the pyramid collet161. The pressure applied could damage the edge of chip162. From the above it follows that the pyramid collet161is not suitable for suction holding of a thin chip. Therefore, in the case of the flip chip bonding, as shown inFIG. 25A, flat collet160to achieve surface contact with the chip surface is used to hold the chip162under suction, and the heat and pressure from heater block164is applied to the chip162.

However, the use of the flat collet160results in bringing the entire chip surface of the chip162into contact with the flat collet160. If the entire chip surface to become the light-incident surface is in contact with the flat collet160to be exposed to pressure and heat, regions corresponding to impurity diffused layers forming the photodiodes, on the chip surface could suffer physical damage. The damage on the chip surface will pose problems of appearance failure and characteristic degradation (increase in dark current and noise or the like).

The present invention has been accomplished in view of the above-described respects and an object of the invention is to provide a photodiode array and production method thereof capable of preventing the characteristic degradation while preventing the damage on the regions corresponding to the photodiodes during mounting, and to provide a radiation detector.

In order to achieve the above object, a photodiode array according to the present invention is a photodiode array comprising a semiconductor substrate, wherein a plurality of photodiodes are formed in array on an opposite surface side to an incident surface of light to be detected, in the semiconductor substrate, and wherein a depression with a predetermined depth more depressed than a region not corresponding to regions where the photodiodes are formed, is formed in regions corresponding to the regions where the photodiodes are formed, on a side of the incident surface of the light to be detected, in the semiconductor substrate.

In the photodiode array according to the present invention, the region not corresponding to the regions where the photodiodes are formed projects relative to the regions corresponding to the regions where the photodiodes are formed, on the incident surface side of the semiconductor substrate. In the case where the flat collet is used in mounting, the region not corresponding to the regions where the photodiodes are formed, creates a clearance between the flat collet and the regions corresponding to the regions where the photodiodes are formed. For this reason, the flat collet is kept out of direct contact with the regions corresponding to the photodiodes, whereby the regions corresponding to the photodiodes are prevented from being damaged by pressure and heat. In consequence, it is feasible to effectively prevent the characteristic degradation due to noise, dark current, and so on.

Preferably, the depression comprises a plurality of depressions, and adjacent depressions are in communication with each other. Preferably, the depression comprises a plurality of depressions formed corresponding to the respective photodiodes, and adjacent depressions are in communication with each other. In either of these cases, adjacent depressions are in communication with each other; therefore, in a case of applying a resin (e.g., an optical resin for mounting of a scintillator panel) onto the incident surface of the photodiode array, it becomes easier for the resin to spread into each of the depressions. It is also feasible to suppress generation of voids in each depression.

Preferably, the semiconductor substrate is provided with an impurity region between the photodiodes adjacent to each other, for separating the photodiodes from each other. In this case, the impurity region prevents occurrence of surface leak, and thus adjacent photodiodes can be electrically separated from each other with certainty.

Preferably, a high-impurity-concentration layer of the same conductivity type as the semiconductor substrate is formed on the incident surface side of the light to be detected, in the semiconductor substrate. In this case, carriers generated near the light-incident surface inside the semiconductor substrate efficiently migrate into each photodiode, without being trapped. This results in enhancing photodetecting sensitivity.

Preferably, a plurality of depressions having a predetermined depth are formed in array on the opposite surface side to the incident surface of the light to be detected, in the semiconductor substrate, and each photodiode is formed in a bottom portion of the associated depression. In this case, the distance becomes shorter between the incident surface of the light to be detected and the photodiodes in the semiconductor substrate, and thus recombination of carriers is suppressed in migration of carriers generated with incidence of the light to be detected. This results in improving the photodetecting sensitivity.

A photodiode array production method according to the present invention is a method of producing a photodiode array, the method comprising: a step of preparing a semiconductor substrate comprised of a semiconductor of a first conductivity type; a step of forming a plurality of impurity diffused layers of a second conductivity type on one surface side of the semiconductor substrate to form a plurality of photodiodes each comprised of the impurity diffused layer and the semiconductor substrate, in array; and a step of forming a depression with a predetermined depth more depressed than a region not corresponding to regions where the photodiodes are formed, in regions corresponding to the regions where the photodiodes are formed, on another surface of the semiconductor substrate.

The photodiode array production method according to the present invention permits us to obtain the photodiode array wherein the photodiodes are formed in array on one surface of the semiconductor substrate and wherein the depression is formed in the regions corresponding to the regions where the photodiodes are formed, on the other surface.

Preferably, the method further comprises a step of forming a high-impurity-concentration layer of the first conductivity type on the other surface of the semiconductor substrate, after the step of forming the depression. In this case, the high-impurity-concentration layer of the same conductivity type as the semiconductor substrate is formed on the other surface of the semiconductor substrate. For this reason, carriers generated near the light-incident surface inside the semiconductor substrate efficiently migrate into each photodiode, without being trapped. This results in enhancing the photodetecting sensitivity.

Another photodiode array production method according to the present invention is a method of producing a photodiode array, the method comprising: a step of preparing a semiconductor substrate comprised of a semiconductor of a first conductivity type; a step of forming a plurality of first depressions in array on one surface side of the semiconductor substrate; a step of forming a plurality of impurity diffused layers of a second conductivity type in bottom portions of the first depressions to form a plurality of photodiodes each comprised of the impurity diffused layer and the semiconductor substrate, in array; and a step of forming a second depression with a predetermined depth more depressed than a region not corresponding to regions where the photodiodes are formed, in regions corresponding to the regions where the photodiodes are formed, on another surface of the semiconductor substrate.

The photodiode array production method according to the present invention permits us to obtain the photodiode array wherein the photodiodes are formed in array in the bottom portions of the first depressions formed on one surface of the semiconductor substrate and wherein the second depression is formed in the regions corresponding to the regions where the photodiodes are formed, on the other surface.

Preferably, the method further comprises a step of forming a high-impurity-concentration layer of the first conductivity type on the other surface of the semiconductor substrate, after the step of forming the second depression. In this case, the high-impurity-concentration layer of the same conductivity type as the semiconductor substrate is formed on the other surface of the semiconductor substrate. For this reason, carriers generated near the light-incident surface inside the semiconductor substrate efficiently migrate into each photodiode, without being trapped. This results in enhancing the photodetecting sensitivity.

Preferably, the method further comprises a step of providing an impurity region of the first conductivity type between the impurity diffused layers adjacent to each other. In this case, the method permits us to obtain the photodiode array wherein adjacent photodiodes are electrically separated from each other with certainty.

A radiation detector according to the present invention is a radiation detector comprising: the above-described photodiode array; and a scintillator panel arranged opposite to the incident surface of the light to be detected in the photodiode array, and arranged to emit light with incidence of radiation.

Another radiation detector according to the present invention is a radiation detector comprising the photodiode array produced by the above-described photodiode array production method; and a scintillator panel arranged opposite to the surface where the depression is provided in the photodiode array, and arranged to emit light with incidence of radiation.

Still another radiation detector according to the present invention is a radiation detector comprising the photodiode array produced by the above-described photodiode array production method; and a scintillator panel arranged opposite to the surface where the second depression is formed in the photodiode array, and arranged to emit light with incidence of radiation.

Since each of these radiation detectors according to the present invention comprises the above-described photodiode array, it is feasible to effectively prevent the characteristic degradation due to noise, dark current, and so on.

BEST MODE FOR CARRYING OUT THE INVENTION

The preferred embodiments of the photodiode array and production method thereof, and the radiation detector according to the present invention will be described below in detail with reference to the drawings. The same elements, or elements with the same functionality will be denoted by the same reference symbols in the description, without redundant description.

First Embodiment

FIG. 1is a view showing a sectional configuration of photodiode array1according to an embodiment of the present invention. In the description hereinafter, the incident surface of light L (the upper surface inFIG. 1) will be referred to as a back surface, and the opposite surface (the lower surface inFIG. 1) to it as a front surface. It is noted that the dimensions in each drawing below are altered as occasion may demand, for convenience sake of illustration.

The photodiode array1has a plurality of photodiodes4comprised of pn junctions. The plurality of photodiodes4are two-dimensionally arranged in a vertically and horizontally regular array on the front surface side of the photodiode array1. Each photodiode4functions as a pixel of photodiode array1and the photodiodes4as a whole constitute one photosensitive region.

The photodiode array1has an n-type (first conductivity type) silicon substrate3. The thickness of the n-type silicon substrate3is approximately 30-300 μm (preferably, 100 μm). The impurity concentration in the n-type silicon substrate3is approximately 1×1012-1015/cm3. P-type (second conductivity type) impurity diffused layers5are two-dimensionally arranged in a vertically and horizontally regular array on the front surface side of the n-type silicon substrate3. The thickness of the p-type impurity diffused layers5is approximately 0.05-20 μm (preferably, 0.2 μm). The impurity concentration in the p-type impurity diffused layers5is approximately 1×1013-1020/cm3. The pn junctions formed of the p-type impurity diffused layers5and the n-type silicon substrate3constitute the photodiodes4. A silicon oxide film22is formed on the front surface of the n-type silicon substrate3. A passivation film2is formed on this silicon oxide film22. The passivation film2is made, for example, of SiN or the like.

Electrode wirings9are formed corresponding to the respective p-type impurity diffused layers5(photodiodes4), on the silicon oxide film22. Each electrode wiring9is made of aluminum and has the thickness of about 1 μm. One end of each electrode wiring9is electrically connected through a contact hole formed in the silicon oxide film22, to the corresponding p-type impurity diffused layer5. The other end of each electrode wiring9is electrically connected through a contact hole formed in the passivation film2, to a corresponding under bump metal (UBM)11. A bump electrode12of solder is formed on each UBM11. The UBM11and bump electrode12are electrically connected to each other.

The UBM11is preferably one achieving strong interface bonding to solder and being capable of preventing diffusion of a solder component into aluminum, and is often constructed in multilayer structure. An example of this multilayer structure is nickel (Ni)-gold (Au) by electroless plating. This structure is obtained by depositing a thick nickel plated layer (3-15 μm) on an aluminum-exposed region and depositing a thin gold plated layer (0.05-0.1 μm) thereon. The gold layer is provided for preventing oxidation of nickel. Other available structures include multilayer structures of titanium (Ti)-platinum (Pt)-gold (Au) and chromium (Cr)-gold (Au) formed by liftoff.

An accumulation layer8as a high-impurity-concentration layer is provided on the back surface side of the n-type silicon substrate3. The accumulation layer8is formed in a substantially uniform depth across almost all the back surface. The accumulation layer8has the same conductivity type as the n-type silicon substrate3and the impurity concentration thereof is higher than that of the n-type silicon substrate3. The photodiode array1of the present embodiment has the accumulation layer8, but the photodiode array has the photodetecting sensitivity at a practically acceptable level, without provision of the accumulation layer8.

An AR film24is formed on the accumulation layer8, in order to cover and protect the accumulation layer8and to suppress reflection of light L. The AR film24is made, for example, of SiO2and has the thickness of about 0.01 to several μm. The AR film24may be formed in a multilayer or complex structure with SiN or with an optical film capable of preventing reflection at a desired wavelength, in addition to SiO2.

On the front surface side of the n-type silicon substrate3, a region where each p-type impurity diffused layer5exists is a region where a photodiode4is formed (hereinafter referred to as a “formed region”), and the region except for the formed regions constitutes a region where the photodiodes are not formed. A plurality of depressions6are provided corresponding to the respective photodiodes4, in regions corresponding to the formed regions of the respective photodiodes4(the regions will be referred to hereinafter as “corresponding regions”). Each depression6is formed, for example, in rectangular shape in the size of 1 mm×1 mm and is more depressed than a region not corresponding to the formed regions of the photodiodes4(the region will be referred to hereinafter as a “noncorresponding region”). The depth d of each depression6is, for example, 3-150 μm and preferably approximately 10 μm.

An n+-type impurity region7is provided between adjacent p-type impurity diffused layers5, i.e., between adjacent photodiodes4in the n-type silicon substrate3. The thickness of the n+-type impurity region7is approximately 0.1-several ten μm. The n+-type impurity region7functions as a separating layer for electrically separating adjacent photodiodes4(p-type impurity diffused layers5) from each other. This can securely electrically separate adjacent photodiodes4from each other and reduce crosstalk between photodiodes4. The photodiode array1in the present embodiment, without the n+-type impurity region7, possesses the photodetecting characteristics at a practically acceptable level.

The photodiode array1is of extremely thin plate shape, as shown inFIG. 2. The width W1of photodiode array1is approximately 22.4 mm, and the thickness D of photodiode array1approximately 0.3 mm. The photodiode array1has a number of aforementioned photodiodes4(e.g., a two-dimensional array of 256 (16×16) photodiodes). The pitch W2between adjacent photodiodes4(pixels) is approximately 1.4 mm. The photodiode array1is a chip of a large area (e.g., 22.4 mm×22.4 mm). The top illustration inFIG. 2is one for showing how thin the photodiode array1is, and the details of the photodiode array1are illustrated in enlarged views.

In the photodiode array1, when light L is incident on the back surface, the incident light L passes through the accumulation layer8to reach the pn junctions. Then each photodiode4generates carriers according to the incident light. At this time, the accumulation layer8prevents the carriers generated near the light-incident surface (back surface) inside the n-type silicon substrate3from being trapped at the light-incident surface and the interface to the AR film24. This permits the carriers to efficiently migrate to the pn junctions and thus enhances the photodetecting sensitivity of the photodiode array1. A photocurrent caused by generated carriers is extracted through electrode wiring9and UBM11connected to each p-type impurity diffused layer5, and from bump electrode12. The incident light is detected based on the output from the bump electrode12.

In the present embodiment, as described above, the depressions6are formed in the corresponding regions of the respective photodiodes4on the light-incident surface side of the light L (i.e., the back surface side) in the photodiode array1, and thus the noncorresponding region of each photodiode4projects by the distance corresponding to the depth d relative to the corresponding regions. In the case where the flip chip bonding is carried out with the flat collet holding the photodiode array1under suction, the noncorresponding region of each photodiode4comes into contact with the flat collet and functions to secure a clearance between the flat collet and the corresponding region of each photodiode4. In this configuration, the corresponding region of each photodiode4is protected by the noncorresponding region while being kept out of direct contact with the flat collet. Therefore, the corresponding region of each photodiode4gets rid of direct stress due to pressure and direct stress due to heat, so that the accumulation layer8in the corresponding region is prevented from suffering physical damage. The photodiodes4are thus free of the dark current and noise caused by crystal defects or the like due to such damage. In consequence, the photodiode array1is able to perform photodetection with high accuracy (at high S/N ratios).

In cases except for the flip chip bonding, e.g., in a case where the photodiode array1is integrated with a scintillator to be used as a CT sensor, as described later, the scintillator is kept out of direct contact with the corresponding regions, and it is thus feasible to avoid damage during mounting of the scintillator.

The depressions6are formed corresponding to photodiodes4in such a manner that one depression is provided or each photodiode4. For forming the depressions6in this manner, as shown inFIG. 13A, a wall part13acan be formed in a lattice pattern in the noncorresponding region of the photodiodes4. In another configuration, as shown inFIG. 13B, a plurality of short wall portions13cmay be discontinuously formed in portions except for intersections13bin the noncorresponding region of the photodiodes4. In still another configuration, as shown inFIG. 13C, wall portions13dof cross shape may be formed at intersections13b. In a further configuration, which is not shown, the depressions6may be formed in a plurality of separate regions, for example, by dividing the region into two large areas left and right.

The accumulation layer8is formed on the entire back surface side of the n-type silicon substrate43. The AR film24is formed on the accumulation layer8. This accumulation layer8and the AR film24are similar to those in the aforementioned photodiode array1. A plurality of depressions6(second depressions) are formed corresponding to the respective depressions45in the corresponding regions of the photodiodes4so that one depression is provided for each photodiode4. The depressions6are also similar to those in the aforementioned photodiode array1.

Where the depressions6are formed in this manner, adjacent depressions6are preferably formed in communication with each other, without being partitioned by the noncorresponding region of the photodiodes4. For implementing it, for example, the noncorresponding region may be formed by discontinuously arranging the aforementioned wall portions13c,13d.

It is also possible to adopt a configuration, as shown inFIG. 14A, wherein a wall portion13eof frame shape is formed at the edge of the photodiode array1so that the entire region inside this wall portion13econstitutes one depression6. This wall portion13emay be replaced by wall portions13fof partly cut frame shape, as shown inFIG. 14B. In these cases, the depression6is formed without being partitioned by the noncorresponding region.

The entire noncorresponding region of the photodiodes4does not have to be formed so as to be thicker than the depressions6, but the necessary condition is that a part of the noncorresponding region (the portion where the wall portion13e,13fis formed) is formed so as to be thicker than the depressions6, as shown inFIGS. 14A and 14B. On the other hand, the corresponding regions of the photodiodes4all have to be provided in the depressions6.

In the configurations where adjacent depressions6are in communication with each other without being partitioned, the clearance between adjacent wall portions functions as an escape for resin (e.g., optical resin35used in bonding of scintillator panel31described later). In applying the resin onto the back surface of the photodiode array1, therefore, voids (air bubbles) are unlikely to be made in the depressions6(to decrease the number of voids), whereby the applied resin can uniformly spread into each depression6to be uniformly filled.

It is also possible to continuously provide the wall portions13aand wall portion13e, as shown inFIG. 14C, and in this case, each depression6is partitioned off by the wall portions13aand wall portion13e.

Next, a production method of the photodiode array1according to the present embodiment will be described below on the basis ofFIGS. 3 to 12.

The first step is to prepare an n-type silicon substrate3having the thickness of about 150-500 μm (preferably, 350 μm), as shown inFIG. 3. Next, a silicon oxide film (SiO2)20is formed on the front surface and on the back surface of the n-type silicon substrate3(cf.FIG. 4).

Next, the silicon oxide film20formed on the front surface of the n-type silicon substrate3is patterned with a predetermined photomask to form openings at intended positions for formation of the n+-type impurity regions7. Then the n-type silicon substrate3is doped with phosphorus through the openings formed in the silicon oxide film20, to provide the n+-type impurity regions7in the n-type silicon substrate3. In the present embodiment, the n+-type impurity region7is also formed on the back surface side of the n-type silicon substrate3. This step (impurity region forming step) may be omitted if the n+-type impurity regions7are not provided. Subsequently, a silicon oxide film21is again formed on the front surface and on the back surface of the n-type silicon substrate3(cf.FIG. 5). This silicon oxide film21is used as a mask in the subsequent step of forming the p-type impurity diffused layers5.

Next, the silicon oxide film21formed on the front surface of the n-type silicon substrate3is patterned with a predetermined photomask to form openings at intended positions for formation of the respective p-type impurity diffused layers5. The substrate is doped with boron through the openings formed in the silicon oxide film21, to form the p-type impurity diffused layers5in two-dimensional arrangement of a vertical and horizontal array. This results in forming photodiodes4of pn junctions between each p-type impurity diffused layer5and n-type silicon substrate3in two-dimensional arrangement of a vertical and horizontal array. Each photodiode4becomes a portion corresponding to a pixel. Subsequently, a silicon oxide film22is again formed on the front surface side of the substrate (cf.FIG. 6).

Next, the back surface of the n-type silicon substrate3is polished up to a predetermined thickness (about 30-300 μm) to obtain the n-type silicon substrate3in thin shape (thin plate). Subsequently, a silicon nitride film (SiN)23is formed on the front surface and on the back surface of the n-type silicon substrate3by LP-CVD (or plasma CVD) (cfFIG. 7). Then patterning with a predetermined photomask is carried out to remove the intended regions for formation of the respective depressions6, i.e., the silicon nitride film23from the corresponding regions of the photodiodes4and to leave the portions where the depressions6are not formed, i.e., the silicon nitride film23in the noncorresponding region of the photodiodes4(cf.FIG. 8). In this step, the regions where the silicon nitride film23is left can be appropriately modified, whereby the noncorresponding region (wall portions13a,13c,13d,13e,13f) can be formed in the aforementioned various patterns.

Next, the n-type silicon substrate3is etched by anisotropic alkali etching with potassium hydroxide solution (KOH), TMAH, or the like, using the left silicon nitride film23as a mask, to form the depressions6in the portions not covered by the silicon nitride film23. Thereafter, the left silicon nitride film23is removed. Then an n-type ion species (e.g., phosphorus or arsenic) is allowed to diffuse from the back surface of the n-type silicon substrate3into the depth of about 0.05 to several ten μm, thereby forming the aforementioned accumulation layer8with the impurity concentration higher than that of the n-type silicon substrate3. Furthermore, thermal oxidation is carried out to form the AR film24on the accumulation layer8(cf.FIG. 9).

Next, contact holes extending to the respective p-type impurity diffused layers5are formed in the formed regions of the respective photodiodes4and in the silicon oxide film22by the photoetching technology. Subsequently, an aluminum metal film is formed on the silicon oxide film22by evaporation, and thereafter it is patterned with a predetermined photomask to form electrode wirings9(cf.FIG. 10). Then an SiN film25to become the passivation film2is formed on the silicon oxide film22so as to cover the electrode wirings9. The SiN film25can be formed by sputtering, plasma CVD, or the like. The passivation film2may be one selected from the insulating films of SiO2, PSG, BPSG, etc., polyimide resin, acrylate resin, epoxy resin, fluororesin, composite films and multilayer films thereof, and so on.

Next, contact holes are formed at predetermined positions in the SiN film25to make electrode extracting portions (cf.FIG. 11). Furthermore, the bump electrodes12are to be formed. Where the bump electrodes12are of solder, since the solder has poor wettability to aluminum, UBM11for intervention between each electrode extracting portion and bump electrode12is formed on each electrode extracting portion. Then the bump electrode12is formed over each UBM11(cf.FIG. 12).

Through the above steps, the production method permits us to produce the photodiode array1capable of performing photodetection with high accuracy, without occurrence of noise due to damage in mounting.

The bump electrodes12can be made by placing solder on the predetermined UBM11by the solder ball mounting method or printing method and reflowing the solder. The bump electrodes12are not limited to the solder bumps, but may be gold bumps, nickel bumps, or copper bumps, or may be electroconductive resin bumps containing such metal as an electroconductive filler. The drawings show only the extraction of the anode electrodes, and the cathode (substrate) electrodes can also be similarly extracted from the n+-type impurity regions7(though not shown) in the same manner as the anode electrodes. The drawings show the case where the bump electrodes12of the anode electrodes are formed in the areas of the n+-type impurity regions7, but the bump electrodes12of the anode electrodes may be formed in the areas of the p-type impurity diffused layers5.

Second Embodiment

Next, the second embodiment of the photodiode array and production method thereof will be described.

The present embodiment is directed to a photodiode array41having an n-type silicon substrate43in which depressions45(first depressions) are formed on the opposite surface side (front surface side) to the incident surface of light L, as shown inFIG. 15. Since this photodiode array41has common portions to the photodiode array1, the description below will be given with focus on the differences between them, while omitting or simplifying the description of the common portions.

In the photodiode array41, a plurality of depressions45are formed in two-dimensional arrangement of a vertically and horizontally regular array on the front surface side of the n-type silicon substrate43. Each depression45is made by recessing a predetermined region of the n-type silicon substrate43so as to make it thinner than the region around it, and the depressions45are formed at arrangement intervals of about 1.4-1.5 mm. The aforementioned photodiodes4are formed in respective bottom portions45aof the depressions45, thereby constituting the photodiode array41in which the photodiodes4are two-dimensionally arranged in array.

Each depression45is formed with a rectangular opening, for example, in the size of about 1 mm×1 mm in the front surface of the n-type silicon substrate43so that the aperture size gradually decreases from the opening toward the bottom portion45a(i.e., from the front surface side toward the back surface side). In this configuration, each depression45has a slope side surface45b. The depth from the front surface of the n-type silicon substrate43to the bottom portion45ais, for example, about 50 μm.

Electrode wirings9are formed along side faces45band on the silicon oxide film22. One end of each electrode wiring9is electrically connected through a contact hole formed in the silicon oxide film22, to the corresponding p-type impurity diffused layer5. The other end of each electrode wiring9is electrically connected through a contact hole formed in the passivation film2, to the corresponding UBM11. An n+-type impurity region7is provided between adjacent photodiodes4.

The accumulation layer8is formed on the entire back surface side of the n-type silicon substrate3. The AR film24is formed on the accumulation layer8. This accumulation layer8and the AR film24are similar to those in the aforementioned photodiode array1. A plurality of depressions6(second depressions) are formed corresponding to the respective depressions45in the corresponding regions of the photodiodes4so that one depression is provided for each photodiode4. The depressions6are also similar to those in the aforementioned photodiode array1.

The accumulation layer8permits the carriers generated near the light-incident surface (back surface) inside the n-type silicon substrate43to efficiently migrate to the pn junctions, without recombination. This permits the photodiode array41to have higher photodetecting sensitivity (though the photodiode array41of the present embodiment has the detection sensitivity at a practically acceptable level, without provision of the accumulation layer8).

In the photodiode array41constructed as described above, when light L is incident on the back surface, just as in the case of the photodiode array1, the incident light L passes through the accumulation layer8to reach the pn junctions. Each photodiode4generates carriers according to the incident light. Since each pn junction is provided in the bottom portion45aof depression45, the distance is shorter between the back surface of the n-type silicon substrate43and the pn junction (e.g., approximately 10-100 μm). Therefore, the photodiode array41is configured to prevent a situation in which the carriers generated with incidence of light L annihilate through recombination in the process of migration. In consequence, the photodiode array41is able to maintain high detection sensitivity.

As described above, the photodiode array41of the present embodiment is also provided with the depressions6formed in the corresponding regions of the respective photodiodes4as the photodiode array1was. Where the photodiode array41is held in suction by the flat collet to be subjected to the flip chip bonding, the noncorresponding region of each photodiode4comes into contact with the flat collet and functions to secure the clearance between the flat collet and the corresponding region of each photodiode4. In this configuration, the corresponding region of each photodiode4is protected by the noncorresponding region so as to be kept out of direct contact with the flat collet. Therefore, the corresponding regions of the respective photodiodes4get rid of direct stress due to pressure and direct stress due to heat, whereby the accumulation layer8in the corresponding regions is free of physical damage. In the photodiodes4there is neither dark current nor noise caused by crystal defects or the like due to such damage. In consequence, the photodiode array41is able to perform photodetection with high accuracy (at high S/N ratios).

A photocurrent caused by generated carriers is extracted through electrode wiring9and UBM11connected to each p-type impurity diffused layer5, and from bump electrode12. The incident light is detected based on the output from bump electrode12. This is much the same as in the case of the photodiode array1.

As described above, the photodiode array41of the present embodiment is also provided with the depressions6formed in the corresponding regions of the respective photodiodes4as the photodiode array1was. Where the photodiode array41is held in suction by the flat collet to be subjected to the flip chip bonding, the noncorresponding region of each photodiode4comes into contact with the flat collet and functions to secure the clearance between the flat collet and the corresponding region of each photodiode4. In this configuration, the corresponding region of each photodiode4is protected by the noncorresponding region so as to be kept out of direct contact with the flat collet. Therefore, the corresponding regions of the respective photodiodes4get rid of direct stress due to pressure and direct stress due to heat, whereby the accumulation layer8in the corresponding regions is free of physical damage. In the photodiodes4there is neither dark current nor noise caused by crystal defects or the like due to such damage. In consequence, the photodiode array1is able to perform photodetection with high accuracy (at high S/N ratios).

In cases except for the flip chip bonding, for example, in a case where the photodiode array41is integrated with a scintillator to be used as a CT sensor, as described later, the scintillator is kept out of direct contact with the corresponding regions and it is thus feasible to avoid damage during mounting of the scintillator.

Next, a production method of the photodiode array41according to the present embodiment will be described on the basis ofFIGS. 3 to 6andFIGS. 17 to 22.

First, the steps described withFIGS. 3 to 6are executed in the same manner as in the case of the photodiode array1. Next, the back surface of the n-type silicon substrate3is polished to make the n-type silicon substrate3thinner (into a thin plate) before the thickness of the n-type silicon substrate3becomes a predetermined thickness. Subsequently, a silicon nitride film (SiN)23is formed on the front surface and on the back surface of the n-type silicon substrate3by LP-CVD (or plasma CVD), and thereafter the silicon oxide film22and silicon nitride film23on the front surface side are patterned with a predetermined photomask to form openings at intended positions for formation of the respective depressions45(cf.FIG. 17).

Next, the p-type impurity diffused layer5and n-type silicon substrate3are removed by alkali etching to form a depression45so as to leave a frame peripheral part5aof p-type impurity diffused layer5, for each target of a region where the p-type impurity diffused layer5is formed, in the front surface of the n-type silicon substrate3. This results in obtaining an n-type silicon substrate43. At this time, the frame peripheral part5ais formed as a region resulting from diffusion with a p-type impurity, in the edge part of the opening of each depression45. Each depression45comes to have a side face45band a bottom portion45a. The frame peripheral part5ais not always essential. When the frame peripheral part5ais formed, it provides the effect of preventing noise and dark current due to damage in the edge part formed by etching for formation of the depressions45.FIGS. 15,16, and24show the example without formation of the frame peripheral part5a.

Subsequently, the bottom portion45aof each depression45thus formed is doped with boron or the like. This results in forming a p-type impurity diffused layer5bin the bottom portion45aof each depression45, and the photodiodes4comprised of pn junctions of such p-type impurity diffused layers5band n-type silicon substrate43are formed in two-dimensional arrangement of a vertical and horizontal array. Then a silicon oxide film22is formed on the regions not covered by the silicon nitride film23formed on the front surface. At this time, though not shown, the silicon oxide film is also formed on the silicon nitride film23formed on the back surface.

Next, patterning with a predetermined photomask is conducted on the silicon nitride film23formed on the back surface side of the n-type silicon substrate43, to remove the intended regions for formation of the respective depressions6, i.e., the silicon nitride film23from the corresponding regions of the photodiodes4and to leave the portions where the depressions6are not formed, i.e., the silicon nitride film23in the noncorresponding region of the photodiodes4(cf.FIG. 18). In this step, the regions where the silicon nitride film23is left can be properly modified, whereby the noncorresponding region (wall portions13a,13c,13d,13e,13f) can be formed in the aforementioned various patterns.

Next, the n-type silicon substrate43is etched by anisotropic alkali etching with potassium hydroxide solution (KOH), TMAH, or the like, using the left silicon nitride film23as a mask, to form the depressions6in the portions not covered by the silicon nitride film23. Thereafter, the left silicon nitride film23is removed. Then ion implantation with an n-type ion species or the like is carried out in the same manner as in the first embodiment to form the aforementioned accumulation layer8with the impurity concentration higher than that of the n-type silicon substrate43. Furthermore, thermal oxidation is performed to form the AR film24on the accumulation layer8(cf.FIG. 19).

Then, in the formed region of each photodiode4a contact hole extending up to each p-type impurity diffused layer5bis formed in the silicon oxide film22on the front surface side by photoetching technology. Subsequently, an aluminum metal film is formed on the silicon oxide film22by evaporation and thereafter patterned with a predetermined photomask to form the electrode wirings9(cf.FIG. 20).

Next, an SiN film25to become the passivation film2is formed on the silicon oxide film22so as to cover the electrode wirings9. The SiN film25can be formed by sputtering, plasma CVD, or the like. Subsequently, contact holes are formed at positions corresponding to the respective electrode wirings9in the SiN film25(cfFIG. 21). Subsequently, the UBM11electrically connected with each electrode wiring9through the contact hole is formed by electroless plating or the like in the same manner as in the first embodiment. Then the bump electrode12is formed over each UBM11(cf.FIG. 22).

Through the above steps, the production method permits us to produce the photodiode array41capable of performing photodetection with high accuracy, without occurrence of the noise and dark current due to damage during mounting. The drawings show only the extraction of the anode electrodes, but the cathode (substrate) electrodes can also be extracted from the n+-type impurity regions7(though not shown) in the same manner as the anode electrodes.

Third Embodiment

Next, a radiation detector according to the third embodiment will be described.

FIG. 23is a view showing a sectional configuration of radiation detector50according to the present embodiment. This radiation detector50has a scintillator panel31arranged to emit light with incidence of radiation, and the aforementioned photodiode array1. The scintillator panel31emits light generated with incident radiation, from its light exit surface31a. The scintillator panel31is arranged opposite to the light-incident surface of photodiode array1, i.e., opposite to the surface with the depressions6in the photodiode array1. When the light emerging from the light exit surface31aof scintillator panel31is incident on the light-incident surface, the photodiode array1converts the incident light into electric signals.

The scintillator panel31is mounted on the back surface side (incident surface side) of the photodiode array1. Since the photodiode array1is provided with the aforementioned depressions6, the back surface of the scintillator panel31, i.e., the light exit surface31ais in contact with the noncorresponding region of the photodiodes4, but is kept out of direct contact with the corresponding regions of the photodiodes4. The space between the light exit surface31aof the scintillator panel31and the depressions6is filled with an optical resin35having a refractive index set so as to sufficiently transmit the light. This optical resin35allows the light from the scintillator panel31to efficiently enter the photodiode array1. This optical resin35can be selected from epoxy resin, acrylic resin, urethane resin, silicone resin, fluororesin, etc. with the property of transmitting the light from the scintillator panel31, or may be one of composite materials of these resins.

In an operation of bonding the photodiode array1onto an unrepresented mounting wiring board, the flat collet holds the photodiode array1under suction. However, since the photodiode array1is provided with the aforementioned depressions6, the sticking surface of the flat collet is kept out of direct contact with the corresponding regions of the respective photodiodes4. When the scintillator panel31is mounted, its light exit surface31ais not in direct contact with the corresponding regions of the photodiodes4, either. Therefore, the radiation detector50having such photodiode array1and scintillator panel31is able to prevent occurrence of noise, dark current, etc. due to damage of the corresponding regions in the mounting. In consequence, the radiation detector50is able to accurately perform photodetection and, in turn, to accurately perform radiation detection.

Fourth Embodiment

Next, a radiation detector according to the fourth embodiment will be described.

FIG. 24is a view showing a sectional configuration of radiation detector55according to the present embodiment. This radiation detector55has a scintillator panel31, and the aforementioned photodiode array41. The scintillator panel31is arranged opposite to the light-incident surface of photodiode array41, i.e., opposite to the surface where the depressions6are provided in the photodiode array41.

The scintillator panel31is mounted on the back surface side (incident surface side) of the photodiode array41. Since the photodiode array41is provided with the aforementioned depressions6, the back surface of the scintillator panel31, i.e., the light exit surface31ais kept out of direct contact with the corresponding regions of the photodiodes4. The space between light exit surface31aof scintillator panel31and depressions6is filled with the optical resin35. This optical resin35allows the light from the scintillator panel31to efficiently enter the photodiode array41.

In an operation of bonding the photodiode array41to an unrepresented mounting wiring board, the flat collet holds the photodiode array41under suction. However, since the photodiode array41is provided with the aforementioned depressions6, the sticking surface of the flat collet is not in direct contact with the corresponding regions of the respective photodiodes4. When the scintillator panel31is mounted, its light exit surface31ais also kept out of direct contact with the corresponding regions of the photodiodes4. Therefore, the radiation detector55having such photodiode array41and scintillator panel31is able to prevent occurrence of noise, dark current, etc. due to damage of the corresponding regions in the mounting. In consequence, the radiation detector55is able to accurately perform photodetection and, in turn, to accurately perform radiation detection.

INDUSTRIAL APPLICABILITY

The present invention is applicable to X-ray CT scanners and radiographic image taking systems.