Abstract:
A semiconductor device includes a first semiconductor layer of a first conductivity type having a primary surface on one side thereof and a secondary surface on an opposite side thereof, and having a sensor therein, a second semiconductor layer of a second conductivity type having a circuit element formed therein, the second semiconductor layer being formed at said one side of the primary surface of the first semiconductor layer, an insulating layer formed between the first semiconductor layer and the second semiconductor layer, and being disposed on the primary surface of the first semiconductor layer, and a charge-attracting semiconductor layer of the first conductivity type configured to attract electrical charges generated in the insulating layer when a fixed voltage is supplied to the charge-attracting semiconductor layer.

Description:
CROSS REFERENCE TO RELATED APPLICATION AND INCORPORATION BY REFERENCE 
       [0001]    This is a continuation of U.S. application Ser. No. 15/270,101, filed on Sep. 20, 2016. Furthermore, this application claims the benefit of priority of Japanese application number 2015-186403, filed on Sep. 24, 2015. The disclosures of both of these prior applications are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
     Technical Field 
       [0002]    The present invention relates to a semiconductor device and a manufacturing method of a semiconductor device. 
       Background Art 
       [0003]    Semiconductor devices in which a semiconductor layer having a sensor formed therein and a semiconductor layer having a peripheral circuit formed therein are stacked on the same semiconductor substrate through an insulating film are known. One example of such semiconductor devices is an X-ray sensor in which circuit elements such as a transistor and resistance and a PN diode that functions as a sensor are formed together on the same SOI (silicon on insulator) substrate. 
         [0004]    In the X-ray sensor formed on the SOI substrate, a MOS-FET (metal oxide semiconductor-field effect transistor: will be simply referred to as a transistor below) for the circuit operation and the like are formed in a silicon layer of the SOI substrate, and a pixel sensor is formed adjacent to the substrate. Between the silicon layer and the substrate, a buried oxide (BOX) film is interposed. When X-ray is radiated, the X-ray collides with atoms in the silicon and the oxide film, which form the X-ray sensor, and generates electron-hole pairs. Among them, electrons or holes generated near the substrate are attracted to an electrode due to the field effect, extracted to the outside, and converted to an electric signal. This way, the function of the X-ray sensor is realized. However, when electron-hole pairs are generated in an oxide film such as the buried oxide film, positive charges are trapped and accumulated in the oxide film in some cases. 
         [0005]    As the total amount of X-ray radiated to the X-ray sensor increases, the total amount of positive charges accumulated in the oxide film also increases. In some cases, the accumulated positive charges cause a characteristic change of the transistor, and the degree of the characteristic change may fluctuate depending on the amount of accumulated positive charges. In particular, positive charges trapped in the field oxide film that faces the channel region of the transistor, or positive charges trapped in the buried oxide film might change the threshold voltage or the current amount of the transistor or generate a leak current that is not relevant to the control by the gate. 
         [0006]    One known example of the prior art that achieves an X-ray sensor configured to suppress a leak current is a semiconductor device disclosed in Japanese Patent Application Laid-open Publication No. 2013-069924 (will be referred to as Patent Document 1 below). As shown in FIG. 1 of Patent Document 1, a semiconductor device (100) includes a photodiode (30), a semiconductor region (14), a buried oxide film (10), and a semiconductor layer (9). 
         [0007]    The photodiode (30) has a semiconductor layer (11) of one conductivity type, a first semiconductor region (182) of the other conductivity type that is disposed in a primary surface (151) of the semiconductor layer, semiconductor regions (191) and (192) of the one conductivity type that have a higher impurity concentration than that of the semiconductor layer (11), and a semiconductor region (99). The semiconductor regions (191) and (192) are formed in the primary surface (151) of the semiconductor layer (11) so as to be separated from the semiconductor region (182). The semiconductor region (99) of the one conductivity type has a higher impurity concentration than that of the semiconductor layer (11) and a lower impurity concentration than that of the semiconductor regions (191) and (192). The semiconductor region (99) is formed in the primary surface (151) of the semiconductor layer (11) at least between the semiconductor region (182) and the semiconductor regions (191) and (192). 
         [0008]    The semiconductor region (14) of the other conductivity type is formed in the primary surface (151) of the semiconductor layer (11) and is given a fixed potential. The buried oxide film (10) is disposed on the primary surface (151) of the semiconductor layer (11). The semiconductor layer (9) is formed on the buried oxide film (10) and has a transistor element (40) formed therein. 
         [0009]    In the semiconductor device (100) disclosed in FIG. 1 of Patent Document 1, the semiconductor region (99) functions as an inversion preventing layer of the primary surface (151) of the semiconductor layer (11), which suppresses the generation of a leak current at the interface of the semiconductor layer (11) and the buried oxide film 10. In the transistor element 40 formed in the semiconductor layer (9), the channel region thereof on the side closer to the buried oxide film (10) is not activated due to the effect of the semiconductor region (14), and therefore, it is possible to suppress the generation of a leak current that is not relevant to the control by the gate electrode (15). 
       SUMMARY OF THE INVENTION 
       [0010]    In the X-ray sensor described above, in view of the above-mentioned phenomenon of the positive charges being trapped in the oxide film, there is a demand to suppress the characteristic change of the active element such as a transistor caused by the entrapment of charges in the oxide film (generally, insulating region) so that the accurate operation is ensured. However, an X-ray sensor that takes into consideration the phenomenon described above and that is configured to suppress the characteristic change of the active element caused by such a phenomenon has not been fully studied. The semiconductor device disclosed in FIG. 1 of Patent Document 1 is aiming at suppressing the generation of an unintended current, but the main focus thereof is to suppress a leak current that flows through the surface of the semiconductor layer (11) due to the interface state generated at the interface between the semiconductor layer (11) and the buried oxide film (10). Thus, the semiconductor device disclosed in Patent Document 1 is not designed to suppress the characteristic change of the transistor caused by the entrapment of electric charges in the buried oxide film (10) due to the X-ray radiation. 
         [0011]    The present invention was made in view of the above-mentioned points, and an object thereof is to provide a semiconductor device that can suppress the characteristic change of an active element caused by the entrapment of charges in an insulating region, and a manufacturing method of the semiconductor device. 
         [0012]    According to one aspect of the present invention, a semiconductor device includes a first semiconductor layer of a first conductivity type having a primary surface on one side thereof and a secondary surface on an opposite side thereof, and having a sensor therein, a second semiconductor layer of a second conductivity type having a circuit element formed therein, the second semiconductor layer being formed at said one side of the primary surface of the first semiconductor layer, an insulating layer formed between the first semiconductor layer and the second semiconductor layer, and being disposed on the primary surface of the first semiconductor layer, and a charge-attracting semiconductor layer of the first conductivity type configured to attract electrical charges generated in the insulating layer when a fixed voltage is supplied to the charge-attracting semiconductor layer. 
         [0013]    According to another aspect of the invention, a semiconductor device includes a first semiconductor layer of a first conductivity type having a primary surface and a secondary surface and having a sensor therein, a second semiconductor layer of a second conductivity type having a circuit element formed therein, the second semiconductor layer being formed at a same side of the primary surface of the first semiconductor layer, and an insulating layer formed between the first semiconductor layer and the second semiconductor layer, the insulating layer being disposed on the primary surface of the first semiconductor layer so as to surround the circuit element, the insulating layer including a charge-attracting semiconductor pattern of the first conductivity type that is disposed near the circuit element, the charge-attracting semiconductor pattern being configured to attract electrical charges generated in the insulating layer. 
         [0014]    According to one aspect of the invention, a manufacturing method of a semiconductor device includes preparing a semiconductor substrate that includes a first semiconductor layer of a first conductivity type, a first insulating layer formed on the first semiconductor layer, and a second semiconductor layer of a second conductivity type formed on the first insulating layer, forming, in a portion of the second semiconductor layer, an active region of the second conductivity type so as to be surrounded by a second insulating layer, the second insulating layer being integrally formed with the first insulating layer, and forming a charge-attracting semiconductor pattern in the first insulating layer so as to be adjacent to the active region, the charge-attracting semiconductor pattern being configured to attract electric charges generated in the first insulating layer or the second insulating layer. 
         [0015]    According to another aspect of the invention, a manufacturing method of a semiconductor device includes preparing a semiconductor substrate that includes a first semiconductor layer of a first conductivity type, a first insulating layer formed on the first semiconductor layer, an intermediate semiconductor layer of the first conductivity type formed on the first insulating layer, a second insulating layer formed on the intermediate semiconductor layer, and a second semiconductor layer formed on the second insulating layer, forming, in a portion of the second semiconductor layer, an active region of the second conductivity type so as to be surrounded by a third insulating layer, the third insulating layer being integrally formed with the second insulating layer, and forming a charge-attracting semiconductor pattern in the second insulating layer so as to be adjacent to the active region, the charge-attracting semiconductor pattern being configured to attract electric charges generated in the second insulating layer or the third insulating layer. 
         [0016]    According to the present invention, it is possible to provide a semiconductor device that can suppress the characteristic change of an active element caused by the entrapment of charges in an insulating region, and a manufacturing method of the semiconductor device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  is a vertical cross-sectional view showing an example of the configuration of a semiconductor device of Embodiment 1. 
           [0018]      FIG. 2  is a plan view showing an example of the configuration of a charge-attracting part of Embodiment 1. 
           [0019]      FIG. 3  is a vertical cross-sectional view for explaining how electrical charges are attracted in the semiconductor device of Embodiment 1. 
           [0020]      FIGS. 4A to 4D  are vertical cross-sectional views showing an example of a manufacturing method of the semiconductor device of Embodiment 1. 
           [0021]      FIGS. 5A to 5D  are vertical cross-sectional views showing an example of a manufacturing method of the semiconductor device of Embodiment 1. 
           [0022]      FIGS. 6A to 6D  are vertical cross-sectional views showing an example of a manufacturing method of the semiconductor device of Embodiment 1. 
           [0023]      FIG. 7  is a plan view showing an example of the configuration of a charge-attracting part of Embodiment 2. 
           [0024]      FIG. 8  is a vertical cross-sectional view showing an example of the configuration of a semiconductor device of Embodiment 3. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0025]    Embodiments of the present invention will be explained below with reference to figures. In the respective figures, constituting elements or parts that are identical or equivalent to each other are given the same reference characters, and overlapping descriptions will be omitted as necessary. 
       Embodiment 1 
       [0026]      FIG. 1  is a vertical cross-sectional view showing an example of the configuration of a semiconductor device  100  of this embodiment. The semiconductor device  100  includes a photodiode  11  constituting an X-ray sensor and a transistor  51  that is a circuit element constituting a peripheral circuit. The photodiode  11  and the transistor  51  are formed in an SOI (silicon on insulator) substrate that is formed by stacking a first semiconductor layer  10  made of an n-type semiconductor, a first insulating layer  20 , and a second semiconductor layer  30  made of a p-type semiconductor in this order. 
         [0027]    The photodiode  11  includes an anode  12  made of a high concentration p-type semiconductor and a cathode  13  made of a high concentration n-type semiconductor that are disposed on the surface of the first semiconductor layer  10 , which is made of low-concentration n-type silicon, so as to be separated from each other. The photodiode  11  also includes an anode electrode  74  connected to the anode  12 , a cathode electrode  75  connected to the cathode  13 , and a rear electrode  14  formed on the rear side of the first semiconductor layer  10 . 
         [0028]    The circuit element including the transistor  51  is formed on the second semiconductor layer  30  at a position that does not overlap the photodiode  11 . The transistor  51  includes a channel region  53 , a gate electrode  55  formed on the channel region  53 , source and drain regions  52  made of a high concentration n-type semiconductor and disposed to have the channel region  53  therebetween, and source and drain electrodes  72  connected to the source and drain regions  52  respectively. The surface of the second semiconductor layer  30  is covered by a second insulating layer  40  made of an insulator such as a silicon oxide film (SiO 2  film). 
         [0029]    The semiconductor device  100  also includes a charge-attracting part  60  as a charge-attracting semiconductor pattern. The charge-attracting part  60  of this embodiment includes a buried polysilicon (polycrystalline silicon)  80  as a charge-attracting semiconductor layer, and a buried well  84  as a fixed potential region, and a buried polysilicon electrode  82  is connected to the buried polysilicon  80 , and a buried well electrode  86  is connected to the buried well  84 . The buried polysilicon  80  is made of n-type polysilicon (polycrystalline silicon) as described below. The buried well  84  is a semiconductor region formed by injecting a p-type impurity into the first semiconductor layer  10  as described below. In some cases, the buried well  84  has a contact region (not shown in the figure), to which a high-concentration p-type impurity has been injected, in a region that includes the interface with the buried well electrode  86 . 
         [0030]      FIG. 2  is a plan view of the charge-attracting part  60 . As shown in  FIG. 2 , the buried polysilicon  80  has two regions that are separated from the transistor  51  in the gate width direction of the transistor  51  (direction indicated with the arrow W in  FIG. 2 ) and that extend in the gate length direction (direction indicated with the arrow L in  FIG. 2 ). The buried well  84  is formed so as to include the transistor  51  and the buried polysilicon  80  in a plan view. 
         [0031]    Next, one example of the bias method in using the semiconductor device  100  will be explained with reference to  FIG. 1  again. In order to detect X-ray in the semiconductor device  100 , a reverse bias voltage is applied to the photodiode  11 , thereby depleting the first semiconductor layer  10 . That is, in order to detect X-ray in the semiconductor device  100 , the rear electrode  14  and the cathode electrode  75  are connected to the positive electrode of a power source  200 , and the anode electrode  74  is connected to the negative electrode of the power source  200 , which is connected to the ground potential. The reverse bias voltage applied to the photodiode  11 , or in other words, the voltage of the power source  200 , is several hundred volts, for example. On the other hand, the source and drain electrodes  72  connected to the source and drain regions  52  of the transistor  51  are connected to the positive electrode of a power source  202 . The voltage applied to the source and drain regions, or in other words, the voltage of the power source  202  is several volts, which is 3.3 V or smaller, for example. 
         [0032]    In the semiconductor device  100 , the buried polysilicon electrode  82  connected to the buried polysilicon  80  of the charge-attracting part  60  and the buried well electrode  86  connected to the buried well  84  are connected to the negative electrode of the power source  200 . 
         [0033]    Next, the operation of the charge-attracting part  60  when the semiconductor device  100  biased in the manner described above is activated will be explained with reference to  FIG. 3 . 
         [0034]    When a prescribed bias voltage is applied, a depletion layer is generated in the first semiconductor layer  10  based on the difference in potential between the anode  12  and the cathode  13  of the photodiode  11 . If X-ray is incident on the semiconductor device  100  in this state, electron-hole pairs are generated in the first semiconductor layer  10  of the photodiode  11 , and the generated electrons are attracted to the cathode  13 , and the generated holes are attracted to the anode  12 . The electrons and holes are then taken out and observed. 
         [0035]    When X-ray is incident on the semiconductor device  100  and electron-hole pairs are generated in insulating layers (first insulating layer  20  and second insulating layer  40 ) due to the phenomenon described above, positive charges PC might be trapped in the insulating layer as shown in  FIG. 3 . As the amount of incident X-ray increases, the amount of trapped positive charges also increases, and the accumulated charges in the insulating layer could adversely affect the operation of the transistor  51 . 
         [0036]    In order to solve this problem, in the semiconductor device  100  of this embodiment, a potential (ground potential in this embodiment) lower than the potential applied to the source and drain regions  52  of the transistor  51  (about +3.3V, for example) is applied to the buried polysilicon  80 . Thus, the positive charges PC generated in the insulating layer due to the radiation of X-ray are attracted to the buried polysilicon  80 , which reduces the amount of the positive charges PC below the transistor  51 , in particular. By reducing the amount of positive charges PC near the transistor  51 , the degree of characteristic change of the transistor  51  can be suppressed. 
         [0037]    The semiconductor device  100  of this embodiment is configured to have the p-type buried well  84  applied with a potential lower than the potential applied to the source and drain regions  52  of the transistor  51 , and a depletion layer is formed in the PN junction at the interface between the buried well  84  and the first semiconductor layer  10 . Thus, the potential of the buried well  84  is not affected by the bias voltage applied to the first semiconductor layer  10  via the rear electrode  14 , and is maintained at the same potential as that of the negative electrode side of the power source  200 , which is applied to the buried well  84  (ground potential in this embodiment). This makes it possible for the buried polysilicon  80  to attract the positive charges PC efficiently. In this embodiment, the semiconductor device  100  does not necessarily have to have the buried well  84 , and it is possible to attract the positive charges PC by the buried polysilicon  80  alone depending on the radiation amount of X-ray and the like. 
         [0038]    As described above, the buried well  84  is formed so as to include the buried polysilicon  80  (so as to extend to the outer periphery of the buried polysilicon  80 ) in a plan view. When the buried well  84  is not formed between the buried polysilicon  80  and the first semiconductor layer  10 , it is necessary to ensure a withstand voltage that at least corresponds to the voltage of the power source  200  between the buried polysilicon  80  and the first semiconductor layer  10 , and in order to ensure this withstand voltage, an oxide film having a sufficient thickness needs to be provided, for example. In other words, it is possible to omit the buried well  84  as long as an oxide film that can ensure such a withstand voltage can be formed. 
         [0039]    Next, one example of the manufacturing method of the semiconductor device  100  will be explained with reference to  FIGS. 4 to 6 .  FIGS. 4 to 6  are vertical cross-sectional views showing the manufacturing method of the semiconductor device  100 . 
         [0040]    First, an SOI substrate  1  in which a first semiconductor layer  10  made of an n-type semiconductor, a first insulating layer  20 , and a second semiconductor layer  20  made of a p-type semiconductor are stacked in this order is prepared ( FIG. 4A ). 
         [0041]    Next, a field oxide film  90  is formed in the second semiconductor layer  30  by the LOCOS (local oxidation of silicon) method. The portion of the second semiconductor layer  30  where the field oxide film  90  is not formed is an active region  30 A in which a circuit element such as a transistor is to be formed ( FIG. 4B ). 
         [0042]    Next, the first insulating layer  20  and the field oxide film  90  are etched by photolithography, for example, so as to expose the first semiconductor layer  10  and form openings  91  that reach the first semiconductor layer  10  in regions where a buried polysilicon  80  is to be formed ( FIG. 4C ). The width of the openings  91  is approximately 0.5 μm, for example, and the openings  91  are formed at positions that are approximately 0.3 μm from the active region  30 A, for example. 
         [0043]    Next, the oxidation process is conducted on exposed areas O of the first semiconductor layer  10 , thereby forming an SiO 2  film having a thickness of approximately 10 nm on the surface of the first semiconductor layer  10 . This oxide film is an insulating film to provide insulation between the first semiconductor layer  10  and the buried polysilicon  80 , which will be formed later, and is formed to allow different potentials (several V, for example) to be applied to the buried polysilicon  80  and to the first semiconductor layer  10 , respectively. It is apparent that the formation method of the insulating film is not limited to this. It is also possible to partially remove the first insulating layer  20  in the etching process described above so that a portion thereof remains with a thickness of approximately 10 nm. 
         [0044]    Next, the openings  91  are filled by the CVD (chemical vapor deposition) method using polysilicon, and thereafter, by etching back the deposited polysilicon, the thickness of the polysilicon is adjusted so that the top surface of the polysilicon is above the openings  91  ( FIG. 4D ). In this embodiment, doped polysilicon, which has an impurity doped therein in advance, is used for the polysilicon for forming the buried polysilicon  80 . The doped polysilicon is polysilicon that contains an n-type impurity at a high concentration (approximately 1×10 20  cm −3 , for example), which is formed by supplying a gas including an n-type impurity (P (phosphorus), for example)) during the CVD process using polysilicon. 
         [0045]    Next, a gate oxide film  92  is formed in a region including the surface of the active region  30 A. Then, a region other than a buried well  84  forming region in the first semiconductor layer  10  is entirely covered by a photoresist R and a p-type impurity such as B (boron) is injected, thereby forming the buried well  84  ( FIG. 5A ). In this process, the impurity concentration of the buried well  84  is set to approximately 1×10 17  cm −3 , for example. 
         [0046]    Next, a polysilicon film is deposited on the gate oxide film  92 , and by patterning the polysilicon film by photolithography, a gate electrode  55  is formed ( FIG. 5B ). 
         [0047]    Next, a side wall  56  is formed on each side of the gate electrode  55 . Thereafter, an impurity containing a group V element such as phosphorus or arsenic is injected into the active region  30 A in the second semiconductor layer  30  by the ion injection method, thereby forming source and drain regions  52  made of a high concentration n-type semiconductor at the respective sides of the gate electrode  55 . This way, the transistor  51  is formed ( FIG. 5C ). 
         [0048]    Next, by dry-etching, openings  93  and  94  that respectively reach the first semiconductor layer  10  through the field oxide film  90  and the first insulating layer  20  are formed ( FIG. 5D ). If a contact region is to be formed in the buried well  84 , an opening that reaches the buried well  84  is also formed in this process. 
         [0049]    Next, by injecting an impurity containing a group V element such as phosphorus or arsenic to a portion of the first semiconductor layer  10  that is exposed in the opening  94  by the ion injection method, a cathode  13  made of a high concentration n-type semiconductor is formed on the surface of the first semiconductor layer  10 . Next, by injecting an impurity containing a group III element such as boron to a portion of the first semiconductor layer  10  that is exposed in the opening  93  by the ion injection method, an anode  12  made of a high concentration p-type semiconductor is formed on the surface of the first semiconductor layer  10  ( FIG. 6A ). If a contact region is to be formed in the buried well  84 , an impurity containing a group III element such as boron is injected into a portion exposed in an opening formed in the preceding process. 
         [0050]    Next, by the CVD method, a second insulating layer  40  is formed of an insulator such as an SiO 2  film so as to cover the second semiconductor layer  30  where the circuit element including the transistor  51  is formed. The openings  93  and  94  formed in the preceding process are filled by the second insulating layer  40  ( FIG. 6B ). 
         [0051]    Next, openings  99  that reach the source and drain regions  52  through the second insulating layer  40  and openings  98  that reach the buried polysilicon  80  through the second insulating layer  40  are formed by dry-etching. Also, an opening  97  that reaches the buried well  84  formed in the first semiconductor layer  10  through the second insulating layer  40 , the field oxide film  90 , and the first insulating layer  20  is formed by dry-etching. Furthermore, openings  95  and  96  that reach the anode  12  and cathode  13  formed in the first semiconductor  10 , respectively, are formed through the second insulating layer  40 , the field oxide film  90 , and the first insulating layer  20  by dry-etching ( FIG. 6C ). 
         [0052]    Next, a metal such as aluminum is deposited on the surface of the second insulating layer  40  by spattering. The openings  95 ,  96 ,  97 ,  98 , and  99  are filled by this metal. Thereafter, this metal is patterned into a desired shape. This way, source and drain electrodes  72  connected to the source and drain regions  52 , buried polysilicon electrodes  82  connected to the buried polysilicon  80 , a buried well electrode  86  connected to the buried well  84 , an anode electrode  74  connected to the anode  12 , and a cathode electrode  75  connected to the cathode  13  are formed. Next, a rear electrode  14  is formed on the rear surface of the first semiconductor layer  10  by spattering ( FIG. 6D ). 
         [0053]    The semiconductor device  100  of this embodiment is manufactured by the manufacturing method described above. 
       Embodiment 2 
       [0054]    A semiconductor device  100   a  of this embodiment will be explained with reference to  FIG. 7 .  FIG. 7  is a plan view showing a charge-attracting part  60   a  of the semiconductor device  100   a . The semiconductor device  100   a  differs from the semiconductor device  100  in the shape of the buried polysilicon  80 . Thus, the same configurations as those of the semiconductor device  100  are given the same reference characters, and the detailed descriptions thereof will be omitted. 
         [0055]    As shown in  FIG. 7 , the position in the gate width direction relative to the transistor  51  and the width of the buried polysilicon  80   a  of the charge-attracting part  60   a  of this embodiment are similar to those of the buried polysilicon  80 , but the buried polysilicon  80   a  differs from the buried polysilicon  80  in that the buried polysilicon  80   a  is formed to surround the transistor  51 . 
         [0056]    In the semiconductor device  100  in which the buried polysilicon  80  has two regions separated from each other in the gate width direction of the transistor  51 , if the gate width of the gate electrode  55  is great, a distance between the center of the transistor  51  and the buried polysilicon  80  would be longer, which possibly makes it difficult to efficiently attract the positive charges PC generated near the center of the transistor  51 . In order to solve this problem, in this embodiment, the buried polysilicon  80   a  is formed so as to surround the transistor  51 . With this configuration, the positive charges PC generated near the center of the transistor  51  can be attracted in the gate length direction (direction indicated with the arrow L in  FIG. 7 ), and therefore, it is possible to make the positive charges PC attracted to the buried polysilicon more efficiently. 
       Embodiment 3 
       [0057]    A semiconductor device  100   b  of this embodiment will be explained with reference to  FIG. 8 .  FIG. 8  is a vertical cross-sectional view showing an example of the configuration of the semiconductor device  100   b . The semiconductor device  100   b  differs from the semiconductor device  100  in that a double-SOI (double-silicon on insulator) substrate is used for the substrate and that a fixed potential region is formed using an intermediate semiconductor layer instead of the fixed potential region by the buried well  84  of the semiconductor device  100 . Thus, the same configurations as those of the semiconductor device  100  are given the same reference characters, and the detailed descriptions thereof will be omitted. 
         [0058]    In the double-SOI substrate, a first semiconductor layer  10  made of an n-type semiconductor, a first insulating layer  20 , an intermediate semiconductor layer  32 , a third insulating layer  42 , and a second semiconductor layer  30  made of a p-type semiconductor are stacked in this order. 
         [0059]    The intermediate semiconductor layer  32  made of an n-type semiconductor is formed between the first semiconductor layer  10  having the photodiode  11  formed therein and the second semiconductor layer  30  having the circuit element such as the transistor  51  formed therein. Between the intermediate semiconductor layer  32  and the first semiconductor layer  10 , the first insulating layer  20  made of an insulator such as an SiO 2  film is formed, and between the intermediate semiconductor layer  32  and the second semiconductor layer  30 , the third insulating layer  42  made of an insulator such as an SiO 2  film is formed. 
         [0060]    The intermediate semiconductor layer  32  includes a contact region  88  made of an n-type semiconductor that has a higher concentration than that of the intermediate semiconductor layer  32 . The contact region  88  is connected to an intermediate semiconductor layer electrode  89 , and the intermediate semiconductor layer electrode  89  is connected to the negative electrode of the power source  200 . In the semiconductor device  100   b , the charge-attracting part  60   b  is constituted of the buried polysilicon  80  and the intermediate semiconductor layer  32 . The buried polysilicon  80  of this embodiment may have two regions that are separated from the transistor  51  in the gate width direction of the transistor  51  and that extend in the gate length direction as shown in  FIG. 2 , or may be formed so as to surround the transistor  51  as shown in  FIG. 7 . 
         [0061]    As shown in  FIG. 8 , in the semiconductor device  100   b , the potential applied to the intermediate semiconductor layer  32  disposed below the transistor  51  is adjusted so as to cancel the positive charges PC accumulated in the insulating layer (mainly the third insulating layer  42 ). This makes it possible to further suppress the characteristic change of the transistor  51 . 
         [0062]    The semiconductor device  100   b  of this embodiment can be manufactured according to the manufacturing method of the semiconductor device  100  shown in  FIGS. 4 to 6 . The manufacturing method thereof will be briefly explained below. 
         [0063]    First, a double-SOI substrate is prepared by stacking a first semiconductor layer  10  made of an n-type semiconductor, a first insulating layer  20 , an intermediate semiconductor layer  32 , a third insulating layer  42 , and a second semiconductor layer  30  made of a p-type semiconductor in this order (see  FIG. 4A  for reference). 
         [0064]    Next, a field oxide film is formed in the second semiconductor layer  30  by the LOCOS method. The portion of the second semiconductor layer  30  where the field oxide film is not formed is an active region in which a circuit element such as a transistor is to be formed (see  FIG. 4B  for reference). 
         [0065]    Next, the field oxide film, which was formed in the preceding process, and the first insulating layer  20  are etched by photolithography, for example, so as to expose the intermediate semiconductor layer  32  and form openings that reach the intermediate semiconductor layer  32  in regions where a buried polysilicon  80  is to be formed (see  FIG. 4C  for reference). 
         [0066]    Next, the oxidation process is conducted on the exposed areas of the intermediate semiconductor layer  32 , the openings are filled by the CVD method using doped polysilicon, and the thickness of the polysilicon is adjusted by etching back the deposited polysilicon (see  FIG. 4D  for reference). 
         [0067]    Next, after forming a gate oxide film in a region that includes the surface of the active region, a polysilicon film is deposited on the gate oxide film, and by patterning the polysilicon film by photolithography, a gate electrode  55  is formed (see  FIG. 5B  for reference). 
         [0068]    Next, a side wall is formed on each side of the gate electrode  55 . Next, by injecting an impurity containing a group V element such as phosphorus or arsenic to the active region of the second semiconductor layer  30  by the ion injection method, source and drain regions  52  made of a high-concentration n-type semiconductor are formed at the respective sides of the gate electrode. This way, the transistor  51  is formed (see  FIG. 5C  for reference). 
         [0069]    Next, an opening for forming a contact region  88  is formed so as to reach the intermediate semiconductor layer  32  through the field oxide film and the third insulating layer  42  by dry-etching. Also, openings for forming an anode  12  and a cathode  13  are formed so as to reach the first semiconductor layer  10  through the field oxide film, the third insulating layer  42 , the intermediate semiconductor layer  32 , and the first insulating layer  20  by dry-etching (see  FIG. 5D  for reference). 
         [0070]    Next, by injecting an impurity containing a group V element such as phosphorus or arsenic to a portion of the intermediate semiconductor layer  32  that is exposed in the opening by the ion injection method, the contact region  88  made of a high concentration n-type semiconductor is formed on the surface of the intermediate semiconductor layer  32 . 
         [0071]    Next, by injecting an impurity containing a group V element such as phosphorus or arsenic to a portion of the first semiconductor layer  10  that is exposed in the opening by the ion injection method, a cathode  13  made of a high concentration n-type semiconductor is formed on the surface of the first semiconductor layer  10 . Thereafter, by injecting an impurity containing a group III element such as boron to a portion of the first semiconductor layer  10  that is exposed in the opening by the ion injection method, an anode  12  made of a high concentration p-type semiconductor is formed on the surface of the first semiconductor layer  10  (see  FIG. 6A  for reference). 
         [0072]    Next, by the CVD method, a second insulating layer  40  is formed of an insulator such as an SiO 2  film so as to cover the second semiconductor layer  30  where the circuit element including the transistor  51  is formed. The openings formed in the preceding process are filled by the second insulating layer  40  (see  FIG. 6B  for reference). 
         [0073]    Next, openings that reach the source and drain regions  52  through the second insulating layer  40  and openings that reach the buried polysilicon  80  through the second insulating layer  40  are formed by dry-etching. Also, an opening that reaches the contact region  88  formed in the intermediate semiconductor layer  32  is formed through the second insulating layer  40 , the field oxide film, and the third insulating layer  42  by dry-etching. Furthermore, openings that reach the anode  12  and the cathode  13  formed in the first semiconductor  10 , respectively, are formed through the second insulating layer  40 , the field oxide film, the third insulating layer  42 , and the first insulating layer  20  by dry-etching (see  FIG. 6C  for reference). 
         [0074]    Next, a metal such as aluminum is deposited on the surface of the second insulating layer  40  by spattering. The openings formed in the preceding process are filled by this metal. Thereafter, this metal is patterned into a desired shape. This way, source and drain electrodes  72  connected to the source and drain regions  52 , buried polysilicon electrodes  82  connected to the buried polysilicon  80 , an intermediate semiconductor layer electrode  89  connected to the contact region  88 , an anode electrode  74  connected to the anode  12 , and a cathode electrode  75  connected to the cathode  13  are formed. Next, a rear electrode  14  is formed on the rear surface of the first semiconductor layer  10  by spattering (see  FIG. 6D  for reference). 
         [0075]    The semiconductor device  100   b  of this embodiment is manufactured by the manufacturing method described above. 
         [0076]    In the embodiments described above, the buried polysilicon and the fixed potential region (buried well  84  and intermediate semiconductor layer  32 ) are connected to the negative electrode of the power source  200 , or in other words, to the ground, but the present invention is not limited to this configuration. A different power source from the power source  200  may be connected to the buried polysilicon and the fixed potential region so that a potential is applied independently of the power source  200 , or a negative potential may be applied. The buried polysilicon and the fixed potential region do not necessarily have to have the same potential, and different levels of potential may be applied to the buried polysilicon and the fixed potential region, respectively.