Patent Publication Number: US-2022238825-A1

Title: Semiconductor device

Description:
The present application is a continuation application of International Application No. PCT/JP2020/033236, filed on Sep. 2, 2020, which claims priority to Japanese Patent Application No. 2019-192521, filed on Oct. 23, 2019. The contents of these applications are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a semiconductor device having an optical sensor using a photoelectric conversion element made of an organic material. 
     (2) Description of the Related Art 
     Optical sensors utilizing photoelectric conversion have been used in fields such as biometric authentication as well as recognition of images, and their use has spread. A photoelectric material using an organic material has been developed because it can reduce dark current, improve photoelectric conversion efficiency, and add wavelength selectivity. 
     As an example of an organic material used as a photoelectric conversion element, Patent Document 1 is cited. In addition, Patent Document 2 describes a film structure as a photoelectric conversion element using an organic material. 
     PRIOR ART REFERENCE 
     [Patent Document] 
     
         
         [Patent document 1] WO 2014-054255 A1 
         [Patent document 2] Japanese Patent Application Publication No. 2014-225525 A 
       
    
     SUMMARY OF THE INVENTION 
     In a sensor device of photodiode using an organic photoconductive film (herein after, an organic photoconductive film diode OPD: Organic Photo Diode), a reflective electrode is used to increase the utilization efficiency of light from the outside. Silver having high reflectivity is used as the reflective electrode. This silver is then used as one electrode of an organic photoconductive film diode. A thin film of silver of about 100 nm is used for the reflective electrode, however, such a thin film of silver has high reducing action and is oxidized by coupling with oxygen in the atmosphere immediately after film formation, resulting in a high resistance. 
     On the other hand, since the organic photoconductive film material is weak in moisture, it is necessary to block moisture from the atmosphere. For blocks of moisture, aluminum oxide films (also referred to as “AlOx” hereinafter) have excellent properties. Therefore, a laminated film of the silver film as a reflection electrode and alumina (AlOx) film for blocking moisture is used. 
     However, alumina (AlOx) contains oxygen. Further, since alumina (AlOx) is often formed by reactive sputtering, it contains more oxygen. Thus, silver laminated with alumina (AlOx) is more susceptible to oxidation by oxygen from alumina (AlOx). When silver is oxidized, it becomes high in resistance, which is unsuitable for an electrode, and further, the oxidized silver becomes black or transparent and thus cannot serve as a reflective electrode. 
     A purpose of the present invention is to solve a problem that a silver film is oxidized when an alumina (AlOx) film is laminated when a laminated film of a silver film and an alumina (AlOx) film is used for one electrode of a photodiode. The same applies not only to the case of a photodiode but also to an organic EL display device (OLED) using an organic material, for example. 
     The present invention solves the above problems, and the main specific means thereof are as follows. 
     (1) The semiconductor device having a thin film transistor formed on a substrate including: an electrode formed from a silver film electrically connected to the thin film transistor, a first ITO film formed on the silver film, and an alumina (AlOx) film formed on the first ITO film. 
     (2) The semiconductor device according to (1), in which the semiconductor device includes a photo sensor; the photo sensor includes a photodiode which includes an anode, a photoconductive film, and a cathode, and the electrode is an anode of the photo diode. 
     (3) The semiconductor device according to (2), in which the photoconductive film is an organic photoconductive film. 
     (4) The semiconductor device according to (3), in which a thickness of the first ITO film is 5 to 20 nm. 
     (5) The semiconductor device according to (4), in which a thickness of the silver film is 90 to 200 nm. 
     (6) The semiconductor device according to (5), in which a thickness of the alumina (AlOx) film is 10 to 50 nm. 
     (7) The semiconductor device according to (6), in which the first ITO film is an amorphous ITO film. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of an optical sensor device to which the present invention is to be applied; 
         FIG. 2  is a plan view of sensor elements; 
         FIG. 3  is a cross-sectional view of the optical sensor device according to embodiment 1; 
         FIG. 4A  is a cross-sectional view of a sample A in which only a silver film  201  is formed on a glass substrate; 
         FIG. 4B  is a cross-sectional view of a sample B in which alumina (AlOx) film is formed on the silver film; 
         FIG. 4C  is a cross-sectional view of a sample C in which an ITO film is formed on the silver film; 
         FIG. 4D  is a cross-sectional view of a sample D in which an ITO film  202  is formed on a silver film  201  and an alumina (AlOx) film  203  is formed on the ITO film; 
         FIG. 5  is a table of manufacturing condition of the ITO film; 
         FIG. 6  is a graph to show an effect of ITO film between the silver film and the alumina (AlOx) film; and 
         FIG. 7  is a cross-sectional view of the optical sensor device in the vicinity of the photo conductive film according to embodiment 2. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The contents of the present invention are described using embodiments below. In Embodiment 1, an optical sensor device for receiving light from a lower surface of a sensor array is described, and in Embodiment 2, an optical sensor device for receiving light from an upper surface of a sensor array is described. Further, the present invention is applicable to an organic EL display device (OLED) using an organic material as a light-emitting element. 
     Embodiment 1 
       FIG. 1  is a plan view of an optical sensor device to which the present invention is applied. In  FIG. 1 , sensor elements are formed in a matrix in the sensor region. For example, the sensor area has a lateral diameter xx of 3 cm and a vertical diameter yy of 3 cm. In the sensor region, scanning lines  11  extend in the horizontal direction (x-direction) and are arranged in the vertical direction (y-direction). A detection line  12  and a power supply line  13  extend in the vertical direction and are arranged in the horizontal direction. A region surrounded by a scanning line  11  and a detection line  12 , or a region surrounded by a scanning line  11  and a power line  13  constitutes a sensor element. In each sensor element, a switching TFT  15  and an organic photoconductive film diode  10  are formed. 
     A scanning line driving circuit  20  is disposed in the lateral direction outside the sensor region, a power supply circuit  40  is disposed in the upward direction, and a detection circuit  30  is disposed in the downward direction. The scanning line driving circuit  20  and the detection circuit  30  are formed of TFTs. The scanning line  11  is sequentially selected from the upper direction by the shift register in the scanning line driving circuit  20 . 
     The power supply line  13  is connected to an anode of each photodiode, extends in the vertical direction, and is connected to the same power supply in the power supply circuit  40  above the sensor region. Then, an anode potential is supplied to the power supply line  13 . The detection line  12  is connected to the drain of the switching TFT, and the source of the switching TFT is connected to the cathode of the photodiode  10 . A detection line  12  extends downward from each sensor element, and a photocurrent is detected in the detection circuit  30 . In  FIG. 1 , when light is applied to the sensor element selected by the scanning line  11 , a photocurrent is generated from the photodiode  10 , and this photocurrent is detected by the detection circuit  30  through the detection line  12 . 
       FIG. 2  is a plan view of each sensor element. In order not to complicate the drawing, some electrodes or the like are omitted from  FIG. 2 . The size of each sensor element is, for example, 50 μm in the lateral direction x 1  and 50 μm in the vertical direction y 1 . In FIG.  2 , the scanning lines  11  extend in the horizontal direction and are arranged in the vertical direction. Further, the power supply line  13  and the detection line  12  extend in the vertical direction and are arranged in the horizontal direction. A cathode  126  of a photodiode, an organic photoconductive film  127 , an anode  128 , and the like are formed in a region surrounded by a scanning line  11 , a power supply line  13 , or a scanning line  11  and a detection line  12 . 
     Further, the anode electrode  128  is integrally formed over the entire sensor region. In other words, one anode electrode  128  is provided over the entire sensor region, and a plurality of cathode electrodes  126  overlap the one anode electrode  128 . 
     The semiconductor film  107  extends in the x direction from the detection line  12  through the through hole  135 , then bends, and then passes under the scan line  11 . At this time, a TFT is formed. In this case, the scanning line  11  becomes the gate electrode of the TFT. The semiconductor film  107  extends in the y direction and is connected to the cathode  126  of the photodiode formed of ITO in the through hole  123 . As described in  FIG. 3 , the through hole  123  is formed in the thick organic passivation film  122 , therefore the diameter of the through hole  123  is large. An organic photoconductive film  127  is formed on the cathode  126 , and an anode  128  is formed thereon by a silver film. Thus, an organic photoconductive film diode is formed. Also, the organic photoconductive film  127  is integrally formed on the entire surface of the sensor region, and is not formed in an island shape for each of the plurality of sensor elements in the sensor region. In other words, one organic photoconductive film  127  is provided on the entire sensor area, and one anode electrode  128  and a plurality of cathode electrodes  126  overlap the one organic photoconductive film  127 . 
     In the configuration shown in  FIG. 2 , as described above, the organic photoconductive film  127  and the anode  128  are formed on the entire surface of the sensor region in common with the respective elements. Accordingly, in  FIG. 2 , although only the shape of the cathode  126  is illustrated in the sensor element, the organic photoconductive film  127  and the anode  128  are stacked on the cathode  126 . More specifically, the organic photoconductive film  127  and the anode electrode  128  exist in the region between the cathode electrodes  126  adjacent to each other in the first direction x and the second direction y, that is, in the region where the cathode electrode  126  is not formed. Since the anode electrode  128  is formed of a silver film  128  of small thickness of about 100 nm, the resistance of the entire cathode is reduced by being connected to a plurality of power supply lines  13 . The power supply line  13  may be stacked on the silver film  128  and extend to the power supply circuit  40  as it is, or may extend in the same layer as the drain electrode or the source electrode of the TFT via a through hole formed in the organic passivation film  122  to the power supply circuit  40 . 
       FIG. 3  is a cross-sectional view of the optical sensor device of  FIG. 1 ; In the optical sensor shown in  FIG. 3 , light is input from the substrate  100  side. As shown in  FIG. 1 , a driving circuit formed of a TFT is formed outside the sensor region. Since a polysilicon semiconductor has a large mobility, it is advantageous that the TFT constituting the drive circuit is formed of a polysilicon semiconductor. 
     On the other hand, it is advantageous that the switching TFT formed in the sensor region is formed of an oxide semiconductor (sometimes referred to as an OS: Oxide Semiconductor) having a small leakage current. Therefore, in this embodiment, a hybrid array substrate using both of a polysilicon semiconductor TFT and an oxide semiconductor TFT is used. In  FIG. 3 , the left side is a polysilicon TFT for a peripheral circuit, and the central portion is an organic film photodiode and a switching TFT therefor. 
     Polysilicon is a so-called low-temperature polysilicon in which a-Si (amorphous Silicon) is poly-siliconized by an excimer laser. Nevertheless, since an annealing temperature of a polysilicon semiconductor exceeds a process temperature for forming an oxide semiconductor, a polysilicon semiconductor TFT is formed at first, and then an oxide semiconductor TFT is formed. Thus, a peripheral circuit is manufactured first. 
     In  FIG. 3 , a base film  101  made of a laminated film of silicon nitride (SiN) and silicon oxide (SiO) is formed on a glass substrate  100 . This is for preventing impurities from the glass substrate  100  from contaminating the polysilicon semiconductor  102  and the oxide semiconductor  107 . The thickness of the SiO film is, for example, 200 nm, and the thickness of the SiN film is, for example, 20 nm. 
     On top of this, a polysilicon film  102  is formed. In the polysilicon film  102 , an a-Si film is first formed, and then a-Si is converted into polysilicon by an excimer laser and patterned. The thickness of the polysilicon film  102  is, for example, 50 nm. Note that the SiO film and the SiN film, serving as the base film  101 , and the a-Si film can be continuously formed by CVD. 
     Thereafter, the first gate insulating film  103  is formed of SiO covering the polysilicon semiconductor film  102 . A thickness of the first gate insulating film  103  is, for example, 100 nm. Then, a first gate electrode  104  is formed on the first gate insulating film  103  by metal or metal alloy. The first gate electrode  104  is formed of MoW, for example. Incidentally, the peripheral circuit region and the sensor region are formed simultaneously. At the same time as forming the first gate electrode  104 , a light shielding film  105  is formed of the same material as the first gate electrode  104  in a portion corresponding to the switching TFT in the sensor region. This light shielding film  105  can be used as a bottom gate electrode of an oxide semiconductor TFT to be formed later. 
     A first interlayer insulating film  106  is formed of a stacked film of an SiO film and an SiN film covering the first gate electrode  104  and the light shielding film  105 . For example, the SiN film has a thickness of 300 nm and the SiO film has a thickness of 200 nm. An oxide semiconductor film  107  is formed over the first interlayer insulating film  106 . Examples of the oxide semiconductor include IGZO (Indium Gallium Zinc Oxide), ITZO (Indium Tin Zinc Oxide), ZnON (Zinc Oxide Nitride), and IGO (Indium Gallium Oxide). In this embodiment, IGZO is used as an oxide semiconductor. 
     In order to maintain characteristics of an oxide semiconductor, it is important to maintain an oxygen amount. Therefore, the upper layer of the first interlayer insulating film  106  needs to be a SiO film. This is because SiN supplies hydrogen to reduce the oxide semiconductor. If the SiO film is in contact with the oxide semiconductor film  107 , oxygen can be supplied from the SiO film to the oxide semiconductor. 
     A drain protective electrode  108  is stacked on a drain region of the oxide semiconductor film  107 , and a source protective electrode  109  is formed on a source region thereof. The drain protection electrode  108  and the source protection electrode  109  are formed of a metal, and prevent the oxide semiconductor film  107  from being lost by hydrofluoric acid (HF) in the through holes on the oxide semiconductor TFT side when the through holes in the polysilicon TFT are cleaned with the hydrofluoric acid (HF). 
     A second gate insulating film  110  is formed of a SiO film covering the oxide semiconductor film  107 . The thickness of the SiO film is about 100 nm. A gate alumina film  111  is formed on the SiO film, on which a second gate electrode  112  is formed, for example, of a MoW alloy. By supplying oxygen to the oxide semiconductor film  107  from the second gate insulating film  110  formed of SiO and the gate alumina film  112 , the characteristics of the oxide semiconductor film  107  are stabilized. 
     A second interlayer insulating film  113  is formed of a stacked film of a SiO film and a SiN film covering the second gate electrode  112 . For example, the SiO film is 300 nm and the SiN film is 100 nm. In many cases, a SiO film is disposed on the lower side closer to the oxide semiconductor film  107 . After forming the second interlayer insulating film  113 , through holes  118  and  119  are formed on the polysilicon TFT side of the peripheral circuit, and through holes  120  and  121  are simultaneously formed on the oxide semiconductor TFT side on the sensor region side. 
     The through-holes  118  and  119  on the side of the polysilicon TFT are subjected to hydrofluoric acid (HF) cleaning in order to remove the oxide film. At this time, hydrofluoric acid (HF) is also introduced into the through holes  120  and  121  on the oxide semiconductor TFT side; to countermeasure the hydrofluoric acid (HF), the drain protective electrode  108  and the source protective electrode  109 , which are formed from metal, are used in order to prevent the oxide semiconductor film  107  from being lost. 
     A first drain electrode  114  and a first source electrode  115  are formed corresponding to the through holes  118  and  119  on the polysilicon TFT side; a second drain electrode  116  and a second source electrode  117  are formed corresponding to the through holes  120  and  121  on the oxide semiconductor TFT side. The second drain electrode  116  is connected to the detection line  12 . 
     An organic passivation film  122  is formed of, for example, acrylic resin covering the second interlayer insulating film  113 . Since the organic passivation film  122  also serves as a planarization film, it is formed to have a thickness of about 2 μm. Through hole  123  for connecting the source electrode  117  and the cathode  126  of the photodiode is formed in the organic passivation film  122  corresponding to the source electrode  117  of the TFT. Since the thickness of the organic passivation film  122  is large, the diameter of the through hole  123  is also large. 
     An inorganic passivation film  124  is formed to a thickness of about 20 to 100 nm, for example, with SiN covering the organic passivation film  122 . Impurities such as moisture are released from the organic passivation film  122 ; the inorganic passivation film  124  prevents the organic photoconductive film  127 , formed on upper side, from being contaminated. 
     On the inorganic passivation film  124 , a cathode electrode  126  is formed by an ITO (Indium Tin Oxide) film to a thickness of, e.g., about 50 nm. The ITO film is crystallized by annealing to reduce electric resistance. Through holes  125  are formed in the inorganic passivation film  124  at the through holes  123  of the organic passivation film  122  to connect the cathode electrode  126  with the source electrode  117 . In the present invention, ITO is also used for the upper electrode side, which is the anode electrode  128  side; therefore, the ITO as the cathode electrode  126  is sometimes referred to as cathode ITO  126  in order to distinguish it from the anode side TFT. 
     An organic photoconductive film  127  is formed on the cathode  126  with a thickness of 300 to 500 nm. The organic photoconductive layer  127  is formed by sputtering or vacuum evaporation. Since the organic photoconductive film  127  has a wavelength selectivity as well as superior photoconductive characteristics, it can be used as a so-called bio-recognition sensor, e.g. for vein images. 
     An anode electrode  128  is formed of a silver film on the organic photoconductive film  127 . Silver has an excellent reflectivity when it becomes 90 nm or more. Also, the work function is suitable as the anode electrode  128 , and the conductivity is also excellent. 
     On the other hand, since the organic photoconductive film  127  is vulnerable to impurities such as moisture and the like, it is necessary to block it from outside, therefore, the alumina (AlOx) film  130  of a thickness of approximately 30 nm is formed to cover the silver film  128  which is an anode electrode  128 . The alumina (AlOx) film  130  is formed by sputtering, however, film formation rate is very low; thus, a reactive sputtering is used. Alumina (AlOx)  130  formed by reactive sputtering contains a large amount of oxygen. Incidentally, for the purpose of blocking moisture and the like, the alumina (AlOx) film preferably has a thickness of 10 to 50 nm. 
     However, since silver has strong reducibility, oxygen is removed from the alumina (AlOx) film  130  and the silver film  128  is oxidized. When the silver film  128  is oxidized, electric resistance increases and blackening occurs. Further, when the oxidation is further progressed, the silver film is made transparent. Then, the silver film  128  does not serve as a reflective electrode. 
     A feature of the present invention is to prevent oxidation of the silver film  128  by the alumina (AlOx) film  130  by forming an ITO film  129  between the silver film (or anode electrode) as the reflective electrode  128  and an alumina (AlOx) film  130  for moisture block. The thickness of the alumina (AlOx) film  130  may be about 7 nm. As the thickness of the ITO film increases, crystallization proceeds, and unevenness becomes conspicuous on the surface of ITO, and therefore, even when the thickness is increased, the thickness is preferably about 70 nm. The thickness of the ITO film  129  for this purpose is, for example, from 5 to 70 nm, and more preferably from 7 to 20 nm. 
     The ITO film  129  can be continuously sputtered without breaking the vacuum in a chamber in which the silver film  128  is sputtered. Therefore, it is possible to prevent the silver film  128  from being oxidized by oxygen in the atmosphere. On the other hand, since the alumina (AlOx) film  130  is sputtered in a separate chamber from the silver film  128 , if the ITO film  129  is not present, the silver film  128  is oxidized even by oxygen in the atmosphere before the formation of the alumina (AlOx)  130 . However, in this embodiment, since the silver film  128  is already covered with the ITO film  130 , oxidation due to oxygen in the atmosphere can be prevented. 
     The ITO film  129  itself also contains oxygen. However, the amount of oxygen supplied from the ITO film  129  is much smaller than that from the alumina (AlOx) film  130 . Note that the ITO film  129  on the anode side is formed after the organic photoconductive film  127  is formed. Since the organic photoconductive film  127  is vulnerable to heat, the ITO film  129  on the anode side is formed at a low temperature, e.g., at a substrate temperature of about 30 degrees Celsius. Since the film thickness is also as thin as about 7 nm, the ITO  129  on the anode side is formed in an amorphous state. It can be assumed that such an amorphous ITO thin film  129  does not supply oxygen which oxidizes the silver film  128  as a reflective electrode. 
     Since the ITO film  129  on the anode side has a thin film thickness of about 7 nm, it can be formed by ordinary sputtering rather than by reactive sputtering using oxygen. Also in this respect, the amount of oxygen contained in the ITO film  129  can be suppressed more than usual. 
     In  FIG. 3 , an organic protective film  131  is formed of a resin such as acrylic for mechanical protection on an alumina (AlOx) film  130 . The organic protective film  131  may be omitted depending on a product. 
       FIGS. 4A to 6  show the effects of the present embodiment. In this embodiment, an ITO film  129  for preventing the oxidation of the silver film  128  is formed between the silver film  128  as a reflective electrode and an alumina (AlOx) film  130  as a moisture block on the anode side. In such a configuration,  FIG. 4A  to  FIG. 6  show how an effect can be obtained when the ITO film  129  is formed to have a thickness of about 7 nm. 
       FIGS. 4A to 4D  show samples of various membrane configurations for confirming the effect. In  FIGS. 4A to 4D , the film thickness of the ITO film  202  is 7 nm, and the thickness of the alumina (AlOx) film is 30 nm. As a film thickness of the silver film  201 , a sample whose thickness was changed like a 100 nm, 200 nm, 300 nm, 400 nm, 500 nm was prepared. All of the films are formed by sputtering. 
       FIG. 4A  is a cross-sectional view of a case where only a silver film  201  is formed on a glass substrate  200 .  FIG. 4B  shows a case where alumina (AlOx) film  203  is formed on the silver film  201 .  FIG. 4C  shows a case where an ITO film  202  is formed on the silver film  201 .  FIG. 4D  shows a case where an ITO film  202  is formed on a silver film  201  and an alumina (AlOx) film  203  is formed thereon, which is the film structure according to embodiment 1. 
       FIG. 5  shows the condition for forming the ITO film  202  used for the sample. The ITO film  202  is formed to a thickness of 7 nm by sputtering, and the characteristic is that the sample substrate is maintained at 30 degrees Celsius. In other words, the temperature of the organic photoconductive film in  FIG. 3  is taken into consideration. Further, the oxygen flow rate is 0.05 sccm (standard cubic centimeter per minute) and is very small compared with an argon (Ar) flow rate of 140 sccm. It is considered that the ITO film formed under such conditions is amorphous and has a small oxygen content. 
     In  FIGS. 4B to 4D , the silver film  201  and the ITO film  202  are successively formed in the same chamber, however, the alumina (AlOx) film  203  was formed by exposing the substrate  200  on which an ITO film is formed to an atmosphere and then, sputtering the alumina (AlOx) in another chamber. Since the state of oxidation of the silver film  201  appears remarkably in electric resistance, the state of oxidation of the silver film  201  is measured by measuring the sheet resistance of the silver film  201 . 
     After forming the films of  FIGS. 4A to 4D , the sheet resistance of the silver film  201  was measured by Lowlesta (Product Name). A Lowlesta is used for measuring the sheet resistance using 4 needles, and an alumina (AlOx) film  203 , which is an insulating material on the surface, is penetrated by a needle. Thus, the sheet resistance of the silver film  201  can be measured. If the silver film  201  is oxidized, its sheet resistance becomes very large. 
       FIG. 6  is a graph showing an evaluation result. In  FIG. 6 , the horizontal axis represents the thickness of the silver film  201 , and the vertical axis represents the sheet resistance of the silver film  201 . Since the resistance of the silver film  201  largely changes due to oxidation, the vertical axis is a log scale. In the case where the film thickness of the silver film  201  was 100 nm, 200 nm, 300 nm, all of the samples  4 A to  4 D were prepared and evaluated, and when the thickness of the silver film  201  was 400 nm and 500 nm, only samples  4 A and  4 B were prepared and evaluated. The resistance value was measured immediately after film fabrication. 
     In  FIG. 6 , A corresponds to sample  4 A, B corresponds to sample  4 B, C corresponds to sample  4 C, and D corresponds to sample  4 D. When the thickness of the silver film  201  is 100 nm, a sample B in which an alumina (AlOx) film  203  is stacked on the silver film  201  has a resistance of 9×10 6 , and is much larger than that of other samples. In other words, it is understood that the silver film  201  has been oxidized by alumina (AlOx) over the entire thickness direction. 
     On the other hand, there is only little difference in resistance among the sample A, which is only the silver film  201 , the sample B, in which the ITO film  202  is laminated on the silver film  201 , and the sample D, in which the ITO film  202  and the alumina (AlOx) film  203  are laminated on the silver film  201 . In particular, attention is given to Sample B and sample D, and it is understood that the effect of oxidation of the silver film  201  by the alumina (AlOx) film  203  having a thickness of 30 nm is almost eliminated only by disposing the ITO film  202  having a film thickness of 7 nm between the silver film  201  and the alumina (AlOx) film  203 . 
     This tendency is the same even when the thickness of the silver film  201  is 200 nm. As shown in Sample B, even when the film thickness of the silver film  201  is 200 nm, the sheet resistance is substantially the same as in the case where the film thickness is 100 nm. In other words, it is understood that the effect of the alumina (AlOx) film  203  to oxidize the silver reaches to a thickness of approximately 200 nm of the silver film  201 . 
     On the other hand, when attention is paid to the samples A, C, and D, when the thickness of the silver film  201  is 200 nm, the resistance of the silver film  201  is about half of that when the thickness of the silver film  201  is 100 nm (note the vertical axis of  FIG. 6  is a log scale). Thus, it can be seen that Samples A, C, and D are hardly oxidized. 
     When the film thickness of silver is 300 nm, the sheet resistance value of the silver film  201  decreases to almost the same level as that of the other samples A, C, and D even in Sample B. In other words, it is understood that the influence of the alumina (AlOx) film  203  having a thickness of 30 nm does not reach about 300 nm of the silver film  201 . Accordingly, it is understood that the influence of the alumina (AlOx) film  203  having a thickness of 30 nm extends from the interface between the silver film  201  and the alumina (AlOx) film  203  to about 200 to 300 nm. 
     On the other hand, when Sample C and Sample D are compared with each other, there is little difference in the sheet resistance of the silver film  201 . In other words, since an ITO film  203  of about 7 nm is present between the silver film  201  and the alumina (AlOx) film  203 , it is possible to almost eliminate the influence of the alumina (AlOx) film  203  on the silver film  201 . 
     When the thickness of the silver film  203  is 400 nm and the thickness of the silver film  203  is 500 nm, only the samples A and B are measured. As the thickness of the silver film  201  increases, the influence of the alumina (AlOx) film  203  stacked on the surface of the silver film  203  becomes small. However, in actual products, forming a silver film  201  of 300 nm or more is disadvantageous in terms of cost. In an actual product, the thickness of the silver film  103  is 200 nm or less, more preferably the thickness of the silver film  103  is between 90 and 120 nm. 
     In such a range of thickness of the silver film  202 , it is very effective to form an ITO film  202  between the silver film  201  and the alumina (AlOx) film  203 . With this configuration, it is possible to realize an optical sensor using an organic photoconductive film having excellent reflection characteristics and high reliability. 
     In the above description, the organic photoconductive film and the anode, i.e., the silver film, are commonly formed in the entire sensor region, but the same applies to the case where the organic photoconductive film or the anode is formed for each of the individual sensor elements. Further, in the above description, a case has been described in which a thin film of ITO is disposed between a silver film and an alumina (AlOx) film, but a similar effect can be obtained for a transparent oxide film such as AZO (Antimony Zinc Oxide) or IZO (Indium Zinc Oxide) instead of ITO. 
     Embodiment 2 
     In the optical sensor of Embodiment 1, light L is incident from the substrate  100  side in  FIG. 3 . On the other hand, there is an optical sensor of a type in which light is incident on the opposite side of the substrate  100 , i.e., from the side of the upper electrode  128  of the photoconductive film  127 . When light is incident from the side of the upper electrode  128 , a reflective film is formed on the side of the lower electrode  126 , and the upper electrode  128  becomes a transparent electrode. In a configuration in which light is incident from the side of the upper electrode  128 , a switching TFT or a driving TFT can be formed between the lower electrode  126  and the substrate  100 , which is advantageous in terms of space. 
     Incidentally, when silver becomes a thin film having a film thickness of 50 nm or less, particularly 30 nm or less, it transmits visible light. By utilizing this property, it is possible to realize an optical sensor in which light is incident from the side of the upper electrode  128  without changing the basic structure of the optical sensor described in Embodiment 1 (hereinafter, also referred to as an upper light incident type). 
       FIG. 7  is a cross-sectional view of an organic photodiode portion in Embodiment 2. Since the configuration of the switching TFT and the driving TFT are the same as that described with reference to  FIG. 3 , only the organic photodiode portion is shown in  FIG. 7 . In  FIG. 7 , an inorganic passivation film  124  is formed on an organic passivation film  122 , for example, by a SiN film having a thickness of 100 nm. On the inorganic passivation film  124 , a reflective film  150  made of silver, aluminum, an aluminum alloy or the like is formed to have a thickness of about 100 nm. A cathode  126  is formed from an ITO film of a thickness of about 50 nm thereon.  FIG. 7  differs from  FIG. 3  of Embodiment 1 in that a reflecting film  150  made of metal is formed under the cathode  126 . 
     The organic photoconductive film  127  is formed on the cathode  126  with a thickness of 300 to 500 nm as shown in  FIG. 3 . An anode  128  is formed of a silver film on an organic photoconductive film  127 . In  FIG. 7 , the silver film as the anode  128  does not act as a reflective electrode, but rather needs to pass light. Therefore, the thickness of the silver film  128  is 50 nm or less, preferably 20 to 30 nm. When the thickness of silver film becomes equal to this level, a transmittance of the silver film becomes equal to or higher than that of ITO film. 
     An ITO film  129  for preventing the oxidation of silver is formed on the anode  128  by about 7 nm. The ITO film  129  is formed by low-temperature sputtering in succession to the silver film  128 . As described in Embodiment 1, the ITO film  129  is an amorphous film. However, in the configuration of  FIG. 7 , since the thickness of the ITO film  129  is preferably reduced in order to prevent the attenuation of light, a more preferable thickness is 5 to 20 nm. 
     On the ITO film  129 , an alumina (AlOx) film  130  having a thickness of, e.g., about 10 to 50 nm is formed, the same as Embodiment 1. This is for preventing an organic photoconductive film from being contaminated from external moisture or the like. In the configuration shown in  FIG. 7 , in order not to attenuate the light, a more preferable range of the alumina film  130  is 10 to 30 nm. Since oxygen from the alumina film  130  is blocked by the ITO film  129 , it does not reach the anode  128 , and the silver film  128  is not oxidized, therefore the conductivity of the silver film  128  can be maintained. 
     Although the silver film  128  is thin, as shown in  FIGS. 1 and 2 , since the power supply line  13  extends in the vertical direction, the potential drop of the anode  128  can be prevented. In other words, since the thin silver film  128  only needs to act as a conductive film only at each sensor element, an increase in the resistance value caused by thinning of the silver film  128  does not become a substantial problem unless the silver film  128  is oxidized. 
     As described above, by forming the ITO thin film  129  between the silver film  128  as the anode and the alumina (AlOx)  130  for the moisture block, it is possible to prevent the oxidation of the silver thin film  128 , and thus it is possible to realize an optical sensor having an organic photoconductive film of an upper surface incident type. 
     In the above description, a case in which an organic photoconductive film is used as an optical sensor has been described; however, the present invention is not limited to this, and the present invention can be applied to other optical sensors in the case where silver is used as a cathode or an anode. Further, while the present invention has been described with reference to an optical sensor using an organic photoconductive film, the present invention is not limited thereto and can be used in an organic EL display device using an organic EL film and so forth.