Film type light receiving element and sputter method of manufacturing the same
BACKGROUND OF THE INVENTION 
     The present invention relates to a photoconductive film type light receiving element for converting data on ordinary clerical documents into electrical signals in time-series and to a method of manufacturing the same. 
     More specifically, the invention relates to a film type light receiving element in which a photoconductive non-crystalline film is formed, for instance, by vacuum evaporation on a flat and smooth insulating substrate on which belt-shaped electrodes are arranged in a desired density and in which an optically transmissive electrode is formed on predetermined parts of the belt-shaped electrodes. A film of Au or the like is used for each of the belt-shaped electrodes, an Se-As-Te film is used for the photoconductive non-crystalline film, and a high and low specific resistance layer of the Sn oxide or a film of a target compound SnO 2  or In 2  O 3  --SnO 2  (hereinafter referred to as &#34;ITO&#34;) which is obtained by sputtering under a gaseous pressure 3×10 -3  -4×10 -3  Torr of a mixture of argon and oxygen with the ITO as a target material are used for the optically transmissive electrode. Furthermore, the invention pertains to a method of manufacturing the film type light receiving element. 
     FIG. 1 is a sectional view showing the structure of a conventional film type light receiving element. Belt-shaped electrodes 2, a photoconductive film 3 and an optically transmissive electrode 4 are formed on a flat and smooth substrate 1 in the stated order. 
     For instance, the substrate 1 is made of a highly insulative material such as quartz glass or Vycor™ brand glass, the belt-shaped electrodes are formed of an Au film, the photoconductive film 3 is an Se-As-Te film or a CdSe film, and the optically transmissive electrode 4 is an In-Sn oxide film. 
     In FIG. 2, the curve I indicates specific resistance ρ with oxygen partial pressure P o  for the case where an SnO 2  film is formed on a glass substrate by a magnetron type sputtering device using an SnO 2  sintered substrate as a target. Both the specific resistances ρ and the oxygen partial pressures P o  are indicated by using logarithmic scales. 
     By forming on the flat and smooth substrate of Pyrex™ glass 1 the belt-shaped electrodes, i.e. the Au films 2, the photoconductive film, i.e. the Se-As-Te film 3, and the optically transmissive electrode, i.e. the SnO 2  film 4 in the stated order, a light receiving element having a structure as shown in FIG. 1 is obtained. 
     In the conventional method, the electrical conductivity and the transmissivity of the optical transmissive electrode are considered important factors. However, the conventional method is disadvantageous in that the most important factor of the photoconductive conversion element, namely, the electric current ratio for light and dark thereof has been heretofore thought impossible to increase. 
     In order to eliminate the above-described difficulty, the inventors have proposed a method of manufacturing a film type light receiving element in which metal electrodes and a photoconductive film are provided in the form of layers on a flat and smooth substrate and an optically transmissive electrode film of tin oxide is formed thereon using a magnetron type sputtering technique wherein, by utilizing the fact that the electric current ratio for light and dark input intensities of the light receiving element varies according to the oxygen partial pressure of a sputtering atmosphere in the sputtering process and the ratio has a peak value, the oxygen partial pressure is set close to a value at which the electric current ratio for light and dark input intensities reaches its peak value. This method is described in U.S. patent application Ser. No. 242,736 filed on Mar. 11, 1981. This method is here specifically not described as nor is to be construed as being prior art so far as the instant application is concerned and is mentioned here only to aid in attaining a full understanding of the invention disclosed herein. 
     An Example of that method will be briefly described with reference to the case where the substrate 1 is an insulating glass or ceramic material having a high surface smoothness, the belt-shaped electrodes 2 are formed of an Au film with a Cr base, the photoconductive film 3 is an Se-As-Te series film, and the optically transmissive film 4 is an SnO 2  film which is formed by a magnetron type sputtering device using an SnO 2  sintered substance as a target. 
     The photoelectric conversion characteristic of the film type light receiving element having the afore-described structure depends on the quality of the material of each layer and the quality of each film. Especially the interface between the belt-shaped electrodes 2 and the photoconductive Se-As-Te film 3 and the junction interface between the Se-As-Te film 3 and the optically transmissive SnO 2  film 4 greatly affect the electric current ratio for light and dark input intensities. 
     The Se-As-Te film 3 acts as a P type photoelectric element. Therefore, it is desirable that the light receiving element be operated with the belt-shaped electrode 2 as the negative electrode and with the optically transmissive SnO 2  electrode 4 as the positive electrode. In this connection, it is preferable that the material of the negative electrode be Au, which has a low electron injection property, while the material of the positive electrode is an n type SnO 2  film which has a low Hall injection property. 
     As described in the above-referenced patent application an important factor in manufacturing the SnO 2  film is to use a magnetron type sputtering device with which the temperature of the substrate is increased only a little and the Se-As-Te film is not crystallized. 
     As is apparent from the curve I in FIG. 2, the point A at which the specific resistance ρ is a minimum corresponds to a specific resistance of 2×10 -1  Ω-cm and an oxygen partial pressure of 1.8×10 -4  Torr. In this case, the gaseous pressure of a mixture of argon and oxygen is 3.0×10 -3  Torr for a target of an SnO 2  sintered body. Accordingly, if it is desired only to minimize the electrical conductivity, an optically transmissive electrode having an excellent electrical conductivity can be obtained by forming the electrode under the sputtering conditions defined by the point A. 
     The curve II in FIG. 2 indicates the relations between electric current ratios for light and dark intensities when a predetermined electric field is applied to the light receiving element and for a sputtering gas oxygen partial pressure P o . In FIG. 2, the electric current ratio for light and dark intensities becomes maximum at the point B. In this case, the oxygen partial pressure is 2×10 -4  Torr and the electric current ratio for light and dark intensities is 400. As is clear from FIG. 2, the oxygen partial pressure at the point B is not always equal to that at the point A. 
     If the above-described light receiving element is operated in an accumulation type photoelectric conversion system with a circuit as shown in FIG. 3, a signal as shown in FIG. 4 is produced. Also, if a number of circuits as shown in FIG. 3 are arranged in a row, a one-dimensional image sensor can be provided. 
     As is apparent from the curve II in FIG. 2, a light receiving element which is provided by forming the optically transmissive SnO 2  film 4 on the photoelectric film 3 by using the SnO 2  target under the oxygen partial pressure 2×10 -4  Torr and the gaseous pressure 3.5×10 -4  Torr of the mixture of oxygen and argon has a considerably high electric current ratio for light and dark intensities. 
     If a 90% In 2  O 3  -10% SnO 2  sintered body is used as the target, the sputtering process is carried out with a magnetron type sputtering device under an oxygen partial pressure (2.5×10 -4  Torr) defined by the point B on the curve II in FIG. 5, the light receiving element so produced has a maximum electric current ratio for light and dark intensities. The electric current ratio is about 400. 
     However, if the optically transmissive electrode 4 is formed by sputtering under the oxygen partial pressure which is defined by the point B in FIG. 2 or 5, the specific resistance ρ is increased as a result of which the resistance of the entire optically transmissive electrode 4 is increased. 
     Accordingly, an object of the invention is to provide a light receiving element in which, with the electric current ratio for light and dark intensities maintained large, the resistance of the entire optically transmissive electrode is made low. 
     SUMMARY OF THE INVENTION 
     In accordance with this and other objects of the invention, there is provided a film type light receiving element including an insulating substrate having a flat and smooth surface with metal electrodes and a photoconductive film provided in the form of layers on the flat and smooth surface. An optically transmissive electrode film is formed on the photoconductive film. A portion of the optically transmissive electrode film adjacent the photoconductive film has a high specific resistance while the remaining portion has a low specific resistance. 
     Yet further, the invention can be practiced by a method of manufacturing a film type light receiving element, and by a product manufactured by this method, including the steps of forming metal electrodes and a photoconductive film in the form of layers on a flat and smooth surface of a substrate, and forming an optically transmissive electrode film on the photoconductive film with a magnetron type sputtering technique. A portion of the optically transmissive electrode film adjacent the photoconductive film is formed with a high specific resistance while the remaining portion has a low specific resistance. To accomplish this, during the formation of the portion having a low specific resistance, sputtering is performed at an oxygen partial pressure close to a value at which a minimum specific resistance is obtained while during formation of the portion having a high specific resistance the oxygen partial pressure is higher than the value at which the minimum specific resistance is obtained. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an explanatory diagram showing a sectional structure of a conventional film type light receiving element; 
     FIG. 2 is a graphical representation indicating an example of the relation between sputtered SnO 2  film specific resistance, electric current ratio for light and dark intensities and oxygen partial pressure; 
     FIG. 3 is a circuit diagram showing an example of a drive circuit for driving a film type light receiving element; 
     FIG. 4 is a diagram showing an example of an output signal produced when the film type light receiving element is operated in an accumulation mode; 
     FIG. 5 is a graphical representing indicating an example of the relation between sputtered ITO film specific resistance, electric current ratio for light and dark intensities and oxygen partial pressure; and 
     FIG. 6 is a sectional view of a film type light receiving element according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A preferred embodiment of the invention is illustrated in FIG. 6, in which those components which have been previously described with reference to FIG. 1 are similarly numbered. 
     In FIG. 6, reference character 4A designates an optically transmissive electrode having a high specific resistance and reference character 4B designates an optically transmissive electrode having a low specific resistance corresponding to the point A in FIG. 2. 
     In the light receiving element according to the invention, the specific resistance of the SnO 2  film 4A which is in contact with the P type semiconductor, i.e. the Se-As-Te film 3, is high as described above. Accordingly, the amount of Hall current injected into the Se-As-Te film 3 from the SnO 2  film 4A is small. This means that, when no light is applied, the dark current is decreased. On the other hand, when light is applied to a certain extent, the injection of Hall current is scarcely affected by the presence of the high specific resistance layer 4A and accordingly a high electric current ratio for light and dark intensities is obtained. In addition, the electrical conductivity of the combination of the optically transmissive electrodes 4A and 4B is maintained satisfactory. Thus, according to the invention, both the electric current ratio for light and dark intensities of the light receiving element and the electrical conductivity of the optically transmissive electrodes are maintained high. 
     A method of manufacturing a film type light receiving element according to the invention will be described with reference to specific examples. 
     EXAMPLE 1 
     The surface of a substrate 1 of quartz glass was optically polished. Each belt-shaped electrode 2 was a laminated film having a Cr base layer 700 A in thickness and an Au layer 800 A in thickness. A film of 5% As-95% Se was formed on the electrode 2 to a thickness of 2 μm by vacuum evaporation. Furthermore, a layer of 5% As-20% Te-75% Se and a layer of 5% As-95% Se were formed on the film, in the stated order, to provide a preliminary element. 
     In order to make the size of the light receiving section 100 μm×100 μm, a mask for the optically transmissive electrodes was placed on the element and the element with the mask was placed in a magnetron type sputtering device. An SnO 2  sintered body was used as a target. 
     After the pressure in the vacuum chamber was reduced to about 10 -7  Torr, oxygen was introduced into the vacuum chamber until the pressure was raised to 6×10 -4  Torr. After the oxygen partial pressure was stabilized, a sputtering gas, namely, argon was introduced into the vacuum chamber until the pressure reached 3.5×10 -3  Torr. 
     After pre-sputtering was carried out for a suitable period of time, an SnO 2  film was formed on the Se-As-Te film to a thickness of 200 A. Thereafter, under an oxygen partial pressure of 1.8×10 -4  Torr and an argon pressure of 3.5×10 -3  Torr, an SnO 2  film was formed on the aforementioned SnO 2  film to a thickness of 1000 A. 
     An optically transmissive, compound SnO 2  film composed of the high specific resistance part 4A and the low specific resistance part 4B which were formed as described above had an optical transmissivity higher than 80% and a sheet resistance lower than 100 KΩ. Thus, the compound SnO 2  film satisfied the requirements as the light receiving element. 
     The light receiving element had an electric current ratio for light and dark intensities of 400 and an optical sensitivity of larger than 10 Pa/lux. It can readily be appreciated that the light receiving element of the invention has a much more excellent reproducibility characteristic and electric current ratio for light and dark input intensities than an element manufactured according to the conventional method. In addition, the light receiving element according to the invention has a more excellent stability characteristic and longer service life than a conventional light receiving element. 
     EXAMPLE 2 
     In sputtering the optically transmissive electrode layers, instead of the SnO 2  sintered body as used in Example 1, a 90% In 2  O 3  -10% SnO 2  sintered body was employed as the target. 
     The manufacturing procedure in Example 2 was the same as that of Example 1 except for the following. The pressure in the vacuum chamber of the sputtering device was set to lower than 2×10 -5  Torr before oxygen was introduced thereinto. The oxygen partial pressure in forming the high specific resistance layer and the low specific resistance layer were set to 2.5×10 -4  Torr and 1.5×10 -4  Torr, respectively. The thickness of the low specific resistance optically transmissive electrode layer 4B was 1500 A. 
     The above-described various values were determined according to the relations between the oxygen partial pressure P o  and ITO transmissive film specific resistance ρ and between the oxygen partial pressure P o  and electric current ratio for light and dark intensities in accordance with the graphs shown in FIG. 5. 
     The ITO compound film provided in Example 2 had an optical transmissivity of larger than 75% and a sheet resistance of lower than 1 KΩ thus satisfying the requirements for the light receiving element. The electric current ratio for light and dark intensities and the optical sensitivity of the light receiving element were 500 and more than 10 Pa/lux, respectively. Thus, the light receiving element was excellent in all of the reproducibility characteristic, electric current ratio for light and dark intensities and optically transmissive electrode conductivity compared with a light receiving element manufactured according to the conventional method. 
     As is apparent from the above description, according to the invention, the optically transmissive electrode in contact with the photoconductive layer is a high specific resistance layer. Accordingly, the dark current is greatly reduced as a result of which the effect of one element on adjacent elements is decreased and therefore the background noise is greatly reduced.