1. Field of the Invention
The present invention relates to a radiation detector of a direct conversion type used in the medical field, the industrial field, the atomic field and the like, particularly relates to a technique for improving a withstand voltage property of a semiconductor film of a radiation sensitive type.
2. Description of the Related Art
According to detectors for detecting radiation (for example, X-ray), there are an indirect conversion type detector and a direct conversion type detector. The indirect conversion type detector is adapted to convert radiation (for example, X-ray) firstly into light and thereafter convert converted light into an electric signal by photoelectric conversion. The direct conversion type detector is adapted to directly convert incident radiation into an electric signal by a semiconductor film of a radiation sensitive type.
The latter direct conversion type detector is constructed by a constitution in which radiation is detected by applying predetermined bias voltage to a voltage application electrode formed at a front surface of the radiation sensitive type semiconductor film, collecting carriers generated in accordance with irradiation of radiation by a carrier collection electrode formed at a back surface of the semiconductor film and outputting the carriers as a radiation detection signal.
Further, among the related art radiation detector of the direct conversion type, when a semiconductor thick film such as that of amorphous selenium is used as a semiconductor film, the amorphous semiconductor can be formed as a thick and large film simply by a method of vacuum evaporation or the like. Therefore, the amorphous semiconductor is suitable for constituting a two-dimensional array type radiation detector which needs a large area thick film.
As shown in FIG. 11, the related art two-dimensional array type radiation detector is constituted by an insulating substrate 86, a semiconductor thick film 81, and a voltage application electrode 82. The insulating substrate 86 is formed with a plurality of capacitors Ca for storing charge and switching elements (for example, thin film transistors) 88 in a normally OFF state in an alignment of a vertical/horizontal two-dimensional matrix. The semiconductor thick film 81 is electrically connected respectively to the plurality of charge storage capacitors Ca, and formed on the insulating substrate 86 via a plurality of carrier collection electrodes 87. In semiconductor thick film 81, charge transfer media (carriers) are generated by incidence of radiation. The voltage application electrode 82 is formed on a surface of the amorphous semiconductor thick film 81. Further, each of the carrier collection electrodes 87 is provided with one of the charge storage capacitors Ca and one of the charge reading switching elements 88. Each of sets of carrier collection electrodes 87, the charge storage capacitor Ca and the charge reading switching element 88 form a detecting element DU serving as a radiation detection unit.
Here, when radiation is irradiated to the voltage application electrode 82 in a state of being applied with bias voltage, charge is formed at the amorphous semiconductor thick film 81 and stores at the charge storage capacitor Ca and the stored charge is read as a radiation detection signal by bringing the switching element 88 into an ON state.
When the radiation detector of the two-dimensional array constitution of FIG. 11 is used for, for example, detecting a X-ray fluoroscopic image of an x-ray fluoroscopic imaging apparatus, an X-ray fluoroscopic image is provided based on the radiation detection signal outputted from the radiation detector.
However, according to the related art radiation detector, there is a problem that an electric field is concentrated on an end edge portion of the voltage application electrode 82 formed at the surface of the amorphous semiconductor thick film 81 and dielectric breakdown is liable to cause at the end edge portion. There are two modes of dielectric breakdown caused at the end edge portion. One of the modes is a mode of creeping discharge in which dielectric breakdown is caused at a path to portions 810a, 811a, and 812a, which are exposed on the insulating substrate 86, of a read line 810, a gate line 811 and a ground line 812 from an end edge 82a of the voltage application electrode 82 along a surface of an end edge 81a of the amorphous semiconductor thick film 81.
Other of the breakdown modes is a mode of penetration discharge in which dielectric breakdown is caused at a path to a carrier collection electrode 87a, which is installed right below the end edge 82a of the voltage application electrode 82, from the end edge 82a of the voltage application electrode 82 by penetrating inside of the end edge 81a of the amorphous semiconductor thick film 81.
FIG. 12 is a view enlarging the end edge 82a of the voltage application electrode 82, which is overwritten with a potential distribution when voltage is applied. According thereto, the potential distribution at a vicinity of an end portion of the electrode (down to 10 μm from electrode surface) is calculated and field strength at the vicinity of the end portion of the upper electrode is predicted. As is apparent from the drawing, it is known that on the vicinity of the end portion of the electrode, a change in the potential is large and the electric field is concentrated. Further, when the high bias voltage is continued to be applied in the state in which the electric field is concentrated on the end edge 82a of the voltage application electrode 82, discharge is caused at the end edge portion of the voltage application electrode. When dark current is acquired under this state, there is provided a band-like or a block-like image as shown in FIG. 13B. Here, FIG. 13A shows an image provided from the detector in an initial state of voltage application and FIG. 13B shows an image provided from the detector at the time when 18 hours has passed after voltage application. In the drawings, gray upper portions are produced by dark current right below the electrode and FIG. 13B shows a prestage phenomenon of penetration discharge and the image is whitened due to current by discharge. Further, these noises spread also to other portion and the detector cannot operate in the normal operation. Further, when the high bias voltage is continued to apply to the voltage application electrode for a long period of time, a probability of causing discharge breakdown is rapidly increased.