Patent Number: 
Section: description

Hereinbelow, embodiments according to the present invention will be explained in detail with reference to FIGS. 1 through 3, in which the same or equivalent elements are designated by the same reference numerals. In FIG. 1, numeral 5 shows an incident X-ray beam, and numeral 7 shows an upper face reflection plate which is provided for shielding external light incident to the X-ray detection device and for efficiently reflecting light generated in the scintillators 1 so as to guide the same to the silicon photo diode array 3. Within the X-ray detection device, a two dimensionality arranged X-ray detection element array is constituted by arranging on a circuit substrate 6 a great many X-ray detection elements which are formed by adhering a corresponding number of scintillators 1 on respective light receiving portions provided on the surface of a two dimensional silicon photo diode array 3 in a two dimensional manner, in parallel and at a predetermined pitch, together with isolation walls 2a in a channel direction and with isolation walls or bands 2axe2x80x2 in a slice direction, both of which are made of a material as a metal or an organic material having a high refractive Index. The two dimensionally arranged X-ray detection element array according to the present embodiment is illustrated as including 12 channels and 4 slices. However, an actual multi slice type X-ray detection device includes 40 blocks of two dimensionally arranged X-ray detection element arrays and each array includes X-ray detection elements for covering 24 channels. Accordingly, with an X-ray CT apparatus mounting an X-ray detection device incorporating the two dimensionally arranged X-ray detection element array, a multi slice type X-ray CT apparatus which produces through one scanning at once tomographic images for 4 slices (4 cross sections) can be constituted. Possible electrical and optical cross talk in the thus constituted two dimensionally arranged solid state X-ray detection device is reduced by the means shown in FIGS. 2 and 3 which will be explained herein below. In FIG. 2, numeral 1 is a scintillator which converts incident X-ray into light, numeral 2a is an isolation wall which optically separates and isolates between the neighboring scintillators 1, numeral 3 including 3axcx9c3g is a two dimensionally arranged silicon photo diode array which converts the light converted by the scintillators 1 into electrical signals, numeral 4 is an adhesive layer which couples optically and mechanically the scintillators 1 and the isolation walls 2a with the two dimensionally arranged silicon photo diode array 3. In order to realize a high quantum efficiency and a high speed response, it is preferable for the two dimensionally arranged silicon photo diode array 3 to use a PIN type silicon photo diodes each of which includes an intrinsic semiconductor layer 3b having a reduced impurity density in an intermediate depletion layer expanding region between a p type semiconductor layer 3c and an n type semiconductor layer 3a. Further, in order to improve S/N ratio of the X-ray detection device by reducing capacitances and dark currents of the silicon photo diodes, it is preferable to use an epitaxial substrate for the substrate of the PIN type silicon photo diodes. The refractive index of the p type semiconductor layer 3c serving as the light receiving region is large, such as n≈3.5, and the refractive index of the adhesive layer 4 which is the medium contacting to the surface of the p type semiconductor layer 3c is small, such as n≈1.5, therefore, the difference therebetween becomes large, for this reason, the critical angle with respect to incident light decreases and light reflection at the surface of the p type semiconductor layer 3c increases. Therefore, in order to cause efficient light incident a reflection preventing film 3e such as SiN and Si02 film having a refractive index n of about 2.1xcx9c2.7 is provided on the surface at the light incident side of the p type semiconductor layer 3c. Further, on a part of the surface of the p type semiconductor layer 3c serving as the light receiving region an anode electrode 3d of a material such as A1 thin film for taking out signal currents (photo currents) is provided, and the surface thereof is covered by a protective film 3f such as Si02 for the purpose of corrosion prevention. Now, the present embodiment will be explained more specifically by indicating actual sizes of the respective portions. When assuming an arrangement pitch of the respective X-ray detection elements in the channel direction as 1 mm, it is preferable to provide an opening width of the X-ray detection element, in which the width of the scintillator 1 is 0.9 mm, and the width of the isolation wall 2a is 0.1 mm. On the other hand, with regard to the dimension of the silicon photo diode array to be combined with the scintillators 1 together with the isolation walls 2a, when assuming that the pitch between the neighboring light receiving portions in the form of p type semiconductor layer 3c is 1 mm, it is preferable to set the width of the light receiving porion in the form of p type semiconductor layer 3c in a range of 0.7 mmxcx9c0.8 mm which is narrower than the opening width of 0.9 mm of the X-ray detection element. Further, all of the surface of the light receiving portions in the form of p type semiconductor layer 3c are covered, for the purpose of efficient light incidence, by the reflection preventing film of transparent optical thin film 3e having refractive index n of about 2.1xcx9c2.7 such as SiN film and Si02 film. The width of a dead zone 3bxe2x80x2, where no p type semiconductor layer is formed and an i type semiconductor region remains which is provided between the neighboring light receiving portions in the form of p type semiconductor layer 3c for separating the neighboring X-ray detection elements, is made as 0.3 mm, alternatively, when the width of the light receiving portion is 0.8 mm, the width of the dead zone is made as 0.2 mm, which width is usually larger than the width of the above isolation wall 2a. Further, in order to prevent electrical cross talk and to ensure electrical separation between the neighboring silicon photo diodes an n type semiconductor region 3axe2x80x2, a p type semiconductor region 3cxe2x80x2 and another n type semiconductor region 3axe2x80x2 having a width of 0.03 mmxcx9c0.05 mm are locally formed in the dead zone 3bxe2x80x2 with a predetermined distance, alternatively, when the width of the light receiving portion is 0.8 mm, the width of the locally formed n and p type regions is 0.02 mmxcx9c0.03 mm, further, over the dead zone 3bxe2x80x2 on the substrate a first insulating protection film 3e such as SiN film, a second insulative protection film 3f such as Si02 film and a third light shielding and reflecting film 3g such as Al and Ag film for shielding light incident to the dead zone are provided, with respective width substantially the same as of the dead zone 3bxe2x80x2, i.e., 0.3 mm, as an alternative, when the width of the light receiving portion is 0.8 mm, the width of the first through third films is 0.2 mm. Further, over the third film 3g and at the center portion in the width direction as a fourth layer a light absorbing film 3h such as carbon film having the same or slightly narrower width than that of the isolation wall 2a, i.e., 0.1 mm or slightly less than 0.1 mm, is provided. It is preferable that the width of the light absorbing film 3h never exceeds the width of the isolation wall 2a. Further, at both side portions in the width direction of the light absorbing film of a material such as carbon film, protective films 3f of transparent optical thin film such as SiO2 film are provided in such a manner as to sandwich the light absorbing film 3h in the width direction. When the width of the dead zone 3bxe2x80x2 is 0.3 mm, the width of the protective film 3f of a material such as SiO2 film is equal to 0.1 mm or more, alternatively, when the width of the dead zone 3bxe2x80x2 is 0.2 mm, the width of the protective film 3f is 0.05 mm or more. Since a light reflection rate of the surface of these protective films 3f is high, i.e., more than 80%, even if the width of the dead zone 3bxe2x80x2 is larger than the width of the isolation wall 2a and extends into the region of the scintillator 1, a possible loss of light due to absorption by the surface of the protective films 3f can be reduced. Although it is ideal to use for the light absorbing film 3h, a material that fully absorbs the light coming from the scintillators 1, materials that are black, charcoal gray and dark brown, such as carbon (C) and carbon compounds, are usually used. As film forming methods for the light absorbing film 3h,the following methods can be used; a method of forming a carbon (C) film through PVD or a spattering, a method of forming a silicon carbide (SiC) film through CVD, and a method of forming a silver sulfide (Ag2S) film by reacting with sulfur or a method of forming a silver sulfide (Ag2S) film by acting hydrogen sulfide (H2S) onto a silver (Ag) film, when silver (Ag) is used as an under layer light reflecting film. Other than the above light absorbing materials giving black color, charcoal gray or dark brown, sulfides such as FeS, NiS and Mo2S3 and oxides such as OsO, CrO, SnO, TeO, Pb20, NbO, BiO, MoO and RuO can be used. Further, as the material for the adhesive layer 4 which couples optically and mechanically the scintillators 1 and the isolation walls 2a with the two dimensionally arranged silicon photo diode array 3, an epoxy resin which shows a high transparency, a stability with regard to X-ray and a refractive index of nxe2x89xa71.5, can be used, for example. Such a composition preferably includes a bisphenol A epoxy resin as a main component, a polyamide curing agent of 3, 9-bis (3-aminopropyl) - 2, 4, 8, 10-tetrapyro [5.6] undecane (ATU), a reactive diluent for adjusting viscosity and a silan coupling agent as a resin modifier. The thickness of the adhesive layer 4 affects the degree of optical cross talk, in that if the thickness is too great, cross talk increases, and if the thickness is too little, bubbles tend to be mixed therein and the mechanical strength of the layer decreases. For this reason, thickness selection of the adhesive layer 4 is important, therefore, it is preferable to make the thickness of the adhesive layer 4 less than xc2xd of the width of the isolation wall 2a, so that when the width of the isolation wall 2a is 0.1 mm, it is preferable to make the thickness of the adhesive layer less than 0.05 mm and more practically in a range of 0.01 mmxcx9c0.02 mm. FIG. 3 is a cross sectional view taken along a channel direction of a second embodiment according to the present invention of the two dimensionally arranged multi element X-ray detection device as schematically illustrated in FIG. 1. In FIG. 3, other than the structure of the dead zone for separating photo diode elements located between neighboring light receiving portions in the form of p type semiconductor layer 3c in the two dimensionally arranged silicon photo diode array which is formed by integrating in a matrix shape the light receiving portions in the form of p type semiconductor layer on a common silicon substrate, the structure of the X-ray detection element array is substantially the same as that of the first embodiment as shown in FIG. 2, and the same reference numerals in FIG. 3 as in FIG. 2 designate the same or equivalent portions as in FIG. 2. At the portion of the dead zone 3bxe2x80x2 a plurality of signal leading out wires 3dxe2x80x2, such as Al thin film interconnects, which are to be connected to the anode electrode 3d as Al thin film for taking out the signal currents (photo currents) of the respective light receiving portions in the form of p type semiconductor layer and are also illustrated in FIG. 1, are formed on the substrate surface of the dead zone 3bxe2x80x2 via an insulative oxide film 3e.  The surfaces of the signal leading out wires 3dxe2x80x2 are covered by the insulative film 3f, and the surface of the insulative film 3f is covered by the shielding and light reflecting film 3g of a material such as Al and Ag for shielding the light incident into the dead zone 3b, as in the first embodiment. Further, on the surface of the shielding and light reflecting film 3g the regions covered by the protective film 3f such as Si02 film and the region covered by the light absorbing material, a light absorbing film 3h such as carbon film can be provided. According to the embodiments as shown in FIGS. 2 and 3, the light generated by the scintillators 1 in the X-ray detection element array passes through the scintillators 1 after repeated reflections at the surfaces of the isolation walls 2a and the upper face reflection plate 7 and at the boundaries and the surfaces of the scintillators 1, and is emitted into the adhesive layer 4, further the emitted light passes through the adhesive layer 4 and is introduced into the respective light receiving portions in the form of p type semiconductor layer 3c, in the silicon photo diode array where the received light is photo-electrically converted and is detected as electrical signals (photo currents). Now, all of the light emitted into the adhesive layer is not necessarily directed toward the respective light receiving portions in the form of p type semiconductor layer 3c, some of the light advances in the adhesive layer 4 in the lateral direction, and is directed toward the neighboring elements. The difference in refractive index between the adhesive layer 4 (n≈1.5) and the reflection preventing film 3e (n≈2.1xcx9c2.1) provided on the light incidence surface of the respective light receiving portions is large, thus, the critical angle with respect to incident light onto the respective light receiving portions assumes a small angle below 40xc2x0, therefore, incident light from lateral direction with a large incident angle will mostly be reflected at the surface of the reflection preventing film 3e to prevent incidence into the neighboring light receiving regions, however, light with a small incident angle falling within the critical angle may reach the neighboring light receiving regions to cause optical cross talk. However, according to the embodiments of the present invention, on the surface of the dead zone for separating photo diode elements located between neighboring light receiving portions in the form of p type semiconductor layer 3c in the two dimensionally arranged silicon photo diode array a region covered by the light absorbing material, the light absorbing film 3h is provided. Accordingly, when light generated in the respective isolated scintillators would otherwise leak through the adhesive layer to the respective neighboring scintillators or the respective neighboring light receiving portions on the photo diode array, such light is absorbed by the material having light absorbing property provided on the surface of the isolation wall or band, when the light passes through the respective isolation bands provided between the respective neighboring light receiving portions on the photo diode array. Accordingly, the light which reaches the isolation bands is substantially absorbed by the light absorbing material, and light leakage between the neighboring X-ray detection elements is eliminated, thereby, a possible deterioration of the spatial resolution due to optical cross talks of the device is prevented. Further, at both side portions in the width direction of the light absorbing film 3h, since the protective films 3f of transparent optical thin film having a light reflection rate of more than 80% are provided in such a manner as to sandwich the light absorbing film 3h in the width direction, and because a film of a material such as Al and Ag having a high reflection rate is provided as an under layer for the optical thin film, the reflection rate of the protective film 3f is enhanced, even if the width of the dead zone 3bxe2x80x2 is set larger than the width of the isolation wall 2a and extends into the region of the scintillator 1, a possible loss of light due to absorption by the surface of the protective films 3f can be reduced. Further, with the structure according to the present embodiments, since the necessity of cutting up to the inside of the silicon photo diode array 3 so as to separate the respective X-ray detection elements as has been explained in connection with JP-B-2720159 (1997) is avoided, problems such as of decreasing S/N ratio and characteristic deviation of the respective X-ray detection elements caused by such as increased leakage currents and dark currents in the silicon photo diodes due to microcracks caused when performing the cutting processing, are eliminated. In the above explanation of FIGS. 2 and 3 embodiments, in order to avoid duplicate explanation, only the provision of the dead zones with the light absorbing film 3h between the X-ray detection elements arranged in the channel direction has been explained, however, the same is true with regard to X-ray detection elements arranged in the slice direction, and through the combination of both, a multi slice type X-ray detection device is constituted. As has been explained hitherto, in the multi element solid state X-ray detection device according to the present invention, on the surfaces of the isolation bands for separating photo diode elements located between neighboring light receiving portions in the form of p type semiconductor layer 3c in the silicon photo diode array for multi channels a region covered by the light absorbing material is provided. Accordingly, when light generated in the respective isolated scintillators separated by the respective isolation walls would otherwise leak through the adhesive layer to the respective neighboring scintillators or the respective neighboring light receiving portions on the photo diode array, such light is absorbed by the material having light absorbing property provided on the surface of the isolation wall or bands, when the light passes through the respective isolation wall or bands provided between the respective neighboring light receiving portions on the photo diode array. Accordingly, light which reaches the isolation bands is substantially absorbed by the light absorbing material, and light leakage between the neighboring X-ray detection elements is eliminated, thereby, a possible deterioration of the spatial resolution due to optical cross talks of the device is prevented. Still further, as has been explained in connection with the FIG. 3 embodiment, since the isolation wall or bands are also utilized as the wiring regions, a packing density of the X-ray detection element array is enhanced. Further, according to the present invention, the necessity of cutting up to the inside of the silicon photo diode array 3 so as to separate the respective X-ray detection elements as has been explained in connection with JP-B-2720159 (1997) is avoided, thereby, the problems such as of decreasing S/N ratio and characteristic deviation of the respective X-ray detection elements caused by such as increased leakage currents and dark currents in the silicon photo diodes due to such as microcracks caused when performing the cutting processing, are eliminated. Still further, according to the present invention, when using the multi element solid state X-ray detection device provided with the electrical and optical cross talk reducing means and the packing density enhancing means as has been explained hitherto in an X-ray CT apparatus, in particular, in a multi slice type X-ray CT apparatus which requires great many number of X-ray detection elements, tomographic images with a high quality for a plurality of slices can be obtained concurrently.