Radiation detector and method of producing the same

[0001] The present invention relates to a radiation detector used for radiation imaging and a manufacturing method thereof and, more particularly, to a compact dental radiation detector used upon insertion into an oral cavity and a manufacturing method thereof.

[0002] X-ray image sensors using a CCD instead of an X-ray sensitive film are becoming popular as a medical X-ray diagnostic apparatus. In such a radiation imaging system, a radiation detection device having a plurality of pixels acquires two-dimensional image data by radiation as an electrical signal, and this signal is processed by a processor and displayed on a monitor.

[0003] As a dental radiation detector used upon insertion into an oral cavity, a radiation detector which is disclosed in JP 10-282243 A has been known. In this radiation detector, an FOP (Fiber Optical Plate) with a scintillator is stuck on the light-receiving surface of a CCD. The radiation detector has a mechanism to convert incident radiation into light by the scintillator, guide the light to the CCD by the FOP, and detect the light.

[0004] A dental radiation detector used upon insertion into an oral cavity requires a larger imaging area while downsizing and thinning the whole detector. In the above apparatus, thinning is limited in the presence of the FOP. As disclosed in WO98/36291, a scheme for thinning the detector by directly forming a scintillator on the light-receiving surface of an imaging element is considered. However, when a light-receiving portion is formed on the entire light-receiving surface, it is difficult to uniformly form the scintillator on the entire light-receiving portion, thus decreasing the output and resolution of end portions. This makes it difficult to attain the large imaging area.

[0005] It is an object of the present invention to provide a radiation detector which can attain both a small, thin structure and a large imaging area, and can be easily manufactured, and manufacturing method thereof.

[0006] To solve the above problems, a radiation detector according to the present invention comprises (1) a solid-state imaging element having a light-receiving portion where a plurality of photoelectric conversion elements are arranged, and electrode pads electrically connected to the photoelectric conversion elements, (2) a scintillator formed on a surface of the light-receiving portion of the solid-state imaging element, and (3) a substrate having a support surface for mounting the solid-state imaging element thereon, and a positioning portion which is adjacent to the support surface and projects higher than the support surface to position the solid-state imaging element with a side wall, wherein the positioning portion is formed such that the light-receiving surface of the solid-state imaging element projects toward a light incident side higher than an upper surface of the positioning portion.

[0007] On the other hand, a radiation detector manufacturing method according to the present invention comprises the steps of: (1) preparing a solid-state imaging element having a light-receiving portion where a plurality of photoelectric conversion elements are arranged, and electrode pads electrically connected to the photoelectric conversion elements, and a substrate having a positioning portion which is adjacent to a support surface, and projects higher than the support surface, (2) mounting and fixing the solid-state imaging element on the support surface of the substrate such that the light-receiving surface of the solid-state imaging element projects higher than an upper surface of the positioning portion by using a side wall of the positioning portion, and (3) forming a scintillator on the surface of the light-receiving portion of the solid-state imaging element.

[0008] In this manner, in the radiation detector according to the present invention, the solid-state imaging element is accurately positioned and fixed on the support surface of the substrate by using the positioning portion formed on the substrate. After positioning and fixing the solid-state imaging element, the scintillator is formed on the surface of the light-receiving portion of the solid-state imaging element by vapor deposition or the like. At this time, the surface of the light-receiving portion projects the most toward the light incident side. Hence, no projecting portion which becomes an obstacle is formed on the light-receiving portion in forming the scintillator, thereby uniformly forming the scintillator on the entire light-receiving portion. This can ensure the resolution and attain the large area of the effective light-receiving portion. The FOP is not used, thus facilitating thinning the detector.

[0009] The substrate further has the external connection electrodes and the electrode pads which are arranged on the upper surface of the positioning portion, and electrically connected to the external connection electrodes, and further has the wiring lines which connect the electrode pads of the solid-state imaging element and the electrode pads of the substrate, respectively. This facilitates connecting connection lines to external devices.

[0010] FIG. 1 is a perspective view showing an embodiment of a radiation detector according to the present invention, FIG. 2 is a partial enlarged view thereof, and FIG. 3 is a side view thereof;

[0011] FIG. 4 is a perspective view of a substrate used for the apparatus of FIG. 1;

[0012] FIGS. 5 to 7 are side or perspective views for explaining the manufacturing process of the apparatus in FIG. 1;

[0013] FIGS. 8 and 9 are sectional views of a vapor deposition substrate holder used in the manufacturing process of the apparatus in FIG. 1;

[0014] FIGS. 10 to 15 are side or perspective views for explaining the following process of the manufacturing process of the apparatus of FIG. 1;

[0015] FIGS. 16A and 16B are enlarged sectional views of wiring portions shown in FIG. 14 in two embodiments;

[0016] FIG. 17 is a perspective view of the radiation detector according to the present invention before forming the layer of a scintillator in still another embodiment, and FIG. 18 is a side view of this radiation detector;

[0017] FIG. 19 is a perspective view of a radiation detector according to the present invention before forming the layer of a scintillator in still another embodiment, and FIG. 20 is a side view of this radiation detector; and

[0018] FIG. 21 is a perspective view of a radiation detector according to the present invention before forming the layer of a scintillator in still another embodiment, and FIG. 22 is a side view of this radiation detector.

[0019] Preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings. To facilitate the comprehension of the explanation, the same reference numerals denote the same parts, where possible, throughout the drawings, and a repeated explanation will be omitted. The size and shape of each drawing need not always be the same as the actual design. Some parts are exaggerated for convenience of understanding.

[0020] FIG. 1 is a perspective view showing an embodiment of a radiation detector according to the present invention, FIG. 2 is an enlarged view thereof, and FIG. 3 is a sectional view thereof. A radiation detector 100 of this embodiment is constituted by mounting a solid-state imaging element 2 on a ceramic substrate 1. The substrate 1 has in the center of the upper surface a cavity 10 where the solid-state imaging element 2 is set and stored. Projections 14 to 16 are formed along the three sides around the cavity 10. On a top surface 15a of the projection 15, a plurality of electrode pads 11 are arrayed along the boundary between the cavity 10 and the projection 15. These electrode pads 11 are electrically connected to external connection electrode terminals 12 on the lower surface of the substrate 1 via wiring lines 13 extending through the substrate 1. Note that the projection height of each of the projections 14 to 16 (the distance between a bottom surface 10a of the cavity 10 and the top surfaces 14a to 16a of the projections 14 to 16) is set to be less than the thickness of the solid-state imaging element 2 (to be described later).

[0021] The solid-state imaging element 2 is comprised of a CCD image sensor, and a portion where photoelectric conversion elements 21 are arrayed forms a light-receiving portion. Each photoelectric conversion element 21 is electrically connected via a signal line (not shown) to a corresponding one of electrode pads 22 arranged on one side of the solid-state imaging element 2. The solid-state imaging element 2 is mounted on the substrate 1 such that corresponding electrode pads 11 and 22 are located close to each other. Corresponding electrode pads 11 and 22 are electrically connected by wiring lines 4. Since the height of each of the projections 14 to 16 on the substrate 1 is set as described above, an incident surface 2a of the solid-state imaging element 2 is set at the higher position than the top surfaces 14a to 16a of the projections 14 to 16.

[0022] A columnar scintillator 3 which converts incident radiation into light of a waveband sensed by the photoelectric conversion element 21 is formed on the light-receiving portion of the solid-state imaging element 2. The scintillator 3 can be made of various materials, and can preferably adopt Tl-doped CsI with high emission efficiency.

[0023] The protective film 5 covers the surfaces of the solid-state imaging element 2 and substrate 1. The protective film 5 transmits X-rays and cuts off water vapor. The protective film 5 is formed by stacking an electrical insulating first organic film 51, metal thin film 52, and electrical insulating second organic film 53 on the substrate 1.

[0024] The first and second organic films 51 and 53 are preferably made of a polyparaxylylene resin (Parylene available from Three Bond Co., Ltd.), and particularly polyparachloroxylylene (Parylene C also available from Three Bond Co., Ltd.). The parylene coating film hardly transmits water vapor and gas. This film has high repellency, high chemical resistance, and high electrical insulating property even as a thin film. In addition, the parylene coating film is transparent to radiation and visible light. These properties are suitable for the organic films 51 and 53. The metal thin film 52 can be a metal thin film of gold, silver, aluminum, or the like. The metal thin film 52 functions as a mirror which increases the detection sensitivity of the detector by reflecting, of light emitted by the scintillator 3, light traveling toward not the solid-state imaging element 2 but the radiation incident plane.

[0025] A protective resin layer 6 is so formed as to further cover the protective film 5 which covers the electrode pads 11 and 22 and the wiring lines 4. The protective resin layer 6 is preferably made of a resin having a good adhesion property with a protective film 5, e.g., WORLD ROCK No. 801-SET2 (70,000, cP type) available from Kyoritsu Chemical & Co., Ltd. as an acrylic adhesive.

[0026] The manufacturing method of the radiation detector according to the present invention will be explained in detail with reference to FIGS. 4 to 15. First, the substrate 1 as shown in FIG. 4 is prepared. The external connection electrode terminals 12 are arrayed on the lower surface of the substrate 1 while the electrode pads 11 are arrayed on the upper surface of the substrate 1 as described above. The projections 14 to 16 are formed along the three sides on the upper surface side. The cavity 10 is formed in a portion interposed by the projections.

[0027] The solid-state imaging element 2 is set and fixed in the cavity 10 with the light-receiving surface of the photoelectric conversion element 21 facing up, as shown in FIG. 5, such that the electrode pads 22 are directed to the electrode pads 11 of the substrate 1. At this time, positioning the solid-state imaging element 2 by using the projections 14 to 16 facilitates fixing the solid-state imaging element 2. The incident surface 2a of the solid-state imaging element 2 is set at the higher position than the top surfaces 14a to 16a of the projections 14 to 16. Then, the electrode pads 11 and 22 are electrically connected via the wiring lines 4 by wire bonding (see FIGS. 6 and 7).

[0028] Next, the substrate 1 on which the solid-state imaging element 2 is mounted is set into a vapor deposition substrate holder 200. FIGS. 8 and 9 show the sectional views after setting. At this time, by supporting the corresponding projections 14 and 16 with 200d of the vapor deposition substrate holder 200 as shown in FIG. 8, the substrate 1 is stored and supported in a storing portion 200c. As shown in FIGS. 8 and 9, the incident surface 2a of the solid-state imaging element 2 is set near a vapor deposition-side surface 200a of the vapor deposition substrate holder 200, and preferably set such that the incident surface 2a projects toward a vapor deposition chamber 201. This structure can be implemented because the top surfaces 14a to 16a of the projections 14 to 16 are lower than the incident surface 2a. A cover portion 200a formed on the projection 15 side of the vapor deposition substrate holder 200 covers the electrode pads 11 and 12 and wiring lines 4.

[0029] The vapor deposition substrate holder 200 is set in the vapor deposition apparatus in this state, and Tl-doped CsI is grown as columnar crystals to a thickness of about 200 m by vacuum deposition on the light-receiving portion on the incident surface 2a of the solid-state imaging element 2, thereby forming the layer of a scintillator 3 (see FIGS. 10 and 11). Only the cover portion 200a projects toward the vapor deposition chamber 201 along the incident surface 2a around the light-receiving portion on the incident surface 2a of the solid-state imaging element 2 which is arranged in the vapor deposition substrate holder 200. Hence, the layer of the scintillator 3 with substantially uniform thickness can be formed to the peripheral portion of the light-receiving portion except the electrode pad 22 side.

[0030] CsI is high deliquescent, and if CsI is kept exposed, it absorbs water vapor in air and dissolves. To protect the scintillator, as shown in FIG. 12, the overall substrate 1 with the solid-state imaging element 2 bearing the scintillator 3 is covered with a 10-m thick parylene film by CVD (Chemical Vapor Deposition), thus forming a first organic film 51.

[0031] More specifically, coating by deposition is performed in vacuum, similar to vacuum deposition of a metal. This process includes the step of thermally decomposing a diparaxylylene monomer serving as a raw material and quenching the product in an organic solvent such as toluene or benzene to obtain diparaxylylene called a dimer, the step of thermally decomposing the dimer to produce stable radical paraxylylene gas, and the step of absorbing and polymerizing the produced gas in a raw material to form a polyparaxylylene film with a molecular weight of about 500,000 by polymerization.

[0032] There is a gap between CsI columnar crystals, but parylene enters this narrow gap to a certain degree. The first organic film 51 tightly contacts the layer of the scintillator 3 and tightly seals the scintillator 3. The first organic film 51 is formed not only on the electrode pads 11 and 22 but also around the wiring lines 4 which connect them, and covers the wiring lines 4. This structure increases the adhesion and mechanical strengths of the wiring lines 4, and the wiring lines 4 can be easily handled in subsequent steps. By parylene coating, a uniform-thickness precise thin film coating can be formed on the corrugated layer surface of the scintillator 3. CVD formation of parylene can be achieved at room temperature with a lower degree of vacuum than in metal deposition, and parylene can be easily processed.

[0033] As shown in FIG. 13, a 0.15m thick Al film is formed on the incident surface of the first organic film 51 by vapor deposition, thereby forming a metal thin film 52. In forming the metal thin film 52, a proper mask (not shown) is desirably arranged in front of the layer of the scintillator 3 so as to form the metal thin film 52 only immediately above the layer of the scintillator 3 on the first organic film 51. Even if the mask is arranged, metal vapor may slightly flow outside the mask in vapor deposition. Especially at a small interval between the light-receiving portion and the electrode pad, it is difficult to form the metal thin film 52 only immediately above the layer of the scintillator 3. The metal may be deposited on the wiring lines 4 and the electrode pads 11 and 22. According to the present invention, the metal thin film 52 is not directly formed on the wiring lines 4 and the electrode pads 11 and 22 because the wiring lines 4 and the electrode pads 11 and 22 are covered with the first organic film 51. Short-circuiting of the wiring lines 4 and the electrode pads 11 and 22 by the metal thin film 52 can be effectively prevented.

[0034] Also when no mask is arranged in vapor deposition of the metal thin film 52, the metal thin film 52 is formed on the wiring lines 4 and the electrode pads 11 and 22. However, the wiring lines 4 and the electrode pads 11 and 22 are covered with the first organic film 51, and short-circuiting can be prevented. The humidity resistance can be further increased by widely forming the metal thin film 52 which covers the wiring lines 4 and the electrode pads 11 and 22 via the first organic film 51.

[0035] After that, parylene is applied to a thickness of 10 m on the entire surface of the substrate by CVD again, thus forming a second organic film 53 (see FIG. 14). The second organic film 53 prevents contamination, peeling, or oxidization of the metal thin film 52 by handling or the like. In this manner, a protective film 5 as a multilayered structure of the first organic film 51, metal thin film 52, and second organic film 53 is formed.

[0036] When the metal thin film 52 is also formed on the wiring lines 4 and the electrode pads 11 and 22 via the first organic film 51, the thin film 52 and second organic film 53 are formed not only on the electrode pads 11 and 22 but also around the wiring lines 4 which connect them, and the wiring lines 4 are triple-covered with the first organic film 51. This structure increases the adhesion and mechanical strengths of the wiring lines 4. When the metal thin film 52 is formed using a mask, as described above, the wiring lines 4 and the electrode pads 11 and 22 are double-coated with the first and second organic films 51 and 53.

[0037] Subsequently, a resin is applied to the protective film 5 on the electrode pads 11 and 12 so as to surround the wiring lines 4 covered with the protective film 5. This resin is cured to form a protective resin layer 6. The wiring lines 4 project on the protective film 5 covering the electrode pads 11 and 12, and when the metal thin film 52 is also formed on the wiring lines 4, the periphery of each wiring line 4 is triple-coated with the protective film 5 made up of the organic films 51 and 53 and the metal thin film 52 (see FIG. 16A). If no metal thin film 52 is formed on the wiring lines 4, the wiring lines 4 are double-coated with the organic films 51 and 53 (see FIG. 16B). In either case, the protective resin layer 6 surrounds the covered wiring lines 4 and pots them, as shown in FIG. 15 and FIGS. 16A and 16B. The wiring lines 4 are further protected, and damage to the wiring lines 4 in use can be effectively prevented. The protective resin layer 6 need not always be arranged, but is preferably formed in terms of protection of the wiring lines 4.

[0038] Of the formed protective film 5, the protective film 5 on the lower surface of the substrate 1 is removed to expose the external connection electrode terminals 12 on the lower surface of the substrate 1. As a result, the radiation detector shown in FIGS. 1 to 3 is obtained.

[0039] The operation of this embodiment will be described with reference to FIGS. 1 to 3. X-rays (radiation) coming from the incident surface reach the scintillator 3 through the protective film 5, i.e., all the second organic film 53, metal thin film 52, and first organic film 51. X-rays are absorbed by the scintillator 3, which radiates light proportional to the quantity of X-rays. Of the radiated light, light opposite to the X-ray incident direction passes through the first organic film 51 and is reflected by the metal thin film 52. Hence, almost all light generated by the scintillator 3 enters the solid-state imaging element 2. This realizes high-sensitivity measurement with high efficiency.

[0040] In the radiation detector according to the present invention, the layer of the scintillator 3 can be formed with a substantially uniform thickness on the entire light-receiving portion. Hence, the maximum effective pixel area whose output characteristics are substantially uniform can be obtained, and the light-receiving portion can be formed on the entire imaging element. When the light-receiving portion is the same as the conventional one, the detector itself can be downsized.

[0041] Each photoelectric conversion element 2 generates an electrical signal corresponding to the quantity of visible light by photoelectric conversion, and accumulates the signal for a predetermined time. The quantity of visible light corresponds to the quantity of incident X-rays. That is, the electrical signal accumulated in each photoelectric conversion element 2 corresponds to the quantity of incident X-rays, and an image signal corresponding to an X-ray image can be obtained. The image signal accumulated in the photoelectric conversion element 2 is output from a signal line (not shown) and finally from the electrode terminal 12 via the electrode pad 22, wiring line 4, electrode pad 11, and wiring line 13. The image signal is externally transferred, and processed by a predetermined processing circuit to display an X-ray image.

[0042] The use of this structure can decrease the thickness of the radiation detector to about 2.5 mm, which is almost half a 5-mm thickness of a radiation detector with a conventional FOP (Fiber Optical Plate). The scintillator can be uniformly formed on the light-receiving portion of the solid-state imaging element in which the light-receiving portion is formed on the entire imaging element. Hence, the downsized radiation detector having the largest light-receiving portion of the imaging element can be implemented. As a result, while ensuring almost the same area of the light-receiving portion as that of the prior art (JP 10-282243A), the compact (about 90% the prior art) area on the incident surface side of the radiation detector can be implemented. This is greatly advantageous to a dental radiation detector used upon insertion into an oral cavity.

[0043] In the above description, the radiation detector has a structure with the protective film 5. However, when another protective means is used, i.e., when the radiation detector is used in a humidity-resistant case or when humidity-resistant material is used as a scintillator, the radiation detector need not have any protective film 5. Even if the protective film 5 is provided, the radiation detector may have various structures, e.g., a single-layered structure of an organic or inorganic film or a multilayered structure of a combination of them. The solid-state imaging element 2 may be formed from an amorphous silicon photodiode (PD) array and thin-film transistor (TFT), or may be a MOS image sensor.

[0044] Subsequently, other embodiments of the radiation detector according to the present invention will be described. In an embodiment shown in FIGS. 17 and 18, six projections 14 to 19 are formed around a cavity 10 which is formed at the center of the upper surface of a substrate 1. The top surfaces of these projections are set to be lower than an incident surface 2a of a solid-state imaging element 2 which is mounted in the cavity 10.

[0045] An embodiment shown in FIGS. 19 and 20 is different from that shown in FIGS. 1 to 3 in that projections 14 and 16 are set to be lower than the projection 15. An embodiment shown in FIGS. 21 and 22 is different from the embodiment shown in FIGS. 1 to 3 in that a projection 17 is formed along a side opposing a projection 15 in a substrate 1.

[0046] In these embodiments, the top surfaces of the projections are also set to be lower than the incident surface of the light-receiving portion of the mounted solid-state imaging element. Hence, the uniform layer of a scintillator can be formed on the entire light-receiving portion after mounting the solid-state imaging element on the substrate 1 and wiring.

[0047] Industrial Applicability

[0048] The present invention can be preferably used as a thin and small radiation detector and a manufacturing method thereof and, more particularly, as a compact dental radiation detector used upon insertion into an oral cavity and a manufacturing method thereof.