Abstract:
The present invention relates to a method of fabricating a radiation detector comprising a photosensitive sensor assembly ( 1, 4 ), a scintillator ( 6 ) that converts the radiation into radiation to which the photosensitive sensor assembly ( 1, 4 ) is sensitive, the scintillator ( 6 ) being fastened by adhesive bonding to the sensor assembly, the sensor assembly comprising a substrate ( 4 ) and several attached sensors ( 1 ), the sensors ( 1 ) each having two faces ( 11, 12 ), a first face ( 11 ) of which is bonded to the substrate ( 4 ) and a second face ( 12 ) of which is bonded to the scintillator ( 6 ). The method consists in linking the following operations:
       the sensors ( 1 ) are deposited via their second face ( 12 ) on an adhesive film ( 13 ); and   the sensors ( 1 ) are bonded via their first face ( 11 ) to the substrate ( 4 ).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/EP2008/056312, filed May 22, 2008, which claims priority to foreign French Application No. FR 07 55204, filed May 23, 2007, the disclosure of each application is hereby incorporated by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to a method of fabricating a radiation detector comprising a photosensitive sensor associated with a radiation converter. The fields of application of this type of detector are notably the detection of X-rays used in radiology: radiography, fluoroscopy and mammography, and also for nondestructive testing. The invention will be described in relation to an X-ray detector. Of course, the invention may be implemented with any type of detector for which the photosensitive sensor is not directly sensitive to the radiation to be detected, and for which it is therefore necessary to interpose a radiation converter between an entry window of the detector and the photosensitive sensor. 
     Such radiation detectors are known for example from French patent FR 2 605 166, in which a sensor formed from amorphous silicon photodiodes is associated with a radiation converter. 
     The operation and the structure of such a radiation detector will now be briefly reviewed. 
     The photosensitive sensor is generally fabricated from solid-state photosensitive elements arranged in a matrix. The photosensitive elements are fabricated from semiconductor materials, usually single-crystal silicon in the case of CCD or CMOS sensors, polycrystalline silicon or amorphous silicon. A photosensitive element comprises at least one photodiode, at least one phototransistor or at least one photoresistor. These elements are deposited on a substrate, generally a glass plate. 
     In general, these elements are not directly sensitive to radiation of very short wavelength, such as X-rays or gamma rays. This is why the photosensitive sensor is associated with a radiation converter, which comprises a layer of a scintillating substance. This substance has the property, when it is excited by such radiation, of emitting radiation of longer wavelength, for example visible or near-visible light, to which the sensor is sensitive. The light emitted by the radiation converter illuminates the photosensitive elements of the sensor, which perform a photoelectric conversion and deliver electrical signals that can be exploited by appropriate circuits. The radiation converter will be called a scintillator in the rest of the description. 
     Certain scintillating substances of the family of alkali metal halides or rare-earth metal on/sulfides are frequently employed for their good performance. 
     Among alkali metal halides, cesium iodide doped with sodium or with thallium, depending on whether it is desired for the emission to be at around 400 nanometers or around 550 nanometers respectively, is known for its strong X-ray absorption and for its excellent fluorescence yield. It takes the form of fine needles which are grown on a support. These needles are approximately perpendicular to this support and they partly confine the light emitted toward the sensor. Their fineness determines the resolution of the detector. Lanthanum oxysulfide and gadolinium oxysulfide are also widely used for the same reasons. 
     However, some of these scintillating substances have the drawback of being not very stable—they partially decompose when exposed to moisture and their decomposition releases chemical species that migrate either toward the sensor or away from the sensor, these species being highly corrosive. Notably, cesium iodide and lanthanum oxysulfide have this drawback. 
     As regards cesium iodide, its decomposition gives cesium hydroxide Cs + OH −  and free iodine I 2 , which can then combine with iodide ions to give the complex I 3   − . 
     As regards lanthanum oxysulfide, its decomposition gives hydrogen sulfide H 2 S, which is chemically very aggressive. 
     Moisture is extremely difficult to eliminate. The ambient air, in which the sealing operation is carried out, always contains moisture. 
     One of the important aspects when producing these detectors is to minimize the amount of moisture initially present within the detector, and in contact with the scintillator, and to prevent said moisture from diffusing into the sensor during its operation. 
     In a first configuration, called an “added-scintillator” configuration, the scintillating substance is deposited on a support through which the radiation to be detected has to pass before reaching the sensor. The assembly is then bonded to the sensor. 
     In a second configuration, called a “direct-deposition” configuration, the sensor serves as support for the scintillating substance, which is therefore in direct and intimate contact with the sensor. The scintillating substance is then covered with a protective coating. The two configurations each have advantages and disadvantages. 
     One advantage of the first, attached-scintillator, configuration is that the sensor and the scintillator are joined together only if they have been successfully tested, thereby improving the overall fabrication yield. 
     Other advantages of this configuration will be apparent on reading French patent application FR 2 831 671. 
     The aim of the invention is to improve the production of a large detector requiring the attachment of several smaller photosensitive sensors, the radiation detector being fabricated in the first configuration. 
       FIGS. 1   a  to  1   d  describe an operating method for producing attached assemblies. 
     Small sensors  1  are aligned and positioned individually. A seal  2  is positioned between the sensors  1  and then a liquid adhesive  3  is spread over the sensors  1 . The seal  2  prevents the adhesive  3  from flowing between the sensors  1 . A substrate  4  common to the various sensors  1  is then bonded to the sensors  1  by means of the adhesive  3 . The material of the seal  2  must have a high viscosity so as to prevent it from flowing between the sensors  1 . However, this viscosity must be low enough for it to perfectly match a portion of the space that the adhesive  2  must fill between the sensors  1 . 
     As for the material of the adhesive  3 , this must have a viscosity low enough to migrate over all the surfaces of the sensors  1  in order to be bonded to the entire surface. However, too fluid an adhesive  3  would have a tendency to extend beyond the periphery of the sensors  1 . 
     With this operating method, it is impossible to develop a single adhesive acting both as adhesive  2  and as adhesive  3 . Such a single adhesive would have to reconcile the contradiction of being both fluid enough to migrate over all the surfaces of the sensors  1 , and achieve complete bonding, and viscous enough to prevent it from flowing between the sensors  1 . 
     In a radiation detector embodiment, the following operation consists in depositing, on the attached assembly thus fabricated, a layer of liquid adhesive  5  intended to bond a scintillator  6  to the attached assembly. The adhesive  5  is a material that has to be optically transparent so as not to absorb the light emitted by the X-ray-illuminated scintillator  6 . The material of the adhesive  5  must have a viscosity low enough for it to penetrate between the sensors  1  and to completely fill the residual space between the sensors  1  left free by the adhesive  2  deposited during the preceding operation. It must also have a viscosity high enough for it not to flow excessively over the periphery of the sensors  1 . 
     The adhesive  5  must be thin enough to allow good image quality. This is because the light generated by the scintillator  6  must pass through this adhesive layer before being absorbed by the sensors  1 . This light will be scattered less the thinner the adhesive  5 . 
     For example, liquid adhesives  5  have been used from the families of silicone, epoxy and acrylic adhesives, or any other family of adhesives. These liquid adhesives must be deposited beforehand on one of the two surfaces to be bonded, or on both of them. This adhesive dispensing must be very uniform in terms of thickness and this thickness must be perfectly controlled so as to achieve image quality, and notably resolution, which is uniform at all points on this image. 
       FIGS. 2   a  to  2   d  describe four types of defects encountered when producing the detectors. These fabrication defects result in defects in the image produced by the detector when irradiated with X-rays. 
     In  FIG. 2   a , the first defect is characteristic of too low a viscosity of the adhesive  2 , which completely interfered in the space available between the sensors  1  and comes into direct contact with the scintillator  6  when the latter is bonded by means of the adhesive  5 . This first fabrication defect results in a defect in the image obtained by the detector. The amount of light reaching the sensors  1  will be too great at the junction between the sensors  1 , the sensitivity of the detector in these places being too high. 
     In  FIG. 2   b , the second defect is characteristic either of too high a viscosity of the adhesive  2  or of too high a viscosity of the adhesive  5 . These two adhesives cannot come into contact with each other, so that an air-filled cavity persists between the sensors  1 . This may result in a break in the optical continuity of the light paths at the interfaces when the light coming from the scintillator  6  passes through them. As a result, there is a loss of light, and too dark an image at the location of these air-filled cavities. 
     This type of defect may evolve and create the third defect, shown in  FIG. 2   c , in which the volume of trapped air may expand and create delaminations at the bonded interfaces. 
     The fourth defect shown in  FIG. 2   d  may be observed when the sensors  1  have not been correctly aligned in the horizontal plane—there is thus a height discrepancy. A mechanical step when passing from one sensor  1  to its neighbor may result in a break in continuity of the adhesive  5 . This type of defect could be resolved by an adhesive  5  having a viscosity low enough to follow the break of the mechanical step. However, too low a viscosity of the adhesive  5  may result in poor lifetime of the optical contact at the step break. 
     Thus, the four fabrication defects described above always result in breaks in the continuity of the adhesive  5  and absences of material or the presence of air cavities that will result in faults in optically coupling the scintillator  6 . At the place of these faults, the image may be exaggeratedly bright (first defect) or exaggeratedly dark (second, third and fourth defects). 
     Resolution of these defects is faced with having to have the viscosity of the adhesives  2 ,  3  and  5  which require contradictory properties. It is in fact required of the adhesives  2 ,  3  and  5  to have sufficiently low viscosities or sufficiently high viscosities for fulfilling contradictory functions. 
     SUMMARY OF THE INVENTION 
     The aim of the invention is to alleviate all or some of the problems mentioned above by providing a method of fabricating a radiation detector comprising several attached sensors without a specific seal between the sensors. 
     For this purpose, the subject of the invention is a method of fabricating a radiation detector comprising a photosensitive sensor assembly, a scintillator that converts the radiation into radiation to which the photosensitive sensor assembly is sensitive, the scintillator being fastened by adhesive bonding to the sensor assembly, the sensor assembly comprising a substrate and several attached sensors, the sensors each having two opposed faces, a first face of which is bonded to the substrate and a second face of which is bonded to the scintillator, characterized in that the following operations are linked: 
     the sensors are deposited via their second face on an adhesive film; and 
     the sensors are bonded via their first face to the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The invention will be better understood and other advantages will become apparent on reading the detailed description of embodiments given by way of example, the description being illustrated by the appended drawing in which: 
         FIGS. 1   a  to  1   d  show a known method of fabricating a radiation detector; 
         FIGS. 2   a  to  2   d  show various defects due to the method described using  FIGS. 1   a  to  1   d;    
         FIGS. 3   a  to  3   d  show a first alternative way of implementing the method of the invention; 
         FIGS. 4   a  to  4   c  show a second alternative way of implementing the method of the invention; 
         FIGS. 5   a  to  5   c  show a third alternative way of implementing the method of the invention; and 
         FIGS. 6   a  to  6   d  show a fourth alternative way of implementing the method of the invention. 
     
    
    
     For the sake of clarity, identical elements will bear identical references in the various figures. 
       FIGS. 1   a  to  1   d  and  FIGS. 2   a  to  2   d  have already been described above. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIG. 3   a  shows a first step in fabricating a radiation detector comprising several photosensitive sensors  1  advantageously comprising a matrix of photodiodes or phototransistors of the TFT type that are fabricated on an amorphous silicon wafer.  FIG. 3   a  shows only two sensors  1 , but of course the invention may be implemented for a larger number of attached sensors  1 . The sensors  1  both have two opposed faces  11  and  12 . A first step of a method according to the invention consists in depositing the sensors  1  via their face  12  on an adhesive film  13 . The adhesive film  13  is used as reference plane when depositing and bonding the sensors  1  thereto. This makes it possible to improve the alignment of the faces  12  of the various sensors  1 . 
     A second step of the method consists in bonding the sensors  1  via their face  11  to a substrate  4 . This step is illustrated with the aid of  FIGS. 3   b  and  3   c . In a first phase of this second step, a liquid adhesive  16  is deposited on the face  11  of the sensors  1  and in interstices  15  separating the sensors  1 . A low-viscosity adhesive  16  is chosen so as to fill the entire space left at its disposal, notably the interstices  15 . In a second phase, the sensors  1  and the substrate  4  are bonded by means of the liquid adhesive  16 . The film  13  prevents the liquid adhesive  16  from flowing via the interstices  15 . The use of the adhesive film  13  as reference plane also allows the space between the sensors  1  and the substrate  4  to be properly filled with the adhesive  16 . Notably, the adhesive film  13  ensures that the interstices  15  are properly sealed in order for the liquid adhesive  16  to be contained therein and in particular for the liquid adhesive  16  to be prevented from being able to slip between the adhesive film  13  and the sensors  1  on the face  11 . There is no longer a requirement to use a seal  2 , as described in  FIGS. 1   a  to  1   d.    
     This space has to be properly filled so as to obtain spatially continuous and homogeneous optical bonding between sensors  1  and substrate  4 . This filling is necessary for correct operation of detectors, which require spatially continuous and homogeneous illumination through the substrate  4  (the illumination not being shown). 
     A third step of the method, illustrated in  FIG. 3   d , consists in bonding a scintillator  6  to the face  12  of the sensors  1  by means of the adhesive film  13 . 
     In the first step of the method, the sensors  1  are aligned and deposited on the adhesive film  13  under a vacuum or by lamination, so as to prevent air bubbles from being trapped between the adhesive film  13  and the sensors  1 . The adhesive film  13  by its continuous nature and its constant thickness provides a perfectly planar surface after the assembly has been attached and will present this defect-free planar surface for optically bonding the scintillator  6 . 
     The fact of carrying out the first step before the second ensures that there is good flatness between the faces  12  of the various sensors  1 . 
     A variant of the second step, illustrated in  FIGS. 4   a  to  4   c , consists in placing, on the periphery of the sensors  1 , a ring  20  for retaining the liquid adhesive  16 . Thanks to this ring  20 , it is possible to use an adhesive  16  of very low viscosity so as to ensure that the interstices  15  are properly filled, while preventing any overflow of the adhesive  16  beyond the periphery of the sensors  1 . Apart from retaining the adhesive  16  on the periphery of the sensors  1 , the ring  20  may also strengthen an assembly formed by the sensors  1  and the adhesive film  13 . 
       FIG. 4   a  illustrates the first phase of the second step, during which the liquid adhesive  16  is deposited.  FIG. 4   b  illustrates the second phase during which the sensors  1  and the substrate  4  are bonded.  FIG. 4   c  illustrates the third step of the method during which the scintillator  6  is bonded to the face  12  of the sensors  1  by means of the adhesive film  13 . 
       FIGS. 5   a  to  5   c  illustrate a variant of the method of the invention in which the liquid adhesive  16  is replaced with a second adhesive film  25  for bonding the sensors  1  to the substrate  4 . In  FIG. 5   a , the adhesive film  25  is deposited on the substrate  4 . In  FIG. 5   b , the assembly formed by the sensors  1  and the film  13  is bonded to the substrate  4 . In  FIG. 5   c , the scintillator  6  is bonded to the face  12  of the sensors  1  by means of the adhesive film  13 . 
     In this embodiment, it is possible for the interstices  15  to be filled by means of a liquid adhesive having a very low viscosity in order to ensure good optical continuity between the scintillator  6  and the various sensors  1 . 
       FIGS. 6   a  to  6   d  illustrate another variant of the method of the invention in which the bonding of the scintillator  6  to the second face  12  of the sensors  1  by means of the adhesive film  13  takes place before the bonding of the sensors  1  to the substrate  4 . 
     In  FIG. 6   a , the adhesive film  13  is deposited on the scintillator  6 . In  FIG. 6   b , the sensors  1  are bonded to the scintillator  6  by means of the film  13 . In  FIG. 6   c , the assembly formed by the sensors  1  and the scintillator  6  is bonded to the substrate  4  by means of the adhesive film  25 . In  FIG. 6   d , the interstices  15  are filled by means of a liquid adhesive having a very low viscosity. 
     Of course, it is possible to replace the bonding of the sensors  1  using the film  25  by bonding using the adhesive  16 , as described in  FIGS. 3 and 4 . 
     The adhesive films  13  and  25  may be raw transparent adhesives available between two protective peel-off membranes. They may be transparent films coated on both their sides with adhesive layers, which are themselves also protected by protective membranes. They may also be hot-melt films, the adhesion of which is developed by a heat treatment, as will be described below. 
     Advantageously, the adhesive films  13  and  25  are not adherent at room temperature. This makes it easier to position the scintillator  6  and the substrate  4  relative to the sensors  1  before the actual bonding with the adhesive films  13  and  25  is carried out. The absence of room-temperature adhesion has another advantage in the case of a scintillator comprising gadolinium oxysulfide. This is because the needle structure of scintillators based on cesium iodide allows air trapped between the sensors  1  and the scintillator  6  to be removed when they are brought together. However, scintillators based on gadolinium oxysulfide use a binder of the plastic type that gives them a smooth surface that prevents the air trapped between the sensors  1  and the scintillator  6  from being removed, as in the case of a needle structure. Consequently, the absence of adhesion enables the air trapped between the sensors  1  and the scintillator  6  to be removed when the scintillator  6  is being positioned relative to the sensors  1  before the actual bonding using the adhesive film  13  is carried out. 
     Advantageously, the adhesive film  13  is a hot-melt film. In other words, the bonding by means of the adhesive film  13  is carried out by raising the temperature of the assembly formed by the sensor  1 , the adhesive film  13  and the scintillator  5  once they have been positioned relative to one another. 
     Advantageously, the adhesive film  13  comprises polyethylene or polypropylene or polyurethane or polyamide or polyvinyl butyral or ethylene/vinyl acetate. 
     Advantageously, the adhesive film  13  has a thickness between 20 and 40 μm. This thickness is less than that which is possible to achieve with means for using a known optical cement available in liquid or gel form. For example screen printing is used to deposit the cement as uniformly as possible. This means of implementation does not make it possible in practice to obtain a uniform layer of adhesive with a thickness of less than 40 μm, which thickness makes it possible for the scintillator optical coupling function to be achieved. In contrast, it is possible to obtain adhesive films of perfectly controlled thickness equal to 25 μm by lamination. 
     It will be readily seen by one of ordinary skill in the art that embodiments according to the present invention fulfill many of the advantages set forth above. After reading the foregoing specification, one of ordinary skill will be able to affect various changes, substitutions of equivalents and various other aspects of the invention as broadly disclosed herein. It is therefore intended that the protection granted hereon be limited only by the definition contained in the appended claims and equivalents thereof.