Patent Publication Number: US-9846246-B2

Title: Method and apparatus with tiled image sensors

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. Ser. No. 14/828,772, filed on Aug. 18, 2015, entitled “METHOD AND APPARATUS WITH TILED IMAGE SENSORS”, in the names of Bradley S. Jadrich et al., which is incorporated herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention generally relates to the field of medical radiographic imaging systems and, in particular, to digital radiography (DR) X-ray detectors sometimes referred to as flat panel detectors (FPD), and methods of making flat panel detectors. 
     Solid-state, ionizing radiation based detectors used in projection radiography typically require direct or indirect conversion image sensors. Direct conversion image sensors, such as made using selenium, directly capture X-rays in a photo-conductive material to produce electrical signals in an array of pixels. Indirect sensors such as made using amorphous silicon (a-Si), and complementary metal oxide semiconductor (CMOS), use a scintillating material to convert X-rays to visible light in the pixel array. To fabricate large area flat panel detectors, smaller planar, rectangular sensor arrays may be tiled together in an M×N two dimensional arrangement to form larger panels. Thus, accurate alignment and assembly of such smaller image sensor tiles may be desired for particular imaging applications. Embodiments of the presently disclosed invention are intended to provide simple and superior methods to advantageously assemble a plurality of image sensor tiles. 
     The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     BRIEF DESCRIPTION OF THE INVENTION 
     An array of sensor tiles may be attached to a substrate using a compliant film that includes an adhesive. A thickness of the compliant film varies depending on a thickness of the sensor tiles so that outward facing sides of the sensor tiles are coplanar. 
     In one embodiment, an imaging device may have an array of sensor tiles attached to a substrate on a bottom side of the sensor tiles. A sheet is disposed between the array of sensor tiles and the substrate, the sheet being made from a compressible, compliant material. An adhesive is also disposed between the array of sensor tiles and the substrate on both sides of the sheet. 
     In another embodiment, a method of fabricating a tiled sensor array includes providing a substantially flat surface, aligning a plurality of sensor tiles using the flat surface, placing a compliant film on a substrate using an adhesive therebetween, pressing the compliant film against the back sides of the plurality of sensor tiles including an adhesive therebetween, and removing the flat surface to release the tiled sensor array and substrate having the compliant film adhered therebetween. 
     In another embodiment, an apparatus includes an array of photosensitive tiles and a substrate facing a bottom side of the tiles. A compliant film is placed between the array and the substrate, wherein the film includes adhesive. A thickness of the compliant film is different between the substrate and a first one of the tiles as compared to the film between the substrate and a second one of the tiles. The top sides of the sensor tiles are coplanar. 
     The summary descriptions above are not meant to describe individual separate embodiments whose elements are not interchangeable. In fact, many of the elements described as related to a particular embodiment can be used together with, and possibly interchanged with, elements of other described embodiments. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. The drawings below are intended to be drawn neither to any precise scale with respect to relative size, angular relationship, relative position, or timing relationship, nor to any combinational relationship with respect to interchangeability, substitution, or representation of a required implementation. 
     This brief description of the invention is intended only to provide a brief overview of subject matter disclosed herein according to one or more illustrative embodiments, and does not serve as a guide to interpreting the claims or to define or limit the scope of the invention, which is defined only by the appended claims. This brief description is provided to introduce an illustrative selection of concepts in a simplified form that are further described below in the detailed description. This brief description is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the features of the invention can be understood, a detailed description of the invention may be had by reference to certain embodiments, some of which are illustrated in the accompanying drawings. It is to be noted, however, that the drawings illustrate only certain embodiments of this invention and are therefore not to be considered limiting of its scope, for the scope of the invention encompasses other equally effective embodiments. The drawings are not necessarily to scale, emphasis generally being placed upon illustrating the features of certain embodiments of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views. Thus, for further understanding of the invention, reference can be made to the following detailed description, read in connection with the drawings in which: 
         FIG. 1  is a schematic diagram showing an exemplary radiographic imaging system using a DR detector; 
         FIG. 2  is a diagram of an exemplary M×N two dimensional arrangement of a plurality of die with alignment orientation; 
         FIG. 3A  is a side view showing a plurality of die in an exemplary tiled image sensor assembly; 
         FIG. 3B  is a top view showing the plurality of die in the exemplary tiled image sensor assembly of  FIG. 3A ; 
         FIG. 4  is a diagram of another exemplary M×N two dimensional arrangement of a plurality of die with alignment orientation; 
         FIG. 5  is an exploded isometric view of an exemplary tiled image sensor assembly using two die in an M×N two dimensional arrangement with glass flat and alignment markers; 
         FIG. 6  is a flow chart of an exemplary method of making the tiled image sensor assembly of  FIGS. 3A-3B  and  FIG. 5 ; 
         FIG. 7  is a side view showing a step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 8  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 9  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 10  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 11  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 12  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 13  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; 
         FIG. 14  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly; and 
         FIG. 15  is a side view showing another step of the exemplary method of making the exemplary tiled image sensor assembly. 
         FIGS. 16A-16B  are perspective views illustrating a pressure direction applied to the sensor tiles and a representative deflection in the sensor tiles caused thereby. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A schematic diagram of one exemplary projection X-ray system  100  is shown in  FIG. 1 . The system  100  may be a stationary fixed exam room system or a mobile X-ray imaging system. The X-ray source  112  and the DR detector  114  may be part of a rotatable system such as a gantry driven system, a C-arm, or table system, for example. The detector  114  is positioned diametrically opposite the x-ray source  112  and an object  110  under examination is positioned therebetween, whereby x-rays  104  pass through the object  110  and are detected by a two-dimensional array of imaging elements, or pixels, in the detector  114 . Rotation of the source  112  and detector  114  components in either direction indicated by arrow  102 , while maintaining the object  110  at a rotational axis of the source  112  and detector  114 , may be used to enable cone beam computed tomography (CBCT) and 3-D image reconstruction applications, such as in medical and dental applications, in the x-ray imaging system  100 . The x-ray generator  116  causes the x-ray source  112  to fire one or a sequence of pulses of x-ray radiation  104 , which firing may be controlled and synchronized with activation of the detector  114  using detector control circuits  118 . Operation and control of the x-ray system  100  components just described may be centralized in a computer system  106 . The exemplary DR detector  114  and the image sensor assembly  115  therein, as shown in  FIG. 1 , will be described in more detail herein. The orientation of x-ray system  100  components as illustrate in  FIG. 1  may be varied. The object  110  under examination may be a human or animal patient, or another object, and may be lying on an examination table, standing, sitting upright, or positioned with respect to the source  112  and detector  114  in some other suitable orientation. 
       FIG. 2  is a diagram of an M×N array  200  of image sensor tiles  201 - 204 , as used in one or more disclosed embodiments of the image sensor assembly  115  ( FIG. 1 ) of the present invention. Each of the sensor tiles  201 - 204  is substantially planar and generally rectangular in shape as illustrated herein. Each of the sensor tiles  201 - 204  may be said to have a major surface referred to herein as a top side, or sensor side, and a major surface opposite the top side referred to herein as a bottom side, or back side. In the example embodiment of  FIG. 2 , M=2 and N=2, although other embodiments are possible, and are considered as alternative embodiments herein, whereby M and N are counting numbers (i.e., positive integers 1, 2, 3 . . . ) having the same or different value, wherein at least one of M or N must be greater than 1. The sensor tiles  201 - 204  may also be referred to herein individually as “a die” or in plural as “die”, since the sensor die are typically precision cut or “diced” from a larger piece of thin-film transistor (TFT) glass or silicon wafer. The plurality of die need to be aligned to each other in the X, Y, and Theta-Z (θ z ) axes as indicated by the arrows  208  in  FIG. 2 . The gap indicated by arrows  206  between adjacent die is intended to be minimized as the die are finally positioned, and may comprise a distance of about half the width (or length) of a pixel, or less, wherein a pixel width (or length) may be defined and measured by reference to one or more pixels formed on one or all of the die  201 - 204 , such as by reference to a mean pixel width (or length), a designed width (or length) of the pixels, or a mean dimension of the pixels which may be a mean length or width. The alignment is accomplished in such a way as to also minimize the number of “dead zone” pixels between die that may otherwise appear in a radiographic image captured by the detector  114 . An integer number of dead zone pixels between die is desired, with a one (1) pixel width dead zone being preferred. Such alignment between the die may be typically controlled to a tolerance of about 1/10 th  (0.1) pixel. Alignment to this precision may be necessary to produce artifact-free projection radiographic images. 
     Electrical contacts or bond pads  210  on the die  201 - 204  are disposed adjacent to one edge of each die  201 - 204 . This enables what is known to those skilled in the art as a 3-side buttable configuration. While an arrangement of four die are shown in  FIG. 2 , the present invention is not limited to four die, as additional die can be aligned in the X-direction, maintaining the 3-side buttable configuration. 
       FIGS. 3A and 3B  show side and top views, respectively, of one embodiment of an image sensor assembly  115  of the present invention. After alignment of the image sensor array  200  ( FIG. 2 ), a rigid substrate  302  is mated to the M×N arrangement of die  200  using a compliant layer, support, sheet, or film  306 . Printed circuit boards (PCBs)  308 ,  309 , with analog and/or digital detector electronics can be added to the substrate  302  using adhesive  508 ,  509  ( FIG. 5 ), respectively, and electrically connected to the image sensor arrangement  200  as an integral part of the image sensor assembly  115 . Electrical contacts between the image sensor tiles  201 - 204  and the PCBs  308 ,  309  may include wire bonding  310 , anisotropic conductive film (ACF) bonding through flex circuits, or other suitable electrical contact means and methods. The major surfaces of the die  201 - 204  facing outward from the page of  FIG. 3B  are the top sides, or top surfaces, of the sensor tiles  201 - 204 , which face away from the substrate  302 . The top sides, or sensor sides, are generally positioned by an operator of the imaging system  100  to face the x-ray source during examinations using the DR detector  114 . The top surfaces  311 ,  312 ,  313 , and  314  of the die  201 ,  202 ,  203 , and  204 , respectively, are illustrated in the side and top views of  FIGS. 3A and 3B . 
     For simplicity and ease of understanding of the details of the present invention, the two die  201 ,  202 , M×N configuration  400  as shown in  FIG. 4 , where M=1 and N=2, will be described herein as an exemplary embodiment used in making the image sensor assembly  115 . Other M×N configurations of various sizes are considered to be within the scope of the claims appended hereto and of the following description. 
       FIG. 5  shows an exploded isometric view of the components used in the present invention as described above. Also shown in  FIG. 5  is a glass flat  502  used during the die alignment process as described herein. The glass flat  502  is a planar, rigid device having a plurality of alignment markers  504  (six markers in the example of  FIG. 5 ) on its flat, top surface which faces the sensor side of each die  201 ,  202 . The glass flat  502  also includes a plurality of holes  506  formed therethrough corresponding to each of the die. In one embodiment, one of the holes  506  corresponds to each of the die  201 ,  202 . As described herein, the holes  506  are used to secure the die  201 ,  202 , to the glass flat using a vacuum (suction) source in communication with the holes  506  so that the holes  506  act as channels for the vacuum source. The compliant film  306  may have perforations  307  formed therethrough corresponding to a pattern  309  of adhesive  1108  ( FIG. 11 ) applied therein, as will be explained below. 
       FIG. 6  is a flow chart of an exemplary method of making a plurality of embodiments of the present invention. The steps illustrated in  FIG. 6  will be explained in more detail with reference to  FIGS. 7-15 . Steps  602 - 606  will now be described with respect to  FIG. 7 . At step  602 , a die  201  is selected and placed upon the glass flat  502  alignment fixture having alignment markings  504  thereon to align the die  201  thereto. A protective layer (not shown) may be applied to at least the sensor side of the die  201  prior to or during the procedure described herein, which may be removed later as desired. 
     In this example, to place the die  201  on the glass flat  502 , the die  201  is first held against a transfer plate  702  which has at least one hole  703  formed therethrough, whereby a vacuum source  710  is applied to the hole  703  to hold the die  201  against the plate  702  while moving and positioning the die  201  above selected ones of the alignment markers  504  on the glass flat  502 . A vacuum (suction) source  710  is in communication with the hole  703  so that the hole  703  may act as a channel for the vacuum. Alternatively, a clamp may be used in place of the transfer plate+vacuum source. A mechanism (not shown) may be attached to the transfer plate  702  to facilitate movement and positioning of the die  201 . At step  604 , a vision camera  706  and optics  704  may be used to image features of the die  201  (such as an edge of the die  201 ) and selected alignment markers  504  through the glass flat  502 . Thus, the glass flat  502  is advantageously made from a suitably transparent material sufficient to allow use of the camera  706  for viewing therethrough. The optics  704  may be adjustable to allow adequate focusing of either or both the die  201  and selected alignment markers  504 . At step  606 , alignment of the die  201  in the X, Y, θ z , directions is performed. 
     After a final alignment of the die  201  is achieved, with reference to  FIG. 8  and step  608 , a vacuum source  810  is applied to the bottom of the hole  506  in the glass flat  502  to engage the die  201  against the top surface of the glass flat  502 . Vacuum  710  to the transfer plate  702  is then removed to disengage the transfer plate  702  from the die  201 , leaving the die  201  secured against glass flat  502 . At decision step  610 , with reference to  FIGS. 9-10 , a second die  202  is then aligned adjacent to, or abutting, as desired, the first die  201  using selected alignment markers  504  on the glass flat  502 . The second die  202  is aligned using the same process steps  602 - 608  as used to align the first die  201 , described above. After alignment of the second die  202 , returning to decision step  610 , with reference to  FIGS. 9-10 , third and/or additional die, as desired, may then be aligned adjacent to, or abutting, as desired, the first or second die  201 ,  202 , using selected alignment markers  504  on the glass flat  502  to form a completed aligned arrangement of die  1000  on the glass flat  502 . The third and/or additional die may be aligned using the same process steps  602 - 608  as used to align the first and second die  201 ,  202 . Because of die-to-die variations primarily due to thickness, runout, and wedge, the back sides (i.e., the surfaces facing away from the glass flat  502 ) of both die  201 ,  202  and any additional die, may not be co-planar. As an example, a 200 mm diameter silicon wafer that is diced to fabricate the die  201 ,  202  illustrated herein, may have thickness and flatness variations across its diameter as large as 0.050 mm. It is therefore desired that attachment of a substrate, as will be explained herein, to the plurality of die  201 ,  202  allow for these variations in thickness by using the compliant film  306  as described herein. 
     After the desired number of die  201 ,  202  are aligned to the glass flat  502  as described herein, decision step  610  may be followed by steps  612  and  620 , with reference to  FIG. 11 , wherein a compliant film  306 , having a plurality of perforations  307  formed therethrough, is positioned on one surface of the substrate  302 . The surface  1104  of the complaint film  306  that faces the surface of the substrate  302  may be fabricated to have, or may be later treated to provide, a higher coefficient of friction than its opposite side  1106  to facilitate a tackier engagement with the surface of the substrate  302 . The opposite surface  1106  of the compliant film  306  that faces away from the substrate  302  may either have a lower tack for better repositioning capability against the dies  201 ,  202  ( FIG. 12 ) or no tack at all. At step  620 , a viscous, high tack, adhesive  1108  is dispensed into the perforations  307  in the compliant film  306  using a dispenser  1110  that places the adhesive  1108  at least into the perforations  307  so that the adhesive  1108  makes contact at least with the surface of the substrate  302 , as shown in  FIG. 11 . The adhesive  1108  may be a UV, thermal, or room temperature curing adhesive, as desired. In one embodiment, the adhesive  1108  is a UV curing adhesive having a fast cure time and also minimizes strain and stress between the substrate and die during the curing operation ( FIG. 12 ). In one embodiment, a compliant film  306  may be selected that has significantly lower stiffness and elastic modulus compared with the viscous adhesive  1108  after the adhesive is UV cured. One exemplary compliant film is manufactured and sold by the 3M company of St. Paul, Minn. under the name of VHB4914, which is 0.100 mm thick with elastic modulus E f =0.6 MPa. Compliant films of greater thickness can be used as well, allowing for greater die-to-die flatness or thickness variation. An exemplary viscous adhesive  1108  is manufactured and sold by Dymax Corporation of Torrington, Conn., under the name of OP-61 which is a UV cure type adhesive with cured elastic modulus Ev=16,000 MPa and viscosity at 160,000 cP. This particular adhesive has low shrinkage at 0.4%, as well as low coefficient of thermal expansion at 43 parts per million per ° C. The compliant film  306  and adhesive  1108  properties cited herein are adequate to achieve thermal and long term stability of the tiled image sensor assembly  115 . 
     With reference to  FIG. 12  and step  622 , the substrate  302  and the compliant film  306 , having the viscous adhesive  1108  dispensed therein, are inverted and pressed against the aligned arrangement of die  1000 , wherein the adhesive  1108  in the perforations  307  contacts the back sides of the die  201 ,  202 . In one embodiment, using a curing adhesive, a curing process may be performed, at step  624 . The substrate may be optically transparent to UV light in this embodiment. In one embodiment, a UV curing adhesive may be used and a curing source  1202 , such as an ultraviolet (UV) light, may be used in a curing process at step  624 . In one embodiment, a different adhesive may be used wherein no separate curing step is required. After adhesion of the substrate  302  to the aligned arrangement of die  1000  is complete, the vacuum source  810  applied to the holes  506  is turned off and, with reference to  FIG. 13  and step  626 , the tiled sensors  1300  are removed from the glass flat  502 . At this point, the die  201 ,  202 , top surfaces  311 ,  312 , respectively, are coplanar, which is particularly critical at the tile seam  1302  so that adequate image sharpness and uniformity is achieved. The compliant film  306  is compressed to a smaller thickness  1304  between the substrate  302  and the die  201  as compared to its thickness between the substrate  302  and the die  202 . This is due to the variation in thickness of the die  201 ,  202  (the die  201  is thicker) when the die  201 ,  202 , are pressed against the compliant film  306 . 
     After the desired number of die  201 ,  202  are aligned to the glass flat  502  as described herein, decision step  610  may alternatively be followed by steps  612  and  616 , with reference to  FIG. 11 , wherein a compliant film  306 , having a plurality of perforations  307  formed therethrough, is positioned on and adhered to one surface of the substrate  302 . Although not shown, an alternative compliant film without perforations may be positioned and adhered on one surface of the substrate  302 . The compliant film  306  with or without perforations may be coated, treated, layered, impregnated, with an adhesive on one or both its major surfaces. Alternatively, the compliant film  306  may not be used in the method described herein and only an adhesive may be placed between the substrate and die  201 ,  202 . The adhesive described herein may be a UV, thermal, or room temperature curing adhesive, as desired. In one embodiment, the adhesive is a UV curing adhesive having a fast cure time and also minimizes strain and stress between the substrate and die during the curing operation ( FIG. 12 ). 
     With reference to  FIG. 12  and step  618 , the substrate  302  and the compliant film  306 , with or without perforations, having adhesive thereon, as described herein, are inverted and pressed against the aligned arrangement of die  1000 . In one embodiment using a curing adhesive, the adhesive compliant film  306  contacts the back sides of the die  201 ,  202 , and a curing process may be performed. In one embodiment, a UV curing adhesive may be used and a curing source  1202 , such as an ultraviolet (UV) light, may be used in a curing process. In one embodiment, a different adhesive may be used wherein no separate curing step is required. After adhesion of the substrate  302  to the aligned arrangement of die  1000  is complete, the vacuum source  810  applied to the holes  506  may be turned off and, with reference to  FIG. 13  and step  626 , the tiled sensors  1300  are removed from the glass flat  502 . At this point, the die  201 ,  202 , top surfaces  311 ,  312 , respectively, are coplanar, which is particularly critical at the tile seam  1302  so that adequate image sharpness and uniformity is achieved. The compliant film  306 , with or without perforations, is compressed to a smaller thickness  1304  between the substrate  302  and the die  201  as compared to its thickness between the substrate  302  and the die  202 . This is due to the variation in thickness of the die  201 ,  202  (the die  201  is thicker) when the die  201 ,  202 , are pressed against the compliant film  306 . 
     After the step  626  is complete, with reference to  FIG. 14 , respective PCBs  308 ,  309 , can be attached to the substrate  302  using adhesives  508 ,  509 , respectively, pressure sensitive adhesive (PSA), fasteners, or other means. With reference to  FIG. 15 , electrical connection between the PCBs  308 ,  309 , and the die  201 ,  202  may be formed using wire bonds  310 , or other means as described herein. A scintillator  1506  for use as an x-ray wavelength converter, in embodiments using indirect conversion image sensors, may be applied to the top surfaces of the tiled sensors,  201 ,  202 , to form the image sensor assembly  115 . Application of the scintillator  1506  to the tiled sensors  201 ,  202 , can be performed via pressure, optical coupling adhesive (OCA) attachment, or similar optical coupling gels or adhesives. 
     Finite Element Analysis (FEA) simulation was performed to measure the effect of attaching a scintillator against the tiled sensor assembly, with and without the compliant film  306 . For this simulation, uniform pressure  1601  of 0.5 MPa was applied on top of both die  201 ,  202 .  FIG. 16A  shows a representation of the measured peak-to-peak deformation  1605  of the die  201 ,  202 , in the Z-direction  1603 , i.e., deviation from a planar shape in the x-y plane, when only a viscous adhesive is adhered between the die  201  and substrate  302 .  FIG. 16B  shows a representation of the measured deformation  1607  of the die  201 ,  202 , in the Z-direction  1603  when both a viscous adhesive  1108  and compliant film  306  are adhered between the die  201 ,  202 , and substrate  302 . Maximum deformation  1605  for the case with only the viscous adhesive  1108  was measured as 0.325 mm, and the deformation  1607  for the case with both the viscous adhesive  1108  and the compliant film  306  was measured at 0.015 mm. Thus, the deformation is significantly reduced, by over 20×, when using the compliant film  306 . This will result in superior scintillator attachment and imaging performance. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.