X or Gamma ray indirect image detector with fiber optic plate (FOP) stand-offs and method of assembly

Stand-offs are attached around the periphery of the fiber optic plate (FOP) to ensure a certain minimum thickness between the FOP and the imaging sensor to reduce shear stress and the risk of delamination due to shear stress in an X or Gamma ray detector. A coupling material fills the gap between the FOP and the imaging sensor.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to X or Gamma indirect image detectors that incorporate a fiber optic plate (FOP) to transfer visible light to the imaging sensor, and more particularly to a structure and method of assembling of the FOP to reduce shear stress and the possibility of localized delamination that would produce artifacts in the detected image.

Description of the Related Art

With indirect flat image detectors, the X or Gamma ray radiation penetrates through an object to be examined and encounters a scintillator layer that converts the X or Gamma ray radiation into visible light. If CCD or CMOS imaging sensors are used to detect the light, a fiber optic plate (FOP) is used as an intermediate layer. The FOP allows the converted visible light to pass through but blocks the X or Gamma radiation and thus protects the sensitive sensors. The FOP consists of many individual optical fibers aligned in parallel through which the light is guided. The FOP transfers an image from one end of the fiber to the other without any distortions.

The detector is assembled by forming a coupling layer of adhesive or coupling oil approximately 5 to 100 microns thick on the surface of the imaging sensor and then directly contacting the FOP to that coupling layer. The adhesive or coupling oil may contain small glass spheres that act as spacers. The spheres are intended to provide a defined space, following the macro non-flatness of the FOP and imaging surfaces. The adhesive or coupling oil are suitably index matched to reduce optical distortion and maintain the modulation transfer function (MTF). If the coupling layer is too thick, the optical losses will reduce the MTF.

The coupling layer serves to physically attach the FOP to the imager. This creates shear stress due to TCE (Thermal Coefficient of Expansion) mismatch between the FOP (5-8 ppm/C) and the imaging sensor (approximately 3 ppm/C). The shear stressses may cause localized delamination of the adhesive or coupling oil (air pockets) producing artifacts in the detected image. The coupling oil produces less shear stress but is disfavored as it may contaminate the entire detector and tends to dry out over times.

To maintain the detector's MTF, the thickness of the coupling layer is on the order of the surface flatness of the imaging sensor (e.g. the Silicon). The imaging sensor may have a specified peak-to-valley variation of, for example, 55 microns and the coupling layer may be nominally the same thickness. When the FOP is mounted onto the coupling layer, the layer may be very thin at certain points and the FOP may be in direct contact with the surface of the imaging sensor at its highest peaks. Direct contact of the FOP to the active pixels (imaging area) or amplifier circuitry can damage those components. These points have a higher stress, and thus higher risk of delamination as well. The use of the spherical balls as spacers is typically ineffective as they tend to cluster in the valleys on the surface of the imaging sensor and thus do not effectively space the FOP from the imaging sensor. These clusters also tend to inhibit flow of the adhesive or coupling oil and can produce optical artifacts. The glass spheres may also reduce product life time as they can cause point stresses in the detector.

SUMMARY OF THE INVENTION

The present invention provides for the use of stand-offs around the periphery of the fiber optic plate (FOP) to ensure a certain minimum thickness between the FOP and the imaging sensor to reduce shear stress and the risk of delamination due to shear stress in an X or Gamma ray detector.

This is accomplished by attaching multiple stand-offs around the periphery of the FOP. The stand-offs contact the non-imaging area of the imaging sensor to form a gap that ensures a certain minimum distance between the FOP and the surface of the imaging sensor. A coupling material fills the gap between the FOP and the imaging sensor.

In an embodiment, the stand-offs are attached to the FOP by first applying a film to the underside of the FOP to set the height of the gap. The thickness of the film is selected based on the surface flatness specification of the die that makeup the imaging sensor. The FOP is mounted onto a reference plate that is very flat. The stand-offs are attached around the periphery of the FOP in contact with the surface of the reference plate. The FOP is taken off of the reference plate and the film is removed. The stand-offs extend from the underside of the FOP at a precise and uniform distance set by the thickness of the film. This approach provides uniformity among detectors.

In an embodiment, the stand-offs are attached to the FOP by first applying a film to the underside of the FOP to set the height of the gap. The FOP is mounted onto the imaging sensor. The stand-offs are attached around the periphery of the FOP in contact with non-imaging areas on the surface of the imaging sensor. The FOP is taken off of the imaging sensor and the film is removed. The stand-offs extend from the underside of the FOP by an amount determined by the local “peaks” on the surface of the imaging sensor plus the thickness of the film. The thickness of the film is selected to provide the certain minimum distance between the peaks and the FOP. This approach tailors the stand-offs for each detector.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a structure and method of assembly to ensure a certain minimum thickness of coupling material between the FOP and the imaging sensor to reduce shear stress and the risk of delamination due to shear stress in an X or Gamma ray detector, and does so without introducing spacers or other structure in the optical path that might produce artifacts in the detected image.

The approach is applicable to X or Gamma ray imaging detectors in which an FOP (and conversion layer) is mounted on a visible-band imaging sensor. The conversion layer may be formed as a coating on the topside of the FOP or as a separate “scintillator” that mounted to the FOP. The imaging sensor may be a single or multi-tile configuration in which each tile includes one or more CMOS or CCD die configured to detect visible light. The detector core may be configured with the imaging sensor mounted on the substrate (e.g. PCB, ceramic, stainless steel substrate), mounted side-by-side with a PCB on a tile carrier, or connected via a flex cable to external electronics. The detector core may be housed in a sealed package or an open air package.

Referring now toFIGS. 1athrough 1c, an embodiment of a detector core10comprises an imaging sensor12mounted side-by-side with a PCB14on a tile carrier16. Imaging sensor12includes one or more CMOS or CCD die18, each die having an imaging area configured to detect visible light and a non-imaging area. Together the dies provide an active imaging area19for the image sensor. Tile carrier16is stepped to accommodate the different thicknesses of the imaging sensor and PCB so that the two components are approximately co-planar. If the imaging sensor and PCB have the same thickness the carrier does not need to be stepped. Wire bonds20are formed between electrical contact pads on the non-imaging area of the die and the PCB.

A FOP22and conversion layer24are mounted over and optically coupled to the imaging area of imaging sensor12using a coupling material26. Conversion layer24may be a coating formed on the topside of FOP22or a separate optical component. Coupling material26is suitably an index-matched adhesive (UV, thermal or anerobically cured) or a coupling oil. The coupling material is devoid of any spacer material.

The thickness of coupling material26is nominally equal to the surface flatness specification28of the die plus a certain minimum thickness30between the FOP22and the surface of the imaging sensor12(i.e. peaks32on the surface of the die). Surface flatness is specified as the maximum peak-to-valley variation on the surface of the die. Typically, CCD die may have a surface flatness of between 50 and 100 microns. By comparison the surface of the FOP is very flat having a surface flatness of approximately 3-7 microns. The certain minimum thickness30may range from 10 to 100 microns to reduce localized sheer stress and the risk of delamination. If the coupling material is too thick the detector's MTF may be degraded. The stand-offs also provide the capability to accurately set the gap and thereby adjust the MTF.

To ensure that the coupling material26does provide the certain minimum thickness, a plurality of stand-offs34are attached about the periphery36of FOP22extending out from a bottom surface of the FOP outside the imaging area of the FOP. Stand-offs34contact the non-imaging area of the one or more die to form a gap36between the FOP and the imaging sensor that is filled by coupling material26. The stand-offs may or may not be specifically attached to the die. The coupling material tends to adhere the FOP to the imaging sensor. The stand-offs may be formed from glass, silicon, or cyanoacrylates or any other sufficiently rigid material. The number of stand-offs depends on the aspect ratio of the FOP and the size of the stand-offs.

Referring now toFIGS. 2athrough 2d, different embodiments for attaching the stand-offs to the FOP use a reference plate to provide uniformity over a class of detectors or use the actual imaging sensor in-situ to tailor the stand-offs to that sensor.

As shown inFIGS. 2aand 2b, an adhesive film50is attached to the underside of a FOP52and placed on the surface of a reference plate54. The reference plate has a surface flatness of a few microns, essentially flat as shown in the inset illustration. Adhesive film50is selected with a nominal thickness equal to the surface flatness of the die plus the certain minimum thickness. For example, if the dies have a surface flatness of 80 microns and the minimum thickness is 20 microns the film would have a 100 micron thickness. A plurality of stand-offs56are attached around the periphery58of FOP52in contact with the surface of reference plate54. The reference plate54and then the adhesive film50are removed, leaving the FOP52with stand-offs56around the periphery that extend out from the backside of the FOP by a precisely controlled amount.

As shown inFIGS. 2cand 2d, an adhesive film60is attached to the underside of a FOP62and placed on the surface of an imaging sensor64that will be packaged together with the FOP. The imaging sensor has a surface flatness of tens of microns as best shown in the inset illustration. Adhesive film60is selected with a nominal thickness equal to the certain minimum thickness. A plurality of stand-offs66are attached around the periphery68of FOP62in contact with the non-imaging area on the surface of imaging sensor64. The imaging sensor64and then the adhesive film60are removed, leaving the FOP62with stand-offs66around the periphery that extend out from the backside of the FOP by a precisely controlled amount nominally equal to the surface flatness of the imaging sensor plus the certain minimum thickness.

Referring now toFIGS. 3, 4aand4b, an embodiment of an X-ray imaging detector100and a method of assembly are depicted. This embodiment is directed to a multi-tile configuration of CMOS imaging die for X-ray detection. The architecture of the detector, method of assembly and in particular the configuration of the glass cap and method of assembly for protecting the wire bonds is applicable to single-tile configurations, CCD image die and for Gamma ray detection.

Assembly of the imaging detector100starts at the Wafer level with the performance of wafer-level testing of the individual CMOS dies (step102). The individual dies are sawed from the wafer (step106). Each die has a non-imaging area that includes electrical contact pads and an imaging area configured to detect visible light. In an embodiment, the individual die may be 100 mm×100 mm. The CMOS dies may be fabricated using Silicon or InGaAs technology.

Tile-level assembly of each tile108comprises attaching one or more of the individual die that together form an image sensor124to a tile carrier126(step128). A tile PCB130is attached to tile carrier126adjacent image sensor124(step132) forming a trench there between. Tile PCB130has a plurality of electrical contact pads that are electrically connected to read out or other processing circuitry for processing the detected image. Wire bonds138are formed from the die contact pads to the PCB contact pads spanning trench (step139). The surfaces of the image sensor124and tile PCB130are preferably substantially co-planar. Often the tile PCB is thicker than the image sensor in which case the tile carrier126would be “stepped” such that the image sensor and tile PCB are coplanar. The tile carrier may, for example, be formed of a Silicon, InGaAs, Kovar or stainless steel material. FOP-level assembly comprises attaching stand-offs140around the periphery of a FOP142(step144). Optionally, a stiffening layer may be formed over the conversion layer145on FOP142(step146). In a multi-tile configuration the span of the FOP may be large enough that sagging could be a problem. The stiffening layer maintains the overall flatness of the FOP until it can be attached to the imaging sensor and supported by the coupling material.

Detector-level assembly comprises attaching one or more tiles108to a detector carrier148(step150). As shown, the exposed backsides of the imaging sensors that extend laterally from the tile carriers are mounted on the top surface of detector carrier148to form a multi-tile array in which the tile PCBs are arranged around the periphery of that array. The FOP142is aligned and attached to the imaging side of the imaging sensors (step152). Optionally, the stand-offs140may be used as an alignment feature to align the FOP to the imaging sensors. A coupling material154is applied between the FOP and tiles to fill the gap (step155). The coupling material may be a coupling oil or adhesive that can be injected via capillary action between the FOP and tiles. Alternately, the oil or adhesive may first be applied to one or both of the FOP and tiles and then compressed together until the stand-offs contact the non-imaging areas of the imaging sensor. If an adhesive is used, the adhesive is cured (UV, thermal or anerobic) (step156). If a stiffening layer was used it is removed (step158). This forms a core detector162.

The core detector162and a camera PCB164are suitably mounted to opposite sides of a base plate166(step169). This assembly is mounted in a detector housing168(step170). Alternately the core detector and camera PCB may be directly mounted to the detector housing. The camera PCB164is electrically connected to the one or more tile PCBs by, for example, flex connectors171and an external connector172(step174). A lid176configured to allow transmission of X rays (e.g., a carbon based material) is attached to the detector housing168(step177) to complete X-ray imaging detector100. Final X ray testing is performed on detector100(step178).