Abstract:
This invention describes an imaging system based on an array of semiconductor photosensitive elements with isolating structure between elements (pixels) of the array. The isolated pixels of the array may be photodiodes and they provide excellent imaging capabilities that are important for many applications. The isolated photosensitive pixels may be comprised also by photoconductors, avalanche photodiodes, photosensitive IC, or other similar solid-state devices. The fields of possible application include but are not limited to the detector modules for homeland security, medical imaging systems (CT, SPECT, and PET including), fundamental and applied research, etc.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 61/057,603 filed May 30, 2008. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present application relates to an imaging system in which radiation is received and then converted into electron-hole pairs by radiation-sensitive semiconductor detectors. 
         [0004]    2. Prior Art 
         [0005]    In many imaging applications, 1D or 2D detector arrays are used. In the case of 2D detectors the term “slice” is used to describe a detector row in the direction perpendicular to the scanned object (z-axis). The invention may be useful for medical imaging systems, such as CT, SPECT, PET scanners, and x-ray fluorography, as well as for other applications like baggage inspection and similar security systems. 
         [0006]    As an example, a multi-slice x-ray detector may be composed of a series of individual pixels in the z-axis (slices) and a series of individual pixels in the x-axis (called channels). Each individual pixel may be composed of a scintillating crystal coupled to a primary photodetector. See  FIG. 1  as an example, in which the photodiode array is used as a primary photodetector. The x-ray deposited in the scintillator pixel is converted to visible light. The light quanta generated in the scintillator are then propagated up to the surface of the primary photodetector where they are absorbed and converted to electron-hole charge. 
         [0007]    In a different application a uniform (or quasi-uniform) scintillator material is deposited on an array of primary semiconductor photodetectors ( FIG. 2 ). The uniform (quasi-uniform) scintillator material may be a film, a polycrystalline structure, or other micro-crystalline material. The scintillator material is not specified herein. 
         [0008]    The electrical signal of each primary semiconductor photodetector pixel is individually routed to a corresponding pre-amplifier channel. The pre-amplifiers (or other readout electronics) may be attached to the primary photodetector array either directly ( FIG. 3 ) or via the substrate with the re-routed signals from each individual pixel ( FIG. 4 ). The further connection to the data acquisition system is made in one of the common for the industry ways. 
         [0009]    Many imaging applications, including CT, SPECT, and PET scanners require clear separation of signals between adjacent pixels. However, the primary photodetector arrays used in the contemporary imaging systems do not provide good isolation between pixels. The electrical and optical crosstalk is imminent in such systems, which significantly deteriorate the image quality. To handle this problem, sophisticated filtering has to be applied, which could not solve the problem completely anyway. See U.S. Pat. Nos. 6,426,991, 6,510,195, 6,760,404, 7,003,076 and 7,439,516. 
         [0010]    Recently, the light-sensitive primary photodetector (photodiode) array having isolated pixels was described in U.S. Pat. Nos. 6,762,473 and 7,112,465, U.S. Patent Application Publication No. 2005/0221541 and U.S. patent application Nos. 11,368,041, 11/636,026, 11/811,121, 11/786,385 and 12/188,829 the disclosures of which are hereby incorporated herein by reference. 
         [0011]    Such primary photodetector arrays with isolated pixels are characterized with a very low electrical crosstalk between adjacent pixels. The optical crosstalk is also reduced significantly in imaging systems with such isolated pixel arrays. As a result, a less noisy and higher quality image can be obtained using imaging systems with isolated primary photodetector pixels. Moreover, less noisy signals require less exposure time and consequently less total radiation dose to a subject to obtain an image of the same quality. These findings are especially important for medical imaging applications. These topics were discussed thoroughly in a series of our recent publications. See “Silicon PIN Photodiode Array for Medical Imaging Applications: Structure, Optical Properties and Temperature Coefficients” (Goushcha et al., IEEE Nuclear Science Symposium Conference Record, 2005), “Temperature coefficients and noise performance and studies for the back-illuminated arrays for medical imaging applications” (Goushcha et al., Proceedings of SPIE,  2006 , Vol. 6142) and “Optical and Electrical Crosstalk in PIN Photodiode Array for Medical Imaging Applications” (Goushcha et al., “IEEE Nuclear Science Symposium Conference Record, 2007), the disclosures of which are hereby incorporated herein by reference. 
         [0012]    The present invention contemplates an apparatus and method to incorporate a primary photodetector with isolated pixels in imaging systems and their subcomponents, including CT, PET, and SPECT scanners. The invention allows overcoming many of the above listed problems and to build imaging systems with less noise, higher imaging quality, and potentially lower exposure dose. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  (prior art), schematically illustrates a cross section of a conventional detector array consisting of the array of back-illuminated primary photodetectors (e.g. photodiodes) and attached pixilated scintillator array. 
           [0014]      FIG. 2  (prior art) schematically illustrates a cross section of a conventional detector array consisting of the array of back-illuminated primary photodetectors (e.g. photodiodes) and attached or deposited uniform (or quasi-uniform) scintillator material  202 . 
           [0015]      FIG. 3  (prior art) illustrates that the electrical signal of each primary photodetector pixel is individually routed to a corresponding pre-amplifier channel on a readout electronics chip. 
           [0016]      FIG. 4  (prior art) schematically illustrates that each pixel of the primary photodetector array is attached to the pre-amplifier and other readout electronics chip  420  via the substrate  410 , which uses re-routed signals from the primary photodetector array. 
           [0017]      FIG. 5  schematically illustrates a main concept of the present invention, in which the primary photodetector of the imaging system is the array of isolated photosensitive pixels. 
           [0018]      FIG. 6  schematically illustrates a radiation detector in which the scintillator material of the imaging system is the uniform (or quasi-uniform) scintillator material. 
           [0019]      FIG. 7  schematically illustrates a direct conversion imaging detector, in which the detector of the imaging system does not have a scintillator attached to the isolated pixels primary photodetector array  520 . 
           [0020]      FIG. 8  schematically illustrates another embodiment of the present invention, in which the electrical signal from each isolated pixel of the primary photodetector is individually routed to a corresponding pre-amplifier channel on a readout electronics chip. 
           [0021]      FIG. 9  schematically illustrates another embodiment, in which each isolated pixel of the primary photodetector array is attached to the pre-amplifier and other readout electronics chip or substrate (layer) via the substrate (layer), which uses individual re-routed signals from the isolated pixels of the primary photodetector array. 
           [0022]      FIG. 10  shows schematically another embodiment, in which a switch for selecting a photosensitive element of the array and a data acquisition chip are shown as parts of either or both substrates (layers) attached to the primary photodetector array. 
           [0023]      FIG. 11  schematically illustrates another embodiment, in which the substrate attached to the primary photodetector array consists of the two substrate layers, the first disposed parallel to the photodetector array and the second disposed perpendicular to and in support of the first substrate layer. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0024]    In one embodiment of the invention, the imaging system for medical imaging or other applications includes a radiation sensitive detector with a pixellated scintillator array optically coupled to the isolated pixels semiconductor photo-sensitive device (primary photodetector). A plurality of isolated pixels of the semiconductor photodetector array is connected to the readout electronics either by directly contacting the pre-amplifiers or via routings through the support substrate. The connection to the readout electronics may be provided on either side of the isolated pixels primary photodetector array. 
         [0025]    In accordance with another embodiment, the isolated pixels of the primary photodetector are connected individually to the first polarity electrodes and further to the readout electronics of the imaging system. The isolation area between pixels may be connected to the opposite polarity electrode of the readout electronics. 
         [0026]    In accordance with another embodiment of the present invention, the isolation between the pixels of semiconductor devices of the imaging system is made by the matrix of through diffusions of the same polarity dopants as the substrate. The diffusions can penetrate through the whole thickness of the primary photodetector array. The through diffusion areas may not necessarily be of uniform concentration across the whole thickness of the semiconductor device. 
         [0027]    In accordance with another embodiment of the present invention, the isolated pixels of the semiconductor array are separately connected to the readout electronics either by direct contacting the pre-amplifiers or via the routing through the support substrate. 
         [0028]    In accordance with another embodiment of the present invention the isolated photodetector pixels can be photodiodes. 
         [0029]    In accordance with another embodiment of the present invention the isolated photodetector pixels can be photoconductors. 
         [0030]    In accordance with the other embodiment the isolated pixels of a primary photodetector can be avalanche photodiodes or silicon photomultipliers. 
         [0031]    In accordance with another embodiment of the present invention each isolated pixel of a primary photodetector that is a part of imaging system can contain an integrated pre-amplifier. 
         [0032]    In accordance with another embodiment of the present invention the whole detector module with isolated pixel primary photodetector array is used as a detector for the medical imaging system, such as CT, SPECT, PET, or similar. 
         [0033]      FIG. 1  (prior art), item  100 , exemplifies schematically a cross section of a conventional detector array consisting of the array of primary photodetectors (e.g. photodiodes) and attached pixilated scintillator array. In particular,  FIG. 1  shows the back-illuminated primary photodiode array  101 . 
         [0034]    The primary photodetector array  101  is an array of photo-sensitive elements, each converting the optical quanta into electrical signal. The features and structure of the primary photodetector array are not the embodiments of the current invention.  FIG. 1  shows a conventional back-illuminated photodiode array  101  as an example of a primary photodetector array.  110 ,  111 , and  112  are anodes of the elements of the primary photodetector array,  140  are the cathode contact diffusions,  160  is the cathode backside diffusion,  130  are the metal pads, and  120  are the solder balls or stud bumps. The photosensitive pixels of the primary photodetector array  101  in  FIG. 1  are not isolated. 
         [0035]    The trenches (gaps) between scintillator pixels  102  may be filled with epoxy containing reflective particles (for example TiO2), item  103  in  FIG. 1 . The epoxy provides also mechanical integrity to the whole scintillator array, keeping the scintillator pixels together. X-ray photons  190  are deposited on scintillator array creating optical photons  180 . The optical photons  180  generated inside the scintillator pixels  102  travel towards the primary photodetector  101  and create electron-hole pairs  170  via absorption mechanism, generating electrical signal in the elements of the primary photodetector array. In  FIG. 1 , the primary photodetector array is attached to the printed circuit board  104  and electrical signals from each isolated pixel are routed via PCB  104  to the readout electronics (not shown in  FIG. 1 ). 
         [0036]      FIG. 2  (prior art), item  200 , exemplifies schematically a cross section of a conventional detector array consisting of the array of primary photodetectors (e.g. photodiodes) and attached or deposited uniform (or quasi-uniform) scintillator material  202 . In Particular,  FIG. 2  shows the back-illuminated primary photodiode array  201 , which may be similar to the item  101  in  FIG. 1. 210 ,  211 , and  212  are anodes of the elements of the primary photodetector array,  240  are the cathode contact diffusions,  250  is the cathode backside diffusion,  230  are the metal pads, and  220  are the solder balls or stud bumps. The scintillator material  202  may be coupled to the primary photodetector array  201  using optical cement (not shown in  FIG. 2 ). Alternatively, the scintillator material may be directly deposited in the surface of the photodetector array. The photosensitive pixels of the primary photodetector array  201  in  FIG. 2  are not isolated. 
         [0037]    X-ray photons  290  are deposited in scintillator material  202  creating optical quanta  280 , which travel towards the primary photodetector  201  and create electron-hole pairs  270  via absorption mechanism. 
         [0038]    In  FIG. 2 , the primary photodetector array  201  is attached to the printed circuit board  203  and electrical signals from each isolated pixel are routed via PCB  203  to the readout electronics (not shown in  FIG. 2 ). 
         [0039]    In  FIG. 3  (prior art), item  300 , the electrical signal of each primary photodetector pixel is individually routed to a corresponding pre-amplifier channel on a readout electronics chip  310 . The readout electronics (pre-amplifier) chip is attached to the primary photodetector array  301  directly.  320  are the metal pads on the chip  310 .  330  is the scintillator material, which may be the same as item  102  in  FIG. 1  or item  202  in  FIG. 2 . In the prior art, the primary photodiode array  301  consisted of not isolated pixels. 
         [0040]    In  FIG. 4  (prior art), item  400 , each pixel of the primary photodetector array  301  is attached to the pre-amplifier and other readout electronics chip  420  via the substrate  410 , which uses re-routed signals from the primary photodetector array  301 . The substrate  410  may be either PCB or ceramic, or other known in the industry substrate. The further connection to the data acquisition system is made in one of the common for the industry ways.  440  are the contact pads on the chip  420 .  430  are the solder balls or studs on the contact pads. The scintillator material  450  may be the same as item  330  in  FIG. 3 . The photosensitive pixels of the primary photodetector array  301  in  FIG. 4  are not isolated. 
         [0041]      FIG. 5  demonstrates the main idea of the current invention, in which the primary photodetector  520  of the imaging system is an array of isolated photosensitive pixels. The isolation structures  510  between pixels of the array may span across the whole thickness of the semiconductor crystal that forms the primary photodetector array  520 . The isolation structures  510  may consist either partially or completely of diffusion of n- or p-type. The diffusions  510  may be of the same conductivity type as the semiconductor substrate. Diffusions  510  may not necessarily be uniform in concentration across the semiconductor crystal thickness. The doping concentration in the diffusion region may be at least one order of magnitude higher than the doping concentration of the semiconductor material used to manufacture the primary photodetector array. The pixilated scintillator array may in some embodiments be attached to the primary photodetector array using optical coupler  150 . Pixels  102  of the scintillator array may be separated from each other with a reflective material (septa)  103 . Metal pads  540  used to contact the downstream electronics on a ceramic or PCB support substrate  550  via the solder balls or studs  530 . The downstream electronics may contain a data acquisition chip for acquiring data output from the array of photosensitive elements and a switch for selecting elements of the array. 
         [0042]      FIG. 6 , item  600 , describes one of the ideas of the current invention, in which the scintillator material  610  of the imaging system may be a uniform (or quasi-uniform) scintillator material. The scintillator material may be coupled to the primary photodetector array  520  with optical coupler (not shown in  FIG. 6 ). Alternatively, the scintillator material may be directly deposited on the surface of the isolated pixel primary photodetector array. The photosensitive pixels of the primary photodetector array  520  in  FIG. 6  are isolated from each other by the through structures  510 . The structures  510  may be formed by the diffusions that penetrate through the semiconductor substrate from either both surfaces or one surface of the said semiconductor substrate. Such diffusions  510  may not necessarily be uniform across the array thickness. As in the previous case of  FIG. 5 , the doping concentration in the diffusion region  510  may be at least one order of magnitude higher than the doping concentration of the semiconductor material used to manufacture the isolated pixels primary photodetector array. 
         [0043]      FIG. 7 , item  700 , describes another idea of the current invention, in which the detector of the imaging system may not have a scintillator attached to the isolated pixels primary photodetector array  520 . This is a direct conversion imaging detector. 
         [0044]      FIG. 8 , item  800 , describes another embodiment of the current invention, in which the electrical signal from each isolated pixel of the primary photodetector array may be individually routed to a corresponding pre-amplifier channel on a readout electronics chip or substrate layer  830 . The readout electronics (pre-amplifier) chip is attached to the primary photodetector array directly using bonding pads  820  on the readout electronics chip. The readout electronics chip  830  may contain a data acquisition chip for acquiring data output from the array of photosensitive elements and a switch for selecting elements of the array. Item  810  can be either a pixilated scintillator array like item  102  in  FIG. 5  or a (quasi)uniform scintillator material like item  610  in  FIG. 6 . Item  810  is coupled to the isolated pixels primary photodetector array. The scintillator material  810  may be coupled to the primary photodetector array  520  with an optical coupler (not shown in  FIG. 8 ) or it can be deposited on the surface of the array  520 . 
         [0045]      FIG. 9 , item  900 , describes another embodiment, in which each isolated pixel of the primary photodetector array  520  is attached to the pre-amplifier and other readout electronics chip or support substrate layer  930  via the support substrate  920 , which uses individual re-routed signals from the isolated pixels of the primary photodetector array  520 . The readout electronics may contain a data acquisition chip for acquiring data output from the array of photosensitive elements and a switch for selecting elements of the array. The support substrate  920  may be either PCB or ceramic, or other known in the industry substrate. The further connection to the data acquisition system is made in one of the common for the industry ways. Item  910  is a scintillator material, which may be the same as item  810  in  FIG. 8 . Note that the detector of the imaging system may not have a scintillator at all. In this case each isolated diffusion pixel of the primary photodetector array performs as a direct conversion primary detector. 
         [0046]      FIG. 10 , item  1000 , shows another embodiment of the invention, in which a switch  1010  for selecting a photosensitive element of the array and a data acquisition chip  1020  are shown as parts upon the first support substrate (layer)  920 . A switch  1030  and a data acquisition chip  1040  may be parts upon the second support substrate (layer)  930 . 
         [0047]    In still another embodiment,  FIG. 11 , item  1100 , the support substrate that may be attached to the primary photodetector array may consists of two support substrate layers, the first, item  1110 , disposed parallel to the photodetector array  520  and the second, item  1120 , disposed at any angle to and in support of the first support substrate layer  1110 . In  FIG. 11  the second support substrate, item  1120  is shown perpendicular to the first support substrate  1110  however it should be obvious to one skilled in the arts that in fact item  1120  may be deployed at some angle other than 90 degrees relative to the first support substrate  1110 . The first support substrate layer  1110  may be the same as item  920  in  FIG. 9 . As well, for reference, there may be electronic switches,  1130  and data acquisition chips  1140  on this embodiment as well. 
         [0048]    While certain preferred embodiments of the present invention have been disclosed and described herein for purposes of illustration and not for purposes of limitation, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.