Patent Publication Number: US-7897929-B2

Title: Reduced cost pixel design for flat panel x-ray imager

Description:
BACKGROUND 
     The invention relates generally to imaging systems. In particular, the invention relates to an imaging panel for use in a digital x-ray detector. 
     Non-invasive imaging broadly encompasses techniques for generating images of the internal structures or regions of a person or object that are otherwise inaccessible for visual inspection. For example, non-invasive imaging techniques are commonly used in the industrial field for inspecting the internal structures of parts and in the security field for inspecting the contents of packages, clothing, and so forth. One of the best known uses of non-invasive imaging, however, is in the medical arts where these techniques are used to generate images of organs and/or bones inside a patient which would otherwise not be visible. 
     One class of non-invasive imaging techniques that may be used in these various fields is based on the differential transmission of X-rays through a patient or object. In the medical context, a simple X-ray imaging technique may involve generating X-rays using an X-ray tube or other source and directing the X-rays through an imaging volume in which the part of the patient to be imaged is located. As the X-rays pass through the patient, the X-rays are attenuated based on the composition of the tissue they pass through. The attenuated X-rays then impact a detector that converts the X-rays into signals that can be processed to generate an image of the part of the patient through which the X-rays passed based on the attenuation of the X-rays. Typically the X-ray detection process utilizes a scintillator, which generates optical photons when impacted by X-rays, and an array of photosensor elements, which generate electrical signals based on the number of optical photons detected. 
     A significant fraction of the cost of a digital x-ray detector is the imaging panel that contains the two dimensional pixel array composed of light sensitive photo-diodes and field effect transistors (FET)s. This panel is composed of a series of patterned thin film layers deposited on a thin glass substrate using standard semi-conductor processing techniques. Each additional layer adds to the total manufacturing time (cycle time) and the final yield of the detector; and both cycle time and yield add to the total cost of the detector. 
     One known technique for reducing the cycle time includes processing larger glass substrates, each containing multiple x-ray detectors, which effectively reduces the cycle time. Another known technique for reducing the cycle time includes investing in larger manufacturer capacity. Manufacturers of digital x-ray imaging systems are constantly working on incremental yield improvements via root cause analysis of failure mechanisms and fine-tuning the manufacturing process. Panel and pixel designs are generally chosen based on a trade-off of cost and performance. 
     A need therefore exists for an imaging panel for use in a digital x-ray detector that reduces both the cost and cycle time associated with the production of a digital x-ray imaging system. 
     BRIEF DESCRIPTION 
     According to one embodiment, an X-ray imager comprises: 
     a flat panel detector configured to generate electrical signals in response to incident X-rays, the flat panel detector comprising:
         an array of pixels, each pixel comprising a silicon FET and a silicon diode, each FET comprising a gate dielectric layer and a silicon layer, wherein the full, as-deposited, thickness of the FET gate dielectric and at least part of the FET silicon layer remains beneath the diode silicon across the entirety of the flat panel detector in which the edges or perimeters of the diode silicon features are self-aligned to the underlying FET silicon features.       

     According to another embodiment, a pixel structure for an X-ray imager comprises:
         a silicon diode; and   a silicon FET comprising:
           a gate dielectric layer; and   a silicon layer deposited on top of the gate dielectric layer,   
               

     wherein the full, as-deposited, thickness of the gate dielectric layer and at least part of the silicon layer remains beneath the diode silicon across the entirety of the pixel structure in which the edges or perimeters of the diode silicon features are self-aligned to the underlying FET silicon features. 
     According to yet another embodiment, a method of manufacturing a detector for use in an imaging system comprises: 
     forming an array of pixels, each pixel comprising a silicon FET and a silicon diode, each FET comprising a gate dielectric layer and a silicon layer, wherein the full, as-deposited, thickness of the FET gate dielectric and at least part of the FET silicon layer remains beneath the diode silicon across the entirety of the detector in which the edges or perimeters of the diode silicon features are self-aligned to the underlying FET silicon features. 
     According to still another embodiment, a method of manufacturing a pixel element for use in an imaging system, the method comprising: 
     forming a silicon FET and a silicon diode, each FET comprising a gate dielectric layer and a silicon layer, wherein the full, as-deposited, thickness of the FET gate dielectric and at least part of the FET silicon layer remains beneath the diode silicon across the entirety of the pixel element in which the edges or perimeters of the diode silicon features are self-aligned to the underlying FET silicon features. 
     According to still another embodiment, a method of manufacturing a pixel element for use in an imaging detector panel comprises:
         depositing a gate dielectric layer;   depositing a silicon layer on top of the gate dielectric layer;   depositing a molybdenum layer on top of the silicon layer;   depositing a silicon oxide layer on top of the molybdenum layer;   depositing a diode silicon layer on top of the silicon oxide layer, wherein all layers are deposited uniformly over the imaging detector panel; and   selectively patterning and etching each layer subsequent to its deposition to determine where the deposited layer material remains, such that the full, as-deposited, thickness of the gate dielectric layer and at least part of the silicon layer remains beneath the diode silicon across the entirety of the pixel element in which the edges or perimeters of the diode silicon features are self-aligned to the underlying silicon features.       

     According to still another embodiment, a method of manufacturing a pixel element for use in an imaging detector panel comprises:
         depositing a gate dielectric layer;   depositing a silicon layer on top of the gate dielectric layer;   depositing a molybdenum layer on top of the silicon layer;   depositing a silicon oxide layer on top of the molybdenum layer;   depositing a diode silicon layer on top of the silicon oxide layer, wherein all layers are deposited uniformly over the imaging detector panel; and   simultaneously etching the diode silicon layer and the silicon layer such that the full, as-deposited, thickness of the gate dielectric layer and at least part of the silicon layer remains beneath the diode silicon across the entirety of the pixel element in which the edges or perimeters of the diode silicon features are self-aligned to the underlying silicon features.       

     According to still another embodiment, a method of manufacturing a detector panel for use in an imaging system comprises:
         depositing a gate dielectric layer;   depositing a silicon layer on top of the gate dielectric layer;   depositing a molybdenum layer on top of the silicon layer;   depositing a silicon oxide layer on top of the molybdenum layer;   depositing a diode silicon layer on top of the silicon oxide layer, wherein all layers are deposited uniformly over the detector panel; and   simultaneously etching the diode silicon layer and the silicon layer to form isolated pixel structures such that the full, as-deposited, thickness of the gate dielectric layer and at least part of the silicon layer remains beneath the diode silicon across the entirety of the detector panel in which the edges or perimeters of the diode silicon features are self-aligned to the underlying silicon features, wherein all layers are deposited uniformly over the detector panel.       

    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatic representation of an exemplary X-ray imaging system; 
         FIG. 2  is a cut-away perspective view of a detector; 
         FIG. 3  is a plan view illustrating a portion of a flat panel x-ray detector, in accordance with one aspect of the present invention; 
         FIG. 4  is a cross-sectional view of a flat panel x-ray detector constructed using a conventional mask process; and 
         FIG. 5  is a cross-sectional view of the flat panel x-ray detector shown in  FIG. 3  constructed using a mask process according to one aspect of the present invention. 
     
    
    
     While the above-identified drawing figures set forth alternative embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an X-ray imaging system designated generally by a reference numeral  10 . According to one embodiment, the X-ray imaging system  10  is designed to acquire and process image data as described in greater detail below. The X-ray imaging system  10  includes an X-ray source  12  positioned adjacent to a collimator  14 . In one embodiment, the X-ray source  12  is a low-energy source and is employed in low energy imaging techniques, such as fluoroscopic techniques, or the like. Collimator  14  permits a stream of X-ray radiation  16  to pass into a region in which a target  18 , such as a human patient, is positioned. A portion of the radiation is attenuated by the target  18 . This attenuated radiation  20  impacts a detector  22 , such as a fluoroscopic detector. As will be appreciated by one of ordinary skill in the art, the detector  22  may be based on scintillation, i.e., optical conversion, direct conversion, or on other techniques used in the generation of electrical signals based on incident radiation. For example, a scintillator-based detector converts X-ray photons incident on its surface to optical photons. These optical photons may then be converted to electrical signals by employing photodiodes. Conversely, a direct conversion detector directly generates electrical charges in response to X-ray&#39;s; and the electrical signals are stored and read out from storage capacitors. As described in detail below, these electrical signals, regardless of the conversion technique employed are acquired and processed to construct an image of the features within the target  18 . 
     The X-ray source  12  is controlled by power supply/control circuitry  24  which furnishes both power and control signals for examination sequences. Moreover, detector  22  is coupled to detector acquisition circuitry  26 , which commands acquisition of the signals generated in the detector  22 . Detector acquisition circuitry  26  may also execute various signal processing and filtration functions, such as, for initial adjustment of dynamic ranges, interleaving of digital, and so forth. 
     In the depicted exemplary embodiment, one or both of the power supply/control circuitry  24  and detector acquisition circuitry  26  are responsive to signals from a system controller  28 . In some exemplary systems it may be desirable to move one or both of the detector  22  or the X-ray source  12 . In such systems, a motor subsystem may also be present as a component of the system controller  28  to accomplish this motion. In the present example, the system controller  28  also includes signal processing circuitry, typically based upon a general purpose or application specific digital computer. The system controller  28  may also include memory circuitry for storing programs and routines executed by the computer, as well as configuration parameters and image data, interface circuits, and so forth. 
     Image processing circuitry  30  is also present in the depicted embodiment of the X-ray imaging system  10 . The image processing circuitry  30  receives acquired projection data from the detector acquisition circuitry  26  and processes the acquired data to generate one or more images based on X-ray attenuation. 
     One or more operator workstations  32  are also present in the depicted embodiment of the X-ray imaging system  10 . The operator workstation  32  allows an operator to initiate and configure an X-ray imaging examination and to view the images generated as part of the examination. For example, the system controller  28  is generally linked to operator workstation  32  so that an operator, via one or more input devices associated with the operator workstation  32 , may provide instructions or commands to the system controller  28 . 
     Similarly, the image processing circuitry  30  is linked to the operator workstation  32  such that the operator workstation  32  may receive and display the output of the image processing circuitry  30  on an output device  34 , such as a display or printer. The output device  34  may include standard or special purpose computer monitors and associated processing circuitry. In general, displays, printers, operator workstations, and similar devices supplied within the system may be local to the data acquisition components or may be remote from these components, such as elsewhere within an institution or hospital or in an entirely different location. Output devices and operator workstations that are remote from the data acquisition components may be linked to the image acquisition system via one or more configurable networks, such as the internet, virtual private networks, and so forth. As will be appreciated by one of ordinary skill in the art, though the system controller  28 , image processing circuitry  30 , and operator workstation  32  are shown distinct from one another in  FIG. 1 , these components may actually be embodied in a single processor-based system, such as a general purpose or application specific digital computer. Alternatively, some or all of these components may be present in distinct processor-based systems, such as a general purpose or application specific digital computers, configured to communicate with one another. For example, the image processing circuitry  30  may be a component of a distinct reconstruction and viewing workstation. 
     Flat panel x-ray detectors are typically composed of a light sensitive imaging panel, associated electronics to operate this panel, a scintillator screen, and a protective cover. The light sensitive panel is composed of a series of thin films of metal, dielectric, and semi-conductor material deposited on a thin glass substrate. For each layer, the desired material is deposited, lithographically patterned, and then etched to form the intended two-dimensional design. In order to fabricate a field effect transistor (FET) at each pixel, several layers are needed including a gate metal layer, a FET silicon layer, a source/drain layer, and a FET passivation layer. In order to fabricate an additional photodiode at each pixel, a diode silicon layer, a diode passivation layer, and a common electrode metal layer are typically required. Additional layers may also be needed in order to optimize the performance and/or manufacturability of the process. 
     Referring now to  FIG. 2 , a scintillation-based detector  35  suitable for use as the detector  22  depicted in  FIG. 1  is described herein below. Though the scintillation-based detector  35  of  FIG. 2  is discussed herein as an example for use with the present technique, it should be remembered that this is only one example. Discussion of the scintillation-based detector  35 , therefore, should be understood to be merely exemplary and presented for the purpose of illustrating the principles of operation for one type of detector which may benefit from the present technique. 
     Turning again to  FIG. 2 , an exemplary physical arrangement of the components of a scintillation-based detector  35  is presented in accordance with one embodiment. The detector  35  includes a glass substrate  36  on which the components described below are disposed. 
     In the depicted embodiment, the scintillation-based detector  35  includes an array of photosensor elements  38 . In one implementation, the photosensor elements  38  are photodiodes formed from silicon. In the exemplary embodiment of  FIG. 2 , the photodiodes are arranged in an array of rows and columns that define the pixels, or picture elements, read out by the detector acquisition circuitry  26  shown in  FIG. 1 . Each photodiode includes a photosensitive region  40 , and a thin film transistor (TFT)  42  such as an FET, which may be selectively activated using data lines  48  and scan lines  50 . 
     Further, the scintillation-based detector  35  includes a scintillator  44 , which, when exposed to X-rays, generates the optical photons detected by the photosensitive regions  40 . As illustrated in this embodiment, a conductive layer  54  disposed on a dielectric layer  56  is disposed between the scintillator  44  and the array of photosensor elements  38 . Vias  58  electrically couple the conductive layer  54  to the top surface of each element of the array of photosensor elements  38  to allow a common bias to be applied to each photosensor element. 
     Referring now to  FIG. 3 , a plan view illustrates two-pixels within a portion of a flat panel x-ray detector structure  60 , in accordance with one aspect of the present invention. Flat panel x-ray detector structure  60  comprises a two-dimensional pixel array including light sensitive photo-diode islands  62 ,  64  and field effect transistor islands  66 ,  68  and is constructed using a series of patterned thin film layers deposited on a thin glass substrate (i.e.  36  in  FIG. 2 ). The edges of photo-diode island  62  are coincident with the edges of FET island  66 , while the edges of photo-diode island  64  are coincident with the edges of FET island  68 . A FET SiOx hard mask layer  69  functions to protect the FET silicon in the respective circular FET areas  74 ,  79 . 
     The desired material for each layer is deposited, lithographically patterned, and then etched to form the intended two-dimensional structure. Several layers are required to fabricate a FET at each pixel. These include, but are not limited to, a gate metal layer, a FET silicon layer, a source/drain metal layer, and a FET passivation layer. A diode passivation layer and a common electrode metal layer are typically required to also fabricate a photodiode at each pixel. Additional layers may also be needed to optimize performance and/or manufacturability of the process. 
       FIG. 4  is a cross-sectional view showing one portion of a pixel structure  70  for a flat panel x-ray detector structure in which the pixel structure  70  is fabricated with a conventional mask process. The conventional mask process commences by first depositing, and then patterning and etching a gate metal (not shown). Subsequent to the above initial gate metal deposition, patterning, and etching level, pixel structure  70  is constructed by first depositing a tri-layer comprising a gate dielectric (SiN) layer  61  deposited on top of the gate metal layer, a FET silicon layer  63  deposited on top of the SiN layer  61 , and a molybdenum (Mo) layer  65  deposited on top of the FET silicon layer  63  as shown in level  1 ). 
     Subsequent to the foregoing level  1  deposition process, patterning and etching of the Mo layer  65  and patterning and back channel etching (BCE) FET silicon layer  63  are performed in level  2 ). 
     Next, patterning and wet etching of the Mo layer  65 , and patterning and dry etching via a reactive ion etch (RIE) process of the FET silicon layer  63  are then performed as shown in level  3 ). 
     Following level  3 ) processing, a FET silicon oxide layer  67  is deposited, patterned, and wet etched as shown in level  4 ). 
     Finally, the diode silicon  69  is deposited, patterned and dry etched via RIE as shown in level  5 ) to form and complete pixel structure  70 . 
     Keeping the pixel structure  70  fabricated with a conventional mask process as described above in mind,  FIG. 5  is a cross-sectional view of the flat panel x-ray detector structure  60  shown in  FIG. 3  constructed using a mask process according to one aspect of the present invention. Unlike the pixel structure  70  shown in  FIG. 4  that is produced when using a conventional mask process, the pixel structure  80  shown in  FIG. 5  is produced by patterning and etching both the FET silicon and diode silicon at the same time. Because the diode silicon and FET silicon are deposited in subsequent steps, simultaneous etching of both layers requires that the FET silicon layer be underneath the photodiode silicon everywhere on the detector structure  60 . This mask process requires removal of any corresponding FET passivation layer from areas where the FET and diode silicon are simultaneously removed, to allow etching to penetrate to the FET silicon layer between all pixels. In this way, the mask process eliminates the FET silicon photolithograph steps, the FET silicon dry etch step, and the associated cleaning steps. Removing one silicon etch step advantageously improves the overall yield by eliminating small areas of un-etched silicon that cause cluster type defects. 
     With continued reference now to  FIG. 5 , a mask process according to one aspect of the present invention commences by first depositing, and then patterning and etching a gate metal (not shown). Subsequent to the above initial gate metal deposition, patterning, and etching level, pixel structure  70  is constructed by first depositing a tri-layer comprising a gate dielectric (SiN) layer  61  deposited on top of the gate metal layer, a FET silicon layer  63  deposited on top of the SiN layer  61 , and a molybdenum (Mo) layer  65  deposited on top of the FET silicon layer  63  as shown in level  1 ). 
     Subsequent to the foregoing tri-layer deposition process, patterning and etching of the Mo layer  65  and patterning and back channel etching (BCE) FET silicon layer  63  are performed as shown in level  2 ). Etching in level  2 ) for this embodiment is implemented completely through Mo layer  65  and partially into the FET silicon layer  63  to expose a first portion  82  of the FET silicon layer  63 . 
     Patterning and wet etching of the Mo layer  65 , and patterning and dry etching via a reactive ion etch (RIE) process of the FET silicon layer  63  that is required during conventional level  3 ) mask processing such as described above with reference to  FIG. 4 , is not required for this mask process since the FET silicon layer  63  must remain beneath the diode silicon everywhere on the detector structure  60 . 
     Following level  2 ) processing, a FET silicon oxide layer  67  is deposited, patterned, and wet etched as shown in what is depicted as level  4 ). Deposition, patterning and wet etching of the FET silicon oxide layer  67  in level  4 ) is implemented to expose a second portion  84  of the FET silicon layer  63 . 
     Finally, at what is now level  5 ), the photodiode (diode) silicon  69  is deposited over the entire pixel structure  80 ; and both the FET silicon layer  63  and the diode silicon  69  are patterned and RIE etched at the same time to provide the desired pixel structure  80  that includes exposed gate dielectric (SiN) layer  61  and exposed FET silicon layer  63  as shown in what is depicted as level  5 ). 
     The mask process described above with reference to  FIG. 5  also requires removal of any corresponding FET passivation layer from areas where the FET and diode silicon are removed, to allow etching to penetrate to the FET silicon layer  63  between all pixels. This mask process advantageously eliminates the FET silicon photolithograph steps, the FET silicon dry etch step, and the associated cleaning steps, as stated herein before. Removing one silicon etch step improves the overall yield by eliminating small areas of un-etched silicon that can cause cluster type defects, as also stated herein before. 
     The full, as-deposited, thickness of the FET gate dielectric  61  and (at least) part of the FET silicon layer  63  remains underneath the diode silicon  69  across the entirety of the flat panel detector  60  due to the resultant pixel structure  80  that is provided using the foregoing process in which the diode silicon  69  and the FET silicon  63  are simultaneously etched to form isolated structures (array photodiodes, I/O elements, and so on) in which the edges or perimeters of the diode silicon  69  features are self-aligned to the underlying FET Si  63  features. 
     The above process advantageously reduces the cycle time and increases the yield associated with the manufacture of flat panel x-ray detectors in a manner that does not require addition to or modification of existing production equipment and/or manufacturing facilities. 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.