Abstract:
This disclosure is directed at a photoconductive element for a digital X-ray imaging system which consists of a detector element comprising a semiconducting layer for absorbing photons, an insulator layer on at least one surface of said semiconducting layer and at least two electrodes on one surface of said insulator layer; and a switching element wherein at least one layer within said switching element is in the same plane as at least one said layer within said detector element.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    This invention relates to radiography imaging systems, and more particularly to a photoconductive detector using an insulating contact layer in a radiography imaging system. 
       BACKGROUND OF THE DISCLOSURE 
       [0002]    Traditionally, X-ray diagnostic processes record x-ray image patterns on silver halide films. These systems direct an initially uniform pattern of impinging X-ray radiation through the object to be studied, intercept the modulated pattern of X-ray radiation with an X-ray radiation intensifying screen, record the intensified pattern on a silver halide film, and chemically transform the latent pattern into a permanent and visible image called a radiograph. 
         [0003]    Radiographs have been produced by using layers of radiation sensitive materials to directly capture radiographic images as modulated patterns of electrical charges. Depending on the intensity of the incident X-ray radiation, electrical charges generated either electrically or optically by the X-ray radiation within a pixelized area are quantized using a regularly arrange array of discrete solid state radiation sensors. 
         [0004]    Recently, there has been rapid development of large area, flat panel, digital X-ray imagers for digital radiology using active matrix technologies used in large are displays. An active matrix consists of a two-dimensional array (of which, each element is called a pixel) of thin film transistors (TFTs) made with a large area compatible semiconductor material including among others, amorphous silicon, polycrystalline silicon, sputtered metal oxides, and organics. There are two general approaches to making flat-panel x-ray detectors, direct or indirect. The direct method primarily uses an amorphous selenium photoconductor as the X-ray to electric charge converting layer coupled directly to the active matrix. In the indirect method, a phosphor screen or scintillator (e.g. CsI, GdOS etc) is used to convert X-rays to light photons which are then converted to electric charge using an additional pixel level light sensor fabricated with the TFT on the active matrix array. 
         [0005]    In prior art imaging systems, the pixel level light sensor disclosed is a vertical photodiode or alternately, a lateral metal-semiconductor-metal (MSM) photoconductor as taught in U.S. Pat. No. 6,373,062 B1: Interdigital photodetector for indirect x-ray detection in a radiography imaging system. 
         [0006]    The key challenges with fabricating a vertical photodiode are the modifications required to the thin film transistor fabrication process specifically, thick amorphous silicon layers, specialized p doped contact layer and a complex RIE sidewall etching process to prevent optical crosstalk. These challenges reduce the fabrication yield and drive up the cost of manufacture. The key challenges with fabricating a lateral MSM photoconductor include the high dark currents at higher electric fields and photoresponse non-uniformity due to a non-uniform electric field. Both issues degrade performance, which is the key reason why MSM devices are not used in industry today for large area digital X-ray imaging. Accordingly, a need exists for a photodetector for use in flat-panel radiographic detectors that can mitigate the process yield and cost issues with vertical photodiodes while overcoming the device performance limitations in the MSM structures. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention meets this need and provides system and method aspects for a lateral Metal-Insulator-Semiconductor-Insulator-Metal (or MISIM) photoconductive element integrated with a switching element for a radiography imaging system. The photoconductive element includes a semiconducting layer for absorbing photons, an insulator layer placed on the semiconducting layer and two electrodes placed on the insulating layer. 
         [0008]    The insulator layer placed between the semiconducting layer and lateral sensor contacts performs the function of reducing the dark current of photoconductor even when a high electric field is applied across the sensor contacts. Applying the high electric field enables the MISIM photoconductive element to operate at a faster speed than conventional metal-semiconductor-metal (MSM) photoconductor designs and also to increase the collection efficiency of the electron hole pairs created by the photons impinging on the semiconducting layer. The structure of the present invention is simpler and correspondingly less expensive to manufacture in comparison to a traditional photodiode structure. Moreover, unlike traditional MSM photoconductors, the structure of the present invention yields higher performance and can be realized in a large area display electronics manufacturing process. These and other advantages of the aspects of the present invention will be understood in conjunction with the following detailed description and accompanying drawings. 
         [0009]    The concept of the MISIM detector, which led to this invention, is described in the following article: (1) S. Ghanbarzadeh, S. Abbaszadeh and K. S. Karim, “Low dark current amorphous silicon Metal-Semiconductor-Metal photodetector for digital imaging applications”, IEEE Electron Device Letters (2014). However, the inventive aspects of the actual implementation of the MISIM photoconductive element for large area digital radiography are not described and form the basis of the present application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  illustrates a general diagram of a radiographic imaging environment in accordance with the present invention. 
           [0011]      FIG. 2  illustrates a two-dimensional active matrix imaging array structure in accordance with the present invention. 
           [0012]      FIG. 3  illustrates a pixel circuit architecture in accordance with the present invention. 
           [0013]      FIG. 4  illustrates a cross-section of a Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) detector in a top electrode and bottom electrode configuration in accordance with the present invention. 
           [0014]      FIG. 5  illustrates a cross-section of a bottom-gate and top-gate thin film transistor (TFT) configuration in accordance with the present invention. 
           [0015]      FIG. 6  illustrates a cross-section of a pixel implementation of an MISIM detector co-planar with a TFT switch in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The present invention relates to a novel photoconductive element that includes a MISIM detector integrated with a switching element for a radiography imaging system. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment will be readily apparent to those skilled in the art and the generic principles herein may be applied to other embodiments. Thus, the present invention is not intended to be merely limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0017]      FIG. 1  illustrates a general diagram of a radiographic imaging environment in accordance with the present invention. As shown, an X-ray source  10  generates an X-ray beam  11  that is transmitted toward an object  12 , e.g., a patient&#39;s hand, for imaging by a radiography detector system  14  and viewing on a Computer  16 . For some radiography detector systems  14 , synchronization hardware  18  is necessary to get the correct timing between the X-ray source  10  and the radiography detector system  14  that is sampling the impinging X-ray beam  11 . In the present invention, the radiography detector system  14  utilizes a large area, flat panel detector based on active matrix technologies to achieve the imaging of object  12 . In general, the object  12  to be imaged is positioned between the radiation source  10  and the radiography detector system  14 . X-rays, which pass through the object  12  interact with the radiography detector system  14 . In direct imaging, the x-rays generate electronic charge within the radiography detector system  14  and there is no need for the Scintillator  15 . In indirect imaging, the x-rays generate light photons as they pass through a phosphor screen or Scintillator  15 , such as CsI, GOS or CaWO4 (Calcium Tungsten Oxide). These indirectly generated light photons then further generate electronic charge within the radiography detector system  14 . 
         [0018]      FIG. 2  shows the components of the radiography detector system  14 . An active matrix pixel array  20  comprises of a two-dimensional matrix of pixel elements where electronic charges generated directly or indirectly by incident x-rays are sensed and stored. To access the stored charge at each pixel, Gate lines  21  are driven typically sequentially by a Row Switching Control  22  causing all pixels in one row to output their stored charge onto Data lines  23  that are coupled to Charge amplifiers  24  at the end of each active matrix pixel array  20  column. The charge amplifiers  24 , that may perform a multiplexing function in addition to their typical amplifying function, send the pixel charge data to analog to digital converters (A/D&#39;s)  26 , where the analog signal is converted to a digital representation which is then be stored in memory  28  awaiting transmission to the Computer  16  at a time determined by the Control Logic  29 . 
         [0019]      FIG. 3  shows the pixel level circuit for one pixel in the active matrix pixel array  20  described in  FIG. 2 , which typically contains a plurality of pixels. Within each pixel is a two terminal MISIM detector  30  that absorbs the incident photons and generates electronic charge, a two terminal optional capacitor  32  to stored the converted electronic charge and a switch  34 , typically a three electrode thin film transistor (TFT) switch for transferring the electronic charge off the pixel. One electrode of the MISIM detector  30  is connected to a high potential Bias terminal  33  that is shared with other pixels in the active matrix pixel array  20  and one electrode of the Capacitor  32  is connected to a low potential Ground terminal  35  which is also shared with other pixels in the active matrix pixel array  20 . The drain electrode of the TFT switch  34  is connected to both, the second electrode of the MISIM detector  30  and the second terminal of the Capacitor  32 . The source electrode of the TFT  34  is connected to the Pixel Data line  36  which is coupled to one of the plurality of Data lines  23  described in  FIG. 2 . The gate electrode of the TFT  34  is connected to the Pixel Gate line  38  which is coupled to one of the plurality of Gate lines  21  described in  FIG. 2 . 
         [0020]      FIG. 4   a  shows a cross-section of the MISIM detector  30  in a top electrode configuration. In this instance, there is a substrate  40  (typically glass or plastic) with the following layers deposited in sequence: an antireflective layer  42 , a semiconducting layer  44 , an insulator layer  46  and a patterned contact layer yielding at least two contacts, one bias contact  48  and one sense contact  49 . Note that the anti-reflective layer is optional and not necessary for correct working of the MISIM detector  30 . However, in indirect conversion imaging, it typically enhances performance by increasing the percentage of light photons impinging on the semiconducting layer  44  where photons are absorbed. In  FIG. 4   b , a cross-section of the MISIM detector  30  in a bottom electrode configuration is shown. Here the sequence is reversed: first there is the patterned contact layer containing the bias and sense contacts followed by an insulator layer, semiconducting layer and the optional antireflective layer. 
         [0021]    Dark current is a key problem with traditional MSM detectors because it reduces the detector dynamic range and image quality and is a function of the electric field applied on the bias contact  48 . A large electric field is necessary for charge separation of the electronic carriers generated from the impinging photons on the semiconducting layer  44 . If photocurrent can be maintained at a high level while dark current is reduced or alternately, a higher electric potential can be applied to the bias contact  48  to increase charge separation efficiency and correspondingly the photocurrent, without increasing the dark current, then a larger photo-to-dark current ratio is possible which equates to better dynamic range, higher contrast, higher quantum efficiencies and better digital images. Neither ohmic nor blocking contacts for the bias  48  and sense  49  contacts have to date been able to achieve the dark current densities necessary for sensitive medical radiography imaging applications (around 10 pA/mm 2  or less). In addition, insulating contacts are typically not considered viable because of the anticipated slow response times and the potential for charge build-up on the insulating layer that can lead to reliability concerns. 
         [0022]    The present invention uses a specific insulator layer  46  that simultaneously: (1) reduces dark currents when there are no photons impinging on the semiconducting layer  44  and (2) enables high photocurrents when photons impinge on the semiconducting layer  44 . To achieve these two goals, the material of the insulator layer  46  must be carefully selected to both, provide a good interface with the semiconducting layer and to have a dielectric strength such that it can be operated in soft (reversible) breakdown during device operation repeatably when the applied bias and insulator layer  46  thickness are optimized to take into account both the dark conductivity and photoconductivity of the semiconducting layer  44  which is also a function of semiconducting layer  44  thickness, applied electric bias and material properties. 
         [0023]    When photons are impinging on the semiconducting layer  44  thereby causing the resistivity of the semiconducting layer  44  to decrease, the insulator layer  46  operates in soft (i.e. reversible) breakdown mode allowing a vertical conduction path from bias  48  and sense contacts  49  through the insulator layer  46  to the semiconducting layer  44 . Operating in soft breakdown allows for conduction through the insulator layer  46  which can overcome the response time challenge while still maintaining a low dark current by limiting bias  48  and sense  49  contact injection currents. Using an insulator layer  46  that is too thick or with a high dielectric breakdown strength can yield poor results or alternately, choice of an incompatible insulator layer  46  material can yield a poor interface with the semiconducting layer  44  so that traps and defects cause a drop in MISIM detector  30  quantum efficiency. 
         [0024]    For example, we determined that using a 450 nm amorphous silicon semiconducting layer  44  works well with a 300 nm polyimide insulator layer  46  and yields a good quality interface with high external quantum efficiency (above 65%) for green light. Alternately, if high external quantum efficiency is required for blue light, then, for the same amorphous silicon and polyimide material system, the semiconducting layer  44  thickness may need to be reduced which requires a corresponding re-optimization of the insulator layer thickness  46 . If the semiconducting layer  44  is changed from amorphous silicon to a metal oxide like IGZO or even polysilicon, both of which have different material properties and absorption coefficients, the choice of insulator layer material (for interface purposes), thickness and maximum bias voltage applied need to be re-optimized. 
         [0025]    Moreover, it must be emphasized that using an insulator layer  46  as opposed to simply patterning the insulating contacts above the bias  48  and sense  49  contacts can lead to a better overall interface with the semiconductor layer  44  with fewer defects and traps as well as encapsulating the semiconducting layer  44  in the longterm thus maintaining higher quantum efficiency. A patterning process (e.g. of the bias  48  or sense  49  contacts or the insulator layer  46 ) may degrade the semiconducting layer  44  interface because of exposure to air and chemicals during the patterning process. 
         [0026]    Furthermore, if the bias  48  and sense  49  contacts are made using transparent materials, the insulator layer  46  can serve an additional function such as being an anti-reflective layer, which will allow additional photons to reach the semiconducting layer  44  through the now transparent contacts. 
         [0027]      FIG. 5   a  shows a bottom gate, inverted staggered thin film transistor (TFT) structure where a substrate  50  (e.g. glass or plastic) contains a patterned gate electrode  52 , followed by a gate insulator  54 , a semiconducting layer  56  and a patterned contact layer defining the source  58  and drain  59  contacts.  FIG. 5   b  shows a top gate, inverted staggered TFT structure with the layers in a reverse configuration. Both are implementations of amorphous silicon TFTs in use by the display industry today. Similar cross-sections can be drawn for IGZO and polysilicon TFTs as understood by one skilled in the art. 
         [0028]      FIG. 6  shows a cross-section of a photoconductor element using a co-planar implementation. The element components can be mapped to the pixel level circuit shown in  FIG. 3 , which consists of an amorphous silicon MISIM detector  30 , a capacitor  32  and an amorphous silicon TFT switch  34 . In  FIG. 6 , the MISIM detector cross-section  74  contains Bias electrodes  67  and Sense electrodes  66  in a commonly known comb electrode configuration along with a polyimide insulator layer  68  (which could be, among others, amorphous silicon nitride, amorphous silicon oxide, amorphous silicon oxynitride, benzocyclobutene (BCB), or polystyrene), a semiconducting layer of amorphous silicon  69  (or alternately, one or more of molybdenum sulphide, Indium Gallium Zinc Oxide, polycrystalline silicon, amorphous selenium, mercuric iodide, lead oxide, microcrystalline silicon, nanocrystalline silicon, crystalline silicon, PTCBI, or CuPc), an amorphous silicon nitride antireflective layer  70  and a further amorphous silicon nitride passivation layer  71 . The capacitor cross-section  73  shows the bottom plate shared with the sense electrode  60  along with a top capacitor plate connected to ground  65 , typically a low electric potential. The capacitor dielectric in this case is amorphous silicon nitride  70 , and is shared with the anti-reflective layer in the MISIM detector cross-section  74 . The TFT cross-section  72  consists of a source electrode  61  connected to the pixel data line  36  from  FIG. 3 . Also shown is a gate electrode  63  connected to the pixel gate line  38  in  FIG. 3 . The drain electrode  62  is connected to the sense electrodes  66  and forms one plate of the capacitor shown in the capacitor cross-section  73 . For the TFT cross-section  72 , an amorphous silicon layer  69  is the active layer and this is shared with the MISIM detector cross-section  74 . The TFT gate dielectric is formed by an amorphous silicon nitride layer  70 , shared with the anti-reflective layer shown in the MISIM detector&#39;s cross-section  74  and the capacitor&#39;s dielectric layer. 
         [0029]    One of the benefits of the co-planar design shown in  FIG. 6  allows for shared uses of multiple layers, for example, the TFT gate dielectric can serve as an anti-reflective coating for the MISIM detector  30 . In contrast, in a PIN diode, the unique amorphous silicon PIN isolation process and the thick semiconductor layer required to absorb green photons typically precludes sharing of any layers except metal contacts. In addition, the PIN diode sidewalls need to be etched carefully and passivated to prevent excess leakage current. In the MISIM detector  30 , because the conduction path is horizontal, the horizontal interface is primarily important. As described earlier, using the insulator layer  46  helps protect the interface to the semiconducting layer  44 . Thus, device performance remains stable in the long term even if the MISIM detector  30  is built in a standard TFT switch  34  manufacturing process. 
         [0030]    Since the MISIM does not have a p+ doped layer like the PIN photodiode, blue light emitting phosphors can work. The MISIM detector  30  disclosed can use any one of a number of insulating layers between the contact electrodes and the amorphous silicon, i.e. polyimide, polystyrene, amorphous silicon nitride, amorphous silicon oxide or amorphous silicon oxynitride. Similarly, the antireflecting layer can be a layer of the same set as described above. 
         [0031]    To detect X-rays indirectly, the scintillating phosphor  15  can be placed on either side of the radiography detector system  14 . In the device architecture shown in  FIG. 6 , it is preferable for the scintillator  15  to be placed on top i.e. adjacent to the passivation layer  71 . This is because the semiconducting layer  44  would now be exposed fully to incident light from the scintillator  15  resulting in a higher absorption of incident light and thus, better quantum efficiency. If the scintillator  15  is placed on the bottom (i.e. adjacent to the glass  60 ), then there could be a loss of spatial resolution due to the thickness of the glass  60 , additional process complexity due to need for a TFT back gate (not shown) to prevent incident light from the scintillator falsely turning the TFT ON, and loss in quantum efficiency if the electrodes are opaque and block light from reaching the amorphous silicon  69  semiconducting layer. 
         [0032]    The implementation shown in  FIG. 6  uses a top gate TFT switch  34  and a bottom electrode MISIM detector  30 . It should be noted that additional implementations are possible that use a combination of either a top or bottom gate TFT switch and a top or bottom electrode MISIM detector. The ideal combination would have the drain electrode  62  of the TFT and the sense electrodes  66  of the MISIM on the same plane (as shown in  FIG. 6 ) to save on having an additional space consuming via that runs through the semiconducting layer of amorphous silicon  69 . Moreover, if transparent drain  62 , source  61 , sense  66  and bias  67  electrodes are used, a bottom gate TFT switch  34  along with a top electrode MISIM detector  30  can potentially achieve similar performance as the implementation in  FIG. 6 .