Abstract:
This disclosure is directed at a photoconductive element for a digital X-ray imaging system which consists of a detector element comprising at least one semiconducting layer for absorbing photons, a first electrode coupled to a surface of said semiconducting layer, a second electrode coupled to a surface of said semiconducting layer, wherein said first electrode and said second electrode are separated horizontally, and at least one of said electrodes is electrically isolated from said semiconducting layer by an insulating layer; a readout circuit element coupled to said detector element; and a dielectric layer between said detector element and said readout circuit element.

Description:
FIELD OF THE DISCLOSURE 
       [0001]    This invention relates to radiography imaging systems, and more particularly to a photoconductive element that includes a radiation detector element integrated with a readout circuit element 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. Csl, 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 as taught in U.S. Pat. No. 7,615,731 B2: High fill-factor sensor with reduced coupling. Alternately, a metal-semiconductor-metal (MSM) photoconductor was 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. In addition, the lateral MSM photoconductor is not space efficient leading to low effective quantum efficiency (EQE). Each of these issues degrades imager 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 dark current, non-uniformity and EQE 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 novel photoconductive element. The photoconductive element consists of a lateral Metal-Insulator-Semiconductor-Insulator-Metal (or MISIM) detector element coupled to a readout circuit element through a via in a dielectric layer that sits between the detector element and readout circuit element. The MISIM detector element comprises a semiconducting layer to absorb incident photons and two electrodes coupled to the semiconducting layer separated in the horizontal plane, where at least one of the electrodes is electrically isolated from the semiconducting layer using an insulator. The insulated contact, also typically under high voltage bias, maintains a low dark current even under high electric field conditions. Applying the high electric field enables the MISIM detector element to operate at a faster speed than conventional metal-semiconductor-metal (MSM) photoconductor designs and also to increase the collection efficiency (and hence EQE) of the electron hole pairs created by the photons impinging on the semiconducting layer. However, the presence of a high bias voltage near the readout circuit can lead to lower reliability and higher dark currents in the photoconductive element and requires use of a dielectric to yield a novel integrated photoconductive element design. 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 because the readout circuit element can be embedded under the MISIM detector element yielding a larger area for light absorption. Moreover, the entire photoconductive element can be realized in a large area TFT display manufacturing process, which is more reliable and easier to access than the specialized process for a PIN photodiode. These and other advantages of the aspects of the present invention will be understood in conjunction with the following detailed description and accompanying drawings. 
         [0008]    Some of the ideas behind the MISIM detector element, 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 implementation of the complete photoconductive element (i.e. with the readout circuit element) for large area digital radiography are not described and form the basis of the present application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates a general diagram of a radiographic imaging environment in accordance with the present invention. 
           [0010]      FIG. 2  illustrates a two-dimensional active matrix imaging array structure in accordance with the present invention. 
           [0011]      FIG. 3  illustrates a pixel circuit architecture in accordance with the present invention. 
           [0012]      FIG. 4  illustrates a cross-section of a Metal-Insulator-Semiconductor-Insulator-Metal (MISIM) detector element in a top electrode and bottom electrode configuration in accordance with the present invention. 
           [0013]      FIG. 5  illustrates a cross-section of a bottom-gate and top-gate thin film transistor (TFT) configuration in accordance with the present invention. 
           [0014]      FIG. 6  illustrates a cross-section of a photoconductive element implementation using a MISIM detector element placed on top of a readout circuit element in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present invention relates to a novel photoconductive element that includes a MISIM detector element integrated with a readout circuit 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. 
         [0016]      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 Csl, GOS or CaWO4 (Calcium Tungsten Oxide). These indirectly generated light photons then further generate electronic charge within the radiography detector system  14 . 
         [0017]      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 . 
         [0018]      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 element  30  that absorbs the incident photons and generates electronic charge, a two terminal optional capacitor  32  to stored the converted electronic charge and a readout circuit element, usually a three electrode thin film transistor (TFT) switch  34  as shown for transferring the electronic charge off the pixel. One electrode of the MISIM detector element  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 . 
         [0019]      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 separated horizontally, a bias contact  48  and a 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. 
         [0020]    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. 
         [0021]    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. 
         [0022]    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. 
         [0023]    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 EQE (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 (Indium Gallium Zinc Oxide) 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. Additional improvements in EQE are possible if an optional anti-reflective layer like amorphous silicon nitride is used on top of the semiconducting layer directly in the path of the incident photons. 
         [0024]    Moreover, it must be noted that it is possible to pattern the insulator layer  46  and use either insulating contacts for both the bias  48  and sense  49  contacts or alternately, use an insulating contact for just one contact (e.g. either for the bias  48  contact or for the sense  49  contact depending on the bias used). A patterning process (e.g. of the bias  48  or sense  49  contacts or the insulator layer  46 ) can also potentially degrade the semiconducting layer  44  interface because of exposure to air and chemicals during the patterning process. Typically though, as shown in  FIG. 4 , an insulator layer running across both bias  48  and sense  49  contacts leads 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 long term thus maintaining higher quantum efficiency. However, MISIM detector elements where only one the bias  48  or sense  49  contacts are insulating remain a feasible alternative if careful semiconductor processing is undertaken. Moreover, it is also possible to place the bias  48  ad sense  49  contacts, one each on opposite sides of the semiconducting  44  layer as long as they are separated by a horizontal distance so that photon absorption and transport remains in the horizontal (lateral) direction. Furthermore, if bias  48  and sense  49  contacts are made using transparent materials, both the top electrode or bottom electrode configuration can detect light photons equally well from either direction. 
         [0025]      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 CMOS (complementary metal-oxide-semiconductor), IGZO and polysilicon transistors as understood by one skilled in the art. 
         [0026]      FIG. 6  shows a cross-section of a photoconductor element implemented using a readout circuit element embedded physically underneath the amorphous silicon MISIM detector element. The MISIM detector element consists of sense  66  and bias  67  electrodes in a comb configuration, a polyimide insulator layer  71  (or alternately, among others, amorphous silicon nitride, amorphous silicon oxide, amorphous silicon oxynitride, benzocyclobutene (BCB), parylene or polystyrene) that covers both the sense  66  and bias  67  electrodes, an amorphous silicon (a-Si:H) semiconducting layer  70  (or alternately, molybdenum sulphide, Indium Gallium Zinc Oxide, polycrystalline silicon, amorphous selenium, mercuric iodide, lead oxide, microcrystalline silicon, nanocrystalline silicon, crystalline silicon, pentacene, PTCBI, CuPc, small molecule organic semiconductor, or polymer organic semiconductor) and an optional anti-reflective coating layer  69  e.g. amorphous silicon nitride (a-SiNx:H). 
         [0027]    The readout circuit element shown employs a bottom gate amorphous silicon TFT acting as a switch. The TFT comprises an amorphous silicon nitride (a-SiNx:H) gate dielectric layer  72 , an amorphous silicon (a-Si:H) semiconducting layer  73 , an a-SiNx:H  74  passivation layer and a doped ohmic contact layer  62 . 
         [0028]    Alternately, the readout circuit element could employ a variety of active pixel sensor readout circuits (see, for example, Active pixel sensor for digital imaging, WO 2002067337 A2). Active pixel circuits include as a key element, an on-pixel preamplifier circuit in place of the TFT switch circuit  34  shown in  FIG. 3 . 
         [0029]    The MISIM detector element has a lower intrinsic capacitance than a comparably sized PIN photodiode because this intrinsic capacitance arises between the sense  66  and bias  67  electrodes, which are placed further apart (e.g. 5 um) in contrast to a PIN photodiode (e.g. 1 um). In particular, the lower capacitance of the MISIM detector element (here around 0.2 pF for a 100 micron pixel) as compared to PIN photodiodes (typically around 1 pF for a 100 micron pixel) makes the combination of a MISIM detector element with an active pixel sensor readout element superior in terms of signal-to-noise ratio (SNR). The SNR improvement occurs because the input charge to voltage gain of the MISIM detector element is proportionally higher than if a PIN diode is employed due to the proportionally lower capacitance of the MISIM detector element. 
         [0030]    Embedding the readout circuit element underneath the MISIM detector element also has the advantages of maximizing the light absorption area. This becomes more important because active pixel sensor circuits typically use more than one transistor in the readout circuit element in contrast to a switch  34  that requires just one TFT. In WO 2002067337 A2, a three-transistor active pixel sensor circuit is disclosed. Thus, in general, embedding the readout circuit element under the MISIM detector element is beneficial to maximize performance and EQE. 
         [0031]    The TFT drain electrode  76  is connected by a via  64  in an interlevel Dielectric  65  to one of the sense electrodes  66  where the dielectric  65  physically separates the MISIM detector element and the readout circuit element. The dielectric can be chosen from a variety of materials including amorphous silicon nitride, amorphous silicon oxide, amorphous silicon oxynitride, polyimide, benzocyclobutene (BCB), parylene, acrylic, and polystyrene. 
         [0032]    The choice of the dielectric  65  is important particularly because using a MISIM detector element requires the use of potentially high voltages due to insulating contacts. A high voltage on the bias  67  or sense  66  electrodes can give rise to high vertical electric field between the TFT electrodes (e.g. back gate  75 , source  61  or drain  76 ) leading to local breakdown of the dielectric  65  and catastrophic failure. 
         [0033]    However, each material has a different dielectric strength and breakdown voltage and correspondingly requires tuning of layer thickness. This design for high voltage resilience is in addition to the traditional design process undertaken to optimize an interlevel dielectric to serve as a planarization layer and as a low-k dielectric to reduce parasitic coupling capacitance. For example, if BCB is used for the dielectric  65  with a breakdown voltage of 1 MV/cm, and the bias  67  electrodes are set to a potential of 500V, then at least 5 um of BCB are necessary to prevent accidental dielectric  65  breakdown. The thickness of BCB required is now well beyond the thickness used typically for an interlevel dielectric in the TFT industry. Using very thick layers of dielectric  65  requires overcoming integration challenges between the detector element and readout circuit element. 
         [0034]    The amorphous silicon MISIM detector element shown in  FIG. 6  works well if the bias  67  and sense  66  electrode layers are made thin (e.g. 50-100 nm) to avoid step coverage issues for the follow-on insulator  71  and semiconducting  70  layers. Here, a 5 um thick dielectric  65  layer underneath the MISIM detector element can cause a functional (EQE loss) and reliability (poor connectivity) problem if the via is made in the traditional process with steep sidewall angles. So, to allow for proper continuity and coverage, the via  64  in the Dielectric  65  can have a sloped or angled sidewall. For BCB, an angle of 45 degrees or shallower was discovered to work appropriately for this task although other sidewall angles and sense  66  and bias  67  electrode thickness combinations can also work by proper design. 
         [0035]    The photoconductive element implementation shown in  FIG. 6  can be mapped onto the pixel circuit shown in  FIG. 3 , where the transistor gate electrode  63  is connected to the Pixel Gate line  38 , the source electrode  61  is connected to the Pixel Data line  36  (see  FIG. 3 ) and the bias electrodes  67  are connected to the Bias node  33 . Since the MISIM detector element has an intrinsic internal capacitance between the sense  66  and bias  67  electrodes as discussed earlier, the capacitor  32  shown in  FIG. 3  is optional. 
         [0036]    One additional challenge with placing the TFT readout circuit element underneath the MISIM detector element is that the normal operating voltage on the bias  67  and/or sense  66  electrodes can influence TFT operation especially if a bottom gate TFT configuration is employed as is the case in  FIG. 6 . Here, a back gate  75  (e.g. preferentially coupled to the Gate electrode  63  to minimize leakage current) is included to ensure the TFT does not conduct inadvertently due to one of the electrodes on top. If a top gate TFT configuration is employed, the need for the back gate  75  can be mitigated since the top gate will act as an electrostatic shield and prevent the bias  67  or sense  66  electrodes from inadvertently biasing the TFT ON. 
         [0037]    To detect X-rays indirectly, the scintillator  15  can be placed on either side of the radiography detector system  14 . In the device architecture shown in  FIG. 6 , the scintillating layer  68  (akin to the scintillator  15 ) is deposited or placed on top of the MISIM detector element. This is because the semiconducting layer  70  is exposed fully to incident light from the scintillating layer  68  resulting in a higher absorption of incident light and thus, better EQE. If the scintillating layer  68  is deposited or 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  and loss in EQE if the electrodes are opaque and block light from reaching the amorphous silicon  70  semiconducting layer. Also, since the photoconductive element disclosed does not use a p+ doped layer like the PIN photodiode, blue light emitting scintillating phosphors can work. 
         [0038]    The implementation shown in  FIG. 6  uses a bottom gate TFT and a bottom electrode MISIM detector. 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. In addition, it is possible to fabricate the readout circuit element on top of the MISIM detector element for example, using a top electrode MISIM detector and a top gate TFT. Moreover, use of transparent sense  66  and bias  67  electrodes can also enable top and bottom electrode MISIM detectors to be used interchangeably with comparable performance.