Patent Publication Number: US-9419046-B2

Title: Integrated scintillator grid with photodiodes

Description:
TECHNICAL FIELD 
     The present disclosure relates to X-ray imaging and, more particularly, to an integrated scintillator grid with photodiodes. 
     BACKGROUND 
     Solid-state digital X-ray detectors, also referred to as X-ray sensors, of an X-ray imaging system can be constructed by employing either of two physical detection methods, so-called direct and indirect conversion methods. Generally, a direct conversion method makes use of direct production of electrons by X-rays in elemental compounds such as amorphous silicon or selenium, lead oxide, lead iodide, thallium bromide, or various gadolinium compounds. In this case, the electrons are collected via electric fields and electrodes attached to thin film transistors. On the other hand, an indirect conversion method employs conversion of X-ray interactions to flashes of light in well-known scintillating materials such as thallium-activated cesium iodide or gadolinium oxysulfate. In this case, the light flashes are sensed by photodiodes, and the resulting electron currents are again collected by attached transistor electronics. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present disclosure are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified. Components in the figures are not drawn to scale and may be drawn out of proportion to facilitate ease of understanding of embodiments of the present disclosure. 
         FIG. 1  is a cross-sectional view of a structure of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with an embodiment of the present disclosure. 
         FIG. 2  is a cross-sectional view of a structure of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with another embodiments of the present disclosure. 
         FIG. 3A  is a cross-sectional view of a structure of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with yet another embodiments of the present disclosure. 
         FIG. 3B  is an enlarged view of the dash-lined portion of the structure of  FIG. 3A . 
         FIG. 4A  is a cross-sectional view of an optical guide structure in accordance with an embodiment of the present disclosure. 
         FIG. 4B  is an enlarged view of the dash-lined portion of the optical guide structure of  FIG. 4A . 
         FIG. 5A  shows a cross-sectional view of a structure of an integrated scintillator grid with photodiodes of an X-ray imaging system during a fabrication process in accordance with an embodiment of the present disclosure. 
         FIG. 5B  shows a cross-sectional view of the structure of  FIG. 5A  in a subsequent stage of the fabrication process in accordance with an embodiment of the present disclosure. 
         FIG. 5C  shows a cross-sectional view of the structure of  FIG. 5B  in a subsequent stage of the fabrication process in accordance with an embodiment of the present disclosure. 
         FIG. 5D  shows a cross-sectional view of the structure of  FIG. 5C  in a subsequent stage of the fabrication process in accordance with an embodiment of the present disclosure. 
         FIG. 6  is a top view and a cross-sectional view of a structure of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with still another embodiments of the present disclosure. 
         FIG. 7A  is a cross-sectional view of a conventional X-ray imaging system in operation. 
         FIG. 7B  is a cross-sectional view of another conventional X-ray imaging system in operation. 
         FIG. 8  is a cross-sectional view of an X-ray imaging system in accordance with an embodiment of the present disclosure in operation. 
         FIG. 9  is a flowchart of a fabrication process of a structure of an integrated scintillator grid with photodiodes in accordance with an embodiment of the present disclosure. 
     
    
    
     In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustrating specific exemplary embodiments in which the disclosure may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the concepts disclosed herein, and it is to be understood that modifications to the various disclosed embodiments may be made, and other embodiments may be utilized, without departing from the scope of the present disclosure. The following detailed description is, therefore, not to be taken in a limiting sense. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a structure  100  of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with an embodiment of the present disclosure. As shown in  FIG. 1 , structure  100  includes a silicon wafer  102 . Silicon wafer  102  may include a device layer  104 , an insulator layer  106  and a handle substrate  108 . In some embodiments, silicon wafer  102  may be a silicon-on-insulator (SOI) wafer with a layer of buried oxide, which may be insulator layer  106 . In some embodiments, thickness of the handle substrate  108  may be approximately 500 microns. In some embodiments, thickness of the buried oxide of the SOI wafer may be approximately 2500 angstroms. In some embodiments, device layer  104  may be an epi layer and may have a thickness of approximately 6 microns. 
     Device layer  104  may have an array of photodiodes constructed therein. That is, an array of photodiodes may be provided in device layer  104  on a first side (e.g., the top side of silicon wafer  102  shown in  FIG. 1 ) of silicon wafer  102 . The following description of a given photodiode of the array of photodiodes applies to each photodiode of the array. As shown in  FIG. 1 , each photodiode of the array of photodiodes may include a p region  126  formed in an n epi region  128  to form a PN diode. A contact diffusion of p+ impurities may be formed as a p+ region  124 . In some embodiments, n+ isolation regions may be placed in n epi region  128  to isolate a depletion region  130  formed around the PN diode. In  FIG. 1 , depletion regions  130  are indicated by dashed lines around the p region  126 . Structure  100  may additionally include an array of metal contacts  120  each of which is aligned with and corresponds to a respective photodiode of the array of photodiodes. Additionally, contacts  122  may be formed between the metal contacts  120  and photodiodes so that each photodiode is electrically connected to the respective metal contact  120 . 
     Handle substrate  108  may have an array of grid holes  116  constructed therein. That is, an array of grid holes  116  may be provided in handle substrate  108  on a second side (e.g., the bottom side of silicon wafer  102  shown in  FIG. 1 ) of silicon wafer  102 . Grid holes  116  may be etched into the second side of silicon wafer  102  by any suitable process. In some embodiments, width of each grid hole  116  may be approximately 90 microns or more and thickness of sidewalls  114  of grid holes  116  may be approximately 10 microns or more. A layer of scintillating material  110  may be disposed over the array of grid holes  116  on the second side of silicon wafer  102  to cover up the array of grid holes  116 . Additionally, a layer of reflective material  112  may be disposed on the layer of scintillating material  110 . In some embodiments, scintillating material  110  may include CsI:Ta or Gaddox. In some embodiments, reflective material  112  may include aluminum (Al). 
     In operation, an object may be placed between an X-ray source and an X-ray imaging system in which the structure  100  of an integrated scintillator grid with photodiodes is implemented. Incident X-rays on the X-ray imaging system will mirror the nature of the object. Dense parts of the object tend to absorb significant amount of the X-rays and lighter parts of the object tend to let through most of the X-rays. As a result, spatial distribution of the X-ray dose represents the image of the object. 
     As shown in  FIG. 1 , X-rays  140  coming through an object are radiated on the second side (e.g., the bottom side) of silicon wafer  102 . X-rays  140  will go through the layer of reflective material  112  and go into the layer of scintillating material  110  up to a distance D measured from an interface between the layer of reflective material  112  and the layer of scintillating material  110 . Distance D depends on the type of scintillating material  110  used. When CsI:TA is utilized in scintillating material  110  the distance D may be approximately 300 microns. Thickness of the scintillating material  110  in  FIG. 1  is larger than D so that when X-rays  140  are converted into light  142  most of the X-rays are absorbed to give highest X-ray-to-light conversion efficiency. Light  144  travels through grid holes  116  and the layer of insulator  106  (e.g., buried oxide), and travel into the silicon of device layer  104  including the PN diodes, which may be in reverse bias of −2V at p+ contact and 0.0V at n+ isolation layer common to all the PN diodes. In some embodiments, each of the PN diodes may be designed such that most of the light is absorbed in the depletion layer  130  above and around the PN diode. Light photon in the depletion layer  130  will generate electron-hole pairs. The electric field in the depletion region  130  will cause holes  146  (shown as “h” in  FIG. 1 ) to travel towards p region  126 , p+ region  124  and contact  122 , thereby generating photocurrent at a fixed bias voltage. This current can be sensed and can be related to incident X-ray radiation. 
     An alternative way to sense X-ray photons is to pre-charge the PN diodes with a fixed voltage, such as 2.0V for example, through a MOS transistor prior to radiating with X-rays and turn off the MOS transistor, before radiating with X-rays. Light generated by the scintillating material  110  will be converted to hole-electron pairs and the PN diodes will start discharging to a voltage level corresponding to the incident light  144  or X-rays  140 . Now, by adding one more MOS transistor, the voltage on the PN diode can be sensed and amplified using more MOS transistors. Sensing circuits that perform such functions are well known in industry and, thus, in the interest of brevity a detailed description thereof is not provided herein. 
     A sense chip having an array of sensing circuits as described above may be formed over the first side (e.g., top side) of silicon wafer  102  such that each sensing circuit of the array of sensing circuits is configured to sense an electrical signal in a respective photodiode of the array of photodiodes corresponding to electron-hole pairs in the respective photodiode caused by light absorbed by the respective photodiode. Each sensing circuit along with its corresponding photodiode may form a pixel. Silicon wafer  102 , which an array of photodiodes in the device layer  104  and an array of grid holes  116  in the handle substrate  108 , may be referred to as a photo chip hereinafter. Each pair of photodiode and its corresponding grid hole  116  may be referred to as a photo pixel hereinafter. The circuit needed to pre-charge and sense the electrical signal in a corresponding photodiode may be referred to as sense pixel hereinafter. An array of sense pixels along with peripheral circuits to select and sense the electrical signals, e.g., voltage, generated by photodiodes is placed on top of the array of photo pixels so that all the photodiodes under the sense chip are addressed by all the sense pixels in the sense chip. Each sense pixel may include a contact pad that is aligned to a contact pad in the corresponding photo pixel that is connected to the respective photodiode. Thus, a sense pixel is connected to photo pixels. 
     When X-rays  140  are radiated as shown in  FIG. 1 , it is converted into light  142  by the layer of scintillating material  110  and in turn light  144  traveling through each grid hole  116  is converted to electron-hole pairs in the respective photodiode under the grid hole  116 . Electron-hole pairs will recombine in the photodiodes and a voltage corresponding to X-rays  140  or light  144  in the grid hole will be generated. Sense chip may be designed to sense electrical signals, e.g., voltage, from all the photodiodes, convert the sensed electrical signals into digital data and stream the digital data out to a digital signal processor (DSP) for signal processing and generation of an image of the object. 
     In view of the above, it will be appreciated by those of ordinary skill in the art that the proposed scheme provides a number of advantages over conventional designs of the X-ray detector or sensor in an X-ray imaging systems. Firstly, structure  100  as shown in  FIG. 1  does not require filling of the grid holes  116 . Secondly, while the X-ray-to-light conversion efficiency is about 6% for conventional designs in which grid holes are filled with scintillating material, the proposed scheme can result in an X-ray-to-light conversion efficiency of more than 50%. Moreover, the proposed scheme can achieve very high modular transfer function or contrast. 
     For proof of concept, a chip with through-holes of 90-micron and 10-micron walls were fabricated. A scintillator sheet was placed on one side of the chip. X-rays were radiated on the scintillator sheet. The observed X-ray-to-light conversion efficiency was more than 60%. 
       FIG. 2  is a cross-sectional view of a structure  200  of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with another embodiments of the present disclosure. Structure  200  may be similar or identical to structure  100  in various aspects and, thus, detailed description of structure  200 , except for any difference, is not provided in the interest of brevity. In structure  200 , the grid holes are filled with a filler material  216 . In some embodiments, filler material  216  may be a transparent solid material such as, for example, silicon dioxide (SiO 2 ), polyimide coatings or any transparent glass. This feature provides firmness to the silicon wafer of structure  200  for mechanical handling. 
     Alternatively, filler material  216  may be a scintillating material such as, for example, CsI:Ta. The scintillating material, e.g., CsI:Ta, may be deposited in the grid holes by evaporation or by melting powder CsI:Ta. Once grid holes are filled, an additional layer of CsI:Ta may be deposited over the array of filled grid holes. In some embodiments, about 300 microns of CsI:Ta may be deposited in the grid holes and additional 300 microns of CsI:Ta may be deposited on the top of the array of filled grid holes. One main advantage of this feature is that CsI:Ta in the grid holes will pick up X-rays that have not been absorbed in the CsI:Ta layer that is deposited over the array of filled grid holes. 
       FIG. 3A  is a cross-sectional view of a structure  300  of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with yet another embodiments of the present disclosure.  FIG. 3B  is an enlarged view of the dash-lined portion  302  of structure  300 . Structure  300  may be similar or identical to structure  100  in various aspects and, thus, detailed description of structure  300 , except for any difference, is not provided in the interest of brevity. In structure  300 , silicon sidewalls  304  of the grid holes are coated with a cladding material  306  with suitable refractive index. The grid holes are filled with appropriate glass  308  to form a structure similar to fiber optic cable. This design will improve the light transmission through the grid holes significantly. 
       FIG. 4A  is a cross-sectional view of an optical guide structure  400  in accordance with an embodiment of the present disclosure.  FIG. 4B  is an enlarged view of the dash-lined portion  402  of optical guide structure  400 . As shown in  FIG. 4A , optical guide structure  400  includes a silicon wafer  402 . Silicon wafer  402  may have an array of grid holes  406  constructed therein. Grid holes  406  may be etched into silicon wafer  402  by any suitable process. In some embodiments, width of each grid hole  406  may be approximately 90 microns or more and thickness of sidewalls  404  of grid holes  406  may be approximately 10 microns or more. A layer of scintillating material  408  may be disposed over the array of grid holes  406  to cover up the array of grid holes  406 . Additionally, a layer of reflective material  410  may be disposed on the layer of scintillating material  408 . In some embodiments, scintillating material  408  may include CsI:Ta or Gaddox. In some embodiments, reflective material  410  may include aluminum. 
     As shown in  FIG. 4B , in some embodiments, silicon sidewalls  404  of grid holes  406  are coated with a cladding material  412  with suitable refractive index. Grid holes  406  may be filled with appropriate glass  414  to form a structure similar to fiber optic cable. This design will improve the light transmission through the grid holes  406  significantly. 
       FIGS. 5A-5D  show a cross-sectional view of a structure  500  of an integrated scintillator grid with photodiodes of an X-ray imaging system during a fabrication process in accordance with an embodiment of the present disclosure. Structure  500  may be similar or identical to structure  100  (grid holes not filled) or structure  200  (grid holes filled with filler material) in various aspects and, thus, detailed description of structure  500 , except for any difference, is not provided in the interest of brevity. 
     As shown in  FIG. 5A , structure  500  includes a silicon wafer. Silicon wafer may include a device layer, an insulator layer and a handle substrate. In some embodiments, silicon wafer may be a SOI wafer with a layer of buried oxide, which may be insulator layer. Device layer may have an array of photodiodes constructed therein. That is, an array of photodiodes may be provided in device layer on a first side (e.g., the top side of silicon wafer shown in  FIG. 5A ) of silicon wafer. Handle substrate may have an array of grid holes constructed therein. That is, an array of grid holes may be provided in handle substrate on a second side (e.g., the bottom side of silicon wafer shown in  FIG. 5A ) of silicon wafer. 
     As shown in  FIG. 5B , a layer of scintillating material, such as CsI:Ta or another scintillating material, is deposited on the back side of the silicon wafer. In some embodiments, thickness of the layer of scintillating material may be 300 microns. 
     As shown in  FIG. 5C , the layer of scintillating material is etched to form an array of scintillators (or islands of scintillating material) isolated from each other such that each scintillator covers a respective grid hole of the array of grid holes. In some embodiments, a layer of poly(p-xylylene) polymers (not shown), such as Parylene, may be deposited to protect the scintillating material from moisture. Parylene is the trade name for a variety of chemical vapor deposited poly(p-xylylene) polymers used as moisture and dielectric barriers. 
     As shown in  FIG. 5D , a layer of reflective material, such as aluminum or another suitable material, is deposited over the array of scintillators. There are several reasons for using aluminum as the reflective material. Firstly, aluminum can reflect light back into grid holes thereby increasing the light that enters the grid holes. Secondly, aluminum reduces loss of light going sideways. Thirdly, aluminum blocks the light in a given grid hole going to neighboring grid hole(s). 
       FIG. 6  shows a top view and a cross-sectional view of a structure  600  of an integrated scintillator grid with photodiodes of an X-ray imaging system in accordance with still another embodiments of the present disclosure. Structure  600  may be similar or identical to structure  100  (grid holes not filled) or structure  200  (grid holes filled with filler material) in various aspects and, thus, detailed description of structure  500 , except for any difference, is not provided in the interest of brevity. 
     In the example shown in  FIG. 6 , thickness “a” of handle substrate of the SOI wafer is approximately 300 microns, and thickness “b” of the layer of scintillating material is approximately 200 microns. A holding ring may be utilized to contain the layer of scintillating material, e.g., CsI:Ta, therein. 
       FIG. 7A  is a cross-sectional view of a conventional X-ray imaging system  710  in operation. As shown in  FIG. 7A , an X-ray pulse in the scintillator may scatter in a wide scattering angle (represented by the two angled lines with arrows) and be sensed by any one of the photo sensors within the scattering angle. As a result, the detected image of an object may have less-than ideal resolution and may be blurry. 
       FIG. 7B  is a cross-sectional view of another conventional X-ray imaging system  720  in operation. As shown in  FIG. 7B , an X-ray pulse in the scintillator within sidewalls of a collimating structure, e.g., grid holes, may scatter in a narrow scattering angle (represented by the two angled lines with arrows) and be sensed by one photo sensor that corresponds to the particular grid hole. This design, however, tends to have a less-than-optimal X-ray-to-light conversion efficiency. 
       FIG. 8  is a cross-sectional view of an X-ray imaging system  800  in accordance with an embodiment of the present disclosure. X-ray imaging system  800  may utilize a structure similar to any of structures  100 ,  200 ,  300  and  500  as well as optical guide structure  400  as described above. As shown in  FIG. 8 , each of incoming X-ray pulses has a relatively wider scattering angle when in the scintillator and a relatively narrower scattering angle when within the collimating structure, e.g., grid holes. This design results in optimal X-ray-to-light conversion efficiency. 
       FIG. 9  is a flowchart of a fabrication process  900  of a structure of an integrated scintillator grid with photodiodes in accordance with an embodiment of the present disclosure. Process  900 , wholly or partially, may be implemented to fabricate any of structures  100 ,  200 ,  300  and  500  as well as optical guide structure  400  as described above. Further, process  900  may include one or more operations, actions, or functions depicted by one or more blocks  910 ,  920 ,  930 ,  940 ,  950  and  960 . Although illustrated as discrete blocks, various blocks may be divided into additional blocks, combined into fewer blocks, or eliminated, depending on the desired implementation. 
     At  910 , process  900  may form an array of photodiodes on a first side of a silicon wafer. For example, process  900  may form an array of photodiodes, which are electrically isolated from each other, in a device layer on a first side of a silicon wafer. 
     At  920 , process  900  may form an array of grid holes on a second side of the silicon wafer opposite the first side. For example, process  900  may form an array of grid holes on a second side of the silicon wafer opposite the first side thereof, with each grid hole of the array of grid holes aligned with a respective photodiode of the array of photodiodes. 
     At  930 , process  900  may dispose a layer of scintillating material over the array of grid holes on the second side of the silicon wafer. 
     At  940 , process  900  may dispose a layer of reflective material on the layer of scintillating material. 
     At  950 , process  900  may form an array of sensing circuits configured to sense electrical signals in the photodiodes. For example, process  900  may form an array of sensing circuits such that each sensing circuit of the array of sensing circuits is configured to sense an electrical signal in a respective photodiode of the array of photodiodes corresponding to electron-hole pairs in the respective photodiode caused by light absorbed by the respective photodiode. 
     At  960 , process  900  may bond the array of sensing circuits to the array of photodiodes. For example, process  900  may bond the array of sensing circuits to the array of photodiodes on the first side of the silicon wafer such that each sensing circuit of the array of sensing circuits is electrically connected to the respective photodiode. 
     In some embodiments, the silicon wafer may include a silicon-on-insulator (SOI) wafer. 
     In some embodiments, the grid holes on the second side of the SOI wafer may reach an insulator of the SOI wafer. 
     In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the reflective material may include aluminum (Al). 
     In some embodiments, in disposing the layer of scintillating material over the array of grid holes, the method may perform operations including: filling the grid holes with the scintillating material; and depositing the layer of scintillating material over the array of grid holes which are filled with the scintillating material. 
     In some embodiments, a thickness of the layer of scintillating material may be greater than or equal to a depth of the grid holes. 
     In some embodiments, the method may further include filling the grid holes with a filler material. 
     In some embodiments, the filler material may include a transparent solid material. In some embodiments, the transparent solid material may include silicon dioxide (SiO 2 ), polyimide, or glass. 
     Alternatively, the filler material may include a scintillating material. In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the method may further include, prior to filling the grid holes with the filler material, coating a layer of refractive material on sidewalls of the grid holes. 
     In some embodiments, the method may further include, prior to disposing the layer of reflective material on the layer of scintillating material, etching the layer of scintillating material into an array of scintillators such that each scintillator of the array of scintillators is physically separate from each other and covers a respective grid hole of the array of grid holes. 
     In some embodiments, the method may further include depositing a layer of polyp-xylylene) polymers on the array of scintillators. 
     Highlights of Features 
     In view of the above, features of various embodiments of the present disclosure are highlighted below. 
     In one aspect, a structure implemented in an X-ray imaging system may include a silicon wafer. The silicon wafer may include a first side and a second side opposite the first side. The silicon wafer may also include an array of photodiodes on the first side of the silicon wafer with the photodiodes electrically isolated from each other. The silicon wafer may further include an array of grid holes on the second side of the silicon wafer. Each grid hole of the array of grid holes may be aligned with a respective photodiode of the array of photodiodes. The structure may also include a layer of scintillating material disposed over the array of grid holes on the second side of the silicon wafer. The structure may further include a layer of reflective material disposed on the layer of scintillating material. 
     In some embodiments, the silicon wafer may include a silicon-on-insulator (SOI) wafer. 
     In some embodiments, the grid holes on the second side of the SOI wafer may reach an insulator of the SOI wafer. 
     In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the reflective material may include aluminum (Al). 
     In some embodiments, the grid holes may be filled with a filler material. 
     In some embodiments, the filler material may include a transparent solid material. In some embodiments, the transparent solid material may include silicon dioxide (SiO 2 ), polyimide, or glass. 
     Alternatively, the filler material may include a scintillating material. In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, a thickness of the layer of scintillating material may be greater than or equal to a depth of the grid holes. 
     In some embodiments, the structure may further include a layer of refractive material on sidewalls of the grid holes. 
     In some embodiments, the layer of scintillating material may include an array of scintillators such that each scintillator of the array of scintillators is physically separate from each other and covers a respective grid hole of the array of grid holes. 
     In some embodiments, the structure may further include a layer of poly(p-xylylene) polymers deposited on the array of scintillators. 
     In some embodiments, the structure may further include an array of sensing circuits. Each sensing circuit of the array of sensing circuits may be electrically connected to the respective photodiode. Each sensing circuit of the array of sensing circuits may be configured to sense an electrical signal in a respective photodiode of the array of photodiodes corresponding to electron-hole pairs in the respective photodiode caused by light absorbed by the respective photodiode. 
     In another aspect, a structure implemented in an X-ray imaging system may include a silicon wafer. The silicon wafer may include a first side and a second side opposite the first side. The silicon wafer may also include an array of grid holes on the second side of the silicon wafer. Each grid hole of the array of grid holes may be aligned with a respective photodiode of the array of photodiodes. The structure may also include a layer of scintillating material disposed over the array of grid holes on the second side of the silicon wafer. The structure may further include a layer of reflective material disposed on the layer of scintillating material. 
     In some embodiments, the silicon wafer may include a silicon-on-insulator (SOI) wafer. 
     In some embodiments, the grid holes on the second side of the SOI wafer may reach an insulator of the SOI wafer. 
     In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the reflective material may include aluminum (Al). 
     In some embodiments, the grid holes may be filled with a filler material. 
     In some embodiments, the filler material may include a transparent solid material. In some embodiments, the transparent solid material may include silicon dioxide (SiO 2 ), polyimide, or glass. 
     Alternatively, the filler material may include a scintillating material. In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, a thickness of the layer of scintillating material may be greater than or equal to a depth of the grid holes. 
     In some embodiments, the structure may further include a layer of refractive material on sidewalls of the grid holes. 
     In some embodiments, the layer of scintillating material may include an array of scintillators such that each scintillator of the array of scintillators is physically separate from each other and covers a respective grid hole of the array of grid holes. 
     In some embodiments, the structure may further include a layer of poly(p-xylylene) polymers deposited on the array of scintillators. 
     In one aspect, a method of fabricating an X-ray imaging system may include:
         forming an array of photodiodes, which are electrically isolated from each other, in a device layer on a first side of a silicon wafer; forming an array of grid holes on a second side of the silicon wafer opposite the first side thereof, each grid hole of the array of grid holes aligned with a respective photodiode of the array of photodiodes; disposing a layer of scintillating material over the array of grid holes on the second side of the silicon wafer; disposing a layer of reflective material on the layer of scintillating material; forming an array of sensing circuits such that each sensing circuit of the array of sensing circuits is configured to sense an electrical signal in a respective photodiode of the array of photodiodes corresponding to electron-hole pairs in the respective photodiode caused by light absorbed by the respective photodiode; and bonding the array of sensing circuits to the array of photodiodes on the first side of the silicon wafer such that each sensing circuit of the array of sensing circuits is electrically connected to the respective photodiode.       

     In some embodiments, the silicon wafer may include a silicon-on-insulator (SOI) wafer. 
     In some embodiments, the grid holes on the second side of the SOI wafer may reach an insulator of the SOI wafer. 
     In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the reflective material may include aluminum (Al). 
     In some embodiments, in disposing the layer of scintillating material over the array of grid holes, the method may perform operations including: filling the grid holes with the scintillating material; and depositing the layer of scintillating material over the array of grid holes which are filled with the scintillating material. 
     In some embodiments, a thickness of the layer of scintillating material may be greater than or equal to a depth of the grid holes. 
     In some embodiments, the method may further include filling the grid holes with a filler material. 
     In some embodiments, the filler material may include a transparent solid material. In some embodiments, the transparent solid material may include silicon dioxide (SiO 2 ), polyimide, or glass. 
     Alternatively, the filler material may include a scintillating material. In some embodiments, the scintillating material may include CsI:Ta. 
     In some embodiments, the method may further include, prior to filling the grid holes with the filler material, coating a layer of refractive material on sidewalls of the grid holes. 
     In some embodiments, the method may further include, prior to disposing the layer of reflective material on the layer of scintillating material, etching the layer of scintillating material into an array of scintillators such that each scintillator of the array of scintillators is physically separate from each other and covers a respective grid hole of the array of grid holes. 
     In some embodiments, the method may further include depositing a layer of polyp-xylylene) polymers on the array of scintillators. 
     Additional Notes 
     Reference throughout this specification to “one embodiment,” “an embodiment,” “one example,” or “an example” means that a particular feature, structure, or characteristic described in connection with the embodiment or example is included in at least one embodiment of the present disclosure. Thus, appearances of the phrases “in one embodiment,” “in an embodiment,” “one example,” or “an example” in various places throughout this specification are not necessarily all referring to the same embodiment or example. Furthermore, the particular features, structures, databases, or characteristics may be combined in any suitable combinations and/or sub-combinations in one or more embodiments or examples. In addition, it should be appreciated that the figures provided herewith are for explanation purposes to persons ordinarily skilled in the art and that the drawings are not necessarily drawn to scale. 
     As used to describe such embodiments, terms such as “above”, “below”, “upper”, “lower”, “top”, “bottom”, “horizontal”, “vertical” and “side”, for example, describe positions relative to an arbitrary axis of an element. In particular, the terms “above” and “below” refer to positions along an axis, where “above” refers to one side of an element while “below” refers to an opposite side of an element. Relative to terms “above” and “below”, the term “side” refers to a side of an element that is displaced from an axis, such as the periphery of the element, for example. Further, it is understood that such terms do not necessarily refer to a direction defined by gravity or any other particular orientation reference. Instead, such terms are merely used to identify one portion versus another portion. Accordingly, “upper” and “lower” may be equivalently interchanged with “top” and “bottom”, “first” and “second”, “right” and “left”, and so on. The term “horizontal” may refer to an orientation perpendicular to an axis while the term “vertical” may refer to an orientation parallel to the same axis, and vice versa. 
     The terms “and”, “and/or” and “or” as used herein may include a variety of meanings that are expected to depend at least in part upon the context in which such terms are used. Typically, the term “or” as well as “and/or”, if used to associate a list, such A, B or C, is intended to mean A, B and C, used in the inclusive sense, as well as A, B or C, used in the exclusive sense. In addition, the term “one or more” as used herein may be used to describe any feature, structure, or characteristic in the singular or plural sense to describe some combination of features, structures or characteristics. Though, it should be noted that this is merely an illustrative example and the claimed subject matter of the present disclosure is not limited to this example. 
     Although the present disclosure is described in terms of certain preferred embodiments, other embodiments will be apparent to those of ordinary skill in the art, given the benefit of this disclosure, including embodiments that do not provide all of the benefits and features set forth herein, which are also within the scope of this disclosure. It is to be understood that other embodiments may be utilized, without departing from the scope of the present disclosure.