Abstract:
An image sensor array includes image sensors having photo TFTs to generate photocurrent in response to received images. The photo TFTs each have their respective gate electrodes shorted to source electrodes to increase generated photocurrent. Storage capacitors are coupled to each photo TFT and discharged upon generation of a photocurrent. Each storage capacitor is coupled to a readout TFT that passes a current from the storage capacitor to a data line. Data lines indicate location of the received image on the image sensor array.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part and claims the benefit under 35 U.S.C. § 120 of U.S. patent application Ser. No. 10/217,798, filed Aug. 12, 2002, and U.S. patent application Ser. No. 10/307,106, filed Nov. 27, 2002. In addition, this application claims the benefit under 35 U.S.C. § 19(e) of U.S. Provisional Patent Application No. 60/359,263, filed Feb. 20, 2002. 
    
    
     TECHNICAL FIELD 
     The present invention relates to image sensors and, more specifically, to flat panel image sensors employing photosensitive thin film transistors. 
     BACKGROUND OF THE INVENTION 
     Flat panel image sensors are commercially successful products that are able to effectively detect electromagnetic radiation in and near the visible light spectrum. Flat panel image sensors are fabricated by depositing and patterning various metals, insulators, and semiconductors on glass substrates as is done in flat panel displays. Such sensors commonly employ photosensitive elements, such as amorphous silicon (a-Si) PIN diodes. The photosensitive element is coupled to a readout switch, such as thin film transistor (TFT), that provides data indicative of received light. 
     A common use for flat panel image sensors is for medical and industrial applications to detect X-rays. The image sensor includes a phosphorescent screen that overlays an array of image sensing elements. The phosphorescent screen converts received X-rays to visible light. The array receives the visible light and generates a photocurrent responsive to the light. The photocurrent is read out as data indicative of the sensed light. 
     The arrays are difficult to manufacture since separate process steps are required to construct the PIN diodes and the TFTs. The total mask count may be 8 or more which is burdensome while the yields are low. Furthermore, a-Si PIN diodes are not a standard device in flat panel display processing which increases manufacturing expense. It would therefore be advantageous to use a standard process to greatly reduce the cost of flat panel image sensors. 
     Manufacturing TFTs for flat panel applications is a common process. A common use for TFTs is in active matrix liquid crystal displays (AMLCDs). Each TFT functions as a switch for a pixel in a matrix display. The voltage across each pixel is controlled independently and at a high contrast ratio. TFTs may be fabricated by depositing and patterning metals, insulators, and semiconductors on substrates through methods well known in the art. TFTs typically employ a-Si, polycrystalline silicon, or CdSe film as the semiconductor material. A-Si is typically used in flat panel applications as it is easily deposited on large area glass substrates at temperatures below 350 centigrade. 
     TFTs are more economical to fabricate than a-Si PIN diodes and are well suited for flat panel applications. The present inventors have recognized that if both the image sensing element and the readout switch of an image sensor array were incorporated as TFTs, fewer photomasks would be required and manufacturing costs would be greatly reduced. 
     TFTs have not commonly been used as photosensitive elements. U.S. patent application Publication Nos. 2001/0055008 and 2001/0052597, both to Young et al. (hereinafter the “Young applications”), disclose the use of TFTs as light sensing elements for a display device. The light sensing elements provide feedback to progressively adjust the current flow through display elements to control light output. However, the use of TFTs exclusively for an image sensor is not disclosed. Since a TFT is more economical to manufacture and has already been successfully incorporated into flat panel applications, the present inventors have recognized that it would be advantageous to employ TFTs in image sensors. 
     SUMMARY OF THE INVENTION 
     An image sensor array includes image sensors disposed on a substrate and arranged to receive and sense an image. Each image sensor represents a pixel for a received image. The image sensors each include a photo TFT that generates a photocurrent in response to the image. The photo TFT may be manufactured using common processes for TFTs in flat panel applications. The photo TFT has a gate electrode which is shorted to its source electrode to obtain a photocurrent that is substantially independent of source-drain bias. The photo TFT may also be configured with interdigitated source and drain electrodes to increase the photosensitivity. 
     Each photo TFT is coupled to a bias line to enable operation and a storage capacitor to store a charge and discharge upon generation of a photocurrent. The storage capacitor is coupled to a readout TFT that is also manufactured using known processes. The readout TFT passes a charge from the storage capacitor to a data line. Operation of the readout TFT is enabled by a select line which is coupled to the gate electrode of the readout TFT. A light shield may be disposed over the channel of the readout TFT to prevent a charge leaking through the readout TFT. 
     The photo TFTs can provide an effective and economical alternative to conventional photodiodes. Photo TFTs may be manufactured with corresponding readout TFTs using conventional methods thereby reducing mask counts and costs. Photo TFTs may further yield photocurrents greater than that of photodiodes. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments thereof, which proceeds with reference to the accompanying drawings. 
     Additional aspects and advantages of this invention will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-exhaustive embodiments of the invention are described with reference to the figures in which: 
         FIG. 1A  is a schematic diagram of an embodiment of a sensor element; 
         FIG. 1B  is a schematic diagram of an alternative embodiment of a sensor element; 
         FIG. 2  is a schematic diagram of an array of sensor elements; 
         FIG. 3  is a plan view of an embodiment of sensor elements for use in an image sensor array; 
         FIG. 4  is a cross-sectional view of one embodiment of a photo TFT; 
         FIG. 5  is a cross-sectional view of one embodiment of a readout TFT and a storage capacitor; and 
         FIG. 6  is a graph comparing the photocurrent for a PIN photodiode with the photocurrent for a photo TFT. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference is now made to the figures in which like reference numerals refer to like elements. For clarity, the first digit or digits of a reference numeral indicates the figure number in which the corresponding element is first used. 
     Throughout the specification, reference to “one embodiment” or “an embodiment” means that a particular described feature, structure, or characteristic is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. 
     Furthermore, the described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. Those skilled in the art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or not described in detail to avoid obscuring aspects of the invention. 
     Referring to  FIG. 1A , an embodiment of a sensor element  10  is shown suitable for use in an image sensor matrix array. The sensor element  10  includes a photo TFT  12  that generates a photocurrent in response to received light. The photo TFT  12  is easily fabricated using common thin film layers. In the embodiment shown, the photo TFT  12  is a gated device having similarly doped contact regions and an intrinsic semiconductor region disposed between. The photo TFT  12  has a gate electrode  14  that is coupled directly to the source electrode  16 . The coupling of the gate and source electrodes  14 ,  16  creates a generated photocurrent that, for short channel lengths, exceeds that of an a-Si PIN photodiode. The photocurrent is a secondary photocurrent and has a potential gain of more than one whereas an a-Si PIN photodiode typically has a gain of less than one. The source electrode  16  is coupled to a bias line  17  which is common to all photosensitive elements in an array. In operation, the bias line  17  may be coupled to a negative voltage. 
     The source and drain electrodes  16 ,  18  of the photo TFT  12  are coupled to a storage capacitor  20  which is discharged when the photo TFT  12  is exposed to light. The storage capacitor  20  is coupled to the source electrode  22  of a readout TFT  24 . The charge on the storage capacitor  20  is read out periodically through the readout TFT  24  and a data line  26 . As shown, the gate electrode  28  of the readout TFT  24  is coupled to a select line  30  to enable the readout TFT  24 . A drain electrode  32  is coupled to the data line  26  to readout a charge. 
     The photo TFT  12  and the readout TFT  24  may be manufactured using common TFT manufacturing methods, such as in AMLCD applications. The TFTs  12 ,  24  generally include substantially co-planar source and drain electrodes, a semiconductor layer between the source and drain electrodes, and a gate electrode in proximity to the semiconductor layer but electrically insulated by a gate insulator. Current flow between the source and drain electrodes is controlled by the application of a voltage to the gate electrode. The voltage to the gate electrode produces an electric field which accumulates a charged region near the semiconductor-gate insulator interface. This charged region forms a current conducting channel in the semiconductor layer through which the device current is conducted. 
     Referring to  FIG. 1B  an alternative embodiment of a sensor element  33  is shown. The sensor element  33  differs from the previously shown embodiment in that the gate electrode  14  is coupled directly to the drain electrode  18 . In operation, the bias line  17  has a positive voltage bias. Operation of the sensor element  33  is similar to that of sensor element  10 . The storage capacitor  20  is discharged when the photo TFT  12  is exposed to light. The charge on the storage capacitor  20  is read out periodically through the readout TFT  24  and the data line  26 . 
     Referring to  FIG. 2 , a schematic is shown of an image sensor array  34  that includes regularly-spaced sensor elements  10  of  FIG. 1A . Although elements  10  of  FIG. 1A  are shown, sensor elements of  FIG. 1B  may be used as well. The sensor elements  10  are arranged at intersections between rows and columns with each element corresponding to an individual pixel. The rows are designated by the select lines  30  and the columns are designated by the data lines  26 . Individual sensor elements respond to received light and generate a data signal that is transmitted on an associated data line  26 . As data signals pass through the data lines  26  to a control circuit, an image sensor array  34  determines the location of a received image on the array  34 . 
     The array  34  is for illustrative purposes only as an image sensor would have far more sensor elements. The select lines  30  may be in communication with an address circuit to address individual readout TFTs. 
     Referring to  FIG. 3 , a plan view of four sensor elements  10  is shown. The photo and readout TFTs  12 ,  24  may be embodied as various structures which may be manufactured using processes similar to that for TFTs in an AMLCD. The plan view provides a view of components relative to one another. As shown, the photo TFT  12  has a source electrode  16  and drain electrode  18  configured with extending members. The extending members are disposed relative to one another to form an interdigitated pattern. 
     Referring to  FIG. 4 , a cross-sectional view of one embodiment of a photo TFT  12  of  FIG. 3  is shown. The photo TFT  12  may be used in either sensor elements  10 ,  33  of  FIG. 1A  or  1 B. The photo TFT  12  includes a gate electrode  14  deposited and patterned on an insulating transparent substrate  40 , such as glass, quartz, sapphire, or the like. The gate electrode  14  may include metals such as Cr, Cu, Al, Ta, Ti or the like. A gate insulator  42  insulates the gate electrode  14  from a semiconductor layer  44 . The gate insulator  42  may include various materials well known in the art, such as silicon nitride, silicon oxide, or the like. 
     A semiconductor layer  44  is stacked on the gate insulator  42  above the gate electrode  14  and may include a-Si, p-Si, amorphous silicon carbide (SiC), tellurium (Te), selenium (Se), cadmium sulfide (CdS), cadmium selenide (CdSe), or the like. However, a-Si is well suited to large area fabrication on glass substrates at temperatures below 350 centigrade and exhibits a very high, dark resistivity. The semiconductor layer  44  may be deposited in accordance with methods known in the art including sputtering or deposition, such as plasma-enhanced chemical vapor deposition. The semiconductor layer  44  may be patterned through known methods, such as by use of a mask and photolithographic processing. 
     The semiconductor layer  44  may include an n+ layer that contacts the source and drain electrodes  16 ,  18 . The n+ layer may be deposited on opposing ends of the semiconductor layer  44  where contact exists with the source and drain electrodes  16 ,  18 . The n+ layer provides a low resistance contact for the source and drain electrodes  16 ,  18 , and suppresses hole injection at negative gate voltage. 
     The source and drain electrodes  16 ,  18  are patterned at a certain distance from one another so that the electrodes  16 ,  18  are separated by a co-planar region of semiconductor material  44  in order to form a gate controlled current channel. The semiconductor layer  44  may be configured as shown in  FIG. 4 , to provide contact between alternating source and drain electrode extending members  16 ,  18 . As such, the semiconductor layer  44  and source and drain electrode extending members  16 ,  18  are patterned and etched to form multiple channels  46 . The interdigitated pattern increases the photosensitivity of the photo TFT  12 , although one of skill in the art will appreciate that the photo TFT  12  may have an alternative configuration such as a simple inverted staggered structure, trilayer type inverted staggered structure, or other known structures. 
     In one embodiment, a passivation layer  48  is formed on an upper surface of the photo TFT  12  to cover and protect the channels  46 . The passivation layer  48  may include silicon nitride, silicon oxide, and combinations thereof. The passivation layer  48  may extend and cover the electrodes  16 ,  18  as well. 
     The present invention has particular application in detecting x-ray emissions in the industrial and medical industries. The photo TFT  12  may be used to detect x-ray emissions by covering the TFT  12  with a screen  50  to convert x-rays to light. The screen  50  includes scintillator material that absorbs x-rays and converts the energy to visible light. Scintillator material may yield many light photons for each received x-ray photon. The scintillator material usually consists of a high-atomic number material, which has high x-ray absorption, and a low-concentration activator that provides direct band transitions to facilitate visible photon emission. 
     Acceptable scintillator materials include granular like phosphors or crystalline like cesium iodide (CsI). Phosphors glow when exposed to x-rays. Various grain sizes and chemical mixtures may be used to produce a variety of resolution and brightness varieties. CsI provides a better combination of resolution and brightness. Because cesium has a high atomic number, it is an excellent x-ray absorber and is very efficient at converting x-ray to visible light. The scintillator material may be mixed with a glue binder and coated onto plastic sheets to form the screen  50 . In one embodiment the scintillator material includes relatively low cost external phosphor such as Kodak® LANEX, which has a Gd 2 O 2 S:Tb layer to convert X-rays to green light with a wavelength of 544 nm. 
     In operation, the dark current of the photo TFT  12  may be significant and create noise in an image. In order to compensate, the dark image may be stored in a memory and subtracted from the light image. The dark current may also exhibit some drift over time, which affects the gray scale accuracy and image quality. This may be minimized by periodically, e.g. every minute, interrupting the light exposure and retaking the dark reference image. 
     Referring to  FIG. 5 , a cross-sectional view of an embodiment of a storage capacitor  20  and a readout TFT  24  coupled to one another is shown. The storage capacitor  20  may be embodied as a stacked capacitor having multiple layers and is deposited on a substrate  40 . A stacked capacitor increases the dynamic range of the sensor element  10  and requires less space thereby increasing the space available for the photo TFT  12 . In one embodiment, the capacitor  20  has a top electrode  52 , bottom electrode  54 , and a center electrode  56 . The center electrode  56  is separated from the top and bottom electrodes  52 ,  54  by first and second dielectric layers  58 ,  60 . The center electrode  56  may include Ni or a Ni alloy and, in the embodiment shown, couples to the source electrode  22  of the readout TFT  24 . 
     The readout TFT  24  may have a conventional structure and a manufacturing process similar to that of a TFT in an array for an AMLCD. The TFT  24  includes a gate electrode  28  deposited on the upper surface of the insulating substrate  40 . The gate electrode  28  may include Cu, Cr, Al, Ta, Ti, or combinations thereof and is deposited through known methods such as sputtering or vapor deposition. The gate electrode may then be patterned by photolithography or other known methods. A gate insulator  62  covers the gate electrode  28  and may include silicon nitride, silicon oxide, tantalum oxide, or combinations thereof. The gate insulator  62  may be the same layer that serves as a dielectric layer  60  for the storage capacitor  20 . 
     A semiconductor layer  64  is deposited on the gate insulator  62  above the gate electrode  28  using known sputtering and deposition methods. The semiconductor layer  64  may include a-Si, p-Si, or the like and further include a doping layer, such as a n+ layer, that contacts the source and drain electrodes  22 ,  32 . The source and drain electrodes  22 ,  32  are deposited and patterned using known methods and, with the semiconductor layer  64 , form a channel  66 . 
     A passivation layer  68 , which may also be the same layer as passivation layer  48 , covers and protects the channel  66 . The passivation layer  68  may include silicon nitride, silicon oxide, or other suitable dielectric material and may also extend and serve as a dielectric layer  58  for the storage capacitor  20 . 
     In one embodiment, which departs from conventional TFT structures, a light shield  70  may be disposed on the upper surface of the passivation layer  68  to cover the channel  66 . The readout TFT  24  may be exposed to the same light level as the photo TFT  12 . Shielding prevents a charge from leaking from the storage capacitor  20  through the readout TFT  24 . The light shield  70  is opaque and may include Cr or other suitable material. In an alternative embodiment, the light shield  70  may also extend and serve as the top electrode  52  or as an additional electrode for the storage capacitor  20 . An additional electrode increases the value of the storage capacitor  20 . 
     Referring to  FIG. 6 , a graph is shown illustrating the relative photocurrents of an a-Si photodiode and an a-Si photo TFT as an inverse function of channel length (L). The photo TFT is configured with its gate electrode  14  coupled to its source electrode  16  as taught by the present invention. As illustrated, the resulting photocurrent of the photo TFT exceeds that of the photodiode for certain values of L. In the photodiode, the photocurrent  72  is proportional to the linear dimension L, whereas, in the photo TFT, the photocurrent  74  is proportional to 1/L. 
     At a certain value of L, which is approximately 10 to 20 μm, the conversion efficiency of photons into current becomes equal for the two devices. For smaller values of L, the photo TFT becomes more efficient. This is because the photocurrent  72  in the photo diode is a primary photocurrent with a gain that cannot exceed unity. One photon generates not more than one electron-hole pair, since the photo diode has blocking contacts for electrons and holes when reverse biased. Therefore, only one charge carrier can contribute to the photocurrent per absorbed photon. In the photo TFT, the source and drain contacts may be embodied as n+ layers so that electrons can be injected from the contacts and replenish the photocurrent. Thus, a secondary electron photocurrent occurs which can have a gain more than 1 when the electron lifetime exceeds the transit time from source to drain. 
     According to the present invention, there is provided an image sensor array having a substrate with addressable pixels. Each pixel defined by a sensor element  10  that has a photo TFT  12 , storage capacitor  20 , and a readout TFT. Each sensor element  10  is in electrical communication with a control circuit (not shown) to operate the sensor elements. The photo TFT  12  includes a doped semiconductor material that generates a current channel in response to received light and effectively discharges the storage capacitor  20 . 
     TFT manufacturing for flat panel applications is a standard process which reduces the cost of the image sensor of the present invention. Whereas PIN diodes and TFTs require separate processes to manufacture and can require 8 or more photomasks, an image sensor of the present invention can be constructed with 3 to 5 photomasks. A photo TFT can yield photocurrents that exceed that of a PIN diode. 
     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments of this invention without departing from the underlying principles thereof. The scope of the present invention should, therefore, be determined only by the following claims.