Abstract:
A light block material disposed over the photosensitive region of a switching device (e.g., TFT) of a radiation imager is disclosed. The light block material prevents optical photons emitted from a scintillator from passing into the switching device and being absorbed. Cross-talk and noise in the imager are thereby reduced. Also, non-linear pixel response and spurious signals passing to readout electronics are avoided. Optionally, opaque caps comprising the same light block material may be included in the imager structure. The caps cover contact vias filled with a common electrode and located in the contact finger region of the imager. The integrity of the filled vias is thereby maintained during subsequent processing. Also disclosed is a radiation imager containing these structures.

Description:
This application claims priority to the provisional application Serial No. 60/163,043, filed Nov. 2, 1999, assigned to the assignee herein, which provisional application is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to radiation imagers, and more particularly, to the incorporation of a light block material to reduce cross-talk caused by persistent transistor photoconductivity, nonlinear pixel response, and poor contact via integrity. 
     Radiation imagers are typically coupled with a scintillator, wherein radiation (such as an x-ray beam, for example) absorbed in the scintillator emits optical photons which in turn pass into a light sensitive region of the imager. The imager typically comprises a significantly flat substrate (e.g., glass) on which a two dimensional array of light-sensitive pixels is disposed. Each pixel comprises a light-sensitive imaging element (photosensor), such as a photodiode, and an associated switching element, such as a thin film transistor (TFT). Both photodiodes and TFTs preferably comprise hydrogenated amorphous silicon (doped or undoped) or alloys thereof, due to the advantageous characteristics and relative ease of fabrication associated with these materials. Hydrogenated amorphous silicon is commonly referred to as “amorphous silicon” or “a-Si”, and the light sensitive pixel array discussed above is typically referred to as being “active”. Also contained in the active area of the imager are metal address lines electrically connected to the pixels. 
     A reverse-bias voltage is applied across each photodiode. Charge generated in the photodiode as a result of the absorption of light photons from the scintillator is collected by the contacts, thereby reducing the bias across the diode. This collected charge is read when the TFT switching device in the array couples the photodiode to readout electronics via an address line. 
     The address lines of the active array are electrically contiguous with contact fingers extending away from the active pixel region toward the edges of the substrate, where they are in turn electrically connected to contact pads, typically through contact vias. Electrical connection to external scan line drive and data line read out circuitry is made at the contact pads. 
     In the active region, optimal spatial resolution and contrast in the signal generated by the array is achieved when incident optical photons from the scintillator are absorbed substantially only in the photodiodes directly in line with the region of the scintillator in which the optical photons are generated. However, optical photons from the scintillator are often scattered, passing into the TFT switching devices or the address lines. Such scattering and absorption present problems of increased cross-talk and noise in the array. Cross-talk reduces the spatial resolution of the array, and absorption of optical photons in TFT switching devices can result in spurious signals being passed to the readout electronics. 
     Thus, although light from the scintillator discharges the reverse bias of the diode (the desired signal), the same light also impinges on the a-silicon in the TFT, producing a photocurrent, which is driven by the high source-drain voltage. This photocurrent persists even after the light is terminated. Hence, when pixels in the area under the object are read, even after the x-ray beam or other radiation is turned off, some of the persistent photocurrent from pixels not being read is integrated by the readout amplifier. Thus, an undesirable type of long range spatial cross-talk in the image is produced. An additional impact of the photocurrent is the production of a non-linear response by the pixel, which occurs because charge is lost from the photodiode due to this leakage current. 
     It is therefore clear that TFT photosensitivity can degrade the performance of an a-Si radiation imager, such as an x-ray imager. Furthermore, at high, but clinically relevant, x-ray exposure levels, for example, photodiodes in pixels which do not have the x-ray signal attenuated by the object under examination develop the highest field effect transistor (FET) source-drain voltages, thus increasing the FET photocurrent. 
     In addition to the aforementioned problems, another concern is that the integrity of the contact vias, which are located in the contact fingers outside the active region of the array, is often compromised during fabrication and processing. The contact vias are filled with a common electrode material comprising a light-transmissive conducting oxide, typically indium tin oxide (ITO), tin oxide, indium oxide, zinc oxide, or the like. External electrical connection from the contact pads to the underlying metals of the address lines extending from the active array is facilitated through the common electrode in the contact vias. 
     However, the relatively thin (about 100 nm) common electrode layer must form a continuous layer over an underlying, relatively thick (1 to 2 μm) light-transmissive dielectric layer in the contact vias. Because the ITO is so thin and because it may be porous due to its polycrystalline nature, contact to the underlying address line material in the contact fingers may be degraded during processing, particularly by chemical attack of the underlying metal and ITO-metal interface. 
     It is therefore clear that an imager array in which the TFTs are shielded from incident optical photons is desirable. Furthermore, improvement in the electrical yield and mechanical robustness of the contact vias is also desirable. A need therefore exists for a means to reduce TFT photosensitivity without otherwise degrading imager performance, while also preserving or improving the integrity of the common electrode, the contact vias, and other imager structures. 
     SUMMARY OF THE INVENTION 
     The present invention includes a structure for a radiation imager comprising an opaque shield overlying substantially all of a photosensitive region of a switching device, which is disposed on a substrate. The photosensitive region comprises a light sensitive portion of a semiconductor layer, and this light sensitive portion overlies a bottom conductive metal layer, but is free of a first top conductive metal layer and a second top conductive metal layer, which overlie the semiconductor layer. Furthermore, the structure includes a light transmissive dielectric layer overlying the photosensitive region and a common electrode disposed between the light transmissive dielectric layer and the opaque shield. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a radiation imager, in accordance with the present invention. 
     FIG. 2 is an exploded view of a segment of the active array in the imager of FIG.  1 . 
     FIGS. 3 and 4 are cross-sectional views taken along lines  2 — 2  of a portion of the array segment of FIG. 2 during fabrication. 
     FIG. 5 is an exploded view of a segment of a contact finger in the imager of FIG.  1 . 
     FIGS. 6 a-c  are cross-sectional views of exemplary embodiments taken along lines  5 — 5  of a portion of contact finger segment of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses the problems of cross-talk caused by transistor photosensitivity, nonlinear pixel response, and contact via integrity. By incorporating a light block material into the imager structure, optical photons are prevented from passing into the switching device and being absorbed. Furthermore, if the light block material is also used to cover filled contact vias in the contact finger region of the imager, the constitution of the vias remains intact during subsequent processing. 
     FIG. 1 is a top plan view of an exemplary radiation imager  10  in accordance with the present invention. However, for simplicity, the light blocking aspect of the present invention is not shown in FIG.  1 . Imager  10  is typically formed on a generally flat substrate  12  usually made of glass. Imager  10  includes active array  14  comprising photosensors, i.e. light-sensitive imaging elements (not shown), which are preferably photodiodes. Each imaging element has an associated switching element (not shown), preferably a thin-film transistor (TFT). The combination of each photosensor and switching device (e.g., photodiode and TFT) is referred to as a “pixel”. The pixels are arranged in a matrix of rows and columns, and each pixel is typically positioned in active array  14  so as to be exposed to optical photons passing from a scintillator (not shown) that is optically coupled to the imager. 
     It should be noted that the invention described herein is not limited to the use of TFTs and photodiodes, which are discussed throughout for exemplary purposes only. Other suitable switching devices and/or photosensors may be used instead, as will be known to those skilled in the art. 
     Radiation imager  10  also includes a plurality of address lines  11  for addressing individual pixels in photosensor array  14 . Each address line  11  is either a data line  32  oriented substantially along a first axis  101  of imager  10  or a scan line  36  oriented substantially along a second axis  102  of imager  10 . The aforementioned first and second axes are perpendicular to one another. For ease of illustration in FIG. 1, only a few data lines  32  and scan lines  36  are shown extending across array  14 , although each set of such address lines  11  would typically extend across the array. Scan and data lines are arranged in rows and columns, respectively, so that any one pixel in active array  14  is addressable by one scan line and one data line. In operation, the voltage on row scan lines  36  is switched on, and thereby the TFTs, allowing the charge on each scanned line&#39;s photodiode, to be read out via the column data lines  32 , which are connected to external amplifiers. Address lines  11  comprise a conductive material, such as chromium, molybdenum, aluminum, tantalum, tungsten, or the like. 
     As illustrated, address lines  11  (i.e. scan lines  36  and data lines  32 ) are disposed in the active region of the pixel array  14 , with contact fingers  20  extending from the active region toward the edge of substrate  12 . Each contact finger  20  is typically disposed in imager  10  such that a metal extension  112  electrically extends from a corresponding address line  11 . In addition, each metal extension  112  of each contact finger  20  is electrically connected to a corresponding contact pad  18 , which can be electrically coupled to an external device, as indicated as drive and read circuits in FIG.  1 . However, contact pad  18  is typically disposed so as to be electrically insulated from metal extension  112 , and electrical coupling thereto generally occurs through contact vias (not shown) on contact finger  20 . Contact pads  18  comprise a conductive material such as aluminum, molybdenum, chromium, indium tin oxide, or the like, or alternatively, multiple layers of conductive material, such as indium tin oxide over molybdenum. Contact pads and connections through a common electrode to the array are discussed in commonly assigned U.S. Pat. No. 5,389,775 to Kwasnick et al. 
     FIG. 2 is an enlarged plan view of a representative pixel region  140  of active array  14  of FIG.  1 . Pixel region  140  includes switching device  130 , typically a TFT, electrically coupled to a respective photosensor  160 , typically a photodiode, and a couple of address lines  11 . 
     In accordance with the present invention, FIG. 2 shows opaque shield  195  (the perimeter of which is indicated by dashed lines), which comprises a “light block” or opaque material, substantially covering (about 90% or more) a photosensitive region ( 250  in FIGS. 3 and 4) of switching device (e.g. TFT)  130 . This photosensitive region includes light sensitive portion  251  of semiconductor layer  136 , typically a-Si. Light sensitive portion  251  is shown in cross-hatch and overlaps bottom conductive metal layer  132 , e.g. gate electrode. Overlapping semiconductor layer  136 , but not covering light sensitive portion  251 , are first top conductive metal layer (e.g. source electrode)  144  and second top conductive metal layer (e.g. drain electrode)  142 . Thus, light sensitive portion  251  is free of first top conductive metal layer, source  144 , and second top conductive metal layer, drain  142 . The term “free” is used conventionally herein and refers to portion  251  not being obstructed by overlying first and second top conductive metal layers  142  and  144 . Furthermore, the semiconductor material underlying source  144  and drain  142  is not deemed part of light sensitive portion  251  because source  144  and drain  142  are opaque. Channel region  145 , which is part of photosensitive region  250 , laterally separates first top conductive metal layer  144  (source) from second top conductive metal layer  142  (drain). Gate metal  132  also forms the scan (row) line  36  (address line  11 ), and drain metal  142  also forms data (column) address line  32  (or  11 ). 
     The light block material from which opaque shield  195  is formed is preferably a light-absorbing conductive metal, such as molybdenum (Mo), chromium (Cr), tantalum (Ta), aluminum (Al), or the like. However, the material may instead be a semiconductor material, such as amorphous silicon, or alternatively, a light-absorbing nonconductive material, such as a dye/organic layer or carbon on metal oxide particles dispersed in a matrix. As used herein, the terms “light block” and “opaque” both refer to a material having an absorbance greater than 1, but preferably greater than 2. “Absorbance”, as used herein, is a measure of the light absorption characteristics of the material, and is determined by the negative log (base  10 ) of the transmittance (transmittance being the fraction of incident light passing through a sample). 
     Bottom conductive metal layer  132  (gate electrode) is a conductive material such as aluminum, chromium, molybdenum, tantalum, tungsten, or the like. Likewise, first top conductive metal layer (source electrode)  144  and second top conductive metal layer (drain electrode)  142  are also made of a conductive metal such as aluminum, chromium, molybdenum, tantalum, tungsten, or the like. 
     FIG. 3 illustrates a cross-sectional view  300  of region  140  of FIG. 2 taken along lines  2 — 2 . However, in FIG. 3, light block material layer  190  has not yet been patterned to form opaque shield  195 . In FIG. 3, representative pixel  120  from the active region of the imager, comprises TFT  130  electrically coupled to photodiode  160  by first top conductive metal layer  144 , e.g. source electrode. 
     In the exemplary pixel  120  shown in FIG. 3, TFT  130  is disposed on substrate  12 . Gate dielectric layer  134 , which is typically an inorganic dielectric material such as silicon oxide, silicon nitride, silicon-oxy-nitride, or the like, is disposed over gate electrode  132  (i.e bottom conductive metal layer). Semiconductor layer  136  comprising amorphous silicon or the like is disposed over gate dielectric layer  134 . Doped semiconductor layer  138 , typically comprising n+ doped a-Si, is disposed over a-Si layer  136 . These layers over gate electrode layer  132  are patterned to form the body of TFT  130 . TFT  130  further comprises first top conductive metal layer, e.g. source electrode,  144  and second top conductive metal layer, e.g. drain electrode  142 , which are disposed over the body of the TFT and laterally separated by channel region  145 . Channel region  145  is formed by removal of n+ silicon layer  138  and part of semiconductor layer  136  in the region between source electrode  144  and drain electrode  142 . TFT passivation layer  150  typically comprising an inorganic dielectric material such as silicon oxide, silicon nitride, silicon-oxy-nitride, or the like is disposed over TFT  130 . TFT passivation layer  150  typically has a thickness in the range between about 0.1 μm and 1 μm. Photosensitive region  250  of TFT  130  includes light sensitive portion  251  of a-Si semiconductor layer  136  overlying gate electrode area  132 , as illustrated in FIGS. 2,  3 , and  4 . Light sensitive portion  251  is free of the conductive metal that forms source and drain electrodes  144  and  142 . The elements comprising photosensitive region  250  are all vertically aligned over substrate  12 . 
     Photodiode  160  is disposed on substrate  12  and is electrically coupled to TFT  130  via source electrode  144 . Photodiode  160  comprises bottom contact electrode  162  disposed on substrate  12  and formed from the same electrically conductive material that comprises source electrode  144 . Photosensor island  164  is disposed in electrical contact with bottom contact electrode  162 . Photosensor island  164  is typically mesa-shaped, having sidewalls sloping upwardly and inwardly from substrate  12  toward an upper surface  165 . Photosensor island  164  typically comprises a-Si or combinations of a-Si and other materials, for example a-Si carbide or a-Si germanium. Photosensor island  164  typically further has relatively thin bands of an n-type doped region (not shown) and p-type doped region (not shown) at the bottom and top, respectively, of the island structure to enhance electrical contact with the adjoining electrode and to form the p-i-n diode structure. The thickness of photosensor island  164  is typically between about 0.1 micron and 10 microns, although in some arrangements the thickness may be greater than 10 microns. To form the desired island structure (as well as other features included in the imager), amorphous silicon and related materials are typically deposited by plasma enhanced chemical vapor deposition (PECVD) or similar methods and then patterned, for example, by etching. 
     In accordance with the present invention, light transmissive dielectric layer  170  is then deposited over the entire imager, including each pixel  120  of the active array and areas outside the array, such as the contact fingers (not shown). The thickness of light transmissive dielectric layer  170  typically ranges from about 0.1 μm to about 2 μm. As used herein, the term “light transmissive” means allowing at least about 10% of the incident light to pass through, but more preferably about 90% to 100%. Light transmissive dielectric layer  170  comprises either a thermally stable organic dielectric material, an inorganic dielectric material, or a combination of organic and inorganic dielectric materials (dual dielectric). Examples of suitable organic dielectrics include polyimides, polyamides, polycarbonates, polyesters, polyphenylene ether, acrylics, and blends thereof. Exemplary inorganic dielectric materials include silicon nitride and/or silicon oxide. The aforementioned dual dielectrics are discussed in commonly assigned U.S. Pat. No. 5,233,181 to Kwasnick et al. 
     When dielectric layer  170  is an organic polymer, such as a polyimide, conventional coating methods may be used to apply the dielectric layer to the structure. Such methods include “spinning” or “meniscus coating”. Inorganic dielectrics, such as silicon oxide, silicon nitride, or silicon-oxy-nitride, can be deposited, for example, by plasma-enhanced chemical vapor deposition. A dual dielectric, typically comprising an inorganic diode dielectric and a polymer layer in various configurations, can be applied using a combination of methods. 
     After deposition, light transmissive dielectric layer  170  is conventionally patterned (using commonly known techniques) to uncover a plurality of contact areas. Each contact area  165  lies atop a respective photosensor or photodiode  160 , as shown in FIG.  3 . 
     After light transmissive dielectric layer  170  has been patterned, common electrode layer  180  is deposited over the imager such that it is disposed over light transmissive dielectric layer  170  and is in electrical contact with contact area  165  of photodiode  130 . The thickness of common electrode layer  180  ranges from about 50 nm to about 200 nm, but is usually about 100 nm. Common electrode  180  typically comprises a light-transmissive conducting oxide ( 180 ), such as indium tin oxide, tin oxide, indium oxide, zinc oxide, or the like. 
     Layer  190 , which comprises the previously described opaque or “light block” material, is then deposited, typically by sputtering or PECVD, over common electrode  180 , covering both active and inactive regions of imager  10 . The thickness of opaque layer  190  disposed on common electrode  180  preferably ranges from about 20 to about 200 nm, but is usually about 50 nm. However, a thinner or thicker layer  190  can be employed ranging from about 10 nm to about 500 nm. Relative to the opaque polymers disclosed in commonly assigned U.S. Pat. No. 5,517,031 to Wei et al., the light absorption of the present light block materials useful in forming light block layer  190  is greater than that of opaque polymers, so thinner layers, which are easier to work with, can be used. 
     Opaque layer  190  is then patterned using conventional methods of photolithography including etch and photoresist strip steps to form opaque shield  195 , as depicted in FIGS. 2 and 4. The steps of depositing and patterning opaque layer  190  can be performed before or after deposition and/or patterning of common electrode  180 . However, these steps (i.e. depositing and patterning opaque layer  190 ) are preferably performed after the deposition of common electrode  180 , but prior to the patterning of common electrode  180 . As shown in FIG. 4, opaque shield  195  substantially covers (about 90% or more) of photosensitive region  250 , overlying substantially all (about 90% or more) of light sensitive region  251  of a-Si layer  136 , where it overlies bottom conductive metal layer (gate electrode)  132  in the TFT region. However, shield  195  usually extends about 2 μm or more past the edges of the a-Si region  251  where it overlaps gate  132 . This overlap allows for misalignment during photolithography, which is common for large area substrates. This extension is also depicted in FIG. 2, where opaque shield  195  is shown extending past region  251 . 
     As stated above, light transmissive dielectric layer  170  is deposited over contact fingers  20 , as well as active region  14  of imager  10  (FIG.  1 ). Thus, when light transmissive dielectric layer  170  is patterned to uncover contact areas  165  (FIG.  3 ), at least one contact via may also be also be formed in the dielectric layer covering each contact finger. As shown in FIGS. 6 a-c,  which are described in more detail below, each contact via  21  extends through light transmissive dielectric layer  170  to metal extension  112 . Furthermore, at the same time common electrode  180  is deposited onto light transmissive dielectric layer  170 , as previously described, it is also deposited into each contact via  21 . Similarly, light block material layer  190  is deposited atop common electrode layer  180  overlying the entire imager structure  10 , including contact fingers  20 . 
     Optionally, the light block layer can be patterned simultaneously (along with the TFT opaque shield described above) to form opaque caps  198  over the filled contact vias  21 . Such a cap improves the reliability of electrical contact in the via and also protects the via from attack during etch of the light block material during fabrication of the TFT opaque shields. 
     In accordance with the present invention, FIG. 5 illustrates such an opaque cap  198  and depicts an enlarged portion  500  of contact finger  20  from FIG.  1 . For simplicity, the substrate on which contact finger  20  is disposed is not shown in FIG.  5 . Portion  500  is a top plan view of contact via region  510  of contact finger  20  after light block material has been deposited and patterned. As shown in FIG. 5, contact via region  510  comprises metal extension  112  (e.g., molybdenum, chromium, or aluminum), which is an extension of an address line  11  from the active array  14  shown in FIG.  1 . Address line  11  extends from gate electrode  232  or alternatively from drain electrode  142  of FIG.  2 . Over metal extension  112  is previously described light-transmissive dielectric layer, which may be a single or double layer (dual dielectric). As shown in FIG. 5, the dielectric layer is a dual dielectric comprising inorganic dielectric layer  171  (e.g., silicon oxide, silicon nitride, or silicon-oxy-nitride) and overlying organic dielectric layer  172  (e.g., polyimide), which have been patterned to form (at least one) contact via  21 . After contact via  21  has been formed, light-transmissive common electrode  180 , preferably ITO, is deposited over dielectrics  171  and  172  and into contact via  21 . The filled contact via  21  permits electrical connection between metal extension  112  of finger  20  and contact pad  18  (FIG.  1 ), which is the same material as common electrode  180  (e.g., ITO). Above common electrode  180  is opaque cap  198  (the perimeter of which is shown as dashed lines) which comprises the light block material previously discussed. Opaque cap  198  is patterned to cover substantially all (about 90% or more) of contact via  21  through which common electrode  180  connects underlying metal extension  112  (from a corresponding address line  11 ) and contact pad  18  (FIG.  1 ). Preferably, cap  198  extends about 5 μm or more past the edges of contact via  21 , due to varied slope at the via edge. However, opaque cap  198  generally does not extend past the edges of the common electrode (ITO) pattern (shown in FIG. 6) to minimize the risk of shorts between neighboring contact fingers. 
     FIGS. 6 a-c  show alternative embodiments, each depicting a cross-sectional view  600  of portion  500  taken along lines  5 — 5  in FIG.  5 . As shown in FIGS. 6 a-c,  metal extension  112 , which is an extension of address line  11  (FIG.  1 ), from the active array is disposed over substrate  12 . Over metal extension  112  is light-transmissive dielectric  170 , which, in these depictions, is a dual dielectric comprising inorganic dielectric layer  171  (e.g., silicon oxide, silicon nitride, or silicon-oxy-nitride) and organic dielectric layer  172 , which overlies all (FIG. 6 a ) or part (FIG. 6 c ) of layer  171 . FIG. 6 b  shows an embodiment wherein inorganic dielectric layer  172  overlies layer  171  and extends over metal extension  112 . Alternate embodiments include structures wherein both layers  171  and  172  are inorganic dielectrics and wherein  171  is an organic dielectric and  172  is an inorganic dielectric. In FIGS. 6 a-c  dielectric  170  (inorganic dielectric  171  and organic dielectric  172 ) has been patterned to form (at least) a contact via  21 , and light transmissive common electrode  180  has been conformally deposited (that is, extends across the topography of the underlying area) over dielectric  170  and filling contact via  21 . Electrical connection between underlying metal extension  112  and a contact pad  18  of FIG. 1 is thereby facilitated. Opaque cap  198  overlies substantially all of metal filled via  21 , as previously described. 
     After opaque shield  195  (FIGS.  2  and  4 ), and optionally, opaque cap  198  (FIGS. 5 and 6) have been formed by photoresist patterning and etching, the patterning photoresist is removed by conventional processes, such as by plasma ashing in O 2  or by wet stripping, which are both well known in the art. When the light block material used to form opaque shield  195  and optional cap  198  is molybenum, then it can be wet-etched using standard etchants (e.g. Cyantek 12-S, comprising nitric and phosphoric acids), to which ITO is substantially impervious. Common electrode  180  is subsequently patterned as in prior art disclosures describing the imager process using HCl-containing etchants. 
     The radiation imager is completed by formation of a barrier layer (not shown) on common electrode  180 , shield  195 , and cap  198 , followed by coupling to a scintillator (not shown). The scintillator is disposed over the active array and comprises a scintillating material selected to have a high absorption cross section for radiation of the type it is desired to detect. For example, in imagers designed for detection of x-rays, the scintillator material is typically cesium iodide doped with thallium (CsI:Th), or sodium (CsI:Na), or alternatively, the scintillator material may be a powder of gadollium oxide-sulfate (GdOS) crystals. 
     While the invention has been particularly shown and described with reference to preferred embodiment(s) thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.