Abstract:
A method and apparatus for reducing vertical leakage current in a high fill factor sensor array is described. Reduction of vertical leakage current is achieved by eliminating Schottky junction interfaces that occur between metal back contacts and intrinsic amorphous silicon layers. One method of eliminating the Schottky junction uses an extra wide region of N doped amorphous silicon to serve as a buffer between the metal back contact and the intrinsic amorphous silicon layer. Another method of eliminating the Schottky junction completely replaces the metal back contact and the N doped amorphous silicon layer with a substitute material such as N doped poly-silicon.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to the structure of a high fill factor image array with a continuous sensor layer and its method of manufacture. More particularly, the present invention describes a high fill factor image array that reduces vertical leakage current by reducing contact injection current. 
     BACKGROUND OF THE INVENTION 
     A conventional image sensor array is typically formed from a plurality of photosensitive elements or pixels arranged in rows or columns. FIG. 8 illustrates an example of a typical PIN (P+/Intrinsic/N+ layered) photosensitive element  10  used in the image sensor array. 
     Each photosensitive element  10  includes a contact pad  14  positioned over a substrate  11 . A photosensor island  12  of doped amorphous silicon (a-Si), includes a P+ doped region  24  and an N+ doped region  22  that covers contact pad  14 . Photons that enter photosensor island  12  generate electrons in the a-Si. An applied voltage generates an electric field between transparent upper conductive layer  16  of indium tin oxide (ITO) and contact pad  14 . The electric field moves the generated electrons to contact pad  14 . Passivation layer  18  separates transparent upper conductive layer  16  from substrate  11  except where the ITO contacts an upper surface of phosphor island  12 . Passivation layer  18  typically includes an oxynitride layer  26  and a polyamide layer  28 . 
     Conventional image arrays that use the photosensor element illustrated in FIG. 8 include spaces between adjacent elements. These spaces do not detect light. A ratio of element areas that detect light to space areas occupied by the pixel is defined as a pixel fill factor. Method&#39;s of defining fill factor are described in an article entitled “High Efficiency X-Ray Imaging Using Amorphous Silicon Flat-Panel Arrays” by J. Rahn, F. Lemmi, J. P. Lu, P. Mei, R. B. Apte, and R. A. Street published in IEEE Trans. Nucl. Sci. (USA), IEEE Transactions on Nuclear Science (June 1999) vol.46, no.3, pt.2 p.  457-61 and hereby incorporated by reference.    
     High fill factor image arrays greatly improve the pixel fill factor such that an increased area of the sensor array detects light. FIG. 9 illustrates a high fill factor image array  40  that uses a continuous P+ doped amorphous silicon layer  52  deposited over a continuous intrinsic amorphous silicon layer  50 . The continuous layers allow light detection across the entire sensor surface. 
     A voltage difference between upper electrode  54  and a plurality of source-drain metal contacts  44  on substrate  42  creates an electric field through amorphous silicon layers  50 . Upper electrode  54  is typically made of a transparent ITO while source-metal drain contacts  44  are made of an electrically conductive material such as a tri-layer TiW/A 1 /Cr. The electric field moves the generated electrons to contacts  44 . Each contact communicates with switching and processing circuits (not shown) that generate an image based on the charge on each contact. 
     Patterned back contact collection electrodes  46  coupled to each source-drain contact  44  increases the area of electron collection. An N+ doped amorphous silicon layer  48  is deposited over each source-drain contact  44  to form a PIN structure with continuous layers  50 ,  52 . A conventional passivation layer, typically an approximately one micron thick oxynitride layer, serves as an insulator between adjacent mushroom electrodes. 
     One problem with conventional image arrays are leakage currents that arise due to material defects. Leakage currents include lateral leakage current between adjacent mushroom contacts and vertical or intrinsic leakage currents that occur along the direction of arrow  58 . Lateral leakage currents reduce image resolution. A typical 60×60 square micrometer of PIN sensor may include up to 0.3 pico-amps (pA) of lateral leakage current. A system for minimizing lateral leakage current is described in a patent application entitled Dual Dielectric Structure for Suppressing Lateral Leakage Current in High Fill Factor Arrays by Jeng Ping Lu, Ping Mei, Francesco Lemmi, Robert Street and James Boyce, Ser. No. (D/99215) 09/419,293 hereby incorporated by reference. a system for minimizing lateral leakage current is described in a patent application entitled dual dielectric structure for suppressing lateral leakage current in high fill factor arrays by jeng ping lu, ping mei, francesco lemmi, robert street and james boyce, Ser. No. 09/419,293, hereby incorporated by reference. 
     Vertical leakage current also degrades image quality by introducing noise. The introduced noise reduces image contrast and/or gray scale. A typical 60×60 square micrometer of a PIN sensor may include about 20 femto-amps (fA) of intrinsic leakage current at five volt contact voltages. 
     Thus a method and apparatus for reducing vertical leakage current is needed. 
     SUMMARY OF THE INVENTION 
     One problem with high fill factor image arrays is that vertical leakage currents reduce image contrast thereby degrading the quality of images output. One source of vertical leakage current is a contact injection current that results when a metal comes in contact with intrinsic amorphous silicon. 
     During fabrication of a typical back contact, a single mask is used to form both the metal back contact and an N+ doped amorphous silicon layer over the metal contact. However, the use of a single mask for both the N+ doped amorphous silicon layer and the metal back contact results in an edge of the metal being exposed to the intrinsic amorphous silicon. Despite the small size of the exposed area, it has been found that under certain reverse bias conditions, the area is sufficient to generate a large amount of vertical leakage current. In order to avoid vertical leakage current, one embodiment of the invention uses a second mask to generate a wider N+ amorphous silicon layer that seals in the metal portion of the back contact and prevents all direct contact between the intrinsic amorphous silicon layer and the metal back contact. An alternative embodiment of the invention replaces both the metal back contact and the N+ amorphous silicon layer with a single alternative material such as an N+ doped poly-silicon contact. Still a third embodiment of the invention retains the N+ amorphous silicon layer and replaces only the metal back contact with another conducting material that does not generate significant injection current when brought into contact with intrinsic amorphous silicon. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention will be more readily obtained and understood by referring to the following detailed description and the accompanying drawings. 
     FIG. 1 is a schematic cross sectional view of a high fill factor sensor in which the N doped amorphous silicon layer has been extended to eliminate direct contact between the intrinsic amorphous silicon layer and a metal back contact. 
     FIG. 2 is an energy band diagram showing the Fermi level across a p-i-n doped amorphous silicon and metal interface. 
     FIG. 3 is an energy band diagram showing the Fermi level across a p-i amorphous silicon and metal interface. 
     FIG. 4 is a graph that shows the contact injection current as a function of voltages applied across a Schottky junction. 
     FIG. 5 is a schematic cross sectional view of a full fill factor sensor in which the N+ amorphous silicon layer and the metal back contact has been replaced with a single N+ doped poly-silicon layer. 
     FIG. 6 is a graph that shows the change in sheet resistivity of a section of N doped amorphous silicon as a function of laser exposure. 
     FIG. 7 is a schematic cross sectional view of a full fill factor sensor in which the N+ amorphous silicon layer is retained, but the metal back contact is replaced with a non-metal film. 
     FIG. 8 is a schematic cross sectional view of a prior art PIN photosensitive element. 
     FIG. 9 is a schematic cross sectional view of a prior art full fill factor sensor array. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows a sensor structure in which an N+ doped amorphous silicon layer completely eliminates contact between a metal back contact and an intrinsic amorphous silicon layer. As used herein, a “back contact” is defined as a sensor contact that is designed to increase charge collection from a sensor medium, such as amorphous silicon, by increasing the surface area of a charge collecting surface. Typically such back contacts are “mushroom shaped” to provided a larger surface area on a side exposed to the sensor medium and a smaller contact point surface area that couples the collected charge to detection electronics. A center portion of the mushroom shape typically forms a “U” with the bottom of the “U” forming the contact point and extensions from the top of the “U” providing additional surface area to collect charge. The extensions are typically oriented perpendicular to the top of the “U” and parallel to the collection surface. Such a structure will be illustrated and described further in FIG.  1 . 
     In FIG. 1, a P+ doped amorphous silicon layer  52 , an intrinsic silicon layer  50 , and an N+ doped amorphous silicon layer  48  together form a PIN structure. A transparent conducting ITO upper layer  54  and a metal back contact  46  bias the PIN structure. Photons pass through the transparent conducting ITO layer and generate free electrons and holes in the intrinsic amorphous silicon layer  50 . The voltage applied to the contacts generates an electric field that moves the electrons to metal back contact  46 . In the illustrated embodiment, metal back contact  46  is mushroom shaped to from a metal mushroom contact. 
     As used herein, a mushroom shaped contact typically includes a “U” shaped portion including a conducting bottom segment  65  and two conducting upright segments  67 ,  69 . A first end of each upright segment  67 ,  69  is coupled to corresponding ends of bottom segment  65 . Each upright segment  67 ,  69  is oriented approximately perpendicular to bottom segment  65 . A second end of each upright conducting segment  67 ,  69  is coupled to an end of a corresponding conducting extension segment  71 ,  73 . Each, extension segment  71 ,  73  is oriented approximately parallel to bottom segment  65  and approximately perpendicular to upright conducting segments  67 ,  69 . 
     As described, each mushroom contact includes a larger charge collection surface  49  created by extension segments  71 ,  73 . Charge collection surface  49  receives charge from the sensor medium. A contact point  51  transfers the collected charge from the mushroom contact to underlying electronics via line  104 . Line  104  couples the charge to support or readout electronics. In the illustrated embodiment, line  104  is a data line that couples to a gate of a thin film transistor TFT transistor  108 . 
     TFT transistor  108  may be formed using a number of different methods. One method of integrating the sensor with detection or imaging electronics that include TFT transistor  108  is to form the TFT transistor from a Complimentary Metal Oxide Structure (CMOS) and to form the sensor medium including the amorphous silicon layer  50  directly over the CMOS electronics. A reference that describes the formation of a CMOS structure for use in a sensor is “Image Sensors in TFA Technology- Status and Future Trends” by Bohm, M. et al. presented on Pages 327-338 of the conference paper of the Amorphous and Microcrystalline Silicon Technology 1998 Symposium held in San Francisco on Apr. 14-17, 1988. 
     Formation of the mushroom structure in prior art metal mushroom contacts was convenient because a single mask is used for forming the metal back contact  46  and the doped N+ layer  48 . However, formation using a single mask leaves edges  112 ,  116  of metal back contact  46  in direct contact with the intrinsic amorphous silicon layer. Under reverse bias conditions, a leakage current resulting from contact injection would occur at the interface of the intrinsic amorphous silicon layer and the edges  112 ,  116  of metal back contact  46 . The amount of leakage current depends on an injection barrier height and the electric field from the applied bias voltage. A discussion of the reasons for the leakage current and factors that affect the leakage current will be provided with the discussion accompanying FIG. 2, FIG.  3  and FIG.  4 . 
     To reduce the vertical leakage current, one embodiment of the invention uses a first mask to form the metal back contact and a second mask to form the N+ amorphous silicon layer. The area of the N+ amorphous silicon layer is slightly larger than the area of the metal back contact to create an overhang area  120  of N+ amorphous silicon that covers the edges  112 ,  116  of metal back contact  46 . The overhang area  120  eliminates the direct contact between metal and intrinsic amorphous silicon by sealing off metal portions of the back contact from the intrinsic amorphous silicon. 
     To understand the advantage of eliminating the Schottky contact, FIG. 2 illustrates the Fermi level of a reverse biased PIN amorphous silicon to metal interface while FIG. 3 illustrates the Fermi level of a PI amorphous silicon to metal Schottky interface. In a PIN amorphous silicon sensor, a dark reverse-bias current results from  1 ) bulk generation,  2 ) contact injection and  3 ) edge leakage. The contact injection portion of the dark reverse bias current arises from carrier injection across a junction barrier between a metal electrode and the amorphous silicon. The contact injection current may be expressed as I=I init  exp(−qΦ/kT) where Φ is the junction barrier height, k is boltzmann&#39;s constant, T is temperature, I init  is a current pre-factor, and q is electronic charge. 
     As illustrated in FIG. 2, when the doping in the N+ layer is very high and the PIN diode is reversed-biased, the energy band profile or Fermi level  200  is relatively flat. The barrier height  220  for hole generation, Φ is the potential difference near interface  208  between a Fermi level  200  in the N+ layer and the valence band  204  in the N+ layer. The relatively large value of Φresults in a relatively small injection current due to the I=I init  exp(−q Φ/kT) relationship. 
     FIG. 3 illustrates a Schottky diode in which an intrinsic amorphous silicon contacts a metal. In a Schottky diode, the Fermi level  304  in the intrinsic amorphous silicon layer spreads slightly but is typically pinned at a center of bandgap  308  near the silicon to metal interface  312 . The junction barrier height for hole generation, Φ is the potential difference between Fermi level  304  and valence band  316  near the silicon to metal interface  312 . FIG. 3, and FIG. 2 together illustrate that the Schottky diode barrier height  320  is significantly smaller than the barrier height  220  when a heavily doped N+ layer interfaces with a metal. The smaller barrier height results in a larger injection current. 
     FIG. 4 plots the injection current density in a Schottky diode as a function of an applied bias voltage. The current density is plotted along vertical axis  408  and the bias voltage is plotted along horizontal axis  404 . Each curve  412 ,  416 ,  420 ,  424  plots the current density versus bias voltage characteristic for a different type of metal. 
     To avoid edge contact injection current and still fabricate a mushroom contact, the metal can be removed in FIG. 1 leaving only the N+ amorphous layer deposited in a mushroom form. However, the sheet resistance of N doped amorphous silicon is high, typically on the order of 6 MegaOhms per square. The high sheet resistance slows down read-out time. 
     In order to maintain a reasonable read-out time, FIG. 5 illustrates one embodiment of the invention in which both the metal back contact  46  and the N doped silicon layer  48  of FIG. 1 are replaced by a single N doped poly-silicon back contact  504 . The poly-silicon back contact  504  serves as a good conductor to collect and transport charge to line  104 . The N doping of poly-silicon back contact  504  also provides the desired PIN structure. As illustrated, the poly-silicon back contact  504  also maintains a mushroom shape. 
     Various methods may be used to form a doped poly-silicon back contact. One method of formation includes depositing an N doped amorphous silicon layer in a mushroom configuration. Laser Crystallization converts the N doped amorphous silicon layer into an N doped poly-silicon layer. To protect TFT  108  and metal line  104  during laser crystallization, a selective laser process may be used that does not completely melt the N doped amorphous silicon. Complete melting of the amorphous silicon is not needed to achieve the low resistance desired as will be illustrated in the graph of FIG.  6 . Passivation layer  56  also serves as a buffering layer further protecting TFT  108 . 
     FIG. 6 is a graph that shows the change in sheet resistivity of a 100 nanometer section of amorphous silicon doped with phosphor atoms as a function of laser exposure. Sheet resistivity is plotted along vertical axis  604  while laser energy density is plotted along horizontal axis  608 . As illustrated in FIG. 6, moderate laser energies of around 200 mJ/cm 2  are sufficient to achieve sheet resistances below one KiloOhm per square. 
     A second method of forming a doped poly-silicon back contact includes depositing intrinsic amorphous silicon. Ion implantation or laser doping is used to deposit N+ ions in the intrinsic amorphous silicon. A laser anneal after the ion implantation or laser doping converts the amorphous silicon into poly-silicon. Poly-silicon back contact  504  may be exposed to several minutes of plasma hydrogenation prior to deposition of intrinsic amorphous silicon to assure a high quality n-i junction  508  between the poly-silicon and the intrinsic amorphous silicon. 
     Alternate materials besides poly-Silicon may be used to replace the combination of metal back contact  46  and N+ doped amorphous silicon layer  48  of FIG.  1 . One example of such a material is doped microcrystalline Silicon. A PECVD ( Plasma Enhance Chemical Vapor Deposition) process may be used to deposit the doped miro-crystalline silicon with hydrogen dilute silane and phosphine. Another suitable material is doped poly-SiC (poly-silicon carbide). One benefit of using poly-SiC is that the wide energy band gap of the poly-SiC provides a high potential barrier that reduces hole injection and minimizes contact current. 
     FIG. 7 illustrates an alternate embodiment of the invention that continues to utilize two layers for a mushroom contact, including the N+ amorphous silicon layer  48  of FIG.  1 . In FIG. 7, only metal mushroom contact  112  is replaced with a substitute material  700 . Suitable substitute materials should be a semiconductor conductor with a low sheet resistance (lower than 100 kilo Ohms per square) which allows direct contact with the intrinsic amorphous silicon layer without generating large amounts of contact current. Suitable substitute materials include doped poly-silicon, doped SiC, doped SiGe and doped Ge films. The dual structure of an N+ layer and a substitute material maintains the nature of an amorphous silicon PIN junction while utilizing the substitute material to conduct charge and minimize contact injection current. Because some contact with the intrinsic amorphous silicon is allowed, a single mask is used in the fabrication of both the N doped amorphous silicon layer and the substitute material of the back contact 
     While the invention has been described in terms of a number of specific embodiments, it will be evident to those skilled in the art that many alternatives, modifications, and variations are within the scope of the teachings contained herein. For example, variations in dopant materials used to replace a metal mushroom contact and variations in laser power used to convert amorphous silicon to poly-silicon are possible. Accordingly, the present invention should not be limited by the embodiment used, to exemplify it, but rather should be considered to be within the spirit and scope of the following claims, and equivalents thereto, including all such alternatives, modifications, and variations.