Abstract:
Disclosed is an image capture sensor including a light detection transistor having a light sensitive layer which conducts electricity in response to detection of a predetermined amount of light and a switch interconnected to the light detection transistor and responsive to detection of light by the light detection transistor. A glass substrate is layered over both the light detection transistor and switch. The glass substrate provides a durable and smooth surface upon which a patterned object to be imaged in placed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to provisional patent application Ser. No. 60/405,604 filed Aug. 21, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a imaging of a patterned object such as a fingerprint. More specifically, this invention relates to patterned object capture sensors including thin-film transistors.  
         [0004]     2. Background  
         [0005]     As known to those skilled in the art, fingerprint recognition is a kind of technology for granting an access authorization to systems such as a computer, an access control system, a banking system, etc. Fingerprint recognition systems are generally classified into two types: optic type system using a lens and a prism, and non-optic type system using a semiconductor or thin-film transistor (TFT), not a lens. A TFT fingerprint capture device is a kind of contact image sensor using photosensitivity of a-Si:H, and has high photosensitivity due to its relatively thin structure.  
         [0006]     The structure of the fingerprint capture sensor is shown in  FIG. 1 .  FIG. 1  is a vertical sectional view showing a unit cell of a conventional fingerprint capture sensor.  FIG. 1  illustrates a conventional thin film transistor (TFT) image acquisition sensor which may be used to image a fingerprint for use with equipment and software providing identity verification. Such an image acquisition device is disclosed in co-pending U.S. patent application Ser. No. 10/014,290 filed Dec. 10, 2001, which is hereby incorporated by reference in its entirety.  FIG. 1  is a sectional view showing a unit cell of a conventional fingerprint capture sensor. In the fingerprint capture sensor  10  a light sensing unit  12  and a switching unit  13  are horizontally arranged on a transparent substrate  11 . Under the transparent substrate  11 , a back light (not shown) irradiates light upward to be passed through the fingerprint capture sensor  10 . A source electrode  12 -S of the light sensing unit  12  and a drain electrode  13 -D of the switching unit  13  are electrically connected to each other through a first electrode  14 . A gate electrode  12 -G of the light sensing unit  12  is connected to a second electrode  15 .  
         [0007]     In the above structure, a photosensitive layer  12 -P such as amorphous silicon (a-Si:H) is formed between the drain electrode  12 -D and source electrode  12 -S of the light sensing unit  12 . Then, when more than a predetermined quantity of light is received, current flows through the drain electrode  12 -D and the source electrode  12 -S.  FIG. 2  illustrates how sensor  10  operates to capture a ridge  22  of a fingerprint  20 . Light  24  generated from the back light under the transparent substrate  11  is reflected on a fingerprint pattern and received by the photosensitive layer  12 -P of the light sensing unit  12 , thus causing electricity to flow in the light sensing unit  12 . Referring again to  FIG. 1 , an upper surface ranging from the drain electrode  13 -D to the source electrode  13 -S is covered with a light shielding layer  13 - sh  such that external light cannot be received by the switching unit  13 . Preferably, an insulating layer  17  is formed over first electrode  14  and a passivation layer  18  is formed over insulating layer  17 . Passivation layer  18  can be formed of silicon-nitride (SiNx) and is provided to electrically and physically protect the remainder of capture sensor  10 . As is understood by those skilled in the art, an array of capture sensors such as capture sensor  10  can be formed to image an entire fingerprint.  
         [0008]     Regarding capture sensor  10 , however, passivation layer  18  may not be durable enough to withstand many repeated uses of sensor  10 . Additionally, it may be difficult to make the surface of passivation layer  18  relatively smooth. And, irregularities in the surface of passivation layer  18  can distort a fingerprint image which sensor  10  is acquiring.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     An image capture sensor in accordance with the present invention includes a glass layer on which an object to be imaged is placed. Unlike the passivation layer discussed above in the background section, a glass layer can be made thick enough to be relatively durable and is relatively smoother than the passivation layer of the prior art. Accordingly, an image capture sensor in accordance with the present invention includes a light detection transistor having a light sensitive layer which conducts electricity in response to detection of a predetermined amount of light and a switch interconnected to the light detection transistor and responsive to detection of light by the light detection transistor. A glass substrate is layered over both the light detection transistor and switch. The glass substrate is the surface upon which a patterned object to be imaged in placed.  
         [0010]     In another aspect of the invention, the glass substrate include fiber-optic strands, allowing the glass substrate to be thicker and, thereby, advantageously more durable. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a sectional view of a prior art thin-film transistor object capture sensor which includes a light sensing transistor and a switch and which can be used to detect a patterned object such as a fingerprint.  
         [0012]      FIG. 2  is an illustration showing the operation of the object capture sensor shown in  FIG. 1 .  
         [0013]      FIG. 3  is a sectional view of an object capture sensor including a glass substrate on which an object to be patterned is to be placed in accordance with the present invention.  
         [0014]      FIG. 4   a  is an illustration of the operation of the object capture sensor shown in  
         [0015]      FIG. 4   b  is an illustration showing detail of the operation of the object capture sensor shown in  FIGS. 3 and 4   a.    
         [0016]      FIG. 5  is a sectional view of a second embodiment of an object capture sensor including a conducting layer adjacent to a glass substrate on which an object to be patterned is to be placed in accordance with the present invention.  
         [0017]      FIG. 6  is a sectional view of a third embodiment of an object capture sensor—including fiber-optic strands in a glass substrate on which an object to be patterned is to be placed in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     An image capture sensor in accordance with the present invention is shown in  FIG. 3 . Capture sensor  100  includes a passivation layer  118 , which can be formed of SiNx. On top of passivation layer  118 , a storage capacitor layer is formed including first electrode  115 . This storage capacitor layer is preferably formed from indium tin oxide (ITO), which is conductive and transparent. On top first electrode  115 , a insulating layer  117  is formed, preferably of SiNx. Over insulating layer  117 , a second electrode  114  is formed, preferably of tin oxide. First electrode  115 , insulating layer  117  and second electrode  114  together form the storage capacitor. Over second electrode  114 , another insulating layer  116  is formed, which can be formed from SiNx. A layer of glass layer  111  is placed over insulating layer  116 . A fingerprint to by imaged is placed on glass layer  111 , which may be referred to herein as the imaging surface.  
         [0019]     A light sensing unit  112 , which is preferably a thin-film transistor, and a switching unit  113 , which is also preferably a thin-film transistor, are horizontally arranged on a passivation layer  118 . Under passivation layer  118 , a back light  120  irradiates light upward to be passed through the fingerprint capture sensor  100 . As shown in  FIG. 3 , back light  120  is separated from a lower, exposed surface of passivation layer  118 . It is also considered, however, that backlight  120  be placed against lower surface of passivation layer  118 . Backlight  120  can be an LED or any other type of light source as is understood in the art. A source electrode  112 -S of the light sensing unit  112  and a drain electrode  113 -D of the switching unit  113  are electrically connected through second electrode  114 . A gate electrode  112 -G of the light sensing unit  112  is connected to first electrode  115 . Additionally, a first light shielding layer  113 - sh  is placed between insulating layer  117  and passivation layer  118  at switching unit  113 . As detailed below, first light shielding layer  113 - sh  blocks light from backlight  120  from reaching swithing unit  113 . Additionally, second light shielding layer  122  is positioned between glass layer  111  and insulating layer  116  at switching unit  113  to shield switching unit  113  from light passing through or reflected from glass layer  111 .  
         [0020]     In the above structure, a photosensitive layer  112 -P such as amorphous silicon (a-Si:H) is formed between the drain electrode  112 -D and source electrode  112 -S of the light sensing unit  112 . As is understood in the art, photosensitive layer  112 -P allows current to flow in response to a predetermined amount of light striking a surface of photosensitive layer  112 -P. In this way, when more than a predetermined quantity of light is received at a surface of photosensitive layer  112 -P, current flows through the drain electrode  112 -D and the source electrode  112 -S.  
         [0021]      FIGS. 4   a  and  4   b  illustrate the operation of sensor  100  discussed above.  FIG. 4   a  illustrates a fingerprint  130  placed against glass layer  111 .  FIG. 4   b  is a detailed view of a portion of  FIG. 4   a  showing a single ridge of fingerprint  130   a  placed against glass layer  111  of sensor  100 . Light  150 , generated from back light  120  beneath passivation layer  118 , is reflected from fingerprint ridge  130   a  and received by the photosensitive layer  112 -P of the light sensing unit  112 , thus causing electricity to flow in the light sensing unit  112 . Gate electrode  112 -G of light sensing unit  112  serves to block light  150  directly emitted by light source  120  from reaching light sensing unit  112  through a lower face thereof. Additionally, as discussed above, a portion of switching unit  113  from the drain electrode  113 -D to the source electrode  113 -S is covered with a light shielding layer  113 - sh  such that external light cannot be received by the switching unit  113 .  
         [0022]     When light photosensitive layer  112 -P of light sensing unit  112  allows current to flow, the current passes through electrode  114  and into drain electrode  113 -D of switching unit  113 . This causes switching unit  113  to be activated, thereby indicating that a portion of a fingerprint ridge is above the location of sensor  100  in a fingerprint sensor array (not shown). If a fingerprint valley is above the location of sensor  100 , then incident light from backlight  120  will be reflected back into sensor  100  to a far smaller degree than if a ridge is above the location of sensor  100 . As such, photosensitive layer  112 -P will not receive sufficient light to begin conducting sufficient current to activate switching unit  113 . In this way, an array of image capture sensors such as image capture sensor  100  can be used to determine the contours of fingerprint ridges and valleys of a fingerprint placed on the imaging surface of such an array.  
         [0023]     As discussed above, a glass surface, which is relatively durable, is used as the imaging surface for capture sensor  100 . As such a relatively high degree of protection is provided to the rest of capture sensor  100 . Also, the glass imaging surface can be relatively smooth, causing relatively little distortion in a captured image. Additionally, no extra coating over the surface of a capture sensor in accordance with the present invention is necessary.  
         [0024]     Referring again to  FIG. 3 , in a method of fabricating capture sensor  100 , a second light shielding layer  122  is first placed on glass layer  111  via evaporation, sputtering or any other method. Glass layer  111  is preferably between about 5 and 10 um, though may be either thicker or thinner. Light shielding layer  122  is preferably formed from a metal such as aluminum, but may be formed from any suitable light blocking material. Next, insulating layer  116  is formed on top of glass layer  111  and second light shielding layer  122 . As noted above, insulating layer  116  is preferably formed from SiNx. Photosensitive layer  112 -P is then formed over insulating layer  116 . As discussed above, photosensitive layer  112 -P is preferably formed from a-Si:H. Source electrode  112 -D of light sensing unit  112 , second electrode  114  and drain electrode  113 -D of switching unit  113  are next formed over insulating layer  116 . Source electrode  112 -D, second electrode  114  and drain electrode  113 -D are each preferably formed of ITO, but may be formed of any suitable conductor. Next, insulating layer  117  is formed and over insulating layer  117  first electrode  115  is formed. Insulating layer  117  is preferably formed from SiNx and first electrode  115  is preferably formed of ITO but may be formed of any suitable conductor. Next, gate electrode  112 -G of light sensing unit  112  and light shield  113 - sh  are formed. Preferably, gate electrode  112 -G and light shielding layer  113 - sh  are each formed of ITO, but may be formed of any suitable material and light shielding layer  113 - sh  does not need to be formed from the same material as gate electrode  112 -G. Next, passivation layer  118 , which is preferably formed from SiNx, is formed over first electrode  115 , gate electrode  112 -G and light shielding layer  113 - sh . As discussed above, backlight  120  can either be attached to the lower, exposed surface of passivation layer  118  or separately supported in a known manner.  
         [0025]     A second embodiment of an image capture sensor in accordance with the present invention is illustrated in  FIG. 5 . Image capture sensor  200  has substantially the same structure as capture sensor  100  except that conductive ITO layer  230  is placed beneath glass layer  211  and an insulating layer  232 , which can be formed of SiNx, is placed below ITO layer  230 . Because ITO layer  230  is conductive, electrostatic charge built up on glass layer  211  can be discharged by connecting ITO layer to a ground in a known manner. This can advantageously prevent damage to capture sensor  200 . Image capture sensor can be fabricated in substantially the same manner as image capture sensor  100  except that ITO layer  230  is formed over glass layer  211  and insulating layer  232  is formed over ITO layer  230  prior to forming light shielding layer  222  over insulating layer  232 .  
         [0026]     A third embodiment of an image capture sensor in accordance with the present invention is shown in  FIG. 6 . Image capture sensor  300  has substantially the same structure as capture sensor  100 . Specifically, capture sensor  300  includes a light sensing unit  312 , which is substantially the same and light sensing unit  112 , and switching unit  313 , which is substantially the same as switching unit  113 , formed between an insulating layer  316  and a passivation layer  318 . However, above insulating layer  316  capture sensor  300  includes a substrate layer  330  having a plurality of fiber-optic strands  330   a  running in a direction perpendicular to a surface of substrate layer  330 . Preferably, the diameter of the fiber-optic strands  330   a  forming substrate layer  330  is from about 4 um to about 8 um in diameter and more preferably about 6 um in diameter, though larger or smaller diameters can also be used. Substrate layer  330  can be formed from glass fiber optic strands  330   a  or fiber optic strands of other substantially transparent materials including polymers. Fiber optic sheets which can be used to form substrate layer  330  are known in the art and available from, for example, Schott Fiber Optics of Southbridge Mass.  
         [0027]     In operation, as shown in  FIG. 6 , a fingerprint  320  including a fingerprint ridge  322  to be imaged is placed on an exposed surface of fiber-optic layer  330 . Incident light from backlight  320 , which can be substantially the same as backlight  120  of capture sensor  100 , passes into fiber-optic layer  330  and can either directly pass through fiber-optic layer  330  as shown by arrow  340 , or pass through fiber-optic layer  330  by undergoing total internal reflection (TIR) from the sides of a fiber-optic strand  330   a , as shown by arrow  342 . In either case, if the incident light from backlight  320  strikes a fingerprint ridge  322 , it will scatter back through fiber-optic layer  330  either directly or, as shown by arrow  344 , undergoing TIR to reach photosensitive layer  312 -P of light sensing unit  312 . Because light scattered from a fingerprint ridge  322  can undergo total internal reflection to pass through fiber-optic layer  330 , fiber-optic layer  330  can be relatively thicker than a glass layer such as glass layer  111  without degrading the performance of capture sensor  300 . As such, fiber-optic layer is preferably 0.8 mm to 1.0 mm but may be either thicker or thinner. Because, as described above, fiber-optic layer can be relatively thick, a fiber-optic layer such as fiber-optic layer  330  can provide relatively more protection for an image capture sensor such as image capture sensor  300 . Image capture sensor  300  can be fabricated in substantially the same manner as image capture sensor  100  except that fiber-optic layer  330  is used in place of glass layer  111 . It is also considered that glass layer  211  of image capture sensor  200  be replaced by a fiber-optic layer such as fiber-optic layer  330 .  
         [0028]     Although particular embodiments have been described in detail, various modifications to the embodiments described herein may be made without departing from the spirit and scope of the present invention, thus, the invention is limited only by the appended claims.