Abstract:
A method and apparatus for sensing a fingerprint has an array of sensors, each sensor having a sensing surface for receiving the pressure of a finger and having an ITO layer that has an intrinsic variable resistance characteristic that varies because of the varying ridges and valleys of a finger. The intrinsic variable resistance characteristic is converted to a variable voltage for a given pixel based on the pressure applied by the finger, and the fingerprint is determined based on the variable voltage readings for each pixel.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to apparatus for identification of fingerprints. In particular, this invention relates to a sensor for sensing a fingerprint in order to enter corresponding electrical information into a fingerprint authentication device. Still more particularly, this invention relates to a fingerprint authentication device having a surface for pressing a finger thereto and having pressure-sensitive means for reading the ridges and valleys of the finger when pressed against the sensing surface. 
         [0003]    2. Description of the Prior Art 
         [0004]    In a fingerprint sensor, the finger under investigation is usually pressed against a flat surface, such as one side of a glass plate, and the ridge-valley pattern of the finger tip is sensed by some sensing element such as an interrogating light beam if a laser technique is used. 
         [0005]    Fingerprint authentication devices of this nature are generally used to control the access of individuals to information (information access control), for instance, computer terminals, or to buildings (physical access control). 
         [0006]    One of the problems associated with fingerprint sensors concerns the reliable and accurate transformation of the ridge-valley pattern of the finger tip into electrical signals. Optical techniques which are widely used require a high amount of sophisticated equipment. Simple electromechanical sensors are sometimes not sensitive enough. 
         [0007]    The condition of the finger can also attribute to inaccurate readings. For example, certain sensors may not accurately read a wet, oily or dirty finger that is pressed on the sensor. Also, the speed at which a finger is slid on to the sensor may impact the accuracy of the reading. 
         [0008]    One form of sensor that has been used is a capacitive sensor, as described in U.S. Pat. No. 4,353,056 to Tsikos, where the sensing member contains a plurality of small capacitors. When a finger is pressed against the sensing surface, the capacitances of the capacitors are locally changed in accordance with the ridges and the valleys. The information about the capacitance distribution is transformed into an electrical signal that is used for processing. Unfortunately, the sensitivity of capacitors may still be insufficient to ensure accurate reading and authentication of fingerprints under all circumstances. 
         [0009]    Therefore, there is a need for a fingerprint sensor which is adapted to reliably sense the fingerprint relief and transform the sensed information into electrical signals. 
       SUMMARY OF THE DISCLOSURE 
       [0010]    In order to accomplish the objectives of the present invention, the present invention provides a fingerprint authentication device having a sensor which uses variable voltage to detect the ridge-valley pattern of a finger tip. The variable voltage for each pixel in an array is obtained by the intrinsic variable resistance characteristic of an indium-tin-oxide (ITO) layer based on the pressure applied by the finger. 
         [0011]    The fingerprint authentication device according to one embodiment of the present invention has an array of sensors, each comprising an E-sheet having a sensing surface for receiving the pressure of a finger, a TFT pixel spaced from the E-sheet and having an ITO layer that has an intrinsic variable resistance characteristic that varies because of the varying ridges and valleys of a finger, and means coupled to the TFT pixel for converting the intrinsic variable resistance characteristic to a variable voltage for a given pixel based on the pressure applied by the finger. The device further includes means for determining the fingerprint based on the variable voltage readings for each pixel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is an electrical diagram illustrating an array of sensors for use in detecting a fingerprint according to one embodiment of the present invention. 
           [0013]      FIG. 2  is a single fingerprint sensor according to the present invention. 
           [0014]      FIG. 3  is a diagram of the structure of a silicon TFT pixel according to the present invention. 
           [0015]      FIG. 4  illustrates the operation of the fingerprint sensor of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]    The following detailed description is of the best presently contemplated modes of carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating general principles of embodiments of the invention. The scope of the invention is best defined by the appended claims. 
         [0017]    According to  FIG. 1 , the fingerprint authentication device  10  according to the present invention contains a two-dimensional array  12  of sensors  14 . In one non-limiting embodiment of the present invention, the array  12  can be comprised of 256×256 sensors  14 . 
         [0018]      FIGS. 2 and 3  illustrate a single sensor  14  of the array  12 . The finger F is adapted to be pressed on an E-sheet  20  layer that is spaced from an ITO (indium-tin-oxide) layer  16 . The E-sheet  20  represents a flat sensing surface. The circuit of the sensor  14  includes a silicon TFT (thin-film-transistor) pixel  18 .  FIG. 3  is a diagram of the structure of a single silicon TFT pixel  18 . The thickness between B and A is less than or equal to  1  micrometer. The TFT pixel  18  has a drain that is coupled to the ITO layer  16 , and a source that is coupled to the E-sheet  20 . As such, the E-sheet  20  behaves as a switch that connects the source to ground via a 10 k ohm resistor. 
         [0019]    Referring to  FIG. 4 , the E-sheet  20  is comprised of three layers, which when combined is preferably soft and flexible enough to conform to the ridges and valleys of a fingerprint. The E-sheet  20  includes a bottom layer  50  that directly faces the TFT pixel  18 , a top layer  52  that is adapted to contact a finger, and a middle layer  54 . In the preferred embodiment, the bottom layer  50  is a very flexible Au (gold) layer, the middle layer  54  is a flexible PET layer, and the top layer  52  is a silicon anti-scratch coating. The combined thickness of the three layers  50 ,  52 ,  54  is about 25 micrometers. The spacing between the bottom layer  50  and the TFT pixel  18  is about 30 micrometers. 
         [0020]    Referring back to  FIG. 2 , a current-limiting resistor  22  is positioned between the E-sheet  20  and ground. In addition, the TFT pixel  18  is driven by a 12V voltage that is applied to the gate of the TFT pixel  18 .  FIG. 2  illustrates a parasitic resistor, which is actually the intrinsic impedance of the TFT gate, and not an actual resistor. The current at the gate of the TFT pixel  18  varies depending on the finger pressure applied to the E-sheet  20 , and the current is proportional to the pressure. An intrinsic resistance Rv is generated at the gate of the TFT pixel  18 . 
         [0021]    A voltage divider is formed by the resistors  22 ,  24  and  26 , and the TFT pixel  18 (Rv), with the resulting voltage Vd provided to a sample-and-hold (S/H) circuit  30 . A gate  28  is provided for the reset circuit. When the gate  28  is on, the gate  28  forms a short between its “source” and “drain”, thereby providing a path to set the point where the S/H circuit  30  is to the initial voltage. The resistor  24  is coupled to the drain of the gate of the TFT pixel  18 . The resistor  24  is actually an intrinsic impedance of the gate of the TFT pixel  18 , and not an actual resistor. 
         [0022]    The ITO layer  16  turns out the characteristics of the ITO layer&#39;s variable resistance (which is proportional to pressure), which is then converted to a (variable) voltage reading, and then converted from analog to digital format so that the resulting digital signals can be processed by an MCU (processor) (not shown) to determine the fingerprint. 
         [0023]      FIGS. 2 and 4  illustrate the operational concept of the present invention, which uses the intrinsic resistance characteristic of the ITO layer  16  to obtain a voltage reading that varies because of the varying ridges and valleys of a fingerprint. In this regard, the TFT pixel  18  can be viewed as generating an intrinsic resistance Rv which can also be represented by the notation R(drain+source). Then, the RC&#39;s equivalent circuit is formed by the internal capacitance times R(total), where: 
         [0000]      V(s&amp;h)=(Rv+10 K+R 1 )/(Rv+10 K+R 1 +1 M) 
         [0024]    R(total)=(R 1 +1 M)//(Rv+10 K)≈(Rv+10 K), since 1 M is comparatively huge and hence can be ignored. 
         [0000]    Thus, the E-sheet  20  actually behaves as an on-off switch for the TFT pixel  18 , so that the imposed pressure on the TFT pixel  18  results in a resistance that is proportional to it, which turns out to be the V(s&amp;h) with a R(total)×C discharge time constant. 
         [0025]    As illustrated by  FIG. 1 , the fingerprint authentication device  10  scans the array  12  using a well-known recursive X-axis and Y-axis scan to detect the pressure at each sensor  14 . The analog signal from each sensor  14  is then provided to a plurality of S/H circuits  30  in the form of a voltage. The S/H circuits are coupled to an ADC (ahalog-to-digital converter)  32  that converts the analog signals to  8 -bit grey scale digital signals. Since the surface and patterns of a fingerprint are not flat, the grey scale can sense the light and dark areas of a finger&#39;s pattern. These digital signals can be stored in a memory (not shown) of the device  10 . The information contained in these digital signals represents the fingerprint. If desired, the information stored in the memory can be read out, and can also be displayed on a display device, such as a screen, printed out, or plotted on a chart. 
         [0026]    While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.