Abstract:
A compact sensor which maps fingerprints for identification purposes. The sensor consists of an array of pixels with each pixel configured with one or more pickup conductive electrodes surrounded by voltage electrodes of different phases. This configuration performs capacitive differencing to eliminate the large background capacitance without the need for complex sensor circuitry. In addition, the readout lines are electrically shielded from the input voltage lines by an intermediate grounded conductive layer, thereby eliminating the parasitic capacitance and allowing the detection of minute capacitance variation of the finger surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to two dimensional mapping of fingerprint patterns for identification purposes utilizing a capacitive circuit array. 
     2. Description of the Prior Art 
     Fingerprint sensing and the associated identification systems which include data bases and a match algorithm processor have been available in the prior art. In the area of the fingerprint sensor, the primary prior art systems utilize an optical scanning method which is relatively bulky and expensive due to the optics, lasers and the CCD array utilized. Due to this reason, there have been attempts to develop electronic means of sensing fingerprint patterns. All electronic fingerprint sensors can be categorized as follows; tactile pressure sensors, thermal sensors and capacitive sensors. The first two categories are complex and expensive to make, and thus most of recent development activities are in the area of capacitive sensors. 
     One group of capacitive sensors rely on thin deformable membranes with metal electrodes (one electrode per pixel) coated underneath each membrane. If the membrane is very thin and can follow the finger surface deformation, the distance of the metal electrodes can be measured through capacitance means. However, such thin membranes are not durable, and hence, are not yet marketable. 
     Another group of capacitive fingerprint sensors read the capacitance difference from the rigid sensor electrodes to the finger surface ridges directly without relying on membranes. In this case, the capacitance difference due to the finger surface variation is minute, with a typical order of a few fF (femto farad), and is imbedded in the larger background and parasitic capacitance from the sensor structure and the readout lines. Therefore, somewhat complex circuitry such as those disclosed by Tartagni et al. in an article entitled “Fingerprint Sensor Based on the Feedback Capacitive Sensing Scheme”, IEEE Journal of Solid State Circuits, Vol. 33, p. 133 (January 1998) and U.S. Pat. No. 5,835,141 to Ackland et al. have been implemented to filter out the background capacitance. The circuitry utilized in Tartagni et al and Ackland et al involve several transistors, an amplifier or charge accumulation and transfers in every pixel, making the sensor array complex. The prior art all have one or two metal electrodes per pixel at the top most layer of the sensor. 
     What is thus desired is to provide a fingerprint sensor which maps fingerprints for identification purposes which is reliable, compact and is less expensive than existing fingerprint identification systems. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention utilizes sensor electrodes, having three or more metal electrodes/lines per pixel at the topmost layer of the sensor; at least two input voltage lines of different phases and one or more of sensor pickup electrodes. This combination nulls out the DC background capacitance and thus no electronic circuitry below the sensor electrodes is required. The sensor array can be manufactured without the need for expensive and slow vacuum systems such as CVD&#39;s (chemical vapor deposition) and diffusion machines, etc. In particular, the sensor array can be manufactured entirely on a low cost substrate, such as circuit boards, glass or ceramics, with thick/thin film processing methods requiring only 15-20 micron feature sizes. The array readout is achieved externally without requiring transistor switches or CCD&#39;s in the pixel itself making the sensor device very economical to manufacture. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For better understanding of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawing therein: 
     FIG. 1 illustrates a prior art fingerprint sensing device; 
     FIG. 2 is a first embodiment of the present invention implemented as one dimensional array; 
     FIG. 3 is a preferred embodiment implemented as a full two dimensional array; 
     FIG. 4 shows the multi-layer profile of the sensor shown in FIG. 3 in cross-section; 
     FIG. 5 illustrates the use of all geometric configurations satisfying a criterion; 
     FIG. 6 illustrates another embodiment of the preferred embodiment of the present invention; and 
     FIG. 7 illustrates a method of picking up the signal electrodes in the bottom layer of FIG. 4 to minimize the inter-pixel cross-talk. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a prior art fingerprint sensing device  1 . The circuitry for each pixel (only one illustrated) comprises amplifier  15 , switch  14  and two capacitors  16  (a pixel is defined as an elementary set of patterns which repeats in the array and consists of pick up voltage electrodes which are necessary to perform sensing of finger ridge lines at a point or at a small point-like region). The capacitive readout between the two pickup electrodes, or conductive layers,  13  is passed through the circuit to remove the large background and parasitic capacitance. A thin protective coating  12  is disposed between the pickup electrodes  13  and the finger  11 . The circuitry requires fabrication of many transistors per pixel with sub-micron resolution processing which in turn requires expensive and slow semiconductor process equipment. In addition, the silicon wafer and other processing material costs are also high with a limited wafer size per process, yielding relatively small numbers of sensor dies which must be of approximate size 1×1 inch. 
     FIG. 2 shows an embodiment of the fingerprint sensing device  2  of the present invention used for a one dimensional array. On the left side of the figure, alternating positive and negative polarity voltages  21  and  22 , respectively, are input to the device  2  via conductors  27  and  29  while the conductor readout lines  23  are positioned on the right side of the figure. The readout lines  23  are disposed in the middle of a pair two voltage conductors, or lines,  27  and  29  such that the background capacitance subtracts out if a uniform surface is placed on the top surface. This construction thus intrinsically detects the finger surface variation only. To prevent the input and output lines from reading parasitic capacitance, the active sensor area is raised by a dielectric spacer  25  (note that the lower part of FIG. 2 is a side elevation view of the device shown in the upper portion of the figure). A thin protective layer  24  of preferably low dielectric constant is disposed on top of the conductors  23 ,  27  and  29  with a thin layer in the active area  26 . This configuration detects finger surface variation along the vertical direction only and requires the user to scan the finger in the horizontal direction. 
     Device  2  requires only one metal deposition and patterning on the substrate  30  which can be virtually any flat dielectric material, making the whole device extremely economical to manufacture. For an extra measure of caution to prevent parasitic capacitance, a grounded conductive layer can be disposed on top with an opening only over the active area  26 . 
     FIG. 3 shows a two dimensional implementation  3  of the present invention. A plurality of pickup, or sensor, electrodes  31  are placed in the center of two opposite polarity voltage lines, or conductors,  32  and  33 . The opposite polarity voltage lines are shaped as a comb and are interlocked with the pickup electrodes  31  in between the voltage lines as illustrated. In this embodiment, the user simply places his/her finger on the surface of the sensor array. 
     The layer structure of the two dimensional embodiment of FIG. 3 is shown in FIG.  4 . The signal which is detected at the metal pickup electrode  31  manifests as a current which flows to the bottom metal electrode layer  37  and is read out from bottom substrate  38  by conventional means (not shown). Grounded intermediate metal layers  34  are provided to shield the readout lines  37  from the fringe fields generated by metal voltage electrodes  32  and  33 . A hole  39  is formed between pickup conductor electrode  31  and shielding layer  34  to prevent a short therebetween which would prevent signal pickup by electrode  31 . Electrode  31  is connected to readout electrode  37  via metal fill material  41 . 
     The two dimensional sensor array shown in FIGS. 3 and 4 satisfies one of the two criteria below. The first criteria is pictorially shown in FIG.  5 . The criteria are as follows; 
     1. If two phase voltages (180° phase angles applied to electrodes  53  and  54 ) are used, the straight line  52  passing through the center of the pickup electrode  31  must intersect the opposite voltage lines at equal distances at almost any angle. This enables equal and opposite electric field lines to terminate at the pickup electrodes  31  such that the net readout from the pickup electrode  31  is zero when the medium above the electrode is zero, i.e. a user&#39;s finger is not positioned on the sensor array (in essence, if the straight line  52  was rotated in the direction of arrow  55 , the centerpoint  57  of pickup electrode  31  would be substantially equidistant from the opposite voltage lines  53  and  54 ). 
     2. If more than two phase voltages are used (three phase voltages, for example, would have 120° phase angles applied to three different voltage lines), a calculation of the integral of (distance) times (voltage) over all angles would be zero from the origin of the coordinate placed at the center of the pickup electrode. 
     In the illustration, the distance is the distance from the center of the pick up electrode  31  to the intersection of the straight line and the electrodes  53 ,  54 . In the first criteria, “almost any angle” is stated because the configuration shown in FIG. 6, for example, where small gaps exist in the chevron patterns of the voltage lines, will work since it is unlikely that the fingerprint ridge lines will run exactly along the narrow region connecting the centers of the pickup electrodes  31 . However, if the voltage lines  61  and  62  are straight, the electrodes  31  will not work because all fingerprint ridge lines running along the vertical direction will not be detected, i.e. it becomes a one dimensional sensor. Voltage lines  61  and  62  are equivalent to the hair comb patterns shown in FIG.  3 . In essence, these different polarity voltage lines generate electric fields above the surface of the finger surface. The positive and negative field lines terminate at the pickup electrode  31  such that the net readout from the electrode  31  is zero unless there is an irregular surface (i.e. a fingerprint) above which disrupts the field lines. In the case of an irregular surface, the pickup electrode  31  will register some signal (i.e. field lines are not canceled) and a current will flow to the bottom layers for readout. 
     The sensor readout is done much like the external matrix addressing of display devices. Referring to FIG. 4, the readout lines  37  are disposed in the orthogonal direction from the electrode lines  32  and  33 . At any given time, only the nearest one pair of opposite polarity voltage lines are activated while the appropriate readout line is interrogated. In this way, a row is read out one at a time, and the voltage lines are scanned down in pairs to read the whole array. The whole readout process takes a short time, typically a fraction of a second. Therefore, there is no need for a complex circuitry for every pixel, and the sensor array can be manufactured with three metal deposition and patterning. In order to reduce the cross talk between the nearest pixels, the readout lines  71  and  72  can connect every other pickup electrodes  74  as shown in FIG.  7 . FIG. 7 illustrates how the signal electrodes can be connected in the bottom layer  37  (FIG.  4 ). Conductive lines  72  terminate at the edges of the fingerprint sensor where readout driver chips are connected. 
     The present invention thus provides a relatively simple and cost effective capacitive circuit array for sensing fingerprints. 
     While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from its essential teachings.