Abstract:
A low cost capacitive fingerprint sensor which can be fabricated on various substrates, such as large glass or plastic substrates. The sensor is made by depositing and patterning alternate layers of conductive and insulation materials. A pixel of the sensor is comprised of a pick up pad and a plurality of voltage electrodes symmetrically placed around the pick up pad. The symmetry is such that only when a finger surface ridge is present, the pick up pad registers a signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to two dimensional mapping of finger print 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 utilizes 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 from the rigid sensor electrodes to the finger surface ridges directly without relying on membranes. In this case, the capacitance variation due to the finger surface variation is minute, with a typical order to a few femto farads, and the signal is imbedded in the larger background and parasitic capacitance from the finger and the sensor structure. 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 (1/1998) and U.S. Pat. No. 5,835,141 to Aukland et al, have been implemented to filter out the background capacitance. The circuitry utilized in Tartagni et al, and Aukland et al, involve several transistors, an amplifier or charge accumulation and transfers in every pixel, making the sensor array essentially a large silicon chip and expensive to manufacture. 
     Copending application Ser. No. 09/550626 describes a new way of sensing with simple low cost fabrication by applying external fields which are diametrically opposed in respect to a pick up pad in the center. When a uniform finger surface is in contact, the fields cancel and pick up pad reads a null signal, but if there is a surface ridge, the fields are imbalanced and the pick up pad picks up a signal. Although the circuit described therein performs satisfactorily, it is desired to provide an improved system wherein the sensor is more sensitive and substantially immune from misreadings. 
     SUMMARY OF THE INVENTION 
     The present invention provides top electrode patterns applied on a substrate to which four or more phase external fields are applied at a given sensor scan time (frame time) or at two consecutive scan times. The readout lines and the sensor structure are symmetrical such that the parasitic capacitance from the sensor structure cancel out. The advantage of being able to fabricate the sensor on any substrate, such as glass, polymer and ceramics, is retained, the sensor being more sensitive and substantially immune from false readings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For better understanding of the present invention as well other objects and further features thereof, reference is made the following description which is to be read in conjunction the accompanying drawing therein: 
     FIGS.  1 ( a ) and  1 ( b ) show the basic concept of the present invention; 
     FIG. 2 shows a pattern of the top layer of the sensor which is in close proximity to the finger surface; 
     FIG. 3 is a further example of possible patterns for the top layer; 
     FIG. 4 is an example of address lines for the external voltage drivers which are made symmetrical with respect to the pick up pads; 
     FIG. 5 illustrates the sensor where each pad serves at different scan times as either a pick-up pad or a voltage pad; and 
     FIG.  6 ( a ) and FIG.  6 ( b ) show a complete sensor with voltage driver IC&#39;s and signal pick up IC&#39;s mounted on the sensor substrate. 
    
    
     DESCRIPTION OF THE INVENTION 
     FIGS.  1 ( a ) and  1 ( b ) show a simplified version of the basic sensor device  10  of the present invention. Diametrically opposing fields  11  and  12  are generated within a pixel  10  (one point of the array) of the sensor array across pick up pad  13  by external voltage drivers, and when the contact surface  14  is uniform, the charges induced on the pick up pad  13  are zero or close to zero as shown in FIG. 1 a  (note that although only one pixel is illustrated, the actual device comprises a two-dimensional array of pixels). However, if the surface in contact  15  is non-symmetric, as shown in FIG.  1 ( b ), then the fields become distorted and the induced charges on the pick up pad  13  do not cancel, and the pick up pad  13  registers a signal which is interpreted as the presence of a finger surface ridge. Pixel  10  further comprises shield  16 , signal lines  17 , substrate  19 , insulation layers  21  and  23  and electrode pads  27  and  29  (a positive voltage is applied to pad  27 ; a negative voltage is applied to pad  29 ) formed on conductive layer  2 . Shield layer  16  serves to block fields emanating from electrodes  27  and  29  such that readout lines  17  are not effected by the staying fields. Substrate  19  can be formed of an insulating material or a semiconductive substrate with an insulation coating. 
     The entire structure of the sensor is essentially alternating layers of conductors and insulators, with conductive layers patterned in certain ways and the insulator layers patterned with holes. Therefore, the fabrication of the sensor requires simple inexpensive machines with no active elements in the sensor array itself. This makes the sensor immune to the high voltage static damage unlike the silicon chip sensors of the prior art. In addition, the sensor can be made on large inexpensive substrates which makes the sensor die cost inexpensive. An example of a structure having additional layers of conductors and insulators would be, for example, when the drive lines are placed beneath the top surface of insulation layer  23 . 
     FIG. 2 shows a layout of the top most conductor layer  2  of the sensor which is in contact with the finger surface. As an option, the sensor can be protected by a protective thin insulating layer, in which case the pattern of FIG. 2 will be the second layer but still the top most conductive layer. The shaded squares  27  are the electrodes which are applied with positive voltages, the cross hatched squares  29  are the electrodes which are applied with negative voltages, and the open squares  13  are the pick up pads. There are two pairs of positive and negative electrodes and one pick up pad per pixel, but the voltage electrodes are shared with the neighbor pixels (electrodes  27   a,    27   b,    27   c  and  27   d  form one pixel). At any given time, a pair of rows of voltage electrodes  27  and  29  are activated and the pick up pads  13  are read out. Therefore, there are two readings per pixel, although four voltages could be applied simultaneously. During the reading, a detected signal indicates ridges in the surface of a finger are present on surface  2 . A processor combines (superimposes) the two readouts and produces a complete reading of the fingerprint. 
     FIG. 3 shows that the top conductive layer pattern can take a variety of forms using the basic principle of the present invention. The rectangles are the positive and negative voltage electrodes  31  and  32  respectively, and the squares  33  are the pick up pads. 
     FIG. 4 shows a typical pattern of the second (third if there is a shield layer) conductive layer  4  (corresponds to the drive line layer) which has the address lines  41  and  42  for the external applied voltages. These lines are orthogonal to the signal lines underneath (not shown), the signal (readout) lines and addresses lines being on different layers so that they do not intersect. The pick up pad goes through the signal area  43  to the lower signal lines via holes. Note that the address lines  41  and  42  are symmetrically placed around the signal area  43  such that the applied voltage influences cancel out at this layer level. The areas  44  and  45  are where the voltage electrodes  27  and  29 , respectively, of the top conductive layer  2  come down through via holes in the intermediate insulation layer  23 . In summary, the entire structure of the sensor is constructed such that the applied fields through the sensor structure alone generate null signals, and only the finger surface ridges will generate a signal, the driver lines being symmetric with respect to the readout pads. 
     The multiple conductive layers in the sensor structure could be slightly misaligned during the fabrication giving rise to a fixed parasitic signal pattern in the array even when the external impinging surface is uniform. In this case, the fixed parasitic pattern can be stored in the processor and subtracted from the real readout. 
     FIG. 5 shows an example of a conductive layer  5  structure, similar to that shown in FIGS.  1 ( a ) and  1 ( b  ), where electrodes serve as voltage pads at one time and as pick up pads at another time. In this way, the number of layers is reduced, simplifying the fabrication process. At the first scan period, the electrodes with horizontal or vertical shades  51  and  52  respectively, serve as pick up pads while the electrodes with diagonal shades  53 ,  54  function as voltage applying pads with alternating +/− voltages row to row as shown. In the next scan period, the electrodes with horizontal or vertical shades  51 ,  52  serve as voltage applying pads with the alternating voltage signs as shown, while electrodes  53 ,  54  serve as pick up pads. In this way, essentially the same information as the four phase conductive layer  2  can be obtained albeit two scans instead of one. The structure  5  can be made with as few as two metal layers with one metal layer with the pad patterns and the connection of one set of pads, for example  51 ,  52 ; while the other set of pads are connected at the lower metal layer. These connection lines are connected to IC&#39;s (integrated circuits) at the periphery of the sensor as shown in FIG.  6 . 
     FIGS.  6 ( a ) and  6 ( b ) show typical packaging of the sensor device  10  (FIG.  6 ( a ) illustrates the TAB package; FIG.  6 ( b ) the surface mount). The central area  61  is for the sensor device array discussed hereinabove. The horizontal rows are the applied voltage lines, which enter into the central area  61  from both sides, one positive voltage  62  and one negative voltage  63 . The driver IC&#39;s  64  are synchronized to drive a pair of positive and negative lines at a time. The signal lines extend from  65  and  66  of the sensor array area  61 . Each signal line  65  and  66  connects every other pick up pad vertically to reduce cross talk (dotted lines in FIG.  2 ). The amplifier IC&#39;s  67  activate one column signal line at a time and amplifies the signal before ending it to the algorithm processor (not shown). Driver IC&#39;s  64  and Amplifier IC&#39;s  67  are formed on tape portions  69  as illustrated. 
     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.