Abstract:
A scanning fingerprint detection system includes an array of capacitive sensing elements, the array having a first dimension greater than the width of a fingerprint and a second dimension less than the length of a fingerprint. Each of the capacitive sensing elements has first and second conductor plates connected across an inverting amplifier, the conductor plates forming capacitors with the ridges and valleys of a fingerprint of a finger pressed against a protective coating above the array, the inverting amplifier generating a signal indicative of a ridge or valley. Circuitry is provided for scanning the array to capture an image of a portion of fingerprint and for assembling the captured images into a fingerprint image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is continuation of commonly assigned Application Ser. No. 09/006,670, filed Jan. 13, 1998; now U.S. Pat. No. 6,317,508. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to methods of and systems for capturing fingerprint images, and more particularly to a semiconductor capacitive fingerprint scanning device. 
     BACKGROUND OF THE INVENTION 
     Fingerprint recognition has been suggested for use in many security applications such as controlling access to buildings, computers, or the like. Fingerprint recognition systems enable a user to access the controlled facility without a device such as a key or smart card or without having to memorize a password or other personal identification number. 
     The sensing device is an important part of a fingerprint recognition system and the quality of the representation of the fingerprint that the device produces will affect recognition capability and the amount of processing required for verification of the fingerprint. Various technologies have been proposed for use in fingerprint sensing devices. One commonly proposed technology involves optical detection. Examples of optical fingerprint detection devices are disclosed in Jensen, U.S. Pat. No. 4,784,484; Fishbine, et al., U.S. Pat. No. 5,467,403; and Giles, et al., U.S. Pat. No. 5,548,394. 
     Optical detectors include a glass surface upon which a subject places his finger to be recognized. Optical detectors may present recognition problems when the glass surface or the subject&#39;s finger is wet. The optics of the detectors are constructed based upon the indices of refraction of air and glass. When water or perspiration is between the glass and the surface of the finger, the operation of the detector is affected. 
     In addition to optical sensors, various electrical sensor systems have been proposed, as for example in Knapp, U.S. Pat. No. 5,325,442; Tamori, U.S. Pat. No. 5,400,662; and Tamori, U.S. Pat. No. 5,429,006. The electrical detection devices typically comprise an array of sense elements. The individual sense elements respond with an output that depends upon whether a fingerprint ridge or valley is located over the sense element. 
     The electrical detection devices offer advantages over the optical detection device. However, an electrical detector that is large enough to detect a fingerprint is a large and expensive semiconductor device. For example, the TouchChip™ Silicon Fingerprint Sensor (STFP2015-50) available from STMicroelectronics, Inc. has an active sensor surface measuring 19.2 mm by 12.8 mm that includes a 384 by 256 sensor array. Accordingly, electrical detection devices tend to be more expensive than optical detectors. 
     It is an object of the present invention to provide a fingerprint detecting device that overcomes the shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a scanning fingerprint detection system that includes an array of capacitive sensing elements lying beneath the top surface of a protective layer. The array has a first dimension about the width of a fingerprint and a second dimension substantially less than the length of a fingerprint. Each of the capacitive sensing elements has first and second thin conductor plates, which are closely spaced relative to each other, and an inverting amplifier having an input connected to the first conductor plate and an output connected to the second conductor plate, the amplifier generating a signal indicative of a ridge or a valley of a fingerprint of a finger pressed against the top surface of the protective layer. Circuitry is provided for scanning the array to capture an image of a portion of fingerprint and for assembling the captured images into a fingerprint image as a fingerprint is moved over the array parallel to the second dimension. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system according to the present invention. 
     FIG. 2 is a block diagram of a sensor array according to the present invention. 
     FIG. 3 illustrates the physical structure and electrical operation of individual sensor cells according to the present invention. 
     FIG. 4 illustrates a sequence of partial fingerprint images captured according to the present invention. 
     FIG. 5 illustrates a fingerprint image assembled according to the present invention from the partial images of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the drawings, and first to FIG. 1, a fingerprint scanner according to the present invention is designated generally by the numeral  11 . Fingerprint scanner  11  includes a scanning array  13 , which captures an image of a fingerprint, and a suitable output  15 . Scanning array  13  is preferably fabricated on a single semiconductor chip. 
     Scanning array  13  is rectangular in shape and has a width about the width of the surface of a finger  17  that contacts scanning array  13 . In the preferred embodiment, scanning array  13  is about one-half inch or 12.8 mm wide. The length of scanning array  13  is substantially less than the length of the end of finger  17 , and in the preferred embodiment, the length of scanning array  13  is about one-tenth inch or 2.5 mm. As will be described in detail hereinafter, fingerprint scanner  11  captures a fingerprint image as finger  17  is swept over scanning array  13 . 
     Referring now to FIG. 2, there is shown a block diagram of scanning array  13 . Scanning array  13  is preferably integrated into a single chip, and it includes a rectangular array  27  of cells  29  of the type illustrated in FIG. 3 hereof. Each cell  29  is smaller than the width of a fingerprint ridge. 
     In the preferred embodiment, cells  29  are on a pitch of 50 μm, which corresponds to a resolution of about 508 dpi. The exact number of rows needed depends upon the capabilities of the image regeneration software as well as the maximum finger speed and the frame rate at which array  27  is scanned. The number of rows must be sufficient so that, when the finger is moving at its maximum speed, a pair of consecutive frames has enough rows in common for them to be aligned by the regeneration algorithm. The more image rows in common from one frame to the next, the more exactly the regeneration algorithm can combine two frames into a single larger frame. In the preferred embodiment, array  27  comprises about twenty to fifty rows of cells in the shorter dimension and about 250 columns of cells in the longer dimension. 
     The fingerprint scanner  11  includes a horizontal scanning stage  31  and a vertical scanning stage  33 . Scanning stages  31  and  33  enable one cell  29  at a time according to a predetermined scanning pattern. The scanning rate depends upon the maximum finger speed and the amount of blurring that can be tolerated. In the preferred embodiment, each cell  29  is scanned at a rate once each one to ten millisecond to produce a frame rate of 100 to 1,000 frames per second. 
     The fingerprint scanner  11  includes a power supply and scan control unit  35 . Power supply and scan control unit  35  supplies a reference voltage to each cell  29  of array  27 . Power supply and scan control  35  also operate scanning stages  31  and  33  to produce the desired scanning of cells  29 . 
     An A/D converter  37  is connected to receive the output of each cell  29  of array  27 . The output of A/D converter  37  is connected to output logic  39 . Output logic  39  processes the output of A/D converter  37  to capture successive images of a portion of the fingerprint of the user. Output logic  39  compares successive images to detect movement of the fingerprint. If output logic  39  detects movement, output logic computes the displacement of the fingerprint ridges over the scanning period, which in the preferred embodiment is one to ten milliseconds, and assembles the captured images into a complete fingerprint image. 
     Referring now to FIG. 3, there is illustrated the structure and operation of a cell  29  according to the present invention. The cell of the preferred embodiment of the present invention is of type disclosed in Tartagni, U.S. Pat. No. 6,114,862, entitled Capacitive Distance Sensor, the disclosure of which is incorporated herein by reference. Each cell  29  includes a first conductor plate  47  and a second conductor plate  49  supported on a semiconductor substrate, which is preferably a conventional silicon substrate that may have a conventional shallow epitaxial layer defining an upper surface region thereof. The top surface of the substrate includes an insulating layer  41 . Insulating layer  41  is preferably an oxide layer, which may be a conventional thermally grown silicon dioxide layer. Conductor plates  47  and  49  are covered by a protective coating  51  of a hard material, which protects cell  29  from moisture, contamination, abrasion, and electrostatic discharge. 
     Each cell  29  includes a high gain inverting amplifier  53 . The input of inverter  53  is connected to a reference voltage source V REF  through an input capacitor  54 . The output of inverter  53  is connected to an output V OUT . The input of inverter  53  is also connected to conductor plate  47  and the output of inverter  53  is also connected to conductor plate  49 , thereby creating a charge integrator whose feedback capacitance is the effective capacitance between conductor plates  47  and  49 . 
     When a finger  23  is placed on the surface of protective layer  51 , the surface of the skin over each sensor acts as a third capacitor plate separated from adjacent conductor plates  47  and  49  by a dielectric layer that includes protective coating  51  and a variable thickness of air. Because fingerprint valleys or pores  55  will be farther from conductor plates  47  and  49  than finger ridges  57 , sensors  29  beneath valleys or pores will have more distance between their conductor plates  47  and  49  and the skin surface than sensors under ridges. The thickness of this dielectric layer will modulate the capacitance coupling between plates  47  and  49  of each cell  29 . Accordingly, sensors  29  under valleys or ports will exhibit a different effective capacitance than sensors  29  under ridges. As shown in FIG. 3, the effective capacitance of sensor  29   a  is different from the effective capacitance fo sensor  29   b.    
     Sensors  29  work in two phases. During the first phase, the charge integrator is reset with a switch  59  by shorting the input and output of inverter  53 . This causes inverter  53  to settle at its logical threshold. During the second phase a fixed charge is input to the charge integrator, causing an output voltage swing inversely proportional to the feedback capacitance, which is the effective capacitance between conductor plates  47  and  49 . For a fixed amount of input charge, the output of inverter  53  will range between two extremes depending on the effective feedback capacitance value. The first extreme is a saturated voltage level if the effective feedback capacitance is very small. The second extreme is a voltage close to the logical threshold, which is the reset value, when the effective feedback capacitance is large. Since the distance between the skin and the sensor changes the effective feedback capacitance of the charge integrator, the output of sensor  29   a  under ridge  57  will be different from the output of sensor  29   b  under valley  55 . 
     The operation of the present invention to capture a fingerprint image is illustrated with respect to FIGS. 4 and 5. FIG. 4 illustrates a sequence of partial fingerprint images  61 - 83  captured according to the present invention. FIG. 5 illustrates a fingerprint image  85  assembled according to the present invention from partial images  61 - 83 . In FIG. 4, partial image  61  is captured first and partial image  62  is captured an instant later. It will be noted that partial images  61  and  62  share a number of common fingerprint features. Similarly, partial images  63  through  83  are captured at sequentially later instants of time and they each share fingerprint features with their sequentially adjacent partial images. Output logic  39  of FIG. 2 compares successive partial images  61 - 83  to detect movement of the fingerprint. If output logic  39  detects movement, output logic computes the displacement of the fingerprint ridges over the scanning period, which in the preferred embodiment is one to ten milliseconds, and assembles the captured images into a complete fingerprint image  85 . 
     From the foregoing, it may be seen that the present invention is well adapted to overcome the shortcomings of the prior art. The capacitive sensors of the present invention enable the device to be scanned at a high frame rate. The high frame rate enables a finger to be moved quickly over the device. Additionally, the high frame rate reduces the number of rows needed to capture the successive images. The device of the present invention is thus small in size, an it may be fabricated on a single integrated circuit chip. The present invention provides the advantages of electrical fingerprint detection at a cost lower than optical systems. 
     Although the present invention has been illustrated and described with respect to a presently preferred embodiment, it is to be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention as defined by the appended claims.