Abstract:
Within a capacitive fingerprint detection device, finger detection is provided by a plurality of resistive grids overlying the fingerprint sensor electrodes to measure the resistance of the finger placed on the sensor surface. A finger placed on the sensor surface connects the resistive rids and allows the skin resistivity to be measured. The measured resistance is compared to a reference resistance or range of resistances to determine whether the measured resistance matches the expected bio-characteristics of living skin tissue. The finger detection thus provides anti-spoofing protection for the fingerprint detection device.

Description:
RELATED APPLICATIONS 
     The present invention is related to the subject matter of commonly assigned, U.S. patent application Ser. No. 09/648,968 entitled “CAPACITIVE FINGER DETECTION FOR FINGERPRINT SENSOR” and filed Aug. 28, 2000. The content of the above-referenced application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to fingerprint detection and recognition and in particular to finger detection within integrated circuit devices employed for fingerprint-based identification and security mechanisms. 
     2. Description of the Prior Art 
     Fingerprint detection and recognition has become a widely implemented identification and security (verification) measure for a variety of applications, including credit card usage, governmental licensing and registration, and premises access control. Integrated circuit fingerprint sensors used in such detection usually contain a two-dimensional array of sensing electrodes at a pitch substantially smaller than the size of the lines on the skin surface of a fingertip, allowing an image of the fingerprint pattern to be acquired for processing, analysis and comparison. 
     The structure of a typical electronic fingerprint sensor is depicted in FIG.  4 . Fingerprint sensor  402  includes a planar array of sensing electrodes  404  conductively coupled to detection and image capture circuitry (not shown). Sensing electrodes  404  may be covered by a protective layer  406  against which the skin surface  408  of the fingertip is placed. Ridges  410  and valleys  412  formed by the lines on the fingertip skin surface  408  are then detected utilizing sensing electrodes  404 . 
     Fingerprint detection is most commonly achieved at least partially through capacitive coupling with the finger. In such embodiments, skin surface  408  and sensing electrodes  404  each form a capacitor having a capacitance proportional to the distance between a respective sensing electrode  404  and the overlying skin surface  408 . Thus, for example, the capacitance measured by sensor  402  at a point near the top of a fingerprint ridge  410  by a sensing electrode separated from skin surface  408  by a distance d x,y  (where x and y denote the position of the respective sensing electrode within the two-dimensional sensor array) will differ from the capacitance measured through another sensing electrode at a point near the bottom of a fingerprint valley  412 , where the intervening distance d x,y+2  is greater. In this manner an electronic image of the fingerprint may be captured for further processing. 
     Fingerprint detection normally involves a relative or comparative process, in which a characteristic such as capacitance) at one sensing electrode within the array is processed relative to similar measurements by the other sensing electrodes. As a result, the fingerprint sensor is subject to attack by “spoofing,” or presentation of the appropriate fingerprint pattern by an unauthorized individual. For example, the simplest spoofing technique might involve use of a finger severed from an authorized individual for that purpose. 
     It would be desirable, therefore, to protect against unauthorized uses of fingerprint patterns being employed in order to circumvent fingerprint-based identification and security mechanisms. 
     SUMMARY OF THE INVENTION 
     Within a capacitive fingerprint detection device, finger detection is provided by a plurality of resistive grids overlying the fingerprint sensor electrodes to measure the resistance of the finger placed on the sensor surface. A finger placed on the sensor surface connects the resistive rids and allows the skin resistivity to be measured. The measured resistance is compared to a reference resistance or range of resistances to determine whether the measured resistance matches the expected bio-characteristics of living skin tissue. The finger detection thus provides anti-spoofing protection for the fingerprint detection device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, and further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 depicts a fingerprint sensor circuit employing resistive finger detection in accordance with a preferred embodiment of the present invention; 
     FIGS. 2A-2B are diagrams of fingerprint sensor arrays including resistive finger detection in accordance with a preferred embodiment of the present invention; 
     FIG. 3 depicts a circuit diagram for a finger detection circuit employed within a fingerprint sensor in accordance with a preferred embodiment of the present invention; and 
     FIG. 4 is a diagram of a known capacitive fingerprint detection structure. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description details the structure, application and features of the present invention, but it will be understood by those of skill in the art that the scope of the invention is defined only by the issued claims, and not by any description herein. The process steps and structures described below do not form a complete process flow for manufacturing integrated circuits. The present invention can be practiced in conjunction with integrated circuit fabrication techniques currently used in the art, and only so much of the commonly practiced process steps are included as are necessary for an understanding of the present invention. The figures representing cross-sections of portions of an integrated circuit during fabrication are not drawn to scale, but instead are drawn so as to illustrate the important features of the invention. 
     With reference now to the figures, and in particular with reference to FIG. 1, a fingerprint sensor circuit employing resistive finger detection in accordance with a preferred embodiment of the present invention is depicted. FIG. 1 depicts a block diagram of the sensor circuit  102 , which is formed as an integrated circuit on a single die. A suitable sensor circuit  102  and its operation are described more fully in commonly assigned, application Ser. No. 09/040,261, entitled “CAPACITIVE DISTANCE SENSOR” and filed May 9, 1998, which is incorporated herein by reference. 
     Sensor circuit  102  includes I 2 C interface and control device  104 , which provides a bidirectional communication protocol enabling sensor circuit  102  to communicate with a controller, such as a microcontroller. I 2 C interface and control device  104  exchanges digital signals via control line  106  with controller circuitry (not shown) external to sensor circuit  102  in the example illustrated, although alternatively such control circuitry may also be integrated within sensor circuit  102 . Sensor circuit  102  also includes an array  108  of capacitive sensors for fingerprint acquisition by sensing distances between capacitive electrodes within the sensor array  108  and ridges and grooves on a finger placed in contact with sensor array  108 . 
     Sensor array  108  is coupled to the external controller circuit via synchronization line  110 . Through synchronization line  110  sensor array  108  provides synchronization signals allowing detected voltages representative of the capacitive value of individual capacitive electrodes within sensor array  108 , and therefore representative of the distance between the capacitive electrode and the portion of the epidermal layer contacting sensor array  108  in the region of the capacitive electrode, to be properly interpreted by the external controller. Sensor circuit  102  also includes an analog-to-digital (A/D) converter  112  processing analog voltage measurements received from sensor array  108  and generating digital representations recognized by the external controller as distance measurements of the analog measured voltages from individual capacitive electrodes within sensor array  108 . A/D converter  112  transmits these digital signals to the external controller on output bus  114 . 
     Sensor array  108  includes a plurality of cells  116  each containing one or more capacitive electrodes. Sensor array  108  in the exemplary embodiment contains square cells approximately 45-50 μm on a side, forming a 250×350 array of contiguous cells  116  within sensor array  108 . Sensor array  108  is covered by a passivation material overlying the capacitive electrodes within each cell  118 . Other active devices required to form sensor circuit  102  are formed below the capacitive electrodes. 
     Referring to FIGS. 2A and 2B, diagrams of sensor arrays including resistive finger detection in accordance with a preferred embodiment of the present invention are illustrated. The cross-section depicted in FIG. 2A is a cross-section of sensor array  108  taken at section line A—A depicted in FIG. 1 . Cells  116  within sensor array  108  each include at least one capacitive fingerprint sensor electrode  202 , which is employed to detect the fingerprint in accordance with the known art. A protective layer  204 , which may be a combination of insulating and passivating layers, overlies the fingerprint sensor electrodes  202  and provides a surface on which the finger is placed. 
     In the present invention, sensor array  108  also includes a plurality of resistive grids  206   a - 206   b  employed to sense the resistance of the finger when placed on the surface of the sensor array  108 . This sensed resistance is then compared to a predefined “sample” or threshold value or range. If the resistance of the finger is greater than the predefined value (or within the range), a finger is detected on the surface of sensor array  108 . 
     Resistive finger detection grids  206   a - 206   b  are formed over the fingerprint sensor detection electrodes  202 , on the surface of fingerprint detection device  102  on which the finger is to be placed. Relative capacitive measurements between fingerprint sensor detection electrodes  202  will not be skewed by the presence of resistive grids  206   a - 202   b . Resistive sensing grids  206   a - 206   b  are isolated from each other, but may come in contact with a finger touching the surface of fingerprint detection device  102 . When the finger is on the sensing surface of the fingerprint detection device  102 , the skin resistivity of the finger provides a resistive path from one grid  206   a  to the other  206   b . Measuring the resistance between the two grids provides a means for measuring the skin resistivity of the finger. A plan view of a suitable geometry for grids  206   a - 206   b  is illustrated in FIG.  2 B. 
     With reference now to FIG. 3, a circuit diagram for a finger detection circuit employed within a fingerprint sensor in accordance with a preferred embodiment of the present invention is depicted. Within finger detection circuit  302 , resistive grids  206   a - 206   d  are coupled to a resistance measurement and comparison unit  308 . As noted above, the resistance of a finger placed on the sensor device  102  is measured and utilized to detect a finger. This resistance will depend on the bio-characteristics of the skin on the finger, which is employed to prevent spoofing by determining whether the finger placed on the fingerprint sensor device  102  provides a resistance within a predefined range or above a predefined value. 
     The resistance measurement is accomplished by connecting the variable resistances representing the finger connecting resistive grids  206   a - 206   b , through amplifiers  304  and  306 , to a resistance measurement and comparison unit  308 , which compares the resistance between grids  206   a - 206   b to a reference resistance or a range of resistances. The difference in resistances is determined by the comparator portion of resistance measurement and comparison unit  308 , which generates an output signal  310  indicated whether a finger has been detected. 
     Depending upon the processing and memory capabilities available, the measured resistance may be compared to absolute thresholds or ranges for the entire populace, or to specific ranges for particular individuals identified by their fingerprint, with the individual ranges of authorized individuals being dynamically updated over time to accommodate aging and seasonal variations in the relevant bio-characteristics. In addition to providing anti-spoofing protection, fingerprint acquisition by the fingerprint sensor may also be concurrently triggered by the finger detection mechanism of the present invention. 
     The present invention employs measurement of bio-characteristics of skin, specifically resistance of a finger placed on a sensor for fingerprint detection, to provide anti-spoofing protection and real finger detection. Other bio-characteristics of living skin tissue may also be employed in conjunction with the present invention. The resistive finger detection mechanism may be combined with the capacitive finger detection mechanism described in the related application, either utilizing separate capacitive and resistive grids or utilizing a single grid for both capacitive and resistive measurements. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.