Abstract:
A fingerprint sensor in accordance with the invention includes a non-conductive substrate providing a first surface onto which a user can apply a fingerprint to be sensed. A sensor circuit is applied to a second surface of the non-conductive substrate opposite the first surface to sense a fingerprint when juxtaposed proximally thereto. An electrostatic discharge conductor is applied to the non-conductive surface and is located between an area where a fingerprint is swiped and the sensor circuit. The electrostatic discharge conductor discharges electrostatic charge resulting from a user swiping a fingerprint across the first surface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to electrostatic discharge (ESD) protection and more particularly to ESD protection for fingerprint sensing electronics. 
     2. Description of the Related Art 
     Electrostatic discharge (ESD) is a serious problem when dealing with many types of solid state electronics, such as integrated circuits (ICs), due to its ability to damage sensitive circuitry. Electronic components such as ICs may be exposed to ESD from various different sources, the most common of which is the human body. A body capacitance of approximately 150 pF can hold a charge of approximately 0.6 μC, which can generate potentials of several kV. Contact between the body and a grounded IC can generate large enough currents through the IC to significantly damage internal components. 
     As transistors and other components of ICs continue to shrink in accordance with Moore&#39;s law, ESD damage becomes an even greater risk because of the smaller components&#39; inability to withstand large currents. For this reason, many recommend touching or connecting the body to ground prior to touching or handling sensitive electronic components. 
     The effects of ESD create special problems when dealing with electronics intended for touching by the body. For example, electronic fingerprint sensors allow a user to swipe or press a finger over some portion of the circuit in order to read the user&#39;s fingerprint. It would be infeasible as well as inconvenient for a user to have to ground his or her body prior to touching the sensor in order to dissipate an electrostatic charge. 
     Problems with ESD may be especially pronounced with conventional fingerprint sensors that allow a user to directly touch a piece of silicon. Nevertheless, ESD may also be a concern with newer more advanced “flexible” fingerprint sensors. These sensors may include circuits printed or otherwise applied to flexible, non-conductive materials, such as Kapton® (i.e., polyimide) substrates or other flexible non-conductive materials. These sensors may enable a user to swipe a finger across the polyimide surface without directly contacting the sensor circuitry. Unfortunately, the fact that polyimide is a good insulator allows electrostatic charge to build up on the polyimide surface as a user swipes his or her finger. This charge will continue to increase in potential until the path of least resistance is found and the charge dissipated. In certain cases, the charge may discharge to the sensor circuitry, causing damage to sensitive electronic components such as IC I/O cells. 
     In view of the foregoing, what is needed is an apparatus and method to safely discharge the electrostatic charge that accumulates on the non-conductive portions of circuits and electronic devices, including those intended for human touch, such as fingerprint sensing circuits. 
     SUMMARY OF THE INVENTION 
     The present invention provides an apparatus and method for dissipating the electrostatic charge that accumulates on circuits such as fingerprint sensing circuits. The features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter. 
     In a first aspect of the invention, a fingerprint sensor in accordance with the invention includes a non-conductive substrate providing a first surface onto which a user can apply a fingerprint to be sensed. A sensor circuit is applied to a second surface of the non-conductive substrate opposite the first surface to sense a fingerprint when juxtaposed proximally thereto. An electrostatic discharge conductor is applied to the non-conductive surface and is located between an area where a fingerprint is swiped and the sensor circuit. The electrostatic discharge conductor discharges the electrostatic charge resulting from a user swiping a fingerprint across the first surface. 
     In a second aspect of the invention, an ESD-protected circuit includes a non-conductive surface onto which an electrostatic charge can accumulate. A circuit which is sensitive to electrostatic discharge is coupled to the non-conductive surface. An electrostatic discharge conductor is coupled to the non-conductive surface and is positioned to discharge electrostatic charge from the non-conductive surface and thereby protect the circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings, in which: 
         FIG. 1  is a perspective view of one embodiment of a fingerprint sensing circuit having a substrate and an ESD conductor coupled to a first side of the substrate; 
         FIG. 2  is a perspective view of another embodiment of a fingerprint sensing circuit having a substrate and an ESD conductor coupled to the opposite side of the substrate; 
         FIG. 3  is a perspective view of another embodiment of a fingerprint sensing circuit having a substrate and an ESD conductor coupled to an edge of the substrate; 
         FIG. 4A  is a perspective view of an embodiment of a fingerprint sensing circuit having multiple layers; 
         FIG. 4B  is a perspective view of the fingerprint sensing circuit of  FIG. 4A  showing the layers sandwiched together; 
         FIG. 5  is a flow diagram of one embodiment of a method for dissipating electrostatic charge in a fingerprint sensing circuit; and 
         FIG. 6  is a flow diagram of one embodiment of a method for producing an ESD-protected fingerprint sensing circuit in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     It will be readily understood that the components of the present invention, as generally described and illustrated in the Figures herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of apparatus and methods in accordance with the present invention, as represented in the Figures, is not intended to limit the scope of the invention, as claimed, but is merely representative of certain examples of presently contemplated embodiments in accordance with the invention. The presently described embodiments will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
     Referring to  FIG. 1 , a fingerprint sensing circuit  10  in accordance with a first embodiment of the invention may include a flexible non-conductive substrate  11  having a circuit side  12  and a sensing side  14 . In certain embodiments, the substrate  11  may be constructed of a flexible polyimide material marketed under the trade name Kapton® and with a thickness of between about 25 and 100 μm. The Kapton® polymer allows the fingerprint sensing circuit  10  to be applied to products such as touchpads and molded plastics having a variety of shapes and contours while at the same time providing exceptional durability and reliability. Nevertheless, the invention is not limited to this type of substrate  11  but may include other flexible or rigid substrates  11  suitable for applying a circuit thereon. 
     In certain embodiments, the fingerprint sensing circuit  10  may include an image sensor  16  to detect the ridges and valleys of a fingerprint as it moves across the sensor  16 . Optionally, the fingerprint sensing circuit  10  may include a velocity sensor  18  to detect the speed of a finger moving across the image sensor  16 . The image sensor  16  and velocity sensor  18  may include conductive traces printed or otherwise applied to the circuit side  12  of the substrate  11  using any suitable lithographic or application technique. In certain embodiments, the image sensor  16  may be implemented as an array of capacitive sensors capable of sensing the ridges and valleys of a finger as it travels over the sensor  16 . Similarly, the velocity sensor  18  may by implemented using two or more capacitive detectors  18  at intervals along the direction of travel of the finger. 
     The above-mentioned conductive traces may connect the image sensor  16  and velocity sensor  18  to one or more sensor ICs  20  connected to the circuit side  12  of the substrate  11 . A sensor IC  20  may contain drive and sense electronics for detecting and reading fingerprints passed over the image sensor  16 . In certain embodiments, the sensor IC  20  may be connected to one or more interconnect pads which enable the fingerprint sensing circuit  10  to interface with a processor or other external system. The sensor IC  20  may be bonded to the flexible substrate  11  using any suitable technique such as a chip-on-flex (COF) process. This process may be used to electrically connect the sensor IC  20  to the image sensor  16 , velocity sensor  18 , and interconnect pads  22  to form the fingerprint sensing circuit  10 . In selected embodiments, the fingerprint sensing circuit  10  may be designed with an open architecture in order to utilize the most recent matching algorithms. Such a feature may enable users to fine tune security vs. convenience tradeoffs by selecting a suitable matching algorithm. 
     Additional information related to the implementation of fingerprint sensing circuits  10  in accordance with the invention is disclosed in U.S. Pat. No. 7,146,024 and entitled “Swiped Aperture Capacitive Fingerprint Sensing Systems and Methods,” which is herein incorporated by reference. Other information for implementing fingerprint sensing circuits in accordance with the invention may be found in U.S. Patent Pub. No. 2005/0244038 and entitled “Finger Position Sensing Methods and Apparatus” and U.S. Patent Pub. No. 2006/0083411 and entitled “Fingerprint Sensing Assemblies and Methods of Making,” which are also incorporated by reference. The fingerprint sensors disclosed in the above-identified applications are examples of fingerprint sensors that may be used with the ESD protection apparatus disclosed herein and do not represent an exhaustive list. Indeed, the invention disclosed herein may be used with many different types of fingerprint sensors including conventional sensors using silicon to contact and read a user&#39;s fingerprint. 
     One benefit of the fingerprint sensing circuit  10  illustrated in  FIG. 1  is that a user&#39;s finger is isolated from the image sensor  16 , velocity sensor  18  and sensor IC  20 . The user&#39;s finger is swiped along the polyimide surface of the sensing side  14  of the flexible substrate  11  as opposed to the circuit side  12 . The image sensor  16  and velocity sensor  18  are able to detect changes in capacitance as the finger is swiped across the sensing side  14  of the circuit  10 . Thus, the polyimide substrate electrically and mechanically isolates the user&#39;s finger from the image sensor  16 , velocity sensor  18  and sensor IC  20 , thereby providing some degree of protection from ESD and mechanical abrasion. 
     Despite its advantages, however, the flexible polyimide substrate may be susceptible to electrostatic buildup on the sensing side  14  of the substrate  11 . This occurs as a result of rubbing two non-conductive surfaces (i.e., a finger and the polyimide substrate) together. Although the polyimide substrate initially provides an effective shield between the fingerprint sensing circuitry and electrostatic charge, the electrostatic charge may continue to build up until the path of least resistance is found and the charge is dissipated. In certain cases, the charge may follow a path around the edge of the substrate  11  until it reaches the circuit side  12  of the substrate. There, the charge may discharge to the sensor circuitry  16 ,  18 ,  20 , causing damage to sensitive electronic components such as IC I/O cells. 
     In selected embodiments in accordance with the invention, an ESD conductor  24  may be placed between the sensing side  14  of the fingerprint sensor  10  and the fingerprint sensing circuitry  16 ,  18 ,  20 . This ESD conductor  24  may be connected to a known potential (e.g., ground) and may be used to safely dissipate electrostatic charge accumulated on the sensing side  14 . In certain embodiments, the ESD conductor  24  may be connected to one or more interconnect pads  22 , which may be connected to a known potential. 
     Because the ESD conductor  24  is positioned between the sensing side  14  and the fingerprint sensing circuitry  16 ,  18 ,  20 , the shortest path, and thus the path of least resistance, is the path between the sensing side  14  and the ESD conductor  24 . In selected embodiments, the ESD conductor  24  may encircle the sensor circuitry  16 ,  18 ,  20  to eliminate discharge paths between the sensing side  14  and the sensor circuitry  16 ,  18 ,  20 . Nevertheless, the ESD conductor  24  is not limited to this shape but may include various continuous and non-continuous shapes and may, in certain embodiments, only partially surround the circuitry  16 ,  18 ,  20 . Similarly, the ESD conductor  24  may be placed at or near a perimeter of the substrate  11  to maximize the space available for the circuitry  16 ,  18 ,  20 . 
     The ESD conductor  24  may be made of various conductive materials including, for example, aluminum, gold, nickel, copper, or the like. In selected embodiments, the ESD conductor  24  is made of the same conductive material as that used for traces and other conductors of the fingerprint sensing circuit  10 . Thus, the ESD conductor  24  may be manufactured at low cost using standard chip on flex (COF) or other suitable manufacturing processes. 
     Referring to  FIG. 2 , in other embodiments, an ESD conductor  24  may be placed on the sensing side  14  of the substrate  11  (In the illustrated embodiment, the fingerprint sensing circuit  10  is flipped over such that the sensing side  14  faces upwards). For example, an ESD conductor  24  may be configured to encircle the perimeter of the sensing side  14 . Such an embodiment may be advantageous in certain situations. For example, some application may require interconnect pads or pins that reach the edge of the substrate  11  in order to properly interface or mate with a host system. In such applications, it may be infeasible to place an ESD conductor  24  around the sensor components  16 ,  18 ,  20  on the circuit side  12  of the substrate  11 . Thus, in certain embodiments, an ESD conductor  24  may be placed around the perimeter of the sensing side  14  to provide a similar function. 
     Referring to  FIG. 3 , in other embodiments, an ESD conductor  24  may be provided along an edge of the substrate  11  to prevent migration of electrostatic charge from one side of the substrate  11  to the other. 
     Referring to  FIGS. 4A and 4B , in another embodiment, a fingerprint sensing circuit  10  in accordance with the invention may include several layers  11   a ,  11   b , or substrates  11   a ,  11   b , sandwiched together. For example, in one embodiment, fingerprint sensor components  16 ,  18 ,  20  may be printed or otherwise attached to a first non-conductive substrate  11   a . The substrate  11   a  may include various flexible or rigid substrate materials suitable for receiving a conductive circuit. In certain embodiments, the substrate  11   a  and circuit components  16 ,  18 ,  20  are provided in the form of a conventional printed circuit board (PCB). 
     A second non-conductive layer  11   b  or substrate  11   b  may be placed over the circuit components  16 ,  18 ,  20  of the first layer  11   a . For example, a flexible polyimide layer  11   b  such as a layer  11   b  of Kapton® may be used to cover the circuit components  16 ,  18 ,  20 , sandwiching the circuit components  16 ,  18 ,  20  between the two layers  11   a ,  11   b . In selected embodiments, interconnect pads  22  may be made accessible on an underside  26  of the substrate  11   a  or through apertures in the polyimide layer  11   b.    
     To read a fingerprint, a user&#39;s finger may be swiped across a sensing side  14  of the layer  11   b  without directly touching the circuit components  16 ,  18 ,  20 . Thus, the user&#39;s finger may be electrically and mechanically isolated from the circuit components  16 ,  18 ,  20 . The sensors  16 ,  18  beneath the layer  11   b  may read the fingerprint by detecting changes in capacitance as the finger is swiped across the layer  11   b.    
     To prevent electrostatic charge from building up on the surface  11   b  and discharging to the circuit components  16 ,  18 ,  20 , an ESD conductor  24  may be provided on the substrate  11   a . In selected embodiments, the ESD conductor  24  may be printed on the substrate  11   a  with the conductive traces of the sensor components  16 ,  18 ,  20 . If electrostatic charges builds up to a point where it attempts to discharge around the edge of the layer  11   b , the ESD conductor  24  may be used to dissipate the charge to a known potential. In other contemplated embodiments, the ESD conductor  24  may be placed on the second layer  11   b.    
     Referring to  FIG. 5 , in certain embodiments, a method  30  for dissipating electrostatic charge in a fingerprint sensing circuit  10  may include initially activating  32  the fingerprint sensor. This may include, for example, activating the image sensor  16 , velocity sensor  18 , and sensor IC  20 . The fingerprint sensor  10  may then be used to sense  34  a user&#39;s fingerprint as it is swiped across the sensor  10 . As mentioned, this may cause an electrostatic charge to accumulate  36  on the sensor  10  which may then be dissipated through the ESD conductor  24 . 
     Referring to  FIG. 6 , in certain embodiments, a method  40  for producing an ESD-protected fingerprint sensing circuit  10  in accordance with the invention may include providing  42  a non-conductive substrate  11  and applying  44  a fingerprint sensing circuit to the substrate  11  using any suitable lithographic or application technique. An ESD conductor  24  may be applied  46  to the substrate  11  before, concurrently with, or after applying  44  the fingerprint sensing circuit components. The fingerprint sensing circuit  10  may then be connected  48  to a processor or other host system. 
     It should be understood that the ESD conductor  24  disclosed herein is not limited to fingerprint sensing technology, to flexible substrates, or to any single manufacturing process. An ESD conductor  24  in accordance with the invention may be applied to various non-conductive surfaces that can accommodate a conductive pattern, including but not limited to COF, ICs, flexible circuit boards, printed circuit boards, or the like. Furthermore, the invention is not limited to devices intended for human touch but may be used to protect circuitry coming into contact with any static generating body such as humans, assembly equipment, animals, or the like. Nevertheless, the invention may be particularly useful to protect sensitive circuitry associated with devices intended for human touch, including but not limited to touch pads, touch screens, touch panels, keyboards, keypads, mice, joysticks, trackballs, or the like. 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.