Abstract:
A capacitive fingerprint sensor is fabricated on a plastic substrate ( 363 ) with an embedded integrated circuit chip ( 380 ). The invention describes a way to create two or three dimensional forms for electrode structures ( 321, 322, 325, 365, 366 ) that can be used to optimize the performance of the sensor. When the three dimensional structure is designed to follow the shape of a finger, a very small pressure is required when sliding the finger along the sensor surface. This way the use of the sensor is ergonomic and the measurement is made very reliable. The inventive fabrication method describes the way, how to connect and embed an integrated circuit containing measurement electronics with a batch processed larger scale electrode configuration that is used for capturing the capacitive image of the fingerprint.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority from Finnish application serial number 20030102 filed Jan. 22, 2003.  
       TECHNICAL FIELD OF THE INVENTION  
       [0002]     The invention relates to an arrangement for authentication of a person, for example for authentication of a user of a mobile terminal. In particular the invention relates to a fingerprint sensor arrangement. The invention also relates to a manufacturing method of the inventive fingerprint sensor.  
       BACKGROUND ART OF THE INVENTION  
       [0003]     There is a need of providing sensors in mobile terminals in order to make the mobile terminal capable of sensing its ambient conditions. The information can be used for context awareness applications where the ambient information is used for controlling the user interface profile and different settings of the mobile terminal user interface. Fingerprint sensors are also needed for authenticating the user of the terminal.  
         [0004]     There exist several kinds of fingerprint sensors: skin impedance based sensor, thermal sensors, and optical sensors. The most practical solution for authentication of a user of small appliances, such as mobile terminals, is based on capacitive impedance measurement. The basic idea of the capacitive fingerprint sensor to measure the change of skin impedance is described in  FIGS. 1A and 1B . An array of sensors  120  measure the skin impedance values when a finger  101  is gradually pulled over the array of sensors. The capacitance between the electrode surface and the conductive saline layer inside the skin surface varies as a function of distance to the conductive layer. The varying air gap and the dead horny cells in the surface of the skin form the capacitance  125  to the conductive layers  121 ,  122  forming the electrodes of the capacitive sensor.  
         [0005]      FIG. 2  shows a rough equivalent circuit of the skin impedance and the impedance measurement principle. Skin has a fixed resistive tissue component  202 , and a fixed resistive surface component  203 . The measurement capacitance also has a fixed component  272  and a component  271  that varies according to the surface form of the finger. The capacitive fingerprint sensor measures the varying capacitive component by applying an alternating voltage  281  to a drive electrode  222  and measuring a signal from a sensor electrode  221 . The signal is amplified with a low noise amplifier  282 , and the phase difference between driver and sensing electrodes is measured,  283 . Interference can be suppressed with a guard electrode, which is kept in the same potential as the signal input using a buffer  285 .  
         [0006]     A fingerprint sensor also requires a signal processing circuit, which is preferably a silicon-based integrated circuit. One solution for providing a fingerprint sensor would be to use an integrated circuit, which would serve both as capacitive measurement electrodes and as signal processing electronics. This integrated circuit would then be mounted on the surface of the appliance. However, the area needed for the capturing the capacitive image of the fingerprint is roughly in the scale of one square centimeter. This is a very large area for using a silicon integrated circuit as measurement electrodes. Furthermore, the measurement consists of hundreds of capacitive pixels that are arranged in a row or in a matrix depending on the measurement principle. A lot of wiring is needed and the measurement electrodes need to be isolated from the integrated circuits. Therefore a cost efficient method for connecting the capacitive electrodes to the signal processing silicon integrated circuit is needed.  
         [0007]     One typical prior art solution is described in patent documents U.S. Pat. No. 5,887,343 and U.S. Pat. No. 6,067,368. The problem is solved by using a separate insulating planar substrate to form the measurement electrode. This substrate contains the interconnecting wiring and the vias through the substrate. The substrate is connected to the silicon integrated circuit containing the signal and data processing capabilities. However, this solution is complicated to manufacture because a large number of interconnecting wiring must be connected within a small space. Such wiring also is not very robust, which tends to make the structure easy to break in mobile use.  
         [0008]     Another prior art solution is to create the measurement electrodes directly on top of the silicon wafer. This leads to a simple configuration of interconnecting wiring but the solution requires a large silicon surface due to the large area needed for the electrodes.  
         [0009]     The prior art solutions also have a disadvantage that relates to security. It is possible to make external connections to the wiring between the capacitive measurement electrodes and the integrated circuit, and by using such a connection it is possible to “record” signals that relate to a certain finger when the finger is measured. It is then later possible to input these recorded signals to the integrated circuit and thus a positive authentication result can be achieved electrically without any finger.  
         [0010]     A further disadvantage with the prior art solutions relates to the ergonomics of the sensor. A finger must be pressed rather heavily against the flat sensor in order to achieve sufficient contact area between the sensor and the finger. Therefore the measurement may often fail when the finger is not pressed and slid properly along the sensor surface.  
       SUMMARY OF THE INVENTION  
       [0011]     The purpose of the invention is to provide a capacitive fingerprint sensor with improvements related to the aforementioned disadvantages. The invented arrangement for fingerprint measurement facilitates good suitability to serial production, good security properties and ergonomics. Hence, the invention presents a substantial improvement to the cost efficiency and reliability of the fingerprint sensors, especially in mobile applications.  
         [0012]     A fingerprint sensor arrangement according to the present invention, comprising at least one driver electrode and at least one sensor electrode for a capacitive measurement, and an integrated signal processing circuit for the measurement of signals from the electrodes, and interconnecting wiring between the electrodes and the integrated circuit, is characterized in that the at least one driver electrode, the at least one sensor electrode, said signal integrated circuit and said interconnecting wiring are embedded within an integrated module.  
         [0013]     The invention also concerns a mobile terminal, which comprises a fingerprint sensor arrangement according to the invention.  
         [0014]     A method according to the present invention for producing a fingerprint sensor, is characterized in that the method comprises the following steps: 
        providing a signal processing integrated circuit,     providing at least one driver electrode,     providing at least one sensor electrode,     encapsulating said integrated circuit, said at least one driver electrode and said at least one sensing electrode into an integrated module.        
 
         [0019]     One essential idea in implementing this invention is to fabricate a capacitive fingerprint sensor into a plastic substrate with an embedded integrated circuit chip. The inventive fabrication method describes how to connect and embed an integrated circuit containing measurement electronics, to a batch processed larger scale electrode configuration that is used for capturing the capacitive image of the finger print. The inventive concept can most advantageously be realized using one or several of the following technical details: 
    1) Attachment of the silicon integrated circuit on a carrier substrate with interconnecting wiring;     2) Different methods for connecting the integrated circuit electrically to the carrier substrate: 
        wire bonding,     via connection using electroplated, electroless, or thin film metallization, and     direct electrical contact from the integrated circuit to the carrier substrate (laser holes, etc.);    
        3) Molding of a two or three dimensional plastic structure on top of the carrier and the IC to form a substrate for the measurement electrode;     4) Deposition of the electrode metallization on top of the three dimensional structure with preferably 5-10 μm resolution, and     5) Encapsulation of the structure in plastic.    
 
         [0028]     Alternatively, if the integrated circuit has a large surface, it is also possible to use an embodiment in which the electrodes and insulating/protective polymer layers are deposited directly on the integrated circuit.  
         [0029]     It is also possible to integrate other types of sensors to the fingerprint sensor unit. For example, in one embodiment of the invention a light emitting diode and a light sensitive detector are placed on the opposite sides of the finger groove in order to measure light absorption through the finger. The wavelength of the light used is such that blood in a live finger causes maximal absorption signal. This way oxidized hemoglobin can be detected from the user&#39;s finger. Thus by this method also the heartbeat rate can be monitored. This makes the usage of any artificial fingers for identification falsification very difficult. In addition, other sensors such as temperature TS and light LS sensors can be integrated within the finger groove and embedded into the fingerprint sensor module.  
         [0030]     The present invention offers important advantages over the prior art solutions. The fabrication process is very simplified, and the invention can be applied to existing fingerprint measurement concepts and electronics to make the fabrication of the device more cost efficient.  
         [0031]     Due to the embedded structure the sensor structure is very secure. It is practically not possible to make any external connections to the wiring between the sensor electrodes and the signal processing circuits. Therefore there is a minimal risk of recording signals from actual finger measurements and using them fraudulently.  
         [0032]     The invention also describes a way to create two- or three-dimensional electrode surface structures that can be used to optimize the performance of the sensor. When the at least two-dimensionally formed structure is designed to follow the shape of a finger, a very small pressure is required when sliding the finger along the sensor surface. This way the use of the sensor is ergonomic and the measurement is made very reliable.  
         [0033]     An at least two-dimensionally formed structure of the sensor surface is preferably achieved by integrating the sensor electrodes and the measurement electronics such as ASICs into a three-dimensional module using chip-on-flex technology. The chip-on-flex technology is based e.g. on the use of flexible Kapton® film as the substrate for wiring and attachment of sensor and ASIC chips. The integrated circuits and sensors are protected using molded polymer cover on top. When using the flexible circuit board for the creation of 2D or 3D structures the sensors and electronics can be a part of the device case.  
         [0034]     Preferred embodiments of the invention are described below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]     Next the invention will be described in greater detail with reference to exemplary embodiments in accordance with the accompanying drawings, in which  
         [0036]      FIG. 1A  illustrates using a capacitive fingerprint sensor,  
         [0037]      FIG. 1B  illustrates the operating principle of a prior art capacitive fingerprint sensor,  
         [0038]      FIG. 2  illustrates a block diagram describing the measurement of skin impedance using active guarding,  
         [0039]      FIG. 3  illustrates a cross section of an exemplary arrangement according to the invention, in which wire bonding is applied between a substrate and an ASIC,  
         [0040]      FIG. 4  illustrates a cross section of an exemplary arrangement according to the invention, in which metallization connection is applied between a substrate and an ASIC,  
         [0041]      FIG. 5  illustrates a cross section of an exemplary arrangement according to the invention, in which an ASIC and electrodes are located on opposite sides of a substrate,  
         [0042]      FIG. 6  illustrates a cross section of an exemplary arrangement according to the invention, in which a flexible substrate is bent to serve as a surface for electrical connections of the unit,  
         [0043]      FIG. 7A  illustrates a cross section of an exemplary arrangement according to the invention, in which metallizations between the ASIC and the electrodes are located on the substrate,  
         [0044]      FIG. 7B  illustrates a top view of an exemplary arrangement according to the invention, in which metallizations between the ASIC and the electrodes are located on the substrate and in which guard rings are used,  
         [0045]      FIG. 8  illustrates a cross section of an exemplary arrangement according to the invention, in which different metallized layers are used for electrodes and guard rings  
         [0046]      FIG. 9  illustrates further cross sections and a top view of an exemplary arrangement according to the invention, in which different metallized layers are used for electrodes and guard rings  
         [0047]      FIG. 10  illustrates a cross section of an exemplary arrangement according to the invention, in which a flexible substrate is bent to serve as electrodes and a surface for electrical connections of the unit,  
         [0048]      FIG. 11  illustrates cross section of a further exemplary arrangement according to the invention, in which a flexible substrate is applied to serve as a surface for electrical connections of the unit,  
         [0049]      FIG. 12  illustrates a cross section of an exemplary arrangement according to the invention, in which electrodes and external connections are provided directly on an ASIC,  
         [0050]      FIG. 13  illustrates a cross section of an exemplary arrangement according to the invention, in which electrodes are provided directly on an ASIC, and wire bonding is used between the ASIC and a substrate,  
         [0051]      FIG. 14  illustrates a cross section of an exemplary arrangement according to the invention, in which electrodes are provided directly on an ASIC, and metallized connections are used between the ASIC and a substrate,  
         [0052]      FIG. 15  illustrates a cross section of an exemplary arrangement according to the invention, in which electrodes and external connections are provided directly on an ASIC using metallization of grooves in a polymer,  
         [0053]      FIG. 16A  illustrates cross sections of a production sample after phases  160 - 164  in an exemplary process to produce a unit according to the invention, wherein small ASIC is used,  
         [0054]      FIG. 16B  illustrates cross sections of a production sample after phases  165 - 169  in an exemplary process to produce a unit according to the invention,  
         [0055]      FIG. 17A  illustrates cross sections of a production sample after phases  170 - 174  in an exemplary process to produce a unit according to the invention, wherein a large ASIC is used, and  
         [0056]      FIG. 17B  illustrates cross sections of a production sample after phases  175 - 179  in an exemplary process to produce a unit according to the invention. 
     
    
     DETAILED DESCRIPTION  
       [0057]      FIGS. 1A, 1B  and  2  were explained above in the description of prior art.  
         [0058]      FIG. 3  illustrates a cross section of an exemplary arrangement according to the invention. The arrangement comprises a substrate  363 , which is e.g. Kapton® film. The ASIC processing/measurement circuit  380  is attached on the carrier  363 . The unit is connected to a printed circuit board by soldering from its soldering balls  377  and  378 . The ASIC circuit is coupled electrically to the soldering balls by wire bonding  361 ,  362  to metallizations  375 ,  376  on the substrate. The driver electrode  321  and the sensing electrodes  322  are connected to the ASIC circuit with wires made by metallizations and vias,  323  and  324 . The electrodes are made closer to the surface of the unit by producing polymer bumps  365  and  366  to the microreplicated polymer layer  367 . The thickness of the bumps is e.g. 100-200 μm. On top of the unit there is encapsulation  325 .  
         [0059]      FIG. 4  illustrates a cross section of another exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment of  FIG. 3 , but the contacts from the ASIC to the soldering balls are made with metal film connection vias  461 ,  462  to the printed metallizations  475 ,  476  of the substrate. This arrangement requires a thinned ASIC circuit so that the vias  461 ,  462  do not need to be very deep.  
         [0060]      FIG. 5  illustrates a cross section of a third exemplary arrangement according to the invention. The arrangement is similar with the previous embodiments, but in this arrangement the ASIC is electrically coupled directly to a flexible substrate or “flex”  563 . The electrodes  521 ,  522  and the ASIC  580  are on opposite sides of the substrate. The electrodes are connected to the ASIC with vias  523 ,  524 , which extend to the surface of the ASIC through holes on the substrate. The unit is preferably connected to other electronics with a connector at the end of the flex  563 . The flexible substrate is preferably Kapton® film.  
         [0061]      FIG. 6  illustrates a cross section of a fourth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment of  FIG. 5 , but in this arrangement the unit is soldered to other electronics. This is achieved by bending  669  and attaching the flex  663  under the unit, and further connecting soldering balls  679  to the flex  663 . The layers  625  and  668  are produced by encapsulation.  
         [0062]      FIG. 7A  illustrates a cross section of a fifth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment of  FIG. 5 , but in this arrangement the metallizations  723 ,  724  between the ASIC  780  and the electrodes  721 ,  722  are located on the flex substrate  763  thus avoiding one layer of microreplicated polymer and reducing the depth of the vias.  FIG. 7B  illustrates a top view of the arrangement shown in  FIG. 7A , without the top capsulation.  FIG. 7B  shows the drive electrode  721 , which is located on the polymer layer  765 . The driver electrode is connected through the via  723  to the ASIC. The Figure also shows the array of sensing electrodes  722  with guard rings  729 . The sensing electrodes and guard rings are wired by metallizations to array of the vias  724 .  
         [0063]      FIG. 8  illustrates a cross section of a sixth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment of  FIG. 7 , but in this arrangement there are guard electrodes  827 ,  828  under the sensing electrode metallizations. The guard electrodes and sensing electrodes are both connected to the ASIC with vias,  826 ,  824 .  
         [0064]      FIG. 9  illustrates top and cross section views of exemplary sensing electrodes  922  and guard electrodes  928  on a substrate  963 . The guard electrodes  928  are located under the sensing electrodes  922  with an insulating layer  929  between the electrodes. In this embodiment the guard electrodes have larger surface. A buffer amplifier  985  keeps the guard electrodes in the same potential as the sensor electrodes and thus the sensor electrodes are less loaded by the adjacent materials, or interference.  
         [0065]      FIG. 10  illustrates a further modification of an arrangement where the connection to other electronics is made by bending a flexible printed wired board (PWB) or film substrate  1063  to under the unit, and attaching soldering balls  1078  to the flex. In this embodiment the other end of the flex is bent above the unit in order to use the other end of the flex as electrodes. The wiring to the electrodes  1022  and to the soldering balls  1078  is provided using two-sided metallization  1030 ,  1034  of the flex film and vias  1023 ,  1024 . On the electrode end of the flex one metallized surface  1030  serves as sensing electrode and the second metallized surface  1034  of the flex serves as a guard electrode. This construction enables a two- or three-dimensional form of the electrode-finger interface.  
         [0066]      FIG. 11  illustrates another embodiment where one end of a flexible printed, wired substrate is used for electrodes  1122 , and other part of the flex  1163  for external connection. The connections between the metallized surfaces and the ASIC  1180  can be made similar to the embodiment of  FIG. 10 . This construction also enables a two- or three-dimensional form of the electrode-finger interface. This arrangement can be directly molded into a cover  1168  of e.g. mobile phone.  
         [0067]      FIG. 12  illustrates a cross section of a further exemplary arrangement according to the invention. In this case the ASIC circuit  1280  is large with respect to the needed electrode structure, and therefore it is possible to create a two- or three-dimensional form of the electrode structure directly on the ASIC. The sensing electrodes  1222  and the driver electrode  1221  are produced on polymer bumps  1266 . The connections to the soldering balls  1277  and  1278  are also made using similar polymer bumps and metallizations. There is a protecting polymer layer  1225  on the ASIC and electrodes.  
         [0068]      FIG. 13  illustrates a cross section of a still further exemplary arrangement according to the invention. This embodiment is similar to the arrangement of  FIG. 12 , but in this arrangement there is a substrate  1363  and bonding wires  1361 ,  1362  for creating connections from the ASIC  1280  to the metallized pads of the substrate  1363  and the soldering balls  1377 ,  1378 .  
         [0069]      FIG. 14  illustrates a cross section of a further exemplary arrangement according to the invention. This embodiment is similar to the arrangement of  FIG. 13 , but instead of bonding wires the connections  1461 ,  1462  between the ASIC  1480  and the substrate  1463  are made using metallization.  
         [0070]      FIG. 15  illustrates a cross section of a further exemplary arrangement according to the invention. This embodiment is similar to the arrangement of  FIG. 12 , but this embodiment includes a polymer layer  1525  with preferably 100-150 μm deep grooves  1565 . The soldering balls  1577 ,  1578  are first attached to holes in the polymer, and metallizations  1521 ,  1522  that form the electrodes are made on the grooves in the opposite side of the polymer. The polymer substrate  1525  is then attached on the ASIC,  1580 .  
         [0071]      FIGS. 16A and 16B  illustrate an exemplary process for manufacturing a unit of  FIG. 4  according to the invention. The Figures show a cross section of the unit to be manufactured after the concerned manufacturing phase has been executed. First in phase  160  an ASIC circuit is glued on a flexible substrate that includes wiring. The ASIC is preferably a thinned type component with height of only 50-100 μm. The flex sheet substrate may be large for attachment of several components. In step  161  a polymer layer is cast on top of the attached ASIC. In step  162  vias are opened through the polymer layer until the wiring of the flex substrate and to the ASIC pads. The metallization is then electroplated and patterned in step  163 . The polymer layer is injection molded using micro replicated mold, step  164 .  
         [0072]     Next illustrated in  FIG. 16B , vias are opened through the polymer layer to the ASIC pads in step  165 . This may also be made by cavity molding during the previous step. In step  166  the electroplated metallization is patterned to form the electrode structure. A protective polymer layer is cast on top of the device in step  167 . The solder areas of the substrate are then opened, step  168 , and finally the solder bumps are processed and diced in step  169 .  
         [0073]      FIGS. 17A and 17B  illustrate another exemplary process, which is for manufacturing a unit of  FIG. 12  according to the invention. First in phase  170  two or three dimensionally formed structures are fabricated on top of the ASIC surface with roughly 100-200 μm of height. The thin film metallization is deposited on top of the ASIC and the 3D polymer structures in phase  171 . The metallization can be made using e.g. Cr—Au. In step  172  a photoresist is electroplated on top of the metallization. The photoresist is then patterned, step  173 , and the metal layers are etched in step  174 .  
         [0074]     Next illustrated in  FIG. 17B  the photoresist is removed in step  175 . A protective polymer layer is cast in step  176 , and contact areas are opened for the flip-chip process, step  177 . The soldering balls are then attached with flip-chip bump process in step  178 , and finally the produced unit is attached to a cover, step  179 .  
         [0075]     The invention has been explained above with reference to the aforementioned embodiments, and several industrial advantages of the invention have been demonstrated. It is clear that the invention is not only restricted to these embodiments, but comprises all possible embodiments within the spirit and scope of the inventive thought and the following patent claims. For example, the inventive idea of the authentication arrangement is not restricted to be used in mobile terminals, but it can be applied also in many other components and purposes. The invention is not either restricted to use of the mentioned materials.