Arrangement for authentication of a person

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.

CROSS REFERENCE TO RELATED APPLICATION

This application claims priority from Finnish application serial number 20030102 filed Jan. 22, 2003.

TECHNICAL FIELD OF THE INVENTION

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

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.

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 inFIGS. 1A and 1B. An array of sensors120measure the skin impedance values when a finger101is 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 capacitance125to the conductive layers121,122forming the electrodes of the capacitive sensor.

FIG. 2shows a rough equivalent circuit of the skin impedance and the impedance measurement principle. Skin has a fixed resistive tissue component202, and a fixed resistive surface component203. The measurement capacitance also has a fixed component272and a component271that varies according to the surface form of the finger. The capacitive fingerprint sensor measures the varying capacitive component by applying an alternating voltage281to a drive electrode222and measuring a signal from a sensor electrode221. The signal is amplified with a low noise amplifier282, 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 buffer285.

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.

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.

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.

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.

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

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.

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.

The invention also concerns a mobile terminal, which comprises a fingerprint sensor arrangement according to the invention.

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.

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, anddirect 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, and5) Encapsulation of the structure in plastic.

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.

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'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.

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.

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.

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.

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.

Preferred embodiments of the invention are described below.

DETAILED DESCRIPTION

FIGS. 1A,1B and2were explained above in the description of prior art.

FIG. 3illustrates a cross section of an exemplary arrangement according to the invention. The arrangement comprises a substrate363, which is e.g. Kapton® film. The ASIC processing/measurement circuit380is attached on the carrier363. The unit is connected to a printed circuit board by soldering from its soldering balls377and378. The ASIC circuit is coupled electrically to the soldering balls by wire bonding361,362to metallizations375,376on the substrate. The driver electrode321and the sensing electrodes322are connected to the ASIC circuit with wires made by metallizations and vias,323and324. The electrodes are made closer to the surface of the unit by producing polymer bumps365and366to the microreplicated polymer layer367. The thickness of the bumps is e.g. 100-200 μm. On top of the unit there is encapsulation325.

FIG. 4illustrates a cross section of another exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment ofFIG. 3, but the contacts from the ASIC to the soldering balls are made with metal film connection vias461,462to the printed metallizations475,476of the substrate. This arrangement requires a thinned ASIC circuit so that the vias461,462do not need to be very deep.

FIG. 5illustrates 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 electrodes521,522and the ASIC580are on opposite sides of the substrate. The electrodes are connected to the ASIC with vias523,524, 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 flex563. The flexible substrate is preferably Kapton® film.

FIG. 6illustrates a cross section of a fourth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment ofFIG. 5, but in this arrangement the unit is soldered to other electronics. This is achieved by bending669and attaching the flex663under the unit, and further connecting soldering balls679to the flex663. The layers625and668are produced by encapsulation.

FIG. 7Aillustrates a cross section of a fifth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment ofFIG. 5, but in this arrangement the metallizations723,724between the ASIC780and the electrodes721,722are located on the flex substrate763thus avoiding one layer of microreplicated polymer and reducing the depth of the vias.FIG. 7Billustrates a top view of the arrangement shown inFIG. 7A, without the top capsulation.FIG. 7Bshows the drive electrode721, which is located on the polymer layer765. The driver electrode is connected through the via723to the ASIC. The Figure also shows the array of sensing electrodes722with guard rings729. The sensing electrodes and guard rings are wired by metallizations to array of the vias724.

FIG. 8illustrates a cross section of a sixth exemplary arrangement according to the invention. The arrangement is similar with the previous embodiment ofFIG. 7, but in this arrangement there are guard electrodes827,828under the sensing electrode metallizations. The guard electrodes and sensing electrodes are both connected to the ASIC with vias,826,824.

FIG. 9illustrates top and cross section views of exemplary sensing electrodes922and guard electrodes928on a substrate963. The guard electrodes928are located under the sensing electrodes922with an insulating layer929between the electrodes. In this embodiment the guard electrodes have larger surface. A buffer amplifier985keeps 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.

FIG. 10illustrates a further modification of an arrangement where the connection to other electronics is made by bending a flexible printed wired board (PWB) or film substrate1063to under the unit, and attaching soldering balls1078to 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 electrodes1022and to the soldering balls1078is provided using two-sided metallization1030,1034of the flex film and vias1023,1024. On the electrode end of the flex one metallized surface1030serves as sensing electrode and the second metallized surface1034of the flex serves as a guard electrode. This construction enables a two- or three-dimensional form of the electrode-finger interface.

FIG. 11illustrates another embodiment where one end of a flexible printed, wired substrate is used for electrodes1122, and other part of the flex1163for external connection. The connections between the metallized surfaces and the ASIC1180can be made similar to the embodiment ofFIG. 10. This construction also enables a two- or three-dimensional form of the electrode-finger interface. This arrangement can be directly molded into a cover1168of e.g. mobile phone.

FIG. 12illustrates a cross section of a further exemplary arrangement according to the invention. In this case the ASIC circuit1280is 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 electrodes1222and the driver electrode1221are produced on polymer bumps1266. The connections to the soldering balls1277and1278are also made using similar polymer bumps and metallizations. There is a protecting polymer layer1225on the ASIC and electrodes.

FIG. 13illustrates a cross section of a still further exemplary arrangement according to the invention. This embodiment is similar to the arrangement ofFIG. 12, but in this arrangement there is a substrate1363and bonding wires1361,1362for creating connections from the ASIC1280to the metallized pads of the substrate1363and the soldering balls1377,1378.

FIG. 14illustrates a cross section of a further exemplary arrangement according to the invention. This embodiment is similar to the arrangement ofFIG. 13, but instead of bonding wires the connections1461,1462between the ASIC1480and the substrate1463are made using metallization.

FIG. 15illustrates a cross section of a further exemplary arrangement according to the invention. This embodiment is similar to the arrangement ofFIG. 12, but this embodiment includes a polymer layer1525with preferably 100-150 μm deep grooves1565. The soldering balls1577,1578are first attached to holes in the polymer, and metallizations1521,1522that form the electrodes are made on the grooves in the opposite side of the polymer. The polymer substrate1525is then attached on the ASIC,1580.

FIGS. 16A and 16Billustrate an exemplary process for manufacturing a unit ofFIG. 4according 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 phase160an 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 step161a polymer layer is cast on top of the attached ASIC. In step162vias 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 step163. The polymer layer is injection molded using micro replicated mold, step164.

Next illustrated inFIG. 16B, vias are opened through the polymer layer to the ASIC pads in step165. This may also be made by cavity molding during the previous step. In step166the electroplated metallization is patterned to form the electrode structure. A protective polymer layer is cast on top of the device in step167. The solder areas of the substrate are then opened, step168, and finally the solder bumps are processed and diced in step169.

FIGS. 17A and 17Billustrate another exemplary process, which is for manufacturing a unit ofFIG. 12according to the invention. First in phase170two 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 phase171. The metallization can be made using e.g. Cr—Au. In step172a photoresist is electroplated on top of the metallization. The photoresist is then patterned, step173, and the metal layers are etched in step174.

Next illustrated inFIG. 17Bthe photoresist is removed in step175. A protective polymer layer is cast in step176, and contact areas are opened for the flip-chip process, step177. The soldering balls are then attached with flip-chip bump process in step178, and finally the produced unit is attached to a cover, step179.

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.