THE BIOMETRIC IMAGING DEVICE

Field of the Invention

The present invention relates to a biometric imaging device and to a method of manufacturing the biometric imaging device. In particular, the invention relates to a fingerprint sensor module comprising a thin-film- transistor (TFT) fingerprint sensor.

Background of the Invention

As the development of biometric imaging devices for identity verification, and in particular of fingerprint sensing devices, has led to devices which are made smaller, cheaper and more energy efficient, the range of applications for such devices is increasing.

In particular, capacitive fingerprint sensing devices have been adopted more and more in for example consumer electronic devices due to small form factor, relatively beneficial cost/performance factor and high user acceptance. There is also an increasing interest in using fingerprint sensors in smartcards to enable biometric identification in a card such as a bank card where other types of biometric systems are not applicable.

Capacitive fingerprint sensors can be manufactured in large numbers and at low cost using a thin film transistor (TFT) process. However, current TFT-circuits are often very thin, having a thickness of about 5 pm, and a total thickness of about 30 pm including a carrier substrate, which requires careful handling of the TFT-circuits during manufacturing of a fingerprint sensing module to prevent wrapping, cracking or other damages.

Previous solutions have included arranging the TFT-circuit on a thick substrate such as a flexible substrate or a glass substrate. However, such additional substrates increase the overall thickness of the sensing module, while in fingerprint sensing modules suitable for smartcard integration it is desirable to minimize the thickness. Moreover, in applications where fingerprint sensing takes place through the substrate, it is even more important to minimize the thickness of the substrate on which the TFT-circuit is arranged.

Accordingly, it is desirable to provide improved manufacturing methods for fingerprint sensing modules comprising a capacitive fingerprint sensing device based on a TFT-process.

Summary

In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved method for manufacturing a biometric imaging module based on thin-film transistor technology.

According to a first aspect of the invention, it is provided a method for manufacturing a biometric imaging device. The method comprises: providing a glass carrier; arranging a plastic film on the glass carrier; forming a thin-film transistor circuit, TFT-circuit, on the plastic film, the TFT-circuit being configured to form a capacitive fingerprint sensing device having a sensing side facing towards the plastic film, the fingerprint sensing device comprising a plurality of electrically conductive contact pads for connecting the fingerprint sensing device to external circuitry, the contact pads facing away from the plastic film; arranging at least one component on the TFT-circuit; arranging a cover layer covering the TFT-circuit; and removing the glass carrier.

In the present context, the glass substrate can be made from any type of glass suitable for use in microelectronic manufacturing and assembly, such as quartz or silica glass.

Moreover, the plastic film can be a polyimide film which will be acting as an outer surface of the completed biometric imaging device. Polyimide is a preferable material since it can withstand higher temperatures compared to many other plastic materials which may be required during subsequent steps of the manufacturing process. Furthermore, a film is in the present context a thin layer of material or materials which is flexible, and which can be provided as sheets or on a roll or as a liquid deposited on a surface where the liquid is subsequently hardened to form a film. Moreover, a film may also comprise a plurality of different layers.

The present invention is based on the realization that it is preferable to provide a biometric imaging device comprising a capacitive fingerprint sensing device based on TFT-technology where a finger does not have to be sensed through connection layers of the TFT-circuit. The claimed method provides biometric imaging device where only a plastic film separates the outer surface of the biometric imaging device and an active portion of the capacitive fingerprint sensing device, thereby minimizing the distance between the active portion and the finger which in turn improves the performance of the biometric imaging device.

According to one embodiment of the invention, arranging a cover layer on the TFT-circuit comprises providing a supporting film having openings at locations corresponding to the contact pads and to locations of the at least one component; and laminating the supporting film to the TFT-circuit. The supporting film helps to increase the stiffness of the biometric imaging device which makes it less sensitive to breaking during handling and integration in a final device. The supporting film has pre-formed openings which correspond to contact pads and components already placed on the TFT-circuit so that the contact pads and components are accessible after lamination of the supporting film. The supporting film can for example be made from epoxy glass, or from polymer materials such as PVC, PET or PLA where the choice of supporting film is preferably based on the intended implementation of the biometric imaging device.

According to one embodiment of the invention a thickness of the supporting film is lower than a height of the at least one component. By selecting an appropriate material for the supporting film, sufficient stiffness of the biometric imaging device can be achieved without increasing the thickness of the device beyond the height of the one or more components arranged on the TFT-circuit. Moreover, the difference in thickness between the cover layer and the component can be used to form a biometric imaging device having a T-shaped cross section which can be advantageous in some applications.

According to one embodiment of the invention, arranging a cover layer on the TFT-circuit comprises arranging an encapsulation layer on the TFT- circuit. The encapsulation can be referred to as a mold layer which is deposited on the surface of the TFT-circuit and the encapsulant can be arranged to fully or partially cover components arranged on the TFT-circuit. Accordingly, it is not required that the components are fully covered by the encapsulation layer. In other words, the thickness of the encapsulation layer can be lower than a height of components arranged on the TFT-circuit as long as sufficient overall stiffness of the biometric imaging device is achieved.

According to one embodiment of the invention, the method comprises forming openings in the encapsulation layer at the locations of the contact pads to expose the contact pads, thereby enabling connection of the biometric imaging device to external circuitry.

According to one embodiment of the invention, the method further comprises removing a portion of the encapsulation layer on at least two opposing sides of the biometric imaging module such that the biometric imaging module has a T-shaped cross-section. In embodiments where the encapsulation layer has a thickness which is equal to or higher than a height of the one or more components, a T-shape cross section profile of the biometric imaging device can be achieved by removing selected portions of the encapsulation layer.

According to one embodiment of the invention, the method further comprises arranging a connecting element on the connection pad. The connecting element can be a solder ball, a conductive plate, a conductive pillar, a PCB plate, a via connection or a similar conductive structure. The connecting element can be arranged on the contact pad either before or after arranging the cover layer on the TFT-circuit, depending on both the type of cover layer and the type of connecting element used. The connecting element is primarily intended to reduce the height difference between the contact pad and the top surface of the cover layer in order to facilitate an electric connection between the biometric imaging device and external circuitry. The connecting element may also be configured to protrude above the cover layer.

According to one embodiment of the invention, the method further comprises arranging the biometric imaging device in an opening of a smartcard and forming an electrical connection between a conductive layer of the smartcard and the contact pads of the biometric imaging device.

A smartcard can be considered to be any card comprising functionality such as biometric sensing, and smartcards may be used as payment cards, identification cards, access cards and in other applications where a card with built-in functionality is desirable. In the present context, the smartcard comprises a biometric imaging device.

According to a second aspect of the invention, it is provided a biometric imaging device comprising: a plastic film forming an outer surface of the biometric imaging device; a thin-film transistor, TFT-circuit comprising a capacitive fingerprint sensing device having a sensing side on a first side of the TFT-circuit facing towards the plastic film, the fingerprint sensing device comprising electrically conductive contact pads on a second side of the TFT- circuit facing away from the plastic film; at least one component arranged on the second side of the TFT-circuit; and a cover layer arranged to cover the second side of the TFT-circuit, the cover layer comprising openings at the locations of the contact pads.

There is also provided a smartcard comprising a card body comprising a biometric imaging device according to any one of the preceding embodiments arranged in the card body; and a conductive layer electrically connected to the contact pads of the biometric imaging device.

Effects and features of the second aspect of the invention are largely analogous to those described above in connection with the first aspect of the invention.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing an example embodiment of the invention, wherein:

Figs. 1 A-J schematically illustrate steps of a method of manufacturing a biometric imaging device according to an embodiment of the invention;

Fig. 2 is a flow chart describing steps of a method of manufacturing a biometric imaging device according to an embodiment of the invention;

Fig. 3 schematically illustrates a biometric imaging device according to an embodiment of the invention;

Figs. 4A-B schematically illustrate a biometric imaging device according to embodiments of the invention;

Fig. 5 schematically illustrates a biometric imaging device according to an embodiment of the invention;

Fig. 6 schematically illustrates a biometric imaging device according to an embodiment of the invention;

Fig. 7 schematically illustrates a biometric imaging device according to an embodiment of the invention;

Fig. 8 schematically illustrates details of capacitive fingerprint sensing device in a biometric imaging device according to an embodiment of the invention; and

Figs. 9A-B schematically illustrates a smartcard comprising a biometric imaging device according to an embodiment of the invention.

Detailed Description of Embodiments

In the present detailed description, various embodiments of a biometric imaging device and a method of manufacturing the biometric imaging device according to the present invention are mainly discussed with reference to a biometric imaging device comprising a capacitive fingerprint sensing device.

Figs. 1A-1 J schematically illustrate steps of a method for manufacturing a biometric imaging device according to an embodiment of the invention and Fig. 2 is a flow chart outlining general steps of the method. The steps of Fig. 2 will be described with reference to the Figs. 1A-1 J.

First a glass carrier 100 is provided 200 followed by arranging 202 a plastic film 102 on the glass carrier 100 illustrated in Figs. 1 A and 1 B, respectively. The plastic film 102 is preferably a polyimide film 102 which is deposited as a liquid coating on the glass substrate 100 followed by heat treatment to solidify the polyimide film 102.

Next, a thin-film transistor, TFT, circuit 104 is formed 204 on the plastic film 102 as illustrated in Fig. 1 C. The TFT-circuit 104 is configured to form a capacitive fingerprint sensing device 106 having a sensing side 108 on a first side 108 of the TFT-circuit 104 facing towards the plastic film 102. Here, conventional TFT manufacturing processes are used and the steps of manufacturing a TFT-based capacitive fingerprint sensing device 106 are generally known and will not be described in further detail herein. That the sensing side 108 of the fingerprint sensing device 106 is arranged to face the plastic film 102 means that connection layers of the TFT-circuit are arranged above an active sensing portion of the sensing elements in the TFT-circuit 104.

The capacitive fingerprint sensing device 106 comprises an array of electrically conductive sensing elements connected to thin-film transistors. A capacitive fingerprint sensing device 106 should be understood to further comprise sensing circuitry connected to the sensing elements for reading a signal from the sensing elements. The sensing circuitry may in turn comprise internal readout circuitry or be connected to external readout circuitry for providing a result of the sensing elements to an external device for further processing, which in the present case may be included in the biometric imaging device. The capacitive fingerprint sensing device 106 further comprises a plurality of electrically conductive contact pads 110 for connecting the fingerprint sensing device 106 to external circuitry. The contact pads 110 are arranged on a second side 111 of the TFT-circuit 104 facing away from the plastic film 102 and are exposed on an upper surface of the TFT-circuit 104 as illustrated in Fig. 1 D. A base metal layer of the contact pads 110 can be formed together with a connection layer which is a part of the TFT-circuit 104. Depending on the application, further surface metallization of the contact pads 110 such as electro-plating of Ni and Au may be required to prevent oxidation and to facilitate interconnection.

The method further comprises arranging 206 at least one component 112 on the TFT-circuit 104 illustrated in Fig. 1 E. The component 112 arranged on the TFT-circuit can be a surface mounted discrete component 112 such as a decoupling capacitor or the component may be an integrated circuit such as a readout circuit for the fingerprint sensing device, a secure element (SE) or micro control unit (MCU). Direct soldering of extra components onto the TFT-circuit provides a short connection length and good electrical performance. Moreover, the integration of components and circuits in the biometric imaging device limits the total number of separate devices that need to be integrated together with the biometric imaging device, which is advantageous for example in a smartcard implementation where manufacturing and assembly can be simplified by reducing the number of separate components. The described biometric imaging device could of course also be used without additional components arranged on the TFT- circuit.

In the next step, a cover layer 114 is arranged 208 to cover the TFT- circuit 104. Fig. 1 F illustrates a top view of a cover layer 114 in the form of a supporting film 114 having openings at locations 116 corresponding to the contact pads 110 and at locations 118 of the at least one component 112. The cross section views of the biometric imaging device during different stages of the manufacturing process are taken along the dashed line 119, i.e. through a central portion of the device. The supporting film 114 can be laminated to the TFT-circuit 104 and Fig. 1 G illustrates a cross section view of the biometric imaging device 120 with the support film attached to the TFT- circuit 104. Moreover, depending on the application, the supporting film 114 may comprise epoxy glass (epoxy resin), or polymer materials such as PVC (polyvinyl chloride), PET (Polyethylene terephthalate) or PLA (polylactide).

For applications where the biometric imaging device is integrated in a smartcard or the like it is preferable to use a supporting film material which has a similar thermal expansion coefficient as the material of the smartcard body in order to reduce the risk of warping of the smartcard and/or of the imaging device.

The final step illustrated in Fig. 1 H comprises removing 210 the glass carrier 100 which can be done by means of a laser lift-off debonding process where a UV-wavelength laser illuminates the interface between the plastic film 102 and the glass carrier 100 through the glass carrier 100 to release the adhesion between the polyimide film 102 and the glass carrier 100.

Fig. 11 illustrates the completed biometric imaging device 120 and Fig. 1 J is a top view of the same device 120 illustrating the exposed contact pads 110 and centrally arranged components 112 which may comprise both one or more integrated circuits and one or more discrete components. The illustrated side in Fig. 1 J is thus the side opposite the sensing side of the biometric imaging device 120.

Fig. 3 schematically illustrates an embodiment of the biometric imaging device 300 where a thickness of the supporting film 114 is lower than a height of the components 112. The biometric imaging device 300 will then have a T- shaped cross section profile which can be advantageous in some implementations. Moreover, by lowering the distance between the top of the supporting film and the contact pad 110 it may be easier to form an electrical connection between external circuitry and the contact pad 110.

Figs. 4A-B schematically illustrate an embodiment where the cover layer is provided as an encapsulation layer 402 deposited on the TFT-circuit 104, as seen in Fig 4A. The encapsulation layer 402 can be made to be lower than the height of the contact pads 110 such that the contact pads are exposed. In applications where the encapsulation layer 402 is thicker than the height of the contact pads 110 as illustrated in Fig. 4A, the method may also comprise forming openings 404 in the encapsulation layer at the locations of the contact pads 110 to thereby expose the contact pads 100, illustrated in Fig. 4B. The contact pads 110 may also be accessed by forming via connections through the encapsulation layer 402. The encapsulation layer can for example be commonly used epoxy mold compounds or other liquid filing materials. Laser ablation can be used to form openings in the encapsulation layer.

Fig. 5 schematically illustrates an embodiment where the encapsulation layer 402 comprises first and second area portions 502, 504 at the edge of the biometric imaging device 500 and at two opposing sides of the biometric imaging device 500 where the encapsulation layer 402 has a thickness which is lower than a thickness of a third area portion 506 located between the first and second area portion 502, 504 so that the biometric imaging device 500 has a T-shaped cross-section profile. The third area portion 506 is then a central area portion of the biometric imaging device 500.

Fig. 6 schematically illustrates a biometric imaging device 600 comprising a connecting element 602 arranged on the connection pad 110. The connecting element 602 is an electrically conductive element such as a solder ball or the like which is used to simplify the formation of an electrical connection between external circuitry and the biometric imaging device 600. The properties of the connecting element 602 can be configured and selected depending on the application and on the device in which the biometric imaging device 600 is to be integrated. The connecting element 602 may for example protrude above the cover layer or it may be lower than the cover layer. Similarly, the connecting element 602 may protrude above the components 112 or it may be lower than the components 112 arranged on the biometric imaging device. In Fig. 6, the connecting element 602 is illustrated in an embodiment where the cover layer is a supporting film 114. However, connection elements 602 may equally well be used in embodiments where the connection elements 602 are located in openings of an encapsulation layer.

Fig. 7 schematically illustrates a biometric imaging device 700 where connecting elements 602 have been embedded in an encapsulation layer 402 and where the height of the encapsulation layer 402 has been reduced at two opposing area portions 502, 504 of the biometric imaging device 700 to expose the connecting elements 602 and to form a T-shaped cross section. Thereby, also the height of the connecting elements 602 is reduced.

Different types of connecting elements can in principle be used together with both types of described cover layers, i.e. with a supporting film or with an encapsulant and the choice of connecting element can be based on what is most suitable for a specific implementation.

Fig. 8 is a schematic view of different layers of the TFT-circuit 104 which is arranged on the plastic film 102. The lower layer 802 arranged closest to the polyimide film 102 is here described as an active layer 802 where the transistors are formed, illustrated by the active sensing portion 808 of the TFT-circuit 104. In practice, the active layer 802 consist of a plurality of conducting and insulating layers to form the desired transistor and capacitive sensing functionality. On top of the active layer 802 in Fig. 8 follows first and second connection layers 804, 806 where electrical connections to the active layer 802 are formed. It should be noted that the number of connection layers can be selected based on the requirements of a given application, and that the two connection layers in the presently described TFT-circuit should be seen as an illustrative example. The contact pads 110 are not shown in Fig. 8 but they may be formed in the second connection layer 806 and in the same process steps as when conductive layers of the topmost connection layer is formed. The sensing side 108 of the capacitive fingerprint sensing device formed by the TFT-circuit 104 is thus defined as the side of the TFT-circuit 104 where the active region 808 is closest to the surface of the circuit. In a biometric imaging device according to various embodiments of the invention, the finger 810 to be imaged is placed on the outer surface of the plastic film 102 to thereby minimize the distance between the finger 810 and the active region 808.

The total thickness of the TFT-circuit 104 can be approximately 5 pm or below and the TFT-circuit 104 therefore requires structural support in the form of the plastic film 102 and the cover layer 114, 402 in order to be manageable in an assembly process.

Fig. 9A schematically illustrates a biometric imaging device 120 integrated in the body of a smartcard 900. Here it can be seen that only an outer layer of the smartcard 900 separates the finger 810 from the biometric imaging device 120. Electrical connections are formed between the biometric imaging device 120 and a conductive layer 902 of the smartcard 900. As illustrated in Fig. 9A, the electrical connection is formed by forming a vertical connection 904 between the conductive layer 902 and the contact pad 110. However, different configurations of the electrical connection are possible depending on if a connecting element is used or not, and if so, which type of connecting element. Accordingly, with a connecting element reaching the top surface of the biometric imaging device, an electrical connection to the smartcard can be simplified. The biometric imaging device can be connected to functionality in the smartcard such as a secure element, SE, used in fingerprint authentication and/or to a contact plate 906 of the type commonly used in credit cards and as illustrated in Fig. 9B.

Fig. 9B schematically illustrates a smartcard 900 comprising a biometric imaging device according to an embodiment of the invention. The smartcard 900 is provided with means for wireless communication with a smartcard reader such as a point-of-sale (POS) terminal 908 and with a contact plate 906 for communication via physical contact with the terminal 906.

Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Also, it should be noted that parts of the biometric imaging device may be omitted, interchanged or arranged in various ways, the biometric imaging device yet being able to perform the functionality of the present invention.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.