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1. (WO2019004906) METHOD FOR MANUFACTURING A FINGERPRINT SENSOR MODULE
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METHOD FOR MANUFACTURING A FINGERPRINT SENSOR MODULE

Field of the Invention

The present invention relates to a method for manufacturing a fingerprint sensor module comprising a fingerprint sensor device having electrically conductive via connections.

Background of the Invention

Various types of biometric systems are used more and more in order to provide increased security and/or enhanced user convenience. In particular, fingerprint sensing systems have been adopted in, for example, consumer electronic devices, thanks to their small form factor, high performance, and user acceptance.

Among the various available fingerprint sensing principles (such as capacitive, optical, thermal etc.), capacitive sensing is most commonly used, in particular in applications where size and power consumption are important issues. All capacitive fingerprint sensors comprises a sensing array to provide a measure indicative of the capacitance between each of several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor for each sensing element of the sensing array and thereby forming a fingerprint image.

For a fingerprint sensor module to be easy to integrate in an electronic device such as a smartphone or the like, or in a smart card, it is important to provide a convenient way of electrically connecting the fingerprint module to external circuitry. On example is to form wire bonds from connection pads located adjacent to the sensing array on the front side of the sensor device down to corresponding connection pads on a substrate on which the sensor device is arranged.

However, wire bonds protruding above the sensing array on the front side of the sensor device may add to the overall distance between the sensing array and a sensing surface.

Another more attractive solution would be to connect the fingerprint sensor device to the substrate by means of through-silicon-via connections (TSVs) to avoid wire bonding. However, a via connection manufacturing process instead is more complicated, making it more expensive and difficult to optimize.

In view of the above, it is desirable to provide an improved

manufacturing method for forming a fingerprint sensor module comprising via connections.

Summary

In view of above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide and improved method for manufacturing a fingerprint sensor module comprising via connections.

According to a first aspect of the invention, there is provided a method for manufacturing a fingerprint sensor module. The method comprises:

providing a fingerprint sensor wafer comprising a plurality of fingerprint sensor chips, wherein each sensor chip is configured to acquire an image of a finger placed on a sensing surface of the fingerprint sensor module; forming at least one via connection opening through the fingerprint sensor chip; performing chip singulation, dividing the wafer into separate chips such that the edges of each fingerprint sensor chip are exposed after singulation; depositing an electrically conductive material in the at least one via connection opening, thereby forming an electrically conductive via connection reaching through the fingerprint sensor chip; and in the same process step, depositing a protective material on the electrically conductive material, on the backside of the fingerprint sensor chip, and on the chip edges.

The fingerprint sensor chip can be considered to comprise a fingerprint sensing array configured to capture a fingerprint. The fingerprint sensor chip may also be referred to as a die, device or the like. The fingerprint sensor chip may also comprise associated readout circuitry for forming a fingerprint image and for communicating with external circuitry. The sensing array of the fingerprint sensor device is an array comprising a plurality of individual

sensing elements, which may also be referred to as pixels. In a capacitive fingerprint sensing device, each sensing element comprises an electrically conductive plate and associated sensing and readout circuitry for detecting a capacitive coupling between each sensing element and a finger placed on a sensing surface of the fingerprint sensor module. It should however be noted that various embodiments of the present invention are equally applicable for other types of fingerprint sensor devices, such as optical, thermal and ultrasonic fingerprint sensor devices.

The wafer comprising the plurality of fingerprint sensor chips may be a semiconductor wafer, such as a silicon wafer used in CMOS-compatible manufacturing processes.

The protective material is a material which provides both mechanical and environmental protection for the fingerprint sensor chip edges.

The present invention is based on the realization that an improved method for manufacturing a fingerprint sensor module can be achieved by combining steps which are typically performed separately. As is common in existing manufacturing methods, the via connection passivation and protection steps are performed in connection to the formation of the via connection openings. Here, a protective material is deposited which acts to protect the electrically conductive via connections as well as the chip edges. This means that after the protective material has been deposited, the fingerprint sensor module is ready to be mounted in an electronic device, smartcard or the like. The fingerprint sensor chip may for example comprise a ball grid array (BGA) located on the backside of the fingerprint sensor chip for mounting and connecting the chip to external circuitry. The protective material, which is an insulating material, is also deposited on the backside of the fingerprint sensor chip, thereby providing insulation and protection also for the connections of the BGA.

Moreover, in many available manufacturing methods for semiconductor chips, the resulting produced chips are not intended to be as exposed as fingerprint sensor chips are. As fingerprint sensor chip must be accessible by a finger, and the fingerprint sensor chip will thereby be exposed to the

environment. In comparison, conventional semiconductor chips are often packaged in capsules and the like, thereby not requiring a protective layer on the side edges of the chip, in which case the protective layer may be deposited directly after the electrically conductive via connection has been formed.

According to one embodiment of the invention, forming the at least one via connection opening and chip singulation is performed simultaneously in the same process step. Accordingly, two method steps which are typically performed at different stages in the manufacturing process are here performed simultaneously, thereby further improving the efficiency of the manufacturing method.

In one embodiment of the invention, forming the at least one via connection opening and chip singulation is performed using a plasma etch process. The plasma edge process may advantageously be the same type of process which is commonly used for formation of via connection openings. Such a process may be a deep reactive ion etch (DRIE) process using SF6/C4F8/Ar as an etch gas mixture. Using a plasma edge process for chip singulation instead of the commonly used dicing method using a diamond saw prevents dust and dirt from contaminating the fingerprint sensor chip, thereby further simplifying the manufacturing method. Moreover, using a plasma etch process both for chip singulation for forming the via connection openings, the step of depositing the electrically conductive material in the via connection openings can in principle be performed directly after chip singulation. If the wafer is diced by sawing, it is required that the via connection openings are protected before sawing to prevent dirt from entering the via connection openings. Accordingly, using a plasma etch process both for forming via connection openings and for chip singulation provides advantageous synergy effects which simplifies the overall manufacturing process.

According to one embodiment of the invention, the protective material may be a polyimide or silicone based material. Different applications for the fingerprint sensor module may requires different protective materials.

Possible material include materials based on silicone, epoxy, PI (polyimide), PBO (polybenzoxazole), PET (Polyethylene terephthalate), PVDF

(Polyvinylidene fluoride) and all relevant polymers known to those skilled in the art. Silicone based materials can be very elastic and thereby good for mechanical protection. Silicone based materials are also good at protection against water vapor and chemicals.

According to one embodiment of the invention, chip singulation advantageously comprises leaving a mechanical connection bridge between adjacent fingerprint sensor chips. The mechanical connection bridge is thereby a remaining portion of the wafer substrate. Moreover, the mechanical connection bridge may be as narrow as about 30pm. By means of the mechanical connection bridge, current paths between adjacent fingerprint sensor chips can be provided during subsequent electroplating steps, thereby improving the electroplating process.

According to one embodiment of the invention, the protective material deposited on the chip edges preferably has a thickness above 5pm. The required thickness for providing sufficient mechanical protection depends on the properties of the selected material. The thickness of the material is also selected based on the application for which the fingerprint sensor module is to be used. For instance, in applications which require flexibility, such as for a fingerprint sensor module in a smartcard, the protective material is preferably be very elastic. On the other hand, in applications that require good water/chemical protection, the material should have good encapsulation properties, which for example silicone has, and the thickness of the layer of protective material is not as important. For other applications, the adhesion between the protective material and the materials used in a downstream process, i.e. underfill in SMT process, should also be considered to avoid delamination or other problems in the interface between the protective material and other materials used.

According to one embodiment of the invention, the method may further comprise performing secondary chip singulation comprising cutting through the protective material located between adjacent fingerprint sensor chips.

Depending on the deposition technique used, the deposited thickness of the protective material and the distance between adjacent fingerprint sensor chips after singulation, the protective material may completely fill the gap between adjacent fingerprint sensing chips. Accordingly, an additional singulation step may be required.

Cutting of the protective material may for example be performed using a saw. However, it is also possible use an etching process or laser cutting to cut through the protective material and to separate adjacent fingerprint sensor chips from each other.

According to one embodiment of the invention, secondary chip singulation may comprise singulation of a carrier on which the fingerprint sensor wafer is arranged, wherein a sensing array of the fingerprint sensor chip faces the carrier. A carrier may advantageously be used so that the fingerprint sensor chips remain in place after singulation.

The carrier may for example be a glass wafer or an adhesive tape. In the case of glass wafer carrier, the glass remaining attached to the fingerprint sensor chip after singulation may act as a protective plate for the fingerprint sensor module. Accordingly, a fingerprint sensor module having a protective plate may be formed in the second singulation step.

According to one embodiment of the invention, depositing an

electrically conductive material in the via connection opening may be performed using a metal plating process. The metal plating process may advantageously be lithographically patterned.

According to one embodiment of the invention, the method may further comprise forming a redistribution layer on the backside of the fingerprint sensor chip using the redistribution layer process.

There is also provided a fingerprint sensor module manufactured using the method according to any one of the aforementioned embodiments.

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:

Fig. 1 is a flow chart outlining the general steps of a method according to an embodiment of the invention;

Fig. 2 schematically illustrates a fingerprint sensor wafer;

Figs. 3A-E schematically illustrates steps of a method according to an embodiment of the invention;

Fig. 4 schematically illustrates a fingerprint sensor module according to an embodiment of the invention;

Figs. 5A-B schematically illustrate manufacturing steps according to an embodiment of the invention;

Fig. 6 schematically illustrates a fingerprint sensor module according to an embodiment of the invention; and

Fig. 7 schematically illustrates a wafer comprising fingerprint sensor modules according to an embodiment of the invention.

Detailed Description of Example Embodiments

In the present detailed description, various embodiments of the system and method according to the present invention are mainly described with reference to a capacitive fingerprint sensor module. However, various embodiments of the invention are equally applicable also to other types of fingerprint sensors, such as optical, ultrasound and thermal fingerprint sensors.

Fig. 1 is a flow chart outlining the general steps of a method for manufacturing a fingerprint sensor module according to an embodiment of the invention. The method will be described with further reference to Fig. 2 illustrating a semiconductor wafer 200 comprising a plurality of fingerprint sensor chips 202, and to Figs. 3A-E illustrating various method steps.

The method first comprises providing 100 a fingerprint sensor wafer 200 comprising a plurality of fingerprint sensor chips 201 . The fingerprint sensor wafer 200 is illustrated in Fig. 2 and further in Fig. 3A. The fingerprint sensor wafer 200 is typically a silicon wafer comprising fingerprint sensor chips 201 manufactured according to CMOS-compatible process methods. Each sensor chip 201 is configured to acquire an image of a finger placed on a sensing surface of the fingerprint sensor module. As can be seen in Fig. 2, the fingerprint sensor chip 201 comprises a plurality of sensing elements 202 arranged in a sensor array 204. The fingerprint sensor chip 201 further comprises connection pads 206 for example forming a power supply interface and a communication interface. The sensor array 204 comprises a large number of sensing elements 202, each sensing element 202 being

controllable to sense a distance between an electrically conductive sensing structure comprised in the sensing element and the surface of a finger contacting a sensing surface of the fingerprint sensor module. In the present context, the sensing surface of the fingerprint sensor chip 201 will be the outer surface of the finalized fingerprint sensor module.

In Fig. 3A, the fingerprint sensor wafer 200 is arranged on a carrier 300 and attached by means of a temporary adhesive, such as a temporary glue. The carrier may for example be a glass wafer, but it is also possible to attach the fingerprint sensor wafer 200 to an adhesive tape or the like.

The top side of the fingerprint sensor chip 201 comprising the sensing array 204 is also covered by a coating layer 301 such as a polyimide layer. It should be noted that the coating layer 301 may comprise an encapsulant. The coating layer 301 may further comprise a plurality of layers, such as an adhesive, a pigment layer, a dielectric layer and a top surface coating. The fingerprint sensor chip 201 may also comprise a protective plate in the form of a glass or ceramic plate.

The next step, illustrated in Fig. 3B, comprises forming 102 at least one via connection opening 302 through the fingerprint sensor chip 201 . The

location of the via connection opening 302 correspond to the location of the connection pad 206. The via connection opening 302 is formed from the backside of the fingerprint sensor chip 201 , i.e. from the side opposite the side of the fingerprint sensor array 204.

Furthermore, chip singulation is performed 104, dividing the wafer 200 into separate fingerprint sensor chips 201 such that edges 304 of each fingerprint sensor chip 201 are exposed after singulation as can be seen in Fig. 3B.

The via connection openings 302 are preferably formed in the same process step and simultaneous with chip singulation, for example using a plasma etch process commonly used for etching via connections in silicon, e.g. a deep reactive ion etch (DRIE) process. Different feasible via connection formation methods may be referred to a via-first, via-middle and via-last processes.

Once the via connection openings 302 are formed, an electrically conductive material 306 together with a passivation material is deposited in the at least one via connection opening 302, thereby forming an electrically conductive via connection reaching through the fingerprint sensor chip 201 as illustrated in Fig. 3C. The passivation material may for example be silicon dioxide which is deposited on the silicon sidewalls prior to deposition of the electrically conductive material. The electrically conductive material 306 can for example be copper deposited by metal plating. Here, the electrically conductive material is illustrated as an electrically conductive layer covering the sidewalls of the via connection opening 302 without completely filling the opening 302. However, it is equally possible to form a via connection where the via connection opening 302 is completely filled with an electrically conductive material.

In the next step, illustrated in Fig. 3D, a layer of protective material 308 is deposited 108 on the electrically conductive material 306, on the backside of the fingerprint sensor chip, and on the chip edges 304. The protective material 308 may be deposited using spin coating, spray coating or dry film lamination.

The method may further comprise performing a second chip

singulation step where the protective material 308 is cut to separate the fingerprint sensor chips 201 from each other as illustrated in Fig. 3E, thereby forming the final fingerprint sensor modules 310. The second chip singulation step may also comprise cutting the carrier 300. However, it is also possible to remove the fingerprint sensor wafer from the carrier prior to the second chip singulation step so that the fingerprint modules 310 are ready to be used after the second singulation step is performed. In some embodiments, the carrier may in itself form part of the fingerprint sensor module 310 as illustrated in Fig. 3E.

Fig. 4 illustrates a fingerprint sensor module 400 without a carrier or protective plate attached the top of the fingerprint sensor module 400. The illustrated module 400 is thus a completed module with the chip edges protected. The fingerprint sensor module 400 is thereby ready to be integrated in an electronic device such as a smartphone or in a smartcard. The fingerprint sensor module may also be configured to be connected to the display glass of a smartphone and similar electronic devices.

Figs. 5A-B schematically illustrate examples of dry film lamination processes for depositing the protective material 308. The dry film protective material 308 may be a silicone-based photo-sensitive polymer and the thickness of such a dry film is typically in the range of 20pm to 750pm.

In Fig. 5A, a film comprising the protective material 308 is arranged on the back surface of the fingerprint sensor wafer 200 and pressed against the surface using a pressing tool 500. The protective material 308 can thereby fill the via connection openings as well as the gaps between adjacent sensor chips in order to protect the edges 304 of each fingerprint sensor chip 201 .

Fig. 5B illustrates an alternative method where a rolling element 502 is used to press the protective material into the gap and openings on the backside of the fingerprint sensor wafer.

It is also possible to form a BGA (Ball Grid Array), LGA (Land Grid

Array), Cu pillars or other commonly known forms of bump connection pads

on the backside of the fingerprint sensor chip 201 for connecting the fingerprint sensor chip 201 to external circuitry.

Fig. 6 schematically illustrates a fingerprint sensor module 400 comprising a connection pad in the form of a bump 600 of a BGA for connecting the fingerprint sensor module 400 to external circuitry.

Fig. 7 schematically illustrates a fingerprint sensor wafer after a chip singulation step the where the tracks 700 between adjacent fingerprint sensor chips comprises bridges 702 mechanically connecting adjacent fingerprint sensor chips 210. The bridges consist of remaining wafer material, e.g.

silicon, which is not removed during the singulation step. The formation of such bridges 702 is facilitated by the use of plasma etching for chip

singulation. It would not be possible, or at least not convenient, to form such bridges 702 if saw dicing would be used for chip singulation of the fingerprint sensor wafer. The bridges 702 may for example be cut or removed in the secondary singulation step.

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 method may be omitted,

interchanged or arranged in various ways, the method 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.