Patent Publication Number: US-9842244-B2

Title: Fingerprint sensing device with interposer structure

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/168,591 filed on May 31, 2016, issued as U.S. Pat. No. 9,672,407, which claims the benefit of Swedish Patent Application No. 1550748-6 filed Jun. 8, 2015, and Swedish Patent Application No. 1551288-2 filed Oct. 7, 2015. The disclosures of the above applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a fingerprint sensing device. In particular, the present invention is related to a fingerprint sensing device comprising an interposer structure for enhancing the performance in the sensing device, and to a method for manufacturing a fingerprint sensor comprising such an interposer structure. 
     BACKGROUND OF THE INVENTION 
     As the development of biometric devices for identity verification, and in particular of fingerprint sensing devices, has lead to devices which are made smaller, cheaper and more energy efficient, the possible applications for such devices are increasing. 
     In particular fingerprint sensing has 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. 
     Capacitive fingerprint sensing devices, built based on CMOS technology for providing the fingerprint sensing elements and auxiliary logic circuitry, are increasingly popular as such sensing devices can be made both small and energy efficient while being able to identify a fingerprint with high accuracy. Thereby, capacitive fingerprint sensors are advantageously used for consumer electronics, such as portable computers, tablet computers and mobile phones, e.g. smartphones. 
     A fingerprint sensing chip typically comprises an array of capacitive sensing elements providing a measure indicative of the capacitance between several sensing structures and a finger placed on the surface of the fingerprint sensor. The sensing chip may further comprise logic circuitry for handling addressing of the array of sensing elements. 
     A typical fingerprint sensor is protected so that the finger does not come in physical contact with the sensing elements. In particular, it may be desirable to arrange a glass plate on top of the sensor for protecting the sensor, or to arrange the sensor behind a display glass. By arranging elements between the sensing surface and the sensing elements, the distance between the sensing surface and the sensing elements increases which reduces the capacitive coupling between a finger placed on a sensing surface of the device and the capacitive sensing elements. 
     With increased distance and reduced capacitive coupling, an increasing sensitivity is required of the sensing elements, i.e. the sensing elements must be able to measure a lower capacitance. With the sensing elements being pushed to the limit with regard to the minimum measurable capacitance, it is increasingly important to ensure that the fingerprint sensor measures uniformly over the entire sensing area of the sensor. 
     In view of the above, it is desirable to improve the performance of a fingerprint sensor having a low capacitive coupling between a finger placed on the sensing surface and the sensing elements. 
     Many attempts are made at improving the capacitive coupling, for example, US2013/0201153 discloses a fingerprint sensing device where electrically conductive strands are arranged between the sensing surface and the sensing elements of a fingerprint sensing device. An insulating material is arranged between conductive strands. However, a direct electrical contact between the finger and the pixel may cause problems related to electrostatic discharge (ESD). Moreover, the metallic portions of the surface may oxidize, resulting in undesirable aesthetic effects. 
     SUMMARY OF THE INVENTION 
     In view of above-mentioned desirable properties of a fingerprint sensing device, and drawbacks of prior art, it is an object of the present invention to provide a fingerprint sensing device and a method for manufacturing a fingerprint sensing device which improves capacitive measurements for capacitances near the limit of what is measureable. 
     According to a first aspect of the invention, there is provided a fingerprint sensing device comprising: a sensing chip comprising an array of sensing elements, the sensing elements being configured to be connected to readout circuitry for detecting a capacitive coupling between each of the sensing elements and a finger placed on a sensing surface of the sensing device, wherein a surface of the sensing elements define a sensing plane; a plurality of interposer structures arranged on the sensing chip extending above sensing plane, wherein the plurality of interposer structures have substantially the same height above the sensing plane; and a protective plate attached to the sensing chip by means of an adhesive arranged on the sensing chip, wherein the protective plate rests on the interposer structures such that a distance between the protective plate and the sensing plane is defined by the height of the interposer structures. 
     The sensing chip should in the present context be understood as a chip comprising a plurality of sensing elements in the form of conductive plates or pads, typically arranged in an array, which are capable of forming a capacitive coupling between each sensing element and a finger placed on an exterior surface of the fingerprint sensing device. Through readout of the capacitive coupling for each sensing element, ridges and valleys of a fingerprint can be detected as a result of the distance dependence of the capacitive coupling. To achieve a fingerprint image with sufficient resolution, the sensing elements are typically substantially smaller than the features (ridges and valleys) of the finger. In general, a chip may also be referred to as a die. 
     The protective plate typically comprises a dielectric material in order to provide a good capacitive coupling between a finger placed on the plate and the sensing elements of the sensing chip. In particular, the protective plate may advantageously comprise a glass or ceramic material, such as a chemically strengthened glass, ZrO 2  or sapphire. The aforementioned materials all provide advantageous properties in that they are hard and thereby resistant to wear and tear, and in that they are dielectric thereby providing a good capacitive coupling between a finger placed on the surface of the protective plate and the sensing element of the sensing device. The protective plate described herein commonly forms the outer surface of the fingerprint sensing device, hereinafter referred to as the sensing surface. 
     The sensing chip according to various embodiments of the invention may subsequently be arranged on a conventional rigid PCB substrate or it may be implemented using a flexible type of substrate comprising readout circuitry to form the fingerprint sensing device. 
     The present invention is based on the realization that a more homogeneous distance distribution between the sensing surface and the sensing plane can improve the performance of a fingerprint sensing device. In particular, when a protective plate is used and the distance between the sensing surface and the sensing plane is increasing to the degree that the difference in capacitance between a fingerprint ridge and a valley is barely discernable by the sensing chip, it is increasingly important that the capacitive coupling is as homogeneous as possible over the entire surface of the sensing device. 
     Moreover, since a wafer coating material is sometimes provided on the sensing chip to protect and cover the sensing elements, it has been realized that the wafer coating material, if formed with a high degree of accuracy, can be used as an interposer structure defining the distance between the sensing plane and the sensing surface. The thickness of the protective plate can be controlled to a high degree of accuracy, whereas the adhesive attaching the protective plate to the sensing chip typically is more difficult to deposit evenly, thereby exhibiting a more uneven surface. Thus, the distance between the sensing surface and the sensing plane has previously been defined, at least in part, by the adhesive attaching the protective plate, which may lead to some inhomogeneity in the distance and in subsequent readout of a fingerprint image. 
     According to embodiments of the present invention, interposer structures exhibiting high thickness uniformity are provided on the sensing elements such that they have substantially the same height above the sensing plane. Accordingly, with the protective plate attached to the sensing chip by means of an adhesive, while resting on the interposer structures, the distance between a finger placed on the protective plate and the sensing elements can be controlled with a high degree of accuracy. In such a situation, optimal settings can be applied across the whole sensing area thereby facilitating acquisition of a fingerprint image with high quality. Furthermore, the interposer structures are preferably arranged and configured to provide sufficient mechanical support for the protective plate to avoid movement or flexing of the protective plate when the fingerprint sensor is in use. 
     The interposer structures can in principle be made from any commonly used wafer coating material, which may refer to any material which is arranged to cover the sensing chip and in particular the sensing elements. 
     According to one embodiment of the invention, a variation in height between the plurality of interposer structures may advantageously be less than 1 μm. 
     According to one embodiment of the invention, the plurality of interposer structures may comprise parallel lines, which would provide an assembly of interposer structures which is simple to manufacture and which could provide sufficient mechanical support for the protective plate. The specific configuration of lines, such as the length, width, pitch and orientation can be selected based on the specific configuration of the fingerprint sensor. For example the parallel lines may be aligned with an edge of the sensing chip and have a length substantially similar to the side of the sensing chip, thereby providing interposer structures which define the distance between the sensing plane and the sensing surface over the full area of the sensing chip, including at the edges of the sensing chip to avoid that any edge effects occur. 
     Alternatively, according to one embodiment of the invention, the plurality of interposer structures may comprise one interposer structure centered on each of said sensing elements. Thereby, it can be ensured that the distance from each individual sensing element to the sensing surface is well defined and homogeneous over the full area of the sensing chip. It should be noted that many different configurations of interposer structures are possible, while still providing the advantageous effects described above. 
     In one embodiment of the invention, in the case where one interposer structure is located on each of said sensing elements, the interposer structures may be formed in a material having a dielectric constant higher than a dielectric constant of the adhesive. An improved capacitive coupling between a finger and a sensing element can be achieved by providing an interposer structure having a dielectric constant which is higher than the surrounding adhesive. Thereby, the electric field between a finger placed on the sensing surface and the sensing element can be focused towards the respective sensing elements by means of the difference in dielectric constant. Accordingly, a further improved fingerprint sensing device can be provided. 
     According to one embodiment of the invention, the plurality of interposer structures may be arranged in alignment with boundaries between the sensing elements, such that a central portion of each sensing element is not covered by an interposer structure. For the above configuration of interposer elements, where at least a central portion of the sensing element is not covered by the interposer structure, the interposer structures may be formed in a material having a dielectric constant lower than a dielectric constant of the adhesive in order to achieve the above described effect of improved capacitive coupling between the sensing element and a finger. The adhesive will then fill the space between interposer structures such that an inhomogeneous layer is formed with respect to the dielectric constant of the materials, and where the higher dielectric constant of the adhesive provides the improved capacitive coupling. 
     According to one embodiment of the invention, the interposer structures may be arranged on the sensing chip at locations outside of a sensing area of the sensing chip, the sensing area being defined by the array of sensing elements. Thus, the interposer can be arranged to form a frame partially surrounding the array of sensing elements. The adhesive can then be arranged primarily within the frame formed by the interposer elements 
     According to one embodiment of the invention, the plurality of interposer structures may comprise a photoresist. By using a photoresist, the interposer structures can be formed using conventional photolithography and development processes, which simplifies the overall process flow. Moreover, a photoresist can easily be tailored to have a specific dielectric constant so that a desired ratio of dielectric constants can be achieved. 
     According to one embodiment of the invention, the sensing device may further comprise a bond wire arranged between a bond pad on the sensing chip and a substrate on which the sensing chip is arranged, wherein a bond loop height of the bond wire is lower than the interposer structure. It may also be possible to have a bond wire loop height which is slightly higher than the height of the interposer structure, in which case the protective plate may push down on the bond wire when the protective plate is attached to the sensing chip. 
     In one embodiment of the invention, the sensing device may further comprise a via connection through said sensing chip to electrically connect the sensing chip to a substrate. A via connection, commonly referred to as a through-silicon via (TSV) connection, can be used to electrically connect the sensing chip to readout circuitry on a substrate without using wire bonding. TSV connection is for example used when wire bonding is undesirable or unpractical, such as when the desired height of the interposer structures is lower than a height of the wire bond. This situation could for example occur when the sensor shall be utilized with a very thick protective plate such as e.g. a cover glass in a mobile phone. 
     According to a second aspect of the invention, there is provided a method for manufacturing a fingerprint sensing device, the method comprising; providing a sensing chip comprising an array of sensing elements, the sensing elements being configured to be connected to readout circuitry for detecting a capacitive coupling between each of the sensing elements and a finger placed on a sensing surface of the sensing device, wherein a surface of the sensing elements define a sensing plane; forming an interposer layer on the sensing chip; forming a plurality of interposer structures from the interposer layer, wherein the plurality of interposer structures have substantially the same height above the sensing plane; providing a liquid adhesive on the sensing chip; and arranging a protective plate on the sensing chip such that the adhesive is filling out spaces between the plurality of interposer structures, and such that the protective plate rests on the interposer structures. 
     Through the above described manufacturing method, a fingerprint sensing device can be manufactured which exhibits the advantages described above in connection with the first aspect of the invention. 
     That the adhesive is liquid should be interpreted to mean that it has a fluidity which is sufficiently high to allow redistribution of the adhesive when the protective plate is arranged on the sensing chip. The protective plate is pressed down onto the sensing chip until it rests on the interposer structures, thereby defining the distance between the sensing elements and the sensing surface. Through the redistribution of the adhesive it can be assured that voids between interposer structures are filled and that superfluous adhesive is pushed out to the sides of the sensing chip. 
     According to one embodiment of the invention, the interposer layer may be deposited by spin coating or by spray coating which are methods that allow a high degree of accuracy and thickness uniformity when depositing the interposer layer. Moreover, spin and spray coating represent established methods which are compatible with conventional CMOS processing technologies, thereby providing a controllable manufacturing process which is easily integrated in existing manufacturing processes. 
     In one embodiment of the invention, providing the adhesive may comprise dispensing a liquid adhesive on and in between said interposer structures. A liquid adhesive can easily be dispensed onto the sensing chip, over and in between the interposer structures. The uniformity when dispensing the adhesive is not crucial since the adhesive will be redistributed when the protective plate is pressed down onto the adhesive. 
     According to one embodiment of the invention, the interposer layer may advantageously comprise a photoresist, and the plurality of interposer structures can thus be formed using photolithography, which is a well established manufacturing technique that can be performed with high accuracy, uniformity and repeatability. 
     According to one embodiment of the invention, the method may comprise the steps of depositing a hard mask on the interposer layer, patterning the hard mask, patterning the interposer layer according to the hard mask pattern, and removing the hard mask. Thereby, an alternative manufacturing technique is provided. The hard mask can for example be made from SiN and conventional photolithography may be used to form a pattern in the hard mask which is subsequently transferred to the interposer layer. 
     According to a third aspect of the invention, there is provided a method for manufacturing a fingerprint sensing device, comprising: providing a semiconductor wafer, on the semiconductor wafer forming a plurality of fingerprint sensing chips, each sensing chip comprising an array of sensing elements, the sensing elements being configured to be connected to readout circuitry for detecting a capacitive coupling between each of the sensing elements and a finger placed on a sensing surface of the sensing device, wherein a surface of the sensing elements define a sensing plane; forming an interposer layer on the semiconductor wafer, the interposer layer covering the plurality of fingerprint sensing chip sub-areas; forming a plurality of interposer structures from the interposer layer, wherein the plurality of interposer structures have substantially the same height above the sensing plane; dicing the semiconductor wafer into a plurality of individual sensing chips; arranging a sensing chip on a substrate comprising readout circuitry; electrically connecting the sensing elements of the sensing chip to the readout circuitry; providing a liquid adhesive on the sensing chip; and arranging a protective plate on the sensing chip such that the adhesive is filling out spaces between the plurality of interposer structures, and such that the protective plate rests on the interposer structures. 
     An advantage of the above described method is that an interposer layer can be deposited on a full wafer with a high uniformity using established deposition techniques such as spin coating, thereby providing a more effective manufacturing method where a large number of sensing chips can be prepared simultaneously. Moreover, using spin coating or spray coating also allows the process to be easily modified with respect to the desired thickness of the interposer layer. 
     Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled person will 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  schematically illustrates a handheld electronic device comprising a fingerprint sensing device according to an embodiment of the invention; 
         FIGS. 2A-B  schematically illustrate a fingerprint sensing device according to an embodiment of the invention; 
         FIGS. 3A-B  schematically illustrate a fingerprint sensing device according to an embodiment of the invention; 
         FIGS. 4A-B  schematically illustrates fingerprint sensing devices according to embodiments of the invention; 
         FIG. 5  is a flow chart outlining the general steps of a method for manufacturing a fingerprint sensing device according to an embodiment of the invention; 
         FIGS. 6A-G  schematically illustrate a method for manufacturing a fingerprint sensing device according to an embodiment of the invention; and 
         FIG. 7  schematically illustrates a fingerprint sensing device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     In the present detailed description, various embodiments of a fingerprint sensing device according to the present invention are mainly discussed with reference to a capacitive fingerprint sensing device. A method for manufacturing a fingerprint sensing device is also discussed. 
       FIG. 1  is a schematic illustration of a handheld device  100  comprising a fingerprint sensing device  102  comprising a touchscreen display  104 . A fingerprint sensing device  102  can be used in for example a mobile phone, a tablet computer, a portable computer or any other electronic device requiring a way to identify and/or authenticate a user. 
       FIG. 2A  is a schematic illustration of a fingerprint sensing device  200  according to an embodiment of the invention, as seen in a top view. In particular,  FIG. 2A  illustrates the outline of a plurality of interposer structures  202 , where each interposer structure  202  is arranged on a sensing element  204  of the sensing device  200 . 
       FIG. 2B  illustrates the sensing device  200  in further detail where a sensing chip  206  comprising an array of sensing elements  204  is shown. Here it can be seen that an adhesive  208  is arranged in the space between interposer structures  202 , and that a protective plate  210  is attached to the sensing device  200  by means of the adhesive  208 . The protective plate  210  can be manufactured with a high degree of accuracy, and the variation in thickness over the sensing area is typically less than 2 μm. Moreover, the distance between the surface of the sensing elements  204  and the surface  212  of the protective plate  208 , is defined by the height of the interposer structures  202 . The height of the interposer structures is typically in the range of 5 to 50 μm. The height the of the interposer structures for a specific application can for example be selected based on the bonding technique used to connect the sensing chip to a substrate, or on the type of adhesive used. The surface plane of the sensing elements  204  is defined as the sensing plane, and the surface  212  of the protective plate  210  is defined as the sensing surface  212 . 
     The protective plate  210  may also be the cover glass in a device comprising a touch screen, and a cover glass covering the fingerprint sensing device may also be covering the display and touchscreen portions of the handheld device. In principle, the protective plate may be any structure which acts to cover and protect the sensing device while still allowing a capacitive coupling between a finger placed on the surface of the protective plate and the sensing elements. 
     The sensing elements  204  are here shown arranged in a square array, the sensing elements having a size of about 50×50 μm and a distance between adjacent elements is about 5 μm. The sensing elements  204  are electrically conductive, typically metallic, and can as a general approximation be considered to act as one plate in a parallel plate capacitor, where a finger placed on a sensing surface  212  of the fingerprint sensing device  200  represents the other plate. Each sensing element  204  is connected to readout circuitry (not shown) for detecting a capacitive coupling between each of said sensing elements  204  and a finger placed on the sensing surface  212 . 
       FIG. 3A  is a schematic illustration of a fingerprint sensing device  300  according to an embodiment of the invention where the interposer structures  302  are provided in the form of parallel lines, or ridges, aligned with the boundaries between sensing elements  204 . 
     In  FIG. 3B  it can be seen that the adhesive is arranged to fill out the space between the ridges  302  and to attach the protective plate  210  to the sensing chip  206 . 
       FIG. 4A  is schematic illustration of an alternative embodiment of a fingerprint sensing device  400  where interposer structures  402  are arranged aligned with boundaries between adjacent sensing elements  204  but with a gap or opening  306  in the interposer structures  402  between adjacent interposer structures  402 . 
       FIG. 4B  is schematic illustration of an alternative embodiment of a fingerprint sensing device  410  where interposer structures  412  are arranged on the sensing chip outside of the array of sensing elements  204 , which defines the sensing area. Thereby, the adhesive  208  can be arranged to completely cover the sensing elements  204 , providing a homogeneous covering layer. As can be seen in  FIG. 4B , there is a space between adjacent interposer structures  412  enabling a liquid adhesive to flow out through the gaps formed between the structures when the adhesive is deposited. Thereby, any additional adhesive can be pushed out outside of the interposer structures  412  when the protective plate  210  is arranged on the sensing device  410  so that the protective plate  210  rests on the interposer structures  412 . It should be noted that the configuration illustrated in  FIG. 4B  is one of many possible configurations for the interposer structures, and that many different configurations of the interposer elements are possible, where the main feature is that the interposer structures are all of the same height and that they provide mechanical support for the protective plate. 
     An advantage of providing interposer structures as illustrated in  FIGS. 2-4  above is that when a liquid adhesive is being dispensed, the adhesive can easily flow out and fill all the spaces between interposer structures, thereby forming a homogeneous layer without air gaps, which in turn leads to a well defined dielectric structure between the sensing elements  204  and the sensing surface  212 . The adhesive may for example have the same dielectric constant as the material from which the interposer structures are made, thereby making the layer comprising the interposer structures and the adhesive behave as a uniform layer in a dielectric perspective. 
     Alternatively, it is possible to use an adhesive and an interposer structure having different dielectric constants, in which case the difference in dielectric constant can be used to focus the electric field towards the sensing elements. This requires that the material located directly above each sensing element should have a higher dielectric constant than the surrounding material. Taking the sensing device  200  of  FIGS. 2A-B  as an example, the interposer structures  202  would need to have a dielectric constant which is higher than the dielectric constant of the adhesive  208  to achieve the field focusing effect. 
       FIG. 5  is a flow chart outlining the general steps of a manufacturing method according to an embodiment of the invention. The manufacturing method will be discussed also with reference to  FIGS. 6A-G . 
     First, in step  502  illustrated in  FIG. 6A , a circular wafer  600  comprising a plurality of sensing chips  602  is provided. The wafer  600  may for example be a silicon wafer where sensing chips  602  have been formed using conventional CMOS-compatible processing. By using a full size circular wafer, large scale processing advantages can be achieved. 
     Next  504 , as illustrated in  FIG. 6B , an interposer layer  604  is formed on the surface of the wafer  600 , thereby covering the sensing chips  602  to form a uniform layer on the surface of the wafer  600 . The interposer layer  604 , which may also be referred to as a coating layer, is formed to have a uniform thickness and to cover the entire area of the wafer  600 . The interposer layer  604  can for example be a photoresist deposited by spin-coating or spray coating, and the photoresist may be either a positive or a negative photoresist. Spin- and spray-coating are well established manufacturing techniques which can be performed with high accuracy on wafer scale, thereby providing an interposer layer  604  having a uniform thickness over the surface of the wafer  600 . To achieve a high homogeneity in the deposited layer, spray coating can advantageously be performed using ultrasonic nozzles. Furthermore, it is also possible to use methods such as inkjet printing or 3D-printing to form the interposer structures. 
     In the next step  506 , illustrated in  FIG. 6C , interposer structures  606  are formed on the surface of the wafer using photolithography. Here, the interposer structures are provided in the form of individual structures where each structure  606  is centered on a corresponding sensing element  608 . The cross section of the interposer structure  606  is illustrated as being substantially hexagonal. However, the cross section of the interposer structure may in principle be selected arbitrarily, and it may be circular, quadratic or have any polygonal shape there between. Moreover, it is not required that the interposer structure is symmetric, nor does it need to be symmetric in the vertical direction. 
     As an alternative to using a photoresist to form the interposer structures as described above, it is also possible to form the interposer structures in another material. As an example, a hard mask may be formed on the wafer, for example a SiN mask, after which the hard mask is patterned using photolithography, patterning and subsequent deep reactive ion etching (DRIE). It is also possible to use laser ablations to remove material in selected areas to form the desired patterns of interposer structures. Moreover, it is also possible to use additive techniques for the purpose of fabricating the interposer structures and associated geometries, such as e.g. inkjet printing or 3D printing. 
     After forming the interposer structures, the interposer layer may be treated in a plasma cleaning process in order to improve adhesion between the interposer structures and the subsequently deposited adhesive. The plasma cleaning may for example comprise oxygen mixed with an inert gas such as nitrogen or argon. 
     After forming the interposer structures  606  on the wafer, the wafer is diced  508  into separate sensing chips  602  illustrated in  FIG. 6D .  FIG. 6D  further illustrates a bond pad  609  on the sensing chip  602  subsequently used to electrically connect the sensing chip  602  to a substrate. As illustrated in  FIG. 6E , the individual sensing chips  602  are subsequently arranged  510  onto corresponding substrates  612  comprising readout circuitry (not shown) for reading out the information from the sensing elements  608  of the sensing chip  602  in order to form a fingerprint image. The sensing chip  602  is connected  512  to the substrate  612  by means of a bond wire  614  reaching from a first bond pad  616  on the sensing chip to a second bond pad  618  on the substrate. Here, only one bond wire is illustrated to avoid cluttering the drawings. 
     As a next step  514 , illustrated in  FIG. 6F , a liquid adhesive  620  is provided  514  by dispensing the adhesive  620  onto the interposer layer so that the adhesive  208  fills the spaces between the interposer structures  606 . 
     In the final step  516  as illustrated in  FIG. 6G , a protective plate  210  is attached to the sensing device by means of the adhesive  208 . The protective plate  210  is arranged onto the adhesive  620  and a certain pressure is applied so that the adhesive  620  is redistributed to fill all the spaces between adjacent interposer structures  606 . The protective plate is pressed down until it rests on the interposer structures. After the step of applying the adhesive on the sensing chip, there could be a drying step involved (sometimes referred to as beta stage curing) to partially dry the adhesive. In case of curing, the protective plate can be attached to the partially cured/dried adhesive in a subsequent assembly step by applying heat and pressure. 
       FIG. 7  illustrates a sensing device where the electrical connection between the sensing elements and the substrate  612  is formed using a via connection  702  through the sensing chip  602 . Such a via connection  702  may also be referred to as a through silicon via (TSV) connection. 
     Even though the above method is illustrated as starting from a full wafer, it is equally possible to form the interposer structures on an already diced sensing chip. 
     Moreover, the protective plate may also be provided with a frame, which may also be referred to as a bezel, surrounding the sensing chip once the protective plate is in place. The bezel may for example help to protect the bond wires between the sensing chip and the substrate. The bezel may also be a conductive structure acting as a drive element for a finger, and/or functioning as an ESD discharge node. 
     It should be noted that the general aspects of the invention discussed herein are not limited to the specific dimensions and sizes disclosed in the present description. The above description merely provides an example embodiment of the inventive concepts as defined by the claims. 
     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 device and method may be omitted, interchanged or arranged in various ways, the device and 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.