FINGERPRINT SENSING DEVICE AND METHOD FOR MANUFACTURING A FINGERPRINT SENSING DEVICE

Abstract

There is provided a capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, said capacitive fingerprint sensing device comprising: a protective top layer to be touched by said finger; a first metal layer comprising a two-dimensional array of sensing structures arranged underneath said top layer; a second metal layer, arranged underneath said first metal layer, comprising a plurality of conductive structures a dielectric layer arranged between the first and second metal layers to electrically insulate the first metal layer from the second metal layer, the dielectric layer comprising a low-k material; and readout circuitry arranged underneath said second metal layer and coupled to each of the electrically conductive sensing structures by means of via connections to receive a sensing signal indicative of a distance between said finger and said sensing structure. There is also provided a method for manufacturing such a device.

Claims

1. A capacitive fingerprint sensing device for sensing a fingerprint pattern of a finger, said capacitive fingerprint sensing device comprising: a protective top layer to be touched by said finger; a first metal layer comprising a two-dimensional array of sensing structures arranged underneath said protective top layer; a second metal layer, arranged underneath said first metal layer, comprising a plurality of conductive structures; a dielectric layer arranged between the first and second metal layers to electrically insulate the first metal layer from the second metal layer, the dielectric layer comprising a low-k material configured to reduce a capacitive coupling between the first and second metal layer, thereby reducing capacitive crosstalk between the sensing structures and the conductive structures; and readout circuitry arranged underneath said second metal layer and coupled to each of the electrically conductive sensing structures by means of via connections to receive a sensing signal indicative of a distance between said finger and said sensing structure.

2. The sensing device according to claim 1, wherein the dielectric layer comprises an organic polymer.

3. The sensing device according to claim 1, wherein the dielectric layer has thickness of at least 3 μm.

4. The sensing device according to claim 1, wherein the dielectric constant k of the low-k material dielectric layer is in the range of 1<k<3.9.

5. The sensing device according to claim 1, wherein the dielectric layer is Polybenzobisoxazole (PBO), Polyimide or Benzocyclobutene (BCB).

6. The sensing device according to claim 1, wherein the dielectric layer is provided in the form of a dry film.

7. The sensing device according to claim 1, comprising a metal structure of said second metal layer arranged directly below a sensing structure of the first metal layer and having an area smaller than the area of the sensing structure.

8. The sensing device according to claim 7, wherein a thickness and a dielectric constant of the dielectric layer is selected based on a desired capacitance between a sensing structure of the first layer and the underlying metal structure of the second metal layer.

9. A method for manufacturing a fingerprint sensing device, the method comprising; providing a substrate comprising a plurality of metal structures arranged in a metal layer on the surface of the substrate and fingerprint readout circuitry located within the substrate; providing a low-k dielectric layer covering the substrate; forming via connection openings in the dielectric layer; depositing a top metal layer on the dielectric layer forming via connections such that each sensing structure is connected to corresponding readout circuitry by means of said via connections; and patterning the top metal layer to form a two dimensional array of sensing structures, wherein the low-k layer is configured to reduce a capacitive coupling between the first and second metal layer, thereby reducing capacitive crosstalk between the sensing structures and the conductive structures.

10. The method according to claim 9, wherein the dielectric layer is deposited using spin coating or spray coating.

11. The method according to claim 9, wherein the dielectric layer is deposited using dry film lamination.

12. The method according to claim 9, wherein the dielectric layer comprises an organic polymer.

13. The method according to claim 9, wherein the metal layer is deposited using sputtering.

14. The method according to claim 9, further comprising electrodeposition of metal to form the top metal layer.

15. The method according to claim 9, wherein the openings in the dielectric layer are formed using photolithography or direct laser structuring.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] 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:

[0039] FIG. 1 schematically illustrates a mobile phone comprising a fingerprint sensing device;

[0040] FIG. 2 schematically illustrates a fingerprint sensing device according to an embodiment of the invention;

[0041] FIG. 3 schematically illustrates part of a fingerprint sensing device according to embodiments of the invention;

[0042] FIG. 4 is a flow chart outlining the general steps of a method according to an embodiment of the invention; and

[0043] FIGS. 5A-D schematically illustrate a manufacturing method according to embodiments of the invention.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0044] In the present detailed description, various embodiments of the device and method according to the present invention are mainly described with reference to a capacitive fingerprint sensing device and to a method for manufacturing such a device.

[0045] FIG. 1 schematically illustrates an application for a fingerprint sensing device 102 according to an example embodiment of the present invention, in the form of a mobile phone 100 with an integrated fingerprint sensing device 102. The fingerprint sensing device is illustrated here as being arranged underneath a cover glass of the mobile phone 100. The fingerprint sensing device 102 may also be arranged in a button, on the side or on a backside of a phone.

[0046] The fingerprint sensing device 102 may, for example, be used for unlocking the mobile phone 100 and/or for authorizing transactions carried out using the mobile phone, etc. A fingerprint sensing device 102 according to various embodiments of the invention may also be used in other devices, such as tablet computers, laptops, smart cards or other types of consumer electronics.

[0047] FIG. 2 is schematic illustration of a part of a capacitive fingerprint sensing device 200 for sensing a fingerprint pattern of a finger 202. The fingerprint sensing device comprises a protective top layer 204 to be touched by the finger, a first metal layer comprising a two-dimensional array of sensing structures 208 arranged underneath the top layer 204, a second metal layer, arranged underneath the first metal layer, comprising a plurality of conductive structures 212, a dielectric layer 216 arranged between the first and second metal layers to electrically insulate the first metal layer from the second metal layer, the dielectric layer 216 comprising a low-k material. The second metal layer also comprises small metal structures 222, sometimes referred to as “landing pads”, for the via connections to contact. The sensing device 200 further comprises readout circuitry 218 arranged underneath the second metal layer and coupled to each of the electrically conductive sensing structures 208 by means of via connections 214 to receive a sensing signal indicative of a distance between the finger 202 and the sensing structure 208. The via connections reach through the dielectric layer 216 between the sensing structure 208 and the landing pad 222.

[0048] The readout circuitry 218 can be considered to be comprised in a substrate 220. The substrate 220 may for example be a silicon substrate and the fingerprint sensing device 200 may be manufactured using conventional silicon-compatible manufacturing techniques. The finger 202 is here illustrated as a fingerprint ridge in contact with the sensing surface of the sensing device 200.

[0049] It should be noted that even though the substrate 220 and the top layer 204 are here illustrated as single layers, both may comprise a plurality of layers, i.e. consist of a stack of layers.

[0050] FIG. 3 schematically illustrates the capacitance 302 between a finger 202 placed on the sensor and the sensing structure 208. There is also a parasitic capacitance 304 between the sensing structure 208 and underlying metal structures in the second metal layer. One reason for the presence of conductive metal structures in the second layer may in part be related to the minimum allowable amount of metal in the underlying metal layer dictated by manufacturing restrictions, where a metal layer comprising too little metal may not be reliably manufactured. There may also be structures in the second metal layer related to other functionality of the fingerprint sensing device. This is illustrated by the capacitance 306 between the sensing structure 208 and the underlying metal structure 208.

[0051] To reduce the parasitic capacitance 304, a low-k dielectric layer 216 is arranged between the first 206 and second metal layer 210. Since the parasitic capacitance is proportional to the dielectric constant, i.e. k-value, of the dielectric layer 216 and inversely proportional to the thickness of the dielectric layer 216, it is desirable to minimize the dielectric constant and maximize the thickness of the dielectric layer 216.

[0052] A commonly used dielectric material in semiconductor processing is SiO.sub.2 having a dielectric constant of 3.9, and the lowest possible dielectric constant is 1 which is the dielectric constant of air. The low-k material will thus have a k-value between 1 and 3.9. It has been found that organic polymers, and in particular Polybenzobisoxazole (PBO), or Polyimide are advantageous to use to form the dielectric layer, where PBO has a k-value in the range of 2.9 to 3.2 and Polyimide has a k-value in the range of 2.8 to 3.3. Further organic dielectric materials to be used may include Benzocyclobutene (BCB) having a k-value of 2.65 and Polytetrafluoroethylene (PTFE) having a k-value of 2.1.

[0053] The thickness of the dielectric layer is at least 3 μm, and preferably at least 10 μm. This should be seen in relation to the distance between the sensing structure 208 and the finger 202 which may be several hundreds of micrometers, such as at least 500 μm if the sensing device is arranged under a cover glass or display glass of an electronic device. Moreover, the size of the sensing structure is typically about 50 μm and the diameter of the via connection is about 5 μm.

[0054] The flow chart of FIG. 4 describes the method according to an embodiment of the invention which will be discussed with reference to FIGS. 5A-E schematically illustrating the steps of the manufacturing method.

[0055] First, a substrate 220 is provided 402 comprising a plurality of metal structures 212 and landing pads 222 as illustrated in FIG. 5A. The metal structures 212 are arranged in the second metal layer, as seen from the top of the device, i.e. from the sensing surface of the sensing device 200. The substrate 220 is further assumed to comprise the associated readout circuitry required for forming a fingerprint image.

[0056] Next, a low-k dielectric layer 216 is provided 404 to cover the substrate 220, including the metal structures 212 as illustrated in FIG. 5B. The low-k dielectric layer 216 may for example be deposited by spin coating, spray coating or by dry film lamination. Here, it is important to achieve a good thickness uniformity of the dielectric layer 216 in order to conform to other process tolerances.

[0057] After the dielectric layer 216 has been deposited, FIG. 5C illustrates that openings 502 are formed 406 in the dielectric layer 216 to prepare for via connections to the underlying readout circuitry. The openings are formed at the locations corresponding to the landing pads 222, reaching through the dielectric layer 216 to the landing pads. Next, the top metal layer, i.e. the first metal layer, is deposited 408 such that the openings 502 are filled to form via connections 214 between the first metal layer and the readout circuitry. The top metal layer may for example be deposited using sputtering and/or electrodeposition. Accordingly, the metal for forming the via connections 214 and the sensing structures 208 is deposited in the same process step.

[0058] Finally the top metal layer is patterned 410 using photolithography or laser etching to form a two dimensional array of sensing structures 208, which may also be referred to as a pixel array. Each sensing structure is connected to corresponding readout circuitry 218 as illustrated in FIG. 5D.

[0059] Furthermore, the pixel array may be covered by additional layers, such as a mold layer for protecting the sensing structures 208. The sensing device may also comprise a protective plate or cover glass attached by means of an adhesive, such that the protective plate forms the outer surface of the sensing device.

[0060] 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.

[0061] 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.