ACOUSTIC BIOMETRIC IMAGING SYSTEM WITH ACOUSTIC IMPEDANCE MATCHED OPAQUE MASKING LAYER, AND MANUFACTURING METHOD

Abstract

An acoustic biometric imaging system comprising: a transparent device member having a first face to be touched by a finger surface of a user, and a second face opposite the first face, the transparent device member having a first acoustic impedance; a first ultrasonic transducer acoustically coupled to the second face of the transparent device member in a first transducer region for receiving acoustic signals conducted by the transparent device member from a finger touch region laterally spaced apart from the first transducer region, the first ultrasonic transducer having a second acoustic impedance; and an opaque masking layer arranged between the transparent device member and the first ultrasonic transducer in the first transducer region, the opaque masking layer having a third acoustic impedance between the first acoustic impedance and the second acoustic impedance.

Claims

1. An acoustic biometric imaging system comprising: a transparent device member having a first face to be touched by a finger surface of a user, and a second face opposite the first face, said transparent device member having a first acoustic impedance; a first ultrasonic transducer acoustically coupled to the second face of said transparent device member in a first transducer region for receiving acoustic signals conducted by said transparent device member from a finger touch region laterally spaced apart from said first transducer region, said first ultrasonic transducer having a second acoustic impedance; and an opaque masking layer arranged between said transparent device member and said first ultrasonic transducer in said first transducer region, said opaque masking layer having a third acoustic impedance between said first acoustic impedance and said second acoustic impedance.

2. The acoustic biometric imaging system according to claim 1, wherein said third acoustic impedance is greater than 8 MRayls and less than 24 MRayls.

3. The acoustic biometric imaging system according to claim 1, wherein said opaque masking layer has a thickness of less than 10 m.

4. The acoustic biometric imaging system according to claim 1, wherein said opaque masking layer is a vacuum deposited layer.

5. The acoustic biometric imaging system according to claim 1, wherein said opaque masking layer is an oxide layer.

6. The acoustic biometric imaging system according to claim 5, wherein said opaque masking layer comprises silicon and zirconium.

7. The acoustic biometric imaging system according claim 1, further comprising an attachment layer between said opaque masking layer and said first ultrasonic transducer.

8. The acoustic biometric imaging system according to claim 7, wherein said attachment layer is a Bismuth-based alloy.

9. The acoustic biometric imaging system according to claim 7, further comprising a metallic layer between said opaque masking layer and said attachment layer.

10. The acoustic biometric imaging system according to claim 9, wherein said metallic layer is a vacuum deposited layer.

11. The acoustic biometric imaging system according to claim 1, wherein said first ultrasonic transducer is a ceramic transducer.

12. The acoustic biometric imaging system according to claim 1, wherein said first ultrasonic transducer is a shear wave transducer.

13. The acoustic biometric imaging system according to claim 1, further comprising: transducer control circuitry connected to said first ultrasonic transducer for receiving, from said first ultrasonic transducer, electrical signals indicative of the acoustic signals conducted by said transparent device member from said finger touch region.

14. The acoustic biometric imaging system according to claim 13, further comprising processing circuitry connected to said transducer control circuitry and configured to form a representation of said finger surface based on signals from said transducer control circuitry.

15. An electronic device comprising: the acoustic biometric imaging system according to claim 14; and a controller configured to: acquire the representation of said finger surface from the acoustic biometric imaging system; authenticate a user based on said representation; and perform at least one user-requested process only if said user is authenticated based on said representation.

16. A method of manufacturing an acoustic biometric imaging system, comprising the steps of: providing a transparent device member assembly including a transparent device member having a first face and a second face, and an opaque masking layer deposited on a portion of the second face of said transparent device member; and attaching a first ultrasonic transducer to the opaque masking layer on the second face of said transparent device member, wherein the transparent device member has a first acoustic impedance, the first ultrasonic transducer has a second acoustic impedance, and the opaque masking layer has a third acoustic impedance between said first acoustic impedance and said second acoustic impedance.

17. The method according to claim 16, wherein the step of providing said transparent device member assembly comprises the steps of: providing said transparent device member; and vacuum depositing said opaque masking layer on the second face of said transparent device member.

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 is an illustration of an exemplary electronic device comprising an acoustic biometric imaging system according to an embodiment of the present invention, in the form of a mobile phone;

[0040] FIG. 2A is a schematic cross-section view of the acoustic biometric imaging system in FIG. 1, with the section taken along the line A-A in FIG. 1;

[0041] FIG. 2B is an enlarged view of a portion of the acoustic biometric imaging system in FIG. 2A;

[0042] FIG. 3 is a partial cross-section view of the ultrasonic transducer array included in the acoustic biometric imaging system in FIG. 2A, with the section taken along the line B-B in FIG. 1;

[0043] FIG. 4 is a flow-chart illustrating an example embodiment of the manufacturing method according to the present invention; and

[0044] FIGS. 5A-E schematically illustrate the result of the respective method steps in the flow-chart in FIG. 4.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

[0045] In the present detailed description, various embodiments of the acoustic biometric imaging system according to the present invention are mainly described with reference to an acoustic biometric imaging system comprising a cover glass for a mobile communication device, with an ultrasonic transducer array attached thereto. It should be noted that acoustic biometric imaging systems with many other configurations also fall within the scope defined by the claims. For instance, the transparent device member need not necessarily be a cover glass, and/or the ultrasonic transducer array included in the acoustic biometric imaging system may include fewer or more piezoelectric elements. Moreover, the first and second transducer electrodes may be connectable from the same or different sides of the ultrasonic transducer device.

[0046] The acoustic biometric imaging system according to embodiments of the present invention may be included in various electronic devices. FIG. 1 schematically illustrates a representative electronic device, in the form of a mobile phone 1, comprising an acoustic biometric imaging system 3 according to an embodiment of the present invention.

[0047] As is schematically indicated in FIG. 1, the acoustic biometric imaging system 3 may comprise an ultrasonic transducer array 5, and a controller 9 connected to the ultrasonic transducer array 5.

[0048] The ultrasonic transducer array 5 is acoustically coupled to a transparent device member, here cover glass 11, of the electronic device 1 in a first transducer region, corresponding to the extension of the ultrasonic transducer array 5. The user touch, which takes place in a finger touch region 14 laterally spaced apart from the first transducer region 5, is indicated by the thumb 13 in FIG. 1. An exemplary near zone limit of the finger touch region 14 is schematically indicated by the dashed line 16 in FIG. 1.

[0049] When the acoustic biometric imaging system 3 is in operation, the controller 9 controls one or several piezoelectric element(s) comprised in the ultrasonic transducer array 5 to transmit an acoustic transmit signal S.sub.T, indicated by the block arrow in FIG. 1. Further, the controller 9 controls the ultrasonic transducer array 5 to receive acoustic interaction signals S.sub.In, indicated by the dashed arrows in FIG. 1. The acoustic interaction signals S.sub.In are indicative of interactions between the transmit signal S.sub.T and the interface between the cover glass 11 and the skin of the user (thumb 13). The acoustic interaction signals S.sub.In are transformed to electrical signals by the receiving piezoelectric elements in the ultrasonic transducer array 5, and the electrical signals are processed by the controller 9 to provide a representation of the fingerprint of the user.

[0050] The acoustic interaction signals S.sub.In are presently believed to mainly be due to so-called contact scattering at the contact area between the cover glass and the skin of the user (thumb 13).

[0051] The acoustic transmit signal S.sub.T may advantageously be a pulse train of short pulses (impulses), and the acoustic interaction signals S.sub.In, which may be measured for different angles by different receiving piezoelectric elements, may then be impulse responses. The impulse response data carried by the acoustic interaction signals S.sub.In can be used to reconstruct a representation of the contact area (the fingerprint) using a reconstruction procedure similar to methods used in ultrasound reflection tomography.

[0052] It should be understood that the representation of the fingerprint of the user may be any information extracted based on the received acoustic interaction signals S.sub.In, which is useful for assessing the similarity between fingerprint representations acquired at different times. For instance, the representation may comprise descriptions of fingerprint features (such as so-called minutiae) and information about the positional relationship between the fingerprint features. Alternatively, the representation may be a fingerprint image, or a compressed version of the image. For example, the image may be binarized and/or skeletonized. Moreover, the fingerprint representation may be the above-mentioned impulse response representation.

[0053] FIG. 2A is a schematic cross-section view of the acoustic biometric imaging system 3 in FIG. 1, with the section taken along the line A-A in FIG. 1, and FIG. 2B is an enlarged view of a portion of the acoustic biometric imaging system 3 in FIG. 2A.

[0054] Referring first to FIG. 2A, the transparent device member 11, here cover glass, has a first face 12a to be touched by a finger surface of a user, and a second face 12b opposite the first face 12a. The ultrasonic transducer array 5 comprises a plurality of ultrasonic transducers 15, each comprising a piezoelectric element 19, a first transducer electrode 31, and a second transducer electrode 33. Each of the ultrasonic transducers 15 is acoustically coupled to the second face 12b of the transparent device member 11. As can be seen in FIG. 2A, the acoustic biometric imaging system 3 further comprises an opaque masking layer 18 arranged between the second face 12b of the transparent device member 11 and the ultrasonic transducers 15 in the ultrasonic transducer array 5. The opaque masking layer 18 render the ultrasonic transducer array 5 invisible from the first face 12a of the transparent device member 11, and can be colored as desired.

[0055] Referring now additionally to FIG. 2B, the exemplary acoustic biometric imaging system 3 in FIG. 2A further comprises a metal layer 22 deposited on the opaque masking layer 18, and an attachment layer 24 deposited on the metal layer 22. The top electrode 31 of the ultrasonic transducer 15 is bonded to the transparent device member 11 using the attachment layer 24.

[0056] As is also indicated in FIG. 2B, the transparent device member 11 has a first acoustic impedance Z.sub.1, the ultrasonic transducer 15 (the piezoelectric element 19) has a second acoustic impedance Z.sub.2, the opaque masking layer 18 has a third acoustic impedance Z.sub.3, and the attachment layer 24 has a fourth acoustic impedance Z.sub.4.

[0057] To provide for a good acoustic coupling between the ultrasonic transducer 15 and the transparent device member 11, the third acoustic impedance Z.sub.3 and the fourth acoustic impedance Z.sub.4 should both have values between the value of the first acoustic impedance Z.sub.1 and the second acoustic impedance Z.sub.2. Since the metal layer 22 and the first transducer electrode 31 can be made very thin (such as less than 1 m), these layers can be disregarded in view of the acoustic coupling between the ultrasonic transducer 15 and the transparent device member 11.

[0058] An ultrasonic transducer 15 comprising a piezoelectric element 19 made of PZT has an acoustic impedance of about 23.6 MRayls for longitudinal waves, and about 14.4 MRayls for shear waves.

[0059] Chemically modified glass (such as so-called gorilla glass), which is a suitable material for the transparent device member 11 for use in, for example, a mobile communication device 1, has an acoustic impedance of about 13.7 MRayls for longitudinal waves, and about 8.8 MRayls for shear waves.

[0060] For a first exemplary acoustic biometric imaging system using longitudinal waves, the first acoustic impedance Z.sub.1 may thus be around 23.6 MRayls, and the second acoustic impedance Z.sub.2 may be around 13.7 MRayls. This means that the third acoustic impedance Z.sub.3 should be higher than about 13.7 MRayls, and less than about 23.6 MRayls.

[0061] For a second exemplary acoustic biometric imaging system using shear waves, the first acoustic impedance Z.sub.1 may be around 14.4 MRayls, and the second acoustic impedance Z.sub.2 may be around 8.8 MRayls. This means that the third acoustic impedance Z.sub.3 should be higher than about 8.8 MRayls, and less than about 14.4 MRayls.

[0062] Neither of these ranges can be achieved using conventional polymer ink, which typically has an acoustic impedance of less than 1 MRayls for both longitudinal waves and shear waves.

[0063] According to embodiments of the biometric acoustic imaging system 3 of the present invention, the opaque masking layer 18 is instead a vacuum deposited oxide layer formed by a mix of silicon oxide and zirconium oxide.

[0064] Silicon oxide (silicon dioxide) has (depending on the density) an acoustic impedance of about 13 MRayls for longitudinal waves, and about 8.5 MRayls for shear waves.

[0065] Zirconium oxide (zirconium dioxide) has (depending on the density) an acoustic impedance of about 30 MRayls for longitudinal waves, and about 19 MRayls for shear waves.

[0066] By vacuum depositing silicon oxide and zirconium oxide in suitable proportions, it is clear that an opaque layer can be achieved that is within the desired acoustic impedance ranges for longitudinal waves as well as for shear waves. Using Non-Conductive Vacuum Metallization (NCVM), which is, per se, well known in the art for other applications, silicon oxide and zirconium oxide can be provided in suitable proportions. A desired appearance (such as color) of the opaque masking layer 18 can, for example, be achieved by tuning the thickness of the layer.

[0067] The attachment layer 24 may suitably, for example, consist of a SnBi alloy, which has an acoustic impedance of 11.3 MRayls for shear waves.

[0068] Before turning to an exemplary embodiment of the manufacturing method according to the present invention, an example configuration of the ultrasonic transducers 15 in the biometric acoustic imaging system 3 will be described with reference to FIG. 3.

[0069] As is indicated in FIG. 3, the piezoelectric element 19 has a first face 25, a second face 27, and side edges 29 extending between the first face 25 and the second face 27. The first transducer electrode 31 can be shaped to directly interconnect the first face 25 of the piezoelectric element 19 with a conductive via 26. As can also be clearly seen in FIG. 3, the edges 29 of the piezoelectric element 19 are completely covered by the embedding dielectric material 23, and as the embedding dielectric material 23 and the piezoelectric element 19 have been thinned in the same thinning process, the embedding dielectric material 23 is co-planar with the first face 25 of the piezoelectric element 19, at least at the side edges 29 of piezoelectric element. Moreover, the integrated circuit 20, which may, for example be an ultrasound driver circuit for driving at least one piezoelectric element with a relatively high voltage signal, such as 12 V or more, and/or an ultrasound receiver circuit, is completely embedded by the dielectric material 23. As can also be seen in FIG. 3, the spacers 37a-b define a spacer plane, represented by the line 40 in FIG. 3, which is spaced apart from the first transducer electrodes 31 and parallel with a plane defined by the first face 25 of the piezoelectric element 19.

[0070] An example method of manufacturing the acoustic biometric imaging system 3 according to embodiments of the invention will now be described with reference to the flow-chart in FIG. 4, and the accompanying illustrations in FIGS. 5A-E.

[0071] In a first step 101, a transparent device member, here cover glass 11 having a first face 12a and a second face 12b is provided.

[0072] In the subsequent step 102, an opaque masking layer 18 is vacuum deposited on a portion of the second face 12b of the cover glass 11, using NCVM. As described further above, the proportions of silicon oxide and zirconium oxide, or other suitable oxides or nitrides, are tuned so that the opaque masking layer 18 exhibits an acoustic impedance in a desired range as exemplified further above. As the NCVM process is, per se, well known for other purposes, details of NCVM processing are not provided herein.

[0073] In the next step, 103, a thin metal layer 22 is deposited on the opaque masking layer 18, using, for example PVD. The metal layer 22 is provided as an interface layer between the opaque masking layer 18 and the attachment layer 24, that is deposited in step 104.

[0074] In the final step 105, an ultrasonic transducer array 5 comprising a plurality of ultrasonic transducers 15 is bonded to the second face 12b of the cover glass 11, using the attachment layer 24.

[0075] 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 measured cannot be used to advantage.