U.S. patent application number 15/397038 was filed with the patent office on 2017-09-14 for capacitive fingerprint sensing device and method for capturing a fingerprint using the sensing device.
The applicant listed for this patent is Fingerprint Cards AB. Invention is credited to Farzan Ghavanini.
Application Number | 20170262692 15/397038 |
Document ID | / |
Family ID | 59786902 |
Filed Date | 2017-09-14 |
United States Patent
Application |
20170262692 |
Kind Code |
A1 |
Ghavanini; Farzan |
September 14, 2017 |
CAPACITIVE FINGERPRINT SENSING DEVICE AND METHOD FOR CAPTURING A
FINGERPRINT USING THE SENSING DEVICE
Abstract
The present invention relates to a capacitive fingerprint
sensing device for sensing a fingerprint pattern. The sensing
device comprises a protective dielectric top layer having an outer
surface forming a sensing surface to be touched by the finger; a
two-dimensional array of electrically conductive sensing structures
arranged underneath the top layer; readout circuitry coupled to
each of the electrically conductive sensing structures to receive a
sensing signal indicative of a distance between the finger and the
sensing structure; and an electroacoustic transducer arranged
underneath the top layer and configured to generate an acoustic
wave, and to transmit the acoustic wave through the protective
dielectric top layer towards the sensing surface to induce an
ultrasonic vibration potential in a ridge of finger placed in
contact with the sensing surface.
Inventors: |
Ghavanini; Farzan;
(Goteborg, SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fingerprint Cards AB |
Goteborg |
|
SE |
|
|
Family ID: |
59786902 |
Appl. No.: |
15/397038 |
Filed: |
January 3, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 9/0002 20130101;
G06K 9/00087 20130101; G06K 9/00114 20130101; G06K 9/00899
20130101 |
International
Class: |
G06K 9/00 20060101
G06K009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2016 |
SE |
1650342-7 |
Claims
1. A capacitive fingerprint sensing device for sensing a
fingerprint pattern of a finger, said capacitive fingerprint
sensing device comprising: a protective dielectric top layer having
an outer surface forming a sensing surface to be touched by said
finger; a two-dimensional array of electrically conductive sensing
structures arranged underneath said top layer and configured for
use in capturing a fingerprint image based on a capacitive coupling
with the finger; readout circuitry coupled to each of said
electrically conductive sensing structures to receive a sensing
signal, based on a capacitive coupling between the finger and a
respective sensing structure, indicative of a distance between said
finger and said sensing structure; and an electroacoustic
transducer arranged underneath said top layer and configured to
generate an acoustic wave, and to transmit the acoustic wave
through the protective dielectric top layer towards the sensing
surface to induce an ultrasonic vibration potential in a ridge of
finger placed in contact with the sensing surface.
2. The sensing device according to claim 1, wherein the
electroacoustic transducer is an ultrasonic transmitter configured
to generate an ultrasonic wave.
3. The sensing device according to claim 1, wherein the
electroacoustic transducer is a planar electroacoustic
transducer.
4. The sensing device according to claim 1, wherein the
electroacoustic transducer is configured such that the transmitted
acoustic wave is a plane wave.
5. The sensing device according to claim 1, wherein the top layer
is configured to have an acoustic impedance matching an acoustic
impedance of a finger.
6. The sensing device according to claim 1, wherein the
electroacoustic transducer is a piezoelectric transducer.
7. The sensing device according to claim 6, wherein the
electroacoustic transducer is a Piezoelectric Micromachined
Ultrasonic Transducer, PMUT.
8. The sensing device according to claim 1, wherein the
electroacoustic transducer is a Capacitive Micromachined Ultrasonic
Transducer, CMUT.
9. The sensing device according to any claim 1, wherein the
electroacoustic transducer is of the same size as the array of
sensing structures.
10. The sensing device according to claim 1, comprising a plurality
of electroacoustic transducers, each electroacoustic transducer
having an area corresponding to an area of a sub-array of the array
of sensing structures.
11. The sensing device according to claim 1, wherein the array of
sensing structures is arranged between the transducer and the
protective dielectric top layer.
12. The sensing device according to claim 11, further comprising a
delay layer arranged between the electroacoustic transducer and the
array of sensing structures.
13. The sensing device according to claim 12, wherein the delay
layer comprises a plastic material or PMMA.
14. The sensing device according to claim 1, further comprising a
shielding layer arranged between the electroacoustic transducer and
the array of sensing structures to electrically shield the array of
sensing structures from the electroacoustic transducer.
15. The sensing device according to claim 14, wherein the shielding
layer comprises an electrically conductive structure connected to
ground potential.
16. The sensing device according to claim 1, wherein the
electroacoustic transducer is arranged between the array of sensing
structures and the protective top layer.
17. A method for controlling a capacitive fingerprint sensing
device comprising: a protective dielectric top layer having an
outer surface forming a sensing surface to be touched by a finger;
a two-dimensional array of electrically conductive sensing
structures arranged underneath said top layer; readout circuitry
coupled to each of said electrically conductive sensing structures
to receive a sensing signal, based on a capacitive coupling between
the finger and a respective sensing structure, indicative of a
distance between said finger and said sensing structure; and an
electroacoustic transducer arranged underneath said top layer, the
method comprising: activating the electroacoustic transducer,
generating an acoustic wave, and transmitting the acoustic wave
through the protective dielectric top layer towards the sensing
surface to induce an ultrasonic vibration potential in a ridge of a
finger placed in contact with the sensing surface; and capturing a
main fingerprint image by reading out a capacitive coupling between
the finger and the sensing structures by means of the readout
circuitry.
18. The method according to claim 17, further comprising: before
the step of activating the electroacoustic transducer, capturing an
initial fingerprint image; comparing the initial fingerprint image
with the main fingerprint image; and if the difference between the
initial and the main fingerprint image is larger than a
predetermined threshold, determining that the fingerprint image
originates from an authentic finger.
19. The method according to claim 17, further comprising: before
the step of activating the electroacoustic transducer, capturing an
initial fingerprint image; comparing the initial fingerprint image
with the main fingerprint image; and if the difference between the
initial and the main fingerprint image is smaller than a
predetermined threshold, determining that the fingerprint image
originates from a fake finger.
20. A method for controlling a capacitive fingerprint sensing
device comprising: a protective dielectric top layer having an
outer surface forming a sensing surface to be touched by a finger;
a two-dimensional array of electrically conductive sensing
structures arranged underneath said top layer; readout circuitry
coupled to each of said electrically conductive sensing structures
to receive a sensing signal, based on a capacitive coupling between
the finger and a respective sensing structure, indicative of a
distance between said finger and said sensing structure; and an
electroacoustic transducer arranged underneath said top layer, the
method comprising: activating the electroacoustic transducer,
generating an acoustic wave, and transmitting the acoustic wave
through the protective dielectric top layer towards the sensing
surface to induce an ultrasonic vibration potential in a ridge of a
finger placed in contact with the sensing surface; deactivating the
electroacoustic transducer; and while the ultrasonic vibration
potential in the finger is detectable, capturing a fingerprint
image by reading out a capacitive coupling between the finger and
the sensing structures by means of the readout circuitry.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a fingerprint sensing
device. In particular, the present invention relates to a
capacitive fingerprint sensing device comprising an electroacoustic
transducer, and to a method for capturing a fingerprint using the
sensing device.
BACKGROUND OF THE INVENTION
[0002] Various types of biometric systems are used more and more in
order to provide for 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.
[0003] 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.
[0004] All capacitive fingerprint sensors 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.
[0005] Since a capacitive sensor detects a finger based on the
capacitance between the finger and the sensor, the distance between
the sensing surface and the sensing structures directly influence
the contrast and the resolution of the fingerprint image captured
by the measurement. This traditionally did not pose a problem as
the thickness of the cover material could be chosen with little
design pressure. However, according to new design trends it is
desirable to place the sensor under thick cover glass and to
eventually integrate the fingerprint sensor within a display
arrangement.
[0006] This presents a challenging problem. The source of this
problem is not only related to weakening of the capacitive signal
by increased finger-to-sensor distance. Commercially available
capacitive touch sensors may function well through thick cover
glasses. However, a problem is related to the loss of resolution
and image contrast as the finger-to-sensor distance is increased.
This is caused by the fact that distinguishing minute capacitance
variations due to finger corrugations from a large background
"average" that comes from the sum of all the ridges and valleys
"visible" to a pixel becomes extremely difficult at large
finger-to-sensor distances.
[0007] Accordingly, it is desirable to provide a fingerprint sensor
overcoming some of the above described difficulties associated with
capacitive sensing through thick cover layers.
SUMMARY
[0008] In view of above-mentioned and other drawbacks of the prior
art, it is an object of the present invention to provide an
improved fingerprint sensing device for capacitive fingerprint
measurement.
[0009] According to a first aspect of the invention, there is
provided a capacitive fingerprint sensing device for sensing a
fingerprint pattern of a finger, the capacitive fingerprint sensing
device comprising: a protective dielectric top layer having an
outer surface forming a sensing surface to be touched by the
finger; a two-dimensional array of electrically conductive sensing
structures arranged underneath the top layer; readout circuitry
coupled to each of the electrically conductive sensing structures
to receive a sensing signal indicative of a distance between the
finger and the sensing structure; and an electroacoustic transducer
arranged underneath the top layer and configured to generate an
acoustic wave, and to transmit the acoustic wave through the
protective dielectric top layer towards the sensing surface to
induce an ultrasonic vibration potential in a ridge of finger
placed in contact with the sensing surface.
[0010] In the present context, the protective dielectric top layer
may be a single layer or it may comprise a plurality of stacked
layers. Moreover, that the layer is dielectric in the present
context means that it is non-conductive, and that it can be
representative of a dielectric in a parallel plate capacitor where
the two plates are represented by a finger placed on the outer
surface of the sensing device and each of the electrically
conductive sensing structures. Accordingly, that the
two-dimensional array of electrically conductive sensing structures
is arranged underneath the top layer does not exclude the
possibility that there may be additional layers arranged between
the sensing structures and the outer surface of the sensing device.
Moreover, that the electroacoustic transducer is arranged
underneath the top layer is in the present context interpreted to
mean that the electroacoustic transducer is arranged below, or
beneath, the top layer as seen from an outer sensing surface of the
sensing device. Thereby, additional layers may be arranged between
the electroacoustic transducer and the top layer, as will be
described in the following.
[0011] An electroacoustic transducer converts an electric signal to
an acoustic signal to provide an acoustic wave having a frequency
which is typically in the ultrasound range, i.e. a frequency above
the audible range. When a finger is placed on the surface of the
fingerprint sensing device, the ridges of the fingers are in
contact with the surface while the valleys of the finger are not.
The portion of the acoustic wave reaching an interface between the
top layer and the ridge of the finger will penetrate into the
finger, whereas the portion of the acoustic wave reaching an
interface between the top layer and air will be reflected due to
the large difference in acoustic impedance between the top layer
and air. Next, the portion of the acoustic wave penetrating the
finger gives rise to an induced ultrasonic vibration potential,
which can be detected by the capacitive fingerprint sensing device.
The mechanisms behind the generation of the ultrasonic vibration
potential in the finger will be discussed in further detail in the
detailed description section.
[0012] Accordingly, the present invention is based on the
realization that capacitive fingerprint sensing can be improved by
providing a fingerprint sensing device capable of inducing an
ultrasonic vibration potential in the finger by means of an
electroacoustic transducer, thereby creating an electric potential
which is detected by the sensing structures. Thereby, an improved
capacitive coupling between ridges of the finger and the sensing
structures is achieved, and the influence from the valleys in the
capacitive measurement, i.e. the background influence, is reduced.
This is in contrast with existing technologies where a potential is
controllably introduced into the finger through a galvanic or
capacitive coupling to the finger, hence the entire finger is
placed at the same potential level, i.e. both ridges and valleys.
In comparison, the above described sensing device only induces a
potential in the ridges of the finger, thereby providing a larger
contrast between ridges and valleys, which in turn improves the
contrast of the capacitive measurement.
[0013] The above described inventive concept is also applicable as
an enhancement to existing capacitive fingerprint sensing
technologies where non-acoustic means for potential generation in
the finger are already used. Furthermore, the present invention
opens up new opportunities relating to the sensing device
architecture since these non-acoustic means for introducing a
potential in the finger may be eliminated.
[0014] According to one embodiment of the invention, the
electroacoustic transducer may be an ultrasonic transmitter
configured to generate an ultrasonic wave. Ultrasonic transmitters
are a commonly used type of electroacoustic transducers which
convert an electric signal into an ultrasonic wave, and the
properties of ultrasonic transmitters are well known, facilitating
integration in a fingerprint sensing device.
[0015] Moreover, the electroacoustic transducer may advantageously
be a planar electroacoustic transducer, providing the advantage
that it is easily integrated in a planar sensor structure.
[0016] In one embodiment of the invention, the electroacoustic
transducer may be configured such that the transmitted acoustic
wave is a plane wave. By providing the acoustic wave as a plane
wave, all the parts of the finger in contact with the surface of
the sensing device, i.e. all the finger ridges in contact with the
surface of the sensing device, are simultaneously excited by the
penetrating acoustic wave and hence exhibit an ultrasonic
vibrational potential. Consequently, an image of the entire
fingerprint can be taken at once by simultaneously measuring the
capacitive coupling of all sensing structures. Moreover, a plane
wave provides a uniformity of the magnitude of the induced
ultrasonic vibration potential in the finger over the entire sensor
area, which in turn simplifies the capacitive measurement since it
can be assumed that the influence from the ultrasonic vibration
potential is the same for all parts of the finger in contact with
the sensor.
[0017] In one embodiment of the invention the top layer may be
configured to have an acoustic impedance matching an acoustic
impedance of a finger. The portion of the acoustic wave
transitioning over the interface between the top layer and the
finger is determined by the difference in their acoustic
impedances, where a large difference in acoustic impedance results
in that a large portion of the acoustic wave is reflected, whereas
a small difference means that the wave travels across the
interface. Accordingly, it is desirable to select the top layer
from a material having an acoustic impedance which is as similar as
possible to the acoustic impedance of the finger. The difference in
acoustic impedance between air and a solid material is typically
several orders of magnitude. However, even though many solid
materials would provide a large contrast in acoustic impedance
compared to air, it is still desirable to select a material having
an acoustic impedance which is as similar as possible to that of a
finger to avoid or reduce reflection losses at the interface.
[0018] According to one embodiment of the invention, the transducer
may be a piezoelectric transducer, such as Piezoelectric
Micromachined Ultrasonic Transducer, PMUT. The piezoelectric
transducer may include piezoelectric crystals, piezoelectric
ceramics, or piezoelectric polymer. Moreover, the electroacoustic
transducer may also be a Capacitive Micromachined Ultrasonic
Transducer, CMUT.
[0019] According to one embodiment of the invention the
electroacoustic transducer may be of the same size as the array of
sensing structures, meaning that the transducer has the same
surface area as the overall surface area array of sensing
structures. Thereby, the transducer can transmit an acoustic wave
which induces an ultrasonic vibration potential in the finger over
the entire active surface of the fingerprint sensing device.
[0020] According to one embodiment of the invention, the
fingerprint sensing device may comprise a plurality of
electroacoustic transducers, where each electroacoustic transducer
has an area corresponding to an area of a sub-array of the array of
sensing structures. A sub-array of the array of sensing structures
can for example be a regular n.times.m array, where n and m can be
selected based on the desired number of electroacoustic
transducers. Moreover, the plurality of electroacoustic transducers
may be individually controllable such that it can be selected which
transducers are active at any given time. This may be advantageous
for example if a finger is placed only on a portion of the area of
the fingerprint sensing device, or if there are specific regions
where the contrast needs to be enhanced. Accordingly, the plurality
of electroacoustic transducers offers an increased flexibility in
acquiring a fingerprint image. The total area of the plurality of
electroacoustic transducers may correspond to the total area of the
array of the sensing structures, or the plurality of
electroacoustic transducers may be arranged to only cover selected
portions of the sensing device area. Furthermore, in a fingerprint
sensing device comprising a plurality of electroacoustic
transducers, it can be selected which transducers are used at a
specific measurement, which in turn leads to reduced energy
consumption compared to a sensing device comprising one transducer
covering the entire sensing area.
[0021] According to one embodiment of the invention, the array of
sensing structures may be arranged between the transducer and the
protective dielectric top layer. Thereby, there is no increase in
the distance between the sensing array and the finger as compared
to a conventional capacitive fingerprint sensing device. Moreover,
by forming a stack of layers where the electroacoustic transducer
is located at the bottom, beneath the sensing structures, a
capacitive fingerprint sensing device can be manufactured according
to known methods, without having to modify the manufacturing
process to accommodate for the electroacoustic transducer. The
electroacoustic transducer may thus be readily integrated in
existing manufacturing schemes. Furthermore, since the acoustic
wave as such does not disturb or influence the capacitive sensing,
the acoustic wave can pass through the sensing structures and
associated circuitry without adverse effects, or with only
negligible adverse effects.
[0022] According to one embodiment of the invention, the sensing
device may further comprise a delay layer arranged between the
electroacoustic transducer and the array of sensing structures. The
delay layer increases the time it takes for the generated acoustic
wave to reach the finger. The delay layer is advantageously placed
between the electroacoustic transducer and the array of sensing
structures, i.e. below the array of sensing structures. The delay
layer may for example comprise a plastic material or PMMA. Effects
and advantages related to the delay layer will be discussed in
further detail below in relation to a method for controlling the
fingerprint sensing device.
[0023] According to one embodiment of the invention, the sensing
device may further comprise a shielding layer arranged between the
electroacoustic transducer and the array of sensing structures to
electrically shield the array of sensing structures from the
electroacoustic transducer. Even though there is no or limited
electromagnetic distortion from the acoustic wave as such, the
electroacoustic transducer may generate an electromagnetic field
during generation of the acoustic wave, in turn influencing the
sensing structures during capacitive sensing. Accordingly, a
shielding layer can reduce or eliminate the influence of the
electromagnetic field on the sensing structures.
[0024] According to one embodiment of the invention, the shielding
layer may comprise an electrically conductive structure connected
to ground potential, thereby electromagnetically shielding the
sensing structures from the electroacoustic transducer. The
electrically conductive structure can be a continuous layer,
individual structures, a grid, an array of structures etc.
[0025] According to one embodiment of the invention, the
electroacoustic transducer may be arranged between the array of
sensing structures and the protective top layer. Since the
electroacoustic transducer can be made from a non-conductive
material, such as a piezoelectric material it is possible to place
the electroacoustic transducer between the array of sensing
structures and the protective top layer as long as there are no
conductive structures of the transducer shielding the sensing
structures from the finger.
[0026] According to a second aspect of the invention, there is
provided a method for controlling a capacitive fingerprint sensing
device comprising: a protective dielectric top layer having an
outer surface forming a sensing surface to be touched by the
finger; a two-dimensional array of electrically conductive sensing
structures arranged underneath the top layer; readout circuitry
coupled to each of the electrically conductive sensing structures
to receive a sensing signal indicative of a distance between the
finger and the sensing structure; and an electroacoustic transducer
arranged underneath the top layer, the method comprising:
activating the electroacoustic transducer, generating an acoustic
wave, and transmitting the acoustic wave through the protective
dielectric top layer towards the sensing surface to induce an
ultrasonic vibration potential in a ridge of a finger placed in
contact with the sensing surface; and capturing a main fingerprint
image by reading out a capacitive coupling between the finger and
each of the sensing structures by means of the readout
circuitry.
[0027] The above described method outlines the capture of a
fingerprint using a capacitive fingerprint sensing device
comprising an electroacoustic transducer generating an acoustic
wave to induce an ultrasonic vibration potential in the ridges of
the finger placed in contact with the sensing surface. In a sensing
device where no additional potential reference is connected to the
finger, the described method can be seen as an enhanced direct
capacitive measurement method.
[0028] According to one embodiment of the invention, the method may
further comprise, before the step of activating the electroacoustic
transducer, capturing an initial fingerprint image; comparing the
initial fingerprint image with the main fingerprint image; and if
the difference between the initial and the main fingerprint image
is larger than a predetermined threshold, determining that the
fingerprint image originates from an authentic finger. By capturing
an initial fingerprint image before the activation of the
electroacoustic transducer, a reference image is acquired where the
finger is not influenced by an acoustic wave, and where no
ultrasonic vibration potential is present in the finger. Due to the
mechanisms responsible for inducing an ultrasonic vibration
potential, it is required that the substance placed on the
fingerprint sensor is an ionic or colloidal substance, such as a
finger. Thereby, an inorganic material, such as rubber or a plastic
material, placed on the finger would not give rise to an ultrasonic
vibration potential when the electroacoustic transducer is active.
Accordingly, for a fake fingerprint made from rubber or the like,
there would not be any detectable difference between the images
captured before and after the electroacoustic transducer is
activated. Thereby, a fake fingerprint can be detected to prevent
fingerprint spoofing.
[0029] Accordingly, when the reference image is compared to the
main image captured when the electroacoustic transducer is active
and when an ultrasonic vibration potential is induced in the
fingerprint ridges, a difference between the two images can be seen
as a difference in contrast between ridges and valleys of the
finger. Thus, the predetermined threshold can for example be a
predetermined average difference in contrast between ridges and
valley of the fingerprint.
[0030] If a difference between the initial image and the main image
is larger than the predetermined threshold, e.g. if there is a
noticeable difference in contrast, it can be determined that the
fingerprint image originates from an authentic finger.
[0031] According to one embodiment of the invention, the method may
further comprise, before the step of activating the electroacoustic
transducer, capturing an initial fingerprint image; comparing the
initial fingerprint image with the main fingerprint image; and if
the difference between the initial and the main fingerprint image
is smaller than a predetermined threshold, determining that the
fingerprint image originates from a fake finger. Analogously to
what is described above, if the difference between the initial
image and the main image is lower than a predetermined threshold,
it can be determined that the fingerprint image originates from a
fake finger. The skilled person realizes that the threshold can be
defined in many different ways, and that the threshold also may be
determined empirically for different types of sensing devices and
for different applications.
[0032] Accordingly, in addition to the improved contrast between
ridges and valleys of the fingerprint, the described sensing device
and method also provides efficient spoofing protection/liveness
detection.
[0033] Additional effects and features of the second aspect of the
invention are largely analogous to those described above in
connection with the first aspect of the invention.
[0034] According to a third aspect of the invention, there is
provided a method for controlling a capacitive fingerprint sensing
device comprising: a protective dielectric top layer having an
outer surface forming a sensing surface to be touched by the
finger; a two-dimensional array of electrically conductive sensing
structures arranged underneath the top layer; readout circuitry
coupled to each of the electrically conductive sensing structures
to receive a sensing signal indicative of a distance between the
finger and the sensing structure; and an electroacoustic transducer
arranged underneath the top layer, the method comprising:
activating the electroacoustic transducer, generating an acoustic
wave, and transmitting the acoustic wave through the protective
dielectric top layer towards the sensing surface to induce an
ultrasonic vibration potential in a ridge of a finger placed in
contact with the sensing surface; deactivating the electroacoustic
transducer; and while the ultrasonic vibration potential in the
finger is detectable, capturing a fingerprint image by reading out
a capacitive coupling between the finger and each of the sensing
structures by means of the readout circuitry.
[0035] By means of the above described method, a fingerprint image
can be captured when the induced ultrasonic vibration potential in
the finger is detectable, but when the electroacoustic transducer
is deactivated, thereby eliminating the risk that an
electromagnetic field from the transducer's activity disturbs the
capacitive measurement.
[0036] The principle of the method is based on the difference in
propagation velocity between the acoustic wave and the
electromagnetic field. Accordingly, in a simplified description,
the acoustic wave is generated and transmitted by the transducer,
after which the transducer is deactivated. When the transducer is
deactivated, the electromagnetic field from the transducer can be
considered to go to zero instantaneously. In the meantime, the
acoustic wave can be seen as still propagating towards the finger.
When the acoustic wave penetrates the finger, an ultrasonic
vibration potential is induced and a fingerprint image can be
captured by means of the readout circuitry. The timing of the
capacitive measurement must be controlled such that the time
between deactivation of the transducer and the image capture is
sufficiently short so that the influence of the ultrasonic
vibration potential in the finger is still detectable.
[0037] The described method may advantageously be employed in a
fingerprint sensing device comprising a delay layer as described
above. The delay layer creates a time margin within which the
acoustic wave is still penetrating the finger while the transducer
is deactivated, hence eliminating the adverse effects of the
electromagnetic field generated by the transducer on the capacitive
sensing structures. The delay layer can be arranged and configured
in many different ways as long as the generated acoustic wave
passes through the delay layer on its way to the finger.
[0038] Additional effects and features of the third aspect of the
invention are largely analogous to those described above in
connection with the first and second aspect of the invention.
[0039] Further features of, and advantages with, the present
invention will become apparent when studying the appended claims
and the following description. The skilled person realizes 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
[0040] 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:
[0041] FIG. 1 schematically illustrates a mobile phone comprising a
fingerprint sensing device;
[0042] FIG. 2 schematically illustrates a fingerprint sensing
device according to an embodiment of the invention;
[0043] FIGS. 3A-D schematically illustrate the displacement of
charge carriers resulting from an acoustic wave;
[0044] FIG. 4 is a schematic illustration of a portion of the
readout circuitry in a fingerprint sensing device according to an
embodiment of the invention;
[0045] FIG. 5 schematically illustrates a fingerprint sensing
device according to an embodiment of the invention;
[0046] FIG. 6 schematically illustrates a fingerprint sensing
device according to an embodiment of the invention;
[0047] FIGS. 7A-B are schematic illustrations of sensing devices
according to embodiments of the invention; and
[0048] FIGS. 8A-C are flow charts outlining the general steps of
methods according to embodiments of the invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0049] 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 sensing device
suitable for being arranged in an electronic device such as a
mobile phone. It should however be noted that various embodiments
of the fingerprint sensing device may be adapted for use also in
other applications.
[0050] 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.
[0051] 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.
[0052] FIG. 2 is a schematic cross section of a portion of the
fingerprint sensing device 102 according to an embodiment of the
invention, with a finger 104 placed on an outer surface of the
sensing device 102. The fingerprint sensing device 102 comprises a
protective dielectric top layer 106 having an outer surface forming
a sensing surface 105 to be touched by a finger. A two-dimensional
array of electrically conductive sensing structures 108 is arranged
underneath the top layer 106, and readout circuitry is coupled to
each of the electrically conductive sensing structures 108 to
receive a sensing signal indicative of a distance between the
finger and the sensing structure 108. The array of sensing
structures 108 is here illustrated as being arranged on a substrate
110, where the substrate may include at least a portion of the
readout circuitry. The substrate 110 may for example be a silicon
substrate and the fingerprint sensing device 102 may be
manufactured using conventional silicon-compatible manufacturing
techniques.
[0053] Furthermore, the sensing device 102 comprises an
electroacoustic transducer 112 arranged underneath the top layer
106. In FIG. 2, the electroacoustic transducer 112 is arranged
underneath the substrate 110. The electroacoustic transducer 112 is
configured to generate an acoustic wave, and to transmit the
acoustic wave through the protective dielectric top layer 106
towards the sensing surface 105 to induce an ultrasonic vibration
potential in a ridge 116 of finger 104 placed in contact with the
sensing surface 105. In the illustrated embodiment, the acoustic
wave passes through the substrate 110 before it reaches the
protective dielectric top layer 106. It should be noted that even
though the substrate 110 and the top layer 106 are illustrated as
single layers, both may comprise a plurality of layers, i.e.
consist of a stack of layers, as will be discussed in further
detail in relation to various embodiments of the invention.
[0054] The electroacoustic transducer 112 may be a plane wave
generator. FIG. 2 shows an electroacoustic transducer 112
comprising a sheet of piezoelectric material sandwiched between a
first metallic electrode layer 118a and a second metallic electrode
layer 118b. The piezoelectric sheet may be made of piezoelectric
ceramics, piezoelectric crystals, or piezoelectric polymers. The
metallic electrodes 118a-b can be deposited or attached on either
sides of the piezoelectric sheet in a number of different ways
known to a person skilled in the art. By applying an electrical
signal to the electrodes 118a-b of the plane wave generator 112 as
described above, an acoustic wave is generated that emanates in a
planar fashion, i.e. the acoustic energy is distributed uniformly
over the wavefront.
[0055] The electroacoustic transducer 112 may be a piezoelectric
transducer based on, a Piezoelectric Micromachined Ultrasonic
Transducer, PMUT, or a Capacitive Micromachined Ultrasonic
Transducer, CMUT. The electroacoustic transducer 112 can in some
cases also be referred to as an ultrasonic transmitter. As an
example, the frequency of the acoustic wave is in the range of 10
MHz to 100 MHz.
[0056] The present fingerprint sensing device 102 is utilizing an
induced ultrasonic vibration potential in the finger. The
mechanisms that lead to the generation of the ultrasonic vibration
potential in a body are described in the following.
[0057] It has long been known that the propagation of longitudinal
ultrasonic waves through an electrolytic solution result in the
generation of alternating potential differences within the
solution. These alternating potentials were first predicted for
simple ionic solutions. In the presence of a longitudinal sound
wave, any differences in the effective mass or friction coefficient
between anions and cations would result in different displacement
amplitudes. In turn, this difference in displacement would create
an alternating electric potential between points within the
solution. This phenomenon is sometimes referred to as an "Ion
Vibration Potential" and is a type of ultrasonic vibration
potential.
[0058] The mechanism of the generation of an ion vibration
potential is schematically shown in FIG. 3A illustrating
displacements at a particular instant represented on the Y-axis and
distance in the direction of propagation on the X-axis. For the
conditions represented here region A will be charged positively
relative to region B. For example, if inert metal probes are placed
at positions A and B, an alternating potential difference will be
observed since the curve representing displacement may be
considered as traveling in the positive direction at the speed of
sound in a progressive sound field. The frequency of the
alternating potentials corresponds to that of the sound field.
[0059] It has been shown that an ion vibration potential is
generated in every instance where ultrasonic waves are propagated
through a solution containing ionic species, however complex these
species may be, as for instance proteins or poly-ions in solutions
of polyelectrolytes.
[0060] Ultrasonic vibration potential has also been shown to arise
in colloidal suspensions. Colloids are suspensions of charged
particles in a liquid with a counter charge distributed in the
fluid around each particle as illustrated in FIG. 3B. The counter
charge, which is normally a spherical distribution around the
particles, gives the solution overall charge neutrality and
stabilizes the suspension against particle agglomeration. When
sound propagates through a suspension where the particles have
either a higher or a lower density than that of the surrounding
fluid, the amplitude and phase of the particle motion, owing to the
difference in inertia between the particle and the volume of fluid
it displaces, differs from that of the fluid so that fluid flows
back and forth relative to the particle on alternating phases of
the acoustic cycle. Since the counter charge is carried by the
fluid, the oscillatory motion of the fluid relative to the particle
distorts the normally spherical counter charge distribution
creating an oscillating dipole at the site of each particle which
results in a macroscopic voltage. This type of ultrasonic vibration
potential is referred to as "Colloid vibration potential". The
generation of a colloid vibration potential is schematically shown
in FIGS. 3C-D.
[0061] FIG. 3C illustrates colloidal particles and countercharge in
the presence of an acoustic wave, where two dipoles oscillate out
of phase to each other. At the point in time illustrated in FIG.
3C, region A will be negatively charged relative to region B.
[0062] FIG. 3D illustrate the following half period of the acoustic
wave, where the dipoles have moved to the opposite phase, making
region A positively charged relative to region B. It can thus be
understood that a periodic ultrasonic vibration potential is
formed, having the same frequency as the frequency of the acoustic
wave.
[0063] The human body is a relatively good conductor of
electricity. This is due to the electrolytic nature of the fluids
in the human body. For example, sodium chloride in water is
decomposed to positively charged sodium ions and negatively charged
chlorine. The ionic nature of the liquids in the body makes it
possible to create a vibration potential by exposing the body to
ultrasonic waves.
[0064] Moreover, the strongest ultrasonic vibration potential
signals that have been detected so far in biological samples are
from blood. This is due to the fact that blood is both colloidal,
as a result of the presence of red blood cells, and ionic, from
dissolved electrolytes, leading to the generation of larger
vibration potentials. This can be exploited to develop a more
secure fingerprint sensor where the presence of organic tissue and
blood can be used to induce an ultrasonic vibration potential in
the finger.
[0065] As illustrated in FIG. 2, a longitudinal acoustic wave is
generated by the ultrasonic transducer 112. The generated wave
travels toward the finger. When the acoustic wave arrives at the
interface between the top layer 106 and the finger 104 two possible
scenarios may occur. If the interface is made to a fingerprint
valley 114 then most of the energy of the arriving acoustic wave
will be reflected because of the large mismatch between the
acoustic impedance of air and the top layer 106. On the other hand,
most of the arriving acoustic energy will penetrate into the finger
104 at the portions of the interface where finger ridges 116 are in
direct contact with the top layer 106.
[0066] The passage of the ultrasonic wave through the finger tissue
at the ridges 116 will generate a periodic potential inside the
tissue, i.e. an ultrasonic vibration potential. This in turn causes
a periodic electrical field to appear between the fingerprint ridge
116 and the sensing structure 108 placed beneath the ridge 116,
which is held at a fixed potential level. This time-varying
electric field is then sensed by the sensing structure 108 and
registered by the readout circuitry, schematically illustrated in
FIG. 4.
[0067] FIG. 4 is a schematic cross section and a circuit schematic
of a portion of a fingerprint sensing device 102 according to an
embodiment of the invention, with a fingerprint ridge 116 located
above a sensing structure 108. The fingerprint sensing device
comprises a plurality of sensing elements 402, each comprising a
protective dielectric top layer 106, a conductive sensing structure
108, here in the form of a metal plate 108 underneath the
protective dielectric top layer 106, a charge amplifier 404. As
illustrated in FIG. 4, a ridge 116 of the finger 104 is located
directly above the sensing structure 108 indicating the minimum
distance between the finger 104 and the sensing structure 108,
defined by the dielectric top layer 106.
[0068] The charge amplifier 404 comprises at least one amplifier
stage, here schematically illustrated as an operational amplifier
(op amp) 406 having a first input (negative input) 408 connected to
the sensing structure 108, a second input (positive input) 410
connected to ground (or to another reference potential), and an
output 412. In addition, the charge amplifier 404 comprises a
feedback capacitor 414 connected between the first input 408 and
the output 412, and reset circuitry, here functionally illustrated
as a switch 416, for allowing controllable discharge of the
feedback capacitor 414. The charge amplifier 404 may be reset by
operating the reset circuitry 416 to discharge the feedback
capacitor 414.
[0069] As is often the case for an op amp 406 in a negative
feedback configuration, the voltage at the first input 408 follows
the voltage at the second input 410. Depending on the particular
amplifier configuration, the potential at the first input 408 may
be substantially the same as the potential at the second input 410,
or there may be a substantially fixed offset between the potential
at the first input 408 and the potential at the second input 410.
In the configuration of FIG. 4, the first input 408 of the charge
amplifier is virtually grounded.
[0070] When a finger is placed on the sensing surface, a potential
difference occurs between the sensing structure 108 and the
fingerprint ridge 116. The induced change in potential difference
between the fingerprint ridge 116 and the reference sensing
structure 108 in turn results in a sensing voltage signal Vs on the
output 412 of the charge amplifier 404, where the amplitude of the
voltage is a function of the capacitive coupling between the
fingerprint ridge 116 and the sensing structure, and thereby
indicative of the existence of an induced vibration potential. The
sensing voltage signal V.sub.S is in turn provided to readout
circuitry 418 where sensing voltage signals from the array of
sensing elements together form a fingerprint image.
[0071] As described above in reference to FIG. 2, the
electroacoustic transducer 112 generates an acoustic wave, which is
transmitted through the protective dielectric top layer 106 towards
the sensing surface to induce an ultrasonic vibration potential in
a ridge 116 of finger, thereby creating an acoustic field in the
finger. The acoustic field gives rise to an ultrasonic vibration
potential in the ridge of the finger placed in contact with the
sensing surface according to the mechanisms described above.
Thereby, the induced ultrasonic vibration potential is detectable
by the charge amplifier 404 and a fingerprint image may be captured
also in a situation where solely the difference in capacitive
coupling of finger ridges and valleys to the sensing structures is
not sufficient for the generation of an accurate fingerprint image.
This may be the case for thick top layers, such as a cover glass or
display glass. Accordingly, the electroacoustic transducer takes no
part in the readout of the sensing signal.
[0072] In FIG. 4, the charge amplifier 404 and the readout
circuitry is illustrated as being located primarily in the
substrate 110. However, the charge amplifier and selected portions
of the readout circuitry may also be located underneath the
electroacoustic transducer, where electrical connections such as
via connections can be used to connect the charge amplifiers to the
sensing structures.
[0073] FIG. 5 is a schematic illustration of an example embodiment
of a fingerprint sensing device 500, where a delay layer 502 is
arranged between the electroacoustic transducer 112 and the array
of sensing structures 108. The purpose of the delay layer 502 is to
increase the time it takes for the generated acoustic wave to reach
the finger 104, and consequently to increase the time between the
generation of an acoustic wave to the generation of an acoustic
vibration potential in the finger. It is desirable that the delay
layer 502 has low acoustic attenuation to reduce the losses in
energy of the acoustic wave as it travels through the delay layer
502. It is also preferable that the acoustic impedance of the delay
layer 502 is similar to the acoustic impedance of the adjacent
layer, here the substrate 110, to reduce reflections at the
interface between the delay layer 502 and the substrate 110. The
delay layer 502 can comprise a plastic material, PMMA or a
dielectric material having known acoustic properties. For example,
the delay layer may be selected such that at least 10% of the
incident energy passes through the delay layer, preferably 50%, and
more preferably 90%. The advantages of the delay layer will be
discussed further below in relation to a method for controlling a
fingerprint sensing device.
[0074] FIG. 6 is a schematic illustration of an example embodiment
of a fingerprint sensing device 600. In the fingerprint sensing
device of FIG. 6, the stack of layers arranged over the array of
sensing structures 108 comprises an encapsulation layer 602, or a
cap layer, arranged to protect the sensing structures, and adhesive
layer 604 for attaching a protective plate 606 such as a cover
glass to the sensing device, and an outermost layer 608 which may
be a colored or patterned coating layer providing a desired
aesthetic appearance of the fingerprint sensor 600. Accordingly,
all of the aforementioned layers together comprise the dielectric
top layer 106. The skilled person realizes that the described stack
of layers may be varied in many different ways to form embodiments
not explicitly described herein. Moreover, the layers described
herein are typically continuous and substantially homogeneous
uniform layers.
[0075] The outermost layer 608 may also be a matching layer,
configured to match the acoustic impedance of the underlying layer,
here the protective plate 606, to the acoustic impedance of the
finger 104. The acoustic impedance of the matching layer may
advantageously be the geometric average of the acoustic impedance
of the finger and the acoustic impedance of the underlying layer.
By matching the acoustic impedances, the portion of the acoustic
wave which is reflected at the interface between the outermost
layer and the finger can be minimized, there by maximizing the
induced ultrasonic vibration potential.
[0076] FIG. 7A is a schematic illustration of a sensing device 102
comprising a single electroacoustic transducer 112 of the same size
as the array of sensing structures 108. In comparison, FIG. 7B is a
schematic illustration of a sensing device 700 comprising a
plurality of electroacoustic transducers 702a-b, where each
electroacoustic transducer 702a-b has an area corresponding to a
subarea of the array of sensing structures 108. Each
electroacoustic transducer can thus be controlled individually such
that only selected transducers are activated. For clarity, the
sensing devices in FIGS. 7A-B are illustrated without a top
layer.
[0077] FIG. 8A is a flow chart outlining the general steps of a
method for controlling a fingerprint sensing device according to
embodiments of the invention. In a fingerprint sensing device
according to any of the above described embodiments, the
electroacoustic transducer is activated 802, by providing a supply
voltage which is converted such that an acoustic wave is generated
804. Next the acoustic wave is transmitted 806 to the finger such
that an ultrasonic vibration potential is induced in the finger.
Once the ultrasonic vibration potential is induced, a fingerprint
image is captured 808 by reading out the capacitive coupling
between the finger and the sensing structures of the sensing
array.
[0078] Since the ultrasonic vibration potential is a periodic
potential, having a frequency corresponding to the frequency of the
acoustic wave, it is preferable to capture the fingerprint image
when the potential is at or near its maximum amplitude in the
regions of the finger closest to the sensing surface.
[0079] FIG. 8B is a flow chart outlining the general steps of a
method for controlling a fingerprint sensing device according to
embodiments of the invention. First, with the electroacoustic
transducer deactivated, an initial fingerprint image is captured
810. Next 812, the electroacoustic transducer is activated, an
acoustic wave is generated, transmitted into the finger, and a
second, main, fingerprint image is captured 814. The initial image
is compared 816 to the main image, and if the difference is larger
than a predetermined threshold 818, it is determined 820 that the
captured fingerprint originates from an authentic finger. If the
difference between the initial image and the main image is lower
than a predetermined threshold, it is determined 822 that the
captured fingerprint originates from a fake finger. It should also
be noted that the above described method could be combined with
other means for liveness detection for even further spoofing
protection.
[0080] FIG. 8C is a flow chart outlining the general steps of a
method for controlling a fingerprint sensing device according to
embodiments of the invention. In a first step 812, the
electroacoustic transducer is activated, an acoustic wave is
generated and transmitted into the finger. Next the electroacoustic
transducer is deactivated 824 and while the ultrasonic vibration
potential in the finger is detectable, a fingerprint image is
captured 826 by reading out a capacitive coupling between the
finger and the sensing structures. The described method is
advantageously employed in a sensing device 500 comprising a delay
layer 502, as illustrated in FIG. 5. By tuning the thickness and
acoustic properties of the delay layer, the delay layer can be
configured to delay the acoustic wave such that the electroacoustic
transducer is deactivated when the fingerprint image is captured,
while the ultrasonic vibration potential in the finger is still
detectable. Thereby, the distortion from an electromagnetic field
generated by the electroacoustic transducer can be avoided. In
practice, the fingerprint image is preferably captured as soon as
possible after the electroacoustic transducer is deactivated due to
the decay of the ultrasonic vibration potential.
[0081] The method described in relation to FIG. 8C can also be used
in combination with the method illustrated by FIG. 8B. Accordingly,
in the method for determining if the fingerprint is authentic or
fake, the main fingerprint image can be captured using the method
outlined in FIG. 8C, where a fingerprint is captured after the
electroacoustic transducer has been deactivated but while there
still is a detectable ultrasonic vibration potential in the
finger.
[0082] Moreover, it should be noted that even though the present
invention is described with reference to a capacitive sensing
device, the technique described herein utilizing an electroacoustic
transducer can be integrated in any type of sensing device capable
of directly or indirectly detecting an induced potential in the
finger. Such sensing devices include electric field sensing devices
and the like.
[0083] 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 fingerprint sensing device and method may be omitted,
interchanged or arranged in various ways, the fingerprint sensing
device yet being able to perform the functionality of the present
invention.
[0084] 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.
* * * * *