U.S. patent application number 14/997689 was filed with the patent office on 2016-12-08 for fingerprint sensing device with heterogeneous coating structure comprising an adhesive.
This patent application is currently assigned to Fingerprint Cards AB. The applicant listed for this patent is Fingerprint Cards AB. Invention is credited to Karl LUNDAHL.
Application Number | 20160358004 14/997689 |
Document ID | / |
Family ID | 56670872 |
Filed Date | 2016-12-08 |
United States Patent
Application |
20160358004 |
Kind Code |
A1 |
LUNDAHL; Karl |
December 8, 2016 |
FINGERPRINT SENSING DEVICE WITH HETEROGENEOUS COATING STRUCTURE
COMPRISING AN ADHESIVE
Abstract
A fingerprint sensing device comprises a sensing chip comprising
an array of capacitive sensing elements. The sensing device
comprises a coating material arranged in a layer on top of the
array of sensing elements, the coating material comprising a
plurality of cavities filled with an adhesive; wherein locations of
the cavities correspond to locations of the sensing elements, such
that a cross-section area of a cavity covers at least a portion of
an area of a corresponding sensing element; and wherein a
dielectric constant of the adhesive is higher than a dielectric
constant of the coating material; and a protective plate attached
to the sensing chip by means of the adhesive. Another sensing
device is disclosed, where the coating layer comprises trenches
filled with an adhesive, and where the coating has a higher
dielectric constant than the adhesive. Associated methods of
manufacturing are also disclosed.
Inventors: |
LUNDAHL; Karl; (Goteborg,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fingerprint Cards AB |
Goteborg |
|
SE |
|
|
Assignee: |
Fingerprint Cards AB
Goteborg
SE
|
Family ID: |
56670872 |
Appl. No.: |
14/997689 |
Filed: |
January 18, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G06K 9/00053 20130101;
G03F 7/16 20130101; G06K 9/0002 20130101; G06K 9/00 20130101 |
International
Class: |
G06K 9/00 20060101
G06K009/00; G03F 7/16 20060101 G03F007/16 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 8, 2015 |
SE |
1550748-6 |
Claims
1. A fingerprint sensing device comprising: a sensing chip
comprising an array of sensing elements, said sensing elements
being configured to be connected to readout circuitry for detecting
a capacitive coupling between each of said sensing elements and a
finger placed on a sensing surface of said sensing device; a
coating material arranged in a layer on top of said array of
sensing elements, said coating material comprising a plurality of
cavities filled with an adhesive; wherein locations of said
cavities correspond to locations of said sensing elements, such
that a cross-section area of a cavity covers at least a portion of
an area of a corresponding sensing element; and wherein a
dielectric constant of said adhesive is higher than a dielectric
constant of said coating material and a dielectric constant of said
coating material is in the range of 2-5; and a protective plate
attached to said sensing chip by means of said adhesive.
2. The fingerprint sensing device according to claim 1, wherein
said coating material comprises one cavity for each sensing
element.
3. The fingerprint sensing device according to claim 1, wherein a
dielectric constant of said adhesive is in the range of 5-100.
4. (canceled)
5. The fingerprint sensing device according to claim 1, wherein a
ratio between said dielectric constant of said adhesive and said
dielectric constant of said coating material is equal to or larger
than 2:1.
6. The fingerprint sensing device according to claim 1, wherein
said adhesive comprises filler particles having a dielectric
constant higher than an average dielectric constant of said
adhesive.
7. The fingerprint sensing device according to claim 6, wherein
said filler particles comprises a ferroelectric material, such as
barium titanate (BaTiO.sub.3).
8. The fingerprint sensing device according to claim 1, wherein
each of said cavities comprises at least one lateral opening
connecting said cavity to at least one adjacent cavity, enabling a
flow of said adhesive between adjacent cavities when depositing
said adhesive.
9. The fingerprint sensing device according to claim 1, wherein
said coating material is a photoresist.
10. A method for manufacturing a fingerprint sensing device, said
fingerprint sensing device comprising: a sensing chip comprising an
array of sensing elements, said sensing elements being configured
to be connected to readout circuitry for detecting a capacitive
coupling between each of said sensing elements and a finger placed
on a sensing surface of said sensing device; a coating material
arranged in a layer on top of said array of sensing elements, said
coating material comprising a plurality of cavities filled with an
adhesive; wherein locations of said cavities correspond to
locations of said sensing elements, such that a cross-section area
of a cavity covers at least a portion of an area of a corresponding
sensing element; and wherein a dielectric constant of said adhesive
is higher than a dielectric constant of said coating material and a
dielectric constant of said coating material is in the range of
2-5; and a protective plate attached to said sensing chip by means
of said adhesive, said method comprising; providing said sensing
chip; depositing said layer of said coating material covering said
array of conductive sensing elements; forming said plurality of
cavities in said coating material; providing said adhesive to fill
said cavities; and attaching said protective plate to said sensing
device by means of said adhesive.
11. The method according to claim 10, wherein said coating layer is
deposited by spin coating or by spray coating.
12. The method according to claim 10, further comprising plasma
cleaning of said coating material prior to the step of providing
said adhesive.
13. The method according to claim 10, wherein providing said
adhesive comprises dispensing a liquid adhesive on said layer of
coating material and in said cavities.
14. The method according to claim 10, wherein said coating material
is a photoresist, and wherein forming a plurality of cavities in
said coating material comprises patterning said layer of coating
material by photolithography.
15. A fingerprint sensing device comprising: a sensing chip
comprising an array of sensing elements, said sensing elements
being configured to be connected to readout circuitry for detecting
a capacitive coupling between each of said sensing elements and a
finger placed on a sensing surface of said sensing device; a
coating material arranged in a layer on top of said array of
sensing elements, said coating material comprising a plurality of
trenches filled with an adhesive; wherein said trenches are aligned
with areas between said sensing elements; and wherein a dielectric
constant of said adhesive is lower than a dielectric constant of
said coating material; and a protective plate attached to said
sensing chip by means of said adhesive.
16. A method for manufacturing a fingerprint sensing device, said
fingerprint sensing device comprising: a sensing chip comprising an
array of sensing elements, said sensing elements being configured
to be connected to readout circuitry for detecting a capacitive
coupling between each of said sensing elements and a finger placed
on a sensing surface of said sensing device; a coating material
arranged in a layer on top of said array of sensing elements, said
coating material comprising a plurality of trenches filled with an
adhesive; wherein said trenches are aligned with areas between said
sensing elements; and wherein a dielectric constant of said
adhesive is lower than a dielectric constant of said coating
material; and a protective plate attached to said sensing chip by
means of said adhesive, said method comprising; providing said
sensing chip; depositing said layer of said coating material
covering said array of sensing elements; forming said plurality of
trenches in said coating material; providing said adhesive to fill
said trenches; and attaching said protective plate to said sensing
device by means of said adhesive.
17. A fingerprint sensing device comprising: a sensing chip
comprising an array of sensing elements, said sensing elements
being configured to be connected to readout circuitry for detecting
a capacitive coupling between each of said sensing elements and a
finger placed on a sensing surface of said sensing device; a
coating material arranged in a layer on top of said array of
sensing elements, said coating material comprising a plurality of
cavities filled with an adhesive; wherein locations of said
cavities correspond to locations of said sensing elements, such
that a cross-section area of a cavity covers at least a portion of
an area of a corresponding sensing element; and wherein a
dielectric constant of said adhesive is higher than a dielectric
constant of said coating material and a ratio between said
dielectric constant of said adhesive and said dielectric constant
of said coating material is equal to or larger than 2:1; and a
protective plate attached to said sensing chip by means of said
adhesive.
18. The fingerprint sensing device according to claim 17, wherein
said coating material comprises one cavity for each sensing
element.
19. The fingerprint sensing device according to claim 17, wherein a
dielectric constant of said adhesive is in the range of 5-100.
20. The fingerprint sensing device according claim 17, wherein a
dielectric constant of said coating material is in the range of
2-5.
21. The fingerprint sensing device according to claim 17, wherein
said adhesive comprises filler particles having a dielectric
constant higher than an average dielectric constant of said
adhesive.
22. The fingerprint sensing device according to claim 21, wherein
said filler particles comprises a ferroelectric material, such as
barium titanate (BaTiO.sub.3).
23. The fingerprint sensing device according to claim 17, wherein
each of said cavities comprises at least one lateral opening
connecting said cavity to at least one adjacent cavity, enabling a
flow of said adhesive between adjacent cavities when depositing
said adhesive.
24. The fingerprint sensing device according to claim 17, wherein
said coating material is a photoresist.
25. A method for manufacturing a fingerprint sensing device, said
fingerprint sensing device comprising: a sensing chip comprising an
array of sensing elements, said sensing elements being configured
to be connected to readout circuitry for detecting a capacitive
coupling between each of said sensing elements and a finger placed
on a sensing surface of said sensing device; a coating material
arranged in a layer on top of said array of sensing elements, said
coating material comprising a plurality of cavities filled with an
adhesive; wherein locations of said cavities correspond to
locations of said sensing elements, such that a cross-section area
of a cavity covers at least a portion of an area of a corresponding
sensing element; and wherein a dielectric constant of said adhesive
is higher than a dielectric constant of said coating material and a
ratio between said dielectric constant of said adhesive and said
dielectric constant of said coating material is equal to or larger
than 2:1; and a protective plate attached to said sensing chip by
means of said adhesive, said method comprising; providing said
sensing chip; depositing said layer of said coating material
covering said array of conductive sensing elements; forming said
plurality of cavities in said coating material; providing an said
adhesive to fill said cavities; and attaching said protective plate
to said sensing device by means of said adhesive.
26. The method according to claim 25, wherein said coating layer is
deposited by spin coating or by spray coating.
27. The method according to claim 25, further comprising plasma
cleaning of said coating material prior to the step of providing
said adhesive.
28. The method according to claim 25, wherein providing said
adhesive comprises dispensing a liquid adhesive on said layer of
coating material and in said cavities.
29. The method according to claim 25, wherein said coating material
is a photoresist, and wherein forming a plurality of cavities in
said coating material comprises patterning said layer of coating
material by photolithography.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of Swedish Patent
Application No. 1550748-6 filed Jun. 8, 2015. The disclosure of the
above application is incorporated herein by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a coating structure for a
fingerprint sensor. In particular, the present invention related to
a heterogeneous coating structure for enhancing the performance in
a fingerprint sensor.
BACKGROUND OF THE INVENTION
[0003] 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.
[0004] 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.
[0005] 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.
[0006] 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.
[0007] 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 a sensing surface of
the device and the capacitive sensing elements. This in turn leads
to an image blurring effect. As a function of an increased distance
between a finger and any given pixel, each pixel is starting to
receive signals from areas that are not immediately located
vertically on top of said pixel resulting in image blurring
negatively impacting the capabilities of the sensors to resolve
fine features in a fingerprint.
[0008] In view of the above, it is desirable to improve the
capacitive coupling between a finger placed on the sensing surface
and the sensing elements.
[0009] 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
[0010] 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 provides an improved capacitive coupling between a finger
placed on a sensing surface and the sensing elements of the sensing
device.
[0011] 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; a coating
material arranged in a layer on top of the array of sensing
elements, the coating material comprising a plurality of cavities
filled with an adhesive; wherein locations of the cavities
correspond to locations of the sensing elements, such that a
cross-section area of a cavity covers at least a portion of an area
of a corresponding sensing element; and wherein a dielectric
constant of the adhesive is higher than a dielectric constant of
the coating material; and a protective plate attached to the
sensing chip by means of the adhesive.
[0012] 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.
[0013] 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.sub.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, also referred to as the sensing
surface.
[0014] The sensing device according to various embodiments of the
invention may be formed on a conventional rigid PCB substrate or it
may be implemented using a flexible type of substrate.
[0015] An improved capacitive coupling between a finger and a
sensing element can be achieved by forming a heterogeneous coating
layer where portions of the layer above the sensing elements have a
higher dielectric constant than surrounding portion, thereby
focusing the electric field towards the respective sensing element.
Furthermore, the present invention is based on the realization that
the adhesive used to attach a protective plate to the sensing
device can be used to achieve this effect by selecting or forming
an adhesive having a dielectric constant which is higher than the
surrounding material. Thereby, an improved capacitive coupling can
be achieved without substantial alterations of the material stack,
meaning that conventional manufacturing processes may be used.
[0016] That a cross-section area of a cavity covers at least a
portion of an area of a corresponding sensing element means should
be interpreted to mean that the cavity may or may not cover the
complete area of the sensing element. Moreover, it is not required
that the cavity is centered over the sensing element, although it
very well may be.
[0017] Furthermore, it is important to note that the cavity should
be understood as a cavity in the coating material, which is
subsequently filled with an adhesive.
[0018] The coating material may refer to any material which is
arranged to cover the sensing chip and in particular the sensing
elements. The coating material is often referred to as wafer
coating, and it may also function as an interposer structure.
[0019] According to one embodiment of the invention, the coating
material may comprise one cavity for each sensing element. Although
it is not strictly required that there is a 1:1 ratio of the number
of cavities to the number of sensing elements, this is most likely
how the greatest improvement in capacitive coupling can be
achieved. However, there may be instances where it is desirable to
only have cavities over some of the sensing elements. For example,
for various reasons it may be difficult to separate adjacent
cavities, in which case a pattern where cavities are only located
above a select number of sensing elements can be utilized.
[0020] According to one embodiment of the invention, the dielectric
constant of the adhesive may be in the range of 5-100 and the
dielectric constant of the coating material may be in the range of
2-5. The specified ranges are should be seen as exemplary ranges
providing the desired effect. The adhesive and the coating material
may have dielectric constants outside of the specified ranges
within the scope of various embodiments of the present
invention.
[0021] Furthermore, the ratio between the dielectric constant of
the adhesive and the dielectric constant of said coating material
may advantageously be selected to be equal to or larger than 2:1.
With respect to the focusing effect, it is the ratio between the
two dielectric constants which determines the amount of focusing,
where a higher ratio provides a better focus. It should be noted
that the above mentioned dielectric constants and ratio is merely
an example, and that a desired advantageous effect can be achieved
with in principle any ratio higher than 1, although the effect is
increasing with increasing ratio.
[0022] In one embodiment of the invention, the adhesive may
advantageously comprise filler particles having a dielectric
constant higher than an average dielectric constant of the
adhesive, which is one way of tailoring the average dielectric
constant of the adhesive. The filler particles may be referred to
as dielectric filler particles or high-k filler particles. Thereby,
the dielectric constant of the adhesive can be selected so that a
desirable ratio can be achieved for different choices of coating
material. Moreover, one and the same adhesive material can be used
while providing different dielectric constants depending on what is
required for a particular application. This simplifies the
manufacturing process since there is no need to adjust the process
for different adhesive.
[0023] According to one embodiment of the invention, the filler
particles may advantageously comprise a ferroelectric material,
such as barium titanate (BaTiO.sub.3). There are a range of
ferroelectric materials which have a high dielectric constant, and
which may be suitable for use as filler material. Other filler
particles may of course also be used, such as aluminum oxide
(Al.sub.2O.sub.3). One desirable property is that the filler
material should be possible to be provided in a form which may be
evenly mixed with an adhesive, and that the filler material does
not agglomerate in the adhesive since it is important that the
dielectric constant of the adhesive is at least approximately
homogeneous over the entire surface of the sensing device.
[0024] In one embodiment of the invention, each of the cavities may
advantageously comprise at least one lateral opening connecting the
cavity to at least one adjacent cavity, enabling a flow of the
adhesive between adjacent cavities when depositing the adhesive.
During manufacturing of the fingerprint sensing device, the
adhesive is typically provided in the form of a liquid adhesive
onto the coating structure comprising cavities. It is desirable to
achieve a homogeneous thickness distribution of the adhesive when
the protective plate is being attached to the sensing device, both
to provide good adhesion and to provide uniformity in measurements
over the entire sensing surface. By means of the lateral openings
in the coating layer, fluidly connecting adjacent cavities, the
adhesive can flow between the cavities to form an even distribution
as the protective plate is being places onto the sensing
device.
[0025] According to one embodiment of the invention, the coating
material may advantageously be a photoresist. By using a
photoresist, the cavities 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. Furthermore, a
photoresist can be deposited on a full wafer with a high degree of
accuracy and thickness uniformity, using for example spin coating
or spray coating.
[0026] 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; depositing a layer of a
coating material covering the array of conductive sensing elements;
forming a plurality of cavities in the coating material, wherein
locations of the cavities correspond to locations of the sensing
elements such that a cross-section area of a cavity covers at least
a portion of an area of a corresponding sensing element; providing
an adhesive to fill the cavities, the adhesive having a dielectric
constant higher than a dielectric constant of the coating material;
and attaching a protective plate to the sensing device by means of
the adhesive.
[0027] The coating material is preferably arranged in a homogeneous
layer on the sensing chip to cover the sensing elements.
[0028] According to one embodiment of the invention the coating
layer may advantageously be deposited by spin coating or by spray
coating, which can be done on a full wafer thereby providing a
large-scale efficient process. Using spin coating or spray coating
also allows the process to be easily modified with respect to the
desired thickness of the coating layer.
[0029] In one embodiment of the invention the, method may further
comprise plasma cleaning of the coating material prior to the step
of providing the adhesive. The plasma cleaning of the surface of
the coating material provides a surface with improved adhesion to
the adhesive. Thereby, a better adhesion between the sensing chip
and the sensing chip and the protective plate is achieved.
[0030] According to one embodiment of the invention, providing the
adhesive may advantageously comprise dispensing a liquid adhesive
on the layer of coating material and in said cavities. A liquid
adhesive is advantageous in that the cavities are easily filled and
in that a homogeneous thickness distribution can be achieved.
However, it is equally possible to deposit an adhesive by spin
coating or spray coating or in the form of a film. Moreover, the
adhesive may be deposited on a full wafer comprising a plurality of
sensing chips, or the adhesive may be deposited in a single sensing
chip after dicing of the wafer.
[0031] According to one embodiment of the invention, the coating
material may advantageously be a photoresist, and forming a
plurality of cavities in the coating material may then comprise
patterning the layer of coating material by means of
photolithography.
[0032] Additional advantages, 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.
[0033] According to a third 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; a coating
material arranged in a layer on top of the array of sensing
elements, the coating material comprising a plurality of trenches
filled with an adhesive; wherein the trenches are aligned with
areas between the sensing elements; and wherein a dielectric
constant of the adhesive is lower than a dielectric constant of the
coating material; and a protective plate attached to the sensing
chip by means of the adhesive.
[0034] The trenches in the coating material can be considered to
follow the alignment of the border between sensing elements.
Typically, the sensing elements are arranged in a square array with
a certain pitch, here defined as the center-to-center distance of
the sensing elements, where the pitch is larger than the size of
the sensing element, thereby forming an unoccupied area between
adjacent sensing elements.
[0035] By providing an adhesive in the trenches, where the
dielectric constant of the adhesive is lower than a dielectric
constant of the coating material, a heterogeneous coating layer is
provided and the focusing effect discussed in relation to the first
aspect of the invention is achieved.
[0036] Additional advantages, effects and features of the third
aspect of the invention are largely analogous to those described
above in connection with the first aspect of the invention.
[0037] According to a fourth 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; depositing a layer of a
coating material covering the array of sensing elements; forming a
plurality of trenches in the coating material, wherein the trenches
are aligned with areas between the sensing elements; providing an
adhesive to fill the trenches, the adhesive having a dielectric
constant lower than a dielectric constant of the coating material;
and attaching a protective plate to the sensing device by means of
the adhesive.
[0038] Additional advantages, effects and features of the fourth
aspect of the invention are largely analogous to those described
above in connection with the first, second and third 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 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
[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 handheld electronic
device comprising a fingerprint sensing device according to an
embodiment of the invention;
[0042] FIGS. 2a-b schematically illustrate a fingerprint sensing
device according to an embodiment of the invention;
[0043] FIGS. 3a-b schematically illustrate a fingerprint sensing
device according to embodiments of the invention;
[0044] FIG. 4 is a flow chart outlining the general steps of a
method for manufacturing a fingerprint sensing device according to
an embodiment of the invention;
[0045] FIGS. 5a-c schematically illustrate a method for
manufacturing 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] FIG. 7 is a flow chart outlining the general steps of a
method for manufacturing a fingerprint sensing device according to
an embodiment of the invention;
[0048] FIGS. 8a-c schematically illustrate a method for
manufacturing a fingerprint sensing device according to an
embodiment of the invention;
[0049] FIG. 9 schematically illustrates a fingerprint sensing
device according to an embodiment of the invention; and
[0050] FIGS. 10a-b schematically illustrate details of a
fingerprint sensing device according to embodiments of the
invention.
DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS
[0051] 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.
[0052] 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.
[0053] FIG. 2a is a schematic illustration of a fingerprint sensing
device 200 according to an embodiment of the invention. The
exterior surface 201 of the sensing device 200 is referred to as
the sensing surface, since that is the surface where a finger will
be placed for capturing a fingerprint image. The fingerprint
sensing device is based on a sensing chip 202 comprising an array
of sensing elements 204. The sensing elements 204 are here shown
arranged in a square array, the sensing elements having a size of
about 50.times.50 .mu.m and a distance between adjacent elements is
about 5 .mu.m. 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 201 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 201.
[0054] A coating material 205 is arranged in a layer on top of the
array of sensing elements 204, and the coating material comprises a
plurality of cavities 206 which are filled by an adhesive 208 which
is used to attach the protective plate 210 to the sensing chip. The
protective plate 210 may for example be a sapphire plate having a
thickness in the range of 100-1000 .mu.m. The protective plate may
also be the cover glass in a handheld 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.
[0055] The cavity 206 may also be referred to as an opening, or a
recess, in the coating material 205. The purpose of the cavities
206 is to allow an adhesive to be arranged directly above the
sensing elements 204, so that the adhesive 208 is arranged between
the sensing element 204 and the sensing surface 201. The adhesive,
which has a dielectric constant that is higher than a dielectric
constant of the coating material, will then act as a focusing
element helping to focus the electromagnetic field lines between a
finger and the sensing element 204 towards the sensing element 204.
This effect is further illustrated in FIG. 2b showing a side view
of the fingerprint sensing device 200 where a ridges and valleys of
a finger 212 are located on the sensing surface 201. It can be seen
that the field lines 214 originating in a position on the sensing
surface not located directly above a sensing element 204 are curved
towards the cavities in the coating comprising the adhesive 208 due
to the higher dielectric constant of the adhesive 208. Moreover,
the coating material having a lower dielectric constant than the
dielectric act as a blocking structure in order to reduce or
prevent field lines from a fingerprint ridge from reaching a
sensing element 204 not located directly beneath the ridge.
Accordingly, the patterned coating layer helps to prevent blurring
of a captured image, since the non-vertical coupling between the
finger and sensing elements is reduced. In FIG. 2b, the field is
lower in the coating material compared to in the adhesive, due to
the difference in dielectric constant.
[0056] In principle, it is the ratio between the dielectric
constants of the coating material and the adhesive which determines
the distribution of the field lines. Already a ratio of 2:1
provides an advantageous effect, whereas a ratio in the range of
1:10 to 1:20 is more preferable. The dielectric constants of the
materials discussed herein are the average relative dielectric
constants of the material. The respective materials may for example
be compositions and comprise particles having individually
different dielectric constants, which together with the bulk
material provide a resulting average dielectric constant. For
example, an adhesive with an increased dielectric constant can be
achieved by using a conventional adhesive and add particles of a
ferroelectric material such as barium titanate (BaTiO3) which in
itself has a dielectric constant above 1000. By selecting the type
and concentration of the added material, and adhesive, and also a
coating material, can be tailored to have the desired dielectric
constant within a reasonable range, such as between 2 and 100. The
resulting dielectric constant .di-elect cons..sub.eff for a mixture
of components having different dielectric constants .di-elect
cons..sub.1, .di-elect cons..sub.2, can be determined according to
the Lichtenecker model as
log .di-elect cons..sub.eff=v.sub.1 log .di-elect
cons..sub.1+v.sub.2 log .di-elect cons..sub.2
where v.sub.1 and v.sub.2 are empirically determined constants.
[0057] From FIGS. 2a and 2b it can also be seen that the adhesive
208 has the same thickness as the coating layer 205, so that the
protective plate 210 rests on the coating layer 205. This has the
advantageous effect that the protective plate 210 can be arranged
to rest on a surface which is substantially even over the area of
the sensing chip. This is a result of the good thickness uniformity
which can be achieved when depositing the coating layer 205, for
example using spin coating. It is of course also possible that the
adhesive 208 may reach slightly higher than the depth of the
cavities 206, so that all or major portions of the area of the
sensing chip is covered by the adhesive 208. This will provide
improved adhesion between the sensing chip and the protective plate
210 with only marginal influence on the capacitive coupling between
the finger and the sensing element 204.
[0058] FIG. 3a is a schematic illustration of a fingerprint sensing
device according to an embodiment of the invention where adjacent
cavities 206 in the coating material are connected via channels
306, or openings 306, in the side walls of the cavities. The
channels 306 allow a liquid adhesive to flow between cavities
during deposition of the adhesive, as will be discussed in further
detail in relation the method for manufacturing a fingerprint
sensing device. The openings 306 between adjacent cavities are
configured to be larger than the particle size of any filler
particles present in the adhesive 208, so that the adhesive can
flow freely between the cavities without the risk of filler
particles clogging the openings. Preferably, the openings have a
size larger than a maximum size of the filler particles. A typical
maximum particle size may be in the range of 1-3 .mu.m for
ferroelectric particles such as BaTiO.sub.3 particles. However,
filler particles having a high dielectric constant may also be
provided in the form of nanoparticles having a sub-.mu.m diameter.
Accordingly, the openings 306 between adjacent cavities can be
selected based on the size of the filler particles and based on the
method for patterning the coating layer, and a practical size of
the openings 306 may be in the range of 5-10 .mu.m. Furthermore,
the adhesive may comprise additional filler particles in order to
tailor parameters such as the viscosity and the thermal expansion
coefficient of the adhesive. The openings may be adapted to have a
size larger than a maximum size of also such filler particles.
However, it is prioritized to ensure that dielectric particles
influencing the dielectric constant of the adhesive can flow freely
so that a homogeneous dielectric constant can be achieved in the
adhesive over the full area of the sensing chip.
[0059] FIG. 3b is a schematic illustration of a fingerprint sensing
device according to an embodiment of the invention where openings
308 connecting adjacent cavities 206 in the coating material are
located at the corners of the sensing elements 204. It should be
understood that the openings connecting adjacent cavities may be
configured in many different ways to achieve the desired effect of
allowing the adhesive to flow between adjacent cavities.
[0060] FIG. 4 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. 5a-c.
[0061] First, in step 402, a sensing chip 202 is provided and a
coating layer is deposited 404 onto the sensing chip 202. The
coating layer typically has a uniform thickness and is arranged to
cover the entire area of the sensing chip. The coating layer can
for example be a photoresist deposited by spin-coating, and the
photoresist may be either a positive or a negative photoresist.
Moreover, spin- and spray-coating typically provides a homogeneous
thickness of the coating layer which simplifies subsequent adhesion
of the protective plate.
[0062] Cavities 206 are formed 406 in the coating layer 205 by
means of conventional photolithography and subsequent development
to form cavities having the desired shape and distribution, as
exemplified in FIG. 5a. Typically, the cavities are configured to
reach through the coating layer to expose the sensing element.
Moreover, the sensing element may be covered by a silicon
nitride-based passivation layer (not shown) which is well known in
the field of CMOS-processing. However, a certain small thickness of
the coating material remaining in the cavities would not
substantially influence the overall properties of the sensing
device 200. In general, each cavity 206 is centered above a
corresponding sensing element 204, having the same shape as the
sensing element 204, and the size of the cavity is preferably as
close as possible to the size of the sensing element 204. However,
the remaining side walls between cavities must be sufficiently
thick so as to maintain structural stability. As an example, for
sensing elements having a size of 50.times.50 .mu.m, the coating
layer has a thickness of approximately 30 .mu.m and the cavities
preferably have a size in the range of 30.times.30 to 40.times.40
.mu.m.
[0063] After forming the cavities, the coating layer may be treated
in a plasma cleaning process in order to improve adhesion between
the coating and the subsequently deposited adhesive. The plasma
cleaning may for example comprise oxygen mixed with an inert gas
such as nitrogen or argon.
[0064] As a next step, a liquid adhesive 208 is provided 408 by
dispensing the adhesive 208 onto the coating layer 205 so that the
adhesive 208 fills the cavities, as illustrated in FIG. 5b.
[0065] In the final step as illustrated in FIG. 5c, a protective
plate 210 is attached 410 to the sensing device by means of the
adhesive 208. After the adhesive has been applied on the wafer,
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.
[0066] FIG. 6 is a schematic illustration of a fingerprint sensing
device 600 according to another embodiment of the invention. The
fingerprint sensing device is based on a sensing chip 202
comprising a square array of sensing elements 204. In many aspects,
the sensing device 600 of FIG. 6 is similar to the sensing device
of FIG. 2a. However, the sensing device 600 comprises a coating
layer 602 having a plurality of trenches 604 filled with an
adhesive 606. The trenches 604 are aligned with areas between the
sensing elements 204. Moreover, the dielectric constant of the
adhesive 606 is lower than a dielectric constant of the material of
the coating layer 602. The coating 602, will then act as a focusing
element helping to focus the electromagnetic field lines between a
finger and the sensing element 204 towards the sensing element 204
in a similar manner as discussed in relation to FIGS. 2a and 2b.
Moreover, the dielectric constant of the coating can be tailored
using dielectric filler particles in the same manner as discussed
above for the adhesive.
[0067] FIG. 7 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. 8a-c.
[0068] First, in step 702, a sensing chip 202 is provided and next
a coating layer is deposited 704 onto the sensing chip 202. The
coating layer typically has a uniform thickness and is arranged to
cover the entire area of the sensing chip including the sensing
elements 204. The coating layer can for example be a photoresist
deposited by spin-coating, and the photoresist may be either a
positive or a negative photoresist. In order to achieve a coating
material having a dielectric constant higher than the dielectric
constant of the adhesive material, filler particles may be mixed
with the coating material. The filler particles can be similar to
the filler particles discussed above in relation to the embodiment
illustrated by FIGS. 2a-b.
[0069] Trenches 604 are formed 706 in the coating layer by means of
conventional photolithography and subsequent development to form
trenches having the desired shape and orientation, as exemplified
in FIG. 8a. In general, trenches are aligned with areas between the
sensing elements 204. The remaining coating 602 thus form square
structures arranged on top of and aligned with the sensing elements
204.
[0070] After forming the trenches in the coating layer, the
adhesive 606 is provided 708, for example by dispensing a liquid
adhesive, so that the adhesive 606 fills the trenches 604.
[0071] Finally, the protective plate 210 is attached 710 to the
sensing chip by means of the adhesive 606 so that the exterior
surface 201 of the protective plate acts as a sensing surface of
the fingerprint sensing device 600, as illustrated in FIG. 8c.
[0072] FIG. 9 schematically illustrates a fingerprint sensing
device 900 according to an embodiment of the invention. In most
respects, the sensing device 900 is similar to the sensing device
illustrated in FIG. 2a. However, in the sensing device of FIG. 9,
the cavities 901 are smaller, meaning that the side walls 902
surrounding the cavities 901 are thicker, and that they extend out
over a portion of the sensing elements 204. In order to ensure
sufficient structural stability of the side walls 902, it may be
desirable to have side walls 902 which are thicker than the
distance between adjacent elements. Furthermore, the advantageous
effects relating to the higher dielectric constant of the adhesive
in the cavities remains also for smaller cavities, although the
effect is approximately proportional to the size of the
cavities.
[0073] The above example embodiments have been described using a
photoresist as the coating layer. However, various advantages of
the present inventive concept are achievable using another coating
material. For example, the coating material may comprise a
deposited hard mask which is subsequently patterned by for example
deep reactive ion etching (DRIE).
[0074] FIG. 10a is a schematic illustration of a sensing element
204 of a sensing device. Here, a rectangular cuboid structure 910
representing either coating or adhesive according to the various
embodiments discussed above is arranged on the sensing element 204.
In FIG. 10b, a cylindrical structure 920 representing either
coating or adhesive according to the various embodiments discussed
above is arranged on the sensing element 204. FIGS. 10a-b are meant
to illustrate that the portion located above the sensing element,
and which has a higher dielectric constant that the dielectric
constant of a surrounding material, may in principle have an
arbitrary shape. The shape may for example be selected based on
what is most desirable from a manufacturing perspective.
[0075] 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.
[0076] 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.
[0077] 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 measured cannot be used to
advantage.
* * * * *