Field of the Invention

The present invention relates to a fingerprint sensing system for sensing a finger surface of a finger, and to an electronic device comprising such a fingerprint sensing system.

Background of the Invention

Fingerprint sensing systems are widely used as means for increasing the convenience and security of electronic devices, such as mobile phones etc. In various electronic devices having a display, it may be desirable to provide for fingerprint sensing within the area occupied by the display. It may also be desirable to provide for fingerprint sensing across a relatively large area of the display. Also for other applications, such as smart cards, it would be desirable to provide for a larger fingerprint sensor without increasing cost.

A fingerprint sensing system suitable for such applications may have a configuration with a plurality of conductive row lines arranged in parallel to each other; a plurality of conductive column lines arranged in parallel to each other and crossing the row lines; row circuitry coupled to each row line in the plurality of row lines, and controllable to provide signals on at least a subset of the row lines; column circuitry coupled to each column line in the plurality of column lines, and controllable to read out signals from at least a subset of the column lines; and an array of pixel elements arranged in rows and columns, each pixel element being formed at an intersection between a respective set of the row lines including at least a first control row line, and a respective set of the column lines including a readout column line.

This sensor configuration can be at least partly realized using relatively cost-efficient materials and technologies, such as TFT (thin film transistor) technology on a glass or plastic substrate. TFT-panels in general typically include control signal providing circuitry in the form of so-called gate-on-array (GOA) drivers to allow selection of an active row without having one input pin per row. Such existing GOA-drivers, however, generally occupy a rather large surface area and are also inflexible (typically only capable of generating one pulse per row and then proceed to the next row). This may be acceptable for display panel applications etc., but is undesirable for a fingerprint sensing system, which may be considerably smaller and may also require more flexible control.

Summary

It is an object of the present invention to provide an improved fingerprint sensing system, in particular a fingerprint sensing system that is more cost-efficient.

According to the present invention, it is therefore provided a fingerprint sensing system for sensing a finger surface of a finger, comprising: a plurality of row lines arranged in parallel to each other; a plurality of column lines arranged in parallel to each other and crossing the row lines; row circuitry coupled to each row line in the plurality of row lines, and controllable to provide signals on at least a subset of the row lines; column circuitry coupled to each column line in the plurality of column lines, and controllable to read out signals from at least a subset of the column lines; and an array of pixel elements arranged in rows and columns, each pixel element being formed at an intersection between a respective set of the row lines including at least a first control row line, and a respective set of the column lines including a readout column line, each pixel element in the plurality of pixel elements being controllable, by a first control signal on the first control row line in the respective set of the row lines, and configured to provide to the read-out line in the respective set of the column lines, a sensing signal indicative of a distance between the pixel element and the finger surface, wherein the row circuitry comprises, for each row of pixel elements, control signal providing circuitry including: a first control signal input for receiving a first control signal common to a plurality of rows of pixel elements; a first control signal output connected to the first control row line; a first controllable switch between the first control signal input and the first control signal output, the first controllable switch having a first switch control terminal to allow control of the first controllable switch from a non-conductive state in which the first control signal input and the first control signal output are conductively separated to a conductive state in which the first control signal input and the first control signal output are conductively connected, through application of a voltage having a magnitude greater than a predefined first threshold magnitude on the first switch control terminal; at least a first bias voltage input for receiving a first bias voltage; a second controllable switch between the first bias voltage input and the first switch control terminal, the second controllable switch having a second switch control terminal to allow control of the second controllable switch from a non-conductive state in which the first bias voltage input and the first switch control terminal are conductively separated to a conductive state in which the first bias voltage input and the first switch control terminal are conductively connected; a second control signal input coupled to the second switch control terminal; and a first controllable capacitor coupled between the first control signal input and the first switch control terminal, the first controllable capacitor being controllable between: a first capacitance being sufficiently high for a sum of the first bias voltage and a coupling voltage resulting from capacitive coupling by the first controllable capacitor of the first control signal to have a magnitude greater than the predefined first threshold magnitude; and a second capacitance, lower than the first capacitance, being sufficiently low for the coupling voltage resulting from capacitive coupling by the first controllable capacitor of the first control signal to have a magnitude less than the predefined first threshold magnitude.

It should be understood that the above-mentioned first threshold magnitude is in relation to a reference potential of the fingerprint sensing system, such as ground or a negative or positive supply potential.

The fingerprint sensing system defined above can provide one control signal per row of pixel elements. For pixel element designs requiring additional independent control signals, the control signal providing circuitry can easily be expanded by duplicating the inputs, outputs, switches, and controllable capacitor for each row of pixel elements.

Each pixel element in the array of pixel elements may be responsive to a physical property that differs in dependence on the topography of the finger surface. Examples of such physical properties include capacitive coupling, mechanical coupling, thermal coupling, and optical reflection. As is well known to those of ordinary skill in the art, various pixel element configurations exist, that are suitable for sensing these physical properties indicative of the interaction between the finger and the pixel element. In the case of capacitive coupling, the pixel element may, for example, include a conductive plate where charge can be accumulated; in the case of mechanical coupling, the pixel element may, for example, have piezo-electric properties; in the case of thermal coupling, the pixel element may, for example, include a resistor or other circuit element that can be controlled to generate heat; and in the case of optical reflection, the pixel element may, for example, include a photodiode that generates a photocurrent indicative of an amount of incident light.

The present invention is based on the realization that selective output of a control signal to a row of pixel elements can be achieved by combined action of two control signals and a bias voltage input, using a controllable capacitor. Compared to an existing GOA-driver, this operational principle requires less circuitry and may provide for improved controllability.

According to various embodiments, the control signal providing circuitry, for each row of pixel elements, may comprise a second controllable capacitor coupled between the first switch control signal terminal and a first reference potential of the fingerprint sensing system, the second controllable capacitor being controllable between a first capacitance and a second capacitance, lower than the first capacitance; and the second controllable capacitor may be configured to be controlled to its second capacitance when the first bias voltage has a magnitude greater than the predefined second threshold magnitude and the second controllable switch is controlled to its conductive state by the second control signal.

The first reference potential may be provided by a separate signal line, which may be common to the control signal providing circuitry of all rows, or it may be achieved by sampling a signal line, such as a first control signal line providing the first control signal input, at a suitable sampling time/suitable sampling times.

Through the provision of the second controllable capacitor, the robustness of the control signal providing circuitry can be improved, so that the risk of unwanted “leakage” of the first control signal through the first controllable switch can be reduced.

For automatic progression through the sensor array, the second control signal input of the control signal providing circuitry, for each row of pixel elements, may be connected to a control signal output of control signal providing circuitry for another row of pixel elements. This allows the output from the other row to pre-bias the first controllable switch control terminal, by passing the first bias voltage to the first controllable switch control terminal. The next pulse on first control signal input will then be allowed to pass to the first control signal output of the control signal providing circuitry. In embodiments where the first control signal output of the immediately preceding row is coupled to the second control signal input, the rows of pixel elements will be sequentially activated.

According to embodiments, the first controllable capacitor may be a MOSFET-structure, with its gate connected to one of the first control signal input and the first switch control terminal, and its source and drain connected to the other one of the first control signal input and the first switch control terminal.

Analogously, in embodiments comprising the above-mentioned second controllable capacitor, the second controllable capacitor may be a MOSFET- structure, with its gate connected to one of the first reference potential and the first switch control terminal, and its source and drain connected to the other one of the first reference potential and the first switch control terminal. In embodiments where the MOSFET-structure is NMOS, it would be preferable to use the (positive) supply potential as the first reference potential, and in embodiments where the MOSFET-structure is PMOS, it would be preferable to use the ground potential as the first reference potential.

The fingerprint sensing system according to embodiments of the present invention may advantageously be implemented using TFT- technology, providing for a cost-efficient fingerprint sensing system exhibiting a large sensing area.

For improved performance, some functionality of the fingerprint sensing system may be provided using CMOS technology, advantageously in the form of an ASIC coupled to a TFT-module including at least the row lines, the column lines, the pixel elements, and the row circuitry. In particular, at least a portion of the column circuitry may be comprised in a CMOS- component, such as an ASIC, which may provide the advantage of more compact circuitry with more well-controlled and less temperature-sensitive properties.

The fingerprint sensing system according to embodiments of the present invention may be included in an electronic device further comprising processing circuitry coupled to the fingerprint sensing system, and configured to perform an authentication based on the sensing signal provided on the readout column line for each column of pixel elements.

In summary, the present invention thus relates to a fingerprint sensing system comprising a plurality of conductive row lines; a plurality of conductive column lines crossing the row lines; row circuitry controllable to provide signals on at least a subset of the row lines; and an array of pixel elements formed at intersections between the row lines and the column lines. The row circuitry comprises, for each row of pixel elements, control signal providing circuitry controllable to allow passage of a control signal through the control signal providing circuitry using capacitive coupling of the control signal via a first controllable capacitor.

Brief Description of the Drawings

These and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing example embodiments of the invention, wherein:

Fig 1 is an illustration of an exemplary electronic device, in the form of a mobile phone, comprising a fingerprint sensing system according to an embodiment of the present invention;

Fig 2 is a schematic block diagram of the electronic device in fig 1 ;

Fig 3 schematically illustrates a fingerprint sensing system according to an example embodiment of the present invention; Fig 4 is a schematic illustration of row circuitry according to a first example configuration, including control signal providing circuitry for each row of pixel elements;

Fig 5 is a functional illustration of the control signal providing circuitry in fig 4;

Fig 6 schematically shows an example embodiment of the control signal providing circuitry in fig 4;

Fig 7 is a schematic illustration of row circuitry according to a second example configuration, including control signal providing circuitry for each row of pixel elements; and

Fig 8 schematically shows an example embodiment of the control signal providing circuitry in fig 7.

Detailed Description of Example Embodiments

Fig 1 is an illustration of an exemplary electronic device comprising a fingerprint sensing system according to an embodiment of the present invention, in the form of a mobile device 1 . The mobile device 1 in fig 1 has an integrated in-display fingerprint sensing system 3 and a display panel 5 with a touch screen interface 7. The fingerprint sensing system 3 may, for example, be used for unlocking the mobile device 1 and/or for authorizing transactions carried out using the mobile device 1 , etc.

The fingerprint sensing system 3 is here shown to be smaller than the display panel 5, but still relatively large, e.g. a large area implementation, In another advantageous implementation the fingerprint sensing system 3 may be the same size as the display panel 5, i.e. a full display solution. Thus, in such a case the user may place his/her finger anywhere on the display panel for biometric authentication. The fingerprint sensing system 3 may in other possible implementations be smaller than the depicted fingerprint sensing system, such as providing a hot-zone implementation.

Preferably and as is apparent to the skilled person, the mobile device 1 shown in fig 1 may further comprise a first antenna for WLAN/Wi-Fi communication, a second antenna for telecommunication communication, a microphone, a speaker, and a phone control unit. Further hardware elements are of course possibly comprised with the mobile device.

Fig 2 is a schematic block diagram of the electronic device 1 in fig 1 . The electronic device 1 comprises a transparent display panel 5 and a fingerprint sensing system 3, which may comprise light-sensitive pixel elements, conceptually illustrated to be arranged under the transparent display panel 5. Furthermore, the electronic device 1 comprises processing circuitry such as control unit 9 coupled to the fingerprint sensing system 3, and configured to perform an authentication based on sensing signals provided by the fingerprint sensing system 3. The control unit 9 may be standalone control unit of the electronic device 9, e.g. a device controller. Alternatively, the control unit 9 may be comprised in the fingerprint sensing system 3.

It should be noted that the above-described mobile device 1 is only one non-limiting example of an electronic device comprising the fingerprint sensing system 3 according to embodiments of the present invention. Embodiments of the fingerprint sensing system 3 according to the present invention may beneficially be included in other kinds of electronic devices, such as smart watches, laptops, tablet computers, and smart cards etc. Depending on the electronic device, and the chosen integration of the fingerprint sensing system 3, the fingerprint sensing system 3 may comprise pixel elements configured for different sensing principles. For instance, it may be beneficial with a fingerprint sensing system 3 using capacitive sensing in a smart card application.

A fingerprint sensing system 3 according to an example embodiment of the present invention will now be described with reference to fig 3. As is schematically indicated in fig 3, the fingerprint sensing system 3 comprises structures formed on a substrate or carrier 11 . In particular in embodiments where the fingerprint sensing system 3 is primarily manufactured using TFT- techniques, which are perse known to those skilled in the art, the carrier 11 may advantageously be made of glass or plastic. On the substrate 11 are formed a plurality of conductive row lines 13 and a plurality of conductive column lines 15. The row lines 13 are arranged in parallel to each other, and the column lines 15 are arranged in parallel to each other and crossing the row lines 13. The row lines 13 are conductively separated from the column lines 15, typically by a dielectric layer deposited between a first conductive layer including the row lines 13 and a second conductive layer including the column lines 15.

The fingerprint sensing system 3 additionally comprises row circuitry 17 coupled to each row line 13 in the plurality of row lines, and column circuitry 19 coupled to each column line 15 in the plurality of column lines. The row circuitry 17 is controllable to provide signals on at least a subset of the row lines 13, and the column circuitry 19 is controllable to read out signals from at least a subset of the column lines 15.

As is further schematically indicated in fig 3, the fingerprint sensing system 3 comprises an array of pixel elements 21 (only one of these is indicated by a reference numeral in fig 3 to avoid cluttering the drawings). Each pixel element 21 is formed at an intersection between a respective set of the row lines 13, including at least a first control row line, and a respective set of the column lines 15, including a readout column line.

Each pixel element 21 in the plurality of pixel elements is controllable by the row circuitry 17 and configured to provide to its readout column line, a sensing signal indicative of a distance between the particular pixel element 21 and the finger surface. Based on the sensing signals from the pixel elements 21 , a fingerprint representation can be formed, which can be used to authenticate the user.

The configuration of the row circuitry 17 in the fingerprint sensing system according to embodiments of the present invention will now be described in greater details with the aid of enlarged schematic views, in fig 4 and fig 7, of a portion 23 of the fingerprint sensing system 3. Reference will also be made to the functional electronic circuit configurations in fig 5, fig 6, and fig 8.

Fig 4 is a schematic illustration of row circuitry 17 according to a first example configuration, including control signal providing circuitry 25 for each row of pixel elements 21 . Referring to fig 4, the control signal providing circuitry 25 has a first control signal input 27, a first control signal output 29, a first bias voltage input 31 , and a second control signal input 33. In the first example configuration of fig 4, the second control signal input 33 is coupled to the first control signal output of the control signal providing circuitry for the preceding row. Furthermore, the first control signal input 27 of the control signal providing circuitry for each row of pixel elements 21 is provided on a common first control signal input line 35, and the first bias voltage input 31 of the control signal providing circuitry for each row of pixel elements 21 is provided on a common first bias voltage input line 37.

Fig 5 is a functional illustration of the control signal providing circuitry 25 in fig 4. Referring to fig 5, the control signal providing circuitry 25 comprises a first controllable switch 39, a second controllable switch 41 , a first controllable capacitor 43, and an optional second controllable capacitor 45.

As can be seen in fig 5, the first controllable switch 39 is coupled between the first control signal input 27 and the first control signal output 29. The first controllable switch 39 has a first switch control terminal 47 to allow control of the first controllable switch 39 from a non-conductive state in which the first control signal input 27 and the first control signal output 29 are conductively separated, to a conductive state in which the first control signal input 27 and the first control signal output 29 are conductively connected. The first controllable switch 39 is configured in such a way that it can be controlled from the non-conductive state to the conductive state through application of a voltage having a magnitude greater than a predefined first threshold magnitude on the first switch control terminal 47.

With continued reference to fig 5, the second controllable switch 41 is coupled between the first bias voltage input 31 and the first switch control terminal 47. The second controllable switch 41 has a second switch control terminal 49 to allow control of the second controllable switch 41 from a non- conductive state in which the first bias voltage input 31 and the first switch control terminal 47 are conductively separated, to a conductive state in which the first bias voltage input 31 and the first switch control terminal 47 are conductively connected. As can be seen in fig 5, the second switch control terminal 49 is coupled to the second control signal input 33 of the control signal providing circuitry 25.

The first controllable capacitor 43 is coupled between the first control signal input 27 and the first switch control terminal 47, and is controllable between a first capacitance C11 and a second capacitance C12, lower than the first capacitance C11 . The first controllable capacitor 43 is configured such that the first capacitance C11 is sufficiently high for a sum of the first bias voltage on the first bias voltage input 31 and a coupling voltage resulting from capacitive coupling by the first controllable capacitor 43 of the first control signal on the control signal input 27 to have a magnitude greater than the predefined first threshold magnitude needed to control the first controllable switch 39 from its non-conductive state to its conductive state. The controllable capacitor 43 is further configured such that the second capacitance C12 is sufficiently low for the coupling voltage resulting from capacitive coupling by the first controllable capacitor 43 of the first control signal on the control signal input 27 to have a magnitude less than the predefined first threshold magnitude.

This configuration allows a control signal pulse on the first control signal input 27 to control passage of itself to the first control signal output 29, provided that the first switch control terminal 47 has been pre-biased by a first bias voltage having a magnitude greater than a predefined second threshold magnitude on the first bias voltage input 31 . This can be achieved when the second controllable switch 41 is controlled to its conductive state by the second control signal on the second control signal input 33. If the first switch control terminal 47 has not been pre-biased, the first controllable switch will remain non-conductive, even if there is a control signal pulse on the first control signal input 27. This principle of operation provides for a very spaceefficient and compact circuit configuration.

For improved performance, the control signal providing circuitry 25 may be provided with a second controllable capacitor 45 coupled between the first switch control signal terminal 47 and a first reference potential Vref of the fingerprint sensing system 3. The second controllable capacitor 45 is controllable between a first capacitance C21 and a second capacitance C22, lower than the first capacitance C21. The second controllable capacitor 45 is configured to be controlled to its second capacitance C22 when the first bias voltage on the first bias voltage input 31 has a magnitude greater than the predefined second threshold magnitude and the second controllable switch 41 is controlled to its conductive state by the second control signal on the second control signal input 33. The combination the first controllable capacitor 43 having its relative low second capacitance C12 and the second controllable capacitor 45 having its relatively high first capacitance C22 increases the operational margin, so that the risk can be reduced of the first signal unintentionally passing from the first control signal input 27 to the first control signal output 29.

Fig 6 schematically shows an example embodiment of the control signal providing circuitry 25 in fig 4, with substantially the same functionality as has been described above with reference to fig 5. In fig 6, the first controllable switch 39 and the second controllable switch 41 described above are realized using transistors, in this case NMOS-transistors. Furthermore, the first controllable capacitor 43 is, in the example embodiment, realized as a MOSFET-structure (here NMOS), with its gate connected to the first switch control terminal 47, and its source and drain connected to the first control signal input 27. Similarly, the second controllable capacitor 45 is, in the example embodiment, realized as a MOSFET-structure (here NMOS), with its gate connected to the reference potential Vref, and its source and drain connected to the first switch control terminal 47.

In addition to the functionality of the control signal providing circuitry 25 in fig 5, the control signal providing circuitry 25 in fig 6 comprises a third controllable switch 51 , here realized using a transistor, which allows conductive connection of the first control signal output 29 to a second reference potential, in this case illustrated as ground (GND). Hereby, the first control signal output 29 can be reset when the particular row of pixel elements is no longer active, for example.

Fig 7 is a schematic illustration of row circuitry 17 according to a second example configuration, including control signal providing circuitry 25 for each row of pixel elements. The row circuitry 17 in fig 7 has additional functionality in relation to the row circuitry 17 described above with reference to fig 4. In the second example configuration of the row circuitry 17 in fig 7, the control signal providing circuitry 25 comprises a second bias voltage input 53 provided on a common second bias voltage input line 55.

Fig 8 schematically shows an example embodiment of the control signal providing circuitry 25 comprised in the row circuitry 17 in fig 7. The control signal providing circuitry 25 configuration in fig 8 substantially corresponds to that described above with reference to fig 6. For simplification, the optional reset-functionality realized using the third controllable switch 51 in fig 6 is not present in fig 8. Furthermore, a fourth controllable switch 57 (here realized as an NMOS-transistor) has been added. The fourth controllable switch 57 is coupled between the second bias voltage input 53 and the first switch control terminal 47, and comprises a fourth switch control terminal 59. The fourth switch control terminal 59 allows control of the fourth controllable switch 57 from a non-conductive state in which the second bias voltage input 53 and the first switch control terminal 47 are conductively separated to a conductive state in which the second bias voltage input 53 and the first switch control terminal 47 are conductively connected. The fourth switch control terminal 59 may, in embodiments, be coupled to the first control signal output 29 of the control signal providing circuitry 25. In embodiments where the control signal providing circuitry 25 has an additional second control signal output, the fourth switch control terminal 59 may be coupled to such a second control signal output.

To enable control of the number of pulses to be provided to an active row, before proceeding to another row, a first pulsing control signal may be provided on the first bias voltage input line 37 and a second pulsing control signal may be provided on the second bias voltage input line 55. The second pulsing control signal may be an inverse (a logical inverse) of the first pulsing control signal. Thus, when the first pulsing control signal is high, the second pulsing control signal may be low, and vice versa. In this manner, pulses (received via the first control signal line 35) may be provided to a particular active row until there is a pulse is on the first bias voltage input line 37 (and the corresponding inverse signal is provided on the second bias voltage input line 55). This functionality provides for increased flexibility in the control of the pixel elements 21 .

The relatively simple example configurations of the control signal providing circuitry described so far allow the provision of one control signal per pixel element 21 . Based on the teachings herein and their own general knowledge, it would, however, be straight-forward to one of ordinary skill in the art to extend the described configurations to multiple control signals per pixel element 21 . This can, for example, be done by simply duplicating the presented circuitry for two control signals, etc. Further options and variations also exist, where the first control signal output could be used as input for generation of the second control signal output, and vice versa.

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.