GB2621409A

GB2621409A - Sensors and methods

Info

Publication number: GB2621409A
Application number: GB2211850.9A
Authority: GB
Inventors: Sakai Toru; Van Lier Wilhelmus; Derckx Henricus
Original assignee: Touch Biometrix Ltd
Current assignee: Touch Biometrix Ltd
Priority date: 2022-08-12
Filing date: 2022-08-12
Publication date: 2024-02-14
Also published as: GB202211850D0

Abstract

A capacitive fingerprint (skin) sensor 10 wherein each sensor pixel stack comprises: a thin film transistor (TFT), capacitor electrode 114 and a colour filter layer 14 to provide a coloured sensor surface. Reflective layers 21 may be located below the colour filter in the stack. The reflective layers may be a capacitive sensing electrode 114, shield layer 113 or any conductive component of the stack. The colour filter may be provided between a passivation layer 101 and a hydrophobic coating 103. The filter may provide a variable aperture ratio to the sensor pixels, where the ratio is determined by the transmissive middle section (311, Fig.3) of a filter and an opaque border section (312, Fig.3). The colour filter may compromise two filters of different colour. The filter may be backlit by a light source 22. The colour filter layer may be provided by a photolithography method.

Description

Sensors and Methods

Technical Field

The present disclosure relates to the field of capacitive biometric skin contact sensors, as well as methods of designing capacitive biometric skin contact sensors, and methods of designing capacitive biometric skin contact sensors.

Background

Biometric skin sensors typically utilise optical sensors. Such optical sensors rely on being able to image a user's skin. The optical sensor will obtain an image of skin interacting with a sensing surface of the sensor. Typically this will involve the person placing their fingertip on the sensing surface so that image data of their fingertip is obtained. That image data is compared against reference data (i.e. known fingertip data) to try to identify that person. If the image data of the person's fingertip matches reference data of a known authorised user, then person with their fingertip on the sensing surface is an authorised user. In particular, the optical sensor will identify if the contours of the person's skin (ridges and valleys in their skin) are the same as those of the authorised user. Such optical biometric sensors may be included in electronic devices or other suitable equipment to provide biometric authentication. However, for such optical biometric sensors to work, it is essential that they can obtain images of a person's skin on the sensing surface. As a result, for such an optical sensor to be used in any given device, that device will need to include a transparent region that is aligned with the contact surface. That way, a user interacting with the device can place their finger on the transparent portion of the device, and the optical sensor may image their finger for biometric authentication. The inclusion of such optical biometric sensors in these devices therefore places constraints as to the design and appearance of those electronic devices.

Summary

Aspects of the disclosure are set out in the independent claims and optional features are set out in the dependent claims. Aspects of the disclosure may be provided in conjunction with each other, and features of one aspect may be applied to other aspects.

In an aspect, there is provided a capacitive biometric skin contact sensor comprising a sensor array of sensor pixels, wherein: each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode; a top surface of the sensor array provides a contact surface for contacting by an object to be sensed; and each sensor pixel is formed of a plurality of layers including an optical colour filter layer arranged to filter one or more colours of light.

Embodiments may enable biometric sensing to be provided by a non-transparent sensor. The capacitive sensor may obtain biometric skin contact data for the object to be sensed, and the sensor will have a colour filter layer deposited thereon. Due to colour filtering by the colour filter layer, the sensor will appear coloured to a user of the sensor, where that colour is based on properties of the colour filter chosen for each sensor pixel in the sensor array. The provision of a colour filter layer may enable greater design freedom for the appearance of the biometric sensor, while still enabling the sensor to perform capacitive biometric skin contact sensing for a user interacting with that sensor.

Sensors of the present disclosure may be configured so that light travelling from the sensor towards a user of the sensor will travel through the optical colour filter layer of the sensor. The optical colour filtering layer may be configured to provide optical colour filtering of that light so that the sensor appears coloured to the user interacting with the sensor (e.g. where the particular appearance is governed by one or more properties of the chosen colour filter). The sensor may be a reflective light colour filtering sensor, a transmissive light colour filtering sensor or a combination thereof (e.g. in which both reflective and transmissive colour filtering is provided). For a reflective light colour filtering sensor, the sensor may comprise at least one optically reflective component configured to reflect light back towards the user (and through the colour filter). For the transmissive light colour filtering sensor, the sensor may be provided with a backlight which is arranged to direct light through the colour filter of the sensor pixel and to the user. In either case, light will be colour filtered as it passes through the optical colour filter layer, thereby causing the sensor to appear coloured to the user. A combined transmissive and reflective light colour filtering sensor may be provided in which at least some light is reflected (e.g. from an optically reflective portion of the sensor) which will travel through the colour filter, and at least some light will be transmitted (e.g. from the backlight) which will travel through the colour filter.

The plurality of layers may include an optically reflective layer located below the optical colour filter layer and comprising an optically reflective element. For example, there may be one or more different layers separating the optically reflective layer from the colour filter layer. The optically reflective element may comprise an electrical conductor. For example, electrically conductive and optically reflective material may be used for the electrical conductor. The optically reflective layer may comprise a metallization layer in which the metal is optically reflective. It may comprise multiple such layers.

The capacitive sensing electrode may provide the optically reflective element for each sensor pixel. For each sensor pixel, the optically reflective layer may be a first optically reflective layer, and the sensor pixel may also include a second optically reflective layer comprising an optically reflective element. The optical colour filter layer may be located above the reflective elements of both reflective layers. Each sensor pixel may comprise an electrical shield layer comprising an electrical shield. The electrical shield layer may provide a first plate of a reference capacitor for the sensor pixel and the capacitive sensing electrode may provide a second plate of the reference capacitor. As another example, a reference capacitor may be provided by a plate in a first layer (e.g. a gate or source/drain layer) and a plate in a second layer (e.g. the other of the gate or source/drain layer). Other layers could be used to provide reference capacitor plates. The capacitive sensing electrode may be located above the electric shield. Both the capacitive sensing electrode and the electric shield may provide optically reflective elements for each sensor pixel. The colour filter layer may be located above the capacitive sensing electrode and the electric shield.

Some or all electrically conductive components of the sensor pixel may provide optically reflective elements. Each electrically conductive component of the sensor pixel may be made of an optically reflective material, such as an aluminium alloy (other example materials also include molybdenum or titanium). For each sensor pixel, the optical colour filter layer may be provided on a top surface of the capacitive sensing electrode. For each sensor pixel, a passivation layer may be provided on a top surface of the capacitive sensing electrode. The optical colour filter layer may be provided on a top surface of the passivation layer. For each sensor pixel, a hard coat may be provided on top of the sensor pixel. The optical colour filter layer may be provided on a top surface of the hard coat. A hydrophobic coating may be provided on a top surface of the sensor pixel.

For at least some of the sensor pixels, an aperture ratio for the colour filter may be varied to provide a selected property for the colour filtering provided by said pixels. For at least some of the sensor pixels, the colour filter layer for the sensor pixel may include a first colour filter and a second colour filter. The first colour filter may be for a different colour to the second colour filter. The first colour filter may be white or black. The second colour filter may another colour, such as red, green or blue. A ratio of the first colour filter to the second colour filter on the sensor pixel may be selected to a provide a selected colour property to the sensor pixel.

For example, the area of the sensor pixel covered by the first colour filter (as compared to the second colour filter) may be selected to provide a selected property to the colour of the sensor pixel. The selected colour property may comprise a brightness and/or a darkness. In other words, each sensor pixel may be designed to provide a certain colour for the colour filtering, e.g. where that colour contains a desired grayscale effect for that particular colour.

For a reflective colour filtering sensor (and/or the combination of transmissive and reflective), the sensor may include an optically reflective layer located below the optical colour filter layer.

The optically reflective layer may comprise an optically reflective element. Any non-transparent components of the sensor pixel in layers between the optically reflective layer and the colour filter layer may be spatially arranged to inhibit blocking (e.g. attenuation) of light travelling between the optical reflective layer and the optical colour filter layer. For example, said non-transparent components of the sensor pixel may be arranged to be laterally offset from an area of the sensor underneath the colour filter. For each sensor pixel, non-transparent components of the sensor pixel located above the optically reflective layer may be spatially arranged to inhibit blocking (e.g. attenuation) of light travelling between the optical reflective layer and the optical colour filter layer. For example, said non-transparent components of the sensor pixel may be laterally offset from a region of the sensor pixel covered by the optical colour filter. Said non-transparent components may be aligned with regions of the sensor array in between adjacent capacitive sensing electrodes. For example, said non-transparent components may be located underneath regions of the sensor pixel which are not covered by a colour filter.

For a transmissive colour filtering sensor (and/or the combination of transmissive and reflective), the sensor may be arranged to be backlit by a transmitting element located below the optical colour filter layer, and wherein any non-transparent components of the sensor pixel in layers between the transmitting element and the colour filter layer are spatially arranged to inhibit blocking (e.g. attenuation) of light travelling between the transmitting element and the optical colour filter layer. For example, said non-transparent components of the sensor pixel may be arranged to be laterally offset from an area of the sensor underneath the colour filter. For each sensor pixel, non-transparent components of the sensor pixel located above the transmitting elements may be spatially arranged to inhibit blocking (e.g. attenuation) of light travelling between the transmitting element and the optical colour filter layer. For example, said non-transparent components of the sensor pixel may be laterally offset from a region of the sensor pixel covered by the optical colour filter. Said non-transparent components may be aligned with regions of the sensor array in between adjacent capacitive sensing electrodes. For example, said non-transparent components may be located underneath regions of the sensor pixel which are not covered by a colour filter.

For each sensor pixel, the capacitive sensing electrode may be at least partially optically transparent. For example, a reflective element (and/or a backlight) may be provided beneath the capacitive sensing electrode (and light may pass therefrom and through the partially transparent capacitive sensing electrode). For example, a colour filter may be located beneath the capacitive sensing electrode (or this could be located above the capacitive sensing electrode). Each sensor pixel may comprise at least one of: (i) an optically reflective electric shield; (ii) optically reflective source and/or drain conductive elements; (iii) an optically reflective gate conductive element; (iv) a substrate onto which the sensor pixel is built, wherein a surface of the substrate is optically reflective, optionally wherein the surface is a bottom surface of the substrate; thereby to provide the optically reflective element of the sensor pixel.

In an aspect, there is provided an apparatus comprising any sensor of the present disclosure. The apparatus includes a light transmitting element, wherein the light transmitting element is arranged beneath the colour filter layer of the sensor pixels of the sensor. The light transmitting element may be part of the sensor or may be provided by a separate component to the sensor.

The apparatus may include a controller configured to control operation of the light emitting element. For example, the controller may be configured to selectively activate the transmitting element to display information to a user of the sensor, where the information is displayed using the one or more colour filters of the sensor array.

In an aspect, there is provided a method of manufacturing a capacitive biometric sensor, the sensor comprising a sensor array of sensor pixels, wherein a top surface of the sensor array provides a contact surface for contacting by an object to be sensed, and wherein each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode. The method comprises: for each sensor pixel, providing a plurality of layers for that sensor pixel including an optical colour filter layer. Providing the optical colour filter layer for the sensor pixels of the array may comprise use of a photolithography method. Providing the plurality of layers may comprise providing the capacitive sensing electrode for the sensor pixel. The capacitive sensing may be optically reflective and provided in a layer beneath the optical colour filter layer, thereby to provide an optically reflective layer beneath the colour filter layer.

Providing the optical colour filter layer for each sensor pixel may comprise depositing an optical colour filter above the capacitive sensing electrode.

In an aspect, there is provided a method of designing a capacitive biometric sensor, the sensor comprising a sensor array of sensor pixels, wherein a top surface of the sensor array provides a contact surface for contacting by an object to be sensed, wherein each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode, and wherein each sensor pixel is formed of a plurality of layers including an optical colour filter layer arranged to filter one or more colours of light. The method comprises: obtaining an indication of a selected appearance for the top surface of the sensor; and selecting at least one optical property for the optical filter of each sensor pixel of the array based on the selected appearance for the top surface of the sensor. The selected appearance for the top surface of the sensor may comprise a spatial distribution of one or more colours across the sensor array.

Selecting the at least one optical property for the optical filter of each sensor pixel of the array may comprise selecting a colour for said optical filter, wherein said colour may be selected according to the spatial distribution of colours for the sensor array. For example, one or more pixel dithering techniques could be applied to vary particular colours for each optical filter.

Aspects of the present disclosure may provide one or more computer program products comprising computer program instructions configured to control operation of a sensor manufacturing assembly to manufacture any capacitive biometric skin contact sensor as disclosed herein and/or to control operation of a processor to design any capacitive biometric skin contact sensor disclosed herein.

Figures Some examples of the present disclosure will now be described, by way of example only, with reference to the figures, in which: Figs. la to lc show schematic diagrams of capacitive biometric skin contact sensors. Fig. 2 shows a schematic diagram of a capacitive biometric skin contact sensor.

Fig. 3 shows a portion of a sensor array of sensor pixels for a capacitive biometric skin contact sensor.

Figs. 4a and 4b show example sensor pixel designs for a capacitive biometric skin contact sensor.

Fig. 5 shows a schematic diagram of a capacitive biometric skin contact sensor.

In the drawings like reference numerals are used to indicate like elements.

Specific Description

The present disclosure relates to the use of an optical colour filter in a capacitive biometric skin contact sensor. The capacitive biometric skin contact sensor includes a sensor array comprising a plurality of sensor pixels. Each sensor pixel has a plurality of different layers. This includes an optical colour filter layer, in which a colour filter has been deposited on another layer of the sensor pixel. The sensor is designed for light to pass through the colour filter layer towards a user of that sensor. The colour filter layer of each sensor pixel will filter certain light to provide a selected appearance for that sensor pixel, as seen by the user of the sensor. For this, the sensor may be provided in a transmissive colour filtering arrangement or in a reflective colour filtering arrangement (or a combination of both). For the transmissive colour filtering arrangement, the sensor may be 'backlit', so that a light emitter is arranged behind the sensor. The light emitter will direct light from behind the sensor, through the colour filter of the sensor, and to the user. For the reflective colour filtering arrangement, the sensor may include one or more reflective elements. The reflective elements are arranged beneath the colour filter. Light from a user side of the sensor (e.g. ambient) will therefore reflect off the reflective element and back to the user of the sensor, having also passed through the colour filter. A combination of the two arrangements may be provided in which the sensor is backlit and also includes at least one reflective element, so that both backlit light and reflected light may pass through the colour filter.

Figs. la to lc show example capacitive biometric skin contact sensors. Each of Figs. la to lc is shown in cross-section to illustrate different layers of the sensor.

The capacitive biometric skin contact sensors of the present disclosure include a sensor array formed of a plurality of sensor pixels. Each sensor pixel includes at least one thin film transistor (TFT') and a capacitive sensing electrode. The sensor pixels are built up over multiple layers. For example, one layer may provide a substrate on top of which other layers of the pixels are stacked. In a 'forward stack', this substrate may be a base layer (on the opposite side of the sensor from the user) onto which the other layers are deposited. In a 'reverse stack', this substrate may be on a user-facing side of the sensor, and the other layers may be deposited onto a surface of this substrate facing away from the user of the sensor. The TFT may be provided over a plurality of different layers (e.g. to provide source/drain connections, a gate connection, and a semiconductor region). The capacitive sensing electrode is usually provided in one of the uppermost layers (so that it is closer to the user interacting with the sensor). For example, for a forward stack, the capacitive sensing electrode may be one of the last layers to be deposited, and in a reverse stack, the capacitive sensing electrode may be one of the first layers to be deposited. Additional examples of sensor pixel designs and sensor pixel stacks will be described in more detail below.

As will be appreciated, a user will interact with the sensor by placing a portion of their body into contact (or near to) the sensor. The sensor may provide a contact surface which the user will contact for sensing. For example, this may comprise the user placing their hand or finger (e.g. fingertip) on the contact surface. The capacitive biometric skin contact sensors of the present disclosure are configured to provide capacitive biometric skin contact sensing for that portion of the user's skin which is interacting with the contact surface. As such, a 'user side' of the sensor may be defined as the side of the sensor which is closest to the user of the sensor (e.g. the side of the sensor with which the user interacts, i.e. the side which provides the contact surface). An 'opposite side' may be defined as the side of the sensor which is furthest away from the user side of the sensor, i.e. the side of the sensor which is opposite to the user side of the sensor. For simplicity, the user side will be referred to as being 'above' the opposite side (and with the user themselves being 'above' the user side). However, it will be appreciated that this use of 'above' and 'below' does not require the sensor to always be provided horizontally with the user vertically above the sensor. Rather, the use of 'above' and 'below' is to describe whether features are closer to the user, or further away from the user (e.g. a first component will be closer to the user than a second component if the first component is described as being above the second component).

Fig. la shows a capacitive biometric skin contact sensor 10. The sensor 10 of Fig. la is a reflective colour filtering sensor. The sensor 10 includes a plurality of different layers. The sensor 10 includes an optically reflective layer 21 and an optical colour filter layer 14. The sensor 10 will also include additional layers (e.g. for including a capacitive sensing electrode and one or more TFTs), but these are not shown in Fig. la.

The optically reflective layer 21 is located beneath the colour filter layer 14. The optically reflective layer 21 comprises at least one optically reflective element. The optical colour filter layer 14 comprises one or more optical colour filters. The one or more colour filters may span a portion of each sensor pixel (e.g. they may cover the majority of a surface of each sensor pixel).

The optically reflective element is configured to reflect light incident on its top surface. In other words, the optically reflective layer 21 is configured so that, incident light that has travelled from a user-side of the sensor 10 (i.e. ambient light which travelled from above the optically reflective layer 21) will be reflected by the optically reflective element back towards the user (i.e. reflected upwards). The colour filter layer 14 is arranged above the optically reflective layer 21 so that light reflected by the optically reflective layer 21 will pass through the colour filter layer 14. The colour filter layer 14 will also filter light which travels from above the sensor 10 towards the optically reflective layer. This double filtering of light may reduce filtering requirements for the colour filter (e.g. a thinner layer of colour filter to be used), as light will pass through twice the thickness of colour filter. The one or more colour filters of the colour filter layer 14 are configured to filter light. Each individual colour filter may cause the light to appear a certain colour due to that colour filter filtering out other colours of light.

An example path for light through the sensor 10 is shown in Fig. la. The light travels from above the sensor 10 (on a user side of the sensor 10) towards the optically reflective layer 21.

As can be seen, on this trajectory, this light will also pass through the colour filter layer 14 as it travels down towards the optically reflective layer 21. The light is then incident on a top surface of the optically reflective layer 21, where the optically reflective element causes reflection of that light. The reflected light then travels upwards and through the one or more colour filters of the colour filter layer 14, and then on towards the user of the sensor 10. This colour filtering will cause the sensor 10 to appear a certain colour to the user. That particular appearance will be controlled based on the choice of colour filter.

For Fig. la, the sensor 10 is arranged so that the optical colour filter of the optical colour filter layer 14 is located between: (i) the optically reflective element of the optically reflective layer 21, and (ii) a user interacting with the sensor 10. As such, the sensor 10 will provide optical colour filtering so that reflected light from the optically reflective element appears coloured to the user according to the one or more colours filtered by the optical colour filter layer 14.

Fig. lb shows another example of a capacitive biometric skin contact sensor 10. The sensor of Fig. lb is a transmissive colour filtering sensor. The sensor 10 includes a plurality of different layers. The sensor 10 includes an optical colour filter layer 14. The sensor 10 will also include additional layers (e.g. for including a capacitive sensing electrode and one or more TFTs), but these are not shown in Fig. lb. Also included is a transmitting element 22. The transmitting element 22 may be a backlight, e.g. a component which may illuminate the sensor stack from behind. The backlight may be part of the sensor 10. For example, the backlight may be provided by a component on a bottom surface (e.g. attached to a bottom layer) of the sensor 10. Alternatively, the backlight may be provided by another component located behind the sensor 10 (such as a light source), e.g. a component which is separate to the sensor 10.

The optical colour filter layer 14 is provided as part of the sensor 10, i.e. it is provided as one of the layers within the multi-layer sensor stack.

The colour filter layer 14 of Fig. lb may be similar to that described above in relation to Fig. la. The colour filter layer 14 of Fig. lb may be thicker that of Fig. la, such as being double the thickness (e.g. as light may travel through the filter 14 in one direction, not two). The optical colour filter layer 14 includes one or more optical colour filters configured to provide optical filtering of light according to the selected colour(s) for that colour filter. The backlight is configured to direct light towards the colour filter. The backlight is arranged so that it will direct light through the colour filter and towards a user of the sensor 10. For example, the backlight may be a light emitting element (e.g. an LED, such as a white LED). In other words, the backlight may be provided by a component located underneath the colour filter (e.g. behind the colour filter, on a side of the colour filter opposite to the side on which the user interacting with the sensor 10 is located).

An example path for light travelling through the sensor 10 is shown in Fig. lb. The light is emitted by the backlight (on the opposite side of the sensor 10 to the user) towards the optically reflective layer 21. As can be seen, on this trajectory, this light will pass through the colour filter layer 14 as it travels up towards the user of the sensor 10. This colour filtering will cause the sensor 10 to appear a certain colour to the user. That particular appearance will be controlled based on the choice of colour filter.

For Figs. la and lb, other components of the sensor pixel stack are not shown. In these Figs., the optical colour filter layer 14 is located so that light which reaches the user's eyes from the sensor pixel will have passed through the colour filter(s) of that pixel (and will thus appear coloured as per the one or more optical colour filters in that optical colour filter layer 14).

In relation to Fig. la, the sensor 10 includes an optically reflective layer 21 comprising an optically reflective element. The sensor 10 may include a plurality of different optically reflective layers, each comprising one or more optically reflective elements. The sensor pixel may be designed so that a majority of the surface area of the sensor pixel (when viewed from above) is covered by an optically reflective element. For example, the sensor pixel may be arranged so that a majority (if not all) of the surface area covered by the colour filter is also covered by one or more optically reflective elements (located beneath the colour filter).

Each optically reflective element may be provided by an existing component in the sensor pixel design. That is, the optically reflective element may be provided by using an optically reflective material to provide a component of the sensor pixel. In particular, electrically conductive materials may be used which are also optically reflective. That is, the electrical functionality of those components of the sensor pixel may not be altered, but those components may then also be optically reflective.

Each sensor pixel includes a capacitive sensing electrode. The capacitive sensing electrode for each sensor pixel will typically span a large surface area, e.g. the majority of the surface area (when viewed from above) of each sensor pixel. Each sensor pixel may also include one or more other conductive elements which span a large area of the sensor pixel, such as an electrical shield and/or a reference capacitor. Each sensor pixel may also include a plurality of electrical conductors for electrically connecting different components of the sensor pixel (e.g. for connecting capacitive sensing electrode to a gate region of the TFT etc.). Any or all of these different electrical components of the sensor pixels may be provided by an optically reflective material. Examples of optically reflective electrical conductors include Aluminium, Aluminium alloys (such as AINd), titanium, gold, silver, molybdenum. For example, different combinations of materials could be used, e.g. one conductive layer of the stack may be formed from one material, and another layer in the stack may be formed from another material.

Additionally, or alternatively, the sensor pixel may include one or more additional components, e.g. additional pieces of material, which have been included to provide the optical reflectivity for the sensor pixel.

An example capacitive biometric skin contact sensor 10 is shown in Fig. 1c. The sensor 10 is formed of a plurality of different layers. As shown in Fig. 1c, the sensor 10 includes a substrate 11, a TFT 12, a capacitive sensing electrode 13 and a colour filter 14.

The sensor 10 of Fig. lc is a reflective colour filtering sensor. The colour filter 14 is provided on the top-most layer of the sensor 10 shown in Fig. 1c. However, it will be appreciated that additional layers may be included which are not shown, such as a hard coat and/or hydrophobic layer, which are provided on top of the colour filter layer 14.

The capacitive sensing electrode 13 is arranged to provide an optically reflective element for each sensor pixel. For example, the capacitive sensing electrode 13 may be made of an optically reflective aluminium alloy conductive material. The colour filter 14 for each sensor pixel may overlie the capacitive sensing electrode 13 for that pixel. That is, the capacitive sensing electrode 13 may be horizontally aligned with, and located under, the colour filter 14. The contact surface for the sensor 10 will be the top-most surface, so the colour filter 14 is located between the capacitive sensing electrode 13 and the user. The sensor 10 is arranged so that light which reflects off the optically reflective capacitive sensing electrode 13 may then pass through the colour filter 14 (to be optically colour filtered) before reaching the user's eyes.

For each sensor pixel, the capacitive sensing electrode 13 and the colour filter 14 may each span a majority of the cross-section (when viewed from above) of that sensor pixel. For example, this may maximise the aperture ratio for the colour filter (and e.g. to maximise the amount of filtered light output). As such, by using an optically reflective capacitive sensing electrode 13, and a colour filter 14 above that electrode 13, the desired colour filtering for the sensor 10 may be provided without the need to alter the sensor pixel design for the portion of the sensor 10 (and relevant components) located beneath the capacitive sensing electrode 13. Also, by placing the optically reflective element (in this case the capacitive sensing electrode 13) higher up in the sensor pixel stack, there are fewer intervening components which may attenuate the incident/reflected light.

As shown in Fig. 1 c, the capacitive sensing electrode 13 may be the highest electrical component of the sensor pixel. The remaining electrical components of the sensor 10 are provided in layers beneath the capacitive sensing electrode 13. By providing the capacitive sensing electrode 13 in a higher layer, the capacitive response of the sensing electrode 13 to a conductive object (e.g. skin) contacting the contact surface of the sensor 10 will be greater (due to the decreased separation distance between the conductive object and the sensing electrode 13). Layers of the sensor pixel beneath the electrode 13 may also include conductive elements. For instance, in the TFT layer(s), which are below the electrode 13, there may be a plurality of different electrical conductors.

Some or all of the components in the lower layers of the sensor pixel may also be provided by an optically reflective material. For instance, any or all of the electrical conductors in the lower layers of the sensor pixel may be optically reflective. This may enable greater reflectivity for the light incident on the sensor pixel (from the user side of the sensor 10). As such, more light will be reflected back through the colour filter 14 and towards the user. For example, the components of the sensor pixel may be spatially arranged to maximise the coverage of optically reflective material across the sensor pixel. For instance, optically reflective elements (e.g. conductors) in layers beneath the sensing electrode 13 may be arranged to at least partially occupy regions of the sensor pixel which are not beneath the reflective electrode 13.

In other words, the sensor pixel may be arranged to maximise the area of the sensor pixel that is covered with an optically reflective material.

As shown in Fig. 1 c, the capacitive sensing electrode 13 is located above the TFT 12. The TFT 12 may span across several layers. A bottom most layer shown in Fig. lc is the substrate 11 on which the other components of the sensor pixel are provided. One or both surfaces of the substrate 11 may have optically reflective material thereon. For example, at least a portion of (e.g. the entire) lowermost surface of the substrate 11 may be covered with an optically reflective material, and/or a separate reflective component could be provided beneath the substrate. This may further increase the amount of light which is reflected back through the colour filter 14 (especially in areas beneath regions of the sensor pixel which are not covered by the optically reflective electrode 13 and/or other optically reflective components of the sensor pixel). Also, by providing an optically reflective material on the opposite side of the substrate 11 (i.e. the side facing away from the user), this material may be less likely to interact with any components of the electronic circuitry of the sensor 10.

A larger sensor pixel stack will now be described with reference to Fig. 2.

Fig. 2 shows a sensor pixel stack for a capacitive biometric skin contact sensor 10. The stack shown in Fig. 2 includes more layers than those shown in Figs. la to 1c. Fig. 2 is intended to show a number of options for different layers which could be included in the sensor pixel stack, but it will be appreciated that this stack should not be considered limiting. Some of these layers need not be included, and other additional layers not shown may also be included. Also, the particular ordering of the different layers should not be considered limiting. For example, the arrangement of the one or more TFTs of the sensor pixel could be altered (e.g. the source/drain and gate layers could be swapped). Fig. 2 is just intended to show a number of different example locations in a stack where a reflective element 21, transmitting element 22, and/or colour filter layer 14 may be included.

As with the example shown in Fig. 1c, the stack of Fig. 2 has a substrate 100. The substrate 100 forms a base of the stack on which other layers are provided.

The stack includes a plurality of layers which may carry one or more electrical conductors. These may be metallization layers. In the stack of Fig. 2, four metallization layers are included: (i) a first metallization layer ('M1'), 00 a second metallization layer (M2'), (iii) a third metallization layer ('M3') and, (iv) a fourth metallization layer ('M4'). The M4 layer is the highest of the four, and the M1 layer is the lowest.

In the example stack of Fig. 2, the uppermost of these layers (M4) may be a capacitive sensing electrode layer 114, which may provide the capacitive sensing electrode for the sensor pixel. The next uppermost of these layers (M3) may be a shield layer 113, which may provide an electrical shield for the sensor pixel (e.g. to electrically shield the capacitive sensing electrode, such as from parasitic capacitances associated with components beneath the shield layer 113 in the stack). The two lowermost of these layers (M1 and M2) may be used to provide connections to one or more TFTs in the sensor pixel stack. In the example shown in Fig. 2, the M2 layer is a source and drain layer 112 and the M1 layer is a gate layer 111 (although the ordering for these layers could be reversed). The source and drain layer 112 may provide electrical connections to the source and drain regions of the one or more TFTs of the sensor pixel. The gate layer 111 may provide electrical connections to the gate region(s) of the one or more TFTs of the sensor pixel.

The electrically conductive layers (e.g. M1 to M4) may be separated from each other by additional layers of the sensor stack. These separating layers may be designed to electrically insulate components in adjacent metallization layers. A number of such insulating layers are shown in Fig. 2. These may include a gate insulator ('GI') layer 120, a first inner layer (IL1') 121, a second inner layer ('IL2') 122 and a third inner layer ('IL3') 123. The stack may also include a semiconductor layer ('SC') 130 for the TFT(s). The stack may also be covered by a passivation layer ('PU) 101, a hard coat ('HC') 102 and/or a hydrophobic layer ('HP') 103.

The stack may be arranged with the substrate 100 as the base layer. The other layers are provided on top of the substrate 100. As shown in Fig. 2, the M1 gate layer 111 may be provided on the substrate 100. The M1 gate layer 111 may be separated from the semiconductor layer 130 by the gate insulator layer 120. The M2 source and drain layer 112 may be separated from the semiconductor layer 130 by the first inner layer 121. Although not shown, it will be appreciated that some of the layers may be connected to other non-adjacent layers. For example, one or more conductive vias may extend between the different layers, such as between the M2 layer and the semiconductor 130, or between the M4 layer and other M layers. The M2 source and drain layer 112 may be separated from the M3 shield layer 113 by the second inner layer 122. The M3 shield layer 113 may be separated from the M4 capacitive sensing electrode layer 114 by the third inner layer 123. The passivation layer 101 may be provided on top of the capacitive sensing electrode. The passivation layer 101 may comprise Silicon Nitride. A hard coat layer 102 may be provided on the passivation layer 101. A hydrophobic layer 103 may be provided on top of the stack.

The sensor pixel may be provided as a transmissive colour filtering pixel (which includes a transmitting element 22) or a reflective colour filtering pixel (with a reflective element 21). Where a reflective colour filtering pixel is used, the colour filter will be located above the reflective element 21 in the sensor 10. Where a transmissive colour filtering pixel is used, the colour filter will be located above the transmitting element 22.

The colour filter layer 14 may be provided in a plurality of different positions within the stack. The colour filter layer 14 may be provided by depositing optical colour filter material on another layer within the stack. The colour filter layer 14 may be provided on top of the passivation layer 101 (which covers the capacitive sensing electrode). The colour filter layer 14 may be provided on top of the hard coat 102.

The stack may be built up one layer at a time. Building the stack may involve sequentially building (e.g. depositing) layers on top of the substrate 100 and working up. For example, this would start by depositing the M1 gate layer 111 on the substrate 100, before depositing layers above that. Additionally, or alternatively, building the stack may involve building (e.g. depositing) layers onto the hard coat 102 or hydrophobic layer 103 and working down. For example, this may start by depositing the passivation layer 101 and then capacitive sensing electrode onto the hard coat 102. In some examples, when building down (e.g. from a hard coat 102), that layer may itself be temporarily affixed (e.g. glued) to a carrier substrate, which may then be subsequently removed later in the manufacturing process. For example, when building down, the layer (e.g. hard coat 102) onto which the subsequent layers are deposited may itself be a thin film. That thin film may be coupled to a carrier substrate to support it during the manufacturing process, before subsequently removing that carrier substrate.

In other words, the layers of the stack may each be built up sequentially (and separately). The colour filter layer 14 may be provided in the stack by depositing a colour filter onto the relevant layer during this process of building the stack. Depositing colour filter onto each sensor pixel may be done using photolithographic techniques. For example, the stack may be built up from the substrate 100 until the capacitive sensing electrode is provided, and the passivation layer 101 is deposited over the capacitive sensing electrode. The same process may be performed for all of the sensor pixels in the sensor array. Colour filters may then be deposited photolithographically over all of the different sensor pixels of the sensor array. The different sensor pixels may have different colour filters deposited thereon. A hard coat 102 and/or hydrophobic layer 103 may then be provided on top of the colour filter layer 14 (which would lie on top of the passivation layer 101 in this example). The colour filter layer 14 could be provided on top of other suitable layers, such as on top of the hard coat 102.

Where the stack provides a reflective colour filtering pixel, one or more of the existing layers in the stack may provide the optically reflective layer 21. The stack may include a plurality of optically reflective layers. Each optically reflective layer 21 may include at least one element which is optically reflective (so that light incident on that element from above is reflected back upwards towards the colour filter). Each optically reflective layer 21 may be provided by making the components in that layer out of an optically reflective material. For example, any of the metallization layers may utilise an optically reflective electrically conductive material, such as an aluminium alloy. For example, all of the metallization layers may be formed of an optically reflective material. The capacitive sensing electrode may be made of an optically reflective material. The electrical shield may be made of an optically reflective material. Any conductive elements in the M1 and M2 layers may be made of an optically reflective material.

Additionally, or alternatively, the optically reflective element 21 may be provided in a standalone optically reflective layer 21 to be included in the stack. Such an optically reflective layer 21 may be provided at any suitable location in the stack. For example, the optically reflective layer 21 may be provided on an under surface of the substrate 100 and/or as a separate layer beneath the substrate 100. That is, the surface of the substrate 100 on the opposite side of the user may be coated in an optically reflective material, and/or a separate reflective layer could be provided underneath the substrate 100.

Where the stack provides a transmissive colour filtering pixel, a transmitting element 22 may be provided. The transmitting element 22 may be in the form of a backlight for lighting the sensor stack from behind. The transmitting element 22 may be part of the sensor 10, or the transmitting element 22 may be a separate component to the sensor 10. For example, the transmitting element 22 may be a separate light emitting element onto which sensor stack is to be mounted. The transmitting element 22 may be a light emitter, such as an LED, which may be mounted onto an opposite side of the substrate 100 to the user. The transmitting element 22 may itself form the substrate 100 on top of which the sensor stack is provided. The transmitting element 22 may be arranged so that light may be transmitted, from the transmitting element 22, through the colour filter layer 14 (and towards a user of the sensor 10).

In these examples, each sensor pixel may be provided by a multi-layered stack. Within the layers of the stack, there will be at least one optical colour filter. Each sensor pixel is arranged so that some light will pass through that colour filter as that light travels towards a user of the sensor 10. The sensor pixel may therefore appear coloured to the user, where the colouring of the pixel is set by the particular colour filter chosen for that pixel (e.g. based on which colour(s) are used in the filter, and/or a ratio of colour filter to black or white colour filtering). In the reflective colour filtering pixel design, at least one of the layers in the sensor pixel stack will be an optically reflective layer 21 which includes one or more optically reflective elements.

For example, any layers which contain a large coverage of electrical conductor, such as the M4 capacitive sensing electrode layer 114 or the M3 shield layer 113, may use an optically reflective electrical conductor to provide the one or more optically reflective elements for that layer. In the transmissive colour filtering pixel design, a transmitting element 22 is included, either as part of the sensor pixel stack, or as a component behind the stack. In either case, the result will be that light may pass through the colour filter of the sensor pixel towards the user.

In Fig. 2, there are a plurality of different layers of electrical conductors shown (M1 to M4). These layers may be provided by different materials. For example, for a reflective design, one or all of these layers may be made of an optically reflective material. The capacitive sensing electrode 114 (M4) may be made of a more optically reflective material than other layers. For example, the capacitive sensing electrode 114 (M4) may be made of an aluminium alloy, such as AINd (or other suitable alloy) or silver, and/or M1 could be Molybdenum, M2 could be AINd and/or M3 could be AINd. One or more of the layers may be made of titanium, especially for lower reflective layers. For the transmissive design, M3 and/or M4 may be made of an at least partially transparent conductor, such as ITO. Non transparent components of the sensor, and/or relevant components within M1 and/or M2 layers may be aligned under a black colour filter component of each pixel.

An example sensor array for a capacitive biometric skin contact sensor will now be described with reference to Fig. 3.

Fig. 3 shows a sensor array 300. The sensor array 300 is formed of a plurality of sensor pixels 310. The sensor array 300 may be a rectangular array comprising a plurality of rows of sensor pixels 310 and a plurality of columns of sensor pixels 310. Each sensor pixel 310 in the sensor array 300 may be similar. The sensor pixel design, as well as stack layup, for each sensor pixel 310 may be the same. However, the sensor pixels 310 may differ in the colour filtering they apply to their respective sensor pixel 310. The sensor array 300 may comprise an active matrix sensor array. For this, each sensor pixel 310 in a row may be connected to a gate drive channel for that row. Each sensor pixel 310 in a column may be connected to a read-out channel for that column. The sensor may be configured to apply a gate drive signal to one gate drive channel at a time. In response, each sensor pixel 310 in that row may output a read-out signal to the read-out channel for its row. The sensor may provide capacitive biometric skin contact sensing based on these read-out signals.

Fig. 3 shows a plan view of the sensor array 300. Inset A of Fig. 3 shows a zoomed in view of one sensor pixel 310 in the array 300. Each sensor pixel 310 is formed of two portions: a first portion 311 and a second portion 312. The first portion 311 is in a central region of the sensor pixel (e.g. the first portion 311 is a central portion of the sensor pixel 310). The second portion 312 is located around the edge of the sensor pixel 310 (e.g. the second portion 312 is a border portion). The second portion 312 may surround (e.g. completely circumscribe) the first portion 311. In other words, the first portion 311 occupies a central region of the pixel 310 and the second portion 312 occupies a perimeter region of the pixel 310.

Each of the first portion 311 and the second portion 312 may be covered by optical filtering material. For the first portion 311, this may comprise a colour filter (e.g. red, green, blue, white, black). For the second portion 312, the optical filtering material may filter substantially all colours of light. For example, the second portion 312 may utilise a black colour filtering material. The material which overlays the second portion 312 may be a light blocking material, such as a black colour filter. For example, the sensor array 300 may have a grid of black matrix material (e.g. a black colour filter which blocks light from passing therethrough), wherein the black material defines a series of rows and columns corresponding to the sensor pixels 310 of the array 300. In other words, this array of black matrix material may define a border region containing a black colour filter material for each sensor pixel 310 of the array 300. This border region may provide the second portion 312 of that pixel 310.

The first portion 311 and the second portion 312 may each provide respective colour filtering. The colour filtering provided by the first portion 311 will depend on what colour is used for the colour filter that overlays the first portion 311. The second portion will be overlayed by black colour filtering material, and so this will block light from passing therethrough. To maximise the transmission of colour filtered light through the first portion 311, the area of the first portion 311 within the sensor pixel 310 may be maximised. For example, the second portion 312 may provide a thin border around the edge of the pixel (relative to the area of the first portion 311 within a central region of that pixel). Surrounding each first portion 311 with the black material (e.g. with a black colour filtering second portion 312) may reduce the amount of light leakage associated with that pixel, which may enable a sharper static image to be shown by the sensor.

Each pixel may be square. Each pixel may be a 50 x 50 micron square. The first portion 311 of each sensor pixel 310 may be square, but is to be appreciated that this is just illustrative, the first portion 311 could be any shape. The first portion 311 may cover a majority of the sensor pixel area.

As described above, for each sensor pixel 310 of the sensor array 300, a colour filter layer 14 is included in the sensor pixel stack, and that colour filter will provide optical colour filtering of light passing through that colour filter. That colour filter will overlie the first portion 311 of the pixel 310. The sensor pixel 310 will then appear coloured, where the appearance of that sensor pixel 310 is dictated by the particular colour filter used. Such colour filtering may be provided for each sensor pixel 310 in the sensor array 300. In turn, this will provide a much larger area over which such colour filtering is performed, thereby giving rise to a much larger area of colour filtering. In this sense, each sensor pixel 310 of the sensor array 300 may also provide a pixel which influences the appearance of the sensor (e.g. due to the relevant colour filtering performed by that pixel 310).

The particular colour filter applied to each sensor pixel 310 may be selected to provide a selected overall appearance for the sensor array 300. For example, an image may be provided on the sensor array 300. The image will be a static image. The static image may be formed by the filtering performed by each colour filter of each sensor pixel 310. This includes colours, as per the different colours being filtered (e.g. R, G, B, VV), as well as grayscaling for those colours (e.g. with K, black, filtering) As such, the sensor pixels 310 may effectively also provide pixels of the static image. Each pixel of the static image will appear (e.g. will be coloured) according to the colour filter(s) of the corresponding sensor pixel 310 which provides that pixel of the static image. To provide a desired appearance for the sensor, an arrangement of colour filters may be chosen for each of the different sensor pixels 310 in the array 300 so that such an arrangement of colour filters will appear to provide a corresponding static image.

As will be appreciated in the context of the present disclosure, the particular appearance chosen for the sensor array 300 should not be considered limiting.

The colour filter(s) to be applied to the sensor pixels 310 may be chosen to provide colour matching. For example, the colour filter(s) to be applied to the sensor pixels 310 of the sensor array 300 may be selected to match surroundings to that sensor (e.g. to provide colour matching between the sensor array 300 and the surroundings to that sensor array 300, when installed in its intended location). The sensor array 300 may be monochromatic. Each sensor pixel 310 in the sensor array 300 may comprise the same colour filter. The sensor could be used to provide a touchpad, or a portion of a touchpad, (e.g. for a laptop). The sensor may provide capacitive biometric skin contact sensing functionality for that touchpad. Additionally, when the touchpad is installed, it may be surrounded by material (e.g. part of the laptop) which is all of a certain colour, and the colour filters applied to the sensor pixels 310 of the sensor array 300 may be colour matched to the colour of the material which surrounds the touchpad. That way, there may be a uniformity to the appearance of the device (e.g. laptop) which houses that touchpad (even though the touchpad may also provide capacitive biometric skin contact sensing functionality).

As described above, each pixel 310 may have a first portion 311 for providing colour filtering according to a selected colour, and a second portion 312 which blocks light around the perimeter of the pixel 310. The colour filters to be applied to the sensor array 300 may include more than one different colour. For example, the sensor array 300 could provide a static image, such as a photo, a logo, text (e.g. coloured text), by applying relevant different colour filters to the first portions 311 of individual sensor pixels 310 in the sensor array 300. Similarly, text may be shown on the sensor array 300 by using different colour filters for the sensor pixels 310 to show the writing. The colour filter material chosen may be selected to have certain thermal response properties. For example, a material may be used which changes colour when touched (e.g. in response to increased localised heating due to contact with a person's finger). As such, the sensor array 300 may be configured to respond to interaction with a user, such as to illuminate a certain portion of the sensor which was touched.

As will be appreciated, the sensor pixel resolution for the sensor array 300 will be much greater than that which the human eye can resolve. In other words, the human eye will not be able to resolve (unaided) the colour filtering being provided by each individual pixel 310 in the array 300. As a result, sensor arrays of the present disclosure may utilise only a few different colour filters for the sensor pixels 310 of the array 300. For example, blue, green, red, black and white filters may be used, and these different colour filters may be combined to provide all of the different colours and shades for the sensor array. Blue, green and/or red colour filters may be combined to provide additional colour filtering (e.g. where the combination of two or more different colours makes another colour). Black and white colour filters may be combined with one or more other colours to provide grayscaling effects, e.g. to provide different shades of the same colour (by combining one colour with black/white to make a different shade of that colour). For example, variable amounts of black colour filter may be used to control how dark another colour appears (e.g. a higher proportion of black resulting in a darker colour).

Two examples of combining different colour filtering options will now be described with reference to Insets B and C in Fig. 3. Inset B shows an aperture ratio control technique being applied to the sensor pixels 310, and Inset C shows a pixel dithering technique being applied to the sensor pixels 310.

Inset B of Fig. 3 shows a column of three sensor pixels 310. For these three pixels, an aperture ratio for the colour filter in each pixel is varied. As described above, the sensor pixel 310 may have a first portion 311 (over which a colour filter may be placed) and a second portion 312 (over which a light blocking material may be placed). In Inset B, the first portion includes two different regions: first region 320a and second region 320b, and the second portion surrounds those two regions.

Within the first portion 311 of the pixel 310, two different colour filters may be applied (one to the first region 320a and one to the second region 320b). To provide maximum amount of colour filtering the full area of the first portion 311 of the pixel 310 is covered with a colour filter (R, G, B, \AO and a minimum amount of black filter material is used (i.e. just that in the second portion 312 of the pixel 310). The first region 320a may be configured to filter a different colour to the second region 320b. In Inset B, the first region 320a is shown as surrounding the second region 320b. For example, the first region 320a may form an outer perimeter which surrounds an inner region (the second region 320b). The second region 320b may be a square which lies inside (the centre of) the first region 320a (which may have a square perimeter). However, other shapes may be utilised (e.g. circular, rectangular or other polygonal shapes), and/or the first and second regions could be arranged in a number of different ways with respect to each other.

The colour filter for the first portion 320a and the (different) colour filter for the second portion 320b may be selected so that they combine to provide a desired outcome colour. For example, by using two different colour filters, the sensor pixel 310 may appear according to a combination of those two colour filters. Any combination of colour filters may be provided.

Alternatively, one of the first portion 320a and the second portion 320b will be covered with a black filter, and the other of the portions will be covered with a chosen colour filter. The ratio of the area of the first portion 320a relative to the area of the second portion 320b may be selected to provide a desired colour combination for the pixel 310. For example, where one of the portions is covered with black colour filter, by varying the ratio of pixel area covered by black filter to pixel area covered by other colour filter, a grayscaling (or darkness) may be chosen for that pixel.

This arrangement of varying the aperture ratio may find particular utility for providing colour grayscaling. For this, the aperture ratio for each pixel may be varied by utilising a black colour filter for one of the first and second regions of the sensor pixel 310. For example, the first region 320a may be black, and the second region 320b may be another filter colour (e.g. red, green, blue, white), or vice-versa. In examples where a black colour filter overlies the first region 320a, this black colour filter may be provided in combination with a black colour which overlies the second portion 312 of the sensor pixel 310. That is, as described above, the second portion 312 of each sensor pixel 310 may be overlayed by a black colour filter (such as black matrix material), and the border this second portion 312 provides around the colour filter may effectively be thickened so that this black colour filtering material also overlies the first region 320a of the first portion 311 of the sensor pixel 310. In other words, the first portion 311 of each sensor pixel 310 may represent the maximum amount of area per that pixel 310 to which a colour filter can be applied. As such, the maximum aperture ratio for each pixel 310 involves applying the colour filter(s) to cover the entirety of the first portion 311. This aperture ratio may be reduced by covering some of the first portion 311 with a black colour filter (i.e. effectively shrinking the area of the pixel 310 which is covered by a colour filter). The more this area is shrunk, the darker the light filtering provided by that pixel 310 will be.

For the sensor pixels in Inset B, the aperture ratio may be varied by varying the thickness of this black filtering portion of the pixel 310 (i.e. by varying the thickness of black colour filter material surrounding the colour filter of the pixel). For example, in Inset B, the first region 320a will be black and the second region 320b will be red (and also the second portion 312 which surrounds the first region 320a will also be black). The appearance (e.g. colour grayscale) of red will get darker down the column. In the topmost pixel shown in Inset B, there is not much black surrounding the red, and so the pixel would have a relatively strong red colour (a more saturated red colour). For the two pixels beneath it, there is an increasing proportion of black colour filter per pixel, and so these pixels will appear increasingly darker red (e.g. the red grayscale will be darker). For the lower pixels, there is a smaller aperture ratio, as more of the light will be filtered out by that black filter, and less will only pass through the red filter. In other words, the ratio of the area of the pixel 310 covered by black colour filtering material (e.g. light blocking material, as applied to the second portion 312 and the first region 320a) to the area of the pixel 310 covered by another colour filtering material (e.g. as applied to the second region 320b) is selected to provide a desired colour grayscale for that another colour.

In Inset B, different colour filters are applied within a single sensor pixel to provide a desired colouring effect for that pixel. Inset C shows an additional or alternative approach in which different colour filters are applied to different adjacent pixels to provide a desired colouring effect for a particular region of the sensor array.

Inset C shows a 2 x 3 grid of sensor pixels 310. A plurality of different colour filters are shown in inset B (four different colour filters are shown, but of course there could be more or fewer used). A pixel dithering technique is employed in which a desired overall appearance for a particular region of the sensor array is achieved by selecting colour filtering to be provided within each of a plurality of different pixels within that particular region. For instance, different colour filters may be used for different pixels within a cluster of pixels in the array, and the particular colour filtering to be provide by each pixel in that cluster is selected to provide a desired overall appearance for a region of the sensor array which is bigger than just one pixel.

To illustrate example pixel dithering functionality, four different colour filter colours are shown in Inset C (first to fourth colours, 321 to 324). Each pixel may also have black colour filtering provided in the second portion 312 which traces the perimeter of that pixel. The sensor pixels 310 are adjacent to each other, and they each individually cover a small area, so that a region of the sensor array 300 containing the 2 x 3 cluster of pixels will appear coloured according to a combination of the different colour filtering being performed by each pixel. Although a 2 x 3 grid is shown, it will be appreciated that this pixel dithering may be provided over a much larger area of the sensor array 300.

To provide pixel dithering, different colour filtering may be performed by different pixels within a cluster of pixels. For example, a first colour filter 321 may be applied to one pixel in the cluster, and a second (different) colour filter 322 may be applied to one or more adjacent pixels (two are shown as second colour filter 322 in Inset C). As the human eye may only resolve the colour filtering as provided by a larger area than the size of one pixel (i.e. a cluster of many pixels), colour filtering performed by each of a plurality of neighbouring pixels may effectively combine (e.g. to appear as though one single colour filter has been used). In other words, the cluster of pixels may appear according to a combination of the different types of colour filtering being performed by individual pixels within that cluster (e.g. according to first and second colour filtering).

By using such a pixel dithering approach, a greater number of different possible combinations of colour filters may be provided. For example, a higher resolution of colour grayscaling could be provided. That is, in order to provide a chosen colour (e.g. a chosen shade, or darkness etc.), different colour filters could be provided to each of a plurality of different neighbouring pixels so as to obtain the chosen colour when viewing the region as a whole. For example, within a particular region of the sensor, the amount of different colour filters applied can be varied, e.g. so that the ratio of different colour filters used within that region provides the desired overall appearance for that region as a whole. As an example, where varying the aperture ratio of a pixel (between black filtering and colour filtering) can vary the darkness for colour filtering by that pixel, the same effect may be achieved by instead applying the colour filter to some of the pixels (e.g. to the whole area of each said pixel) and applying the black filter to other pixels, where the ratio of colour to black filtering is the same for both (but this is imparted on a pixel by pixel basis rather than a sub-pixel basis. Of course, the two techniques could also be combined (as shown by the bottom two pixels in Inset C).

The pixel dithering may therefore provide more freedom for a selected static image of the sensor. For instance, within the small cluster of pixels, a third colour filter 323 could be used. Likewise, pixels may be provided which each contain two or more different colour filters (e.g. 322a and 324b, and 323a and 324b). For example, such pixels may utilise the variable pixel aperture ratio approach described above in relation to Inset B. Thus, the scope for varying the appearance of each region within the sensor array may be increased by providing a greater variety of colour filtering performed within a cluster of pixels in that region. That is, different colour filters may be applied to different pixels within a region, where the different colour filters applied (and the amount of pixels to which they are applied) are controlled so that the resulting combination from the different colour filtering performed by the different pixels, when viewing that region as a whole (as the human eye will), appears according to a selected appearance.

The pixel dithering or variable aperture ratio techniques may be used to provide blended transitions between different portions of the sensor array 300 (where different colours are filtered). For example, the pixels may extend from a first region where one colour filter is used through to a second region where a different colour filter is used. Between those two regions may be a transition region which includes pixels of both colour filters arranged to gradually change the colour from the colour of the first region through to the colour of the second region.

The pixel dithering or variable aperture ratio techniques may be used to provide colour grayscaling for the sensor array 300. For this, black or white colour filters may be used for some of the sensor pixels 310 in an area (or for some regions within sensor pixels), and the other sensor pixels 310 in that area (or other regions of sensor pixels within that area) may be of one or more selected colour(s). By varying the spatial density of black/white colour filtering in that area, a brightness for the overall colour seen in that region may be controlled. For example, for each cluster of pixels, there may be a certain proportion of black/white colour filtered pixels, as well as the relevant other colour filtered pixels (e.g. red pixels), and/or for each pixel there may be a certain proportion of black/white relative to said another colour. The darkness of that relevant other colour (e.g. red) may be controlled by selecting that spatial are of black colour filtering in that region (with more black pixels, and/or more black area, leading to a darker shade of that colour being shown).

For the pixel dithering approach, the same colour filter may be applied to more than one pixel in the same region. For example, different colour filters may be applied to clusters of pixels rather than just individual pixels. For example, a colour filter may be applied to each pixel in a 2 x 2 grid of pixels, and the arrangement of the different clusters of pixels (i.e. of different 2 x 2 grids of pixels) may be controlled to provide the desired colouring effect (e.g. to provide a desired colour or a desired grayscale effect for the colour in that area). Some pixels may not have colour filters at all. The number, and/or arrangement, of pixels without colour filters may be selected to provide a desired appearance property to the sensor.

It is to be appreciated in the context of the present disclosure that the different examples could be combined, or used interchangeably. For example, two different colours may be provided on the same pixel to provide another colour. Similarly, each pixel may have two different colour filters, but those pixels could also be used for pixel dithering with other adjacent pixel colour filters (which may have one or two colour filters). A pigment density and/or colour filter layer thickness may be selected to provide a desired saturation for the colour filtering (e.g. where increasing either will increase the colour saturation).

In view of the above, a capacitive biometric skin contact sensor may be designed to have a selected appearance. For example, that selected appearance may be a particular colour (or a plurality of particular colours), or it may show a certain static image or message. Once the selected appearance for the sensor is known, a corresponding arrangement for the colour filters of the sensor array 300 may be determined. The colour filters may be chosen so that they will cause the image to appear according to the selected appearance due to the optical filtering of light passing through those colour filters to a user of the sensor.

The colour filters may be applied so that the colour filter layer 14 is less than one micron thick.

The material for the colour filter may be selected to have a high dielectric constant. Additionally, or alternatively, the different colour filter materials for the colour filter layer 14 (e.g. for the different colours) may be matched. That way, the range/performance of each sensor pixel may be the same (despite different colour filtering). The capacitance to be sensed using the capacitive sensing electrode may therefore remain relatively unchanged as compared to a corresponding sensor pixel 310 which did not include a colour filter layer 14. Nevertheless, a calibration method may be performed for a sensor which has been manufactured to include a colour filter layer 14 in the sensor pixel stack. For example, an object with a known expected capacitive response may be sensed by the sensor. To the extent that the read-out from any of the sensor pixels 310 deviates from the expected values for those sensor pixels 310, a calibration may be applied to account for this deviation. For example, the sensor may comprise a controller which stores calibration data for read-out signals. The calibration data may be for some or all (e.g. for each) of the sensor pixels 310 in the sensor array 300. The calibration data may provide a mapping between an observed value, as measured using that sensor, and an adjusted value to which that observed value should correspond. For example, a black noise image calibration could be used. For this, the sensor may be operated to obtain capacitance data when no object is in proximity to the sensor array 300. The resulting capacitance data may be stored as a calibration data. That calibration data may then be used to calibrate subsequently obtained data (e.g. to subtract the influence of the black noise data from newly obtained data for an object). For example, this may facilitate calibration of the sensor to account for any differences in dielectric properties associated with the colour filter materials (e.g. differences between the R, G, B, W, K filter materials).

In examples described herein, a colour filter layer 14 is provided above a reflective element 21 and/or a transmissive element 22. That way, light travelling from said element towards a user of the sensor will be optically colour filtered according to the colour filter layer 14 in that pixel. As shown in Inset A of Fig. 3, each sensor pixel 310 may include a first portion 311 and a second portion 312. Light will be blocked by the second portion 312, so that light may only pass through the first portion 311 of the colour filter layer 14 (which may thus provide colour filtering thereof). The appearance of the sensor array 300 may be improved with a greater amount of light travelling through the colour filter (as well as a greater proportion of all light which reaches the user from the sensor travelling through a colour filter). The first portion 311 may take up a relatively large portion of the sensor pixel 310 (e.g. as large a portion as possible, except where aperture ratio techniques are applied with a greater proportion of black filter material provided to darken the apparent colour of that pixel).

Each sensor pixel 310 itself may be arranged to increase the amount of light which passes through the colour filter. For a transmissive colour filtering sensor pixel, light may take a path from the transmitting element 22 to the colour filter and through the colour filter to the user. For a reflective colour filtering sensor pixel, light may take a path into the sensor pixel 310 (from above) and through the sensor pixel 310 (and colour filter) until it reflects of a reflective element 21 of the sensor pixel 310, where that light will then be reflected and travel back upwards through the sensor pixel 310 (and colour filter) towards the user. For both types of sensor pixel 310, any non-transparent components of the sensor may be arranged so that they do not lie in these paths for light travelling through the sensor pixel 310. For example, any non-transparent components of the sensor may be provided in layers of the sensor pixel 310 beneath the reflective element 21 (or beneath the transmitting element 22). Additionally, or alternatively, any non-transparent components of the sensor may be provided underneath the second portion 312 of the sensor pixel 310 (where light will in any case be blocked, e.g. by black matrix material in the colour filter layer 14). As such, a greater proportion of available light may pass through the colour filter 14 (rather than being blocked by intervening components within the sensor pixel circuitry).

At least some of the components of the sensor could be provided by optically transparent materials. For example, electrical conductors could utilise a suitable transparent electrical conductor, such as Indium Tin Oxide (1T0'). This arrangement may enable light to pass through such conductive elements of the sensor without any substantial attenuation (e.g. with a minimal amount of the light being blocked). For example, the capacitive sensing electrode could be provided by a transparent material (rather than e.g. an optically reflective one). The colour filter layer 14 could for example be provided beneath the capacitive sensing electrode layer 114. The reflective or transmitting element 22 could be provided beneath the capacitive sensing electrode layer 114. For example, any non-transparent components, such as the one or more TFTs of the sensor pixel 310 could be located either beneath the transmitting/reflecting element(s) and/or underneath the second portion 312 of the sensor pixel 310. For example, in transmissive colour filter arrangements, the capacitive sensing electrode may be transparent (so too may be the electric shield, if included). Other non-transparent components (e.g. in M1 or M2) could be located beneath the second portion 312. For example, in the reflective colour filter arrangements, all of the layers (M1 to M4) may be provided by an optically reflective electrically conductive material.

In examples with a light transmitting element 22, the sensor may be configured to control operation of the light transmitting element 22. For example, the sensor may comprise a controller configured to selectively turn on or off one or more light emitting regions of the light transmitting element 22. The light transmitting element 22 may comprise a plurality of individual elements, each of which may be arranged to direct light towards a portion of the sensor array 300. The controller of the sensor may be configured to select which portion of the sensor array is illuminated by elements of the light transmitting element 22. For example, the controller may control operation of the transmitting element 22 so that some, but not all, of the sensor array is illuminated. This may mean that colour filtering only appears for a portion of the sensor array 300.

The sensor array 300 may be arranged so that the colour filters in different portions of the array 300 are different. The different colour filters may provide an indication of some information about operation of the sensor. For example, a green filter may indicate positive feedback (e.g. success at verification etc.), whereas a red filter may indicate an issue. The controller may be configured to selectively operate the light transmitting element 22 to direct light to a relevant portion of the sensor array. For example, in response to determining that biometric authentication was successful, light may be directed through a first portion of the sensor array (to use a first subset of the colour filtered pixels), and in response to unsuccessful biometric authentication, light may be directed through a second portion of the sensor array (to use a second subset of the colour filtered pixels). For example, the light may be directed through a certain subset of colour filters to indicate to a user that they should interact with that subset of sensor pixels in the sensor array.

The present disclosure may provide a capacitive biometric skin contact sensor formed of a plurality of sensor pixels, each having a colour filter layer 14 for providing a desired appearance to the sensor. The particular componentry for each sensor pixel should not be considered limiting. Typically, each sensor pixel will include a capacitive sensing electrode (for sensing proximity of a conductive object to be sensed) and at least one TFT (to give a readout signal proportional to the capacitance sensed by the electrode). For example, the TFT may be operated to selectively provide a read-out signal indicative of the capacitance of the sensing electrode. The pixel may include a reference capacitor (or electrical shield) for shielding the sensing electrode from parasitic capacitances in the sensor array 300. The pixel may include other TFTs, such as a TFT for selectively activating individual sensor pixels, and/or a TFT for controlling biasing and/or reset circuitry for that pixel. The sensor array 300 may include a plurality of gate drive channels for providing gate drive signals to sensor pixels, and a plurality of read-out channels for receiving read-out signals from sensor pixels.

For such sensor pixels, the capacitive sensing electrode may be a first electrode (e.g. plate) which is arranged to effectively form a capacitor in response to proximity to the first electrode of a conductive body to be sensed (e.g. skin of a user, such as of their finger). By detecting changes in capacitance of the capacitive sensing electrode brought about by the proximity to the electrode of the conductive body, biometric data may be obtained for that conductive body. Biometric data may comprise any suitable biometric marker, such as an indication of the arrangement of ridges and valleys in the user's skin, and/or other skin markers such as sweat pores or glands etc. Two example sensor pixel designs will now be described with reference to Figs. 4a and 4b.

Fig. 4a shows a sensor pixel 420 for a capacitive biometric skin contact sensor. The sensor pixel 420 of Fig. 4a includes a capacitive sensing electrode 424 and a voltage-controlled impedance shown as a thin film transistor ('TFT') and referred to hereon in as 'sense TFT 430'. The capacitive sensing electrode 424 is shown with a variable capacitor symbol. It will be appreciated that the capacitive sensing electrode 424 is formed of one electrode (e.g. a plate), and the variable capacitance for this will effectively be provided by a user interacting with that one electrode (e.g. due to the proximity of a portion of the user's skin proximal to the electrode). As will be appreciated, the capacitance associated with this capacitive sensing electrode 424 will vary in dependence on the proximity of the user's skin to the capacitive sensing electrode 424. The sensor pixel also includes a reference capacitor 422 (e.g. an electrical shield, as described above). For example, for the reference capacitor 422, the capacitive sensing electrode 424 may provide one plate of the reference capacitor 422, and an electrical shield layer 113 may provide a second plate of the reference capacitor 422. A first gate drive channel 411 is shown in Fig. 4a, as is a first read-out channel 421. The sensor pixel 420 of Fig. 4a also includes a select TFT 440, a select reference connection 442, a reset TFT 450, a first reset reference connection 452, and a second reset reference connection 454.

A first region of the sense TFT 430 is connected to the select TFT 440. A second region (e.g. a control terminal) of the sense TFT 430 is coupled to the capacitive sensing electrode 424. A third region of the sense TFT 430 is coupled to the first read-out channel 421. The second region of the sense TFT 4130 may be a gate region. The first region of the sense TFT 430 may be a drain region and the third region of the TFT 430 may be a source region. A first electrode of the reference capacitor 422 is coupled to the first gate drive channel 411. A second electrode of the reference capacitor 422 is coupled to the capacitive sensing electrode 424 and the second region of the sense TFT 430. As such, a connection between the second region of the sense TFT 430 and the capacitive sensing electrode 424 is also connected to the second electrode of the reference capacitor 422.

The select TFT 440 is coupled to the sense TFT 430 to selectively inhibit the sense TFT 430 from outputting a read-out signal to the first read-out channel 421. The select TFT 440 has a conductive channel connected in series between a reference signal supply and the sense TFT 430. A first region of the select TFT 440 is arranged to receive the reference signal supply (via the select reference connection 442). The second region of the select TFT 440 is coupled to the first gate drive channel 411, and a third region of the select TFT 440 is coupled to the first region of the sense TFT 430.

Additionally, reset circuitry is also coupled to the second electrode of the reference capacitor 422 (and thus the second region of the sense TFT 430 and the capacitive sensing electrode 424). The reset circuitry is configured to selectively tune the second region of the sense TFT 430 to a reference voltage (e.g. to provide a selected sensitivity for the pixel 420). A first region of the reset TFT 450 is coupled to the second electrode of the reference capacitor 422, the capacitive sensing electrode 424, and the second region of the sense TFT 430. A second region of the reset TFT 450 is arranged to receive a reset voltage (e.g. via the first reset reference connection 452). The first reset reference connection 452 may be connected to a preceding gate drive channel of the sensor. The reset circuitry is arranged so that, in response to the second region of the reset TFT 450 receiving the reset voltage, a conductive channel is opened between the first and third regions of the reset TFT 450. Current may flow either way through this channel (e.g. it could be arranged to permit current flow in either direction). For example, current may flow into the pixel 420 to charge the second region of the sense TFT 430 to a selected voltage (e.g. to tune its sensitivity), or current may flow away from the pixel to discharge the second region of the sense TFT 430. The second reset reference connection 454 thus connects the reset TEl 450 to provide relevant current flow (e.g. it is either connected to a reset reference voltage, or to distribute current elsewhere away from the pixel 420).

Fig. 4b shows a sensor pixel 420 for a capacitive biometric skin contact sensor. As with Fig. 4a, the sensor pixel 420 includes a capacitive sensing electrode 424, a reference capacitor 422 (e.g. an electrical shield layer 113 in combination with the capacitive sensing electrode 424), a sense TFT 430, a select TFT 440, a select reference connection 442, a reset TFT 450, and a first reset reference connection 452. As shown, the pixel 420 is connected to a first gate drive channel 411 and a first read-out channel 421. Also, the pixel 420 of Fig. 4b includes biasing circuitry comprising a bias TFT 460, and a bias reference connection 462.

The sensor pixel 420 shown in Fig. 4b is described in more detail in the Applicant's pending application GB 2013864.0. The structural arrangement of this sensor, the function of the sensor and the individual components of the sensor, and the method of operation as described in GB2013864.0 is incorporated herein by reference for all purposes.

The sensor pixels 420 of Figs. 4a and 4b are similar in that they may each receive a gate drive signal from a gate drive channel which in turn gives rise to a capacitive potential divider arrangement involving the capacitive sensing electrode 424 and the reference capacitor 422.

Likewise, this capacitive potential division controls operation of a TFT (sense TFT 430) to regulate the current output from the pixel in dependence on the proximity to the capacitive sensing electrode 424 of a conductive body to be sensed.

The sensor pixel 420 of Fig. 4b includes biasing circuitry comprising a one-way conduction path from a bias voltage connection to a control terminal of the sense TFT 430 so that current flows from the bias voltage towards the control terminal of the sense TFT 430 in response to the control terminal voltage of the sense TFT 430 dropping below a floor value. In other words, the biasing circuitry of the sensor pixel 420 is arranged to ensure that prior to making a measurement, the voltage at the control terminal (e.g. gate region) of the sense TFT 460 is at a selected value (e.g. a predefined voltage). The bias voltage may be varied to provide a selected voltage at the gate region of the sense TFT 430 (e.g. to provide a selected level of sensitivity for the sensor, or a define operation point for starting operation of the pixel). As shown in Fig. 4b, the biasing circuitry comprises a connection to the bias voltage (via bias reference connection 462) and the bias TFT 460. The bias TFT 460 is connected in diode configuration to provide the one-way conduction path. The drain of the bias TFT 460 is coupled to each of the second electrode of the reference capacitor 422, the capacitive sensing electrode 424 and the gate region of the sense TFT 430. As with Fig. 4a, the sensor pixel 420 of Fig. 4b includes reset circuitry selectively operable to provide a reference voltage on the reference capacitor 422. Similarly, the sensor pixel 420 of Fig. 4b may include select circuitry to selectively couple the sense TFT 430 to the supply voltage.

It will be appreciated in the context of the present disclosure that other sensor pixel designs could also be used. Also, sensor pixels have been described as being square, but it is to be appreciated that other shapes could be used. For example, the sensor pixel shapes may be selected with particular shapes or geometries based on the static image which will appear due to the colour filtering provided by those pixels. That is, the shape of each sensor pixel may provide the shape of each pixel of the static image, and so the sensor pixel may be shaped based on the desired display static image pixel shape. For example, the pixels may be triangular, rectangular, or any other suitable shape.

Figs. la and lb show optically reflective and optically transmissive sensors respectively.

However, it will be appreciated that the present disclosure also provides sensors which utilise both aspects in combination. One such sensor is shown in Fig. 5.

Fig. 5 shows a sensor which has both reflective elements 21 and a light emitting element 22, as well as colour filter 14. The sensor may be arranged to maximise an amount of light which may pass through colour filter 14. For this, intervening components of the sensor located between the light emitting element 22 and the colour filter 14 may be made of transparent materials and/or they may be clustered to try to minimise the amount of light from the light emitting element 22 that they block. For example, such elements may be located around the perimeter of the pixel (e.g. where they may be located underneath a black filtering portion of the pixel). Optically reflective material may also be used in such regions to increase the amount of light reflected by the sensor pixel (thereby to provide additional light to that emitted from the light emitting element 22). In other words, each pixel may be arranged so that light passing through the colour filter 14 to the user will include both reflected light from the reflective elements 21 and transmitted light emitted from the light emitting element 22. The arrangement of the reflective elements 21 and the emitting element 22 may be selected to maximise the amount of filtered light leaving each pixel.

It will be appreciated from the discussion above that the examples shown in the figures are merely exemplary, and include features which may be generalised, removed or replaced as described herein and as set out in the claims. With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein.

As will be appreciated by the skilled reader in the context of the present disclosure, each of the examples described herein may be implemented in a variety of different ways. Any feature of any aspects of the disclosure may be combined with any of the other aspects of the disclosure. For example method aspects may be combined with apparatus aspects, and features described with reference to the operation of particular elements of apparatus may be provided in methods which do not use those particular types of apparatus. In addition, each of the features of each of the examples is intended to be separable from the features which it is described in combination with, unless it is expressly stated that some other feature is essential to its operation. Each of these separable features may of course be combined with any of the other features of the examples in which it is described, or with any of the other features or combination of features of any of the other examples described herein. Furthermore, equivalents and modifications not described above may also be employed without departing from the invention.

Certain features of the methods described herein may be implemented in hardware, and one or more functions of the apparatus may be implemented in method steps. It will also be appreciated in the context of the present disclosure that the methods described herein need not be performed in the order in which they are described, nor necessarily in the order in which they are depicted in the drawings. Accordingly, aspects of the disclosure which are described with reference to products or apparatus are also intended to be implemented as methods and vice versa. The methods described herein may be implemented in computer programs, or in hardware or in any combination thereof. Computer programs include software, middleware, firmware, and any combination thereof. Such programs may be provided as signals or network messages and may be recorded on computer readable media such as tangible computer readable media which may store the computer programs in non-transitory form. Hardware includes computers, handheld devices, programmable processors, general purpose processors, application specific integrated circuits (ASICs), field programmable gate arrays (FPGAs), and arrays of logic gates.

Other examples and variations of the disclosure will be apparent to the skilled addressee in the context of the present disclosure.

Claims

Claims 1. A capacitive biometric skin contact sensor comprising a sensor array of sensor pixels, wherein: each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode; a top surface of the sensor array provides a contact surface for contacting by an object to be sensed; and each sensor pixel is formed of a plurality of layers including an optical colour filter layer arranged to filter one or more colours of light.
2. The sensor of claim 1, wherein the plurality of layers includes an optically reflective layer located below the optical colour filter layer and comprising an optically reflective element.
3. The sensor of claim 2, wherein for each sensor pixel, the capacitive sensing electrode provides the optically reflective element.
4. The sensor of any of claims 2 to 3, wherein, for each sensor pixel, the optically reflective layer is a first optically reflective layer, and the sensor pixel also includes a second optically reflective layer comprising an optically reflective element, wherein the optical colour filter layer is located above the reflective elements of both reflective layers.
5. The sensor of any preceding claim, wherein each sensor pixel comprises an electrical shield layer comprising an electrical shield.
6. The sensor of claim 5, wherein the capacitive sensing electrode is located above the electric shield.
7. The sensor of claim 6, wherein both the capacitive sensing electrode and the electric shield provide optically reflective elements for each sensor pixel, and wherein the optical colour filter layer is located above the capacitive sensing electrode and the electric shield.
8. The sensor of any preceding claim, wherein all electrically conductive components of the sensor pixel provide optically reflective elements, optionally wherein each electrically conductive component of the sensor pixel is made of an optically reflective material, such as an aluminium alloy.
9. The sensor of any preceding claim, wherein, for each sensor pixel, the optical colour filter layer is provided on a top surface of the capacitive sensing electrode.
10. The sensor of any preceding claim, wherein, for each sensor pixel, a passivation layer is provided on a top surface of the capacitive sensing electrode, and wherein the optical colour filter layer is provided on a top surface of the passivation layer.
11. The sensor of any preceding claim, wherein, for each sensor pixel, a hard coat is provided on top of the sensor pixel, and wherein the optical colour filter layer is provided on a top surface of the hard coat, optionally wherein a hydrophobic coating is provided on a top surface of the sensor pixel.
12. The sensor of any preceding claim, wherein for at least some of the sensor pixels, an aperture ratio for the colour filter is varied to provide a selected property for the colour filtering provided by said pixels.
13. The sensor of any preceding claim, wherein, for at least some of the sensor pixels, the colour filter layer for the sensor pixel includes a first colour filter and a second colour filter, and wherein the first colour filter is for a different colour to the second colour filter.
14. The sensor of any preceding claim, wherein at least one of: the sensor includes an optically reflective layer located below the optical colour filter layer and comprising an optically reflective element, wherein any non-transparent components of the sensor pixel in layers between the optically reflective layer and the colour filter layer are spatially arranged to inhibit blocking of light travelling between the optical reflective layer and the optical colour filter layer; and the sensor is arranged to be backlit by a transmitting element located below the optical colour filter layer, and wherein any non-transparent components of the sensor pixel in layers between the transmitting element and the colour filter layer are spatially arranged to inhibit blocking of light travelling between the transmitting element and the optical colour filter layer.
15. The sensor of claim 14, wherein said non-transparent components of the sensor pixel are arranged to be laterally offset from an area of the sensor underneath the colour filter.
16. The sensor of any preceding claim, wherein, for each sensor pixel, the capacitive sensing electrode is at least partially optically transparent.
17. The sensor of claim 2, or any claim dependent thereon, wherein each sensor pixel comprises at least one of: (i) an optically reflective electric shield; (ii) optically reflective source and/or drain conductive elements; (iii) an optically reflective gate conductive element; (iv) a substrate onto which the sensor pixel is built, wherein a surface of the substrate is optically reflective, optionally wherein the surface is a bottom surface of the substrate; thereby to provide the optically reflective element of the sensor pixel.
18. An apparatus comprising the sensor of any preceding claim and a light transmitting element, wherein the light transmitting element is arranged beneath the colour filter layer of the sensor pixels of the sensor, optionally wherein the light transmitting element is part of the sensor or is provided by a separate component to the sensor.
19. A method of manufacturing a capacitive biometric sensor, the sensor comprising a sensor array of sensor pixels, wherein a top surface of the sensor array provides a contact surface for contacting by an object to be sensed, and wherein each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode, the method comprising: for each sensor pixel, providing a plurality of layers for that sensor pixel including an optical colour filter layer.
20. The method of claim 19, wherein providing the optical colour filter layer for the sensor pixels of the array comprises use of a photolithography method.
21. The method of claim 19 or 20, wherein providing the plurality of layers comprises providing the capacitive sensing electrode for the sensor pixel, wherein the capacitive sensing is optically reflective and provided in a layer beneath the optical colour filter layer, thereby to provide an optically reflective layer beneath the colour filter layer.
22. The method of any of claims 19 to 21, wherein providing the optical colour filter layer for each sensor pixel comprises depositing an optical colour filter above the capacitive sensing electrode.
23. A method of designing a capacitive biometric sensor, the sensor comprising a sensor array of sensor pixels, wherein a top surface of the sensor array provides a contact surface for contacting by an object to be sensed, wherein each sensor pixel comprises at least one thin film transistor, TFT, and a capacitive sensing electrode, and wherein each sensor pixel is formed of a plurality of layers including an optical colour filter layer arranged to filter one or more colours of light, the method comprising: obtaining an indication of a selected appearance for the top surface of the sensor; and selecting at least one optical property for the optical filter of each sensor pixel of the array based on the selected appearance for the top surface of the sensor.
24. The method of claim 23, wherein the selected appearance for the top surface of the sensor comprises a spatial distribution of one or more colours across the sensor array; and wherein selecting the at least one optical property for the optical filter of each sensor pixel of the array comprises selecting a colour for said optical filter, wherein said colour is selected according to the spatial distribution of colours for the sensor array.
25. A computer program product comprising computer program instructions configured to control operation of a sensor manufacturing assembly to manufacture a capacitive biometric sensor according to the method of any of claims 19 to 22 and/or to control operation of a processor to design a capacitive biometric sensor according to the method of claims 23 and 24.