WO2021094705A1

WO2021094705A1 - Display device for displaying a pattern, method of manufacturing a display device

Info

Publication number: WO2021094705A1
Application number: PCT/GB2020/052515
Authority: WO
Inventors: Clement TALAGRAND; Peiman HOSSEINI
Original assignee: Bodle Technologies Ltd
Priority date: 2019-11-12
Filing date: 2020-10-09
Publication date: 2021-05-20
Also published as: GB201916438D0

Abstract

The disclosure relates to a display device for displaying a pattern. In one arrangement, the display device has a front plane portion comprising a switching layer with a plurality of pixel regions and a back plane portion with a plurality of individually addressable drive elements. Each drive element comprises a heater and an interaction between each drive element and a respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer. A pattern defined by a spatially inhomogeneous thermal conductivity of the heat transmission layer is either non-periodic or periodic with a unit cell that is smaller in at least one direction within the plane of the heat transmission layer than a unit cell of a pattern defined by the plurality of drive elements.

Description

DISPLAY DEVICE FOR DISPLAYING A PATTERN, METHOD OF MANUFACTURING A DISPLAY DEVICE

The present invention relates to a display device and methods of manufacturing a display device. The display device may be configured for use in a range of contexts, including consumer electronics, industrial signage and user interfaces.

Phase change material (PCM) based reflective display devices, in which an optical switching effect in a front plane portion is thermally modulated, and a light modulating portion of the front plane portion is electrically isolated from drive elements (e.g. heaters) and electronics in a back plane portion of the device, are disclosed in WO2017134506A1 and related publications ‘Garcia Castillo, S. et al., “57-4: Solid State Reflective Display (SRD®) with LTPS Diode Backplane”, SID Digest, 50, 1, pp 807-810 (2019)’ and ‘Broughton, B. et al., “38-4: Solid-State Reflective Displays (SRD®) Utilizing Ultrathin Phase-Change Materials”, SID Digest, 48, 1, pp 546-549 (2017)’.

W02018109430A1 further discloses that a display device of the above type can be fabricated using a method whereby the front plane portion and back plane portion of the device are manufactured separately and subsequently joined using a lamination or other gluing process. This approach is desirable because it allows components of the front plane portion (e.g. a thin-film optical stack) to be manufactured more flexibly, economically and/or quickly. However, it can be more difficult to achieve accurate registration between the front plane portion and the back plane portion in comparison to alternative manufacturing processes based on lithography.

It is an object of the invention to improve manufacture of display devices.

According to an aspect, there is provided a display device for displaying a pattern, comprising: a layered structure having a front plane portion and a back plane portion, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; the heat transmission layer has a spatially inhomogeneous thermal conductivity to constrain lateral spreading of heat within a plane of the heat transmission layer; and a pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is either non-periodic or periodic with a unit cell that is smaller in at least one direction within the plane of the heat transmission layer than a unit cell of a pattern defined by the plurality of drive elements in the back plane portion.

Thus, a display device is provided in which a front plane portion comprising optical elements can be connected to a back plane portion comprising heaters for switching the optical elements without careful registration of the two portions during the connection process being required. Despite the reduced need for registration, the pattern of the heat transmission layer still provides effective localization of heat produced by each heater to a particular pixel region of the front plane portion over the heater, while also allowing heat to spread efficiently underneath the pixel region to produce a complete and uniform switching of the pixel region.

In an embodiment, the spatially inhomogeneous thermal conductivity of the heat transmission layer comprises one or more regions having a first thermal conductivity and one or more regions have a second thermal conductivity, lower than the first thermal conductivity; and material forming at least one of the regions of first thermal conductivity extends integrally through a plurality of the unit cells of the spatially inhomogeneous thermal conductivity. This approach provides an improved balance between acceptable constraining and uniformity of heating provided by the patterned heat transmission layer and acceptably low levels of optical dead space.

In an embodiment, the pattern defined by the spatially inhomogeneous thermal conductivity comprises a pattern of crack lines within the heat transmission layer. Crack lines can be provided in a non-periodic (e.g. random) form which is less visible optically than periodic alternatives. Additionally, crack lines can be produced efficiently (e.g. by mechanical flexing of the front plane portion 1) and thinly (so as to minimize optical dead space).

According to an alternative aspect, there is provided a method of manufacturing a display device for displaying a pattern, comprising: connecting a back plane portion of the display device to a front plane portion of the display device, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; the heat transmission layer has a spatially inhomogeneous thermal conductivity to constrain lateral spreading of the heat within a plane of the heat transmission layer; and a pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is either non-periodic or periodic with a unit cell that is smaller in at least one direction within the plane of the heat transmission layer than a unit cell of a pattern defined by the plurality of drive elements in the back plane portion.

According to an alternative aspect, there is provided a method of manufacturing a display device for displaying a pattern, comprising: connecting a back plane portion of the display device to a front plane portion of the display device, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; and the method comprises: transforming each of a plurality of localized regions of an intermediate layer, between the heaters and the heat transmission layer, by heating each localized region with a respective one of the heaters; and removing material above a region or regions of the intermediate layer outside of the transformed localized regions to provide the heat transmission layer with a spatially inhomogeneous thermal conductivity effective to constrain lateral spreading of heat within a plane of the heat transmission layer.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a side sectional view of a portion of a display device;

Figure 2 is a side sectional view depicting connection of a front plane portion to a back plane portion where the front plane portion is mounted on a flexible front plane support substrate;

Figure 3 is a top sectional view of a portion of a display device showing unit cells of a pattern defined by drive elements in a back plane portion; Figure 4 shows the arrangement of Figure 3 overlaid with a misaligned heat transmission layer;

Figure 5 shows the arrangement of Figure 3 overlaid with an alternative heat transmission layer having a smaller unit cell;

Figure 6 is a side sectional view of a portion of a display device having the alternative heat transmission layer of Figure 5;

Figure 7 is a top view of a further alternative heat transmission layer having integrally connected unit cells;

Figure 8 is a top view of a further alternative heat transmission layer having crack lines; and

Figures 9-11 are side sectional views of a portion of a display device showing successive stages in a method of manufacture using thermally induced transformation of localized regions in an intermediate layer to define a pattern in a heat transmission layer.

Throughout this specification, the terms "optical" and "light" are used, because they are the usual terms in the art relating to electromagnetic radiation, but it is understood that in the context of the present specification they are not limited to visible light. It is envisaged that the invention can also be used with wavelengths outside of the visible spectrum, such as with infrared and ultraviolet light.

Figure 1 depicts a display device manufactured using sequential deposition followed by lithographic patterning. Embodiments of the present disclosure relate particularly (but not exclusively) to manufacturing display devices that broadly have the functionality of Figure 1 but which do not require the same approach of sequential deposition followed by lithographic patterning for manufacture. The display device of Figure 6 is one example of such a display device, which in this case can be manufactured by laminating a pre-formed front plane portion 1 onto a pre-formed backplane portion 2.

Each type of display device (e.g. Figure 1 and Figure 6) comprises a layered structure having a front plane portion 1 and a back plane portion 2. The front plane portion 1 is connected to the back plane portion 2. The front plane portion 1 is between the back plane portion 2 and a viewing side of the device (the upper side in the orientation of Figures 1 and 6).

The front plane portion 1 comprises a switching layer 13. The switching layer 13 comprises a plurality of pixel regions. Each pixel region is switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions. The pattern may be visible pattern (e.g. visible unaided by the human eye or via a microscope) or may be a machine -readable pattern (e.g. containing elements that are too small for the human eye or which need to be viewed using wavelengths of light that are not visible to the human eye). The switching layer 13 may be provided as part of a thin film optical stack. As will be described below, the thin film optical stack may further comprise a reflective heat transmission layer 11, an optically transparent spacer layer 12 and/or an optically transparent capping layer 14.

The back plane portion 2 comprises a plurality of individually addressable drive elements. Each drive element interacts with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states. Each drive element comprises a heater 22 and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater 22 to the pixel region through a heat transmission layer 11.

Each pixel region in the switching layer 13 lies over a corresponding one of the heaters 22 and is of a similar size. In the arrangement of Figure 1, three stacks of layers of the front plane portion 1 are shown (separated from each other by the gaps 15). The portion of the switching layer 13 in each stack is an example of one distinct pixel region. In Figure 6, the front plane portion 1 is not divided into three stacks of layers but the pixel regions have a similar size, shape and position to the pixel regions in Figure 1 because the heaters 22 have the same size, shape and position. In both Figures 1 and 6, three example pixel regions are thus present, each directly above a respective one of the three heaters 22.

The individual addressability may be provided by any of various known techniques for driving individual pixels in an array of pixels. Each drive element may, for example, comprise an electronic unit 32 (e.g. a thin film electronic selector element such as a thin-film transistor or diode, typically formed for example from doped amorphous silicon, polysilicon or crystalline silicon) which, when addressed by signals from row and column lines 31, 33 intersecting at the electronic unit 32, drives the heater 22 of the drive element. The signals may be generated by external control electronics. Activation of the pixels (by switching the pixel regions) either sequentially and/or simultaneously in a controlled manner allows a desired pattern (e.g. an observable image) to be written to the display device. The components forming the back plane portion 2 may be mounted on a substrate 34. The front plane portion 1 is connected to the back plane portion 2 via an intermediate layer 23. The intermediate layer 23 may act as a barrier layer and/or planarization layer. The intermediate layer 23 will typically be an electrical insulator that is thermally conductive such that the intermediate layer 23 electrically insulates the heaters 22 from the pixel regions of the switching layer 13, but allows heat from the heaters 22 to pass through the intermediate layer 23 to the switching layer 13 to switch the pixel regions. In example embodiments the intermediate layer 23 comprises one or more of the following: SiN, AIN, S1O2, silicon carbide (SiC), and diamond (C). In the example shown in Figure 6 the intermediate layer 23 comprises an adhesive material for joining the front plane portion 1 to the back plane portion 2. The adhesive material may be provided on the front plane portion 1 only, on the back plane portion 2 only, or on both of the front plane portion 1 and the back plane portion 2, prior to and/or during the joining. In some embodiments at least a portion of the adhesive material is introduced to a region between the front plane portion 1 and the back plane portion 2 during the joining process.

The heat transmission layer 11 is provided between the drive elements 22 and the pixel regions of the switching layer 13. The heat transmission layer 11 will typically be a reflective layer, which may also be referred to as a mirror layer. The heat transmission layer 11 may have a higher average (e.g. spatially averaged) optical reflectivity with respect to visible light than each layer of the device on the same side of the heat transmission layer 11 as the switching layer 13. The heat transmission layer 11 may be substantially optically opaque on average. In some embodiments, the heat transmission layer 11 is metallic. Metals are known to provide good reflectivity (when sufficiently thick) and also have high thermal and electrical conductivities. The heat transmission layer 11 may have an average reflectance of 50% or more, optionally 90% or more, optionally 99% or more, with respect to visible light, infrared light, and/or ultraviolet light. The heat transmission layer 11 may comprise a thin metal film, composed for example of Au, Ag, Al, or Pt. If this layer is to be partially reflective then a thickness in the range of 5 to 15 nm might be selected, otherwise the layer is made thicker, such as 100 nm, to be substantially totally reflective.

The heat transmission layer 11 has a spatially inhomogeneous thermal conductivity to constrain lateral spreading of heat within the heat transmission layer 11 during actuation of a pixel element by heat propagating through the heat transmission layer 11 to the pixel element from a heater 22 corresponding to the pixel element. In some embodiments, the thermal conductivity is inhomogeneous when viewed perpendicularly to a plane of the heat transmission layer (e.g. from above in the orientation of Figures 1 and 6). The thermal conductivity may thus vary as a function of a position within the plane of the heat transmission layer 11 and be uniform in the depth direction of the heat transmission layer 11. The constraining of lateral spreading of heat reduces spreading of heat in directions within the plane of the heat transmission layer (e.g. in directions perpendicular to the vertical direction in the orientation of Figures 1 and 6). In the embodiment of Figure 1, the spatially inhomogeneous thermal conductivity is provided at least partially by a separation 15 between the stacks of layers 11-14 in the front plane portion 1. In the embodiment of Figure 6, the spatially inhomogeneous thermal conductivity is provided at least partially by regions 41 of lower thermal conductivity provided in the plane of the heat transmission layer 11. Heat may thus spread relatively easily within the selected portions of the heat transmission layer 11 but will not pass efficiently between those selected portions (e.g. across the separation 15 between different stacks in Figure 1 or across the regions 41 of lower thermal conductivity in Figure 6).

Constraining lateral spreading of heat is desirable for ensuring that actuation of a heater 22 reliably causes switching of only the pixel region of the switching layer 13 that corresponds to that heater 22, as well as for promoting uniform and effective switching of the pixel region. The constraining of the lateral spreading also reduces the overall power needed to effectively switch the pixel regions as desired.

Advantages of arrangements such as that of Figure 6 relative to Figure 1 and other alternatives will now be described by referring to Figures 2-4.

As mentioned above, the arrangement of Figure 1 could be manufactured using sequential deposition followed by lithographic patterning, which allows accurate alignment to be achieved between the pixel regions in each stack and the heaters 22. In this case, the intermediate layer 23 may be an electrically insulating planarization layer. This approach is relatively time-consuming and costly however.

Figure 2 depicts an alternative approach in which a front plane portion 1 is laminated onto a back plane portion 2 via an intermediate layer 23 in the back plane portion. In this case, the intermediate layer 23 may comprise a coatable and/or conformable glue layer. Due to difficulties with alignment, the front plane portion in prior approaches of this type has been restricted to stacks of optical layers 11-14 that are unpattemed spatially, as exemplified in Figure 2. Accurate registration between elements in the back plane portion 2 (e.g. heaters 22) and corresponding elements in the front plane portion 1 (e.g. pixel regions) is not therefore necessary. However, advantages associated with having patterned layers in the stack of optical layers 11-14, such as a patterned heat transmission layer 11 to constrain lateral spreading of heat, are lost in such an approach.

Figure 3 is a top view depicting an example arrangement of heaters 22 in a layer of a display device such as is depicted in Figure 1 and Figure 6. Figure 3 depicts nine example heaters 22, which would ideally be provided in spatial registration with regions of a heat transmission layer 11 corresponding to intended pixel regions in a switching layer 13 (e.g. aligned vertically as shown in Figure 1 for example).

Figure 4 is a top view illustrating the effect of laminating a front plane portion 1 with a patterned heat transmission layer 11 of the type depicted in Figure 1 onto an arrangement of heaters 22 in a back plane portion as depicted in Figure 3, without correct registration (e.g. using a lamination method such as that depicted in Figure 2). It can be seen that the constraining of lateral heat spreading in the patterned heat transmission layer 11 will not function optimally. Heat produced by electrical signals delivered to a particular heater 22 will not be effectively contained, and efficiently spread, within a corresponding pixel region in the switching layer 13 of the front plane portion. Adapting the method of Figure 2 to achieve highly accurate registration would detract from the ease, and therefore the speed and economy of the manufacturing process.

Embodiments of the present disclosure provide ways of manufacturing a display device in which a front plane portion 1 and a back plane portion 2 can be manufactured separately and connected together during manufacture without needing accurate alignment between them (as in Figure 2), in a case (unlike in Figure 2) where the display device comprises a heat transmission layer 11 that is patterned to achieve constraining of lateral spreading of heat within the heat transmission layer 11 (as discussed above with reference to Figure 1).

In some embodiments, this is achieved by providing a display device as described above with reference to Figure 1, but in which the heat transmission layer 11 is configured such that a pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer 11 is either non-periodic (an example of which is described below with reference to Figure 8) or periodic (examples of which are described below with reference to Figures 5-7). In cases where the pattern is periodic, a unit cell of the pattern is arranged to be smaller than a unit cell of a pattern defined by the plurality of drive elements in the back plane portion 2, optionally to be less than 75% of the size of the unit cell, optionally to be less than 50% of the size of the unit cell, optionally to be less than 25% of the size of the unit cell, optionally to be less than 10% of the size of the unit cell, optionally to be less than 5% of the size of the unit cell, in at least one direction, optionally in all directions, within a plane of the heat transmission layer 11. In some embodiments, an area of the unit cell of the pattern is less than 50%, optionally less than 10%, optionally less than 5%, of an area of the unit cell of the pattern defined by the plurality of drive elements in the back plane portion 2. Thus, a periodicity in the heat transmission layer 11 is finer than a periodicity of the heaters 22. Figure 3 depicts an example pattern defined by the plurality of drive elements in the back plane portion 2. In this case, a unit cell of the pattern (e.g. the smallest sub-pattem that can be repeated to build up the whole pattern) has a square shape and contains one of the heaters 22. Figure 3 depicts nine such unit cells. The heater 22 in each unit cell comprises a meandering electrically conductive track extending within a plane parallel to the plane of the heat transmission layer 11 (not shown in Figure 3). In the example shown, the track extends from a lower left portion of each unit cell to a lower right portion of each unit cell. Heating is provided in the heater 22 by driving an electrical current along the track to cause Joule heating in the track.

Figure 5 depicts superposition over the pattern of drive elements depicted in Figure 3 of an example heat transmission layer 11 in which the pattern defined by the spatially inhomogeneous thermal conductivity is spatially periodic. Figure 6 is a side view of a portion of an example display device using such a heat transmission layer 11. In this embodiment, a unit cell of the pattern in the heat transmission layer 11 is square. Figure 5 depicts 72 of these unit cells. Each unit cell of the pattern in the heat transmission layer 11 is smaller than a unit cell of the pattern of drive elements. The 72 unit cells of the heat transmission layer 11 cover approximately the same area as just nine of the unit cells of the pattern of drive elements.

In some embodiments, the spatially inhomogeneous thermal conductivity of the heat transmission layer 11 comprises one or more regions 40 having a first thermal conductivity and one or more regions 41 have a second thermal conductivity, lower than the first thermal conductivity. In embodiments of this type the thermal conductivity may be substantially constant in the regions 40 (i.e. the first thermal conductivity) and in the regions 41 (i.e. the second thermal conductivity). In some embodiments, a heat transmission layer 11 having such a pattern may be formed by providing an initially continuous heat transmission layer 11 (having the first thermal conductivity uniformly throughout the heat transmission layer 11) and, in a subsequent step, selectively replacing or otherwise processing portions of the heat transmission layer 11 to provide the regions 41 having the second thermal conductivity. Figure 5 depicts an arrangement of this type in which only the regions 40 having the first thermal conductivity are filled in. Although not shown in Figure 5, regions 41 in between the regions 40 would typically be filled with a solid material having the second thermal conductivity. In the arrangement of Figure 6, for example, an adhesive material fills gaps in the plane of the heat transmission layer 11 to provide the low thermal conductivity regions 41. In other embodiments, the regions 41 are provided by air or vacuum gaps in the heat transmission layer 11.

As can be seen by comparing Figures 4 and 5, the smaller unit cells of the heat transmission layer 11 in Figure 5 provide a more localized constraining of lateral heat spreading in the heat transmission layer 11. This reduces or removes any negative effects from misalignment between the heat transmission layer 11 and the underlying pattern of drive elements. The extent to which heat from a given selected heater 22 spreads into a portion of the heat transmission layer 11 underneath a pixel region of the switching layer that it is not intended to be switched is limited by the reduced size of the unit cells of the heat transmission layer 11. Any negative effects on switching efficiency or accuracy caused by misalignment between the front plane portion and the back plane portion are thus reduced or eliminated. Manufacturing alignment tolerances can thus be relaxed and efficiency, speed, yield and/or equipment costs can be improved.

In the example of Figures 5 and 6, the pattern in the heat transmission layer 11 comprises a plurality of regions of high thermal conductivity (e.g. reflective regions) that are isolated from each other by regions of lower thermal conductivity (e.g. non-reflective regions). This arrangement provides highly localized constraining of lateral heat spreading in the heat transmission layer 11 but may involve a relatively large proportion of the heat transmission layer 11 being made to have low thermal conductivity and/or low reflectance.

As the fineness of the pattern in the heat transmission layer 11 increases, a proportion of optically inactive dead space between reflective regions may thus increase to an undesirable level. An intermediate level of fineness (pitch) may therefore be needed to achieve a suitable balance between acceptable constraining and uniformity of heating provided by the patterned heat transmission layer 11 and acceptably low levels of dead space. In other embodiments, a higher proportion of the heat transmission layer 11 can be retained in the high thermal conductivity and/or reflective state by arranging for material forming at least one of the regions 40 of first thermal conductivity to extend integrally through (i.e. without interfaces) a plurality of the unit cells of the spatially inhomogeneous thermal conductivity in the heat transmission layer 11. Thus, fewer or no regions of high thermal conductivity/reflectivity may be provided that are isolated from each other by regions of lower thermal conductivity/reflectivity. An example of a heat transmission layer 11 of this type is depicted in Figure 7. Each cross-shaped region 41 represents a region of low thermal conductivity and the remaining region 40 of the heat transmission layer has higher thermal conductivity and/or is reflective. The cross-shaped regions 41 are arranged to provide relatively narrow channels of the higher thermal conductivity region 40 that integrally connect different unit cells together. The narrowness of these channels restricts flow of heat between different unit cells and thereby provides thermal behaviour similar to that of the heat transmission layer 11 of Figures 5 and 6, while the integral connection between unit cells increases the total amount of the high thermal conductivity/reflectance material provided, thereby improving optical performance.

In an alternative approach, the pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is arranged to be non-periodic. The non periodic arrangement may be pseudo-random or random. Non-periodic variations (e.g. pseudo-random or random) in reflectivity may have a smaller visible effect than otherwise similar periodic variations, thereby improving optical performance.

In an embodiment, as depicted schematically in Figure 8, the pattern defined by the spatially inhomogeneous thermal conductivity comprises a pattern of crack lines within the heat transmission layer 11. Each crack line represents a narrow region 41 of lower thermal conductivity relative to other regions 40 of the heat transmission layer. Crack lines can easily be made very narrow, which minimises the amount of optical dead space introduced by the crack lines. In an embodiment, the pattern of crack lines creates a plurality of crack-isolated regions in the heat transmission layer 11. Each crack-isolated region is surrounded by a crack line at least at a surface of the heat transmission layer 11 closest to the switching layer 13.

The crack-isolated regions may function in an analogous way to the regions 40 in the heat transmission layer 11 depicted in Figure 5. In some embodiments, an average area (e.g. viewed perpendicular to a plane of the heat transmission layer 11 and/or along an intended viewing direction for the display device) of the crack-isolated regions is smaller than, optionally less than 50% of, optionally less than 10% of, optionally less than 5% of, the area of the unit cell of the pattern defined by the plurality of drive elements in the back plane portion 2. Making the crack-isolated regions smaller than the drive elements provides a more localized constraining of lateral heat spreading in the heat transmission layer 11, similar to that achieved by arrangements of the type discussed above with reference to Figures 5 and 6. In an embodiment, a method of manufacturing the display device comprises applying mechanical stress to a heat transmission layer 11 (e.g. by flexing the front plane portion 1) to introduce crack lines and thereby provide the heat transmission layer 11 having the pattern of crack lines. In an embodiment, the mechanical stress (e.g. flexing) is applied before the front plane portion 1 is connected to the back plane portion 2 during manufacture. In an embodiment, each pixel region in the switching layer 13 comprises a PCM. The PCM is thermally switchable between a plurality of stable refractive index states having different refractive indices relative to each other. The switching may be reversible. In an embodiment, the PCM in each pixel region controls the colour of a pixel corresponding to the pixel region. The PCM in each pixel region may be switchable between a set of optical states comprising at least two optical states which cause the pixel to have different colours. In an embodiment, the different colours include red and white, blue and white, or green and white.

Further optical elements may optionally be provided, such as optical elements that control the overall intensity of light coming from each pixel region (e.g. to control grey scale levels). Each further optical element may, for example, comprise a liquid crystal display (LCD) element, comprising for example one or more of the following: an LCD with polarizer, a polarizer-free LCD, a dye-doped LCD. Alternatively or additionally, the further optical elements may comprise an electrowetting optical element or a MEMS element. Any other optical element providing the desired optical properties (e.g. grey scale control) may be used.

Each PCM may be provided in a stack comprising other layers that effect the optical effects achieved by the PCM, e.g. by interference effects. The stack comprises the PCM in the pixel region of the switching layer 13. The PCM may be provided as a continuous layer spanning across multiple pixels (such that the PCM of different pixel regions is integrally connected together in the same switching layer 13), or a separate unit of PCM may be provided for each pixel region. Each pixel region comprises a portion of PCM that is thermally switchable at least predominantly independently of the portion of PCM of any of the other pixel regions (although there may be some cross-talk between pixel regions where heating intended to switch the PCM of one pixel region also causes a degree of heating in the PCM of a neighbouring pixel region).

Each stable state of the PCM has a different refractive index (optionally including a different imaginary component of the refractive index, and thereby a different absorbance) relative to each of the other stable states. In an embodiment all layers in each stack are solid- state and configured so that their thicknesses as well as refractive index and absorption properties combine so that the different states of the PCM result in different, visibly and/or measurably distinct, reflection spectra. Optical devices of this type are described in Nature 511, 206-211 (10 July 2014), WO2015/097468A1, WO2015/097469A1, EP3203309A1 and PCT/GB2016/053196. In an embodiment the PCM comprises, consists essentially of, or consists of, one or more of the following: an oxide of vanadium (which may also be referred to as VOx); an oxide of niobium (which may also be referred to as NbOx); an alloy or compound comprising Ge, Sb, and Te; an alloy or compound comprising Ge and Te; an alloy or compound comprising Ge and Sb; an alloy or compound comprising Ga and Sb; an alloy or compound comprising Ag, In, Sb, and Te; an alloy or compound comprising In and Sb; an alloy or compound comprising In, Sb, and Te; an alloy or compound comprising In and Se; an alloy or compound comprising Sb and Te; an alloy or compound comprising Te, Ge, Sb, and S; an alloy or compound comprising Ag, Sb, and Se; an alloy or compound comprising Sb and Se; an alloy or compound comprising Ge, Sb, Mn, and Sn; an alloy or compound comprising Ag, Sb, and Te; an alloy or compound comprising Au, Sb, and Te; and an alloy or compound comprising A1 and Sb (including the following compounds/alloys in any stable stoichiometry: GeSbTe, VOx, NbOx, GeTe, GeSb, GaSb, AglnSbTe, InSb, InSbTe, InSe, SbTe, TeGeSbS, AgSbSe, SbSe, GeSbMnSn, AgSbTe, AuSbTe, and AlSb). Preferably, the PCM comprises one of Ge2Sb2Te5 and Ag3hi4Sb76Tei7. It is also understood that various stoichiometric forms of these materials are possible: for example Ge_xSb_yTe_z; and another suitable material is Ag3hi4Sb76Tei7 (also known as AIST). Furthermore, any of the above materials can comprise one or more dopants, such as C or N. Other materials may be used.

PCMs are known that undergo a drastic change in both the real and imaginary refractive index when switched between amorphous and crystalline phases. There is a substantial change in the refractive index when the material is switched between amorphous and crystalline phases. The material is stable in either state. Switching can be performed an effectively limitless number of times.

Although some embodiments described herein mention that the PCM is switchable between two states such as crystalline and amorphous phases, the transformation could be between any two solid phases, including, but not limited to: crystalline to another crystalline or quasi-crystalline phase or vice-versa; amorphous to crystalline or quasi-crystalline/semi- ordered or vice versa, and all forms in between. Embodiments are also not limited to just two states.

In an embodiment, the PCM comprises Ge2Sb2Tes (GST) in a layer less than 200 nm thick. In another embodiment, the PCM comprises GeTe (not necessarily in an alloy of equal proportions) in a layer less than 100 nm thick.

The PCM in each pixel region of the switching layer 13 is switched by applying heating via a corresponding heater 22 in the back plane portion 2. Applying different thermal heating profiles (i.e. different variations of heating as a function of time) allows selective switching between different phases, such as to selectively switch the PCM from amorphous to crystalline or from crystalline to amorphous. For example, a control signal comprising a current pulse of relatively low amplitude and long duration may be effective for switching the PCM from an amorphous state to a crystalline state, the resulting heating profile being such that the PCM is heated to a temperature higher than the crystallization temperature Tc of the PCM, but less than the melting temperature T_M of the PCM. The temperature is maintained above the crystallization temperature Tc for a time sufficient to crystallize the PCM. A control signal comprising a current pulse of higher amplitude but shorter duration may be effective for switching the PCM from a crystalline state to an amorphous state, the resulting heating profile being such that the PCM is heated to a temperature that is higher than the melting temperature T_M, causing melting of the PCM, but is cooled sufficiently quickly that re-crystallization does not occur excessively and the PCM freezes into an amorphous state. After the heating of the PCM has finished the PCM remains in the stable state selected (e.g. amorphous or crystalline) until further heating is applied. Thus, when based on PCM the pixel region is naturally held in a given optical state without application of any signal, and can thus operate with significantly less power than other display technologies. Switching can be performed an effectively limitless number of times. The switching speed is also very rapid, typically less than 300 ns, and certainly several orders faster than the human eye can perceive.

In an embodiment, the stack in the front plane portion 1 further comprises a spacer layer 12 (which may be provided as depicted in Figure 6 for example). The spacer layer 12 is between the switching layer 13 (e.g. PCM layer) and the heat transmission layer 11 (e.g. reflective layer).

In an embodiment, the stack in the front plane portion 1 further comprises a capping layer 14 (which may be provided as depicted in Figure 6 for example). The pixel region of the switching layer 13 (and, where present, the PCM) is between the capping layer 14 and the heat transmission layer 11 (e.g. reflective layer). The upper surface of the capping layer 14 may face towards a viewing side of the display device and the heat transmission layer 11 may act as a back-reflector. Light enters and leaves through the viewing surface (from above in Figure 6). However, because of interference effects which are dependent on the refractive index of the pixel region of the switching layer 13 (e.g. the PCM) and the thickness of the spacer layer 12, the reflectivity varies significantly as a function of wavelength. The spacer layer 12 and the capping layer 14 are both optically transmissive, and are ideally as transparent as possible.

Each of the capping layer 14 and spacer layer 12 may consist of a single layer or comprise multiple layers having different refractive indices relative to each other (i.e. where the capping layer 14 or spacer layer 12 consists of multiple layers at least two of those layers have different refractive indices relative to each other). The thickness and refractive index of the material or materials forming the capping layer 14 and/or spacer layer 12 are chosen to create a desired spectral response (via interference and/or absorption). Materials which may be used to form the capping layer 14 and/or spacer layer 12 may include (but are not limited to) ZnO, T1O2, S1O2, S13N4, TaO, ITO, and ZnS-SiCk.

In a particular embodiment, a PCM of a pixel region comprises GST, is less than 100 nm thick, and preferably less than 10 nm thick, such as 6 or 7 nm thick. The spacer layer 12 is grown to have a thickness typically in the range from 10 nm to 250 nm, depending on the colour and optical properties required. The capping layer 14 is, for example, about 20 nm thick.

Embodiments of the disclosure provide a method of manufacturing a display device (e.g. according to any of the arrangements discussed above with reference to Figures 5-8) in which a back plane portion 2 of the display device is connected to a front plane portion 1 of the display device during manufacture. As mentioned above, the back plane portion 2 of the display device may comprise a pattern defined by a spatially inhomogeneous thermal conductivity of a heat transmission layer 11.

In some embodiments, the pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer 11 is present before the front plane portion 1 is attached to the back plane portion 2. This facilitates manufacturing of the pattern.

In some embodiments, the connecting of the front plane portion 1 to the back plane portion 2 is performed while the front plane portion 1 is supported by a front plane support substrate 44 (as exemplified in Figure 2) and the front plane support substrate 44 is removed (e.g. by delamination, etching or laser lift-off process) after the front plane portion 1 has been connected to the back plane portion 2, or the front plane support substrate 44 may be left in place to act as an encapsulation and/or protection film. The front plane support substrate 44 (and the front plane portion 1 connected thereto) may be flexible to allow lamination to be achieved progressively as depicted in Figure 2. This approach can be implemented efficiently and reliably, with minimal risk of lamination problems such as trapped gas bubbles.

Similarly, in some embodiments, the connecting of the front plane portion 1 to the back plane portion 2 is performed while the back plane portion 2 is supported by a back plane support substrate (not shown) and the back plane support substrate is removed (e.g. by delamination, etching or laser lift-off process) after the front plane portion 1 has been connected to the back plane portion 2 or is left in place. The back plane support substrate (and the back plane portion 2 connected thereto) may be flexible. Alternatively, both the front plane portion 1 and the back plane portion 2 may be rigid or both flexible while still being joinable.

Figure 9-11 depict successive stages in an alternative method of manufacture using thermally induced transformation of localized regions in an intermediate layer 23 to define a pattern in a heat transmission layer 11. This approach allows the pattern in the heat transmission layer 11 to be defined after a front plane portion 1 has been attached (e.g. laminated to) to a back plane portion 2 and therefore does not require a high degree of accuracy in the attachment process while still providing the advantages associated with having a patterned heat transmission layer 11.

In an embodiment of this type, the method comprises connecting a back plane portion 2 of the display device to a front plane portion 1 of the display device. The front plane portion 1 is between the back plane portion 2 and a viewing side of the device, as in the embodiments discussed above. The connecting of the front plane portion 1 to the back plane portion 2 may provide an arrangement such as that depicted in Figure 9. As in the embodiments described above, the front plane portion 1 comprises a switching layer 13 having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions. The back plane portion 2 comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states. Each drive element comprises a heater 22 and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater 22 to the pixel region through a heat transmission layer 11 between the drive elements and the pixel regions. However, in contrast to the embodiments of Figure 5-8, the heat transmission layer 11 is not necessarily patterned at this stage.

In a subsequent step, a plurality of localized regions 23 A of an intermediate layer 23 between the heaters 22 and the heat transmission layer 11 are transformed by heating each localized region 23A with a respective one of the heaters 22 to provide the arrangement of Figure 10. The intermediate layer 23 at this stage thus comprises a combination of localized regions 23A that have been transformed and regions 23B outside of these localized regions 23A that have not been transformed. The intermediate layer 23 may comprise a thermally curable material (e.g. a glue) for example. The transformation in the localized regions 23A may thus comprise curing of the thermally curable material. The transformation in the localized regions 23A is effective to make these regions more rugged or otherwise resistant to subsequent process steps.

In a subsequent step, material above a region 23B or regions 23B of the intermediate layer 23 outside of the transformed localized regions 23A is removed (e.g. by an etching or lift-off process) to provide the heat transmission layer 11 with a spatially inhomogeneous thermal conductivity effective to constrain lateral spreading of heat within the heat transmission layer 11, as depicted in Figure 11. The arrangement is similar to that of Figure 1, in that plural stacks of layers separated by gaps 55 (which may be filled with transparent thermally insulating material) are provided. Again, each pixel region of the switching layer 13 may comprises a PCM that is thermally switchable between a plurality of stable refractive index states having different refractive indices relative to each other. The PCM and other layers in each stack may take any of the configurations described above with reference to Figures 1 and 6.

In an embodiment, the approach of Figures 9-11 is repeated to sequentially form sub pixels of different types. The different types of sub-pixel may comprise a sub-pixel type that reflects a first colour (e.g. red), a sub-pixel type that reflects a second colour (e.g. green) and a sub-pixel type that reflects a third colour (e.g. blue). In an example method of this type, the approach of Figures 9-11 is performed firstly with layers in the front plane portion 1 being configured to reflect the first colour. A plurality of regions 23A of the intermediate layer 23 are then transformed to provide an arrangement such as that shown in Figure 10 but instead of transforming all possible regions (i.e. every region in front of a heater 22) only a subset of the possible regions are transformed that corresponds to one of the sub-pixel types (in this example, the sub-pixel type that reflects the first colour). In the case where the sub-pixel types correspond to three different colours, one third of the possible regions may be transformed. The resulting arrangement is then processed to provide an arrangement of the type shown in Figure 11 but with a stack of layers being retained only on a subset of the heaters 22 (e.g. on a third of the heaters 22). In a subsequent step, a new stack of layers is formed over the back plane portion 2 (at least in regions where the previous front plane portion 1 has been removed). The new stack of layers corresponds to a front plane portion 1 configured to reflect the second colour. A plurality of different regions 23A corresponding to positions where the second colour is to be reflected (e.g. a different third of the possible regions) are then transformed and the processing of Figure 11 is repeated again. Finally, a further stack of layers corresponding to a front plane portion 1 configured to reflect the third colour is formed over the back plane portion 2 (at least in regions where the previous front plane portion 1 has not been removed). A plurality of further different regions 23A corresponding to positions where the third colour is to be reflected (e.g. a final third of the possible regions) are then transformed and the processing of Figure 11 is repeated again to provide a plurality of pixels that each comprise three sub-pixels of different colour.

Claims

1. A display device for displaying a pattern, comprising: a layered structure having a front plane portion and a back plane portion, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; the heat transmission layer has a spatially inhomogeneous thermal conductivity to constrain lateral spreading of heat within a plane of the heat transmission layer; and a pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is either non-periodic or periodic with a unit cell that is smaller in at least one direction within the plane of the heat transmission layer than a unit cell of a pattern defined by the plurality of drive elements in the back plane portion.

2. The device of claim 1, wherein the heat transmission layer is a reflective layer having higher average optical reflectivity with respect to visible light than each layer of the device on the same side of the heat transmission layer as the switching layer.

3. The device of claim 1 or 2, wherein an area of the unit cell of the spatially inhomogeneous thermal conductivity is less than 50% of an area of the unit cell of the plurality of drive elements in the back plane portion.

4. The device of any preceding claim, wherein: the spatially inhomogeneous thermal conductivity of the heat transmission layer comprises one or more regions having a first thermal conductivity and one or more regions having a second thermal conductivity, lower than the first thermal conductivity; and material forming at least one of the regions of first thermal conductivity extends integrally through a plurality of the unit cells of the spatially inhomogeneous thermal conductivity.

5. The device of any preceding claim, wherein the pattern defined by the spatially inhomogeneous thermal conductivity comprises a pattern of crack lines within the heat transmission layer.

6. The device of claim 5, wherein the pattern of crack lines creates a plurality of crack- isolated regions in the heat transmission layer, each crack-isolated region being surrounded by a crack line at least at a surface of the heat transmission layer closest to the switching layer.

7. The device of claim 6, wherein an average area of the crack-isolated regions is smaller than the area of the unit cell of the pattern defined by the plurality of drive elements in the back plane portion.

8. The device of any preceding claim, wherein: the spatially inhomogeneous thermal conductivity of the heat transmission layer comprises one or more regions having a first thermal conductivity and one or more regions having a second thermal conductivity, lower than the first thermal conductivity; and the second thermal conductivity is provided by air or vacuum gaps in the heat transmission layer or by an adhesive material joining the front plane portion to the back plane portion.

9. The device of any preceding claim, wherein each pixel region comprises a phase change material thermally switchable between a plurality of stable refractive index states having different refractive indices relative to each other.

10. A method of manufacturing a display device for displaying a pattern, comprising: connecting a back plane portion of the display device to a front plane portion of the display device, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; the heat transmission layer has a spatially inhomogeneous thermal conductivity to constrain lateral spreading of the heat within a plane of the heat transmission layer; and a pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is either non-periodic or periodic with a unit cell that is smaller in at least one direction within the plane of the heat transmission layer than a unit cell of a pattern defined by the plurality of drive elements in the back plane portion.

11. The method of claim 10, wherein the pattern defined by the spatially inhomogeneous thermal conductivity of the heat transmission layer is present before the front plane portion is attached to the back plane portion.

12. The method of claim 10 or 11, wherein the connecting of the front plane portion to the back plane portion is performed while the front plane portion is supported by a front plane support substrate.

13. The method of claim 12, wherein the front plane support substrate is removed after connection of front plane portion to the back plane portion.

14. The method of claim 12 or 13, wherein the front plane portion is flexible.

15. The method of any of claims 10-14, wherein the back plane portion is flexible.

16. The method of any of claims 10-15, wherein the heat transmission layer is a reflective layer having higher optical reflectivity with respect to visible light than each layer of the device on the same side of the heat transmission layer as the switching layer.

17. The method of any of claims 10-16, wherein an area of the unit cell of the spatially inhomogeneous thermal conductivity is less than 50% of an area of the unit cell of the plurality of drive elements in the back plane portion.

18. The method of any preceding claim, wherein: the spatially inhomogeneous thermal conductivity of the heat transmission layer comprises one or more regions having a first thermal conductivity and one or more regions have a second thermal conductivity, lower than the first thermal conductivity; and material forming at least one of the regions of high thermal conductivity extends integrally through a plurality of the unit cells of the spatially inhomogeneous thermal conductivity.

19. The method of any of claims 10-16, wherein the pattern defined by the spatially inhomogeneous thermal conductivity comprises a pattern of crack lines within the heat transmission layer.

20. The method of claim 19, comprising applying mechanical stress to a heat transmission layer to introduce crack lines and thereby provide the heat transmission layer having the pattern of crack lines.

21. The method of claim 20, wherein the mechanical stress is applied before the front plane portion is connected to the back plane portion.

22. The method of any of claims 19-21, wherein the pattern of crack lines creates a plurality of crack-isolated regions in the heat transmission layer, each crack-isolated region being surrounded by a crack line at least at a surface of the heat transmission layer closest to the switching layer.

23. The method of claim 22, wherein an average area of the crack-isolated regions is smaller than the area of the unit cell of the pattern defined by the plurality of drive elements in the back plane portion.

24. A method of manufacturing a display device for displaying a pattern, comprising: connecting a back plane portion of the display device to a front plane portion of the display device, wherein: the front plane portion comprises a switching layer having a plurality of pixel regions, each pixel region being switchable between a plurality of refractive index states to allow a pattern to be defined by the plurality of pixel regions; the back plane portion comprises a plurality of individually addressable drive elements, each drive element being configured to interact with a respective one of the pixel regions to switch the pixel region between different ones of the refractive index states; each drive element comprises a heater and the interaction between each drive element and the respective pixel region occurs by propagation of heat from the heater to the pixel region through a heat transmission layer; and the method comprises: transforming each of a plurality of localized regions of an intermediate layer, between the heaters and the heat transmission layer, by heating each localized region with a respective one of the heaters; and removing material above a region or regions of the intermediate layer outside of the transformed localized regions to provide the heat transmission layer with a spatially inhomogeneous thermal conductivity effective to constrain lateral spreading of heat within a plane of the heat transmission layer.

25. The method of any of claims 10-24, wherein each pixel region comprises a phase change material thermally switchable between a plurality of stable refractive index states having different refractive indices relative to each other.