A method for fabricating a frusto-conical micro-lightguide for collimation of light emitted from micro-LEDs. The method comprises depositing a layer of UV-curable material onto a substrate. A first part of the layer is selectively cured using UV light having a conical irradiation profile to define a shape of the frusto-conical micro-lightguide. The UV-curable material is developed to remove one of the first part of the layer and a second part of the layer, wherein the second part of the layer is uncured.

FIELD OF THE DISCLOSURE

The disclosure relates to the field of light emitting diodes (LEDs). More particularly, the present disclosure relates to methods of improving emission efficiency of LEDs.

BACKGROUND

LEDs convert electrical energy into optical energy. In semiconductor LEDs this usually occurs via electron-hole transitions when recombination of electrons, from an n-doped semiconductor layer, and holes, from a p-doped semiconductor layer, occurs. The area in which the main light emission takes place may be referred to as the active region. The light generated at a quantum well in an LED may be emitted in all directions but the change in refractive index at a boundary of the LED material means that only emitted light rays with an angle of incidence within a critical angle range (an escape range) can be emitted. Even some light within the escape range may be lost due to small Fresnel losses with change of angle. If the angle of incidence is outside the escape range, total internal reflection may occur. One major challenge in LED production is to improve extraction efficiency, and to capture as much of the emitted light as possible.

Some LEDs emit directly to air. An emission efficiency may be said to be the number of photons that escape the LED into the air relative to the total number of photons generated. The refractive index of the substrate material is generally much higher than that of air, so only light that is incident at an angle close to the normal of the exit surface can escape. Often LEDs are coupled to light collection devices such as projection lenses instead of directly to air. In such cases there may be further losses at an interface between the LED and the light collection device due to some light emitted by the LED diverging at an angle such that it does not reach the interface with the light collection device. Emission efficiency then depends both on the proportion of photons that escape the LED and on the proportion of those escaped photons that are captured by the light collection device.

The efficiency in capturing the escaped photons may depend on size of the divergent light angle (a solid angle formed by a half-power beam width of the emitted light) compared to the light collection angle (a solid angle through which at least half of the available photons are captured by a light collection device). LEDs emit light in an angular distribution close to a Lambertian emission with a full-width half maximum (FWHM) of 120 degrees. The acceptance angle of a lens may be determined by its F number, which for a typical projection lens might be F/2.5 or F/3 giving acceptance angles 11.3° and 9.5° respectively. Only 2.7% of light emitted by a Lambertian LED is within ±9.5°, so 97.3% of light is lost by virtue of not passing into the lens. Thus there is a need to increase efficiency of emission from the LED and to collimate the emitted light.

Existing solutions may rely either on precise etching of the LED semiconductor material or shaping of the chip mesa of an LED device. The shape of a mesa may be designed such that the light emitted from the active region is reflected towards the emission surface in such a way that more photons have angles of incidence which allow them to be transmitted, and may also be selected to focus the beam. For example, an integrated transparent conductive layer may be formed integrally with the LED structure during fabrication, and etched to form a cap which enhances light extraction (US2015008392 A1). A convex optical structure may also be formed by laser ablation on the side of the substrate opposite to the light emitting region, which reflects light towards the light emitting surface such that it is transmitted and collimated (US2018083170 A1). Rather than shaping the LED material itself, the chip mesa can be shaped into a parabolic structure in which the active layer sits, so that light incident on the sidewalls is reflected towards the light emitting surface opposed to the mesa (US2015236201 A1 and US2017271557 A1). Etching the mesa risks damaging the active layer, and it may be difficult to achieve a smooth enough finish for high degrees of collimation.

Micro-LEDs are used for high-resolution displays, and with ever decreasing dimensions it may be increasingly difficult to etch features with sufficient precision to collimate light effectively. The inherently small dimensions of the semiconductor materials used to collimate the emitted light may also result in poor levels of luminance uniformity. It is an object of the present invention to provide a scalable design which provides an accurate emission angle as well as high levels of angular and brightness uniformity.

SUMMARY OF THE DISCLOSURE

Against this background, there is provided:

A method for fabricating a frusto-conical micro-lightguide for collimation of light emitted from micro-LEDs, comprising:depositing a layer of UV-curable material onto a substrate;selectively curing a first part of the layer using UV light having a conical irradiation profile to define a shape of the frusto-conical micro-lightguide;developing the UV-curable material to remove one of the first part of the layer and a second part of the layer, wherein the second part of the layer is uncured.

In this way, it is possible to fabricate precise micro-lightguides at small scales in order to collimate light emitted from a micro-LED so as to increase a proportion of extracted light.

The second part of the layer may be removed and the first part of the layer may comprise the frusto-conical micro-lightguide.

Advantageously, using the first part of the UV-curable material as the frusto-conical micro-lightguide requires relatively few processing steps and is a scalable process.

The first part of the layer may be removed and the second part of the layer may comprise a frusto-conical recess that defines the shape of the frusto-conical micro-lightguide.

Advantageously, a negative resist may be used instead of a positive resist so the process is flexible.

The method may further comprise depositing a lightguide material in the frusto-conical recess and removing the second part of the layer, such that the lightguide material comprises the frusto-conical micro-lightguide.

In this way, a wider range of materials may be used for the frusto-conical micro-lightguide since the lightguide material is not necessarily UV-curable.

The frusto-conical micro-lightguide may comprise a first planar surface and a second planar surface, wherein the first planar surface has a smaller area than the second planar surface.

In this way, light that is transmitted through the first planar surface may be incident on a sidewall of the frusto-conical micro-lightguide and may be reflected such that the angle of the reflected light ray to a central axis of the frusto-conical micro-lightguide may be smaller than the angle of the incident light ray to a central axis of the frusto-conical micro-lightguide. Thus a light beam transmitted through the first planar surface is collimated and a narrower light beam is emitted from the second planar surface.

The method may further comprise fabricating an array of micro-lightguides.

In this way, light from each micro-LED in an array of micro-LEDs may be collimated.

The conical irradiation profile may take a form of a substantially inverted cone and may be achieved by transmitting the UV light through a mask that is moving in a circular trajectory, such that the first planar surface of the frusto-conical micro-lightguide is proximate the substrate.

In this way, a part of the UV-curable material that is frusto-conical in shape may be cured.

The substrate may be a processed wafer comprising a plurality of micro-LEDs.

Advantageously, the frusto-conical micro-lightguides are fabricated directly onto the micro-LEDs and so the frusto-conical micro-lightguides do not need to be aligned with the micro-LEDs after fabrication.

The mask may comprise one or more circular apertures.

In this way, the irradiation profile of light transmitted through the mask may have a conical profile when the mask is moved in a circular trajectory.

The conical irradiation profile may be achieved by collimation of the UV light such that the second planar surface of the frusto-conical micro-lightguide is proximate the substrate.

In this way, the frusto-conical micro-lightguide may be fabricated separately to the micro-LEDs.

The collimation may be achieved using one or more micro-lenses.

In this way, the UV light that is transmitted through the micro-lenses has a conical profile.

The substrate may be a transparent material such as glass or sapphire.

Advantageously, the frusto-conical micro-lightguide may then be coupled to a micro-LED and the substrate does not need to be removed.

The micro-lightguides may be coupled to an array of micro-LEDs.

In this way, light from each micro-LED in an array of micro-LEDs may be collimated.

The angle of a sidewall of the frusto-conical micro-lightguide to a central axis of the frusto-conical micro-lightguide may preferably be between 10° and 18°, wherein the central axis of the frusto-conical micro-lightguide passes through a central point of the first planar surface and a central point of the second planar surface.

In this way, the light that is transmitted through the frusto-conical micro-lightguide may be collimated.

The central axis of the frusto-conical micro-lightguide may be aligned with a central axis of a micro-LED.

In this way, efficiency of light collection by the frusto-conical micro-lightguide is increased.

The micro-lightguide may further comprise a reflective coating.

In this way, light cross-talk between adjacent frusto-conical micro-lightguides is reduced.

The UV-curable resist material may be deposited by spin coating.

Advantageously, this process is scalable and achieves a layer that is uniform.

The first planar surface has a characteristic dimension that may be 50% of a characteristic dimension of the second planar surface.

In this way, an appropriate angle of the sidewall of the frusto-conical micro-lightguide is achieved.

The characteristic dimension of the second planar surface may be equal to a characteristic dimension of the frusto-conical micro-lightguide that is parallel to the central axis of the frusto-conical micro-lightguide.

In this way, an appropriate level of collimation of a light beam is achieved.

The characteristic dimension of the first planar surface may be 60 % larger than a characteristic dimension of the micro-LED.

The characteristic dimension of the first planar surface may preferably be 70 % larger than the characteristic dimension of the micro-LED.

In this way, efficiency of light collection by the frusto-conical micro-lightguide is increased.

DETAILED DESCRIPTION

According to an embodiment of this disclosure, a micro-lightguide100for collimation of light emitted by a micro-light emitting diode (micro-LED) is provided. A method for fabricating the micro-lightguide100is also provided.

With reference toFIG.1, a micro-lightguide100comprises a first planar surface110and a second planar surface120opposing the first planar surface110, wherein the first planar surface110has a smaller area than the second planar surface120. The first planar surface110and second planar surface120may both be circular. The micro-lightguide100may comprise a sidewall130extending between the first planar surface110and the second planar surface120. The sidewall130may be curved. Thus, the micro-lightguide100may have a frusto-conical shape. The micro-lightguide100may be fabricated from a material that is transparent to light in the visible spectrum. An incident light ray that is incident on the first planar surface110may be transmitted through the first planar surface110. Depending on the angle of incidence of the incident light ray on the first planar surface110, the transmitted light ray may transmit through the micro-lightguide so as to be incident on the second planar surface120. Transmission through the micro-lightguide100may either be direct without reflecting off any part of the sidewall130or may include one or more reflections off the sidewall130by total internal reflection.

First, second and third exemplary transmitted light rays141,151and161are shown inFIG.1. The first transmitted light ray141is incident on the second planar surface120without having reflected off the sidewall130and is transmitted through the second planar surface120, thus exiting the micro-lightguide100. The second and third transmitted light rays151and161are reflected from the sidewall130via total internal reflection. The second transmitted light ray151may be reflected off the sidewall130as a first reflected light ray152. The first reflected light ray152may be incident on the second planar surface120and may be transmitted, exiting the micro-lightguide100. The angle of the first reflected light ray152to the central axis of the micro-lightguide100may be smaller than the angle of the second incident light ray151to the central axis of the micro-lightguide100, wherein the central axis of the micro-lightguide100passes through the centre of the first planar surface110and through the centre of the second planar surface120. The third transmitted light ray161may be incident on the sidewall130and reflected off the sidewall130as a second reflected light ray162. The second reflected light ray162may then be incident on the sidewall130and reflected again as a third reflected light ray163. The third reflected light ray163may then be incident on the second planar surface120and transmitted through the second planar surface120to exit the micro-lightguide100. The angle of the third reflected light ray163to the central axis of the micro-lightguide100may be smaller than the angle of the third incident light ray161to the central axis of the micro-lightguide100

With reference toFIG.2, the micro-lightguide100may be coupled to a light source210. The light source210may be proximate to the first planar surface110such that light emitted by the light source is incident on the first planar surface110. The light source may emit light in any direction and may emit light from any point on the light source, but inFIG.2, for clarity of explanation, only two exemplary light rays221are shown which are emitted from a single point only. The light rays221are transmitted through the first planar surface110and are incident on the sidewall130. The light rays221are reflected from the sidewall130via total internal reflection and are incident on the second planar surface120, and transmitted through the second planar surface120to exit the micro-lightguide100. The angle of the reflected light ray222to the central axis of the micro-lightguide100may be smaller than the angle of the incident light ray221to the central axis of the micro-lightguide100. There may be a plurality of micro-lightguides100arranged in an array200such that each micro-lightguide100is coupled to a separate light source210. The light source210may be a micro-LED.

With reference toFIG.3, the array200of micro-lightguides100may be coupled to a substrate310comprising an array of light sources210. The central axis of each micro-lightguide100may be aligned with the central axis of each light source210. The micro-lightguides100may be arranged at a constant pitch.

With reference toFIG.4, the light source210may comprise a pixel400comprising a micro-LED. The micro-LED may comprise a substrate410, a semiconductor material420provided on the substrate410and a capping material430provided on the semiconductor material420. The semiconductor420is configured to emit light in response to an electric current, which may be applied using electrodes440and450. In a certain embodiment, the substrate410may comprise a complementary metal-oxide-semiconductor (CMOS) and the semiconductor material420may comprise a monolithic InGaN LED. The monolithic InGaN LED may emit blue light, in which case the capping material430may be a clear transparent material for a blue pixel, and a colour converting material such as quantum dots or phosphor for red and green pixels. With reference also toFIG.5, a plurality of pixels400may be provided and arranged in an array, and each pixel400of the plurality of pixels400may be coupled to a micro-lightguide100. There may be a light blocking material460between each capping material430to prevent light cross talk between pixels. The light-blocking material460may absorb visible light and be photo-definable. A simplistic representation of an example of such an array is shown in plan view inFIG.6.

In a certain embodiment, the characteristic dimension of the micro-lightguide100along its central axis may be 5 µm, a characteristic dimension of the first planar surface110may be 2.5 µm and a characteristic dimension of the second planar surface120may be 5 µm. The angle of the sidewall130to the central axis of the micro-lightguide is then 14°. The first planar surface110and second planar surface120may be circular, so the characteristic dimension of the first and second planar surfaces may be a diameter. The characteristic dimension of the first planar surface110may preferably be 60% larger than the characteristic dimension of the light-source210. More preferably, the characteristic dimension of the first planar surface110may be 70% larger than the characteristic dimension of the light-source210. The pitch of an array200of micro-lightguides100may be 8 µm, and the pitch of an array of light sources210may be 8 µm.

With reference toFIG.7, the profile of light emitted from the second planar surface120of the micro-lightguide100is shown. The data plotted inFIG.7are derived from simulation results for a micro-lightguide100with a characteristic dimension along its central axis of 5 µm, a diameter of the circular first planar surface110of 2.5 µm and a diameter of the circular second planar120surface of 5 µm. The light from the light source210that is incident on the first planar surface110has a Lambertian distribution with a full width half maximum (FWHM) of 120°, and the light emitted from the second planar surface120has a full width half maximum of 57°.FIG.7Ashows the intensity of the light plotted with angle on a polar graph, andFIG.7Bshows the intensity plotted against angle on a linear scale.

With reference toFIG.8, the micro-lightguides100might be coated with a reflective material810to prevent light cross-talk between micro-lightguides100. In a certain embodiment, the reflective material810may be Aluminium or Silver.

With reference toFIG.9, the micro-lightguide100may be fabricated by depositing a layer of UV-curable material910onto a substrate920(FIG.9A). UV light930or940having a conical irradiation profile is incident on a first surface950of the UV-curable material910such that the UV light930,940selectively cures a first part912or914of the UV-curable material910. The UV-curable material910is then developed to remove one of the first, cured part (912or914) and a second, uncured part (911or913) of the layer such that the remaining part defines the shape of a frusto-conical micro-lightguide. In the example shown inFIG.9, the second, uncured part911or913of the UV-curable material910is removed such that the first part912or914of the UV-curable material910remains as a frusto-conical micro-lightguide100. The frusto-conical micro-lightguide may have its central axis perpendicular to the plane of the substrate920, but may have either its first planar surface110adjacent to the substrate920(resulting from a conical irradiation profile similar to an inverted cone) or its second planar surface120adjacent to the substrate920(resulting from a conical irradiation profile similar to a cone).FIGS.9B and9Cshow the process wherein the UV light930has a conical irradiation profile such that the irradiation profile is narrower at the substrate920than at first surface950of the UV-curable material910, so after fabrication the first planar surface110is adjacent to the substrate920.FIGS.9D and9Eshow the process wherein the UV light940has a conical irradiation profile such that the irradiation profile is wider at the substrate920than at first surface950of the UV-curable material910, so after fabrication the second planar surface120is adjacent to the substrate920. In a certain other embodiment, the cured part912or914of the UV-curable material910may be removed such that the remaining uncured part911or913comprises a recess that defines a shape of the frusto-conical micro-lightguide. A lightguide material may be deposited in the recess and the uncured part911or913of the UV-curable material910may be removed, such that the remaining lightguide material comprises a frusto-conical micro-lightguide100.

In a first embodiment, the substrate920may comprise an array310of light-sources210. The UV-curable material910may be deposited directly onto an array of light-sources210. With reference toFIG.10Athe light-sources210are shown as pixels400comprising micro-LEDs, as shown inFIG.4. The UV-curable material910may be deposited by spin coating. With reference toFIG.10B, UV light1011is incident on a mask1020at a normal to the plane of the mask1020, wherein the plane of the mask1020is parallel to the substrate920and to the UV-curable material910. The mask may have a plurality of apertures1021, the central axis of each aperture1021being aligned with a central axis of a pixel400. Each aperture1021may be circular. The mask1020is moved with a circular trajectory1030that is in the plane of the mask1020, such that the UV light1011that is transmitted through the mask has an inverted conical irradiation profile. The UV light1011is incident on the first surface950of the UV-curable material910. The inverted conical irradiation profile has highest intensity at the centre of the profile and lowest intensity at the edge of the profile. The highest intensity is constant over an area equal to the area of the first planar surface, and the central axis of the conical irradiation profile is aligned with the central axis of the pixel400. The widest part of the irradiation profile has the same area as the second planar surface120. The penetration depth of the curing of the UV-curable material910is a function of the intensity of the radiation profile, so the cured part912of the UV-curable material has a frusto-conical shape. The cured part912of the UV-curable material has a cross-sectional area adjacent to the substrate920equal to the area of the first planar surface110and a cross-sectional area at the first surface950equal to the area of the second planar surface. In a certain embodiment, the un-cured part911of the UV-curable material910is removed and the cured part912of the UV-curable material comprises frusto-conical micro-lightguides that are left on the substrate920, as shown inFIG.10C. In a certain embodiment, the cured part912of the UV-curable material910is removed, leaving frusto-conical recesses in the uncured part911, and a lightguide material is deposited in the frusto-conical recesses. The uncured part911of the UV-curable material910is then removed and the remaining lightguide material comprises frusto-conical micro-lightguides.

In a second embodiment, the substrate920may comprise a transparent material that is transparent to electromagnetic radiation with wavelengths in the visible spectrum. In a certain embodiment, the transparent material may be glass or sapphire. With reference toFIG.11A, UV light is collimated using a micro-lens1110, such that the UV light1120that is incident on the first surface950of the UV-curable material910has a conical profile within the UV-curable material910. The conical profile has an area equal to the first planar surface110at the first surface950of the UV-curable material910, and has an area equal to the second planar surface at the substrate920. The UV-curable material910that is within the conical profile is cured, and the uncured part913may be removed. The cured part914has a frusto-conical shape and may be left on the substrate. In an embodiment, there may be an array of micro-lenses1110such that there is an array of cured parts914. With reference toFIG.12, a perspective view of a schematic of an array of cured parts914on a substrate920is shown, wherein the cured part914is a micro-lightguide100. With reference toFIG.13, the substrate920may be placed on an array of light-sources210such that each micro-lightguide100is coupled to a light-source210. The central axis of a micro-lightguide 10 is aligned with the central axis of a light source210. The first planar surface110is proximate to the light source210. In an embodiment, the light source210may be a pixel400comprising a micro-LED. In a certain embodiment, the distance between the first planar surface110and the light source210may be smaller than 20% of an area of a light emitting area of the light source210.

In a certain embodiment, the UV-curable material910may be absorbing of electromagnetic radiation with wavelengths in the UV region, and transparent to electromagnetic radiation with wavelengths in the visible spectrum. In a certain embodiment, the arithmetic average of the roughness profile of a surface of the micro-lightguide100may be less 20 nm. In a certain embodiment, the UV-curable material may have a refractive index of 1.555 for light with a wavelength of 589 nm. The UV-curable material may comprise or consist of OrmoClear®FX.

The UV-curable material910may be spin-coated onto the substrate920. The thickness of the UV-curable material910may depend on the duration of the spin coating.

In a certain embodiment, in which the substrate920may comprise an array310of light sources201, the substrate920may be spin-coated with the UV-curable material910and then baked at 80° C. for 2 minutes to improve adhesion to the substrate920. The UV-curable material910may be exposed to UV light that is transmitted through a moving mask, wherein the mask moves in a circular trajectory such that the irradiance profile is similar to an inverted cone. The dose of IV exposure may be below 1000 mJ cm-2in order to achieve appropriate resolution. The UV-curable material910is developed to remove the uncured part911. The substrate920and the cured parts912may be baked at 120° C. for 10 minutes in order to increase adhesion of the micro-lightguides100to the substrate920.

In a certain embodiment, in which the substrate920may be a transparent material, the substrate920may be spin-cleaned with acetone/2-propanol and then baked at 200° C. for 5 minutes and cooled to room temperature before spin coating with UV-curable material910. Alternatively, the substrate920may be cleaned via plasma cleaning with oxygen or ozone. After spin coating the UV-curable material910on to the substrate920, the substrate920may be baked at 80° C. for 2 minutes to improve adhesion. The UV-curable material910may then be exposed to UV light with a conical irradiation profile. In a certain embodiment the UV light is collimated using an array of micro-lenses1110such that the UV light incident on the UV-curable material has a conical irradiation profile. The dose of IV exposure may be below 1000 mJ cm-2in order to achieve appropriate resolution. After exposure the UV-curable material910is developed to remove the uncured part913. The substrate920and the cured parts914may be baked at 120° C. for 10 minutes in order to increase adhesion of the micro-lightguides100to the substrate920.