Improved μ-LED projection device and method for manufacturing an LED projection device

Abstract

The invention provides a projection device for an LED array, comprising an LED array (1) on which a number of micro LEDs (1a, 1b, 1c) are arranged at regular intervals, a projection optics (4) spaced apart from the LED array (1) capable of receiving the light emitted by the array (1) and projecting it onto a projection surface, and a light-collecting structure (2) with individual protrusions (3). According to the invention, the protrusions (3) are optically coupled to the individual LEDs (1a, 1b, 1c) of the LED array (1) so as to limit the angular space of the light emitted from them and / or to homogenize the emission. The manufacturing of the projection device is similarly described.

Description

Technical Field

[0001] This application claims the priority of German Patent Application No. 102021113695.4 filed on May 27, 2021, and incorporates its disclosure herein by reference. The present invention starts from a conventional μ-LED projection device, i.e., a projection device equipped with known very small light-emitting diodes (hereinafter, μ-LED, light emitting diodes).

Background Art

[0002] Conventionally, μ-LEDs are still not frequently used for projection applications in the consumer field. Instead, other technologies such as DLP ("Digital Light Processing") technology using conventional LEDs or (transmissive or reflective) LCD technology are mainly used.

[0003] In DLP (Digital Light Processing) technology, light is imaged on a projection surface via a DMD ("Digital Micro-mirror Device"), which is a chip on which up to 2.2 million microscopically small mirrors arranged at right angles are placed. These mirrors are moved by tiny electrodes below each mirror. Each mirror is attached to a joint and can thus be rotated in the direction towards the light source or away from the light source.

[0004] To generate a colored display, the mirror array is sequentially illuminated over time by three primary colors R, G, and B, and the resulting images are superimposed on the projection surface. To achieve this, in many cases, a white light source emits radiation to the DMD via a fast-rotating "ColorWheel" or color wheel having three different segments that each pass light of a given color (red, green, or blue). This rotation is synchronized with the tilting of the micromirrors so that the desired images of each individual color are generated.

[0005] In addition to DLP, the second widely used projection technology is LCD technology. LCD projectors generate images using transparent liquid crystal displays (so-called "LCDs", as represented by the name Liquid Crystal Display). LCD projectors function similarly to conventional slide projectors. One difference is that LCD projectors incorporate one or three LCDs (one for each RGB color) instead of slides. The orientation of the LCD crystals changes according to the applied electric field, allowing more or less light to pass through. Since LCD displays are relatively inexpensive today, a separate display is provided for each primary color (red, green, blue). This means that there is no time-shifted projection of multiple primary colors here (no rainbow effect occurs). The three light emissions exit simultaneously from the projector's lens system via dichroic mirrors.

[0006] One of its variants uses a reflective LCD. In the case of a reflective LCD, the change in the polarization of light upon reflection can be utilized to extract light. For example, Patent Document 1 shows such a variant of the principle described above, along with the advantages in the optical path configuration.

[0007] Even in the automotive field, an increasing number of projection applications are being used, for example, in dynamic in-vehicle projections such as so-called "head-up displays" or in dynamic "welcome lights" in the out-of-vehicle area. However, certain drawbacks of conventional white light projectors have an impact here. That is, classical bright light sources such as incandescent bulbs are relatively sensitive to vibration, and furthermore, mechanical components such as color wheels are also noisy, which can be a problem, especially in future electric vehicles, in view of the quieter interior spaces in the luxury automotive segment. Additionally, such configurations are relatively large. This is because it is necessary to provide three radiating paths for three LCDs and their recombination, or to provide space for a color wheel, so reducing the structural dimensions faces fundamental difficulties.

[0008] In all of these methods, white light is decomposed into a plurality of primary colors (usually red, green, blue), and then, depending on each primary color and the desired image, it is transmitted or blocked to the projection surface to generate the desired image, or (in the case of DMD) deflected from the projection radiation path (and subsequently absorbed). Therefore, these systems are subtractive systems that cannot fully utilize the generated light structurally. In other words, light must be absorbed to display dark / black pixels.

[0009] As an alternative, for example, an LED array that forms the basis for conventional flat panel displays can be considered. Recent developments have made it possible to further miniaturize LEDs, and thus, μLEDs or micro-LEDs have become a topic of discussion. In this case, a plurality of μLEDs are arranged as a matrix or an array, and the length of one side of the LED chip itself is in the range of μm (one millionth of a meter). The pixel pitch is, for example, several μm to several tens of μm. Such a matrix can be composed of RGB-LEDs, but in other applications, a single-color (one-color) matrix can also be considered.

[0010] However, the brightness of these μ-LEDs is too low in the conventional configuration. This is because these μ-LEDs emit light at a wide half-space angle, so the conventional imaging optical system connected downstream cannot absorb and project sufficient light.

[0011] On the other hand, in order to improve optical extraction, it is known to combine LEDs with a light condensing structure. For example, Non-Patent Document 1 describes improving the efficiency of GaN-LEDs (gallium nitride-based light-emitting diodes) by using a patterned sapphire with silica array (PSSA) as a base on which the LEDs are constructed. As a result, the light that was reflected in the PSS structure and would otherwise have been emitted backward in the optical path could be better utilized, enabling a 16.5% improvement in the light extraction efficiency of the LEDs.

[0012] Non-Patent Document 2 describes a transfer molding method for silicon-based LEDs equipped with an aspherical lens and a microlens matrix. In this regard, microelectromechanical technology, that is, substantially conventional ablation technology, is used, but on a miniature scale, to manufacture the mold. This mold is then transferred by pressing it into an epoxy cover and a silicone gel cover of the wafer, thereby achieving accurate and reproducible alignment of the objective lens structure and the reflector with respect to the silicon wafer. The increase in brightness and filling rate using only one aspherical lens is described as 16 - 26% compared to an unstructured curved surface. Furthermore, the illumination is normalized.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Non-Patent Literature

[0014]

Non-Patent Literature 1

Non-Patent Literature 2

Summary of the Invention

Problems to be Solved by the Invention

[0015] The problem of the present invention is to provide an improved projection device for LEDs, especially for micro-LED arrays. However, the use of the present invention is not basically limited to micro-LEDs.

Means for Solving the Problems

[0016] The above problems are solved by the LED projection device according to claim 1 and the method for manufacturing the LED projection device according to claim 7.

[0017] Particularly advantageous embodiments are set forth in the dependent claims.

[0018] A projection device for an LED array according to the present invention includes, for example, an LED array in which a plurality of micro LEDs are placed at regular intervals, and a projection optical system spaced apart from the LED array that can receive the light emitted from the array and project it onto a projection surface. The projection device according to the present invention further includes a light condensing structure having individual protrusions, and the protrusions are optically coupled to the individual LEDs so as to limit the angular space of the light emitted from the individual LEDs of the LED array and / or to homogenize the radiation.

[0019] Such a projection device according to the present invention includes a light condensing structure, which is usually placed directly above the (micro) LEDs and actually condenses all the light emitted from the individual LEDs or the LED array, for example, from an RGB array of LEDs. Thus, this structure reduces the radiation angle before the light enters the projection optical system. Thus, a higher light collection rate is obtained, which makes the LED projection brighter, and thereby for the first time makes it usable for many applications. Furthermore, the light condensing structure can be manufactured with high precision and low cost by a well-known method such as photochemical etching. Furthermore, the configuration of this structure in the optical path is very short, so that the corresponding projection optical system can be constructed much more compactly. The structure according to the present invention enables the use of LEDs for bright projectors, so the structure according to the present invention can be constructed even more simply compared to the prior art. Thus, compared to the three LCDs or DMDs and color wheels described with respect to the prior art, only one LED array is required, so that the precise synchronization between mechanical components (color wheel and DMD) and the precise alignment and synchronization of the three optical paths (by three LCDs) are also unnecessary. Thus, the device according to the present invention can be attached to a high-performance projection optical system at a lower cost.

[0020] Particularly preferably, the light collecting structure has a conical protrusion. In this case, the uppermost surface of the conical protrusion is coupled to the LED or to the portion of the array that emits the light points of the image to be projected. When the protrusion is formed in a conical shape, i.e., a frustum shape, the light from one LED or from one array of multiple LEDs can be expanded into a wider light point at the lower side of the cone. This makes it possible to make completely invisible within the image, at the upstream of the projection optical system already, the spacing required between the LEDs for wiring or the like. Such a cone functions in the same way as the taper in a fiber optic system when connecting two fibers of different thicknesses. In this case, it is particularly advantageous that the spacing required between the LEDs can be maintained relatively large without degrading the imaging quality.

[0021] If the projection device is arranged in a medium having a refractive index lower than that of the light collecting structure, for example, in air or water having a refractive index lower than the typical refractive index of silicon oxide of about 1.5, and further, if the cone angle is appropriately adapted to the maximum emission angle of the corresponding LED, all the light of the LED is retained by total internal reflection within the cone and directed towards the exit surface of the light collecting structure located opposite to the LED. Alternatively, the outside of the cone can be mirrored, for example, by evaporation or sputtering, whereby it is guaranteed that in an uncertain environment, all the incident light exits from the light collecting structure on the side opposite to the LED and is available for projection. By both of these means, it becomes possible to further optimize the light collection efficiency.

[0022] The protrusion or taper can cover one (micro)LED or a plurality of (micro)LEDs so as to be coupled to one (micro)LED or a plurality of (micro)LEDs. When one protrusion covers a plurality of LEDs, in the case of a colored array (for example, an array including red, green, and blue LEDs), it is preferable to cover each one LED array that generates white light as a whole. In such a case, it is possible to generate one point of any color having an optimal luminance for each protrusion from the light condensing structure and form an image by a projection device. In the case of a single-color LED array, such a device has an advantage that, in addition to (conventional) control by pulse width modulation, the luminance can be stepwise controlled by turning on or off the LEDs.

[0023] Normally, the protrusion of the light condensing structure may be flatly placed on the flat surface of the corresponding array of LEDs in order to achieve an ideal incidence. However, it is also conceivable that the light condensing structure has a curved and thus optically effective surface in order to condense the radiation emitted from the corresponding LED or, among other things, to provide an imaging step already at this location. By doing so, in some cases, the light collection rate can be further increased.

[0024] In addition to the device, a method for manufacturing a device having the features mentioned above is also a subject of the claims.

[0025] Hereinafter, the present invention will be described in more detail based on one (or a plurality) of a plurality of embodiments.

Brief Description of the Drawings

[0026]

Figure 1

Figure 2

Figure 3a

Figure 3b

Figure 4a

Figure 4b

Figure 4c

Figure 4d

[0027] Description of Embodiment FIG. 1 shows an example of a micro-LED array or a μ-LED array as an example of an LED array 1 (hereinafter abbreviated as an array) for use in a projection device according to the present invention. In this embodiment, one colored array in which LEDs of three primary colors, red (R), green (G), and blue (B), are arranged in a predefined pattern is utilized. In this figure, for illustration in black and white, hatching from bottom left to top right is selected as a symbol for red light, hatching from top left to bottom right is selected as a symbol for blue light, and cross-hatching is selected as a symbol for green light.

[0028] Such an array emits colored light according to each drive, and thereby, a bright image can be directly generated additively. Many flat panel displays utilize a similar device equipped with not only micro-LEDs but also conventional LEDs or OLEDs (organic LEDs). In this case, possible arrangements of the LEDs are equivalent even if the scales are different. Therefore, possible arrangements of colored LEDs (or μ-LEDs or micro-LEDs) are well known and will not be described. Of course, a single-color μ-LED array may be used in the case of simple projection.

[0029] Figure 2 shows an example of a light condensing structure according to the present invention in an example of a PSS (patterned sapphire substrate) structure or a PSSA (patterned sapphire with silica array) structure 2. The manufacture of such a structure is basically known (see the documents cited as the prior art), and will only be described roughly here. First, the sapphire substrate is covered with a protective layer made of, for example, SiO2. This layer is coated with a photoresist paint, and then the photoresist paint is exposed through a lithographic optical system in a desired pattern. Subsequently, the unexposed portions of the paint are removed, and the exposed SiO2 and the sapphire layer located thereunder are etched away. This method itself is known from semiconductor manufacturing, and it is also possible to perform multiple coating steps, exposure steps, and etching steps to generate more complex geometries. In this way, a highly precise structure can be generated. In the case of this specification, the purpose is to form a structure such that the completed substrate has a plurality of protrusions 3 that match each of the micro LEDs on the μ-LED array 1. This means that the mutual distance between the center points of the plurality of protrusions of the light condensing structure is the same as the mutual distance between the center points of the plurality of micro LEDs, in other words, the protrusions exactly overlap the micro LEDs.

[0030] In the next step, the light condensing structure 2 is optically coupled to the μ-LED array 1. That is, in the simplest case, the light condensing structure 2 and the μ-LED array 1 are overlapped and pressed together (for example, using an external clip or the like). Instead of this, pasting or other forms of assembly are also possible. What is important is to align the array and the light condensing structure so that each protrusion is optically coupled to the corresponding μ-LED. The corresponding alignment can be achieved, for example, by providing protrusions and recesses that fit into each other on components 1 and 2 outside the optically effective surface. When these protrusions and recesses are overlapped, they fit into each other to align the light condensing structure 2 and the μ-LED array 1 with each other.

[0031] Then, in this way, each one protrusion 3 of the light condensing structure will be located on one μ-LED of the array 1. Therefore, each one protrusion 3 of the light condensing structure and one μ-LED of the array 1 are optically coupled to each other. As shown by two exemplary emissions in FIG. 2, in this figure, the light emission output in an arbitrary direction upward in the half space, indicated by a bent arrow here, is condensed by total internal reflection in the protrusion 3 formed in a tapered (conical) shape here and output to the projection optical system 4.

[0032] Therefore, all the light output from the μ-LED array is received in the PSS structure as the light condensing structure 2 and sent to the projection objective lens. Thereby, a much higher light collection rate is achieved than the light collection rate that might occur without using this light condensing structure 2.

[0033] In the prior art, the PSS structure is used to improve the optical extraction in LEDs. Figure 3a shows the PSS structure used as a reflective light-collecting structure. Here, an LED having an n-type doped GaN layer and a p-type doped GaN layer is attached to the PSS. In Figure 3b, which visually represents the radiation emitted from a given point shown in Figure 3a three-dimensionally, the angle θ (as the radiation angle when not using a specially patterned structure) C PSS and the angle θ (as the radiation angle when using the illustrated patterned PSS structure) C CSS As is clear in the comparison, the light generated from the active layer between the p-type doped layer and the n-type doped layer is first emitted in all directions, but is reflected so that the radiation angle is restricted in the pattern. Note in Figure 3a that the refraction of the radiation at the transition from the LED into the air is ignored. This is because such refraction of the radiation affects only the reduction of the radiation angle with a fixed coefficient depending on the refractive index of the GaN surface (here p-type doped) and without depending on the relevant action here.

[0034] Figs. 4a to 4d show again an example of the shape of the protrusions of the PSS structure as a light condensing structure, which can be fabricated by lithography and subsequent etching as described above. As can be seen when looking from top to bottom, first (Fig. 4a) a photoresist coating 6 is applied, structured and exposed, and subsequently, the unexposed portions of the photoresist coating 6 are etched away to produce a (here tapered) structure of the protrusions of the sapphire substrate 5 (Figs. 4b and 4c). As a final result, a structure exemplified in Fig. 4d can be formed, which is then placed over the micro LED array as shown in Fig. 2 to produce a projection device according to the present invention. These structures can be manufactured with high precision and high quality and in a well-controllable form as described above, whereby these structures can be well adapted to the requirements regarding the geometry determined by the optical system.

[0035] In this embodiment, an example is shown in which the surface on which the LED is placed is a flat surface. However, it is also conceivable to manufacture the light condensing structure so that it penetrates into the intermediate space between the micro LEDs of the micro LED array 1, for example, and completely surrounds the LEDs 1a, 1b, 1c. This can make it possible to make the output light incident even better, and furthermore, it can simplify the alignment of the two components with each other. Further, the surface of the light condensing structure that is coupled to the micro LEDs 1a, 1b, 1c can also be curved so as to already function as a "first lens". Even in this way, the light collection efficiency of the projection device can be further increased. Therefore, the corresponding shape of the surface of the light condensing structure 2 that is coupled to the micro LEDs 1a, 1b, 1c is not limited to the flat surface shown in this embodiment and can be adapted according to each application case.

[0036] In a certain application case, when it is recognized that total internal reflection within the light condensing structure 2 does not sufficiently act and excessive light has already left the light condensing structure from the outer surface of the protruding portion, it is also possible to mirror-finish this structure as shown in the example from the prior art in Fig. 3a. This enables more scattered light to be condensed and utilized.

Explanation of Reference Numerals

[0037] 1 Micro LED array 1a, 1b, 1c Micro LEDs (red, blue, green) 2 Light condensing structure (PSS structure) 3 Protruding portion (taper) 4 Projection optical system 5 PSS structure 6 Photoresist paint

Claims

1. A projection device for an LED array, comprising: an LED array (1) on which a plurality of micro LEDs (1a, 1b, 1c) are mounted at regular intervals; a projection optical system (4) spaced apart from the LED array (1), capable of receiving light emitted from the LED array (1) and projecting it onto a projection surface; a light condensing structure (2) provided with a substrate having individual protrusions (3); the protrusions (3) are optically coupled to the individual LEDs (1a, 1b, 1c) so as to limit the angular space of the light emitted from the individual LEDs (1a, 1b, 1c) of the LED array (1) and / or to equalize the radiation; the light condensing structure (2) optically couples a plurality of micro LEDs (1a, 1b, 1c) to one common protrusion (3) of the light condensing structure (2), a projection device for an LED array.

2. The projection device for an LED array according to claim 1, wherein the light condensing structure (2) has conical protrusions (3).

3. The projection device is disposed in a medium having a refractive index lower than that of the light condensing structure (2), the cone angle of the protrusion (3) is adjusted in accordance with the maximum radiation angle of the LED (1a, 1b, 1c) such that the incident light of the LED (1a, 1b, 1c) exits the light condensing structure (2) only on the side opposite to the LED (1a, 1b, 1c) by total internal reflection within the protrusion (3), a projection device for an LED array according to claim 2 having a combination of a cone angle and a refractive index.

4. The projection device for an LED array according to claim 1, wherein the plurality of micro LEDs (1a, 1b, 1c) each have a different color emission.

5. The projection device for an LED array according to claim 1, wherein all surfaces of the light condensing structure (2) facing the LEDs (1a, 1b, 1c) are provided with a specular coating except for the surfaces that can be coupled to the micro LEDs.

6. The projection device for an LED array according to claim 1, wherein the surfaces of the protrusions (3) that can be coupled to the LEDs (1a, 1b, 1c) have curvature.

7. A method for manufacturing a projection device for an LED array according to claim 1, comprising: - a step of providing an LED array (1); - providing a light collecting structure (2) comprising a substrate having protrusions (3) adapted to the LEDs (1a, 1b, 1c) on the LED array (1); - connecting the light collecting structure (2) to the LED array (1) such that each protrusion (3) is optically coupled to at least one LED (1a, 1b, 1c). A method comprising:

8. The method according to claim 7, comprising the step of fabricating the protrusion (3) in a tapered shape that tapers conically towards the LED array (1).

9. A further step of disposing the projection device in a medium having a refractive index lower than that of the light collecting structure (2); The method according to claim 8, further comprising adjusting the cone angle of the protrusion (3) such that the incident light of the corresponding LED (1a, 1b, 1c) exits the light collecting structure (2) only on the side opposite to the LED (1a, 1b, 1c) by total internal reflection within the protrusion (3), in accordance with the maximum emission angle of the LED (1a, 1b, 1c) and the refractive index of the material of the protrusion (3).

10. The method according to claim 7, comprising the step of coupling one protrusion to a plurality of corresponding micro LEDs.

11. The method according to claim 7, further comprising the step of mirroring all the surfaces of the light collecting structure facing the micro LED array, except for the surfaces that can be coupled to the micro LEDs.

12. The method according to claim 7, further comprising the step of fabricating the protrusion such that the surface thereof that can be coupled to the micro LED has a curvature.

Images