DISPLAY WITH COLOR CONVERSION LAYER AND ISOLATION WALLS

Abstract

A multi-color display includes a backplane having backplane circuitry, an array of micro-LEDs electrically integrated with backplane circuitry of the backplane, a first color conversion layer over each of a first plurality of light emitting diodes, a second color conversion layer over each of a second plurality of light emitting diodes, and a plurality of isolation walls separating adjacent micro-LEDs of the array. The micro-LEDs of the array are configured to generate illumination of the same wavelength range, the first color conversion layer converts the illumination to light of a first color, and the second color conversion layer converts the illumination to light of a different second color.

Claims

1. A multi-color display, comprising: a backplane having backplane circuitry; an array of semiconductor micro-LEDs electrically integrated with backplane circuitry of the backplane, the semiconductor micro-LEDs of the array configured to generate illumination of the same wavelength range; a first color conversion layer over each of a first plurality of light emitting diodes from the array of semiconductor micro-LEDs to convert the illumination from the first plurality of light emitting diodes to light of a first color, the first color conversion layer comprising a polymer cured by illumination of the wavelength range generated by the emitted by the of semiconductor micro-LEDs; a second color conversion layer over each of a second plurality of light emitting diodes from the array of semiconductor micro-LEDs to convert the illumination from the second plurality of light emitting diodes to light of a different second color, the second color conversion layer comprising polymer cured by illumination of the wavelength range generated by the emitted by the of semiconductor micro-LEDs; and a plurality of isolation walls that separate adjacent micro-LEDs of the array and that substantially block illumination from the first plurality of light emitting diodes from reaching the second color conversion layer over each of the second plurality of light emitting diodes, the plurality of isolation walls laterally spaced from the micro-LEDs and extending from the backplane to above respective top surfaces of the first color conversion layer and the second color conversion layer.

2. The display of claim 1, further comprising a third color conversion layer over each of a third plurality of light emitting diodes from the array of semiconductor micro-LEDs to convert the illumination from the third plurality of light emitting diodes to light of a different third color, and the plurality of isolation extend respective top surfaces of the third color conversion layer.

3. The display of claim 2, wherein the first color, second color and third color are selected from blue, green and red.

4. The display of claim 1, wherein the semiconductor micro-LEDs of the array of semiconductor micro-LEDs are configured to generate ultraviolet light.

5. The display of claim 1, wherein the array of semiconductor micro-LEDs includes a third plurality of light emitting diodes, and semiconductor micro-LEDs of the array of semiconductor micro-LEDs are configured to generate visible light of a different third color.

6. The display of claim 5, wherein no color conversion layer is formed over the third plurality of light emitting diodes.

7. The display of claim 5, wherein the light emitting diodes of the array of light emitting diodes are configured to generate blue or violet light.

8. The display of claim 7, wherein the first color and second color are selected from green and red.

10. The display of claim 9, wherein the first color is green and the second color is red.

11. The display of claim 1, wherein the isolation walls comprise a photoresist.

12. The display of claim 1, wherein the isolation walls comprise a metal.

13. The display of claim 1, wherein material of the isolation wall is absent from between the backplane and the semiconductor micro-LEDs.

14. The display of claim 1, comprising a ultraviolet blocking layer over the array of light emitting diodes.

15. The display of claim 1, wherein the semiconductor micro-LEDs comprise III-V semiconductor LEDs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a schematic top view of a micro-LED array that has already been integrated with a backplane.

[0023] FIG. 2A is a schematic top view of a portion of a micro-LED array.

[0024] FIG. 2B is a schematic cross-sectional view of the portion of the micro-LED array from FIG. 2A.

[0025] FIGS. 3A-3H illustrate a method of selectively forming color conversion agent (CCA) layers over a micro-LED array.

[0026] FIGS. 4A-4C illustrate formulations of photo-curable fluid.

[0027] FIGS. 5A-5E illustrate a method of fabricating a micro-LED array and isolation walls on a backplane.

[0028] FIGS. 6A-6D illustrate another method of fabricating a micro-LED array and isolation walls on a backplane.

[0029] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0030] As noted above, selective deposition of color conversion agents can be performed using use high-resolution shadow masks or controllable inkjet or aerosol jet printing. Unfortunately, shadow masks are prone to problems with alignment accuracy and scalability, whereas inkjet and aerosol jet techniques suffer from resolution (inkjet), accuracy (inkjet) and throughput (aerosol jet) problems. In order to manufacture micro-LED displays, new techniques are needed to precisely and cost-effectively provide color conversion agents for different colors onto different pixels on a substrate, such as a large area substrate or flexible substrate.

[0031] A technique that may address these problems is to coat a layer of photo-curable fluid containing a color conversion agent (CCA) for a first color on a substrate having an array of monochrome micro-LEDs, then turn on selected LEDs to trigger in-situ polymerization and immobilize the CCA in the vicinity of the selected subpixels. The uncured fluid over the non-selected subpixels can be removed, and then the same process can be repeated with CCAs for different colors until all subpixels on the wafer are covered with CCAs of the desired colors.

[0032] This technique may overcome the challenges in alignment accuracy, throughput and scalability.

[0033] FIG. 1 illustrates a micro-LED display 10 that includes an array 12 of individual micro-LEDs 14 (see FIGS. 2A and 2B) disposed on a backplane 16. The micro-LEDs 14 are already integrated with backplane circuitry 18 so that each micro-LED 14 can be individually addressed. For example, the backplane circuitry 18 can include a TFT active matrix array with a thin-film transistor and a storage capacitor (not illustrated) for each micro-LED, column address and row address lines 18a, column and row drivers 18b, etc., to drive the micro-LEDs 14. Alternatively, the micro-LEDs 14 can be driven by a passive matrix in the backplane circuitry 18. The backplane 16 can be fabricated using conventional CMOS processes.

[0034] FIGS. 2A and 2B illustrate a portion 12a of the micro-LED array 12 with the individual micro-LEDs 14. All of the micro-LEDs 14 are fabricated with the same structure so as to generate the same wavelength range (this can be termed “monochrome” micro-LEDs). For example, the micro-LEDs 14 can generate light in the ultraviolet (UV), e.g., the near ultraviolet, range. For example, the micro-LEDs 14 can generate light in a range of 365 to 405 nm. As another example, the micro-LEDs 14 can generate light in the violet or blue range. The micro-LEDs can generate light having a spectral bandwidth of 20 to 60 nm.

[0035] FIG. 2B illustrates a portion of the micro-LED array that can provide a single pixel. Assuming the micro-LED display is a three-color display, each pixel includes three sub-pixels, one for each color, e.g., one each for the blue, green and red color channels. As such, the pixel can include three micro-LEDs 14a, 14b, 14c. For example, the first micro-LED 14a can correspond to a blue subpixel, the second micro-LED 14b can correspond to a green subpixel, and the third micro-LED 14c can correspond to a red subpixel. However, the techniques discussed below are applicable to micro-LED displays that use a larger number of colors, e.g., four or more colors. In this case, each pixel can include four or more micro-LEDs, with each micro-LED corresponding to a respective color. In addition, the techniques discussed below are applicable to micro-LED displays that use just two colors.

[0036] In general, the monochrome micro-LEDs 14 can generate light in a wavelength range having a peak with a wavelength no greater than the wavelength of the highest-frequency color intended for the display, e.g., purple or blue light. The color conversion agents can convert this short wavelength light into longer wavelength light, e.g., red or green light for red or green subpixels. If the micro-LEDs generate UV light, then color conversion agents can be used to convert the UV light into blue light for the blue subpixels.

[0037] Vertical isolation walls 20 are formed between neighboring micro-LEDs. The isolation walls provide for optical isolation to help localize polymerization and reduce optical crosstalk during the in-situ polymerization discussed below. The isolation walls 20 can be a photoresist or metal, and can be deposited by conventional lithography processes. As shown in FIG. 2A, the walls 20 can form a rectangular array, with each micro-LED 14 in an individual recess 22 defined by the walls 20. Other array geometries, e.g., hexagonal or offset rectangular arrays, are also possible. Possible processes for back-plane integration and isolation wall formation are discussed in more detail below.

[0038] The walls can have a height H of about 3 to 20 μm. The walls can have a width W of about 2 to 10 μm. The height H can be greater than the width W, e.g., the walls can have an aspect ratio of 1.5:1 to 5:1. The height H of the wall is sufficient to block light from one micro-LED from reaching an adjacent micro-LED.

[0039] FIGS. 3A-3H illustrate a method of selectively forming color conversion agent (CCA) layers over a micro-LED array. Initially, as shown in FIG. 3A, a first photo-curable fluid 30a is deposited over the array of micro-LEDs 14 that are already integrated with the backplane circuitry. The first photo-curable fluid 30a can have a depth D greater than a height H of the isolation walls 20.

[0040] Referring to FIG. 4A, the first photo-curable fluid 30a includes at least cross-linkable groups 32, a photo-initiator 34 to trigger polymerization under illumination of a wavelength corresponding to the emission of the micro-LEDs 14, and color conversion agents 36a.

[0041] The cross-linkable groups 32 will increase the viscosity of the fluid 30a when subjected to polymerization, e.g., the fluid 30a can be solidified or form gel-like network structures. The cross-linkable groups 32 can be provided by monomers that form a polymer when cured, e.g., acrylate, methacrylate and acrylamide. The cross-linkable groups 32 can be provided by a negative photoresist, e.g., SU-8 photoresist.

[0042] Examples of the photo-initiator 34 include Irgacure 184, Irgacure 819, Darocur 1173, Darocur 4265, Dacocur TPO, Omnicat 250 and Omnicat 550.

[0043] The color conversion agents 36a is a material that can convert the shorter wavelength light from the micro-LED 14 into longer wavelength light corresponding to one of the three colors. In the example illustrated by FIGS. 3A-3H, the color conversion agent 36 converts the UV light from the micro-LED 14 into blue light. The color conversion agent 36 can include quantum dots, nanostructures, organic or inorganic florescence molecules, or other suitable materials.

[0044] Optionally, the first photo-curable fluid 30a can include a solvent 37, e.g., water, ethanol, toluene or methylethylketone, or a combination thereof. The solvent can be organic or inorganic. The solvent can be selected to provide a desired surface tension and/or viscosity for the first photo-curable fluid 30a. The solvent can also improve chemical stability of the other components.

[0045] Optionally, the first photo-curable fluid 30a can include one or more other functional ingredients 38. As one example, the functional ingredients can affect the optical properties of the color conversion layer. For example, the functional ingredients can include nano-particles with a sufficiently high index of refraction that the color conversion layer functions as an optical layer that adjusts the optical path of the output light, e.g., provides a microlens. Alternately or in addition, the nano-particles can have an index of refraction selected such that the color conversion layer functions as an optical layer that reduces total reflection loss, thereby improving light extraction. As another example, the functional ingredients can be a surfactant to adjust the surface tension of the fluid 30a.

[0046] Returning to FIG. 3A, the first photo-curable fluid 30a can be deposited on the display over the micro-LED array by a spin-on, dipping, spray-on, or inkjet process. An inkjet process can be more efficient in consumption of the first photo-curable fluid 30a.

[0047] Next, as shown in FIG. 3B, the circuitry of the backplane 16 is used to selectively activate a first plurality of micro-LEDs 14a. This first plurality of micro-LEDs 14a correspond to the sub-pixels of a first color. In particular, the first plurality of micro-LEDs 14a correspond to the sub-pixels for the color of light to be generated by the color conversion components in the photo-curable fluid 30a. For example, assuming the color conversion component in the fluid 30a will convert light from the micro-LED 14 into blue light, then only those micro-LEDs 14a that correspond to blue sub-pixels are turned on. Because the micro-LED array is already integrated with the backplane circuitry 18, power can be supplied to the micro-LED display 10 and control signals can be applied by a microprocessor to selectively turn on the micro-LEDs 14a.

[0048] Referring to FIGS. 3B and 3C, activation of the first plurality of micro-LEDs 14a generates illumination A (see FIG. 3B) which causes in-situ curing of the first photo-curable fluid 30a to form a first solidified color conversion layer 40a (see FIG. 3C) over each activated micro-LED 14a. In short, the fluid 30a is cured to form color conversion layers 40a, but only on the selected micro-LEDs 14a. For example, a color conversion layer 40a for converting to blue light can be formed on each micro-LED 14a.

[0049] In some implementations, the curing is a self-limiting process. For example, illumination, e.g., UV illumination, from the micro-LEDs 14a can have a limited penetration depth into the photo-curable fluid 30a. As such, although FIG. 3B illustrates the illumination A reaching the surface of the photo-curable fluid 30a, this is not necessary. In some implementations, the illumination from the selected micro-LEDs 14a does not reach the other micro-LEDs 14b, 14c. In this circumstance, the isolation walls 20 may not be necessary.

[0050] However, if the spacing between the micro-LEDs 14 is sufficiently small, isolation walls 20 can affirmatively block illumination A from the selected micro-LED 14a from reaching the area over the other micro-LEDs that would be within the penetration depth of the illumination from those other micro-LEDs. Isolation walls 20 can also be included, e.g., simply as insurance against illumination reaching the area over the other micro-LEDs.

[0051] The driving current and drive time for the first plurality of micro-LEDs 14a can be selected for appropriate photon dosage for the photo-curable fluid 30a. The power per subpixel for curing the fluid 30a is not necessarily the same as the power per subpixel in a display mode of the micro-LED display 10. For example, the power per subpixel for the curing mode can be higher than the power per subpixel for the display mode.

[0052] Referring to FIG. 3D, when curing is complete and the first solidified color conversion layer 40a is formed, the residual uncured first photo-curable fluid is removed from the display 10. This leaves the other micro-LEDs 14b, 14c, exposed for the next deposition steps. In some implementations, the uncured first photo-curable fluid 30a is simply rinsed from the display with a solvent, e.g., water, ethanol, toluene or methylethylketone, or a combination thereof. If the photo-curable fluid 30a includes a negative photoresist, then the rinsing fluid can include a photoresist developer for the photoresist.

[0053] Referring to FIGS. 3E and 4B, the treatment described above with respect to FIGS. 3A-3D is repeated, but with a second photo-curable fluid 30b and activation of a second plurality of micro-LEDs 14b. After rinsing, a second color conversion layer 40b is formed over each of the second plurality of micro-LEDs 14b.

[0054] The second photo-curable fluid 30b is similar to the first photo-curable fluid 30a, but includes color conversion agents 36b to convert the shorter wavelength light from the micro-LEDs 14 into longer wavelength light of a different second color. The second color can be, for example, green.

[0055] The second plurality of micro-LEDs 14b correspond to the sub-pixels of a second color. In particular, the second plurality of micro-LEDs 14b correspond to the sub-pixels for the color of light to be generated by the color conversion components in the second photo-curable fluid 30b. For example, assuming the color conversion component in the fluid 30b will convert light from the micro-LED 14 into green light, then only those micro-LEDs 14b that correspond to green sub-pixels are turned on.

[0056] Referring to FIGS. 3F and 4C, optionally the treatment described above with respect to FIGS. 3A-3D is repeated yet again, but with a third photo-curable fluid 30c and activation of a third plurality of micro-LEDs 14c. After rinsing, a third color conversion layer 40c is formed over each of the third plurality of micro-LEDs 14c.

[0057] The third photo-curable fluid 30c is similar to the first photo-curable fluid 30a, but includes color conversion agents 36c to convert the shorter wavelength light from the micro-LEDs 14 into longer wavelength light of a different third color. The third color can be, for example, red.

[0058] The third plurality of micro-LEDs 14c correspond to the sub-pixels of a third color. In particular, the third plurality of micro-LEDs 14c correspond to the sub-pixels for the color of light to be generated by the color conversion components in the third photo-curable fluid 30c. For example, assuming the color conversion component in the fluid 30c will convert light from the micro-LED 14 into red light, then only those micro-LEDs 14c that correspond to red sub-pixels are turned on.

[0059] In this specific example illustrated in FIGS. 3A-3F, color conversion layers 40a, 40b, 40c are deposited for each color sub-pixel. This is needed, e.g., when the micro-LEDs generate ultraviolet light.

[0060] However, the micro-LEDs 14 could generate blue light instead of UV light. In this case, the coating of the display 10 by a photo-curable fluid containing blue color conversion agents can be skipped, and the process can be performed using the photo-curable fluids for the green and red subpixels. One plurality of micro-LEDs is left without a color conversion layer, e.g., as shown in FIG. 3E. The process shown by FIG. 3F is not performed. For example, the first photo-curable fluid 30a could include green CCAs and the first plurality 14a of micro-LEDs could correspond to the green subpixels, and the second photo-curable fluid 30b could include red CCAs and the second plurality 14b of micro-LEDs could correspond to the red subpixels.

[0061] Assuming that the fluids 30a, 30b, 30c included a solvent, some solvent may be trapped in the color conversion layers 40a, 40b, 40c. Referring to FIG. 3G, this solvent can be evaporated, e.g., by exposing the micro-LED array to heat, such as by IR lamps. Evaporation of the solvent from the color conversion layers 40a, 40b, 40c can result in shrinking of the layers so that the final layers are thinner.

[0062] Removal of the solvent and shrinking of the color conversion layers 40a, 40b, 40c can increase concentration of color conversion agents, e.g., quantum dots, thus providing higher color conversion efficiency. On the other hand, including a solvent permits more flexibility in the chemical formulation of the other components of the photo-curable fluids, e.g., in the color conversion agents or cross-linkable components.

[0063] Optionally, as shown in FIG. 3H, a UV blocking layer 50 can be deposited on top of all of the micro-LEDs 14. The UV blocking layer 50 can block UV light that is not absorbed by the color conversion layers 40. The UV blocking layer 50 can be a Bragg reflector, or can simply be a material that is selectively absorptive to UV light. A Bragg reflector can reflect UV light back toward the micro-LEDs 14, thus increasing energy efficiency.

[0064] FIGS. 5A-5E illustrate a method of fabricating a micro-LED array and isolation walls on a backplane. Referring to FIG. 5A, the process starts with the wafer 100 that will provide the micro-LED array. The wafer 100 includes a substrate 102, e.g., a silicon or a sapphire wafer, on which are disposed a first semiconductor layer 104 having a first doping, an active layer 106, and a second semiconductor layer 108 having a second opposite doping. For example, the first semiconductor layer 104 can be an n-doped gallium nitride (n-GaN) layer, the active layer 106 can be a multiple quantum well (MQW) layer 106, and the second semiconductor layer 107 can be an p-doped gallium nitride (p-GaN) layer 108.

[0065] Referring to FIG. 5B, the wafer 100 is etched to divide the layers 104, 106, 108 into individual micro-LEDs 14, including the first, second and third plurality of micro-LEDs 14a, 14b, 14c that correspond to the first, second and third colors. In addition, conductive contacts 110 can be deposited. For example, a p-contact 110a and an n-contact 110b can be deposited onto the n-GaN layer 104 and p-GaN layer 108, respectively.

[0066] Similarly, the backplane 16 is fabricated to include the circuitry 18, as well as electrical contacts 120. The electrical contacts 120 can include first contacts 120a, e.g., drive contacts, and second contacts 120b, e.g., ground contacts.

[0067] Referring to FIG. 5C, the micro-LED wafer 100 is aligned and placed in contact with the backplane 16. For example, the first contacts 110a can contact the first contacts 120a, and the second contacts 110b can contact the second contacts 120b. The micro-LED wafer 100 could be lowered into contact with the backplane, or vice-versa.

[0068] Next, referring to FIG. 5D, the substrate 102 is removed. For example, a silicon substrate can be removed by polishing away the substrate 102, e.g., by chemical mechanical polishing. As another example, a sapphire substrate can be removed by a laser liftoff process.

[0069] Finally, referring to FIG. 5E, the isolation walls 20 are formed on the backplane 16 (to which the micro-LEDs 14 are already attached). The isolation walls can be formed by a conventional process such as deposition of photoresist, patterning of the photoresist by photolithography, and development to remove the portions of the photoresist corresponding to the recesses 22. The resulting structure can then be used as the display 10 for the processed described for FIGS. 3A-3H.

[0070] FIGS. 6A-6D illustrate another method of fabricating a micro-LED array and isolation walls on a backplane. This process can be similar to the process discussed above for FIGS. 5A-5E, except as noted below.

[0071] Referring to FIG. 6A, the process starts similarly to the process described above, with the wafer 100 that will provide the micro-LED array and the backplane 16.

[0072] Referring to FIG. 6B, the isolation walls 20 are formed on the backplane 16 (to which the micro-LEDs 14 are not yet attached).

[0073] In addition, the wafer 100 is etched to divide the layers 104, 106, 108 into individual micro-LEDs 14, including the first, second and third plurality of micro-LEDs 14a, 14b, 14c. However, the recesses 130 formed by this etching process are sufficiently deep to accommodate the isolation walls 20. For example, the etching can continue so that the recesses 130 extend into the substrate 102.

[0074] Next, as shown in FIG. 6C, the micro-LED wafer 100 is aligned and placed in contact with the backplane 16 (or vice-versa). The isolation walls 20 fit into the recesses 130. In addition, the contacts 110 of the micro-LEDs are electrically connected to the contacts 120 of the backplane 16.

[0075] Finally, referring to FIG. 6D, the substrate 102 is removed. This leaves the micro-LEDs 14 and isolation walls 20 on the backplane 16. The resulting structure can then be used as the display 10 for the processed described for FIGS. 3A-3H.

[0076] Terms of positioning, such as vertical and lateral, have been used. However, it should be understood that such terms refer to relative positioning, not absolute positioning with respect to gravity. For example, laterally is a direction parallel to a substrate surface, whereas vertically is a direction normal to the substrate surface.

[0077] It will be appreciated to those skilled in the art that the preceding examples are exemplary and not limiting. For example: [0078] Although the above description focuses on micro-LEDs, the techniques can be applied to other displays with other types of light emitting diodes, particularly displays with other micro-scale light emitting diodes, e.g., LEDs less than about 10 microns across. [0079] Although the above description assumes that the order in which the color conversion layers are formed is blue, then green, then red, other orders are possible, e.g., blue, then red, then green. In addition, other colors are possible, e.g., orange and yellow.

[0080] It will be understood that various modifications may be made without departing from the spirit and scope of the present disclosure.