Ultra-smooth sidewall pixelated array LEDs

Abstract

Pixelated array light emitters are formed with closely-spaced pixels having ultra-smooth sidewalk. In methods for making such pixelated array light emitters, a converter layer of phosphor particles dispersed in a binder is disposed on a carrier, and then singulated by saw cuts or similar methods to form an array of phosphor pixels. The binder is fully cured prior to singulation of the converter layer. Further, the carrier is rigid rather than flexible. As a consequence of fully curing the binder and of using a rigid carrier to support the converter layer, singulation results in phosphor pixels having smooth side walls. The array of phosphor pixels is subsequently attached to a corresponding array of LEDs with an adhesive layer, separate from the binder used to form the converter layer. The pixel sidewalls may be formed with controlled morphology, for example at acute or obtuse angles with respect to the carrier.

Claims

1. A method of fabricating a wavelength converter structure, the method comprising: depositing on a first surface of a carrier substrate a converter layer comprising phosphor particles dispersed in a binder; fully curing the binder after depositing the converter layer on the first surface of the carrier substrate; depositing an adhesive layer on the converter layer after fully curing the binder; and after depositing the adhesive layer, forming trenches entirely through the adhesive layer and the converter layer by removing portions of the adhesive layer and the converter layer to define a plurality of spaced-apart phosphor pixels each comprising a remaining portion of the adhesive layer.

2. The method of claim 1, wherein forming trenches comprises forming trenches partially through the carrier substrate.

3. The method of claim 1, wherein the binder is or comprises a silicone and fully curing the binder comprises thermally curing the silicone.

4. The method of claim 1, wherein the adhesive is or comprises a silicone.

5. The method of claim 1, wherein the trenches define phosphor pixel side walls having a roughness parameter Ra of less than or equal to about 100 nanometers.

6. The method of claim 1, wherein the trenches define phosphor pixel side walls that are oriented at acute or obtuse angels with respect to the first surface of the carrier substrate.

7. The method of claim 1, wherein forming the trenches comprises cutting through the adhesive and converter layers with a saw.

8. The method of claim 7, wherein the saw comprises a tapered blade that cuts the trenches to define phosphor pixel side walls that are oriented at acute or obtuse angels with respect to the first surface of the carrier substrate.

9. The method of claim 1, comprising: removing a sufficient portion of the carrier substrate from a second surface of the carrier substrate, opposite from the first surface, to reach and open the trenches and thereby expose phosphor pixel side walls defined by the trenches; and coating the exposed phosphor pixel side walls with a reflective or scattering material.

10. The method of claim 9, wherein the reflective or scattering material comprises DBR structures.

11. The method of claim 1, wherein; forming the trenches comprises cutting through the adhesive and converter layers with a saw; the saw comprises a tapered blade that cuts the trenches to define phosphor pixel side walls that are oriented at acute or obtuse angels with respect to the first surface of the carrier substrate; and the phosphor pixel side walls have a roughness parameter Ra of less than or equal to about 100 nanometers.

12. The method of claim 1 wherein the phosphor pixels are rectangular.

13. A method of fabricating an array of phosphor converted LEDs, the method comprising the method of claim 1, and using the remaining portion of adhesive layer on each phosphor pixel to attach a corresponding LED to each of the phosphor pixels.

14. The method of claim 1, comprising: removing all of the carrier substrate after removing forming trenches entirely through the adhesive layer and the converter layer.

15. The method of claim 1, wherein the carrier substrate is rigid.

16. The method of claim 1, wherein the carrier substrate is formed from a glass.

17. The method of claim 1, wherein the converter layer has a thickness of about 50 microns to about 200 microns.

18. The method of claim 1, wherein forming trenches comprises forming the trenches to have a width of about 5 microns to about 200 microns.

19. The method of claim 1, wherein the adhesive layer has a thickness of about 0.1 microns to about 5 microns.

20. The method of claim 9, wherein the reflective or scattering material comprises TiO.sub.2 particles embedded in silicone.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic cross-sectional view of an example pcLED.

(2) FIGS. 2A and 2B show, respectively, cross-sectional and top schematic views of an array of pcLEDs.

(3) FIG. 3A shows a schematic top view an electronics board on which an array of pcLEDs may be mounted, and FIG. 3B similarly shows an array of pcLEDs mounted on the electronic board of FIG. 3A.

(4) FIG. 4A shows a schematic cross sectional view of an array of pcLEDs arranged with respect to waveguides and a projection lens. FIG. 4B shows an arrangement similar to that of FIG. 4A, without the waveguides.

(5) FIG. 5A, FIG. 5B, FIG. 5 FIG. 5D, FIG. 5E, FIG. 5F, and FIG. 5G are partial cross-sectional views that schematically show stages in an example method for fabricating a pixelated array of light emitters with closely-spaced pixels and ultra-smooth sidewalls.

DETAILED DESCRIPTION

(6) The following detailed description should be read with reference to the drawings, in which identical reference numbers refer to like elements throughout the different figures. The drawings, which are not necessarily to scale, depict selective embodiments and are not intended to limit the scope of the invention. The detailed description illustrates by way of example, not by way of limitation, the principles of the invention.

(7) FIG. 1 shows an example of an individual pcLED 100 comprising a semiconductor diode structure 102 disposed on a substrate 104, together considered herein an “LED”, and a phosphor layer 106 disposed on the LED. Semiconductor diode structure 102 typically comprises an active region disposed between n-type and p-type layers. Application of a suitable forward bias across the diode structure results in emission of light from the active region. The wavelength of the emitted light is determined by the composition and structure of the active region.

(8) The LED may be, for example, a III-Nitride LED that emits blue, violet, or ultraviolet light. LEDs formed from any other suitable material system and that emit any other suitable wavelength of light may also be used. Other suitable material systems may include, for example, III-Phosphide materials, III-Arsenide materials, and II-VI materials.

(9) Any suitable phosphor materials may be used, depending on the desired optical output from the pcLED.

(10) FIGS. 2A-2B show, respectively, cross-sectional and top views of an array 200 of pcLEDs 100 including phosphor pixels 106 disposed on a substrate 202. Such an array may include any suitable number of pcLEDs arranged in any suitable manner. In the illustrated example the array is depicted as formed monolithically on a shared substrate, but alternatively an array of pcLEDs may be formed from separate individual pcLEDs. Substrate 202 may optionally comprise CMOS circuitry for driving the LED, and may be formed from any suitable materials.

(11) As shown in FIGS. 3A-3B, a pcLED array 200 may be mounted on an electronics board 300 comprising a power and control module 302, a sensor module 304, and an LED attach region 306. Power and control module 302 may receive power and control signals from external sources and signals from sensor module 304, based on which power and control module 302 controls operation of the LEDs. Sensor module 304 may receive signals from any suitable sensors, for example from temperature or light sensors. Alternatively, pcLED array 200 may be mounted on a separate board (not shown) from the power and control module and the sensor module.

(12) Individual pcLEDs may optionally incorporate or be arranged in combination with a lens or other optical element located adjacent to or disposed on the phosphor layer. Such an optical element, not shown in the figures, may be referred to as a “primary optical element”. In addition, as shown in FIGS. 4A-4B a pcLED array 200 (for example, mounted on an electronics board 300) may be arranged in combination with secondary optical elements such as waveguides, lenses, or both for use in an intended application. In FIG. 4A, light emitted by pcLEDs 100 is collected by waveguides 402 and directed to projection lens 404. This arrangement may be suitable for use, for example, in automobile headlights. In FIG. 4B, light emitted by pcLEDs 100 is collected directly by projection lens 404 without use of intervening waveguides. This arrangement may particularly be suitable when pcLEDs can be spaced sufficiently close to each other, and may also be used in automobile headlights as well as in camera flash applications. A microLED display application may use similar optical arrangements to those depicted in FIGS. 4A-4B, for example. Generally, any suitable arrangement of optical elements may be used in combination with the pcLEDs described herein, depending on the desired application.

(13) Converter layers comprising phosphor particles dispersed in a silicone binder may be laminated onto a semiconductor LED surface to make a phosphor-converted LED emitting white light, or light of a desired color. The nature of the silicone binder allows for adhesion to the semiconductor surface during curing of the binder at the same time the silicone binder hardens during cross-linking. However, curing the binder results in shrinkage of the total volume (˜1% to ˜3%) of the converter layers. For large LEDs the shrinkage is inconsequential. However, for closely spaced pixel arrays, for example in microLEDs, this shrinkage can lead to uncontrolled separation between pixels and poor sidewall structure such as rough topography or slanted or slumped walls.

(14) As summarized above, the methods disclosed in this specification separate the step of cross-linking the silicone or other binder to form a solid binder for the phosphor particles from the step of attaching the phosphor pixels to the semiconductor LED surfaces, and also support the converter layer on a rigid carrier during singulation of the converter layer to form phosphor pixels. This results in much improved sidewall surface quality and angle. Further, the pixels may be formed with a tapered saw blade to have angled side walls that do not change from the sawn shape after the converter layer is attached. An array of phosphor pixels formed in this manner and supported by a carrier may be attached to a corresponding array of LEDs. That is, the LEDs may be attached by parallel placement in a single step rather than by pick-and-place of individual LEDs or phosphor pixels. Pixels in the resulting pcLED pixelated array may have a spacing of, for example, about 5 microns to about 200 microns, for example less than or equal to about 20 microns, with good uniformity in pixel spacing and side wall angles.

(15) The partial cross-sectional views of FIGS. 5A-5G schematically show stages in an example method for making a pixelated array of light emitters with closely-spaced pixels and ultra-smooth sidewalls. In FIG. 5A, a phosphor layer 500 is deposited on a phosphor carrier substrate 505. Phosphor layer 500 comprises phosphor particles dispersed in a binder such as a silicone binder, for example. Any suitable phosphor material may be used. Suitable phosphor materials may include, for example, YAG, LuAG, silicates, BOSE, β-SiAlON, SCASN, BSSN, KSiF:Mn, SLA, and quantum dots. Phosphor layer 500 may have a thickness perpendicular to carrier substrate 505 of, for example, about 20 microns to about 500 microns.

(16) Conventionally a phosphor layer 500 may be deposited on a flexible support, such as a tape for example. In such cases, during a singulation step as described below the flexible support may flex, stretch, or otherwise deform sufficiently to degrade the shape and the smoothness of the phosphor pixel sidewalls formed by singulation of the phosphor layer.

(17) In contrast, in the methods disclosed in this specification phosphor carrier 505 is rigid. By rigid, this specification means that carrier 505 does not flex, stretch, or otherwise deform significantly during the singulation step. Phosphor carrier 505 may be formed, for example, from a glass or from any other suitable rigid material. Phosphor carrier 505 may be formed from a borosilicate glass, for example. Phosphor carrier 505 may have a thickness of, for example, about 50 microns to about 200 microns, for example of about 100 microns. Any suitable thickness for phosphor carrier 505 may be used.

(18) After phosphor layer 500 is deposited on phosphor carrier 505, the binder in phosphor layer 500 is fully cured by, for example, thermal curing. All or essentially all shrinkage of phosphor layer 500 resulting from curing occurs at this step.

(19) Subsequently, as shown in FIG. 5B, an adhesive layer 510 is deposited on a top surface of phosphor layer 500 to be attached to LEDs. Adhesive layer 510 may be deposited, for example, by spin-coating or thin film coating. Adhesive layer 510 may be, for example, a silicone glue layer. Such a silicone glue may be identical to a silicone material used as binder in phosphor layer 500. However, adhesive layer 510 is used specifically to attach phosphor layer 500 to LEDs.

(20) Adhesive layer 510 may have a thickness of, for example, about 100 nanometers to about 5 microns. Preferably, the adhesive layer thickness is about the minimum necessary to enable attachment of phosphor pixels formed form the phosphor layer to an array of pixels on the semiconductor device. Adhesive layer 510 may be subjected to heat or UV light (B-staging) to remove the majority of solvent to form a dry film without fully curing the adhesive layer.

(21) Subsequently, as shown in FIG. 5C, individual phosphor pixels are defined (singulated) by forming trenches 520 through adhesive layer 510 and phosphor layer 500 to form an array of phosphor pixels 515. This may be done by sawing, for example, but any suitable singulation method may be used. The phosphor pixels may be formed in a rectangular array, for example, by intersecting trenches formed in phosphor layer 500. Optionally, as shown, trenches 520 may penetrate into but not entirely through phosphor carrier substrate 505.

(22) Because phosphor layer 500 has been fully cured and because phosphor carrier 505 is rigid, rather than a flexible tape for example, the singulation step defines the final shape of the sidewalls of the phosphor pixels and the sidewalls may be very smooth. The sidewalls of the phosphor pixels may have a roughness parameter Ra of, for example, less than or equal to about 10 nanometers, less than or equal to about 100 nanometers, or less than or equal to about 200 nanometers.

(23) Sawing a fully cured phosphor layer on a rigid support also helps keep the saw blade clean.

(24) Furthermore, the angle of the saw blade can be used to control the angle of the phosphor pixel sidewalls relative to the top light-output surface of the phosphor pixels. For example, a tapered saw blade may be used to form angled phosphor pixel sidewalls, as shown in FIG. 5C for example. Such angled phosphor pixel sidewalls may advantageously help to extract or direct light out of the phosphor pixels.

(25) Trenches 520 may have a width of for example, about 5 microns to about 200 microns. Trenches having tapered sides (FIG. 5C) may have a narrowest width of for example, about 5 microns to about 200 microns, and a greatest width of, for example, about 5 microns to about 200 microns. Trenches 520 may be spaced apart from each other by, for example, about 5 microns to about 200 microns.

(26) Subsequently, as shown in FIG. 5D, each phosphor pixel 515 in the array of phosphor pixels is aligned with and attached to a corresponding LED 525 in an array of semiconductor LEDs. LEDs 525 may be supported by a substrate 530, for example, which may be formed from any suitable material. The LEDs are attached to the phosphor pixels by adhesive layer 510, which is fully cured in the attachment step to bond the LEDs and the phosphor pixels to each other.

(27) Subsequently, as shown in FIG. 5E, a sufficient thickness of phosphor carrier 505 is removed so that phosphor pixels 515 are no longer interconnected by the phosphor carrier and side walls 535 of the phosphor pixels and of the LEDs are exposed for side coating, described next. Optionally, each individual phosphor pixels retains a portion 537 of carrier 505 on its upper light output surface as shown in FIG. 5E. Alternatively, all of phosphor carrier 505 is removed at this step.

(28) Subsequently, as shown schematically in FIG. 5F, a reflective or scattering material 540 is deposited on the top and sides of the phosphor pixels and the sides of the LEDs. Reflective or scattering material 540 may be or comprise, for example, a light scattering material such as TiO.sub.2 particles embedded in silicone, one or more reflective metal layers, or one or more Distributed Bragg Reflector (DBR) structures formed from a stack of alternating layers of high and low refractive index material. Reflective metal layers may be deposited by vapor deposition or sputtering, for example. DBR structures may be deposited by atomic layer deposition, for example.

(29) Subsequently, as shown in FIG. 5G, excess reflective or scattering material 540 is removed from the upper light output surfaces of the phosphor pixels by any suitable method, for example by mechanical grinding or polishing. If portion 537 of carrier 505 had been retained on the top surface of the phosphor pixel it can be entirely removed at this step. Alternatively, a portion of carrier 505 may be retained on the top light-output surface of each phosphor pixel through this step as well. Remaining portions of reflective or scattering material 540 form side walls 550 on the phosphor pixels, the glue layer, and optionally on the LEDs, optically isolating individual pcLED pixels from each other.

(30) Advantageously, the smooth phosphor pixel sidewalls formed by the methods disclosed in this specification facilitate formation of highly reflective DBR structures.

(31) Depending on the intended application for the pcLEDs and their dimensions, the resulting pixelated array shown in FIG. 5G may be maintained essentially intact, transferred as an array to another substrate, or divided into separate pcLEDs.

(32) The improvement in phosphor pixel sidewall smoothness resulting from the methods disclosed in this specification was demonstrated by measurements made on two comparative examples. In one example, the sidewalk of phosphor pixels prepared by sawing a partially cured phosphor layer supported on a flexible tape were measured to have a roughness parameter Ra of about 300 nanometers. In contrast, in an otherwise similar example in which the phosphor layer was fully cured and supported on a rigid carrier the sidewalls of the pixels were measured to have a roughness parameter Ra of about 100 nanometers.

(33) This disclosure is illustrative and not limiting. Further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.