Patent Publication Number: US-2021184085-A1

Title: Sidewall scattering for radiation pattern control of leds

Description:
FIELD OF THE INVENTION 
     The invention relates generally to phosphor-converted light emitting diodes. 
     BACKGROUND 
     Semiconductor light emitting diodes and laser diodes (collectively referred to herein as “LEDs”) are among the most efficient light sources currently available. The emission spectrum of an LED typically exhibits a single narrow peak at a wavelength determined by the structure of the device and by the composition of the semiconductor materials from which it is constructed. By suitable choice of device structure and material system, LEDs may be designed to operate at ultraviolet, visible, or infrared wavelengths. 
     LEDs may be combined with one or more wavelength converting materials (generally referred to herein as “phosphors”) that absorb light emitted by the LED and in response emit light of a longer wavelength. For such phosphor-converted LEDs (“pcLEDs”), the fraction of the light emitted by the LED that is absorbed by the phosphors depends on the amount of phosphor material in the optical path of the light emitted by the LED, for example on the concentration of phosphor material in a phosphor layer disposed on or around the LED and the thickness of the layer. 
     Phosphor-converted LEDs may be designed so that all of the light emitted by the LED is absorbed by one or more phosphors, in which case the emission from the pcLED is entirely from the phosphors. In such cases the phosphor may be selected, for example, to emit light in a narrow spectral region that is not efficiently generated directly by an LED. 
     Alternatively, pcLEDs may be designed so that only a portion of the light emitted by the LED is absorbed by the phosphors, in which case the emission from the pcLED is a mixture of light emitted by the LED and light emitted by the phosphors. By suitable choice of LED, phosphors, and phosphor composition, such a pcLED may be designed to emit, for example, white light having a desired color temperature and desired color-rendering properties. 
     SUMMARY 
     This specification discloses LEDs and pcLEDs comprising a top light emitting surface, an oppositely positioned bottom surface, and side walls connecting the top and bottom surfaces. Sidewall reflector structures disposed on the sidewalls comprise a thin specular reflection layer and a light scattering layer disposed between the sidewall and the specular reflection layer. 
     These sidewall reflector structures are more diffusively reflective than a specular reflector, yet maintain high reflectivity. LEDs and pcLEDs comprising such sidewall reflector structures can exhibit a narrower and more Lambertian radiation output pattern from the top (light emitting surface) than devices employing only specularly reflective sidewall reflectors. In addition, such sidewall reflector structures may be thinner than conventional volume scattering sidewall reflectors, allowing LEDs and pcLEDs employing the sidewall reflector structures disclosed herein to be placed closer to each other than would be the case if volume scattering sidewall reflectors were used. 
     These LEDs and pcLEDs may be advantageously employed in arrays of closely spaced devices, for example in automobile headlights. 
     Other embodiments, features and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic cross-sectional view of an example pcLED. 
         FIGS. 2A and 2B  show, respectively, cross-sectional and top schematic views of an array of pcLEDs. 
         FIG. 3A  shows a schematic top view of 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 . 
         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. 
         FIG. 5  shows a schematic cross-sectional view of an example pcLED employing volume scattering sidewall reflectors. 
         FIG. 6  shows a schematic cross-sectional view of an example pcLED employing thin film side coat reflectors. 
         FIG. 7A  shows a schematic cross-sectional view of an example pcLED employing sidewall reflectors that comprise a specularly reflective layer and a scattering layer disposed between the specularly reflective layer and the sidewalls of the device. 
         FIG. 7B  shows another schematic cross-sectional view of an example pcLED employing sidewall reflectors that comprise a specularly reflective layer and a scattering layer disposed between the specularly reflective layer and the sidewalls of the device. 
         FIGS. 8A, 8B, and 8C  schematically show steps in an example process for fabricating sidewall reflectors as in  FIG. 7A  and  FIG. 7B . 
         FIGS. 9A and 9B  schematically show steps in another example process for fabricating sidewall reflectors as in  FIG. 7A  and  FIG. 7B . 
         FIGS. 10A, 10B, 10C, and 10D  schematically show steps in another example process for fabricating sidewall reflectors as in  FIG. 7A  and  FIG. 7B . 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  shows an example of an individual pcLED  100  comprising a light emitting semiconductor diode structure  102  disposed on a substrate  104 , together considered herein an “LED”, and a phosphor layer  106  disposed on the LED. Light emitting 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. 
     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. 
     Any suitable phosphor materials may be used, depending on the desired optical output from the pcLED. 
       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. 
     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. 
     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 . Projection lens  404  may be a Fresnel lens, for example. 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. 
     As summarized in the summary section above, this specification discloses thin sidewall reflector structures for LEDs and pcLEDs that can provide an improved radiation output pattern compared to devices employing only specularly reflective sidewall reflectors, yet maintain high reflectivity with a relatively thin reflector structure. 
       FIG. 5  shows a schematic cross-sectional view of a pcLED  500  comprising a light emitting semiconductor diode  502  disposed on a transparent substrate  503 , and a wavelength converting structure  506  disposed on the transparent substrate opposite from the light emitting semiconductor diode. Upon application of a suitable voltage across contacts  505 A and  505 B semiconductor diode  502  emits light, which may be transmitted through the transparent substrate to the wavelength converting structure. The wavelength converting structure absorbs some or all of the light from the semiconductor diode and in response emits longer wavelength light. Light output through top surface  508  of pcLED  500  may comprise any suitable mixture of light emitted by the light emitting semiconductor diode and light emitted by the wavelength converting structure. 
     Light emitted by the semiconductor diode or by the wavelength converting structure that is incident on a side wall  510  of the wavelength converting structure (e.g., light ray  514 ) or on a sidewall  512  of the semiconductor diode and transparent substrate is generally diffusively reflected by volume scattering sidewall reflectors  516  back into and/or out of the device (e.g., light rays  518 ), so that essentially all light output by pcLED  500  is output through top surface  508 . 
     Light emitting semiconductor diode  502  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. 
     Transparent substrate  503  is transparent to light emitted by semiconductor diode  502  and to light emitted by wavelength converting structure  506 . Transparent substrate  503  may be a sapphire substrate, for example. 
     Wavelength converting structure  506  may comprise any suitable phosphor material, and may for example be a ceramic phosphor plate or other ceramic phosphor structure formed by sintering phosphor particles. Such a ceramic phosphor plate or structure may exhibit little or no scattering of light emitted by the semiconductor diode or emitted by the wavelength converter. 
     Volume scattering reflectors  516  may comprise Titanium Oxide particles dispersed in a silicone binder, for example. Volume scattering reflectors  516  may have a thickness of about 30 microns to about 300 microns, for example, measured perpendicularly to the sidewalls. 
     An advantage of using volume scattering sidewall reflectors as in device  500  is that they are diffusively reflected as illustrated in  FIG. 5  and discussed above. As a consequence, the output radiation pattern from top surface  508  is typically Lambertian, which is desirable. A disadvantage to using volume scattering sidewall reflectors is that the thickness of these reflectors required to provide sufficiently high reflection/scattering makes the lateral dimensions of the device significantly larger than those of the light emitting semiconductor diode. This increases the minimum spacing between such devices. 
       FIG. 6  shows a schematic cross-sectional view of a pcLED  600  that differs from pcLED  500  ( FIG. 6 ) by substituting thin film side coat reflectors  616  for volume scattering reflectors  516 . The thin film side coat reflectors  616  may be a reflective metal coating or a Distributed Bragg Reflector (DBR), for example. This film side coat reflectors  616  may have a thickness of about 0.1 microns to about 10 microns, for example. 
     An advantage to using thin film side coat reflectors  616  is that they may be sufficiently thin that the lateral dimensions of device  600  are not significantly larger than those of the light emitting semiconductor diode. This allows for closer spacing of such devices than is possible for devices employing volume scattering sidewall reflectors. 
     A disadvantage is that such thin film side coat reflectors are typically specularly reflective rather than diffusively reflective—they reflect light rays at an angle of reflection equal to the angle of incidence (e.g., ray  614 ). As a consequence, the output radiation pattern from top surface  608  of the device is typically wider than the preferred Lambertian distribution, especially if the wavelength converting structure (for example, a ceramic phosphor structure) does not strongly scatter light. The wider output radiation pattern is commonly disadvantageous when the device is combined with external optics causing low coupling efficiency, meaning increasing unused light. One way of improving the radiation pattern is to increase volume scattering in the wavelength converting structure, but this method typically increases scattering loss in the wavelength converter. 
     A further disadvantage of using such thin film side coat reflectors is that for light generated in a low-scattering wavelength converter such as a ceramic phosphor platelet, the light can become trapped circulating in an optical cavity due to the specularly reflective nature of the sidewall reflectors. 
     A difficulty with using thin film side coat reflectors is that typically they must be formed on a smooth supporting surface. This is particularly true for Distributed Bragg Reflectors, which must be formed on a sufficiently smooth supporting surface to minimize transmission loss. 
       FIG. 7A  shows a schematic cross-sectional view of a pcLED  700 A in which sidewall reflectors  716  comprise a diffusive or diffractive scattering layer  716 A positioned between the sidewall of the wavelength converter and a specularly reflective thin film side coat reflector  716 B. Reflective layer  716 B is disposed on a smooth outer surface of diffusive or diffractive scattering layer  716 A. The smooth outer surface of layer  716 A may have for example a surface roughness of Ra≤1 micron, ≤750 nm, or ≤500 nm. Scattering layer  716 A may have a thickness of about 1 micron to about 30 microns, for example. Reflector layer  716 B may have a thickness of about 0.1 microns to about 10 microns, for example. 
     The combination of scattering layer  716 A and specularly reflective layer  716 B maintains high reflectivity while providing a more diffusive reflector, allowing for light to be directed in a random direction or preferentially forward (towards the light output surface) direction. This improves the radiation output pattern through top surface  708  of the device, making the radiation pattern more Lambertian, and also helps prevent optical cavity trapping of light in the wavelength converter. 
     Specularly reflective layer  716 B may be or comprise a reflective metal layer or a Distributed Bragg Reflector, for example. Scattering layer  716 A may be or comprise, for example, a porous columnar structure that provides optically diffusive characteristics in the plane perpendicular to the columns, while maintaining a physically smooth outer layer. Alternatively, scattering layer  716 A may be or comprise, for example, Titanium Oxide particles dispersed in a silicone binder. 
     As shown in  FIG. 7B , a schematic cross-sectional view of a pcLED  700 B, sidewall reflectors  716  may extend to cover side walls of the semiconductor LED and the transparent substrate. 
     Referring now to  FIGS. 8A-8C , in some variations light scattering layers  716 A may be formed on sidewalls of a wavelength converting structure  506  (e.g., a ceramic phosphor plate) by depositing and then anodizing an aluminum layer. As shown in  FIG. 8A , a layer of aluminum  820  is formed on a sidewall of the wavelength converting structure, for example by RF sputtering. The aluminum layer may be formed on a transparent conductive layer (Indium Tin Oxide, for example) that is first deposited on the sidewall, to facilitate the following anodization step. As shown in  FIG. 8B , layer  820  may then be anodized to form a porous alumina light scattering layer  716 A comprising columnar structures with long axes oriented away from the sidewall. The physical dimensions of the columnar structures may be slightly smaller than the wavelengths of light emitted by the semiconductor diode and the wavelength converting structure, for example on the order of hundreds of nanometers. The columnar structures may have diameters of 100 nm to 200 nm, for example. Optionally, sputtered SiO 2  can be applied to the porous structures in layer  716 A to help seal up the pores and smooth the outer surface of the layer. Fabrication examples of such porously anodized layers possessing columnar structure have been published in Surface and Coatings Technology 169-170 (2003) 190-194, though not for the use and purpose described here. The outer surface of layer  716 A is smooth, as described above. As shown in  FIG. 8C , specularly reflective layer  716 B is then formed on light scattering layer  716 A by conventional methods, for example. 
     Referring now to  FIGS. 9A-9B , in other variations a diffusive light scattering layer  716 A is applied to a wavelength converting structure by spray coating or dip coating, optionally followed by a surface smoothening solution based coating. The diffusive light scattering layer may comprise Titanium Oxide particles in a silicone binder, for example. Any suitable light scattering composition may be used. The surface smoothening solution based coating may be implemented, for example, as a conventional sol-gel technique. As shown in  FIG. 9A , the light scattering layer may initially be deposited on the sidewalls and top surface of the wavelength converting structure, and subsequently removed from the top surface by conventional methods. The outer surface of layer  716 A is smooth, as described above. As shown in  FIG. 9B , specularly reflective layer  716 B is then formed on light scattering layer  716 A by conventional methods, for example. 
     In other variations a diffusive light scattering layer  716 A is applied to a wavelength converting structure by vapor phase deposition, for example pulsed laser deposition, of a dielectric layer comprising light-scattering (e.g., air-filled) voids. The outer surface of layer  716 A is made smooth, as described above. Similarly to as shown in  FIG. 9B , specularly reflective layer  716 B is then formed on light scattering layer  716 A by conventional methods, for example. 
       FIGS. 10A-10D  illustrate another variation of methods by which smooth-walled diffusive light scattering layers  716 A can be fabricated on a ceramic phosphor plate wavelength converting structure. A ceramic phosphor wafer  1000  on a temporary support  1010  as shown in  FIG. 10A  is sawn into ceramic phosphor platelet wavelength converting structures  506  with, for example, a saw producing wide streets (saw cuts)  1015  as shown in  FIG. 10B . The streets may be  100  microns wide, for example, but any suitable street width may be used. As shown in  FIG. 10C , the streets are then filled with a light scattering composition  1020 , for example phosphor particles and/or Titanium Oxide particles in a silicone binder. This may be done by overmolding with the light scattering mixture, for example. After curing, the filled streets are then re-sawn with a narrower saw to form narrower streets  1025 , leaving light scattering layers  716 A on sidewalls of the ceramic phosphor platelet wavelength converters as shown in  FIG. 10D . 
     Saw cuts through a ceramic phosphor plate typically leave rough edges, but the saw cuts through the filled streets that define layers  716 A leave sufficiently smooth outer surfaces on these layers to support deposition of specular reflective layers  716 B, which may be subsequently deposited. 
     Sidewall reflectors  716  may be formed on a wavelength converting structure  506 , and then the wavelength converting structure subsequently attached to a light emitting semiconductor diode or to a transparent substrate on a light emitting semiconductor diode. 
     Alternatively, sidewall reflectors  716  may be formed on a wavelength converting structure after the wavelength converting structure is attached to a light emitting semiconductor diode or to a transparent substrate on a light emitting semiconductor diode. In that case the sidewall reflectors  716  may be formed to extend to cover sidewalls of the transparent substrate and the light emitting semiconductor diode, as shown for example in  FIG. 7B . 
     Sidewall reflectors  716  may also be formed on sidewalls of a light emitting semiconductor diode and/or side walls of a transparent substrate attached to the light emitting semiconductor diode with no wavelength converting structure attached to the device. 
     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.