Patent Publication Number: US-2020303450-A1

Title: Pixelated led array with optical elements

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/358,085 filed Mar. 19, 2019, which claims benefit of priority to U.S. Provisional Patent Application No. 62/787,006 filed Dec. 31, 2018, each of which is incorporated herein by reference in its entirety. 
    
    
     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 phosphor-converted LED arrays in which the pcLED pixels each comprise an optical element on the light-emitting surface above the phosphor layer, as well as methods for making such arrays. In the disclosed methods, a thin layer of a sacrificial phosphor carrier substrate is retained as the optical element on the output surface of the phosphor pixels upon completion of the fabrication process. 
     This remaining thin layer of the phosphor carrier substrate allows for more tolerance in end point detection during removal of the sacrificial carrier substrate without impacting optical properties of the pcLED such as color point, for example. This can result in higher manufacturing yield. 
     Further, the carrier substrate material and the texture (finish) and geometry of the carrier substrate surface can be used to affect the appearance and output of the pcLED in a desired manner. The carrier substrate material may be, for example, a transparent glass, a transparent crystal, a tinted glass, or a translucent material. The portion of the phosphor carrier substrate retained on the phosphor pixel may be patterned or otherwise configured or selected to function, for example, as a micro or meta-lens, a light extraction element, a dichroic filter, an off-state white diffuser layer, a polarizer, or as any other suitable optical element. 
     The retained portion of the sacrificial carrier substrate may have a thickness of, for example, about 5 microns to about 50 microns perpendicular to the light-emitting surface on the phosphor layer. 
     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. 5A ,  FIG. 5B ,  FIG. 5C ,  FIG. 5D ,  FIG. 5E , and  FIG. 5F  are partial cross-sectional views that schematically show stages in an example method for fabricating a pixelated pcLED array in which the pcLED pixels each comprise an optical element on the light emitting surface above the phosphor pixel, formed from a retained portion of a sacrificial phosphor carrier substrate. 
     
    
    
     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 semiconductor diode structure  102  disposed on a substrate  104 , together considered herein an “LED”, and a phosphor layer or structure  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. 
     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, examples of which are referred to in greater detail below, 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 above, this specification discloses pixelated pcLED arrays in which the pcLED pixels each comprise an optical element on the light emitting surface above the phosphor pixel, formed from a retained portion of a sacrificial phosphor carrier substrate used during the manufacturing process. Much or most of the sacrificial phosphor carrier substrate is removed during the manufacturing process. 
     The partial cross-sectional views of  FIGS. 5A-5F  schematically show stages in an example method for fabricating such a pixelated pcLED array. In  FIG. 5A , a phosphor layer  500  is deposited on a phosphor carrier substrate  505 . Phosphor layer  500  may comprise phosphor particles dispersed in a binder such as a silicone binder, for example. In such cases the binder may be partially or fully cured at this stage. Any suitable phosphor materials 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 of about 20 microns to about 500 microns, for example. 
     Phosphor carrier substrate  505  may be formed, for example, from a transparent glass, a tinted (colored) glass, a crystalline material, a translucent material, a polarizing material, a polarization rotating material, or a material that functions as a dichroic filter. If the resulting array is to be used as a mobile phone flash, having a translucent layer on top of an otherwise vivid-looking phosphor may desired by the user. Optionally, phosphor carrier  505  may be formed from a material permeable to oxygen, allowing oxygen to diffuse into and through the phosphor pixel in the completed pcLED pixels. Suitable oxygen permeable materials may include, for example, porous materials such as porous glasses, perforated glasses, and glass-fiber reinforced plastics. 
     Phosphor carrier substrate  505  may have a thickness of about 50 microns to about 200 microns, typically about 100 microns, for example. Any suitable thickness for phosphor carrier  505  may be used. 
     The surface of phosphor carrier substrate  505  on which the phosphor layer is deposited may be pre-patterned to provide a desired optical effect for the resulting pcLED pixels. For example, the pre-patterning may form a lens (e.g., a Fresnel lens) for each of the resulting pcLED pixels, provide features (e.g., grooves, ridges, protrusions, or other texturing features) that enhance light extraction from the pcLED pixels into air, or scatter light to provide an off-state white diffuser layer for each pcLED pixel. 
     Subsequently, as shown in  FIG. 5B , individual phosphor pixels are defined (singulated) by forming trenches  520  through phosphor layer  500  and into, but not entirely through, phosphor carrier substrate  505  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 (e.g., square) array, for example, by intersecting trenches formed in phosphor layer  500 . 
     Trenches  520  may have a width of, for example, about 5 microns to about 200 microns. Trenches  520  may penetrate phosphor carrier substrate  505  to a depth of, for example, about one quarter or more of the thickness of the substrate but must be shallower than the thickness of the substrate to keep the pixels unseparated. Trenches  520  may be spaced apart from each other by, for example, about 5 microns to about 200 microns. 
     Subsequently, as shown in  FIG. 5C , each phosphor pixel  515  in the array of phosphor pixels is 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. Attachment of the phosphor pixels to the LEDs may be by any suitable method. If the phosphor pixels comprise phosphor particles dispersed in a binder, they may for example be attached to the individual LEDs by curing or further curing the binder to form bonds to the LEDs. Alternatively, or in addition, a separate adhesive layer may be used to attach the phosphor pixels to the LEDs. 
     Subsequently, as shown in  FIG. 5D , 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. Each individual phosphor pixels retains a portion  537  of phosphor carrier  505  on its upper light output surface. For example, a thickness of about 50 microns to about 200 microns of phosphor carrier  505  may be removed at this step. Retained portion  537  of phosphor carrier  505  may have a thickness of, for example, about 5 microns to about 50 microns. 
     Subsequently, as shown in  FIG. 5E , 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 2  particles embedded in silicone, one or more reflective metal layers, or one or more 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. 
     Subsequently, as shown in  FIG. 5F , 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. Optionally, a further portion of phosphor carrier  505  (for example, about 1 micron to about 45 microns) is removed along with the excess portion of material or structure  540 . As noted above, a portion  545  of phosphor carrier  505  is retained on the upper light output surface of each phosphor pixel. Retained portion  545  may have a thickness of, for example, about 5 microns to about 10 microns, or any other suitable thickness. 
     Remaining portions of reflective or scattering material  540  form side walls  550  on the phosphor pixels and LEDs, optically isolating individual pcLED pixels from each other. 
     Depending on the intended application for the pcLEDs and their dimensions, the resulting pixelated array shown in  FIG. 5F  may be maintained essentially intact, transferred as an array to another substrate, or divided into separate pcLEDs. 
     Retaining phosphor carrier portions  545  on the top output surface of the phosphor pixels relaxes manufacturing tolerances compared to a process in which all of sacrificial phosphor carrier  505  is removed to expose the top of the phosphor pixels. This is because variations in the thickness of retained phosphor carrier portions  545  typically have little effect on the performance of the pcLED pixels. In contrast, variations in the thickness of the phosphor pixels may significantly affect the output of the pcLED pixels, because as explained above the amount of light emitted by the LEDs that is absorbed and converted to longer wavelength light by the phosphor pixels depends on the path length of the LED light through the phosphor pixels and thus on the height of the phosphor pixels. Consequently, if all of the sacrificial phosphor carrier  105  is removed, the end point of the removal process must be controlled with sufficient precision so as not to affect the phosphor pixels. That may be challenging. 
     Further, in a process in which all of sacrificial phosphor carrier  505  is removed, the finish surface of the phosphor pixels cannot be easily controlled to achieve a desired effect such as, for example, affecting light extraction or output radiation pattern. 
     As explained above, retaining phosphor carrier portion  545  on the top output surface of the phosphor pixel allows incorporating a primary optic into the pcLED pixel during the manufacturing process for the pcLED. This may be advantageous compared to a process in which such a primary optic is separately prepared and subsequently attached to a pcLED pixel. For example, subsequently attaching a primary optic to a pcLED pixel may require additional process steps, may require use of an adhesive or glue that can spill onto other portions of the pcLED pixel, and would require aligning the primary optic with the pcLED pixel. Using retained phosphor carrier portion  545  as a primary optic avoids these difficulties. 
     Depending on the viscosity of the phosphor material, the retained phosphor carrier portion  545  (the optical element) may retain air structures between the optical element and the film and thereby enhance refractive index contrast and maximize optical performance. 
     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.