Patent Publication Number: US-10790417-B2

Title: Wavelength converted semiconductor light emitting device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Non-Provisional application Ser. No. 15/679,897 filed Aug. 17, 2017, which is a divisional of U.S. Non-Provisional application Ser. No. 14/903,727 filed Jan. 8, 2016, which is the U.S. National Stage, under 35 U.S.C. § 371, of International Application No. PCT/IB2014/062813 filed Jul. 3, 2014 and which claims the benefit of U.S. Provisional Patent Application No. 61/843,466 filed on Jul. 8, 2013, all of which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a wavelength converted semiconductor light emitting device. 
     BACKGROUND 
     Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions. 
       FIG. 1  illustrates a lighting apparatus described in more detail in US patent application 2011/0227477. The device of  FIG. 1  includes a submount  100  with a light source (e.g., an LED) mounted thereon. Paragraph 54 of US patent application 2011/0227477 teaches “First emissive layer  110  is disposed above the second emissive layer  115  and receives at least a portion of the radiation emitted from the light source  105 . Second emissive layer  115  is disposed between the conventional base LED  105  and the first emissive layer  110 . The second emissive layer  115  receives at least a portion of the radiation emitted from the light source  105 . An optional encapsulant resin  120  is placed over the light source  105 , the first emissive layer  110  and the second emissive layer  115 . In some embodiments, the first emissive layer  110  and the second emissive layer  115  are fixed together to form a composite.” 
     Paragraph 60 of US patent application 2011/0227477 teaches “The lighting apparatus can include a first emissive layer having a first garnet phosphor and a second emissive layer having a second garnet phosphor . . . . The emissive layers may, in some embodiments, be ceramic plates . . . . The ceramic plates may be fixed together to form a composite.” 
     SUMMARY 
     It is an object of the invention to provide a wavelength converted semiconductor light emitting device suitable for applications that may require high drive current and/or high operating temperatures. 
     In embodiments of the invention, a light emitting device includes a semiconductor structure including a light emitting layer disposed between an n-type region and a p-type region. A first wavelength converting layer is disposed in a path of light emitted by the light emitting layer. The first wavelength converting layer may be a wavelength converting ceramic. A second wavelength converting layer is fused to the first wavelength converting layer. The second wavelength converting layer may be a wavelength converting material disposed in glass. 
     A method according to embodiments of the invention includes forming a wavelength converting element. Forming a wavelength converting element includes forming a first wavelength converting layer, which may be a wavelength converting ceramic, and fusing a second wavelength converting layer to the first wavelength converting layer. The wavelength converting element is diced into a plurality of platelets. After dicing, one or more platelets are attached to a single semiconductor light emitting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a lighting apparatus including an LED and two ceramic phosphor plates. 
         FIG. 2  illustrates a semiconductor light emitting device. 
         FIG. 3  is a cross sectional view of a portion of a wavelength converting wafer. 
         FIG. 4  illustrates the structure of  FIG. 3  diced into individual platelets. 
         FIG. 5  illustrates the structure of  FIG. 3  diced into individual platelets with shaped sides. 
         FIG. 6  illustrates the platelets illustrated in  FIG. 4  attached to individual LEDs. 
         FIG. 7  illustrates the structure of  FIG. 6  after forming a reflective material layer. 
         FIG. 8  illustrates the structure of  FIG. 7  after etching back the reflective material layer to reveal the tops of the wavelength converting platelets. 
         FIG. 9  illustrates the structure of  FIG. 3  diced into individual platelets with shaped sides. 
         FIG. 10  illustrates the structure of  FIG. 3  with a roughened, patterned, or textured interface. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the invention are directed to wavelength converted semiconductor light emitting devices with more than one wavelength converting material. Embodiments of the invention may be used in applications that may require high drive current and/or high operating temperatures, such as, for example, automotive head lamps. 
     Though in the examples below the semiconductor light emitting devices are III-nitride LEDs that emit blue or UV light, semiconductor light emitting devices besides LEDs such as laser diodes and semiconductor light emitting devices made from other materials systems such as other III-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, or Si-based materials may be used. 
       FIG. 2  illustrates a III-nitride LED that may be used in embodiments of the present invention. Any suitable semiconductor light emitting device may be used and embodiments of the invention are not limited to the device illustrated in  FIG. 2 . 
     The device of  FIG. 2  is formed by growing a III-nitride semiconductor structure on a growing substrate as is known in the art. The growth substrate (not shown in  FIG. 2 ) may be any suitable substrate such as, for example, sapphire, SiC, Si, GaN, or a composite substrate. The semiconductor structure includes a light emitting or active region sandwiched between n- and p-type regions. An n-type region  14  may be grown first and may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, and/or layers designed to facilitate removal of the growth substrate, which may be n-type or not intentionally doped, and n- or even p-type device layers designed for particular optical, material, or electrical properties desirable for the light emitting region to efficiently emit light. A light emitting or active region  16  is grown over then-type region. Examples of suitable light emitting regions include a single thick or thin light emitting layer, or a multiple quantum well light emitting region including multiple thin or thick light emitting layers separated by barrier layers. A p-type region  18  may then be grown over the light emitting region. Like the n-type region, the p-type region may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers. 
     After growth, a p-contact is formed on the surface of the p-type region. The p-contact  20  often includes multiple conductive layers such as a reflective metal and a guard metal which may prevent or reduce electromigration of the reflective metal. The reflective metal is often silver but any suitable material may be used. After forming the p-contact  20 , a portion of the p-contact  20 , the p-type region  18 , and the active region  16  is removed to expose a portion of then-type region  14  on which an n-contact  22  is formed. The n- and p-contacts  22  and  20  are electrically isolated from each other by a gap  25  which may be filled with a dielectric such as an oxide of silicon or any other suitable material. Multiple n-contact vias may be formed; the n- and p-contacts  22  and  20  are not limited to the arrangement illustrated in  FIG. 2 . The n-contacts and p-contacts may be redistributed to form bond pads with a dielectric/metal stack, as is known in the art. 
     In order to attach the LED to a mount  12 , one or more interconnects  24  are formed on or electrically connected to then- and p-contacts  22  and  20 . Interconnects  24  electrically and physically connect the LED to mount  12 . Interconnects  24  may be, for example, gold stud bumps, old layers, or any other suitable structure. Gold stud bumps may be, for example, between 60 μm and 100 μm in diameter. Individual LEDs are diced from a wafer of devices, for example after forming interconnects  24 . 
     Interconnects  26  may be formed on mount  12 . Mount  12  may be any suitable material including, for example, metal, ceramic, or silicon. Interconnects  26  on mount  12  align with interconnects  24  on the LED. Either of interconnects  24  or  26  may be omitted such that interconnects are formed on only one of the LED and the mount, not on both the LED and the mount. Vias may be formed within the mount or traces formed on the surface of the mount to electrically connect the top side of the mount, on which the LED is mounted, to the bottom side of the mount, which may be attached to another structure. 
     An individual LED is flipped over relative to the growth direction of the semiconductor structure and attached to mount  12 . The LED may be attached to the mount by, for example, ultrasonic bonding, thermosonic bonding, solder attach, or any other suitable bonding technique. 
     Before or after bonding to mount  12 , an underfill material  30  is disposed between the LED and the mount  12 . Underfill  30  supports the semiconductor structure during later processing. Underfill  30  may fill gaps  28  between neighboring interconnects  24 . Underfill  30  may be introduced between the LED and mount  12  by injection or any other suitable method. Underfill  30  may be, for example, silicone, epoxy, or any other suitable material. Underfill  30  may be injected in a liquid form then cured to form a solid. Excess underfill material may be removed by any suitable technique such as microbead blasting. 
     The growth substrate may be removed by any suitable technique. A sapphire substrate is often removed by laser melting, where laser light is shined through the substrate and melts the layer of semiconductor material in direct contact with the substrate, releasing the substrate from the semiconductor structure. Other substrates may be removed by, for example, etching or mechanical techniques such as grinding. Removing the substrate exposes the surface  32  of n-type region  14 . Surface  32  may be patterned, textured, or roughened, for example by photoelectrochemical etching or any other suitable technique, which may increase light extraction from surface  32 . 
       FIG. 2  illustrates an LED supported by an underfill and thick metal interconnects, from which the growth substrate has been removed. Any other suitable LEDs may be used. In some embodiments, an LED where the growth substrate remains attached to the semiconductor structure is used. For example, an LED semiconductor structure may be grown on a sapphire substrate that remains attached to the semiconductor structure. The sapphire may be thinned after growth, for example to a thickness less than 100 11 m thick, though it need not be. Because the sapphire mechanically supports the semiconductor structure, underfill is not required for mechanical support, though underfill may be included. Thick metal interconnects are not required, though they may be included. The LED may be attached to a mount by any suitable technique such as soldering. 
     Separate from the LED, a wavelength converting member is formed as illustrated in  FIGS. 3, 4, and 5 . The wavelength converting member absorbs light emitted by the LED and emits light of one or more different wavelengths. Unconverted light emitted by the LED is often part of the final spectrum of light extracted from the structure, though it need not be. Examples of common combinations include a blue-emitting LED combined with a yellow-emitting wavelength converting material, a blue-emitting LED combined with green- and red-emitting wavelength converting materials, a UV-emitting LED combined with blue- and yellow-emitting wavelength converting materials, and a UV-emitting LED combined with blue-, green-, and red-emitting wavelength converting materials. Wavelength converting materials emitting other colors of light may be added to tailor the spectrum of light emitted from the structure. 
     In  FIG. 3 , a wavelength converter  38  with two wavelength converting layers  40  and  42  is formed. Wavelength converting layers  40  and  42  typically include different wavelength converting materials, though they need not. Wavelength converting layers  40  and  42  often emit different colors of light, though they need not. Wavelength converting layers  40  and  42  may be any material that can withstand high temperature and high current operating conditions. For example, 
     wavelength converting layers  40  and  42  may be designed to withstand the operating conditions associated with currents up to 2.5 A and operating temperatures up to 240° C. 
     In some embodiments, wavelength converting layer  40  is a luminescent ceramic such as a powder phosphor sintered into a ceramic layer. In some embodiments, wavelength converting layer  42  is a glass or other suitable transparent material loaded with one or more wavelength converting materials such as conventional phosphors, organic phosphors, quantum dots, organic semiconductors, II-VI or III-V semiconductors, II-VI or III-V semiconductor quantum dots or nanocrystals, dyes, polymers, or other materials that luminesce. 
     In one example, wavelength converting layer  40  is a ceramic phosphor that absorbs blue light and emits red light. Suitable ceramic phosphors that emit red light include but are not limited to (Ba 1−x−y−z Sr x Ca y Eu z ) 2 Si 5−a−b Al a N 8−a−4b O a+4b  with 0.5≤x≤0.9, 0≤y≤0.1, 0.003≤z≤0.03, 0≤a≤0.05 and 0≤b≤0.2, (Ca 1−x−y−z−y M II   v Si 1+x−z Al 1−x+z N 3−x O x :Eu y ,Ce z ; M II =Sr, Ba, Mg with 0≤x≤0.05, 0≤y≤0.01, 0&lt;z≤0.04, 0≤v≤0.85. Wavelength converting layer  40  may be at least 5 μm thick in some embodiments, no more than 400 μm thick in some embodiments, at least 20 μm thick in some embodiments, and no more than 200 μm thick in some embodiments. 
     In one example, wavelength converting layer  42  is a phosphor disposed in glass. The phosphor in wavelength converting layer  42  absorbs blue light and emits green light. Suitable phosphors that emit green light include but are not limited to (Lu 1−x−y Y x Ce y ) 3 Al 5 O 12  with 0≤x≤1, 0.0015≤y≤0.04, or Sr 1−x−y M II   x Eu y Si 2 O 2 N 2 ; M=Ca, Ba with 0≤x≤0.5, 0.002≤y≤0.04. Wavelength converting layer  42  may be at least 5 μm thick in some embodiments, no more than 400 μm thick in some embodiments, at least 20 μm thick in some embodiments, and no more than 200 μm thick in some embodiments. Any combination of thicknesses of wavelength converting layers  40  and  42  may be used which meets the color point requirements of a given application. 
     The structure illustrated in  FIG. 3  may be formed by first forming the ceramic wavelength converting layer  40 , for example by pressing and sintering a powder phosphor or by any other suitable process. In some embodiments, wavelength converting layer  40  is thinned after sintering, for example by a mechanical process such as grinding or by any other suitable technique. Wavelength converting layer  40  may be thinned for example from at least 800 μm thick to no more than 300 μm thick. After thinning, wavelength converting layer  40  is mated with wavelength converting layer  42 . Each of wavelength converting layer  40  and wavelength converting layer  42  may be thinned before or after mating, as described herein. These thinning processes are optional and are not illustrated in the figures. 
     Wavelength converting layer  42  may be formed by, for example, mixing a selected green phosphor with molten glass to a predetermined phosphor loading. The mixture may be rolled into sheets, cut to fit onto a disc of wavelength converting layer  40 , then fused to wavelength converting layer  40 , for example by heating to a temperature greater than the reflow temperature of the glass. Alternatively, the mixture of glass and phosphor may be deposited directly onto wavelength converting layer  40  while hot (for example, while above the reflow temperature) then spread out evenly to form a glass layer of substantially uniform thickness. In some embodiments the reflow temperature may be as low as 320° C. or 
     as high as 1500° C. depending on the glass material. The phosphors used may tolerate temperatures of 1700° C. in some embodiments and of 1800° C. in some embodiments. Any suitable glass or other transparent material may be used. The glass may be a low refractive index glass, for example having an index of refraction less than 1.7 in some embodiments, less than 1.6 in some embodiments, and 1.52 in some embodiments, to improve extraction. The phosphor material, loading amount, and final thickness post-thinning (described below) for wavelength converting layer  42  are selected to match the blue light emitted from the light emitting device such that the light exiting the combined structure of the light emitting device and the wavelength converter meets the targeted specifications for color point and lumens for a given application. 
     In some embodiments, the surface of wavelength converting layer  40  that is mated with wavelength converting layer  42  is roughened, patterned, or textured, which may increase the surface area of the layer and thereby improve the strength of the bond between wavelength converting layer  42  and wavelength converting layer  40 . Roughening, patterning, or texturing the interface (see interface  43  shown in  FIG. 10 ) between wavelength converting layer  42  and wavelength converting layer  40  may also improve light extraction from wavelength converting layer  40  into wavelength converting layer  42 , and may reduce or prevent reflection of light from wavelength converting layer  42  into wavelength converting layer  40 . 
     In some embodiments, a fusing agent or pre-treatment is applied to one or both of wavelength converting layer  40  and wavelength converting layer  42 , to improve bonding between the two materials. For example, one or both of a silicon nitride barrier layer and a silicon oxide layer may be disposed on a ceramic wavelength converting layer  40 , to improve the interface between ceramic wavelength converting layer  40  and glass wavelength converting layer  42 , and/or to prevent ceramic oxidation during heating. The silicon nitride and/or silicon oxide layers may be formed on wavelength converting layer  40  by any suitable technique, including for example, megatron sputtering, chemical vapor deposition, plasma enhanced chemical vapor deposition, and evaporation. 
     In some embodiments, to fully fuse a glass wavelength converting layer  42  with a ceramic wavelength converting layer  40 , the combined structure  38  must be heated to a temperature greater than the reflow temperature of the glass in wavelength converting layer  42  under inert environment. In some embodiments the reflow temperature may be as low as 320° C. or as high as 1500° C. depending on the glass material. After heating to above the reflow temperature during fusing, the glass wavelength converting layer  42  may not be flat or of sufficiently uniform thickness. In some embodiments, to improve the center-to-edge thickness uniformity of wavelength converting structure  38  and/or to meet the color point of a given application, the glass wavelength converting layer  42  may be thinned after being fused to wavelength converting layer  40 , for example by grinding the top surface  41  of the wafer illustrated in  FIG. 3 , or by any other suitable technique. 
     In some embodiments, both the glass wavelength converting layer  42  and the ceramic wavelength converting layer  40  may be thinned after fusing, for example by grinding or any other suitable technique. In some embodiments, only the ceramic wavelength converting layer  40  may be thinned after fusing. In some embodiments, the ceramic wavelength converting layer  40  is thinned after fusing from at least 300 μm to no more than 120 μm. Ceramic wavelength converting layer  40  may be thinned before fusing (for example, from a thickness of 800 μm to a thickness of 300 μm, as described above) by a coarse grinding technique, then thinned after fusing (for example from a thickness of 300 μm to no more than 120 μm, as described above) using a more refined grinding/polishing technique. 
     Thinning generally occurs while the wavelength converting structure  38  is still in wafer form, before the structure of  FIG. 3  is diced into individual platelets, as described below. A ceramic wavelength converting layer  40  typically transfers heat more readily than a glass wavelength converting layer  42 . Accordingly, in some embodiments, the structure  38  is oriented such that wavelength converting layer  40  is disposed adjacent to the light emitting device and wavelength converting layer  42  is the top layer from which light is extracted. In some embodiments, the top surface of wavelength converting layer  42  is roughened, patterned, or textured during thinning to improve light extraction from structure  38  and to reduce or prevent back reflection. 
     The wavelength converter  38  is diced into platelets that are sized for a single light emitting device or a tile of multiple light emitting devices.  FIGS. 4, 5, and 9  illustrate diced wavelength converting platelets. 
     In  FIG. 4 , the platelets  44  are diced with a saw blade oriented perpendicular to the wafer surface. The platelets  44  in  FIG. 4  have substantially vertical sides. The platelets may be the same shape and size as the light emitting device, slightly larger than the light emitting device (for example a platelet 1.06×1.06 mm 2  may be disposed on an LED die 1×1 mm 2 ), or slightly smaller than the light emitting device (for example a platelet 0.965×0.965 mm 2  may be disposed on an LED 1×1 mm 2 ). 
     In  FIG. 5 , the platelets  46  have angled sides  48 . The angle of the sidewall is consistent through the entire thickness of the platelet  46 . Platelets  46  may be formed by cutting the platelets from the wafer  38  using an angled saw blade. For example, the saw blade may be angled at an angle of no more than 80° relative to a normal to the top surface of the wafer in some embodiments, at least 30° relative to a normal to the top surface of the wafer in some embodiments, no more than 65° relative to a normal to the top surface of the wafer in some embodiments, and at least 45° relative to a normal to the top surface of the wafer in some embodiments. 
     In  FIG. 9 , the platelets  70  have side walls with more than one surface. The sidewalls have a first surface  72  with a first orientation and a second surface  74  with a second orientation that is different from the first orientation. In the structure of  FIG. 9 , the first section  72  is substantially vertical and the second section  74  is angled. In other examples, the first section may be angled relative to a normal to the top surface of the wafer and the second section may be angled at a different angle than the first section. The interface between the first and second sections may be within wavelength converting layer  40 , within wavelength converting layer  42  as illustrated in  FIG. 9 , or at the interface between wavelength converting layers  40  and  42 . The structure illustrated in  FIG. 9  is formed by sawing the wavelength converting wafer  38  shown in  FIG. 3  in two passes. In one embodiment an angled blade cuts the angled portion  74 . Next the wafer is cut with a straight blade that forms vertical section  72 . In one embodiment the second cut fully separates the platelets  70  from each other. Either surface may be formed first. When platelets  46  have angled sides as in  FIGS. 5 and 9 , the thickness of the angled region in combination with the angle of the sidewall determines the area of the bottom surface of the platelet, i.e. the surface disposed next to the LED die. For example, for an LED die that is 1×1 mm 2 , if the top surface of the platelet is 1.06×1.06 mm 2 , possible bottom dimensions for the platelet include 0.98×0.98 mm 2 , 0.965×0.965 mm 2 , 0.94×0.94 mm 2  and other dimensions which are equal to or smaller than the 1×1 mm 2  LED die. The bottom surface of the platelet is often the same size as the LED die or smaller, though it need not be. 
       FIGS. 6-8  illustrate assembling the wavelength converting light emitting device. In  FIG. 6 , individual LEDs  10 , which may be the devices illustrated in  FIG. 2  or any other suitable device, are attached to a mount  12 . Individual wavelength converting platelets, such as one of the structures illustrated in  FIG. 4, 5 , or  9 , are pick-and-place attached to each LED  10 . In some embodiments, a layer of adhesive 
       50  is cast, dispensed, jetted, or otherwise disposed onto the LED  10  prior to attaching the wavelength converting platelet to the LED. Any suitable adhesive, such as silicone, may be used. In  FIGS. 6-8 , the wavelength converting platelets are mounted such that the ceramic wavelength converting layer  40  is attached to LED
 
 10  and glass wavelength converting layer  42  is the top layer of the structure. In alternative embodiments, glass wavelength converting layer  42  may be attached to LED  10  and ceramic wavelength converting layer  40  may be the top layer of the structure. In some embodiments, the structure may be heated to cure adhesive  50 . A bond pad  80  on mount  12  provides electrical connection for multiple LEDs  10 .
 
     In  FIG. 7 , a reflective material  52  is pressed into the spaces  56  between neighboring devices. Reflective material  52  is shown crosshatched in the figures to distinguish it from other layers. Reflective material  52  may be, for example, titanium oxide particles disposed in a transparent material such as silicone. Reflective material  52  between individual devices may be necessary to meet specifications for given applications, for example a light and dark contrast specification for automotive headlamps. Reflective material may be disposed over individual LEDs  10 , as illustrated by reflective material  54 , in addition to between neighboring devices. 
     In  FIG. 8 , reflective material  52  is thinned to remove the material over individual devices, such that light may be emitted through the top surface  55  of wavelength converting layer  42  in each device. In some embodiments, one or more bond pads  80  on mount  12 , which provide electrical connection for multiple LEDs 
       10 , are covered by reflective material  52  in  FIG. 7 , which is removed in  FIG. 8 . Excess reflective material may be removed by any suitable technique. In some embodiments, excess reflective material is removed by a dry bead blast or a wet bead blast. For example, in a dry bead blast, a stream of air and baking soda particles with an average diameter of 80 μm may be directed at the surface of the reflective material to remove the excess reflective material. In one example of a wet bead blast, plastic particles with an average diameter of 180 μm in a water slurry are directed at the surface of the reflective material to remove the excess reflective material. In some embodiments, the top surface  55  of wavelength converting layer  42  is roughened to improve light extraction during removal of excess reflective material illustrated in  FIG. 8 . 
     Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.