Patent Publication Number: US-2019172983-A1

Title: Led with ceramic green phosphor and protected red phosphor layer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/170,442 filed on Jun. 1, 2016 and titled “LED WITH CERAMIC GREEN PHOSPHOR AND PROTECTED RED PHOSPHOR LAYER”, which is a continuation of U.S. patent application Ser. No. 14/414,719 filed on Jan. 14, 2015 and titled “LED WITH CERAMIC GREEN PHOSPHOR AND PROTECTED RED PHOSPHOR LAYER”, which is a § 371 application of International Application No. PCT/IB2013/055752 filed on Jul. 12, 2013, which claims priority to U.S. Provisional Patent Application No. 61/673,810 filed on Jul. 20, 2012. U.S. patent application Ser. No. 15/170,442, U.S. patent application Ser. No. 14/414,719, International Application No. PCT/IB2013/055752, and U.S. Provisional Patent Application No. 61/673,810 are each incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of light emitting devices, and in particular to a light emitting device that includes a light emitting element, a ceramic green-wavelength conversion element, and a red-wavelength conversion layer. 
     BACKGROUND OF THE INVENTION 
     Wavelength converters, such as phosphors, are commonly used to provide light comprising a plurality of wavelengths from a single wavelength light emitting element. The composite light output is commonly a combination of the light emitted from the converters upon absorption of some of the light emitting from the light emitting element, and the remainder of the non-absorbed light from the light emitting element. 
     In an example embodiment such as illustrated in  FIG. 1A , the light emitting device  101  includes a light emitting element  110  that emits blue light and a ceramic element  120  above it that includes blue-to-green wavelength converter material. For ease of reference, a wavelength converter that emits green light, irrespective of the wavelength of the light absorbed by the converter, is hereinafter termed a green converter, the individual elements, or particles, that effect the conversion being termed green converter material. When the green converter material in the ceramic element  120  absorbs the blue light from the light emitting element  110 , they emit green light. The composite light is closer to ‘white’ light than the blue light that is emitted from the light emitting element, and may be used for select applications, such as automotive headlights, that currently allow for a cool white illumination. 
     The ceramic element  120  allows for a fairly uniform and consistent distribution of green converter material through the element  120 . The rigid nature of the ceramic composition of element  120  serves to provide permanence to this distribution of green converter material in element  120 . The surface  125  of the ceramic element  120  may also be roughened to enhance the light output efficiency without significantly affecting the structure and characteristics of the element  120 . The rigidity of the ceramic element  120  also facilitates the handling of the element for processes such as a pick-and-place process that situates the ceramic element  120  atop the light emitting element  110 . 
     The light emitting element  110  with ceramic element  120  is encased in a reflective material  140 , such as silicone loaded titanium oxide, TiO. The reflective material  140  serves to protect the light emitting device  101  and to reflect side emitted light from light emitting element  110  and ceramic element  120  to the surface  125  of the ceramic element  120 , to provide a higher projection contrast (well defined boundary between illuminated and non-illuminated regions), which is preferred in certain applications, such as automotive lighting. 
     In a typical operating environment, the light emitting device  102  is coupled to a heat sink  160 , which absorbs and dissipates the heat generated by the light emitter  110  and the green converters in the ceramic element  120 . The ceramic element  120  is an efficient thermal conductor, but the surrounding atmosphere is not, and therefore only a small amount of the heat that is generated in the ceramic element  120  is dissipated. The heat that is generated by the conversion of blue light to green light will be conducted through the glue layer  115  and blue emitter  110  to the heat sink  160 . 
     Although the composite light output of the light emitting device  101  is closer to a white output than the blue light from the light emitting element  110 , some applications require a light output having a ‘warmer’ color temperature. 
     To provide a light that appears ‘more white’ (having a warmer color temperature) than the cool white light output of the device  101 , a wavelength conversion element that emits red light (‘red converter’) may be added, as illustrated in the device  102  of  FIG. 1B . Current technology, however, does not allow for red conversion material to be embedded in a ceramic element, and therefore a separate coating, such as a red converter material loaded silicone layer  130 , is applied between the (blue) light emitting element  110  and the ceramic (green) converter element  120 . 
     Although this arrangement of blue-red-green emitting elements of device  102  has a number of advantages, including providing a durable external surface  125 , and a relatively high red converter efficiency, being adjacent the blue light emitting element  110 , it exhibits poor thermal dissipation efficiency. 
     As noted above, although the ceramic layer  120  may dissipate some heat through the upper surface  125  to the surrounding atmosphere, the thermal transfer efficiency at this interface is poor. Compounding the matter, red converters generate more heat than green converters, having to effect a larger wavelength conversion. Although some of this heat will be transferred to the ceramic element  120 , the ceramic element cannot efficiently dissipate this additional heat. 
     Accordingly, most of the conversion-generated heat must travel through the red converter layer  130  and the glue layer  115  to the emitter  110  and heat sink  160 . The red converter layer  130  and the glue layer  115  are commonly silicone based, and silicone has a relatively low thermal transfer efficiency. Thus, the layers  130  and  115  will act as a thermal barrier to this required heat transfer. Thus, the expected operating temperature of the device of  FIG. 1B  will be high, increasing the likelihood of failure and reducing the device&#39;s life span. 
     SUMMARY OF THE INVENTION 
     It would be advantageous to provide a thermal efficient light emitting device that produces a warmer color temperature than the conventional blue light emitting element with ceramic green converter element. It would also be advantageous to provide a method of producing such thermal efficient light emitting devices that provides for a consistent color temperature among the devices. 
     To better address one or more of these concerns, in an embodiment of this invention, a green converter ceramic element is coated with a red converter loaded silicone and placed above a blue light emitting element such that the ceramic element is attached to the light emitting element, thereby providing an efficient thermal coupling from the red and green converters to the light emitting element and its associated heat sink. To protect the red converter coating from the effects of subsequent processes, a sacrificial clear coating is created above the red converter element. This clear coating may be provided as a discrete layer of clear material, or it may be produced by allowing the red converters to settle to the bottom of its suspension material, thereby forming a converter-free upper layer that can be subjected to the subsequent fabrication processes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in further detail, and by way of example, with reference to the accompanying drawings wherein: 
         FIGS. 1A-1B  illustrate example prior art light emitting devices with ceramic converter elements. 
         FIG. 2  illustrates an example light emitting device with ceramic converter element and protected red converter layer. 
         FIGS. 3A-3C  illustrate an example process for creating a protected red converter layer on a ceramic green converter element. 
         FIGS. 4A-4D  illustrate an example process for creating light emitting devices with a ceramic green converter element and protected red converter layer. 
         FIGS. 5A-5E  illustrate an example alternative process for creating light emitting devices with a ceramic green converter element and protected red converter layer. 
     
    
    
     Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention. 
     DETAILED DESCRIPTION 
     In the following description, for purposes of explanation rather than limitation, specific details are set forth such as the particular architecture, interfaces, techniques, etc., in order to provide a thorough understanding of the concepts of the invention. However, it will be apparent to those skilled in the art that the present invention may be practiced in other embodiments, which depart from these specific details. In like manner, the text of this description is directed to the example embodiments as illustrated in the Figures, and is not intended to limit the claimed invention beyond the limits expressly included in the claims. For purposes of simplicity and clarity, detailed descriptions of well-known devices, circuits, and methods are omitted so as not to obscure the description of the present invention with unnecessary detail. 
       FIG. 2  illustrates an example light emitting device with a light emitting element  110 , a ceramic green converter element  120 , a protected red converter layer  230 , and a protective reflective coating  140 . In this example embodiment, because the ceramic green converter element  120  is a significantly better thermal conductor than the environment above the red converter layer  230 , and because there is no glue, per se, between the red converter layer  230  and the green converter element  120 , heat from the red converter layer  230  will be efficiently conducted into the green converter element  120 . This heat, and the heat caused by the conversion in the green converter element  120  are conducted through the glue layer  115  and the light emitting element  110  into the heat sink  160  upon which the device will be mounted. 
     As noted above, and as detailed further below, after attaching the converter element  120  and converter layer  230  to the light emitting element  110 , the light emitting device is subject to subsequent fabrication processes, such as the removal of any residual materials on the surface  235  of the red converter layer  230 , with a corresponding roughening of the surface  235  to increase the light extraction efficiency. These processes include, for example, micro-bead blasting, which removes surface material by projecting small particles onto the surface at high velocity. Currently a ‘dry’ blasting process is commonly used, wherein air and particles of baking soda (about 80 microns in diameter) are projected onto the surface, although ‘wet’ blasting, wherein a slurry of water and plastic particles (about 180 microns in diameter) is projected onto the surface. 
     When a durable material, such as the ceramic element  120 , is subjected to micro-bead blasting, the surface is gradually worn away; this gradual reduction allows for the micro-bead blasting to be controlled to a fairly high precision. When a less durable material, such as a silicone layer  230  that is embedded with converter elements is subjected to micro-bead blasting, the surface is worn away quickly, allowing for only a relatively coarse control of the degree of reduction. 
     If the red converter layer  230  is subjected to micro-bead blasting with only coarse control of the degree of reduction, the variance in light output among devices will be large. In some devices, only a small amount of the red converters will be removed by the micro-bead blasting; in other devices a larger amount of red converters will be removed by the micro-bead blasting. The quantity of red converter material that remains in the red converter layer  230  will determine the amount of red light that will be produced, and thereby determine the color point of the composite light output. If the quantity of remaining red converter material varies significantly among devices due to the lack of control of the micro-bead blasting effects on the converter loaded silicone, the color point among the devices will correspondingly vary. 
     In a preferred embodiment of this invention, the red converter layer  230  is formed such that the amount of red converters in the layer  230  is fairly consistent among devices, despite the lack of precise control of the effects of micro-bead blasting, and other processes, applied to the device. This consistent amount of red converter material is maintained by providing a sacrificial converter-free (clear) layer to protect the layer that contains the red converter material from damage during subsequent processes. When the micro-bead blasting, or other process, is applied that wears down or otherwise damages the upper surface, the reduction in material will not introduce a corresponding reduction in the amount of red converter material, because the material will be removed from the non-converter-containing sacrificial layer. Typically, the sacrificial layer will be transparent to the any of the wavelengths of light generated by the converters  120 ,  230  or the emitter  110 . One of skill in the art will recognize, however, that the sacrificial layer may be sized to provide a remainder layer after it is worn down that serves to provide other effects, such as scattering, and need not be clear, per se. 
       FIGS. 3A-3C  illustrate an example process for creating a protected red converter layer  330  on a ceramic green converter element  120 . In  FIG. 3A , a red converter layer  330  is applied to the ceramic element  120  using any of a variety of application techniques, including lamination of blade coated film, screen printing, stencil printing, spin cast, or any other suitable method, screen printing typically being preferred. The red converters in the layer  330  may include, for example, BR102C, a-Sialon, BSSN2.0, BSSN2.6, and BSSNE2.6, and the green converters in the ceramic element  120  may include YAG, LuYAG, NYAG1, 2, and so on. The red converters may be suspended in any suitable material, including any variety of silicone, silicon oxide sol gel, and others known in the art. For ease of reference, the term ‘silicone’ is used herein to include such materials. 
     Although the aforementioned ‘fluid’ techniques for directly applying a red converter layer  130  on the green converter element are very well suited for this application, one of skill in the art will recognize that other techniques may be used as well. For example, U.S. Pat. No. 7,344,952, “Laminating Encapsulant Film Containing Phosphor Over LEDs”, issued 3 Jul. 2008 to Haryanto Chandra, and incorporated by reference herein, discloses a technique for laminating a phosphor film to a set of light emitting devices on a submount. A similar technique may be used for creating the red converter layer  330  as a preformed film, then laminating this film to a set of ceramic elements  120 . In like manner, although the use of a glue will reduce the thermal efficiency somewhat, one of skill in the art will recognize that preformed red converters may be glued to the ceramic element  120 . 
     In an embodiment of this invention, the thickness of the converter layer  330  may be between 5 and 50 microns, typically about 30 microns, and the concentration of the red converters in this layer may be between 1% and 70%, typically between 7% and 20%. If silicone is used as the suspension material, a schedule of curing may be 1 hr at 80° C., followed by 1 hr at 120° C., and then followed by 4 hr at 150° C. 
     In  FIG. 3B , a clear non-converter containing sacrificial layer  340  is created above the red converter layer  330 . This sacrificial layer  340  may include, for example, silicone compounds KJR9222A/B and KRJ9226D, as well as other materials. Preferably, the sacrificial layer  340  has the same refraction index as the red converter layer  330 , to avoid losses at the boundary between the layers  330 ,  340 . The sacrificial layer  340  may be formed over the red converter layer  330  using any of a variety of techniques, including, for example, screen printing, lamination, overmolding, casting, or any other suitable method. 
     To facilitate the formation of the sacrificial layer  340  over the converter layer  330 , the converter layer  330  may be subjected to treatments, such as oxygen plasma, UV ozone, and the like, typically for between 2 and 30 minutes. To maximize the effectiveness of this treatment, the delay between the treatment and the formation of the sacrificial layer, if any, should not exceed a few hours. 
     The thickness of the sacrificial layer  340  will be dependent upon the expected degree of control of the effects of micro-bead blasting, or other processes that reduce the thickness, and may range from 2 to 100 microns. Assuming conventional processing techniques, a sacrificial layer thickness of 20-40 microns will generally be sufficient. If silicone is used as the sacrificial material, a schedule of curing may be 1 hr at 80° C., followed by 1 hr at 120° C., and then followed by 4 hr at 150° C. 
     The concentration of red and green converter material in the layer  330  and ceramic element  120 , respectively, and the thickness of the layer  330  and ceramic element  120  will primarily determine the resultant color point of the composite light output. Accordingly, the variance of color point among devices will primarily be determined by the degree of control of these concentrations and thicknesses, which is typically substantially greater than the degree of control of the effects of other processes, such as micro-bead blasting. 
     After forming the red converter layer  330  and sacrificial layer  340  on the ceramic element  120 , the combination is diced, or ‘singulated’, to produce individual ceramic based wavelength converter elements  350 . As noted above, an advantage of a ceramic based structure is that it is rigid enough to facilitate efficient picking and placing of these structures as required during subsequent processes. 
       FIGS. 4A-4D  illustrate an example process for creating light emitting devices with a ceramic green converter element and protected red converter layer. 
     Having formed a plurality of ceramic based wavelength converter elements  350 , these converter elements  350  are attached to a plurality of light emitting elements  110 , as illustrated in  FIGS. 4A-4B . To facilitate this process and subsequent processes, the light emitting elements  110  may be mounted on a substrate  410 . This substrate  410  may include electrical circuitry that facilitates external connections to the light emitting elements  110 . 
     After the ceramic based converter elements  350  are attached to the light emitting elements  110 , protective and reflective material  450 , such as silicone loaded with TiO, may be pressed into the space between these elements  110 ,  350  using an over moulding technique. To facilitate manufacture, the material  450  may be applied as a coating that covers all of the elements  110 ,  350  and fills the space between them, as illustrated in  FIG. 4C . 
     To remove the covering reflective material  450  from the light emitting surface  345 , the devices on the substrate  410  may be subjected to a micro-bead blasting process, or other process that damages the exposed surfaces of the device. In the context of this disclosure, the term ‘damage’ includes any wearing away or abrasion on the exposed surface, even though this damage may have a beneficial effect. For example, depending upon the size of the micro-beads and other factors, the process may provide a roughened surface  345   a ,  345   b ,  345   c  on the exposed layers  340   a ,  340   b ,  340   c , which may improve the light extraction efficiency through this surface. 
     As noted above, such a process is also likely to have different effects on the exposed layers  340   a ,  340   b ,  340   c  of the devices, due to the coarse nature of the control of these effects on relatively non-durable material, such as silicone, as illustrated in  FIG. 4D . 
     It is significant to note that although this subsequent processing has differing effects on the different layers  340   a ,  340   b ,  340   c , the red converter layers  330   a ,  330   b ,  330   c , are unaffected by this subsequent processing, being protected from damage by the sacrificial layers  340   a ,  340   b ,  340   c . Accordingly, the color point among the devices will not be dependent upon the variances associated with these subsequent processes. 
       FIGS. 5A-5E  illustrate an example alternative process for creating light emitting devices with a ceramic green converter element and protected red converter layer. 
       FIG. 5A  illustrates an example application of a thick red converter layer  530  upon a ceramic green converter element  120 . The thickness of the layer  530  will generally range between 30 and 150 microns, 50-80 microns being most common. Red converter layer  530  may consist of suspension material  540  which is embedded with red converter material  560 . In accordance with an aspect of this invention, the suspension material  540  is viscous, and the red converter material  560  is able to ‘sink’ or settle down to the bottom of the layer before the suspension material  540  is fully cured, as illustrated in  FIG. 5B . This sinking, or settling, of the red converter material  560  create an upper region of the suspension material  540  that does not contain red converter material  560 , and a lower region that contains the original red converter material  560  in a denser form. 
     The sedimentation time to achieve a given depth of the non-converter region will be dependent upon the viscosity of the suspension material  540 , the density of the red converter material  560 , the concentration of the red converter material  560  in the suspension material  540 , the charge of the particles of red converter material  560 , and other factors. The concentration of red converter material  560  in the suspension material  540  will generally be between 1% and 70%, although concentrations below 25% are most common. 
     The amount of red converter material  560  needed to convert the light to the correct color point is based on the target color point and the light emitted from the blue emitter and the green converter material. The amount of red converter material  560  in the converter layer  530 , and thus the degree of red conversion, can be controlled by controlling the total thickness of the converter layer  530 , and the sedimentation time. 
     When the material  560  sinks to create an appropriately deep region (e.g. 20-40 microns) of non-converter material, the suspension material  540  is cured, thereby stabilizing the distribution of red converter material  560  in the suspension material  540 . If silicone is used as the suspension material  540  in the converter layer  530 , a schedule of curing may be 1 hr at 80° C., followed by 1 hr at 120° C., and then followed by 4 hr at 150° C., although a cure as short as 25-60° C. for one hour may be sufficient, depending upon the particular makeup of the converter layer  530 . 
     One of skill in the art will recognize that trace amounts of the converter material  560  may exist in the upper region of the converter layer  530 . In the context of this disclosure, a layer/region with a concentration that is an order of magnitude less than the concentration in the lower layer/region with settled converter material is considered to be a non-converter layer/region. 
     The ceramic elements  120  with converter layer  530  are attached to light emitting elements  110 , at  FIG. 5C , and a protective and reflective material  450  is applied to fill the spaces between the elements  110  with ceramic converter element  120  and layer  530 . 
     The excess of material  450  is removed, typically via a micro-bead blasting or other material removal process. This removal process may result in different removal effects (damage) on each of the converter layers  530   a ,  530   b ,  530   c , as illustrated by their different heights and patterns of roughness in  FIG. 5D . However, provided that the removal process only affects the non-converter region of these converter layers  530   a ,  530   b ,  530   c , these removal differences will have minimal, if any, effect on the color point produced by the red converter material  560   a ,  560   b ,  560   c  in the remaining portion of layers  530   a ,  530   b ,  530   c.    
     As illustrated in  FIG. 5E , the devices on the substrate  410  are diced/singulated to provide individual light emitting devices on substrates  410   a ,  410   b ,  410   c . As noted above, although these individual light emitting devices have differing converter layers  530   a ,  530   b ,  530   c , the converter concentration within each converter layer  530   a ,  530   b ,  530   c , is substantially the same, being unaffected by the variances associated with the processes applied after the converter layers  530   a ,  530   b ,  530   c  are attached to the ceramic elements  120  and light emitting elements  110 . 
     In operation, the substrate  410  is coupled to a heat sink  160 , so that the heat generated at the red converter layer  530  and the green converter element  120  is conducted through the ceramic green converter element  120 , through the glue layer  115 , the light emitting element  110 , and the substrate  160 , and is dissipated through the heat sink  160 . Optionally, the substrate  410  may be removed, or the substrate  410  may include the heat sink  160 . 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
     Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.