Abstract:
LED dies are partially singulated while on an unthinned depth growth substrate. Slots are made through the streets separating the LED dies, but not through the growth substrate, leaving the now separated LED dies on the growth substrate. A secondary support is attached to the LED dies on the opposite surface from the growth substrate, and the growth substrate is thinned or removed, leaving the LED dies on the secondary support. Because the LED dies are separated while on the unthinned growth substrate, the likelihood of distortion before slicing is virtually eliminated, and the width of the streets between the LED dies may be correspondingly reduced.

Description:
CROSS-REFERENCE TO PRIOR APPLICATIONS 
     This application is the U.S. National Phase application under 35 U.S.C. §371 of International Application No. PCT/IB2013/052534, filed on Mar. 29, 2013, which claims the benefit of U.S. Patent Application No. 61/620,480, filed on Apr. 5, 2012. These applications are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of semiconductor fabrication, and in particular to the singulation of thin-film light emitting device (LED) dies during a process that includes thinning or removal of the growth substrate. 
     BACKGROUND OF THE INVENTION 
     The use of solid state light emitting devices (LEDs) for conventional lighting applications, such as vehicle light bulbs, interior and exterior lighting, and so on, continues to increase, due primarily to their expected useful life, and their efficiency. 
     In a conventional fabrication process, light emitting devices may be formed/grown on a first growth substrate or wafer, covered with a second, typically thinner, substrate or support material, and then the growth substrate is thinned or removed, effectively transferring the wafer-formed light emitting devices onto the second support material for subsequent processing. This subsequent processing may include the application of protective or functional materials, such as phosphor-embedded silicone, and the eventual dicing, or singulation, of the light emitting structures into individual light emitting devices comprising one or more of these structures. 
       FIG. 1A  illustrates an example flow diagram for a conventional fabrication of a thin-film LED device with a phosphor coating, and  FIG. 1B  illustrates the structures formed during the corresponding fabrication stages. 
     At  110 , light emitting device structures (LED dies)  101  are formed on a substrate (growth layer)  102 , using techniques common in the art, which generally include forming at least an n-type layer, an active layer, and a p-type layer, and other layers, on a sapphire substrate or any suitable substrate, for example, silicon, silicon carbide, GaN, and so on. In this example, the LED dies  101  are structured to emit light through the ‘lower’ surface (typically the n-type layer) that is attached to the growth layer  102 , and to have connections/pads for receiving power at the ‘upper’ surface (to and through the p-type layer), opposite the growth layer  102 . 
     At  120 , a secondary support structure  103  is attached to the upper surface of the LED dies  101 . This secondary support structure may be a relatively thick sacrificial layer of removable material, or a film of removable ‘dicing tape’ on a frame that serves to hold the LED dies in place after the LED dies  101  are singulated. 
     At  130 , the growth layer  102  is thinned or removed, to reduce interference to the light that will be emitted from the LED dies  101 . To further facilitate light extraction from the LED dies  101 , the light emitting surface  104  is finished, at  140 , typically by roughening the surface  104  to reduce internal reflections. 
     At  150 , a phosphor coating  105  is applied to cause a wavelength conversion of some or all of the light emitted by the LED die  101 . In this manner, light output of a desired color is produced by the combination of wavelengths produced by the LED-phosphor combination. Obviously, if the LED emits light of the desired color directly, there is no need for this phosphor coating  105 . Other coatings, such as protective coatings may also be applied. 
     At  160 , the LED dies  101  with coating  105  are ‘diced’, or ‘singulated’, to provide individual devices that may subsequently be mounted on structures that facilitate handling and connection to a lamp or other illumination device. This dicing may be performed by laser or saw, the laser typically being preferred for its thinner kerf width, allowing for improved area efficiency by minimizing the required space between devices. 
     At  170 , the secondary support material  103  is removed, allowing access to the connections to the LED die  101  on the now ‘lower’ surface, opposite the phosphor coating  105 . If connection to the LED die  101  does not require access to the lower surface, or if the support material  103  provides the connections to the LED die  101 , the support material  103  may not be removed. 
     When the LED dies  101  are formed on the growth substrate  102 , the growth process and the combination of different materials, typically having significantly different thermal expansion characteristics, introduce stress within and between the LED dies. Accordingly, the growth substrate is purposely selected to be substantially rigid to avoid distortions, such as bowing, due to this stress. 
     However, the secondary support  103  is generally not as rigid, and when the growth substrate  102  is removed, at  130 , these stresses cause distortions in the structure of the LEDs on the secondary support  103 . These distortions will introduce curvatures in the streets between the rows and columns of the LED dies  101  that are used for dicing the dies  101 . Accordingly, either additional steps must be taken to counteract this distortion, or allowances for this distortion must be made in the spacing between the LED dies  101 , decreasing the area efficiency. 
     The typical kerf width of a laser cut is about ten microns, and, in the case of non-thinned growth substrate, the typical street width to accommodate for this kerf width and manufacturing tolerances is about thirty microns. In a structure formed by a six inch wafer that is thinned or removed, however, the distortion introduced by the growth stresses may be greater than thirty microns. Accordingly, either the yield will be decreased as the LED dies are mistakenly cut, or, the street widths must be significantly increased, often by a factor of two or more. 
     Additionally, some lamp assembly processes rely on the outer edges of the LED die to provide optical alignment with the light emitting surface; if a die is offset from the nominal center line of the street due to the distortion, the optical alignment will be similarly offset. 
     SUMMARY OF THE INVENTION 
     It would be advantageous to improve the area efficiency of wafers formed for processes that include growth substrate thinning or removal. It would also be advantageous to increase the cutting accuracy in processes that include growth substrate thinning or removal 
     To better address one or more of these concerns, in an embodiment of this invention, the LED dies are partially singulated while on the full depth growth substrate. Slots are made through the streets separating the LED dies, but not through the growth substrate, leaving the now separated LED dies on the growth substrate. A secondary support is attached to the LED dies on the opposite surface from the growth substrate, and the growth substrate is thinned or removed, leaving the LED dies on the secondary support. Because the LED dies are separated while on the full depth growth substrate, the likelihood of distortion is virtually eliminated, and the width of the streets between the LED dies may be correspondingly reduced. 
    
    
     
       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 an example flow diagram and corresponding structures for a conventional fabrication of light emitting devices with thinned or removed growth substrates. 
         FIGS. 2A-2B  illustrate an example flow diagram and corresponding structures for a fabrication of light emitting devices with thinned or removed growth substrates in accordance with aspects of this invention. 
     
    
    
     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. 2A  illustrates an example flow diagram and  FIG. 2B  illustrates corresponding structures for a fabrication of light emitting devices with thinned or removed growth substrates in accordance with aspects of this invention. 
     At  210 , the light emitting device (LED) dies  101  are formed on a substrate  102 , similar to  110  in  FIG. 1A . As in  FIG. 1A , the LED dies  101  are structured to emit light through the ‘lower’ surface that is attached to the growth layer  102 , and to have connections/pads for receiving power at the ‘upper’ surface, opposite the growth layer  102 . 
     As noted above, the formation of the LED dies  101  introduces stress during the fabrication process, but the growth substrate  102  is designed/selected to be rigid enough to withstand the stress without distortion. Accordingly, the LED dies  101  will remain accurately situated on the growth layer  102  through the formation process. 
     Instead of singulating the LED dies  101  after completion of the remaining fabrication processes, as in the conventional process of  FIG. 1A , in accordance with an aspect of this invention, the LED dies  101  are “partially-singulated” while on the full depth growth substrate  102 , at  215 , thereby allowing for precision cutting while the dies  101  are fixed on the rigid substrate  102 . As illustrated in  FIG. 2B , the slots, or scribe lines,  201  between the dies  101  extend through the streets between the dies  101 , but do not completely penetrate through the substrate  102 , thereby isolating the dies  101  from each other except through the substrate  102 . These slots  201  are illustrated as having a rectangular profile, but other shapes may be formed, including slopes or curves, typically dependent upon the process or tool used to form these slots 
     After partially singulating the LED dies  101  on the substrate  102 , a secondary support  103  is attached to the LED dies  101  on an opposite surface from the growth substrate  102 , at  220 , similar to  120  in  FIG. 1 . In accordance with an aspect of this invention, the support  103  may be a stretchable dicing film/tape on a frame, the film including an adhesive for attaching the film to the surfaces of the LED dies  101 . Alternatively, the support  103  may be a discrete rigid member, or an additional sacrificial layer formed on the LED dies  101 . 
     The growth surface is thinned or removed, at  230 , similar to the thinning or removal of  130  of  FIG. 1A . However, as contrast to the thinning or removal of the conventional process, the prior isolation of the LED dies  101  on the substrate  102  at  215  relieves the stress between the LED dies  101 , so that when the growth substrate is thinned or removed, the stress induced distortion is minimized. To enable separation of the LED dies  101  when the substrate  102  is thinned, rather than removed, the depth of the slots  201  extends below the thickness of the substrate  102  after thinning. Thus, the thinning step completes the singulation of the LED dies  101 . 
     At  240 , the light emitting surface  104  of the LED dies  101  is optionally finished to enhance the light extraction efficiency, typically by roughening the surface to reduce internal reflections, similar to  140  of  FIG. 1A . 
     The invention is disclosed hereafter using the paradigm of a phosphor coated light emitting device, and, in particular with regard to a technique disclosed in copending U.S. patent application 61/612,427, “SINGULATION OF LIGHT EMITTING DEVICES BEFORE AND AFTER APPLICATION OF PHOSPHOROUS”, filed Mar. 19, 2012 for Frank Wei, and incorporated by reference herein. One of skill in the art will recognize, however, that the principles presented herein are not limited to this example technique, as disclosed further below. 
     In accordance with aspects of this copending application, the LED dies  101  are placed on a stretchable film  103 , such as a film of dicing tape mounted on a frame, at  230 . At  245 , this stretchable film is subsequently stretched to increase the space  202  between the singulated LED dies  101  on the film  103 . This additional space  202  allows for the use of mechanical saws to singulate phosphor coated light emitting elements, avoiding the phosphor damage associated with laser cutting through phosphor, as the phosphor reacts to the laser light. 
     After increasing the space  202  between LED dies  101 , the phosphor coating  105  is applied, at  250 ; and at  260 , these phosphor  105  coated LED dies  101  are singulated, typically by mechanical sawing. This phosphor coating  105  may be applied as a paste-like compound that is cured over the LED dies  101 , as a preformed sheet that is laminated over the LED dies  101 , or in other forms of application, common in the art. 
     At  270 , the support material  103  may be removed, to allow access to the electrical contacts to the light emitting device  101 , similar to  170  of  FIG. 1A . 
     As noted above, an embodiment of this invention need not be limited to the above described stretching-coating-cutting technique that is disclosed in the aforementioned copending application. For example, if a phosphor coating is not to be applied, or is to be applied after singulation, the LED dies  101  on the support  103  after the optional surface finishing at  240  may be singulated by merely removing the support  103 . 
     If the growth substrate  102  has been thinned, rather than removed, the LED dies  101  will be attached via this thinned growth substrate if the depth of the slots  201  does not extend at least as deep as the remaining thinned substrate  102 . If the LED dies  101  remain so attached, the LED dies are singulated by performing a second cut through this thinned substrate  102 . However, because little if any distortion will have been introduced after the LED dies  101  are partially singulated on the full thickness substrate  102 , this second cutting may be performed with high accuracy. 
     Also, although the film stretching process is particularly well suited for increasing the space  202  between the LED dies  101 , at  245 , and subsequently applying the phosphor layer  103 , at  250 , the phosphor layer  103  may be applied at  250  without the intermediate spacing process at  245 . 
     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. 
     For example, it is possible to operate the invention in an embodiment wherein the dies are ‘pre-processed’ before application of the phosphor layer at  250  of  FIG. 2A . For example, the light emitting dies may be tested before the application of phosphor, and only properly operating dies are subsequently processed. In such an embodiment, faulty light emitting dies may be removed from the secondary support  103  and replaced by operational light emitting dies, or by ‘slugs’ that are taller than the light emitting dies, the extra height preventing the application of phosphor in these areas. Alternatively all of the light emitting dies  101  may be removed from the support  103 , the faulty dies discarded, and the non-faulty dies mounted on a new support  103  for further processing at  250 . 
     In like manner, the testing of the light emitting dies  101  may include more than a merely fault/no-fault determination. For example, one or more of the principles presented in U.S.PA 2008/0157103, “Laminating Encapsulant Film Containing Phosphor Over LEDs”, filed 17 Mar. 2008 for Haryanto Chandra, incorporated by reference herein, may be applied to optimize the performance of the combination of particular light emitting dies and phosphor coatings. In this copending application, the light emitting dies are tested and sorted (‘binned’) based on their light output characteristics. Thereafter, a particular phosphor composition is selected to be applied to each group of light emitting dies with similar characteristics so that the combination of the particular light emission of the light emitting dies and wavelength conversion of the selected phosphor provide a desired composite light output. By pairing a group of similarly performing light emitting dies with a phosphor composition that is selected based on the particular characteristics of the group, the variance of the composite light output is substantially reduced. 
     In such an embodiment, at  240  of  FIG. 2A , the light emitting dies  101  may be tested, removed from the substrate  103 , and stored in bins based on their determined light output characteristics. Thereafter, light emitting dies  101  from a given bin of similarly performing dies are situated on a new substrate  103 , at  245 , prior to the application of the phosphor layer  105 , at  250 . The phosphor layer  105  may be a paste compound having a particular combination of phosphor elements, or a preformed phosphor sheet that is selected based on its actual performance when coupled with the particular group of similarly performing light emitting dies. 
     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.