Patent Publication Number: US-2013240834-A1

Title: Method for fabricating vertical light emitting diode (vled) dice with wavelength conversion layers

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of Ser. No. 13/227,335, filed Sep. 7, 2011; which is a continuation-in-part of Ser. No. 13/191,235, filed Jul. 26, 2011; which is a continuation-in-part of Ser. No. 11/530,128, filed Sep. 8, 2006, U.S. Pat. No. 8,012,774; which is a continuation-in-part of Ser. No. 11/032,853, filed Jan. 11, 2005, U.S. Pat. No. 7,195,944; all of which are incorporated by reference. 
    
    
     BACKGROUND 
     This disclosure relates generally to light emitting diode (LED) dice having wavelength conversion layers and to methods for fabricating vertical light emitting diode (VLED) dice with wavelength conversion layers. 
     Light emitting diode (LED) dice have been developed that produce white light. In order to produce white light, a blue (LED) die can be used in combination with a wavelength conversion layer, such as a phosphor layer formed on the surface of the (LED) die. The electromagnetic radiation emitted by the blue (LED) die excites the atoms of the wavelength conversion layer, which converts some of the electromagnetic radiation in the blue wavelength spectral region to the yellow wavelength spectral region. The ratio of the blue to the yellow can be manipulated by the composition and geometry of the wavelength conversion layer, such that the output of the light emitting diode (LED) die appears to be white light. 
     The present disclosure is directed to a method for fabricating vertical light emitting diode (VLED) dice configured to produce white light and to a vertical light emitting diode (VLED) die fabricated using the method. 
     SUMMARY 
     A method for fabricating vertical light emitting diode (VLED) dice includes the steps of: forming a light emitting diode (LED) die having a multiple quantum well (MQW) layer configured to emit electromagnetic radiation in a first spectral region; forming a confinement layer on the multiple quantum well (MQW) layer; forming an adhesive layer on the confinement layer; and forming a wavelength conversion layer on the adhesive layer configured to convert the electromagnetic radiation in the first spectral region to output electromagnetic radiation in a second spectral region. 
     A vertical light emitting diode (VLED) die fabricated using the method includes a multiple quantum well (MQW) layer configured to emit electromagnetic radiation in a first spectral region and a confinement layer on the multiple quantum well (MQW) layer. The (VLED) die also includes an adhesive layer on the confinement layer, and a wavelength conversion layer on the adhesive layer configured to convert the electromagnetic radiation in the first spectral region to output electromagnetic radiation in a second spectral region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments are illustrated in the referenced figures of the drawings. It is intended that the embodiments and the figures disclosed herein are to be considered illustrative rather than limiting. 
         FIG. 1  illustrates a plurality of vertical light emitting diode dice (VLED) on a wafer during a wafer level fabrication method; 
         FIG. 2  illustrates the formation of a phosphor layer on the vertical light emitting diode dice (VLED) during the fabrication method; 
         FIG. 3  illustrates patterning of the phosphor layer using a photoresist masking layer; 
         FIG. 4  illustrates formation of a metal contact layer on the patterned phosphor layer; 
         FIG. 5  illustrates formation of bond pads using the metal contact layer; 
         FIG. 6  illustrates a plurality of vertical light emitting diode dice (VLED) following singulation of the wafer; and 
         FIGS. 7A-7C  are schematic cross sectional views illustrating steps in a method for fabricating an alternate embodiment vertical light emitting diode die (VLED). 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that when an element is stated as being “on” another element, it can be directly on the other element or intervening elements can also be present. However, the term “directly” means there are no intervening elements. In addition, although the terms “first”, “second” and “third” are used to describe various elements, these elements should not be limited by the term. Also, unless otherwise defined, all terms are intended to have the same meaning as commonly understood by one of ordinary skill in the art. 
     Referring to  FIG. 1 , initially a plurality of vertical light emitting diode (VLED) dice  10  can be provided on a LED wafer. Each vertical light emitting diode (VLED) die  10  includes a metal substrate  12 , which can be made using a suitable process such as a laser lift-off process. In addition, a p-electrode  14  can be formed on the metal substrate  12 . Further, a p-contact  16  and a p-GaN layer  18  can be formed on the p-electrode  14 . An active region  20  (including a multi-quantum (MQW) can also be formed, and an n-GaN layer  22  can also be formed on the active region  20 . The n-GaN layer  22  has an exposed surface  24 . 
     The vertical light emitting diode (VLED) dice  10  can be formed using techniques that are known in the art. For example, the vertical light emitting diode (VLED) dice  10  can be formed by depositing a multilayer epitaxial structure above a carrier substrate such as sapphire; depositing at least one metal layer above the multilayer epitaxial structure to form the metal substrate  12 ; and removing the carrier substrate leaving the metal substrate  12 . The metal layers can be deposited using electro chemical deposition, electroless chemical deposition, chemical vapor deposition (CVD), metal organic CVD (MOCVD), plasma enhanced CVD (PECVD), atomic layer deposition (ALD), physical vapor deposition (PVD), evaporation, or plasma spray, or any combination of these techniques. In addition, the metal substrate  12  can comprise a single or multi-layered structure, and can comprise any of various suitable metals, such as at least one of silver (Ag), aluminum (Al), titanium tungsten (TiW) tungsten (W), molybdenum (Mo), tantalum (Ta), tantalum nitride (TaN), or alloys thereof. In one embodiment, Ag/Pt or Ag/Pd or Ag.Cr can be used as a mirror layer. Nickel (Ni) can be used as a barrier for gold (Au) and as a seed layer for copper (Cu) plating, which is used as the bulk substrate. A mirror layer (comprising Ag, Al, Pt, Ti, or Cr, for example) can be deposited, and then a barrier layer comprising any of various suitable materials (such as TiN, TaN, TiWN with oxygen) can be formed above the mirror layer before the electro or electroless chemical deposition of a metal, such as Ni or Cu. For electrochemical deposition of copper, a seed layer can be deposited using CVD, OCD, PVD, ALD, or evaporation process; exemplary seed materials for copper are tungsten (W), Au, Cu, or Ni, among others. 
     The sapphire substrate can be removed using a laser lift-off (LLO) technique. The multilayer epitaxial structure can have a reflective metal layer coupled to the metal plating layer. A passivation layer  26  can also be formed on the sidewalls of the vertical light emitting diode (VLED) dice  10 . 
       FIG. 2  illustrates the formation of a wavelength conversion layer in the form of a phosphor layer  28  on the vertical light emitting diode (VLED) dice  10 . Since the LED wafer is substantially smooth and planar, the phosphor layer  28  can be substantially uniform and parallel to the emitting LED surface  24 . Therefore, color rings on the field patterns of the vertical light emitting diode (VLED) dice  10  are minimized because the blue light emitted from the active layers travels the same distance or light path through the phosphor layer  28 . 
     The phosphor layer  28  can be formed using a spin coater. The phosphor layer  28  can be coated by the spin-coater spinning between 500 to 3000 rpm to control the layer thickness on the n-side-up LED wafer. In addition to the spin coat method, other methods such as screen printing, roller method, or dipping method can be used to form the phosphor layer  28 . After the phosphor layer  28  is deposited on the substrate, the spin coated film can be dried. The drying method is not limited as long as moisture contained in the film is evaporated. Thus, various methods including using a heater, dried air, or surface treatment such as a radiant heat lamp can be used. Alternatively, the spin coated film can be dried by leaving it in a room temperature environment for an extended period of time. 
     To make the material for the phosphor layer  28 , a phosphor powder composition can be prepared. For example, a dispersing agent can be dispersed in purified water. The dispersion can then be stirred with a mixer and placed in the purified water in which the dispersing agent has been dispersed, and the mixture can be stirred. In the phosphor powder composition, water can be used as a dispersing medium. The phosphor powder composition can contain alcohol as a dispersing agent (or a retaining agent) and ammonium bi-chromate can be used as a photosensitive polymer. The phosphor powders can also be surface-treated during the manufacturing process, to improve the dispersion and adhesion properties thereof. The phosphor coating material can comprise phosphor elements mixed in organic chemicals such as alcohol, aerosol, binder material or resin epoxy to tune the viscosity of the coating material. The thickness can be tuned by the material viscosity and spin rate reproducibly to change the resulting CIE coordination of the white light LEDs. 
     Referring to  FIG. 3 , a photoresist layer  30  can be applied and exposed with a contact pattern, and the phosphor layer  28  can be etched to form a patterned phosphor layer  28 . The patterned phosphor layer  32  can be formed on the exposed n-GaN surface  24  and patterned using a dry etching process. The result of the etching is a plurality of openings  34  configured as contact openings for later depositing a contact metal layer  36  such as Ni/Cr. 
     Referring to  FIG. 4 , the contact metal layer  36  can comprise a metal such as Ni/Cr (Ni is in contact with n-GaN) deposited on the photoresist layer  30 , in contact with the n-GaN layer  22 . The contact metal layer  36  can be deposited using a suitable process such as CVD, PVD, or ebeam evaporation. 
     Referring to  FIG. 5 , bond pads  38  can be formed on the patterned phosphor layer  32  in contact with the n-GaN layer  22 . The bond pads  38  can be formed by lift-off techniques during the removal of the photoresist layer  30  using an aqueous solution such as diluted KOH. The processes for phosphor coating and bonding pad formation can be performed in a different order. For example, the contact metal layer  36  can be patterned, dry etched and protected first by the photoresist layer  30  before the phosphor layer  28  is applied and patterned by lift-off technique. 
     Referring to  FIG. 6 , the LED wafer can be diced into a plurality of vertical light emitting diode (VLED) dice  10  using a suitable process. As indicated by the arrows in  FIG. 6 , the dice  10  are configured to emit white light. Although a single phosphor layer  28  is described above, multiple phosphor layers can also be used. For example, a red photosensitive phosphor powder composition (phosphor slurry) can be applied, exposed to light and developed. Then, a green photosensitive phosphor powder composition (phosphor slurry) can be applied, exposed to light and developed, and then a blue photosensitive phosphor powder composition (phosphor slurry) can be applied, exposed to light and developed. 
     Referring to  FIGS. 7A-7C , steps in a method for fabricating an alternate embodiment vertical light emitting diode (VLED) die  40  are illustrated. For simplicity, various elements of the vertical light emitting diode (LED) die  40  are not illustrated. However, this type of vertical light emitting diode (VLED) die is further described in U.S. Pat. Nos. 7,195,944 and 7,615,789, both of which are incorporated herein by reference. Although the vertical light emitting diode (VLED) die  40  is described, it is to be understood that the concepts described herein can also be applied to other types of light emitting diode (LED) dice, such as ones with planar electrode configurations. In addition, although the method is shown being performed on a single die, it is to be understood that the method can be performed at the wafer level on a wafer containing multiple dice, which can be singulated into individual dice following the fabrication process. 
     Initially, as shown in  FIG. 7A , the method includes the step of forming (or alternately providing) the vertical light emitting diode (VLED) die  40  with a conductive substrate  42 , and an epitaxial stack  44  on the conductive substrate  42 . The epitaxial stack  44  includes an n-type confinement layer  46 , a multiple quantum well (MQW) layer  48  in electrical contact with the n-type confinement layer  46  configured to emit electromagnetic radiation, and a p-type confinement layer  50  in electrical contact with the multiple quantum well (MQW) layer  48 . 
     The n-type confinement layer  46  preferably comprises n-GaN. Other suitable materials for the n-type confinement layer  46  include n-AlGaN, n-InGaN, n-AlInGaN, AlInN and n-AlN. The multiple quantum well (MQW) layer  48  preferably includes one or more quantum wells comprising one or more layers of InGaN/GaN, AlGaInN, AlGaN, AlInN and AlN. The multiple quantum well (MQW) layer  48  can be configured to emit electromagnetic radiation from the visible spectral region (e.g., 400-770 nm), the violet-indigo spectral region (e.g., 400-450 nm), the blue spectral region (e.g., 450-490 nm), the green spectral region (e.g., 490-560 nm), the yellow spectral region (e.g., 560-590 nm), the orange spectral region (e.g., 590-635 nm) or the red spectral region (e.g., 635-700 nm). The p-type confinement layer  50  preferably comprises p-GaN. Other suitable materials for the p-type confinement layer  50  include p-AlGaN, p-InGaN, p-AlInGaN, p-AlInN and p-AlN. 
     Still referring to  FIG. 7A , the vertical light emitting diode (VLED) die  10  also includes an n-bond pad  54  on the n-type confinement layer  46  and a reflector layer  56  on the conductive substrate  42 . The n-bond pad  54  can have a size, peripheral shape and location suitable for wire bonding. In addition, the n-bond pad  54  can comprise a conductive wire bondable material, such as a single layer of a metal such as Al, Ti, Ni, Au, Pt, Ag or Cr, or a metal stack such as Ti/Al/Ni/Au, Al/Ni/Au, Ti/Al/Pt/Au or Al/Pt/Au. The reflector layer  56  can comprise a single layer of a highly reflective material such as Ag, Si or Al, or multiple layers, such as Ni/Ag/Ni/Au, Ag/Ni/Au, Ti/Ag/Ni/Au, Ag/Pt, Ag/Pd or Ag/Cr. All of the elements of the vertical light emitting diode (VLED) die  40  described so far can be fabricated using techniques that are known in the art. 
     Next, as shown in  FIG. 7B , the method includes the step of forming an adhesive layer  52  on the n-type confinement layer  46  leaving the n-bond pad  54  exposed. The adhesive layer  52  can comprise a suitable adhesive formed using a suitable process such as dispensing, screen-printing, spin coating, nozzle deposition, spraying and applying a pressure sensitive adhesive (PSA). Suitable adhesives include silicone, epoxy and acrylic glue. A thickness Ta of the adhesive layer  52  can be selected as required with from 200 Å to 50 μm being representative. 
     Next, as shown in  FIG. 7C , the method includes the step of forming a wavelength conversion layer  58  on the adhesive layer  52 . The wavelength conversion layer  58  can comprise a layer of phosphor formed using a spin coater substantially as previously described. Other suitable processes for forming the wavelength conversion layer  58  include lithography, dip-coating, dispensing using a material dispensing system, printing, jetting, spraying, chemical vapor deposition (CVD), thermal evaporation and e-beam evaporation. The wavelength conversion layer  58  can also include an opening  60  aligned with the n-bond pad  54  for providing access to the n-bond pad  54 . The opening  60  can be formed by etching the wavelength conversion layer  58  using a photomask substantially as previously described. 
     The wavelength conversion layer  58  can have a peripheral shape that substantially matches the peripheral shape of the vertical light emitting diode (VLED) die  40 . The wavelength conversion layer  58  is configured to convert at least some of the electromagnetic radiation emitted by the multiple quantum well (MQW) layer  48  into electromagnetic radiation having a different wavelength range, such as a higher wavelength range. For example, if the multiple quantum well (MQW) layer  48  emits electromagnetic radiation in a blue spectral range, the wavelength conversion layer  58  can be configured to convert at least some of this radiation to a yellow spectral range, such that the output of the vertical light emitting diode (VLED) die  40  appears to be white light. 
     While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and subcombinations thereof. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.