Patent Publication Number: US-2007120463-A1

Title: Phosphor plate and light emitting device having same

Description:
The present application is based on Japanese patent application No. 2005-311624, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to a phosphor plate to radiate a wavelength-converted light by being excited by light emitted from a light emitting element and to a light emitting device having the same.  
      2. Description of the Related Art  
      A light emitting device is practically used which is operable to generate white light by the mixture of light emitted from an LED (=light emitting diode) element and a wavelength-converted light radiated from a phosphor being excited by the emitted light.  
      JP-A-2005-93712 discloses a light emitting device that comprises: a package having a case opened on the light extraction side; an LED element housed in the case; and a sealing material which seals the LED element and contains a phosphor.  
      In the light emitting device, when using as the LED element a blue LED element to emit blue light and as the phosphor a yellow phosphor to radiate yellow light by being excited by the blue light, white light can be generate by the mixture of the blue light emitted from the LED element and yellow wavelength-converted light from the yellow phosphor.  
      However, the light emitting device of JP-A-2005-93712 has following problems (1)-(3).  
      (1) A part of light emitted from the LED element is absorbed in the sealing material to cause light absorption loss. Therefore, the light extraction efficiency lowers.  
      (2) Since the phosphor-containing sealing material is in contact with the LED element, the phosphor is easy to deteriorate by heat generated from the LED element during the operation. Therefore, the wavelength conversion efficiency of the phosphor lowers so that the light emitting device cannot have high-brightness emission over a long time.  
      (3) Since the phosphors are suspended in the sealing material filled in the case, a part of the phosphors may sink down toward the bottom of the case. Therefore, the distribution state of the phosphors deteriorates, and the light path length of light radiated in various directions from the LED element is varied in the phosphor-containing sealing material. As a result, the wavelength conversion in the sealing material cannot be uniformly conducted to cause unevenness in emission color.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide a phosphor plate that is capable of enhancing the light extraction efficiency, having high-brightness emission over a long time, and improving the unevenness in emission color.  
      It is a further object of the invention to provide a light emitting device having the phosphor plate.  
      (1) According to one aspect of the invention, a phosphor plate comprises:  
      a wavelength conversion plate member comprising a base material comprising a phosphor, the phosphor being operable to radiate a wavelength-converted light by being excited by a light emitted from a light emitting element,  
      wherein the base material further comprises a converter of light travel direction.  
      In the above invention (1), the following modifications and changes can be made.  
      (i) The converter comprises a refractive index of n 1 , the base material comprises a refractive index of n 2 , and n 1 ≦n 2  is satisfied.  
      (ii) The light emitting element comprises a light emitting diode element.  
      (iii) The phosphor comprises a phosphor to radiate a white light from a light output surface of the plate member.  
      (iv) The converter comprises an air bubble.  
      (v) The converter comprises an particle.  
      (2) According to another aspect of the invention, a light emitting device comprises:  
      a case comprising an internal space opened on a light extraction side thereof;  
      the phosphor plate as defined in (1);  
      a light emitting element housed in the case and disposed on an opposite side to the light extraction side of the case,  
      wherein the phosphor plate is disposed on the light extraction side of the case.  
      &lt;Advantages of the Invention&gt; 
      In the invention, the light extraction efficiency can be enhanced, the high-brightness emission can be obtained over a long time, and the unevenness in emission color can be improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The preferred embodiments according to the invention will be explained below referring to the drawings, wherein:  
       FIG. 1A  is a cross sectional view showing a light emitting device in a first preferred embodiment according to the invention;  
       FIG. 1B  is a cross sectional view showing an LED element in  FIG. 1A ;  
       FIG. 2  is a cross sectional view showing a light emitting device in a second preferred embodiment according to the invention;  
       FIG. 3A  is a cross sectional view showing a light emitting device in a third preferred embodiment according to the invention; and  
       FIG. 3B  is a perspective view showing a phosphor plate in  FIG. 3A . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     First Embodiment  
       FIG. 1A  is a cross sectional view showing a light emitting device in the first preferred embodiment according to the invention.  FIG. 1B  is a cross sectional view showing an LED element in  FIG. 1A .  
      Light Emitting Device  
      As shown in  FIG. 1A , the light emitting device  1  comprises: a package  2  for housing an LED element  3 ; the LED element  3  housed in the package  2 ; a sealing material  8  which is filled in the package  2  and seals the LED element  3 ; a phosphor plate  4  which is disposed on the light extraction side of the LED element  3  while covering the sealing material  8 .  
      Package  
      As shown in  FIG. 1A , the package  2  comprises a case  5  which houses the LED element  3 , and an element mounting substrate  6  which covers an opening on one side (i.e., downside in  FIG. 1A ) of the case  5 .  
      Case  
      As shown in  FIG. 1A , the case  5  comprises an internal space  5 A which is formed circular in plain view and opened in the direction from the substrate side to the light extraction side. The case  5  is, as a whole, box-shaped and formed of a ceramic material such as alumina (Al 2 O 3 ), silicon (Si) and aluminum nitride (AlN), or a white resin. In the case  5 , a slope  5 B is provided to reflect light emitted from the LED element  3  toward the light extraction side. On the top of the slope  5 B, a step surface  5 C is provided to attach the phosphor plate  4  to the case  5 . The sealing material  8  is filled in the internal space  5 A.  
      Element Mounting Substrate  
      The element mounting substrate  6  is formed of a ceramic material such as alumina (Al 2 O 3 ), silicon (Si) and aluminum nitride (AlN), or a white resin. On the light extraction side (i.e., the top side) of the element mounting substrate  6 , first wiring patterns  14 ,  15  are formed which are electrically connected through Au (gold) bonding wires  12 ,  13  to a p-side electrode  3 A and an n-side electrode  3 B (See  FIG. 1B ). On the mounting side (i.e., the bottom side) of the element mounting substrate  6 , second wiring patterns  16 ,  17  are formed which serve to supply power to the LED element  3 . The first wiring pattern  14  and the second wiring pattern  16  are electrically connected to each other through a via pattern  22  filled in a via hole  19  formed penetrating the element mounting substrate  6 . The first wiring pattern  15  and the second wiring pattern  17  are electrically connected to each other through a via pattern  23  filled in a via hole  20  formed penetrating the element mounting substrate  6 . The first wiring patterns  14 ,  15  and the second wiring patterns  16 ,  17  are formed of a high-melting point metal such as tungsten (W) and molybdenum (Mo) and integrally formed with the via patterns  22 ,  23 .  
      Optionally, on the surface of the first wiring pattern  15  and the second wiring pattern  17 , a metal layer can be provided which comprises a single layer or a multilayer formed of nickel (Ni), aluminum (Al), platinum (Pt), titanium (Ti), gold (Au), silver (Ag) and copper (Cu), or a solder material.  
      Sealing Material  
      The sealing material  8  is formed of a transparent resin material such as silicone, epoxy resin and inert gas such as N 2 , Ar. The sealing material  8  is disposed between the element mounting substrate  6  and the phosphor plate  4  to seal the LED element  3  in the case  5 .  
      LED Element  
      The LED element  3  comprises a face-up type blue LED element with the p-side electrode  3 A and the n-side electrode  3 B as shown in  FIG. 1B , sealed with the sealing material  8  as shown in  FIG. 1A , and connected through the bonding wires  12 ,  13  to the first wiring patterns  14 ,  15 . As shown in  FIG. 1B , the LED element  3  comprises, grown sequentially on a sapphire (Al 2 O 3 ) substrate  24 , an AlN buffer layer (not shown), an n-type semiconductor (n-GaN) layer  25 , a light emitting layer  26  and a p-type semiconductor (p-GaN) layer  27 . The dimensions of the LED element  3  can be, e.g., about 1 mm in length and width (i.e., plane size).  
      Phosphor Plate  
      The phosphor plate  4  comprises, as shown in  FIG. 1A , a base material  4 A and a phosphor  4 B, and is attached to the step surface  5 A to define the internal space  5 A of the case. The phosphor plate  4  has a uniform thickness in the range of 100 to 500 μm.  
      The base material  4 A is shaped like a thin circular plate and formed of a transparent material with a refractive index of n 2  (N 2 =1.2 to 1.5) greater than n 1  (n 1 =1.0) of the air. The transparent material of base material  4 A can be an organic material such as silicone and acryl, an inorganic material such as glass, or a mixed material of the organic material and the inorganic material. The base material  4 A includes an air bubble  9  (with the refractive index of n 1 ) which serves as a converter of light traveling direction to convert the traveling direction of light (blue light) emitted from the LED element  3 . The air bubble  9  is, e.g., about 100 μm in diameter and accounts for 2 to 3% to the total volume of the phosphor plate  4 .  
      The phosphor  4 B is included in the base material  4 A. The phosphor  4 B is formed of a material such as YAG (yttrium aluminum garnet) to radiate a wavelength-converted light by being excited by the blue light emitted from the LED element  3 .  
      Operation of Light Emitting Device  
      When a voltage is applied from a power source (not shown) through the second wiring patterns  16 ,  17 , the via patterns  22 ,  23 , and the first wiring patterns  14 ,  15  to the LED element  3 , the light emitting layer  26  of the LED element  3  emits blue light. The light is emitted from the light extraction surface of the LED element  3  to the sealing material  8 .  
      Then, the light emitted from the LED element  3  is irradiated through the sealing material  8  to the phosphor plate  4 . In this case, a part of the emitted light from the LED element  3  is irradiated through the sealing material  8  to the phosphor plate  4 , and the other part thereof is irradiated through the sealing material  8 , reflected on the slope  5 B of the case  5 , again through the sealing material  8 , to the phosphor plate  4 .  
      The phosphor plate  4  (i.e., the phosphor  4 B) is excited by the inputted blue light to emit a yellow wavelength-converted light. Thus, the blue light emitted from the LED element  3  is mixed with the yellow light radiated from the phosphor  4 B to generate white light. In this case, a part of the blue light inputted in the phosphor plate  4  is irradiated to the air bubble  9  in the phosphor plate  4 , and then reflected on the interface of the air bubble  9  to change the traveling direction thereof to be then irradiated to the phosphor  4 B. Otherwise, the blue light irradiated to the air bubble  9  is refracted in the air bubble  9 , again refracted when outputted from the air bubble  9 , and irradiated to the phosphor  4 B. Thus, the part of the blue light can be wavelength-converted without being absorbed in the base material  4 A, i.e., to suppress the light absorption loss, to generate more white light. Then, the generated white light is outputted outside from the light output surface of the phosphor plate  4 .  
      Effects of the First Embodiment  
      The following effects can be obtained by the above first embodiment.  
      (1) A part of the blue light emitted from the LED element  3  can be suppressed from being absorbed in the base material  4 A. Therefore, the light absorption loss can be reduced and thereby the light extraction efficiency can be increased about 20 to 30%.  
      (2) The phosphor  4 B can be prevented from deterioration due to heat generated from the LED element  3 . Therefore, the wavelength conversion efficiency of the phosphor  4 B can be kept high to obtain high-brightness emission over a long time.  
      (3) The distribution state of the phosphor  4 B in the phosphor plate  4  (or the base material  4 A) can be kept stable. Further, the path length of light radiated in various directions from the LED element  3  can be kept constant in the phosphor plate  4 . Therefore, the wavelength conversion in the phosphor plate  4  can be uniformed to improve the unevenness in emission color.  
     Second Embodiment  
       FIG. 2  is a cross sectional view showing a light emitting device in the second preferred embodiment according to the invention. In  FIG. 2 , like components are indicated by using the same numerals as in  FIGS. 1A and 1B  and the detailed explanations are omitted below.  
      As shown in  FIG. 2 , the light emitting device  31  has the feature that a bead  32  is used as the converter of light traveling direction.  
      The base material  4 A includes the beads  32  (with a refractive index of n 3 =1.4), which is formed of silica (SiO 2 ) etc., to covert the traveling direction of blue light emitted from the LED element  3 . The bead  32  is, e.g., about 20 to 100 μm in diameter and accounts for 2 to 3% to the total volume of the phosphor plate  4 .  
      Effects of the Second Embodiment  
      The same effects (1) to (3) as the first embodiment can be obtained by the above second embodiment.  
     Third Embodiment  
       FIG. 3A  is a cross sectional view showing a light emitting device in the third preferred embodiment according to the invention.  FIG. 3B  is a perspective view showing a phosphor plate in  FIG. 3A . In  FIGS. 3A and 3B , like components are indicated by using the same numerals as in  FIGS. 1A and 1B  and the detailed explanations are omitted below.  
      As shown in  FIG. 3A , the light emitting device  41  has the feature that it comprises a phosphor plate  42  with a through hole  42  formed therein.  
      The through hole  42 A of the phosphor plate  42  can be a circular hole opened on the light input surface and the light output surface of the phosphor plate  42 . An air layer inside of the through hole  42 A serves as a converter of light traveling direction.  
      In operation, a part of the blue light inputted in the phosphor plate  42  from the LED element  3  is reflected on the interface of the air layer inside of the through hole  42 A and the base material  4 A to change the traveling direction thereof to be then irradiated to the phosphor  4 B. Otherwise, a part of the blue light inputted in the air layer of the through hole  42 A is refracted in the air layer, again refracted when outputted from the air layer, and irradiated to the phosphor  4 B.  
      Alternatively, the through hole  42 A may be filled with a material such as glass with a lower refractive index than the base material  4 A.  
      Effects of the Third Embodiment  
      The same effects (1) to (3) as the first embodiment can be obtained by the above third embodiment.  
      Although in the above embodiments the phosphor plates  4 ,  42  are used to radiate yellow light by being excited by blue light emitted from the LED element  3 , a phosphor plate can be used to radiate a white wavelength-converted light by being excited ultraviolet light (with a wavelength of 370 to 390 nm) emitted from an LED element.  
      Although in the above embodiments the face-up type LED element  3  is used, a face-down LED element  3  can be used. In this case, the LED element  3  is flip-mounted on the first wiring patterns  14 ,  15 .  
      Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.