Abstract:
A method is described for etching ceramic phosphor converters. The method includes contacting a surface of the converter with a solution of phosphor acid for a time sufficient to etch the converter. The method is applicable to ceramic phosphor converters comprising a phosphor having a general formula M x Al y O z :RE wherein M is a metal and RE is a rare earth element.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/666,391, filed Jun. 29, 2012. 
    
    
     TECHNICAL FIELD 
     This invention is related to light emitting diodes (LEDs) and in particular LEDs that utilize solid ceramic phosphor converters. 
     BACKGROUND OF THE INVENTION 
     It is desirable in lighting applications that rely on the forward projection of light, e.g. automotive and information projection systems, to improve the starting etendue and directionality of the light source. It is also desirable to consider energy losses when designing any light source. One specific problem that arises in phosphor-conversion LEDs (pc-LEDs) that utilize flat, smooth-surfaced ceramic phosphor converter plates, particularly those that are highly transparent, is that beneficial forward light can be lost through unwanted side emission from the converter plate. In particular, light emitted at each point in the ceramic at angles larger than the Total Internal Reflection (TIR) critical angle has an increased probability to be lost to side emission or absorption. The enhanced side emission or absorption in the solid ceramic converters may lead to several problems in directional lighting systems employing such pc-LEDs, e.g., decreased optical efficiency, increased etendue or issues with color consistency. 
     SUMMARY OF THE INVENTION 
     Roughening (or texturing) the surface of a ceramic converter can increase the ratio of forward-to-side emitted light. This invention provides a method for etching ceramic phosphor converters so that more light can be emitted in a forward direction. In particular, the method of this invention uses phosphoric acid to etch the surface of solid ceramic phosphor converters comprising a phosphor with a general formula M x Al y O z :RE wherein M is a metal and RE is a rare earth element. More preferably, the ceramic converter comprises a garnet phosphor having a general formula M 3 Al 5 O 12 :Ce, wherein M is Lu, Tb or Y (e.g. Lu 3 Al 5 O 12 :Ce, Y 3 Al 5 O 12 :Ce, Tb 3 Al 5 O 12 :Ce, (Lu,Y) 3 Al 5 O 12 :Ce, Y 3 (Al,Ga) 5 O 12 , or Y 3 (Al,Si) 5 (O,N) 12 :Ce). 
     The primary advantage is that the invention provides a way to increase overall lumen output and significantly increase forward lumen output in flat ceramic converters at angles near normal to the surface. The surface of the ceramic is modified by etching into the grain structure. The etching can be controlled to some extent by altering time, temperature, and acid concentration. No special equipment is needed to texture the surfaces and the raw materials are relatively inexpensive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of a side view of a ceramic phosphor converter having a smooth primary light emitting surface. 
         FIG. 2  is an illustration of a side view of a ceramic phosphor converter having a roughened primary light emitting surface. 
         FIG. 3  is a photograph comparing bare and etched ceramic phosphor converters. 
         FIG. 4  shows photomicrographs of the surface of a ceramic phosphor converter before and after etching with phosphoric acid. 
         FIG. 5  compares the spectra generated by bare and etched ceramic phosphor converters when excited by a blue-emitting LED. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     For a better understanding of the present invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims taken in conjunction with the above-described drawings. 
     In general, the refraction of light at the interface between two different substances is governed by Snell&#39;s Law. With reference to  FIG. 1 , a flat ceramic converter plate  100  is shown. In a phosphor conversion LED application, the converter  100  would be positioned such that its bottom surface  108  would face the LED and light emitted by the LED would enter the converter  100  through its bottom surface  108 . In the embodiment shown in  FIG. 1  the converter has a primary light emitting surface  110  which is substantially smooth. A light ray  105  within the converter is subject to TIR and may be reflected several times within the converter to be emitted out a side of the converter  103  rather than from the primary light emitting surface  110 . 
     To increase the amount of light that is emitted from the primary light emitting surface  110 , the surface  110  may be roughened as shown in  FIG. 2 . The roughened surface of the ceramic converter creates many new interface angles, which in turn change the TIR conditions. According to the method of this invention, the surface  110  may be roughened by acid etching in which case the size and shape of the surface features are usually determined by the grain size of the ceramic. Larger grains typically result in larger surface features while smaller grains result in smaller features. As a result of the surface features, the light ray  105  has a greater probability of being emitted from the primary light emitting surface  110  as opposed to side emission as shown in  FIG. 1 . In a preferred embodiment, both the primary light emitting surface  110  and the bottom surface  108  of the converter are etched. 
     The thin, flat ceramic converter may be made by a conventional ceramic forming process, e.g., injection molding, pressing, tape casting, or extrusion. The converter is fired at a high temperature to sinter the phosphor particles together to form a solid ceramic piece. For example, powders of cerium-activated yttrium aluminum garnet, Y 3 Al 5 O 12 :Ce (hereinafter YAG:Ce), or lutetium aluminum garnet, Lu 3 Al 5 O 12 :Ce (hereinafter LuAG:Ce) can be mixed with a binder material, formed into the desired shape and sintered to form a solid ceramic converter of YAG:Ce or LuAG:Ce. The size of the ceramic converter can vary, but typical dimensions would be lengths and widths on the order of millimeters and depths on the order of 100 micrometers. 
     In one embodiment, the converter is immersed in a solution of hot, preferably boiling, phosphoric acid in order to roughen the surface of the ceramic converter. The etching time, acid temperature and concentration can be varied to produce the desired amount of surface roughening. The converter is then removed from the acid solution and washed cleaned by rinsing with water. 
     In another embodiment, the phosphoric acid may be mixed with sulfuric acid. Preferably the mixture of sulfuric (H 2 SO 4 ) and phosphoric (H 3 PO 4 ) acids has a proportion in the range between 1:1 and 3:1 sulfuric to phosphoric acid. 
     When a solution of 85% acid is used, the desired amount of etching usually occurs within tens of seconds to a few minutes. The ceramic is removed from the boiling acid and then rinsed with water and dried.  FIG. 3  is a picture showing what happens when a smooth ceramic converter with high inline transmission is etched according to the invention. In this case, a YAG:Ce ceramic converter is shown before and after etching with hot H 3 PO 4 . The surface texturing causes the material to become opaque. Electron microscopy images of the bare and etched ceramic converters that are shown in  FIG. 3  are shown in  FIG. 4 . 
     Photometry measurements were made that compare a bare YAG:Ce ceramic converter to the same converter after going through the acid etching procedure. Briefly, the textured surface increases the overall amount of light emitted from the ceramic converter. Perhaps more importantly, the roughened surface texture significantly changes the pattern of light emission; much more light is emitted normal to the primary light emitting surface. 
     In particular,  FIG. 5  shows that more yellow light from the converter is emitted when the surface has been etched. This is supported by the measurements provided in Tables 1-4 which compare the photometry results of the “bare” and “etched” converters. 
     
       
         
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 BARE 
                 ETCH 
                 ETCH/BARE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 LPW 
                 201.0 
                 261.2 
                 1.08 
               
               
                   
                 Power 
                 0.1538 
                 0.1566 
                 1.02 
               
               
                   
                 Photons (×10 16 ) 
                 4.17 
                 4.28 
                 1.03 
               
               
                   
                 Cx 
                 0.3251 
                 0.3341 
                 1.03 
               
               
                   
                 Cy 
                 0.3561 
                 0.3810 
                 1.07 
               
               
                   
                   
               
             
          
         
       
     
     In Table 2, the lumens per blue optical watt versus angle for a bare converter and that same converter after acid etching. The table shows the percentage gain/loss at four different angles. Table 3 shows the total photons detected versus angle for a bare converter and that same converter after acid etching at four different angles. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Angle 
                 BARE 
                 ETCH 
                 ETCH/BARE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 147.7 
                 181.2 
                 1.23 
               
               
                   
                 30 
                 159.4 
                 181.4 
                 1.14 
               
               
                   
                 60 
                 150.3 
                 149.8 
                 1.00 
               
               
                   
                 80 
                 33.6 
                 26.2 
                 0.78 
               
               
                   
                   
               
             
          
         
       
     
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Angle 
                 BARE 
                 ETCH 
                 ETCH/BARE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 4.16 × 10 6   
                 4.88 × 10 6   
                 1.17 
               
               
                   
                 30 
                 4.16 × 10 6   
                 4.54 × 10 6   
                 1.09 
               
               
                   
                 60 
                 3.41 × 10 6   
                 3.30 × 10 6   
                 0.97 
               
               
                   
                 80 
                 6.35 × 10 5   
                 5.06 × 10 5   
                 0.80 
               
               
                   
                   
               
             
          
         
       
     
     In Table 4, the C x  chromaticity values versus angle are compared for a bare converter and that same converter after acid etching. It is worth noting that the change in Cx with angle, ΔC x , is almost the same at 60° from normal for the bare and etched converters (0.055 and 0.056 respectively). 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Angle 
                 BARE 
                 ETCH 
                 ETCH/BARE 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 0 
                 0.2914 
                 0.2972 
                 1.020 
               
               
                   
                 30 
                 0.3082 
                 0.3157 
                 1.024 
               
               
                   
                 60 
                 0.3463 
                 0.3532 
                 1.020 
               
               
                   
                 80 
                 0.3896 
                 0.3887 
                 0.998 
               
               
                   
                   
               
             
          
         
       
     
     While there have been shown and described what are at present considered to be preferred embodiments of the invention, it will be apparent to those skilled in the art that various changes and modifications can be made herein without departing from the scope of the invention as defined by the appended claims.