Abstract:
High power and high brightness light emitting diode (LED) assemblies emitting white light are disclosed. The present invention also discloses methods for cost effective mass production of the high power and high brightness LED assemblies with high throughput.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     (1) Field of the Invention  
         [0002]     The present invention relates to high power and high brightness white light emitting diode (LED) assemblies and method for mass production of the same.  
         [0003]     (2) Prior Art  
         [0004]     Extensive efforts have been devoted to develop white LEDs and white LED assemblies by: (1) using wavelength converter materials including fluorescent materials, photon-recycling semiconductors, and dye, patents for this method include U.S. Pat. No. 6,635,987 by Wojnarowski, et al, U.S. Pat. No. 6,642,618 by Yagi, et al; (2) integrating red, green, and blue LEDs (RGB LEDs) into a LED lamp; (3) growing more than one epitaxial layers emitting lights of different wavelengths on the same substrate, patents for this method include U.S. Pat. No. 6,163,038 by Chen, et al.; and (4) stacking two LED chips of different wavelengths, patents include U.S. Pat. No. 6,633,120 by Salam.  
         [0005]     There are drawbacks for the above mentioned white LEDs: (1) The life time of fluorescent materials is not as long as that of LEDs; (2) The control system for RGB LEDs is complicated and expensive; (3) White LEDs manufactured by growing two or more epitaxial layers of different wavelengths on a substrate do not have high brightness comparing with the existing LEDs of blue and other colors, the growing process is complicated, and the requirement of lattice match limits the selections of material systems for the epitaxial layers; and (4) The stacking two LED chips emitting lights of different wavelengths to manufacturing high brightness white LEDs is an economic way. Salam disclosed white LED assemblies and each comprises two LED chips of different wavelengths stacked to each other. As shown in  FIG. 1 , the drawbacks of Salam&#39;s white LED assemblies are the following: (a) LED  110  and LED  120  are bonded at chip level; (b) wire bonding pads are on both sides of the LED assemblies; (c) two LEDs are either skewed or shifted one relative to the other for wire bonding, thus wasting the material of the active layers and lower the output light intensity. Therefore the mass production for Salam&#39;s stacking white LED assemblies is difficult and expensive, throughput and yield are very low.  
         [0006]     Therefore there is a need for new high brightness and high power white LED assemblies and for methods of mass production to provide high power, high efficiency, high brightness, and economic white LED assemblies.  
       BRIEF SUMMARY OF THE INVENTION  
       [0007]     In the present invention, new high brightness high power white LED assemblies and method of manufacturing the same are disclosed.  
         [0008]     The high brightness white LED assemblies of the present invention comprise a first epitaxial layer emitting a light of a first wavelength which is disposed on an electrically conductive submount, a second epitaxial layer emitting a light of a second wavelength disposed on the first epitaxial layer, and an electrode connected to the exposed surface of the second epitaxial layer.  
         [0009]     The most bright commercially available LEDs of different colors are always selected for manufacturing the high brightness white LED assemblies of the present invention, therefore, the high brightness white LED assemblies of the present invention provide brighter white light.  
         [0010]     One of embodiments of the high brightness white LED assemblies of the present invention comprises a second epitaxial layer which is a blue GaN epitaxial layer grown on a sapphire substrate which is then removed, and a first epitaxial layer is a yellow AlGaInP epitaxial layer grown on a GaAs or GaP substrate which is then removed.  
         [0011]     Some embodiments of the high brightness white LED assemblies of the present invention have two electrodes, one is contacted to the second epitaxial layer, the other is the bottom side of the submount which is electrically connected to the first epitaxial layer, i.e., two epitaxial layers are electrically connected in serial. There is only one wire bonding pad needed.  
         [0012]     Some embodiments of the high brightness white LED assemblies of the present invention have three electrodes, the second and the first electrodes having the same polarity are electrically connected to the second epitaxial layer and the bottom side of the submount respectively, the third electrode having opposite polarity is sandwiched between the first epitaxial layer and the second epitaxial layer, i.e., the two LED chips are electrically controlled separately, so the colors of the output lights emitted by two epitaxial layers may be controlled to certain degree. In this embodiment, there are two wire bonding pads on the same side of the high brightness white LED assemblies.  
         [0013]     Some of embodiments of the present invention have two active layers directly bonded to each others, therefore two epitaxial layers are grown for a shorter time to lower production cost.  
         [0014]     The high brightness high power white LED assemblies of the present invention have the following advantages. 
        1. Always select commercially available most bright and high power monochromatic LEDs to manufacture white LED assemblies, thus the provided white light is brighter.     2. The bonding process of the high brightness white LED assemblies are at the wafer level, instead of at the chip level, so the mass production is practical.     3. Since either the two wire bonding pads are on the same side of the high brightness white LED assemblies or there is only one wire bonding pad, the manufacture process of wire bonding is much easier and throughput and yield are much higher.     4. The high brightness white LED assemblies of the present invention have all of the advantages of flip chip technique, such as fast heat dissipation.     5. Since there is less wire bonding area comparing to prior art and two LED chips are not skewed or shifted one relative to the other, the material of the active layers is utilized to maximum.     6. The second epitaxial layer is exposed, the second electrode may be so patterned and arranged that to reduce the current crowding effect, fully utilize the material of active layer, and distribute the current more evenly.     7. The current density may be higher, thus the high brightness white LEDs are brighter.     8. For one of embodiments, GaN LEDs as the second epitaxial layer, the sapphire substrate has been removed at wafer level, so the cost of the wafer dicing process is much lower.     9. For a lamp of the high brightness white LED assemblies, after removing the substrate the second epitaxial layer grown on, the second epitaxial layer is directly exposed to a dome material that covers the high brightness white LED assemblies and has the same refractive index as that of the second epitaxial layer, which results in eliminating totally internal reflections when light incidents from the second epitaxial layer to the substrate which has lower refractive index and from the substrate to the dome material.     10. The shape and diameter of the dome is so determined that there is no totally internal reflection when light incidents from the dome to air. Therefore there is no light trapped in the dome for the high brightness white LED assemblies of the present invention.     11. The high power and high brightness LED assemblies and methods for mass production of the same of the present invention may be applied to other LED assemblies emitting light of any desired mixing colors.        
 
         [0026]     The primary object of the present invention is to provide new LED assemblies for providing high brightness high power white light and to have fast thermal dissipation, higher light extraction efficiency, reduced current crowding effect, and higher current density.  
         [0027]     The second object of the present invention is to provide new methods for cost effective mass production of the high brightness high power white LED assemblies with high throughput.  
         [0028]     The third object of the present invention is to provide new high brightness high power white LED assemblies and lamps to significantly improve the extraction efficiency by eliminating the totally internal reflection.  
         [0029]     The fourth object of the present invention is to provide high brightness high power white LED assemblies to eliminate the lattice mismatch to improve the internal efficiency.  
         [0030]     Further objects and advantages of the present invention will become apparent from a consideration of the ensuing description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS  
       [0031]     The novel features believed characteristic of the present invention are set forth in the claims. The invention itself, as well as other features and advantages thereof will be best understood by referring to detailed descriptions that follow, when read in conjunction with the accompanying drawings.  
         [0032]      FIGS. 1   a  and  1   b  are top and cross sectional views of prior art.  
         [0033]      FIGS. 2   a  and  2   b  are cross sectional views of white LED assemblies of the present invention with one wire bonding pad.  
         [0034]      FIGS. 3   a  and  3   b  are cross sectional views of white LED assemblies of the present invention with two wire bonding pads.  
         [0035]      FIGS. 4   a  and  4   b  are cross sectional views of white LED assemblies of the present invention with two active layers directly bonded and having one wire bonding pad.  
         [0036]      FIGS. 5   a  and  5   b  are cross sectional views of white LED assemblies of the present invention with two active layers directly bonded and having two wire bonding pads.  
         [0037]      FIGS. 6   a  and  6   b  are cross sectional view of white LED assemblies of the present invention with MQBW and one wire bonding pad.  
         [0038]      FIGS. 7   a  and  7   b  are cross sectional view of white LED assemblies of the present invention with MQBW and two wire bonding pads.  
         [0039]      FIGS. 8   a  and  8   b  are flow charts of manufacturing white LED assemblies of the present invention with one and two wire bonding pads respectively.  
         [0040]      FIGS. 9   a  to  9   d  are top views of different patterned electrodes on the exposed second epitaxial layer with one or two wire bonding pads.  
         [0041]      FIG. 10  is a sectional view of LED lamp of prior art.  
         [0042]      FIGS. 11   a  and  11   b  are sectional views of two high brightness white LED lamps with high extraction efficiency.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0043]     While embodiments of the present invention will be described below, those skilled in the art will recognize that other LED assemblies, LED lamps and mass production processes are capable of implementing the principles of the present invention. Thus the following description is illustrative only and not limiting.  
         [0044]     Reference is specifically made to the drawings wherein like numbers are used to designate like members throughout.  
         [0045]     Note the followings that are applied to all of embodiments of high brightness high power white LED assemblies of the present invention: 
        (1) The dimensions of all of drawings are not to scale.     (2) The intensities and wavelengths of two LED epitaxial wafers are selected, according to the chromaticity diagram, so that two mixed lights provide desired color.     (3) Material systems of a first epitaxial layer of a first LED epitaxial wafer emitting light of longer wavelength are selected from a group comprising: AlGaInP, InGaN, GaInNP, GaNP, InGaP, GaP:N, AlInP, AlGaAs, and GaAsP.     (4) Material systems of a second epitaxial layer of a second LED epitaxial wafer emitting light of shorter wavelength are selected from a group comprising: GaInN, AlGaInN, GaN, BeZnCdSe, BeZnCdTe, ZnSe, ZnCdSe, and ZnSeTe.     (5) The material systems of multiple quantum barrier-well (MQBW) layers are determined by the material systems of the first and second epitaxial layers respectively. The multiple quantum barrier layers and multiple quantum well layers are laminated alternately and cyclically.     (6) A submount for the LED assemblies is selected from a group comprising electrically conductive Si, SiC, and thin films of Cu and Al. The submounts have high thermal conductivity for fast heat dissipation.     (7) Materials for a reflective/Ohmic layer sandwiched between a submount and the first epitaxial layer are selected from agroup comprising Ag, Al, Au, In, Ni, Ti, Pd, Pt, and alloys of above metals.     (8) A first electrode on the bottom side of the submount comprises Au/Sn.     (9) Electrodes sandwiched between first and second epitaxial layers or between first and second MQBW layers are transparent for, at least, light of longer wavelength.     (10) Electrodes of different polarities are electrically isolated.     (11) The first epitaxial layer is always bonded to the submount, a second epitaxial layer is stacked on the top of the first epitaxial layer. The second epitaxial layer is transparent for light of longer wavelength.     (12) To bond two epitaxial layers, conductive epoxy, a thin layer of Indium, ITO, and other eutectic materials may be employed. Bonding layers are transparent for, at least, longer wavelength light emitted by the first epitaxial layer.     (13) The LED assemblies in  FIGS. 2   a  and  2   b ,  FIGS. 3   a  and  3   b ,  FIGS. 4   a  and  4   b ,  FIGS. 5   a  and  5   b ,  FIGS. 6   a  and  6   b , and  FIGS. 7   a  and  7   b  have the same structure respectively, except that N and P are switched. Therefore only  FIG. 2   a , FIG,  3   a , FIG,  4   a , FIG,  5   a , FIG,  6   a , and  FIG. 7   a  are described in detail below.        
 
         [0059]      FIGS. 1   a  and  1   b  show a prior art of two LEDs of different wavelengths stacked to each other. LED  110  and LED  120  are bonded together at chip level. Only octagonal overlap area  130  emitting lights. There are two of wire bonding pad  150  on LED  110  and two of wire bonding pad  140  on LED  120 . Normal size for wire bonding pad is about 100×100 micrometer, therefore significant material of active areas of LED  110  and LED  120  is wasted. Also as shown in  FIG. 1   b,  wire bonding pad  140  and  150  are on different sides of LED assembly, therefore the wire bonding process is very difficult and time consuming.  
         [0060]      FIG. 2   a  shows an embodiment of the present invention. Reflective/Ohmic layer  213  and N electrode  212  are respectively disposed on submount  211  that is electrically conductive. First epitaxial layer  240  comprising first N-type cladding layer  214 , first P-type cladding layer  216 , and first active layer  215  sandwiched between first N-type cladding layer  214  and first P-type cladding layer  216 , is disposed on reflective/Ohmic layer  213 . Second epitaxial layer  250  comprising second N-type cladding layer  217 , second P-type cladding layer  219 , and second active layer  218  sandwiched between second N-type cladding layer  217  and second P-type cladding layer  219 , is disposed on first epitaxial layer  240 . P electrode  220  is disposed on second P-type cladding layer  219 . N electrode  212  is the conductive layer disposed on submount  211 . First and second epitaxial layer  240  and  250  are electrically connected in serial.  
         [0061]     In this embodiment only one wire bonding pad is needed.  
         [0062]     This embodiment allows controlling the color of mixed lights to some degree by choosing the intensities and wavelengths of lights emitted by first and second epitaxial layer  240  and  250  respectively.  
         [0063]      FIG. 3   a  shows first and second epitaxial layer  340  and  350  are electrically controlled separately. Reflective/Ohmic layer  313  and first N electrode  312  are respectively disposed on submount  311  that is electrically and thermally conductive to improve thermal dissipation. First epitaxial layer  340  comprising first N-type cladding layer  314 , first P-type cladding layer  316 , and first active layer  315  sandwiched between first N-type cladding layer  314  and first P-type cladding layer  316 , is disposed on reflective/Ohmic layer  313 . Second epitaxial layer  350  comprising second N-type cladding layer  320 , second P-type cladding layer  318 , and second active layer  319  sandwiched between second N-type cladding layer  320  and second P-type cladding layer  318 , is disposed on first epitaxial layer  340 . Second N electrode  321  is disposed on second N-type cladding layer  320 . First N electrode  312  is a conductive layer disposed on submount  311 . P electrode  317  is sandwiched between first P-type cladding layer  316  and second P-type cladding layer  318 . A pre-determined area of second epitaxial layer  350  is etched down until P electrode  317  is exposed. Then P contact pad  322  is disposed on P electrode  317 . First and second epitaxial layer  340  and  350  are electrically controlled separately.  
         [0064]     For this embodiment, there are two wire bonding pads, second N electrode  321  and P contact pad  322 , on the same side of the white LED assemblies and easier to wire bonding.  
         [0065]     This embodiment provides additional controls on the color of mixed lights of the white LED assemblies by controlling the applied voltages and currents to first and second epitaxial layers respectively.  
         [0066]      FIG. 4   a  shows an economic way for manufacturing white LED assemblies. Reflective/Ohmic layer  213  and N electrode  212  are respectively disposed on submount  211 . First epitaxial layer  440  comprising N-type cladding layer  412  and first active layer  413 , is disposed on reflective/Ohmic layer  213 . Second epitaxial layer  450  comprising P-type cladding layer  414  and second active layer  415 , is disposed on bonding layer  411  that is disposed on first epitaxial layer  440 . Bonding layer  411  may be conductive epoxy or a layer of either an adhesive metal or alloy comprising Indium. Bonding layer  411  is so thin that it is transparent for light. P electrode  220  is disposed on P-type cladding layer  414 . N electrode  212  is the conductive layer disposed on submount  211 . First and second epitaxial layer  440  and  450  are electrically connected in serial.  
         [0067]      FIG. 5   a  shows another economic way for manufacturing white LED assemblies with more control on the color of mixing lights. Reflective/Ohmic layer  313  and first N electrode  312  are respectively disposed on submount  311 . First epitaxial layer  540  comprising first N-type cladding layer  513  and first active layer  512 , is disposed on reflective/Ohmic layer  313 . Second epitaxial layer  550  comprises second N-type cladding layer  515  and second active layer  514 . P electrode  511  is sandwiched between first active layer  512  and second active layer  514 . Second N electrode  321  is disposed on second epitaxial layer  550 . A pre-determined area of second epitaxial layer  550  is etched down until P electrode  511  is exposed. Then P contact pad  322  is disposed on P electrode  511 . First and second epitaxial layer  540  and  550  are electrically controlled separately.  
         [0068]      FIG. 6   a  shows a similar white LED assemblies as that of  FIG. 4   a , except the following. First epitaxial layer  640  comprises N cladding layer  412 , first active layer  413 , and first multiple quantum barrier-well (MQBW) layer  612 . Second epitaxial layer  650  comprises P cladding layer  414 , second active layer  415 , and second MQBW layer  613 . First and second MQBW layer  612  and  613  are bonded by bonding layer  411 . First and second MQBW layer  612  and  613  are grown on first and second active layer  413  and  415  respectively during wafer growth process and for improving the performance.  
         [0069]      FIG. 7   a  shows a similar white LED assemblies as that of  FIG. 5   a,  except additional MQBW. First epitaxial layer  740  comprises first N-type cladding layer  513 , first active layer  512 , and first MQW  612 . Second epitaxial layer  750  comprises second N-type cladding layer  515 , second active layer  514 , and second MQW  613 . Second N electrode  321  is disposed on second epitaxial layer  750 . First and second epitaxial layer  740  and  750  are electrically controlled separately. P electrode  511  is sandwiched between first MQBW  612  and second MQBW  613 . A pre-determined area of second epitaxial layer  750  is etched down until P electrode  511  is exposed. Then P contact pad  322  is disposed on P electrode  511 .  
         [0070]      FIGS. 8   a  and  8   b  are two slightly different flow charts of manufacturing different embodiments of high brightness high power white LED assemblies.  
         [0071]     Process step  801  and  802  are, according to the complementary wavelengths and power ratio, preparing/selecting two LED epitaxial wafers with shorter and longer wavelengths respectively. The preparation of LED epitaxial wafers also needs to take into account methods of removing substrates, since different removing methods require different epitaxial layer growth processes. Embodiments of white LED assemblies of  FIGS. 2 and 3  are conventional LEDs and only need to consider requirements on wavelength and power ratio. For embodiments of white LED assemblies of  FIG. 4 , a epitaxial layer grown on a substrate comprises one type cladding layer and an active layer, the complementary epitaxial layer grown on another substrate comprises the other type cladding layer and an active layer. For embodiments of white LED assemblies of  FIG. 5 , a epitaxial layer grown on a substrate comprises one type cladding layer and an active layer, the complementary epitaxial layer grown on another substrate comprises the same type cladding layer and an active layer. For embodiments of white LED assemblies of  FIG. 6 , a epitaxial layer grown on a substrate comprises one type cladding layer, an active layer, and a multiple quantum barrier-well (MQBW) layer, the complementary epitaxial layer grown on another substrate comprises the other type cladding layer, an active layer, and a MQBW layer. For embodiments of white LED assemblies of  FIG. 7 , a epitaxial layer grown on a substrate comprises one type cladding layer, an active layer, and a MQW layer, the complementary epitaxial layer grown on another substrate comprises the same type cladding layer, an active layer, and a MQW layer.  
         [0072]     The rest of the process steps are self-explanatory, listed below.  
         [0073]     Step  803 , bonding two selected LED epitaxial wafers to form a combined LED epitaxial wafer.  
         [0074]     Step  804 , removing the substrate of the longer wavelength LED wafer by selective etching, mechanical lapping/polishing, or combination of both. Then the first epitaxial layer of longer wavelength is exposed.  
         [0075]     Step  805 , coating a reflective/Ohmic layer to the exposed first epitaxial layer.  
         [0076]     Step  806 , bonding an electrically conductive submount with high thermal conductivity to the reflective/Ohmic layer.  
         [0077]     Step  807 , removing the substrate of the shorter wavelength LED wafer. For an embodiment of the present invention, the substrate is sapphire which may be removed by mechanical lapping/polishing or laser melting. Then the second epitaxial layer of shorter wavelength is exposed.  
         [0078]     Step  808 , disposing and/or patterning an electrode/contact pad on the exposed second epitaxial layer.  
         [0079]     Step  809 , dicing the combined LED epitaxial wafer into individual discrete LED assemblies.  
         [0080]     Step  810  of  FIG. 8   b , disposing a third electrode on, at least, one of the LED epitaxial layers of the LED epitaxial wafers before step  803 .  
         [0081]     Step  811  of  FIG. 8   b , disposing a contact pad on the third electrode by first etching through the second epitaxial layer before step  809 .  
         [0082]      FIGS. 9   a  and  9   b  show an embodiment of a patterned electrode having ring-grid-shape and disposed on second epitaxial layer  900 . The patterned electrode comprises ring  901 , grid  902 , and wire bonding pad  903 , which are electrically connected. Ring  901  and grid  902  evenly spread the current introduced by wire bonding pad  903  through second epitaxial layer  900  of white LED assembly.  FIG. 9   b  shows a second wire bonding pad  904  which is contacted with P electrode  317  of  FIG. 3   a , P electrode  511  of  FIG. 5   a  and  FIG. 7   a.    
         [0083]      FIGS. 9   c  and  9   d  show an embodiment of patterned electrodes having plus-multi-ring-shape and disposed on second epitaxial layer  900 . The patterned electrode comprises a plurality of ring  901 , plus  905 , and wire bonding pad  906 , which are electrically connected. The plurality of ring  901  and plus  905  evenly spread the current introduced by wire bonding pad  906  through second epitaxial layer  900  of white LED assembly.  FIG. 9   d  shows a second wire bonding pad  907  which is contacted with P electrode  317  of  FIG. 3   a , P electrode  511  of  FIG. 5   a  and  FIG. 7   a.    
         [0084]      FIG. 10  shows a LED lamp of prior art. Light  1002  and light  1005  emitted from active layer  1003  are respectively reflected back by the interfaces between active layer  1003  and transparent substrate  1001  and the interface between substrate  1001  and dome material  1000 . Light  1006  is totally reflected back by the interface between dome material  1000  and air.  
         [0085]     Note that a conventional LED lamp has a reflective cup  1004  surrounded by dome material, therefore, there are  3  types of totally internal reflections at interfaces: between active layer and substrate, between substrate and dome, and between dome and air, therefore the light extraction efficiency is low.  
         [0086]      FIG. 11   a  is an embodiment of a lamp of white LED assemblies of the present invention. A LED assembly of the present invention comprises epitaxial layer  1104  and active layer  1103  attached on base  1105 . Dome material  1001  covers the LED assembly. Anti-reflection coating  1107  is coated on surface  1106  of dome material  1101 . Epitaxial layer  1104  and active layer  1103  have either the same or similar refractive index, therefore there is no totally internal reflection at the interface between the two. Epitaxial layer  1104  and dome material  1101  doped with nano-particles have either the same or similar refractive index, therefore, the totally internal reflection at the interface between epitaxial layer  1104  and dome material  1101  is eliminated.  
         [0087]     From Snell&#39;s law, it can be shown that when 
 
R≧nd, 
 
 where, R is the diameter of hemisphere-shaped dome, n is the refractive index of dome material, and d is the size of the LED, there is no totally internal reflection at the interface between dome and air. Therefore it is easily to eliminate the totally internal reflection between dome and air by employing large enough hemisphere shaped dome. 
 
         [0088]     Therefore all of three types of the totally internal reflections are completely eliminated.  
         [0089]      FIG. 11   b  is a lamp for white LED assemblies. Transparent cover  1120  seals the lamp. White LED assembly  1117  is disposed on heat sink  1110  which has a neck portion  1112  for holding dome  1118 . Pole  1114  is electrically connected with the LED. Wire bonding  1111  connects LED  1117  to pole  1115  which is through hole  1113 . Cover  1120  sits on reflective cup  1116  which directs emitted light to the desired direction.  
         [0090]     Although the description above contains many specifications and embodiments, these should not be construed as limiting the scope of the present invention but as merely providing illustrations of some of the presently preferred embodiments of the present invention.  
         [0091]     Therefore the scope of the present invention should be determined by the claims and their legal equivalents, rather than by the examples given.