Abstract:
A wire-bonding free packaging structure for light emitting diode (LED) is provided. Prepare a silicon sub-mount having a backside bulk micromachining reach-through U-shape cavity for accommodating a flip-chip LED. This stack-integrated packaging module with solder bumps on the surface is than bonded to an aluminum PC board with flip-chip surface mount packaging or bump technology. This gives very good heat conduction to the heat sink of the PC board and can endure more current to enhance light intensity of the LED. This stack-integrated packaging module can also be bonded on a lead frame with two leg packaging, which can also increase heat conduction.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a packaging structure of light emitting diode (LED). In particular, relates to a wire-bonding free packaging structure for LED mounted into a sub-mount.  
         [0003]     2. Description of the Related Art  
         [0004]     As a good light source and device made by III-V or II-VI semiconductor material, LEDs possesses advantages of small size, long life time, low driving voltage, rapid response and good oscillation-proof, etc.  
         [0005]     By changing the semiconductor materials and device structure of LEDs, different visible and invisible light can be achieved, wherein AlGaAs, InGaAlP and InGaN are suitable for producing LEDs with high luminance of more than 1000 mcd.  
         [0006]     In order to increase the light intensity of a LED, the compound materials used has widely studied, also the structure of the device can be modified such as Double Hetero Junction (DH), quantum well (QW) or multi-quantum wells (MQW), etc., the intensity has increased more than  1  order these years, hence the application area of LED is more and more, from indicator to traffic signal, light source of LED printer head, LED display, even LED illumination. However, the light intensity is limited to junction breakdown, which is mostly due to over heating of the junction. It results that heat transfer becomes very important for enhancing light intensity of a LED.  
         [0007]     Package structure has affected strongly on heat transfer of a LED. It is commonly bonding the LED chip on a lead frame by die bonding, then connect the positive and negative electrode to the positive and negative legs, respectively. It results not only the route of heat transfer is too long, but also the conduction area of the gold wire is too small, thus results very bad heat transfer. In operation, the maximum endure current is choose to balance the heat produced by operation and the heat transfer. This maximum current limits the maximum light intensity. This is the cause that LED is still difficult to be used in illumination, such as the head light of a car, room illumination, etc. Thus it is still need a LED light source to replace the electric light bulb or fluorescent lamp with high energy consumption.  
         [0008]     It is then further need to improve the brightness or light intensity other than discovering of new materials or active area structure of LED device, package technique is also an important area. How to improve the heat transfer of LED packaging, such that the current of operation can be increased and would not cause breakdown or burn out of the PN junction, so that the light intensity may improve and the brightness may increase with the same LED device structure and material used.  
         [0009]     Therefore there exists a need to improve significantly the packaging technique, so that the heat transfer capability can be improved to increase the light intensity. The present invention will give a solution to meet this requirement.  
       SUMMARY OF THE INVENTION  
       [0010]     The object of the present invention is to provide a wire-bonding free packaging structure for light emitting diode (LED). A silicon sub mount with a reach-through U-shape cavity is used to accommodate a flip-chip LED, and form a stack-integrated packaging module with solder bump on the surface, the module is then bonded to an aluminum PC board with flip-chip surface mount packaging technology, thus the LED will have very good heat transfer and the light intensity will be enhanced.  
         [0011]     Another object of the present invention is to provide a wire-bonding free packaging structure for light emitting diode (LED). A silicon sub mount with a reach-through U-shape cavity is used to accommodate a flip-chip LED, and form a stack-integrated packaging module with solder bump on the surface, the module is then bonded to a common lead frame with flip-chip surface mount packaging technology, thus the LED will have good heat transfer and the light intensity will be enhanced.  
         [0012]     In order to achieve the above objects, a first aspect of the present invention teaches a packaging structure of light emitting diode (LED), the LED chip is bonding into the U-shape cavity of a silicon sub-mount by flip-chip bonding to form a cascaded packaging module, this module is then packaged by flip-chip surface mount on an aluminum PC board with heat-sink, This including a silicon sub-mount, forming solder bumps of positive and negative electrode on the front side; then by etching to form a reach-through U-shape cavity on the back-side to accommodate the LED chip. A positive electrode, a negative electrode, and reflective metals are evaporated with a native mask; A light emitting diode (LED) chip, can be any chip produced by a conventional technology, having a substrate, an active light emitting area, a positive and a negative electrode on the front-side is used to form the module. A PC board, having an anodic oxide layer, a printed circuit, and a heat-sink device, is used to bond the module. The LED chip is bonding into the silicon sub-mount by flip-chip die-bonding, the positive and negative electrodes of the LED chip are aligned to the positive and negative electrodes of the silicon sub-mount, respectively, to form a cascaded packaging module. Then bonds the cascaded packaging module to the PC board by flip-chip surface mount, and finally forms a micro lens on the surface of the LED. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]      FIG. 1  (A) is the step of forming an align mark and contact via holes in cross sectional view.  
         [0014]      FIG. 1  (B) is a cross sectional view of the steps for forming solder bumps.  
         [0015]      FIG. 1  (C) is a cross sectional view of the step for forming a U-shape cavity.  
         [0016]      FIG. 1  (D) is an example of a third mask.  
         [0017]      FIG. 1  (E) is another example of a third mask.  
         [0018]      FIG. 1  (F) is a cross sectional view of the step for performing aluminum evaporation.  
         [0019]      FIG. 1  (G) is a cross sectional view of the silicon sub-mount after evaporation.  
         [0020]      FIG. 2  is a cross sectional view of the LED chip.  
         [0021]      FIG. 3  (A) is a cross sectional view of a hybrid packaging module after bonding the LED chip into the U-shape cavity of the silicon sub-mount by flip-chip packaging.  
         [0022]      FIG. 3  (B) is the relative position between the positive and negative metal electrodes of the silicon sub-mount and the positive and negative electrodes of the LED chip in accordance with one embodiment of the present invention.  
         [0023]      FIG. 3  (C) is the relative position between the positive and negative metal electrodes of the silicon sub-mount and the positive and negative electrodes of the LED chip in accordance with another embodiment of the present invention.  
         [0024]      FIG. 4  is a cross sectional view of forming a focus lens with transparent polymer material.  
         [0025]      FIG. 5  is a cross sectional view of flip-chip bonding the module of the LED chip and the silicon sub-mount to an aluminum PC board.  
         [0026]      FIG. 6  is a cross sectional view of flip-chip bonding the module of the LED chip and the silicon sub-mount to an ordinary PC board.  
         [0027]      FIG. 7  shows the structure of a display in according to one embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     The foregoing and other advantages of the invention will be more fully understood with reference to the description of the best embodiment and the drawing as followed description.  
         [0029]     The manufacturing procedure of the packaging structure in according to the present invention can be understood by referring to  FIG. 1  through  FIG. 7 .  FIG. 1  illustrates the manufacturing steps of a silicon sub-mount in according to one embodiment of the present invention. First, as shown in  FIG. 1  (A),  FIG. 1  (A) is the step of forming an align mark and contact via holes in cross sectional view. Prepare the p-type silicon wafer  102  which is ( 100 ) orientation, any doping, even a reclaimed substrate. A layer of silicon nitride  104  is deposited on both sides of the wafer by LPCVD. On the front side, a first mask is used in lithography to form a negative via hole  106 , a positive via hole  108  and an align mark for the stepper. Refer to  FIG. 1  (B).  FIG. 1  (B) is a cross sectional view of the step for forming solder bumps. A negative solder bump  116 , a positive solder bump  118 , a seal  114  of the align mark  110  for the stepper and a back side align mark (BSA)  112  are formed by using a second mask in lithography and etching, or by electroplating copper/tin. Refer to  FIG. 1  (C).  FIG. 1  (C) is a cross sectional view of the step for forming a U-shape cavity. An etch-window  120 , a window for negative electrode area  120 - 1 , a window for positive electrode area  120 - 2  is opened by using a third mask in lithography and etching for the next step to perform reach-through etching and form a U-shape cavity. Now a native shadow mask  122  is remained to form an isolation mask in deposition of aluminum metal, thus avoid an etching step. Then perform an anisotropic bulk micromachining etching to form a reach-through U-shape cavity  121  in the silicon wafer. This U-shape cavity  121  will be used to accommodate a LED chip. Now the silicon nitride layer  104  on the front side of the silicon wafer forms a membrane to support the solder bumps  116  and  118 . The silicon nitride layer of the native shadow mask  122  on the rear side of the silicon wafer forms a membrane to be a mask in deposition of aluminum.  FIG. 1  (D) is an example of a third mask. This mask has a native shadow mask  122 , reflection metal area  120 , negative electrode area  120 - 1  and positive electrode area  120 - 2 .  FIG. 1  (E) is another example of a third mask. This mask has a native shadow mask  122 , reflection metal area  120 , a round shaped negative electrode area  120 - 1  and a round shaped positive electrode area  120 - 2 . The round shaped electrodes are used to form a cylinder negative electrode and a cylinder positive electrode to eliminate the effect of thermal expansion on the package. Refer to  FIG. 1  (F).  FIG. 1  (F) is a cross sectional view of the step for performing aluminum evaporation. The evaporation is preferred for E-gun evaporation and can not use sputtering or chemical vapor deposition, otherwise cross deposition will cause the electrodes connected together and isolation will fail. E-gun evaporation is a point source. The direction of evaporation is  128  only and form isolated positive electrode  124 , negative electrode  126  and a reflection metal mirror  130 , no other direction. The evaporated aluminum  129  will stay on the shadow mask  122 , under the shadow area of the native shadow mask  122  would not evaporate and no aluminum there. As shown in  FIG. 1  (G),  FIG. 1  (G) is a cross sectional view of the silicon sub-mount after evaporation. A reflection metal mirror  130  is formed under the reflection metal area  120  of the third mask, a negative electrode  124  is formed under the negative electrode area  120 - 1  of the third mask, a positive electrode  126  is formed under the positive electrode area  120 - 2  of the third mask, an isolation  104  is formed under the native shadow mask  122  of the third mask. After evaporation, the native shadow mask  122  can be removed by mechanical method. The silicon sub-mount  100  with U-shape cavity for accommodating a LED chip is then completed.  
         [0030]     Refer to  FIG. 2 .  FIG. 2  is a cross sectional view of the LED chip. The LED chip such as red, blue, green or other color LED is produced by a traditional technique. The substrate of a LED chip  200  is sapphire or other substrate like GaAs. There is an active light emitting area form by a PN junction or quantum well. A positive electrode  208  is formed on the P-type layer. A negative electrode  206  is formed on the N-type layer with the P-type material removed by etching. Thus form a flip-chip condition.  
         [0031]     Refer to  FIG. 3  (A).  FIG. 3  (A) is a cross sectional view of a hybrid packaging module after bonding the LED chip into the U-shape cavity of the silicon sub-mount by flip-chip packaging. Upside down the LED chip  200 , so that the positive electrode  208  of the LED is aligned to the positive electrode  126  of the silicon sub-mount  100 , the negative electrode  206  of the LED is aligned to the negative electrode  124  of the silicon sub-mount  100 , then forms a hybrid packaging module by thermal bonding. The negative solder bump  116 , positive solder bump  118  of the silicon sub-mount can be packaged on the PC board by flip-chip bonding. The light  302  emitted from the LED will transmit through the transparent substrate  602 . The light  304  transmit to the reflection metal mirror  130  will reflect out to enhance the brightness.  FIG. 3  (B) is the relative position between the positive and negative metal electrodes of the silicon sub-mount and the positive and negative electrodes of the LED chip in accordance with one embodiment of the present invention. The positive electrode  124  and the negative metal electrode  126  of the silicon sub-mount are inter-digital electrodes with larger area, while the positive electrode  208  and the negative electrode  206  of the LED are inter-digital electrodes with narrower area.  FIG. 3  (C) is the relative position between the positive and negative metal electrodes of the silicon sub-mount and the positive and negative electrodes of the LED chip in accordance with another embodiment of the present invention. The positive electrode  124  and the negative metal electrode  126  of the silicon sub-mount are cylinder array electrodes, while the positive electrode  208  and the negative electrode  206  of the LED are inter-digital electrodes with narrower area.  
         [0032]     Refer to  FIG. 4 ,  FIG. 4  is a cross sectional view of forming a focus lens with transparent polymer material. Drop transparent polymer  402  into the gap of the U-shape cavity  121  to make the LED chip  200  integrates with the silicon sub-mount  100 . In order to focus the light in front of the LED, the transparent polymer material forms a micro lens  404 , this micro lens may be a semi-sphere or paraboroid to form a focus lens, such that the light may be focused and transmit forwardly. Finally, scribe the 300 μm of the transparent substrate  602 , the silicon nitride  104  on the front side and the rear side, the negative solder bumps  116  and positive solder bumps  118  on the rear side by a scriber to cut the wafer into chips.  
         [0033]     Refer to  FIG. 5 ,  FIG. 5  is a cross sectional view of flip-chip bonding the module of the LED chip and the silicon sub-mount to an aluminum PC board. Aluminum PC board is used in this years for its advantage of good heat transfer. An aluminum PC board  502  with a layer of native aluminum oxide  506  formed natively or by anodic treatment to be an isolation layer. Then forming printed circuits on the native aluminum oxide  506 , such as positive electrode circuit  508 , negative electrode circuit  518 , both are thick film copper circuit. There are heat sink device on the back side of the aluminum PC board  502 , such as multiple of extended fins  504 . The solder bump  118  and  116  of the module of the LED chip  200  and the silicon sub-mount  100  is then clip-chip bonding to the positive electrode circuit  508  and the negative electrode circuit  518  of the aluminum PC board  502 , the positive and negative electrodes are connected to the positive and negative electric source (not shown) of the control circuit (not shown) by bonding pads  510  and  512 , bonding wires  514  and  516 . LED is then emitting light under control. Since the distance from the PN junction to the heat sink is very short, it results very good heat transfer, and endure more current as compare to the conventional package without significantly temperature rising. Thus increases the light intensity.  
         [0034]     Refer to  FIG. 6 ,  FIG. 6  is a cross sectional view of flip-chip bonding the module of the LED chip and the silicon sub-mount to an ordinary PC board. The module is packaging on an ordinary PC board  602  with metal via holes  604  on. The solder bump  118  and  116  of the module of the LED chip  200  and the silicon sub-mount  100  is then flip-chip bonding to the positive electrode circuit  608  and the negative electrode circuit  618  of the aluminum PC board  602 . The LED chip  200 , the silicon sub-mount  100  and the PC board  602  is then bonded on the aluminum heat sink device  620 . There are multiple of extended fins  624  on the aluminum heat sink device  620 . The positive and negative electrodes of the PC board are connected to the positive and negative electric source (not shown) of the control circuit (not shown) by bonding pads  610  and  612 , bonding wires  614  and  616 . LED is then emitting light under control. The metal via holes  604  also conducts heat quickly, also results very good heat transfer, and endure more current as compare to the conventional package without significantly temperature rising. Thus increases the light intensity.  
         [0035]     The flip-chip bonding module of the LED chip and the silicon sub-mount can also bond on a common lead frame. By using the limited ability of heat transfer, the light intensity can also be increased.  
         [0036]     Finally, refer to  FIG. 7 ,  FIG. 7  shows the structure of a display in according to one embodiment of the present invention. Reach through U-shape cavity array is formed by etching on a silicon substrate. Then packages the red, yellow and blue LED chips  702 ,  704  and  706  with flip chip packaging into the U-shape cavity array to form a structure of display.  
         [0037]     Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.