Abstract:
Systems and methods are disclosed for producing vertical LED array on a metal substrate; evaluating said array of LEDs for defects; destroying one or more defective LEDs; forming good LEDs only LED array suitable for wafer level package.

Description:
[0001]     This invention relates to semiconductor manufacture and more particularly to a method and apparatus for manufacturing light emitting diodes (LEDs) array.  
         [0002]     One of the fastest growing segments of the semiconductor industry is the manufacture of multi-chip modules (MCM). Multi-chip modules are being increasingly used in computers to form PC chip sets and in telecommunication items such as modems and cellular telephones. In addition, consumer electronic products such as watches and calculators typically include multi-chip modules.  
         [0003]     With a multi-chip module, non-packaged or LEDs (i.e., chips) are secured to a substrate (e.g., printed circuit board) using an adhesive. Electrical connections are then made directly to the bond pads on each LED and to electrical leads on the substrate.  
         [0004]     In an effort to minimize the cost and maximize the quality of assembled packages, steps are typically taken to ensure that only LEDs which are found to be functional are assembled with one another. Therefore, prior to the LED attachment process, LEDs and carrier substrates are typically tested for optical and electrical defects, contamination, and other irregularities. LEDs in the array are found to be defective are typically marked in a manner so as to distinguish them from known good components.  
         [0005]     Thus, with unpackaged LEDs, semiconductor manufacturers are required to supply LED arrays that have been tested and certified as known good LED (KGD). On a parallel note, light-emitting diodes (LEDs) are playing an increasingly important role in our daily life. Traditionally, LEDs are become ubiquitous in many applications, such as communications and other areas, such as mobile phones, appliances and other electronic devices. Recently, the demand for nitride based semiconductor materials (e.g., having Gallium Nitride or GaN) for opto-electronics has increased dramatically for applications such as video displays, optical storage, lighting, medical instruments, for-example. Conventional blue light-emitting diodes (LEDs) are formed using semiconductor materials of nitride, such as GaN, AlGaN, InGaN and AlInGaN. Most of the semiconductor layers of the aforementioned-typed light emitting devices are epitaxially formed on electrically non-conductive sapphire substrates. Since the sapphire substrate is an electrically insulator, electrodes cannot be directly formed on the sapphire substrate to drive currents through the LEDs. Rather, the electrodes directly contact a p-typed semiconductor layer and an n-typed semiconductor layer individually, so as to complete the fabrication of the LED devices. However such configuration of electrodes and electrically non-conductive nature of sapphire substrate represents a significant limitation for the device operation. For example, a semi-transparent contact needs to be formed on the p-layer to spread out the current from p-electrode to n-electrode. This semi-transparent contact reduces the light intensity emitted from the device due to internal reflectance and absorption. Moreover, p and n-electrodes obstruct the light and reduce the area of light emitting from the device. Additionally, the sapphire substrate is a heat insulator (or a thermal insulator) and the heat generated during the device operation can not be effectively dissipated, thus limiting the device reliability.  
         [0006]      FIG. 1  shows one such conventional LED. As shown therein, the substrate is denoted as  1 . The substrate  1  can be mostly sapphire. Over the substrate  1 , a buffer layer  2  is formed to reduce the lattice mismatch between substrate  1  and GaN. The buffer layer  2  can be epitaxially grown on the substrate  1  and can be AlN, GaN, AlGaN or AlInGaN. Next, an n-GaN based layer  3 , a multi-quantum well (MQW) layer  4 , and a p-GaN layer  5  are formed in sequence. An etching method is employed to form an exposing region  6  on the n-GaN based layer  3 . An electrical conductive semi-transparent coating is provided above the p-GaN layer  5 . Finally, the n-electrode  9  and p-electrode  8  are formed on selected electrode areas. The n-electrode  9  is needed on the same side of device as p-electrode to inject electrons and holes into the MQW active layer  4 , respectively. The radiative recombination of holes and electrons in the layer  4  emits light. However, limitations of this conventional LED structure include: (1) Semi-transparent contact on p-layer  5  is about 70% transparent at best and can block the light emitted from layer  4 ; (2) current spreading from n-electrode to p-electrode is not uniform due to position of electrodes; and (3) heat is accumulated during device operation since sapphire is a thermal and electrical insulator.  
         [0007]     To increase available lighting area, vertical LEDs have been developed. As shown in  FIG. 2 , a typical vertical LED has a substrate  10  (typically silicon, GaAs or Ge). Over the substrate  10 , a transition metal multi-layer  12 , a p-GaN layer  14 , an MQW layer  16 , a n-GaN layer  18  are then formed. The n-electrode  20  and the p-electrode  22  are then formed on selected areas as electrodes.  
         [0008]     US patent Application No. 20040135158 shows one way to realize vertical LED structure by (a) forming a buffering layer over a sapphire substrate; (b) forming a plurality of masks over said buffering layer, wherein said substrate, said buffering layer and said plurality of masks jointly form a substrate unit; (c) forming a multi-layer epitaxial structure over said plurality of masks, wherein said multi-layer epitaxial structure comprises an active layer; extracting said multi-layer epitaxial structure; (d) removing said remaining masks bonding with a bottom side of said multi-layer epitaxial structure after extracting; (e) coating a metal reflector over said bottom side of said multi-layer epitaxial structure; (f) bonding a conductive substrate to said metal reflector; and (g) disposing a p-electrode over an upper surface of said multi-layer structure and an n-electrode over a bottom side of said conductive substrate.  
       SUMMARY  
       [0009]     In one aspect, systems and methods are disclosed for producing vertical LED including forming an array of LEDs on a metal substrate; evaluating said array of LEDs for defects; destroying or removing one or more defective LEDs and then forming arrays containing only good LEDs. These good LEDs only array then can be packaged including at the wafer level to serve the purpose of multi chip power LED device.  
         [0010]     Implementations of the above aspect can include one or more of the following. The destroying includes vaporizing a defective LED, or alternatively includes applying a laser beam on a defective LEDs or using laser cutting to cut thru the metal substrate to remove them. Electrical functionality can be done for testing each of said plurality of LED to identify a satisfactorily nondefective LED. Nondefective LEDs are then supplied in form of array ready for packaging including wafer level packaging.  
         [0011]     Advantages of the system may include one or more of the following. The above system provide manufacturing processes suitable for fabricating and testing or unpackaged vertical LEDs on metal substrate. The present method of manufacturing and checking LEDs is suitable for the burn-in and checking in practice all kind of LEDs, especially the vertical LED on metal substrate described in this invention. It is highly economical because it provides testing before final component fabrication, making the present method highly reliable in comparison with conventional methods. In addition to enhancing the standard manufacturing testing for LEDs, the system can be a major development for producing good LEDs array Such improvements can improve-packaging assembly, screening, and assembly yields, dramatically reducing costs. Additionally, overall product failure rates can potentially be reduced, thereby improving system and life cycle costs, minimizing program delays and cost associated with component fails late within system integration.  
         [0012]     These and other embodiments, aspects, advantages, and features of the present invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art by reference to the following description of the invention and referenced drawings or by practice of the invention. The aspects, advantages, and features of the invention are realized and attained by means of the instrumentalities, procedures, and combinations particularly pointed out in the appended claims.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     To better understand the other features, technical concepts and objects of the present invention, one may clearly read the description of the following preferred embodiments and the accompanying drawings, in which:  
         [0014]      FIG. 1  shows a prior art conventional LED.  
         [0015]      FIG. 2  shows a prior art vertical LED.  
         [0016]      FIGS. 3-8  show operations in an exemplary process to fabricate vertical LED on metal substrate.  
         [0017]      FIG. 9  shows the way how to remove a bad LED using laser beam. The bad LED is either evaporated or removed by cutting through the metal substrate along the kerfs bordering the bad LED.  
         [0018]      FIG. 10  shows the empty space where the bad LED was positioned before removal.  
     
    
     DESCRIPTION  
       [0019]     In reading the detailed description, the accompanying drawings may be referenced at the same time and considered as part of the detailed description.  
         [0020]     Referring to FIGS.  3  to  8 , a manufacturing method for vertical LEDs on metal substrate is illustrated therein. In the description, the reference numerals given for the inventive device structure will be also used in the recitation of the steps of the inventive manufacturing method.  
         [0021]     The process described below is for one embodiment with InGaN LEDs initially grown on sapphire. Electro or Electroless Chemical plating is then used to deposit a thick metal substrate for electrical and thermal conduction for the resulting LED device. Electro or Electroless Chemical plating is used in lieu of wafer bonding. The process can be applied to any optoelectronic device where bonding was used to attach the epilayer to a new host substrate for improvement of optical, electrical and thermal properties.  
         [0022]     Turning now to the diagrams,  FIG. 3  shows a multi-layer epitaxial structure of an exemplary InGaN LED on a carrier  40 , which can be a sapphire substrate in one embodiment. The multi-layer epitaxial structure formed above the sapphire substrate  40  includes an n-GaN based layer  42 , an MQW active layer  44  and a contact layer  46 . The n-GaN based layer  42  having a thickness of about 4 microns, for example.  
         [0023]     The MQW active layer  44  can be an InGaN/GaN (or AlInGaN/GaN) MQW active layer. Once an electric power is fed between the n-GaN based layer  42  and the contact layer  46 , the MQW active layer  44  may be excited and thus generates a light. The produced light can have a wavelength between 250 nm to 600 nm. The p-layer can be a p + -GaN based layer, such as a p + -GaN, a p + -InGaN or a p + -AlInGaN layer and the thickness thereof may be between 0.01-0.5 microns.  
         [0024]     Next, as shown in  FIG. 4 , a mesa definition process is performed and p-type contacts  48  are formed above the contact layer  46 . The contacts  48  above the multi layer epitaxial structure can be Indium Tin Oxide (ITO), Ag, Al, Cr, Ni, Au, Pt, Pd, Ti, Ta, TiN, TaN, Mo, W, a refractory metal, or a metal alloy, or a composite of these materials (for example Ni/Au), among others. In addition, direct reflected Ag deposition as a metal contact could be also formed. In  FIG. 4 , individual LED devices are formed following mesa definition. Ion coupled plasma etching is used to etch GaN into separate devices.  
         [0025]     Next, as shown in  FIG. 5 , a passivation layer  50  is deposited and reflective metal deposition is performed to form a reflective metal  52  such as Al, Ag, Ni, Pt and Cr, among others, in a window etched into the passivation layer  50  to allow the reflective metal  52  to contact layer  46 . The passivation layer  50  is non-conductive. The reflective metal  52  forms a mirror surface.  
         [0026]      FIG. 6  shows that a thin metal layer or a multi-metal layer  53  (Cr, Pt, Pd, Pt/Au, Cr/Au, Ni/Au, Ti/Au, TaN/Au among others) is deposited over the structure to serve as a barrier/seed layer for the electro/electroless chemical plating process. However the depositing operation is not needed if an electroless chemical process, sputtering or magneto-sputtering process is used in lieu of electroplating. A metal substrate layer  60  is deposited thereon.  
         [0027]     Turning now to  FIG. 7 , the multi-layer epitaxial structure is coated with a metal plating layer  60  using techniques such as electro and electroless chemical plating. With electroless chemical plating, the sapphire substrate  40  is protected using a organic or polymer layer or a coating that can be easily removed without damaging the sapphire or the electroless chemical plated metal of a relatively thick metal such as Ni, Cu, Ag, W, Mo, Pd, Pt, among others.  
         [0028]     Next, the sapphire substrate  40  is removed. In one embodiment shown in  FIG. 8 , a laser lift-off (LLO) operation is applied to the sapphire substrate  40 . Sapphire substrate removal using laser lift-off is known, reference U.S. Pat. No. 6,071,795 to Cheung et al, entitled, “Separation of Thin Films From Transparent Substrates By Selective Optical Processing,” issued on Jun. 6, 2000, and Kelly et al. “Optical process for liftoff of group III-nitride films”, Physica Status Solidi (a) vol. 159, 1997, pp. R3-R4). Furthermore, highly advantageous methods of fabricating GaN semiconductor layers on sapphire (or other insulating and/or hard) substrates are taught in U.S. patent application Ser. No. 10/118,317 entitled “A Method of Fabricating Vertical Devices Using a Metal Support Film” and filed on Apr. 9, 2002 by Myung Cheol Yoo, and in U.S. patent application Ser. No. 10/118,316 entitled “Method of Fabricating Vertical Structure” and filed on Apr. 9, 2002 by Lee et al. Additionally, a method of etching GaN and sapphire (and other materials) is taught in U.S. patent application Ser. No. 10/118,318 entitled “A Method to Improve Light Output of GaN-Based Light Emitting Diodes” and filed on Apr. 9, 2002 10 by Yeom et al., all of which are hereby incorporated by reference as if fully set forth herein. As shown in  FIG. 8 , an n-type electrode  70  such as Cr/Ni is patterned on the top of n-GaN layer  42  to complete the vertical LED.  
         [0029]     At this stage, all the LEDs on metal substrate are probed and mapped for defects. Wafer mapping is performed to test the gross functionality of the LEDs on the wafer. Normally for each LED, wavelength, brightness, forward voltage at certain driving current and leakage current at certain reverse bias are recorded in the mapping data. The nonfunctional LEDs are mechanically marked or mapped in software. Positions of each bad LED could be traced back for the purpose of removing them for subsequent separation into known-good LEDs only arrays.  
         [0030]     A laser is used to destroy non-functioning LEDs. As shown in the example of  FIG. 9 , the middle LED is defective and the laser burns up the middle LED structure prior to the next operation. The laser can be an UV-Diode Pump Solid State (DPSS) laser or excimer laser with wavelength at 266 nm or 355 nm or 248 nm, for example. All laser wavelengths are strongly absorbed by GaN and any metal used as metal substrate. This total absorption results in total energy transfer from laser pulse into the defective LED and elevate its temperature to higher temperature than evaporation temperature of GaN. When burning a defective LED on the wafer level, a laser pulse is radiated and repeated until the LED is totally evaporated.  
         [0031]     A second way to remove a bad LED or a cluster of bad LEDs is using the laser beam to cut the LED off the metal substrate. In this case the laser beam is guided along kerfs bordering the LED. The laser beam can be stationary while the metal substrate moves to achieve the same effect. A third way to remove a bad LED or a cluster of bad LEDs is using diamond saw cut.  
         [0032]     In addition to performing the functions outlined above, the system may include computer hardware and software capable of monitoring, controlling and collecting process data. This data collection capability allows process monitoring and permits real-time traceability of devices. This permits faster internal process feedback specific to device performance to be generated without introducing final packaging process variations.  
         [0033]     In association with vertical LED on metal substrate, this invention can be used for producing known good LED (KGD) arrays, ready for wafer level packaging.  
         [0034]     While the invention has been described with reference to certain preferred embodiments, as will be apparent to those skilled in the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims.