Patent Document

CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International Application No. PCT/SG2013/000542 filed Dec. 19, 2013, claiming priority based on U.S. Provisional Application No. 61/757,931, filed Jan. 29, 2013, and U.S. Provisional Application No. 61/847,295, filed Jul. 17, 2013, the contents of each of which are incorporated herein by reference. 
    
    
     FIELD 
     This invention relates to method of fabricating semiconductor device&#39;s. It is particularly, though not exclusively, applicable to fabrication of vertical LEDs. 
     BACKGROUND 
     Semiconductor devices are ubiquitous in modern society and semiconductor manufacturers, for example manufacturers of solid state lighting devices, are constantly seeking to improve the performance of their products. 
     Recently, devices based on gallium nitride (GaN) have found a wide range of application. In particular, high brightness LEDs based on GaN/InGaN have been widely used, for example in backlighting of LCDs, traffic signals, full color displays and street lights. GaN/InGaN LEDs have also recently started to enter the general lighting market. 
     In order to be more effective in general lighting applications, the performance of InGaN/GaN LEDs has to be further improved. For instance, it is generally thought that the power conversion efficiency of GaN/InGaN LEDs during high power operation must be greatly increased (to at least 50%) in order for them to replace current fluorescent lamps (power conversion efficiency ˜20%) and provide the benefits of better consumer experience and cost effectiveness. 
     Typically, high power LED devices are grown on a sapphire substrate. Sapphire substrate-based LEDs have certain disadvantages which limit the degree to which their power conversion efficiency can be improved. Due to the electrically insulating nature and poor thermal conductivity (41.9 W/(m·K)) of the substrate, sapphire-based LEDs generally suffer from poor light extraction, poor thermal dissipation, high junction temperature (&gt;100° C.) and large efficiency droop with increasing junction temperature (&gt;40%). These drawbacks provide serious difficulties for further improving the LED efficiency under high power operating conditions. 
     To attempt to overcome some of those difficulties, the vertical LED concept has previously been proposed. The principle of the proposal is to remove the sapphire substrate and attach the LED to a substitute substrate which has good electrical and thermal conductivities. The substitute substrate serves as an electrode to conduct current, and as an effective heat dissipation path. 
     Previous methods of fabricating semiconductor devices have implemented a vertical device topology by various means. Typically prior art processes involve the process of final substrate dicing or scribing/cracking. Since in most cases the final substrates are made of metals, the dicing/scribing process may cause metal contamination of the LED devices. This may give rise to leakage current, and may cause device failure or reliability issues. For processes in which the whole LED wafer is attached to a metal final substrate, the mismatch between the LED wafer and the metal substrate may cause large stress generation and wafer bowing after the removal of the original growth substrate, thus potentially giving rise to device failure and reliability issues. 
     There may be a need for a method of fabricating semiconductor devices which can alleviate one or more of the above-mentioned difficulties, or at least provide a useful alternative. 
     SUMMARY 
     In general terms, the present invention proposes methods of fabricating semiconductor devices without dicing or scribing. 
     According to a first specific expression of the invention, there is provided a method of fabricating semiconductor devices, the method comprising:
         providing a device layer on a substrate, the device layer comprising a semiconductor material;   masking a portion of the device layer or the substrate;   depositing a conductive material on an unmasked portion to form a plurality of conductive substrates on the device layer, each conductive substrate being in electrical contact with a respective portion of the device layer; and   separating the device layer into a plurality of semiconductor devices, wherein the separation is substantially within the masked portion.       

     According to a second specific expression of the invention, there is provided a method of fabricating vertical LEDs, comprising:
         providing an LED wafer;   defining dies on the wafer using a thick patterning structure with a thickness in the range 10 μm to 500 μm;   depositing metal islands at die level on the wafer using the patterning structure; and   separating the dies automatically or by applying a slight mechanical force on the wafer.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a flow chart of a semiconductor fabrication process according to a first embodiment; 
         FIGS. 2 to 11  show a layered semiconductor-containing structure at various stages during the process of  FIG. 1 ; 
         FIG. 12  is a flow chart of a semiconductor fabrication process according to a second embodiment; and 
         FIGS. 13 to 23  show a layered semiconductor-containing structure at various stages during the process of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments are described with reference to their application to fabrication of vertical LEDs. However, it will be appreciated that the methods described below can be used (with any required adaptations being straightforward for the skilled person) for fabrication of many types of semiconductor device, including but not limited to: optoelectronic devices such as high power LEDs, photodetectors, laser diodes; and microelectronic devices such as bipolar transistors. 
     Referring to  FIGS. 1 to 11 , a first embodiment of a semiconductor fabrication process comprises providing an LED layer  220  on a growth substrate  210  (step  10 ), as shown in the cross-sectional view of  FIG. 2 . The LED layer  220  comprises a layer of a p-type material, a layer of an n-type material, and an active light-emitting layer between the n-type and p-type layers. In one example, the p-type material is p-GaN, the n-type material is n-GaN, and the active layer comprises an InGaN/GaN multiple quantum well. In the following discussion it will be convenient to refer to the process of  FIG. 1  as applied to this exemplary configuration. It will be appreciated, however, that the process has general application regardless of the precise nature of the materials of LED layer  220 . The growth substrate  210  will typically be sapphire, and LED layer  220  may be grown epitaxially on the sapphire substrate  210  in a manner known in the art. 
     As shown in  FIG. 2 , LED layer  220  is disposed on the substrate  210  such that n-GaN surface  224  faces the substrate  210  and p-GaN surface  222  faces away from it. In other embodiments, the LED layer  220  may be grown on substrate  210  with the opposite orientation, i.e. with p-GaN surface  222  facing substrate  210 . 
     At step  15 , a metallic layer  230  is applied to the p-GaN surface  222  of LED layer  220 . Metallic layer  230  acts as an ohmic contact layer. The p-GaN surface  222  of LED layer  220  may be pre-cleaned using organic solvents and/or acids before the metal contact layer  230  is deposited. The metal contact layer  230  may include Ni/Au, Ni/Ag, Pt, Pd, W, Mo, Ta, TaN, a refractory metal, a metal alloy, ITO or any other suitable metals, or a composite of these materials. The thickness of the metal contact layer  230  is typically in the range from about 3 nm to about 20 nm. 
     The deposition of the metal contact layer  230  can be performed using a number of deposition methods, including but not limited to: electron beam deposition, sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma-enhanced CVD (PECVD), ion beam deposition, or electro-chemical deposition. 
     In some embodiments, the metal contact layer  230  may be subjected to annealing at a temperature ranging from about 400° C. to 700° C. The annealing may be conducted in N 2  ambient, in N 2 /O 2  ambient, or in air. The annealing can be carried out for a time in the range from about 1 minute to about 10 minutes. 
     At step  20 , a mirror layer  240  is deposited on top of the metal contact layer  230  ( FIG. 3 ). The mirror layer  240  acts to increase the light output by the LED, and preferably has a reflectance of 90% or more. The mirror layer  240  may comprise a metal selected from the group consisting of Al, Ag, Ti, Pt, Cr, and Pd. The thickness of the mirror layer  240  is typically in the range from about 50 nm to about 200 nm. 
     In some embodiments, a seed metal layer (not shown) may be deposited onto the mirror layer  240 . The seed layer is useful to enhance the strength of adhesion to a subsequently deposited metal layer, for example. The seed layer materials may be selected from the group consisting of Ni, W, Au, and TaN. The thickness of the seed layer may be in the range from about 10 nm to about 10 um. Advantageously, the seed layer may be made relatively thick to ensure reliable bonding between the mirror layer and the subsequently deposited metal layer, or if no mirror layer is present, between the contact layer and a subsequently deposited metal layer. In this regard, preferred thicknesses are in the range from about 1 um to about 10 um. 
     The mirror layer  240  and the seed layer (if present) can be deposited using a variety of different deposition methods, including but not limited to: electron beam deposition, sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma-enhanced CVD (PECVD), ion beam deposition, or electro-chemical deposition. 
     LED layer  220 , contact layer  230  and mirror layer  240  can collectively be termed a device layer, since they will form part of the final LED devices once processing is complete. The device layer, as referred to herein, may also include other components such as additional contact layers. 
     At step  25 , a patterning structure  250  is applied to mirror layer  240  ( FIG. 4( b ) ). The patterning structure  250  comprises a series of walls  255  laid out in a grid pattern, as best shown in the top plan view of  FIG. 4( a ) . The walls  255  define partitions  258 , each of which is aligned with and overlies a portion of the LED layer  220 . The positions of walls  255  coincide with the boundaries between LED dies as will later be described. Accordingly, the patterning structure  250  comprises a series of partitions or voids  258  separated by walls  255 , the partitions or voids  258  being laid out in a desired pattern according to which material is to be deposited on the device layer. 
     Patterning structure  250  preferably comprises an electrically insulating material, and in certain embodiments, can be applied using photolithography. In one example, a layer of photoresist is spin-coated onto the surface of the mirror layer  240 . The photoresist layer thickness may be in the range from about 10 μm to about 500 μm. Following spin coating, the photoresist is exposed through a mask such that regions of the photoresist corresponding to partitions  258  can be removed, leaving sidewalls  255  of patterning structure  250 . 
     Other methods of applying the patterning structure  250  are also possible. For example, a layer of material may be applied, and then selectively removed by masked etching or imprinting. Exemplary imprinting methods include hot embossing methods, in which a mould carrying the inverse of the desired pattern is heated and pressed into a polymer layer (applied by spin coating, for example), or soft embossing (e.g., UV embossing) methods in which a mould is pressed into a radiation-curable material (e.g., an oligomer or polymer precursor) while the material is soft, and the radiation-curable material then cured with the mould in place to produce the desired structure. In some examples a transparent mould may be used, and the curing performed through the transparent mould. 
     In other embodiments, the sidewalls  255  may be applied directly by deposition of material through a mask. 
     In yet further embodiments, the patterning structure  250  may be prefabricated with the desired pattern, and then releasably secured or otherwise applied to the mirror layer  240 . 
     In certain embodiments, the partitions  258  are square in top plan view, and may have a width in the range between about 100 μm and 2 mm. A wide variety of other top plan view shapes, for example rectangles, triangles, hexagons or circles, are also possible depending on the shape which is desired for the final substrate for the LED dies. 
     After the application of the patterning structure  250 , the partitions  258  are filled in (step  130 ) by deposition of a metallic material (step  30 ). Following the metal layer deposition, islands  260  of metal are formed on the device layer, specifically on mirror layer  240 . The metal islands  260 , when the LED dies are separated from each other, form conductive substrates for the respective LED dies. Each metal island  260  serves as an electrode as well as a heat sink. Accordingly, the metal islands  260  should preferably have both excellent thermal conductivity and excellent electrical conductivity. Copper, silver and other metals possessing these properties can be used to form the metal islands  260 . For example, copper electroplating can be used to plate copper into the partitions  258 . It will be appreciated that the metal islands  260  can also be deposited using other metal deposition methods such as electron beam evaporation, thermal evaporation, PVD, CVD sputtering deposition, etc. The thickness of the metal layer used to form the islands  260  may be in the range between about 10 μm and 500 μm. 
     After the metal island plating  130 , the patterning structure  250  can be removed (step  35 ) to leave the metal islands  260 , as shown in the top plan view of  FIG. 6( a )  and the cross-sectional view of  FIG. 6( b ) . 
     At step  40  the semiconductor-containing structure is bonded to a temporary supporting carrier  275  using temporary bonding adhesive  270  as shown in  FIG. 7 . The temporary supporting carrier  275  acts as a supporting structure when the original sapphire substrate  210  is removed. The temporary supporting carrier  275  can be any suitable material which provides the necessary mechanical support for the device layer. Particular examples of suitable materials are Si, quartz, glass, and sapphire. The bonding adhesive  270  should be strong enough to bond the device layer to the temporary substrate  275 , and should also be readily removable at completion of the process. In certain embodiments the adhesive  270  can be a low melting temperature adhesive, for example a low melting point metal, and the temporary substrate  275  can be removed by heating. The temporary substrate  275  may alternatively be removed by exposing adhesive  270  to a suitable solvent, for example. 
     At step  45 , the sapphire substrate  210  is removed. A preferred removal method is UV laser liftoff, in which the LED layer  220  is irradiated with a UV laser  300  from the substrate side (since the sapphire substrate  210  is UV-transparent). In preferred embodiments, the entire LED layer  220  is irradiated in order to remove the substrate  210 , for example by scanning across the wafer as described below. 
     Laser liftoff relies on the fact that at sufficiently high temperatures, the stability of GaN is limited by the decomposition of the crystal into nitrogen gas and liquid gallium: 2GaN (s)→N2 (g)+2Ga (1). The flux of nitrogen molecules leaving the crystal surface in vacuum exponentially increases with temperature when the temperature exceeds the critical sublimation temperature of 830° C. The decomposition rate reaches approximately one monolayer per second at a temperature of 930° C. Thus, GaN can be removed very efficiently via thermal decomposition by methods which enable a controlled local heating of the sample to temperatures above 900° C. 
     One way to locally decompose GaN is by absorption of intense light with photon energies above the bandgap of GaN (3.42 eV), e.g. the 355 nm (3.49 eV) third harmonic of a Nd:YAG pulsed laser with a pulse width of t=6 ns, or the 248 nm (4.99 eV) line of a KrF excimer laser with a pulse width of t=38 ns. Because of the much longer pulse duration in the case of the KrF laser, a higher pulse energy of typically 600 mJ/cm 2  is necessary to heat the GaN above the sublimation threshold, whereas pulse energies of 300 mJ/cm 2  are sufficient in the case of the Nd:YAG laser, as disclosed in U.S. Pat. No. 6,071,795 (Cheung et al); Kelly et al. (1997), “Optical Process for Liftoff of Group III-Nitride Films”,  Physica Status Solidi  ( a ) Vol. 159, pp. R3-R4; and Wong et al., “Ubiquitous Blue LEDs: The Integration of GaN Thin Films with Dissimilar Substrate Materials by Wafer Bonding and Laser Lift-off”,  Compound Semiconductor , November/December 1999, pp. 54-56; the contents of each of which are incorporated herein by reference. 
     In certain embodiments, a high power UV laser beam is patterned into a 3 mm by 3 mm square beam using a beam homogenizer. The beam homogenizer converts a Gaussian-like laser beam to a flat plateau-like laser beam which provides improved beam uniformity. Large areas may be exposed by scanning the laser beam across the whole wafer. The laser output power can be varied using attenuators. The UV laser is typically pulsed in the range of 1˜10 Hz with one pulse typically being sufficient to achieve decomposition of the GaN layer. The patterned laser beam  300  is directed onto the LED layer  220  through the sapphire substrate  210  side and is scanned across the whole wafer  220 . When the laser power density is larger than a critical value, the GaN layer near the interface of sapphire/GaN will be decomposed into Ga metal and nitrogen. The sapphire substrate  210  can be removed directly, or after heating the wafer to 40° C. or more. The nitrogen is automatically released into the ambient. The Ga metal may be removed using hydrochloric acid. 
     Once the sapphire substrate  210  is removed, the exposed GaN layers of LED layer  220  will typically comprise buffer and coalescence layers. These are generally unintentionally doped and of low crystal quality and should be removed (step  50 ), for example by a dry etching technique such as ICP or RIE  310  as shown in  FIG. 8 . Optionally, before the dry etching, the side walls of the device layer may be protected by passivation with a material such as SiO 2 , SiN, or photoresist. The GaN may be removed to a predetermined depth, typically of the order of 2 μm˜4 μm. Once the desired amount of excess GaN has been removed, n-type GaN is exposed. In order to improve the light extraction efficiency, the exposed n-GaN surface may be roughened, for example by a patterning technique such as nanoimprinting or nanosphere lithography. The pattern can be in a variety of shapes, for example cones, pyramids, pillars, and domes. The patterned elements may have pitch, diameter and height each in the range between about 100 nm and about 5 μm. 
     At step  55 , after the patterning of the n-GaN surface  224 , an n-metal contact layer  278  comprising contacts  280  is deposited on the patterned surface of LED layer  220  as shown in  FIG. 9 . The contacts  280  can have various layouts, for example dots, cross lines, a lattice or grid of lines, inter-digitated fingers, etc. The layout is preferably chosen in order to optimally spread the current whilst minimising blocking and/or absorption or light by the metal contact layer  278 . The metal contact layer  278  can be made of various conductive materials, including but not limited to: Ti/Al, Cr, Ti/Au, ITO, or other metals or conductive metal oxides. 
     The metal contact layer  278  can be deposited by various methods including electron beam deposition, sputtering, physical vapour deposition (PVD), chemical vapour deposition (CVD), plasma-enhanced CVD (PECVD), ion beam deposition, or electro-chemical deposition. The metal contact layer  278  may be subjected to annealing at temperatures from about 500° C. to about 600° C. in N 2  ambient for about 5 to 10 minutes. At step  60  a mounting tape in the form of stretchable adhesive tape  285  is affixed to n-metal contact layer  278  as shown in  FIG. 10 . The adhesive tape  285  preferably has moderate adhesiveness, high flexibility and stretchability, and is preferably resistant to acid and base erosion. As shown in  FIG. 10 , the temporary supporting carrier  275  and temporary bonding adhesive  270  are also removed at this point, for example by exposing the adhesive  270  to a suitable solvent. 
     At step  65 , the LED dies are separated from each other, for example by stretching the mounting tape in direction  288  as shown in  FIG. 11 , to form LED dies  290  each having a metal substrate  260 . Alternatively, the dies  290  can be separated by applying mechanical pressure on the LED wafer. 
     Referring now to  FIGS. 12 to 23 , in another embodiment processing may be performed on a device layer comprising individual LED dies provided on a substrate  101 , separated by trench regions  120 . The trench regions  120  may be formed by etching into a single contiguous epitaxial layer  110  (comprising n-GaN layer  110   c , active layer  110   b  and p-GaN layer  110   a ) which is provided on the substrate  101  (step  1010 ). The epitaxial layer  110  is etched, at step  1015 , to a depth sufficient to reach the substrate  101 , such that LED islands  112  are formed ( FIG. 13 ). Preferably, the islands  112  have angled side walls. Advantageously, angled side walls enable extraction of more light from the device and improve the device efficiency. 
     At step  1020 , a p-electrode layer  130  is applied to the surface of p-doped layer  110   a  of each LED island  112 , as shown in  FIG. 14 . The p-electrode  130  acts as a light reflector, current conductor, and ohmic contact with the p-doped layer  110   a . Thus, the p-electrode  130  preferably comprises a transparent conducting layer, a reflective layer, and a conductive supporting layer. 
     The transparent conducting layer forms an ohmic contact with p-doped layer  110   a , and is preferably formed from a transparent conductive oxide such as indium-tin-oxide (ITO), or is a semi-transparent metal thin film comprising Pd, Ni, Cr, Al, or Ag. 
     The reflective layer is formed on the transparent conducting layer and may comprise Al or Ag, for example. 
     The conductive supporting layer may be formed from Pt, Ni, Pd, Ag, Ti, Al, Au, W, Cr, Cu, or the like, and is formed on the reflective layer. The conductive supporting layer acts as a supporting layer to protect the reflective layer and also acts as a connection layer between the p-electrode  130  and a seed layer (connection metal layer)  132  ( FIG. 16 ). Generally, it is preferred that the p-electrode  130  be patterned so that the edge of the p-doped layer  110   a  will not be covered by the p-electrode. This can help to greatly reduce the leakage current and improve the device reliability. A gap may be left around the edge of the p-electrode  130  such that it does not reach the edge of p-doped layer  110   a.    
     At step  1025 , a passivation layer  131  is applied to the surfaces of LED islands  112 , in order to protect each island  112  as shown in  FIG. 15 . The passivation layer  131  separates the n-doped layer  110   c  from the p-doped layer  110   a  so as to prevent current leakage. The material used for the passivation layer  131  may be an insulative inorganic material such as silicon oxide (SiO x ) or silicon nitride (SiN x ), or photoresist such as SU-8. 
     After the passivation layer  131  is applied, the p-electrode  130  may be partly covered by the passivation layer  131 . In other embodiments, the edge of passivation layer  131  only reaches the p-doped layer  110   a  and does not obscure the p-electrode  130 . 
     At step  1030 , a connection metal layer  132  is formed on the upper surfaces of LED islands  112  as shown in  FIG. 16 . The p-electrode  130  and the passivation layer  131  may be entirely or partly covered by the connection metal layer  132  for preparation for metal plating or wafer bonding. Connection metal layer  132  may be formed from a metal selected from the group Ti, Cu, Au, Ni, Ag, Sn, and In, or a combination of two or more such metals. The connection metal layer  132  is used as a supporting or adhesion layer in subsequent processing. 
     At step  1032  a patterning structure  141  is formed on the connection metal layer  132  as shown in  FIG. 17 . The patterning structure  141  is used to partition the device layer into individual LED die regions, each region containing one of the LED islands  112 . Patterning structure  141  comprises a series of side walls, similar to the grid structure shown in  FIG. 4( a ) , and is formed such that the side walls are within trenches  120 . Patterning structure  141  may be applied by photolithography, as described above for example. 
     The patterning structure  141  is applied such that the sidewalls are high enough for metal plating to be performed. The patterning structure  141  typically has a height in the range between about 50 μm and about 200 μm, depending on the thickness of metal plating layer  142  which is to be applied. 
     At step  1035 , metal plating  142  is selectively formed on the device layer by electroplating into the voids defined by the sidewalls of patterning structure  141 . Due to the partitioning provided by the patterning structure  141 , once electroplating has been completed a series of metal islands  143  is formed on the device layer, each metal island being in electrical contact with one of the LED islands  112 . The metal plating layer  142  may comprise copper, nickel, silver or the like. As before, the metal islands  143 , when the LED dies are separated from each other, form conductive substrates for the respective LED dies and serve as electrodes as well as heat sinks. Also as before, the metal islands  143  can be deposited using other metal deposition methods such as electron beam evaporation, thermal evaporation, PVD, CVD sputtering deposition, etc. 
     Next, as shown in  FIG. 18 , a flexible temporary supporting carrier  151  is formed on the surface of metal plating layer  142  (step  1040 ). Optionally, the patterning structure  141  may be removed prior to applying the temporary carrier  151 . The flexible temporary supporting carrier  151  may comprise photoresist, resin, an organic film or the like, and is preferably applied by spin coating or adhesion. The flexible temporary supporting carrier  151  is preferably selected so as to be soluble in a particular solvent, such as water, which will not significantly react with adhesives used in the rest of the process. An additional material having similar solubility in the particular solvent may be added for better adhesion between the flexible temporary supporting carrier  151  and the metal plating layer  142 . 
     A protection layer  152 , for example a film comprising a resin, photoresist or a metal, may be formed on the flexible temporary supporting carrier  151  for protection and support. The protection layer  152  can be used as a support during a subsequent surface texturing process, if the flexible temporary supporting carrier  151  might not survive the texturing process. Alternatively, or in addition, a semiconductor wafer or glass can be formed on the backside of the device (i.e., on the same side as carrier  151 ) for protection and support. 
     At step  1045  the growth substrate  101  is removed by a chemical or mechanical method  160  as shown in  FIG. 19 , to expose n-GaN layer  110   c  of respective LED islands  112 . During removal, the metal plating layer  142 , flexible temporary supporting carrier  151 , and protection layer  152  collectively act as a supporting layer. The substrate removing process  160  can be laser lift-off (LLO), chemical lift-off, etching, mechanical lapping or the like. 
     At step  1055  an n-electrode layer  171  is formed on the exposed surface of n-GaN layer  110   c , as shown in  FIG. 20 . The exposed surface may be rinsed and partially etched (step  1050 ) for better contact and current spreading. The n-electrode layer  171  may have ohmic contact with n-doped layer  110   c . Preferably, texturing of the exposed n-GaN surface (i.e., that portion not obscured by n-contact  171 ) is performed to produce surface texture  172  in order to improve light extraction efficiency. The surface texturing process  172  may be carried out by wet etching or dry etching, and the pattern can be regular or irregular. Other methods of texturing the surface, such as nanoimprinting or nanosphere lithography, can also be employed. The n-electrode layer  171  may be protected to avoid it being damaged during the surface texturing process. 
     At step  1060 , the device is transferred from the temporary supporting carrier  151  to a mounting tape in the form of adhesive tape  181  ( FIG. 21 ). Before the adhesive tape  181  is adhered, the protection layer  152  is removed. The protection layer  152  may be removed chemically, e.g. using an organic solvent, or mechanically, e.g. by peeling. It is preferred that the protection layer  152  be removed before applying the adhesive tape  181  because the solvent used in a chemical removal process may dissolve the adhesive tape, or the rigidity and hardness of the protection layer  152  may cause damage to the adhesive tape if a mechanical process such as peeling is used. 
     At step  1065  the flexible temporary supporting carrier  151  is removed. As mentioned above, the flexible temporary supporting carrier  151  is soluble in a solvent which does not damage the adhesive tape  181 . Removal of the temporary carrier  151  essentially automatically separates the individual LED dies, each of which comprises LED island  112 , p-contact  132 , n-electrode  171 , and conductive (metal island) substrate  143 , with the adhesive tape  181  being a substrate on which the LED dies are disposed as shown in  FIG. 22 . 
     In order to make n-electrodes  171  upward facing for die sorting, the LED devices may be attached to a second adhesive tape  191 , prior to removal of the first adhesive tape  181  ( FIG. 23 ). If the patterning structure  141  was not previously removed, it advantageously provides better connection and support to the individual devices during the transfer, and can be removed following the transfer (step  1070 ). 
     Embodiments may have one or more of the following advantages: 
     Improved Light Extraction Efficiency 
     In certain embodiments, after the original sapphire substrate is removed, the exposed GaN surface is roughened or patterned, thereby enhancing light extraction efficiency. A highly reflective mirror may be applied between the LED layer and the metal substrate, also helping to enhance the light extraction efficiency. 
     Effective Heat Dissipation 
     The original sapphire substrate is a poor thermal conductor (thermal conductivity of 41.9 W/(m·K)) and the LEDs can consequently suffer from serious, efficiency droop due to the junction temperature increase caused by the large heat resistance of the sapphire substrate. By using a metal substrate with high thermal conductivity, such as copper (thermal conductivity of 401 W/(m·K)), the heat generated by the LEDs can be effectively dissipated and efficiency droop can be avoided. 
     Improved Yield and Reliability 
     In previously known fabrication processes, it has been necessary to apply whole wafer-level metal substrate deposition and metal substrate dicing processes. Stress and bowing are consequently generated in the LED wafer, causing cracks and damages in LED films after removing the original sapphire substrate. This will potentially result in failure of LEDs, and lower yield and reliability. In addition, during metal substrate dicing, it is common for metal particles to be generated. Metal particles are serious contaminants for LED devices and can cause current leakage and short-circuit issues. The mechanical shock imparted to the wafer during the dicing process may also cause damage to LEDs. Together these factors will lower the yield and cause reliability issues. 
     On the contrary, embodiments of the presently disclosed processes involve deposition of metal islands, i.e. the metal substrate is formed at the die level and not at whole-wafer level. This effectively suppresses the generation of stress and bow. In addition, because dies can be separated from the wafer without dicing, this avoids the metal particle contamination problem. Accordingly, yield and reliability can be greatly improved. 
     Cost Effective and Material/Process Efficient 
     In the prior art, in order to dice the wafer, a significant fraction of the wafer area is sacrificed to be used as the dicing area. The dicing process can also be extremely time-consuming. In the present invention, die separation occurs by a simple dicing-free process, thus making dicing equipment redundant. This may be important in an industrial-scale operation, where the space freed up by the lack of dicing equipment can be used for other processing steps, thus potentially increasing the rate at which units are fabricated. In addition, the grid used to partition the dies can be made much thinner than the lanes used for scribing/dicing. The present method may therefore be more cost effective and material/process efficient. 
     Although certain embodiments of the invention have been described in detail, many variations are possible within the scope of the invention, as will be clear to a skilled reader.

Technology Category: 5