Abstract:
A method for fabrication of a semiconductor device on a substrate, the semiconductor having a wafer. The method includes the steps:(a) applying a seed layer of a thermally conductive metal to the wafer;(b) electroplating a relatively thick layer of the conductive metal on the seed layer, and(c) removing the substrate. A corresponding semiconductor device is also disclosed.

Description:
CROSS-REFERENCE TO OTHER APPLICATIONS 
     This is a National Phase of International Application No. PCT/SG2003/000223, filed on Sep. 19, 2003. 
     FIELD OF THE INVENTION 
     The present invention relates to the fabrication of semiconductor devices and refers particularly, though not exclusively, to the plating of a heat sink on the semiconductor device. 
     BACKGROUND TO THE INVENTION 
     As semiconductor devices have developed there has been a considerable increase in their operational speed, and a reduction in overall size. This is causing a major problem of heat build-up in the semiconductor devices. Therefore, heat sinks are being used to help dissipate the heat from the semiconductor device. Such heat sinks are normally fabricated separately from the semiconductor device and are normally adhered to the semiconductor device just prior to encapsulation. 
     There have been many proposals for the electroplating of copper onto surfaces of semiconductor devices during their fabrication, particularly for use as interconnects. 
     The majority of current semiconductor devices are made from semiconductor materials based on silicon (Si), gallium arsenide (GaAs), and indium phosphide (InP). Compared to such electronic and optoelectronic devices, GaN devices have many advantages. The major intrinsic advantages that GaN have are summarised in Table 1: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Band Gap 
                 BFOM 
                   
               
               
                   
                   
                 (eV)/ 
                 (power 
                 Maximum 
               
               
                   
                 Mobility μ 
                 wavelength 
                 transistor 
                 Temperature 
               
               
                 Semiconductor 
                 (cm 2 /Vs) 
                 (nm) 
                 merit) 
                 (C.) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 Si 
                 1300 
                  1.1/1127 
                 1.0 
                 300 
               
               
                 GaAs 
                 5000 
                 1.4/886 
                 9.6 
                 300 
               
               
                 GaN 
                 1500 
                 3.4/360 
                 24.6 
                 700 
               
               
                   
               
             
          
         
       
     
     From Table 1, it can be seen that GaN has the highest band gap (3.4 eV) among the given semiconductors. Thus, it is called a wide band gap semiconductor. Consequently, electronic devices made of GaN operate at much higher power than Si and GaAs and InP devices. 
     For semiconductor lasers, GaN lasers have a relatively short wavelength. If such lasers are used for optical data storage, the shorter wavelength may lead to a higher capacity. GaAs lasers are used for the manufacture of CD-ROMs with a capacity of about 670 MB/disk. AlGaInP lasers (also based on GaAs) are used for the latest DVD players with a capacity of about 4.7 GB/disk. GaN lasers in the next-generabon DVD players may have a capacity of 26 GB/disk. 
     GaN devices are made from GaN wafers that are typically multiple GaN-related epitaxial layers deposited on a sapphire substrate. The sapphire substrate is usually two inches in diameter and acts as the growth template for the epitaxial layers. Due to lattice mismatch between GaN-related materials (epitaxial films) and sapphire, defects are generated in the epitaxial layers. Such defects cause serious problems for GaN lasers and transistors and, to a lesser extent, for GaN LEDs. 
     There are two major methods of growing epitaxial wafers: molecular beam epitaxy (MBE), and metal organic chemical vapour deposition (MOCVD). Both are widely used. 
     Conventional fabrication processes usually include these major steps: photolithography, etching, dielectric film deposition, metallization, bond pad formation, wafer inspection/testing, wafer thinning, wafer dicing, chip bonding to packages, wire bonding and reliability testing. 
     Once the processes for making LEDs are completed at the full wafer scale, it is then necessary to break the wafer into individual LED chips or dice. For GaN wafers grown on sapphire substrates, this “dicing” operation is a major problem as sapphire is very hard. The sapphire first has to be thinned uniformly from about 400 microns to about 100 microns. The thinned wafer is then diced by diamond scriber, sawed by a diamond saw or by laser grooving, followed by scribing with diamond scribers. Such processes limit throughput, cause yield problems and consume expensive diamond scribers/saws. 
     Known LED chips grown on sapphire substrates require two wire bonds on top of the chip. This is necessary because sapphire is an electrical insulator and current conduction through the 100-micron thickness is not possible. Since each wire bond pad takes about 10-15% of the wafer area, the second wire bond reduces the number of chips per wafer by about 10-15% as compared to single-wire bond LEDs grown on conducting substrates. Almost all non-GaN LEDs are grown on conducting substrates and use one wire bond. For packaging companies, two wire bonding reduces packaging yield, requires modification of one-wire bonding processes, reduces the useful area of the chip, and complicates the wire bonding process and thus lowers packaging yield. 
     Sapphire is not a good thermal conductor. For example, its thermal conductivity at 300 K (room temperature) is 40 W/Km. This is much smaller than copper&#39;s thermal conductivity of 380 W/Km. If the LED chip is bonded to its package at the sapphire interface, the heat generated in the active region of the device must flow through 3 to 4 microns of GaN and 100 microns of sapphire to reach the package/heat sink. As a consequence, the chip will run hot affecting both performance and reliability. 
     For GaN LEDs on sapphire, the active region where light is generated is about 3-4 micron from the sapphire substrate. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred form of the present invention, there is provided a method for fabrication of a semiconductor device on a substrate, the semiconductor device having wafer with a device layer; the method including the steps:
         (a) electroplating a layer of a thermally conductive material onto a surface of the wafer remote from the substrate and close to the device layer; and   (b) removing the substrate.       

     The semiconductor device may be a silicon-based device. 
     In accordance with another form, there is provided a method for fabrication of a light emitting device on a substrate, the light emitting device having wafer with an active layer; the method including the steps:
         (a) electroplating a layer of a thermally conductive material onto a surface of the wafer remote from the substrate and close to the active layer; and   (b) removing the substrate.       

     For both forms, the thermally conductive layer may be as a heat sink, and may be of a thickness in the range of from 3 microns to 300 microns, preferably 50 to 200 microns. 
     In a further form, the present invention provides a method for fabrication of a semiconductor device on substrate, the semiconductor device having a wafer; the method including the steps:
         (a) applying a seed layer of a thermally conductive metal to a first surface of the wafer remote from the substrate;   (b) electroplating a relatively thick layer of the thermally conductive metal on the seed layer; and   (c) removing the substrate.       

     Prior to the seed layer being applied, the wafer may be coated with an adhesion layer. Before the electroplating of the relatively thick layer the seed layer may be patterned with photoresist patterns; the relatively thick layer being electroplated between the photoresists. 
     The seed layer may be electroplated without patterning and with patterning being performed subsequently. Patterning may be by photoresist patterning and then wet etching. Alternatively, it may be by laser beam micro-machining of the relatively thick layer. 
     Between steps (b) and (c) there may be performed the additional step of annealing the wafer to improve adhesion. 
     Preferably, the photoresists are of a height of at least 15 to 500 microns, more preferably 50 to 200 microns, and have a thickness in the range 3 to 500 microns. More preferably, the photoresists have a spacing in the range of 200 to 2,000 microns, preferably 300 microns. 
     The relatively thick layer may be of a height no greater that the photoresist height Alternatively, the conductive metal layer may be electroplated to a height greater than the photoresist and be subsequently thinned. Thinning may be by polishing or wet etching. 
     After step (c) there may be included an extra step of forming on a second surface of the wafer remote from the relatively thick layer, a second ohmic contact layer. The contact layer may be a second ohmic contact layer. The second ohmic contact layer may be one of opaque, transparent, and semi-transparent, and may be either blank or patterned. Ohmic contact formation and subsequent process steps may be carried out The subsequent process steps may include deposition of wire bond pads. The exposed second surface of the wafer layer may be cleaned and etched before the second ohmic contact layer is deposited onto it The second ohmic contact layer may not cover the whole area of the second surface of the wafer. 
     The semiconductor devices may be tested on the wafer, and the wafer may be subsequently separated into individual devices. 
     The semiconductor devices may be fabricated without one or more of lapping, polishing and dicing. 
     The wafer may include epitaxial layers and, on the epitaxial layers remote from the substrate, first ohmic contact layers. The first ohmic contact layers may be on p-type layers of the epitaxial layers; and the second ohmic contact layer may be formed on n-type layers of the expitaxial layers. 
     After step (c), dielectric films may be deposited on the epitaxial layers. Openings may then be cut in the dielectric and second ohmic contact layer and bond pads deposited on the epitaxial layers. Alternatively, after step (c), electroplating of a thermally conductive metal (or other material) on the epitaxial layers may be performed. 
     The invention is also directed to a semiconductor device fabricated by the above method. The invention, in a preferred aspect, also provides a light emitting diode or a laser diode fabricated by the above method. 
     In a further aspect, the present invention provides a semiconductor device comprising epitaxial layers, a first ohmic contact layer on a first surface of the epitaxial layers, a relatively thick layer of a thermally conductive metal on the first ohmic contact layer, and a second ohmic contact layer on a second surface of the epitaxial layers; the relatively thick layer being applied by electroplating. 
     There may be an adhesive layer on the first ohmic contact layer between the first ohmic contact layer and the relatively thick layer. 
     The relatively thick layer may be at least 50 micrometers thick; and the second ohmic contact layer may be a thin layer in the range of from 3 to 500 nanometers. The second ohmic contact layer may be transparent, semi-transparent or opaque; and may include bonding pads. 
     For all forms of the invention, the thermally conductive metal may be copper. 
     There may be a seed layer of the thermally conductive metal applied to the adhesive layer. 
     The semiconductor device may be one of a light emitting diode, a laser diode, and a transistor device. 
     In yet another form, there is provided a semiconductor device comprising epitaxial layers, a first ohmic contact layer on a first surface of the epitaxial layers, an adhesive layer on the first ohmic contact layer, and a seed layer of a thermally conductive metal on the adhesive layer. 
     There may be further included a relatively thick layer of the thermally conductive metal on the seed layer. 
     A second ohmic contact layer may be provided on a second surface of the epitaxial layers; the second ohmic contact layer being a thin layer in the range of from 3 to 500 nanometers. The second ohmic contact layer may comprise bonding pads; and may be one of opaque, transparent, and semi-transparent 
     The thermally conductive metal may comprise copper; and the epitaxial layers may comprise GaN-related layers. 
     The semiconductor device may be a light emitting device. 
     In a penultimate form, the present invention provides a method of fabrication of a semiconductor device, the method including the steps:
     (a) on a substrate with a wafer comprising multiple GaN-related epitaxial layers, forming a first ohmic contact layer on a first surface of the wafer;   (b) removing the substrate from the wafer; and   (c) forming a second ohmic contact layer on a second surface of the wafer, the second ohmic contact layer having bonding pads formed thereon.   

     The second ohmic contact layer may be for light emission; and may be opaque, transparent, or semitransparent The second ohmic contact layer may be blank or patterned. 
     In a final form, there is provided a semiconductor device fabricated by the above method. 
     The semiconductor device may be a light emitting diode or a laser diode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may be better understood and readily put into practical effect there shall now be described by way of non-limitative example only a preferred embodiment of the present invention, the description being with reference to the accompanying illustrative (and not to scale) drawings in which: 
         FIG. 1  is a schematic representation of a semiconductor device at a first stage in the fabrication process; 
         FIG. 2  is a schematic representation of the semiconductor device of  FIG. 1  at a second stage in the fabrication process; 
         FIG. 3  is a schematic representation of the semiconductor device of  FIG. 1  at a third stage in the fabrication process; 
         FIG. 4  is a schematic representation of the semiconductor device of  FIG. 1  at a fourth stage in the fabrication process; 
         FIG. 5  is a schematic representation of the semiconductor device of  FIG. 1  at a fifth stage in the fabrication process; 
         FIG. 6  is a schematic representation of the semiconductor device of  FIG. 1  at a sixth stage in the fabrication process; 
         FIG. 7  is a schematic representation of the semiconductor device of  FIG. 1  at the seventh stage in the fabrication process; and 
         FIG. 8  is a flow chart of the process. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For the following description, the reference numbers in brackets refer to the process steps in  FIG. 8 . 
     To refer to  FIG. 1 , there is shown the first step in the process—the metallization on the p-type surface of the wafer  10 . 
     The wafer  10  is an epitaxial wafer with a substrate and a stack of multiple epitaxial layers  14  on it The substrate  12  can be, for example, sapphire, GaAs, InP, Si, and so forth. Henceforth a GaN sample having GaN layer(s)  14  on sapphire substrate  12  will be used as an example. The epitaxial layers  14  (often called epilayers) are a stack of multiple layers, and the lower part  16  (which is grown first on the substrate) is usually n-type layers and the upper part  18  is often p-type layers. 
     On GaN layers  14  is an ohmic contact layer  20  having multiple metal layers. To ohmic contact layer  20  is added an adhesion layer  22 , and a thin copper seed layer  24  ( FIG. 2 ) (step  88 ) of a thermally conductive metal such as, for example, copper. The thermally conductive metal is preferably also electrically conductive. The stack of adhesion layers may be annealed after formation. 
     The ohmic layer  20  may be a stack of multiple layers deposited and annealed on the semiconductor surface. It may not be part of the original wafer. For GaN, GaA, and InP devices, the epitaxial wafer often contains an active region that is sandwiched between n-type and p-type semiconductors. In most cases the top layer is p-type. For silicon devices, epitaxial layers may not be used, but just the wafer. 
     As shown in  FIG. 3 , using standard photolithography ( 89 ), the thin copper seed layer  24  is patterned with relatively thick photoresists  26 . The photoresist patterns  26  are preferably of a height in the range of 3 to 500 micrometers, preferably 15 to 500 micrometers; and with a thickness of about 3 to 500 micrometers. They are preferably separated from each other by a spacing in the range of 200 to 2,000 microns, preferably 300 microns, depending on the design of the final chips. The actually pattern depends on device design. 
     A patterned layer  28  of copper is then electroplated onto layer  24  ( 90 ) between photoresists  26  to form a heat sink that forms a part of the substrate. The copper layer  28  is preferably of a height no greater than that of the photoresists  26  and is therefore of the same or lesser height than the photoresists  26 . However, the copper layer  28  may be of a height greater than that of the photoresists  26 . In such a case, the copper layer  28  may be subsequently thinned to be of a height no greater than that of the photoresists  26 . Thinning may be by polishing or wet etching. The photoresists  26  may or may not be removed after the copper plating. Removal may be by a standard and known method such as, for example, resin in the resist stripper solution, or by plasma aching. 
     Depending on the device design, processing of the epitaxial layers  14  follows using standard processing techniques such as, for example, cleaning, ( 80 ), lithography ( 81 ), etching ( 82 ), device isolation ( 83 ), passivation ( 84 ), metallization ( 85 ), thermal processing ( 86 ), and so forth. ( FIG. 4 ). The wafer  10  is then annealed ( 87 ) to improve adhesion. 
     The epitaxial layer  14  is usually made of n-type layers  16  on the original substrate  12 ; and p-type layers on the original top surface  18  which is now covered with the ohmic  20 , adhesion  22  and copper seed layers  24  and the electroplated thick copper layer  28 . 
     In  FIG. 5 , the original substrate layer  12  is then removed ( 91 ) using, for example, the method of Kelly [M. K Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, and M. Stutzmann, phys. stat sol. (a) 159, R3 (1997)]. The substrate may also be removed by polishing or wet etching. 
       FIG. 6  is the penultimate step and is particularly relevant for light emitting diodes where a transparent ohmic contact layer  30  is added beneath epitaxial layers  14  for light emission. Bonding pads  32  are also added. The Ohmic contact layer  30  is preferably transparent or semi-transparent It is more preferably a thin layer and may be in the range of 3 to 50 nm thick. 
     Prior to adding ohmic contact layer  30 , known preliminary processes may be performed. These may be, for example, photolithography ( 92 ,  93 ), dry etching ( 94 ,  95 ), and photolithography ( 96 ). 
     Annealing ( 98 ) may follow the deposition of ohmic contact layer  30 . 
     The chips/dies are then tested ( 99 ) by known and standard methods. The chips/dies can then be separated ( 100 ) ( FIG. 7 ) into individual devices/chips  1  and  2  without lapping/polishing the substrate, and without dicing. Packaging follows by standard and known methods. 
     The top surface of the epitaxial layer  14  is preferably in the range of about 0.1 to 2.0 microns, preferably about 0.3 microns, from the active region. For silicon-based semiconductors, the top surface of the semiconductor is preferably in the range 0.1 to 2.0 microns, preferably about 0.3 microns, from the device layer. As the active layer/device layer in this configuration is close to az relatively thick copper pad  28 , the rate of heat removal is improved. 
     Additionally or alternatively, the relatively thick layer  28  may be used to provide mechanical support for the chip. It may also be used to provide a path for heat removal from the active region/device layer, and may also be used for electrical connection. 
     The plating step is performed at the wafer level (i.e., before the dicing operation) and may be for several wafers at the one time. 
     The fabrication of GaN laser diodes is similar to the fabrication of GaN LEDs, but more steps may be involved. One difference is that GaN laser diodes require mirror formation during the fabrication. Using sapphire as the substrate compared to the method without sapphire as the substrate, the mirror formation is much more difficult and the quality of the mirror is generally worse. 
     After sapphire is removed, the laser will have better performance. An example of a typical GaN laser epitaxial wafer structure is shown in Table 2. 
     
       
         
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Mg doped p-type GaN contact layer 
                 0.15 μm 
               
               
                 Mg doped p-type Al 0.15 Ga 0.85 N cladding layer 
                 0.45 μm 
               
               
                 Mg doped p-type GaN wave guiding layer 
                 0.12 μm 
               
               
                 Mg doped p-type Al 0.2 Ga 0.8 N electron blocking layer 
                 200 
               
               
                 In 0.03 Ga 0.97 N/In 0.16 Ga 0.74 N 3-period MQWs active layer 
                   
               
               
                 In 0.10 GaN 0.90 N well layer 
                  35 
               
               
                 In 0.03 Ga 0.97 N barrier layer 
                  50 
               
               
                 Si doped n-type GaN wave guiding layer 
                 0.12 μm 
               
               
                 Si doped n-type Al 0.15 Ga 0.85 N cladding layer 
                 0.45 μm 
               
               
                 Si doped n-type In 0.1 Ga 0.9 N 
                 500 
               
               
                 Si doped n-type GaN contact layer 
                   3 μm 
               
               
                 Un-doped n-type GaN 
                   1 μm 
               
               
                 Un-doped n-type ELO GaN layer 
                   6 μm 
               
               
                 Un-doped GaN template layer/Si 3 N 4  mask 
                   2 μm 
               
               
                 GaN buffer 
                 300 
               
               
                 Sapphire substrate 
                  450 μm 
               
               
                   
               
             
          
         
       
     
     For standard commercial GaN LEDs, about 5% light generated in the semiconductor is emitted. Various ways have been developed to extract more light out from the chip in non-GaN LEDs (especially red LEDs based on AlGaInP, not GaN). 
     The first ohmic contact layer  20 , being metal and relatively smooth, is quite shiny and therefore highly reflective of light. As such, the first ohmic contact layer  20 , at its junction with the epitaxial layers  14 , also is a reflective surface, or mirror, to improve light output. 
     Although reference is made to copper, any other platable material may be used m provided it is electrically and/or heat conductive, or provides the mechanical support for the semiconductor device. 
     Whilst there has been described in the foregoing description a preferred form of the present invention, it will be understood by those skilled in the technology that many variations or modifications in design, construction or operation may be made without departing from the present invention.