Abstract:
A method for separating semiconductor devices is disclosed. The method includes providing a substrate having one or more epitaxial layers formed thereon, forming trenches in the one or more epitaxial layers, forming scribe lines in a surface of the substrate, wherein the locations of the scribe lines correspond to the locations of the trenches, and separating the semiconductor devices by cracking the wafer along the scribe lines.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a technique for separating semiconductor dies from a wafer and, in particular, to a technique for separating gallium nitride light emitting diode devices on lattice-mismatched substrates.  
         BACKGROUND  
         [0002]    Light emitting diodes (LEDs) such as gallium nitride (GaN) LEDs are used in a wide variety of applications. GaN LEDs are commonly fabricated by forming one or more GaN epitaxial layers over a top surface of a lattice mismatched wafer substrate, such as a sapphire substrate. The bottom surface of the substrate is then scribed using a scribing tool. The scribing process creates scribe lines which define the dimensions of the LED devices. The substrate is then broken along the scribe lines, yielding individual LED devices.  
           [0003]    The conventional scribe and break process results in devices that have rough edges, and thus a poor visual appearance. The process can also result in devices that have damaged electrical contacts or bond pads due to randomly propagating cracks. During the breaking process, cracks develop at the scribe lines on the surface of the substrate. The cracks then propagate through the substrate. When the cracks reach the substrate/epitaxial layer interface, the cracks tend to propagate in unanticipated and undesirable directions. This is due to the high stress and large number of defects at this interface, caused by the mismatched lattice patterns of the substrate and the GaN layer. Finally, the cracks propagate through the epitaxial layer and break through the top surface of the epitaxial layer. At the very least, this separation method results in individual devices which have rough or jagged edges. However, the propagating cracks frequently break out of the scribe line areas into the active areas of the device, damaging electrical contacts or bond pads located therein.  
           [0004]    What is needed is a technique for separating GaN devices on lattice-mismatched substrates which overcomes the disadvantages mentioned above.  
         SUMMARY  
         [0005]    A technique which overcomes the disadvantages mentioned above is disclosed.  
           [0006]    In one embodiment, a method for separating semiconductor devices is disclosed. The method includes providing a substrate having one or more epitaxial layers thereon, forming trenches in the one or more epitaxial layers, forming scribe lines in a surface of the substrate, wherein the locations of the scribe lines correspond to the locations of the trenches, and separating the semiconductor devices by cracking the substrate along the scribe lines.  
           [0007]    In another embodiment, a semiconductor device is disclosed. The semiconductor device includes a substrate, one or more epitaxial layers disposed over the substrate, trenches formed in the one or more epitaxial layers, and scribe lines formed in the substrate in areas corresponding to the trenches  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:  
         [0009]    [0009]FIG. 1 shows a cross section of a portion of a wafer having two LED devices, in accordance with the present invention.  
         [0010]    [0010]FIG. 2 shows the wafer of FIG. 1 after a mask layer and a photoresist layer have been deposited thereon, in accordance with the present invention.  
         [0011]    [0011]FIG. 3 shows the wafer of FIG. 2 after the photoresist layer has been exposed and developed, in accordance with the present invention.  
         [0012]    [0012]FIG. 4 shows the wafer of FIG. 3 after the mask layer has been etched, in accordance with the present invention.  
         [0013]    [0013]FIG. 5 shows the wafer of FIG. 4 after the epitaxial layer has been etched, in accordance with the present invention.  
         [0014]    [0014]FIG. 6 shows the wafer of FIG. 5 after the photoresist and the mask layers have been removed, in accordance with the present invention.  
         [0015]    [0015]FIG. 7 shows the wafer of FIG. 6 after the substrate has been back thinned, in accordance with the present invention.  
         [0016]    [0016]FIG. 8 shows the wafer of FIG. 7 after the substrate has been scribed, in accordance with the present invention.  
         [0017]    [0017]FIG. 9 shows the wafer of FIG. 8 after the two LED devices have been separated, in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0018]    The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 9 of the drawings. Like numerals are used for like and corresponding parts of the various drawings.  
         [0019]    A method for separating GaN devices on lattice-mismatched substrates in accordance with the present invention is now described. The method is described with reference to FIGS. 1 through 9.  
         [0020]    In step  1 , a portion of a wafer having two LED devices formed thereon is provided as shown in FIG. 1. A gallium nitride (GaN) epitaxial layer  12  having a top surface  12   a  is disposed on a sapphire substrate  10 . Sapphire substrate  10  includes a top surface  10   a , a bottom surface  10   b , and a thickness T 1 . N contacts  14  and  18  and P contacts  16  and  20  are then formed on GaN epitaxial layer  12 . N contact  14  and P contact  16  provide electrical to a first LED device  22 , and N contact  18  and P contact  20  provide electrical connections to a second LED device  24 . The structure shown in FIG. 1 can be formed by techniques well known in the art.  
         [0021]    In addition to sapphire, other suitable materials such as silicon carbide or GaP can be used for substrate  10 . Other suitable III-V materials such as ALGaN, InGaN, AlInGaN, AlGaInP can be used for epitaxial layer  12 . Such layers may also include P and N-type dopants. Although epitaxial layer  12  is shown as a single layer, it should be recognized that epitaxial layer  12  can include multiple layers. It should also be noted that wafers having the structures of FIG. 1 formed thereon can be tested prior to performing the processing steps described below. Such testing ensures that only wafers with acceptable yield are further processed.  
         [0022]    One example of forming an LED is found in U.S. Pat. No. 6,133,589 by Michael Krames et al., incorporated herein by reference.  
         [0023]    In step  2 , a mask material and a photoresist layer are formed over the structure of FIG. 1  as shown in FIG. 2. First, a mask layer  26  is formed over top surface  12 a of epitaxial layer  12 , N contacts  14  and  18 , and P contacts  16  and  20 . Then, a photoresist layer  28  is formed over mask layer  26 . Suitable mask materials includes metals, dielectrics, photoresist, and combinations of multiple layers of all of these materials.  
         [0024]    In step  3 , the photoresist layer is exposed and developed as shown in FIG. 3. First, a photomask  30  is used to expose portions of photoresist layer  28  to a light source corresponding to where openings  32  are to be formed. Then, the exposed areas of photoresist layer  28  are developed resulting in the formation of openings  32 . The formation of openings  32  expose a top surface  26   a  of mask layer  26 . A negative photoresist layer may be used instead, requiring an opposite exposure photomask  30 .  
         [0025]    In step  4 , the mask layer is etched as shown in FIG. 4. In this step, a conventional etching solution for the particular mask material is used to remove portions of mask layer  26  exposed by openings  32 . This etching process continues until top surface  12   a  of epitaxial layer  12  is exposed in areas corresponding to openings  32 .  
         [0026]    In step  5 , the epitaxial layer is etched as shown in FIG. 5. In this step, etchant is used to remove portions of epitaxial layer  12  corresponding to openings  32  (FIG. 4). This etching process continues until top surface  10   a  of substrate  10  is exposed, thereby forming trenches  34 . This step can be performed by placing the wafer into a reactive ion etching (RIE) machine for an appropriate time, which is determined by the thickness of the epitaxial layer and the exact etching conditions used. A wide variety of etchants such as chlorine based etchants can be used to etch the epitaxial layer.  
         [0027]    In step  6 , the photoresist and the mask layers are removed as shown in FIG. 6. The remaining portions of photoresist layer  28  and mask layer  26  are removed using techniques well known in the art. The remaining structure is similar to the structure of FIG. 1 except that trenches  34  have been formed in epitaxial layer  12 .  
         [0028]    In step  7 , the bottom surface of the substrate is thinned or polished as shown in FIG. 7. Bottom surface  10   b  of substrate  10  can be thinned by various techniques such as lapping or grinding such that thickness of the substrate is optimal. Note that the thickness T 2  of substrate  10  as shown in FIG. 7 is less than the thickness Ti of substrate  10  prior to the thinning process. In one embodiment, an original substrate thickness of 430 microns is thinned to a thickness of 105 microns.  
         [0029]    In step  8 , the substrate is scribed using a scribe tool as shown in FIG. 8. A scribe tool  38  is lowered into trench  34  generating a force on top surface  10   a  of substrate  10 . Scribe tool  38  is then moved across top surface  10   a  of substrate  10 , using conventional equipment, causing a scribe line  36  to be formed in top surface  10   a  of substrate  10 . Alternatively, scribe line  36  can be formed on bottom surface  10   b  of substrate  10  opposite trench  34  (not shown).  
         [0030]    In step  9 , the first LED device and the second LED device are separated along scribe line  36 . First LED device  22  and second LED device  24  can be separated using well known breaking techniques, such as adhering the wafer to an elastic layer then bending the wafer around a curved surface. Using such techniques, cracks originate at top surface  10   a  of substrate  10  where scribe line  36  is formed. The cracks then propagate through substrate  10  until they reach bottom surface  10   b  of substrate  10  where they break through, thereby separating the LED devices. Alternatively, if scribe line  36  is formed on bottom surface  10   b  of substrate  10 , cracks originate at bottom surface  10   b  of substrate  10  where scribe line  36  is formed and then propagate through substrate  10  until they reach top surface  10   a  of substrate  10  where they break through, thereby separating the LED devices.  
         [0031]    Thus a method for separating LED devices on lattice-mismatched substrates has been described. According to the method of the present invention, the cracks which form in the break step propagate through the substrate only. The cracks do not propagate through the epitaxial layer or the interface between the epitaxial layer and the substrate layer as they would according to conventional separation techniques. Since the cracks do not propagate through the epitaxial layer or the interface between the epitaxial layer and the substrate layer, the cracks do not travel in unanticipated and undesirable directions, as they would according to conventional separation techniques. As a result, the dimensions of the LED devices formed in accordance with the present invention are easier to control and have greater definition.  
         [0032]    The disclosed method for separating LED devices on lattice-mismatched substrates provides several noteworthy advantages. First, fewer devices are rejected during the manufacturing process due to poor visual appearance. This is because the edges of the devices fabricated according to the present invention are relatively smooth rather than being jagged. Second, fewer devices are rejected during the manufacturing process due to damaged electrical contacts or bond pads. This is because the propagation of the cracks are more easily controlled and do not propagate randomly into the electrical contact and bond pad areas of the device. Third, the light emitting areas of the LED devices are more uniform. Since the edges of the device are smoother, and the devices have more uniform dimensions, the active portion of each device is approximately equal. As a result, the current density for each device at a fixed current is constant. This improves the uniformity of injection efficiency, operating forward voltage, and the light output. Fourth, since the dimensions of each device are approximately equal, assembly yield and speed is improved since fewer devices will be rejected due to nonuniformity. Fifth, radiation patterns are improved since the edges of the devices are more vertical and uniform. Sixth, the quality of white light products having a phosphor coating can be improved since the phosphor coating covers the same light emitting area in every device. Using conventional processes, the phosphor coating would be thicker on a smaller device, resulting in a more yellowish color, whereas the phosphor coating would be thinner on a larger device, resulting in a more bluish color. Seventh, the present invention allows scribing on the top (device) side as well as on the bottom side of the substrate.  
         [0033]    While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appended claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention.