Patent Publication Number: US-11038082-B2

Title: Method of separating light emitting devices formed on a substrate wafer

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 16/134,441, filed on Sep. 18, 2018, which is a continuation of U.S. patent application Ser. No. 14/906,539, filed on Jan. 20, 2016, which issued on Sep. 18, 2018 as U.S. Pat. No. 10,079,327, which was filed as the U.S. National Stage, under 35 U.S.C. § 371, of International Application No. PCT/IB2014/062784 filed Jul. 2, 2014, which claims the benefit of U.S. Provisional Application No. 61/856,857 filed on Jul. 22, 2013, the contents which are hereby incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to methods for separating light emitting devices grown on a substrate wafer. 
     BACKGROUND 
     Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions. 
     In some LEDs the growth substrate remains part of the final device structure, for example to provide mechanical stability to the semiconductor structure. A significant amount of light may be emitted through the sides of the growth substrate. Side light emission from the substrate is undesirable in applications that require or prefer that most or all of the light be emitted from the top of the device. 
     US 2010/0267219 describes a method of thinning the growth substrate. According to the abstract, the method includes “a sapphire substrate grinding step of grinding the back side of the sapphire substrate; a modified layer forming step of applying a laser beam to the sapphire substrate from the back side thereof to thereby form a modified layer in the sapphire substrate along each street, . . . and a wafer dividing step of breaking the sapphire substrate along each street where the modified layer is formed”. 
     SUMMARY 
     It is an object of the invention to provide a method of separating light emitting devices grown on a substrate by forming notches in the substrate, then thinning the substrate to expose the notches. 
     A method according to embodiments of the invention includes providing a light emitting semiconductor structure grown on a substrate. The substrate has a front side and a back side opposite the front side. Notches are formed in the substrate. The notches extend from the front side of the substrate into the substrate. After forming notches in the substrate, the back side of the substrate is thinned to expose the notches. 
     A method according to embodiments of the invention includes growing on a first surface of a sapphire substrate a semiconductor structure including a light emitting layer disposed between an n-type region and a p-type region. The semiconductor structure is formed into a plurality of LEDs. Cracks are formed in the sapphire substrate. The cracks extend from the first surface of the sapphire substrate and do not penetrate an entire thickness of the sapphire substrate. After forming cracks in the sapphire substrate, the sapphire substrate is thinned from a second surface of the sapphire substrate. The second surface is opposite the first surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates one example of a III-nitride LED. 
         FIG. 2  illustrates a portion of a wafer of LEDs formed on a substrate. 
         FIG. 3  illustrates the structure of  FIG. 2  after attaching the wafer to handling tape and thinning the substrate. 
         FIG. 4  illustrates the structure of  FIG. 3  after scribing the substrate. 
         FIG. 5  illustrates the structure of  FIG. 4  after thinning the substrate. 
         FIG. 6  illustrates the structure of  FIG. 5  after stretching the handling tape to separate the LEDs. 
         FIG. 7  illustrates a mask applied to a portion of a substrate. 
         FIG. 8  illustrates the structure of  FIG. 7  after etching notches in the substrate. 
         FIG. 9  illustrates the structure of  FIG. 8  after stripping the mask. 
         FIG. 10  illustrates the structure of  FIG. 9  after forming LEDs on the notched substrate. 
         FIG. 11  illustrates the structure of  FIG. 10  after attaching the wafer to handling tape and thinning the substrate to separate the LEDs. 
         FIG. 12  illustrates a portion of a substrate including partially formed LEDs. 
         FIG. 13  illustrates the structure of  FIG. 12  after forming notches in the substrate. 
         FIG. 14  illustrates the structure of  FIG. 13  after finishing the LEDs. 
         FIG. 15  illustrates the structure of  FIG. 14  after attaching the wafer to handling tape and thinning the substrate to separate the LEDs. 
         FIG. 16  illustrates a portion of a wafer of LEDs formed on a substrate, with cracks formed between neighboring LEDs. 
         FIG. 17  illustrates the structure of  FIG. 16  after thinning the substrate. 
     
    
    
     DETAILED DESCRIPTION 
     In embodiments of the invention, a sapphire or other growth substrate remains part of the final device structure, but is thinned to reduce or eliminate light emission from the sides of the growth substrate. In embodiments of the invention, the wafer is first partially separated by forming separation zones, which are often notches or cracks in the substrate, through at least part of the thickness of the substrate. The wafer is then fully separated by thinning the substrate until the separation zones are reached. Embodiments of the invention are particularly suited to applications that require all or a significant portion of light to be emitted from the top surface of a device, such as some automotive applications. 
     Though in the examples below the semiconductor light emitting devices are III-nitride LEDs that emit blue or UV light, semiconductor light emitting devices besides LEDs such as laser diodes and semiconductor light emitting devices made from other materials systems such as other III-V materials, III-phosphide, III-arsenide, II-VI materials, ZnO, or Si-based materials may be used. 
       FIG. 1  illustrates a single III-nitride LED  12  that may be used in embodiments of the present invention. Any suitable semiconductor light emitting device may be used and embodiments of the invention are not limited to the device illustrated in  FIG. 1 . The device of  FIG. 1  is formed by growing a III-nitride semiconductor structure on a portion of a growth substrate  10  as is known in the art. The growth substrate is often sapphire but may be any suitable substrate such as, for example, SiC, Si, GaN, or a composite substrate. The semiconductor structure includes a light emitting or active region sandwiched between n- and p-type regions. An n-type region  14  may be grown first and may include multiple layers of different compositions and dopant concentration including, for example, preparation layers such as buffer layers or nucleation layers, and/or layers designed to facilitate removal of the growth substrate, which may be n-type or not intentionally doped, and n- or even p-type device layers designed for particular optical, material, or electrical properties desirable for the light emitting region to efficiently emit light. A light emitting or active region  16  is grown over the n-type region  14 . Examples of suitable light emitting regions include a single thick or thin light emitting layer, or a multiple quantum well light emitting region including multiple thin or thick light emitting layers separated by barrier layers. A p-type region  18  may then be grown over the light emitting region  16 . Like the n-type region  14 , the p-type region  18  may include multiple layers of different composition, thickness, and dopant concentration, including layers that are not intentionally doped, or n-type layers. 
     After growth, a p-contact  20  is formed on the surface of the p-type region. The p-contact  20  often includes multiple conductive layers such as a reflective metal and a guard metal which may prevent or reduce electromigration of the reflective metal. The reflective metal is often silver but any suitable material or materials may be used. After forming the p-contact  20 , a portion of the p-contact  20 , the p-type region  18 , and the active region  16  is removed to expose a portion of the n-type region  14  on which an n-contact  22  is formed. The n- and p-contacts  22  and  20  are electrically isolated from each other by a gap  25 , shown hatched, which may be filled with a dielectric such as an oxide of silicon or any other suitable material. Multiple n-contact vias may be formed; the n- and p-contacts  22  and  20  are not limited to the arrangement illustrated in  FIG. 1 . The n- and p-contacts may be redistributed to form bond pads with a dielectric/metal stack, as is known in the art. 
     In order to form electrical connections to the LED  12 , one or more interconnects  26  and  28  are formed on or electrically connected to the n- and p-contacts  22  and  20 . Interconnect  26  is electrically connected to n-contact  22  in  FIG. 1 . Interconnect  28  is electrically connected to p-contact  20 . Interconnects  26  and  28  are electrically isolated from the n- and p-contacts  22  and  20  and from each other by dielectric layer  24 , shown hatched, and gap  27 . Interconnects  26  and  28  may be, for example, solder, stud bumps, gold layers, or any other suitable structure. Many individual LEDs  12  may be formed on a single wafer then diced from a wafer of devices, as described below. 
     Though the embodiments below show separating a wafer into individual LEDs  12 , the techniques described may be used to separate a wafer into groups of LEDs. Though the embodiments below refer to a sapphire growth substrate, the techniques described may be applied to any suitable substrate. 
     One embodiment of the invention is illustrated in  FIGS. 2, 3, 4, 5, and 6 . In the embodiment illustrated in  FIGS. 2-6 , the substrate  10  is thinned, then scribed, then thinned again. 
     In  FIG. 2 , an exemplary group of several LEDs  12  is formed on a substrate  10 . For example, LEDs  12  may be the devices illustrated in  FIG. 1  or any other suitable device. Although six LEDs  12  are shown, there is no expressed limit to the number of LEDs that may be created on a single substrate, nor are these LEDS required to be in a group. The LEDs in the figure are simply examples of a “some” LEDs on a portion of a substrate  10  or a complete substrate  10 . 
     In  FIG. 3 , wafer handling tape  34  is attached to LEDs  12 . A portion  30  of the thickness of the growth substrate  10  is removed by any suitable technique such as, for example, mechanical techniques such as grinding leaving a remaining portion  32 . The substrate  10  is thinned to a thickness which accommodates the scribing described in  FIG. 4 . The thickness of the substrate  10  in  FIG. 2  before thinning may be, for example, at least 300 μm thick in some embodiments and no more than 1500 μm thick in some embodiments, though the substrate may be thicker than 1500 μm in some embodiments. The remaining portion  32  of substrate  10  may be, for example, no more than 300 μm thick in some embodiments, no more than 275 μm thick in some embodiments, and no more than 250 μm thick in some embodiments. 
     In  FIG. 4 , the regions  38  between individual LEDs  12  or groups of LEDs  12  are scribed to form cracks or notches  40  in the remaining portion of substrate  32 . The cracks  40  are localized in a portion of the thickness of the substrate  32  that is closest to LEDs  12 . The cracks  40  do not fully penetrate the remaining portion of substrate  32 . Cracks  40  may be formed by, for example, laser scribing, where a laser  36  is shined through the substrate  32  in regions  38 , or stealth dicing, where a modified layer in the substrate is formed by focusing a laser inside the substrate. For example, a femto-second laser with wavelengths between 266 and 355 nm may be used for laser scribing and a laser with wavelengths between 800 and 1100 nm may be used for stealth dicing. 
     In  FIG. 5 , the remaining portion  32  of the sapphire substrate  10  is then thinned to expose the tops of cracks  40  formed in  FIG. 4 . The substrate may be thinned by any suitable technique including mechanical techniques such as grinding. The thickness of the removed portion  42  of remaining portion  32  may be at least 100 μm thick in some embodiments and no more than 200 μm thick in some embodiments. The cracked portion  44  remaining after thinning may be, for example, no more than 60 μm thick in some embodiments, no more than 50 μm thick in some embodiments, and no more than 40 μm thick in some embodiments. The cracked portion  44  remaining after thinning is sufficiently thick in some embodiments to mechanically support the semiconductor structure. After the thinning illustrated in  FIG. 5 , some or all of the cracks  40  extend through the entire remaining thickness of cracked portion  44 . Preferably all of the cracks  40  extend through the entire remaining thickness of cracked portion  44 . 
     In  FIG. 6 , tape  34  may be stretched to separate individual LEDs  12  or groups of LEDs  12  in the gaps  46  where cracks  40  were formed. Each LED  12  or group of LEDs  12  has a small piece of substrate  10  (cracked portion  44 ) attached to the top of the semiconductor structure. The cracked portion  44  may be thick enough to mechanically support the semiconductor structure. The cracked portions  44  may have smooth or rough edges. 
     Another embodiment is illustrated in  FIGS. 7, 8, 9, 10, and 11 . In the embodiment illustrated in  FIGS. 7-11 , the substrate is etched, then thinned. 
     In  FIG. 7 , a mask  50  is formed over a sapphire substrate  10  and patterned to form openings  52  aligned with areas where the substrate is later separated. The openings  52  may correspond to the edges of a single LED or multiple LEDs in a group. The semiconductor structure may be grown on substrate  10  before or after forming openings  52 . The semiconductor structure may be patterned such that the semiconductor structure is removed from regions between LEDs. 
     In  FIG. 8 , the sapphire substrate  10  is etched to form notches  54  in substrate  10  in the openings  52  in mask  50 . In some embodiments, notches  54  are at least one micron deep. In some embodiments, notches  54  are at least one micron wide. The substrate  10  is etched by any suitable technique such as, for example, dry etching or wet etching. 
     In  FIG. 9 , the mask  50  is stripped, leaving substrate  10  with notches  54  formed in regions  52 . 
     As an alternative to the masking, etching, and stripping technique illustrates in  FIGS. 7, 8, and 9 , in some embodiments, before growing the semiconductor structure of LEDs  12 , notches are formed in the substrate  10  by a technique other than etching. For example, notches  54  may be formed by laser scribing or stealth dicing, as described above, by laser dicing with a UV laser with a wavelength between 266 and 355 nm, or by mechanical dicing, for example using a blade. Such techniques may not require first masking the substrate, though a patterned mask may be used. 
     In  FIG. 10 , LEDs  12  are formed in the regions between notches  54 . For example, the semiconductor structures grown on substrate  10  may be formed into LEDs by etching and forming metal layers to form contacts and interconnects, as described above in reference to  FIG. 1 . 
     In  FIG. 11 , the LEDs  12  are connected to wafer handling tape  34 . Substrate  10  is thinned by any suitable technique, as described above. The portion  56  of the substrate  10  that is removed is sufficiently thick that some or all of notches  54  are exposed, separating individual LEDs  12  or groups of LEDs. Preferably all notches  54  are exposed. A portion  50  of substrate  10  remains attached to each LED  12  or group of LEDs. Portion  50  is sufficiently thick in some embodiments to mechanically support the semiconductor structure of LEDs  12 . 
     Another embodiment is illustrated in  FIGS. 12, 13, 14, and 15 . In the embodiment illustrated in  FIGS. 12-15 , the LEDs are partially formed, the substrate is notched, the LEDs are completed, then the substrate is thinned. 
     In  FIG. 12 , LEDs are partially formed on substrate  10 . For example, the semiconductor structure of the LEDs may be grown on substrate  10 . 
     In  FIG. 13 , notches  54  are formed in the substrate  10  in the regions  52  between individual LEDs or between groups of LEDs. Notches  54  may be formed by any suitable technique, including, for example, etching, sawing, or laser scribing. 
     In  FIG. 14 , the LEDs  12  are completed for example by etching and forming metal layers to form contacts and interconnects, as described above in reference to  FIG. 1 . 
     In  FIG. 15 , the LEDs  12  are connected to wafer handling tape  34 . Substrate  10  is thinned by any suitable technique, as described above. The portion  56  of the substrate  10  that is removed is sufficiently thick that some or all of notches  54  are exposed, separating individual LEDs  12  or groups of LEDs. Preferably all notches  54  are exposed. A portion  50  of substrate  10  remains attached to each LED  12  or group of LEDs. Portion  50  is sufficiently thick in some embodiments to mechanically support the semiconductor structure of LEDs  12 . 
     Another embodiment is illustrated in  FIGS. 16 and 17 . In  FIG. 16 , LEDs  12  are grown on a substrate  10 . For example, LEDs  12  may be the devices illustrated in  FIG. 1  or any other suitable device. Regions  38  between individual LEDs  12  or groups of LEDs  12  are laser scribed from the side of the substrate  10  on which LEDs  12  are formed. The scribing forms cracks or notches  40  in a portion of the thickness of substrate  10 . The cracks  40  are localized in a portion of the thickness of the substrate that is closest to LEDs  12 . Cracks  40  may be, for example, at least 30 μm deep in some embodiments, and no more than 100 μm in some embodiments. 
     LEDs  12  is then mounted on a structure  34  such as a frame, support wafer, or handling tape, as shown in  FIG. 17 . A portion  30  of the thickness of the growth substrate  10  is removed by any suitable technique such as, for example, mechanical techniques such as grinding. The thickness of the substrate  10  before thinning may be, for example, between 300 and 2000 μm. In some embodiments, the substrate is thinned beyond a thickness where some or all of the tops of cracks  40  (in the orientation illustrated in  FIG. 17 ) are reached. Preferably all of the tops of cracks  40  are reached. The remaining portion  32  of substrate  10  is sufficiently thick to mechanically support the semiconductor structure of LEDs  12  in some embodiments. 
     Light emitting devices formed by the techniques described above may have several advantages. Because some of the substrate remains attached to the final device, the fragile semiconductor structure is supported by the substrate, which may reduce the occurrence of failure due to cracking, may eliminate the need for expensive and complex thick metal interconnects that support the semiconductor structure, and may eliminate the need for underfill or other structures to support the semiconductor structure. The device can be solder mounted. In addition, because the substrate is thinned, the amount of light that escapes through the sides of the substrate may be reduced as compared to a device where the entire thickness of the substrate remains attached to the semiconductor structure. Accordingly, devices formed by the techniques described above may avoid or reduce an efficiency penalty typically associated with devices where the substrate remains attached to the semiconductor structure. 
     Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the spirit of the inventive concept described herein. Therefore, it is not intended that the scope of the invention be limited to the specific embodiments illustrated and described.