Abstract:
A method of cleaving a substrate is disclosed. A species, such as hydrogen or helium, is implanted into a substrate to form a layer of microbubbles. The substrate is then annealed a pressure greater than atmosphere. This annealing may be performed in the presence of the species that was implanted. This diffuses the species into the substrate. The substrate is then cleaved along the layer of microbubbles. Other steps to form an oxide layer or to bond to a handle also may be included.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to the provisional patent application entitled “Pressurized Treatment of Substrates to Enhance Cleaving Process” filed Aug. 7, 2009 and assigned U.S. Application No. 61/232,020, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    This invention relates to substrate cleaving, and, more particularly, to a process that forms microbubbles that are used to cleave a substrate. 
       BACKGROUND 
       [0003]    An ion implanter includes an ion source for converting a gas or a solid material into a well-defined ion beam. The ion beam typically is mass analyzed to eliminate undesired ion species, accelerated to a desired energy, and implanted into a target. The ion beam may be distributed over the target area by electrostatic or magnetic beam scanning, by target movement, or by a combination of beam scanning and target movement. The ion beam may be a spot beam or a ribbon beam having a long dimension and a short dimension. 
         [0004]    Implantation of an ion species may allow a substrate to be cleaved. The species forms microbubbles in the substrate material. These microbubbles are pockets of a gas or regions of an implanted species below the surface of the substrate that may be arranged to form a weakened layer or porous layer in the substrate. A later process, such as heat, fluid, chemical, or mechanical force, is used to separate the substrate into two layers along the weakened layer or porous layer. 
         [0005]    Ostwald ripening may occur in substrates that have microbubbles. Ostwald ripening is a thermodynamic process where larger particles grow by drawing material from smaller particles because larger particles are more stable than smaller particles. Any atoms or molecules on the outside of a particle, which may be, for example, a microbubble, are energetically less stable than the more ordered atoms or molecules in the interior of a particle. This is partly because any atom or molecule on the surface of a particle is not bonded to the maximum possible number of neighboring atoms or molecules, and, therefore, is at a higher energy state than those atoms or molecules in the interior. The unsatisfied bonds of these surface atoms or molecules give rise to surface energy. A larger particle, with a greater volume-to-surface ratio, will have a lower surface energy. To lower surface energy, atoms or molecules on the surface of smaller, less stable particles will diffuse and add to the surface of the larger, more stable particles. The shrinking of smaller particles will minimize total surface area and, therefore, surface energy. Thus, smaller particles continue to shrink and larger molecules continue to grow. 
         [0006]      FIG. 1  is a view of Ostwald ripening in a substrate.  FIG. 1  is merely an illustration and is not to scale. A species that forms the microbubbles  100  in the substrate  138  makes smaller microbubbles  101  and larger microbubbles  102 . Due to their greater volume-to-surface ratio and lower surface energy, the larger microbubbles  102  will be more stable than the smaller microbubbles  101 . To lower their surface energy, the smaller microbubbles  101  will diffuse to the larger microbubbles  102  (as illustrated by the dotted lines in  FIG. 1 ). Overall, the smaller microbubbles  101  may shrink and the larger microbubbles  102  may grow. Some of the species in the microbubbles  100  also may diffuse out of the substrate  138  altogether. Ostwald ripening and diffusion of the species out of the substrate  138  will affect the substrate  138  when it is cleaved along the weakened layer or porous layer represented by the dashed line  103 . 
         [0007]    Previous methods have implanted hydrogen or a combination of hydrogen and helium to cleave a substrate. This typically requires a dose of hydrogen of greater than approximately 2E16 cm −2 , such as approximately 6E16 cm −2 , or a co-implant of hydrogen and helium with a dose of approximately 1E16 cm −2  each. Such high doses during implant make this cleaving process expensive and time-consuming. Accordingly, there is a need in the art for an improved process to cleave a substrate and, more particularly, a process that will form microbubbles that are used to cleave a substrate. 
       SUMMARY 
       [0008]    According to a first aspect of the invention, a method of cleaving a substrate is provided. The method comprises implanting a species into a substrate to form a layer of microbubbles. The substrate is annealed at a pressure greater than 1 atm in an environment of the species. The substrate is cleaved along the layer of microbubbles. 
         [0009]    According to a second aspect of the invention, a method of cleaving a substrate is provided. The method comprises implanting a first dose of a species into a substrate to form a layer of microbubbles. The species may be H or He. The substrate is annealed at a pressure greater than 1 atm in an environment of the species. This annealing diffuses the species into the substrate and forms a second dose of the species in the substrate that is larger than the first dose. The substrate is cleaved along the layer of microbubbles. 
         [0010]    According to a third aspect of the invention, a method of cleaving a substrate is provided. The method comprises forming an oxide layer on a substrate. A species is implanted into the substrate to form a layer of microbubbles. The species may be H or He. The substrate is annealed at a pressure greater than 1 atm in an environment of the species, bonded to a handle, and cleaved along the layer of microbubbles. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]    For a better understanding of the present disclosure, reference is made to the accompanying drawings, which are incorporated herein by reference and in which: 
           [0012]      FIG. 1  is a cross-sectional view of Ostwald ripening in a substrate; 
           [0013]      FIG. 2  is a simplified block diagram of a beam-line ion implanter; 
           [0014]      FIG. 3  is a cross-sectional view of an embodiment of an implanted substrate with a layer of microbubbles; 
           [0015]      FIGS. 4A-4E  are cross-sectional views of an embodiment of cleaving with diffusion; and 
           [0016]      FIGS. 5A-5H  are cross-sectional views of an embodiment of silicon-on-insulator (SOI) substrate fabrication that uses substrate cleaving with diffusion. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 2  is a simplified block diagram of a beam-line ion implanter. Those skilled in the art will recognize that the beamline ion implanter  200  is only one of many examples of differing beamline ion implanters. In general, the beamline ion implanter  200  includes an ion source  280  to generate ions that are extracted to form an ion beam  281 , which may be, for example, a ribbon beam or a spot beam. The ion beam  281  may be mass analyzed and converted from a diverging ion beam to a ribbon ion beam with substantially parallel ion trajectories in one instance. The beamline ion implanter  200  may further include an acceleration or deceleration unit  290  in some embodiments. 
         [0018]    An end station  211  supports one or more workpieces, such as the substrate  138 , in the path of the ion beam  281  such that ions of the desired species are implanted into substrate  138 . In one instance, the substrate  138  may be a semiconductor wafer having a disk shape, such as, in one embodiment, a 300 mm diameter silicon wafer. However, the substrate  138  is not limited to a silicon wafer. The substrate  138  also could be, for example, a flat panel, solar, or polymer substrate. The end station  211  may include a platen  295  to support the substrate  138 . The end station  211  also may include a scanner (not shown) for moving the substrate  138  perpendicular to the long dimension of the ion beam  281  cross-section, thereby distributing ions over the entire surface of substrate  138 . 
         [0019]    The ion implanter  200  may include additional components known to those skilled in the art such as automated workpiece handling equipment, Faraday sensors, or an electron flood gun. It will be understood to those skilled in the art that the entire path traversed by the ion beam is evacuated during ion implantation. The beamline ion implanter  200  may incorporate hot or cold implantation of ions in some embodiments. 
         [0020]    One skilled in the art will recognize other systems and processes involved in semiconductor manufacturing, other systems and processes involved in plasma treatment, or other systems and processes that use accelerated ions that may perform the process described herein. Some examples of this, for example, are a plasma doping tool, an ion shower, or a plasma immersion tool. Other semiconductor processing equipment known to those skilled in the art that can accelerate species and implant species into a substrate also may be used. Thus, this process is not limited solely to beam-line ion implanters. 
         [0021]      FIG. 3  is an embodiment of an implanted substrate with a layer of microbubbles. A species  300 , which may be at least one chemical element in this particular embodiment, is implanted into the substrate  138 . In some embodiments, hydrogen may be implanted at approximately 6E16 cm −2  or helium and hydrogen co-implants may be implanted at approximately 1E16 cm −2  to produce a layer of microbubbles  301  below the surface of the substrate  138 . The substrate is later cleaved along this layer of microbubbles  301 . In other embodiments, oxygen, nitrogen, other rare or noble gases, or a combination of gases are used to form the layer of microbubbles  301 . This may be performed in one implant or a series of implants. Other species known to those skilled in the art also may be used to form the layer of microbubbles  301 . Greater implant energy of the species  300  generally will result in a greater implant depth of microbubbles  301 . Greater implant dose of the species  300  generally will result in a greater concentration of the species  300  that form the microbubbles  301 . 
         [0022]      FIGS. 4A-4E  are cross-sectional views of an embodiment of cleaving with diffusion. Embodiments of this process may be applied to, for example, silicon-on-insulator (SOI) or 3D integrated circuit (IC) or stacked chip configurations. This process also may be applicable to the fabrication of substrates that are used in, for example, flat panels, thin films, solar cells, LEDs, other thin metal sheets, or other devices. The substrate that is cleaved using this process may be, for example, Si, SiC, GaN, GaP, GaAs, polysilicon, Ge, quartz, or other materials. 
         [0023]    In fabricating a cleaved workpiece, a substrate  138  is provided (A). The substrate  138  may be referred to as a donor substrate. At least one species  300 , such as hydrogen, helium, or hydrogen and helium, for example, is implanted (B) into the substrate  138  to form a layer of microbubbles  301  (as illustrated by the dotted line in  FIG. 4B ). Forming the microbubbles  301  with the species  300  also may include creating damage sites where the microbubbles  301  grow either during implant or a later processing step. Other species such as oxygen, nitrogen, other rare or noble gases, or a combination of gases also may be implanted. This may be a low-dose implant of approximately 1E14 cm −2  to approximately 2E15 cm −2  in one instance. The layer of microbubbles  301  are a distance (Rp) below the surface of the substrate  138 . The layer of microbubbles  301  initiates a defect plane at the desired depth, which depends on the implant energy. Compared to a dose previously used to cleave a substrate  138  without further implant or diffusion steps, this particular implant to form the microbubbles  301  may use a lower implant dose. 
         [0024]    In one particular embodiment, the temperature of the substrate  138  is increased during the implant. This may be from about 100° C. to about 400° C. If the dose of the species  300  is above the amorphizing threshold, the end-of-range defect density (defect slip lines) tends to be higher. This may remove the need for an annealing step (C) in one instance. The substrate  138  may be heated with lamps or using the platen  295  as seen in  FIGS. 2-3 . The substrate  138  may be heated during implant or pre-heated prior to implant. 
         [0025]    Following formation of the microbubbles  301 , the defect planes are formed during an anneal (C) of, for example, between approximately 400° C. and approximately 600° C. The annealing will grow the microbubbles  301  using Ostwald ripening. In one instance, the anneal is for approximately 500-600° C. for about 5-10 minutes. The temperature and duration of the anneal is optimized for the type of the substrate  138 . 
         [0026]    Following the anneal (C), the substrate  138  is annealed in a low-temperature, high-pressure ambient of species  500  that is diffused into the substrate  138  (D). The species  500  may be, for example, hydrogen, helium, or hydrogen and helium. The species  500  may be mixed with a dilutant gas, such as nitrogen. The temperature during diffusion may be, for example, between approximately 200° C. and approximately 400° C., though other temperatures are possible. The high-pressure process provides an abundant source of the species  500  at the surface of the substrate  138 . This forces the species  500  to diffuse into the substrate  138  and to decorate the defects in the substrate  138  caused by the microbubbles  301 . Thus, the dose of the species that forms the microbubbles  301  increases from the initial implanted dose to a second, higher dose. In one embodiment, the second, higher dose is approximately 7E16 cm −2  for hydrogen in a silicon substrate  138 . The second, higher dose may be an order of magnitude greater for a GaN substrate  138 . 
         [0027]    In one instance, the species  500  matches the species  300 , though the species  500  and species  300  also may be different. In one embodiment, the species  300  and species  500  may both be hydrogen. This enables growth of the microbubbles  301  without any interactions. In another embodiment, the species  300  and species  500  may both be a combination of hydrogen and helium. In yet another embodiment, the species  300  is hydrogen or nitrogen and the species  500  is helium or neon. If the substrate  138  is silicon, the species  300  may be chemically-reactive to assist in stabilizing the wall of the individual microbubbles  301  while the species  500  may diffuse through silicon. The species  500  and species  300  may be selected to stabilize and fill the microbubbles  301 . 
         [0028]    This higher pressure in step D may be, for example, approximately 2× to several 100× greater than atmospheric pressure because at a lower pressure, the species  500  tends to diffuse to the ambient rather than into the substrate  138 . In one particular embodiment, the pressure is approximately 10× to 20× greater than atmospheric pressure. The temperature during step D is configured to increase the diffusion of the species  500  into the substrate  138  and increase the solubility of the species  500  in the substrate  138 . In one particular instance, this temperature is between 400° C. and 800° C. The duration of this step D is determined by the type of substrate  138  and the amount of species  500  that is needed for cleaving. 
         [0029]    This species  500  will cause Ostwald ripening of the largest microbubbles  301 . The species  300  formed nucleus cavities that hold the species  500  during the anneal. These nucleus cavities caused by species  300  may be damage to the substrate  138 . Crystalline silicon, for example, has all its bonds satisfied. If the bonds are broken, the hydrogen, for example, will preferentially attach to the dangling bonds and form the nucleus cavities. Helium is a noble gas and not as reactive as hydrogen, but may in one instance “stuff” a nucleus cavity formed by hydrogen. Other species may do the same. 
         [0030]    During the thermal diffusion, an anneal, or another thermal process, the substrate  138  fractures or cleaves along the layer of microbubbles  301  (E). In another embodiment, a mechanical, chemical, or fluid force is used to fracture or cleave the substrate  138  along the layer of microbubbles  301 . The remaining substrate  138  that is cleaved off may be reused in some embodiments. In another particular embodiment, the substrate  138  is bonded to another workpiece, such as a handle, prior to fracturing or cleaving the substrate  138  along the layer of microbubbles  301 . The substrate  138  may require polishing in one instance. 
         [0031]    The diffusion of species  500  into the substrate  138  reduces the dose of the species  300  required to cleave the substrate  138 . This significantly reduces the cost of the cleaving process because the entire dose of the species  300  does not need to be implanted into the substrate  138 . In an alternate embodiment, the anneal (C) and the diffusion (D) are performed at least partially simultaneously. This combined anneal (C) and diffusion (D) is a high-pressure, high-temperature process. In yet another embodiment, the species  500  is diffused into the substrate  138  during a plasma-enhanced chemical vapor deposition (PECVD) process. 
         [0032]      FIGS. 5A-5H  are cross-sectional views of an embodiment of SOI substrate fabrication that uses substrate cleaving with diffusion. Embodiments of this process are not solely limited to SOI substrates. Embodiments of this process are applicable to other cleaving implants such as 3D IC or stacked chip configurations. This process also may be applicable to the fabrication of substrates that are used in, for example, flat panels, thin films, solar cells, LEDs, other thin metal sheets, or other devices. The substrate that is cleaved using this process may be, for example, Si, SiC, GaN, GaP, GaAs, polysilicon, Ge, quartz, or other materials known to those skilled in the art. 
         [0033]    In fabricating an SOI substrate, a substrate  138  is provided (A). The substrate  138  may be referred to as a donor substrate. The substrate  138  has a thermal oxide layer  400  formed on at least one surface (B). At least one species  300 , such as hydrogen or helium, for example, is then implanted (C) into the silicon of the substrate  138  to form a layer of microbubbles  301  (as illustrated by the dotted line in  FIG. 5C ). Other species such as oxygen, nitrogen, other rare or noble gases, or a combination of gases also may be implanted. 
         [0034]    The substrate is subject to an anneal (D) similar to step C in  FIG. 4 . A species  500  is then diffused into the substrate  138  (E) similar to step D of  FIG. 4 . The species  500  may be, for example, hydrogen, helium, or hydrogen and helium. In one instance, the species  500  matches the species  300 , though the species  500  and species  300  also may be different. This species  500  will cause Ostwald ripening of the largest microbubbles  301 . Of course, the anneal (D) is optional and may be removed if the implant (C) is performed at about 100° C. to about 400° C. 
         [0035]    This substrate  138  is then flipped over, bonded to a handle  401 , and annealed (F). In some embodiments, the substrate  138  is cleaned prior to bonding it to the handle  401 . During the anneal or another thermal process, the substrate  138  fractures or cleaves along the layer of microbubbles  301  (G). The formed SOI substrate  402 , including the thermal oxide layer  400  and silicon overlayer  403 , may require polishing to make the surface smooth enough for device manufacture (H). In another embodiment, a mechanical, chemical, or fluid force is used to fracture or cleave the substrate  138  along the layer of microbubbles  301 . The remaining substrate  138  may be reused in some embodiments. 
         [0036]    For any of the embodiments of  FIGS. 5A-5H , the dose of species  300  during ion implantation is lowered compared to a dose of a species  300  implant without diffusion of species  500 , leading to cost savings. In an alternate embodiment, the anneal (D) and the diffusion (E) are performed at least partially simultaneously. This combined anneal (D) and diffusion (E) is a high-pressure, high-temperature process. In yet another embodiment, the species  500  is diffused into the substrate  138  during a PECVD process. 
         [0037]    The surface roughness of the SOI substrate  402  and the silicon overlayer  403  after cleaving depends on the size of the microbubbles in the layer of microbubbles  301 . Smaller microbubbles in the layer of microbubbles  301  will lead to a smoother surface of the SOI substrate  402  and the silicon overlayer  403  after cleaving. This may eliminate or limit the polishing step in some embodiments. 
         [0038]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Furthermore, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the present disclosure as described herein.