Abstract:
A wire saw for cutting hard materials includes a carbon nanotube fiber wire spun from carbon nanotubes. The carbon nanotube fiber wire may be made from a plurality of fibers, each fiber being spun from carbon nanotubes, the fibers being twisted together to form the wire. Furthermore, the wire may also include diamond particles, silicon carbide particles and/or extra carbon nanotubes to enhance the abrasive properties of the wire. A method is provided for slicing a silicon boule including: linearly translating a carbon nanotube fiber wire between rotating drums while maintaining the wire under tension; using a fixture, moving the silicon boule onto the moving tensioned wire, whereby the wire cuts into the silicon; delivering lubricating fluid to the surface of the silicon where contact is made with the wire; and collecting the lubricating fluid after it leaves the surface of the silicon.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to wire saws for cutting hard materials, and more particularly to a wire saw with a carbon nanotube fiber wire for cutting silicon boules. 
       BACKGROUND OF THE INVENTION 
       [0002]    Wire saws are used to cut hard and brittle materials. They are used to cut silicon wafers from silicon boules/ingots for the semiconductor and photovoltaic industries. Schematic diagrams of wire saws used for cutting silicon wafers are shown in  FIGS. 1 ,  2  &amp;  3 .  FIG. 1  shows a prior art wire saw with a single wire  105  fed between two reels  110 . The wire  105  passes over four drums  115  multiple times, forming a web of wires  120  for cutting a hard material  125  held by a fixture  130 . For the sake of simple illustration, a web  120  of only three cutting wires is shown. However, in practice a web for cutting a silicon boule will contain a large number of wires. The number of wires being determined by the number of wafers to be cut simultaneously from the boule. The drums  115  are used both to move the wire  105  linearly, as indicated, and to help maintain a proper tension in the wire  105 . (The wire may be tensioned by two tensioning devices positioned between the first reel and the drums and the second reel and the drums. Such tensioning devices are not shown in the figure.) The drums  115  rotate about their central axes. The hard material  125  is fixed to a fixture  130 . The fixture  130  is configured to move in a direction perpendicular to the web of wires  120 , such that the hard material  125  can be moved onto the web of wires  120  and cut by the moving wires. An example of such a wire saw and further details are provided in U.S. Pat. No. 5,829,424. 
         [0003]      FIG. 2  shows a prior art wire saw with a multiplicity of closed loop wires  205 . Three wires  205  form a web of wires  220  for cutting a hard material  125  held by a fixture  130 . For the sake of simple illustration, a web  220  of only three cutting wires is shown. However, in practice a web for cutting a silicon boule will contain a large number of wires. The number of wires being determined by the number of wafers to be cut simultaneously from the boule. The drums  215  are used both to move the wires  205  linearly, as indicated, and to tension the wires  205 . The drums  215  rotate about their central axes. In order to tension the wires  205 , the separation of the drums  215  can be adjusted. The hard material  125  is fixed to a fixture  130 . The fixture  130  is configured to move in a direction perpendicular to the web of wires  220 , such that the hard material  125  can be moved onto the web of wires  220  and cut by the moving wires. An example of such a wire saw and further details are provided in U.S. Pat. No. 6,550,364. 
         [0004]      FIG. 3  shows a prior art variation on the wire saw of  FIG. 2 . In  FIG. 3  the hard material  125  is fixed to a fixture  355  which allows for reciprocating motion of the hard material  125  relative to the wire  205 . The fixture  355  also allows for the hard material  125  to be moved vertically, as shown, perpendicular to the wires  205 . This combination of vertical and oscillating motions allows for cutting of the hard material  125  on stationary wires  205 . Although, in practice the wires  205  may also be moved as indicated either intermittently or continuously. The fixture is comprised of a first part  330  which is able to move laterally relative to a second part  335 . The second part  335  is able to rotate about an axis defined by the shaft  336 , as shown. The second part  335  is coupled to a third part  340  by the shaft  336 . The third part  340  is able to move vertically relative to a fixed fourth part  345 , as shown. The vertical and lateral movements are facilitated by bearings  350 . An example of such a wire saw and further details are provided in U.S. Pat. No. 6,886,550. 
         [0005]      FIG. 4  shows a schematic of a cross section through a silicon boule  425  during the process of being cut by a wire saw, such as shown in  FIGS. 1 ,  2  and  3 . The plane of the section is perpendicular to the length of the wires  405 . Thus in a wire saw with wires that move relative to the silicon boule  425 , the wires  405  in  FIG. 4  will move linearly in a direction perpendicular to the plane of the section and the silicon boule  425  will be move down onto the cutting wires  405 , as shown. The wires  405  cut into the silicon  425 , forming slots  426 . As cutting continues, the slots  426  are cut deeper into the silicon boule  425 . Such a slot is referred to as a kerf. The kerf is typically wider than the diameter of the cutting wire  405 . On completion of cutting, the silicon remaining between the slots  426  will be silicon wafers. The cutting process relies on either: (1) a cutting fluid containing abrasive particles in the slots  426 ; or (2) wires  426  covered with abrasive particles, such as silicon carbide. Furthermore, a lubricating fluid is required to conduct away heat generated during cutting and to remove silicon debris from the slots  426 . 
         [0006]    The photovoltaic industry has a high demand for thin wafers, currently less than 200 microns thick and expected soon to reach 100 microns thickness. In order to efficiently produce silicon wafers with ever diminishing thickness the following issues must be addressed: (1) there is a need to reduce the loss of silicon from the kerf when cutting wafers; (2) there is a need to reduce the viscosity of the cutting fluid in order to maintain throughput for wafer cutting as the wafer thickness is reduced; and (3) there is a need to be able to efficiently recapture silicon lost from the kerf, to be recycled into silicon boules. 
         [0007]    The loss of silicon from the kerf can be reduced by using cutting wires with smaller diameters. Currently, cutting wires are no smaller than 120-150 microns in diameter, primarily limited by the strength of available steel wire. Clearly, without a reduction in wire diameter, this will soon lead to a kerf which is wider than the wafer being cut, as the industry requires thinner and thinner wafers. The problem with thinner wires made of steel and similar materials is that they do not have the mechanical strength required for the current sawing process. Consequently, there is a need for thinner sawing wires with better mechanical properties. 
         [0008]    The viscosity of the lubricating/cutting fluid must be reduced in order to maintain the current throughput and efficiency for cutting a boule, as the wafer thickness is reduced. As the width of the kerf is reduced this also requires the viscosity of the lubricating/cutting fluid to be reduced to allow for the same throughput and efficiency. The introduction of abrasive wires—metal wires coated with diamond particles—has allowed for reduced viscosity of the cutting fluid, since there is no longer a need for abrasive particulates in the cutting fluid. (Although, currently the processes used to coat wires with diamond do not produce sufficiently uniform wires of the lengths required for cutting silicon wafers for photovoltaic applications.) In this case the fluid becomes primarily a lubricating fluid. However, current lubricating fluids based on glycol and similar chemicals will be too viscous as kerf widths are reduced to approach 100 microns. The viscosity of current lubricating fluids will require a reduction in the speed of the wire as these small kerf widths are approached. Furthermore, in order to increase throughput, higher wire speeds are required, which will require lower viscosity lubricating fluids. Consequently, there is a need for lower viscosity lubricating fluids. 
         [0009]    In order to capture the silicon lost from the kerf, and to be able to recycle the silicon into semiconductor grade silicon boules, the following must be addressed. First, there must be an efficient means for collecting the silicon lost from the kerf. Most of the silicon ends up in the lubricating fluid in the form of particulates which must be filtered out. This can only easily be achieved when using lubricating fluids which do not contain abrasive particles such as silicon carbide. Second, the lubricating fluids must be free from metal contaminants which can render the silicon unusable for making semiconductor grade silicon boules. The use of metal cutting wires results in metal contaminants getting into the lubricating fluid and onto the silicon particulates lost from the kerf. Consequently, there is a need for cutting/lubricating fluids from which silicon particulates can efficiently be separated and there is a need for cutting wires that do not contaminate the silicon lost from the kerf. 
         [0010]    Therefore, there remains a need for tools and methods that can meet the wafer cutting requirements of the semiconductor and photovoltaic industries while allowing for cost reduction and increased efficiency. 
       SUMMARY OF THE INVENTION 
       [0011]    The concepts and methods of the invention allow the cost and complexity of cutting hard materials to be reduced by providing a wire saw with a wire having better mechanical properties than for metal wires. Furthermore, the invention provides a cutting tool and method which do not produce metal contamination in the cutting lubricant/slurry. This reduces the cost and complexity of recycling silicon kerf loss from the cutting lubricant after slicing silicon ingots. This can reduce the cost for broad market applicability as well as providing yield improvements. According to aspects of the invention, these and other advantages are achieved with the use of carbon nanofiber and carbon nanotube fiber wires, instead of the metal wires used in the prior art. As such, this invention contemplates a wire saw for cutting hard materials, the wire saw including a carbon nanotube fiber wire spun from carbon nanotube filaments. The carbon nanotube fiber wire may be made from a plurality of fibers, each fiber being spun from carbon nanotubes, the fibers being twisted together to form the wire. Furthermore, the wire may also have diamond particles, silicon carbide particles and/or extra carbon nanotubes incorporated into the wire to enhance the abrasive properties of the wire. The diamond particles, silicon carbide particles and/or carbon nanotubes may be incorporated during the process of twisting together the fibers to form a wire. 
         [0012]    According to further aspects of the invention, a method is provided for slicing a silicon boule including the following steps: linearly translating a carbon nanotube fiber wire between two rotating drums while maintaining the wire under tension; using a fixture, moving the silicon boule onto the moving tensioned wire, whereby the wire cuts into the silicon; delivering lubricating fluid to the surface of the silicon where contact is made with the wire; and collecting the lubricating fluid after it leaves the surface of the silicon. The collected lubricating fluid is then available for recycling, which may include recovering silicon from the fluid. Furthermore, the recycling may include recovering carbon nanotubes from the lubricating fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: 
           [0014]      FIG. 1  is a schematic of a prior art wafer wire saw with a single wire fed between two spools; 
           [0015]      FIG. 2  is a schematic of a prior art wafer wire saw with closed wire loops; 
           [0016]      FIG. 3  is a schematic of a prior art wafer wire saw with reciprocating motion between the hard material and the wire; 
           [0017]      FIG. 4  is a schematic representation of a cross-section through a silicon boule being cut on a wafer wire saw; 
           [0018]      FIG. 5  illustrates a plied carbon nanotube fiber wire of the invention; 
           [0019]      FIG. 6  illustrates a schematic cross-section of a carbon nanotube fiber wire with incorporated diamond grit, according to the invention; 
           [0020]      FIG. 7  illustrates a schematic cross-section of a carbon nanotube fiber wire with incorporated carbon nanotubes, according to the invention; 
           [0021]      FIG. 8  is a schematic of a wafer wire saw of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. 
         [0023]    In general, the present invention contemplates incorporation of carbon nanotube fiber wires in wire saws used for cutting hard materials, in particular silicon wafers. Wire saws are used widely in industries such as the semiconductor and photo-voltaic industries. For example, see U.S. Pat. Nos. 5,829,424, 6,550,364, and 6,886,550, all of which are incorporated by reference herein. Wire saws include reel-to-reel wire saws, such as shown in  FIG. 1 , requiring very long wires, and closed-loop wire saws, such as shown in  FIGS. 2 and 3 , which need only relatively short wires. The present invention contemplates incorporating carbon nanotube fiber wires into both reel-to-reel and closed-loop wire saws. 
         [0024]    Carbon nanotubes are nanometer-scale cylinders with walls formed of graphene—single atom thick sheets of graphite. Nanotubes may be either single-walled (cylinder wall composed of a single sheet of graphene, referred to as SWNTs) or multi-walled (cylinder wall composed of multiple sheets of graphene, referred to as MWNTs). Single-walled nanotubes have a diameter of the order of one nanometer. Nanotubes exhibit extraordinary mechanical properties, most notably exceptional strength. Carbon nanotubes can be spun into fibers and these fibers can then be plied (twisted) together to form multi-ply yarns. These fibers and yarns can be in excess of one meter in length and exhibit tensile strength in the range of 150-460 MPa. See Zhang et al., Science 306, 1358-(2004) and Li et al., Science 304, 276 (2004). The present invention contemplates using SWNTs and/or MWNTs to form the fibers in the carbon nanotube fiber wires. 
         [0025]      FIG. 5  shows an illustration of a magnified view of a carbon nanotube fiber wire  505 , according to the invention. Such a carbon nanotube fiber wire  505  replaces the metal wires currently used in wire saws. In  FIG. 5 , a two-ply wire is shown—the wire  505  is comprised of two spun fibers  506 , 10 microns in diameter, twisted together to form a 20 micron diameter wire. Spinning the carbon nanotubes together to form the fibers  506 , and then twisting together the fibers to form the wire  505  adds strength to the wire  505 . Note that a 10 micron diameter fiber will contain of the order of 10 6  nanotubes spun together. The wire is not restricted to fibers of a particular diameter, and is not limited to a specific number of plied fibers. For example, four 8 micron diameter fibers could be plied together to form an approximately 24 micron diameter wire. Furthermore, a large number of smaller diameter fibers can be plied together to form a wire. By analogy to the ancient processes of spinning and plying thread and yarn, there is no limit to the length of wire that can be formed. Various methods for forming carbon nanotube fibers and plying such fibers are known to those skilled in the art of carbon nanotubes. 
         [0026]    The surface of the carbon nanofiber or carbon nanotube fibers is decorated with the ends of individual component carbon nanotubes. This makes the surface of nanotube fibers somewhat abrasive, and thus provides an abrasive cutting wire. The abrasive properties can be enhanced with diamond-phase carbon on the surface of the fibers. The diamond-phase carbon can be deposited on the fiber surface or grown on the fiber surface using chemical vapor deposition (CVD) or related techniques. The abrasive properties of carbon nanotube fiber wires can also be enhanced by incorporating abrasive particles such as silicon carbide or diamond particles into the wires. Incorporation of these abrasive particles can be accomplished by a variety of techniques. For example: abrasive particles can be introduced while plying together the fibers in a solution with a suspension of the particles; individual fibers can be coated with abrasive particles and then the fibers can be plied together; the wire can be coated with abrasive particles using vapor phase deposition, or electrochemical deposition methods; etc.  FIG. 6  shows a representation of a carbon nanotube fiber wire incorporating abrasive particles.  FIG. 6  shows a cross-section along 5-5 of the two-ply wire  505  shown in  FIG. 5 , with the addition of abrasive particles  607 . The particles  607  are shown on the surface of the fibers  506 . The density of abrasive particles and their size will be varied to suit the type of cutting required. Particle dimensions will typically be a small percentage of the final cutting wire diameter. For example, if the wire diameter is 50-70 microns, the abrasive protuberances should be approximately 2-5 microns. 
         [0027]    The abrasive properties of the wire may also be enhanced by incorporating extra carbon nanotubes into the wire—the objective being to substantially increase the density of nanotube ends on the surface of the wire. Incorporation of these extra nanotubes may be accomplished using techniques such as those described above for abrasive particles and as part of the carbon fiber fabrication process.  FIG. 7  shows a representation of a carbon nanotube fiber wire incorporating extra carbon nanotubes.  FIG. 7  shows a cross-section along 5-5 of the two-ply wire  505  shown in  FIG. 5 , with the addition of carbon nanotubes  708 . The nanotubes  708  are shown on the surface of the fibers  506 . The density of nanotubes, their size and their type (SWNTs or MWNTs) will be varied to suit the type of cutting required. Typically, nanotubes will be incorporated into carbon nanotube fiber wires at the level of 5-10% by weight. 
         [0028]    As with metal wires, a lubricating fluid is required for use of the carbon nanotube fiber wire of the invention in a wire saw. The lubricating fluid may contain an abrasive such as silicon carbide particles. However, it is preferred to use the carbon nanotube fiber wires without an abrasive in the lubricating fluid. 
         [0029]    Carbon nanotube fiber wires can be made with smaller diameters than metal wires due to their superior mechanical properties. This allows for cutting thinner wafers, conceivably down to 50 microns thick. However, in order to reduce the thickness of wafers being cut without reducing the speed of cutting, lower viscosity lubricating fluids are required. This will require a move away from glycol-based and oil-based lubricants to lower viscosity lubricants, such as water-based lubricants. Ultimately the wire should work with any suitable lubricant or cutting fluid. Additionally, the carbon nanotube fiber wires may be coated with a passivation layer as described in U.S. Pat. No. 6,902,947. 
         [0030]    When cutting silicon wafers with a wire saw a majority of the silicon lost from the kerf ends up in the lubricating fluid. Metal cutting wires contaminate the silicon in the lubricating fluid, making recycling very difficult and expensive. However, utilizing carbon nanotube fiber wires in wire saws eliminates the major source of metal contamination and allows cost effective recycling of silicon from the lubricating fluid.  FIG. 8  shows a wire saw of the invention, configured for recycling silicon from the lubricating fluid. In  FIG. 8 , lubricating fluid is delivered to the hard material, in this case a silicon boule  425 , where it meets the cutting wires  205 . The lubricating fluid is pumped from a container  860  by a pump  861  through conduits  862  to the silicon surface being cut. As the lubricating fluid leaves the silicon surface it is captured by a tray  865  and drained into a reservoir  866  for storing. In some embodiments the reservoir  866  and the container  860  are connected, and in other embodiments the reservoir  866  and the container  860  are one and the same. 
         [0031]    The lubricating fluid containing silicon lost from the kerf is available from the reservoir  866  for recycling. When abrasive particles are not used in the lubricant, the used lubricant is filtered to remove the silicon particulates lost from the kerf. These particulates can then be used in the manufacture of more silicon boules. 
         [0032]    The lubricating fluid in the reservoir  866  may also contain carbon nanotubes lost from the wire. These carbon nanotubes can be reclaimed from the lubricating fluid. 
         [0033]    Although the present invention has been particularly described with reference to the preferred embodiments thereof, it should be readily apparent to those of ordinary skill in the art that changes and modifications in the form and details may be made without departing from the spirit and scope of the invention. It is intended that the appended claims encompass such changes and modifications.