Technical Field

The present invention relates, generally, to methods and apparatus for polishing or planarizing workpieces such as semiconductor wafers, and more particularly, to a device for conditioning the polishing pads used for the planarization of the workpieces utilizing cubic boron nitride cutting elements.

Background Art and Technical Problems

The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high- precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. As the size of the integrated circuits continues to decrease and the density of microstructures per integrated circuit increases, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.

For a discussion of chemical mechanical planarization (CMP) processes and apparatus, see, for example, Aral, el al.. U.S. Patent No. 4,805,348, issued February, 1989; Ami, el al.. U.S. Patent No. 5.099.614. issued March, 1992; Karlsrudet al, U.S. Patent No. 5.329,732, issued July, 1994; Karlsrud. U.S. Patent No. 5,498,196, issued March, 1996; and Karlsrud et al, U.S. Patent No. 5,498,199, issued March, 1996.

Such polishing is well known in the art and generally includes attaching one side of the wafer to a flat surface of a wafer carrier or chuck and pressing the other side of the wafer against a flat polishing surface. In general, the polishing surface comprises a horizontal polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, fumed/precipitated silica or other particulate abrasives. Polishing pads can be formed of various materials, as is known in the art, and which are available commercially. Typically, the polishing pad may be a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Arizona. The hardness and density of the polishing pad depends on the material that is to be polished.

During the polishing or planarization process, the workpiece (e.g., wafer) is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated back and forth over the surface of the polishing pad. It is well known that polishing pads tend to wear unevenly during the polishing operation, causing surface irregularities to develop on the pad. To ensure consistent and accurate planarization and polishing of all workpieces, these irregularities should either be removed or accounted for.

One method of removing the surface irregularities which develop in the polishing pad is to condition or dress the pad with some sort of roughing or cutting means. Generally this truing or dressing of the polishing pad can occur either while the wafers are being polished (in-situ conditioning), or between polishing steps (ex-situ conditioning). An example of ex-situ conditioning is disclosed in Cesna, el al., U.S. Patent No. 5,486.131 , issued on January 23, 1996, and entitled Device for Conditioning Polishing Pads. An example of in-situ conditioning is disclosed in Karlsrud, U.S. Patent Application Serial No. 08/487,530, filed on July 3, 1995, and entitled Polishing Pad Conditioning. Both the Cesna, el al. patent and the Karlsrud application are incorporated herein by reference.

Generally, in the semiconductor wafer polishing and planarization context, small roughing or cutting elements, such as diamond particles, are used to condition the polishing pads. As shown in the Cesna, et al. patent and the Karlsrud application, both in-situ and ex-situ conditioning apparatus utilize circular ring conditioners which have these cutting elements secured to a bottom flange of the ring. Generally, these cutting elements are secured to the bottom surface of the flange of the carrier ring by an electroplating process or brazing process. Electroplating produces a simple mechanical entrapment of the cutting elements on the carrier ring by depositing metal, for example in a layer-by-layer fashion around the cutting elements until they are entrapped. However, one problem with the electroplating process is that the electroplating bond holding the cutting elements to the ring surface is relatively weak and the cutting elements occasionally become dislodged from the conditioning ring and embedded in the polishing pad. Further, because the electroplating bond is susceptable to shearing forces, a substantial amount of bonding material is needed to hold the cutting elements in place. As a result, the bonding material actually covers most if not all of many cutting elements, compromising the conditioning capacity of the conditioning ring. Thus, the previously mentioned brazing process is preferred. A detailed discussion of the brazing process is discussed herein as weil as in Holzapfel, et al, U.S. Patent Application Serial No. 08/683,571 , filed July 15, 1996, incorporated herein by reference.

In the Holzapfel, et al. application, it if mentioned that diamonds, poly crystal line chips/slivers, silicon carbide particles, and the like can be used as the cutting elements. However, during the brazing process, the cutting elements are heated to extreme temperatures, which tend to make the diamond, crystalline and silicon carbide particles more brittle, thus tending to cause the particles to fracture during the conditioning operation.

A new polishing pad conditioning device utilizing a more heat tolerant cutting element is thus needed which overcomes the limitations of the prior art.

Summary of the Invention

The present invention provides methods and apparatus for conditioning polishing pad devices which overcome many of the shortcomings of the prior art.

In accordance with one aspect of the present invention, a polishing pad conditioning device for conditioning a polishing pad by contact with the pad is configured with braze bonded cubic boron nitride (CBN) cutting elements. The conditioning device also suitably includes a mechanism for engaging the conditioning ring with the polishing pad and for rotating and oscillating the conditioning ring over the top surface of the polishing pad.

In accordance with a further aspect of the present invention, an operating arm is configured to move the conditioning device into and out of operative engagement with the top surface of the pad, and for oscillating the conditioning device radially over the top surface of the pad. Further, the conditioning device comprises a carrier element configured in the shape of a ring, and having CBN cutting elements attached to the bottom surface of the carrier element.

In accordance with yet a further aspect of the present invention, the carrier element may include a flange which extends about the periphery of the ring, with the CBN cutting elements being attached to the flange.

In accordance with yet a further aspect of the present invention, the flange includes cut-out portions to permit materials to escape from the interior of the carrier ring. In accordance with this aspect of the invention, the CBN cutting elements are distributed substantially uniformly along the flange and the elements are braze bonded to the flange with a brazed metal alloy. Preferably, the brazed metal alloy will only cover about 25% to 75%, and preferably about 40 to 60%, and most preferably about 50% of the height of the cutting elements. For example, for cutting elements (e.g. , CBN particles) having an average height (/ ^' . e. , diameter) in the range of 50 to 200 micrometers and most preferably about 150 micrometers, the brazed metal alloy should preferably cover each cutting element up to about 50% of its height, or up to about 75 micrometers.

The present inventor has determined that particle sizes in the range of 50 to 200 U.S. mesh, and most preferably about 100 to 120 U.S. mesh are particularly well adapted to the present invention.

In accordance with a further aspect of the present invention, covering less than 25% to 40% of the height of respective CBN cutting elements with braze may result in an insufficiently secure bond, such that the CBN elements may break away from the braze, liberating the CBN cutting element and perhaps damaging the workpieces. On the other hand, covering the CBN elements with braze in excess of 60% to 80% of the height of the element may impede the ability of the elements to properly dress or condition a pad. Thus, the present inventor has determined that an optimal range involves covering the CBN cutting elements in braze up to about 50% of the height of the elements.

In accordance with yet a further aspect of the present invention, the conditioning device may be configured to condition the polishing pad at the same time workpieces are being polished. In accordance with this aspect of the invention, the conditioning device preferably is configured to mount to a moveable carrier element, which holds the workpieces during polishing.

In accordance with a further aspect of the present invention, the conditioning device comprises a ring configured about the outer perimeter of the workpiece carrier element, wherein the CBN cutting elements are securely attached to the bottom surface of the ring.

In accordance with yet a further aspect of the present invention, the CBN cutting elements may be attached to a flange which extends about the periphery of the ring. In addition, the flange preferably may include cut-out portions to permit materials to escape from the interior of the ring.

Brief Description of the Drawing Figures

The present invention will hereinafter be described in conjunction with the appended drawing figures, wherein like numerals denote like elements, and: Figure 1 is a perspective schematic view of a semiconductor wafer polishing and planarization machine currently known in the art;

Figures 2 and 3 are top cross-sectional views of the wafer cleaning machine shown in Figure 1 illustrating different parts of the machine at different times in the polishing process; Figure 4 is a side cross-sectional view of a semiconductor wafer carrier element with an in-situ polishing pad conditioning ring connected thereto;

Figure 5 is a top view of the in-situ polishing pad conditioning ring shown in Figure 4;

Figure 6 is a side view of the in-situ conditioning ring shown in Figures 4 and 5;

Figure 7 is a perspective view of the polishing surface of the polishing machine shown in Figure 1 with an ex-situ polishing pad conditioning apparatus in operative engagement with the polishing surface;

Figure 8 is a side cross-sectional view of the ex-situ polishing pad conditioning ring holder shown in Figure 7; and

Figure 9 is a top view of an ex-situ polishing pad conditioning ring. Figure 10 is a cross-sectional view of cutting elements which have been braze bonded to a conditioning ring;

Figure 11 is a cross-sectional view of cutting elements which have been electroplated to a conditioning ring.

Detailed Description of Preferred Exemplary Embodiment The subject invention relates to an improved apparatus for conditioning workpiece polishing pads, and an improved method for securing cutting elements to minimize the risk that the cutting elements may dislodge from the apparatus and damage the workpieces being polished. While this invention may be used to condition a large variety of polishing pads which may be used to polish a variety of different types of workpieces, the preferred exemplary embodiments discussed herein relate to polishing pad conditioning apparatus used to condition semiconductor wafer polishing pads. It will be understood, however, that the invention is not limited to any particular workpiece polishing pad conditioning environment.

Referring now to Figures 1 through 3, a wafer polishing apparatus 100 is shown embodying the present invention. Wafer polishing apparatus 100 suitably comprises a multiple head wafer polishing machine which accepts wafers from a previous processing step, polishes and rinses the wafers, and reloads the wafers back into wafer cassettes for subsequent processing. Discussing now the polishing apparatus 100 in more detail, apparatus 100 comprises an unload station 102. a wafer transition station 104, a polishing station 106, and a wafer rinse and load station 108. In accordance with a preferred embodiment of the invention, one or more (e.g., two) cassettes 110, each holding a plurality of wafers, are loaded into the machine at unload station 102. Next, a robotic wafer carrier arm 112 removes the wafers from cassettes 1 10 and places them, one

at a time, on a first wafer transfer arm 1 14. Wafer transfer arm 1 14 then sequentially lifts and moves each wafer into wafer transition section 104. That is, transfer arm 114 suitably places an individual wafer on one of a plurality of wafer pick-up stations 116 which reside on a rotatable table 120 within wafer transition section 104. Rotatable table 120 also suitably includes a plurality of wafer drop-off stations 1 18 alternately interposed between respective pick-up stations 1 16. After a wafer is deposited on one of the plurality of pick-up stations 1 16, table 120 rotates so that a new station 1 16 aligns with transfer arm 1 14. Transfer arm 1 14 then places the next wafer on the new empty pick-up station 116. This process continues until all pick-up stations 1 16 are filled with wafers. In the illustrated embodiment of the invention, table 120 includes five pick-up stations 1 16 and five drop-off stations 1 18.

Next, a wafer carrier apparatus 122, comprising individual wafer carrier elements 124, suitably aligns itself over table 120 so that respective carrier elements 124 are positioned directly above the wafers which reside in respective pick-up stations 1 16. The carrier apparatus 122 then drops down and picks up the wafers from their respective stations and moves the wafers laterally such that the wafers are positioned above polishing station 106. Once above polishing station 106, carrier apparatus 122 suitably lowers the wafers, which are held by individual elements 124, into operative engagement with a polishing pad 126 which sits atop a lap wheel 128. During operation, lap wheel 128 causes polishing pad 126 to rotate about its vertical axis. At the same time, individual carrier elements 124 spin the wafers about their respective vertical axes and oscillate the wafers back and forth across pad 126 (substantially along arrow 133) as they press against the polishing pad. In this manner, the undersurface of the wafer is polished or planarized.

After an appropriate period of time, the wafers are removed from polishing pad 126, and carrier apparatus 122 transports the wafers back to transition station 104. Carrier apparatus 122 then lowers individual carrier elements 124 and deposits the wafers onto drop-off stations 118. The wafers are then removed from drop-off stations 1 18 by a second transfer arm 130. Transfer arm 130 suitably lifts each wafer out of transition station 104 and transfers them into wafer rinse and load station 108. In the load station 108, transfer arm 130 holds the wafers while they are rinsed. After a thorough rinsing, the wafers are reloaded into cassettes 132, which then transport the subsequent stations for further processing or packaging. During this polishing and planarization process, the polishing pad will wear and thus become less effective. Therefore, it is important to buff or condition polishing pad 126 to remove any surface irregularities that may develop during polishing. Generally, there are two ways to

condition the polishing pad; in-situ and ex-situ conditioning. In-situ conditioning takes place during the wafer polishing process, while ex-situ conditioning occurs in between polishing steps. Referring now to Figures 2 through 4, in-situ conditioning will first be discussed. In accordance with a preferred embodiment of the present invention, in-situ conditioning generally occurs by connecting an in-situ conditioning element 200 to each individual carrier element 124. Therefore, as carrier elements 124 rotate and move the wafers over the polishing pad, conditioning elements 200 will also contact the polishing pad, thus conditioning the pad while the wafers are being polished.

Referring now to Figure 4, the configuration of conditioning element 200 and carrier element 124 will now be discussed. As previously mentioned, carrier element 124 holds and presses the wafers against the polishing pad during the polishing operation. As is well known in the art, carrier element 124 may comprise a number of different embodiments. However, for purposes of discussing the present invention, carrier element 124 will be discussed in accordance with the embodiment shown in Figure 4. In accordance with a preferred embodiment of the present invention, carrier element

124 preferably comprises a pressure plate 140. a protective layer 142, a retaining ring 144, and a rotation drive shaft 146. Pressure plate 140 applies an equally distributed downward pressure against the backside of a wafer 10 as it is pressed against polishing pad 126. Protective layer 142 will preferably reside between pressure plate 140 and wafer 10 to protect the wafer during the polishing process. Protective layer 142 may be any type of semi-rigid material that will not damage the wafer as pressure is applied; for example, a urethane-type material. Wafer 10 may be held against protective layer 142 by any convenient mechanism, such as, for example, by vacuum or by wet surface tension. Circular retaining ring 144 preferably is connected around the periphery of protective layer 142 and prevents wafer 10 from slipping laterally from beneath the protective layer as the wafer is polished. Retaining ring 144 is generally connected to pressure plate 140 by bolts 148.

Also connected to pressure plate 140 is conditioning element 200 which, in accordance with a preferred embodiment of the invention, is a ring formed of a rigid material, such as metal. As shown in Figures 4 and 6, conditioning element 200 preferably includes a downwardly extending flange 202 which terminates in a substantially flat bottom surface 204 having cutting elements 205 attached thereto. The flange 202 is of sufficient length so that bottom surface 204 with attached cutting elements 205 will contact the polishing pad during processing. Further, conditioning element 200 preferably will be loosely connected to pressure plate 140 by bolts 206.

This relatively loose connection between pressure plate 140 and conditioning element 200 allows limited vertical movement but restricts lateral movement of conditioning element 200. The vertical movement of the conditioning element 200, which occurs between nuts 208 and 210 (Figure 4), is permitted so that the cutting elements 205 contact pad 126 by virtue of the weight of conditioning element 200, rather than by pressure applied by carrier element 124. If needed, additional weighted rings 212 may be added to conditioning element 200 to increase the weight of the ring and thus the conditioning pressure on the pad.

In accordance with a further aspect of the preferred embodiment of the present invention, flange 202 may include cut-out portions 214 which permit sworf and fluids to escape from the interior of conditioning element 200. Accordingly, as shown in Figures 5 and 6 as dimension "A", cut-out portions 214 may be in the range of about 0.75 to 1.25 inches and more preferably in the range of about 0.875 to 1.125 inches. The remaining portions of flange 202, which have cutting elements 205 attached thereto, are shown in Figures 5 and 6 as elements 216. The size of the remaining flange portions 216 are illustrated in Figure 5 as dimension "B" and are in the range of about 0.75 to 1.25 inches and more preferably in the range of about 0.875 to 1.125 inches.

In accordance with yet another aspect of the present invention, cutting elements 205 may be any hard cutting material useful for conditioning pads, such as, for example, diamond particles, polycrystalline chips/slivers, cubic boron nitrite particles, cubic boron nitride particles, silicon carbide particles, and the like. Preferably, however, for reasons stated below, cubic boron nitride particles or other substances having similar physical, chemical, and mechanical properties are used for the cutting elements. Further, cutting elements 205 may be secured to bottom surface 204 of flange 202 by a brazed bonding process which creates an extremely secure bond. This bonding process will be discussed in more detail below. During operation of apparatus 100, wafer 10 held by carrier element 124 is brought into contact with polishing pad 126 which is secured to lap wheel 128. Preferably, to optimize the polishing process, an abrasive slurry is introduced between polishing pad 126 and wafer 10. Various types of abrasive slurries can be used, as is known in the art. As wafer 10 contacts pad 126, both lap wheel 128 and carrier element 124 rotate, thus facilitating the polishing and planarization of the wafer. In addition, as carrier element 124 lowers wafer 10 onto the pad, conditioning element 200, which is connected to carrier element 124, will be lowered into contact with the pad. As lap wheel 128 and carrier element 124 rotate, cutting elements 205 will rough-up and, thus, condition polishing pad 126 at the same time the wafers are being polished.

In accordance with an alternate embodiment of the present invention, the ex-situ conditioning device of apparatus 100 will now be discussed. As briefly mentioned above, ex-situ conditioning generally occurs between polishing steps. That is, after a set of wafers has been polished and removed from the polishing pad, a separate conditioning device is introduced against polishing pad 126 to condition the pad. It should be noted, however, that apparatus 100 does not have to utilize both in-situ and ex-situ conditioning. One skilled in the art will appreciate that apparatus 100 may include either in-situ conditioning or ex-situ conditioning, or apparatus 100 may include both.

Referring now to Figures 7 through 9, an ex-situ conditioning device 300 preferably comprises a circular conditioning ring carrier element 302 made of a rigid material, such as metal. In accordance with this aspect of the present invention, ring carrier element 302 preferably has a downwardly extending flange 304 which, during operation, contacts and conditions the polishing pad. In accordance with a further aspect of this embodiment of the invention, flange 304 may be interrupted by a plurality of cut-outs 306 which permit sworf and fluids to escape from the interior of conditioning device 300 during operation.

As with the in-situ conditioning ring and as illustrated in Figure 9, cutting elements 308 may be secured to the bottom surface of flange 304. Similarly, cutting elements 308 may comprise a variety of materials, but, in accordance with the preferred embodiment, cutting elements 308 suitably comprise cubic boron nitride particles. As discussed in detail below, cutting elements 308 may be attached to the bottom portion of flange 304 by a unique braze bonding process.

In accordance with this preferred embodiment of the invention, conditioning device 300 preferably is attached to an operating arm 310 which is configured to raise and lower conditioning device 300 into and out of engagement with polishing pad 126. The vertical movement of operating arm 310 is controlled by a pressure cylinder 312. In addition, operating arm 310 may also be adapted for moving conditioning device 300 back and forth across the top of pad 126. thus ensuring that the entire top surface of the pad is conditioned equally. Various means may be employed to connect conditioning element 300 to operating arm 310. For example, as illustrated in Figure 8, ring 302 may be secured to a bearing housing 314 by shoulder bolts 316. In accordance with this configuration, a shaft 318 may be configured to engage a chuck in the head of operating arm 310, thus holding the housing and ring assembly in operative engagement with the arm.

During processing, when it is desired to condition polishing pad 126, arm 310 is activated to bring conditioning device 300, and more particularly cutting elements 308, into contact with the top surface of polishing pad 126. In addition, lap wheel 128 rotates (e.g., counter¬ clockwise) and, at the same time, operating arm 310 oscillates, causing conditioning element 300 to traverse back and forth across the surface of polishing pad 126. The downward pressure that the conditioning device exerts on the polishing pad surface and the length of time that the conditioning element is in contact with the pad may vary as necessary to achieve the desired conditioning results.

Referring now to Figures 10 and 11, the subject method of attaching the cutting elements and, in particular, cubic boron nitride particles to the conditioning rings will now be discussed. In accordance with a preferred embodiment of the present invention, cutting elements 402 may be attached to a carrier ring 400 by a direct brazing technique which creates a very strong, reliable bond between the cutting elements and the ring surface. The brazing method of the present invention utilizes readily available, very hard and durable brazing alloys to create the secure bond. The brazing alloys utilized generally comprise nickel-chromium or cobalt-nickel- chromium combinations. It has been found that this family of brazing alloys creates superior chemical/mechanical bonds because the alloys tend to cling to the cutting element surfaces rather than flow away from them during the treatment process. Thus, greater surface contact between the cutting elements and the alloy are achieved. The process of bonding cutting elements 402 to the conditioning ring surface 400 will now be discussed. In accordance with one aspect of the present invention, cutting elements 402 and braze alloy particles 404 are suitably placed on the metal ring surface in a predetermined fashion. To hold the cutting elements and braze alloy particles in place, a temporary binding agent may be used, such as, for example, a resinous compound dissolved in a suitable organic solvent, or the like. Upon proper distribution of the cutting elements and alloy particles, the ring assembly is then placed in a furnace having a reducing atmosphere or vacuum and heated until the braze flows and wets the cutting elements and metal ring surface. Finally, the braze is cooled, securely bonding the cutting elements to the ring surface.

In accordance with another aspect of the present invention, the brazing process may be performed in two steps rather than one as discussed above. In the two-step process, the brazing alloy is first applied to the ring surface in a manner similar to that described above, however, the cutting elements are not present. After the braze alloy is fused to the ring surface, the cutting elements are then attached to the layer of braze alloy on the ring surface by using a temporary

binder. After the cutting elements are properly positioned, the ring assembly again is placed in the furnace until the braze remelts and surrounds the cutting elements. This two-step process generally achieves the same bonding strength as the one-step method, but the two-step process allows for greater control of surface uniformity of the cutting elements on the ring surface. A more detailed discussion of a brazing process useful in the context of the present invention is discussed in Lowder et. al, U.S. Patent Nos. 3,894.673 and 4,018,576 issued on July 15, 1975 and April, 19, 1977 respectively, both of which are incorporated herein by reference.

In accordance with a further aspect of the present invention, the subject process of braze bonding the cutting elements to the carrier ring surface exhibits superior performance compared to the conventional electroplating bond currently known in the art. Improvements such as the ability to control the amount of plating, the ability to control the amount and placement of the cutting elements on the ring, better adhesion of the cutting elements to the ring surface, the ability to have predictable and repeatable conditioning rings, and better pad management due to the control of the cutting elements, plating, and spacing of the elements are achieved. All such improvements are related to the fact that the invention provides for better bonding of the cutting elements to the conditioning ring surface with less bond metal than has been previously possible. In this regard, the brazing method provides optimal support for each and every cutting element on the ring because during the fusing process, the braze alloy encompasses the side and bottom surfaces of each element, thus forming the solid bond. This aspect of the invention is shown in Figure 10 which depicts a cross-section of cutting elements 402 brazed to the surface of conditioning ring 400. The bond surface 404 is characterized as "concave," i.e.. the alloy metal bond depth is at a minimum at a point intermediate adjacent elements. A cross-section of cutting elements electroplated to the conditioning ring in accordance with prior art techniques is shown in Figure 11. As distinguished from Figure 10, the surface contour of the bonding metal 410 is inherently convex in the electroplated device, thus providing minimal support for cutting elements

412 for a given depth of bond metal. Therefore, with the electroplating process, the bond is weaker even though more bond metal is used. In fact, as much as 50% to 100% of the cutting elements may be covered by the bond metal with the electroplating process. However, with the braze process, the cutting elements can be bonded with as little as 25% to 40% of the cutting element being covered with bond, therefore allowing greater sworf clearance, faster cutting and reduced heat build-up.

In accordance with a further aspect of the present invention, cutting elements having an aspect ratio in the range of 0.5; 1.0 to 1.5: 1.0, and most preferably about 1.0: 1.0 are suitably

employed; that is, in a particularly preferred implementation of the present invention, the height of the cutting elements is approximately equal to the width of the cutting elements. In this way, the effectiveness of the subject bonding technique, as well as the effectiveness of the various cutting elements in the pad dressing operation are substantially independent of the orientation of the cutting elements.

It should be noted, that this braze bonding process can be used to attach cutting elements exhibiting different material properties. For example, as discussed above, cutting elements may comprise diamond particles, polycrystalline chips/slivers, cubic boron nitrite particles, cubic boron nitride particles, silicon carbide particles, and the like. However, because of the extreme heat the particles encounter during the brazing process, most of the particle types (i.e., diamonds, polycrystalline chips/slivers, and silicon carbide particles) tend to fracture during the polishing process. Although CBN is the second hardest substance known next to diamonds, the physical properties of CBN make it less susceptible to fracture when exposed to extremely high temperatures. The heating during the brazing process tends to make the diamond crystalline structure brittle, whereas the crystalline structure of CBN maintains its integrity at the high temperatures. Therefore, in accordance with the preferred embodiment of the invention, CBN is suitably used as the cutting element.

It will be understood that the foregoing description is of preferred exemplary embodiments of the invention and that the invention is not limited to the specific forms shown or described herein. Various modifications may be made in the design, arrangement, and type of elements disclosed herein, as well as the steps of making and using the invention without departing from the scope of the invention as expressed in the appended claims.