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The brazing of diamond
Int. Journal of Refractory Metals & Hard Materials 27 (2009) 382–393
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
The brazing of diamond James C. Sung a,b,c,*, Michael Sung d a
KINIK Company, 64, Chung-San Rd., Ying-Kuo, Taipei Hsien 239, Taiwan, ROC National Taiwan University, Taipei 106, Taiwan, ROC National Taipei University of Technology, Taipei 106, Taiwan, ROC d Advanced Diamond Solutions, Inc., 351 King Street Suite 813, San Francisco, CA 94158, USA b c
a r t i c l e
i n f o
Article history: Received 30 October 2008 Accepted 16 November 2008
Keywords: Diamond brazing Pad conditioner Chemical mechanical planarization
a b s t r a c t When the braze melts, the carbide formers tend to migrate toward diamond to form carbide at the interface. This reaction may be excessive as to degrade diamond’s integrity. In this case, a pre-coating of diamond may be needed to moderate the reactivity between diamond and braze. When diamond is brazed on the surface of a substrate, the melt tends to pull the grits closer together that may thicken the braze layer locally. The clustering of grits can reduce the cutting effectiveness of the diamond tool. A diamond grid design is necessary to maintain the uniform thickness of the braze layer. Moreover, the controlled melting of braze alloy can form a gentle slope around each diamond grit. Such a massive support can allow aggressive cutting of the diamond tool with a low power consumption. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Diamond grit can be bonded to a tool substrate by electroplating of nickel or by brazing of an alloy (e.g. Ni–Cr–B–Si). The electroplated nickel holds diamond grit in place by mechanical entrapment. The brazed alloy may bond diamond with carbide formation at the interface. Due to the presence of chemical bonding, the brazed diamond tools can cut much faster and last significantly longer than electroplated tools. Although, there have been ample research literature on the subject of diamond brazing, this report documents the first time many aspects of industrial practice. For examples, the particle size effect of the alloy on the melting behavior of the braze, the diffusion of carbide formers toward the surface of diamond, the control of wetting profile of the molten liquid on diamond etc. 2. Diamond bonding in tool matrix Diamond is the hardest material known, it is also one of the most inert substance. Due to this low reactivity, diamond grits can cut other materials clean, but they are also notorious difficult to be attached in the tool matrix. Consequently, diamond pullouts from the matrix have been the major limitation of the tool longevity. Among the three types of materials that are used for bonding diamond grits, resinous (polymeric) bonds are too weak to hold diamond firm, vitreous (glassy) matrices are too brittle to with* Corresponding author. Address: KINIK Company, 64 Chung-San Rd., Ying-Kuo, Taipei Hsien 239, Taiwan, ROC. Tel.: +886 2 8678 0880; fax: +886 2 8677 2171. E-mail address: [email protected] (J.C. Sung). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.011
stand impacts, metal bonds, on the other hand, can lock up diamond without yielding. However, because of this mechanical engagement, diamond grits must be buried deep in the matrix, consequently, the protrusion of diamond grit for cutting is limited. As a result, the cutting speed is low. Worse still, the rubbing of metal against the work object can generate much heat that may damage both materials. For single layered diamond tools, conventional technologies are trapping diamond grits by electroplating with nickel. Due to the insulating character of diamond, nickel layer form a depression around the diamond. As a result, diamond can be pulled out even more easily (Fig. 1). There has been research works published in literature on the brazing of diamond [1–3]. But no details about the brazing phenomena were provided. Beginning in 1975, Abrasive technology introduced nickel brazed diamond tools with a single layer of diamond grit [4]. The brazing could form chemical bond of carbide at the interface so diamond was joined with the metal seamlessly. Due to the strong bonding, the diamond grit protruded high to sustain the impact stress (Fig. 2). As the result of reduced frictional contact between the diamond tool and the working object, the power consumed in cutting was reduced. Although, the chemical bonding of the braze is strong but the brazing layer is often very thin, so the diamond may be knocked off form the base along with the attached braze (Fig. 3). 3. The brazing of diamond grid with massive support In order to avoid the clustering of the diamond grits during the brazing process, diamond grits must be planted with a pre-determined pattern. In 1997, Kinik (under the license of the author)
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Fig. 3. The rupture of braze at the base during the aggressive cutting action of diamond grits.
support around each grit. In contrast, electroplated nickel or sintered alloy cannot form such a profile of massive support (Fig. 5). Contrary to conventional diamond tools that diamond losses are inevitable during use, diamond pullouts cannot happen with brazed diamond grids although, the crystals may be shattered if the impact force is excessive. Due the exceptional bonding strength with carbide interface and braze slope, the brazed diamond grids may cut twice as fast as comparable designs of electroplated tools with the longevity of the tool life that may also be doubled (Fig. 6). 4. Diamond pad conditioners for making semiconductors Fig. 1. Electroplated nickel forms cavity around every grit so retention force of diamond is weak. As a result, diamond grits are often pulled out to leave cavities.
introduced the world first products with brazed diamond grid. Such products include wire saw pearls, forming wheels and others. The author displayed these tools in Verona Exhibition in Italy in 1998 (Fig. 4). The brazed diamond features a massive support of braze profile around each diamond grit. In additional to the chemical bonding, the attachment of diamond is further reinforced by the mechanical
Diamond pad conditioners are the most valuable diamond grit tools. A typical diamond pad conditioner is a flat disk of about 100 mm in diameter (e.g. for Applied Materials polishers). It has diamond grits (e.g. 150 microns) brazed on the surface. Such a tool, if it is used to grind a stone, may be purchased for less than $10. However, for diamond pad conditioners, they can be sold for more than $300 a piece. The pad conditioners are designed to dress delicate polyurethane pads, not hard materials normally encountered by conventional diamond grit tools. The pads are used to polish expensive semiconductor wafers that contain delicate layers of integrate circuits (ICs). As any diamond debris may cause cata-
Fig. 2. The clustering of brazed diamond grits tend to thicken the braze layer locally. Note the diamond is weakly held in braze because the steep rise of the wetting layer that is too thin.
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Fig. 5. Electroplated diamond has a convex profile that cannot hold diamond firm (top left). Sintered diamond has a flat profile that may also lose diamond (top right). Strongly bonded diamond has a massive support profile that will never lose any diamond (bottom diagram).
Fig. 4. Brazed diamond grids for various diamond tools [5,6].
strophic scratches on the wafer, no diamond breakage can be tolerated. Thus, pad conditioners are used with diamond attrition by wear. This is in contrast with conventional diamond tools that allow diamond to chip for sharpening, and pulled out to expose lower lying diamond (Fig. 7). The diamond grid design for pad conditioners stabilized the dressing performance of CMP. Since its debut in 1999, almost all CMP pad conditioners contain diamond grits distributed in a predetermined pattern. In 1999, Kinik pioneered diamond pad conditioners with diamond grits set in a pre-determined pattern (Sung, US Patent 6286,498) [7]. Moreover, each diamond particle is
bonded chemically with a strong braze alloy (Sung, US Patent 6039,641 and 6679,243 [7]; Sung and Lin, US Patent 6368,198 [8]). In addition, every individual diamond grit is further supported mechanically with massive reinforcement (Sung, US Patent 6679,243) [7]. Furthermore, most diamond crystals are oriented with tips or edges facing the pad for sharp cutting. The metal surface of the pad conditioner is also uniquely coated with an acid resistant diamond-like carbon coating (Sung and Lin, US Patent 6368,198) [8]. Such a combination of design features has made DiaGridÒ/DiaTrixTM pad conditioners the new standard for CMP manufacture since 2000 (Fig. 8).
5. Chemical bonding of diamond The braze alloy contains a solvent (e.g. Ni or Cu) that can dissolve active elements (e.g. Cr or Ti) (Fig. 9). During the melting process, the active element will migrate toward diamond where they
Fig. 6. The fracture of diamond bonded by the massive support of braze were left with remnant in the matrix, so the pullout of diamond was not possible.
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Fig. 7. An expensive diamond pad conditioner (left diagram) is compared with a cheap stone grinder (right diagram). Both products contain diamond grits that are set in a pre-determined pattern (Kinik’s DiaGridÒ products). The pad conditioner is further coated with a submicron thick of diamond-like carbon (DLC) for preventing metal from being etched by the chemicals in the polishing slurry.
react to form carbide (e.g. Cr3C2 or TiC). This reaction exhibits the wetting phenomenon of the alloy on the diamond surface. The capillary force often pulls the melt up the slope of the diamond faces. At the interface, the strong carbide bond holds diamond firmly atom-by-atom. The chemical reaction on the diamond surface is evident when the diamond is freed from the braze by acid etching (Fig. 10). The reaction takes place between diamond and the active solute elements (e.g. Cr and Si) to form carbide. It is the formation of carbide that constitutes the strong bonding of diamond.
6. The compromise of bonding strength and impact strength Although, the chemical bonding is very strong, the reaction at the interface may also degrade the diamond integrity. As a result, diamond’s impact strength is reduced. In order to moderate the carbide formation at the interface, a thin (e.g. 10 microns) copper layer may be plated on diamond before it is wetted by the molten braze, the result confirmed that the adherence of the diamond surrounded by the molten braze can be strengthened without compromising the impact strength (Fig. 11). The interface engineering may control the fracture path when the diamond containing matrix is broken. The bare diamond will separate the matrix at the interface due to the lack of chemical bonding. The molten braze coated diamond will fracture through because the crystal is weakened by the brazing reaction. The molten braze coated diamond with the moderation by a thin copper interlayer will tear off the weakest link in the coating itself (Fig. 12).
7. The melting behavior of the braze
Fig. 8. Randomly distributed diamond grits buried in electroplated nickel (top diagram) is compared with a clustered diamond grits wetted by weak alloy (middle diagram), and the patterned diamond grid bonded by massive support (bottom diagram).
The melting kinetics of metal powder is sensitively dependent on its particle size, the smaller the particles, the faster the melting process (Fig. 13). During the brazing process, the melting will start from the finer particles, so if the powder is lack of these fines, the onset of the melting process will be postponed (Fig. 14). During the process of melting, the braze will start from the vicinity of diamond and it is progressed outward until the entire braze layer becomes liquid (Fig. 15). The powders just before melting and after its total liquefaction are compared below. During the melting process, the diamond
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Fig. 9. The scanning electron micro image of diamond showing Cr and Si solute atoms in a Ni solvent diffused preferentially toward diamond and form carbides at the interface. Note the complementary nature of Cr and Si near the vicinity of the diamond.
Fig. 10. The reaction of half of diamond surfaces with the molten braze during the brazing process.
Fig. 11. The optimization of bonding strength at the interface between diamond and braze and the impact strength of the crystal by plating a thin interlayer of copper that moderated the reaction of carbide formation. In the diagram, ABCD refers to active brazed bonded diamond.
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Fig. 12. The fracture paths of three different interfaces that display the weakest link at difference locations. Again, ABCD refers to active braze coated diamond.
Fig. 13. The melting point of gold and copper. Note the rapid plunging of the melting point when the particles are becoming submicron in size.
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Fig. 14. The delayed melting of a braze due to the coarsening of the powder. Note that the powder on the right diagram was not fully melted due to such a delay.
Fig. 15. The melting of the braze layer will begin around each diamond and the liquid will radiate from each diamond until the entire braze layer becomes molten.
crystal sitting on the top of the braze layer is wetted and pulled downward by the capillary force of the liquid (Fig. 16).
stripping of the braze from the substrate could be expedited by increasing of the current density (Fig. 18).
8. The stripping of brazed diamond layer
9. Brazing at reduced temperature
The diamond is bonded chemically with the braze that is coated on the substrate (e.g. stainless steel), such a layer can be stripped by the reverse process of electroplating of anodization. Due to the alloying of the braze and the substrate, the interface is dissolved preferentially during the stripping process so the entire braze layer will fall off from the substrate held by anode (Fig. 17). The above experiments showed a higher potential of the stainless steel substrate compared with the nickel based braze, and the
The brazing of diamond with a nickel based alloy at high temperature (e.g. >1000 °C) often cause thermal damages of diamond. It may be desirable to use a braze with a lower melting point. One way to do so is by adding phosphorus in the nickel–chromium alloy. However, the high vapor pressure of phosphorus may require brazing with an externally applied pressure. The phosphorus containing braze may not adhere diamond firm enough so titanium coated diamond is often used to improve the bonding strength (Fig 19).
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Fig. 16. The contrast of a brazed diamond bonded by molten liquid (left diagram) and a diamond sitting on the top of the sintered metal powder that just began to melt (right diagram).
Fig. 17. The brazed diamond grid before stripping (left diagram) and the top layer fell of from the substrate (right diagram).
Fig. 18. The plots of voltage versus current during the stripping process (left diagram) and the time for removing the braze layer versus current applied (right diagram).
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Fig. 19. A titanium coated diamond was brazed by a phosphorus containing nickel–chromium alloy (left diagram). The EDAX mapping of elements around a titanium coated diamond.
Fig. 20. The peeling off the braze layer from titanium coated diamond.
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Fig. 21. Diamond grits pulled out from the brazed matrix that contained phosphorus with sockets leaving behind.
Fig. 22. The appearance of glass brazed diamond grits.
The phosphorus containing braze may not wet titanium on diamond easily due to the oxidation of titanium (Fig. 20). Although, diamond coated by titanium and bonded by phosphorus containing braze is stronger than that mechanically retained in a sintered metal matrix, it can be dislodged from the braze under a strong blow (Fig. 21).
10. Glass bonded diamond Diamond may also be brazed by a glassy (oxide) material that contains carbide formers (e.g. Li and Na). In this case, diamond is wetted by the molten glass and a massive support slope may also
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Fig. 23. The flaking of DLC from the surface of diamond after rubbing against a polyurethane pad.
Fig. 24. Etched DLC showing remnants of flakes.
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Fig. 25. The repelling of molten aluminium at the surface of diamond that was pre-coated with either silicon or titanium.
be formed. The brazing is conducted under air instead of under vacuum (Fig. 22). 11. DLC coated diamond Diamond-like carbon (DLC) may be used to coat a brazed diamond grid to prevent the corrosion of the braze in an acidic environment. However, DLC so formed also cling on diamond surface and it may be rubbed off (Fig. 23). DLC may also be etched by strong oxidizing agent, such as iron chromate. In this case, DLC may form cracks or even isolated flakes (Fig. 24).
Although, the sciences of diamond wetting and brazing were long established, the detailed phenomena of commercial brazing process has been held as trade secret for decades. In this research, we presented extensive data on brazing of commercial products. The discussed melting curves of the braze and the particle size effect. We also introduced brazing with pre-coating of titanium. In addition, brazing by flowing glass was also described. Furthermore, striping of the braze layer by reverse electroplating was presented. Finally, diamond-like carbon coating of brazed diamond was shown with the adherence problems.
Acknowledgement 12. Conclusions The brazing of diamond is often dependent on the competition between carbide formation and metal oxidation at the interface between diamond and molten alloy. For example, diamond cannot be wetted by molten aluminum at 750 °C that is well above the melting point of 660 °C. This may be due to the oxidation of aluminum before the formation of aluminum carbide. By coating diamond with either silicon or titanium that may have limited solubility with aluminum, the wetting of diamond by molten aluminum is still prevented. Again, the rapid oxidation of either silicon or titanium could be the culprit in this case (Fig. 25). Consequently, the brazing must compromise the kinetics between carbide formation and oxidation at the interface between diamond and liquid. In the case of applying chromium as the active element for brazing diamond, the carbide formation can take place along with chromium oxidation. Consequently, the chemical bonds formed at the interface between diamond and molten alloy are strong.
The authors are grateful for the support of this research by KINIK Company.
References [1] Kizikov ED. Vacuum technology for diamond toolmaking. Ind Diamond Rev 1991;51:20–3. [2] Peterman LM. Diamond tooling in nono metallics. In: Proceedings of the superabrasive conference, Chicago, Illimois; 1985. p. 1–12. [3] Chattopadhyay AK, Chollet L, Hintermann HE. Experimental investigation on induction brazing of diamond with Ni–Cr hardfacing alloy under argon atmosphere. J Mater Sci 1991;26:5093–100. [4] Lowder JT, Tausch EM. US Patent 4018,576 (Filed: May 8, 1975). [5] Sung CM. Brazed beads with a diamond grid for wire sawing. Ind Diamond Rev 1998;4/98:134–6. [6] Sung CM. Brazed diamond grid: a revolutionary design for diamond saws. Diamond Relat Mater 1999;8:1540–3. [7] Sung CM. US Patents 6039,641 (Filed: April 4, 1997), 6286,498 (Filed: September 20, 1999) and 6679,243 (Filed: August 22, 2001). [8] Sung CM, Lin FS. US Patent 6368,198 (Filed: April 26, 2000).
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
The brazing of diamond James C. Sung a,b,c,*, Michael Sung d a
KINIK Company, 64, Chung-San Rd., Ying-Kuo, Taipei Hsien 239, Taiwan, ROC National Taiwan University, Taipei 106, Taiwan, ROC National Taipei University of Technology, Taipei 106, Taiwan, ROC d Advanced Diamond Solutions, Inc., 351 King Street Suite 813, San Francisco, CA 94158, USA b c
a r t i c l e
i n f o
Article history: Received 30 October 2008 Accepted 16 November 2008
Keywords: Diamond brazing Pad conditioner Chemical mechanical planarization
a b s t r a c t When the braze melts, the carbide formers tend to migrate toward diamond to form carbide at the interface. This reaction may be excessive as to degrade diamond’s integrity. In this case, a pre-coating of diamond may be needed to moderate the reactivity between diamond and braze. When diamond is brazed on the surface of a substrate, the melt tends to pull the grits closer together that may thicken the braze layer locally. The clustering of grits can reduce the cutting effectiveness of the diamond tool. A diamond grid design is necessary to maintain the uniform thickness of the braze layer. Moreover, the controlled melting of braze alloy can form a gentle slope around each diamond grit. Such a massive support can allow aggressive cutting of the diamond tool with a low power consumption. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Diamond grit can be bonded to a tool substrate by electroplating of nickel or by brazing of an alloy (e.g. Ni–Cr–B–Si). The electroplated nickel holds diamond grit in place by mechanical entrapment. The brazed alloy may bond diamond with carbide formation at the interface. Due to the presence of chemical bonding, the brazed diamond tools can cut much faster and last significantly longer than electroplated tools. Although, there have been ample research literature on the subject of diamond brazing, this report documents the first time many aspects of industrial practice. For examples, the particle size effect of the alloy on the melting behavior of the braze, the diffusion of carbide formers toward the surface of diamond, the control of wetting profile of the molten liquid on diamond etc. 2. Diamond bonding in tool matrix Diamond is the hardest material known, it is also one of the most inert substance. Due to this low reactivity, diamond grits can cut other materials clean, but they are also notorious difficult to be attached in the tool matrix. Consequently, diamond pullouts from the matrix have been the major limitation of the tool longevity. Among the three types of materials that are used for bonding diamond grits, resinous (polymeric) bonds are too weak to hold diamond firm, vitreous (glassy) matrices are too brittle to with* Corresponding author. Address: KINIK Company, 64 Chung-San Rd., Ying-Kuo, Taipei Hsien 239, Taiwan, ROC. Tel.: +886 2 8678 0880; fax: +886 2 8677 2171. E-mail address: [email protected] (J.C. Sung). 0263-4368/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2008.11.011
stand impacts, metal bonds, on the other hand, can lock up diamond without yielding. However, because of this mechanical engagement, diamond grits must be buried deep in the matrix, consequently, the protrusion of diamond grit for cutting is limited. As a result, the cutting speed is low. Worse still, the rubbing of metal against the work object can generate much heat that may damage both materials. For single layered diamond tools, conventional technologies are trapping diamond grits by electroplating with nickel. Due to the insulating character of diamond, nickel layer form a depression around the diamond. As a result, diamond can be pulled out even more easily (Fig. 1). There has been research works published in literature on the brazing of diamond [1–3]. But no details about the brazing phenomena were provided. Beginning in 1975, Abrasive technology introduced nickel brazed diamond tools with a single layer of diamond grit [4]. The brazing could form chemical bond of carbide at the interface so diamond was joined with the metal seamlessly. Due to the strong bonding, the diamond grit protruded high to sustain the impact stress (Fig. 2). As the result of reduced frictional contact between the diamond tool and the working object, the power consumed in cutting was reduced. Although, the chemical bonding of the braze is strong but the brazing layer is often very thin, so the diamond may be knocked off form the base along with the attached braze (Fig. 3). 3. The brazing of diamond grid with massive support In order to avoid the clustering of the diamond grits during the brazing process, diamond grits must be planted with a pre-determined pattern. In 1997, Kinik (under the license of the author)
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Fig. 3. The rupture of braze at the base during the aggressive cutting action of diamond grits.
support around each grit. In contrast, electroplated nickel or sintered alloy cannot form such a profile of massive support (Fig. 5). Contrary to conventional diamond tools that diamond losses are inevitable during use, diamond pullouts cannot happen with brazed diamond grids although, the crystals may be shattered if the impact force is excessive. Due the exceptional bonding strength with carbide interface and braze slope, the brazed diamond grids may cut twice as fast as comparable designs of electroplated tools with the longevity of the tool life that may also be doubled (Fig. 6). 4. Diamond pad conditioners for making semiconductors Fig. 1. Electroplated nickel forms cavity around every grit so retention force of diamond is weak. As a result, diamond grits are often pulled out to leave cavities.
introduced the world first products with brazed diamond grid. Such products include wire saw pearls, forming wheels and others. The author displayed these tools in Verona Exhibition in Italy in 1998 (Fig. 4). The brazed diamond features a massive support of braze profile around each diamond grit. In additional to the chemical bonding, the attachment of diamond is further reinforced by the mechanical
Diamond pad conditioners are the most valuable diamond grit tools. A typical diamond pad conditioner is a flat disk of about 100 mm in diameter (e.g. for Applied Materials polishers). It has diamond grits (e.g. 150 microns) brazed on the surface. Such a tool, if it is used to grind a stone, may be purchased for less than $10. However, for diamond pad conditioners, they can be sold for more than $300 a piece. The pad conditioners are designed to dress delicate polyurethane pads, not hard materials normally encountered by conventional diamond grit tools. The pads are used to polish expensive semiconductor wafers that contain delicate layers of integrate circuits (ICs). As any diamond debris may cause cata-
Fig. 2. The clustering of brazed diamond grits tend to thicken the braze layer locally. Note the diamond is weakly held in braze because the steep rise of the wetting layer that is too thin.
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Fig. 5. Electroplated diamond has a convex profile that cannot hold diamond firm (top left). Sintered diamond has a flat profile that may also lose diamond (top right). Strongly bonded diamond has a massive support profile that will never lose any diamond (bottom diagram).
Fig. 4. Brazed diamond grids for various diamond tools [5,6].
strophic scratches on the wafer, no diamond breakage can be tolerated. Thus, pad conditioners are used with diamond attrition by wear. This is in contrast with conventional diamond tools that allow diamond to chip for sharpening, and pulled out to expose lower lying diamond (Fig. 7). The diamond grid design for pad conditioners stabilized the dressing performance of CMP. Since its debut in 1999, almost all CMP pad conditioners contain diamond grits distributed in a predetermined pattern. In 1999, Kinik pioneered diamond pad conditioners with diamond grits set in a pre-determined pattern (Sung, US Patent 6286,498) [7]. Moreover, each diamond particle is
bonded chemically with a strong braze alloy (Sung, US Patent 6039,641 and 6679,243 [7]; Sung and Lin, US Patent 6368,198 [8]). In addition, every individual diamond grit is further supported mechanically with massive reinforcement (Sung, US Patent 6679,243) [7]. Furthermore, most diamond crystals are oriented with tips or edges facing the pad for sharp cutting. The metal surface of the pad conditioner is also uniquely coated with an acid resistant diamond-like carbon coating (Sung and Lin, US Patent 6368,198) [8]. Such a combination of design features has made DiaGridÒ/DiaTrixTM pad conditioners the new standard for CMP manufacture since 2000 (Fig. 8).
5. Chemical bonding of diamond The braze alloy contains a solvent (e.g. Ni or Cu) that can dissolve active elements (e.g. Cr or Ti) (Fig. 9). During the melting process, the active element will migrate toward diamond where they
Fig. 6. The fracture of diamond bonded by the massive support of braze were left with remnant in the matrix, so the pullout of diamond was not possible.
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Fig. 7. An expensive diamond pad conditioner (left diagram) is compared with a cheap stone grinder (right diagram). Both products contain diamond grits that are set in a pre-determined pattern (Kinik’s DiaGridÒ products). The pad conditioner is further coated with a submicron thick of diamond-like carbon (DLC) for preventing metal from being etched by the chemicals in the polishing slurry.
react to form carbide (e.g. Cr3C2 or TiC). This reaction exhibits the wetting phenomenon of the alloy on the diamond surface. The capillary force often pulls the melt up the slope of the diamond faces. At the interface, the strong carbide bond holds diamond firmly atom-by-atom. The chemical reaction on the diamond surface is evident when the diamond is freed from the braze by acid etching (Fig. 10). The reaction takes place between diamond and the active solute elements (e.g. Cr and Si) to form carbide. It is the formation of carbide that constitutes the strong bonding of diamond.
6. The compromise of bonding strength and impact strength Although, the chemical bonding is very strong, the reaction at the interface may also degrade the diamond integrity. As a result, diamond’s impact strength is reduced. In order to moderate the carbide formation at the interface, a thin (e.g. 10 microns) copper layer may be plated on diamond before it is wetted by the molten braze, the result confirmed that the adherence of the diamond surrounded by the molten braze can be strengthened without compromising the impact strength (Fig. 11). The interface engineering may control the fracture path when the diamond containing matrix is broken. The bare diamond will separate the matrix at the interface due to the lack of chemical bonding. The molten braze coated diamond will fracture through because the crystal is weakened by the brazing reaction. The molten braze coated diamond with the moderation by a thin copper interlayer will tear off the weakest link in the coating itself (Fig. 12).
7. The melting behavior of the braze
Fig. 8. Randomly distributed diamond grits buried in electroplated nickel (top diagram) is compared with a clustered diamond grits wetted by weak alloy (middle diagram), and the patterned diamond grid bonded by massive support (bottom diagram).
The melting kinetics of metal powder is sensitively dependent on its particle size, the smaller the particles, the faster the melting process (Fig. 13). During the brazing process, the melting will start from the finer particles, so if the powder is lack of these fines, the onset of the melting process will be postponed (Fig. 14). During the process of melting, the braze will start from the vicinity of diamond and it is progressed outward until the entire braze layer becomes liquid (Fig. 15). The powders just before melting and after its total liquefaction are compared below. During the melting process, the diamond
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Fig. 9. The scanning electron micro image of diamond showing Cr and Si solute atoms in a Ni solvent diffused preferentially toward diamond and form carbides at the interface. Note the complementary nature of Cr and Si near the vicinity of the diamond.
Fig. 10. The reaction of half of diamond surfaces with the molten braze during the brazing process.
Fig. 11. The optimization of bonding strength at the interface between diamond and braze and the impact strength of the crystal by plating a thin interlayer of copper that moderated the reaction of carbide formation. In the diagram, ABCD refers to active brazed bonded diamond.
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Fig. 12. The fracture paths of three different interfaces that display the weakest link at difference locations. Again, ABCD refers to active braze coated diamond.
Fig. 13. The melting point of gold and copper. Note the rapid plunging of the melting point when the particles are becoming submicron in size.
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Fig. 14. The delayed melting of a braze due to the coarsening of the powder. Note that the powder on the right diagram was not fully melted due to such a delay.
Fig. 15. The melting of the braze layer will begin around each diamond and the liquid will radiate from each diamond until the entire braze layer becomes molten.
crystal sitting on the top of the braze layer is wetted and pulled downward by the capillary force of the liquid (Fig. 16).
stripping of the braze from the substrate could be expedited by increasing of the current density (Fig. 18).
8. The stripping of brazed diamond layer
9. Brazing at reduced temperature
The diamond is bonded chemically with the braze that is coated on the substrate (e.g. stainless steel), such a layer can be stripped by the reverse process of electroplating of anodization. Due to the alloying of the braze and the substrate, the interface is dissolved preferentially during the stripping process so the entire braze layer will fall off from the substrate held by anode (Fig. 17). The above experiments showed a higher potential of the stainless steel substrate compared with the nickel based braze, and the
The brazing of diamond with a nickel based alloy at high temperature (e.g. >1000 °C) often cause thermal damages of diamond. It may be desirable to use a braze with a lower melting point. One way to do so is by adding phosphorus in the nickel–chromium alloy. However, the high vapor pressure of phosphorus may require brazing with an externally applied pressure. The phosphorus containing braze may not adhere diamond firm enough so titanium coated diamond is often used to improve the bonding strength (Fig 19).
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Fig. 16. The contrast of a brazed diamond bonded by molten liquid (left diagram) and a diamond sitting on the top of the sintered metal powder that just began to melt (right diagram).
Fig. 17. The brazed diamond grid before stripping (left diagram) and the top layer fell of from the substrate (right diagram).
Fig. 18. The plots of voltage versus current during the stripping process (left diagram) and the time for removing the braze layer versus current applied (right diagram).
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Fig. 19. A titanium coated diamond was brazed by a phosphorus containing nickel–chromium alloy (left diagram). The EDAX mapping of elements around a titanium coated diamond.
Fig. 20. The peeling off the braze layer from titanium coated diamond.
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Fig. 21. Diamond grits pulled out from the brazed matrix that contained phosphorus with sockets leaving behind.
Fig. 22. The appearance of glass brazed diamond grits.
The phosphorus containing braze may not wet titanium on diamond easily due to the oxidation of titanium (Fig. 20). Although, diamond coated by titanium and bonded by phosphorus containing braze is stronger than that mechanically retained in a sintered metal matrix, it can be dislodged from the braze under a strong blow (Fig. 21).
10. Glass bonded diamond Diamond may also be brazed by a glassy (oxide) material that contains carbide formers (e.g. Li and Na). In this case, diamond is wetted by the molten glass and a massive support slope may also
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Fig. 23. The flaking of DLC from the surface of diamond after rubbing against a polyurethane pad.
Fig. 24. Etched DLC showing remnants of flakes.
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Fig. 25. The repelling of molten aluminium at the surface of diamond that was pre-coated with either silicon or titanium.
be formed. The brazing is conducted under air instead of under vacuum (Fig. 22). 11. DLC coated diamond Diamond-like carbon (DLC) may be used to coat a brazed diamond grid to prevent the corrosion of the braze in an acidic environment. However, DLC so formed also cling on diamond surface and it may be rubbed off (Fig. 23). DLC may also be etched by strong oxidizing agent, such as iron chromate. In this case, DLC may form cracks or even isolated flakes (Fig. 24).
Although, the sciences of diamond wetting and brazing were long established, the detailed phenomena of commercial brazing process has been held as trade secret for decades. In this research, we presented extensive data on brazing of commercial products. The discussed melting curves of the braze and the particle size effect. We also introduced brazing with pre-coating of titanium. In addition, brazing by flowing glass was also described. Furthermore, striping of the braze layer by reverse electroplating was presented. Finally, diamond-like carbon coating of brazed diamond was shown with the adherence problems.
Acknowledgement 12. Conclusions The brazing of diamond is often dependent on the competition between carbide formation and metal oxidation at the interface between diamond and molten alloy. For example, diamond cannot be wetted by molten aluminum at 750 °C that is well above the melting point of 660 °C. This may be due to the oxidation of aluminum before the formation of aluminum carbide. By coating diamond with either silicon or titanium that may have limited solubility with aluminum, the wetting of diamond by molten aluminum is still prevented. Again, the rapid oxidation of either silicon or titanium could be the culprit in this case (Fig. 25). Consequently, the brazing must compromise the kinetics between carbide formation and oxidation at the interface between diamond and liquid. In the case of applying chromium as the active element for brazing diamond, the carbide formation can take place along with chromium oxidation. Consequently, the chemical bonds formed at the interface between diamond and molten alloy are strong.
The authors are grateful for the support of this research by KINIK Company.
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