Bonding tool having diamond head and method of manufacturing the same

Abstract: A bonding tool (40) has a head portion (45) which consists essentially of vapor-deposited diamond, wherein a principal diamond crystal plane forming a head face (47) is a (111) plane. Such a head face has high hardness, and is hard to wear. In order to improve toughness of a bonding head which is mainly made of vapor-deposited diamond, a portion forming a head face consists essentially of high-purity diamond, and another portion supporting the head face consists essentially of low-purity diamond. The head face has high rigidity, while the portion supporting the head face has high toughness. In order to provide a bonding head portion which is mainly made of vapor-deposited diamond with electrical conductivity, a portion forming a head face consists essentially of polycrystalline diamond containing a relatively small amount of dopant, and another portion supporting this portion contains a large amount of dopant. The head portion having electrical conductivity can be heated by energization. In a tool employing a coating containing vapor-deposited diamond as a bonding head portion, the coating is formed by a complex of a metal or ceramics and vapor-deposited diamond, in order to improve strength of the coating and adhesion to a tool substrate. The metal or ceramics is closer in thermal expansion coefficient to the tool substrate than the diamond.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a TAB (tape automated bonding) type bonding tool which is employed for packaging a semiconductor device, and a method of manufacturing the same. Description of the Background Art TAB system LSI packaging, which can cope with reduction in interconnection pitch as well as reduction of the target apparatus in thickness and weight, has increasingly been employed for packaging an LSI in relation to a liquid crystal television, an electronic notebook, a laptop personal computer, a liquid crystal driver IC for a word processor, ICs for a watch and a hand-held calculator, and the like. The term TAB system refers to a method of batch-bonding a plurality of electrodes which are formed on an LSI chip with lead wires provided on a film carrier tape. Fig. 1 typically illustrates LSI packaging in such a TAB system. This system is adapted to batch-bond hundreds of electrodes (Au) 2 which are provided on each LSI chip 1 at pitches of 70 to 200 µm with lead wires 4 which are formed on each film carrier tape 3 by etching with a bonding tool 10 which is heated to a temperature of 500 to 600°C. In such bonding, the bonding tool 10 presses the lead wires 4 against the electrodes 2 by a heated head face 10a. According to such a TAB system, it is possible to package a relatively large number of electrodes in a short time as compared with a system employing a wire bonding tool. Fig. 2 illustrates the shape of a general bonding tool 20 which is employed in the TAB system. In this bonding tool 20, a head portion forming a head face 20a for bonding is supported by a shank 21 which consists essentially of a thermally unexpandable alloy. Such a bonding tool must have high reliability for withstanding repeated use under a high temperature for a long time, and hence it is necessary to employ a material which is excellent in heat resistance and wear resistance particularly for the head portion. In order to satisfy such requirement, the head of a conventional bonding tool is prepared from a synthetic or natural diamond single crystal, a binderless cBN sintered body or a diamond sintered body. However, a natural diamond single crystal has large dispersion of performance due to a large content of impurities, irregular crystal orientation and the like, while it is difficult to supply a large-sized crystal of at least 7 to 8 mm square. A synthesized diamond single crystal, which is one of materials having most excellent performance, can be applied to only limited use since the same is extremely high-priced as compared with other materials. On the other hand, a binderless cBN sintered body is inferior in wear resistance to a material which is mainly composed of diamond. Further, a head face of a diamond sintered body cannot uniformly pressurize lead wires under a high temperature due to extreme warp caused by a binder contained therein. Thus, none of the aforementioned materials is satisfactory for the user. On the other hand, vapor-deposited polycrystalline diamond can sufficiently provide characteristics of diamond since the same contains no binder. Further, it is possible to stably supply large-sized vapor-deposited diamond at a low cost. Thus, such vapor-deposited diamond has recently been employed as a material which is suitable for a bonding tool. In general, practical vapor-deposited diamond is prepared by chemical vapor deposition (CVD) of decomposing/exciting a raw material gas which is mainly composed of hydrocarbon such as methane and hydrogen under a low pressure. Japanese Patent Laying-Open No. 2-224349 (1990) in the name of the assignee discloses vapor deposition of diamond on a base material of a sintered body consisting essentially of Si 3 N 4 , a sintered body consisting essentially of SiC, a sintered body consisting essentially of AlN, or Si, and brazing of the base material provided with such vapor-deposited diamond to a tool substrate consisting essentially of a metal. In a bonding tool according to this technique, a head portion consisting essentially of vapor-deposited diamond is connected to the tool substrate through the base material consisting essentially of the aforementioned sintered body or Si. Further, a head face for bonding is made of a (100) crystal plane and/or a (110) crystal plane of diamond. Japanese Patent Laying-Open No. 64-5026 (1989) discloses a tool having a shank whose head portion is coated with vapor-deposited diamond. In more concrete terms, the shank consists essentially of a stainless steel material, and diamond is deposited on its head portion by CVD in a thickness of about 5 µm. This literature discloses that the tool warps by difference in thermal expansion coefficient between the stainless steel material and the diamond if the diamond coating exceeds 10 µm in thickness, to remarkably reduce the bonding yield. The literature also discloses that durability of the tool is reduced if the diamond coating is not more than 1 µm in thickness. In such a bonding tool, the head face is preferably as flat as possible. Immediately after vapor deposition, however, the diamond surface is so coarse that the same must be polished to define the head face. In order to facilitate such polishing, therefore, the diamond is so vapor-deposited as to orient the (100) crystal plane and/or the (110) crystal plane thereof substantially in parallel with the surface. However, such a head face formed by the (100) plane and/or the (110) plane cannot be regarded as optimum in view of wear resistance of the bonding tool, although the same is preferable in view of polishing. Thus, awaited is a bonding tool having a diamond head face which is flat and hard to wear. As to the aforementioned conventional bonding tools, the former literature discloses a head portion which consists essentially of diamond having a thickness of 5 to 300 µm, while the latter literature suggests a head portion which consists essentially of a diamond film having a thickness of 1 to 10 µm. The diamond forming such a head portion has high purity. In general, high-purity diamond has high rigidity. In relation to such diamond forming the head portion of a bonding tool, however, sufficient study has not yet been made as to contribution of the structure and composition in the direction of thickness to performance of the bonding tool. As to general bonding tools, known are a constant heating type tool and a pulse heating type tool. In the constant heating type tool having a heater which is built in a tool substrate, stationary heat is transmitted from the heater to a head portion through the tool substrate. In the pulse heating type tool, on the other hand, a substrate consisting essentially of stainless steel, inconel or Mo is heated by instantaneous energization. The aforementioned conventional diamond bonding tool is suitable for constant heating, while the same is rather unsuitable for pulse heating. This is because a tool having a diamond head is inferior in thermal responsibility to a tool consisting of a metal, since the diamond head face is heated by heat transmission from the tool substrate. In order to heat the head face of such a conventional diamond tool to a prescribed temperature in response to a current pulse and cool the same, a longer time is required as compared with a tool consisting of a metal. Thus, it is difficult to reduce a time required for single bonding. Under such circumstances, awaited is a diamond bonding tool which is suitable for pulse heating. The tool having a shank whose head portion is coated with diamond can be manufactured through a small number of steps at a low cost. However, a vapor-deposited diamond film directly coating such a shank is inferior in adhesion strength to the shank. Therefore, the conventional diamond-coated tool is inferior in durability since the diamond coating is easily separated or cracked during use. Thus, awaited is a diamond-coated tool which is superior in durability. SUMMARY OF THE INVENTION An object of the present invention as claimed in claims 1, 7 and 8 is to provide a bonding tool which is superior in wear resistance to the prior art. Another object of the present invention is to make more detailed study as to the structure and composition of vapor-deposited diamond which is applied to a bonding tool, thereby providing a bonding tool which is improved to exhibit higher performance. Still another object of the present invention is to provide a bonding tool, having a working surface made of vapor-deposited diamond, which is higher in thermal responsibility. A further object of the present invention is to provide a bonding tool, having a head provided with a coating, which is superior in durability with substantially no separation or cracking caused in the coating. In EP 0 435 423 a bonding tool is disclosed which is provided at one end with a substrate being coated with polycrystalline diamond. The polycrystalline diamond is deposited by a vapor face synthesis method. In EP 0 449 571 a polycrystalline diamond tool and a method for producing it is disclosed. It is proposed to provide a polycrystalline diamond with a different quality of the surfaces, whereby the diamond near the rake surface is of a better quality than of the fixation surface. EP 0 254 560 reveals a gaseous phase synthesized diamond and a method for sythesizing the same. The raw material gases are thermally activated by a thermo electron-radiating material heated at an extremely high temperature for obtaining a crystalline diamond having a high completeness and hardly containing carbon. In EP 0 279 898 a method for providing a wear resistant layer is disclosed, whereby the layer is formed of different single layers, each of which has a different elastic modulus. The outer single layer is constituted out of diamond crystals. In addition, a polycrystalline diamond tool method for producing the same is known from EP-A-487 292. As this document was published after the priority date of the present application, it is only considered as to the novelty of the present invention. A bonding tool not forming part of the claimed invention comprises a bonding head portion having a head face for bonding and consisting essentially of vapor-deposited diamond and a non-diamond component which is formed in vapor deposition of the diamond, wherein the bonding head portion includes a portion forming the head face and another portion which is in contact with the said portion and containing a larger amount of the non-diamond component than the said portion. The non-diamond component may be amorphous carbon, graphite or glassy carbon. The portion containing a larger amount of the non-diamond component is higher in tenacity and toughness than the diamond forming the head face. In this bonding tool, the head face is supported by a material having high toughness. According to an aspect of the present invention, a bonding tool comprises a bonding head portion having electrical conductivity and consisting essentially of vapor-deposited polycrystalline diamond and a dopant, wherein the bonding head portion includes a portion forming a head face for bonding and another portion which is in contact with the said portion and containing a larger amount of the dopant than the said portion. In this bonding tool, an element such as B, Al, P, Sb, Si, Li, S, Se, Cl, N, W, Ta, Re or Cr can be employed as an impurity. Such an impurity can be doped in vapor deposition of the diamond. It is possible to directly energize the head portion, which has electrical conductivity due to the addition of the impurity, to heat the same by its resistance. According to a further aspect of the present invention, provided is a method of manufacturing a bonding tool having a head portion which consists essentially of vapor-deposited diamond containing an impurity added thereto. According to this method, a base material having a surface for deposition of diamond is prepared first, and then diamond is deposited on this surface by vapor deposition with doping. Then, the diamond is further deposited by vapor deposition while suppressing supply of the impurity in doping. Then, the base material is brazed to a tool substrate so that the deposited diamond defines a head portion, and a flat head face is formed by polishing. According to a further aspect of the present invention, provided is a method of manufacturing a bonding tool having a head portion which consists essentially of vapor-deposited diamond containing an impurity added thereto. According to this method, first prepared is a base material having a mirror-finished surface for deposition of diamond. Then, diamond is deposited on this surface by vapor deposition with doping, and thereafter the base material is separated or removed to obtain a diamond material. Then, the diamond material is brazed to a tool substrate so that a surface, which has been in contact with the base material, of the diamond material defines a head face. According to this method, the amount of impurity supply is reduced in a prescribed period from starting of the vapor deposition as compared with that in a later period. The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a model diagram for illustrating a TAB system; Fig. 2 is a perspective view showing a general shape of a bonding tool; Fig. 3 illustrates a Raman spectrum for evaluating the content of a non-diamond component in the present invention; Figs. 4A and 4B are sectional views schematically showing steps of manufacturing a bonding tool according to Example 1; Fig. 5 is a perspective view showing the bonding tool manufactured according to Example 1; Fig. 6 is a sectional view schematically showing a bonding tool manufactured according to Example 2; Fig. 7A is a sectional view showing a principal part of a bonding tool manufactured according to Example 3; Fig. 7B is a perspective view showing the overall shape of the bonding tool manufactured according to Example 3; Fig. 8 is a sectional view showing a principal part of a bonding tool manufactured according to Example 4; Fig. 9 shows time change in concentration of an impurity added in Example 5 of the present invention; Fig. 10 is a sectional view schematically showing a bonding tool manufactured according to Example 5 of the present invention; Fig. 11 shows impurity distribution in the bonding tool according to Example 5 of the present invention; Fig. 12 illustrates time change in concentration of an impurity added in Example 6 of the present invention; Fig. 13 is a sectional view schematically showing a bonding tool manufactured according to Example 6 of the present invention; Fig. 14 shows distribution of impurity concentration in the bonding tool according to Example 6 of the present invention; Fig. 15 is a sectional view schematically showing a bonding tool manufactured according to Comparative Example 4; Fig. 16 is a model diagram showing an apparatus for manufacturing a bonding tool according to Example 7 of the present invention; Fig. 17A is a sectional view showing a principal part of the bonding tool manufactured according to Example 7 of the present invention; and Fig. 17B is a side elevational view showing the overall shape of the bonding tool manufactured according to Example 7 of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the present invention, diamond can be synthesized by various vapor deposition methods. For example, the vapor deposition methods can include CVD of decomposing/exciting a raw material gas using thermionic emission or plasma discharge, CVD employing a combustion flame, and the like. The raw material gas employed for such CVD can be prepared from a mixed gas mainly composed of hydrocarbon such as methane, ethane or propane, alcohol such as methanol or ethanol or an organic carbon compound such as ester, and hydrogen. In addition, the raw material may contain an inert gas such as argon, oxygen, carbon monoxide, water and the like in a range not inhibiting synthesis of diamond and properties thereof. In a bonding tool having a head surface which is mainly made of a (111) diamond crystal plane, diamond forming a portion of at least up to about 10 µm in depth from the head face preferably has a (111) plane which is oriented substantially in parallel with the head face as a principal plane. In the head face and its proximity (up to about 10 µm in depth from the head face, for example), it is preferable to suppress contamination of crystal planes other than the (111) plane to the minimum. Assuming that the (111) plane is 100 in intensity by X-ray analysis, for example, (220), (311), (400) and (331) planes are preferably not more than about 80, more preferably not more than about 10 in intensity (λ = 1.5450 Å when CuKα 1 is employed by a 2 -  method, for example). In the vapor-deposited diamond forming the head face, further, an FWHM (full width at half maximum intensity) value in a rocking curve of the (111) plane obtained by introducing X rays from the head face is preferably within about 20°, as an index showing the orientation of the (111) plane. On the other hand, the (220) plane preferably has an FWHM value of at least about 20° in such a rocking curve. In the bonding tool having the aforementioned features, the head face is preferably polished. For example, the head face preferably has surface roughness R max of not more than about 0.1 µm. The head face which is mainly made of the (111) plane, having the highest hardness among diamond crystal planes, is hardly worn as compared with the (100) and (110) planes forming conventional head faces, whereby this bonding tool is superior in wear resistance to the prior art. The diamond forming the head portion may have a thickness of about 5 µm to 3 mm, preferably about 5 to 300 µm. Since a (111) plane is hard to polish, considerable devices have been required in order to obtain a practically flat head face which consists essentially of such a (111) plane. The present invention has enabled implementation of a practical tool having a head face which is mainly formed of a (111) plane for the first time by the following two methods: In the first method, a base material having a surface for deposition of diamond is prepared first. The base material is preferably prepared from at least one material selected from the group consisting of a sintered body mainly composed of Si 3 N 4 , a sintered body mainly composed of SiC, a sintered body mainly composed of AlN, Si, and a complex thereof, for example. Such a base material is particularly excellent as to adhesion strength to diamond. Then, diamond is deposited on the base material surface by vapor deposition to orient a (111) plane in parallel with the base material surface. The (111) plane can be generally formed by reducing carbon concentration (with respect to gaseous hydrogen, for example) in the aforementioned CVD. The deposited diamond can be about 5 µm to 3 mm, preferably about 5 to 300 µm in thickness. Then, a diamond layer is further deposited on the vapor-deposited diamond to orient a crystal plane having lower hardness than the (111) plane. The crystal plane having lower hardness than the (111) plane can be generally formed by increasing carbon concentration (with respect to gaseous hydrogen, for example) in the aforementioned CVD. The crystal plane having low hardness is preferably prepared from a (220) plane, for example. The diamond layer having low hardness is formed in a thickness of not more than about 50 µm, preferably about 10 µm, for example. Then, the base material on which the diamond is deposited is brazed to a tool substrate. Such brazing is preferably carried out through a metallization layer consisting essentially of any of metals included in the groups IVa, IVb, Va, Vb, VIa, VIb, VIIa and VIIb of the periodic table and compounds thereof. Then, the diamond layer having low hardness is removed by mirror polishing, to expose the (111) plane. A head face obtained by such polishing preferably has surface roughness R max of not more than about 0.1 µm, for example. Further, it is preferable to suppress contamination of crystal planes other than the (111) plane to the minimum in the head face obtained after the mirror polishing and its proximity (up to about 10 µm in depth from the head face, for example). Assuming that the (111) plane is 100 in intensity by X-ray analysis, for example, (220), (311), (400) and (331) planes are preferably not more than about 80, more preferably not more than about 10 in intensity respectively. When the diamond layer having low hardness is removed to expose the (111) plane, it is possible to provide a head face having high flatness, which is mainly formed of the (111) plane. In the second method, a base material having a mirror-finished surface for deposition of diamond is prepared first. The surface of the base material is preferably mirror-finished to have surface roughness R max of not more than about 10 µm, more preferably not more than about 0.2 µm. The base material is preferably prepared from Si or Mo, for example. Then, the base material on which the diamond is deposited is separated or removed to obtain a diamond material. The base material consisting essentially of Si or Mo can be separated or removed from the diamond deposited thereon by machine work or chemical treatment. The as-obtained diamond material is brazed to a tool substrate. In such brazing, a diamond surface having been in contact with the base material, i.e., a crystal growth start surface, is adapted to define a head face. Therefore, a surface opposite to this surface, i.e., a crystal growth end surface, is employed as a brazing surface. This brazing surface is preferably provided with the aforementioned metallization layer in advance of the brazing. The brazing can be effectively performed through such a metallization layer. According to this method, the surface, which has been in contact with the base material, of the diamond material obtained by separating or removing the base material is a mirror surface. This surface may not be polished, or can be only simply polished, if necessary. The vapor deposition is so performed as to orient the (111) plane in parallel with the base substrate surface, whereby the surface of the diamond material having been in contact with the base material is substantially occupied by the (111) plane. When the diamond material is so brazed to the tool substrate that the surface having been in contact with the base material defines a head face, therefore, it is possible to obtain a bonding tool which is superior in wear resistance to the prior art. According to the aforementioned two methods, it is possible to provide practical bonding tools comprising head faces which are flat and mainly formed of (111) planes. In a bonding tool containing a non-diamond component such as amorphous carbon, graphite or glassy carbon in its head face not forming part of the claimed invention, the content of such a non-diamond component can be evaluated by Raman spectroscopic analysis in the following manner, for example: Referring to Fig. 3, it is assumed that a Raman spectrum shown in this figure is obtained by Raman spectroscopic analysis. In this spectrum, a peak A appearing around 1332 cm -1 is caused by diamond, and another peak B is caused by a non-diamond component such as amorphous carbon, for example. First, a background line L is drawn with respect to such a spectrum. Then, a minimum point Q by the peak B is decided on the basis of the background line L. Then, a tangential line M is drawn from the point Q to a portion close to the base (1050 to 1150 cm -1 ) of the peak A. Assuming that C represents the intersection of a perpendicular AA' drawn from the peak A of the diamond toward the X-axis, the distance (m) between A and C shows peak intensity (Id) of the diamond. Assuming that D represents the intersection of another perpendicular BB' drawn from the peak B of the non-diamond component toward the X-axis, on the other hand, the distance (1) between B and D shows peak intensity (Ii) of the non-diamond component. It is possible to relatively evaluate the content of the non-diamond component by the ratio Ii/Id of the peak intensity Ii to Id shown by the distances l and m . When the ratio Ii/Id is reduced, purity of the diamond is improved with a small content of the non-diamond component. As the ratio Ii/Id is increased, purity of the diamond is reduced with increase in content of the non-diamond component. In the bonding tool according to the present invention, therefore, the vapor-deposited diamond is so structured that the ratio Ii/Id of the vapor-deposited diamond forming the head face is smaller than that of the subsequently formed diamond. According to the present invention, the vapor-deposited diamond forming the head face preferably has a ratio Ii/Id of less than about 0.6, for example, and more preferably less than about 0.2. On the other hand, the diamond subsequently formed on the tool base material side preferably contains a larger amount of the non-diamond component, to have a larger ratio Ii/Id as compared with the vapor-deposited diamond forming the head face. According to the present invention, the content of the non-diamond component can also be evaluated by an FWHM (full width at half maximum) value of the diamond peak in Raman spectroscopic analysis. In general, the peak is increased and the FWHM value is reduced as purity of the diamond is improved with complete crystals. In the bonding tool according to the present invention, the vapor-deposited diamond forming the head face is so formed that its FWHM value is smaller than that of the subsequently formed diamond. In Raman spectroscopic analysis, the vapor-deposited diamond forming the head face preferably has an FWHM value of less than about 10 cm -1 , for example. On the other hand, the subsequently formed vapor-deposited diamond containing a larger amount of the non-diamond component preferably has a larger FWHM value. In such a bonding tool, the high-purity diamond forming the head face preferably has a thickness of about 5 to 80 % of that of the overall diamond, for example. On the other hand, the vapor-deposited diamond containing the non-diamond component, which is provided in continuation to the high-purity diamond, preferably has a thickness of about 20 to 95 % of that of the overall diamond. The thickness of the overall diamond can be about 5 to 300 µm, for example. In this bonding tool, the vapor-deposited diamond can be formed on a base material consisting essentially of a sintered body which is mainly composed of Si 3 N 4 , Si or AlN, or Si, for example. The base material on which the diamond is deposited is brazed to a tool substrate. The brazing can be effectively performed through the aforementioned metallization layer. Alternatively, a head member consisting essentially of high-purity vapor-deposited diamond and vapor-deposited diamond containing a non-diamond component may be directly brazed to a tool substrate. The brazing can be effectively performed through a metallization layer which is formed on the head member. In such a bonding tool, the bonding head face is preferably polished. This head face preferably has surface roughness R max of not more than about 0.1 µm, for example. High-purity vapor-deposited diamond which has generally been applied to a bonding tool has high rigidity, while the same is not necessarily superior in toughness. In other words, high-purity vapor-deposited diamond which is synthesized under uniform conditions as a whole is relatively easily chipped by external force. The inventive bonding tool has been proposed in order to compensate for such a disadvantage. In this bonding tool, the vapor-deposited diamond containing a larger amount of non-diamond component as compared with the head face is superior in tenacity and toughness to the diamond forming the head face. Since the head face is supported by such tough vapor-deposited diamond, the diamond forming this head face is supplemented in toughness, to be hardly chipped. Such a structure reinforces toughness while maintaining the high rigidity of the relatively pure diamond forming the head face. Thus, the head portion of the bonding tool is not only excellent in wear resistance, but is hard to chip. The bonding tool whose head portion has electrical conductivity due to doping can contain an impurity such as B, Al, P, Sb, Si, Li, S, Se, Cl, N, W, Ta, Re or Cr, for example. It is possible to form doped vapor-deposited diamond by adding a proper amount of an impurity gas or a gas containing an impurity to a raw material gas in vapor deposition of the diamond. Alternatively, such an impurity may be added to previously vapor-deposited diamond through well-known diffusion or ion implantation. While electrical conductivity of such polycrystalline diamond containing an impurity added thereto is varied with the type of the added impurity, such electrical conductivity is improved with increase of the content as to the same impurity. In the bonding tool according to the present invention, however, the polycrystalline diamond having electrical conductivity preferably has specific resistivity in a range of 10 -5 to 10 5 ohm·cm, and more preferably in a range of 10 -3 to 10 3 ohm·cm. If the specific resistivity is in excess of 10 5 ohm·cm, it can be difficult to directly energize the polycrystalline diamond to attain resistance heating to a prescribed temperature. If the specific resistivity is less than 10 -5 ohm·cm, on the other hand, strength of the diamond can be reduced although energization is simplified. The impurity preferably has a small atomic radius, since the diamond has an extremely small interatomic distance with a lattice constant of 3.56672 Å (10 Å = 1nm). Metal elements remarkably influence on wear resistance and practical flatness of the polycrystalline diamond. Such metal elements, generally having large atomic radii, hardly enter spaces in the lattice of the diamond or serve as substitutional ones, but segregate in grain boundaries of the diamond to reduce crystallinity. Therefore, the content of the metal elements is preferably minimized. From the aforementioned point of view, the impurity is preferably prepared from boron (B), in particular. Concentration of such an impurity with respect to carbon forming the polycrystalline diamond, which is varied with the type thereof, is preferably set in a range allowing formation of a head face having excellent wear resistance and flatness while maintaining crystallinity of the polycrystalline diamond as well as provision of electrical conductivity capable of being heated by energization. When boron is employed as the impurity, for example, its concentration with respect to carbon is preferably in a range of about 0.1 to 50000 p.p.m. If this concentration is less than about 0.1 p.p.m., specific resistivity can be excessively increased to disable heating by energization. If the concentration is in excess of about 50000 p.p.m., on the other hand, crystallinity of the diamond can be so deteriorated that graphite, amorphous carbon etc. are simultaneously deposited in low-pressure vapor deposition. In this bonding tool, the portion forming the head face and having low impurity concentration preferably has a thickness of about 2 to 80 %, more preferably about 5 to 30 %, of that of the overall diamond, for example. On the other hand, the portion having lower impurity concentration may contain absolutely no impurity. Further, concentration distribution of the impurity may be continuously or gradually changed in a section of the diamond. When such concentration distribution is continuously changed, it is possible to increase the impurity concentration as the distance from the head face is increased. The head face of the tool is preferably flattened to the minimum such that the head face preferably has surface roughness R max of not more than about 0.1 µm, for example. The diamond forming the head portion can be about 5 to 3000 µm in thickness. The polycrystalline diamond containing the impurity added thereto can be heated by energization. The diamond forming the head portion is about 10 to several 100 µm in thickness with small thermal capacity, and has large thermal conductivity of at least 10 W/cm·°C. When only the head portion formed by the diamond is heated by energization, therefore, it is possible to increase the temperature at the head face to a prescribed level in a shorter time as compared with the prior art. When such energization is stopped, on the other hand, it is possible to quickly reduce the temperature of the diamond. Such a bonding tool whose head portion is made of diamond having electrical conductivity is so excellent in thermal responsibility to energization that its working surface can be heated/cooled in a shorter time as compared with the prior art. In this bonding tool, it is possible to reduce a time required for single bonding in a pulse heating type, thereby improving production efficiency of bonding. In this bonding tool, further, the portion forming the head face is suppressed in impurity concentration as compared with other portions. Therefore, the portion forming the head face has high strength and excellent wear resistance. Further, the head face maintains flatness under a high temperature due to the low impurity concentration. Although it is difficult to directly heat the head face by energization due to the low conductivity, the temperature at the head face can be controlled with excellent thermal responsibility by heating the portion having high impurity concentration due to high thermal conductivity of the diamond. Thus, the inventive bonding tool is excellent in thermal responsibility with high strength and flatness in the head face, as well as excellent in durability. A bonding tool having a conductive head can be manufactured in the following two methods: In the first method, a base material having a surface for deposition of diamond is prepared first. The base material is preferably prepared from a material consisting essentially of a sintered body mainly composed of Si 3 N 4 , SiC or AlN, Si, or a complex thereof. Such a material has excellent adhesion strength particularly with respect to diamond. Then, diamond is deposited on the base material surface by vapor deposition with doping, and thereafter the diamond is further deposited by vapor deposition while suppressing supply of an impurity in doping. Finally, the diamond is deposited in a thickness of about 5 to 500 µm, for example. The impurity can be prepared from any of the aforementioned various elements, preferably from boron, in particular. In the aforementioned CVD, it is possible to add a proper amount of impurity to the raw material gas for doping the diamond with the impurity. Alternatively, the impurity may be added to previously deposited diamond by well-known diffusion or ion implantation. The amount of the added impurity may be continuously or gradually reduced. Then, the base material having the doped diamond is brazed to a tool substrate. Then, the head face is flattened by polishing, to obtain a bonding tool which is suitable for pulse heating, as hereinabove described. The head face obtained by the polishing preferably has surface roughness R max of not more than about 0.1 µm. In the second method, a base material having a mirror-finished surface for deposition of diamond is prepared first. The base material is preferably prepared from Si or Mo. It is possible to mirror-finish the surface of the base material to have surface roughness R max of not more than about 1 µm, preferably not more than about 0.2 µm. Then, diamond is deposited on the base material surface by vapor deposition with doping. The diamond is deposited in a thickness of about 200 to 300 µm, for example. In the doping, the amount of impurity supply is reduced in a prescribed period from starting of the vapor deposition as compared with a later period. After completion of the vapor deposition, the base material is separated or removed to obtain a diamond material. Then, the diamond material is brazed to a tool substrate so that a surface having been in contact with the base material defines a head face. Namely, the diamond surface having been in contact with the base material, i.e., a crystal growth starting surface, defines a head face in such brazing. Therefore, a surface opposite to this surface, i.e., a crystal growth end surface, is generally employed as a brazing surface. This brazing surface is preferably provided with the aforementioned metallization layer in advance of the brazing. The brazing can be effectively performed through the metallization layer. According to this method, the diamond is deposited on the mirror-finished base material, whereby the diamond material obtained by separating or removing the base material has a mirror surface on the side having been in contact with the base material. This surface may not be polished, or can be only simply polished, if necessary. The initial amount of impurity supply is suppressed as compared with that in the later stage in the vapor deposition, whereby the portion forming the surface of the diamond material having been in contact with the base material is excellent in strength and wear resistance, as described above. When the diamond material is so brazed to the substrate that this surface defines a head face, it is possible to provide a bonding tool having a head face which is excellent in thermal responsibility as well as wear resistance and flatness. In the bonding tool having a coating which is improved in adhesion strength to the tool substrate, the coating can contain a metal such as Mo, W, Cr, Co, Ni or Fe, for example, or ceramics such as SiC, Si 3 N 4 , WC or BN, for example. As to the material contained in the portion concerning adhesion strength to the tool substrate, a proper one or combination is preferably selected from such metals and ceramics in consideration of adhesion to diamond, adhesion to the tool substrate and the thermal expansion coefficient. As to adhesion to the vapor-deposited diamond, SiC, Si 3 N 4 , WC, BN, Mo and W are preferable, and excellent in this order. As to adhesion to the tool substrate, on the other hand, Mo, W, Cr, Co, Ni and Fe are preferable. In consideration of these points, a portion of the coating closer to the tool substrate can contain a large amount of Mo, Wc, Cr, Co, Ni of Fe, which is excellent in adhesion to the tool substrate, while another portion of the coating which is in contact with a portion containing a larger amount of diamond can contain a large amount of SiC, Si 3 N 4 , WC, BN, Mo or W, for example. In this bonding tool, the coating containing the vapor-deposited diamond can be about 5 to 10000 µm, more preferably about 200 to 1000 µm in thickness, for example. In the coating, the portion consisting essentially of the vapor-deposited diamond and forming the tool head face can have a thickness of about 5 to 95 %, more preferably about 20 to 60 % of that of the overall coating. On the other hand, the portion concerning adhesion to the tool substrate and containing the metal or ceramics preferably has a thickness of about 5 to 95 %, more preferably about 40 to 80 % of that of the overall coating. The coating can contain about 20 percent by volume of the metal or ceramics, for example. The bonding tool having a head portion formed by a coating containing vapor-deposited diamond and a metal or ceramics can be manufactured in the following manner: First, a tool substrate having a surface to be provided with a coating is prepared. The tool substrate can be prepared from a material such as Mo, W, an Fe-Ni alloy such as invar, for example, an Fe-Ni-Co alloy such as super invar, for example, an Fe-Co-Cr alloy such as stainless invar, for example, an Fe-Pt alloy, an Fe-Pd alloy, covar (29 % of Ni, 17 % of Co and a residue of Fe), a Cu-W alloy, a WC-Co alloy, a WC-TiC-Co alloy, a W-Ni alloy or stainless steel, for example. Then, diamond is deposited on the surface of the tool substrate by vapor deposition with supply of powder of at least either a metal or ceramics which is closer in thermal expansion coefficient to the tool substrate than the diamond. The powder of the metal or ceramics can have a particle diameter of about 0.1 to 300 µm, preferably about 0.2 to 60 µm. This powder can be supplied to a flow of a raw material gas employed for vapor deposition of the diamond, to adhere onto the tool substrate with the diamond. At this time, the powder can be temporarily completely melted, semi-melted, or absolutely unmelted. Since a high-temperature plasma jet in the vapor deposition has temperature distribution, it is possible to easily adjust the state of the powder depending on a position of the temperature distribution supplied with the powder. It is also possible to control the powder state by varying the particle diameter of the powder, since its thermal capacity is varied with the particle diameter. The diamond coating can be formed with supply of the powder by well-known CVD, for example, while this coating is more preferably formed along a thermal spraying process employing high-temperature plasma CVD. According to this step, a layer containing the diamond with a metal or ceramics is formed. This layer is excellent in adhesion to the tool substrate, and relieves difference in thermal expansion coefficient between the diamond and the tool substrate. When the powder is supplied in an unmelted state, it is possible to grow the diamond from nuclei of the particles of the powder. In this case, the diamond serves as a binder in the layer containing a mixture of the diamond and the powder, with excellent adhesion therebetween. Then, supply of the metal or ceramics is stopped so that only the diamond is vapor-deposited. After the diamond is deposited in a proper thickness, formation of the coating is completed. A portion consisting essentially of the diamond can form a head face which is excellent in heat resistance, wear resistance and chipping resistance. According to the aforementioned series of steps, it is possible to form a coating which is integrally formed by a composite layer and a diamond layer. This coating strongly adheres to the tool substrate, and has excellent durability. The aforementioned steps are also useful for forming a tool comprising a coating which strongly adheres onto a curved surface of a tool substrate. Further, the aforementioned series of steps can be continuously carried out. For example, it is possible to complete a bonding tool having a coating defining a working surface for bonding which strongly adheres to a tool substrate by simply setting the tool substrate in a CVD apparatus and depositing diamond with a proper time of powder supply. According to this process, it is possible to manufacture a bonding tool which is excellent in durability through simple steps at a low cost. In the coating containing diamond and a metal or ceramics, the portion forming a head face consists essentially of vapor-deposited diamond and has excellent heat resistance, wear resistance and chipping resistance. This portion is stuck to the tool substrate through the portion containing a mixture of a metal or ceramics and diamond. The mixed portion adheres to the tool substrate more strongly than the simple diamond. Further, the vapor-deposited diamond contained in the mixed portion is integrally formed with the diamond defining the head face. In addition, the metal or ceramics contained in the mixed portion is closer in thermal expansion coefficient to the tool substrate than the diamond, whereby the portion containing the metal or ceramics and adhering to the tool substrate has smaller internal stress, which is caused by difference in thermal expansion coefficient in employment, as compared with a conventional diamond-coated tool. The coating structure containing the metal or ceramics is adapted to prevent separation of the coating. Further, the portion containing the metal or ceramics is also adapted to relieve the difference in thermal expansion coefficient between the diamond forming the head face and the tool substrate. Due to this function, the coating is prevented from separation or cracking caused by thermal shock which is repeated in employment. Further, the portion containing the metal or ceramics improves toughness of the coating. This bonding tool has a coating which has superior wear resistance and chipping resistance, and more strongly adheres to the tool substrate. This bonding tool is superior in durability to the conventional diamond-coated tool. Examples are now described. Example 1, not forming part of the claimed invention A base material consisting essentially of an SiC sintered body having an edge of 20 mm and a thickness of 5 mm was prepared. Diamond was deposited on the SiC base material in a thickness of about 100 µm by well-known thermal filament CVD using a tungsten wire of 0.5 mm in diameter and 100 mm in length as a filament under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 1 %, total flow rate of 1000 cc/min. Gas Pressure: 80 Torr (1 Torr = 133.3 Pa) Filament Temperature: 2200°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 920°C As the result of X-ray analysis of the as-formed diamond, a peak intensity ratio I(111)/I(220) of a (111) plane to a (220) plane was about 20, and it was recognized that strong orientation was attained as to the (111) plane. This (111) plane exhibited an FWHM value of about 6° in a rocking curve. On the other hand, the (220) plane exhibited an FWHM value of about 14° in a rocking curve. Then, thermal filament CVD was performed under the following conditions, to further deposit a diamond layer on the as-formed polycrystalline diamond in a thickness of about 40 µm: Raw Material Gas (Flow Rate): CH 4 /H 2 = 8 %, total flow rate of 1000 cc/min. Gas Pressure: 80 Torr Filament Temperature: 2400°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 920°C As the result of thin film X-ray analysis, the newly formed diamond layer exhibited a ratio I(111)/I(220) of about 0.4, and it was clarified that the (220) plane was oriented substantially in parallel with the base material surface. Then, Ti and Ni were directly vapor-deposited on the SiC base material surface, provided with the two diamond layers which were vapor-deposited under different conditions, in thicknesses of about 2 µm respectively to form a metallization layer. Then the SiC substrate was vacuum-brazed to a shank of stainless steel with silver solder through this metallization layer. Fig. 4A shows a principal part of such a bonding tool 40 in a state immediately after the brazing. In this bonding tool 40, an SiC base material 44 is provided on a shank 41 of stainless steel through a brazing layer 42 and a metallization layer 43. Further, diamond 45 of about 100 µm in thickness orienting a (111) plane and a diamond layer 46 of about 40 µm in thickness orienting a (220) plane are successively deposited on the base material 44. Then, the diamond layer 46 orienting a (220) plane shown in Fig. 4A was removed by lapping. Due to such mirror working, it was possible to expose the (111) plane of the previously formed diamond as well as to obtain a mirror surface having surface roughness R max of about 0.1 µm. As the result of X-ray analysis performed on the as-obtained head face, a ratio I(111)/I(220) was about 15, and a principal diamond crystal plane forming the head face was the (111) plane. Fig. 4B shows a principal part of the bonding tool 40 obtained in the aforementioned manner. Tn this bonding tool 40, the diamond 45 orienting a (111) plane is provided on the SiC base material 44 which is brazed to the shank 41. A head face 47 of this tool is flattened, and a most part thereof is formed of the (111) plane. Fig. 5 shows the overall shape of the bonding tool 40, in which the vapor-deposited diamond 45 is provided on the shank 41 through the SiC base material 44. As hereinabove described, a most part of the head face 47 for bonding is formed of the (111) plane. The bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, whereby this tool withstood employment of 2,000,000 times. Example 2, not forming part of the claimed invention An Si base material which was mirror-finished to have surface roughness R max of not more than about 0.2 µm was employed to form polycrystalline diamond on the Si mirror surface in a thickness of about 40 µm by thermal filament CVD using a tungsten filament similar to that in Example 1 under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 1 %, O 2 /CH 4 = 20 %, total flow rate of 1000 cc/min. Gas Pressure: 60 Torr Filament Temperature: 2200°C Filament-to-Substrate Distance: 7 mm Substrate Temperature: 880°C Then, thermal filament CVD was performed under the following conditions to further deposit diamond on the as-formed polycrystalline diamond in a thickness of about 310 µm: Raw Material Gas (Flow Rate): CH 4 /H 2 = 12 %, CO 2 /CH 4 = 10 %, total flow rate of 1000 cc/min. Gas Pressure: 120 Torr Filament Temperature: 2400°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 920°C As the result of X-ray analysis, the precedently prepared diamond exhibited a ratio I(111)/I(220) of about 40, and it was recognized that the (111) plane was strongly oriented in parallel with the base material surface in the first formation of the diamond. On the other hand, the subsequently formed diamond exhibited a ratio I(111)/I(220) of about 0.01, and it was clarified that the (220) was strongly oriented in parallel with the base material surface. Then, the Si base material, provided with the two diamond layers which were vapor-deposited under different conditions, was dissolved/removed by fluoronitric acid, to obtain a diamond material. In the as-obtained diamond material, Ti and Ni were successively stacked on a crystal growth end surface in thicknesses of about 1 µm and about 2 µm respectively to form a metallization layer. Then the diamond material was vacuum-brazed to a shank of stainless steel with silver solder so that the surface provided with the metallization layer defined a joint surface. A head face obtained by such brazing had surface roughness R max of about 0.1 µm. Fig. 6 shows a principal part of the as-obtained bonding tool 60. In this bonding tool 60, a diamond material 65 is provided on a shank 61 of stainless steel through a brazing layer 62 and a metallization layer 63. In the diamond material 65, a portion D 1 of up to about 40 µm in depth from a head face 67 is occupied by diamond orienting a (111) plane, while a portion D 2 of about 40 µm up to about 350 µm in depth is occupied by diamond orienting a (220) plane. When the bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, this bonding tool withstood employment of 3,000,000 times. Comparative Example 1 The same SiC base material as that in Example 1 was employed to be provided with polycrystalline diamond of about 100 µm in thickness by the aforementioned thermal filament CVD under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 2.5 %, total flow rate of 1000 cc/min. Gas Pressure: 80 Torr Filament Temperature: 2100°C Filament-to-Substrate Distance: 6 mm Substrate Temperature: 900°C Then, thermal filament CVD was performed under the following conditions to further deposit diamond on the as-formed polycrystalline diamond in a thickness of about 40 µm: Raw Material Gas (Flow Rate): CH 4 /H 2 = 3.5 % Gas Pressure: 80 Torr Filament Temperature: 2100°C Filament-to-Substrate Distance: 6 mm Substrate Temperature: 920°C As the result of X-ray analysis, the precedently formed diamond exhibited a ratio I(111)/I(220) of about 0.10. On the other hand, the subsequently formed diamond exhibited a ratio I(111)/I(220) of about 0.05. Thus, the (220) plane was strongly oriented in each diamond. Ti and Ni were successively stacked on the surface of the SiC substrate, provided with the two diamond layers which were vapor-deposited under different conditions, in thicknesses of about 1 µm and about 2 µm respectively to form a metallization layer. Then the base material was vacuum-brazed to a shank of stainless steel with silver solder while employing the surface provided with the metallization layer as a joint surface. Thereafter the diamond surface was further polished to expose the surface of the precedently formed diamond, thereby preparing a bonding tool provided with a head face having surface roughness R max of about 0.1 µm. The bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, whereby cracking was caused in the diamond forming the bonding face and the SiC base material upon employment of 50,000 times. Example 3, not forming part of the claimed invention A base material consisting essentially of an SiC sintered body of 25 mm□ having a thickness of 5 mm was prepared. Diamond was deposited on the SiC base material in a thickness of about 200 µm by well-known thermal filament CVD using a linear tungsten filament of 0.5 mm in diameter and 100 mm in length as a thermionic emission material under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 3 %, total flow rate of 2000 cc/min. Gas Pressure: 80 Torr Filament Temperature: 2250°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 930°C The as-obtained vapor-deposited diamond exhibited a ratio Ii/Id of about 0.5 in Raman spectroscopic analysis. Further, a peak of this diamond exhibited an FWHM value of about 14 cm -1 . Then, another diamond was further continuously deposited on this diamond in a thickness of about 50 µm under conditions identical to the above except the raw material condition, which was changed from CH 4 /H 2 = 3 % to CH 4 /H 2 = 0.5 %. The newly deposited diamond exhibited a ratio Ii/Id of about 0.05 in Raman spectroscopic analysis. Further, a peak of this diamond exhibited an FWHM value of about 5.5 cm -1 . Then, Ti and Ni were vapor-deposited on an SiC side of the SiC base material, provided with the two diamond layers which were vapor-deposited under different conditions, in thicknesses of about 2 µm respectively to form a metallization layer. Thereafter the SiC base material was vacuum-brazed to a shank of stainless steel with silver solder through the metallization layer. Thereafter the diamond surface was further polished, to prepare a bonding tool having surface roughness R max of not more than 0.1 µm. Figs. 7A and 7B show the as-obtained bonding tool 70. In this bonding tool 70, an SiC base material 74 is provided on a shank 71 of stainless steel through a brazing layer 72 and a metallization layer 73, as shown in Fig. 7A. Further, a coarse diamond layer 75a of about 200 µm in thickness and a high-purity diamond layer 75b of about 50 µm in thickness are successively deposited on the substrate 74. Referring to Fig. 7B showing the overall shape of the bonding tool 70, vapor-deposited diamond 75 is provided on the shank 71 through the SiC base material 74. As hereinabove described, a head face 77 for bonding is formed of high-purity diamond. The bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, whereby this bonding tool withstood employment of 3,000,000 times. Comparative Example 2 The same SiC base material as that in Example 3 was employed to deposit diamond having a substantially homogeneous composition thereon in a thickness of about 300 µm under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 3 %, total flow rate of 1500 cc/min. Gas Pressure: 60 Torr Filament Temperature: 2100°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 930°C The as-obtained vapor-deposited diamond exhibited a rate Ii/Id of about 1.2 in Raman spectroscopic analysis, and a peak of this diamond exhibited an FWHM value of about 15.2 cm -1 . Then, this base material was metallized similarly to Example 3, and thereafter brazed to a shank of stainless steel to prepare a bonding tool. The as-obtained tool was mounted on a bonding apparatus and subjected to an endurance test, whereby cracking was caused in the diamond forming a bonding face and the SiC base material upon employment of 50,000 times. Example 4, not forming part of the claimed invention An SiC base material of 25 mm□ having a thickness of 5 mm, which was mirror-finished to have surface roughness R max of not more than about 0.2 µm, was prepared. Polycrystalline diamond was deposited on the Si mirror surface in a thickness of about 50 µm by well-known thermal filament CVD using a tungsten filament of 0.5 mm in diameter and 100 mm in length under the following conditions: Raw Material Gas: CH 4 /H 2 = 0.8 %, total flow rate of 2000 cc/min. Gas Pressure: 80 Torr Filament Temperature: 2200°C Filament-to-Substrate Temperature: 5 mm Substrate Temperature: 920°C The diamond formed under the above conditions exhibited a ratio Ii/Id of about 0.1 in Raman spectroscopic analysis, and its peak exhibited an FWHM value of about 5.4 cm -1 . Then, vapor deposition of diamond was continuously performed under the same conditions as the above except the raw material gas condition, which was changed from CH 4 /H 2 = 0.8 % to CH 4 /H 2 = 3.4 %, to deposit diamond in a thickness of about 200 µm. The newly deposited diamond exhibited a ratio Ii/Id of about 0.34 and its peak exhibited an FWHM value of about 12.8 cm -1 . Thereafter the Si base material was dissolved/removed by fluoronitric acid, to obtain a diamond material. Ti and Ni were vapor-deposited on a vapor growth end surface of the as-obtained diamond material in thicknesses of about 2 µm respectively to form a metallization layer. Thereafter the diamond material was vacuum-brazed to a shank of stainless steel with silver solder through the metallization layer. A surface of the diamond material having been in contact with the Si base material was adapted to define a head face of the as-obtained bonding tool. Fig. 8 shows a bonding tool 80 obtained in the aforementioned manner. In this bonding tool 80, a diamond material 85 is provided on a shank 81 of stainless steel 81 through a brazing layer 82 and a metallization layer 83, as shown in Fig. 8. In the diamond material 85, a high-purity diamond layer 85b forming a head face 87 is supported by a coarse diamond layer 85a containing a large amount of a non-diamond component. The bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, whereby this diamond tool withstood employment of 3,500,000 times. Comparative Example 3 The same Si base material as that in Example 4 was employed to deposit diamond having a substantially homogeneous composition in a thickness of about 400 µm under the following conditions: Raw Material Gas: CH 4 /H 2 = 2 %, total flow rate of 3000 cc/min. Gas Pressure: 50 Torr Filament Temperature: 2200°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 930°C The as-obtained vapor-deposited diamond exhibited a ratio Ii/Id of about 1.5 in Raman spectroscopic analysis, and its peak exhibited an FWHM value of about 18.2 cm -1 . Then, the base material was removed by fluoronitric acid similarly to Example 4, and the as-obtained diamond material was subjected to metallization and brazed to a shank of stainless steel to prepare a bonding tool. The as-obtained bonding tool was mounted on a bonding apparatus and subjected to an endurance test, whereby cracking was caused in the diamond forming a bonding face upon employment of 60,000 times. Example 5 A base material consisting essentially of an SiC sintered body having a length of 25 mm, a width of 1.3 mm and a thickness of 2 mm was prepared. Diamond was deposited on the SiC base material in a thickness of about 200 µm with doping by well-known thermal filament CVD using a tungsten wire of 0.5 mm in diameter and 100 mm in length as a filament under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 1.5 %, total flow rate of 1000 sccm Gas Pressure: 80 Torr Impurity Gas: B 2 H 6 Impurity Concentration Range: B/C = 0 to 700 p.p.m. Filament Temperature: 2200°C Filament-to-Substrate Distance: 5 mm Substrate Temperature: 920°C Under the above conditions, the impurity concentration (concentration of boron contained in the raw material gas with respect to carbon) was exponentially reduced from 700 p.p.m. to 0 p.p.m. from start to end of vapor deposition, as shown in Fig. 9. Then, Ti and Ni were vapor-deposited on the surface of the SiC base material, provided with the diamond which was vapor-deposited with addition of the impurity, in thicknesses of about 2 µm respectively, to form a metallization layer. Thereafter the SiC base material was vacuum-brazed to a shank of Mo with silver solder through the metallization layer. Thereafter the surface of the vapor-deposited diamond was lapped into a mirror surface having surface roughness R max of about 0.1 µm. Fig. 10 shows a principal part of the as-obtained bonding tool 100. In this bonding tool 100, an SiC base material 104 is provided on a shank 101 of Mo through a brazing layer 102 and a metallization layer 103. Diamond 105 is deposited on the base material 104 in a thickness of about 200 µm. A head face 107 consisting essentially of diamond is finished in a mirror surface having surface roughness R max of about 0.1 µm. On the other hand, concentration (B/C) of boron with respect to carbon is continuously changed in the diamond 105 as shown in Fig. 11. The concentration B/C is 0 p.p.m. in the head face, and 100 p.p.m. in a joint portion with the SiC base material, as shown in Fig. 11. The concentration B/C is exponentially increased from 0 to 100 p.p.m. in the diamond 105. Example 6 An Si base material which was mirror-finished to have surface roughness R max of not more than about 0.2 µm was employed to form polycrystalline diamond on this mirror surface in a thickness of about 230 µm by thermal filament CVD using a tungsten filament similar to that in Example 5 with doping, under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 1 %, total flow rate of 3000 cc/min. Gas Pressure: 80 Torr Impurity Gas: B 2 H 6 Impurity Concentration Range: 0 to 1000 p.p.m. Filament Temperature: 2180°C Filament-to-Substrate Distance: 7 mm Substrate Temperature: 860°C Under the above conditions, concentration of the impurity added to the raw material gas was exponentially increased from start to end of vapor deposition as shown in Fig. 12. Then, the Si substrate, provided with the diamond which was vapor-deposited thereon, was dissolved/removed by fluoronitric acid, to obtain a diamond material. Ti and Ni were stacked on a crystal growth end surface of the as-obtained diamond material in thicknesses of about 1 µm and about 2 µm respectively, to form a metallization layer. Thereafter the diamond material was vacuum-brazed to a shank of an invar alloy with silver solder so that a surface provided with the metallization layer defined a joint surface. A head face obtained after the brazing had surface roughness R max of about 0.1 µm. Fig. 13 shows a principal part of the as-obtained bonding tool 130. In this bonding tool 130, a diamond material 135 is provided on a shank 131 of an invar alloy through a brazing layer 132 and a metallization layer 133. Fig. 14 shows distribution of impurity concentration (B/C) in a portion of the diamond material 135 from a head face 137 to a joint surface 138. Comparative Example 4 The same SiC base material as that in Example 5 was employed to form polycrystalline diamond thereon in a thickness of about 200 µm with doping by the aforementioned thermal filament CVD under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 2 %, total flow rate of 1000 cc/min. Gas Pressure: 60 Torr Impurity Gas: B 2 H 6 Impurity Concentration: 8000 p.p.m. Filament Temperature: 2400°C Filament-to-Substrate Distance: 10 mm Substrate Temperature: 930°C Under the aforementioned conditions, the impurity was added in a constant amount from start to end of vapor deposition. Ti and Ni were successively stacked on the surface of the SiC base material, provided with the diamond which was vapor-deposited thereon in the above manner, in thicknesses of about 1 µm and about 2 µm respectively, to form a metallization layer. Thereafter the base material was vacuum-brazed to a shank of Mo with silver solder so that a surface provided with the metallization layer defined a joint surface. Thereafter the diamond surface was polished to prepare a bonding tool provided with a head face having surface roughness R max of about 0.1 µm. Fig. 15 shows a bonding tool 150 obtained in the aforementioned manner. In this bonding tool 150, an SiC base material 154 is provided on a shank 151 through a brazing layer 152 and a metallization layer 153. Diamond 155 containing boron in homogeneous concentration over the entire portion is provided on the base material 154 in a thickness of about 200 µm. Comparative Example 5 A bonding tool consisting essentially of Mo was prepared to have the same dimensions and shape as those of the aforementioned bonding tool. In Examples 5 and 6 and Comparative Example 4, energization was made on the diamond portions for measuring times required for heating the head faces to 400°C. In Comparative Example 5, energization was made on the Mo shank for measuring a time required for heating the head face to 400°C. As to the tools according to these Examples and Comparative Examples, times required for reducing the temperatures at the head faces from 420°C to 200°C by air cooling. Further, observation was made on temperature dispersion on the head faces which were heated by energization for prescribed times to reach 420°C. Table 1 shows the results. The temperatures at the head faces were measured with an infrared thermometer. Time required for Heating up to 420°C (sec.) Time required for Cooling from 420°C to 200°C (sec.) Temperature Dispersion (°C) Example 5 0.4 1.5 6 Example 6 0.2 0.9 3 Comparative Example 4 1.2 2.0 10 Comparative Example 5 1.8 3.5 60 The bonding tools according to these Examples and Comparative Examples were mounted on a bonding apparatus and subjected to endurance tests, to exhibit results shown in Table 2. In the endurance tests, the bonding tools were adapted to repeatedly bond ICs each having 1000 pins with head face temperatures of 420°C and bonding times of 0.2 seconds respectively. Bonding State of Wear Example 5 3,000,000 times Rmax of head face: 0.18 µm Example 6 4,000,000 times Rmax of head face: 0.21 µm Comparative Example 4 400,000 times Rendered unusable with Rmax of head face reaching 30µm Comparative Example 5 10,000 times Rendered unusable due to partial wear and warp of 20 µm/10µm It is understood from the results shown in Tables 1 and 2 that the inventive bonding tools are superior in thermal responsibility to the comparative bonding tools, and excellent in durability. On the other hand, the bonding tool according to Comparative Example 4 obtained by homogeneously adding boron to diamond is inferior in wear resistance and durability, although the same has excellent thermal responsibility. Example 7 Fig. 16 shows an apparatus for forming a coating on a tool substrate, in order to manufacture a bonding tool according to the present invention. A method of manufacturing a bonding tool according to the present invention is now described with reference to Fig. 16. The apparatus shown in Fig. 16, which is adapted to carry out high-temperature plasma CVD, has a vacuum chamber 161, being exhaustible through an exhaust port 165, provided with a cathode 162 and an anode 163 for discharge. A power source 164 applies a dc voltage across the cathode 162 and the anode 163. A raw material gas for synthesizing diamond is supplied through the cathode 162 and the anode 163 as shown by arrows in Fig. 16, and decomposed/excited by discharge at a supply port 166 therefor, to form plasma. The pressure in the vacuum chamber 161 is reduced in a series of operation from formation of the plasma to formation of diamond. The as-formed plasma hits a surface of a shank 168 which is provided along the direction of its movement and set in a holder 167, to grow diamond. When fine powder of a metal or ceramics is supplied onto the shank 168 through a pipe 169 simultaneously with such diamond growth, a layer containing a mixture of the metal or ceramics and the diamond is formed on the shank surface. At this time, the powder can be supplied to the base material surface in any one of melted, semi-melted and unmelted states through adjustment of the position for supplying the powder. When the layer reaches a proper thickness, supply of the metal or ceramics is stopped while the diamond is further continuously vapor-deposited, thereby depositing a diamond layer in a proper thickness. Thus formed is a coating having a head face which is formed at least by diamond and a portion adhering to the shank which contains a mixture the metal or ceramics and the diamond. The aforementioned apparatus was employed to form a diamond composite layer on a shank consisting essentially of Mo in a thickness of about 100 µm by well-known high-temperature plasma CVD under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 2 %, Ar/H 2 = 100 %, total flow rate of 3000 cc Pressure: 150 Torr Substrate Temperature: 900°C Discharge Voltage: 60 V Discharge Current: 60 A Supplied Powder: SiC and Mo of 20 to 30 µm in particle diameter Amount of Powder Supply: 1 g/min., 30 g in total At this time, the powder was supplied with Mo and SiC in melted and unmelted states respectively. Then, only supply of the powder was stopped and diamond was deposited in a thickness of about 200 µm under the above conditions. Thereafter the shank coated with the diamond was taken out from the apparatus and the diamond surface forming a head was lapped to form a head face having surface roughness R max of about 0.1. Fig. 17A shows a principal part of a bonding tool 170 formed in the aforementioned manner. In this bonding tool 170, a coating 174 including a diamond composite layer 172 containing SiC and Mo and a diamond layer 173 successively deposited thereon is formed on a shank 171. Fig. 17B shows the appearance of the overall shape of the bonding tool 170, in which the coating 174 is provided on a head of the integral shank 171 consisting essentially of Mo. The bonding tool having the aforementioned structure was mounted on a bonding apparatus and subjected to an endurance test, whereby this bonding tool withstood employment of 1,200,000 times. Comparative Example 6 The same apparatus as the above was employed to form a diamond coating on a shank of the same material with no supply of powder under the following conditions: Raw Material Gas (Flow Rate): CH 4 /H 2 = 3 %, Ar/H 2 = 100 %, total flow rate of 3000 cc Pressure: 200 Torr Substrate Temperature: 880°C Discharge Voltage: 70 V Discharge Current: 62 A An attempt was made to form a diamond coating of about 150 µm in thickness on the shank under the aforementioned conditions, while the as-formed diamond film was separated from the shank consisting essentially of Mo after completion of the coating. According to the present invention, the material for the tool substrate can be prepared from W, another Fe-Ni alloy, an Fe-Ni-Co alloy such as super invar, for example, an Fe-Co-Cr alloy such as stainless invar, for example, an Fe-Pt alloy, an Fe-Pd alloy, covar (29 % of Ni, 17 % of Co and a residue of Fe), a Cu-W alloy, a WC-Co alloy, a WC-TiC-Co alloy, a W-Ni alloy or the like, for example, in addition to the aforementioned materials employed in Examples. Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.

Claims: A bonding tool (100) comprising: a bonding head portion (105) having electrical conductivity and consisting essentially of vapor-deposited polycrystalline diamond and a dopant; and a substrate (101) for holding said bonding head portion (105), said bonding head portion (105) including a third portion forming a head face (107) for bonding and a fourth portion being in contact with said third portion and containing a larger amount of said dopant than said third portion. A bonding tool in accordance with claim 1, wherein said head portion (105) is coupled to said substrate (101) through a base material (104) consisting essentially of a material selected from the group consisting of a sintered body mainly composed of Si3N4, a sintered body mainly composed of SiC, a sintered body mainly composed of AlN, and Si. A bonding tool in accordance with claim 1, wherein said head portion (135) is stuck to said substrate (131). A bonding tool in accordance with claim 1, wherein said head face has surface roughness R max of not more than about 0.1 µm. A bonding tool in accordance with claim 1, wherein said dopant is selected from the group consisting of B, Al, P, Sb, Si, Li, S, Se, Cl, N, W, Ta, Re and Cr. A bonding tool in accordance with claim 5, wherein boron is doped in a concentration range of about 0.1 to 5000 p.p.m. A method of manufacturing a bonding tool (100), comprising: a step of preparing a base material (104) having a surface for deposition of diamond; a step of depositing diamond on said surface by vapor depositing with doping, whereby an electrically active impurity is added; a step of then further depositing said diamond by vapor deposition while suppressing supply of an impurity in said doping to form a head face portion; a step of brazing said base material (104) to a tool substrate (101) so that said deposited diamond defines a head portion (105); and a step of preparing a flattened head face (107) by polishing. A method of manufacturing a bonding tool (130), comprising: a step of preparing a base material having a mirror-finished surface for deposition of diamond; a step of depositing diamond on said surface by vapor deposition with doping, whereby an electrically active impurity is added; a step of separating or removing said base material provided with said diamond being deposited thereon to obtain a diamond material; and a step of brazing said diamond material (135) to a tool substrate (131) for providing a surface of said diamond material (135) having been in contact with said base material on a head face (137) side, an amount of an impurity supplied in a prescribed period upon starting of said vapor deposition being reduced as compared with that supplied later in said doping.

Inventors: FUJIMORI NAOJI, IKEGAYA AKIHIKO, TAKAHASHI TOSHIYA, TANABE KEIICHIRO, FUJIMORI, NAOJI, IKEGAYA, AKIHIKO, TAKAHASHI, TOSHIYA, TANABE, KEIICHIRO, FUJIMORI, NAOJI, C/O ITAMI WORKS OF SUMITOMO, IKEGAYA, AKIHIKO, C/O ITAMI WORKS OF SUMITOMO, Takahashi, Toshiya, c/o Itami Works of Sumitomo, TANABE, KEIICHIRO, C/O ITAMI WORKS OF SUMITOMO
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LIMITED
Application year: 1993
Main classification: H01L21
IPC codes: C04B41, H05K13, B23K20