Method to produce a hard material coated wafer

Abstract: A complex wafer (2) having a soft, fragile substrate and a hard-material film coating on the substrate is difficult to polish, because of the hardness of the film, the fragility of the substrate and the inherent distortion. The complex wafer (2) is stuck in a convex-distorted shape to a holder (3). Preferably, a buffer (11) is inserted between the wafer (2) and the bottom of the holder (3). Further, insertion of a convex spacer is effective to reform a inherently flat wafer (2) or a inherently concave-distorted wafer (2) into a convex-distortion on the holder (3). The holder (3) can incline a little to its own shaft (4). The convex-distorted wafer (2) is in contact with a rotary whetstone turn-table (1). Auxiliary shafts (10) push the periphery of the holder (3) for inclining the holder (3). Slight slanting of the holder (3) changes the contact point on the wafer (2). The slanting angle can be adjusted by the allotment of the force between the main shaft (4) and the auxiliary shafts (10). The roughness of the polished surface is less than Rmax50nm and less than Ra20nm.

Description: This invention relates to a hard-material coated wafer, and a method of producing the same. The hard-material wafer will be utilized for SAWs (surface acoustic wave devices), thermistors, substrates for semiconductor devices, protecting films of discs, and X-ray windows. Here "hard materials" generally indicate diamond, c-BN, or diamond-like carbon. All the materials cited here have a Vickers hardness of more than Hv3000 in the state of bulk materials. The hard materials are endowed with high sound velocity which is determined by the ratio of Young modulus divided by density. Therefore, the velocity of the surface acoustic wave is extremely high. In particular, such hard material-coated wafers now attract attention for the material of the substrate of SAW devices. SAWs have applications for filters, phase shifters or convolvers. Diamond and c-BN which are intrinsically insulators can be converted to semiconductor by doping them with some impurity. These materials are favored with excellent physical and chemical properties. However, these materials have not been utilized in various fields for practical uses, since wide and inexpensive plates or wafers of the materials can not be fabricated so far. Since the hard materials are provided with a lot of physical and chemical advantages, actual applications of the hard materials to various objects are earnestly desired by transplanting the technology of silicon semiconductor devices to the hard materials. What should be prepared in the first place is wide plates (or wafers) of the hard materials. Technologies have already ripened into the definite level capable of producing films of diamond, c-BN (cubic boron nitride) or diamond-like carbon by vapor phase deposition methods. The vapor phase deposition method makes a film of the material by supplying a material gas to a pertinent substrate heated at a suitable temperature, letting the gas react with the hot substrate and depositing a film of the hard material on the substrate in vapor phase. A film of diamond or c-BN is produced by introducing a material gas including hydrogen gas and hydrocarbon gas, or another material gas including hydrogen gas, boron-containing gas and nitrogen-containing gas in the reaction chamber, supplying the material gas on the heated substrate, synthesizing diamond or c-BN by chemical reaction and piling the synthesized material as a film on the substrate. There are some methods for exciting the material gas, that is, a hot filament CVD method, a microwave plasma CVD method, a radio wave plasma CVD method or a DC plasma jet CVD method. Some method is capable of making a wide film of hard materials on a substrate. However, the speed of the synthesis is so slow that the method cannot easily make a thick film at present. A long time of deposition relying on some method can make a considerably thick film on the substrate. Nevertheless, there are still no pure wafer consisting only of a hard material free from a substrate. At present, there is not a diamond wafer or a c-BN wafer in a true meaning, because prior technology has not been able to produce a pure diamond wafer or a pure c-BN wafer. The application of the hard materials, i.e. diamond, c-BN or diamond like carbon to the electronics technology requires wide area wafers of the hard materials. Someone once produced surface acoustic wave devices on very small diamond substrates. However the fact that a new device was fabricated on a quite small diamond substrate, e.g. from 5 mm square to 10 mm square was rather insignificant from the standpoint of industry, even if the device itself exhibited an excellent performance. Since a small substrate allowed to make only small number of devices on it, the productivity was poor. The devices made on the small substrate had little practical significance owing to the poor productivity. What brought about the success to silicon semiconductor devices is the probability of treating with a wide area Si wafer by the same wafer processes at the same time and making a plenty of equivalent devices in short time. The same matter will perhaps hold for the hard materials. If diamond, c-BN or diamond-like carbon wants to obtain a practical importance as the substrates, the material should be formed into wide, round thin plates (wafers). The advent of the wide wafers will enable manufactures to apply the technology which has been grown by the silicon semiconductor to the hard materials. In the case of silicon, big single crystals with a wide section can easily be grown by Czochralski methods, and mainly 8-inch wafers are produced for making devices at present. 12-inch wafers can also be produced now for silicon. However, a diamond or c-BN single crystal cannot be grown by the conventional methods, e.g., Czochralski methods at present. Thus, it is still not promising to produce homogeneous wafers consisting only of a single material of the hard materials unlike silicon (Si) or gallium arsenide (GaAs). It is totally impossible to make wide, homogeneous diamond wafers or c-BN wafers even at present. This invention gives up the attempt of making a homogeneous wide wafer consisting only of a single material of diamond, c-BN or diamond like carbon. Instead of starting from the bulk single crystal, this invention starts from a substrate of other materials for making complex wafers consisting of a non-hard material substrate and hard material films formed on the substrate. The present invention intends to make a hard material film on a commonplace material, e.g., Si, GaAs or so which can easily be produced or obtained. The complex of a substrate plus a hard material film gives manufactures the possibility of making a wide hard material wafer by employing a wide base wafer as a substrate. The base wafer of non-hard material plays the role of the base mount on which the result of the reaction is piled. The film on the base wafer is the principal part which will contribute to the production of semiconductor devices or SAWs. If a homogeneous wafer consisting only of pure diamond or c-BN may be made by forming a very thick film on the substrate and eliminating the substrate by etching. However, it is still unpractical, because it takes very long time and very much material to deposit so a thick layer. A big inner stress would break the film, when the substrate is etched away. Therefore, the production of a freestanding film remains an unpractical object. This invention submits itself to the complex, inhomogeneous wafer having the base wafer as a non-hard material wafer. The remainder base wafer does not cause a problem, since almost all the devices make use only of the nature of the surface of the wafer. This invention employs a inhomogeneous, complex wafer having a Si or GaAs surface on the bottom and a diamond or c-BN surface on the top. The adoption of the complex wafer can overcome the difficulty of making a wide bulk single crystal of the hard materials, because complex wafers can be made by the thin film-formation technology. The term "hard material wafer" in the present invention is quite different from the ordinary wafers produced by slicing an ingot of bulk single crystal. The wafers proposed by the present invention are different from the silicon wafers or the gallium arsenide wafers in the production methods. This invention has another requirement of the hard material wafers. As mentioned before, someone tried to make surface acoustic wave devices on a diamond substrate of 3 mm square or 5 mm square. Such a trial may have a certain significance from the standpoint of technology. However, such a small substrate was useless for making devices on the industrial scale. The industrial production requires circularity, constant thickness, flatness, unbentness, and large diameter for substrate wafers. With regard to the area of a wafer, the application to electronics materials demands wide circular or rectangular wafers of at least an inch diameter (25 mm). Wafers of a diameter less than an inch cannot be treated by the wafer process which has been matured by the silicon semiconductor industries. Furthermore, a two-inch diameter wafer is better than a one-inch diameter wafer. The three-inch diameter wafer is more preferable to the two-inch diameter wafer from the standpoint of the industrial production, which desires large, flat and smooth wafers. Fortunately, the progress of the technology of the vapor phase synthesis enables to make a considerably broad film of diamond or c-BN on a suitable substrate. However, the blunt possibility of forming wide thin films does not give the probability of making wide wafers suitable for the wafer process. The wafers having rugged surfaces are useless. Namely, a smoothness of surface is one of the requisites for wafers. Besides, flatness or unbentness is another matter of significance. Namely the hard material wafer must be a mirror wafer without bending or bow. Here flatness means a long range regularity of a surface. Bending or bow indicates a long range irregularity of a surface. Smoothness is defined as a short range regularity and ruggedness or raggedness signifies a short range irregularity with a lot of convexes or concaves on a surface. The wafer must be mirror-smooth and flat in order to make devices on the wafer by the photolithography. If the wafer is not flat or not mirror-smooth, exact patterns cannot be drawn on the wafer by an optical means. The films which have made by the vapor phase deposition have a lot of micro convexes or concaves on the surface. Wavy morphology is sometimes formed on the surface of the film in a case. In other cases, granular convexes distribute on the surface of the film. In general, the films which have been made by the vapor phase deposition cannot be used as a substrate wafer due to the ruggedness of the surface. If a wafer suffers from a rugged surface, the wafer could be converted into a mirror wafer by eliminating the ruggedness out of the surface with a polishing apparatus. The polishing would remove the convexes, concaves or wavy morphology out of the surface and would make a flat, smooth wafer. Actually in the case of silicon wafers, mirror wafers are made by slicing an Si ingot into a lot of as-cut wafers, etching the as-cut wafers and polishing the etched wafers by a polishing apparatus. Can the same treatment make flat, smooth hard-material coated wafers? No, the same treatment does not make smooth wafers. The polishing of the hard materials has some problems which would have not be anticipated yet. Since the convexes or concaves would be removed from the surface of the hard material-coated wafer by polishing, the starting film must have a sufficient thickness Including a margin of polishing. It takes a long time and much material to produce a thick film of the hard materials. The difficulty is economic one which will be overcome by allotting satisfactory material and time for growing the film. Another big problem is the difficulty of polishing of the hard materials. The hard materials make too hard a film to polish. Diamond and c-BN are the hardest materials. The difficulty of polishing them is far more serious than the polishing of silicon wafers. Diamond or c-BN is polished by diamond powder as an abrasive. The polishing is sometimes called the "together-polishing", because diamond powder shaves diamond by reducing itself in the polishing. A long time together-polishing can polish surfaces of diamond crystals. The problem of hardness can be solved by the together-polishing. However, there is still a more difficult problem. The thermal expansion coefficients are different between the hard material-coated film and the non-hard material substrate. The complex wafer with the film and the substrate will have a large inner stress due to the difference of the thermal expansion coefficients, when the complex wafer is produced in a reaction chamber, and is cooled. The complex wafer will bend due to the release of the inner stress, when it is gotten rid out of the reaction chamber. In one case, the complex wafer bends convexly to the side of the film. In another case, the complex wafer bends concavely to the side of the substrate. In another case, the wafer stays flat as a whole. The directions of bending are now defined with regard to the side of the film. The wafer bending is classified by the directions of bending. The bending which is convex toward the film is called "convex-distortion". The bending which Is concave toward the film is called "concave-distortion". The problem of the distortion is still latent in the case of small plates. No distortion occurs for small complex plates of, e.g., 3 mm square or 5 mm square. The present invention aims at making complex wafers of 1 inch diameter, 2 inch diameter or still bigger diameter. A large bending appears in the complex wafer owing to the difference of the thermal expansion coefficients or the inner stress of the complex film itself. The broadness of the wafer induces a large distortion. The problem of the distortion is quite serious for the hard material coating wafers. In the case of silicon, large, flat mirror- wafers can easily be produced since Si wafers are homogeneous without the complexity of multilayer. On the contrary, the distortion causes a significant problem owing to the inhomogeneity and complexity in the case of hard material-coated wafers. The distortion of wafers causes many difficulties. The distortion prevents the photolithograpy from transcribing mask patterns exactly on the resist coating the wafer. This is a big drawback itself. To begin with, the distortion of a wafer is a large hindrance for polishing the wafer. A polishing machine cannot polish a bending wafer at all. Polishing converts a rugged wafer into a mirror wafer. If polishing is impossible, no mirror wafer can be produced. If a wafer is not mirror-polished, the photolithography is entirely impotent. Thus, it is impossible to make devices on a rugged wafer by the wafer process. Polishing is a fundamental requirement for wafers. Diamond is the hardest material among the materials we can obtain. There is no harder material than diamond. Thus, diamond is now polished by diamond. Namely, diamond plates are polished by an apparatus using diamond powder. There are two methods for polishing diamond plates. One method adopts free polishing powder. The other adopts fixed polishing powder. Skife polishing which has been known as a way of the former polishing method uses an iron polishing plate and diamond free polishing powder. The method polishes a diamond plate by the steps of gluing the diamond plate to a holder, pushing the holder on a rotary polishing turn-table (round whetstone), supplying polishing liquid containing diamond powder, revolving the turn-table, rotating the holder and whetting the object diamond plate by the physical action of diamond granules. The method has a drawback of the big waste of diamond powder since it depends on free diamond powder in polishing. The consumption of diamond powder raises the cost of whetting. Another fault of the method is the slow whetting speed and the poor accuracy of finishing. Another (fixed powder) method grinds diamond plates with a whetstone on which diamond powder is fixed. There are several kinds of diamond whetstones which are classified by the manner of fixing diamond granules on the whetstones. The whetstone on which diamond powder is fixed by phenol resin, polyimide resin or so is called a resin-bonded whetstone. Another whetstone on which diamond powder is fixed by bronze, cobalt, tungsten, iron, nickel and so forth is called a metal-bonded whetstone. The whetstone on which diamond powder is fixed by plating of nickel etc. is called an electrodeposition whetstone. These whetstones using fixed diamond powder have an advantage of avoiding the waste of diamond granules. The fixed granule method polishes the surface of diamond plates by the steps of sticking a diamond plate on a holder, pushing the holder on a rotary whetstone table, revolving the whetstone table, rotating also the holder around own axis and polishing the surface of the diamond by the physical function of the fixed diamond granules. All the methods grinds diamond plates or diamond films by the physical contact with the diamond granules. The methods depend on the physical action of the diamond powder. Because of heightening the physical action, these methods require imposing a heavy load on the polishing faces. The high pressure of the load enables the diamond powder to scuff, scratch, or scrape the surface of the object diamond plates or films. The heavy load defaces also the diamond granules either being fixed on the whetstone or flowing in the liquid as well as the object. Without the heavy load, the object diamond can be ground no more, slipping in vain on the whetstone. Some people suggested a polishing method which dispenses with the heavy load. Japanese Patent Laying Open No.2-26900 (26900/90) proposed a thermochemical method which polishes diamond by chemical reaction by bringing diamond into contact with a heated flat metal table under the oxidizing atmosphere at a high temperature. This chemical method uses no diamond powder. The applicability of the conventional wafer process is indispensable for the exploitation of hard-material coated complex wafers, that is, diamond-coated wafers, diamond-like carbon coated wafers or c-BN coated wafers to the field of electronics, optics or optoelectronics. For example, the technology of photolithography must be able to be applied to the complex wafers for fine processing. Photolithography requires the flatness of object wafers. The various wafer processes have been highly developed in the silicon semiconductor technology. The hard-material coated complex wafers must cope with the conditions requested by the established wafer processes. The hard-material coated complex wafer must satisfy the requirements of the fabrication of semiconductor devices. In general, the fabrication technology demands a diameter of more than 2 inch and a thickness of less than lmm of a wafer. The sizes of electronic devices become smaller year by year. The miniaturization of devices requires the reduction of thickness of wafers. The thinning technique has been established for silicon wafers. Unlike silicon, wide, homogeneous wafers composed of only a single material cannot be produced at present in the case of hard materials, that is, diamond, diamond-like carbon and c-BN, because of the difficulty of making big and long single crystals. These materials can be synthesized Into a thin film. Then complex wafers can be produced by coating a pertinent substrate with a film of the hard-material. Instead of homogeneous wafers, two-component, complex wafers will be produced for the hard-materials. The inhomogeneous wafer consists of a substrate and a hard-material film. The hard-materials are now represented by diamond for avoiding frequent quotation of all the materials. Diamond-coated wafers can be made by the known plasma CVD method, the hot filament CVD method and so forth. Various materials can be adopted as the substrates. The most convenient thing for the substrate is a silicon wafer, because the technology of making and processing silicon wafers has been matured. It is easy to obtain flat silicon wafers at low cost. A complex wafer can be made by depositing a diamond thin film on the substrate by the abovementioned methods. The surface of the film is rugged. Then the rugged surface of the diamond-coated wafer must be polished Into a smooth and flat surface. However, diamond is the hardest material among all the materials obtainable at present. There is no material harder than diamond. Thus, diamond is mechanically polished by a polishing machine using diamond powder as whetting medium. In the polishing, high pressure must be applied to the surface of the object diamond. Thus, a diamond wafer must endure strong stress. However, a diamond wafer consists of a substrate wafer and a thin diamond film. The diamond film owes mechanical strength to the substrate. High load presses mainly the substrate in the long run. Silicon which is used as a substrate is a fragile material. Then when a silicon wafer is adopted as a substrate, the complex wafer is likely to break, in particular when the wafer has a big diameter and thin thickness. If another material is used as a substrate, the problem will not be solved, since the material Is likely to be broken by the high pressure. One problem is the break of a wafer by the high pressure. This problem must be solved in order to make a mirror diamond wafer. There is still another problem. A strong inner stress arises In the complex wafer having a substrate and a film due to the two layered structure. A diamond is synthesized at a high temperature in vapor phase from the excited material gas. Then the wafer is cooled to room temperature. Thermal stress occurs in the complex wafer due to the differences of thermal expansion between the substrate and the film. In addition, a diamond film has inherently intrinsic stress. The thermal stress and the intrinsic stress distort the complex wafer convexly or concavely to a great extent. Conventional polishing apparatuses whet a flat object by a flat holder and a flat polishing turn-table. The conventional machines are entirely inapt at polishing distorted objects. Somebody may have an idea of polishing a distorted object by gluing a distorted object in a forcibly flattened state on a flat holder, pushing the object by the holder upon the turn-table, rotating the holder and revolving the turn-table in a conventional machine. However, such a superficial improvement would be in vain. One problem is the high probability of breakage of wafers. Another problem is the difficulty of whetting the film uniformly. Another difficulty is a large fluctuation of the thickness of the film polished. These problems impede the application of the conventional polishing machine to the two-layered wafers having a substrate and a hard-material film. An object of the present invention is to provide a broad complex wafer having a non-hard material substrate and a hard material film. Another purpose is to provide a method of producing the same. According to the present invention there is provided a hard material-coated wafer having a distorted substrate and a hard material film having a Vickers hardness of more than Hv3000 which has been produced on at least one surface of the substrate by a CVD method, characterized in that: the total thickness of the wafer is in the range from 0.1 mm to 2.1mm; the diameter of the wafer is greater than 25mm; more than 50% of the film is polished to a roughness less than Rmax50nm and Ra20nm and; the wafer is distorted into a convex-shape on the film side with a distortion height H in the range -150µm to -2µm. According to another aspect of the present invention there is provided a method of producing a hard material-coated wafer as per claim 8. The vapor phase method of producing the film can be a filament CVD method, a microwave plasma CVD method, a radio-frequency plasma CVD method, a plasma flame method or so. The pertinent pressure of reaction is 1 Torr to 300 Torr. The material gas is hydrogen gas and hydrocarbon gas for the production of a film of diamond or diamond-like carbon. The material gas is hydrogen gas, boride gas and nitride gas for making a c-BN film. In the formation of a diamond film or a diamond-like film, hydrogen gas and hydrocarbon gas constitute a main part of the material gas. However, the whole of hydrogen gas or a part of the hydrogen gas can be replaced with a rare gas. Hydrocarbon gas can also be replaced with some organic gas or inorganic gas including carbon in the case of diamond or diamond-like carbon. It is preferable to dope the material gas with some organic gas or inorganic gas containing oxygen gas. The condition of the vapor phase synthesis should be selected in order to make a convex-distorted wafer which bends toward the side of the synthesized film. This is an important matter. A flat wafer is rather Inoperative against the common sense, since a substantially flat wafer has a wavy surface unsuitable for polishing. A concave-distorted wafer is also improper. Only convex-distorted wafer can be polished uniformity by the polishing apparatus of this invention. The film of the convex-distorted wafer should be polished till the roughness of the film should be reduced to Rmax less than 50 nm and Ra less than 20 nm. Although the surface is not perfectly smooth, the film of the roughness allows the wafer process to forming electrodes, implanting impurity atoms, diffusing impurities or etching metals, oxides or semiconductors selectively by the photolithography. Fig. 1 shows a sectional view of a hard material-coated wafer in accordance with an embodiment of the present invention. This is a convex-distorted wafer. Partly because of the difference of thermal expansion coefficients and partly because of the big thickness of the film, strong inner stress happens in the complex wafer, when the wafer is cooled to room temperature after the synthesis of the hard material film. The strong inner stress deforms the complex wafer either to a convex-form or to a concave-form. The distortion is now expressed by the height H of the center from the plane including the periphery of the wafer. The sign of the height H is determined now to be positive for concave distortion and to be negative for convex distortion. This invention prefers convex-distorted wafer with negative height H between -2µm and -150µm. Fig.1 shows a convex-distorted wafer of a negative distortion height which is suitable for the polishing of the present invention. The Inventors have found that the distortion of wafers can be controlled by the condition of the vapor phase deposition of the film on the substrate. Some synthesis condition can make a flat wafer. A flat wafer seems to be the best condition. However, this is wrong. A flat wafer actually waves, as shown in Fig.2. The wafer which seems flat is partly concave and partly convex. Namely, a flat wafer is more complex than a convex wafer or a concave wafer as a whole. When a flat wafer with a wavy surface is polished, unpolished portions are left at any effort. Thus this invention avoids flat wafers. This invention does not employ concave-distorted wafers because of the difficulty of polishing. The present invention requires the following conditions of hard materials, substrates and distortion as optima. [A. hard material films] Diamond films, diamond-like films and c-BN films should be provided with the conditions below: 1. Thickness of film: 5 µm to 100 µm; preferably 15 µm to 50 µm. Large thickness of a film raises the cost of formation of the film. A 1000 µm thick film is operative but such a thick film needs too much material and too long time for deposition. Thus, a film thicker than 100 µm is disadvantageous from the economical standpoint. On the contrary, too small thickness of film induces the difficulty of polishing. The base wafer appears at some parts of the surface. Sometimes the wafer is broken by the contact of the base material to the polishing plate. Therefore, this invention employs films with a thickness more than 5 µm. Preferable thickness is 15 µm to 50 µm. 2. Roughness of surface: Rmax is less than 50nm. Ra is less than 20nm. If the surface roughness is bigger than the values, the wafer cannot be utilized as a wafer for electronic devices or a wafer for abrasion resistant tools. If the roughness of a wafer is large, the photolithograpy is unable to form fine patterns of wires of devices on the wafer. Big roughness raises the friction coefficient of the complex wafer. The wafer with high friction cannot be a material of abrasion resistant tools. [B. base wafer (substrate)] The base wafer on which a hard material film is grown is a non-hard material which is suitable for being formed into a wide, thin plate. The material of the base wafer is one of Si, GaAs, GaP, AlN, SiC, Si 3 N 4 , LiTaO 3 , LiNbO 3 , Li 2 B 4 O 7 , PbTiO 3 , PZT(PbZrO 3 -PbTiO 3 ) or quartz. A Si single crystal wafer is, in particular, the most desirable. Furthermore, a (100) Si wafer is the optimum material among various oriented silicon wafers. The optimum thickness depends on the material of the base wafer. In general, the thickness of the base wafer should be 0.1 mmt to 1 mmt in common. A base wafer thinner than 0.1mm would be annoyed by a big distortion and be liable to break down. On the contrary, a wafer thicker than 1 mm cannot be treated with the wafer process. Thus, devices cannot be built on such a thick wafer by the process. If devices were made on the thick wafer, the devices cannot be mounted on packages unless the wafers are thinned by grinding the bottom surface. With regard to the shape, circular wafers are the best for the convenience of handling, e.g., conveyance, fixation, storing, or supporting. However, a rectangular wafer or square wafer is also available as a base wafer. Such wafers are used to some special purposes. In general, circular wafers are most suitable for treating by the common semiconductor process like silicon wafers. The diameter is an arbitrary parameter which will be determined from the purpose. The diameter, however, must be larger than one inch (25 mm) from the efficiency of the treatment in the wafer process. 2-inch wafers, 3-inch wafers, 4-inch wafers, 5-inch wafers or 8-inch wafers are also useful to the base wafers of the invention. [C. distortion] This invention demands convex-distortion for complex wafers after the synthesis of films. Namely the wafer should be distorted convex to the side of the film monotonously from the periphery to the center. The absolute value | H | of the height of distortion must range from 2 µm to 150 µm. The distortion of the wafer is here expressed by the height H of the center from the plane including the peripheral circle. The distortion may be designated by a curvature radius R. R and H are simply related by an equation R = D 2 /8H, where D is the diameter of the wafer. Further, the convex-distortion is allocated with a negative H but the concave-distortion is assigned with a positive H. Thus the requirement of distortion is briefly described by -150 µm ≦ H ≦ -2 µm. Preferably -50 µm ≦ H ≦ -5 µm. The present invention declines the wafer without distortion, that is, H = 0. Distortion less wafers may seem to be the best for the following polishing process. However, if a wafer has no macroscopic distortion, it actually has some waves which invite more complex modes of bending, as shown in Fig.2. Such a wavy wafer cannot be polished perfectly at any way. Unpolished parts or insufficiently polished parts remain at random on the surface. Simpler bending is more suitable for polishing, even though the distortion is pretty large. The minimum of the distortion is determined to be 2 µm for excluding non-distortion H = 0. On the contrary, the maximum of the distortion is 150 µm. A wafer having a distortion height larger than 150 µm will leave some unpolished regions. Wafers should be uniformly polished overall. The uniform polishing is impossible for the wafers which bend more than 150 µm. Thus this invention denies the distortion of more than 150 µm. The polishing step can be carried out as follows. The polishing comprises the steps of sticking a complex wafer having a substrate and a hard film of more than Hv3000 to a center of a bottom face of a holder in a convex-distorted state, pushing the convex film on a rotating turn-table, applying a heavy load on the center of the holder, rotating the holder, inclining the bottom face of the holder to the turn-table, bringing the contact region between the wafer and the turn-table from the center to the peripheral part of the wafer gradually, and moving the polishing region from the center to the periphery of the film. Since the contact region moves from the center to the periphery, the whole of the surface is perfectly polished without unpolished portions. The movement of rotation around a slanting axis whose direction is changing is now called a "precession" or a "swaying movement". In the restrict meaning in physics, the precession requires the rotation in a rigorous cone of the slanting axis itself. However, the word "precession" here means arbitrary rotation movement of the slanting axis of the holder. The locus of the axis includes not only circles but also ellipsoids. The swaying movement means the reciprocal inclinations of the rotational axis in the radial direction. The locus of the axis is a part of a straight line. Thus, swaying movement is a limit of the precession whose locus ellipsoid has a shorter axis of a length of zero. Thus, sometimes both the precession and the swaying movement are represented by precession in short. The advantage of the invention is now explained. Bulk crystals of diamond and c-BN have been synthesized by some methods. All the bulk crystals had a poor practical significance aside from the academic meanings, because they had too narrow area. This invention succeeds in making big-sized wafers of the hardest materials, i.e. diamond, diamond-like carbon and c-BN for the first time. The hard material wafers have a complex structure having a commonplace substrate and a hard film deposited on the substrate. At this point, the wafers are different from Si wafers or GaAs wafers which are homogeneous and entirely composed of a single material. The complex wafers are quite useful for a substrate of making electronic devices, since such devices make the best use of only the surface of the wafers. The chemical vapor phase deposition (CVD) enables this invention to make large sized complex wafers on wide substrate wafers. Supply of large-sized wafers reduces the cost of producing devices by the wafer process. The wafers of the present invention, however, are deformed by the difference of the thermal expansion between the substrate and the film. According to a conventional sense, polishing machines are generally effective only to polish flat objects but are entirely ineffective to polish distorted matters. The Inventors disagree with the common sense. A flat wafer is rather difficult to polish. Convex-distorted wafers with a distortion ranging from -150 µm to -2 µm can uniformly be polished by a polishing machine with a holder which rotates with swaying motion or precession motion. Concave-distorted wafers can be also polished by inserting a convex spacer between the buffer and the wafer. One of the important matters of this invention is the discovery of the possibility of polishing of deformed objects. The possibility of polishing allows this invention to make mirror wafers of hard material for the first time. The machine polishes a convex-distorted wafer from the center to the periphery or vice versa by inclining the holder little by little till all the convex surface is fully polished to a roughness less than Rmax50nm and Ra20nm which enables the photolithograpy to make various devices on the hard-material wafer. A hard-material coated wafer is likely to bend in a convex shape or In a concave shape by the difference of thermal expansion coefficients between the substrate and the hard-material film. Even when a complex wafer is immune from the distortion as a whole, the wafer has waves. In any cases, complex wafers are not flat in a rigorous meaning. Conventional machines could not polish all the surfaces without leaving unpolished portions. This technique succeeds in polishing the whole surface of a complex wafer without leaving unpolished parts by sticking the wafer in convex-distortion to a holder, rotating the holder on a rotary turn-table and giving the holder a swaying motion or a precession which enables all the parts on the surface to come into contact with the turn-table. The polished part has a sufficient smoothness of less than Rmax50 nm and Ra20nm. The present invention can suppress the fluctuation of the film thickness within ± 10%. Two-layered version is also proposed for reducing the fluctuation of thickness of the film polished. This version coats the substrate with a polishing-difficult layer first and deposits another polishing-easy layer till a definite thickness on the polishing difficult layer. When the two-layered wafer is polished, the softer top layer is perfectly polished and the harder undercoat is revealed overall. Thus. the distribution of the thickness becomes uniform in the whole of the surface. Big bulk crystals of the hard material, that is, diamond, diamond-like carbon and c-BN cannot be grown by the present state of technology. Films of the hard materials can be synthesized by the vapor phase deposition methods. However, it will take a very long time and a big cost to make a homogeneous wafer which contains no other material than the object hard material by growing a thick hard-material film on a substrate and eliminating the substrate. A complex wafer having a thin hard-material film is sufficient for the application to the material of semiconductor devices, optical devices or optoelectronic devices. The complex wafers must be mirror-polished for the convenience of photolithography or other wafer processes. However, complex wafers are accompanied by strong distortion owing to big inner stress. This invention enables to mirror-polish the distorted complex wafers for the first time. This is an important invention which leads to effective applications of the hard materials to various fields of technology. The invention will be more fully understood from the following description given by way of example only with reference to the several figures of the accompanying drawings in which, Fig.1 is a brief section of a convex-bending complex wafer having a substrate and a hard material film coated on the substrate. Fig.2 is a brief section of a macroscopically non-bending complex wafer which has a convex bending and a concave-bending as waves, since the distortion is defined by the height of the center from the plane including the periphery. Fig.3 is a section of a base wafer as a substrate at the first step prepared for the processes of making a hard material film and forming electrodes on the film. Fig.4 is a section of the base wafer and a rugged hard material film deposited on the wafer which bends convexly. Fig.5 is a section of the convex-distorted complex wafer having a smooth film which has been polished. Fig.6 is a section of the convex-distorted complex wafer with the base wafer and the hard material film on which an aluminum film has been evaporated. Fig.7 is a section of the complex wafer having the base wafer and the hard material film on which aluminum interdigital electrodes are made by selectively etching the aluminum of Fig.6. Fig.8 is a sectional view of a filament CVD apparatus for making hard material film on a substrate. Fig.9 is a section of a microwave plasma CVD apparatus for producing a hard material film on a substrate. Fig.10 is a sectional view of a plasma jet CVD apparatus for synthesizing a hard material film on a substrate. Fig.11 is a front view of a polishing machine for polishing the surface of the hard material film. Fig. 12 is an enlarged sectional view only of the holder of the polishing machine. Fig.13 is a plan view of the wafer with non-distortion which has been polished with a flat holder by the polishing apparatus. Blank region is a polished part. Dotted region is unpolished parts. Fig.14 is a plan view of the wafer of the concave-distorted film which has been polished with a flat holder by the polishing machine. Fig.15 is a plan view of the wafer of the convex-distorted film which has been polished with a flat holder by the polishing apparatus. To achieve the foregoing objects and in accordance with the purpose of the invention, embodiments will be broadly described herein. Fig.3 to Fig.7 show the steps of an experiment of making a hard material film, e.g., diamond, c-BN or diamond-like carbon on a substrate, polishing the film, coating the film with a metal layer, forming electrodes by etching and inspecting how many wire patterns are broken. Table 1 indicates the condition of forming a film, that is, kind of a substrate, thickness of the substrate, diameters of the substrate, kind of hard material, thickness of the film of samples 1 ○to . For avoiding citation of all hard materials, the hard materials are represented by diamond from here in brief. At the first step 1 ○, a flat circular base wafer is prepared as a substrate which is indicated in Fig.3. At the second step , a diamond film is deposited on the base wafer substrate by the microwave plasma CVD method, the filament CVD method, the plasma jet CVD method or the plasma flame method. The reaction pressure is 1 Torr (133 Pa) to 300 Torr (0.04 MPa). The ratio of methane to hydrogen (CH 4 /H 2 ) is 0. 1 vol% to 10 vol%. A diamond film with a rugged surface is grown on the substrate wafer which is shown in Fig.4. This is a complex wafer having a substrate wafer and a film. The important matter is the distortion of the complex wafer due to the inner stress which originates from the difference of the thermal expansions between the substrate and the film. At the third step, the rugged surface of the diamond film is polished by a mechanical polisher. Since the wafer is distorted, some contrivance is necessary for the polisher to polish such a bending wafer. The contrivance will be described later. Convexes and concaves are eliminated from the film surface. Fig.5 shows the section of the complex wafer. The film becomes smooth. However, the distortion remains. Then an aluminum film is deposited on the hard-material film by evaporation or sputtering. Fig.6 denotes the section of the complex wafer after the fourth step. Micro wire patterns are produced by etching parts of the aluminum film selectively by photolithography. The wires are parallel with each other. The line width of the wires is changed in a range from 0.6 µm and 2 µm. The section of the wafer is shown in Fig.7. The diamond wafer has parallel micro wire patterns. The size of the wires is similar to the size of the interdigital electrodes of SAWs. Then the yield against the breaking of wire patters are investigated. The results are listed in table 1, table 2 and table 3. The Young modulus of the diamond films are measured by the vibration lead method. The result is also denoted in table 3. Substrate, substrate thickness, substrate diameter, kinds of hard material and film thickness NO substrate substrate thickness (mm) substrate diameter (inch) hard material (CVD-method) film thickness (µ m) 1 Si (100) 1 2 diamond 30 2 Si (111) 1 4 diamond 50 3 Si (poly) 0.35 8 diamond 100 4 GaAs 0.1 1 diamond 15 5 AlN 0.8 3 diamond-like carbon 5 6 LiNbO 3 0.5 2 c - BN 30 7 LiTaO 3 0.3 5 diamond 100 8 Quartz 1 4 diamond 5 9 Si(poly) 0.05 2 diamond 150 10 Si (100) 1 8 diamond 2 11 LiNbO 3 3 0.5 diamond 1000 12 GaAs 0.5 3 c - BN 30 Samples 1 ○to 8 ○ are embodiments of the present invention. Samples 9 ○ to are comparison examples. The thickness of the substrate is indicated by a unit of millimeter. The unit of diameters of the substrate is inch. The substrate of sample 1 ○ is a (100) silicon wafer. Sample 2 ○ adopts a (111) silicon wafer of a 4 inch diameter of a thickness of 1 mm. Sample 3 ○ has a poly-silicon wafer as a substrate. Sample 4 ○ employs a GaAs wafer. Sample 5 ○ selects an AlN wafer as a substrate. The substrate of sample 6 ○ is a LiNbO 3 crystal. Sample 7 ○ adopts a LiTaO 3 as a substrate. Sample 9 ○ for comparison has a polycrystalline silicon substrate of a 2 inch diameter of a 0.05 mm thickness. Sample grows a film on an 8-inch Si(100) wafer. Sample has a LiNbO 3 substrate wafer. Sample uses a GaAs single crystal wafer of a 3 inch diameter. The thicknesses of the base wafers are 0.1 mm to 1 mm for all the samples. The diameters of the base wafers range from 1 inch to 8 inches. The unit of the thickness of the hard-material films is µ m (micrometer). The thicknesses of the films are between 2 µm and 1000 µ m. Sample 5 ○ grows a diamond-like carbon film. Sample 6 ○makes a c-BN film. The other samples grow diamond films on the substrate wafers. Fig.8 indicates the schematic structure of a filament CVD apparatus for growing hard-material films on substrates by exciting a gas into plasma by heat. A vacuum chamber (31) contains a susceptor (32) for mounting a substrate wafer (33) on it. The chamber (31) has a gas exhaust outlet (34) which connects to a vacuum pump (not shown in the figure). Electrodes (35) stand from the bottom, sandwiching the susceptor (32). Filaments (37) span the susceptor (32) between the electrodes (35). A material gas including hydrogen gas and a hydrocarbon gas is supplied into the reaction chamber (31) through a gas inlet (38). A vacuum gauge (39) is installed at a point in the vacuum chamber (31). A power source (41) supplies a current to the filaments (37) through the electrodes (35). The current generates heat at the filaments (37). The filament (37) heats the substrate wafer (33) and the material gas. A cooling medium (40) circulates in the inner space of the susceptor (32) for cooling the susceptor and maintains the base wafer (33) at a pertinent temperature. The heat from the filaments (37) excites the material gas and induces a vapor phase reaction. The results of the reaction pile on the heated substrate wafer (33) and form a thin film on the wafer (33). Fig.9 shows the schematic section of a microwave CVD apparatus which excites a gas into plasma by microwaves. The apparatus has a vertically long chamber (42), a susceptor (44) and microwave devices. The chamber (42) is supplied with a material gas which flows from the top to the bottom of the chamber (42). A supporting shaft (43) stands in the chamber (42). The supporting shaft (43) sustains a susceptor (44) of the top. A sample base wafer is laid on the susceptor (44). The material gas which is introduced into the chamber from the top passes in the vicinity of the substrate wafer (45) and goes out of the chamber through an exhaust outlet (not shown in the figure). The parts in which plasma is generated are cooled by a cooling device (48). A magnetron (49) oscillates microwaves. The microwaves propagate in a horizontally long wave-guide (50), transversely pass the flow of the material gas, and excite the gas into plasma (51). A resonance plate (54) which is made from a conductive plate is furnished in an extra tube which faces the wave-guide (50) over the susceptor (44). A piston (52) moves the resonance plate (54) either to the right or to the left for making standing waves of a pertinent mode between the beginning end of the wave-guide and the resonance plate (54). Fig.10 designates a schematic section of a plasma jet CVD apparatus which excites a gas into plasma by DC arc discharge. A reaction chamber (55) has a susceptor (56) in the middle. The susceptor (56) holds a sample wafer substrate (57) on it. A plasma torch (58) is mounted at the top of the reaction chamber (55). The plasma torch (58) has an inner cathode, an outer anode and gas passageway between the cathode and the anode. Gases (40) are supplied through the passageway from outer gas cylinders into the reaction chamber (55). A power source (59) applies electric power between the cathode and the anode. An arc discharge occurs between the cathode and the anode. The DC arc discharge excites the gases into plasma. The gases are a gas for keeping plasma, e.g. argon gas, a carrier gas, e.g. hydrogen gas and a hydrocarbon gas, e.g. methane gas. The apparatuses shown in Fig.8, Fig.9 and Fig.10 are typical CVD apparatuses which enable to excite a material gas and coat a substrate wafer with a hard-material film synthesized from the material gas. Thus, a diamond film, a diamond-like carbon film and a c-BN film can be formed in the apparatuses by supplying a material gas into a reaction chamber, exciting the gas by heat, microwaves, or arc discharge and depositing the result of the reaction on the sample wafer. The material gas is a mixture of hydrogen gas and hydrocarbon gas in the case of making diamond or diamond-like carbon and a mixture of hydrogen gas(H 2 ), borane (B 2 H 6 , B 4 H 10 , B 5 H 9 ,...), ammonia (NH 3 ) or nitrogen (N 2 ) or so in the case of making c-BN. The films of the hard-material are produced here by the microwave plasma CVD method, the filament CVD method or the plasma jet CVD method, but other methods are also available. The roughness Rmax and Ra of the complex wafers having a substrate and a film are measured. The roughness of a surface is defined by the difference of heights of points on the surface. A rugged surface has a plenty of small hills and small valleys. Rmax is the maximum difference between hills and valleys. Ra is the average of the differences between hills and neighboring valleys. The formation of the film generates some distortion in the wafer owing to the inner stress originated from the difference of thermal expansion between the film and the substrate. Thus the distortion is also measured by the height H of the center from the plane including the circumference. Minus sign "-" means the convex-distortion on the film side. Plus sign "+" denotes the concave-distortion on the film side. The distortion height H is not the curvature of distortion. Since the distortion height is defined by the height of the center from the base plane including the circumference, a same distortion height gives different curvatures for different sizes of wafers. R denotes the radius of curvature and D indicates the diameter of the wafer. If the distortion is spherical, the parameters are correlated with each other by the equation H=D 2 /8R. The Table 2 indicates the roughness Rmax and Ra, and distortion heights H for all samples. Roughness and distortion of synthesized hard material wafer NO roughness after synthesis distortion height H (µm) Rmax (µm) Ra (µm) 1 2.5 0.5 - 15 2 3.8 1.1 -50 3 8 3 - 150 4 1 0.2 - 30 5 0.4 0.18 - 5 6 1.6 0.5 -50 7 7 4.2 - 5 8 0.3 0.1 - 50 9 12 0.3 cracks in substrate 10 0.15 0.05 + 200 11 100 4 0 break of substrate 12 2.1 0.9 0 ( Sample 9 generates cracks. Sample 11 breaks. ) Rmax's after the synthesis are dispersed widely from 0.15 µm to 100 µm. Ra's after the film formation range from 0.05 µm to 40µm. The distortions H distribute from +200µm to -150µm. Sample 1 ○which has grown a 30 µm thick diamond film on a 2-inch Si wafer has roughness of Rmax2.5 µm and Ra0.5 µm. This is a smooth surface. The distortion H is -15 µm in a convex-shape on the film side. Sample 2 ○which has used a 4-inch Si wafer is distorted into a convex shape of H = -50µ m. Sample 3 ○ which has produced a 100 µm film on a 8-inch polycrystal silicon wafer has a big convex distortion H = -150µm. Since the substrate is wide, the distortion is also big. Sample 4 ○ having a GaAs substrate and a 15 µm diamond film reveals a moderate distortion of H = -30 µ m. Sample 5 ○which makes a thin diamond-like film on 3-inch AlN substrate has a very small distortion of H = -5µ m. The surface is very smooth. The roughness is Rmax0.4µ m and Ra0.18µ m. Sample 6 has a 2-inch LiNb0 3 substrate and a 30 m thick c-BN film. The roughness is Rmax1.6 µm and Ra0.5µ m. This is smooth enough. The distortion is H=-50µ m. Sample 7 ○produces a thick diamond film of 100 µm on a 5-inch LiTa0 3 substrate. The substrate is wide enough. The film is thick. The distortion is small (H = -5µ m). The surface is rugged (Rmax7 µm and Ra4.2µ m). Sample 8 ○employs a 4-inch quartz substrate of a thickness of lmm and grows a diamond film of a thickness of 5µ m. The surface is smooth (Rmax0.3µ m and Ra0.1µ m). The distortion is convex (H = -50µ m). Samples 1 ○to 8 ○ are embodiments of the present invention. Samples 9 ○ to are comparison examples. Sample 9 ○ consists of a thin 2-inch Si polycrystal wafer (0.05mm = 50 µm) and a thick diamond film of a 150 µm thickness. The roughness is Rmax12 µm and Ra0.3 µm on the diamond film. Cracks occur in the Si substrate. Thus the distortion is not measured. The polishing is omitted for this sample, since the cracks hinder the polishing. The too thin substrate may be defeated by the strong inner stress of the thick film. Thus, too thin substrates or too thick films are inappropriate for making a complex wafer. Sample is composed of a very wide (8-inch) (100) silicon monocrystal wafer (t=1mm) and a very thin diamond of a 2 µm thickness. Sample is featured by the wideness of the substrate and thinness of the film. The surface is smooth enough (Rmax0.15 µm and Ra0.05 µm). However, the distortion is very large (H=+200 µ) in a concave shape. The film is thin but the wafer is too wide. Then sample has a big distortion height. Sample deposits an extremely thick diamond film (1000 µm) on a 3mm thick LiNbO 3 wafer. Sample is characterized by the thick film and the thick substrate. The surface is very rugged with large roughness of Rmax100 µm and Ra40 µm. The substrate is broken. Strong inner stress of the film may destroy the substrate. Sample cannot be polished. This result means too thick films are unsuitable for a complex wafer. Sample adopts a 3-inch GaAs wafer as a substrate and makes a 30µ m c-BN film on the GaAs wafer. The distortion is 0 (H = 0 µ m). This is a flat wafer. The non-distortion may result from the similarity of the thermal expansion coefficients between GaAs and diamond. Furthermore, the surface is smooth (Rmax2.1 µm and Ra0.9 µ m). Thus, sample seems to be a very good complex wafer due to the flatness and the small roughness. The expectation will be betrayed later. A hard-material film grown by the vapor phase CVD method has a rugged surface with large roughness in general. Electronics devices cannot be fabricated on the rugged surface by the photolithography, because the big roughness would perturb the transcription by the light by scattering, reflecting or diffracting the light beams. Polishing is indispensable for the rugged CVD made hard films in order to treat with the complex wafers by the wafer process. However, conventional polishing machines which have been used for polishing flat silicon wafers or flat gallium arsenide wafers are unable to polish a distorted wafer. Prior machines are competent only for flat objects, since the holder is flat and the polishing turn-table is also flat. If distorted wafers are polished by the conventional machines, peripheral portions or intermediate portions are left unpolished, even if the holder presses the wafer to the turn-table with a strong force. A special polishing machine and a new polishing method are required for treating with the inherently-distorted wafers. This special polishing machine is suitable for the distorted hard-material films. The hard-material films on the substrates of samples 1 ○to 8 ○, sample and sample have been mechanically polished by the special machine. The polishing reduces the roughness of the films and changes the distortion of the wafers. Then the roughness and the distortion are again measured for the samples after the polishing. Some samples cannot be perfectly polished because of the concave-distortion or the wavy shape. Then the ratio of the polished part to the whole is also measured. The roughness is reduced by polishing. The distortion either increases or decreases by polishing. Usually, the polishing reduces the distortion. Since the polishing method and apparatus are different from the case of polishing Si or GaAs wafers, the structure of the machine is first explained. Fig.11 indicates the polishing machine which can polish wafers having distortions. Fig.12 is an enlarged section of the holder. A turn-table (41) is supported and driven by a rotary shaft (42). The turn-table (41) is a diamond whetstone. Since the turn-table (41) aims at whetting hard films, the turn-table (41) must be covered with diamond powder on the surface. The diamond granules on the surface of the turn-table (41) polish the object by abrading themselves. It takes several tens to hundreds of hours. An elastic buffer (11) (e.g. rubber disc) is glued to the bottom of the holder (3). The complex wafer (2) is stuck to the buffer (11). The use of the buffer is one of the contrivances. A main shaft (4) is jointed to the center of the holder (3). The main shaft (4) transmits axial force (pressure) and rotation torque to the holder (3). However, the holder (3) is not fixed to the shaft (4). The shaft (4) allows the holder to incline in any direction within a certain range. The holder (3) rotates around its axis (a normal at the center) which sometimes differs from the shaft (4). The angle between the axis and the shaft is called an inclination angle or a slanting angle. The turn-table (disc whetstone) (1) revolves about the rotary shaft (21). An auxiliary shaft (10) presses a point on the holder (3). An arm (6) holds the top ends of the main shaft (4) and the auxiliary shaft (10). An oil pressure cylinder (5) is mounted on the arm (6) for applying strong pressure to the holder shaft (4). The wafer is pressed upon the diamond powder on the turn-table by the strong force from the main shaft (4). The diamond powder abrades the hard film of the wafer by wasting itself. The pressure enhances the abrasion of the film as well as the waste of the diamond powder fixed on the disc whetstone (1). A motor (7) is mounted on the arm (6) for rotating the holder shaft (4). The torque of the motor (7) is transmitted from the output shaft (15) of the motor via a pulley (16) and a belt (8) to the pulley (17) of the main shaft (4). The shaft (4) rotates the holder (3) about own axis which is a normal at the center and is different from the main shaft (4). An oil pressure cylinder (9) is mounted on the arm (6) for applying a strong force upon the auxiliary shaft (10). A circular groove (13) is dug on the upper surface of the holder (3). The bottom end of the auxiliary shaft (10) pushes the groove (13). The main shaft (4) is connected to the holder (3) in a manner which transmits the axial force and the rotation from the main shaft to the holder but allows the relative slanting of the holder (3) to the shaft (4). The bias force of the auxiliary shaft (10) inclines the holder (3) to the turn-table (1). The inclination is represented by the inclination angel between the main shaft (4) and a normal (rotation axis) of the holder (43). The inclination angle depends upon the force of the auxiliary shaft (10). The convex-distortion of the wafer (2) facilitates a smooth inclination of the holder (3). The contact point of the wafer (45) to the turn-table (41) is varied by the inclination angle. Thus, the contact point can be controlled by allocating the forces between the main shaft (4) and the auxiliary shaft (10). The whole surface can be fully polished by moving either the contact point from the center to the periphery or from the periphery to the center of the wafer. As explained before, a hard material coated wafer cannot be entirely polished by conventional machines which have a holder fixed to the main shaft. Some portions are left unpolished. If the wafer were treated by the conventional machine for an extremely long time, unpolished parts might be extinguished. However, if so, other parts of the film would be worn out and the substrate would appear there. The problems of the conventional machines are now clarified by referring to the Fig.13 to Fig.15. The states of wafers after the polishing depend on the states of the wafers after the synthesis. (1) Case of a flat wafer (Fig.13): Somebody may deem a flat wafer after the synthesis of the film would be polished into a flat mirror wafer. But the truth is otherwise. The flat wafer suffers from randomly distributing unpolished parts. Fig.13 indicates the dispersion of the unpolished portions. The unpolished parts are rugged and seem to be dull, since light is scattered randomly and is reflected weakly. An eye observation can easily distinguish unpolished parts from the background polished portion. Thus a flat wafer when it is synthesized is unsuitable for polishing. This is an unexpected result. (2) Case of a concave-distorted wafer (Fig.14; H>0): The central part and the periphery are polished. A ring-shape unpolished part is left unpolished in the intermediate region. (3) Case of a convex-distorted wafer (Fig.15; H<0): The central part and the intermediate part are polished. A periphery is left unpolished. In any cases, the conventional machines are incompetent for mirror-polishing hard-material wafers. By contrast, the present apparatus can polish the whole of the wafer by adopting a holder capable of slanting to the shaft and a convex-shape wafer (H<0). All the surface is mirror-polished by the machine (Fig.11 and Fig.12). The result of the polishing is not indicated in the figures, since the whole of surface becomes mirror-polished. Then samples 1 to 12 are polished by the machine shown in Fig.11 and Fig.12. The roughness, the ratio of the polished area to the whole area and the distortion height are measured. Table 3 denotes the results of the measurement. Roughness (nm), ratio of polished parts (%), distortion (µm) of hard material wafer after polishing NO roughness (nm) ratio of polished parts (%) distortion height H' (µ m) Rmax Ra 1 8 1 100 % polished - 14 2 10 2 100 % polished - 48 3 40 8 100 % polished - 139 4 5 0. 4 100% polished - 30 5 1 0.3 100 % polished - 5 6 3 0. 6 100 % polished - 50 7 50 20 100 % polished - 5 8 1 0.3 100 % polished - 50 9 not polished, since substrate is cracked. 10 70 2 5 50 % unpolished + 200 11 not polished, since substrate is broken. 12 100 40 10% unpolished 0 ( Be careful of the unit (nm) of roughness. 1000nm = 1 µm ) ( Sample 9 and 11 are not polished, because of cracks or break caused in synthesis) The roughness of the samples after polishing ranges from Rmax1nm to Rmax100nm and from Ra0.3nm to Ra40nm. It should be noted that the units of the distortion are different between the pre-polishing wafers and the post-polishing wafers. Table 2 for the pre-polishing samples uses "µm" for expressing roughness. Table 3 for the post-polishing wafers adopts "nm" for signifying roughness. Fig.5 shows a simplified section of the polished wafer. Sample 1 ○which has had intermediate roughness before polishing reduces the roughness to Rmax8nm and Ralnm. The distortion is -14 µm similar to the pre-polishing value. Sample 2 ○had an intermediate value of roughness. The post-polishing roughness of sample 2 ○is Rmax10 nm and Ra2nm. This is an intermediate roughness which enables to transcribe patterns from masks to the resist on the wafer by photolithography. The photolithography requires roughness of less than Rmax50nm and Ra20nm. The distortion is -48 µm which is little different from the pre-polishing value (-50 µm). Sample 3 ○ which had big roughness before polishing has post-polishing roughness of Rmax40nm and Ra8nm. Big pre-polishing roughness invites big post-polishing roughness in general. The distortion is slightly changed from -150 µm to -139 µm after polishing. Sample 4 ○ which has a GaAs substrate and a diamond film with small roughness has quite small roughness of Rmax5nm and Ra0.4nm which is far lower than the critical values of Rmax50nm and Ra20nm. The distortion is -30 µm which is equal to the pre-polishing value. Sample 5 ○has an AlN substrate and a diamond-like carbon film with very small roughness before polishing. The roughness is reduced to Rmax1nm and Ra0.3nm. This is a quite smooth mirror-polished surface. Sample 6 ○is an example which grows a c-BN film. The inherent roughness is small enough (Rmax1600nm, Ra500nm). The polishing reduces the roughness to Rmax3nm and Ra0.6nm which are satisfactory values. The distortion is -50 µm both for pre-polishing and for post-polishing. Sample 7 ○is an example of forming a 100 µm diamond film on a LiTaO 3 substrate. Too thick a film degenerates the surface morphology. Big pre-polishing roughness (Rmax7000nm, Ra4200nm) causes large post-polishing roughness of Rmax50nm and Ra20nm which is equal to the critical value for the application of photolithography. The distortion (-5 µm) is kept constant despite the polishing. Sample 8 ○is a 4-inch quartz substrate and a thin (5µm) diamond film. This is an inherently smooth film (Rmax300nm, Ra100nm). The polishing decreases the roughness to Rmax1nm and Ra0.3nm. Sample 9 ○ which had cracks on the wafer due to too thick a film cannot be polished. Sample had satisfactory small smoothness (Rmax150nm and Ra50nm) but concave-distortion of +200µ m before polishing. Since the roughness is inherently small, it is expected that the polishing would further reduce the roughness. But the fact is otherwise. The post-polishing roughness is Rmax70nm and Ra25nm which is beyond the scope of the application of photolithography. The polishing fails to make a smooth wafer. Furthermore, sample leaves 50% of surface unpolished. Why is the polishing so inactive for sample with little distortion? This is caused by the big concave-distortion. The concave-deformation prevents the concave portions from being in contact with the turn-table in spite of a strong pressure applied from the main shaft. Sample clarifies that the mode of distortion (H<0 or H > 0) has a great influence on the polishing. Sample which had grown a c-BN film on a GaAs substrate had a small roughness of Rmax2.1 µm and Ra0.9 µm. The roughness is still Rmax100nm and Ra40nm after the polishing. 10% of the surface is left unpolished. A complex wafer of H = 0 has wavy deformation with microscopic convexes or concaves. Then unpolished portions are distributed at random after the polishing. In general, the polishing can reduce the roughness to about 1/100 to 1/30 in the case of moderate convex-distortion. The distortion H is slightly decreased to H' by the polishing, but the mode (sign of H) of distortion is maintained. Then an aluminum film is evaporated on the hard films till a thickness of 150nm, as shown in Fig.6. Comb-shaped electrodes are formed on the hard films by etching the Al film selectively by photolithography. Fig.7 shows the section of the wafer. The comb-shaped electrodes are called interdigital transducers which are typical electrodes for surface acoustic wave devices (SAW). The line width of the Al electrodes is varied from 0.6 µm to 2 µm. The ruggedness of the film has a tendency of cutting the narrow wires. The yield against wire-cutting is investigated for all the samples. Table 4 lists the line width and the yield against the wire-cutting. Width and yield against wire cut of aluminium electrodes formed on hard films by evaporation and photolithography NO width of aluminum electrodes (µm) yield against wire cut (%) 1 1 98% 2 0.8 96% 3 1.2 97% 4 1.5 95% 5 0.6 94% 6 1 99% 7 0.8 94% 8 2 97% 9 Polishing and electrode-formation are impossible due to cracks occurring in synthesis. 10 1 3 % 11 Polishing and electrode-formation are impossible due to break of wafer in synthesis. 12 0.6 15% Sample 1 ○ which has grown a 30µ m thick diamond film on a 2-inch (100) Si wafer indicates a yield of 98% for the electrode pattern of a 1 µm line width. This is a satisfactory yield. Sample 2 ○which has produced a 50 µm diamond film on a 4-inch (111) Si wafer exhibits a yield of 96% for the electrodes of a line width of 0.8µ m. This result shows the possibility of making submicron wiring patterns with sufficiently high yield. Sample 3 ○which has piled a 100µm diamond film on a 8-inch polysilicon wafer displays a yield of 97% for the electrode pattern of a 1.2 µm line width. This is a satisfactory result. Sample 4 ○having a 1-inch GaAs wafer and a 15 µm diamond film reveals a yield of 95% for the electrode pattern of a 1.5 µm line width. Sample 5 ○which has covered a 3-inch AlN wafer with a 5 µ m diamond-like carbon film has a good distortion (H=-5 µm) and an excellent smoothness of Rmax1nm and Ra0.3nm. The yield of producing an interdigital electrode of a 0.6 µm line width is 94%. This is the narrowest electrode among the samples. This result is astonishing for such a narrow line width. Sample 6 ○which formed a 30 µm c-BN film on a 2-inch LiNbO 3 substrate with a distortion of -50 µm exhibits a yield of 99 % for making an interdigital electrode of a 1 µm line width. This result indicates such a big distortion allows to make narrow wiring patterns on the film. Further, this sample denotes the possibility of producing a good wafer also from c-BN. Sample 7 ○which made a 100 µm thick diamond film on a 5-inch LiTaO 3 substrate has a poor smoothness of Rmax50nm and Ra20nm. The yield of making 0.8 µm line width electrode patterns, however, is 94%. This is a satisfactory result. Sample 8 ○which consists of a 4-inch quartz substrate and a 5µm diamond film has a good smoothness but a big distortion of -50µm. When comb-shaped electrodes of 2µ m wide lines are fabricated on the film, the yield is 97% in spite of a considerably big distortion of -50µ m. All these embodiments exhibit a high yield more than 90 %. Namely, the embodiments are immune from the breakdown of lines. This is because the films have a smooth surface without convexes and concaves. Furthermore, all the embodiments leave no unpolished parts. 100% of the surface is perfectly polished for all the embodiments. This means that good SAW devices can be produced from the complex wafers of this invention with a high yield. Since the complex wafers allow to make narrow electrode patterns, various devices can be produced from the hard-material wafer of this invention by the technique of the photolithography, when the wafers are used as a substrate wafer of making semiconductor devices. Sample 9 ○, a comparison example, cannot be polished, because the substrate is cracked during the synthesis. Sample cannot be also polished, since it has been broken down during the synthesis. Sample which formed a 2 µm diamond film on a 8-inch (100) Si wafer has a superb smoothness just after the film formation. But sample has a poor smoothness of Rmax70nm and Ra25nm after polishing. 50 % of the surface is left unpolished. The poor property perhaps derives from the big concave-distortion of +200µ m. When the interdigital electrodes are formed on the polished part, the yield is only 3%. Namely, sample is unsuitable to form electrode patterns. This is caused by the concave-distortion and by the imperfection of the polishing. Sample which deposited a 30 µm c-BN film on a 3-inch GaAs wafer has no-distortion. Although the distortion is zero, it has a wavy deformation. Thus sample cannot be polished smooth owing to the wavy deformation. Sample suffers a poor post-polishing roughness of a Rmax100nm and Ra40nm. 10% of the surface is left unpolished. The yield is 15%, when an electrode pattern of 0.6 µ m width is produced on the polished part. This is beyond a practical utility. This poor yield results from the difficulty of polishing.

Claims: A hard material-coated wafer having a distorted substrate and a hard material film having a Vickers hardness of more than Hv3000 which has been produced on at least one surface of the substrate by a CVD method, characterized in that: the total thickness of the wafer is in the range from 0.1mm to 2.1mm; the diameter of the wafer is greater than 25mm; more than 50% of the film is polished to a roughness less than Rmax50nm and Ra20nm and; the wafer is distorted into a convex-shape on the film side with a distortion height (H) in the range - 150µm to -2µm. A hard material-coated wafer as claimed in claim 1, wherein the wafer is circular and is monotonously distorted from the center to the periphery in the convex shape on the film side. A hard material-coated wafer as claimed in claim 2, wherein the ratio (T 1 /T 2 ) of the film thickness (T 1 ) to the substrate thickness (T 2 ) is in the range from 0.05 to 1, the substrate thickness (T 2 ) is in the range 0.1mm to 2mm and the film thickness (T 1 ) is in the range 0.002mm to 0.2mm. A hard material-coated wafer as claimed in claim 3, wherein the hard material is one of diamond, diamond-like carbon and c-BN. A hard material-coated wafer as claimed in claim 4, wherein the substrate is a polycrystal or a single crystal of Si, GaAs, GaP, AlN, SiC, Si3N 4 , LiTaO 3 , LiNbO 3 , Li 2 B 4 O 7 , PbTiO 3 , PZT(PbZrO 3 -PbTiO 3 ) or quartz. A hard material-coated wafer as claimed in claim 5, wherein the substrate is a (100) Si single crystal. A hard material-coated wafer as claimed in claim 5, wherein more than 50% of the film from the center toward the periphery is polished to a roughness less than Rmax50nm and Ra20nm. A method of producing a hard material-coated wafer (2) comprising the steps of: forming a film of hard material on a substrate having a Vickers hardness of more than Hv 3000 which has been produced on at least one surface of the substrate by CVD method and polishing the surface of the films characterised in that the total thickness of the wafer is in the range 0.1mm to 2.1mm, the diameter of the wafer is greater than 25mm and the wafer is distorted into a convex shape on the film side with a distortion height in the range of -2µm to -15µm measured at the center from a plane including the circumference of the substrate; and the method includes fixing the substrate surface of the wafer (2) on a holder (3) of a polishing machine having a whetstone turn-table (1); rotating the holder (3) around its axis; revolving the turn-table (3) around its shaft; inclining the surface of the holder (3) relative to the turn-table (1); and polishing the convex surface of the film (2) either from the center to the periphery or from the periphery to the center so that more than 50% of the film is polished to a roughness less than Rmax50nm and Ra20nm. A method as claimed in claim 8, wherein the film is polished to a roughness less than Rmax50nm and Ra20nm.

Inventors: FUJIMORI NAOJI, IKEGAYA AKIHIKO, SEKI YUICHIRO, TANABE KEIICHIRO, FUJIMORI, NAOJI, IKEGAYA, AKIHIKO, SEKI, YUICHIRO, TANABE, KEIICHIRO, FUJIMORI, NAOJI, C/O ITAMI WORKS OF, IKEGAYA, AKIHIKO, C/O ITAMI WORKS OF, SEKI, YUICHIRO, C/O ITAMI WORKS OF, TANABE, KEIICHIRO, C/O ITAMI WORKS OF
Applicants: SUMITOMO ELECTRIC INDUSTRIES, SUMITOMO ELECTRIC INDUSTRIES, LTD.
Application year: 1995
Main classification: B24B37
IPC codes: B24B41, B24B37, C23C16, H01L21