A COATING FILM AND METHOD AND APPARATUS FOR PRODUCING THE SAME

Abstract: A method and apparatus for providing a substrate (3) with an amorphous carbon coating film having a high hard­ ness, good oxidation resistance, good thermal resistance, good electrical resistance and high thermal conductivity. The coating film is produced by sputtering a graphite target electrode (2) in an atmosphere of hydrogen and/or fluorine at a gas pressure of 6.665 to 666.5 Pa (0.05 to 5.0 Torr) while maintaining the current density for the graphite target elec­ trode (2) 11.3 and 14.7 mA/cm² thereby limiting the H/C ratio in terms of an atomic ratio between 0.5 and 0.9. The app­ aratus includes a coating forming deposition substrate (3) arranged at a position within a sputtering vacuum container (1) where it is not directly exposed to a plasma or it may be mounted on an electron drawing electrode (7) within the con­ tainer with a plasma adjusting electrode positioned in front of the substrate.

Description: The present invention relates to a coating film of amorphous carbon and a method and apparatus for producing the same. Such a film generally has a high hardness, high oxidation resistance, high thermal resistance, high electrical resistance and high thermal conductivity. It is well known that plastics materials and common metals are susceptible to surface scratch so that their surfaces, while initially having excellent luster, gradually lose their luster which damages their appearance. This has been considered to be due to the fact that these plastics material and metals are relatively soft and have low abrasion resistance. Known means for overcoming these disadvantages include the application of a hard thin-film coating to the surface of the material having the soft surface, such as a cutting tool, bite etc. to provide a durable and lustrous surface, and the application industrially of an extremely hard film to the surface to make these materials durable for a long period of time. Recently, studies have been made into the application of coatings of silicon carbide and boron nitride to the surfaces of bite to provide improved surface materials. However, these methods still leave some disadvantages to be overcome. Also, in the case of a material which when subjected to heat or oxygen will decompose or react so changing its properties, and also protective coating films such as an antioxidation coating film applied to the surface of electronic equipment devices (e.g., GaAs semiconductor, ZnS EL device and HgCdTe infrared sensor element) which are required to operate stably for a long period of time, it is essential to have recourse to a material and method which permit the application of the surface coating at relatively low temperatures. In the case of a solar cell comprising amorphous silicon, etc., which contains a large amount of hydrogen or a large amount of hydrogen and fluorine, the cell essentially has a low energy converting efficiency and this is compensated for by means of an increased surface area or the like. However, there still exists a need for the development of materials of higher efficiencies for the surface coating films in view of the necessity to ensure a sufficient amount of incident light to the solar cell. Unfortunately, the abrasion resistant coatings which have been used up to now are insufficient from the standpoint of ensuring a sufficient durability for component parts subjected to severe wear, e.g., the bearings of a rotary member, and gears, and there is a need for coatings of longer durability. In the case of semiconductors, the conventional silicon semiconductors are subject to the limitation that the forbidden band gap may be as narrow as 1.21 eV and they cannot be used at high temperatures, thus giving rise to a need for the development of a material having wider range of working temperatures. The conventional sputtering methods include one in which an argon plasma is produced by applying a voltage across an anode and a cathode in argon gas at a pressure of about 0.133 to 1.33 Pa (10⁻³ to 10⁻² Torr) and a target material is sputtered by the argon ion thereby forming an alloy coating on the substrate. An example of the sputtering apparatus used with the above-mentioned conventional technique is disclosed for example in U.S. Patent NO. 4431562. It has been found that when forming a high-hardness amorphous carbon film of high electrical insulating quality by a sputtering process, increasing the input power is effective in preventing the occurrence of defects in the coating film, and tends to increase the rate of film formation and so on. It is an object of the present invention to provide a coating film of a material which is suited for use in the above-mentioned applications, has high hardness, high oxidation resistance, high thermal resistance, high abrasion resistance, excellent thermal conductivity and high electric resistance. It is a further object to provide a method for its production and an apparatus capable of eliminating the occurrence of defects in coating, thereby producing a homogenous coating film. According to one aspect of the invention, there is provided a method of producing an amorphous carbon coating film on a substrate which comprises sputtering a carbonaceous e.g. graphite target electrode in a vacuum chamber characterised in that the current density between the target cathode and a corresponding anode is maintained at a value between 11.3 and 14.7 mA/cm², the cathode and anode being arranged to create a plasma state region (A) between them, to which region the substrate is not directly exposed, and the substrate is maintained at between room temperature and 100°C, the chamber containing hydrogen and/or fluorine gas at a pressure of between 6.665 and 666.5 Pa (0.05 and 5.0 Torr), thereby producing an amorphous carbon coating film in which the value of H/C or (H + F)/C in terms of an atomic ratio is between 0.5 and 0.9, the current density being defined as the current through the anode per unit area of the target electrode. According to another aspect of the invention, there is provided an amorphous carbon coating film on a substrate, produced by sputtering a carbonaceous, eg graphite, target cathode characterised in that the current density between the target cathode and a corresponding anode is maintained at a value between 11.3 and 14.7 mA/cm², the cathode and anode being arranged to create a plasma state region (A) between them, to which region the substrate is not directly exposed, and the substrate is maintained at between room temperature and 100°C, the chamber containing hydrogen and/or fluorine gas at a pressure of between 6.665 and 666.5 Pa (0.05 and 5.0 Torr), whereby in the carbon coating the value of H/C or (H+F)/C in terms of an atomic ratio is between 0.5 and 0.9, the current density being defined as the current through the anode per unit area of the target cathode. According to another aspect of the invention, there is provided a sputtering apparatus for producing an amorphous carbon coating film on a substrate, the apparatus comprising a vacuum chamber having an atmosphere gas inlet pipe, an exhaust pipe, an electron drawing anode, a carbonaceous, eg graphite, target electrode connected to a power source and positioned opposite to the anode, and means for producing a magnetic field to provide a directionality for a plasma, characterised by means for mounting a substrate on an inner wall of the vacuum chamber in a region (C) which during sputtering is not directly exposed to the plasma, and means for maintaining the substrate at between room temperature and 100°C, and being further characterised in that the power source is a high frequency source. Preferably, a further electrode is arranged between the anode and the target electrode which, in use, is maintained at ground potential or is connected to another power source, the further electrode having a large number of holes for adjusting the amount of the plasma. The present invention has arisen from various studies and investigations made in view of the foregoing disadvantages of the conventional methods, in order to provide an amorphous carbon coating film having a high hardness, high oxidation resistance, high abrasion resistance, high thermal resistance, high thermal conductivity and high electric resistance properties. It has resulted in a production method and an apparatus capable of preventing for example various difficulties assumed to be due to the fact that a substrate is placed in a plasma and the production of a coating film of a material having excellent uniformity. This has led to the resulting discovery that an amorphous carbon coating film can be formed having excellent properties by effecting a sputtering operation including various specific parameters over and above a conventional sputtering method. From the results of the various studies and investigations it has been discovered that the value of the H/C ratio can be reduced by effecting the sputtering operation with an increased gas pressure, that decreasing the H/C ratio of the coating material to 0.5 or less has the effect of reducing the hardness of the coating film thereby making it impractical, and that increasing the H/C ratio to exceed 0.9 is not desirable since the optical band gap or the electric resistance is decreased and hence the insulating capability is deteriorated. In view of this fact, the H/C ratio of the coating film of material is in a range between 0.5 and 0.9. Also, it has been confirmed that the gas pressure is in a range between 6.665 Pa and 666.5 Pa (0.5 to 5 Torr) during the sputtering. It has been found that decreasing the plate current from the power source during the sputtering operation generally tends to increase the hardness of a coating film and it has been found that with a high frequency power source, for example, hardnesses (Vickers) of 2,000 to 1,500 are obtainable with relative current densities of 11.3 mA/cm² and over. Considering this fact, the range of working relative current densities should be selected to be 11.3 to 14.7 mA/cm². A coating film of material produced in accordance with the present invention has a very high hardness, high thermal resistance and high electric resistance, is high in infrared ray transparency and is excellent in chemical resistance, moisture resistance and scratch resistance. More specifically, the coating may enjoy the following effects. (1) The coating film produced in accordance with the present invention has a maximum Vickers hardness of 4,000 which is second to that of diamond and therefore the application of this coating to a metal or plastic not only ensures a long durable lustrous surface which is not susceptible to any scratch but also makes possible the application of the coating to cutting bite, etc., which are used under severe conditions. (2) Since the coating film can be produced at room temperature despite the fact that its electric resistance is on the 10¹⁴ Ω cm level which is close to those of insulators, that it is extremely uniform (compact) and that it is excellent in such environmental resistance properties as chemical resistance, moisture resistance and thermal resistance the coating film is best suited for example as the coating for GaAs semiconductor, HgCdTe element and material susceptible to the effect of heat and oxygen. (3) Since the coating film is transparent in the infrared region, has an excellent infrared transmission factor and has a refractive index of about 1.8 to 2.0 which is substantially equal in value to the square root of the refractive index (n=3.5) of silicon, the coating is well suited for use as a reflection preventive coating film for solar cell (4) Since the coating film can be uniformly applied to materials of all shapes, it is possible to expect the provision of abrasion resistance by the application of the coating to bearings, sliding portions of common machine elements, etc. (5) For example, where the coating film is used to produce an amorphous carbon F type solar cell, there is no danger of any deterioration in the electrical resistance, hardness and optical transparency even if it is used in higher temperature environments due to its improved high thermal resistance. (6) By adjusting the type of gas atmosphere during the sputtering period, it is possible to alternately deposit conducting film and insulating film thereby forming a laminated thin-film capacitor. (7) Where such a technique is utilized to form conducting and insulating films, it is possible to easily produce a hybrid thin-film circuit by using suitable masks and forming any given number of layers and in such a case the insulating film portion can be converted to conducting film through the radiation of a laser beam or electron beam thereby ensuring a wide range of application operations. (8) By adjusting and changing a doped target or atmosphere gas, it is possible to produce such semiconductor devices which are not only wide in energy gap between valence band and conduction band but also high in surface hardness. (9) Due to the arrangement of a deposition substrate in a position which is not exposed to a plasma, the film forming rate of an amorphous carbon film is increased thus making possible the formation of a film which is homogeneous and reduced in dangling bonding. (10) By arranging substrates on the wall portions of the vacuum chamber, it is possible to increase the usable area than previously and therefore it is possible to do much toward decreasing the cost. (11) By providing a mesh electrode between the target electrode and the substrate electrode and maintaining the mesh electrode at ground potential, it is possible to protect the substrate from the plasma and produce a coating film having a reduced spin density, increased electric resistance and excellent optical transparency. (12) Where the mesh electrode is used, it is possible to adjust the rate of film formation by suitably adjusting the distance between the target electrode and the substrate electrode and the distance between the target electrode and the grounded electrode. The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram of a planar magnetron type sputtering apparatus in accordance with the present invention; Fig. 2 is a schematic sectional view of a conventional sputtering apparatus; Fig. 3 is a graph showing the effect of atmosphere gas pressure on coating film of material; Fig. 4 is a graph showing the effect of applied current on the hardness of coating film; Fig. 5 is a sectional view showing in somewhat more detail a sputtering apparatus in accordance with the present invention; and Fig. 6 is a schematic diagram useful for explaining an arrangement in which a grounded electrode is arranged between electrodes. The function of a sputtering apparatus for producing such thin-film coating of material which is smooth and has no surface defects will be described with reference to the schematic diagram shown in Fig. 1. In the Figure, numeral 1 designates a vacuum chamber, 4 an atmosphere gas inlet pipe, and 5 an exhaust pipe connected directly to a vacuum pump which is not shown to maintain a vacuum. Numeral 7 designates an electron drawing counter electrode at the ground potential, and 2 a carbonaceous electrode connected to a high frequency (RF) power source 6 through a matching box 8. Numeral 3 designates deposition substrates arranged inside the container but outside the transport area of a plasma-excited source in contrast to conventional techniques. It is to be noted that hydrogen gas or any other gas is introduced through the atmosphere gas inlet pipe 4. In accordance with the present invention, the interior of the vacuum chamber is considered divided into three regions, i.e. A region, B region and C region as schematically shown in the Figure 1. The A region, represents the area enclosed by curved lines a which is the plasma state environment produced between the electrodes and the vicinity around the electrodes; carbon and hydrocarbon radicals consisting two carbon atoms or less such as C, CH, CH₂, CH₃, CH₄, C₂H₄ and C₂H₆ are generated from the target electrode 2 in this region. The B region represents the area enclosed by a curved line b outside the electrodes 2 and 7 and in this region the above-mentioned particles contained in the plasma are transported at a velocity which is determined by the pressure of the atmosphere gas and the voltage across the electrodes. The C region represents the area other than the A region and the B region and in this region the transported particles are softly deposited on the deposition substrates arranged on the container inner walls. It is to be noted that in the actual operation, charged particles (constituting the majority of the transported particles) are susceptible to the effects of an electric field or the like, therefore it is necessary to take this into consideration so that the C region is maintained at a uniform potential, e.g., around ground potential. In the sputtering apparatus so constructed, during the operation of the sputtering the substrates are never directly exposed to the plasma produced between the target electrode and the counter electrode so that the deposition and bombardment by protons and electrons do not occur simultaneously at the substrate. Therefore there is no danger of causing any defects (depressions) within or in the surface of coating films deposited on the substrates. Also, in these circumstances the rate of film formation can be increased. The following examples describe the present invention in more detail. Example 1. The sputtering was effected by a sputtering apparatus of the type shown in Figure 5. In the Figure, the same reference numerals as used in Figure 1 represent the same or equivalent components. The vacuum chamber 1 is composed of two covers and a cylinder having three flanges which are sealed with O-rings. Numeral 10 designates a metal upper cover sealed with an O-ring. Numeral 12 designates a lower cover sealed by a sealing method which allows it to perform the same function as the upper cover 10. It is constructed so that a hermetic sealing can be provided by means of welding or the like, ignoring a specially provided opening 5. Numeral 13 designates insulating terminals or supports, 14 electrically grounded receptors, 3 deposition substrates made of an insulating material or metal, and 15 substrate supporting members. It will be apparent that while, in this embodiment, the deposition substrates 3 are arranged in positions on the upper and side walls of the chamber which are external to the plasma generating section and the travel path of the plasma, they are not necessarily limited to these positions since it is only necessary that they are located outside the plasma excited source which is indicated as the C region in Fig. 1. Numeral 2 designates a carbonaceous target, 16 an electrode box, 17 a magnetron, 18 a cooling water supplying metal pipe, and 19 a drain pipe. The end of the cooling water pipe 18 outside the vacuum chamber is sealed watertight by welding. Numeral 20 designates a thermocouple. Numeral 7 designates a counter electrode for the target 2 and the target electrode 2 includes a connector which is provided outside the vacuum chamber 1 for connection with a high frequency power source. In addition, the matching box and the high frequency power source 6 shown in Fig. 1 are connected to the connector, although they are not shown. Numeral 4 designates an atmosphere gas inlet pipe connected to the vacuum chamber 1 so as to introduce an atmosphere gas consisting of hydrogen or hydrogen and fluorine. An exhaust pipe 5 is arranged to reduce the internal pressure of the vacuum chamber 1 through a vacuum pump which is not shown. The following table shows the results of a sputtering operating effected for 2 hours by applying a high frequency power of 13.56 MHz and using high-purity hydrogen of 99.999 vol. % as the atmosphere gas in a sputtering apparatus employing the above-mentioned deposition substrate arrangement and a sputtering apparatus having the substrate arranged in the counter electrode position for purposes of comparison. In that case, the sputtering conditions were as follows. i Distance between target and counter electrode 35mm ii Hydrogen pressure 26.7 Pa (0.2 Torr) iii Supply voltage 2.3 kV iv Supply current 0.55 A v Relative current density 12.46 mA/cm² vi Deposition substrate temperature 100 o C It is to be noted that while the mesh electrode may literally be in the form of a mesh, it may also be a metal sheet formed with an innumerable number of pin holes so as to obtain the same effect as the mesh. Also, since the essential point is the adjustment of the amount of plasma, the invention is not limited to the method employing a grounded mesh and the same object can be accomplished by for example applying the potential supplied from a separate power supply to the plasma adjusting electrode. Further, where such a mesh is employed, the amount of plasma may be adjusted by for example sliding the mesh to change its position as shown by the arrows in Fig. 6. Example 2. Example 1 was repeated except that a fluorine gas or a mixed gas of hydrogen and fluorine was used in place of the hydrogen gas. As a result, the thermal resistance (the temperature at which the optical gap starts decreasing) attained by the amorphous carbon film was improved by the use of the mixed gas to from 300°C to 500°C. Also the use of carbon fluoride for the target improved the thermal resistance to 500°C.

Claims: A method of producing an amorphous carbon coating film on a substrate (3) which comprises sputtering a carbonaceous, e.g. graphite, target cathode (2) in a vacuum chamber (1) characterised in that the current density between the target cathode and a corresponding anode (7) is maintained at a value between 11.3 and 14.7 mA/cm², the cathode and anode being arranged to create a plasma state region (A) between them, to which region the substrate is not directly exposed, and the substrate is maintained at between room temperature and 100 o C, the chamber containing hydrogen and/or fluorine gas at a pressure of between 6.665 and 666.5 Pa (0.05 and 5.0 Torr), thereby producing an amorphous carbon coating film in which the value of H/C or (H + F)/C in terms of an atomic ratio is between 0.5 and 0.9, the current density being defined as the current through the anode per unit area of the target electrode. A method as claimed in claim 1 characterised in that the substrate is located on a wall of the vacuum chamber. A method as claimed in claim 1 or 2 characterised by doping the target electrode with a semiconducting material. A method as claimed in claim 3 characterised in that the semiconducting material is boron or phosphorous. A method as claimed in any preceding claim characterised in that the gas in the chamber further includes a semiconducting gas. A method as claimed in claim 5, characterised in that the semiconducting gas is nitrogen, and/or a gas containing boron and/or a gas containing phosphorous. An amorphous carbon coating film on a substrate (3) produced by sputtering a carbonaceous, e.g. graphite, target cathode (2) characterised in that the current density between the target cathode and a corresponding anode (7) is maintained at a value between 11.3 and 14.7 mA/cm², the cathode and anode being arranged to create a plasma state region (A) between them, to which region the substrate is not directly exposed, and the substrate is maintained at between room temperature and 100 o C, the chamber containing hydrogen and/or fluorine gas at a pressure of between 6.665 and 666.5 Pa (0.05 and 5.0 Torr), whereby in the carbon coating the value of H/C or (H + F)/C in terms of an atomic ratio is between 0.5 and 0.9, the current density being defined as the current through the anode per unit area of the target electrode. A sputtering apparatus for producing an amorphous carbon coating film on a substrate (3), the apparatus comprising a vacuum chamber (1) having an atmosphere gas inlet pipe (4), an exhaust pipe (5), an electron drawing anode (7), a carbonaceous, e.g. graphite, target electrode (2) connected to a power source (6) and positioned opposite to the anode, and means (17) for producing a magnetic field to provide a directionality for a plasma, characterised by means for mounting a substrate on an inner wall of the vacuum chamber in a region (C) which during sputtering is not directly exposed to the plasma, and means for maintaining the substrate at between room temperature and 100°C, and being further characterised in that the power source is a high frequency source (6). A sputtering apparatus as claimed in claim 8 in which a further electrode (22) is arranged between the anode and the target electrode which, in use, is maintained at ground potential or is connected to another power source, the further electrode having a large number of holes for adjusting the amount of the plasma. Apparatus as claimed in claim 8 or claim 9 characterised in that the anode is at ground potential. Apparatus as claimed in claim 9 or claim 10 characterised in that the further electrode is a mesh electrode.

Inventors: HAYASHI, MASAO, HIRAKI, AKIO, MIYASATO, TATSURO
Applicants: KABUSHIKI KAISHA MEIDENSHA
Application year: 1985
Main classification: C23C14
IPC codes: C23C14, H01J37