Plasma CVD apparatus and method therefor

Abstract: A plasma CVD apparatus for continuously forming a diamond-like film on a length of magnetic tape (8), which comprises a first vacuum vessel (2) including a plasma generating tube for transforming a gaseous medium (11), introduced into the first vacuum vessel, into a plasma and an electrode device (7) for accelerating ions contained in the plasma to impinge upon a substrate, and a second vacuum vessel (1) fluid-connected with the first vacuum vessel to allow a flow of the gaseous medium therebetween. The first vacuum vessel has a gas flow opening (9) which cooperates with the substrate, disposed inside the second vacuum vessel, to define a flow resistance by which the difference in pressure between the first and second vacuum vessels (1,2) is determined.

Description: BACKGROUND OF THE INVENTION The present invention relates to an apparatus for, and a method of, making a hard carbon film, or a so-called diamond-like carbon film, having various physical properties comparable to those of a diamond. (Description of the Prior Art) Since it has been discovered that a diamond could be synthesized by means of a low-pressure vapor deposition process, numerous reports have been made on various methods for the formation of diamond films and the synthesis of diamond-like carbon films, an example of which include that made by H. Vora and T.J. Moravec (J. Appl. Phys. 52 6151, 1981). Any of those prior art methods comprises the use of a gaseous hydrocarbon such as, for example, CH 4 or C 2 H 6 which is transformed into a plasma so that radicals and/or ions contained in the plasma can be utilized to synthesize a diamond film or a diamond-like carbon film. In recent years, various improved systems and apparatuses have been suggested so that the diamond film or the diamond-like carbon film can be applied in various devices. The inventors have suggested in, for example, U.S. Patent No. 4,645,977, a plasma injection CVD (chemical vapor deposition) method effective to synthesize the diamond-like carbon film on a film-like substrate continuously at a high speed. The plasma injection CVD apparatus used therefor is schematically illustrated in Fig. 9. As shown therein, the apparatus comprises a plasma tube 25, equipped with a plasma generating means, and a vacuum vessel 27 for accommodating a substrate 26. The plasma tube 25 and the vacuum vessel 27 are communicated with each other so that a gaseous medium introduced into the plasma tube can flow into the vacuum vessel 27 through a nozzle 28. The gaseous medium introduced into the plasma tube 25 is transformed into a plasma by means of an RF power applied to an RF coil 30, and the plasma is then blown onto the substrate 26 by the effect of a pressure difference between the plasma tube 25 and the vacuum vessel 27. At this time, ions contained in the plasma are accelerated by the difference in potential between an accelerator electrode 29, installed inside the plasma tube 25, and the substrate 26 to impinge on the substrate 26. Thus, the blow of the plasma of the hydrocarbon gas containing the accelerated ions makes it possible to maximize the use of the ions and radicals in forming the diamond-like carbon film at a high speed. EP-A-0 523 609, published after the priority date of the present application but claiming an earlier priority, and EP-A-0 411 435 disclose plasma CVD apparatuses and plasma CVD methods to form films on magnetic tapes. However, the plasma injection CVD method has been found having the following problem. The speed at which the diamond-like carbon film is synthesized depends on the number of the ions and radicals that reach the substrate. On the other hand, the number of the ions and radicals is determined the pressure of the plasma tube, the type of the gaseous medium used and the electric power invested. Therefore, in order to increase the speed at which the diamond-like carbon film is synthesized, the plasma tube should be evacuated to a pressure as low a vacuum as possible. In addition, since the plasma injection CVD method makes use of the difference in pressure between the plasma tube and the vacuum vessel to blow the radicals of the plasma, this pressure difference should be as large as possible in order to increase the speed at which the diamond-like carbon film is synthesized. However, since the difference in pressure between the plasma tube and the vacuum vessel is limited by the flow resistance imposed by the nozzle, in order for the pressure difference between the plasma tube and the vacuum vessel to be increased while the pressure inside the plasma tube is maintained at the low vacuum, the flow resistance imposed by the nozzle must be increased. Because of this, the opening of the nozzle must be as small as possible and this is limited due to the incompatibility between the increase in speed at which the diamond-like carbon film is synthesized and a large surface area to which the diamond-like carbon film is to be synthesized. Also, according to the plasma injection CVD method, in order to accelerate the ions, a mesh-like electrode is installed within the plasma tube and a positive potential is applied to the electrode. At this time, the plasma exhibits a potential substantially equal to that developed at the mesh-like electrode and the plasma ions are accelerated by the effect of a potential difference between the plasma and the substrate. When the plasma is blown from the nozzle onto the substrate, portion of the plasma diffuses into the vacuum vessel and this may often constitute a cause of an abnormal discharge. The lower the vacuum exhibited by the pressure inside the plasma tube, the greater the amount of the plasma diffused into the plasma tube and, hence, the more often does the abnormal discharge occur. Since the plasma injection CVD method is a system of blowing the plasma onto the substrate, the uniformity of pressure at a film forming portion is limited, and it has been found that the uniformity of the film thickness was limited to ±5% at an area corresponding to the cross-section of the nozzle. Because of this, where application is made to a device in which the uniformity is an important factor (for example, to a protective film in a magnetic tape), a further improvement in uniformity of the film thickness is longed for. SUMMARY OF THE INVENTION The present invention has been devised to substantially eliminate the above discussed problems and is intended to provide an improved plasma CVD apparatus capable of synthesizing a diamond-like carbon film of uniform film thickness and of a relatively large surface area at a high speed and which operates at a stabilized fashion with a minimized possibility of an abnormal discharge. To this end, the present invention provides a plasma CVD apparatus according to claim 1 and a method according to claim 6. Preferred embodiments are claimed in claims 2 to 5 and 7 to 10, respectively. The gas outflow port is so shaped as to conform to the shape of the surface of the substrate, while the second vacuum vessel is accommodated within the first vacuum vessel with the gas outflow port oriented towards and spaced apart from the surface of the substrate with a gap formed therebetween. The gap is chosen to be of a small size enough to permit the pressure of the plasma, discharged from the gas outflow port into the gap, to be uniform at any location within the gap. The apparatus also comprises electric field generating means for directing ions of the plasma, discharged from the gas outflow port into the gap, towards the substrate. The present invention also provides a method of forming, by the use of a plasma CVD technique, a film on a surface of a substrate within a vacuum vessel. This method comprises supplying a higher pressure plasma into the vacuum vessel than the pressure inside the vacuum vessel; developing an electric field acting on the plasma gas within the gap for directing ions of the plasma towards the substrate; and forming the film made of elements contained in the plasma. The plasma supply is carried out by bringing a plasma gas outflow port of a shape, conforming to the shape of the surface of the substrate, in close proximity of the surface of the substrate while leaving a sufficiently small gap left therebetween such that, within an area inwardly delimited by a perimeter of the gap, a higher pressure condition of the plasma gas by at least one figure than the pressure inside the vacuum vessel is created substantially uniformly. According to the present invention, the surface of the substrate on which the film is desired to be formed and the gas outflow port of a shape conforming to the surface of the substrate are spaced a small distance. Since the gas outflow port has a shape conforming to the surface of the substrate, the gap therebetween has a uniform size at any location within the gas outflow port. Since the substrate surface and the gas outflow port are brought close towards each other as discussed above, the amount of the plasma emerging outwardly from the gas outflow port can be controlled to a properly selected small value as compared with the amount of the plasma generated. Therefore, within the gap, a sufficient amount of the plasma gas is accumulated as compared with the amount of the plasma emerging outwardly from the gas outflow port and the plasma gas pressure is increased. In other words, at the time of supply of the plasma towards the substrate surface, a condition can be maintained n which the proper amount of the plasma is accumulated in the gap and, therefore, the stabilized supply of the plasma gas towards the substrate surface can be accomplished and, at the same time, within an area inwardly delimited by the perimeter of the gap, a higher pressure condition of the plasma gas by at least one figure than the pressure inside the vacuum vessel is created substantially uniformly. Therefore, film forming condition required to form the film on the substrate surface can be stabilized. Also, because the rate of flow of the plasma from the plasma outflow port is so controlled as described above, the diffusion of the plasma is advantageously minimized thereby minimizing the possibility of occurrence of the abnormal discharge which would otherwise occur as a result of the diffusion of the plasma. Where the gas pressure of the plasma is chosen to be higher by one or more figures than the pressure inside the vacuum vessel and the pressure inside the vacuum vessel is chosen to be of a value lower than that at which the discharge can be maintained, any undesirable discharge within the vacuum vessel can be avoided. Where a diamond-like carbon film is to be formed with the use of the above discussed plasma CVD method, the radicals of the plasma increase and, therefore, the film quality tends to be deteriorated. While the diamond-like carbon film is generally regarded as a structure in which diamond bonds (SP3 bonds) and graphite bonds (SP2 bonds) are mixed, and the greater the number of the SP3 bonds, the more similar is the characteristic to that of a diamond. Since under ordinary environments the SP2 bonds are rather stable, energies are required to render carbons contained in the plasma to represent the SP3 bonds. In the present invention, an electric field is provided in which the ions of the plasma can be accelerated to increase the energies of ion bombardment thereby to increase a proportion of the SP3 bond for the purpose of providing the diamond-like carbon film of high quality. BRIEF DESCRIPTION OF THE DRAWINGS This and other objects and features of the present invention will become clear from the following description taken in conjunction with preferred embodiments thereof with reference to the accompanying drawings, in which like parts are designated by like reference numerals and in which: Fig. 1 is a schematic diagram showing a plasma CVD apparatus according to one preferred embodiment of the present invention; Fig. 2 is a schematic diagram showing a modified form of the plasma CVD apparatus shown in Fig. 1; Fig. 3 is a schematic diagram showing the plasma CVD apparatus according to a second preferred embodiment of the present invention; Fig. 4 is a schematic diagram showing a modified form of the plasma CVD apparatus shown in Fig. 3; Fig. 5 is a schematic diagram showing another modified form of the plasma CVD apparatus shown in Fig. 3; Fig. 6 is a schematic diagram showing a further modified form of the plasma CVD apparatus shown in Fig. 3; Fig. 7 is a schematic diagram showing the plasma CVD apparatus according to a third preferred embodiment of the present invention; Fig. 8 is a schematic diagram showing a modified form of the plasma CVD apparatus shown in Fig. 7; and Fig. 9 is a schematic diagram showing the prior art plasma CVD apparatus. DETAILED DESCRIPTION OF THE EMBODIMENTS Referring to Fig. 1, the present invention is shown as applied to the formation of a diamond-like carbon film on a magnetic tape and, for this purpose, there is provided a magnetic tape transport system for transporting a continuous length of magnetic tape to a thin film forming unit where a diamond-like film is formed over a magnetic layer of the magnetic tape. Specifically, a first vacuum vessel 1 has a tape transport system accommodated therein and including a supply roller 5 supporting a roll of magnetic tape 8, a can roller 3 adapted to be driven at a peripheral speed substantially equal to the speed of transport of the magnetic tape 8 and around which the magnetic tape 8 drawn from the supply roller 3 is guided, a take-up roller 4 for winding up the magnetic tape 8 having been guided by the can roller 3, and a tensioning roller 6 for imparting a proper tension to the magnetic tape 8 while the latter is guided around the can roller 3. The length of magnetic tape 8 is drawn from a supply roller 5 and is wound around a take-up roller 4 after having travelled around the can roller 3. As a matter of course, the can roller 3 is ordinarily cooled by a known cooling means such as, for example, a water cooling system, to avoid an increase of temperature thereof during an operation of the apparatus. As shown in Fig. 1, the tensioning roller 6 which is electrically grounded contacts the magnetic layer of the magnetic tape 8 to connect the magnetic layer of the magnetic tape 8 electrically to the ground. The other component parts of the tape transport system including the can roller 3 are electrically insulated. In the illustrated embodiment, the can roller 3 has a diameter of about 800 mm and a length of about 280 mm as measured in a direction parallel to the axis of rotation of the can roller 3, that is, in a direction perpendicular to the sheet of Fig. 1. A second vacuum vessel 2 (hereinafter referred to as a plasma generating tube because plasma necessary to form a diamond-like thin film is generated in this vessel) is positioned immediately beneath the can roller 3 and has a gas outflow port 9 defined therein. The gas outflow port 9 in the illustrated embodiment has a width of about 600 mm as measured in a direction conforming to the curvature of the can roller 3 and a length of about 250 mm as measured in a direction parallel to the axis of rotation of the can roller 3. As will be described later, the plasma generating tube 2 is so positioned that the distance of separation between the outer peripheral surface of the can roller 3 and the gas outflow port 9 is uniform over the entire surface area of the gas outflow port 9. The plasma generating tube 2 is supported on a positioning mechanism comprising a motor 12, gears 13, a movable platform 14, a support framework 15, which mechanism is so designed as to adjust the distance of separation between the peripheral surface of the can roller 3 and the gas outflow port 9 of the plasma generating tube 2. The plasma generating tube 2 has an electrode 7 installed therein and connected with an electric power source 10 to produce a potential difference between the electrode 7 and the magnetic layer of the magnetic tape 8. As a raw gaseous material, C 6 H 6 gas is used and is introduced into the plasma generating tube 2 through a gas supply line 11. At this time, the distance of separation between the gas outflow port 9 and the peripheral surface of the can roller 3 and the flow rate of the C 6 H 6 gas are adjusted so that the pressure difference between the plasma generating tube 2 and the vacuum vessel 1 can attain a value in the order of more than a single figure. If this pressure difference is insufficient, not only is an uneven pressure developed at the gas outflow port 9, but also a portion of the plasma generated within the plasma generating tube 2 flows into the vacuum vessel 1, thereby to constitute a cause of a film formation at an unwanted area and also a cause of an abnormal discharge. After the pressure inside the plasma generating tube 2 has been stabilized, an electric potential required to cause the electrode 7 to have a positive potential is applied between the electrode 7 and the magnetic layer of the magnetic tape 8 to cause the plasma generating tube 2 to generate the plasma. Ions contained in the plasma so generated are accelerated by the effect of the potential difference between the electrode 7 and the magnetic layer of the magnetic tape 8 to impinge on the magnetic layer of the magnetic tape 8 thereby to form a diamond-like carbon film together with radicals contained in the plasma. One example of film forming conditions is shown in Table 1. In Table 1, the current density in the magnetic layer refers to the amount of ions flowing into that portion of the magnetic layer of the magnetic tape 8 which is then situated within a gap between the gas outflow port 9 and the outer peripheral surface of the can roller 3. Pressure inside Tube 2 10 to 15 Pa Pressure inside Vessel 1 0.1 Pa C 6 H 6 Flow Rate 12 SCCM Potential Difference between 6 and 7 3 kV Current Density in Magnetic Layer 0.1 mA/cm 2 As a result of the film formation under the above tabulated conditions, over the entire area of the gas outflow port 9 (250 mm in length and 600 mm in width), a film of 3,000 kg/mm 2 in Vickers hardness could be synthesized at a speed higher than 300 nm per minute. Consequently, it has become possible to form the diamond-like carbon film of 10 nm in thickness uniformly (with ±2% in variation in film thickness) over a surface of the length of magnetic tape 8 transported at a speed of 18 m per minute. An important aspect of the present invention lies in that the pressure of the plasma generated from the plasma generating tube 2 and flowing into the first vacuum vessel 1 through the gas outflow port 9 is maintained at a uniform value at any local portion of the gap between the peripheral surface of the can roller 3 and the gas outflow port 9. This can be accomplished by the utilization of the positioning mechanism comprising the motor 12 to adjust the gap between the peripheral surface of the can roller 3 and the gas outflow port 9. If the gap between the gas outflow port 9 and the peripheral surface of the can roller 3 is too large as compared with the flow rate of the gas discharged from the gas outflow port 9, the flow of the plasma emerging outwardly from the gas outflow port 9 will become unstable, making it difficult to achieve a uniform pressure over the entire area of the gap. Accordingly, in order to achieve the uniform pressure over the entire area of the gap, it is necessary to set the distance of separation between the peripheral surface of the can roller 3 and the gas outflow port 9 to a relatively small value. By selecting the distance of separation to be of the small value, the gas flow from the plasma generating tube 2 can be properly controlled to a small value as compared with the amount of the plasma produced within the plasma generating tube 2 and, consequently, not only can a sufficient amount of plasmas be accumulated within the plasma generating tube as compared with the amount of the plasma discharged outwardly from the gas outflow port 9, but the gas pressure of the plasma can also be increased. Therefore, while the proper amount of the plasmas is accumulated within the plasma generating tube 2, the amount of the plasmas produced and the amount of the plasma discharged are balanced with each other, making it possible to accomplish a stabilized supply of the plasma gas to the gap while the gas pressure at the gap is kept uniform. So far as the illustrated embodiment is concerned, the distance of separation is deemed optimum if it is about 0.3 mm, but not exceeding 1 mm. Since in the illustrated embodiment the radius of curvature of the gas outflow port 9 and the curvature of the peripheral surface of the can roller 3 are identical with each other, the distance of separation is not uniform if strictly measured in a direction lengthwise of the can roller 3. However, in terms of the purpose of equalizing the pressure of the plasma gas at any local positions within the gap between the peripheral surface of the can roller 3 and the gas outflow port 9, the distance of separation of about 0.3 mm may be regarded as a uniform gap size. As hereinabove described, the foregoing embodiment of the present invention is effective to form the diamond-like film of uniform film thickness at a high speed. The diamond-like film formed on the surface of the magnetic layer has shown no problem in bondability thereof with the magnetic layer, and has shown an excellent wear resistance, proving that the resultant diamond-like film could be used as a sufficient and effective protective layer. It is to be noted that the magnetic layer of the magnetic tape 8 has been electrically grounded through the tensioning roller 6. However, it may not be held at a ground potential, but it may be connected with a direct current source 16 such as shown in Fig. 2. In the modification shown in Fig. 2, since the potential difference is produced between the magnetic layer and the can roller 3 (ground potential), this potential difference assists the magnetic tape 8 to stick stubbornly stick to the can roller 3. While the can roller 3 is normally water-cooled to avoid an increase of temperature during the film formation and to prevent the magnetic tape 8 from being thermally damaged and an emission of gases therefrom, the application of an electric potential to the magnetic layer to cause the magnetic tape 8 to stick to the can roller 3 is effective to stabilize the film forming conditions. Also, while in the embodiment shown in and described with reference to Fig. 1 a ratio between the pressure inside the first vacuum vessel and that inside the plasma generating tube has been chosen to be in the order of double figures, the ratio of pressure in the order of a single figure is sufficient to accomplish the formation of the diamond-like carbon film at a high speed as compared with the prior art method with the resultant diamond-like film being advantageously usable as a protective layer. Fig. 3 illustrates the plasma CVD apparatus according to a second preferred embodiment of the present invention. While in the foregoing embodiment a direct current power source 17 is employed as means for generating the plasma, the plasma CVD apparatus according to the second embodiment shown in Fig. 3 employs an alternating current of 13.56 kHz in frequency. In this embodiment, the magnetic layer of the magnetic tape is applied with a negative potential from the direct current power source 17 through the bias roller 6. One example of film forming conditions is shown in Table 2 and, as shown in Table 2, the ratio between the pressure inside the plasma generating tube 2 and that inside the vacuum vessel 1 is in the order of double figures. Pressure inside Tube 2 10 Pa Pressure inside Vessel 1 0.1 Pa C 6 H 6 Flow Rate 8 SCCM Potential of Magnetic Layer -2 kV Current Density in Magnetic Layer 0.05 mA/cm 2 AC Power applied to Electrode 7 150 W As a result of the film formation under the conditions tabulated in Table 2, over the entire area of the gas outflow port 9 (250 mm in length and 600 mm in width), a film of 2,500 kg/mm 2 in Vickers hardness could be synthesized at a speed higher than 400 nm per minute. Consequently, it has become possible to form the diamond-like carbon film of 10 nm in thickness uniformly (with ±2% or smaller in variation in film thickness) over a surface of the length of magnetic tape 8 travelling at a rate higher than 24 m per minute. In the practice of the second embodiment of the present invention, the application of the negative potential to the magnetic layer may not be always done from the direct current power source, but the alternating current potential may applied so that a self-biasing effect thereof can be utilized. Also, the frequency of the alternating current to be applied to the electrode 7 is not limited. Similar effects can be obtained even when the AC potential superimposed with a negative DC potential is applied to the electrode 7 such as shown in Fig. 4. This can be accomplished by superimposing a DC potential, supplied from a direct current power source 18 through a low-pass filter 17, on the DC potential supplied from the AC power source 16 and, in this instance, the magnetic layer of the magnetic tape may be grounded. In the practice of the second embodiment of the present invention, a favorable film formation is possible particularly if the pressure ratio between the first and second vacuum vessels is in the order of one or more figures. It is to be noted that the plasma generating tube 2 in its entirety may not be completely enclosed within the vacuum vessel 1, but a portion thereof may be situated outside the vacuum vessel 1 as shown by 2-a in Fig. 5, and even this arrangement bring about no problem. The arrangement in which a portion of the plasma generating tube 2 is situated outside the vacuum vessel 1 as shown in Fig. 5 is particularly advantageous where the AC power source is employed as means for generating the plasma, an example of which is shown in Fig. 6. According to the modification shown in Fig. 6, that portion of the plasma generating tube 2 which protrudes outwardly from the vacuum vessel 1 is exteriorly turned with a high frequency coil 21 which is in turn electrically connected through a matching box 20 with an AC power source for applying an AC potential to the high frequency coil 21. If the plasma is produced by the application of the AC potential while the plasma generating tube 2 is installed completely within the vacuum vessel 1, not only the interior of the plasma generating tube 2, but also the high frequency coil 21 will provide a trigger by which the abnormal discharge is likely to occur. This problem is obviated if the high frequency coil 21 is positioned outside the vacuum vessel 1 such as shown n Fig. 6 and, therefore, while the uniformity of the resultant film is ensured, the possibility of an occurrence of the abnormal discharge which would result in damage to the magnetic tape 8 can be eliminated. Although in describing the preferred embodiments of the present invention the plasma CVD apparatus of the present invention has been described as applied in forming the diamond-like carbon film on the surface of the magnetic layer of the magnetic tape, the apparatus of the present invention may be used in forming the diamond-like carbon film on any other substrate other than the magnetic tape. Also, the present invention is not always limited for use in forming the diamond-like carbon film, but may be equally employed in forming any other thin film provided that the raw gaseous material and the film forming conditions are properly selected. The present invention can also be equally applied to the sputtering of a substrate surface with the use of an inert gas such as Ar gas and the oxidization or nitriding of a surface with the use of O 2 gas or N 2 gas. Furthermore, by modifying the shape of the gas outflow port, the present invention can be used for forming the thin film on a three-dimensional substrate, an example of which will not be described in connection with a third preferred embodiment of the present invention with reference to Fig. 7. In describing the third embodiment of the present invention, reference is made to the synthesis of a diamond-like thin film on a surface of a sliding member employed in a machine for guiding a movement of a movable member. An example of this sliding member includes the one used to guiding a linear motion of a tool carriage in a table-top machine tool. As is well known to those skilled in the art, the linear motion of the tool carriage directly affects the machining precision of a workpiece and is therefore maintained at a high level of preciseness. In view of this, a sliding surface supporting the linear motion of the tool carriage must have a sufficient wear resistance and a sufficiently low coefficient of friction. The diamond-like thin film satisfies these requirements and, if it be formed on the sliding surface, an excellent sliding member can be obtained. Other than the sliding member, a surface of a shaft supported by a slide bearing or any other bearing, a surface of any other tool or any other surface requiring a wear resistance may also be used for the formation of the diamond-like thin film according to the present invention. The plasma CVD apparatus according to the third embodiment of the present invention is shown in Fig. 7, which may be advantageously employed for forming a diamond-like film on the surface of the sliding member shown by 23. The plasma CVD apparatus shown therein comprises a vacuum chamber 1 in which the sliding member 23 is installed, and a plasma generating tube 2. The plasma generating tube 2 has a gas outflow port 22 so designed and so sized in consideration of the size of a film forming portion 23-a that the distance of separation between the sliding member 23 and the gas outflow port 22 is uniform over the entire area of the gas outflow port 22. In the illustrated embodiment, the film forming portion 23-a is flat and rectangular in shape and, therefore, the gas outflow port 22 has a rectangular shape. A substrate to be formed with the diamond-like film in the practice of the third embodiment of the present invention is not a continuous medium such as employed in the previously described embodiments, but a sliding member made of metal and is therefore fixed in position inside the vacuum vessel 1 during the formation of the diamond-like film on the surface of the sliding member 23. Hence, the third embodiment of the present invention does not require the use of the transport system such as required in any one of the foregoing embodiments of the present invention. Where the surface on which the diamond-like film is to be formed is a curved surface such as shown by 24-b in Fig. 8, a gas outflow port 24 must be correspondingly shaped to follow the curved surface 23-b. Settings and operating means employed therein are substantially similar to those used in any one of the foregoing embodiments. As a raw gaseous material, C 6 H 6 gas is used and is introduced into the plasma generating tube 2 through the gas supply line 11. At this time, the distance of separation between the gas outflow port 22 and the sliding member 23 and the flow rate of the C 6 H 6 gas are adjusted so that the pressure difference between the plasma generating tube 2 and the vacuum vessel 1 can attain a value in the order of more than a single figure. If this pressure difference is insufficient, not only is an uneven pressure developed at the gas outflow port 22, but also a portion of the plasma generated within the plasma generating tube 2 flows into the vacuum vessel 1, thereby to constitute a cause of a film formation at an unwanted area and also a cause of an abnormal discharge. After the pressure inside the plasma generating tube 2 has been stabilized, an electric potential required to cause the electrode 7 to have a positive potential is applied between the electrode 7 and the sliding member 23 to cause the plasma generating tube 2 to generate the plasma. Ions contained in the plasma so generated are accelerated by the effect of the potential difference between the electrode 7 and the sliding member 23 to impinge on the film forming portion 23-a of the sliding member 23 thereby to form a diamond-like carbon film together with radicals contained in the plasma. One example of film forming conditions is shown in Table 3. Pressure inside Tube 2 10 to 15 Pa Pressure inside Vessel 1 0.1 Pa C 6 H 6 Flow Rate 12 SCCM Voltage of Power Source 10 3 kV Current Density in Magnetic Layer 0.1 mA/cm 2 As a result of the film formation under the above tabulated conditions, over the entire area of the gas outflow port 22 (50 mm in length and 80 mm in width), a film of 4,000 kg/mm 2 in Vickers hardness could be synthesized at a speed higher than 150 pm per minute. Consequently, it has become possible to form the diamond-like carbon film of 1 µm in thickness uniformly (with ±2% in variation in film thickness) over the film forming portion 23-a of the sliding member 23. It is to be noted that modifications described in connection with any one of the first and second preferred embodiments of the present invention can be equally employed in the practice of the third embodiment of the present invention. In the illustrated embodiment, the potential difference between the electrode within the plasma generating tube 2 and the magnetic layer of the magnetic tape 8 or the sliding member 23 is chosen to be within the range of 0.3 to 5.0 kV and, preferably, within the range of 0.5 to 3.0 kV. If this potential difference is lower than 0.3 kV, the ion bombardment energy will be reduced to result in a film similar to an organic film that is soft and easy to be damaged. On the other hand, if the potential difference is higher than 5.0 kV, the ion bombardment energy will be too excessive enough to sputter and/or damage the formed film. The pressure inside the vacuum vessel is preferably of a vacuum higher than 0.5 Pa and more preferably 0.2 Pa. If the vacuum inside the vacuum vessel is lower than 0.5 Pa, the potential applied to the magnetic layer may provide a cause of the abnormal discharge taking place within the vacuum vessel. Once the abnormal discharge occurs within the vacuum vessel, not only may foreign matter be undesirably deposited on the surface of the magnetic tape 8, but the magnetic layer of the magnetic tape may be damaged. The pressure inside the plasma generating tube is preferably higher than 10 Pa and, more preferably within the range of 10 to 50 Pa. If the pressure inside the plasma generating tube is lower than 10 Pa, the film forming speed will be considerably reduced. On the other hand, if the pressure side the plasma generating tube is higher than 50 Pa, the plasma generating tube is susceptible to an abnormal discharge when the potential is applied to the electrode. Also, an average stroke of free movement of ions is reduced accompanied by a reduction in bombardment energy and, therefore, the quality of the resultant film will be lowered (that is, the SP2 bonds are increased to provide a soft film). Again, although reference has been made to the use of C 6 H 6 gas as the raw gaseous material, the present invention is not always limited thereto, but may employ any gaseous medium provided that it contain carbon elements. The raw gaseous material used in the present invention may or may not contain any one of Ar gas and H 2 gas. While the various embodiments of the present invention have fully been described, the present invention is such that the distance of separation between the gas outflow port and surface of the substrate on which the film is desired to be formed is so chosen that the pressure of the plasma generated by the plasma generating tube 2 and emerging outwardly through the gas outflow port into the first vacuum vessel 1 is uniform over the entire area of the gas outflow port, that is, at any location over the gas outflow port. If the distance of separation is great as compared with the rate of flow of the gas emerging outwardly through the gas outflow port, the flow of the gas emerging from the gas outflow port is so unstable that no uniform pressure can be created in the gap between the gas outflow port and the substrate. Under the condition in which the distance of separation between the gas outflow port and the substrate is chosen to be a proper value, the amount of the gas emerging outwardly from the gas outflow port of the plasma generating tube can be controlled to a properly small value as compared with the amount of the plasma generated within the plasma generating tube and, therefore, not only can the sufficient amount of the plasma be accumulated within the plasma generating tube, but the gas pressure can be increased. Consequently, while the proper amount of the plasma is accumulated in the plasma generating tube, the amount of the plasma generated and the amount of the plasma flowing outwardly therefrom are properly balanced with each other to such an extent that the stabilized supply of the plasma gas to the gap takes place and the gas pressure in the gap is maintained uniform. In the practice of the present invention, the pressure inside the plasma generating tube is higher than that inside the first vacuum vessel. It has, however, been found that, in order to create the uniform gas pressure at any location in the gap between the substrate and the gas outflow port that is required to form the high quality film, the pressure inside the plasma generating tube, that is, the second vacuum vessel, be higher in the order of one, or preferably one to two, figures than the pressure inside the first vacuum vessel. Although the present invention has been described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications are apparent to those skilled in the art. Such changes and modifications are to be understood as included within the scope of the present invention as defined by the appended claims, unless they depart therefrom.

Claims: A plasma CVD apparatus which comprises: a first vacuum vessel enclosing a substrate on which a film is desired to be formed; a second vacuum vessel fluid-connected with a source of a raw gaseous medium and a gas outflow port from which a plasma of the raw gaseous medium introduced into the second vacuum vessel is discharged; plasma generating means for transforming the raw gaseous medium, introduced from the raw gaseous medium source into the second vacuum vessel, into the plasma within the second vacuum vessel, said gas outflow port being so shaped as to conform to the shape of the surface of the substrate, said second vacuum vessel being accommodated within the first vacuum vessel with the gas outflow port oriented towards and spaced apart from the surface of the substrate with a gap formed therebetween, said gap being chosen to be in the range of 0.3 mm to less than 1 mm to permit the pressure of the plasma, discharged from the gas outflow port into the gap, to be uniform at any location within the gap; and electric field generating means for directing ions of the plasma, discharged from the gas outflow port into the gap, towards the substrate. The plasma CVD apparatus as claimed in Claim 1, further comprising movable means for moving the second vacuum vessel so as to vary the gap between the surface of the substrate and the plasma outflow port. The plasma CUP apparatus as claimed in Claim 1, further comprising movable means for moving the substrate so as to vary the gap between the surface of the substrate and the plasma outflow port. The plasma CVD apparatus as claimed in Claim 1, wherein said substrate is a continuous length of magnetic tape and wherein said first vacuum chamber includes a rotatably supported can roller, magnetic tape transport means for transporting the length of magnetic tape around the can roller, and a bias potential applicator for applying an electric bias to a magnetic layer of the magnetic tape, and wherein said gas outflow port is of a shape conforming to an outer peripheral surface of the can roller and said second vacuum vessel includes an electrode installed therein for cooperating with the bias applicator to form an electric field for directing the ions of the plasma towards the magnetic tape. The plasma CVD apparatus as claimed in Claim 4. further comprising movable means for moving the second vacuum vessel so as to vary the gap between the outer peripheral surface of the can roller and the plasma outflow port. A method of forming, by the use of a plasma CVD technique, a film on a surface of a substrate within a vacuum vessel, which method comprises the steps of: supplying a higher pressure plasma into the vacuum vessel than the pressure inside the vacuum vessel; developing an electric field required to direct ions of the plasma towards the substrate; and forming the film made of elements contained in the plasma; said plasma supplying step being carried out by bringing a plasma gas outflow port of a shape, conforming to the shape of the surface of the substrate, in close proximity of the surface of the substrate while leaving a gap of 0.3 mm to less than 1 mm therebetween such that, within an area inwardly delimited by a perimeter of the gap, a higher pressure condition of the plasma gas by at least one figure than the pressure inside the vacuum vessel is created substantially uniformly; said electric field being acted on the plasma gas within the gap. The film forming method as claimed in Claim 8, wherein the pressure of the plasma gas is within the range of 10 to 50 Pa and the pressure inside the vacuum chamber is lower than 0.2 Pa to permit the diamond-like film to be formed on the surface of the substrate. The film forming method as claimed in Claim 6, wherein said substrate is a continuous length of magnetic tape having a magnetic layer formed thereon, said film being formed on the magnetic layer of the magnetic tape, and wherein said vacuum chamber includes a can roller around which the length of magnetic tape is transported and wherein said plasma supplying step is carried out by bringing a plasma gas outflow port of a shape, conforming to the shape of an outer peripheral surface of the can roller, in close proximity of the surface of the substrate while leaving a sufficiently small gap left therebetween such that, within an area inwardly delimited by a perimeter of the gap, a higher pressure condition of the plasma gas by at least one figure than the pressure inside the vacuum vessel is created substantially uniformly. The film forming method as claimed in Claim 8, wherein said plasma is a gaseous version of a gaseous medium containing carbon and wherein said film formed on the magnetic layer of the magnetic tape is a diamond-like film. The film forming method as claimed in Claim 9, wherein the the pressure of the plasma gas at the gap is within the range of 10 to 50 Pa and the pressure inside the vacuum chamber is lower than 0.2 Pa.

Inventors: KUROKAWA HIDEO, MITANI TSUTOMU, NAKAUE HIROKAZU, KUROKAWA, HIDEO, MITANI, TSUTOMU, NAKAUE, HIROKAZU
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1993
Main classification: C23C16
IPC codes: C23C16