Method for forming a film with plasma CVD process

Abstract: In the plasma discharge to form a film on a magnetic tape of the like in a plasma CVD process, a hollow electrode 12 is arranged in a discharge tube 8 or arranged opposed to another electrode 34. A non-polymerizable gas is blown out from the inside of the hollow electrode 12 made from a sintered alloy, and impurities do not adhere to the magnetic tape. Then, small traces of discharge are not generated on the magnetic tape and impurities do not contaminate the magnetic tape, so that a uniform thin film of high quality is manufactured at a high rate in an industrial scale.

Description: The present invention relates to a plasma chemical vapor deposition (CVD) process which is used for example to form a protection film of a magnetic recording medium of thin film type or a superconductor film and an amorphous silicon film for a solar cell. A thin film is produced by a plasma CVD film-deposition process which has been used in many fields. In the fields of semiconductors and liquid crystals, such a film has been developed as a protection film. For example, a SiO 2 film is used as an insulator film of LSI circuits. On the other hand, in the field of magnetic recording medium of thin film type, a diamond-like carbon film or a carbon film is used to improve the reliability of a magnetic disk or a deposition tape. Further, in the field of solar cells, an amorphous silicon film is used practically for a solar cell. The amorphous silicon film is manufactured with a plasma CVD process. However, from the aspect of industrial application, it is a problem that the electrode used in the plasma CVD process has to be cleaned often because it becomes dirty after the plasma discharge of long time. It is a more serious problem that a substance dropped from the dirty electrode makes the plasma discharge unstable and that it is included into the growing film. For example, in the field of magnetic recording medium, manufacturing methods are-developed to balance the productivity and the reliability of a diamond-like carbon film, as disclosed in Japanese patent application Nos. 321698/1989 and 286197/1990 which describe means to solve the anomalous discharge phenomenon accompanied with a large excess current and the improvement of the productivity. However, it has been recognized now that a brown to black substance adheres to the electrode in the discharge tube and to the inner wall of the discharge tube after a long time discharge. The substance peels off to affect the plasma discharge badly and the resultant magnetic tape has small traces of discharge thereon. Especially, this phenomenon is remarkable when a hydrocarbon gas having a large number of carbon atoms and having double bonds is used. Thus, though the film-deposition rate increases to 20 nm/s and the reliability of magnetic tape becomes sufficient, the stability of the production remains as a problem. Further, in a plasma CVD process with use of opposite electrodes of capacitor coupling type, a diamond-like carbon film and an amorphous film as a protection film of magnetic disk are manufactured. In this process, the electrode in the side where no substrate is put becomes dirty after a long discharge time, the deposit to the electrode penetrates into the film. This is a cause of the scattering of the film quality of diamond-like carbon film and the conversion efficiency of solar cell. From JP-A-59086214 the manufacture of amorphous semiconductors is known, in which a plasma CVD apparatus is used comprising a vacuum chamber, a first electrode having small holes and a second electrode opposed to said first electrode and holding a substrate on which a film is to be deposited. Into the first electrode, a non-polymerizable gas such as B 2 H 6 , is introduced, whereas a polymerizable gas such as silicon, is introduced through an inlet port. An electrical voltage from a high frequency generator is applied between the two electrodes. From FR-A-2 397 067 an apparatus is known which also comprises a vacuum chamber with two electrodes, one of which holding substrates on which films are to be deposited whereas the other electrode is a hollow electrode having a porous inner wall into which a gas is introduced. It is an object of the present invention to solve the problem of degradation of the film quality due to the dirt of the electrode in a plasma discharge deposition film such as a SiO 2 protection film for semiconductors and liquid crystals, and depositing films on a magnetic recording medium, so as to be able to manufacture the thin film in an industrial scale. It is a further object of the present invention to solve the degradation of products due to the dirt of the inner wall of the discharge tube caused by a long time plasma discharge. When a protection film of magnetic recording medium, an amorphous silicon film, a SiO 2 protection film or a superconductor film is deposited for a long time in the plasma CVD process, it is desirable to prevent an undesirable substance to disturb the plasma discharge or to prevent the inclusion of such substance to be adhered to the electrode or the inner wall of the discharge tube. In order to solve the object of the invention, a method for forming a film in a plasma CVD apparatus is provided, comprising a vacuum chamber, an insulating discharge tube provided in the vacuum chamber, which discharge tube defines a plasma discharge region together with a substrate to which a film is deposited, which discharge tube has a space between the outer wall and inner wall having narrow holes, and a porous electrode provided in the discharge tube; and which method comprising the steps of: evacuating the vacuum chamber to a prescribed pressure for plasma discharge, introducing a non-polymerizable gas in said porous electrode, introducing the non-polymerizable gas through the space in said discharge tube at the same time as the introduction of the non-polymerizable gas through said discharge tube, introducing a polymerizable gas in said discharge tube to the plasma discharge region, applying an electrical voltage to the porous electrode to cause plasma discharge in the plasma discharge region. Preferable embodiments are defined in the dependent claims. An advantage of the present invention is that a diamond-like carbon film as a protection film of magnetic recording medium can be deposited at a rate of 25 nm/s or more for a long time without traces of discharges on a tape or the like so that the magnetic recording medium of thin film type of high output and high reliability can be manufactured in an industrial scale. Another advantage of the present invention is that the film quality of diamond-like carbon film on a magnetic disk is improved and that the contact start stop characteristic of magnetic disk can be improved eventually. These and other features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings, and in which: Fig. 1 is a schematic sectional view of magnetic tape of metallic thin film; Fig. 2 is a schematic diagram of an apparatus for forming a protection film with a plasma CVD process of an embodiment of the present invention; and Fig. 3 is a graph for showing the stability of the output of representative samples: (a) sample No. 1, (b) sample No. 7 and (c) sample No. 12. Referring now to the drawings, a magnetic tape of thin film type is explained as an example in the field of magnetic recording medium. Fig. 1 shows a schematic sectional view of a magnetic tape of metallic thin film type. The structure of the magnetic tape is explained. The reference numeral 1 denotes a lubricant layer made mainly from a carboxylic acid including fluorine atoms and its thickness is 3 - 5 nm. It may be made from only the carboxylic acid of a mixture with the carboxylic acid ester including fluorine atoms. For example, it includes C 5 F 11 (CH 2 ) 10 COOH or C 5 F 11 (CH 2 ) 10 COOC 8 H 17 . The reference numeral 2 denotes a diamond-like carbon film which has Vickers hardness of about 3000 so as to prevent the damages of magnetic tape together with the lubricant. Its thickness is preferably 10 - 20 nm as to the balance of the reliability and the output. The reference numeral 3 denotes a thin film of ferromagnetic metal of for example Co-Ni-O, Co-O or Co-Cr. Its thickness is generally 50 - 300 nm. The reference numeral 4 denotes a nonmagnetic substrate of a film of polyethylene terephthalate. It may be made of polyethylene naphthalate, polyamide, polyimide or the like. It is preferable that the surface in the magnetic side of the tape has protrusions of 10 - 30 nm in order to reconcile the reliability and the output. The reference numeral 5 designates a back coating layer made from polyurethane, nitrocellulose, polyester and carbon, and calcium carbonate. Its thickness is 500 nm. Fig. 2 shows an example of an apparatus for forming a magnetic recording medium 6, wherein the reference numeral 13 denotes a vacuum chamber which is controlled to have a pressure of 13·10 -3 Pa to 133·10 -3 Pa (10 -4 - 10 -3 Torr). It is evacuated from the evacuation outlet with a vacuum pump (not shown). The reference numeral 7 denotes a can with which a magnetic tape 6 of metallic thin film contacts. As to the magnetic tape 6, the metallic thin film 3 is arranged at the outside with reference to the can 7 and the diamond-like carbon film 2 is deposited on the tape 6 in a plasma discharge area. Water circulates in the inside of the can 7 in order to cool the magnetic tape 6. The reference numeral 9 denotes a feeder of the magnetic tape 6, while the reference numeral 10 denotes a winder of the magnetic tape 6. The reference numeral 11 denotes a roll and it also guides the electric current flowing in a discharge tube 8 via the metallic thin film 3 to the ground. The roll 11 is made for example from a semiconductor material such as SiC, whose resistance is controlled so as not to give damages due to heating to the substrate 4 of polyethylene terephthalate (PET). The reference numeral 8 denotes the discharge tube for depositing a diamond-like carbon film 2 on the magnetic tape 6. A pressure difference is generated between the discharge tube 8 and the can 7 for plasma discharge. The discharge tube 8 consists of inner and outer walls. A gas can be introduced via a pipe 14 inside the discharge tube 8 directly, while another gas can be introduced via a pipe 15 and a valve 16 to a space 17 provided between the two walls. Generally, the outer wall of the discharge tube 8 is made from a heat-resistant polymer material, and the inner wall is made of a porous material of Teflon. Thus, in the apparatus shown in Fig. 2, the another gas can also be blown out to the inside of the discharge tube 8 through the porous inner wall to the inside of the discharge tube 8 if the valve 16 is opened. If it is not needed to introduce gas- through the porous inner wall, the discharge tube is made only from a heat-resistant polymer material. As will be explained later, according to a method of the present invention, hydrocarbon (polymerizable gas) is introduced through the pipe 14 to the inside of the discharge tube 8, while an inert gas (non-polymerizable gas) is introduced through the pipe 15 and the electrode 12 to the inside of the discharge tube 8. According to another method, inert gas is also introduced through the space 17 of the discharge tube 8. A pipe-like electrode 12 made of sintered alloy of stainless steel (SUS 316) is provided inside the discharge tube 8 in order to introduce gas, and an AC and/or DC electric voltage is applied to the discharge tube 12. Only inert gas is introduced to the electrode 12 in this invention. The gas spreads through the pores of about 10 µm diameter of the hollow electrode 12 into the discharge tube 8. Though the electrode 12 is made from a sintered alloy of stainless steel in this embodiment, the material may be any sintered material having narrow holes. Further, the electrode 12 may be a pipe-like electrode of metal having narrow holes of 500 - 50 µm of diameter produced mechanically. The film-deposition in this embodiment is explained below with reference to manufacturing conditions. The nonmagnetic substrate 4 of 500 mm width of polyethylene terephthalate has 10 5 - 10 9 /cm 2 of protrusions of 30 nm height, 200 nm diameter on the surface detected by the scanning tunneling microscope (STM) analysis. A thin film 3 of ferromagnetic metal of Co(80)-Ni(20) is formed up to a thickness of 160 nm on the substrate 4 by introducing oxygen in the oblique vacuum deposition process. Then, a back coating layer 6 is formed up to 500 nm thickness on the other side of the substrate 4 by applying a methyl ketone/toluene/anone solution including 30 % of solid of polyurethane, nitrocellulose and carbon black with a reverse roll coater. Then, the tape is set in the apparatus shown in Fig. 2, and a diamond-like carbon film 2 of the magnetic tape 6 is deposited on the tape. 800 V of AC current and +1000 V of DC current are superposed and applied to the pipe-like electrode 12. The inner pressure of the discharge tube 8 is set to be 13 - 40 Pa (0.10 - 0.30 Torr), while that of the vacuum chamber 13 is 133·10 -3 Pa (0.001 Torr) or less. If the pressure in the vacuum chamber increases more than this value, the discharge starts even in the vacuum chamber. The pressure difference is controlled by a space between the discharge tube 8 and the can 7. The composition ratio of inert gas to hydrocarbon is set to be 1:3 in pressure ratio. The inert gas is for example argon or neon, while the hydrocarbon is for example toluene, methane, propane and benzene. Samples Nos. 1 - 13 are- produced by changing the introduction conditions because the introduction of inert gas and hydrocarbon to the discharge tube 8 and to the electrode 12 affect the stability of long time film deposition of the diamond-like carbon film 2. As to Samples Nos. 1 - 4 according to the present invention, only inert gas is introduced through the electrode 12 while hydrocarbon is introduced in the discharge tube 8. As to Samples Nos. 5, 6, both inert gas and hydrocarbon are introduced through the electrode 12. As to Samples Nos. 7 - 9, both inert gas and hydrocarbon are introduced in the discharge tube 8. As to Samples Nos. 10, 11, hydrocarbon is introduced through the electrode 12, while inert gas is introduced through the discharge tube 8. As to Samples Nos. 12, 13 according to the present invention, inert gas is introduced through the electrode 12 and through the inner wall of the discharge tube 8 while hydrocarbon is introduced in the discharge tube 8. The ratio of the gas blown through the electrode 12 to that introduced in the discharge tube 12 is controlled by the valve 16 to be about 2:1. The feed speed of the tape 6 is adjusted between about 3 and 300 m/min according to the deposition rate. Table 1 compiles the conditions of the introduction of gas and the kind of gas. Table 1 also compiles the number of small discharge traces of 2 - 3 mm of diameter observed on the tape when the diamond-like carbon film 2 of 3000 m long is formed and the deposition rate of the diamond-like carbon film 2. diamond-like carbon film of magnetic tape Sample No. inner pressure in DT (Pa) gas introduction conditions rate of discharge traces (/3000m) deposition rate (nm/s) 1 (0.30) 40 argon: inside EL 0 30 toluene: inside DT 2 (0.20) 26,7 argon: inside EL 0 2 methane: inside DT 3 (0.25) 33,3 argon: inside EL 0 7 propane: inside DT 4 (0.25) 33,3 neon: inside EL 0 26 benzene: inside DT 5 (0.20) 26,7 neon: inside EL 12-22 22 benzene: inside EL 6 (0.20) 26,7 argon: inside EL 8-17 6 toluene: inside EL 7 (0.25) 33,3 argon: inside DT 10-20 26 methane: inside DT 8 (0.30) 40 argon: inside DT 3- 5 3 methane: inside DT 9 (0.30) 40 neon: inside DT 12-25 31 benzene: inside DT 10 (0.30) 40 argon: inside DT 10-20 30 methane: inside EL 11 (0.20) 26,7 argon: inside DT 5-10 7 propane: inside EL 12 (0.30) 40 argon: inside EL and inside the inner space of DT 0 8 propane: inside DT 13 (0.20) 26,7 neon: inside EL and inside the inner space of DT 0 28 benzene: inside DT NB: "EL" denotes the electrode 12, and "DT" denotes the discharge tube 8. A lubricant C 5 F 11 (CH 2 ) 10 COOH dissolved in a solvent is applied with a coater to the various kinds of diamond-like carbon film 2 as grown mentioned above, to apply a lubricant layer 1 of a thickness which becomes 3 nm after the drying to the Samples Nos. 1 - 13. Next, Samples Nos. 1 - 13 are provided for evaluation after cutting by a slitter to give an 8 mm width. The stability of the output in the longitudinal direction of these Samples are examined with an 8 mm video cassette recorder. The output is measured in the longitudinal direction on twenty rolls of 120 minute tapes. Fig. 3 shows the results of Sample No. 1 (this invention), Sample No. 7 and Sample No. 12 (this invention) as representative examples. The data of Table 1 and Fig. 3 show that even if hydrocarbon gas having a large number of carbons and including many double bonds such as benzene or toluene is used, discharge traces due to the adhesion of soot are not generated when a sintered alloy is used as the electrode 12 and inert gas such as argon or neon is blown out. That is, the resultant magnetic recording medium of thin film type has no disturbance of the output, as shown in (a) and (c) in Fig. 3. On the contrary, an shown in (c) in Fig. 3, the comparison sample No. 7 shows drops of the output at the position of discharge traces. It is also found preferable that the inert gas is also blown out from the inner wall of the electrode 12 and the discharge tube 8, as in Samples Nos. 12 and 13. On the contrary, if polymerizable hydrocarbon gas is introduced in the electrode 12 (Samples Nos. 5 and 6) or if inert gas and hydrocarbon are introduced in the discharge tube 8 besides the electrode 12 (Samples Nos. 7 - 10), a soot substance adheres to the surface of the electrode 12 in a long time discharge, and it peels off so that the discharge becomes unstable, and small discharge traces remain on the resultant thin film tape. Thus, the output is lowered. However, when the amount of hydrocarbon as polymerizable gas is relatively small, for example when argon:benzene = 1:1, even if both gases are introduced through the sintered alloy electrode 12, the surface of the electrode does not become so dirty, and large damages are not observed. Thus, when a magnetic recording medium is manufactured, a diamond-like carbon film as a protection film can be deposited at a rate of 25 nm/s or more for a long time without adhering substances to the electrode and the inside wall of the discharge tube and without traces of discharges on the tape.

Claims: A method for forming a film in a plasma CVD apparatus comprising a vacuum chamber (13), an insulating discharge tube (8) provided in the vacuum chamber (13), which discharge tube (8) defines a plasma discharge region together with a substrate (4) to which a film is deposited, which discharge tube (8) has a space (17) between the outer wall and inner wall having narrow holes, and a porous electrode (12) provided in the discharge tube (8); which method comprises the steps of: evacuating the vacuum chamber (13) to a prescribed pressure for plasma discharge; introducing a non-polymerizable gas (15) in said porous electrode (12); introducing the non-polymerizable gas (15) through the space (17) in said discharge tube (8) at the same time as the introduction of the non-polymerizable gas (15) through said discharge tube (8); introducing a polymerizable gas (14) in said discharge tube (8) to the plasma discharge region; applying an electrical voltage to the porous electrode (12) to cause plasma discharge in the plasma discharge region. A method for forming a film with a plasma CVD process according to claim 1, wherein said porous electrode (12) is made from a sintered alloy. A method for forming a film with a plasma CVD process according to claim 1, wherein said porous electrode (12) is made from a metal having fine openings. A method according to claim 3, wherein said polymerizable gas (14) is a hydrocarbon to form a diamond-like carbon film (2) and said non-polymerizable gas (15) is an inert gas. A method according to claim 4, wherein said substrate (4, 3) is a magnetic recording medium and said thin film is a protection film (2) of the magnetic recording medium. A method according to claim 3, wherein said polymerizable gas (14) is a tetrachlorosilane to form an amorphous silicon film and said non-polymerizable gas (15) is hydrogen.

Inventors: MURAI MIKIO, ODAGIRI MASARU, TAKAHASHI KIYOSHI, UEDA HIDEYUKI, MURAI, MIKIO, ODAGIRI, MASARU, TAKAHASHI, KIYOSHI, UEDA, HIDEYUKI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1992
Main classification: C23C16
IPC codes: C23C16, G11B5