JPH06208721A - Magnetic recording medium - Google Patents

Abstract

PURPOSE:To improve adhesive properties with a magnetic material while reducing the irregularities of a medium surface, and to enhance magnetic characteristics by adding a not more than 20 atomic % group IV element to carbon or a film mainly comprising carbon. CONSTITUTION:When a DCL film 13 is formed onto a magnetic recording medium as a protective film or a lubricating film, adhesive properties with a magnetic material can be improved by adding Si. The irregularities of a medium surface can be reduced by forming the DCL film onto the magnetic recording medium, thus making magnetic characteristics better than conventional devices by 1dB or more. Similar adhesive strength can be realized even regarding Ge as another IV group element as an additive for enhancing adhesive properties at that time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a magnetic recording medium such as an audio tape, a video tape, a floppy disk and a hard disk. In particular, the present invention provides a technique for forming a magnetic thin film upper coating for improving the characteristics of the magnetic recording medium.

[0002]

2. Description of the Related Art Magnetic recording media such as audio tapes, video tapes, floppy disks and hard disks are generally constructed by forming a magnetic film on a supporting substrate made of organic resin or metal. The magnetic film is formed by coating or a vacuum process such as vapor deposition or sputtering. Coating is used when cost is prioritized over performance, and vacuum process is used when high performance is required.

In these magnetic recording media, the magnetic head (signal writing / writing
It is in contact with the reading device) or its mechanical parts and the feeding mechanical parts such as the capstein and roller, and receives a mechanical load. For this reason, magnetic properties as well as mechanical durability and low friction are required. Conventionally, as measures to reduce the friction coefficient, measures such as applying a lubricating film on the surface of the magnetic film and providing unevenness on the surface to reduce the substantial contact area have been taken. It wears, the coefficient of friction rises, and the deterioration progresses at an accelerated rate.

Further, in recent years, there is a trend to digitize information which has conventionally been treated as analog signals such as video and audio, and it is desired to improve the recording reliability of magnetic recording media. Against this background, it is necessary for the friction coefficient to be stably maintained at a low level for a long period of time.

For the magnetic thin film, there are a method of forming a layer on a substrate by coating a magnetic material powder mixed with an organic polymer and a method of forming a layer of the magnetic material itself by a vacuum process such as vacuum deposition or sputtering. The magnetic properties are better when the process is used. The magnetic material is ferrite,
Generally, a metal or the like is used. NiCo, CoPtCr, or the like is used as the metal magnetic material.

On the other hand, it is possible to form a coating film containing carbon or carbon as a main component and having a high hardness by using the plasma CVD technique, as disclosed in Japanese Patent Publication No. 3-72711 and Japanese Patent Publication No. 4-27690.
It is disclosed in Japanese Examined Patent Publication No. 4-27691 and is well known. That is, a hydrocarbon gas and a hydrogen gas are mainly introduced as a raw material gas into a decompression container, an electric field of high frequency is usually applied to a pair or more electrodes installed in the container, and the raw material gas is turned into plasma by the electric field. This is a technique for activating particles containing carbon and depositing them on a substrate.

These coatings are called diamond-like carbon (hereinafter abbreviated as DLC) because of their high hardness and diamond-like physical properties. Normally, when forming a DLC film, a self-bias or a bias from an external power source is applied so that a negative bias with respect to the plasma potential is applied to the substrate. As a result, the bond (sp ² hybrid orbital + p orbital) having a graphite-like physical property in the carbon film is etched, and the bond having a diamond-like physical property (sp 3 hybrid orbital) is mainly formed.

The hardness of such a DLC thin film is as high as 2000 kg / mm ² or more in Vickers hardness, and the coefficient of friction is also low, so that it is suitable as a protective film or a lubricating film on the surface of a magnetic film.

[0009]

However, these DL
The C film does not have sufficient adhesiveness with a magnetic material, in particular a metallic magnetic material, and peels off spontaneously after film formation, or peels off due to contact with mechanical parts or sliding, and has an adhesive strength sufficient for practical use. There was a problem not to do.

[0010]

In order to solve the above-mentioned problems, the present invention is to add 20 atom% or less of a Group 4 element to carbon or a coating film containing carbon as a main component formed on a magnetic thin film. is there. Si and Ge can be used as the Group 4 element.

The reason why the adhesion is improved by adding a Group 4 element to DLC can be speculated as follows. In the DLC film, a bond showing physical properties like graphite (sp ² hybrid orbit + p orbit) and a bond showing physical properties like diamond (s)
p ³ hybrid orbital) and 4 σ electrons contribute to the bond in the case of sp ³ hybrid orbital, but in the case of sp ² hybrid orbit + p orbital, 3 σ electrons and 1 π electron form a bond. It Since it is considered that the bond by π electrons has a smaller bonding force than that by σ electrons, a DLC film having a high ratio of carbon atoms in the sp ³ hybrid orbital has higher hardness and exhibits physical properties similar to diamond. This can be similarly considered at the interface with the metal, and sp can be generated near the interface.
^It is considered that the smaller the number of carbon atoms having ^two hybrid orbitals + p orbitals, the better the bond strength, that is, the adhesiveness.

On the other hand, it is known that Si atoms can only have sp ³ hybrid orbital state (four-coordinated state) even in the amorphous state, and if ^three atoms are isolated in four-coordinated elements.
The fact that when an element that coordinates or coordinates 5 is added, it is more thermally stable if it has the same coordination number as the surrounding coordination number, that is, 4-coordination, the fact that when P or B is doped into Si It is well known for its 4-coordination. That is, when a Si atom that can only have a 4-coordination state is arranged as the closest atom of a carbon atom in the sp ² hybrid orbital + p-orbital (3-coordination) state, the carbon atom becomes 4-coordination, resulting in sp
³ The ratio of carbon atoms in a hybrid orbit will increase. Therefore, it is considered that the DLC film containing Si has improved adhesion.

The amount of Si added to the DLC is preferably 20 atomic% or less, more preferably 1 to 15 atomic%. When the amount of Si added is increased, the C—C bond is reduced, and C
The amount of —Si and Si—Si bonds increases. C-Si, S
When the amount of i-Si bonded increases, the physical properties of the film become closer to those of the Si film, the hardness decreases, and the friction coefficient increases. Therefore S
There is an optimum value for the i amount.

In addition to C and Si, H element exists in DLC. This is because H is introduced into the film due to the use of hydrocarbon as the raw material gas, but hydrogen gas may be added to the raw material gas to positively add hydrogen into the film. Hydrogen in the film acts as a terminator of dangling bonds (unbonded hands), and improves the electrical and optical properties of the film and thermal stability. The DLC of the present invention also has 10 to 60 atomic%, preferably 15 to
It contains 40 atomic% of H.

Further, a Group 3 or Group 5 element can be added. B, Al, Ga can be used as the Group 3 element, and N, P, As, or the like can be used as the Group 5 element. These Group 3 or Group 5 elements can act to relieve the internal stress of the DLC film. Adhesiveness is D
Although it largely depends on the adhesive strength at the interface between the LC and the magnetic material, the adhesiveness can be improved by reducing the internal stress of the DLC itself. That is, since the driving force for peeling the film is the internal stress of the DLC itself, it is alleviated.

The concentration of Si atoms contained in DLC may be constant in the depth direction of the film.
It is more preferable that the atomic concentration be reduced. This is Si
This is because the sp ³ property increases as the atomic concentration increases, but the friction coefficient increases. The DLC layer is divided into at least two layers so that the sp ³ property is increased by adding Si in the portion near the interface where adhesion is required and the Si is not added in the portion where a low friction coefficient on the surface is required. It separates functions. As a structure of the DLC film on the magnetic thin film, a function separation type is preferable. For example, the DLC film is
It may have a concentration distribution of impurities (for example, Si, Ge, Sn, Pb) as shown in FIG. 10 (A) or FIG. 10 (B). The impurity concentration decreases from the interface between the magnetic film and the DLC film toward the surface of the DLC film. Further, the impurity concentration of the DLC film near the surface of the DLC film is D near the interface.
It is lower than the impurity concentration of the LC film.

In addition to improving the adhesiveness by the above method, the present inventor further conducted an experiment to confirm the effect of DLC. As a result, the use of DLC eliminates the need for surface irregularities, which was required in the past. Since the distance between the magnetic head and the magnetic material can be shortened, the magnetic characteristics can be greatly improved. The reason is as follows.

Generally, assuming that the magnetization per unit area is constant, the S / N ratio is improved as the distance of the magnetic head is closer to the magnetic material, and conversely, if the distance of the magnetic head is sufficiently short, the S / N ratio is higher than a certain value. The magnetic flux that guarantees the N ratio can be reduced. Therefore, in order to improve the recording reliability, it is necessary to bring the magnetic head as close to the magnetic material as possible.

Conventionally, in order to reduce the surface friction coefficient of the magnetic recording medium, the surface is uneven. This is because if the surface has irregularities, the effective contact area is reduced and the friction coefficient is reduced. However, it can be said that the friction coefficient was prioritized at the expense of magnetic properties.

In the present invention, the above-mentioned contradictory characteristics, that is, the reduction of the friction coefficient between the surface of the magnetic recording medium and various mechanical parts, and the improvement of the recording reliability can be satisfied at the same time.

In the experiments by the inventors, the friction coefficient of the surface on which the DLC is not formed is about 0.4 when the center line average roughness (Ra) is 30 nm, and is 0.8 when Ra = 10 nm.
It can be seen that the practical friction coefficient is 0.4 or less, and Ra = 30 nm or more is required when DLC is not formed. On the other hand, when DLC is deposited, the friction coefficient is Ra = 10 nm.
Even in the case of, it is at a low level of 0.2 to 0.4, and
There was almost no change in the friction coefficient even after repeated movements.

Experiments have shown that the coefficient of friction depends on the thickness of the DLC film, and the larger the film thickness of the DLC, the smaller the coefficient of friction. Practical friction coefficient is 10n
It is obtained at m or more.

That is, it is possible to improve the magnetic characteristics by reducing the surface irregularities while keeping the friction coefficient at a low level.

The method for producing DLC is shown below. As described in the related art, the DLC can be manufactured by using the plasma CVD method. The plasma CVD method may use a general parallel plate electrode or a positive column type plasma CVD using a positive column portion of plasma. In the case of the parallel plate electrode method, the substrate is limited to a flat surface, whereas in the positive column method, a three-dimensional film can be formed, so that the shape of the substrate is not selected, which is advantageous in terms of mass productivity.

When the substrate is flat, it is set in the substrate holder and held in the reaction space. When the substrate is a film, it can be formed into a film by introducing it into a vacuum in a roll state and rolling it from the roll to a roll so as to run in the reaction space.

The reaction gas may use hydrocarbon as a raw material of carbon. Examples of hydrocarbons include saturated hydrocarbons such as methane, ethane, propane and butane, and unsaturated hydrocarbons such as ethylene and acetylene. benzene,
Aromatic compounds such as toluene, adamantane, adamantanol and the like may be used. Alternatively, a halogenated hydrocarbon in which one or more of hydrogen atoms in a hydrocarbon molecule are replaced with a halogen-based element such as F, Cl, or Br may be used.

Hydrogen may be added to the reaction gas in addition to hydrocarbons. When hydrogen is added, the number of hydrogen radicals in the plasma is increased, and the effect of extracting excess hydrogen in the film can be expected. Therefore, a higher quality film can be obtained. The ratio of the hydrogen gas flow rate to the total gas flow rate is 90 to 30.
%, Preferably 70 to 50%. If the ratio of the hydrogen gas flow rate to the total gas flow rate is too high, the film formation rate will decrease, and if it is too low, the effect of extracting excess hydrogen will be lost.

As the gas for adding the Group 4 element, silane, disilane, fluorinated silane or the like can be used for Si. For Ge, germane, fluorinated germane, or the like can be used. For group 3 elements, diborane, boron trifluoride, trimethylboron, etc., and for nitrogen of group 4 elements, N ₂ , ammonia, nitrogen trifluoride, etc.
A gas such as phosphine can be used for P. Although the flow rate ratio of the additive gas depends on other film forming parameters such as pressure and applied power, it is preferable to control the flow rate to be 10% or less with respect to the carbon source gas.

The reaction gas is introduced into the reaction vessel, controlled to a predetermined pressure, and high-frequency power is applied to a pair or a plurality of electrodes installed in the reaction vessel to turn the reaction gas into plasma. The total flow rate of the reaction gas is about 0.0 in the reaction space volume.
In the case of 2 cubic meters, 30 sccm or more, preferably 50 sccm or more is required. The upper limit of the total flow rate depends on the exhaust speed of the exhaust device, but at least the above value is necessary.

The reaction pressure is 5 mTorr to 1000 mT
orr, preferably 10 to 100 mTorr. As the high frequency power, a frequency of 13.56 MHz is usually used.
Power to be applied is 0.01 to 1 / cm ^2, and it is preferably a 0.05 to 0.5 / cm ^2.

The temperature of the substrate may be unheated, which is a great advantage in view of mass productivity.

In the case of DLC film formation, the presence of an electric field in which particles in the plasma, mainly ions, enter the substrate surface is advantageous for improving hardness and densifying the film. In the case of the parallel plate system, there is a method in which a blocking capacitor is installed on the electrode on the RF power feeding side to generate a self-bias on the electrode side, and this is used as an electric field for allowing ions to enter the substrate surface. In this case, the substrate is installed on the RF power feeding side.

In the case of the positive column system, an electric field that ions enter the surface of the substrate is not generated just by setting the substrate in the positive column, so that the electric field must be generated by an external power source. The electric field generated by the external power source is effectively alternating current, and the frequency is preferably set between the ion plasma frequency and the electron plasma frequency in the plasma. The ion plasma frequency is several orders of magnitude smaller than the electron plasma frequency due to the mass difference between ions and electrons. When an external electric field with a frequency higher than the ion plasma frequency and lower than the electron plasma frequency is applied, the movement of the ions cannot follow the external electric field, while the electrons follow the external electric field, so the substrate surface becomes negative. Get charged. As a result, an electric field is generated on the surface of the substrate such that ions are incident on the surface of the substrate, and a hard and dense DLC film is generated by the action of the incident ions. The frequency of the external electric field for applying the bias depends on the electron temperature, electron density, ion temperature, and ion density in the plasma,
1 to 1000 kHz is preferable, and 10 to 50 is preferable.
0 kHz is good. The strength of the electric field is 50 peak to peak.
~ 1000V, preferably 100-400V.

The DLC film of the present invention can be formed using the CVD apparatus shown in FIG. In FIG. 9, the CVD apparatus includes insulators 51, 52, 53, 54, 55 and 56,
Can roll 31 (functions as a cathode electrode),
An RF power source 35 connected to the can roll 31 via an unwinding roll 33, a winding roll 32, a guide roll 34, and a blocking condenser 58, and exhaust pipes 36, 37, 38, 39, 4 connected to a vacuum pump.
2, 45 and 47, the gas supply passage 40 extending to the discharge space 44 through the inside of the anode 43, the DLC film forming space 41, the hydrogen plasma processing space 49, and the anode 46.
The gas supply passage 50 extends into the discharge space 48 through the inside of the. The film substrate 57 (on which the magnetic material layer is provided) is supplied from the unwinding roll 33 and wound up by the winding roll 32. Space 4
9 and the discharge space 48 are connected to the exhaust pipe 4 by a vacuum pump.
7 is evacuated to supply H ₂ gas to the gas supply passage 50.
Supplied from the space 49 and the discharge space 48 have a pressure of 60 to 10
It is maintained at 0 Pa, for example 80 Pa. The space 41 and the discharge space 44 are evacuated by a vacuum pump through the exhaust pipe 42 to supply H ₂ gas and C ₂ H ₄ gas from the gas supply passage 40, and the pressure of the space 41 and the discharge space 44 is 60 to 100.
Pa is maintained at 80 Pa, for example. Anodes 43 and 4
6 is grounded, for example. RF power is supplied to the cathode electrode, that is, the can roll 31 from the RF power source 35 at 13.56 MHz. In this way, a discharge occurs between the can roll (cathode) 31 and the anode 46, and a discharge also occurs between the can roll (cathode) 31 and the anode 43. In this way, the film substrate 57 becomes the discharge space 48.
Is subjected to hydrogen plasma treatment, and a DLC film is formed on the magnetic material layer in the discharge space 44. The distance between the surface of the can roll 31 and the surface 59 of the anode 43 and the distance between the surface of the can roll and the surface 60 of the anode 46 are 20 mm or less, preferably 18 mm or less.
The distance between the surface of the can roll 31 and the surfaces of the insulators 51, 52, 55 and 56 is 5 mm or less, for example 2 mm or less.

[0035]

【Example】

[Embodiment] In Embodiment 1, an example in which a magnetic recording medium is formed on a film substrate will be described. The layer structure of the magnetic tape produced in this example is shown in FIG.

A PET (polyethylene terephthalate) film (11) having a thickness of 7 μm was used as a supporting substrate.
The center line average roughness (Ra) of the substrate surface is 3 nm. The substrate is supplied as a 400 mm wide roll. DLC
The magnetic film (12) was formed in advance before forming the film. The magnetic film (12) used an alloy of CoNi. The film formation was performed by vacuum evaporation, and the film thickness was 200 nm. A DLC film (13) having a thickness of 10 nm to 5 is formed on the magnetic film (12).
The film was formed by changing it between 0 nm.

For the DLC film, a positive column type CVD apparatus was used. The outline of the apparatus is shown in FIG. A pair of electrodes (22) (shown by dotted lines) are installed in a vacuum container (21), and the electrodes (2
When RF power is applied to the 2) from the RF power source (not shown) through the matching box (not shown), the reaction space (2
Plasma is generated in 5). Methane gas of 50 sccm and hydrogen of 50 sccm or methane gas of 30 sccm and hydrogen of 70 sccm were introduced into the reaction space as source gas. The pressure was controlled at 10 mTorr. The applied RF power was 100W.

In order to add Si to the interface between the DLC and the magnetic film, SiH ₄ gas was added to the reaction gas for 1 minute, which is 10% of the reaction time. The flow rate was 2.5 sccm.

The substrate film is supplied from the delivery roll (26) and is wound up after film formation on the winding roll (27). In the reaction space (25), a plurality of turns are repeated by the roller (28) to effectively utilize the reaction space to improve the throughput.

The bias electrode (2
9) was installed and a bias electric field was applied. The bias voltage frequency was 50 kHz, and the bias voltage (peak-to-peak value) was 200V.

At this time, a silicon wafer was set in the same batch for film quality monitoring, and the film quality was evaluated by this sample. The hardness was Knoop hardness of 2500 kg / m.
It was m ² . The results of FT-IR measurement are shown in FIG.
The Raman spectroscopy result is shown in FIG. Stretching vibration of Si-C bonds in the vicinity of 700～800Cm ^-1 from FT-IR measurement result, Si-H stretching vibration bond is observed near 2100 cm ^-1. In addition, from the Raman spectroscopic results, the characteristic of DLC is 1
A broad scattered light peak around 550 cm ^-1 is seen. From this, it is found that Si is added and the DLC structure is maintained.

The friction coefficient of the magnetic tape manufactured in this example was measured. The measuring method is as follows: A tape is wound half way around a stainless steel pin with a diameter of 3 mm, the load is 20 g, and the sliding speed is 428.
mm / min and sliding distance of 50 mm.

A graph of the initial friction coefficient with respect to the film thickness is shown in FIG.
Shown in. When the CH ₄ concentration is 30%, there is film thickness dependence,
When the film thickness is 15 nm, the friction coefficient is as large as 0.44, and the friction coefficient decreases as the film thickness increases. When the film thickness is about 20 nm, the friction coefficient is 0.4 or less, which is a practical friction coefficient. On the other hand, C
When the H ₄ concentration is 50%, there is no film thickness dependence, and the film thickness is 15 n
Even m is 0.3 or less.

FIG. 6 shows changes in the friction coefficient with time with respect to the film thickness. The value of the change was defined as the increase in the friction coefficient after the initial friction coefficient was slid 200 times. From FIG. 6, it can be seen that the film thickness dependence is exhibited regardless of the CH ₄ concentration, and the change in the friction coefficient with time decreases as the film thickness increases. Although the no change with time of friction coefficients in practice state is ideal, if the friction coefficient is the limit that does not exceed 0.4, the change with time of friction coefficients in the case of CH ₄ concentration 50% 0.
1 or less, 0 or less is desirable when the CH ₄ concentration is 30%,
Therefore, the film thickness is 27 nm or more when the CH ₄ concentration is 50%,
It can be seen that 21 nm or more is desirable when the CH ₄ concentration is 30%.

From the change of the initial friction coefficient and the friction coefficient over time, C
27 nm or more when H ₄ concentration is 50%, CH ₄ concentration is 30
It is understood that a film thickness of 21 nm or more is required in the case of%. As for the magnetic characteristics, it is generally known that the signal level decreases by 1 dB when the magnetic head is separated by 10 nm. Since a substrate having a surface roughness of 3 nm was used in this example, if a DLC film that guarantees a friction coefficient necessary for practical use is formed, it is 3 dB at a concentration of 50% and 2.4 dB at a concentration of 30%.
Will decrease.

On the other hand, in the conventional method, the unevenness of 30 nm is required and the lubricating film of 10 nm or more is further required, so the level is lowered by 4 dB or more. Therefore,
When the DLC film of the present invention is used, the level is improved by 1 dB or more.

Comparative Example A DLC film containing no Si was prepared for comparison with the examples. The magnetic tape and the manufacturing method were the same as in the example except that silane gas was not added to the raw material gas. FIG. 7 shows the FT-IR measurement result of the produced film, and FIG. 8 shows the Raman measurement result. The Raman measurement result shows that the film is a typical DLC film, and the FT-IR measurement result shows that Si is not contained in the film. D on magnetic tape
The LC spontaneously peeled off and did not become a film.

[0048]

When the DLC film is formed as the protective film or the lubricating film on the magnetic recording medium, the adhesion with the magnetic material can be improved by adding Si.
Further, by forming the DLC film on the magnetic recording medium, the unevenness of the medium surface can be reduced, and thus the magnetic characteristics can be improved by 1 dB or more as compared with the conventional one.

Although the DLC film of this embodiment was formed using a positive column type CVD apparatus, the effect of the present invention is not limited by the film forming method. Therefore, although not described in the examples, it goes without saying that a parallel plate type CVD apparatus or a film forming method using a carbon ion beam may be used.

Further, although Si was used as an additive for improving the adhesiveness in this embodiment, it goes without saying that the same adhesive strength can be realized with other Group 4 element Ge.

[Brief description of drawings]

FIG. 1 shows a layer structure of a magnetic recording medium.

FIG. 2 shows a schematic view of a positive column type plasma CVD apparatus.

FIG. 3 shows an FT-IR measurement result of a DLC film containing silicon.

FIG. 4 shows a Raman spectroscopic measurement result of a DLC film containing silicon.

FIG. 5 shows DLC film thickness dependence of initial friction coefficient.

FIG. 6 shows DLC film thickness dependence of changes in friction coefficient over time.

FIG. 7 shows an FT-IR measurement result of a DLC film to which silicon is not added.

FIG. 8 shows a Raman spectroscopic measurement result of a DLC film to which silicon is not added.

FIG. 9 shows a schematic view of a roll-to-roll type apparatus for forming a DLC film.

FIG. 10 shows an impurity concentration distribution.

Claims (3)

[Claims] 1. A magnetic recording medium in which carbon or a film containing carbon as a main component is formed on a magnetic thin film formed in contact with a supporting substrate, and the film containing carbon or a carbon as a main component contains a Group 4 element. Content of 20 atomic% or less is added to the magnetic recording medium. 2. The carbon according to claim 1, wherein the thickness of the carbon or carbon-based coating is 50 nm or less, and the center of the surface of the magnetic recording medium after the carbon or carbon-based coating is formed. The magnetic recording medium according to claim 1, wherein the line average roughness (Ra) is 30 nm or less. 3. The carbon according to claim 1, wherein the element of Group 4 added to the coating film containing carbon as a main component is mainly present in the vicinity of an interface between the coating film and the magnetic thin film, and substantially exists on the surface of the coating film. 2. The magnetic recording medium according to claim 1, wherein the added Group 4 element does not exist.