JPS58222439A - Magnetic recording medium and its manufacture - Google Patents

Abstract

PURPOSE:To improve the durability, running properties and weathering resistance of a magnetic recording medium, by laminating a protecting and lubricating layer consisting of a silicon compound, an org. high polymer compound, higher fatty acid and fatty ester on a magnetic metallic thin film layer having a metallic oxide layer. CONSTITUTION:The surface of a base body W delivered from a roller 32 is cleaned in a pre-treating chamber of glow discharge 3 and is activated and the sticking force to a magnetic metallic thin film layer on the next stage is improved. A steam stream V is introduced to evaporate magnetic metal from a magnetic body evaporating source 18 in a vapor depositing chamber 4 of a magnetic metallic thin film and the base body W forms a magnetic metallic thin film A. The magnetic metallic thin film is exposed to oxygen plasma in an oxidation treating chamber 5 and its surface is oxidized to form a metallic oxide layer B. The metallic oxide layer is exposed to glow discharge plasma in a silicic ester glow discharge treating chamber 6 and a silicon compound layer C is formed on its surface. At least, a protecting and lubricating layer D consisting of an org. high polymer compound, higher fatty cid and fatty ester is laminated in a vapor depositing chamber 7 of the protecting and lubricating layer.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording medium comprising a magnetic metal thin film formed by a paper deposition method as a magnetic recording layer. Regarding manufacturing methods.

Conventionally, as a magnetic recording medium, r-
Fe20a, GO doped r-Fe208, Fe5
0. , Fe3O4 doped with Go, lutoride compounds of r-Fe203 and Fe3O4, magnetic powders such as Oro 2, or powder magnetic materials such as ferromagnetic alloy powders, vinyl chloride-vinyl acetate copolymer, styrene-butadiene copolymer Coating-type products have been widely used, in which a material dispersed in an organic resin, such as epoxy resin, polyurethane resin, etc., is applied and dried. In recent years, as the demand for high-density recording has increased, ferromagnetic metal thin films formed by vacuum deposition, sputtering, ion blating, etc., or plating methods such as electroplating and electroless plating are used as recording layers. So-called binder-free magnetic recording media that do not use ζ-ing are attracting attention, and various efforts are being made to put them into practical use.

Conventional coating-type magnetic recording media mainly use metal oxides, which have lower saturation magnetization than ferromagnetic metals, as magnetic materials, so the thinning required for high-density recording leads to a reduction in signal output, which has reached its limit. In addition, the manufacturing process is complicated, and thick t-shirts are used for solvent recovery or pollution prevention.
[It has the disadvantage of requiring additional equipment. In non-binder type magnetic recording media, ferromagnetic metals with higher saturation magnetization than the above-mentioned oxides are formed as thin films without containing non-magnetic substances such as binders. Moreover, the manufacturing process is simple.

111: High coercive force and thinness have been proposed both theoretically and experimentally as one of the requirements for magnetic recording media for high-density recording, and coating-type magnetic recording media. There are great expectations for non-2-in-1 type magnetic recording media, which can be easily made thinner by an order of magnitude and have a higher saturation magnetic flux density.

In particular, the method using the Beno ξ-depositor method has great advantages because unlike plating, there is no need to worry about wastewater treatment, the manufacturing process is simple, and the film deposition rate can be increased.

Furthermore, major problems concerning magnetic recording media made of ferromagnetic metal thin films include strength against corrosion and abrasion, and running stability. During the process of recording, reproducing, and erasing magnetic signals, the magnetic recording medium is subjected to high-speed relative motion with the magnetic head, but at this time, the movement must be smooth and stable, and at the same time, contact with the head must be maintained. Wear or destruction occurs due to [1. There is no c. It is also important that during storage of magnetic recording media, there should be no reduction or disappearance of recorded signals due to changes over time due to rust, etc.':1.
．． .

required. Providing a protective layer has been investigated as a way to improve durability and weather resistance, but there is a restriction that the thickness of the protective layer cannot be increased due to the loss of space between the magnetic layer and the magnetic layer. Therefore, it is necessary that the magnetic film itself has durability and weather resistance.

The protective layer was made of, for example, a hard metal such as rhodium or lithium, or a hard inorganic material such as No, TiO□, or CaF2, or a lubricant or a polymeric agent.

However, by simply providing these protective layers, a magnetic recording medium with sufficiently satisfactory running characteristics and durability cannot be obtained. The main reason for this is that small pieces of the hard metal or hard inorganic material that have peeled off cause scratches on the recording medium.Also, even if the protective layer is made of polymer or lubricant, the metal magnetic thin film and the protective layer The bond at the interface is weak,
This was because the running characteristics and wear resistance characteristics deteriorated significantly over time. Furthermore, JP-A-50-13806 discloses a method of nitriding the vicinity of the surface of the magnetic layer by DC glow discharge in nitrogen; however, simply nitriding the surface of the magnetic layer does not improve the running characteristics.
In order to produce a protective effect by the surface nitrided layer, 1.
It was necessary to perform glow discharge treatment for as long as 0 minutes to 2 hours to sufficiently form the metal nitride layer.

Another method for forming a protective oxide layer by oxidizing the magnetic layer is to place a ferromagnetic metal thin film under appropriate temperature and humidity and oxidize its surface (Japanese Patent Publication No. 42-20
025 publication), a method of bringing an alloy magnetic thin film into contact with nitric acid and then heat-treating it to form an oxide layer on the surface, and then impregnating a lubricant (see British Patent No. 1265175), alloy magnetic thin film surface There has been known a method in which the film is formed by treating with an aqueous solution of an inorganic oxidizing agent and an organic chelating agent, followed by heat treatment in an oxygen atmosphere. With these processing methods, it is difficult to create a thin and uniform oxide layer, and since aqueous solutions are used, it is necessary to break the vacuum after forming the metal thin film, making continuous processes impossible, and the processing time is long. And it was complicated.

The present inventors have made extensive efforts to overcome the drawbacks of the prior art described above and develop a non-binder type magnetic recording medium with excellent durability, runnability, and weather resistance, and as a result, have arrived at the present invention. .

That is, in a method in which an organic protective lubricant layer is formed on a magnetic metal thin film layer to provide a protective lubricant effect, the surface of the magnetic metal thin film layer is heated with oxygen gas before forming the organic protective lubricant layer. by exposure to a glow discharge plasma generated in a vacuum atmosphere containing

It has been discovered that durability, runnability, and weather resistance can be significantly improved by forming an oxide layer and then exposing it to glow discharge plasma generated in a vacuum atmosphere containing silicate ester gas to form a silicon compound layer. This led to the present invention.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and provide a non-ingress type magnetic recording medium that is excellent in durability, runnability, and weather resistance, and a method for manufacturing the same.

The object of the present invention is to provide a magnetic metal thin film layer having a metal oxide layer on the surface, a silicon compound layer, an organic polymer compound, a substance, a higher fatty acid, a fat, 1:', 1, on a non-magnetic substrate. After forming a magnetic metal thin film layer on a non-magnetic substrate by laminating a snare lubricating layer made of a fatty acid ester or a combination of these, the magnetic A metal oxide layer is formed on the surface of the metal thin film layer by exposing it to glow discharge plasma generated in a vacuum atmosphere containing oxygen gas, and then the surface of the metal oxide layer is exposed to a vacuum atmosphere containing silicate ester gas. A silicon compound layer is formed on the surface of the nucleus by exposing it to glow discharge plasma generated under a vacuum atmosphere, and then some polymeric compound, higher fatty acid, fatty acid ester, or a combination thereof is deposited on the silicon compound layer in a vacuum atmosphere. - Achieved by a method for manufacturing a magnetic recording medium characterized by forming a lubricating layer.

Hereinafter, embodiments of the present invention illustrated in the drawings will be described.

As shown in FIG. 1, a magnetic recording special medium manufacturing apparatus 1 for carrying out the method of the present invention includes a substrate delivery chamber/winding chamber 2,! The low discharge pretreatment chamber 3 is successively communicated with through a slit 10 for passage of a substrate limited to the deposition of a magnetic metal thin film layer.

Note that each of the chambers 2 to 7 has an independent exhaust system 8a to 8.
They communicate with f through conduits 9- to 9f, and each can maintain its own degree of vacuum (usually 10-'6 to 10 Torr).

In the delivery and take-up chamber 2, a pull-out roller 11 disposed near a roll 62 made of a rotatably supported non-magnetic and flexible strip substrate W is driven to remove the roll 6.
2, the substrate W is continuously transferred to the glow discharge pretreatment chamber 6.
When the Ar gas is supplied through the Ar gas introduction pipe 12, a glow discharge is generated in a vacuum atmosphere of about 10 Torr by the electrode 13 to which an AC high voltage (for example, about 1 kV) is applied. The surface of the base body W is cleaned and activated to improve adhesion to the magnetic metal thin film layer in the next step.

The substrate W that has passed through the glow discharge pretreatment chamber is moved by a plurality of guide rollers 1 in the magnetic metal thin film deposition chamber 4.
Cooling can 1 rotatably supported via 4
After its running direction is reversed by the lower outer circumferential surface of 5, it is fed into the oxidation treatment chamber and the silicate ester glow discharge treatment chamber 4.

Note that the magnetic metal thin film deposition chamber has a fairly high degree of vacuum (
For example, under an atmosphere maintained at 10 Torr), the magnetic material evaporation source 18 (for example, Co material,
N1, F, material, etc.) is heated and evaporated, and the vapor flow V is usually about 4
Vapor deposition is performed at an incident angle of 0° to 90° to form a magnetic metal thin film (A in FIG. 2).

The incident angle is appropriately set depending on the arrangement positions of the hearth 17 and the mask 19. In this embodiment, a vapor deposition method is shown as a method for forming the magnetic metal thin film layer, but of course a method such as sputtering may also be used.

Oxygen is supplied to each of the oxidation processing chambers through the oxygen introduction pipe 2o at a vacuum level of about 7×10 Torr at a rate of about 20 ee/min. Note that the oxygen plasma is generated by applying a high voltage to the electrode 21 using the DC power supply 22.

The substrate coated with the magnetic metal thin film layer is exposed to the oxygen plasma while passing through the oxidation treatment chamber, so that the surface of the magnetic metal thin film is oxidized and the metal oxide layer (FIG. 6B) is exposed to the oxygen plasma. ) is formed.

The substrate coated with the magnetic metal thin film layer and the metal oxide layer that has passed through the oxidation treatment chamber is sent into the silicate ester glow discharge treatment chamber A.

The silicate ester dark discharge treatment chamber 4 is provided to generate glow discharge plasma in a vacuum atmosphere containing silicate ester gas to form a silicon compound layer on the surface of the magnetic metal thin film. The gas is introduced through the gas introduction pipe 26 at a gas partial pressure or flow rate of . Note that if the partial pressure of the silicate ester gas is lower than the desired vapor pressure, measures such as heating the gas introduction pipe 26 and the silicate ester gas storage container may be taken as appropriate.

In addition, in addition to silicate ester gas, drops and A reactive gas introduction pipe 27 is provided for this purpose.

The glow discharge plasma is generated in a vacuum atmosphere containing silicate ester gas by applying high frequency power (for example, 13.56 MHz, 150 Watt) from the high frequency power source 23 to the coil 25 through the matching box 24. . The coil 25 is installed near the base coated with the magnetic metal thin film layer and the metal oxide layer.

The substrate coated with the magnetic metal thin film layer and the metal oxide layer sent into the silicate ester glow discharge treatment chamber B is exposed to glow discharge plasma generated in a vacuum atmosphere containing the silicate ester gas. Upon movement, a silicon compound layer (C in FIG. 4) is formed on the surface of the metal oxide layer.

Next, in the protective lubricant layer deposition chamber l, 7xlO""T
The vapor flow V' obtained by heating and evaporating an organic polymer compound, higher fat, fatty acid ester, or an evaporation source 28 introducing these with a resistance heating means 29 at a vacuum degree of about The substrate coated with the metal thin film layer, the metal oxide layer, and the silicon compound layer is vapor-deposited C".
A protective lubricant layer (D in FIG. 5) of a desired thickness is laminated on the silicon compound layer B.

Thereafter, the substrate coated with the magnetic metal thin film layer, the metal oxide layer, the silicon compound layer and the protective lubricant layer is returned to the delivery/winding chamber Z and is properly coated by the extractor roller 30. After the wrinkles have been corrected, the rolled 6
1 to complete a series of thin film forming steps.

Note that the substrate running in the protective lubricant layer deposition chamber 1 may be provided with a means such as a cooling can as in the magnetic metal thin film layer deposition chamber 4, or the vapor flow V may be applied to the surface of the substrate. ′ may be obliquely incident.

Note that this embodiment is just one example, and the present invention is not limited to this embodiment.

The magnetic metal materials used in the present invention include Fe, 0O
Metals such as 1N1, or Fe-Go, Fe-Ni,
Go-Ni, Fe-Go-Ni, Fe-Rh, Fe-G
u, Go-Cu, Go-A11. Go-Y%Go-
La, Go-Pr, Go-Ga, Go-8m,
Go-Pt-Ni-Cu, Mn-B1, Mn-sb
, Mn-kl, Fe-0r, Go-Or, Ni
-0r, Fe-Go-Cr, N1-G.

Among the ferromagnetic alloys whose main components are -Or, Fe-Go-N1-C5r, etc., preferred for durability are Go or alloys containing 75% Go.

Furthermore, as additives, W, Mo, Ti, Mg
, Si, AI, etc. may be included. Also, G.
It may also contain a small amount of nonmetallic components such as B, O, N, and P. These may be included in the matrix 1 for thin film formation, or may be added as components of the gas atmosphere during thin film formation.

For example, the formation of the magnetic metal thin film layer in the magnetic metal thin film layer deposition chamber 4 may be carried out in an oxygen atmosphere into which a small amount of oxygen gas is introduced.

The thickness of the magnetic metal thin film layer is generally from about 0.02 μm to 200 μm because it needs to be thick enough to provide sufficient output as a magnetic recording medium and thin enough to perform high-density recording.
00, preferably from 0.05 μm to 1000 OA.

Further, the magnetic metal thin film layer may be a film layer, or may be formed from a multilayer film of two or more layers. Furthermore, in the case of a multilayer structure, a nonmagnetic intermediate layer may be provided between the layers. A nonmagnetic underlayer may be provided on the surfaces of the nonmagnetic substrate and the magnetic metal thin film layer.

A magnetic metal thin film layer may be provided on both sides of the nonmagnetic substrate.

The paper deposition method in the present invention includes, in addition to the normal vacuum evaporation described in the specification of the above-mentioned U.S. Fi. It also includes a method of forming a thin film on a supporting substrate in an atmosphere where the mean free path of evaporated molecules is large, such as the method disclosed in Japanese Patent Application Laid-open No. 149008/1983 by the applicant. Electric field vapor deposition method, such as Japanese Patent Publication No. 43-11525, Japanese Patent Publication No. 46-20484, Japanese Patent Publication No. 47-26579,
Japanese Patent Publication No. 49-45439, Japanese Patent Publication No. 49-33890.

Ionized vapor deposition methods such as those disclosed in JP-A-49-34483 and JP-A-49-54235 can also be used in the present invention, and other methods such as sputtering and plasma polymerization can also be applied.

In addition, the substrate used in the present invention is polyethylene.
zu.

Plastic bases such as lenterephthalate, polyimide, polyamide, polyvinyl chloride, cellulose triacetate, polycarbonate, polyethylene naphthalate are preferred. Furthermore, non-magnetic metals such as A/, Cu, and SUS, or inorganic substrates such as glass and ceramics can also be used. In particular, in the present invention, the surface roughness ra) is 0.
．． Preferably, the flexible plastic base has a thickness of 0.012 μm or less.

Further, as the organic polymer compound used in the time-keeping lubricating layer of the present invention, polyolefin, vinyl resin, vinylidene resin, polyester, polycarbonate, polyamide 9, polyacrylonitrile, polyurethane, polyether, and cellulose resin are preferable. .

Examples of higher fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, (henic acid, oleic acid, linoleic acid, lilunic acid, and arachidonic acid).

Examples of fatty acid esters include methyl stearate, ethyl lumitate, and monoglyceride stearate 1. ″:′;1 The above-mentioned protective lubricant layer can also be formed by arranging a plurality of cooling guns in parallel and depositing various materials. The V width is preferably in the range of 20 to 500A, more preferably 20 to 600A.

The metal oxide layer in the present invention is one obtained by oxidizing the surface of the magnetic metal thin film layer by exposing the surface of the magnetic metal thin film layer to glow discharge plasma generated in a vacuum atmosphere containing oxygen gas.

The thickness of the metal oxide layer in the present invention is preferably 600A or less. Particularly preferably, it is 100A or less.

Glow discharge treatment in oxygen in the present invention refers to introducing oxygen gas into the oxidation treatment chamber (2.5 x 10 to 5 x 107"'ro
It is sufficient to maintain the degree of vacuum at rr and apply glow discharge forming power to the glow discharge electrode. As for electric power, RF, D
A wide range of schemes are used, such as C1. Furthermore, glow discharge electrodes of various shapes can be used.

In the present invention, the silicon compound layer is formed by exposing the surface of the metal oxide layer formed on the magnetic thin film layer to glow discharge plasma generated in a vacuum atmosphere containing silicate ester gas. A silicon compound layer deposited on the surface of a silicon compound, consisting of a silicon oxide such as 8102.

The thickness of the silicon compound layer in the present invention is desirably 100% or less. Particularly preferably, it is 10A or less.

The silicon compound layer in the present invention does not necessarily have to form a uniform continuous film. For this reason, it is difficult to define the film thickness in a strict sense, so the film thickness in this specification means the average film thickness obtained by converting the deposits into film thickness.

Since the silicon compound layer in the present invention is extremely thin, it is difficult to clearly define the state of the coating. It is preferable that the coating state is such that the magnetic element, oxygen, and silicon element, which are the main components of the magnetic element, can be detected simultaneously. That is, from an Auger electron analysis perspective, it is desirable that a surface state is formed in which a magnetic element and oxygen, which are the main components of the metal oxide, and a silicon element are mixed.

Glow discharge treatment in a vacuum atmosphere containing silicate ester gas in the present invention is to mix appropriate amounts of silicate ester gas and reaction gas, maintain the degree of vacuum at 5 x 10 to 5 x 10 "1 Torr, and apply electric power for forming glow discharge. A magnetic metal oxide layer is exposed to glow discharge plasma generated by applying voltage to a discharge electrode to form a silicon compound on the surface.

Preferred examples of the silicic acid ester gas include tetramethoxysilane, tetraethoxysilane, tetrabutoxysilane, tetraphenoxysilane, and tetrabenzyloxysilane. Particularly preferred is tetraethoxysilane.

As a reactive gas, it stabilizes glow discharge and
It is desirable to use a substance that helps decompose the silicate ester, such as oxygen and water vapor.

- Generates a discharge. The 11-pack IC uses high frequency power,
Direct current power, alternating current power, microwave power, etc. are required, but 13.56 MHz high frequency power or direct current power is particularly preferred in view of ease of handling. When high frequency power is used, it is possible to cause glow discharge at a vacuum level of 5 x 10 to 6 x 10 Torr. Further, when direct current power is used, glow discharge occurs at a vacuum degree of about 0.02 to 0.5 Torr.

As the discharge electrode, a coil-shaped, rod-shaped, net-shaped electrode, etc. may be provided in the vacuum chamber. Alternatively, a so-called electrodeless discharge in which an electrode is provided outside the vacuum chamber may be used.

Although the roles of the metal oxide layer and the silicon compound layer in the present invention are not necessarily elucidated, it is thought that the formation of the metal oxide layer suppresses the deterioration of the magnetic metal thin film layer over time. The silicon compound layer also enhances the function of retaining an organic protective layer formed subsequent to the formation of the silicon compound layer on the magnetic metal oxide layer.

In particular, the magnetic metal thin film layer containing a large amount of metal oxide layer obtained by surface treatment BJi by the method of the present invention has an uneven surface; silicon compounds are uniformly precipitated even in the gaps between columnar structures. Therefore, the holding function of the aforementioned protective lubricant layer can be further improved.

In addition, even if the metal oxide layer and silicon compound layer in the present invention are used in extremely small amounts, they can greatly improve various practical properties as a magnetic recording medium, such as durability, running characteristics, and weather resistance.
A magnetic recording medium with a low syngloscus and excellent electromagnetic conversion characteristics can be obtained, and its practical value is significant.

Hereinafter, the novel effects of the present invention will be clearly explained based on Examples.

Example 1 Using a magnetic recording medium manufacturing apparatus 1 as shown in FIG.
A 25μ thick polyethylene terephthalate film substrate was placed in a vacuum evaporation apparatus at an angle of 60' with respect to the evaporation source, and was heated at a vacuum level of 99°C at a vacuum level of I x 10-Torr.
99- cobalt metal was electro-bead-deposited.

Subsequently, the surface of the cobalt vapor deposition layer was 7x10
In an oxygen gas atmosphere of Torr, a voltage of 1 kV was applied to the electrode to form an oxide layer of about 7 OA by exposing the electrode to a DG gucrodischarge zolazma for about 1 minute.

Next, tetraethoxysilane gas and oxygen gas were introduced, and the partial pressure ratio was maintained at approximately 1:1 at 6X10'''Torr.
The surface of the cobalt vapor deposited layer, whose surface was oxidized, was exposed for about 1 minute to glow discharge plasma generated with a high frequency power of 200 Watts in an atmosphere of 100 watts to form an 8102 film equivalent to about 8 people. Thereafter, henic acid was heated to 15O at a rate of 20A/sec by resistance heating in a vacuum of 1X10Tnrr.
A was deposited. Thereafter, it was slit into a 1-inch wide tape, which was then incorporated into a cassette of a VH8 compact VTR and evaluated as a magnetic tape.

That is, the durability, runnability, and weather resistance of the thus produced recording medium were evaluated as follows. Durability was evaluated by measuring the still durability time, that is, the time until the playback output signal is halved after the tape stopped running in a VH8 compact VTR device. The evaluation of running characteristics was 100 for VHS type small VTR equipment.
After repeatedly running the tape, the number of scratches that could be observed with the naked eye in the direction of the tape was counted. The methyl durability time and scratch evaluation are also manufactured by Matsushita Electric Co., Ltd. (7)
An NV-8200 type VTR device was used.

To evaluate the durability, the rate of decrease in magnetization (demagnetization) after being kept in a thermostat at 60 tons and 90 inches RH for two weeks was compared. Table 1 shows still durability time, number of scratches, and demagnetization.

For comparison, Table 1 also shows a case in which the protective lubricant layer is good without treatment (at) and a case in which the protective lubricant layer is untreated (bl). (Configuration of the protective lubricant layer in the case of BL is the same thickness of ihenic acid.Here, untreated means that the metal oxide layer and the silicon compound layer are not provided. Significant improvements in methyl durability, scratch resistance, and weather resistance were observed.

111 As described above, according to the present invention, a magnetic recording medium with excellent durability, runnability, and weather resistance can be obtained, and the effects of its practical application are extremely promising.

In Example 1, mellow discharge plasma was generated by applying high-frequency power, but similar effects were confirmed by glow discharge by applying DC voltage.

[Brief explanation of the drawing]

FIG. 1 is a side view showing a main part of an embodiment of the magnetic recording medium manufacturing apparatus used in the present invention, with section 2r9. ] - FIG. 5 is a schematic cross-sectional view of a magnetic recording medium according to the present invention. 2 is a substrate delivery and winding chamber, 6 is a glow discharge pretreatment chamber, 4 is a magnetic metal thin film layer deposition chamber, 5 is an oxidation treatment chamber, 6 is a silicate ester glow discharge treatment chamber, 7 is a protective lubricant layer deposition chamber, W is the base body. Figure 1

Claims (2)

[Claims] (1) A magnetic metal thin film layer having a metal oxide layer on the surface, a silicon compound layer, and an organic polymer compound on a nonmagnetic substrate,
A magnetic recording medium comprising a protective lubricant layer laminated with a higher fatty acid, a fatty acid ester, or a combination thereof. (2) After forming a magnetic metal thin film layer on a non-magnetic substrate by an enomi-deposition method, the surface of the magnetic metal thin film layer is exposed to glow discharge plasma generated in a vacuum atmosphere containing oxygen gas. A metal oxide layer is formed on the surface, and then a silicon compound layer is formed on the surface by exposing the surface of the metal oxide layer to glow discharge plasma generated in a vacuum atmosphere containing silicate ester gas, and then a silicon compound layer is formed on the surface. A method for manufacturing a magnetic recording medium, characterized in that a protective lubricant layer made of an organic polymer compound, higher fatty acid, fatty acid ester, or a combination thereof is formed on the silicon compound layer.