JPH0886908A - Optical component manufacturing method and optical component manufacturing apparatus - Google Patents

Abstract

(57) [Summary] [Purpose] An undercoat layer and an overcoat layer are provided on a plastic optical component and a metal reflective film, respectively.
In addition to being able to form with a chemically stable film quality such as reducing internal stress and making it less likely to cause cracks, etc., it also makes it difficult to peel off, and in the same system without pretreatment It is possible to reduce the apparatus cost and the like by performing the film forming process. [Structure] A step of forming a Si-Al-O-N compound thin film on one surface of a synthetic resin optical member molded into a desired shape, and then, on the Si-Al-O-N compound thin film. forming and forming the Si-Al-O-N-based compound metal reflective layer having high reflectance than thin, then the oxide ceramic thin film made of Al ₂ O ₃ on the metal reflective film Including and

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing an optical component and an apparatus for manufacturing an optical component, and more particularly, to a reflecting mirror of a light beam scanning system such as a laser beam mounted on a facsimile, a printer or the like, for example, f. It can be applied to the reflection film forming technology of the -θ mirror optical system and the optical thin film forming technology of other optical parts such as plastic optical lenses. In particular, an undercoat layer and a metal reflection film are formed on the plastic optical parts and the metal reflection film, respectively. The overcoat layer can be formed with a chemically stable film quality such that internal stress can be reduced and cracks and the like can be made less likely, and at the same time, a method for manufacturing an optical component and an optical component that can be made difficult to peel off. Manufacturing equipment.

[0002]

2. Description of the Related Art The technique of forming a reflection film or an optical thin film on a plastic (synthetic resin) optical component is an organic substance, so that a metal thin film or a dielectric substance different from the organic substance is formed on the surface of the organic substance of the optical component. It is generally difficult to form an inorganic thin film such as a thin film. Thus, it is difficult to form the inorganic thin film on the organic surface of the optical component, because the organic surface and the inorganic thin film of the optical component are bonded by bonding because of the following reasons. Due to the difference in the thermal expansion coefficient of the inorganic thin film and the presence of internal stress in the inorganic thin film, the bonding interface between the organic optical component and the inorganic thin film is clearly separated, and the bonding interface becomes unclear like bonding by chemical bonding. This is because they do not have a bonding force (adhesive force) that can be countered by external stress.

For this reason, even if an inorganic thin film is bonded to an organic optical component by bending, the bonding strength is weaker than that in the case of bonding by a chemical bond. The inorganic thin film is easily peeled off from the optical component. Unlike optical components made of glass, most of optical components made of plastic or the like cannot be subjected to heating film formation such as ion plating or plasma CVD for removing impurities by surface heating or increasing adhesive force. Is.

When, for example, a polycarbonate resin is used as the plastic optical component, the heat distortion temperature of the polycarbonate resin is as low as about 130 ° C. as compared with glass. Therefore, in the case of plastic optical parts such as polycarbonate, degassing treatment by high temperature heating before film formation to improve the adhesive force on the optical parts, and formation of a thin film having a high adhesive property and film structure by the high temperature process. However, the high temperature process that has been carried out for the conventional glass optical components cannot be directly applied because it does not have heat resistance. Therefore, since the plastic optical parts cannot be subjected to high temperature processing and must be processed at low temperature, the film forming process including film forming pretreatment is
It is a general limitation to carry out at a low temperature of around 00 ° C.

Conventionally, a vacuum vapor deposition method by high temperature film formation is used for forming an optical thin film on a plastic optical component.
For example, in the case of manufacturing a reflecting mirror on a plastic optical component, a metal reflecting film to be a reflecting mirror, for example, an Al metal reflecting film is formed. At this time, SiO, SiO ₂ or the like is formed on the plastic optical component. After forming an inorganic thin film as an undercoat layer, SiO, Si which becomes an undercoat layer
A metal reflection film made of an Al metal thin film is formed as a second layer on an inorganic thin film such as O ₂ . Then, in order to further increase the environment resistance of the Al metal reflection film, an inorganic thin film which is an overcoat layer excellent in corrosion resistance and scratch resistance is formed.

A conventional reflecting mirror constituted by sequentially forming an SiO inorganic undercoat layer, an Al metal reflection film and a SiO inorganic overcoat layer on this plastic optical component has been reported in, for example, Japanese Patent Application Laid-Open No. 2-50104. ing. Here, as an example of the method for forming the optical thin film on the plastic optical component, as described above, the high temperature film formation by the vacuum vapor deposition method is illustrated, but the method can form the film at a lower temperature than the vacuum vapor deposition method. Examples of the method include a sputtering method. A conventional reflecting mirror in which an optical thin film is formed on the plastic optical component by low-temperature film formation by a sputtering method is reported in, for example, Japanese Patent Laid-Open No. 4-176876.

However, in the conventional reflecting mirror in which the optical thin film is formed on the plastic optical component by the former vacuum vapor deposition method described above, the optical thin film formed on the plastic optical component is generally not dense and the optical thin film is not formed. Internal stress tends to be a tensile stress on the plastic optical component. Further, in the conventional reflecting mirror in which the optical thin film is formed on the plastic optical component by the latter sputtering method described above, the film quality becomes dense, but the internal stress of the optical thin film tends to be a compressive stress with respect to the plastic optical component. is there.

In this way, the internal stress of the optical thin film formed on the plastic optical component becomes tensile stress and compressive stress, and the plastic optical component and the optical thin film are affected by the influence of dirt and water on the surface of the plastic optical component. In these conventional mirrors where the bonding interface is not chemically bonded but is clear, the internal stress of the remaining optical thin film,
There is a problem that the optical thin film is peeled off from the plastic optical component due to external thermal and mechanical stress.

Therefore, in the prior art for making it difficult to cause the problem that the optical thin film is peeled off from the plastic optical component, surface treatment with an ion beam is performed before film formation, or the surface of the plastic optical component is plasma cleaned before film formation. A method of activating the surface of the plastic optical component so as to increase the adhesion of the optical thin film to the plastic optical component by performing ion beam cleaning or ion beam cleaning has been investigated. The former ion beam surface treatment is described in, for example, JP-A-5-140.
No. 728 is reported.

[0010]

However, a SiO undercoat layer on the above plastic optical component,
In a conventional reflecting mirror configured by sequentially forming an Al metal reflection thin film and a SiO overcoat layer, SiO that is an undercoat layer and an overcoat layer may be formed depending on film forming conditions when performing film formation by a vacuum deposition method or a sputtering method. There is a problem that the refractive index and the absorption coefficient are easily influenced, it is difficult to form a stable film, and the internal stress is increased, so that cracks and the like are easily generated.

Further, in the case of SiO used as the undercoat layer and the overcoat layer, the internal stress becomes large and S
Since the interface in contact with iO is not chemically bonded but only weakly bonded, there is a problem that it is easily peeled from the underlying film due to external thermal or mechanical stress. For example, it is the current situation that the SiO film is easily peeled off after a high temperature and high humidity test of 60 ° C. and 90% is performed.

Further, in order to make it difficult to peel off the SiO ₂ film from the underlying film, before the SiO film is formed, the underlying plastic optical component and the Al metal reflection film are previously cleaned by plasma cleaning, UV / O ₃ cleaning, and In the conventional method of performing pretreatment such as ion beam cleaning, the pretreatment cannot be performed in the same system as the film forming container and must be performed by so-called outer batch processing, resulting in high equipment cost. In addition to being a hanging
There is a problem that the device cost increases. In addition, since all of these pretreatments are dry treatments, the surface of the plastic optical component may be roughened, which eventually obstructs the adhesive force and makes it impossible to obtain sufficient adhesive force. Charged up was likely to occur on the surface unless the irradiation of the beam was applied.

Therefore, according to the present invention, an undercoat layer and an overcoat layer can be formed on a plastic optical component and a metal reflection film, respectively, so that internal stress can be reduced and cracks can be made less likely to occur chemically. A method of manufacturing an optical component, which can be formed with a stable film quality, can be made difficult to peel off, and can be formed into a film in the same system without pretreatment to reduce apparatus cost and the like. And an optical component manufacturing apparatus.

[0014]

According to the first aspect of the present invention,
A step of forming a Si-Al-O-N based compound thin film on one surface of a synthetic resin optical member molded into a desired shape,
Then, the Si on the Si-Al-O-N-based compound thin film
A step of forming a metal reflection film having a reflectance higher than that of the -Al-O-N compound thin film, and then forming Al on the metal reflection film.
_And a step of forming an oxide-based ceramics thin film made of ₂ O ₃ .

According to a second aspect of the invention, in the invention of the first aspect, the formation of the Si—Al—O—N compound thin film on one surface of the synthetic resin optical member is performed by a magnetron cathode. The sialon target placed on the electrode is sputter-deposited using the RF sputtering method, and the residual stress of the Si-Al-O-N compound thin film is adjusted by changing the oxygen partial pressure during RF magnetron sputtering. It is characterized by doing.

The invention according to claim 3 is the same as claims 1 and 2 above.
In the invention described above, the formation of the metal reflection film on the Si-Al-O-N-based compound thin film is performed by sputter deposition of an Al metal target placed on a non-equilibrium magnetron electrode by a DC magnetron sputtering method. It is a feature. According to a fourth aspect of the invention, in the invention according to the third aspect, when the Al metal reflective film is formed by a DC magnetron sputtering method using the non-equilibrium magnetron electrode, residual stress of the Al metal reflective film is formed. A negative bias is applied to the synthetic resin optical member so as to control

According to a fifth aspect of the present invention, in addition to the first to fourth aspects of the present invention, the Al on the metal reflection film is formed.
The formation of the ₂ O ₃ oxide ceramics thin film is characterized in that the Al metal target placed on the non-equilibrium magnetron electrode is sputter-deposited to an optical film thickness by the DC magnetron reactive sputtering method. According to a sixth aspect of the present invention, in the above-mentioned fifth aspect of the present invention, a differential exhaust device is provided when the Al ₂ O ₃ oxide ceramic thin film is formed on the metal reflection film by a DC magnetron reactive sputtering method. It is characterized in that the partial pressure of the reactive gas at the time of sputtering is constantly monitored by the mass spectrometer provided therein.

The invention according to claim 7 is the above claim 5, 6
In the invention described above, the Al ₂ O is formed on the metal reflection film.
When forming a _3- oxide ceramic thin film by the DC magnetron reactive sputtering method, a negative bias is applied to the synthetic resin optical member so as to control the residual stress of the Al ₂ O ₃ oxide ceramic thin film. It is a feature.

According to an eighth aspect of the invention, in the inventions of the first to seventh aspects, the electron cyclotron resonance plasma is formed before the Si—Al—O—N type compound thin film is formed on the synthetic resin optical member. The optical member made of synthetic resin is exposed to the inside for processing. The invention described in claim 9 is the same as the invention described in claim 8,
When plasma-treating the synthetic resin optical member, impurities that separate from the surface of the synthetic resin optical member are monitored by a mass spectrometer.

According to a tenth aspect of the present invention, a load vacuum chamber for loading the synthetic resin optical member, a cleaning vacuum chamber for plasma-cleaning the synthetic resin optical member, and one surface of the synthetic resin optical member are provided. sequentially forming a RF sputtering vacuum chamber to form a Si-Al-O-N-based compound thin film, a metal reflection film and Al ₂ O ₃ oxide ceramic thin film on the Si-Al-O-N-based compound thin film to DC sputter vacuum chamber, unloading vacuum for unloading the synthetic resin optical member on which the Al ₂ O ₃ oxide ceramic thin film, the metal reflection film and the Si—Al—O—N compound thin film are formed The present invention is characterized in that it has a chamber and a transfer means for sequentially feeding a tray accommodating a plurality of synthetic resin optical members collectively to each vacuum chamber.

The invention according to claim 11 is the above-mentioned claim 10.
In the invention described above, the conveying means is composed of a non-magnetic steel belt. Claim 1
The invention described in 2 is the invention described in claim 11,
The surface of the shaft that drives the steel belt is in contact with the steel belt and is coated with TiN coating.

[0022]

According to the invention described in claim 1, Si-Al-O- is formed on one surface of the synthetic resin optical member molded into a desired shape.
After forming an N-based compound thin film and forming a metal reflective film on the Si-Al-O-N-based compound thin film, which serves as a reflective surface having a higher reflectance than the Si-Al-O-N-based compound thin film, metal reflection is performed. An oxide ceramic thin film made of Al ₂ O ₃ is formed on the film.

For this reason, Si-Al-O is used as an undercoat of an optical component which has a higher fracture toughness value and hardness than the conventional SiO on the surface of a water-absorbing plastic component and has a low water-absorption property and is chemically stable. Since the -N-based compound thin film can be formed by reducing the internal stress, it is possible to prevent cracks in the film and peeling of the film from occurring easily. Then, a metal reflection film is formed on the Si-Al-O-N-based compound thin film, and a conventional Si film is used as an overcoat.
Internal stress of Al ₂ O ₃ oxide ceramics thin film, which has a much smaller absorption coefficient than O, is excellent in environment resistance and scratch resistance, and is less affected by film forming conditions than conventional SiO and has uniform optical properties. Can be formed with reduced
It is possible to prevent cracks in the film and peeling of the film from occurring easily.

Further, since the Si-Al-O-N compound thin film and the Al ₂ O ₃ oxide ceramic thin film can be formed in the same system without pretreatment, the cost of the equipment, etc. Can be reduced. Therefore, it is possible to form an optical component such as a plastic reflecting mirror, which has a very small change over time and is less likely to cause film peeling, at low cost.

According to the second aspect of the invention, the Si-Al-O-N compound thin film is formed on one surface of the optical member made of synthetic resin by the RF sputtering method using the sialon target placed on the magnetron cathode electrode. The residual stress of the Si—Al—O—N based compound thin film is adjusted by changing the oxygen partial pressure during RF magnetron sputtering.

Therefore, a Si-Al-O-N type compound thin film is formed on one surface of the synthetic resin optical member by sputter-depositing the sialon target placed on the magnetron cathode electrode using the RF sputtering method. At this time, the residual stress of the Si—Al—O—N-based compound thin film can be adjusted in the film thickness direction by changing the oxygen partial pressure during RF magnetron sputtering.

Therefore, since the fracture toughness value is large, the compressive stress can be increased on the surface of the synthetic resin optical member with respect to the deformation of the synthetic resin optical member, and the compressive stress becomes closer to the metal reflection film. Since the amount can be reduced, it is possible to efficiently prevent the Si—Al—O—N compound thin film from peeling off from the synthetic resin optical member.
In the invention according to claim 3, the formation of the metal reflection film on the Si-Al-O-N compound thin film is performed by sputter deposition of an Al metal target placed on the non-equilibrium magnetron electrode by the DC magnetron sputtering method. To configure.

Therefore, the ions can be spread radially on the substrate by the ion assist effect of the non-equilibrium magnetron, so that a dense Al metal reflection film in which residual stress is relaxed can be efficiently formed. According to the invention of claim 4, when the Al metal reflective film is formed by the DC magnetron sputtering method using the non-equilibrium magnetron electrode, the synthetic resin optical member is negatively controlled so as to control the residual stress of the Al metal reflective film. It is configured to apply a bias.

Therefore, when the Al metal reflective film is formed by the DC magnetron sputtering method using the non-equilibrium magnetron electrode, a negative bias is applied to the synthetic resin optical member so as to control the residual stress of the Al metal reflective film. Since they can be applied, the plasma ions of the non-equilibrium magnetron can be positively drawn into the synthetic resin optical member to strongly bring out the ion beam assist effect. Therefore, it is possible to efficiently form a dense Al metal reflective film in which residual stress is relaxed.

In the invention according to claim 5, the Al ₂ O ₃ oxide ceramic thin film is formed on the metal reflection film by sputtering the Al metal target placed on the non-equilibrium magnetron electrode by the DC magnetron reactive sputtering method. It is configured to do so. For this reason, the Al metal target placed on the non-equilibrium magnetron electrode was deposited on the metal reflective film by Al ₂ O ₃ by DC magnetron reactive sputtering method.
Since the oxide ceramic thin film can be formed, the ions can be spread radially on the metal reflection film by the ion assist effect of the non-equilibrium magnetron. Therefore, a dense and relaxed residual stress Al ₂ O ₃ oxide can be formed. The ceramic thin film can be formed efficiently.

According to the sixth aspect of the invention, when the Al ₂ O ₃ oxide ceramic thin film is formed on the metal reflection film by the DC magnetron reactive sputtering method, the sputtering is performed by the mass spectrometer having the differential evacuation device. It is configured to constantly monitor the partial pressure of the reactive gas. For this reason, the Al ₂ O ₃ oxide ceramic thin film should be DC-coated on the metal reflection film.
When forming by the magnetron reactive sputtering method, the partial pressure of the reactive gas during sputtering can be constantly monitored by the mass spectrometer with a differential evacuation device, so the proper ratio of the sputtering gas and reactive gas can be determined. Can be maintained.

Therefore, stoichiometric Al ₂ O
_Since it is possible to precisely monitor the formation of the trioxide ceramic thin film and the reactive gas partial pressure, it is possible to control the optical characteristics such as the refractive index and to obtain a hard Al ₂ O ₃ oxide having a small water absorption coefficient. The ceramic thin film can be formed efficiently. In the invention according to claim 7, A
When the l ₂ O ₃ oxide ceramic thin film is formed by the DC magnetron reactive sputtering method using the non-equilibrium magnetron electrode, the synthetic resin optical member is controlled so as to control the residual stress of the Al ₂ O ₃ oxide ceramic thin film. It is configured to apply a negative bias to.

Therefore, when the Al ₂ O ₃ oxide ceramic thin film is formed by the DC magnetron reaction sputtering method using the non-equilibrium magnetron electrode, Al ₂ O _{3 is used.}
Since a negative bias can be applied to the synthetic resin optical member so as to control the residual stress of the oxide ceramic thin film, the plasma ions of the non-equilibrium magnetron are positively attracted to the synthetic resin optical member and the ion beam The assist effect can be brought out strongly. Therefore, it is possible to efficiently form a dense Al ₂ O ₃ oxide ceramic thin film in which residual stress is relaxed.

According to the eighth aspect of the invention, before the Si-Al-O-N compound thin film is formed on the synthetic resin optical member, the synthetic resin is contained in the charged particles and the excited species in the electron cyclotron resonance plasma. The optical member is configured to be exposed and processed. Therefore, Si-on the synthetic resin optical member
Since the synthetic resin optical member can be exposed to the charged particles and excited species in the electron cyclotron resonance plasma before forming the Al-O-N compound thin film,
The synthetic resin optical member can be processed in a highly uniform and stable plasma at a low temperature. As a result, the surface of the synthetic resin optical member can be treated by dry cleaning using a low-temperature process using plasma, so that water and organic substances on the surface of the synthetic resin optical member can be removed and the surface of the synthetic resin optical member can be activated. Can be done at the same time.

Therefore, the surface of the synthetic resin optical member can be appropriately roughened by the impact of plasma particles to improve the adhesiveness, and at the same time, the bonding chains of molecules on the surface of the synthetic resin optical member have high energy in plasma. Activated by being bombarded by ions having a
The bond between (carbon) and an inorganic element such as Si or Al can be protected to improve the adhesion.

According to the ninth aspect of the invention, when the synthetic resin optical member is subjected to plasma treatment, impurities separated from the surface of the synthetic resin optical member are monitored by a mass spectrometer. Therefore, when the synthetic resin optical member is subjected to the plasma treatment, impurities such as H ₂ O that are released from the surface of the synthetic resin optical member can be monitored by the mass spectrometer. As a result, the plasma cleaning end point can be surely grasped based on the information of the plasma cleaning end point of the kind of the synthetic resin optical member which is obtained by monitoring by the mass spectrometer in advance.
Therefore, various synthetic resin optical members can be efficiently plasma-treated, so that productivity can be improved and film-forming quality can be secured.

In a tenth aspect of the present invention, a load vacuum chamber for carrying in the synthetic resin optical member, a cleaning vacuum chamber for plasma-cleaning the synthetic resin optical member, and a Si-on one surface of the synthetic resin optical member. a RF sputtering vacuum chamber Al-O-N system to form a compound thin film, Si-Al-O-N-based metal compound thin film reflective film and Al ₂ O ₃ oxide ceramics film are sequentially formed to DC sputtering vacuum chamber And Al ₂ O
₃ Oxide ceramic thin film, metal reflective film and Si-A
An unloading vacuum chamber for carrying out the synthetic resin optical member on which the l-O-N compound thin film is formed, and a tray for accommodating a plurality of synthetic resin optical members at once in sequence to each vacuum chamber And means.

For this reason, the trays for accommodating a plurality of synthetic resin optical members at a time can be sequentially conveyed to the respective vacuum chambers, so that the trays are not exposed to the atmosphere even once in each film forming process. In addition, it is possible to perform mass batch processing by the in-line method, and it is possible to realize high-quality mass production. In the invention described in claim 11, the conveying means is constituted by a non-magnetic steel belt.

For this reason, since a tray containing a large number of synthetic resin optical members can be conveyed to each vacuum container by the non-magnetic steel belt, the distribution of magnetic force lines during magnetron sputtering is not adversely affected and stable. The tray can be transported. At this time, it is desirable that the tray is made of non-magnetic SUS or the like, in consideration of not adversely affecting the magnetic field line distribution during magnetron sputtering.

According to the twelfth aspect of the present invention, the surface of the shaft for driving the steel belt, which is in contact with the steel belt, is coated with TiN. For this reason, the TiN coating with excellent wear resistance is applied to the surface of the shaft that drives the steel belt in contact with the steel belt, so wear debris due to contact between the steel belt and shaft is generated by the TiN coating with excellent wear resistance. It can be hard to do. Therefore, it is possible to efficiently prevent contamination (contamination) in the vacuum container.

[0041]

Embodiments of the present invention will be described below with reference to the drawings in comparison with comparative examples. First, a method for manufacturing an optical product of a comparative example (conventional example) will be described. FIG. 1 is a sectional view showing the structure of an optical component of a comparative example. The optical component in the illustrated example is applied to a reflecting mirror.

In FIG. 1, 1 is an optical component made of plastic such as polycarbonate resin, 2 is an SiO undercoat layer having a film thickness of about 500 angstrom formed on the plastic optical component by vacuum deposition,
3 is a film thickness 800 formed on the SiO undercoat layer 2
Al metal reflection film of about 1000 angstroms, 4 is an optical film thickness (nd = λ ₀₎ on the Al metal reflection film 3.
The SiO overcoat layer formed in / 2). Here, n is a refractive index, d is an actually deposited film thickness (physical film thickness), and λ ₀ is a wavelength of light. In this reflecting mirror, λ ₀ is the designed dominant wavelength, and the optical film thickness keeps the reflectance of the Al metal reflecting film 3 in a good condition. Si
Since the O undercoat layer 2 only needs to be able to function the Al metal reflection film 3 as a reflection layer, the SiO undercoat layer 2 may have any thickness, but in reality, the stress of the film (of each layer The film thickness is set to about 500 angstroms so that the stress is balanced, the film is not peeled off, and the film is not affected by moisture or the like from the plastic optical component 1.

However, in this conventional method, when polycarbonate is used as the plastic constituting the plastic optical component 1, the coefficient of thermal expansion of the polycarbonate is 1 as compared with that of the inorganic thin film of the SiO undercoat layer 2.
As described above, when the SiO constituting the SiO overcoat layer 4 is vapor-deposited on the plastic optical component 1 by the vacuum vapor deposition method, as described above, the SiO overcoat layer 4 becomes a tensile stress, and the stress applied from the outside causes the SiO overcoat layer 4 to become a tensile stress. There arises a problem that the SiO overcoat layer 4 is easily peeled off from the plastic optical component 1.

Therefore, in this embodiment, in order to solve this problem, the following means are adopted and solved. Hereinafter, a specific description will be given with reference to the drawings. FIG. 2 is a cross-sectional view showing the structure of the optical component of one embodiment according to the present invention. In this embodiment, first, a plastic optical component 1 made of a polycarbonate resin is prepared by using a sialon target placed on a magnetron cathode electrode by an RF magnetron sputtering method.
An Si—Al—O—N based compound thin film 5 is formed thereon by sputtering to have a film thickness of about 500 to 600 angstroms.

The polycarbonate resin constituting the plastic optical component 1 generally expands microscopically due to an increase in external environmental temperature and water absorption, and acts in the direction of pulling the thin film. As shown in FIG. 3, the Si—Al—O—N-based compound thin film 5 has an oxygen gas partial pressure of 1 × 10 ⁻⁵ Tor during sputtering.
When it becomes larger than around r, the stress (compressive stress) starts to decrease significantly. Therefore, near the surface of the plastic optical component 1, the compressive stress of the Si—Al—O—N-based compound thin film 5 is increased and the Al metal thin film formed on the Si—Al—O—N-based compound thin film 5 is formed. By increasing the oxygen partial pressure as it approaches 6, the film is formed so as to reduce the compressive stress.

Next, A placed on the non-equilibrium magnetron electrode
A metal target is sputter-deposited on the Si—Al—O—N-based compound thin film 5 by a DC magnetron sputtering method to form an Al metal reflective film 6 having a film thickness of about 800 to 1000 angstrom. At this time, the magnetron electrode is
A non-equilibrium type magnetron electrode having a structure as shown in FIG. 4 is adopted, and unlike the magnetron electrode of the comparative example shown in FIG. 5, a part of plasma is drawn out toward the substrate 11 by the magnetic field lines 12 a toward the substrate 11. As a result, the effect of ion assist can be effectively brought out.

In FIGS. 4 and 5, 13a and 13b
Is a yoke, 14a and 14b are magnets, and 15 is a target. In the magnetron electrode of the comparative example, as shown in FIG. 5, the magnetic force lines 12b of the magnetron electrode are strong and are restrained by the magnet 14b in the electrode.
In the non-equilibrium magnetron electrode of this embodiment, as shown in FIG. 4, ions flow into the vicinity of the substrate 11. Furthermore, the substrate 1
-100 to -120V to 1 plastic optical component 1
The ion assist effect can be further enhanced by applying a negative bias voltage of a certain degree.

For this reason, the Al target is used as a DC magnet.
For long sputtering (sputtering gas is Ar gas),
Due to this ion assist effect, the remaining Al metal reflection film 6 remains.
The residual stress can be relieved significantly. Next, shown in FIG.
Al target placed on a non-equilibrium magnetron electrode
At the same time as sputtering the
By DC magnetron reactive sputtering method with oxygen addition
Al₂O₃Oxide ceramics film 7 at low temperature
It is formed on the metal reflective film 6. At this time, Al₂O₃Oxidation
The material-based ceramic film 7 has a refractive index substantially equal to that of SiO,
S formed by the vacuum deposition method by the ion assist effect
Formed to be denser and have smaller residual stress than the iO thin film
To be done. This Al ₂O₃Film of oxide-based ceramic film 7
The thickness is the optical film thickness (nd = λ₀/ 2). Also, A
l₂O₃The oxide-based ceramic film 7 is different from SiO.
It has almost no absorption and has a high hardness, so it is resistant to the environment.
The overcoat layer has excellent properties.

Next, a method of manufacturing the optical component of this embodiment will be described using an optical component manufacturing apparatus. FIG. 6 is a schematic diagram showing the configuration of an optical component manufacturing apparatus according to an embodiment of the present invention, and FIG. 7 is a diagram showing a state in which a large number of plastic optical components are set on the tray shown in FIG. Is. In this embodiment, as shown in FIGS. 6 and 7, a large number of plastic optical components 1 are set on the tray 21. The tray 21 on which a large number of plastic optical components 1 are set is conveyed with the surface on which the plastic optical components 1 are set facing downward.

The film forming apparatus of the present embodiment comprises five vacuum chambers, and a load chamber 22 for carrying in a tray 21 on which the plastic optical component 1 is set with the surface on which the plastic optical component 1 is set facing down. And a cleaning chamber 24 for plasma-cleaning the plastic optical component 1 of the tray 21 loaded from the load chamber 22 through the gate valve 23.
An RF sputtering chamber 26 for forming the Si—Al—O—N-based compound thin film 5 on the plasma-cleaned plastic optical component 1 of the tray 21 carried in from the cleaning chamber 24 through the gate valve 25, and an RF sputtering chamber. 26 through the gate valve 27, the Al metal reflective film 6 and the Al ₂ O ₃ oxide ceramic film on the Si—Al—O—N based compound thin film 5 formed on the plastic optical component 1 of the tray 21. DC sputtering chamber 2 for sequentially forming 7
8 and the Al ₂ O ₃ oxide ceramics film 7, the Al metal reflection film 6, and the Si—Al—O—N compound thin film 5 of the tray 21 carried in from the DC sputtering chamber 28 through the gate valve 29 were formed. It comprises an unload chamber 30 for transporting the plastic optical component 1.

Each vacuum container 22, 24, 26, 28, 30
Are evacuated using a vacuum pump such as a turbo molecular pump (not shown). Each vacuum container 22, 24, 26, 2
The gate valves 23, 25, 27, and 29 are connected to each other by 8 and 30 to block or open them. The plastic optical component 1 set on the tray 21 is conveyed by a non-magnetic steel belt installed in each vacuum container 22, 24, 26, 28, 30.

As shown in FIG. 8, the tray 21 is placed on a steel belt 32 stretched around a shaft 31, moved into each of the vacuum containers 22, 24, 26, 28, 30 and conveyed. The shaft 31 includes the vacuum containers 22 and 2.
It is rotated by a drive mechanism (not shown) of 4, 26, 28, 30. As shown in FIG. 8C, the portion A where the steel belt 32 is in contact with the shaft 31 is coated with TiN to maintain wear resistance.

Next, the tray 2 placed in the load chamber 22
1 is sent to the next cleaning chamber 24 through the gate valve 23. In the cleaning chamber 24, before forming the Si—Al—O—N type compound thin film 5 on the plastic optical component 1, E
Oxygen plasma generated from the CR plasma source 41 removes contaminants on the surface of the plastic optical component 1 and activates the surface. Here, 42 and 43 are an ECR plasma generation power source and a plasma generation gas introduction system, which are connected to the ECR plasma source 41, respectively. At this time, 80 ~
The mass spectrometer 45 monitors the desorption state of water and organic impurities (oil component at the time of molding) desorbed from the surface when oxygen plasma is irradiated at a low temperature of 90 ° C. Here, 46 is a valve for introducing gas into the mass spectrometer 45, and 47 is a controller of the mass spectrometer 45. Here, the type and amount of impurities desorbed from the surface of the optical component 1 can be grasped, and it is possible to quantitatively know whether or not the cleaning is surely performed. Therefore, it is possible to monitor the end point of the cleaning process. You can

Next, after performing plasma treatment in the cleaning chamber 24, in the RF sputtering chamber 26, an undercoat layer is formed on the plastic optical component 1 by the RF magnetron sputtering method using a Si-Al-O-N target. Becomes S
The i-Al-O-N type compound thin film 5 is formed. here,
Reference numeral 51 is a magnetron electrode, and a Si-Al-O-N (sialon) target 52a is placed on the magnetron electrode 51.
The RF power source 52 is connected.

Ar gas and O are introduced into the RF sputtering chamber 26.
₂Each gas is introduced by the gas introduction system 53 such as gas
₂The gas introduction ratio can be changed. here
Then, the partial pressure at the time of sputtering is monitored by the mass spectrometer 45.
To be nittered. 10 for sputtering^-3-10^-2Torr
Since the pressure is about the same, the operating pressure of the mass spectrometer 45
It is not within the power range. For this reason, the lowest pressure
In order to reduce the amount of
A gas introduction system 56 having holes (orifices) is used to
The lube 57 is opened and introduced into the mass spectrometer 45. Then S
Al metal reflective film on i-Al-O-N measuring compound thin film 5
6 and Al₂O ₃The oxide ceramic film 7 is a DC spa.
On the non-equilibrium magnetron electrode 61 in the chamber 28.
Target by DC sputtering method or DC reactive sputtering method.
Film formation sequentially. The DC sputter chamber 28 has a reactive spa.
The reaction gas introduction system 62 required at the time of
A DC power source 63 is connected to the balanced magnetron electrode 61.
ing. The tray 21 is an electrode for bias application that can be freely attached and detached.
A negative DC bias is applied by 65. Bias sign
A bias power source 66 is connected to the additive electrode 65.
It Again, the shape of the Si-Al-O-N compound thin film 5
Partial pressure of sputtering gas via valve 67 in the same way as when
Monitor. Then, plasma cleaning before film formation, S
Formation of i-Al-O-N compound thin film 5 and reflection of Al metal
Formation of film 6 and Al₂O₃Of oxide-based ceramic film 7
After the formation and the series of treatments, the tray 21 is
It is unloaded from the unload chamber 30.

Thus, in this embodiment (claim 1),
S on the plastic optical component 1 molded into the desired shape
An i-Al-O-N based compound thin film 5 is formed, and Si-Al is formed.
After forming the Al metal reflection film 6 on the —O—N-based compound thin film 5 as a reflection surface having a higher reflectance than the Si—Al—O—N-based compound thin film 5, Al ₂ is formed on the Al metal reflection film 6. The oxide-based ceramics thin film 7 made of O ₃ is formed.

Therefore, the surface of the optical component 1 made of plastic having water absorbency has a larger fracture toughness value and hardness than conventional SiO, and also has a low water absorbability and is chemically stable as an undercoat of Si. Since the Al—O—N compound thin film 5 can be formed with reduced internal stress, it is possible to prevent cracks in the film and peeling of the film from occurring. Then, a metal reflection film 6 is formed on the Si-Al-O-N-based compound thin film 5, and as an overcoat, an environment resistance having an absorption coefficient much smaller than that of conventional SiO,
Since it is possible to form an Al ₂ O ₃ oxide ceramics thin film 7 which has excellent scratch resistance, is less affected by film forming conditions than conventional SiO, and has uniform optical properties, with reduced internal stress, It is possible to prevent cracks in the film and peeling of the film from occurring easily.

Further, the Si--Al--O--N type compound thin film 5
Since the Al ₂ O ₃ oxide ceramic thin film 7 and the Al ₂ O ₃ oxide ceramic thin film 7 can be formed in the same system without performing pretreatment, the cost of the apparatus can be reduced. Therefore,
It is possible to form an optical component such as a plastic reflecting mirror at a low cost, in which the secular change is very small and the peeling of the film hardly occurs.

In this embodiment (claim 2), the Si-Al-O-N compound thin film 5 is formed on the plastic optical component 1 by using the sialon target 52a placed on the magnetron cathode electrode by the RF sputtering method. The residual stress of the Si—Al—O—N-based compound thin film 5 is adjusted by changing the oxygen partial pressure during RF magnetron sputtering.

Therefore, when the Si-Al-O-N compound thin film 5 is formed on the plastic optical component 1 by sputter-depositing the sialon target 52a placed on the magnetron cathode electrode using the RF sputtering method. , The residual stress of the Si-Al-O-N compound thin film 5 can be adjusted in the film thickness direction by changing the oxygen partial pressure during RF magnetron sputtering.

Therefore, the fracture toughness value is large, the compressive stress against the deformation of the plastic optical component 1 can be increased on the surface of the plastic optical component 1, and the compression stress becomes closer to the Al metal reflection film 6. Since the stress can be reduced, the plastic optical parts 1 to S
It is possible to efficiently prevent the i-Al-O-N compound thin film 5 from peeling off.

In this embodiment (claim 3), Si--Al--O is used.
The formation of the Al metal reflection film 6 on the -N-based compound thin film 5 is
Al metal target placed on non-equilibrium magnetron electrode 1
5 is formed by sputtering vapor deposition by the DC magnetron sputtering method. Therefore, due to the ion assist effect of the non-equilibrium magnetron, Si-Al
Since the ions can be spread radially on the —O—N-based compound thin film 5, it is possible to efficiently form the dense Al metal reflective film 6 in which the residual stress is relaxed.

In this embodiment (claim 4), when the Al metal reflective film 6 is formed by the DC magnetron sputtering method using the non-equilibrium magnetron electrode, the residual stress of the Al metal reflective film 6 is controlled. A negative bias is applied to the plastic optical component 1. Therefore, A
DC using a non-equilibrium magnetron electrode
Since a negative bias can be applied to the plastic optical component 1 so as to control the residual stress of the Al metal reflection film 6 when forming by using the magnetron sputtering method, plasma ions of the non-equilibrium magnetron can be applied to the plastic optical component. The ion beam assist effect can be strongly brought out by positively drawing into 1. Therefore, it is possible to efficiently form the Al metal reflection film 6 that is dense and has a reduced residual stress.

In this embodiment (claim 5), the Al ₂ O ₃ oxide ceramic thin film 7 is formed on the Al metal reflective film 6 by the DC magnetron reaction of the Al metal target 15 placed on the non-equilibrium film magnetron electrode. It is configured to be performed by sputter vapor deposition with an optical film thickness by a reactive sputtering method. Therefore, the Al metal target 15 placed on the non-equilibrium magnetron electrode can be used to form the Al ₂ O ₃ oxide ceramic thin film 7 on the metal reflective film 6 by the DC magnetron reactive sputtering method. Since the ions can be spread radially on the metal reflection film 6 by the ion assist effect, it is possible to efficiently form the dense Al ₂ O ₃ oxide ceramic thin film 7 in which the residual stress is relaxed.

In this embodiment (claim 10), a loading chamber 22 for loading the plastic optical component 1 therein, a cleaning chamber 24 for plasma-cleaning the plastic optical component 1, and a Si-Al on the plastic optical component 1 are mounted. An RF sputtering chamber 26 for forming the -ON-N compound thin film 5, and Si-Al-O.
-Al metal reflective film 6 and Al ₂ O on the N-based compound thin film 5
₃ and DC sputtering chamber 28 sequentially forming an oxide-based ceramic thin films 7, Al ₂ O ₃ oxide ceramic thin film 7, Al metal reflective layer 6 and Si-Al-O-N-based compound thin film 7 is deposited An unload chamber 30 for carrying out the plastic optical parts 1 and a tray 21 for accommodating a large number of plastic optical parts 1 all at once are provided in the respective vacuum chambers 2.
2, 24, 26, 28, 30 are configured to have a transporting means.

For this reason, the trays 21 for accommodating a large number of plastic optical components 1 in a batch are sequentially arranged in the respective vacuum chambers 2.
Since it can be transported to 2, 24, 26, 28 and 30, it is possible to perform a large-scale batch processing by an in-line method without exposing the tray to the atmosphere even once in each film forming process, and to achieve high-quality large-scale processing. Production can be realized. In this embodiment (claim 11), each vacuum container 22, 24, 2
The conveying means installed in 6, 28 and 30 is constituted by a non-magnetic steel belt 32.

Therefore, the tray 21 for accommodating a large number of plastic optical components 1 is attached to the non-magnetic steel belt 32.
Since it can be transferred to each of the vacuum containers 22, 24, 26, 28, and 30 by the use of the vacuum tray 22, the stable tray 21 can be provided without adversely affecting the magnetic field line distribution during magnetron sputtering.
Can be carried. At this time, it is desirable that the tray 21 be made of non-magnetic SUS or the like, in consideration of not adversely affecting the distribution of magnetic force lines during magnetron sputtering.

In this embodiment (claim 12), the steel belt 32 on the shaft 31 for driving the steel belt 32 is used.
The surface contacting with is coated with TiN. For this reason, the surface of the shaft 31 that drives the steel belt 32 in contact with the steel belt 32 is coated with a TiN coating having excellent wear resistance.
The abrasion debris due to the contact between the steel belt 32 and the shaft 31 can be made less likely to occur by the TiN coating having excellent abrasion resistance. Therefore, the vacuum vessels 22, 2
Contamination to 4,26,28,30
Can be effectively prevented.

In the present embodiment (claim 8), the charged particles and the excited species in the electron cyclotron resonance plasma are formed before forming the Si—Al—O—N type compound thin film 5 on the plastic optical component 1. The optical component 1 made of plastic is exposed to and treated. Therefore, the Si-Al-O-N based compound thin film 5 is formed on the plastic optical component 1.
Since the plastic optical component 1 can be exposed to the charged particles and excited species in the electron cyclotron resonance plasma before the formation, the plastic optical component 1 can be processed in a plasma with high uniformity and stability at low temperature. The part 1 can be processed. As a result, the surface of the plastic optical component 1 can be treated by dry cleaning using a low temperature process using plasma, so that water and organic substances on the surface of the plastic optical component 1 can be removed and the surface of the plastic optical component 1 can be activated. Can be done at the same time.

Therefore, the surface of the plastic optical component 1 can be appropriately roughened by the impact of plasma particles to improve the adhesiveness, and at the same time, the bonding chains of molecules on the surface of the plastic optical component 1 have high energy in plasma. It can be activated by being bombarded and cut by ions having a to protect the bond between C (carbon) of the synthetic resin molecule and an inorganic element such as Si or Al to improve the adhesion.

In the present embodiment (claim 9), when the plastic optical component 1 is subjected to plasma treatment, impurities separated from the surface of the plastic optical component 1 are monitored by the mass spectrometer 45. Therefore, when the plastic optical component 1 is subjected to the plasma treatment, impurities such as H ₂ O detached from the surface of the plastic optical component 1 can be monitored by the mass spectrometer 45. As a result, the plasma cleaning end point can be surely grasped based on the information of the plasma cleaning end point of the type of the plastic optical component 1 which is obtained by monitoring with the mass spectrometer 45 in advance. Therefore,
Since various plastic optical components 1 can be efficiently plasma-processed, productivity can be improved and film-forming quality can be ensured.

In this embodiment (claim 6), when the Al ₂ O ₃ oxide ceramic thin film 7 is formed on the Al metal reflective film 6 by the DC magnetron reactive sputtering method, the mass having the differential exhaust system 55 is used. The analysis system 45 is configured to constantly monitor the partial pressure of the reactive gas during sputtering. Therefore, when the Al ₂ O ₃ oxide ceramic thin film 7 is formed on the Al metal reflection film 6 by the DC magnetron reactive sputtering method, the reactive gas at the time of sputtering is generated by the mass spectrometry system 45 having the differential exhaust system 55. Since it is possible to constantly monitor the partial pressure of, it is possible to maintain an appropriate existence ratio of the sputtering gas and the reactive gas.

Therefore, stoichiometric Al ₂ O
₃ it is possible to precisely monitor the formation and reactive gas partial pressure of oxide ceramics thin film 7, low water absorption coefficient while controlling the optical properties such as refractive index, and rigid of the Al ₂ O ₃ oxide The physical ceramics thin film 7 can be efficiently formed. This embodiment (claim 7)
It is, in forming by DC magnetron reactive sputtering using Al ₂ O ₃ oxide-based ceramics film 7 nonequilibrium magnetron electrode 61, so as to control the residual stress of the Al ₂ O ₃ oxide ceramics thin film 7 A negative bias is applied to the plastic optical component 1.

Therefore, Al₂O₃Oxide ceramic
DC thin film 7 using a non-equilibrium magnetron electrode 61
When forming by using the magnetron reactive sputtering method, Al
₂O ₃Controls the residual stress of the oxide ceramic thin film 7.
Bias is applied to the plastic optical component 1 so that
The plasma of a non-equilibrium magnetron can be
Ions are positively drawn into the plastic optical component 1.
It is possible to strongly bring out the ion beam assist effect.
Wear. Therefore, Al that is dense and has relaxed residual stress₂O₃
It is possible to efficiently form the oxide ceramic thin film 7.
it can.

[0075]

According to the present invention, an undercoat layer and an overcoat layer can be formed on a plastic optical component and a metal reflection film, respectively, so that internal stress can be reduced and cracks can be less likely to occur. It is possible to form a film having a stable film quality, to prevent the film from peeling off, and to reduce the cost of the apparatus by performing a film forming process in the same system without performing pretreatment. is there.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of an optical component of a comparative example.

FIG. 2 is a cross-sectional view showing the structure of an optical component according to an embodiment of the present invention.

[Fig. 3] Oxygen gas partial pressure during sputtering is 1 × 10 ^-5 Tor.
It is a figure which shows a mode that the compressive stress of a Si-Al-O-N type compound thin film starts to reduce significantly when it becomes larger than around r.

FIG. 4 is a diagram showing a configuration of a non-equilibrium type magnetron electrode according to an embodiment of the present invention.

FIG. 5 is a diagram showing a configuration of a magnetron electrode of a comparative example.

FIG. 6 is a schematic view showing a configuration of an optical component manufacturing apparatus according to an embodiment of the present invention.

7 is a diagram showing a state in which a large number of plastic optical components are set on the tray shown in FIG.

FIG. 8 is a diagram showing a state in which a steel belt is stretched around a shaft.

[Explanation of symbols] 1 plastic optical component 2 SiO undercoat layer 3, 6 Al metal reflection film 4 SiO overcoat layer 5 Si-Al-O-N based compound thin film 7 Al ₂ O ₃ oxide based ceramic film 11 substrate 12a, 12b Magnetic field lines 13a, 13b Yoke 14a, 14b Magnet 15, 52a Target 21 Tray 22 Load chamber 23, 25, 27, 29 Gate valve 24 Cleaning chamber 26 RF sputtering chamber 28 DC sputtering chamber 30 Unload chamber 31 Shaft 32 Steel belt 41 ECR plasma source 42 ECR plasma generation power source 43 plasma generation gas introduction system 45 mass spectrometer 46, 57, 67 valve 47 controller 51 magnetron electrode 52 RF power source 53, 56 gas introduction system 55 differential exhaust system 61 non-equilibrium magnetron Electric Electrode 62 Reaction gas introduction system 63 DC power supply 65 Bias application electrode 66 Bias power supply

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C23C 14/54 B 8939-4K C 8939-4K

Claims (12)

[Claims] 1. A step of forming a Si—Al—O—N based compound thin film on one surface of a synthetic resin optical member molded into a desired shape, and then the Si—Al—O—N based compound. A step of forming a metal reflection film having a higher reflectance than the Si-Al-O-N compound thin film on the thin film, and then forming an oxide ceramic thin film made of Al ₂ O ₃ on the metal reflection film. The manufacturing method of the optical component characterized by including the process of performing. 2. The formation of the Si—Al—O—N-based compound thin film on one surface of the synthetic resin optical member is performed by using a sialon target placed on a magnetron cathode electrode as a R film.
The residual stress of the Si-Al-O-N compound thin film is adjusted by changing the oxygen partial pressure during RF magnetron sputtering. Item 2. A method of manufacturing an optical component according to Item 1. 3. The formation of the metal reflection film on the Si—Al—O—N compound thin film is performed by sputter deposition of an Al metal target placed on a non-equilibrium magnetron electrode by a DC magnetron sputtering method. The method for manufacturing an optical component according to claim 1, wherein the optical component is manufactured. 4. The synthetic resin optical member is configured to control the residual stress of the Al metal reflective film when the Al metal reflective film is formed by a DC magnetron sputtering method using the non-equilibrium magnetron electrode. The method of manufacturing an optical component according to claim 3, wherein a negative bias is applied. 5. The formation of the Al ₂ O ₃ oxide ceramic thin film on the metal reflection film is performed by sputtering an Al metal target placed on a non-equilibrium magnetron electrode with an optical film thickness by a DC magnetron reactive sputtering method. The method for manufacturing an optical component according to claim 1, wherein the method is performed by vapor deposition. 6. When forming the Al ₂ O ₃ oxide ceramic thin film on the metal reflection film by a DC magnetron reactive sputtering method, a reactive gas at the time of sputtering is generated by a mass spectrometer having a differential evacuation device. The method for manufacturing an optical component according to claim 5, wherein the partial pressure is constantly monitored. 7. When the Al ₂ O ₃ oxide ceramic thin film on the metal reflection film is formed by DC magnetron reactive sputtering method, so as to control the Al ₂ O ₃ oxide ceramic thin film of residual stresses 7. The method of manufacturing an optical component according to claim 5, wherein a negative bias is applied to the synthetic resin optical member. 8. The Si-Al is formed on the synthetic resin optical member.
8. The method of manufacturing an optical component according to claim 1, wherein the synthetic resin optical member is exposed to an electron cyclotron resonance plasma for processing before the -ON compound thin film is formed. 9. The manufacturing of an optical component according to claim 8, wherein when the synthetic resin optical member is subjected to plasma treatment, impurities separated from the surface of the synthetic resin optical member are monitored by a mass spectrometer. Method. 10. A load vacuum chamber for loading a synthetic resin optical member, a cleaning vacuum chamber for plasma-cleaning the synthetic resin optical member, and Si-on one surface of the synthetic resin optical member.
Al-O-N system and the RF sputtering vacuum chamber for forming a compound thin film, the Si-Al-O-N-based metal reflecting film compound on the thin film and the Al ₂ O ₃ DC sputtering vacuum sequentially forming an oxide-based ceramic thin film Chamber, the Al ₂ O ₃ oxide ceramic thin film, the metal reflection film, and the Si-Al-O-N
An unloading vacuum chamber for carrying out the synthetic resin optical member on which the system compound thin film is formed, and a transporting means for sequentially sending a tray accommodating a plurality of synthetic resin optical members to each vacuum chamber in sequence. An optical component manufacturing apparatus characterized by the above. 11. The apparatus for manufacturing an optical component according to claim 10, wherein the conveying means is a non-magnetic steel belt. 12. The apparatus for manufacturing an optical component according to claim 11, wherein a surface of the shaft for driving the steel belt, which is in contact with the steel belt, is coated with TiN coating.