Amorphous carbon, and production method therefor

Abstract

Amorphous carbon having a refractive index of 2.3 or less at a wavelength of 250 nm to 1000 nm.

Description

Amorphous carbon and its manufacturing method

[0001] The present invention relates to amorphous carbon and a method for producing the same.

[0002] While tetrahedral amorphous carbon (ta-C) films have excellent hardness, smoothness, and abrasion resistance, their high refractive index of 2.5 or more in the infrared region makes their development for low-reflection film applications difficult. It is generally known that the refractive index can be reduced by making a film porous, but it is difficult to make dense films such as amorphous carbon films porous.

[0003] Japanese Patent Application Laid-Open No. 2003-301257

[0004] One aspect of the present invention is amorphous carbon having a refractive index of 2.3 or less in the wavelength range of 250 nm or more and 1000 nm or less.

[0005] Another aspect of the present invention is a method for producing amorphous carbon, comprising: a film formation step of forming an amorphous carbon film containing a metal by a PVD method using carbon and metal targets; and a removal step of removing at least a portion of the metal contained in the amorphous carbon film.

[0006] 1 is a schematic diagram showing an example of an FCVA film formation apparatus. It is a bar graph showing the hardness (GPa) of the amorphous carbon of Examples 1 to 3 and the amorphous carbon of Comparative Example 1. It is a graph plotting the refractive index n in the wavelength range of 250 nm or more and 1000 nm or less of the amorphous carbon of Examples 1 to 3 and the amorphous carbon of Comparative Example 1. It is a graph plotting the refractive index n in the wavelength range of 5 μm or more and 30 μm or less of the amorphous carbon of Examples 1 to 3 and the amorphous carbon of Comparative Example 1. It is a cross-sectional STEM image of the amorphous carbon of Example 1.

[0007] An embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described below. The present embodiment is an example for explaining the present invention, and is not intended to limit the present invention to the following content. The present invention can be practiced with appropriate modifications within the scope of its gist.

[0008] <Amorphous Carbon> The amorphous carbon according to this embodiment has a refractive index of 2.3 or less in the wavelength range of 250 nm or more and 1000 nm or less. The amorphous carbon according to this embodiment may also have a refractive index of 2.2 or less in the wavelength range of 1.7 μm or more and 30 μm or less. The lower limit of the refractive index in each wavelength range is, for example, 1.5. By achieving such a low refractive index, application as a low-reflection film is also expected.

[0009] The amorphous carbon according to this embodiment has a porosity of 18% or more and 55% or less, calculated from the density. By having such a high porosity, the porosity of the amorphous carbon can be improved. The method for calculating the porosity will be described in detail later.

[0010] A metal is added to the amorphous carbon according to this embodiment during production to make the film porous. The metal may be silver, zinc, copper, titanium, aluminum, or the like, with silver being preferred. These metals may be contained alone or in combination.

[0011] The amorphous carbon according to this embodiment contains a metal in an atomic percentage (at.%) of 0% to 3.2% inclusive in RBS / HFS analysis. EDX analysis may be used instead of RBS / HFS analysis. The lower limit of the metal content may be 1% in atomic percentage (at.%) in RBS / HFS analysis.

[0012] In addition to carbon and metal, known components may be added in appropriate amounts depending on the application, as long as the effects of the amorphous carbon according to this embodiment can be obtained.

[0013] The amorphous carbon according to this embodiment has a hardness of 0.8 GPa or more and 7.2 GPa or less. The lower limit of the hardness may be 2 GPa or 3 GPa. The upper limit of the hardness may be 10 GPa. By setting the hardness within this range, durability can be improved.

[0014] <Uses of amorphous carbon> Use as low refractive index film, low reflection film As described above, the amorphous carbon according to this embodiment has a low refractive index in the wavelength range of 1.7 nm or more and 30 nm or less. Therefore, it can be used for, for example, near-infrared lenses, infrared windows, sensing systems for autonomous driving of automobiles (LiDAR), low-visibility coatings for fingerprints, etc.

[0015] The amorphous carbon according to this embodiment can be used for semi-prepared wafer holders, molding dies, DNA adsorption, etc. It can also be used as an electrode material for electrochemical capacitors, redox flow batteries, lithium-sulfur secondary batteries, etc. Furthermore, since the pores can support catalysts such as metal nanoparticles, it can be used as a catalyst support.

[0016] <Method for producing amorphous carbon> The method for producing amorphous carbon according to this embodiment includes: a film formation step of forming an amorphous carbon film containing a metal by a PVD method using carbon and metal targets; and a metal removal step of removing at least a portion of the metal contained in the amorphous carbon film.

[0017] Film Formation Step In the film formation step, a metal-containing carbon film is formed by simultaneously or alternately irradiating a substrate with carbon ions and metal ions by a PVD (physical vapor deposition) method.

[0018] PVD methods are conventionally known methods used to form amorphous carbon films, and examples thereof include ion beam deposition, ion beam sputtering, magnetron sputtering, laser deposition, laser sputtering, arc ion plating, filtered cathodic vacuum arc (FCVA), etc. Among these film formation methods, the FCVA is particularly preferred because it provides good adhesion even at room temperature and enables uniform coating even on substrates with complex shapes.

[0019] The FCVA method is a film formation method in which ionized particles are generated by arc discharge on a target, and only these particles are guided to a substrate to form a film. The FCVA film formation apparatus 1 shown in Figure 1 is mainly composed of a first arc plasma generation unit 10a, a second arc plasma generation unit 10b, a first filter unit 20a, a second filter unit 20b, and a film formation chamber unit 30. The first arc plasma generation unit 10a and the second arc plasma generation unit 10b are connected to the film formation chamber unit 30 by the duct-shaped first filter unit 20a and the second filter unit 20b, respectively, and the pressure in the film formation chamber unit 30 is reduced to 10 by a vacuum device (not shown). －５ ~10 －７ The vacuum level is set to about [Torr].

[0020] The first arc plasma generating unit 10a is provided with a first target 11a as a cathode and an anode (striker) (not shown). An arc discharge occurs when the striker is brought into contact with the first target 11a and then immediately released, thereby generating arc plasma. Neutral particles and charged particles generated from the first target 11a by the arc plasma fly through the first filter unit 20a toward the film forming chamber unit 30.

[0021] The first filter section 20a is provided with a first duct 23a around which a first electromagnet coil 21a is wound and a first ion scanning coil 25a. The first duct 23a is bent twice in two perpendicular directions between the first arc plasma generation section 10a and the deposition chamber section 30, and the first electromagnet coil 21a is wound around its outer periphery. Because the first duct 23a has such a bent structure (double bend structure), neutral particles in the first duct 23a are removed by colliding with and depositing on the inner wall surface. When a current is passed through the first electromagnet coil 21a, a Lorentz force acts on charged particles in the first duct 23a, causing the charged particles to concentrate in the central region of the duct cross section, fly along the bend in the duct, and be guided to the deposition chamber section 30. In other words, the first electromagnet coil 21a and the first duct 23a form a narrow-band electromagnetic spatial filter that efficiently passes only charged particles.

[0022] Similarly, second arc plasma generating unit 10b is provided with second target 11b as a cathode and an anode (striker) (not shown). Second filter unit 20b is provided with second duct 23b around which second electromagnetic coil 21b is wound, and second ion scanning coil 25b, and second electromagnetic coil 21b and second duct 23b form a narrow-band electromagnetic spatial filter that passes only charged particles with high efficiency.

[0023] The first ion scanning coil 25a and the second ion scanning coil 25b respectively scan the beam of charged particles that passes through the first duct 23a and the second duct 23b and enters the film formation chamber 30 as described above. A holder 31 is provided in the film formation chamber 30, and a substrate 40 is set on the surface of this holder 31. The holder 31 is rotated by a motor 35. A desired bias voltage can be applied to the holder 31 by a power supply 37. The particle beam scanned by the first ion scanning coil 25a and the particle beam scanned by the second ion scanning coil 25b are incident on the surface of the substrate 40 held by the holder 31, and these particles are uniformly deposited on the substrate 40.

[0024] In this embodiment, a graphite target is used as the first target 11a, and a metal target is used as the second target 11b. As mentioned above, the metal may be silver, zinc, copper, titanium, aluminum, or an alloy containing any of these, but silver is preferably used. Using these targets, carbon and metal films are simultaneously or alternately deposited on the substrate 40. A negative bias voltage is applied to the holder 31 by the power supply 37. This accelerates charged particles incident on the substrate 40. The accelerated charged particles are deposited on the substrate 40, and a dense amorphous carbon film containing the metal is uniformly formed on the substrate 40.

[0025] The amorphous carbon film may be formed only on a part of the substrate 40. In this case, the region of the substrate 40 where the amorphous carbon film is not to be formed may be covered with a mask, and then the substrate 40 may be set on the holder 31 and the amorphous carbon film may be formed.

[0026] The metal content (atomic concentration of metal) deposited on the substrate 40 can be controlled, for example, by changing the current (filter current) of the first electromagnetic coil 21 a and the second electromagnetic coil 21 b. Therefore, by adjusting the value of the filter current, it is possible to form a film in which two or more amorphous carbon films with different metal contents are stacked. Furthermore, by continuously changing the value of the filter current during film formation, it is possible to impart a gradient to the metal content.

[0027] The FCVA method can form an amorphous carbon film having a metal content of 184% or less in atomic percent. The lower limit of the metal content may be 10%, 15%, or 20% in atomic percent. The upper limit of the metal content may be 80%, 75%, or 70% in atomic percent. The higher the metal content of the amorphous carbon film after the film formation process, the lower the refractive index and hardness, and the larger the pore size.

[0028] The thickness of the amorphous carbon film is not particularly limited and is 50 nm to 300 nm. The lower limit of the thickness may be 75 nm or 100 nm. The upper limit of the thickness may be 300 nm, 250 nm, or 200 nm.

[0029] Metal Removal Step In the metal removal step, at least a portion of the metal contained in the amorphous carbon film formed as described above is removed by etching, thereby making the amorphous carbon film porous.

[0030] The etching method employed is electrolysis or immersion in an etching solution. For example, when removing metal by electrolysis, an appropriate potential is applied to an amorphous carbon film containing the metal to dissolve the metal, thereby allowing the metal to be selectively removed. When removing metal by immersion in an etching solution, the metal can be selectively removed by immersing the amorphous carbon film containing the metal in the etching solution to dissolve the metal.

[0031] The etching solution used for immersion is, in the case of an acid, nitric acid, sulfuric acid, hydrofluoric acid, phosphoric acid, chromic acid, etc., preferably nitric acid; in the case of an alkali, sodium hydroxide, potassium hydroxide, etc.; or in the case of a neutral salt, ferric chloride, etc.

[0032] The amorphous carbon film obtained by removing the metal in this manner has a lower refractive index than a normal ta-C film.

[0033] Next, examples of the present invention and comparative examples will be described, but the present invention is not limited to these.

[0034] <Production of amorphous carbon> Examples 1 to 3 A silver-containing carbon film was formed on a quartz substrate by simultaneous or alternating irradiation with carbon ions and silver ions using the FCVA method. ３ in aqueous solution with heating to 70°C for 1 day (Example 3), 1 M HNO ３ The sample was immersed in the aqueous solution for several minutes to 2 hours (Examples 1 and 2) while being heated to 70° C., and Ag etching was carried out.

[0035] Comparative Example 1 ta-C A carbon film was formed on a quartz substrate by irradiating it with carbon ions using the FCVA method, and this was designated as Comparative Example 1.

[0036] Table 1 below shows the Ag concentration (at. %) before etching, the residual Ag concentration (at. %) after etching, and the film density (g / cm ３ ), porosity (%), and film thickness (nm). ３ ) and film thickness (nm) are also shown in Table 1. The Ag concentration before etching in Examples 1 to 3 was a design value calculated from the ratio of the amount of carbon ions and metal ions irradiated in the process of forming the metal-containing carbon film. The residual Ag concentration (at. %) after etching was evaluated by RBS / HFS analysis. The porosity was calculated as follows using the density of the amorphous carbon in Examples 1 to 3 and Comparative Example 1 shown in Table 2: Porosity (%) = (1 - (density of amorphous carbon film in Examples 1 to 3) / (density of amorphous carbon film in Comparative Example 1)) × 100

[0037] Table 2 below shows the nanoindenter hardness (GPa) and the standard deviation σ (GPa) of hardness after etching for Examples 1 to 3. The load in the nanoindentation test was adjusted so that the contact depth was approximately 1 / 5 or less of the film thickness.

[0038]

[0039]

[0040] <Evaluation of Physical Properties> Hardness (Fig. 2) Fig. 2 shows the hardness of Examples 1 to 3 and Comparative Example 1. The hardness was evaluated using a nanoindenter. There is a correlation between hardness and film density, and the lower the hardness, the higher the porosity.

[0041] Refractive index n at wavelengths of 250 to 1000 nm (visible range) (FIG. 3) Figure 3 shows the refractive index at wavelengths of 250 nm or more and 1000 nm or less for Examples 1 to 3 and Comparative Example 1. At wavelengths of 250 nm or more and 1000 nm or less, the more porosity a sample has, the lower the refractive index, and therefore a low reflection effect can be expected.

[0042] Refractive index n in wavelengths of 1.7 to 30 μm (infrared region) (FIG. 4) Figure 4 shows the refractive index in wavelengths of 1.7 μm or more and 30 μm or less for Examples 1 to 3 and Comparative Example 1. As in the visible region, the refractive index is low in the infrared region in Examples 1 to 3, and a low reflection effect can be expected.

[0043] Cross-sectional STEM image (Fig. 5) Fig. 5 shows a cross-sectional STEM image of Example 1. The white areas in the image are voids.

[0044] DESCRIPTION OF SYMBOLS 1...FCVA film formation apparatus, 10a...first arc plasma generation unit, 10b...second arc plasma generation unit, 11a...first target, 11b...second target, 20a...first filter unit, 20b...second filter unit, 21a...first electromagnetic coil, 21b...second electromagnetic coil, 23a...first duct, 23b...second duct, 25a...first ion scanning coil, 25b...second ion scanning coil, 30...film formation chamber unit, 31...holder, 35...motor, 37...power supply, 40...substrate

Claims

1. Amorphous carbon having a refractive index of 2.3 or less at wavelengths of 250 nm or more and 1000 nm or less.

2. The amorphous carbon according to claim 1, having a refractive index of 2.2 or less at wavelengths of 1.7 μm or more and 30 μm or less.

3. The amorphous carbon according to claim 1 or 2, having a porosity of 18% or more and 55% or less, calculated from the density.

4. The amorphous carbon according to any one of claims 1 to 3, containing 0% or more and 3.2% or less of metal in atomic% in RBS / HFS analysis.

5. The amorphous carbon according to claim 4, wherein the metal is silver, zinc or copper.

6. The amorphous carbon according to any one of claims 1 to 5, having a hardness of 0.8 GPa or more and 7.2 GPa or less.

7. A method for producing amorphous carbon, comprising: a film forming step of forming an amorphous carbon film containing a metal by a PVD method using a carbon and metal target; and a metal removing step of removing at least a part of the metal contained in the amorphous carbon film.

8. The method for producing amorphous carbon according to claim 7, wherein the metal is silver, zinc or copper.

9. The method for producing amorphous carbon according to claim 7 or 8, wherein the PVD method is an FCVA method.

10. The method for producing amorphous carbon according to any one of claims 7 to 9, wherein the metal is removed by electrolysis or acid immersion.

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