JP5777962B2 - Method for producing diamond film - Google Patents

Description

  The present invention relates to a diamond film and a manufacturing method thereof.

  Diamond has the highest thermal conductivity among materials, and its dielectric breakdown strength is as high as 10 MV / cm, which is advantageous as a heat dissipation substrate for electronic devices and the like. In recent years, it has become possible to synthesize high quality diamond films at low pressure using chemical vapor deposition (CVD) such as hot filament CVD and microwave CVD.

  For example, the present inventors have proposed that a diamond film can be produced by using nano-diamond particles having a primary particle diameter of about 5 nm as a seed crystal and growing diamond by a CVD method ( Patent Document 1).

JP 2009-209027 A

  According to this method, it was possible to produce a diamond film, but the diamond film produced by this method has low physical properties inherent to diamond, such as thermal conductivity, dielectric breakdown strength, hardness, and elastic modulus. there were.

  The present invention has been made to solve such problems, and an object of the present invention is to provide a diamond film having high physical properties inherent in diamond and a method for producing the same.

The present inventors thought that the reason why the physical properties such as the thermal conductivity of the diamond film formed by using nano-diamond particles as a seed crystal and growing the diamond by the CVD method is due to the small diamond structure. . In addition, it was thought that the reason why the diamond structure was small is that the purity of the nanodiamond particles used as seed crystals was low. That is, it was considered that sp ² carbon was present on the nanodiamond surface. The present invention has been made based on such knowledge.

The invention according to claim 1 of the present invention, "(1) a step of primary particle size to prepare nanodiamond particles 1 to 20 nm, step of depositing the substrate (2) the nanodiamond particles, (3) the A step of heat-treating or plasma-treating the nanodiamond particle-adhering substrate, and (4) a step of growing the nanodiamond particles of the treated nanodiamond particle-adhering substrate by a CVD method to form a diamond film. Manufacturing method of membrane. "

The invention according to claim 1 of the present invention is a diamond film having a low thermal resistance, a diamond film having a thermal resistance of 1.0 × 10 ⁻⁸ (m ² · K / W) or less. In other words, a low thermal resistance means that there are many diamond structures, so that not only the thermal conductivity but also the performance inherent in diamond such as dielectric breakdown strength, hardness and elastic modulus are excellent.

According to a second aspect of the present invention, the nanodiamond particle surface is subjected to heat treatment or plasma treatment before the nanodiamond particles are grown by the CVD method to form a diamond film. ^Since diamonds are grown after removing carbon and increasing the purity of nanodiamond particles, there are many diamond structures and excellent properties inherent in diamond such as thermal conductivity, dielectric breakdown strength, hardness, and elastic modulus. A diamond film can be formed.

The diamond film of the present invention has a low thermal resistance of 1.0 × 10 ⁻⁸ (m ² · K / W) or less, and is a film having many diamond structures. It is excellent in the performance inherent to diamond, such as strength, hardness and elastic modulus. The smaller the value of thermal resistance, the more diamond structure, so the thermal resistance is preferably 9.0 × 10 ⁻⁹ (m ² · K / W) or less, and 8.0 × 10 ⁻⁹ ( m ² · K / W) or less, more preferably 7.0 × 10 ⁻⁹ (m ² · K / W) or less.

This “thermal resistance” is a value calculated from the following equation.
R = d / λ
Here, R means thermal resistance (unit: m ² · K / W), d means film thickness (unit: m), and λ means thermal conductivity (unit: W / m · K).

  This “thermal conductivity” is measured by measuring the thermal diffusion time in the cross-sectional direction of the thin film by the pulsed light heating thermoreflectance method after forming a reflective film (molybdenum film, thickness 100 nm) on the diamond film surface. Calculate the diffusion rate. More specifically, the thermal diffusivity is calculated by the following procedure.

(1) The area thermal diffusion time of the two-layer film of molybdenum film and polycrystalline diamond film is calculated from the temperature history curve obtained by the pulsed light heating thermoreflectance method;
The thermal diffusion of the entire two layers and the thermal penetration into the quartz glass substrate are analyzed by integrating the molybdenum film and the polycrystalline diamond film.

Fitting the obtained temperature history curve with the theoretical formula of the surface temperature response at the time of surface heating shown below, the area thermal diffusion time A of the entire two layers (molybdenum film and polycrystalline diamond film) is determined from the time constant τ _f. Ask.

(2) calculating the thermal diffusivity of the polycrystalline diamond film from the area thermal diffusion time of the two-layer film;
In the area thermal diffusion time method, the area thermal diffusion time A is expressed by the following equation when the interfacial thermal resistance between layers is taken into consideration.

ここで、Ｃは体積熱容量（比熱容量と密度の積で表される）、ｄは膜厚、ｋは熱拡散率、Ｒは層間の界面熱抵抗をそれぞれ意味する。なお、添え字ＤとＭｏはそれぞれ多結晶ダイヤモンド膜とモリブデン膜を表す。

Here, C means volumetric heat capacity (expressed by the product of specific heat capacity and density), d means film thickness, k means thermal diffusivity, and R means interfacial thermal resistance. The subscripts D and Mo represent a polycrystalline diamond film and a molybdenum film, respectively.

Here, since the value R of the interface thermal resistance between the polycrystalline diamond film and the molybdenum film is expected to be considerably smaller than the thermal resistance of the entire film thickness, it is assumed that R = 0 and the thermal diffusion of the polycrystalline diamond thin film is assumed. to calculate the rate _{k D.}

Then, the thermal conductivity of the polycrystalline diamond is calculated from the following formula.
λ = k _D · C _D
Where λ is the thermal conductivity (unit: W / mK), k _D is the thermal diffusivity of diamond (unit: m ² / s), and _CD is the volumetric heat capacity of diamond (product of specific heat capacity and density) (unit : J / m ³ K).

The specific heat capacity and density are as follows.
Density of polycrystalline diamond film: 3515 (kg / m ³ )
Specific heat capacity of polycrystalline diamond film: 706 [J / (kg · K)]
Density of molybdenum film: 10200 (kg / m ³ )
Specific heat capacity of molybdenum film: 249 [J / (kg · K)]

  Further, “film thickness” is a value obtained by taking an electron micrograph in a cross section in the thickness direction of the diamond film and arithmetically averaging the measured thickness values at 10 points.

  The diamond film of the present invention has a diamond structure, and preferably has a diamond peak area ratio of 7% or more by Raman spectrum so that it has excellent thermal conductivity, dielectric breakdown strength, hardness, elastic modulus and the like. % Or more is more preferable, 9% or more is further preferable, 10% or more is further preferable, and 11% or more is further preferable.

This “area ratio of diamond peak by Raman spectrum” is the diamond spectrum (wave number: 1333 cm ⁻¹ ) occupying the peak area of the Raman spectrum (excitation wavelength: 325 nm) of the diamond film using a Gaussian function. The ratio of the area is calculated.

  The thickness of the diamond film of the present invention is not particularly limited, but can be 10 nm or more. Preferably, it is 50 nm-10 micrometers.

Such a diamond film of the present invention includes, for example, (1) a step of preparing nanodiamond particles having a primary particle diameter of 1 to 20 nm, (2) a step of attaching the nanodiamond particles to a substrate, and (3) the above A step of heat-treating or plasma-treating the nanodiamond particle-adhering substrate, and (4) a step of growing the nanodiamond particles of the treated nanodiamond particle-adhering substrate by a CVD method to form a diamond film. it can. In this manner, before the diamond film is formed by growing the nanodiamond particles by the CVD method, the nanodiamond particle surface is subjected to heat treatment or plasma treatment to remove sp ² carbon on the surface of the nanodiamond particles, so that the nanodiamonds are formed. Since diamond is grown after increasing the purity of the particles, a diamond film with many diamond structures and excellent performance inherent in diamond such as thermal conductivity, dielectric breakdown strength, hardness, and elastic modulus should be formed. Can do.

  First, (1) nanodiamond particles having a primary particle diameter of 1 to 20 nm are prepared. In the present invention, since such very fine nanodiamond particles are used, the density when deposited on a substrate can be increased. As a result, it is possible to produce a diamond film that hardly generates pinholes, has excellent thermal conductivity, and has high surface smoothness.

  The nanodiamond particles have a primary particle diameter of 1 to 20 nm so as to exhibit the above-described action, but preferably have a primary particle diameter of 1 to 10 nm and more preferably 2 to 7 nm so as to achieve the above action. More preferably, it is 3 to 5 nm. The “primary particle size” refers to the size of the nanodiamond particles. A nanodiamond particle dispersion solution having a concentration of 2 mass% is prepared, and a concentrated particle size analyzer FPAR-1000 (manufactured by Otsuka Electronics Co., Ltd.) is used. The value obtained by measuring the particle size distribution by dynamic light scattering.

  Such nanodiamond particles can be obtained, for example, by a method in which nanodiamond coarse agglomerates produced by an explosion method are crushed by high-speed rotary wet milling using ceramic beads or metal beads.

  The “explosion method” is a method for collecting soot produced by decomposing oxygen-deficient military explosive composition Composition B in an inert medium such as water and removing amorphous carbon by oxidation with hot concentrated nitric acid. . Therefore, the solvent in the case where nanodiamond is dispersed in the solution is an inert medium such as water used in producing the nanodiamond coarse aggregate. When nanodiamonds are dispersed in the solution as described above, the concentration is preferably 10 mass% or less, more preferably 5 mass% or less, and more preferably 1 mass% or less so that the dispersion solution exists stably. More preferably. Moreover, it is preferably 0.001% by mass or more, more preferably 0.01% by mass or more, and 0.05% by mass or more so that the nanodiamond particles can be attached to the substrate at a high density. More preferably.

  The nanodiamond particles may be in a dry state that is not dispersed in the solution. Such dry nanodiamond particles can be obtained by removing the solvent from the nanodiamond dispersion solution.

  Next, (2) a step of attaching nanodiamond particles to the substrate is performed. In this step, when the nanodiamond particles are dispersed in the solution, for example, a method of drying after immersing the substrate in the dispersion solution, a method of drying after coating the dispersion solution on the substrate by a pulling method, a dispersion solution It can be carried out by a method of drying after spraying on the substrate. From the viewpoint of attaching the nanodiamond particles to the base material with high density, it is preferable to attach the base material by dipping in the dispersion solution and then drying. The drying is not particularly limited as long as the method can be performed while maintaining the state in which the nanodiamond particles are adhered at a high density. For example, a spin dryer, a hot air dryer, an oven, a vacuum oven It can be carried out by microwave irradiation.

  In addition, when a base material is immersed in a dispersion solution, it is preferable to make an ultrasonic wave act so that the dispersion state of the nano diamond particle in a nano diamond dispersion solution can be maintained. The ultrasonic wave is not particularly limited as long as it can maintain the dispersed state of the nanodiamond particles. For example, an ultrasonic cleaning device, a horn type high output ultrasonic device, or the like can be used.

  On the other hand, when the nanodiamond particles are in a dry state, the nanodiamond particles can be adhered to the substrate by rubbing with a cloth or the like.

As a base material that can be used in this step, for example, a heat-resistant flat substrate such as a silicon wafer or silica glass can be used. By using such a heat-resistant flat substrate, even when heat is applied when removing sp ² carbon from nano diamond particles described later or when growing a diamond film by a CVD method, without adverse effects, A diamond film can be produced.

Subsequently, (3) the above-described nanodiamond particle-adhered substrate is subjected to heat treatment or plasma treatment. Usually, since the sp ² carbon exists on the surface of the nano diamond particles as described above, the nano diamond particle-adhered substrate is subjected to heat treatment or plasma treatment in order to remove the sp ² carbon and increase the purity of the diamond. To do.

This heat treatment is not particularly limited as long as sp ² carbon on the diamond particle surface can be removed, but in an oxidizing atmosphere in the presence of an oxidizing gas such as oxygen, ozone, nitrogen dioxide (especially in the presence of air). It is preferable to carry out with. In particular, heat treatment is preferably performed at 400 ° C. to 450 ° C. in an oxidizing atmosphere. This is because if it is lower than 400 ° C., the sp ² carbon cannot be sufficiently removed, and the purity of diamond tends not to be sufficiently increased. On the other hand, when the temperature exceeds 450 ° C., the etching of the nanodiamond itself proceeds, the seed nanodiamond decreases, and the diamond film tends not to be formed. Note that the heat treatment time is a time during which the sp ² carbon can be sufficiently removed and is not particularly limited because it varies depending on the heat treatment temperature. However, it is preferable that the sp ² carbon be removed longer as the heat treatment temperature is lower. For example, when the heat treatment temperature is 400 ° C., the heat treatment is preferably performed for 5 hours or longer, more preferably 10 hours or longer. Moreover, when the heat processing temperature is 425 degreeC, it is preferable to heat-process for 1 hour or more, and it is more preferable to heat-process for 10 hours or more. Furthermore, when the heat treatment temperature is 450 ° C., the heat treatment is preferably performed for 1 hour or longer, more preferably 5 hours or longer.

  Such heat treatment can be performed by, for example, an electric furnace, a heat gun, infrared heating, laser heating, or the like.

On the other hand, the plasma treatment is not particularly limited as long as the sp ² carbon on the surface of the nanodiamond particles can be removed.

  Then, (4) the nanodiamond particles of the nanodiamond particle-adhered base material subjected to heat treatment or plasma treatment are grown by the CVD method to form a diamond film. In the present invention, diamond is grown by the CVD method in a state in which the purity of diamond particles is increased by heat treatment or plasma treatment, so there are many diamond structures, and diamond such as thermal conductivity, dielectric breakdown strength, hardness, elastic modulus, etc. It is possible to form a diamond film that is excellent in the performance inherently. In addition, according to the production method of the present invention, a diamond film having a primary particle size of 1 to 20 nm is grown as a nucleus, so that pinholes are hardly generated, thermal conductivity is excellent, and surface smoothness is high. Can be manufactured.

  The CVD method in this step is not particularly limited as long as diamond can be grown and a diamond film can be produced. For example, a hot filament method, a plasma method (for example, direct current, alternating current, high frequency, microwave, etc.), electron cyclotron A resonance plasma method, a pulse plasma method, a surface wave plasma method, or the like can be used.

  According to the above method, a film-coated base material in which a diamond film is formed on the surface of the base material can be manufactured. However, when the diamond film is used alone, the diamond film is peeled from the film-coated base material. As this peeling method, for example, when the substrate is made of silicon, it is immersed in a hydrofluoric acid (mixed solution of hydrofluoric acid and nitric acid) solution, and when the substrate is made of silica, it is immersed in a hydrofluoric acid solution. The base material can be removed to form a diamond film alone.

  The above is a method for producing a diamond film of the present invention using nanodiamond particles. However, the diamond film of the present invention is not limited to a method using nanodiamond particles. The diamond film of the present invention can also be produced by depositing 0.05 to 0.5 μm microdiamond particles on a substrate, performing heat treatment or plasma treatment, and then growing by CVD.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples.

Example 1
(1) Water-dispersed nanodiamond colloid containing 98.8 mass% of primary particles of 4.6 ± 0.8 nm and 1.2 mass% of aggregates of 59.4 ± 22.8 nm (Nanoamando 5 manufactured by Nanocarbon Laboratory Co., Ltd.) 0.0 mass%) was diluted 100 times to prepare a 0.05 mass% nanodiamond aqueous dispersion.

(2) Next, a quartz glass substrate (size: 1 cm square) is immersed in this nanodiamond aqueous dispersion, and ultrasonic waves are allowed to act for 30 minutes by an ultrasonic cleaning device (Honda Electronics, W-113). It was. Thereafter, the substrate was taken out from the nanodiamond aqueous dispersion, washed with pure water, spin-dried, and nanodiamond particles were adhered to the substrate. The adhesion density was 2.0 × 10 ¹¹ pieces / cm ² .

  (3) The diamond particle-adhered substrate was placed in an electric furnace, and from room temperature to 450 ° C. in air, 3.5 ° C./min. The temperature was increased at a rate of 1 ° C., and a heat treatment was performed at 450 ° C. for 1 hour.

  (4) The heat-treated substrate was set in a microwave plasma CVD apparatus [manufactured by Arios Co., Ltd.] and treated under the following conditions for 3 hours to form a diamond film having a thickness of 300 nm.

(conditions)
Hydrogen gas: 99 sccm, methane gas: 1 sccm, pressure: 1.5 kPa, substrate temperature: 900 ° C., output: 220 W

(Example 2)
(3) The diamond particle-adhered substrate was placed in an electric furnace, and from room temperature to 425 ° C in air, 3.5 ° C / min. A diamond film having a film thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and the heat treatment was held at 425 ° C. for 1 hour.

(Example 3)
(3) The diamond particle-adhered substrate was placed in an electric furnace, and from room temperature to 425 ° C in air, 3.5 ° C / min. A diamond film having a thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and the heat treatment was performed at 425 ° C. for 5 hours.

Example 4
(3) The diamond particle-adhered substrate is placed in an electric furnace, and is heated from room temperature to 400 ° C. in air at 3.5 ° C./min. A diamond film having a film thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and maintained at 400 ° C. for 5 hours.

(Example 5)
(3) A diamond particle-adhered substrate is placed in a CVD apparatus, and hydrogen gas is 100 sccm, pressure is 1.5 kPa, substrate temperature is 600 ° C., output is 150 W, and hydrogen plasma treatment is performed for 0.1 hour. In the same manner as in Example 1, a diamond film having a thickness of 300 nm was formed.

(Comparative Example 1)
A diamond film having a thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment of (3) was not performed.

(Comparative Example 2)
(3) The diamond particle-adhered substrate was placed in an electric furnace, and from room temperature to 375 ° C. in air, 3.5 ° C./min. A diamond film having a film thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and maintained at 375 ° C. for 1 hour.

(Comparative Example 3)
(3) The diamond particle-adhered substrate was placed in an electric furnace, and from room temperature to 475 ° C. in air, 3.5 ° C./min. A diamond film having a film thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and maintained at 475 ° C. for 1 hour.

(Comparative Example 4)
(3) The diamond particle-adhered substrate is placed in an electric furnace, and is heated from room temperature to 400 ° C. in air at 3.5 ° C./min. A diamond film having a film thickness of 300 nm was formed in the same manner as in Example 1 except that the heat treatment was performed at a rate of 5 ° C. and heat treatment was performed at 400 ° C. for 1 hour.

(Measurement of thermal conductivity)
After a reflective film (molybdenum film, thickness 100 nm) is formed on the diamond film surface, the thermal diffusion time in the cross-sectional direction of the thin film is measured by the pulsed light heating thermoreflectance method [using PicoTR (manufactured by Picotherm Co., Ltd.) as an apparatus]. Was measured and the thermal diffusivity was calculated. And the thermal conductivity of diamond was computed from the following formula.
λ = K ・ C
Here, λ is thermal conductivity (unit: W / mK), K is thermal diffusivity (unit: m ² / s), and C is volumetric heat capacity (product of specific heat capacity and density) (unit: J / m ³ K). ).

  These results were as shown in Table 1.

(Raman spectrum measurement)
The diamond film was subjected to Raman measurement using a microscopic laser Raman spectrometer (manufactured by Horiba, Ltd.). For the excitation wavelength, a 325 nm He—Cd laser was used. The obtained Raman spectrum was peak-separated using a Gaussian function, and the ratio of the area of the diamond peak (wave number: 1333 cm ⁻¹ ) in the entire peak area was calculated.

  These results were as shown in Table 1.

In Table 1, A is a heat treatment condition and represents temperature (unit: ° C.) and time (unit: Hr) in order, B represents the thickness of the diamond film (unit: nm), and C represents thermal conductivity (unit: W / mK), D represents the thermal resistance (unit: m ² · K / W), and E represents the diamond peak area percentage (unit:%). In Table 1, # 1 is the condition of hydrogen plasma treatment [in order, temperature (unit: ° C.)-Time (unit: Hr)], # 2 is not heat-treated, and # 3 is a diamond film. It means that it was impossible to measure because it was not.

  As is clear from Table 1, the diamond films of Examples 1 to 5 of the present invention have low thermal resistance, and the diamond peak area percentage is high, so that the diamond film has many diamond structures. Therefore, it can be estimated that the diamond film of the present invention is excellent in performance inherent to diamond, such as dielectric breakdown strength, hardness, and elastic modulus.

In addition, when nanodiamond particles are used, before the diamond is grown by CVD, the sp ² carbon on the surface of the nanodiamond particles is removed to increase the purity of the diamond particles, regardless of whether heat treatment or plasma treatment is used. It was also found that the diamond structure can be increased.

  Since the diamond film of the present invention is excellent in thermal conductivity, dielectric breakdown strength, hardness, elastic modulus and the like, it can be suitably used as a heat dissipation material, an electrical insulating material, a window material, a low friction coating material, an electrode and the like.

Claims (1)

(1) a step of preparing nanodiamond particles having a primary particle diameter of 1 to 20 nm,
(2) a step of attaching the nanodiamond particles to a substrate;
(3) a step of heat-treating or plasma-treating the nanodiamond particle-adhering substrate, and (4) a step of growing the nanodiamond particles of the treated nanodiamond particle-adhering substrate by a CVD method to form a diamond film,
A method for producing a diamond film, comprising: