JPH11189472A - Synthesis of carbon nitride - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for synthesizing carbon nitride capable of efficiently forming carbon nitride. SOLUTION: This method for synthesizing carbon nitride comprises irradiating a target 11 with a laser beam to perform laser ablation and to synthesize carbon nitride. A solid substance containing an ammonium compound and a carbonaceous material is used as the target 11. Molecules of formed impurities are reduced by the laser ablation of the solid substance containing the ammonium compound and the carbonaceous material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for synthesizing carbon nitride, and more particularly, to a method for synthesizing carbon nitride by laser ablation.

[0002]

2. Description of the Related Art Carbon nitride is a compound containing only nitrogen and carbon as constituent elements. Examples of such carbon nitride include α-C ₃ N ₄ crystal, β-C ₃ N ₄ crystal and C ₃ N _{4. -x} amorphous material (0 <x <4) and the like are known. Especially,
β-C ₃ N ₄ has a larger energy gap in the crystalline state than diamond or gallium nitride, and has an ultraviolet laser diode or a vacuum ultraviolet optical window (particularly for vacuum ultraviolet light having a shorter wavelength than light used in LiF crystal). It is thought that it can be used. Also, β
-C ₃ N ₄ crystal is considered more robust than diamond (AYLiu and MLCohen, Science, vol.245 ,
p841 (1989)) and other carbon nitride materials are also known to be extremely hard. For this reason, the carbon nitride material is expected to be applied to various fields such as industrial use, physicochemical use, and medical use, for example, it can be used as a wear-resistant material for semiconductors, mechanical parts, tooth and bone joints, and the like.

As an example of a method for synthesizing this kind of carbon nitride, for example, “Surface Review and Letters, Vol.
No. 1 (1996) 197-199 (PTMurray, MYChen). This method synthesizes a carbon nitride film on graphite by irradiating the graphite with a short pulse laser beam in a nitrogen atmosphere. In order to examine the composition of this film, the film is again irradiated with laser light to perform laser ablation, and the presence of C ₃ N _{4 and the} like has been confirmed by mass spectrum measurement of the substance generated at that time.

[0004]

However, in the above-mentioned conventional method, although carbon nitride such as C ₃ N ₄ is present, the overall composition is CN _0.29 , and the proportion of the nitrogen component is less than that of the carbon component. It was much lower than the ratio. That is, the formation of carbon nitride was insufficient.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a method for synthesizing carbon nitride capable of efficiently producing carbon nitride having a stoichiometric composition of C ₃ N _4. And

[0006]

The present inventors performed laser ablation of a target using the above-described conventional method, and performed spectroscopic measurement at that time. Then, it has been found that impurity molecules such as C ₂ molecules are generated in addition to carbon nitride, the ratio of carbon nitride is reduced, and the ratio of nitrogen component is smaller than the ratio of carbon component as a whole.

The inventors of the present invention have conducted intensive studies to suppress the generation of such impurity molecules, and as a result, depending on the type of target material used for the synthesis of carbon nitride, It has been found that the impurity molecules to be reduced are reduced.

That is, the present invention provides a method for synthesizing carbon nitride, wherein laser ablation is performed by irradiating a target with laser light to synthesize carbon nitride.
A solid material containing an ammonium compound and a carbonaceous material is used as a material forming the target. According to the present invention, by laser ablation of a solid containing an ammonium compound and a carbonaceous material, generated impurity molecules are reduced, and the ratio of nitrogen in carbon nitride generated is increased.

Preferably, the laser beam is a laser beam capable of decomposing an ammonium compound and releasing nitrogen atoms in the ammonium compound. The ammonium compound is preferably solid at a temperature of 20 to 200 ° C and has a vapor pressure at 20 ° C of 10 ^{-4 to 1} Torr.

[0010] In the present specification, carbon nitride means
A compound containing only nitrogen and carbon atoms as constituent elements.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the method for synthesizing carbon nitride according to the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a schematic sectional view showing an example of a carbon nitride synthesizing apparatus for carrying out the carbon nitride synthesizing method of the present invention. As shown in FIG. 1, the carbon nitride synthesizing apparatus 1 includes a vacuum vessel 2, and a flat substrate (for example, a quartz substrate) 4 is provided in the vacuum vessel 2. This substrate 4 is for depositing a target thin film.
The substrate 4 is connected to, for example, a heater 3 provided near the inner surface of the vacuum vessel 2, and is heated to a desired temperature by the heater 3 when depositing a thin film. The synthesis of carbon nitride is performed by introducing a gas into the vacuum vessel 2. This gas is introduced into the vacuum vessel 2 from a gas supply source (for example, a gas cylinder) 5 provided outside the vacuum vessel 2 through a gas supply pipe 6. Such gases include noble gases (eg, helium, neon, argon, etc.), hydrogen, oxygen, nitric oxide, nitrogen dioxide, nitrogen, ammonia, hydrazine, and the like. These may be used in combination with each other. The introduced gas is exhausted by an exhaust device 7 attached outside the vacuum vessel 2.

An opening 2b is formed in one surface 2a of the vacuum container 2, and a light-transmitting window 8 is fitted in the opening 2b in an airtight manner. The light transmitting window 8 is for transmitting the laser light, and is made of, for example, MgF ₂ . This laser beam
The light is emitted from a pulse laser light source 9 provided outside the vacuum vessel 2 and is incident on a light transmitting window 8 via a condenser lens 10. As the pulse laser light source 9, a vacuum ultraviolet / ultraviolet pulse laser light source is used. The laser light emitted from the vacuum ultraviolet / ultraviolet pulse laser light source 9 is, for example, F ₂
Laser light (oscillation wavelength: 157 nm), ArF excimer laser light (oscillation wavelength: 193 nm), KrF excimer laser light (oscillation wavelength: 248 nm), XeCl excimer laser light (oscillation wavelength: 308 nm), or Nd-YAG
A second or higher harmonic (preferably a fifth or lower harmonic) by a solid laser beam such as a laser beam or an Nd-YLF laser beam is preferable. The reason for using such short-wavelength laser light is that smaller particles (eg, atoms and ions) tend to be generated during ablation. Also,
The laser light emitted from the laser light source 9 is preferably a short pulse of 1 μs or less (for example, on the order of femtoseconds). This is because if the pulse width exceeds 1 μs, ablation of the target 11 described later tends to be difficult.

A rotatable holder 12 is mounted in the vacuum vessel 2, and the target 1 is mounted on the holder 12.
1 is attached. The target 11 is arranged to face the substrate 4 and at a position where the surface of the target 11 is irradiated with the laser beam transmitted through the light transmitting window 8.

The target 11 is composed of a solid containing an ammonium compound and a carbonaceous material. For example, the target 11 is a solid obtained by uniformly mixing particles of an ammonium compound and particles of a carbonaceous material at an appropriate ratio. Be composed. Solids are obtained, for example, by pressing and compacting the mixture. By mixing and producing a solid containing C and N in this way, the solid is simultaneously decomposed and recombined by laser ablation, so that the compound of C and N can be formed without using a gas containing nitrogen. It is formed.

The carbonaceous material used in the present invention refers to a material composed of carbon atoms. Examples of such a carbonaceous material include graphite, amorphous carbon, glassy carbon, and diamond. Among them, graphite is particularly preferred in terms of good workability and high purity.

The ammonium compound used in the present invention is
A compound having an ammonium group, which is a solid at a temperature of 20 to 200 ° C. and a vapor pressure sufficiently lower than a gas pressure (for example, 1
And preferably has a 0 ^-4 ~1Torr). When the temperature is out of the above range of the vapor pressure in the above temperature range, the following tends to occur. That is,
First, the target 11 is thermally evaporated without being decomposed into atoms by laser ablation due to heat generated by the target 11 itself during laser ablation or radiant heat from the heated deposition substrate 4 and its vicinity. As a result, it tends to vaporize as molecules or clusters, and carbon nitride is not sufficiently generated. Second, since the melting point of graphite is as high as 3000 ° C. or more, with an ammonium compound having a low melting point, only the ammonium compound tends to evaporate by laser light and only graphite remains.

Examples of such an ammonium compound include ammonium chloride, ammonium carbonate, and ammonium nitrate. These ammonium compounds can be easily decomposed by the above-mentioned ultraviolet or vacuum ultraviolet pulse laser light to release nitrogen atoms in the ammonium compound. For this reason, the released nitrogen atoms react with the carbon atoms of graphite to easily generate CN molecular gas.

In the target used in the present invention, the mixing ratio between the ammonium compound and the carbonaceous material is, for example, such that the molar ratio of carbon atoms in the carbonaceous material to nitrogen atoms in the ammonium compound is 0.1 to 1. It is preferably 2.

When the ammonium compound and the carbonaceous material constituting the target 11 are in the form of particles, it is preferable that each particle diameter is sufficiently smaller than the sectional area of the laser beam to be focused and irradiated. In this case, the particle size of the ammonium compound is preferably 100 μm or less, and the particle size of the carbonaceous material is 50 μm.
m or less.

Next, a method for synthesizing carbon nitride using the above-described carbon nitride synthesizing apparatus will be described.

First, the vacuum vessel 2 is evacuated by the exhaust device 7, and then gas is introduced into the vacuum vessel 2 from the gas supply source 5 through the gas introduction pipe 6. Gas is 0.1 ~
Preferably, the pressure is 10 Torr. Next, the substrate 4 is heated by the heater 3 to a temperature of preferably 200 to 600 ° C. And holder 10 is preferably 10 to
The target 11 is rotated by rotating at a rotation speed of 200 RPM. In this state, laser light is emitted from the pulse laser light source 9, this laser light is condensed by the condenser lens 10, transmitted through the light transmitting window 8, and
Is irradiated with laser light. Here, since the laser light has a short pulse and a short wavelength, the effect of relatively large species such as molecules and clusters evaporating from the surface of the target 11 due to heat generated by the irradiation of the laser light is suppressed. Irrespective of the constituents, laser ablation is uniformly and intensely performed, and is vaporized or excited to generate atoms and ions of carbon and nitrogen, respectively, to generate monocyan CN molecules (carbon nitride).

At this time, nitrogen species and active carbon species are simultaneously mixed and generated on the surface of the target 11 at a high density, and a chemical reaction between carbon and nitrogen occurs strongly. For this reason,
The C ₂ molecular component decreases, the ratio of monocyan CN molecules increases, and carbon nitride is produced efficiently. In particular, when a rare gas, hydrogen, oxygen, nitric oxide, nitrogen dioxide, nitrogen, ammonia, or hydrazine or a combination thereof is used as a gas introduced into the vacuum vessel 2, carbon and nitrogen generated at the time of ablation are used, respectively. The reaction of the atoms and ions is promoted, and more CN molecules are generated. As a result, after the laser irradiation, until the CN molecules reach the substrate 4, after reaching the substrate 4 and before deactivation, the chemical reaction with the nitrogen gas or another nitrogen compound gas is repeated, It is possible to form a carbon nitride thin film (for example, a C ₃ N ₄ crystal film) containing a large amount of nitrogen atoms as a whole on the substrate 4.

Of course, the method of the present invention is not limited to the use of the device shown in FIG. For example, the laser ablation in the method of the present invention may be performed under a vacuum instead of under a gas atmosphere. In this case, the gas supply source 5 and the like are unnecessary.

Next, the present invention will be described more specifically with reference to examples.

[0026]

EXAMPLE 1 Commercially available ammonium chloride microcrystals (trade name: ammonium chloride (manufactured by Kanto Kagaku)) and graphite particles (trade name: graphite powder (manufactured by Kanto Kagaku)) were charged into a mortar and granulated. After finely grinding to a diameter of 50 μm or less, the ratio of the number of nitrogen atoms of the ammonium chloride microcrystal powder to the number of carbon atoms of the graphite particle powder (= (the number of nitrogen atoms of the ammonium chloride microcrystal powder) / (graphite powder Of each of the powders was uniformly mixed in such a ratio that the number of carbon atoms) was about 1. This mixture was press-shaped into a disk shape using a press molding apparatus, and a disk having a diameter of 20φ and a thickness of 5 mm was used as an ablation target.

At the time of ablation, laser ablation was performed at about 2 Torr of nitrogen gas in order to efficiently take in nitrogen atoms by a simple method. An F ₂ laser beam (oscillation wavelength: 157 nm) having a laser power of 50 mJ / pulse and a pulse width of 15 ns is emitted at a repetition frequency of 20 Hz, and this laser beam is projected onto a target by a plano-convex lens at a frequency of 1 Hz.
Light was collected at a density of 0 J / cm ² and irradiated for 4 hours. At the time of laser beam irradiation, the target was rotated at a rotation speed of the holder of about 60 RPM. At this time, the temperature of the quartz substrate was 4
It was kept at 00 ° C.

At this time, the emission spectrum of the target during laser ablation was measured. Figure 2 shows the result.
Is shown by a curve a. As shown in the figure, in addition to light emission by CN molecule, C ₂ molecules by 468 ± 4,510 ± 7,
The emission at 550 ± 10 nm disappeared within the experimental error, indicating that the method according to this example is suitable for increasing the ratio of CN molecules.

A film having a thickness of about 1 μm was obtained on the quartz substrate. The resulting film has an X-ray photoelectron spectrum (E
SCA) and X-ray diffraction measurements confirmed that the film was a carbon nitride film.

Comparative Example 1 KrF laser light (wavelength 248)
Laser ablation was performed in the same manner as in Example 1 except that the graphite target was irradiated with a laser beam of 15 nm (nm, pulse width: 15 ns), and the emission spectrum was measured. The result is shown in FIG. In the figure, 359 ± 3, 383
Each peak at ± 9, 415 ± 4 nm is due to CN molecules, but 468 ± 4 due to C ₂ molecules,
It can be seen that there is a strong emission peak at 510 ± 7, 550 ± 10 nm. This is a light emission peak corresponding to the unnecessary C ₂ molecule. From this, the generated gas contains
It was found that a large number of C ₂ molecules as impurity molecules existed in addition to carbon nitride. As a result, it was found that the relative intensity of the peak corresponding to the C ₂ molecule was larger than the peak intensity corresponding to the CN molecule than in the case of the curve a in FIG.

(Comparative Example 2) Shortest wavelength (wavelength 157 nm)
The a F ₂ laser beam except irradiated to graphite target in a nitrogen gas perform laser ablation in the same manner as in Example 1, the emission spectrum was measured occurring at that time. As shown by the curve b in FIG. 2, the number of C ₂ molecules was relatively small. However, although weak, it is still C
468 ± 4, 510 ± 7, 550 ± 10n by _two molecules
m was observed.

[0032]

As described above, according to the present invention, impurity molecules generated by laser ablation of a target can be reduced, and carbon nitride can be efficiently generated.

[Brief description of the drawings]

FIG. 1 is a schematic cross-sectional view showing one example of a carbon nitride synthesizing apparatus for performing a carbon nitride synthesizing method of the present invention.

FIG. 2 is a curve a showing an emission spectrum measured during laser ablation in the method for synthesizing carbon nitride of the present invention, and a curve b using graphite as a target.
An emission spectrum measured when the laser ablation in the case of using the F ₂ laser beam as the laser beam.

FIG. 3 is an emission spectrum measured during laser ablation in a conventional carbon nitride synthesis method.

[Explanation of symbols]

 11 Target.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Osamu Matsumoto 1126 Nomachi, Hamamatsu City, Shizuoka Prefecture 1 Hamamatsu Photonics Co., Ltd.

Claims (7)

[Claims] 1. A method for synthesizing carbon nitride, wherein laser ablation is performed by irradiating a target with laser light to synthesize carbon nitride, wherein the target comprises an ammonium compound and a carbonaceous material. A method for synthesizing carbon nitride, comprising using a solid. 2. The synthesis of carbon nitride according to claim 1, wherein the laser beam is a laser beam capable of decomposing the ammonium compound and releasing nitrogen atoms in the ammonium compound. Method. 3. The laser beam according to claim 1, wherein the laser beam is F ₂ laser beam, ArF
Excimer laser light, KrF excimer laser light or X
The method for synthesizing carbon nitride according to claim 1, wherein the method is a second or higher harmonic of eCl excimer laser light, Nd-YAG laser light, or Nd-YLF laser light. 4. The method according to claim 1, wherein the ammonium compound is 20 to 20.
Solid at a temperature of 0 ° C and 10 ^-4 to 1 at 20 ° C
The method for synthesizing carbon nitride according to claim 2 or 3, wherein the method has a vapor pressure of Torr. 5. The method according to claim 4, wherein the ammonium compound is ammonium chloride, ammonium carbonate, or ammonium nitrate. 6. The method according to claim 1, wherein the target is placed in at least one gas atmosphere selected from the group consisting of a rare gas, hydrogen, oxygen, nitric oxide, nitrogen dioxide, nitrogen, ammonia, and hydrazine. A method for synthesizing carbon nitride according to any one of claims 1 to 5. 7. The method according to claim 1, wherein the carbonaceous material is graphite, amorphous carbon, or diamond.