JP4150786B2 - Method for forming amorphous thin film of organic optical functional material - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for forming an amorphous thin film used as an optical functional material from a solid low molecular weight organic compound that forms a molecular crystal.
[0002]
[Prior art]
In recent years, efforts have been actively made to use high-molecular or low-molecular organic compounds having optical functions as various optical functional materials such as nonlinear optical materials, photoelectric materials, and light-emitting materials. Among these, as low-molecular organic compounds having an optical function (hereinafter referred to as organic optical functional materials), those that are solid at room temperature are mainly used, and these have the property of easily forming molecular crystals. Have. When an organic optical functional material having such a molecular crystal is used as an element, when the organic optical functional material forms a relatively large single crystal, the single crystal can be used as it is or as desired. It can be cut into a shape and used. However, inconvenience occurs when it is necessary to form the organic optical functional material as a thin film on the substrate. That is, a conventional film forming method such as a spin coat method or a heat evaporation method that is often used for such purposes can be obtained because the organic optical functional material is once decomposed into single molecules and then deposited on a substrate. The film tends to be an aggregate of microcrystals made of an organic optical functional material. When a thin film layer formed of such an assembly of microcrystals is used as an optical element, there is a problem that an optical signal incident on the device is distorted due to a scattering phenomenon at the crystal interface. Under such circumstances, there has been a demand for a method of forming a thin film made of an organic optical functional material on a substrate in a substantially amorphous state.
[0003]
Further, as a method of forming an organic optical functional material for forming a molecular crystal into a thin film in an amorphous state, the organic optical functional material is heated in a vacuum to be released as a single molecule or a small molecule group, A method of forming a thin film made of an organic optical functional material on a substrate by rapidly cooling on the substrate, that is, a so-called vacuum deposition method in a broad sense is known. However, when this method is used, the kinetic energy of particles flying on the substrate is not so large, and in order to obtain an amorphous film, it is necessary to cool and hold the substrate at a very low temperature by using liquefied nitrogen or the like. For this reason, there is a problem that the apparatus becomes large and complicated.
In addition, in the case of using a so-called sputtering method in which target molecules bounced off by accelerated ion collisions are deposited on the substrate, the organic optical functional material is remarkably reduced at the molecular level due to intense ion impact during sputtering. There was a problem that the original chemical structure was hardly maintained and the target optical function was not exhibited.
[0004]
The laser deposition technique known in the art as laser ablation has been applied to the formation of thin films from a wide variety of materials ranging from polymers to semiconductors to insulators. As an application example of organic polymer thin film formation, Japanese Patent No. 1487564 (Japanese Patent Laid-Open No. 60-86266) discloses that a high-energy laser beam is irradiated to the organic polymer to release it once and is reconstructed on a substrate. A method for forming a thin film is disclosed. Since the laser light irradiated here has an energy higher than the chemical bond energy of the polymer chain, the liberated substance generated by the laser light irradiation is substantially a decomposition product generated by the chemical bond breakage of the polymer chain. The decomposition products are bonded to each other on the substrate to form a polymer chain again to form a film. On the other hand, as will be described later, the thin film formation method of the present invention substantially affects the chemical bonding of the raw material molecules by irradiating an organic low molecular weight compound as a film material with an ultraviolet laser in a specific energy range. The basic idea is to release and vaporize as a single molecule or a group of micromolecules having very large kinetic energy without giving them, and to deposit them as a thin film on the substrate with the original molecular structure. Essentially different.
[0005]
Also, Laser Research, Vol. 23 (No. 8), page 625, uses a laser ablation method to make organic optical functional materials 4-dimethylamino-4′-nitro-stilbene (DMANS) and copper phthalocyanine (CuPc). A technique for forming a thin film on a substrate is disclosed. However, this document discloses that a crystalline film can be obtained by film formation at a minimum energy intensity at which deposition of a film on a substrate starts, that is, an irradiation energy intensity very close to a threshold value. In addition, it is stated that the chemical structure of the organic optical functional material has completely changed, although an amorphous film can be obtained by film formation with a very high irradiation energy intensity of 1500 mJ / cm ² . . However, there is no disclosure about a method for forming an amorphous film that maintains the chemical structure of the organic optical functional material.
[0006]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and provides a method for forming an amorphous thin film substantially maintaining the chemical structure of an organic optical functional material on a substrate without using a large-scale apparatus. For the purpose.
[0007]
[Means for Solving the Problems]
The present inventors have continued intensive studies to solve the above problems. As a result, in the thin film forming method using the laser ablation method, it has been found that the above problem can be solved by setting the irradiation energy intensity within a specific range, and the present invention has been achieved. That is, according to the present invention,
A method of forming a thin film by irradiating a target made of a low molecular weight organic compound having an optical function with an ultraviolet laser to vaporize the low molecular weight organic compound and depositing the vaporized low molecular weight organic compound on a substrate. Thus, there is provided a method for forming an amorphous thin film of an organic optical functional material, wherein the irradiation intensity E (mJ / cm ² ) of an ultraviolet light laser satisfies the following conditions.
C × 1.5 ≦ E ≦ C × 4
(However, C: Irradiation intensity of the ultraviolet laser (mJ / cm ² ) where the vaporized low-molecular organic compound starts to be deposited substantially on the substrate)
Furthermore, there is provided a method for forming an amorphous thin film of an organic optical functional material, wherein the ultraviolet laser is a KrF excimer laser.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
A feature of the present invention is that a method of forming an amorphous thin film made of an organic optical functional material on a substrate uses a laser ablation method using an ultraviolet laser, and the irradiation intensity of the ultraviolet laser at that time is adjusted to an organic optical function. It is to set within a specific range according to the characteristics of the material. In this way, even when an organic optical functional material that originally tends to form a molecular crystal is used as a film material, the chemical structure of the organic optical functional material is almost unchanged without using a large substrate cooling device. A maintained amorphous thin film can be formed. The present invention will be described in detail below.
[0009]
The organic optical functional material used in the present invention is not particularly limited as long as it is an organic compound that is a molecular crystalline solid at room temperature and can form an amorphous thin film that exhibits a function as an optical element. Examples of these organic optical functional materials include N, N′-bis (4-nitrophenyl) -urea, N, N′-bis (4-nitrophenyl) -methanediamine, and 3,4-dimethoxy-4′-nitro. Non-linear optical material represented by stilbene etc., represented by copper phthalocyanine or N, N′-diphenyl-N, N′-bis (3-methylphenyl) -1,1′-biphenyl-4,4′-diamine etc. And the like. These organic optical functional materials are molecular crystalline solids at room temperature, and receive energy that matches the crystal energy or intermolecular energy by irradiation with an ultraviolet laser of a specific wavelength in a vacuum, and evaporate and vaporize as single molecules or small molecule groups. To do. In the present invention, the organic optical functional material may contain a trace amount of additives as long as it does not affect the function of the film formed on the substrate.
[0010]
A target is formed from the organic optical functional material described above. As the method, it is preferable to form a powdery organic optical functional material into a plate shape by pressure molding. However, a single crystal plate formed from an organic optical functional material can be used as it is, or a powdered organic optical material can be used. The functional material may be used as it is.
[0011]
In the present invention, the intensity of the ultraviolet laser that irradiates the target organic optical functional material is set on the basis of the so-called threshold value of the irradiation laser intensity at which film deposition starts on the substrate. The threshold varies depending on the chemical structure of the organic optical functional material and the wavelength of the ultraviolet laser. The irradiation laser intensity E (mJ / cm ² ) is set so as to satisfy the following conditions.
C × 1.5 ≦ E ≦ C × 4
(However, C: Irradiation intensity (mJ / cm ² ) of an ultraviolet light laser where vaporized organic optical functional material starts to be substantially deposited on the substrate, so-called threshold value)
By irradiating an organic optical functional material with an ultraviolet laser with an intensity within this range, the chemical structure of the organic optical functional material is substantially maintained, and as a single molecule or small molecule group having a large kinetic energy. It can be vaporized. As a result, the thin film formed on the substrate can maintain almost the original chemical structure of the organic optical functional material and can maintain an amorphous state.
If the irradiation laser intensity E (mJ / cm ² ) is smaller than this range, not only the film formation rate is slowed but also microcrystals are mixed in the film, which is not preferable. On the other hand, when the irradiation laser intensity E (mJ / cm ² ) is larger than this range, the organic optical functional material is destroyed at the molecular level by the energy of the irradiation laser, and the chemical structure of the thin film deposited on the substrate is intended. It becomes different and is not preferable.
[0012]
Next, a method for obtaining this threshold will be described. In laser ablation of an organic optical functional material with an ultraviolet laser in vacuum, the relationship between the irradiation laser intensity and the film deposition rate shows a common pattern for any compound. That is, film deposition hardly occurs on the substrate when the irradiation laser intensity is below a certain value. When the irradiation laser intensity exceeds a certain value, film deposition is started. Subsequently, when the irradiation laser intensity is gradually increased, the film deposition rate increases rapidly, and then gradually decreases after a peak. The irradiation laser intensity at which this film deposition is substantially started is a threshold value. Actually, it can be easily determined by a method of plotting the relationship between the magnitude of the irradiation energy intensity and the deposition rate of the film obtained with the intensity.
[0013]
The type of the light source of the ultraviolet laser used in the present invention can be appropriately selected according to the absorption wavelength of the organic optical functional material as the film material, the degree of absorption of ultraviolet light, etc., but is not particularly limited, but a rare gas-halogen excimer laser is generally used. F ₂ laser (wavelength 157 nm), ArF laser (wavelength 193 nm), KrCl laser (wavelength 222 nm), KrF laser (wavelength 249 nm), XeCl laser (wavelength 308 nm) can be used. Further, harmonics of Nd: YAG laser (wavelength 266 nm) can be used. Among these light sources, the shorter wavelength is advantageous because the absorption coefficient of the film material is high and the deposition rate of the film is increased. However, if the wavelength becomes too short, the energy attenuation due to absorption into the air also increases, and the optical path needs to be purged with an inert gas. From these points, it is most preferable to use a KrF laser (wavelength 249 nm). The oscillation method may be a continuous oscillation method or a pulse oscillation method.
[0014]
Further, the degree of vacuum at the time of film formation is not particularly limited as long as the mean free path is larger than the distance from the target to the substrate, but it is caused by collision with other molecules of the organic compound molecule that flew from the target. From the viewpoint of avoiding a decrease in kinetic energy as much as possible, it is preferably 1 × 10 ⁻³ Torr or less, and more preferably 1 × 10 ⁻⁵ Torr or less.
[0015]
In order to avoid a chemical change of the target due to heat generated during laser light irradiation, it is desirable to scan the target with the laser light, or to rotate or move the target.
[0016]
【Example】
EXAMPLES Hereinafter, although this invention is demonstrated further in detail based on an Example, this invention is not limited to the following Example at all, It can implement by changing suitably.
[0017]
The thin film was formed using the apparatus schematically shown in FIG. That is, first, a fine powder of an organic optical functional material is pressure-molded as a target 13 into a target holder 14 attached to the tip of a shaft of a vacuum motor 15 in a vacuum chamber 12 to form a disk (diameter 20 mm, thickness 0. 5 mm) was attached. In addition, a calcium fluoride disk (diameter 30 mm, thickness 1.5 mm) was attached to the substrate holder 10 as a thin film production substrate 11 with a distance of 27 mm vertically from the target. The inside of the vacuum chamber 12 is evacuated by a vacuum exhaust pump 17 through a valve 16 until the degree of vacuum becomes about 5 × 10 ⁻⁶ Torr, and then a laser from a KrF excimer laser oscillator 1 (LAMBDA PHYSIK LPX-305iS). Light 3 (wavelength 249 nm, pulse width 25 ns, repetition rate 10 Hz) is attenuated to the desired energy by the attenuator 2, passes through the mirror 4 and the diaphragm plate 5 having a hole diameter of 3 mm, and then condensed by the lens 8. After passing through the vacuum sealing window 9, the surface of the target 13 was irradiated for a predetermined time so as to have an irradiation area of 5.2 mm ² . During the laser irradiation, in order to prevent the laser pulse light from concentrating on one place, the vacuum motor was operated and the target was rotated at a speed of 30 rpm. The irradiation intensity E (mJ / cm ² ) on the target surface is obtained by subtracting the attenuation due to the vacuum sealing window 9 from the laser light energy immediately after passing through the lens measured with a power meter prior to film formation. It was determined by dividing the irradiation area 5.2 mm ² in.
[0018]
The deposited amount of the obtained film was measured as the thickness at the center of the film using a surface shape measuring machine (Surfcom 570A manufactured by Tokyo Seimitsu Co., Ltd.) and a repetitive reflection interferometer (manufactured by Anerva Corporation). On the other hand, confirmation of the difference between the chemical structure of the organic optical functional material used as the target and the chemical structure of the thin film formed on the substrate is obtained with the infrared absorption spectrum of the organic optical functional material measured by the KBr tablet method. Comparison was made with the infrared absorption spectrum obtained by the transmission measurement method. In addition, the confirmation of the crystal state of the thin film was determined by whether or not a crystal peak appeared in the X-ray diffraction pattern of the thin film, and also by optical transparency.
[0019]
Under the above experimental methods and conditions, N, N′-bis (4-nitrophenyl) -urea (hereinafter abbreviated as NPU), N, N′-bis (4-nitrophenyl) -methanediamine (hereinafter referred to as NPMDA) And film formation experiments of 3,4-dimethoxy-4′-nitrostilbene (hereinafter abbreviated as MOMONS) were performed by changing the irradiation intensity E in various ways. 2, 3, and 4 are graphs plotting the relationship between the irradiation intensity E and the film deposition rate for each compound. It is clear that all compounds show exactly the same pattern. That is, there is a clear threshold for film deposition, and the rate of film deposition increases rapidly as the initial irradiation laser intensity increases, and then gradually decreases after a peak. From these results, the threshold values for film deposition were determined as NPU: 13 mJ / cm ² , NPMDA: 12 mJ / cm ² , and MOMONS: 11 mJ / cm ² , respectively. The threshold value was determined by extrapolating plot points indicating the relationship between the irradiation intensity E and the film deposition rate, and obtaining the intersection with the irradiation intensity E axis.
[0020]
[Example 1]
Using NPU as a target, laser irradiation is performed for 60 minutes so that the laser irradiation intensity E is 25 mJ / cm ² (intensity 1.9 times higher than the threshold), and the thickness near the center of the substrate is about 0.96 μm yellow. A very transparent film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of NPU measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this example substantially maintained the chemical structure of NPU. Further, no peak indicating the presence of crystals was observed in the X-ray diffraction pattern of the film obtained in this example. Moreover, it was optically transparent. From these results, it is clear that the film obtained in this example is amorphous.
[0021]
[Comparative Example 1]
Using NPU as a target, laser irradiation was performed for 120 minutes with a laser irradiation intensity E of 15 mJ / cm ² (an intensity 1.2 times higher than the threshold), and the thickness near the center of the substrate was about 0.12 μm yellow. A slightly opaque film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of NPU measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this comparative example substantially maintained the chemical structure of NPU. However, a peak was observed in the X-ray diffraction pattern of the film obtained in this comparative example, although it was very small near 2θ = 28 °. Furthermore, since the film is slightly opaque, it is clear that a very small amount of fine crystals are present in the film obtained in this comparative example.
[0022]
[Comparative Example 2]
Using NPU as a target, laser irradiation was performed for 60 minutes with a laser irradiation intensity E of 60 mJ / cm ² (intensity 4.6 times higher than the threshold), and the thickness near the center of the substrate was about 0.46 μm. A clear transparent film was obtained. The X-ray diffraction pattern of the obtained film has no peak indicating the presence of crystals and is optically transparent, so that it is clear that the film obtained in this Comparative Example is amorphous. On the other hand, in the infrared absorption spectrum of the film obtained, ^{1736 cm} -1 was observed in the infrared absorption spectrum of NPU measured by KBr tablet ^method, 1552 cm -1, absorption peaks such as 849cm ^-1 almost disappeared, other The peak of was also small and broad overall. In addition, it is clear that the chemical structure of the film obtained by this comparative example is greatly changed from the fact that the color of the film is changed to black.
[0023]
[Example 2]
Using NPMDA as a target, laser irradiation is performed for 60 minutes so that the laser irradiation intensity E is 25 mJ / cm ² (intensity 2.1 times higher than the threshold), and the thickness near the center of the substrate is about 1.83 μm in yellow A very transparent film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of NPMDA measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this example substantially maintained the chemical structure of NPMDA. Further, no peak indicating the presence of crystals was observed in the X-ray diffraction pattern of the film obtained in this example. Moreover, it was optically transparent. From these results, it is clear that the film obtained in this example is amorphous.
[0024]
[Comparative Example 3]
Using NPMDA as a target, laser irradiation was performed for 60 minutes with a laser irradiation intensity E of 15 mJ / cm ² (intensity 1.3 times higher than the threshold value), and the thickness near the center of the substrate was about 0.54 μm. An opaque yellow film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of NPMDA measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this comparative example substantially maintained the chemical structure of NPMDA. However, in the X-ray diffraction pattern of the film obtained in this comparative example, a peak was recognized although it was very small around 2θ = 26 °. Furthermore, since the film is somewhat opaque, it is clear that there is a very small amount of microcrystals in the film.
[0025]
[Comparative Example 4]
Using NPMDA as a target, laser irradiation was performed for 60 minutes with a laser irradiation intensity E of 60 mJ / cm ² (intensity 5.0 times higher than the threshold), and the thickness near the center of the substrate was about 0.90 μm. A yellowish transparent film was obtained. The X-ray diffraction pattern of the obtained film has no peak indicating the presence of crystals, and is optically transparent. Therefore, it is clear that the film obtained in this comparative example is amorphous. On the other hand, in the infrared absorption spectrum of the film ^obtained, 1531cm -1 were observed in the infrared absorption spectrum of NPMDA measured by KBr tablet ^{method, 1469cm -1, 1042cm -1,} absorption peaks such as 834cm ^-1 Most It disappeared, and the other peaks were small and broad overall. In addition, it is clear that the chemical structure of the film obtained by this comparative example has changed greatly from the fact that the film has turned black.
[0026]
[Example 3]
Using MOMONS as a target, laser irradiation was performed for 40 minutes so that the laser irradiation intensity E was 25 mJ / cm ² (intensity 2.3 times higher than the threshold), and the thickness near the center of the substrate was about 1.67 μm in yellow A very transparent film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of MOMONS measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this example substantially maintained the chemical structure of MOMONS. Further, no peak indicating the presence of crystals was observed in the X-ray diffraction pattern of the film obtained in this example. Moreover, it was optically transparent. From these results, it is clear that the film obtained in this example is amorphous.
[0027]
[Comparative Example 5]
Using MOMONS as a target, laser irradiation was performed for 80 minutes so that the laser irradiation intensity E was 15 mJ / cm ² (1.4 times higher than the threshold), and the thickness near the center of the substrate was about 1.78 μm. An opaque yellow film was obtained. The infrared absorption spectrum of the obtained film almost coincided with the infrared absorption spectrum of MOMONS measured by the KBr tablet method. From this, it was confirmed that the chemical structure of the film obtained in this comparative example substantially maintained the chemical structure of MOMONS. However, in the X-ray diffraction pattern of the film obtained in this comparative example, peaks were recognized although they were extremely small around 2θ = 17 °, 25 °, and 27.5 °. Furthermore, since the film is slightly opaque, it is clear that a very small amount of fine crystals are present in the film obtained in this comparative example.
[0028]
[Comparative Example 6]
Using MOMONS as a target, laser irradiation was performed for 60 minutes such that the laser irradiation intensity E was 50 mJ / cm ² (intensity 4.5 times higher than the threshold), and the thickness near the center of the substrate was 1.14 μm. A yellow transparent film was obtained. The X-ray diffraction pattern of the obtained film has no peak indicating the presence of crystals and is optically transparent, so that it is clear that the film obtained in this Comparative Example is amorphous. On the other hand, in the infrared absorption spectrum of the film ^obtained, 1629cm -1 were observed in the infrared absorption spectrum of MOMONS measured by KBr tablet ^method, 1420 cm ^-1, 1260 cm -1, absorption peaks such as 1140 cm ^-1 is most It disappeared, and the other peaks were small and broad overall. In addition, it is clear that the chemical structure of the film obtained by this comparative example has changed greatly from the fact that the film has turned black.
[0029]
According to the present embodiment, according to the present invention, even when an organic optical functional material showing very high crystallinity represented by NPU, MOMONS, or NPMDA is used as a target, the organic optical functional material is It is clear that an amorphous and optically transparent thin film can be formed on the substrate while substantially retaining the chemical structure.
[0030]
【The invention's effect】
As described above, according to the present invention, there is provided a method for forming an amorphous thin film substantially maintaining the chemical structure of an organic optical functional material on a substrate without using a large-scale apparatus. The optical device obtained by the present invention has the advantages of being economically advantageous because it has a good optical function due to the fact that the thin film is amorphous and can be manufactured without using a large-scale apparatus. is doing.
As described above, the method for forming an amorphous thin film of an organic optical functional material of the present invention is useful in providing a high-quality optical device at low cost, and can be said to be very useful for industry.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram showing an embodiment of a thin film forming apparatus used in the method for forming an amorphous thin film of an organic optical functional material of the present invention.
FIG. 2 is a graph showing the relationship between the deposition rate of the NPU film formed in Examples and Comparative Examples of the present invention and the laser irradiation intensity E;
FIG. 3 is a graph showing the relationship between the deposition rate of the NPMDA film formed in Examples and Comparative Examples of the present invention and the irradiation intensity E of the laser.
FIG. 4 is a graph showing the relationship between the deposition rate of the MOMONS film formed in Examples and Comparative Examples of the present invention and the irradiation intensity E of the laser.
[Explanation of symbols]
1: Laser oscillator 2: Attenuator 3: Laser beam 4: Mirror 5: Aperture plate 6: Power meter 7: Indicator 8: Lens 9: Window 10: Substrate holder 11: Substrate 12: Vacuum chamber 13: Target 14: Target Holder 15: Vacuum motor 16: Valve 17: Vacuum exhaust pump

Claims (2)

A method of forming a thin film by irradiating a target made of a low molecular weight organic compound having an optical function with an ultraviolet laser to vaporize the low molecular weight organic compound and depositing the vaporized low molecular weight organic compound on a substrate. A method for forming an amorphous thin film of an organic optical functional material, wherein the irradiation intensity E (mJ / cm ² ) of an ultraviolet light laser satisfies the following conditions:
C × 1.5 ≦ E ≦ C × 4
(However, C: Irradiation intensity of the ultraviolet laser (mJ / cm ² ) where the vaporized low-molecular organic compound starts to be deposited substantially on the substrate) 2. The method for forming an amorphous thin film of an organic optical functional material according to claim 1, wherein the ultraviolet laser is a KrF excimer laser.

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