JP2573285B2 - Method of forming organic thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Field of Industrial Application) The present invention relates to a method for forming an organic thin film in which an organic molecular film is accumulated by a Langmuir-Blodgett method.

(Prior Art) In recent years, research on organic thin films (LB films) by the Langmuir-Blodgett method has been actively conducted for the purpose of application to various new functional devices. Using an LB film, it is possible to obtain an element that exhibits desired functions by controlling the orientation, stacking structure, and intermolecular distance of functional molecules having a dye skeleton in a uniform and extremely thin film. There is.

Generally, an LB film develops an amphiphilic organic molecule having a highly hydrophilic portion and a highly hydrophobic portion in the molecule on the water surface, and compresses the area so as to show a predetermined surface pressure. After forming a monomolecular film on the water surface by the above method, the monomolecular film is obtained by transferring the monomolecular film to a predetermined substrate. As a specific operation method for transferring the monomolecular film to the substrate, a method of vertically immersing the substrate in the water surface where the monomolecular film is developed (vertical immersion method),
There is a method (horizontal attachment method) in which the substrate is attached to a monomolecular film and pulled up while the substrate is kept parallel to the water surface. What is fundamentally important in forming the LB film is that the condition that the surface pressure is a certain value must always be satisfied. In a normal LB film forming apparatus, when the substrate is pulled up, the decrease in surface pressure due to the decrease of the monolayer on the water surface near the substrate is immediately transmitted to the entire monolayer, and this is measured by a surface pressure gauge placed at an arbitrary position in the water tank. It is assumed that the feedback control is performed so that the detected surface pressure is always constant.
However, according to the study of the present inventors, such a condition can be satisfied only when a very limited number of aliphatic molecules are used. For example, in the case of a dye-containing molecule or a polymer, it was found that the viscoelastic properties of the monomolecular film were large, and a decrease in the surface pressure near the substrate was not immediately transmitted to a surface pressure gauge located away from the substrate. For this reason, it is difficult to maintain a constant surface pressure at all times even when trying to accumulate a monolayer of a dye-containing molecule or a polymer by, for example, the vertical immersion method, which causes a change in the density and molecular orientation of the monolayer. Thus, an LB film having a non-uniform structure and many defects is formed.

On the other hand, when considering the conventional horizontal deposition method, the conventional research examples concentrate on the polymerized film, but no satisfactory accumulated film has been obtained. In general, in the horizontal deposition method, the cumulative structure is X-shaped or Z-shaped.
It is considered a type. X-type or Z-type is a cumulative structure in which the previously deposited layer and the next deposited layer have the same molecular orientation. On the other hand, a structure in which hydrophilic groups are attracted to each other and hydrophobic groups are attracted to each other, that is, a structure in which monomolecular films are alternately inverted is called a Y-type structure. Even when a Y-type cumulative film is to be formed originally using amphiphilic molecules, such a cumulative film has not been obtained by the horizontal deposition method. This is because the process of pulling the substrate is not well controlled, and even if the first layer adheres, the surface pressure near the substrate subsequently decreases and the second layer is removed.
This is probably because the layers are formed only with a disordered structure.

In order to avoid the problem of disorder of the film structure in the formation of the second layer by the horizontal deposition method, after the substrate is deposited, a partition plate is provided around the substrate, and the substrate is pulled up in a state where no monomolecular film flows. Is considered. However, in this case,
A large amount of water adheres to the monomolecular film attached to the pulled-up substrate because the hydrophilic group is directed to the surface, and this causes a disturbance in the structure of the film to be accumulated next. I can't.

(Problems to be Solved by the Invention) As described above, in the conventional LB film accumulating operation by the horizontal deposition method, the accumulating mechanism is not well understood in the first place, and therefore the control of the accumulating process is insufficient. No LB film was obtained.

The present invention has been made in view of the above points, and has as its object to provide a method for obtaining a favorable LB film cumulative film by a horizontal deposition method.

[Configuration of the Invention] (Means for Solving the Problems) The present invention is based on the following findings. According to the thermodynamic considerations of the present inventors, in the horizontal attachment method, when horizontal attachment and pull-up are performed while satisfying thermodynamic equilibrium conditions or conditions close thereto, hydrophilic groups face each other in the pull-up process. A Y-type cumulative film is formed.

The present invention basically reduces the surface pressure of a substrate attached to a monomolecular film developed on a water surface in order to realize accumulation in a state as close as possible to such thermodynamic equilibrium conditions. Using the fact that the meniscus spontaneously starts moving toward the substrate center in order to maintain the meniscus around the substrate determined by the pressure difference at the gas / liquid interface under a certain distance condition, It is characterized in that the substrate is stopped at such a pulling distance, and a Y-type cumulative film is formed in a state where the hydrophilic groups adhere to each other.

One effective method for detecting the pulling point at which the meniscus automatically starts to move, which satisfies the above-mentioned thermodynamic equilibrium condition, is a method of measuring the load applied to the substrate to be pulled. When the meniscus starts moving when the substrate is stopped by holding the substrate in a load measuring mechanism (for example, an electronic balance) and pulling it up a predetermined distance, the measured load value starts to decrease. , Automatically Y
A type accumulation film is obtained.

In the actual pulling operation, it is also effective to make the pulling speed different before and after the critical point at which the meniscus starts to move, and to control the moving speed of the meniscus by controlling the pulling speed after the critical point. Further, it is also useful to stop the substrate around the critical point and control the meniscus moving speed by controlling the surface pressure of the monomolecular film.

(Action) According to the present invention, in the horizontal deposition method, by precisely controlling the pulling speed and the pulling distance of the substrate so as to satisfy the thermodynamic equilibrium condition, the Y-type cumulative film can be formed by a single deposition and pulling operation. Can be obtained.

(Examples) Before describing specific examples, the present invention
The principle of the LB film accumulation method will be described with reference to FIG. As shown in (a), a predetermined organic molecule is spread on a water surface and compressed to form a monomolecular film 1 set at a predetermined surface pressure, and a predetermined substrate 2 kept horizontally is attached thereto. Let it. Then, as shown in (b), the substrate 2 is pulled up at an extremely slow speed so as to keep the surface pressure of the monomolecular film at a certain value or more. Specifically, it is pulled up at a rate of 0.01 to 1 mm / min, more preferably 0.01 to 0.1 mm / min. For the measurement of the surface pressure, a normal surface pressure measurement is performed near the substrate. When the substrate 2 is pulled up to a predetermined height h, a force for maintaining the meniscus formed around the substrate acts on the meniscus formed around the substrate as shown in FIG. Start moving. The conditions will be described later in detail. When the substrate is stopped at this height position, the monomolecular film 1 is attached to the entire surface of the substrate 2 in a state where the hydrophilic groups are automatically attached as shown in (d) and (e) with the movement of the meniscus. Is formed as a Y-type cumulative film.

In the above description of the principle, the conditions under which the meniscus automatically starts moving laterally when the substrate is pulled up by a predetermined distance are as follows:
It is determined as follows. FIGS. 3A and 3B schematically show an elementary process of the lateral movement of the meniscus. The thermodynamic energy change ΔU in this elementary process is
It is given by the following equation.

ΔU = (γw−π) (1-cos θ) lΔx + (γxx ′
−γwx−γwx ′) lΔx (1) where θ is the contact angle between the monolayer and the substrate (shown in FIG. 3), π is the surface pressure of the monolayer, γw is the surface energy of water, γxx ′ Is the interfacial energy between the hydrophilic group (X) of the first layer and the hydrophilic group (X ′) of the second layer, and γwx, γwx ′
Represents a hydrophilic group (X) and water (W), respectively, and a hydrophilic group (X ')
Energy between water and water (W). Δx is the lateral displacement distance of the meniscus, and l is the width of the substrate 2 (the direction perpendicular to the plane of the drawing).

The movement of the meniscus occurs when the condition of ΔU ≦ 0 (2) is satisfied. This relationship is rewritten as follows.

1− (γwx + γwx′−γxx ′) / (γw−π) ≦ cos
θ (3) On the other hand, the height h of the substrate from the water surface and cos θ are related by the following equation.

Here, ρ is the density of water, and g is the gravitational acceleration.

It is known that the surface energy γw of water is 73 dyn / cm, and therefore equation (4) is rewritten as follows.

The value of θ that satisfies the expression (3) depends on the type of hydrophilic group, but in most cases, 0 ° ≦ θ ≦ 90 °. The inventor empirically grasped. Therefore, the height h (mm) from the water surface is In this case, the lateral movement of the meniscus occurs spontaneously.

 Next, specific examples will be described.

Example 1 Using a trough of a 0.5 mM solution of cadmium chloride, a chloroform solution of stearic acid was spread on the water surface, and 25 dy
Compression was performed until a surface pressure of n / cm was exhibited to form a monomolecular film.
As a substrate, using a silicon substrate with a thermal oxide film formed,
After washing and drying, it was left in a gaseous atmosphere of hexamethyldisilazane for 24 hours to perform a hydrophobic treatment. The substrate was lowered while keeping it parallel to the water surface of the water tank, and stopped when it came into contact with the stearic acid monomolecular film. Thereafter, the substrate was pulled up while keeping the substrate parallel to the water surface at a slow speed such that the set surface pressure did not decrease, and the substrate was stopped at a height of 3.3 mm from the water surface. Thereafter, the meniscus formed around the substrate moves very slowly toward the center, and finally the water surface separates from the substrate.

The above operation was repeated five times to form a 10-layer cumulative film of stearic acid on the substrate. As a result of evaluating the accumulated film using X-ray, TEM, and SEM, it was confirmed that the LB film was uniform and defect-free without disturbance of the periodic structure in the stacking direction.

Example 2 Using a trough of pure water, a chloroform solution of a donor molecule (I) represented by the following structural formula having paraphenylenediamine as a skeleton was developed on the water surface.

This was compressed to a surface pressure of 40 dyn / cm to form a monomolecular film. Ten monolayers of this monomolecular film were accumulated on a silicon substrate subjected to a hydrophobic treatment in the same manner as in Example 1.

As a result of a structural evaluation of the obtained accumulated film by X-ray diffraction, TEM, and SEM, it was confirmed that a uniform and defect-free film with little disturbance of the periodic structure in the stacking direction was formed.

Example 3 Using a trough of pure water, an acceptor molecule having a quinodiimine skeleton represented by the following structural formula (I
The chloroform solution of I) was developed.

This was compressed to show a surface pressure of 30 dyn / cm to form a monomolecular film. Ten monolayers of this monomolecular film were accumulated on a silicon substrate subjected to a hydrophobic treatment in the same manner as in Example 1.

As a result of a structural evaluation of the obtained accumulated film by X-ray diffraction, TEM, and SEM, it was confirmed that a uniform and defect-free film with little disturbance of the periodic structure in the stacking direction was formed.

In the above-described embodiment, the film accumulation is performed by monitoring the height at which the substrate is pulled up and stopped, and setting the condition in which the meniscus expressed by Expression (6) moves spontaneously. In this case, as an effective means for detecting a critical point at which the meniscus starts to move automatically, one effective method is to attach a load measuring device (such as an electronic balance or a strain gauge) to the substrate and measure the load.

FIGS. 2 (a) to 2 (e) show the substrate pulling operation in such an embodiment corresponding to FIGS. 1 (a) to 1 (e). An electronic balance 3 is attached to the substrate 2 so that the load of the substrate 2 to be pulled can be measured. If the meniscus does not move when the substrate is stopped after being pulled up by a predetermined distance, the measured load value shows a constant value there. (C)
Under the condition that the meniscus moves in the lateral direction as shown in FIG. 7, since the amount of water lifted with the substrate 2 decreases with time, the measured load value also decreases with time. Therefore, if the weight measurement value is moving when the substrate is pulled up and stopped by a predetermined distance under the condition that the surface pressure is constant, the condition of the above-mentioned expression (6) is satisfied.

When the substrate is pulled up by a predetermined distance h and the meniscus is pinned to the periphery of the substrate, the load F of the substrate is expressed by the following equation, where S is the area of the substrate and L is the peripheral length.

F = Shρg + L (γw−π) sin θ (7) Here, the first term on the right side indicates buoyancy determined by the size of the substrate, and the second term indicates surface tension. Therefore, when the point at which the load applied to the substrate starts to deviate from the constant value of the equation (7) is detected in the process of lifting the substrate while keeping the substrate parallel to the water surface, and the substrate is stopped at that point, the point described above is obtained. Thus, the accumulation of the Y-type films is performed by the spontaneous movement of the meniscus.

Example 5 The LB film accumulation was performed under basically the same conditions as in Examples 1 to 4 described above, while monitoring the load on the substrate and setting the pulling height. As a result, a good cumulative film was obtained.

In the above, after the board is pulled up to a certain height,
The substrate was allowed to stand still, and in that state, monolayer accumulation was performed by automatic meniscus movement. However, in actual operation, it is not always necessary to keep the substrate stationary. By controlling the substrate pulling speed, it is also possible to optimally control the moving speed of the meniscus. That is, the substrate is once stopped at or around the critical point height at which the meniscus starts to move spontaneously, and thereafter, the speed is switched to a speed slower than the previous pulling speed, and the speed of the meniscus is controlled by the speed control. Can be controlled. As a result, for example, it is possible to perform a rapid film formation within a range satisfying the thermodynamic equilibrium condition as compared with a case where the substrate is completely stopped. It is not always necessary to temporarily stop the substrate, and the film formation speed can be controlled by changing the substrate pulling speed stepwise or continuously while taking the critical point into consideration.

Example 6 By changing the pulling speed before and after the critical point height,
The method of controlling the moving speed of the meniscus is described in the first embodiment.
4 were applied. As a result, a good LB film was obtained.

After the substrate is pulled up to or above the critical point height described above, the movement speed of the meniscus can also be controlled by controlling the surface pressure of the monomolecular film.

Example 7 In the same manner as in Example 1, a monomolecular film of stearic acid was formed in a trough, a hydrophobic substrate was attached in parallel with the water surface, and then pulled up in parallel with the water surface. Unlike Example 1, the substrate was stopped before the meniscus started to move, and the surface pressure of the monomolecular film was gradually increased in this state. As a result, the meniscus began to slowly move toward the inside of the substrate, and a Y-type cumulative film of stearic acid was formed as in Example 1. As a result of forming a 10-layer cumulative film and evaluating the structure, it was confirmed that the film was a good film as in Example 1.

[Effects of the Invention] As described above, according to the present invention, in the horizontal deposition method, the phenomenon in which the meniscus spontaneously moves to the center of the substrate under conditions substantially satisfying the thermodynamic equilibrium condition is positively used. This makes it possible to obtain a Y-type cumulative film having a uniform cumulative structure and few defects.

[Brief description of the drawings]

1 (a) to 1 (e) are diagrams for explaining the operation and principle of the film accumulation of the present invention, and FIGS. 2 (a) to 2 (e) are the operation of the film accumulation in the same specific embodiment. FIGS. 3 (a) and 3 (b) are diagrams for explaining how the meniscus moves in the present invention. 1 ... monomolecular film, 2 ... substrate, 3 ... electronic balance.

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-61-291058 (JP, A) JP-A-62-286576 (JP, A) JP-A-62-176574 (JP, A)

Claims (5)

(57) [Claims] 1. A monomolecular film having a predetermined surface pressure is formed by expanding and compressing organic molecules on a water surface, and is attached to the monomolecular film and pulled up while keeping a predetermined substrate horizontal. Thus, in the method of forming an organic thin film on the substrate, the substrate is pulled up a predetermined distance at a speed that does not reduce the surface pressure in the vicinity thereof, utilizing the force to move the meniscus standing around the substrate to the center of the substrate, A method for forming an organic thin film, comprising forming a monomolecular Y-type cumulative film. 2. A monomolecular film having a predetermined surface pressure is formed by expanding and compressing organic molecules on a water surface, and is attached to the monomolecular film and pulled up while keeping a predetermined substrate horizontal. Thus, in the method of forming an organic thin film on the substrate, the substrate is pulled up at a speed that does not lower the surface pressure in the vicinity thereof, and the height h in the range represented by the following equation with respect to the surface pressure π (dyn / cm) (Mm), and the Y-cumulative film of the monomolecular film is formed by utilizing the fact that the meniscus standing around the substrate automatically moves to the center of the substrate at that time. A method for forming an organic thin film. formula 3. A monomolecular film having a predetermined surface pressure is formed by expanding and compressing organic molecules on a water surface, and is attached to the monomolecular film and pulled up while keeping a predetermined substrate horizontal. Thereby, in the method of forming an organic thin film on the substrate, the substrate is pulled up at a speed that does not lower the surface pressure in the vicinity thereof, and the point at which the meniscus standing around the substrate automatically starts moving to the substrate center can be pulled up. A method for forming an organic thin film, comprising: detecting a load applied to a substrate; measuring the load on the substrate; and stopping the substrate at that point, thereby forming the Y-type cumulative film of the monomolecular film. 4. A monomolecular film having a predetermined surface pressure is formed by expanding and compressing organic molecules on a water surface, and is attached to the monomolecular film and pulled up while keeping a predetermined substrate horizontal. Thus, in the method of forming an organic thin film on the substrate, the substrate is pulled up at a speed that does not lower the surface pressure in the vicinity thereof, and the critical point of the height at which the meniscus standing around the substrate automatically starts moving to the substrate center. A method for forming an organic thin film, wherein the Y-cumulative film of the monomolecular film is formed by controlling a pulling speed of the substrate from before and after and controlling a moving speed of the meniscus. 5. A monomolecular film having a predetermined surface pressure is formed by expanding and compressing organic molecules on a water surface, and is attached to the monomolecular film and pulled up while keeping a predetermined substrate horizontal. Thus, in the method of forming an organic thin film on the substrate, the substrate is pulled up at a speed that does not lower the surface pressure in the vicinity thereof, and the critical point of the height at which the meniscus standing around the substrate automatically starts moving to the substrate center. A method for forming an organic thin film, wherein the substrate is stopped before and after, and then the surface pressure is controlled to control the moving speed of the meniscus, thereby forming the monomolecular Y-cumulative film.