JP2947603B2 - Method for producing polyimide thin film - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Object of the Invention] (Industrial application field) The present invention relates to a heat-resistant thin film, an insulating thin film, and a polyimide thin film used for a liquid crystal alignment film and the like. More specifically, the present invention relates to an improvement in a method for producing a polyimide thin film by the LB method (Langmuir-Projet method).

(Prior Art) In recent years, an ultrathin organic film having a thin film of 100 nm or less has attracted attention. Among them, an insulating thin film occupies the mainstream, and is used as an insulating protective film in various elements and a dielectric thin film in a capacitor.

As a method for forming such a thin film, the following wet process and dry process are well known.

The most common method by a wet process is a solvent casting method. This method is a technique for forming a thin film by naturally drying a polymer solution. According to the spin coating method that applies this, that is, a technique in which a polymer solution is spread on a rotating substrate by a spinner and dried to make a thin film, a thin film having a thickness of about 10 nm is formed depending on the type of polymer used. It is also possible. However, in this method, the structure controllability in film formation is poor, and it is difficult to obtain a thin film having a uniform and dense structure and high insulating properties.

As a typical example of the dry process, there is a vacuum deposition method in which a deposition material is heated and evaporated in a vacuum, and the deposition material is attached to a substrate to form a thin film. Although thin films such as polyethylene and polytetrafluoroethylene can be formed by this method, it is very difficult to vacuum-deposit a heat-resistant polymer. Also, a method is known in which two kinds of monomers are vapor-deposited and polymerized on a substrate to form a polyimide having high heat resistance, but it is difficult to optimize the vapor deposition conditions, and a polymer having a sufficient high molecular weight is not available. There is a problem that cannot be obtained.

On the other hand, recently, in addition to the above method, an attempt has been made to form an organic thin film (LB film) on a substrate of a semiconductor, a metal, or the like by the LB method and to use the organic thin film as an insulating film.
Thin Films Magazine, No. 99
Volume, p. 283, 1984, Electronics Letters (Ele
ctronics Letters), 20, 12, 489, 1984). LB
The method is a method of forming a monolayer on a water surface and repeating the process of transferring the monolayer onto a solid substrate, thereby forming a cumulative layer (LB layer) of monolayers. .

The LB method can control the molecular arrangement and orientation in the highest order among the organic ultra-thin film manufacturing methods. Therefore, LB
The film generally has a uniform thickness, has few surface defects,
In addition, there is an advantage that the final film thickness can be controlled by the thickness of the monomolecular film, that is, about 10 °. However, all of these LB films have a critical defect that heat resistance and mechanical strength are small and do not satisfy practical requirements in various devices. For this reason, research on improving the heat resistance and mechanical strength of LB films has been actively pursued recently. For example, an example in which a condensed ring compound having excellent heat resistance is contained in a film-forming organic material (Electronic Letters, No. 20)
Vol. 12, p. 489, 1984), an example in which a non-polymerizable low-molecular compound coexists with a high-molecular compound (Journa of Colloid and Interface Science (Journa)
l of Colloid and Interface Science), Vol. 79, 268
P. 1983), a film formed from a polymerizable low molecular compound and then polymerized (Shin Solid Films, 99th Edition)
Volume, p. 249, 1983) Example of forming a vinyl polymer film (Preprints of the Society of Polymer Science, Vol. 36, No. 10, p. 3218 and p. 3221, 1987)
Year) is disclosed. However, none of the LB films formed by the prior arts described above is still sufficient in terms of heat resistance and mechanical resistance.

The most effective means for solving this problem is to form a polyimide, which has the highest level of organic material in heat resistance and mechanical strength, by the LB method. However, those that can form a film by the LB method are limited to those having both a hydrophilic group and a hydrophobic group in the molecule. For this reason, it is impossible to form a film by the LB method with any of polyimide and polyamic acid as a precursor thereof. In order to obtain a polyimide LB film, the following method has been developed (Preprints of the Society of Polymer Science, Vol. 36, No. 10,
3215). In this method, a long chain alkylamine salt of polyamic acid is used as a polyimide precursor, developed on a water surface, and a formed monomolecular film is transferred onto a substrate.
Thereafter, a polyimide thin film is formed by imidization (Shin Solid Films, 160, 15
P. 1988, Vol. 160, p. 21, p. 1988). Of course,
In this film forming method, the uniformity of the monomolecular film on the water surface,
The accumulation of monomolecular films on solid substrates in the LB method and the LB method significantly affects the uniformity of the polyimide thin film as the final product. However, the viscosity of a film obtained by spreading the polyamic acid salt on the water surface is generally large, and it has been difficult to obtain a polyimide thin film having desired uniformity.

(Problems to be Solved by the Invention) The present invention has been made in view of the above-mentioned problems, and an object of the present invention is to provide a polymer thin film having characteristics satisfying practical requirements in various electronic devices. The method is to form a polyimide thin film having excellent uniformity by using the method.

[Configuration of the invention]

(Means for Solving the Problems) The production of a polyimide thin film in the present invention is performed by transferring a monomolecular film of a polyamic acid derivative formed on a water surface onto a substrate, and then imidizing the monomolecular film. Wherein the polyamic acid derivative has a steroid skeleton introduced into the molecule.

The inventor has succeeded in improving the uniformity of the LB film by introducing a steroid hydrophobic group instead of a long-chain alkyl group into the LB film having a dye skeleton in Japanese Patent Application No. 63-319582. . Therefore, the present invention was completed by conceiving the application of this method to a method for producing a polyimide thin film by the LB method.

Hereinafter, the method for producing a polyimide thin film of the present invention will be described in detail.

The present invention is characterized in that the polyamic acid derivative is used as a precursor of a polyimide thin film. Therefore, the polyamic acid derivative will be described first.

The first polyamic acid derivative of the present invention includes a salt of a polyamic acid having a structure represented by the general formula (I) and an amine having a steroid skeleton.

Here, Ar ¹ is a linking group serving as a skeleton of tetracarboxylic dianhydride, and Ar ² is a linking group serving as a skeleton of diamine, n
Represents a positive integer.

As the amine having a steroid skeleton, steroid amines shown in the following (SA-1) to (SA-13) can be used.

The second polyamic acid derivative of the present invention includes an ester of a polyamic acid having a structure represented by the general formula (I) and an alcohol having a steroid skeleton.

As the alcohol having a steroid skeleton, sterols and the like shown in the following (AL-1) to (AL-8) can be used.

In addition to the first and second derivatives, for example, a derivative in which a steroid group is introduced into the amide nitrogen of polyamic acid can also be used.

On the other hand, the polyamic acid contained in these polyamic acid derivatives can be polymerized by a usual polymerization method of tetracarboxylic dianhydride and diamine, that is, by heating and stirring both components in a solvent.

The tetracarboxylic dianhydride includes pyromellitic dianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride, 3,4,3 ', 4'-biphenyltetracarboxylic dianhydride Bis (3,4-dicarboxyphenyl) methane dianhydride, bis (3,4-dicarboxyphenyl) ether dianhydride, bis (3,4-dicarboxyphenyl) sulfone dianhydride, 2,2- Bis (3,4-dicarboxyphenyl) propane dianhydride, 2,2-bis (3,4-dicarboxyphenyl) -1,1,1,3,3,3-hexafluoropropane dianhydride, 3 , 4,3 ', 4'-benzophenonetetracarboxylic dianhydride, butanetetracarboxylic dianhydride, 1,2,5,6-naphthalenetetracarboxylic dianhydride, thiophene-2,3,
4,5-tetracarboxylic dianhydride, perylene-3,4,9,10
-Tetracarboxylic dianhydride, ethylenetetracarboxylic dianhydride and the like may be used alone or in combination of two or more.

The diamine includes metaphenylenediamine, paraphenylenediamine, paraxylylenediamine, di (4-
Aminocyclohexyl) methane, hexamethylenediamine, 2,2-dimethylpropylenediamine, 1,4-diaminocyclohexane, bis (p-aminophenyl) ether and the like can be used alone or in combination of two or more.

As the solvent used for the synthesis of the polyamic acid, a solvent that dissolves the generated polyamic acid is used.N, N-dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide, propylene carbonate, and the like have a high molecular weight. Is desirable in that it can produce a polymer of Polyamic acid produced by the above method,
The polymerization solution is recovered as a solid by dropping it into a poor solvent such as methanol.

Degree of polymerization n of polyamic acid (I) thus synthesized
Is preferably an integer of 10 to 500. When n is less than 10, the mechanical properties of the film and the adhesive force when the thin film is formed on the substrate are reduced, and when the formed thin film is used as a liquid crystal alignment film, elution into the liquid crystal may occur. Also,
If n is greater than 500, the solubility of the polyamic acid itself will be reduced, which will hinder the formation of the polyamic acid derivative.

The polyamic acid derivative of the present invention can be obtained by reacting the above-mentioned amine compound or alcohol compound having a steroid skeleton with the polyamic acid synthesized by the above method in a solvent. At this time, it is preferable to adjust the content of the steroid skeleton to 0.5 to 4 equivalents to one repeating unit of the polyamic acid.
As the solvent, N, N-dimethylacetamide, N
-Methyl-2-pyrrolidone, dimethyl sulfoxide, tetramethylene sulfone, cyclohexanone, cresol, phenol and the like can be used. Further, benzene, toluene, chloroform and the like may be added for the purpose of improving the solubility of each component. The introduction of the steroid skeleton into the polyamic acid can be all performed by a well-known reaction. For example, steroid esters of polyamic acids are:
The polyamic acid can be easily synthesized by treating it with phosphorus pentachloride or the like to form a carboxylic acid chloride, and then reacting it with an alcohol having a steroid skeleton such as cholesterol or sorestanol.

Thus, the polyamic acid derivative of the present invention is produced. As a specific structure, the first polyamic acid derivative is introduced into the following structural formula (II), the second polyamic acid derivative is introduced into the following structural formula (III), and a steroid group is introduced into the amide nitrogen of the polyamic acid. The derivative thus obtained is exemplified in the following structural formula (IV).

However, pyromellitic dianhydride as tetracarboxylic dianhydride, p-phenylenediamine as diamine, and the above-mentioned (SA-
1) was used respectively. n has the same meaning as in the above formula (I).

However, bis (3,4-
Bis (p-aminophenyl) ether was used as diamine, dicarboxyphenyl) methane dianhydride, and (AL-4) was used as alcohol having a steroid skeleton. n has the same meaning as in the above formula (I).

However, 2,2-bis (3,4-dicarboxyphenyl) -1,1,1,3,3,3-hexafluoropropane dianhydride is used as tetracarboxylic dianhydride and p-phenylenediamine is used as diamine. (AL-1) was used as an alcohol having a steroid skeleton. n has the same meaning as in the above formula (I).

Next, a specific method for forming the polyimide thin film of the present invention using the above polyamic acid derivative will be described.

The formation of the polyimide thin film of the present invention is performed according to a normal LB method, particularly a method called a vertical immersion method. That is, first, the polyamic acid derivative is dropped on a water surface to form a monomolecular film of the derivative. Next, while compressing the monomolecular film to a predetermined surface pressure and controlling the film area to keep the surface pressure constant, the substrate previously subjected to a hydrophobic treatment or the like is vertically moved up and down with respect to the film surface. Let it. Each time the substrate is raised and lowered, a monomolecular film is accumulated on the substrate one by one. Then, the substrate is moved up and down a plurality of times, whereby a layered cumulative film of the polyamic acid derivative is formed on the substrate.

Further, the polyamic acid derivative is imidized on the formed layered cumulative film. For the imidization, a treatment with a solution prepared for the imidization is employed in the same manner as in the method using a long-chain alkylamine described in the section of the prior art. By this treatment, the steroid skeleton portion of the polyamic acid derivative is removed, and the amide acid is closed. Thus, a polyimide thin film having a structure represented by the following general formula (V) can be formed.

However, Ar ¹ , Ar ² , and n have the same meanings as in the general formula (I).

(Function) In the method for producing a polyimide thin film of the present invention, a steroid skeleton is introduced into a polyamic acid derivative which is a polyimide precursor in forming a thin film by the LB method. This steroid skeleton has a lower molecular association force than the long-chain alkylamine conventionally introduced, and suppresses the viscosity of the polyamic acid derivative monomolecular film formed on the water surface by the LB method. Therefore, nonuniformity of the film formed by the compression operation of the monomolecular film in the LB method can be prevented, and a film having excellent uniformity can be formed. Furthermore,
The steroid skeleton is three-dimensionally larger than long-chain alkylamines, and sufficiently functions as a hydrophobic group of the LB film.

Further, this steroid skeleton component can be very easily separated from the polyamic acid derivative by the above-mentioned imidization. Accompanying this, imidization by cyclization of the polyamic acid further proceeds. In addition, since the imidization is performed by the solution treatment, the eliminated steroid skeleton component flows out into the treatment solution. Therefore, the steroid skeleton component does not remain in the formed polyimide film, and does not adversely affect the characteristics of the polyimide film. Thus, formation of a thin film having sufficient performance as polyimide, such as heat resistance, electrical insulation and mechanical properties, is achieved.

(Examples) Hereinafter, examples of the method for producing a polyimide thin film according to the present invention will be described in detail.

Example 1 A 3-inch p-type semiconducting silicon wafer was washed with hydrofluoric acid, followed by water, and then dried at 150 ° C. Next, the surface of the wafer was subjected to a hydrophobic treatment in a vapor of hexamethyldisilazane.

On the other hand, a polyamic acid having a molecular weight of 200,000 synthesized from 3,4,3 ', 4'-biphenyltetracarboxylic dianhydride and paraphenylenediamine, and two equivalents of N, N-dimethyl per repeating unit of the polyamic acid. -Cholesta-3-
Ilamine was dissolved in N, N-dimethylformamide to prepare a 0.5 mg / ml developing solution. This developing solution was dropped on the water surface of an LB trif at a water temperature of 18 ° C. to form a monomolecular film of a polyamic acid steroid amine salt. After compressing the monomolecular film to a surface pressure of 20 dyn / cm, the silicon wafer as a substrate is immersed perpendicularly to the water surface while controlling the film area so as to maintain the surface pressure, and the monomolecular film is formed into a wafer. More accumulated on top. Next, the wafer was pulled above the water surface and accumulated even more. This operation was repeated to accumulate a total of 10 monolayers on the wafer surface.

Next, the wafer on which the above-mentioned cumulative film was formed was placed in a 1: 1: 3 mixed solution of acetic anhydride, pyridine and benzene at room temperature for 6 hours.
After standing for a time, the accumulated film was cured. When the obtained thin film was analyzed by ellipsometry and infrared absorption spectrum, it was found that a polyimide having a thickness of about 50 ° was formed. When the formed thin film was observed with a differential interference optical microscope and a high-resolution scanning electron microscope, no defect was confirmed, and it was found that the thin film was formed uniformly.

Example 2 A method similar to that of Example 1 except that an ITO (Indium Tin Oxide) glass substrate was used as the substrate and that the first layer of the cumulative film was formed by pulling the substrate above the water surface. According to the above, a total of 23 monomolecular films were accumulated, and a polyimide thin film was formed in the same manner. After drying this thin film in a desiccator containing calcium chloride for 2 days, 10 dot-like gold electrodes were vacuum-deposited on the surface.
This film has a resistivity of 10 ¹⁶ Ω at room temperature on any electrode.
cm and a breakdown voltage of 10 ⁶ V / cm. This value is 1
There was almost no change up to 80 ° C.

Example 3 After reacting pyromellitic anhydride and cholestanol, the product was reacted with phosphorus oxychloride at a low temperature to obtain an acid chloride. Next, this acid chloride was reacted with paraphenylenediamine to synthesize a cholestanol ester of a polyamic acid represented by the following general formula (VI).

n has the same meaning as in the above formula (I). The above ester was dissolved in N, N-dimethylformamide to prepare a 0.3 mg / ml developing solution. This developing solution was dropped onto the water surface of an LB trif at a water temperature of 18 ° C. to form a monomolecular film of a polyamic acid ester. Next, after compressing the monomolecular film to a surface pressure of 20 dyn / cm, the silicon wafer used in Example 1 as a substrate was immersed perpendicularly to the water surface while controlling the film area to maintain the surface pressure. The monomolecular film was further accumulated on the wafer. Next, the wafer was pulled above the water surface and accumulated even more. Repeat this operation,
A total of 10 monolayers were accumulated on the wafer surface.

Next, the wafer on which the above-mentioned cumulative film was formed was placed in a 1: 1: 3 mixed solution of acetic anhydride, pyridine and benzene at room temperature for 6 hours.
After standing for a time, the accumulated film was cured. When the obtained thin film was analyzed by ellipsometry and infrared absorption spectrum, it was found that a polyimide having a thickness of about 50 ° was formed. When the formed thin film was observed with a differential interference optical microscope and a high-resolution scanning electron microscope, no defect was confirmed, and it was found that the thin film was formed uniformly.

Example 4 Except that an ITO glass substrate was used as the substrate and that the first layer of the cumulative film was formed by pulling the substrate above the water surface, a total of 23 layers were formed in the same manner as in Example 3.
The monomolecular films of the layers were accumulated, and a polyimide thin film was formed in the same manner. After drying this thin film in a desiccator containing calcium chloride for 2 days, 10 dot-like gold electrodes were vacuum-deposited on the surface. This thin film exhibited a resistivity of 10 ¹⁶ Ωcm and a breakdown voltage of 10 ⁶ V / cm at room temperature for all electrodes. This value hardly changed up to 180 ° C.

Comparative Example 1 Instead of N, N-dimethyl-cholest-3-ylamine
Except that N, N-dimethyl-n-hexadecylamine is used, a total of 11 monolayers are accumulated on a silicon wafer in the same manner as in Example 1, and a polyimide thin film is formed by the same processing. did. The formed thin film was confirmed to have a uniform structure by observation with a differential interference optical microscope, but was observed to have hole-like defects by observation with a high-resolution scanning electron microscope, indicating that the thin film had an uneven structure. all right.

Comparative Example 2 Instead of N, N-dimethyl-cholest-3-ylamine
Except that N, N-dimethyl-n-hexadecylamine was used, a total of 23 monolayers were accumulated on an ITO glass substrate in the same manner as in Example 2, and a polyimide thin film was formed by the same treatment. Formed. After drying this thin film in a desiccator containing calcium chloride for 2 days, 10 dot-like gold electrodes were vacuum-deposited on the surface, and 5 out of 10 electrodes were short-circuited.

Comparative Example 3 Example 2 except that n-hexadecanol was used instead of N, N-dimethyl-cholest-3-ylamine.
A total of 23 monomolecular films were accumulated on the ITO glass substrate in the same manner as in the above, and a polyimide thin film was formed in the same manner. After drying this thin film in a desiccator containing calcium chloride for 2 days, a dot-shaped gold electrode 10 was formed on the surface.
As a result of vacuum evaporation, four out of ten electrodes were short-circuited.

〔The invention's effect〕

As described in detail above, the method for producing a polyimide thin film of the present invention is excellent in uniformity, and has a remarkable effect on extremely easily forming a polyimide thin film having good performance with respect to electrical insulation. .

Claims (1)

(57) [Claims] 1. A method for producing a polyimide thin film by transferring a monomolecular film of a polyamic acid derivative formed on a water surface onto a substrate and then imidating the monomolecular film, wherein the polyamic acid derivative comprises A method comprising using a substance containing a steroid skeleton therein.