JPH0495962A - Photosensitive resin composition - Google Patents

Abstract

PURPOSE:To enhance sensitivity, film strength, and adhesion by introducing polyfunctional groups into partial carboxylic groups of polyamidoacid and dissolving this polyamidoacid ester and a sensitizer into a liquid amido compound. CONSTITUTION:The photosensitive resin composition comprises the polyamidoacid ester represented by formula I, the amide compound liquid at normal temperature, polymerizable by chemical rays, and having a C=C structure, the sensitizer having absorption maximum wavelength lambda max 330 - 500 nm as essential components. In formula I, R1 is a tri- or tetra-valent organic group; R2 is a divalent organic group; R3 is a group of formula II; R4 is a tri- - hxa-valent organic group; R5 is H or methyl; each of (l), (m), and (n) is 0 or 1; (p) is an integer of 2 - 5; and each of (x), (y), and (z) is a percentage of each structural unit, thus permitting adhesion to a substrate at the time of development and the film strength after hardening to be both enhanced.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a photosensitive polyimide resin composition that has extremely high sensitivity and strong film properties, and also has high adhesion to a substrate. It is.

[Conventional technology] Conventionally, surface protection films and interlayer insulating films of semiconductor elements, etc.
Polyimide is used because it has excellent heat resistance, as well as excellent electrical insulation and mechanical strength, but recently technology has been developed to impart photosensitivity to polyimide itself in order to simplify the complicated process of creating polyimide patterns. It is attracting attention.

For example, a polyimide precursor composition to which a photosensitive group is added with an ester group having a structure as shown in the following formula (for example, Japanese Patent Publication No. 55-414
22 Publication), etc. are known.

All of these are dissolved in a suitable organic solvent, applied as a varnish, dried, exposed to ultraviolet light through a photomask, developed and rinsed to obtain the desired pattern, and then heat-treated to form a polyimide. It is a coating.

Using photosensitive polyimide not only simplifies the pattern creation process, but also eliminates the need to use highly toxic etching solutions, making it safer and less polluting. It is expected that this technology will become even more important in the future.

However, when such conventional photosensitization techniques are applied, polyimide precursor compositions with photosensitive groups provided with ester groups have low sensitivity and, moreover, the photosensitive groups are difficult to scatter during heat curing, resulting in poor film quality after curing. Only low strength products can be obtained. Furthermore, peeling occurs from the substrate during development, etc.
It was difficult to use in the semiconductor industry.

In addition, in order to increase the sensitivity and the adhesion with the substrate, (R: -0-CH2C(CH20CH2CH=CH2
)3) It is also known that a photosensitive resin composition with high sensitivity and high adhesion can be obtained by introducing a polyfunctional photosensitive group and further adding an adhesion aid such as vinyl silane (K,
L, Mittal, Polyimides-5ynt
hesis, Characterization an
d Applications Vol, 2. p919
(1984)). However, this technique had many problems, such as a large amount of photosensitive group components, the strength of the cured film was extremely low, and although adhesion strength improved after curing, no improvement was seen during development. .

[Problems to be Solved by the Invention] An object of the present invention is to introduce a polyfunctional acrylic (methacrylic) group into some carboxyl groups in a polyimide acid, and further to introduce this polyamic acid ester and a sensitizer. A photosensitive resin composition with extremely high sensitivity and excellent film strength after curing, as well as improved adhesion during development, etc. by dissolving it in an amide compound having a photoreactable carbon-carbon double bond. It's about providing things.

[Means for Solving the Problems] The present invention provides (1) (A) a polyamic acid ester represented by the following formula CI), (wherein R□: a trivalent or tetravalent organic group, R2: a divalent organic group); Each of the organic groups R+R2!R3HR41R5 is independent and may be the same or different.

1, m, n: 0 or 1, p: an integer from 2 to 5, 1<
x,y<100. o<z <90 and x y+z=i
o.

x, y, z? (Percentage of each structural unit) (B) Carbon-carbon polymerizable by actinic radiation in solution at room temperature
A photosensitive resin composition comprising as essential components an amide compound containing a carbon double bond and (C) a sensitizer having a maximum absorption wavelength (^wax) of 330 to 500n111.

(2) A photosensitive resin composition containing as essential components 50 to 1000 parts by weight of an amide compound (B) and 10 to 10 parts by weight of a sensitizer (c)o, i based on 100 parts by weight of the polyamic acid ester (A). It is.

[Function] The polyamic acid ester represented by the general formula (I) used in the present invention has high sensitivity, and due to the remaining carboxyl component, improves adhesion to the substrate during development and improves film strength after curing. It has excellent performance. In the general formula CI), R1 is a compound having a trivalent or tetravalent organic group, and aromatic tetracarboxylic acids or derivatives thereof and aromatic tricarboxylic acids or derivatives thereof are usually mainly used. For example, trimellitic anhydride, pyromellitic dianhydride, benzene-1,2,3
゜4-Tetracarboxylic dianhydride, 3.3', 4.4
'-benzophenone tetracarboxylic dianhydride, 2.2
',3,3'-benzophenonetetracarboxylic dianhydride, 2,3°3',4'-benzophenonetetracarboxylic dianhydride, naphthalene-2,3,6,7-tetracarboxylic dianhydride, Naphthalene-1,2,5,6-tetracarboxylic dianhydride, Naphthalene-1,2,4,5
-Tetracarboxylic dianhydride, naphthalene-1,4,5
, 8-tetracarboxylic dianhydride, naphthalene-1,2
, 6,7-tetracarboxylic dianhydride, 4.8-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-1.2,5.6-tetracarboxylic dianhydride, 4 ,8
-dimethyl-1,2,3,5,6,7-hexahydronaphthalene-2.3,6.7-tetracarboxylic dianhydride, 2,6-cyclonaphthalene-1.4,5.8- Tetracarboxylic dianhydride, 2,7-cyclonaphthalene-1.4,5.8-tetracarboxylic dianhydride, 2,3,
6.7-titrachloronaphthalene-1.4,5.8-tetracarboxylic dianhydride, 1,4.5゜8-tetrachloronaphthalene-2,3,6,7-tetracarboxylic dianhydride, 3.3', 4.4'-diphenyltetracarboxylic dianhydride, 2.2', 3.3'-diphenyltetracarboxylic dianhydride, 2,3.3', 4'-diphenyltetracarboxylic acid dianhydride, 3.3″, 4.4″
-p-terphenyltetracarboxylic dianhydride, 2.2
″, 3.3″-p-terphenyltetracarboxylic dianhydride, 2,3,3″, 4″-p-terphenyltetracarboxylic dianhydride, 2.2-bis(2,3- dicarboxyphenyl)-propane dianhydride, 2,2-bis(3,4-dicarboxyphenyl)-propane dianhydride, bis(2,3-dicarboxyphenyl)ether dianhydride, bis(3,4- dicarboxyphenyl)ether dianhydride, bis(2,3-dicarboxyphenyl)methanidianhydride, bis(3,4-dicarboxyphenyl)methanidianhydride, bis(2,3-dicarboxyphenyl)
Sulfone dianhydride, bis(3,4-dicarboxyphenyl)sulfone dianhydride, 1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride, 1,1-bis(3,
4-dicarboxyphenyl)ethane dianhydride, perylene-2,3,8,9-tetracarboxylic dianhydride, perylene-3,4,9,10-tetracarboxylic dianhydride, perylene-4,5, 10,11-tetracarboxylic dianhydride, perylene-5,6,11,12-tetracarboxylic dianhydride, phenanthrene-1,2,7,8-tetracarboxylic dianhydride, phenanthrene-1,2 ,6,7-
Tetracarboxylic dianhydride, phenanthrene-1,2,
9,10-tetracarboxylic dianhydride, cyclopentane-1゜2.3.4-tetracarboxylic dianhydride, pyrazine-2,3゜5.6-tetracarboxylic dianhydride, pyrrolidine-2, 3゜4.5-tetracarboxylic dianhydride,
Examples include, but are not limited to, thiophene-2,3°4,5-tetracarboxylic dianhydride. Further, in use, one type or a mixture of two or more types may be used.

R2 used in the present invention is a divalent organic group, and usually aromatic diamine and/or derivatives thereof are used. For example, m-phenylene-diamine, 1-isopropyl-2,
4-phenylene-diamine, p-phenylene-diamine, 4,4'-diamino-diphenylpropane, 3,3'
-Diamino-diphenylpropane, 4,4'-diamino-diphenylethane, 3,3'-diamino-diphenylethane, 4,4'-diamino-diphenylmethane, total;
3'-diamino-diphenylmethane, 4,4'-diamino-diphenyl sulfide, 3,3'-diamino-diphenyl sulfide, 4,4'-diamino-diphenyl sulfone, 3,3'-diamino-diphenyl sulfone, 4
．． 4'-diamino-diphenyl ether, 3,3'-diamino-diphenyl ether, benzidine, 3,3'-
Diamino-biphenyl, 3,3'-dimethyl-4,4'
-Diamino-biphenyl, 3.3'-dimethoxy-benzidine, 4゜4''-diamino-p-terphenyl, 3.
3”-diamino-p-terphenyl, bis(p-amino-cyclohexyl)methane, bis(p-β-amino-t
-butylphenyl)ether, bis(p-β-methyl-
δ-aminopentyl)benzene, p-bis(2-methyl-4-amino-pentyl)benzene, p-bis(1,1
-dimethyl-5-amino-pentyl)benzene, 1,5
-diamino-naphthalene, 2,6-diamino-naphthalene, 2,4-bis(β-amino-t-butyl)toluene, 2,4-diamino-toluene, m-xylene-2,5
-diamine, p-xylylene-2,5-diamine, I-xylylene-diamine, p-xylylene-diamine, 2.
6-diamitupyridine, 2,5-diamino-pyridine, 2,5-diamino-1,3,4-oxadiazole,
1,4-diamino-cyclohexane, piperazine, methylene-diamine, ethylene-diamine, propylene-diamine, 2,2-dimethyl-propylene-diamine, tetramethylene-diamine, pentamethylene-diamine,
Hexamethylene-diamine, 2,5-dimethyl-hexamethylene-diamine, 3-methoxy-hexamethylene-diamine
Diamine, heptamethylene-diamine, 2,5-dimethyl-heptamethylene-diamine, 3-methyl-heptamethylene-diamine, 4,4-dimethyl-heptamethylene-diamine, octamethylene-diamine, nonamethylene-diamine, 5-methyl -nonamethylene-diamine, 2
, 5-dimethyl-nonamethylene-diamine, decamethylene-diamine, 1,10-diamino-1,10-dimethyl-decane, 2.11-diamino-dodecane, 1.12
-Diamino-octadecane, 2,12-diamino-octadecane, 2,17-diaminosiloxane, diaminosiloxane, 2,6-diamino-4-carboxylic benzene, 3,3'-diamino-4,4'-dicarboxylic benzidine Examples include, but are not limited to, these. Further, in use, one type or a mixture of two or more types may be used.

In the general formula CI used in the present invention, R3 is acrylic (
It is a polyfunctional photosensitive group having 2 to 5 (methacrylic) groups. If the number of acrylic (methacrylic) groups is one, the crosslinking density is low and a test with low sensitivity cannot be obtained, which is not preferable. Further, six or more acrylic (methacrylic) groups are not only difficult to produce industrially, but also have a large molecular weight, which reduces compatibility, which is not preferred. Examples of compounds for introducing R3 include pentaerythritol triacrylate, pentaerythritol trimethacrylate, pentaerythritol acrylate dimethacrylate, pentaerythritol diacrylate methacrylate, dipentaerythritol pentaacrylate, dipentaerythritol pentamethacrylate, and glycerol dimethacrylate. Acrylate, glycerol dimethacrylate, glycerol acrylate methacrylate, trimethylolpropane diacrylate, 1,3-diacryloylethyl-5-hydroxyethyl isocyanurate, 1,3-dimethacrylate-5-hydroxyethyl isocyanurate, ethylene glycol-modified pentaerythritol Examples include, but are not limited to, triacrylate, propylene glycol-modified pentaerythritol triacrylate, trimethylolpropane diacrylate, and trimethylolpropane dimethacrylate. When using these, one type or a mixture of two or more types may be used.

In the polyamic acid ester represented by the general formula CI) used in the present invention, the proportion of structural units in which a photosensitive group is introduced into the carboxyl group is X, the proportion of structural units in which a photosensitive group is partially introduced is y, and the proportion of structural units in which a photosensitive group is introduced into the carboxyl group is the group is not substituted with a photosensitive group,
The proportion of different structural units is Z, and three types of structural units are mixed. 1<x, y<100, O
<z<90 and satisfies X+y+Z:100,
x, y, and z indicate the percentage of each structural unit. X
When y is less than 1, or Z is more than 90, the amount of photosensitive groups is small, resulting in low sensitivity and little practicality. If X is 100,
Since it does not have a carboxyl group, it easily peels off from the substrate during development, making it less practical and undesirable.

Examples of amide compounds containing a carbon-carbon double bond that can be polymerized by actinic radiation in solution at room temperature and used in the present invention include N-methylacrylamide, N-ethylacrylamide, N-methylmethacrylamide, and N-ethylmethacrylamide. , N,N-dimethylacrylamide, N,N-
Diethylacrylamide, N, N-dibutylacrylamide, N-acryloylpiperidine, N-acryloylmorpholine, N.

Amides in solution at 10-30°C, such as N-dimethylmethacrylamide, N,N-diethylmethacrylamide, N,N-dimethylaminoethylmethacrylamide, N,N-dimethylaminopropylmethacrylamide, and N-vinylpyrrolidone. Examples include, but are not limited to, compounds. Also, when using one type,
A mixture of two or more types may be used.

These amide compounds containing a carbon-carbon double bond that can be polymerized by actinic radiation are preferably used in an amount of 50 to 1000 parts by weight dissolved in 100 parts by weight of the polyamic acid ester. If it is less than 50 parts by weight, it will not be completely dissolved and it will be difficult to use, and if it is more than 1000 parts by weight, the solution viscosity of the varnish will decrease and it will be difficult to obtain a uniform coating film, making it difficult to use.

Also, as a solvent component, conventional reaction solvents such as N, N
-dimethylformamide, N,N-dimethylacetamide, N,N-diethylformamide, N,N-diethylacetamide, N,N-dimethylmethoxyacetamide, dimethylsulfoxide, hexamethylphosphoamide, N-methyl-2-pyrrolidone, Pyridine, dimethylsulfone, tetramethylenesulfone, dimethyltetramethylenesulfone, methylformamide, N-acetyl-2
- Pyrrolidone, diethylene glycol dimethyl ether, and diethylene glycol dimethyl ether may be used alone or in combination with the amide compound of component (B).

Other solvents that can be used in combination include:
Non-solvents such as benzene, benzonitrile, dioxane, butyrolactone, methyl cellosolve, ethyl cellosolve, butyl cellosolve, xylene, toluene, and cyclohexane are used as dispersion media for raw materials, reaction regulators, volatilization regulators for solvents from products, and film smoothing agents. It may also be used in combination as an agent. Conventional reaction solvents were not sensitive to light. Therefore, in the production of conventional photosensitive resins, a photosensitive component to light has been particularly added. However, since it is diluted with a large amount of reaction solvent, there is a limit to the improvement of the photosensitive group concentration.

According to the method of the present invention using the amide compound as a solvent, since the solvent itself is 100% photosensitive, the concentration of photosensitive groups is significantly high, and therefore, the photosensitivity can also be significantly increased. In addition, since the method for producing photosensitive resin according to the present invention only involves adding and mixing each component,
It was extremely simple, and the variation in quality was significantly reduced.

The sensitizer used in the present invention is a compound having a maximum absorption wavelength (λWaX) in the range of 330 to 500 nm. If λwax is 330 nm or less, light is absorbed by the polyamic acid itself and no photoreaction is possible, which is not preferable. On the other hand, if it is 500 nm or more, it is not preferable because it will undergo a photoreaction with visible light, making it necessary to set up a work place in a shielded room, and the ease of handling will deteriorate. Examples of the sensitizer of the present invention include, but are not limited to, the following. Further, in use, one type or a mixture of two or more types may be used.

Further, the amount of the photosensitizer added is preferably 0.1 to 10 parts by weight per 100 parts by weight of the polyamic acid ester (A).

If it is less than 0.1 part by weight, the amount added is too small and it is difficult to obtain the effect of improving sensitivity. If the amount is more than 10 parts by weight, the sensitizer in the system will undesirably lower the strength of the cured film.

The photosensitive resin composition according to the present invention may contain adhesion aids, inhibitors, leveling agents, and various other fillers.

The polyamic acid ester (A) in the present invention is usually synthesized as follows. First, a compound having an alcohol group for introducing the polyfunctional photosensitive group R3 is dissolved in a solvent, and an excess acid anhydride or a derivative thereof is reacted with the compound. Thereafter, the remaining carboxyl group and acid anhydride group can be synthesized by reacting with diamine. That is, by controlling the reaction molar ratio of the R3 component, the acid anhydride component, and the diamine component, it is possible to easily control the photosensitive group substitution rate of the carboxyl group.

The method of using the photosensitive resin composition according to the present invention is to first apply the composition to a suitable support such as a silicon wafer, ceramic, or aluminum substrate. Coating methods include spin coating using a spinner, spray M coating using a spray coater, dipping, printing, and roll coating. Next, after drying the coating film by prebaking at a low temperature of 60 to 80°C, the desired pattern shape is irradiated with actinic radiation. As the actinic radiation, X-rays, electron beams, ultraviolet rays, visible light, etc. can be used, but those having a wavelength of 200 to 500 nm are preferable.

Next, a relief pattern is obtained by dissolving and removing the unirradiated areas with a developer. As a developer, N-methyl-2
-pyrrolidone, N,N-dimethylacetamide, N,N
- Dimethylformamide, methanol, isopropyl alcohol, water, etc. are used alone or in combination. As the developing method, methods such as spray, paddle, immersion, and ultrasonic waves can be used.

Next, the relief pattern formed by development is rinsed. As the rinsing liquid, methanol, ethanol, isopropyl alcohol, butyl acetate, etc. are used. Next, heat treatment is performed to form imide rings and obtain a final pattern with high heat resistance.

The photosensitive resin composition according to the present invention is useful not only for semiconductor applications but also as an interlayer insulating film for multilayer circuits, a cover coat for flexible copper clad boards, a solder resist film, a liquid crystal alignment film, and the like.

The present invention will be specifically explained below using Examples.

(Example 1) Pyromellitic dianhydride 65.5g (0.30 mol)
and 225.5 g (0.70 mol) of 3°3',4,4'-benzophenonetetracarboxylic dianhydride,
-Hydroxy-1,3-dimethacryloxypropane 11
After esterifying a part of the carboxyl group with 4 g (0,50 mol), 4,4'-diaminodiphenyl ether 1
A polyamic acid ester copolymer was obtained by using 70.2 g (0.85 mol) of dicyclohexylcarbodiimide as a condensing agent. After removing dicyclohexylurea by filtration, it was reprecipitated in ethanol, and the solid matter was filtered and dried under reduced pressure. next,
A photosensitive resin composition was obtained by dissolving 5 parts by weight of Michler's ketone (λmax 365 nm) and 0.1 part by weight of methyl ether hydroquinone in 300 parts by weight of N,N-dimethylacrylamide in 100 parts by weight of this boamimic acid ester.

The obtained solution was applied onto an aluminum plate using a spinner and dried at 60°C for 1 hour using a drier.

This film is coated with Kodak's Photographic Step Tablet No. 2.21 Step (in this gray scale, as the number of steps increases, the amount of transmitted light is 1/1 of that of the previous step).
f2, so the larger the residual stage after development, the better the sensitivity) was stacked, irradiated with ultraviolet rays at 500@J/cm2, and then a developer containing 60% by weight of N-methylpyrrolidone and 40% by weight of xylene was used. When developed by spraying and further rinsed with isopropyl alcohol, the pattern remained up to 13 steps, indicating high sensitivity.

Furthermore, no phenomena such as peeling were observed during development, indicating high adhesion.

Furthermore, after coating on an aluminum plate, exposing the entire surface, developing, rinsing, and heat curing, the aluminum plate was removed by etching.
Got the film. Tensile strength of the obtained film (JIS
K-6760) was 12.0 kg/mm2.

In this way, very excellent effects such as high sensitivity, high adhesion, and high strength were obtained.

(Comparative Example 1) In Example 1, the reaction amount of 2-hydroxy-1,3-dimethacryloxypropane was changed to 547.2 g (2.4 mol), and then the same operation was performed to prepare a resin composition. The same evaluation was conducted. Although the sensitivity was as high as 17 steps, it was observed that the pattern was peeled off at some corners during development. Also, the tensile strength is 5kg/a
It was found that the practicality was low at m2.

(Comparative Example 2) In Example 1, 2-hydroxy-1,3-dimethacryloxypropane was replaced with 2-hydroxyethyl methacrylate and reacted, and then the same operation was performed to obtain a resin composition. We conducted an evaluation. It was found that the sensitivity was as low as 6 steps, making it less practical.

(Comparative Example 3) In Example 1, N,N-dimethylacrylamide was replaced with N,
A resin composition was obtained in place of N-dimethylacetamide, and the same evaluation was performed. It was found that the sensitivity was as low as 4 steps, making it impractical.

(Comparative Example 4) In Example 1, evaluation was performed using 30 parts by weight of N,N-dimethylacrylamide and 270 parts by weight of N,N-dimethylacetamide. It was found that the sensitivity was as low as 5 steps, making it impractical.

(Comparative Example 5) In Example 1, the amount of N,N-dimethylacrylamide added was 1200 parts by weight. A similar evaluation was attempted, but it was found that the coating could not be applied uniformly to the wafer and was not practical.

(Comparative Example 6) In Example 1, Michler's ketone was replaced with 3,3-dimethyl-4-
It was used instead of methoxybenzophenone. Similar evaluation was attempted, but since the max of this sensitizer was 296 nm, the photoinitiation reaction could not be carried out efficiently, and the entire pattern was washed away during development, making it impractical.

(Comparative Example 7) In Example 1, Michler's ketone was replaced with a tetraphenylporphyrin zinc complex. Similar evaluation was attempted, but since the λmax of this sensitizer was 650 nm, a photoreaction occurred during the process, and a pattern could not be obtained by development.

(Comparative Example 8) In Example 1, the amount of Michler's ketone added was 20 parts by weight. A similar evaluation was attempted, but crystals precipitated after prebaking, making it impractical.

(Comparative Example 9) In Example 1, the amount of Michler's ketone added was changed to 0.05 parts by weight, and the same evaluation was performed, but the sensitivity was as low as 1 step tablet, and it was found that it was not practical. .

[Effects of the Invention] Conventionally, a technique has been known in which a photosensitive group is introduced into the carboxyl group of polyamic acid in the form of an ester, but it has been found that this technique has low sensitivity and causes peeling during development.

It was also found that the mechanical strength was low. However, in the present invention, by introducing a polyfunctional photosensitive group into a part of the polyamic acid, the number of crosslinking points is not reduced and the carboxyl group is left.

Furthermore, by blending a sensitizer with λmaX in the range of 330 to 500 nm and dissolving it in an amide compound containing a carbon-carbon double bond that can be polymerized by actinic radiation in a solvent component, it is possible to develop with extremely high sensitivity. It was also found that the adhesion during bonding was significantly improved by the hydrogen bonding of the carboxyl groups. In addition, since the substitution rate of the photosensitive group was reduced and I was able to easily obtain a high molecular weight product, the extremely excellent effect of increasing the strength of the cured film was also obtained. In addition, in the present invention, essential components (A) (B)
There was no problem when additives other than (C) were added to impart various properties.

Claims (2)

[Claims] (1) (A) Polyamic acid ester represented by the following formula [I], ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (In the formula, R_1: trivalent or tetravalent organic group, R_2: divalent organic group, R_3: ▲ There are mathematical formulas, chemical formulas, tables, etc. ▼ R_4: Trivalent to hexavalent organic group, R_5: R_1, R_2, R_3, R_4, and R_5 of each H or CH_3 group are independent and are not the same. l, m, n: 0 or 1, p: an integer from 2 to 5, 1<x, y
<100, 0<z<90 and x+y+z=100x,y
, z: percentage of each structural unit) (B) Carbon-carbon which can be polymerized by actinic radiation in solution at room temperature.
An amide compound containing a carbon double bond, (C) Maximum absorption wavelength (λmax) of 330 to 500 nm
A photosensitive resin composition containing a sensitizer as an essential component. (2) A photosensitive resin composition containing as essential components 50 to 1000 parts by weight of an amide compound (B) and 0.1 to 10 parts by weight of a sensitizer (C) based on 100 parts by weight of the polyamic acid ester (A). .