PROCESSES FOR PRODUCING MULTILAYER FLUORINATED POLYIMIDE FILM

TECHNICAL FIELD

The present invention relates to processes for producing a multilayer fluorinated polyimide film.

BACKGROUND ART As a basic industry in the information society, the electronics industry is now prospering immensely. Particularly, the electronics age persisting today would not have been created without polymeric materials for insulation and for micro processing. Polymeric materials which, by virtue of their main characteristic property of defying the flow of electricity, have supported the advance of industries and technologies of the electronics age can exhibit such various functions as ferroelectricity , high conductivity for electrons and ions, superconductivity, as well as ferromagnetism, which have heretofore been characteristic of metals, semiconductors, and further inorganic materials, if they are properly conditioned for their molecular and cluster structures. In recent years, polymeric materials have been

used in an abundant variety of applications such as interlayer insulating films and passivation materials for transistors, thyristors, and ICs, junction coating materials represented by silicone resins, chip coat-grade buffer materials for relaxing mold stress, α-ray shielding materials for overcoming soft errors in memory devices, die bonding materials, resist materials, semiconductor sealing materials, moisture-proof coating materials for hybrid ICs, chip carrier tapes for TAB (Tape Automated Bonding), and multilayer circuit boards. One of such polymeric materials is polyimide for electronic materials, which has been used for multilayer printed boards, oriented films for liquid crystals, α-ray protecting coats for LSI' s, passivation films, and the like.

To form a polyimide film on an object, generally used is a method of applying a precursor polyamide acid by a spin coating method, a casting method, or the like, followed by baking. In the case of forming a fine pattern of a polyimide film, however, the use of an ordinary polyimide requires complicated processes such as application of a resist onto a polyimide film, peeling of the resist, and etching of the polyimide film. Therefore, the use of photosensitive polyimide has recently been started.

The photosensitive polyimide, however, has a problem that cracks are easily formed in its film after development. Thus, to prevent problems such as generation of cracks in a photosensitive polyimide film after development, Japanese Patent Lai-open

Publication No. 8-8170, for example, discloses a technique for forming a polyimide precursor film by controlling the humidity in a film forming region during the formation of a coated film using a photosensitive polyimide precursor.

Further, as other polymeric materials, fluorine-containing polyimides are attracting much attention in terms of their excellent functions, heat resistance, and the like. For example, wholly fluorinated polyimide formed of repeating units containing only carbon-fluorine bonds (C-F bonds) instead of carbon-hydrogen bonds (C-H bonds) are known to be useful as an optical material (e.g., see Japanese Patent Laid-open Publications No. 5-1148 and No. 2004-269591) . The wholly fluorinated polyimide has heat resistance enough for allowing the production of photoelectronic integrated circuits and has an extremely small optical loss in the near-infrared region, particularly in the optical communication wavelength region (i.e., 1.0 to 1.7 μm) .

Optoelectronic integrated circuits are required to have an optical waveguide structure. To produce an optical waveguide structure by the use of fluorine-containing polyimide, a process is employed in which a multilayer film is produced by repeating at least twice a step of applying fluorine-containing polyamide acid varnish, which is a precursor of polyimide, to a substrate, followed by baking. In this process, if the baking in the first polyimide film forming step is carried out insufficiently, the polyimide in the first layer will dissolve in a solvent contained in the polyamide acid varnish immediately after the second application of polyamide acid, causing a problem that disorder may occur in the interface between the first and second layers or cracks may be formed in the polyimide of the first layer. However, there is disclosed a technique of preventing the occurrence of cracks by converting the polyimide film of the first layer into a polyimide film insoluble in solvents through heat treatment at 380 ⁰ C or higher (e.g., see Japanese Patent Publication No. 3019166). According to the studies by the present inventors, however, an increase in baking temperature leads to an increased degree of coloring of a polyimide film, resulting in an increase in the optical loss of an

optical waveguide. Moreover, an increase in baking temperature produces an increased residual stress in a polyimide film after baking due to a difference in linear heat expansion coefficient from a substrate. This makes difficult the subsequent step for producing a waveguide. As a result, a yield reduction problem is caused. Since multilayer fluorine-containing polyimide films have excellent heat resistance, chemical resistance, dielectric properties, electrical properties, optical properties, and the like, they have been used for various kinds of optical materials. Therefore, a reduction in yield of a multilayer fluorine-containing polyimide film will result not only in an increase in the production cost of the multilayer film, but also in a reduction in the yields of various optical materials such as optical parts, optoelectronic integrated circuits (OEIC), and optical waveguides in optoelectronic mixed mounting wiring boards, and therefore, will directly be linked with a rise in prices. A process for producing a highly reliable multilayer fluorine-containing polyimide film, particularly, the optimization of film formation conditions for quality improvement, has therefore been sought . As a next-generation FTTH (Fiber to the Home) , the

B-PON (Broadband Passive Optical Network) system, which has been standardized as the ITU-T recommendation G.983.1, is regarded as the most hopeful. Therefore, the shift from the existing A-PON (Asynchronous Transfer Mode Passive Optical Network) system to the B-PON system is advancing. In the A-PON system, a wavelength of 1.31 μm and a wavelength of 1.55 μm are used as an uplink optical communication wavelength and a downlink optical communication wavelength, respectively. Fluorinated polyimide is useful as an optical material because of its small optical losses at these two wavelengths. In the B-PON system, a wavelength of 1.49 μm is to be used as a downlink optical communication wavelength. According to the studies by the present inventors, when a multilayer fluorinated polyimide film is formed by a conventional method, an optical absorption at a wavelength of 1.49 μm is observed, which has become a problem (see Comparative Example 4 and Fig. 2 shown later) .

DISCLOSURE OF THE INVENTION

Under these circumstances, it is an object of the present invention to provide a process to be used in the production of a fluorinated polyimide film represented by a fluorine-containing polyimide film,

especially a process for producing a multilayer fluorinated polyimide film, by which a less colored film will be produced, a less optical loss will be caused when an optical waveguide is produced, and neither cracks nor disorder of interfaces will occur through the formation of a multilayer. Another object of the present invention is to provide a process for producing a multilayer fluorinated polyimide film which can suppress optical absorption not only at wavelengths of 1.31 μm and 1.55 μm but also at a wavelength of 1.49 μm for the purpose of coping with the B-PON system. The present inventors have extensively studied to thereby find that when a multilayer film is produced from a fluorinated polyimide precursor containing carbon-fluorine bonds (C-F bonds) , it becomes possible to produce a multilayer film which is insoluble in a solvent and which forms neither disorder of interfaces nor cracks by baking the precursor at a temperature lower than 380°C for at least one hour or by baking the precursor under such a condition that the frequency of ventilation of the atmospheric gas in the baking oven is 0.07 times/min. or more, so that the optical losses at the specific wavelengths of an optical waveguide formed from the multilayer film can be reduced. Thus, they have completed the present invention.

That is, the present invention provides a process for producing a multilayer fluorinated polyimide film comprising repeating at least twice a step of forming a fluorinated polyimide film by applying a fluorinated polyamide acid free of C-H bonds to a substrate and subsequent baking, wherein the baking is carried out at a maximum temperature lower than 380 ⁰ C for at least one hour (this process may hereinafter be referred to as "the first production process of the present invention") . The baking may preferably be carried out in an atmospheric gas having an oxygen concentration of 10% (vol.%) or less.

The present invention further provides a process for producing a multilayer fluorinated polyimide film comprising repeating at least twice a step of forming a fluorinated polyimide film by applying a fluorinated polyamide acid free of C-H bonds to a substrate and subsequent baking of the fluorinated polyamide acid, wherein the baking is carried out under such a condition that a frequency of ventilation of an atmospheric gas in a baking oven is 0.07 times/min. or more (this process may hereinafter be referred to as "the second production process of the present invention") . The baking may preferably be carried out using nitrogen gas having a purity of 99.9% or higher and a dew point of

-60°C or lower as the atmospheric gas in the baking oven while introducing the nitrogen gas into the baking oven so that the frequency of ventilation is achieved. The baking may still preferably be carried out at a temperature lower than 380°C for at least one hour. According to the first production process of the present invention, a reduction in the maximum temperature at the baking makes it possible to prevent the coloring of a fluorinated polyimide film, and therefore, the optical loss of optical waveguides can be reduced. In addition, setting the baking time to at least one hour makes it possible to prevent the occurrence of cracks during the formation of a multilayer film. According to the second production process of the present invention, the control of a frequency of ventilation of an atmospheric gas at the baking makes it possible to reduce the optical loss of optical waveguides, particularly at a wavelength of 1.49 μm, and therefore, optical waveguides for the B-PON system can be provided.

In addition, the first production process of the present invention and the second production process of the present invention may hereinafter be referred to collectively and simply as the "production processes

of the present invention".

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a spectrum showing the wavelength dependency of the optical loss property of the optical waveguide produced in Example 6.

Fig. 2 is a spectrum showing the wavelength dependency of the optical loss property of the optical waveguide produced in Comparative Example 4.

BEST MODE FOR CARRYING OUT THE INVENTION

The first production process of the present invention is a process for producing a multilayer fluorinated polyimide film comprising repeating at least twice a step of forming a fluorinated polyimide film by applying a fluorinated polyimide precursor to a substrate and subsequent baking at a temperature lower than 380 ⁰ C for at least one hour.

The first production process of the present invention is characterized in that the baking of a fluorinated polyimide precursor is carried out at a temperature lower than 380 ⁰ C for at least one hour. When the baking time is one hour or longer, a fluorinated polyimide precursor is converted to a film insoluble in solvents, even if the baking temperature is reduced

to lower than 380°C. Therefore, the application of a fluorinated polyimide precursor to the first layer (an underlayer), followed by similar baking, will provide a multilayer film free of interface disorder and cracks without adversely affecting the first layer. The first production process of the present invention is, therefore, designed so that the baking is carried out at a temperature lower than 380 ⁰ C for at least one hour. The upper limit of the baking temperature may preferably be 360 ⁰ C, more preferably 340 ⁰ C. Too low baking temperatures will need too much time before the film becomes insoluble, and therefore, the lower limit of the baking temperature may preferably be 250 ⁰ C, more preferably 300 ⁰ C. The baking time may usually be from 1 hour to 10 hours, preferably from 3 hours to 10 hours, and more preferably from 5 hours to 10 hours.

The second production process of the present invention is a process for producing a multilayer fluorinated polyimide film comprising repeating at least twice a step of forming a fluorinated polyimide film by applying a fluorinated polyimide precursor to a substrate and subsequent baking under such a condition that a frequency of ventilation of an atmospheric gas in a baking oven is 0.07 times/min. or more .

The second production process of the present invention is characterized in that the baking of the fluorinated polyimide precursor is carried out under such a condition that a frequency of ventilation of an atmospheric gas in a baking oven is 0.07 times/min. or more. An increase in the amount of the atmospheric gas to be introduced into the baking oven can prevent an optical absorption at a wavelength of 1.49 μm. Although the reason for this is not clear, this is probably because, for example, when the flow rate of an atmospheric gas is increased, a substance which absorbs the light of a wavelength of 1.49 μm may be produced during the baking, or even if such a substance is formed, it is possible to prevent the substance from being accumulated in a resulting film. The second production process of the present invention is, therefore, designed so that the baking is carried out under such a condition that a frequency of ventilation of an atmospheric gas in a baking oven is 0.07 times/min. or more. If the frequency of ventilation is less than 0.07 times/min. , an optical loss at a wavelength of 1.49 μm will not reach a level for practical use. The term "frequency of ventilation" as used herein means N/V (times/min.) where the internal volume of a baking oven is V (liter) and the flow rate of an atmospheric gas

is N ( liter/min . ) . Thus, the frequency of ventilation is determined when the flow rate of an atmospheric gas is determined according to the internal volume of a baking oven. The lower limit of a frequency of ventilation may preferably be 0.10 times/min., more preferably 0.14 times/min. The upper limit of a frequency of ventilation is not particularly limited, but it may preferably 0.46 times/min., more preferably 0.28 times/min. from the viewpoint of production cost and the like.

There are many methods of forming a coated film of a fluorinated polyimide precursor on a substrate, and there also are various methods of forming a coated film on a specific kind of substrate. In the following, the production processes of the present invention will be explained using a method of producing a fluorinated polyimide film by forming a coated film of a fluorinated polyimide precursor on a substrate such as a silicon wafer . The method of applying a fluorinated polyimide precursor onto a substrate may appropriately be selected depending on the kind of substrate, the kind of fluorinated polyimide precursor, and the like, and is not particularly limited. Specific examples thereof may include a casting method, spin coating, a

roll coating method, a spray coating method, a bar coating method, a flexographic printing method, and a dip coating method. A polyimide film can be formed on a substrate by applying a fluorinated polyimide precursor onto the substrate using any of these methods, thereby forming a coated film, followed by baking the coated film in an atmosphere of an inert gas such as nitrogen gas, helium gas, or argon gas. In those methods, a spin coating method is suitable in the case where a fluorinated polyimide precursor is applied onto a substrate such as a silicon wafer because a thin film having a uniform thickness can be formed only on the substrate in a short time.

As a substrate onto which a fluorinated polyimide precursor is applied, there may be used conventionally known kinds of substrates, including both inorganic materials and organic materials. From a viewpoint of suppressing thermal deformation at a baking temperature, it is preferred to use silicon wafer; glass substrates such as quartz and Pyrex (registered trademark) ; substrates made of metals such as Al and Cu; metal oxide substrates; substrates made of resins such as polyimide and polyether ketone; organic-inorganic hybrid substrates; and the like. When a fluorinated polyimide precursor is applied

onto a substrate, this precursor may be applied in the form of a solution or a dispersion liquid prepared by dissolving or dispersing the precursor in a solvent. Examples of the solvents which can be used may include polar solvents such as N-methyl-2 -pyrrolidinone , N , N-dimethylacetamide , acetonit rile , benzonitrile , nitrobenzene, nitromethane , dimethylsulfoxide , acetone, methyl ethyl ketone, isobutyl ketone, and methanol; and non-polar solvents such as toluene and xylene. These solvents may be used alone, or two or more kinds of these solvents may also be used in combination. In these solvents, preferred are N-methyl-2 -pyrrol idinone and N , N-dimethylacetamide . The concentration of a fluorinated polyimide precursor in a solvent may preferably be in the range of from 10% to 50% by mass, more preferably from 15% to 45% by mass. The viscosity of the solution or dispersion of a fluorinated polyimide precursor may preferably be in the range of from 10 to 1,000 poises, more preferably from 25 to 150 poises.

When a fluorinated polyimide precursor is applied onto a substrate to form a coated film, and the coated film of the precursor is baked, the precursor is converted into a fluorinated polyimide through a ring closing reaction, so that a first layer of a fluorinated

polyiitiide film is formed. The term "first layer" as used herein refers to a fluorinated polyiitiide film firstly formed on a substrate, and the term "second layer" as used herein refers to a fluorinated polyimide film formed on the first layer. As a heating oven to be use for the baking of a fluorinated polyimide precursor, any of the general-purpose ovens may be selected. To reduce the oxygen concentration during the baking in the first production process of the present invention, it is preferred to use an inert oven or a vacuum baking oven. To increase the frequency of ventilation of an atmospheric gas in the second production process of the present invention, it is preferred to use, for example, an inert oven, a clean oven, or a vacuum baking oven. In the case where an inert oven is used in the first production process of the present invention, the baking may preferably be carried out while introducing an inert gas such as nitrogen gas, helium gas, or argon gas into the inert oven. It is recommended to carry out the baking while adjusting the oxygen concentration in the atmospheric gas up to 10% (vol%) , more preferably up to 5%, and more preferably up to 3%. In the second production process of the present invention, the atmospheric gas is not particularly limited, so long as it can produce a gas

flow, but an inert gas such as nitrogen gas, helium gas, or argon gas is preferred. In view of availability, nitrogen gas is more preferred. The use of nitrogen gas having a purity of 99.99% or more and a dew point of -60 ⁰ C or lower is recommended.

As described above, in the first production process of the present invention, the baking is carried out at a temperature lower than 38O ⁰ C for at least one hour. In the second production process of the present invention, the baking conditions are not particularly limited. However, for reducing the coloring of a fluorinated polyimide film and suppressing the occurrence of cracks, the baking may preferably be carried out at a temperature lower than 380 ⁰ C for at least one hour. The upper limit of the baking temperature may more preferably be 360 ⁰ C, still more preferably 340 ⁰ C. The lower limit of the baking temperature may preferably be 250 ⁰ C, more preferably 300 ⁰ C. The upper limit of the baking time may preferably be 10 hours, and the lower limit of the baking time may more preferably be 3 hours, still more preferably 5 hours.

In the production processes of the present invention, the rate of temperature increase from room temperature to the maximum baking temperature is not

particularly limited, so log as the solvent contained in the fluorinated polyimide precursor can be vaporized and a desired polyimide film can be produced. For example, the temperature may be increased either continuously or stepwise. The rate of temperature decrease from the temperature just after the film formation to room temperature is also not particularly limited, so long as a desired polyimide film can be produced. For example, the temperature may be decreased continuously or stepwise.

In the production processes of the present invention, a multilayer fluorinated polyimide film is formed by forming a first layer fluorinated polyimide film on a substrate, subsequently forming a coated film of a fluorinated polyimide precursor again, and then applying heat treatment to the precursor to form a second layer fluorinated polyimide film. The composition of the second layer may appropriately be selected depending on the applications of a desired multilayer polyimide film, and is not particularly limited. That is, the composition of the second layer may be either the same as, or different from, the composition of the first layer. The second layer can be formed by applying and baking a fluorinated polyimide precursor under the above temperature, time,

and atmosphere conditions. The first layer and the second layer or layers formed after the formation of the second layer may be formed under different conditions, so long as the conditions are within the above temperature, time, and atmosphere conditions. The fluorinated polyimide precursor to be used in the production processes of the present invention is not particularly limited, but a fluorinated polyamide acid having a repeating unit of the formula (1) :

wherein X is a tetravalent organic group and Y is a divalent organic group, provided that X and/or Y has at least one fluorine atom (such a fluorinated polyamide acid may hereinafter be referred to simply as a "fluorinated polyamide acid") is suitable in view of its excellent heat resistance, chemical resistance, water repellency, dielectric properties, electrical properties, and optical properties.

Examples of the tetravalent organic group represented by X in the above formula (1) may include tetravalent halogen-containing aliphatic organic groups derived from cyclic alkyl groups, linear alkyl

groups, olefins, glycols, and the like; and tetravalent halogen-containing aromatic organic groups derived from benzene, biphenyl, biphenyl ether, bisphenylbenzene , bisphenoxybenzene , and the like. It should be noted that such tetravalent organic groups are required to have no C-H bonds and all hydrogen atoms of C-H bonds have been replaced by halogen atoms (any of fluorine, chlorine, bromine, and iodine atoms) . Taking into consideration heat resistance, chemical resistance, water repellency, and low dielectric properties, it is preferred that there be no C-H bonds in a multilayer fluorinated polyimide film, and therefore, in the production processes of the present invention, a fluorinated polyamide acid free of C-H bonds is used as a fluorinated polyimide precursor. Halogen atoms in such tetravalent organic groups may be either the same or different. In these tetravalent organic groups, preferred are tetravalent halogen-containing aromatic organic groups, and particularly preferred are tetravalent wholly- fluorinated aromatic organic groups. Particularly preferred examples of the tetravalent organic groups represented by X in the above formula (1) may include tetravalent organic groups of the formulas:

Each of R ¹ and R ² in the above three formulas is a halogen atom, i.e., a fluorine, chlorine, bromine, or iodine atom, and doesn't include hydrogen atoms. R ¹ and R ² may be either the same or different, but it is preferred that one of R ¹ and R ² be a fluorine atom, and more preferred that all be fluorine atoms.

Further, in the above three formulas, Z may be divalent groups of the formulas:

In the formulas showing "Z", each of Y' and Y" is a halogen atom, i.e., a fluorine, chlorine, bromine, or iodine atom, and preferably, one is a fluorine atom, and more preferably all are fluorine atoms. When Y' and Y" are present in a formula showing "Z", each of Y' and Y" may be either the same or different. In each benzene ring, Y' or Y" may be the same or different in each occurrence. In these divalent groups, preferred are those of the formulas:

In the above formula (1) , Y is a divalent organic group, and when X contains no fluorine atoms, Y must contain at least one fluorine atom. Examples of the divalent organic group represented by Y may include divalent halogen-containing aliphatic groups which may contain a straight chain(s), a branched chain(s), or a ring(s) composed only of carbon-halogen atom bonds; divalent halogen-containing aromatic groups; and divalent halogen-containing organic groups in which two or more of the above aliphatic or aromatic groups are linked via a hetero atom(s) other than a carbon atom(s) , such as an oxygen atom(s) , a nitrogen atom(s) , and a sulfur atom(s) . The halogen atoms in these

divalent organic groups may be either the same or different. Examples of the divalent halogen-containing aliphatic group may include divalent halogen-containing aliphatic Organic groups derived from cyclic alkyl groups, linear alkyl groups, olefins, glycols, and the like; and divalent halogen-containing aromatic organic groups derived from benzene, biphenyl, biphenyl ether, bisphenylbenzene , bisphenoxybenzene , and the like. These divalent organic groups have no C-H bonds and all hydrogen atoms of C-H bonds must have been replaced by- halogen atoms (any of fluorine, chlorine, bromine, and iodine atoms ) .

In the above formula (1) , specific examples of the particularly preferred divalent organic groups represented by Y may include divalent organic groups i) and ii) shown below. Taking into consideration heat resistance, chemical resistance, water repellency, and low dielectric properties, divalent organic groups ii) are the most preferred.

In the production processes of the present invention, the polyamide acid having a repeating unit of the above formula (1) must contain at least one

fluorine atom as previously described. The fluorinated polyamide acid having a repeating unit of the above formula (1) can achieve, due to the presence of such a repeating unit, a desired refractive index (i.e., a refractive index difference Δn relative to existing fluorinated polyimides) in the fluorinated polyimide film of the present invention formed from the fluorinated polyamide acid. In the production processes of the present invention, taking into consideration an optical loss in a near infrared region, particularly an optical communication wavelength region (i.e., 1.0 to 1.7 μm) , used as a polyamide acid which is a starting material of a polyimide film is a polyamide acid having no carbon-hydrogen bonds (C-H bonds), i.e., a polyamide acid in which all hydrogen atoms bonded to carbon atoms forming the above formula (1) have been replaced by halogen atoms (any of fluorine, chlorine, bromine, and iodine atoms) and which contains at least one fluorine atom. That is, such a fluorinated polyamide acid can be a starting material for a fluorinated polyimide film having excellent heat resistance, chemical resistance, water repellency, dielectric properties, electrical properties, and optical properties. The process for producing a fluorinated polyamide

acid will hereinafter be described in detail. From this description, it is considered that the fluorinated polyainide acid is terminated with either an amine or an acid or a derivative thereof, although it may vary with the amounts of a diamine compound and a tetracarboxylic acid or a derivative thereof to be used (molar ratios) . The fluorinated polyamide acid may be formed of the same repeating unit or different repeating units. In the latter case, the repeating units may be either in a block form or in a random form.

The fluorinated polyamide acid may be produced by a combination of conventionally known techniques, and the process of such a production is not particularly limited. A process comprising reacting a diamine compound of the following formula (2) with a tet racarboxylic acid of the following formula (3) or a derivative thereof (e.g., an acid anhydride, an acid chloride, an ester) in an organic solvent may preferably employed. The symbol "Y" in the following formula (2) and the symbol "X" in the following formula (3) have the same definitions as those in the above formula ( 1 ) .

H ₂ N Y NH ₂ ⁽ 2 ⁾

The diamine compound of the above formula (2) is not particularly limited, so long as it has such a structure that reacts with a t etracarboxylic acid of the above formula (3) to give a fluorinated polyamide acid having a repeating unit of the above formula (1) . In view of a preferred structure of the fluorinated polyamide acid, specific examples of the preferred diamine compound may include 4 , 4 ' -diaminodipheny1 ether, 2, 2-dimethyl-4, 4' -diaminobiphenyl, 2, 2-bis [4- (4-aminophenoxy) phenyl] propane, 1, 4-bis (4-aminophenoxy) benzene, 9, 9-bis (4-aminophenyl) fluorene, 5-chloro-l, 3-diamino-2, 4, 6-trifluorobenzene, 2, 4, 5, 6-tetrachloro-l, 3-diaminobenzene, 2, 4, 5, 6-tetrafluoro-1, 3-diaminobenzene, 4, 5, 6-trichloro-l, 3-diamino-2-fluorobenzene, 5-bromo-l, 3-diamino-2, 4, 6-trifluorobenzene, and 2 , 4 , 5 , 6-tetrabromo-l , 3-diaminobenzene . These diamine compounds may be used alone, or two or more kinds of these diamine compounds may also be used in

combination. In these diamine compounds, particularly preferred are

2 , 4 , 5, β-tetrafluoro-1, 3-diaminobenzene and 5-chloro-l, 3-diamino-2, 4 , 6-trifluorobenzene. The tet racarboxylic acid of the above formula (3) or a derivative thereof (e.g., an acid anhydride, an acid chloride, an ester) is not particularly limited, and can be produced by any of the known technique, e.g. , the method disclosed in Japanese Patent Laid-open Publication No. 11-147955, or a combination of such known techniques. Specific examples thereof may include halogenated tet racarboxylic acids of the above formula (3), such as hexafluoro-3 , 3 ' , 4 , 4 ' -bipheny1 tetracarboxylic acid, hexachloro-3 , 3 ' , 4 , 4 ' -biphenyl tetracarboxylic acid, hexafluoro-3 , 3 ', 4 , 4 ' -biphenyl ether tetracarboxylic acid, hexachloro-3 , 3 ', 4 , 4 ' -biphenyl ether tetracarboxylic acid, bis (3, 4 -dicarboxytrifluorophenyl ) sulfide, bis (3, 4-dicarboxytrichlorophenyl) sulfide, 1, 4-bis (3, 4-dicarboxytrifluorophenoxy) tetrafluorobenzene,

1, 4-bis (3, 4-dicarboxytrichlorophenoxy) tetrafluorobenzene, 1, 4-bis (3, 4-dicaraboxytrichlorophenoxy) tetrachlorobenzene , 3 , 6-di fluoropyromellit ic acid,

3 , 6-dichloropyromellitic acid, and

3-chloro- 6-f luoropyromellit i c acid; acid dianhydrides corresponding thereto; acid chlorides corresponding thereto; corresponding esters such as methyl esters and ethyl esters; and the like. These halogenated tetracarboxylic acids or derivatives thereof may be used alone, or two or more kinds of these halogenated tetracarboxylic acids or derivatives thereof may also be used in combination. In these tetracarboxylic acids or derivatives thereof, preferred are hexafluoro-3 , 3 ' , 4 , 4 ' -biphenyl tetracarboxylic acid, hexafluoro-3 , 3 ' , 4 , 4 ' -biphenyl ether tetracarboxylic acid, 1, 4 -bis (3, 4-dicaraboxytrifluorophenoxy) tetrafluorobenzene, 1, 4-bis (3, 4-dicarboxytrifluorophenoxy) tetrachlorobenzene, and acid dianhydrides and acid chlorides corresponding thereto, and particularly preferred are hexafluoro-3 , 3 ', 4 , 4 ' -biphenyl ether tetracarboxyl ic acid, 1, 4-bis (3, 4-dicarboxytrifluorophenoxy) tetrafluorobenzene,

1, 4-bis (3, 4-dicarboxytrifluorophenoxy) tetrachlorobenzene, and acid dianhydrides thereof. A desired polyamide acid can be produced by reacting the diamine compound of the above formula (2)

with the tetracarboxylie acid of the above formula (3) or a derivative thereof in an organic solvent. In this case, these starting materials must be selected so that the resulting polyamide acid will have at least one fluorine atom.

The amount of the diamine compound to be used is not particularly limited, so long as it is an amount such that this compound can efficiently react with a tetracarboxylic acid or a derivative thereof. Specifically, from a stoichiometric point of view, the amount of the diamine compound to be used should be equivalently molar to the tetracarboxylic acid or a derivative thereof. It is, however, preferably in a range of from 0.8 to 1.2 moles, more preferably from 0.9 to 1.1 moles, when the total mole number of the tetracarboxy1 ic acid or a derivative thereof is taken as 1 mole. In this case, if the amount of the diamine compound to be used is smaller than 0.8 moles, much tetracarboxylic acid or a derivative thereof will remain unreacted, possibly complicating the step of purification, or preventing the degree of polymerization from being increased. In contrast, if the amount of the diamine compound to be used is greater than 1.2 moles, much diamine compound will remain unreacted, possibly complicating the step of

purification, or preventing the degree of polymerization from being increased.

The reaction may be carried out in an organic solvent, which is not particularly limited, so long as it can allow the diamine compound and the tet racarboxylie acid or a derivative thereof to react efficiently and it is inert to these starting materials. Examples of the organic solvent which can be used may- include N-methyl-2 -pyrrol idinone, N, N-dimethylacetamide, N , N-dimethyl formamide , dimethylsulfoxide , sulfolane, methyl isobutyl ketone, acetonitrile , and benzonit rile . These organic solvents may be used alone, or two or more kinds of these organic solvents may also be used in combination. The amount of the organic solvent to be used is not particularly limited, so long as it is such an amount that the diamine compound and the tet racarboxylic acid or a derivative thereof can efficiently react with each other. It is preferred to be such an amount that the concentration of the diamine compound in an organic solvent may be in a range of from 1% to 80% by mass, more preferably from 5% to 50% by mass.

The conditions for the reaction between the diamine compound and the tet racarboxylic acid or a derivative thereof are not particularly limited, so

long as they are such conditions that the reaction can be allowed to fully proceed. For example, the reaction temperature may preferably be in a range of from 0 ⁰ C to 100 ⁰ C, more preferably from 20 ⁰ C to 50 ⁰ C. The reaction time may preferably be in a range of from 1 to 72 hours, more preferably from 2 to 48 hours. The reaction may be carried out under either increased pressure, normal pressure, or reduced pressure, but it is preferably carried out under normal pressure. Further, the reaction between the diamine compound and the tetracarboxylic acid or a derivative thereof may preferably be carried out in an atmosphere of a dried inert gas, taking into consideration the reaction efficiency, the degree of polymerization, and the like. The relative humidity of an atmospheric gas for this reaction may preferably be not lower than 10 RH%, more preferably not higher than 1 RH%. Examples of the inert gas may include nitrogen gas, helium gas, and argon gas.

When a resulting fluorinated polyamide acid is subjected to heat-treatment (baking), a corresponding fluorinated polyimide is produced. The heat treatment of a fluorinated polyamide acid may be carried out either in a solvent or in the absence of a solvent, but it may preferably be carried out in a solvent, taking into consideration the reaction efficiency, and the

like. In this case, the fluorinated polyamide acid may be heat-treated in the form of a solution obtained from the reaction of a diamine compound and a tetracarboxylic acid or a derivative thereof by the above production process. Alternatively, the heat treatment may be carried out after separating the fluorinated polyamide acid as a solid from the solution and then dissolving it again in a solvent.

When a fluorinated polyamide acid having a repeating unit of the above formula (1) is used as a fluorinated polyimide precursor, a fluorinated polyimide having a repeating unit of the following formula will be obtained:

wherein X and Y have the same definitions as those in the above formula (1) . A film made of the fluorinated polyimide can exhibit a low optical loss in a specific optical communication wavelength region and have excellent heat resistance, chemical resistance, water repellency, dielectric properties, electrical properties, and optical properties. This film is, therefore, useful for printed circuit boards,

interlayer insulating films for LSI's, sealing materials for semiconductor parts, optical parts, optoelectronic integrated circuits (OEIC) , and various optical materials such as optical waveguides in optoelectronic mixed mounting wiring boards.

Examples

The present invention will be described below in detail by reference to Examples and Comparative Examples, but the present invention is not limited to these Examples. The present invention can be put into practice after appropriate modifications or variations within a range meeting the gists described above and below, all of which are included in the technical scope of the present invention.

First described are Synthesis Examples for the fluorinated polyamide acid used in Examples and Comparative Examples.

<Synthesis Example 1> A 50-mL three-neck flask was charged with 1.80 g (10 mmol) of 1 , 3-diamino-2 , 4 , 5 , 6-tetrafluorobenzene , 5.82 g (10 mmol) of 4 , 4 ' - [ ( 2 , 3 , 5 , 6-tetrafluoro- 1, 4-phenylene)bis (oxy) ] bis (3, 5, 6-trifluorophthalic anhydride) of the formula:

and 10.3 g of N , N-dimethylacetamide . The mixed solution was stirred under an atmosphere of nitrogen gas at room temperature for two days. Thus, a 38-mass% polyamide acid solution was obtained. <Synthesis Example 2>

A 50-mL three-neck flask was charged with 1.97 g (10 mmol) of 5 -chloro-1 , 3-diamino-2 , 4 , 6- t rifluorobenzene , 5.82 g (10 mmol) of 4,4'- [ (2, 3, 5, 6-tetrafluoro-l, 4-phenylene)bis (oxy) ]bis (3, 5 , 6-tri fluorophthalic anhydride), and 11.7 g of N , N-dimethylacetamide . The mixed solution was stirred under an atmosphere of nitrogen gas at room temperature for two days. Thus, a 33-mass% polyamide acid solution was obtained.

The following are Examples 1 to 5 and Comparative Examples 1 to 3 for the first production process of the present invention, and Reference Example.

<Example 1> The 38-mass% polyamide acid solution obtained in Synthesis Example 1 was dropped onto a silicon wafer,

and was spin-coated so that a film after baking will have a thickness of 15 μm. This coated film was then baked at 300 ⁰ C for 10 hours in a baking oven purged with nitrogen gas to form a fluorinated polyimide film as a first layer. The oxygen concentration in the atmospheric gas in the baking oven was 3%.

The 33-mass % polyamide acid solution obtained in Synthesis Example 2 was dropped onto the silicon wafer on which the first layer fluorinated polyimide film had been formed, and was spin-coated so that a film after baking will have a thickness of 10 μm. This coated film was then baked at 300 ⁰ C for 10 hours in a baking oven purged with nitrogen gas to form a fluorinated polyimide film as a second layer. The oxygen concentration in the atmospheric gas in the baking oven was 3% .

The resulting multilayer fluorinated polyimide film was visually observed, and there was found neither disorder of the interface between the first layer and the second layer nor cracks. This follows that an excellent multilayer film was obtained. <Example 2>

The 38-mass % polyamide acid solution obtained in Synthesis Example 1 was dropped onto a silicon wafer, and was spin-coated so that a film after baking will

have a thickness of 15 μm. This coated film was then baked at 300 ⁰ C for 5 hours in a baking oven purged with nitrogen gas to form a fluorinated polyimide film as a first layer. The oxygen concentration in the atmospheric gas in the baking oven was 3%.

The 33-mass % polyamide acid solution obtained in Synthesis Example 2 was dropped onto the silicon wafer on which the first layer fluorinated polyimide film had been formed, and was spin-coated so that a film after baking will have a thickness of 10 μm . This coated film was then baked at 300 ⁰ C for 5 hours in a baking oven purged with nitrogen gas to form a fluorinated polyimide film as a second layer. The oxygen concentration in the atmospheric gas in the baking oven was 3%.

The resulting multilayer fluorinated polyimide film was visually observed, and there was found neither disorder of the interface between the first layer and the second layer nor cracks. This follows that an excellent multilayer film was obtained. <Example 3>

A multilayer fluorinated polyimide film was obtained in the same manner as described in Example 1, except that the baking conditions for forming the first layer and the second layer were changed to 34O ⁰ C for

one hour. There were found neither disorder of the interface between the first layer and the second layer nor cracks. This follows that an excellent multilayer film was obtained. <Example 4>

From the multilayer fluorinated polyimide film obtained in Example 1, a straight waveguide structure was prepared by an RIE (reactive ion etching) method. The polyamide acid solution obtained in Synthesis Example 1 was dropped onto the silicon wafer on which the straight waveguide structure had been formed, and was spin-coated so that a film after baking will have a thickness of 15 μm. This coated film was then baked at 300°C for 10 hours in a baking oven purged with nitrogen gas to form a fluorinated polyimide film as a third layer. Thus, an embedded-type straight waveguide was obtained. The oxygen concentration in the atmospheric gas in the baking oven was 3%.

The optical loss of the resulting embedded-type straight waveguide was measured to be 0.2 dB/cm at a wavelength of 1.55 μm (1,550 nm) . The optical waveguide had a length of 5cm. <Examρle 5> An embedded-type straight waveguide was obtained in the same manner as described in Example 4, except

that the baking conditions for forming the third layer were changed to 340 ⁰ C for one hour. The oxygen concentration in the atmospheric gas in the baking oven was 3% . The optical loss of the resulting embedded-type straight waveguide was measured to be 0.2 dB/cm at a wavelength of 1.55 μm (1,550 nm) . The optical waveguide had a length of 5cm.

<Comparative Example 1> A multilayer fluorinated polyimide film was obtained in the same manner as described in Example 1, except that the baking conditions for forming the first layer and the second layer were changed to 300 ⁰ C for 0.5 hours. This multilayer film was found to have many cracks .

<Comparative Example 2>

A multilayer fluorinated polyimide film was obtained in the same manner as described in Example 1, except that the baking conditions for forming the first layer and the second layer were changed to 340 ⁰ C for 0.5 hours. This multilayer film was found to have many cracks .

<Comparative Example 3>

An embedded-type straight waveguide was obtained in the same manner as described in Example 4, except

that the baking conditions for forming three layers were changed to 380 ^c C for one hour. The optical loss of the resulting embedded-type straight waveguide was measured to be 2 dB/cm at a wavelength of 1.55 μm (1,550 nm) . The optical waveguide had a length of 5cm. <Reference Example>

An embedded-type straight waveguide was obtained in the same manner as described in Example 4, except that the oxygen concentration for forming three layers was changed to 15%. The optical loss of the resulting embedded-type straight waveguide was measured to be 1 dB/cm at a wavelength of 1.55 μm (1,550 nm) . The optical waveguide had a length of 5cm.

The following are Examples 6 to 8 and Comparative Example 4 for the second production process of the present invention. <Example 6>

The 38-mass % polyamide acid solution obtained in Synthesis Example 1 was dropped onto a polyimide substrate, and was spin-coated so that a film after baking will have a thickness of 15 μm . This coated film was then baked at 340 ⁰ C for one hour in a baking oven (an inert oven available from Espec Co., Ltd.; model "STPH-101") purged with nitrogen gas to form a fluorinated polyimide film (an under clad layer) as a

first layer. The internal volume of the baking oven was 216 liters, and the nitrogen gas flow rate was adjusted to 30 liters/min . Thus, the frequency of nitrogen gas ventilation in this case was 0.14 times/min.

The 33-mass % polyamide acid solution obtained in Synthesis Example 2 was dropped onto the polyimide substrate on which the fluorinated polyimide film had been formed as the first layer, and was spin-coated so that a film after baking will have a thickness of 10 μm. This coated film was then baked at 340 ⁰ C for one hour in the same manner as described above, i.e., under such a condition that the frequency of nitrogen gas ventilation was 0.14 times/min., to form a fluorinated polyimide film (a core layer) as a second layer.

The resulting multilayer fluorinated polyimide film was visually observed, and there was found neither disorder of the interface between the first layer and the second layer nor cracks. This follows that an excellent multilayer film was obtained.

From this multilayer film, a straight waveguide structure was prepared by an RIE (reactive ion etching) method, and the 38-mass% polyamide acid solution obtained in Synthesis Example 1 was dropped onto the polyimide substrate on which the straight waveguide

structure had been formed, and was spin-coated so that a film after baking will have a thickness of 15 μm . This coated film was then baked at 340 ⁰ C for one hour in the same manner as described above, i.e., under such a condition that the frequency of nitrogen gas ventilation was 0.14 times/min., to form a fluorinated polyimide film (an upper clad layer) as a third layer. The resulting embedded-type optical waveguide was measured for optical loss, and was found to exhibit extremely low optical loss values at wavelengths of 1.31 μm (1,310 nm) and 1.55 μm (1,550 nm) , as well as at a wavelength of 1.49 μm (1,490 nm) , as shown in Fig. 1. The optical waveguide had a length of 5cm. <Example 7> An embedded-type optical waveguide was obtained in the same manner as described in Example 6, except that the baking conditions for forming three layers were changed to 300 ⁰ C for 10 hours. There were found neither disorder of the interfaces between the layers nor cracks. This follows that an excellent multilayer film was obtained. The resulting embedded-type optical waveguide was measured for optical loss, and was found to exhibit extremely low optical loss values at wavelengths of 1.31 μm (1,310 nm) , 1.49 μm (1,490 nm) , and 1.55 μm (1,550 nm) . The optical waveguide had

a length of 5cm. <Example 8>

An embedded-type optical waveguide was obtained in the same manner as described in Example 6, except that the frequency of nitrogen gas ventilation for forming three layers was changed to 0.07 times/min. There were found neither disorder of the interfaces between the layers nor cracks. This follows that an excellent multilayer film was obtained. The resulting embedded-type optical waveguide was measured for optical loss, and was found to exhibit extremely low optical loss values at wavelengths of 1.31 μm (1,310 nm) , 1.49 μm (1,490 nm) , and 1.55 μm (1,550 nm) . The optical waveguide had a length of 5cm. <Comparative Example 4>

An embedded-type optical waveguide was obtained in the same manner as described in Example 6, except that the frequency of nitrogen gas ventilation for forming three layers was changed to 0.015 times/min. There were found neither disorder of the interfaces between the layers nor cracks, but an optical absorption was observed at a wavelength of 1.49 μm (1,490 nm) as shown in Fig. 2. The optical waveguide had a length of 5cm.

INDUSTRIAL APPLICABILITY

Multilayer fluorinated polyimide films obtained by the first production process of the present invention simultaneously has high light transparency in the entire region of the existing optical communication wavelength zone and high heat resistance, as well as excellent chemical resistance, water repellency, dielectric properties, electrical properties, and optical properties. Further, multilayer fluorinated polyimide films obtained by the second production process of the present invention has high light transparency in the entire region of the optical communication wavelength zone, particularly in the B-PON system, as well as excellent heat resistance, chemical resistance, water repellency, dielectric properties, electrical properties, and optical properties. Therefore, these multilayer fluorinated polyimide films are useful for printed circuit boards, interlayer insulating films for LSI's, sealing materials for semiconductor parts, optical parts, optoelectronic integrated circuit s (OEIC), and various optical materials such as optical waveguides in optoelectronic mixed mounting wiring boards.