JPS6360778B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to the curing of photosensitizable polyamic acid or salt films to form polyimides. More specifically, the present invention relates to the use of ultraviolet radiation to cure films of polyamic acids or salts thereof, and to the photosensitization of such acids or salts by the use of photosensitizers. It is something. Polyimides are typically made by a polycondensation reaction between organic diamines and tetracarboxylic acid dianhydrides and exhibit excellent high temperature properties and high dielectric properties. Because of its properties, polyimide has found wide utility in many areas. Current commercial practice is to purchase polyamic acid in liquid and therefore castable form, either as a polyimide film itself or as a precursor to polyimide. Once this polyamic acid is cast and the solvent is removed, it can be cured or imidized into a polyimide form at elevated temperatures. The curing temperature is in the range of 650°C. The problem with this practical matter is, of course, that the number of substrates available for applying the curable polyamic acid or its salts is somewhat limited, i.e. they must be temperature resistant in nature. That's what it means. Nevertheless, it is often desirable to use substrates in combination with polyimides that are not heat stable at the normal elevated curing temperatures for polyimide formation. Such applicability has not been available in the past. It has now been established that ultraviolet radiation can contribute to curing films of polyamic acid or its salts to polyimide. The polycondensation reactions involved in the curing process can therefore be carried out at much lower temperatures than would normally be considered necessary.
This allows substrates that are not normally stable to excessive temperatures to be used in combination with polyimide. Furthermore, through the use of ultraviolet curing, films of polyamic acid or its salts can be rendered photographically defined, i.e., selected areas of the film of polyamic acid or its salts can be imaged. In order to harden it like this,
A mask can be used selectively, whereby removal of unexposed parts can take place in such a way that an image can also be produced. By including a photoinitiator in the film, the curing reaction to the polyimide is significantly more rapid upon exposure to actinic radiation. According to the present invention, a method is provided for curing a film of polyamic acid or a salt thereof without the presence of externally applied heat to the corresponding polyimide, which comprises exposing at least one side of said film to ultraviolet radiation. curing the film by exposing it to a material for a sufficient period of time and at a sufficient intensity. Further provided is a sensitized film of polyamic acid comprising the above acid and its salt and at least one photoinitiator. Since high temperatures are not required for curing, a variety of substrates that are not thermally stable can be used with the resulting polyimide. By selectively curing in such a way that an image is formed through a suitable original, followed by development, a photographically defined polyimide image is provided. The polyimide contains at least one organic diamine having the structural formula H ₂ N—R ₁ —NH ₂ (wherein
R ₁ is a divalent group containing at least two carbon atoms, and the two amino groups are each attached to a separate carbon atom of this divalent group). at least one tetracarboxylic acid dianhydride having
not more than two carbonyl groups attached to any one carbon atom of this tetravalent group). This reaction initially forms a polyamic acid which, when cured at elevated temperatures or further reacted, forms the corresponding polyimide. The method for producing polyamic acid is patented in the U.S. Patent No.
No. 3179614, No. 3179630, No. 3179631, and No.
It is disclosed in numerous US patents, including No. 3,073,784. It has now been determined that ultraviolet radiation can be used to effect effective curing. Ultraviolet radiation refers to actinic radiation with accompanying infrared energy normally associated with ultraviolet radiation. This infrared energy is not harmful to the curing process, in fact it helps. However, no external application of heat is required to drive the curing. The polyamic acid can be simply solvated in a solvent such as N-methylpyrrolidone, dimethylacetamide, or dimethylformamide to provide a coating or application solution.
A polyamic acid concentration of about 1% to about 30% by weight is satisfactory, with increased solution viscosity and resulting application problems being practically the only limitation on solvent concentration. About 15 to about 20% by weight is optimal for coating applications and is therefore preferred. The thickness of the polyamic acid or salt dry film is limited only by practical considerations regarding ultraviolet light exposure. Exposure to UV radiation can, of course, be applied to both sides of the film simultaneously, thereby maximizing UV penetration and curing of the polyamic acid or salt to the polyimide. The intensity of radiation that causes curing can be varied, with lower intensities requiring longer exposure times.
The polyamic acid or salt should be subjected to irradiation at a sufficient intensity for a sufficient time to cure into the polyimide form. As used herein, the term "photoinitiator" corresponds to the conventional usage of the term, i.e., a compound capable of undergoing photodegradation upon exposure to actinic radiation, which Additional chemical reactions can then be initiated. As explained in the Examples, a wide variety of photoinitiators can be utilized with polyamic acids. Photoinitiator concentration is not critical and the examples clearly illustrate that very small concentrations of initiator are very effective in enhancing the curing reaction. The invention is herein described in further detail using the following examples, which are not meant to be limiting. All parts are by weight unless otherwise specified. The ultraviolet light source utilized in the examples, unless otherwise specified, is an ASHDEE UV12/1 manufactured by the ASHDEE Division of George Kotsuho and Sons. The device is a single mercury lamp, 12 inches long, 200 watts per inch, medium pressure, and has a 12 inch arc length. During exposure of the sample to UV radiation, the sample is clamped to an aluminum plate to securely hold the sample during exposure, then placed at the entrance of the conveyor belt of the exposure device, and then the aluminum plate is placed at the exit of the exposure device. Throw it away. Each such movement in this device is referred to as one pass.
It's called. The sample was sent through the device several times to increase the irradiation dose. When you do something like this,
To help provide uniform exposure across the surface,
The sample was moved lengthwise through the light source during the first pass and perpendicular thereto during the second pass. In all cases, the conveyor speed was 6 feet per minute and the exposure time per pass under the exposure apparatus was approximately 1 second of exposure for the most intense light. This exposure, i.e., one pass through the instrument at the recorded velocity, resulted in a density of 0.22 watts/cm ² as measured using an Atsushiday UltraViolet Power Density Meter with a standard wavelength of 3650 angstroms. equal. Example 1 A commercially available polyamic acid, Pyre ML from Dupont, was utilized in this example. This acid is the reaction product of oxydianiline and pyromellitic acid dianhydride,
It is in the form of a solution suitable for coating. This material was cast with a knife coater onto 1 oz. copper foil to a wet thickness of 6 mils, and then air dried at 275 mm to evaporate the solvent from the film. The material was placed in an exposure device and the degree of hardening or imidization of the material with increasing irradiation dose was then followed using infrared spectroscopy. Samples of uncured film were exposed to UV radiation from one side as recorded in the table below. The copper foil was then etched away using a conventional ferric chloride solution and the polyamic acid film was washed with deionized water. The spectrum of the uncured film shows a dominant absorption band at a wavelength of 3.1 microns due to NH bonds. As imidization progresses, the bond at 3.1 micron wavelength disappears, and the bond at 5.6 micron wavelength disappears.
The characteristic bond of imide at the wavelength of 13.8 microns appears with strong absorption. This progressive picture is shown in Table 1, as observed for samples exposed to increasing numbers of passes of UV radiation.

[Table] Example 2 As in the case of the unsupported film, in order to investigate the effect of curing of polyamic acid by exposure through both sides of the film to ultraviolet irradiation, as in Example 1 was applied onto a copper foil. Samples of polyamic acid film prepared at 300 °C and dried at 300 °C were exposed to ultraviolet light. The copper backing of the film was then etched away and the film was exposed to ultraviolet light from the opposite side, that is, the side in contact with the copper. The infrared spectra of samples exposed on both sides show more rapid curing or imidization than exposure from only one side. As shown in the table below, the samples achieved essentially complete cure by passing through the exposure device twice.

Table: Example 3 Polyamic acid films are readily dissolved in caustic solutions at room temperature, but once hardened, the caustic solutions do not show any significant attack. Therefore, a caustic solution is prepared by dissolving analytical grade sodium hydroxide in water at the rate of 5 g of solid sodium hydroxide in 95 ml of water, followed by mixing the solution and allowing it to reach room temperature. This solution was placed in a beaker equipped with a magnetic stirrer, and a sample of polyamic acid film was suspended therein such that the stirrer provided gentle agitation. For samples prepared as in Example 1, the film, which was not exposed to UV radiation, completely dissolved in this caustic solution in 1 minute. After only one pass through the exposure device, some areas of the sample were eroded after 2 minutes, indicating non-uniform exposure over the surface area of the film. After two passes, partial erosion was observed after 5 minutes in the caustic solution, and the same result was observed after three passes through the exposure device. After four passes through this apparatus, no corrosion was observed in the imidized film even after 6 minutes of immersion in the caustic solution. Example 4 Example 1 was repeated, but a substrate material other than copper foil was used as the coating substrate. In all cases, the degree of imidization or hardening was tested using the caustic solubility test as described in the previous examples. The following backing materials were used: paper, polyester film, aluminum, stainless steel,
Epoxy-fiberglass circuit board materials, solders, and silicone wafers. In all cases, polyamic acid was cured on these various substrates. Example 5 Various polyamic acids were made using various combinations of dianhydrides and diamines to study the curability of other polyamic acids upon exposure to ultraviolet radiation. In each case, diamines were added to N in a jar.
-Dissolved in methyl-pyrrolidone. The dianhydride was placed in a small polyethylene boat and floated on top of the diamine solution. The bottle was capped with polyethylene film and shaken suddenly and vigorously to quickly mix the dianhydride. Generally, a mild exotherm was observed as the solution viscosity increased. The mixture was placed on a shaker for further mixing. Generally, the reaction is about
It took 30 minutes to complete and the solution contained approximately 16% polyamic acid by weight. In this manner, samples were prepared using benzophenonetetracarboxylic acid dianhydride together with hexanediamine, methylene dianiline, m-phenylenediamine, oxydianiline, and methylenebis-cyclohexylamine. Additionally, the corresponding polyamic acid was prepared using pyromellitic acid dianhydride together with methylene dianiline, m-phenylene diamine, and methylene bis-cyclohexylamine. These acids were then cast onto copper foil, dried in a 275° bath, and subsequently cured by exposure under the UV source of Example 1. The progress of the curing cycle after exposure to UV radiation is 3.1 in the infrared spectrum
This was followed by the reduction in the micron suction peak or by the caustic solubility test described above. All samples prepared as described above exhibited curing upon exposure to UV radiation. Example 6 Polyamic acid was prepared in the same manner as in Example 5 using benzophenone tetracarboxylic acid dianhydride and hexanediamine. The polyamic acid thus obtained was cast onto copper foil and dried in a 270° bath for 15 minutes to remove the solvent. The dry film thickness on the copper foil was measured to be 0.1 mil to 0.2 mil. The samples were then exposed to ultraviolet radiation using original images with various designs and various resolution target levels placed on them. A piece of quartz glass was placed over the original paintings to hold them flat. Exposure to the ultraviolet light source of Example 1 for 75 seconds was satisfactory for imparting photographic definition to the film. Samples exposed in this manner show a visible image of the exposed pattern. The exposed areas became dark yellow, which is the typical color of cured polyimide, while the unexposed areas remained unchanged. The sample was then developed with N-methylpyrrolidone and washed with water to remove unexposed areas, followed by drying in air. The developed image provided excellent resolution. Preferred polyamic acids include benzophenonetetracarboxylic acid dianhydride with hexanediamine, oxydianiline, and methylenebis-cyclohexylamine, and that of Example 1. The benzophenonetetracarboxylic acid dianhydride/hexane diamine combination is most preferred as it exhibits the best properties with respect to photographic definition. Example 7 RC-5085 polyamic acid solution commercially available from DuPont - This is a polyamic acid solution obtained by the reaction of an aromatic diamine with an aromatic dianhydride such as pyromellitic dianhydride, according to DuPont documentation. -10g of
was mixed with 0.075 g of diethoxyacetophenone. The mixture was heated to 60° C. for approximately 10 minutes to aid solution formation. The resulting clear solution was knife coated onto a 3 mil thick polyester film and a 7 mil wet film was applied. The coated film was then placed in a forced air circulation bath at approximately 120°C for 15 minutes to remove the solvent from the film. This produced a solvent-free film approximately 1 mil thick. Once this sample was removed from the drying bath, the polyamic acid film was peeled from the polyester film and a clean 12" x 12"
It was placed on an inch hard board. Curing was carried out by passing the plate and the sample mounted thereon through an Atsushiday UV unit. To measure the degree of curing or cross-linking, ie the amount of conversion of polyamic acid to the polyimide state, a Perkin Elmer infrared spectrophotometer model 727B was used. By comparing each sample to a fully cured control sample as well as an uncured control sample, the degree of cure of the test samples was determined to be 95% conversion as determined using this infrared spectral data. Examples 8-48 Example 7 was repeated using different photoinitiators and their concentration levels. The results are shown below.

【table】

[Table] Examples 49-64 Examples 49 to 64 were carried out to determine the relationship between the degree of conversion or degree of cure of polyamic acid to polyimide with respect to photoinitiator concentration; Method Same as Example 7 except for photoinitiator concentration.

[Table] Examples 65-68 To compare the above examples with the use of heat alone without ultraviolet irradiation during curing or cross-linking of polyamic acid, the method of Example 7 was repeated, but However, rather than UV irradiation, thermal heating was used as the curing medium. When curing for 15 minutes at 300°, zero conversion to polyimide was observed. 400 during the same time = 80
% conversion was observed, 90% conversion was achieved in 15 minutes at 500〓, and 90% conversion was achieved in 15 minutes at 650〓.
A conversion of 95% was recorded. Examples 69-74 To confirm the effectiveness of including a photoinitiator in the polyamic acid system, unsensitized polyamic acid films were prepared according to Example 7 for comparative purposes. These desensitized films were subjected to prolonged ultraviolet irradiation as described in the following table. A comparison of the data in the table for a single pass through the UV irradiator with the table clearly shows the effect of the addition of photoinitiator to the polyamic acid system.

[Table] Example 75-84 Benzophenone tetracarboxylic acid dianhydride/m-phenylenediamine (Example 75-84)
80) and benzophenonetetracarboxylic acid dianhydride/oxydianiline (Example 81-
84) was used to prepare polyamic acid. Example 7 was then repeated except that the conveyor speed in the exposure apparatus was 10 feet/minute. Show the results below.

[Table] Muhexafluoroantiminate

Polyamic acid can be applied to substrates that are unstable at elevated temperatures, such as paper, polyester, etc., and can be effectively cured to the polyimide state. Additionally, it has been discovered that converting the polyamic acid to a salt prior to curing provides a product with the same functionality for UV curing and, in fact, provides additional advantages. When utilizing the corresponding salts, a larger width due to exposure and development is observed. It has been observed that the exposure time required to form a cured image is reduced, the resolution of the image is increased, and unexposed areas are removed more easily and without residue. Polyamic acids can be converted into salts by simple mixing with a base, preferably in a solvent using equivalent concentrations. When the resulting cured polyimide is used in electrical applications, for example as an insulating layer, it is preferred to use a volatile, non-metallic base to obtain the salt. Example 85 Polyamic acid was prepared according to Example 6, and the product mixture contained 20% solids polyamic acid. To 20 g of this acid was added 1.84 g of triethylamine, thereby converting the acid into a salt. Coating this salt solution onto aluminum foil,
The solvent was removed by drying in a 130°C bath for 15 minutes.
This dried film is coated with Colite's model DMVLS.
Exposure was performed through a quartz glass photomask for 1 minute to ultraviolet radiation using an exposure device. The imagewise exposed film is developed with N-methylpyrrolidone, thereby removing the unexposed areas of the film. The resulting sample was rinsed with toluene and dried with air. The film was heated at 340° C. for 3 minutes to ensure complete removal of solvent and drive off residual triethylamine. A polyimide image field with excellent resolution was produced.

Claims (1)

[Claims] 1. A method of curing a film of polyamic acid or its salt by exposing at least one side of a film of polyamic acid or a salt thereof to ultraviolet light of sufficient intensity for a sufficient period of time in the absence of thiol. A method of curing the film to the corresponding polyimide without applying heat. 2 Polyamic acid is benzophenonetetracarboxylic acid dianhydride/hexanediamine,
Benzophenonetetracarboxylic acid dianhydride/oxydianiline, benzophenonetetracarboxylic acid dianhydride/methylene bis-
The method of claim 1, wherein the method is selected from the group consisting of cyclohexylamine and pyromellitic acid dianhydride/oxydianiline.