WO1997045497A1 - Photoimageable polyimides coatings based on non-aromatic dianhydrides - Google Patents

Abstract

The present invention relates to polyimides which are derived from at least one non-aromatic component, a process for their manufacture and their uses in electronic applications, particularly as dielectric coatings for printed wiring boards (PWBs), coatings for optical fibers, optical waveguides, gas separation membranes, and alignment layers for liquid crystal displays.

Description

PHOTOIMAGEABLE POLYIMIDES COATINGS BASED ON NON-AROMATIC DIANHYDRIDES

Field Of The Invention

This invention relates to photoimageable polyimides containing an aliphatic component, their preparation, and their use in electronic and other applications.

More particularly, this invention relates to the use of photoimageable low dielectric constant, low moisture uptake, thermally stable polyimides and co-polyimides of 3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA), and/or other aromatic dianhydrides, such as 2,2-bis(3,4- dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), and an unsaturated or saturated non-aromatic dianhydride, or their esters or acids, and selected aromatic diamines as interlevel dielectrics and coatings for on-chip and multi- chip packages in electronic applications and, more particularly, to multi-layer structures for electronics use wherein low dielectric constant, low moisture uptake, thermally stable photoimageable polymers are required for use as protective coatings on substrates and interlevel dielectrics sandwiched between two layers. These polyimides may also find use as gas separation membranes and alignment layers for liquid crystal displays.

Background Of The Invention

The present invention relates to photoimageable, low dielectric constant, low moisture uptake, thermally stable polyimides derived from an aliphatic dianhydride monomer component, an aromatic dianhydride monomer component, and selected aromatic diamines, their method of preparation, and the use of such polyimides in electronic applications, especially as interlevel dielectrics and coatings for on-chip and multi-chip packages in electronic applications and, more particularly, to multilayer structures for electronics use wherein low dielectric constant, low moisture uptake, thermally stable, high T_g polymers are required for use as protective coatings on substrates and interlevel dielectrics sandwiched between two layers. Aromatic polyimides have found extensive use in industry as fibers, films, composites, molded parts and dielectrics due to their ease of coating, toughness, flexibility, mechanical strength and high thermal stability. In the electronic industry, polyimides have proven to be useful due to their low dielectric constant and high electrical resistivity. Such polymers have been used in both film and coating form as advanced materials for such uses as interlevel dielectrics, passivation coatings, insulating coatings, die attach adhesives, flexible circuit substrates, and the like.

The use of polyimides in industry, particularly in the coating art, is not new, and a number of publications describe their preparation and details of their uses. However, the definition of and optimization of the relevant polymer properties, particularly in the use of polyimides for microelectronics, still struggles with defining the polymer properties desired and relating the desired properties to the chemical and physical nature of the polyimide.

Many electronic applications, for example, passivation coatings or interlevel dielectrics, require that vias (or openings) be etched through the polymer coatings to permit access for electrical connections between the substrate and the outside environment.

Etching vias through polyimides, or their polyamic acid precursor, requires a multistep procedure. The polymer is generally dissolved in solution and the resulting solution of polymer is spread on a substrate to form a coating. In the case of polyamic acid, the coating is further coated with a photoresist material which itself is in a solvent and that solvent is substantially removed, typically by heating (also called soft baking). The photoresist material is then shielded with a mask containing a pattern of openings and the photoresist material is exposed to actinic radiation. Thus, a positive-acting photoresist material is photochemically altered such that the areas that were exposed to actinic radiation are soluble and vias (or openings) are created by taking advantage of this selective solubility to develop and remove specific areas of photoresist material. The underlying polyamic acid coating may be etched with the photoresist in a single step or sequentially. After the polyamic acid is etched, forming vias in the polyamic acid coating, the remaining photoresist material is removed. Thereafter, the polyamic acid is imidized, generally by heating, generally in a range of from about 200°C to about 400°C, to form the final coating.

Thus, etching can be viewed as a multistep process to remove selected areas of a coating on a substrate with an appropriate solvent to form vias (or openings) in the coating. However, the number of steps involved in the process would be substantially reduced if photosensitivity could be incorporated into the polymer so that a photosensitive polymer could be applied to the substrate and patterned directly, i.e., without the need for the application and removal of a photoresist material. The number of steps would be further reduced if the polymer coating could be applied as a polyimide, thereby eliminating the imidization step. By reducing the number of process steps, the production cost of each electronic component will be reduced and overall production will be more efficient.

Polymer shrinking is a significant problem which may cause delamination of the film from the substrate because of the internal stress that builds up in the film from the polymer shrinkage. Further, in order to compensate for polymer shrinkage, the mask design must be adjusted so that the final pattern features have the correct dimensions. For example, in order to pattern 10 micron lines from a polymer having a 45 percent thickness loss and a 20 percent loss in line width, one would need a 12 micron pattern. Further, the degree of feature shrinkage can vary from one polymer to another; therefore, the shrinkage characteristics of each new polymer must be determined in order to compensate for that polymer's unique characteristics. Some have tried to overcome the shrinkage problem by providing a cured polyimide with photoimageable properties incorporated into the polymer backbone. Rohde, O., 3rd Annual International Conference on Crosslinked Polymers, Luzern, Switzerland, 197-208 (1989), discloses a photoimageable polyimide prepared from pyromellitic acid dianhydride (PMDA) and 2,2',6,6'- tetramethyl-4,4'-methylenedianiline (TMMA).

The present invention provides polyimides (having an aliphatic component) which are photosensitive and can be photodefined. Further, by selective exposure the polyimides of the present invention can be etched or channeled by wet etch techniques. The polyimides of the present invention are made solvent resistant by actinic radiation and can be fabricated into multi-layer structures by overcoating one polyimide layer over another. The polyimides of the present invention are useful as interlevel dielectrics and coatings for on-chip and multi-chip packages in electronics applications and, more particularly, as dielectric coatings in multi-layer structures for electronics such as printed wiring boards wherein low dielectric constant, low moisture uptake, thermally stable photoimageable polymers are required for use as protective coatings on substrates and interlevel dielectrics sandwiched between two layers.

This invention also relates to the use of low dielectric constant, low moisture uptake, thermally stable polyimides derived from an aromatic dianhydride component, an aliphatic dianhydride component and a diamine component which comprises at least one diamine having alkyl substituents ortho to at least one amine nitrogen, and selected aromatic diamines as interlevel dielectrics and coatings for on-chip and multi-chip packages in electronic applications. More particularly, this invention relates to multilayer structures for electronic use wherein low dielectric constant, low moisture uptake, thermally stable, high T_g polymers such as the polyimides of the present invention, are required for use as protective coatings on substrates and as interlevel dielectrics sandwiched between two layers.

In addition to those requirements stated above, a polymer for such use in the electronics industry must be resistant to processing solvents, form pinhole-free coatings when coated, adhere well to a variety of substrates such as silicon dioxide and silicon nitride, aluminum, copper, other polymers etc., be ionically pure, exhibit appreciable etch rates in an appropriate etching system (plasma etch or reactive ion etch), and have high electrical resistivity. Because of the multiple and severe requirements, new polymers with improved properties (particularly the combination of low dielectric constant, low moisture uptake, thermal stability and photo pattemability) are commercially significant and in great demand.

The use of polyimides in industry, particularly in the coating art, is not new, and a number of publications exist treating their preparation and details of their uses. However, the definition of and optimization of the relevant polymer properties, particularly in the use of polyimides for microelectronics, still struggles with defining the polymer properties desired and relating the desired properties to the chemical and physical nature of the polyimide.

Summary Of The Invention

The present invention relates to polyimides which are derived from at least one non-aromatic component, a process for their manufacture and their uses in electronic applications, particularly as dielectric coatings for printed wiring boards (PWBs), coatings for optical fibers, optical waveguides, gas separation membranes, and alignment layers for liquid crystal displays. The present invention further relates to polyimides and polyimide coatings and interlevel dielectrics prepared therefrom. The polyimides include polymers derived from dianhydrides and aromatic diamines wherein at least one recurring unit of the polyimide is selected from the group consisting of

and

wherein the remainder of recurring units are derived from aromatic dianhydrides, aliphatic dianhydrides, aromatic diamines, and aliphatic diamines.

In preferred polyimides, the remainder of recurring units are derived from aromatic dianhydrides comprising at least one dianhydride containing a photosensitizing moiety, and at least one aromatic diamine which is substituted in the two ortho-positions relative to at least one N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyl.

The present invention also includes photoimageable polyimides and photoimageable polyimide coatings and interlevel dielectrics made from such polyimides wherein the polyimide comprises the following recurring units

φ

(I) wherein R¹ and R² comprise aromatic tetravalent radicals which may be the same or different and wherein at least one of said tetravalent radicals contains a photosensitizing moiety; R³ comprises at least one tetravalent aliphatic radical; Y¹ is at least one divalent radical of an aromatic diamine and comprises at least about 1 mole percent of a divalent radical of an aromatic diamine which is substituted in the two ortho-positions relative to at least one N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyl; and Y² may be the same as or different from Y¹ and is at least one divalent radical of an aromatic or aliphatic diamine.

Preferably from about 1 to about 99 mole percent of the dianhydride residues of the polyimides of the present invention are derived from at least one aliphatic dianhydride. More preferably at least 10 mole percent of the dianhydride residues are derived from at least one aliphatic dianhydride. Preferred aliphatic dianhydrides are 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3- cyclohexene-1 ,2-dicarboxylic anhydride; bicyclo[2.2.2]oct-7-ene-2, 3,5,6- tetracarboxyiic dianhydride; and 5-(2,5-dioxotetrahydrofuryl)-3-methyl- cyclohexane-1 ,2-dicarboxylic anhydride.

Briefly, the polymers of this invention are useful in electronic applications as coatings and can be made into flexible substrates for electrical components, interlevel dielectrics, high density interconnects and the like. In particular, the polymers of the present invention provide a unique photoimageable polyimide coating for microelectronic uses, particularly as dielectric coatings for printed wiring boards. The polyimides of the present invention may also be useful as gas separation membranes, alignment layers for liquid crystal displays, optical waveguides, and coatings for optical fibers. The polyimides of the present invention can also be used, for example, as an alpha particle barrier, ion-implantation mask, high temperature resist material, buffer coating layer, planarization layer, multichip module, interlayer dielectric insulating layer, gas separation membrane, or alignment layer for liquid crystal displays. The use of aliphatic dianhydrides in the photoimageable polyimides of the present invention provides several advantages over current materials used for photoimageable polyimides. The following are some of these advantages:

1. Improved solubility; image rapidly develops in common developing solvents.

2. High photospeed, high contrast and resolution. 3. Incorporation of an aliphatic monomer contributes to low dielectric constant and moisture absorption.

4. Reduced T_g enhances low temperature cure required for PWB relative to wholly aromatic systems. 5. Lower cost relative to 6FDA.

6. Low optical density compared to aromatic dianhydrides.

7. An unsaturated aliphatic dianhydride adds reactive crosslink sites to the dianhydride portion of the polymer.

8. Good adhesion to metallization. The incorporation of aliphatic anhydrides into the polyimides of the present invention promotes good solubility of the polyimides; an image develops rapidly and with good resolution in common developing solvents. Ortho-alkyl substitution alone is not sufficient for good solubility. Thus, a BTDA/DMDE polyimide is not sufficiently soluble to have utility as a photodefinable dielectric coating. However, the use of 6FDA to enhance solubility is expensive and limits anhydride selection to only one monomer. Aliphatic anhydrides also serve to enhance solubility and can be used to replace all or part of the 6FDA in a photodefinable coating.

The aliphatic anhydrides incorporated into the polyimides of the present invention also allow additional derivative chemistry to be used to cure the polyimide. Thus, the unsaturated aliphatic anhydrides can also be cured with additives (e.g., peroxides) which are known to thermally crosslink unsaturated resins. The saturated anhydrides also undergo a thermal crosslinking at lower temperature than the wholly aromatic systems, allowing the possibility of a post-development thermal cure to further enhance the properties of the resin. Lower cure temperatures are important in applications where the thermal stability of the substrate (such as an epoxy PWB) may limit the allowable curing temperature. Detailed Description Of The Invention

The preferred polyimides, polyimide coatings, and interlevel dielectrics of the present invention are photoimageable polyimides derived from a tetravalent aromatic dianhydride component which includes at least one tetravalent photosensitizing moiety, at least one tetravalent aliphatic dianhydride component and a diamine component which includes at least one diamine which is substituted in the two ortho-positions relative to at least one N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyl.

The preferred polyimides, polyimide coatings, interlevel dielectrics of the present invention are those in which the polyimide is a photoimageable polyimide comprising the following recurring units

(I) wherein R¹ is

R² may be the same as or different from R¹ and is at least one tetravalent aromatic radical; R³ comprises at least one tetravalent aliphatic radical; and Y¹ is

or mixtures thereof; X, X¹, X², and X³, independently are alkyl of 1 to 6 carbon atoms; Z, Z¹, Z², and Z³ independently are hydrogen or alkyl of 1 to 6 carbon atoms; and the mole ratios of m, n, and o range from about 99-1 :0-98:1-99.

In the above formula I, m + n + o = 100 mole percent of the dianhydride residues of the polyimide and r + s = 100 mole percent of the diamine residues of the polyimide.

More preferred polyimides of the present invention are those comprising the following recurring units

(I) wherein R¹ is

R² is

R³ is at least one tetravalent aliphatic radical; Y¹ is

^3 or mixtures thereof; X, X¹ , X², and X^"3 independently are methyl or ethyl; and the mole ratios of m, n, and o range from about 90-10:0-60:10-90.

Other preferred polyimides of the present invention are those comprising the following recurring units

(I) wherein R¹ is

R² is

R is selected from

and

or mixtures thereof; Y is

Y² is the same as Y¹ or is a different aromatic diamine or an aliphatic diamine; and the mole ratios of m, n, and o range from about 90-10:0-60:10-90.

More preferably, the mole ratios of m, n, and o in the formula I range from about 40-60:50-30:10-20. Also preferred are polyimides of formula I derived from at least 10 mole percent aliphatic dianhydride and at least 30 mole percent BTDA.

When both R¹ and R² are BTDA radicals (n=0), the aliphatic component R³ is preferably at least about 20 mole percent (o=20%) in order to obtain a polyimide having sufficient solubility to form a good image when exposed to a developing solvent.

More preferred are polyimides of formula I wherein m is from about 40 to about 50 mole percent, n is from about 50 to about 40 mole percent, and o is about 10 mole percent.

Other preferred polyimides of formula I are those wherein m is about 50 mole percent, n is about 40 mole percent and o is about 10 mole percent.

Also included in the invention are polyimides of formula I wherein m is about 45 mole percent, n is about 45 mole percent, and o is about 10 mole percent.

Included in the present invention are polyimides of formula I wherein m is about 55 mole percent and o is about 45 mole percent.

Representative aliphatic dianhydrides which may be used to make the polyimides of the present invention include but are not limited to: 5-(2,5- dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1 ,2-dicarboxylic anhydride

(MCTC) is known from U.S. 4,271 ,079 and is commercially available as

Epiclon B-4400 from Dainippon Ink & Chemicals; bicyclo[2.2.2]oct-7-ene-

2,3,5,6-tetracarboxylic dianhydride (BCOE) available from Swiss Society of

Explosives, Brig, Switzerland; 5-(2,5-dioxotetrahydrofuryl)-3- methylcyclohexane-1 ,2-dicarboxylic anhydride (MCHA); 3,4-dicarboxy-

1 ,2,3,4-tetrahydro-1-naphthalenesuccinic acid dianhydride; tetrahydrofuran-

2,3,4,5-tetracarboxylic dianhydride; ethylenediaminetetraacetic dianhydride; bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic dianhydride and cyclopentane-

1 ,2,3,4-tetracarboxylic dianhydride. Preferred aliphatic dianhydrides are:

(a) 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3-cyclohexene-1 ,2- dicarboxylic anhydride (MCTC)

(b) bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylicdianhydride (BCOE) o o

I I II o

II II 0 o and

(c) 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclohexane-1 ,2- dicarboxylic anhydride (MCHA)

A divalent radical of an aromatic diamine Y¹ or Y² is preferably a divalent, mononuclear or dinuclear phenylene radical. A linear or branched alkyl or alkoxy substituent of Y¹ or Y² can contain 1 to 20, preferably 1 to 6 and in particular 1 to 4, carbon atoms, a linear or branched alkoxyalkyl substituent of Y¹ or Y² can contain 2 to 12, in particular 2 to 6, carbon atoms, an alkylene substituent of Y¹ or Y² can contain 3 or 4 carbon atoms, a cycloalkyl substituent of Y¹ or Y² can contain 5 to 8, in particular 5 or 6, ring carbon atoms and an aralkyi substituent of Y¹ or Y² can contain 7 to 12 carbon atoms. Alkoxyalkyl is preferably alkoxymethyl and aralkyi is preferably benzyl.

Examples of substituents are: methyl, ethyl, propyl, isopropyl, n-butyl, isobutyl, tert.-butyl, pentyl, hexyl, octyl, dodecyl, tetradecyl, eicosyl, cyclopentyl, cyclohexyl, methylcyclohexyl, methoxy, ethoxy, propoxy, butoxy, pentoxy, hexoxy, methoxymethyl, methoxyethyl, ethoxymethyl, propoxymethyl, butoxymethyl, benzyl, methylbenzyl and phenethyl. Preferred radicals are methoxymethyl, ethoxymethyl, methyl, ethyl, isopropyl, trimethylene and tetramethylene. Particularly preferred radicals are isopropyl, ethyl and, especially, methyl. Examples of aromatic tetracarboxylic acid dianhydrides which may be incorporated in the polyimides of the present invention are 2,2-bis(3,4- dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 2,3,9,10- perylenetetracarboxylic acid dianhydride, 1 ,4,5,8-naphthalenetetracarboxylic acid dianhydride, 2,6-dichloronaphthalene-1 ,4,5,8-tetracarboxylic acid dianhydride, 2,7-dichloronaphthalene-1 ,4,5,8-tetracarboxylic acid dianhydride, 2,3,6,7-tetrachloronaphthalene-1 ,4,5,8-tetracarboxylic acid dianhydride, phenanthrene-1 ,8,9,10-tetracarboxylic acid dianhydride, pyromellitic dianhydride, 3,3',4,4'-biphenyltetracarboxylic acid dianhydride, 2, 2', 3,3'- biphenyltetracarboxylic acid dianhydride, 4,4'-isopropylidenediphthalic anhydride, 3,3'-isopropylidenediphthalic anhydride, 4,4'-oxydiphthalic anhydride, 4,4'-sulfonyldiphthalic anhydride, 3,3'-oxydiphthalic anhydride, 4,4'-thiodiphthalic anhydride, 4,4'-ethylidenediphthalic anhydride, 2,3,6,7- naphthalenetetracarboxylic acid dianhydride, 1 ,2,4,5-naphthalene- tetracarboxylic acid dianhydride, 1,2,5,6-naphthalenetetracarboxylic acid dianhydride, benzene-1,2,3,4-tetracarboxylic acid dianhydride, thiophene- 2,3,4,5-tetracarboxylic acid dianhydride, 1-(3',4'-dicarboxyphenyl)-1 ,3,3- trimethylidane-5,6-dicarboxylic acid dianhydride, 1-(3',4'-dicarboxyphenyl)- 1 ,3,3-trimethylindane-6,7-dicarboxylic acid dianhydride, 1-(3',4'-dicarboxy- phenyl)-3-methylindane-5,6-dicarboxylic acid dianhydride, 1-(3',4'- dicarboxyphenyl)-3-methylindane-6,7-dicarboxylic acid dianhydride, 3,3',4,4'- benzophenonetetracarboxylic acid dianhydride and 2,3,3',4'-benzo- phenonetetracarboxylic acid dianhydride.

Representative diamines and dianhydrides and their radicals which may be incorporated in the polyimides of the present invention are contained in United States Patent No. 4,629,777. A preferred sub-group of polyimides according to the invention are those in which Y¹ and Y² independently are:

wherein X, X¹, X² and X³ are CH₃ or C₂H₅. More preferred are polyimides in which both Y¹ and Y² are derived from diaminodurene (DMDE).

As used herein, photosensitizing moiety means a moiety that increases the sensitivity of the polyimide to crosslinking as a result of actinic radiation. In general, moieties which contain a chromophore can function as a photosensitizing moiety. Examples include, but are not limited to,

and the like. Where the photosensitizing moiety is derived from a diamine, the linkage is divalent. Where the photosensitizing moiety is derived from a dianhydride, the linkage is tetravalent. In either case, the present invention also incorporates the isomeric variants of the above-described photosensitizing moieties.

A particularly preferred polyimide is a polyimide having from about 30 to about 90 mole percent BTDA relative to the dianhydride moiety of the polymer. A co-initiator may be included in the photoimageable polyimide coating composition to further increase the photosensitivity of the polymer. These co- initiators may or may not be included in the polymer backbone. Examples include, but are not limited to, anthraquinone, 2-ethylanthraquinone, 2-tert- butylanthraquinone, benzophenone, Michler's ketone, thioxanthone, 3- ketocoumarines, triethylamine, N-methyldiethanolamine, 4-(amino) methylbenzoate, 4-(dimethylamino) methylbenzoate, 4-(dimethy!amino) benzaldehyde, and the like.

Where the photoimageable polyimide coating of the present invention is a copolymer, the copolymer can be either a random copolymer or a block copolymer.

The following abbreviations as used herein are defined as follows:

The photoimageable polyimide of the present invention can be prepared as the polycondensation product of components comprising, for example, 6FDA or another tetravalent dianhydride moiety, and/or an aliphatic tetravalent dianhydride and DMDE, MEDA, and/or another diamine moiety, or derivatives thereof, and includes a component that will contribute a photosensitizing moiety, for example, BTDA. Examples of dianhydrides that will contribute a photosensitizing moiety include, but are not limited to, 3,3', 4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA), 2,3,6,7- anthraquinone tetracarboxylic acid dianhydride, 2,3,6,7-thioxanthone tetracarboxylic acid dianhydride, and the like, as well as isomers thereof. Examples of diamines include, but are not limited to, the various isomers of benzophenone diamine, anthraquinone diamine, thioxanthone diamine, and the like.

Using conventional lithography equipment, a photosensitivity measurement expressed as mJ/cm2 which is lower indicates the polymer is relatively more photosensitive (requires less exposure energy) and will provide a lithographically useful image at a lower exposure dose. The photosensitivity measurement is not, however, an inherent property of the polymer composition. Photosensitivity is strongly influenced by polymer molecular weight, the solvent strength of the developer composition, and polymer film thickness as discussed by Rohde, et al. in Polymer Engineering and Science News. Mid-November 1992, Vol. 32, No. 21 , pages 1623-1629. All relevant factors must be considered when determining whether a polymer has a suitable photospeed for a particular application. Those skilled in the art will realize that it is difficult to determine from the photosensitivity measurement alone whether one polymer composition is inherently more photosensitive than another.

Generally, polyimides are made by mixing a diamine component and a dianhydride component and adding a compatible solvent to form a solution of polyamic acid. The polyamic acid is then imidized by either chemical or thermal methods to form a polyimide. With the polyimides of the present invention a three step process is used which comprises:

(a) reacting the aliphatic dianhydride monomer (or a precursor therefor), with or without a portion of an aromatic dianhydride monomer (or a precursor therefor), with an ortho-alkyl-substituted diamine component to form imide segments using azeotropic dehydration; (b) forming a high molecular weight polyamic acid by adding the remaining monomer component(s) to bring the reaction into stoichiometric balance; and then

(c) chemically imidizing the polymer. The phrase "precursor therefor" means a species that would either form the reactive species under the reaction conditions or lead to the same product. In this case, precursors to the dianhydride would include the tetraacid or the diester-diacid (the reaction products of the dianhydride with water or with alcohol, respectively), which would convert to dianhydride under the reaction conditions.

This three step process avoids gelling which occurs with thermal imidization of aromatic monomers such as BTDA and diamines having alkyl substituents ortho to the N and allows better control of molecular weight.

The aliphatic dianhydride is thermally imidized in the presence of an excess of diamine in step (a) to consume the aliphatic dianhydride. Step (b) forms a high molecular weight amic acid/imide copolymer. This is preferable to direct formation of high molecular weight polyimide by thermal imidization alone, because the molecular weight of the amic acid copolymer can be readily adjusted by further addition of amine or anhydride. Chemical imidization [Step (c)] then locks in the molecular weight and allows a better control of molecular weight than is possible by thermal imidization alone.

Control of molecular weight is particularly important in electronic applications where it is desirable to produce a uniform coating having controlled thickness.

It will be understood by those skilled in the art that a small portion of the polyamic acid may be present in the polyimide product.

The range of aromatic monomer that may be included with the aliphatic monomer in step (a) is from about 0 to about 99 mole percent. Preferably, up to about 50% of the aromatic dianhydride component is added in step (a) with less than 50% more preferred. When making a random copolymer it is preferred that none of the aromatic monomer is included in the azeotropic dehydration step (a). To achieve different polymer structures, one may add a portion of the aromatic dianhydride and/or withhold a portion of the diamine from the thermal imidization step to form different block polymer sequences. When the aliphatic dianhydride is BCOE and the diamine is DMDE it is useful to include a portion of the aromatic dianhydride in step (a) to help alleviate any solubility problems associated with the BCOE/DMDE intermediate. While it may be possible to make the polyimides of the present invention using only thermal imidization, we have found that this results in having less control of the molecular weight of the polymer, incomplete imidization, and poor ability to scale up the reaction to produce the larger quantities of polyimide needed for commercial use due to the heat-transfer problems which occur in a viscous solution. Reaction conditions for chemical imidization are contained in the examples and are well-known to those skilled in the art.

An imidization catalyst may be used in the imidization step of the process. Both basic and acidic catalysts may be used. Examples of basic catalysts are isoquinoline and 1-methylbenzimidazole. Acetic acid may be used as an acidic catalyst. Excess amine may also function as a catalyst.

A composition of a polyimide of the present invention in solution solvent is spread on a substrate to form a coating. Further, multiple layers of polyimide can be interspaced with a conducting material such as metal, or other materials to form a multilayer structure. For example, the polymer layers can act as an interlayer dielectric insulating material where layers of conducting material, for example copper or aluminum patterns, are interspaced with layers of the insulating polymer. Further, many such layers can be assembled with each layer of conducting material separated by a layer of polymer.

If vias (or openings) are desired, the polyimide coating is shielded with a mask containing a pattern of openings, and the polyimide. is exposed to actinic radiation through the openings in the mask. Thus, the polyimide is photochemically altered such that, depending on the particular polymer, the areas that were exposed to actinic radiation are insoluble. Vias (or openings) can be created by taking advantage of this selective insolubility to dissolve the soluble polymer and rinse it away with one or more rinses of one or more rinse compositions, thereby leaving a pattern of insoluble polymer on the substrate (i.e., developing a negative image of the mask). The process of creating vias is generally referred to as etching. The vias (or openings) through the polymer coating are necessary to permit access for electrical connections between layers of conducting material or between the substrate and the outside environment.

Thus, etching can be viewed as a process to dissolve selected areas of polyimide coating on a substrate with an appropriate solvent to form vias (or openings) in the coating. In general, the etching process leaves areas of undissolved polyimide over some of the substrate while dissolving and removing polyimide over other areas of the substrate, to form a pattern of polyimide.

The dianhydride component and diamine component typically are allowed to react in the presence of a polar aprotic solvent to provide a polyamic acid solution. The stoichiometric ratio of the total diamine and the total dianhydride concentrations of the polymer ranges from about a 10 mole percent excess anhydride to about 1 :1 stoichiometric ratio with excess anhydride preferred, preferably 1.0% excess anhydride. For example, a total dianhydride component made up of a tetravalent photosensitizing moiety and an aliphatic dianhydride with or without 6FDA can range from about 99 to about 1 mole percent tetravalent photosensitizing moiety to from about 0 to about 98 mole percent 6FDA with at least about 1 percent of the aliphatic dianhydride.

Suitable polar solvents include, but are not limited to, N- methylpyrrolidone (NMP), N-cyclohexylpyrrolidone (CHP), dimethylsulfone, tetramethylene sulfone. Chlorobenzene and dichlorobenzene may also be used. Cosolvents or catalysts such as acetic acid, phenols, cresols, or tertiary amines may also be included. Azeotropic solvents generally are C₆- C₉ aromatics including, but not limited to, benzene, toluene or xylenes. The reaction between the aliphatic dianhydride and, optionally, a portion of the aromatic dianhydride monomer, and some or all of the diamine is conducted by heating a mixture of the polar aprotic solvent and an azeotropic co-solvent to a reflux and collecting water in a Dean-Stark trap. After cooling to room temperature, the remaining aromatic dianhydride monomer is added, and the condensation reaction takes place, preferably at less than 50°C in about three hours to several days, more preferably from about 5 to about 24 hours.

The resulting viscous polyamic acid is then chemically imidized. Chemical imidization is generally accomplished using dehydrating agents, such as, for example, acetic anhydride or trifluoroacetic anhydride. Other examples of suitable dehydrating agents can be found in Bessonov, M.I., et al., Polvimides - Thermally Stable Polymers. Consultants Bureau, New York, 76-82 (1987), incorporated herein by reference. A particularly suitable chemical imidization composition is the dehydrating agent, acetic anhydride, used in the presence of a catalyst such as pyridine. Also preferred are 1 ,3- dicyclohexylcarbodiimide (DCC), thionyl chloride, phosphorus trichloride, trifluoroacetic anhydride, and the like.

A solid polymer can be isolated from solution by precipitating the polymeric solution in a solvent in which the polyimide is not soluble, such as for example, alkanes such as pentane, hexane, heptane; alcohols such as methanol, ethanol, propanol; water; ethers such as diethyl ether, and the like. Preferably, the polymer is precipitated with methanol or water, washed with the non-solvent, and dried in air or inert atmosphere (such as nitrogen), or vacuum.

The solid polymer is then dissolved in a suitable solution solvent to form a coating composition. This composition is used to apply the polyimide coating to the substrate. Examples of suitable solution solvents are polar aprotic solvents which can be used by themselves or in mixtures of two or more solvents. Suitable solution solvents, for example, include ethers such as dibutyl ether, tetrahydrofuran, dioxane, methylene glycol, dimethylethylene glycol and dimethyltriethylene glycol; halogenated hydrocarbons such as chloroform, dichloromethylene, 1 ,2-dichloroethane, 1 ,1 ,1-trichloroethane and

1 ,1 ,2,2-tetrachloroethane; carboxylic acid esters and lactones such as ethyl acetate, methyl propionate, ethyl benzoate, 2-methoxyethyl acetate, p- valerolactone, gamma-butyrolactone, and pivalolactone; ketones such as acetone, cyclopentanone, cyclohexanone, methyl ethyl ketone; carboxylic acid amides and lactams such as formamide, acetamide, N-methylformamide, N,N-dιethylformamιde, N,N-dimethylacetamιde (DMAC), N,N-dιethyl- acetamide, gamma-butyrolactam, epsilon-caprolactam, N-methylcaprolactam, N-acetylpyrrolidone, N-methylpyrrohdone (NMP), tetramethylurea and hexamethylphosphoric acid amide; sulfoxides such as dimethylsulfoxide, sulfones such as dimethyl sulfone, diethyl sulfone, trimethylene sulfone, tetramethylene sulfone, tπmethylamine sulfone, and tetramethylene sulfone, amines such as tπmethylamine, triethylamine, N-methylpipeπdine, N- methylmorpholine, and substituted benzenes such as chlorobenzene, nitrobenzene, phenols, cresols, and the like. Preferred solution solvents are those that generally have high boiling points and are polar in nature, such as, for example, NMP, dimethylacetamide, diglyme, gamma-butyrolactone, and N-methylformamide and cyclohexanone More preferred solution solvents are NMP, gamma-butyrolactone, and cyclohexanone

Generally, the polyimide solution will be diluted with the solution solvent, such as NMP, based on the requirements of the coating method to attain the desired thickness of the final coating Typically, solutions of the polyimide are applied to the substrate with solids concentrations from about 1 to about 60 weight percent and preferably from about 5 to about 40 weight percent Clean, dry, high-purity solvent (solution solvent) is generally used as the diluent The diluted solution is generally pressure-filtered to a pore size from about 10 microns to about 0.1 microns before further processing, depending on the application requirements

For application by spin-coating, the polyimide solution can be applied either statically or dynamically. In static application, the polyimide solution is dispensed to a nonrotating substrate and spread across the surface by spinning. In dynamic application, the polyimide solution is dispensed to a rotating substrate In either case, the substrate is spun at a spin speed which is determined from the spin curve for the final coating thickness required The coating is typically between about 1 and about 50 microns in thickness Alternatively, the photoimageable polyimide coating can be applied to suitable carriers, or substrates, by other conventional methods, which can include, but are not necessarily limited to, dipping, brushing, casting with a bar, roller-coating, spray coating, dip-coating, whirler-coating, cascade- coating, curtain-coating, or other methods. The solution solvent can be removed, if desired, by methods known to those skilled in the art.

Examples of suitable carriers or substrates are printed circuit boards, plastics, metal and metal alloys, semi-metals, semiconductors, such as Si, Ge, GaAs, glass, ceramics and other inorganic materials, for example, Si0₂ and Si₃N₄. Further, the substrate can be treated with an adhesion promoter, such as 3-aminopropyl triethoxysilane (APES), cleaned appropriately, or dried (dehydration) to remove moisture on the surface of the substrate before the application of the polyimide coating.

Selected areas of the polyimide coating are then shielded, for example, with a mask, and the unshielded polyimide coating is exposed to actinic radiation to effect crosslinking of the polymer. This photocrosslinking is brought about by actinic, or high-energy, radiation, for example, by light within the region of 600 to 200 nm or the deep ultraviolet region, or by X-rays, laser light, electron beams, and the like. The effect of irradiating the exposed polymer is to cause crosslinking which results in a differential solubility in the polymer coating. In this case, the exposed, irradiated polymer is more resistant towards dissolution as compared with the unirradiated portion. The polyimide is photochemically altered such that the areas that were exposed to actinic radiation are insoluble. Channels or vias can be created by taking advantage of this selective insolubility to dissolve the soluble polymer and rinse it away with one or more rinses of one or more rinse compositions (developers) to develop an image. Hence, when the polymer is developed in an etching (developing) composition, the unirradiated parts will be dissolved away to afford a pattern on the coated polymer. The exact composition of the etching composition and the duration for each step of the developing process are generally empirically determined for each polyimide. Preferred developer depends on coating thickness, feature size, and desired resolution.

In addition, photosensitivity is affected by the molecular weight of a particular polyimide. Thus, the molecular weight must be sufficient to produce a photoimageable polymer. In this case, care must be taken in the preparation of the polymer to produce a polyimide having a number molecular weight average above approximately 10,000 g/mol.

Etching composition applications procedures can include dip-etching and spray-etching. In dip-etching, a substrate is dipped into a container of the etching composition and the polyimide is allowed to dissolve (mechanical or ultrasonic agitation may be used). The polymer effectively dissolves as an infinite dilution. In spray-etching, an etching solution is sprayed on the surface of a polyimide-coated substrate. In this manner, fresh etchant is continually delivered to the surface and dissolved polymer is continually being spun off. A particular etching composition may not work equally well in both the dip-etch and spray-etch method. Also, mechanical agitation and temperature will affect the outcome of the dip-etch application procedure. Preferred temperatures are 20°C or greater with 30°C to 50°C more preferred. Suitable etching (developing) compositions which can be used alone, in combination with another etching composition, or in combination with a suitable rinse composition (see below) include: ethers such as dibutyl ether, tetrahydrofuran, dioxane, dipropylene glycol, dimethyl ether, methyl ether, methyl ether acetate, ethylene glycol, dimethyldiethylene glycol, diethyldiethylene glycol, dimethyltrimethylene glycol; halogenated solvents such as methylene chloride, chloroform, 1 ,2-dichloroethane, 1 ,1 ,1- trichloroethane, 1 ,1 ,2,2-tetrachloroethane; esters and lactones such as ethyl acetate, 2-methoxyethyl acetate, gamma-butyrolactone; amides and lactams such as N,N-dimethylformamide, N,N-diethylformamide, N,N- dimethylacetamide, N,N-diethylacetamide, N-methylpyrrolidone, N- acetylpyrrolidone; sulfoxides such as dimethylsulfoxide; derivatives of benzenes such as chlorobenzene, nitrobenzene, cresols; ketones such as acetone, methyl ethyl ketone, cyclopentanone, cyclohexanone, and the like. The preferred etching compositions are N,N-dimethylformamide, N,N- dimethylacetamide, N-methylpyrrolidone, gamma-butyrolactone, cyclopentanone, cyclohexanone, propylene carbonate, and dipropylene glycol methyl ether. The etching composition may also contain a mixture of solvents and non-solvents, including water.

Suitable rinse compositions which can be used alone or in combination include xylenes, toluene, isopropanol, water, dipropylene glycol methyl ether, and the like. The preferred rinse combinations are toluene, isopropanol and dipropylene glycol methyl ether. The most preferred rinse compositions are isopropanol and dipropylene glycol methyl ether.

Other customary additives which do not have an adverse influence on the photosensitivity of the photoimageable polyimide coating can be incorporated in the coating during preparation of the coating composition. Examples of these additives are delustering agents, flow control agents, fine- particled fillers, flameproofing agents, fluorescent brighteners, antioxidants, light stabilizers, stabilizers, dyes, pigments, adhesion promoters and antihalo dyes. If the polyimide is used in a waveguide, additional additives which do not destroy the function of the waveguide can be incorporated into the polyimide. For example, chromophores with non-linear optical properties, chromophores with electro-optic properties, absorbing dyes, particulant fillers, low molecular weight dopants, and the like.

The procedure for preparation of a layered structure in which the instant polyimides are used, for example as an interlayer dielectric insulating material, includes diluting the polymer solution to the proper viscosity and solids level to obtain the desired coating thickness, filtering, substrate cleaning, applying the adhesion promoter, coating the polymer solution on a surface, thermal curing, applying the top layer of metal or inorganic material, photolithographic patterning of the top layer, and wet or dry etching of the top layer.

Dilution of the polymer solution is based on the thickness requirement of the final coating, the viscosity and solids content of the solution. Clean, dry, high-purity solvent should be used as the diluent. The diluted solutions should be pressure filtered before further processing. Final cure temperatures range from about 150°C to about 400°C.

The cured polyimide surface may be modified before the application of the top layer of metal or other material to enhance the adhesion between the polymer and the top layer, for example, by reactive ion etching (RIE) in a plasma, permanganate or modification by treatment with chromate.

Uses of the polyimides and copolyimides include interlayer dielectrics on silicon and gallium arsenide integrated circuits fabricated with multilevel metal schemes and on multilayer thin-film, high-performance packages; dielectrics in flat panel displays; passivating coatings, thermal-mechanical buffer and alpha-particle protection coatings on I.C.s and other circuitry, masks for multi-layer resist processes; negative profile lift-off processes; harsh processes, such as ion implantation or dry etching; and high aspect- ratio masking processes such as plating, and as dielectrics in printed wiring boards (PWBs).

The polyimides of the present invention are particularly useful as an interlevel dielectric in a multilayer circuit device, for example, in printed wiring boards (PWBs) and multilayer circuit boards. The manufacture of such structures is described in U.S. 5,206,091 ; U.S. 5,268,193; U.S. 4,628,022; U.S. 4,601 ,972 ; and Yutaka, et al. "Surface Laminar Circuit Packaging:, 42nd ECTC, P. 22-27, San Diego, CA (1992) , EPA 0 478 313, and EPA0 609 774. For example, the manufacturing process may begin with a conventional substrate having a desired electrical conductor, (e. g., a metal such as copper or aluminum, or an inorganic material) patterned thereon. An energy sensitive material, e.g., a photosensitive epoxy, is applied to the printed wiring board by any method which produces a uniform coating, e.g. curtain coating. Upon exposure to actinic radiation through a mask, the exposed areas of the coating undergo a modification, e.g. a chemical change such as crosslinking, which in turn modifies the solubility properties of the material. Signal via holes are formed by wet-etching the masked (unexposed) regions of the coating. The mechanical and chemical properties of the coating may be further modified by a blanket exposure of UV radiation with an intensity significantly greater than that used for initial patter delineation. A final bake may also be used to complete the desired modification of the coating prior to deposition of the conductive layer onto the coating. A conductor, e.g., copper, aluminum, or an inorganic material, is deposited directly onto the resulting patterned insulating material. For example, when copper is used, the copper fills the voids in the energy sensitive material formed during the previous patterning steps and thus contacts the initial copper pattern. Additionally, the copper forms a layer on the energy sensitive material. Thus, in one step a copper layer is presented for production of a second suitable patterned copper layer and inter¬ connection between copper layers is provided. The desired pattern is formed in the newly deposited copper layers through conventional photolithography. The energy sensitive delineable material, such as the polyimides of the present invention, is chosen so that after patterning an electrical conductor, e.g., copper, deposited thereupon, it has sufficient adherence to produce a durable bond. By depositing an electrical conductor directly and permanently onto the surface of the irradiated photosensitive material a number of previously required processing steps are eliminated. In a multilayer polyimide integrated circuit configuration, the polyimide layer acts as a dielectric material between layers containing thin film circuitry and should have a high Tg, a high thermal stability and hybrid process compatibility. It is preferred that the polyimide material is photodefinable. For use as a dielectric layer in a multilayer hybrid integrated circuit device the polyimides should be capable of being imaged by means of actinic radiation so as to be able to achieve fine line features and an aspect ratio approaching 1. In addition, the polyimides should be tough enough to withstand thermal cycling specifications and the surface of the dielectric should be metallizible so as to form an adherent circuit pattern thereon. In addition, the dielectric material should have chemical resistance to all chemicals used in further processing steps and should be in a form that can be coated reproducibly and efficiently. The polymeric dielectric material should also have good high voltage breakdown characteristics and be compatible with all other components and materials employed. Preferably, for commercial purposes, good shelf stability and shelf life of the uncured polymer is desirable. The polyimides of the present invention meet the above requirements. The polyimides of the present invention are also useful as alignment layers for liquid crystal display devices. A liquid crystal display device comprises a pair of substrates with a transparent electrode and an orientation layer (alignment layer) disposed thereon, and a liquid crystal composition between the substrates. The alignment layer is in contact with the liquid crystal and sets the orientation of the liquid crystal. In general, an alignment (orientation) layer is disposed on each substrate which has been coated with a transparent electrode such as indium tin oxide. The liquid crystal is disposed between the alignment layers. For example, an alignment layer may be applied by spin coating to an indium tin oxide (ITO) coated glass substrate, baked to remove solvent and/or cure the alignment layer, may be irradiated to affect solubility or tilt angle, and rubbed to produce a desirable orientation. A description of the construction of liquid crystal display devices is found in P.A. Penz, et al., "Digital Displays," Kirk-Othmer. Encyclopedia of Chemical Technology. Vol. 7, John Wiley & Sons, Inc. (1979) The present invention includes a liquid crystal display device having at least one alignment layer comprising a polyimide coating of the present invention.

The following examples will serve to illustrate certain embodiments of the herein disclosed invention. These examples should not, however, be construed as limiting the scope of the invention as there are may variations which may be made thereon without departing from the spirit of the disclosed invention, as those of skill in the art will recognize.

EXAMPLES Unless otherwise indicated, all percents used are weight percents.

All monomers were stored under nitrogen atmosphere. Inherent Viscosity (IV)

Inherent viscosity is determined from 0.5% w/v solution of the polyimide in NMP at 25°C.

Photosensitivity

Photosensitivity is measured as the incident input energy (or dose) per unit area at a particular polyimide thickness that is required to effect crosslinking. Evidence of crosslinking, therefore, indicated that a particular polymer was photosensitive. In this case, the photosensitivity of the polymer is determined by calculating the amount of light that affects the photocrosslinking in the polymer sufficiently to prevent cracking when exposed to developer. In order to determine the photosensitivity of the polymer of interest, a substrate is spin-coated with a polymer solution composed of about 14% solid content, by weight of the polymer, in a processable solvent such as gamma-butyrolactone (GBL).

The polymer is soft-baked at approximately 110°C for approximately 10 minutes. Soft baking depends on the thickness of the polymer film. Generally, a thicker film requires a longer heating time and possibly higher heating temperatures. Typically, the preferred ranges are approximately 90- 170°C and 10-60 minutes.

After a soft bake step, the polymer is exposed to actinic radiation through a multidensity resolution target mask for a predetermined length of time to produce a latent image. The latent image is then developed by contacting the polymer with an experimentally determined mixture of etching composition in order to remove the unexposed polymeric areas. The polymeric layer containing the high-resolution images resulting from the photocrosslinking is then dried under a steady stream of nitrogen and then hard-baked. Photosensitivity is affected by the molecular weight of a particular polyimide. For the polyimides of the present invention it is desirable to adjust the molecular weight to produce a photosensitivity measurement of 2000 mJ/cm² or less (approximately 2 micron thick coating) with a photosensitivity measurement of 1000 mJ/cm² more preferred. In general, a number average molecular weight of at least about 10,000 g/mol is desirable. For photoimageable polyimides, a number average molecular weight sufficient to provide an image resolution of at least 3 microns is desirable.

Polymer Cure

Soft-bake cure is accomplished with a hot plate (Solitec). Hard-bake cure is accomplished in a convection oven under nitrogen.

Image Development

Image development is accomplished by manual agitation in a fresh bath of etching (developing) composition or by spray-etching using the spin- coating equipment (Solitec).

Adhesion Promoter

An adhesion promoter (0.05% solution of 2-aminopropyl triethoxysilane (APES) in a mixture of methanol and water in a 95:5 volume ratio) is applied to each of the substrates before spin-coating the polyimide coating on the substrate.

Microlithoαraphy

Microlithography is performed in a clean room using a Karl Suss lithographic system (Model MJB3). The light source is broadband (lambda > 330 nm) where the incident light intensity is automatically regulated to a constant value of 10 mW/cm², as measured at 365 nm. The lithographic mask used is a Ditric Series I (Ditric Optics Inc., Massachusetts). Example 1

0.2 MCTC/0.8 BTDA/1.0 DMDE

A solution containing 5-(2,5-dioxotetrahydrofuryl)-3-methyl-3- cyclohexene-1 ,2-dicarboxylic anhydride (MCTC) (Epiclon B-4400) (2.907 g,

0.011 moles) and diaminodurene (DMDE) (9.034 g, 0.055 moles) in N- methylpyrrolidone (140 g) and toluene (35 g) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA) (14.18 g, 0.044 moles) was added and the resulting solution was mixed overnight. To the resulting viscous solution was added acetic anhydride (19.9 g, 0.195 moles) and pyridine (8.3 g, 0.105 moles); mixing was continued overnight. The polymer solution was precipitated in methanol and dried to give 22.9 grams of off-white solids. The IV of the polymer was determined to be 0.74 dl/g.

A 17.5% solution of the polymer (based on weight) was prepared in gamma-butyrolactone (GBL) and spin-coated on a silicon wafer. The coated wafer was soft-cured on a hotplate for 3 minutes at 100°C to produce a 7.14 micron coating. The polymer was exposed through a Stouffer step-wedge mask to 1000 mJ UV, developed with a mixture of 75% NMP and 25% dipropylene glycol methyl ether (DPM) and was found to be solvent resistant at an exposure energy of 630 mJ/cm².

Example 2

0.45 MCTC/0.55 BTDA/1 0 DMDE

In the same manner as Example 1 , MCTC (Epiclon B-4400) (13.08 g, 0.0495 moles), diaminodurene (18.067 g, 0.11 moles) and BTDA (19.675 g, 0.0611 moles) were reacted in NMP (200 ml) and toluene (40 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (30 g) and pyridine (12.8 g) at 70°C for six hours, then precipitated in methanol and dried to give 48.4 grams of off-white solids. The IV of the polymer was determined to be 0.89 dl/g. The exposure level required for solvent resistance of a 7.06 micron coat was 230 mJ/cm².

Example 3

0.8 MCTC/0.2 BTDA/1.0 DMDE

In the same manner as Example 1 , MCTC (Epiclon B-4400) (23.25 g,

0.088 moles), diaminodurene (18.068 g, 0.11 moles) and BTDA (7.089 g, 0.022 moles) were reacted in NMP (200 ml) and toluene (60 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (27.6 g) and pyridine (11.5 g) overnight at ambient temperature, then precipitated in methanol and dried to give 42.4 grams of off-white solids. The IV of the polymer was determined to be 0.30 dl/g. The exposure level required for solvent resistance of a 4.3 micron coat was 3780 mJ/cm².

Example 4

0.45 BCOE/0.55 BTDA/1.0 DMDE

A solution containing bicyclo[2,2,2]oct-7-ene-2, 3, 5, 6-tetracarboxylic dianhydride (BCOE) (12.41 g, 0.05 moles) and diaminodurene (18.07 g, 0.11 moles) in N,N-dimethylacetamide (DMAC) (200 ml) and toluene (60 ml) with 1-benzyl-2-methylimidazole catalyst (5 ml) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, BTDA (19.33 g, 0.06 moles) was added and the resulting solution was again heated to an azeotropic reflux. The resulting viscous solution was precipitated in methanol and dried to give 37.7 grams of off-white solids. The IV of the polymer was determined to be 0.81 dl/g. The exposure level required for solvent resistance of a 6.4 micron coat was 1380 mJ/cm². Example 5

0.225 MCTC/0.225 BCOE/0.55 BTDA/1.0 DMDE

A solution containing BCOE (6.14 g, 0.025 moles), MCTC (Epiclon B-

4400) (6.54 g, 0.025 moles), BTDA (13.23 g, 0.041 moles) and diaminodurene (18.07 g, 0.11 moles) in NMP (200 ml) and toluene (60 ml) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, BTDA (6.62 g, 0.021 moles) was added and the resulting solution was mixed overnight. The resulting polyamic acid was chemically imidized with acetic anhydride (30 g) and pyridine (12.8 g) at 70°C for four hours, then precipitated in methanol and dried to give 47.2 grams of off-white solids. The IV of the polymer was determined to be 0.43 dl/g. The exposure level required for solvent resistance of a 5 micron coat was 530 mJ/cm².

Example 6

0.1 BCOE/0.45 BTDA/0.45 6FDA/1.0 DMDE

A solution containing BCOE (2.314 g, 0.009 moles) and diaminodurene (15.31 g, 0.093 moles) in NMP (83 g) and toluene (17 g) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, BTDA (13.74 g, 0.043 moles), 6FDA (18.63 g, 0.042 moles) and NMP (96 g) were added and the resulting solution was mixed three days at ambient temperature. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (24.3 g) and pyridine (10.6 g) at 50°C for three hours, then precipitated in methanol and dried to give 41.1 grams of off-white solids. The IV of the polymer was determined to be 0.70 dl/g. The exposure level required for solvent resistance of a 6.5 micron coat was 500 mJ/cm². Example 7

0.45 MCTC/0.55 BTDA/0.5 DMDE/0.5 BAAF

A solution containing MCTC (Epiclon B-4400) (6.54 g, 0.025 moles),

2,2-bis(4-aminophenyl) hexafluoropropane (BAAF) (9.19 g, 0.0275 moles) and diaminodurene (4.517 g, 0.0275 moles) in NMP (125 ml) and toluene (45 ml) with isoquinoline catalyst (2.5 ml) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, BTDA (9.84 g, 0.0305 moles) was added and the resulting solution was stirred at ambient temperature. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (19 g) at 60°C for three hours, then precipitated in methanol and dried to give 30.3 grams of off-white solids. The IV of the polymer was determined to be 0.46 dl/g. The exposure level required for solvent resistance of a 7.4 micron coat was 1980 mJ/cm².

Example 8

0.1 MCTC/0.45 BTDA/0.45 6FDA/0.8 DMDE/0.2 BAAF

A solution containing MCTC (Epiclon B-4400) (1.45 g, 0.0055 moles) and BAAF (3.68 g, 0.011 moles) in NMP (125 ml) and p-xylene (45 ml) with isoquinoline catalyst (2.5 ml) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, DMDE (7.23 g, 0.044 moles), BTDA (7.92 g, 0.025 moles) and 6FDA (11.00 g, 0.025 moles) were added and the resulting solution was stirred at ambient temperature. The resulting viscous polyamic acid solution was chemically imidized with acetic anhydride (19 g) at 50°C for seven hours, then precipitated in methanol and dried to give 31.5 grams of off-white solids. The IV of the polymer was determined to be 0.60 dl/g. The exposure level required for solvent resistance of an 4.9 micron coat was 1980 mJ/cm². Example 9

0.45 MCTC/0.55 BTDA/0.5 DMDE/0.5 DATI

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.54 g,

0.025 moles), BTDA (9.84 g, 0.0305 moles), 3'5-diamino-1 ,3,3,6-tetramethyl-

1-(4'-tolyl)indan (DATI) (8.10 g, 0.0275 moles) and diaminodurene (4.517 g,

0.0275 moles) were reacted in NMP (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (19 g) at 70°C for four hours, then precipitated in methanol and dried to give 27.4 grams of off-white solids. The IV of the polymer was determined to be 0.73 dl/g. The exposure level required for solvent resistance of a 6 micron coat was 3780 mJ/cm².

Example 10

0.45 MCTC/0.55 BTDA/0.5 DMDE/0.5 TM-MDA

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.54 g,

0.025 moles), BTDA (9.84 g, 0.0305 moles), 3,3'5,5'-tetramethyl-4,4'- methylenedianiline (TM-MDA) (7.00 g, 0.0275 moles) and diaminodurene

(4.517 g, 0.0275 moles) were reacted in NMP (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18.7 g) at 60°C for four hours, then precipitated in methanol and dried to give 25.2 grams of off-white solids. The IV of the polymer was determined to be 0.65 dl/g. The exposure level required for solvent resistance of a 4.4 micron coat was 1380 m J/cm². Example 11

0.45 MCTC/Q.55 BTDA/1 0 TM-MDA

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.54 9,

0.025 moles), TM-MDA (13.99 g, 0.055 moles) and BTDA (9.84 g, 0.0305 moles) were reacted in NMP (125 ml) and toluene (40 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (19 g) at 55°C for six hours, then precipitated in methanol and dried to give 29.7 grams of off-white solids. The

IV of the polymer was determined to be 1.00 dl/g. The exposure level required for solvent resistance of a 5.7 micron coat was 1980 mJ/cm².

Example 12

0.45 MCTC/0.55 BTDA/1.0 MEDA

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.28 g, 0.0238 moles), diaminomesitylene (8.26 g, 0.055 moles) and BTDA (9.44 g, 0.0293 moles) were reacted in NMP (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (18.2 g) at 60°C for six hours, then precipitated in methanol and dried to give 21.4 grams of off-white solids. The IV of the polymer was determined to be 0.41 dl/g. The exposure level required for solvent resistance of a 7.5 micron coat was 2970 mJ/cm².

Example 13

0.45 MCTC/0.55 BTDA/0.93 DMDE/0.7 PDMS

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.54 g,

0.025 moles), BTDA (9.84 g, 0.0305 moles), bis(aminopropyl)- polydimethylsiloxane (PDMS) (3.81 g, 0.004 moles) and diaminodurene (8.37 g, 0.051 moles) were reacted in N,N-dimethylacetamide (DMAC) (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18 g) at 60°C for five hours, then precipitated in methanol and dried to give 25.8 grams of off-white solids. The IV of the polymer was determined to be 0.51 dl/g. The exposure level required for solvent resistance of a 5.2 micron coat was 960 mJ/cm².

Example 14

0.45 MCTC/0.55 BTDA/0.5 DMDE/0.5 HMDA

In the same manner as in Example 7, MCTC (Epiclon B-4400) (6.54 g, 0.025 moles), BTDA (9.84 g, 0.0305 moles), hexamethylenediamine (HMDA) (3.20 g, 0.0275 moles) and diaminodurene (4.517 g, 0.0275 moles) were reacted in NMP (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18 g) at 60°C for six hours, then precipitated in methanol and dried to give 22.0 grams of off-white solids. The IV of the polymer was determined to be 0.40 dl/g. The exposure level required for solvent resistance of a 3.9 micron coat was 2000 mJ/cm².

Example 15 0.45 THF-DA/0.55 BTDA/DMDE

In the same manner as in Example 1 , tetrahydrofuran-2,3,4,5- tetracarboxylic dianhydride (THF-DA) (5.25 g, 0.025 moles), diaminodurene (9.03 g, 0.055 moles) and BTDA (9.92 g, 0.031 moles) were reacted in NMP (100 ml) and toluene (40 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (15 g) and pyridine (6.3 g) at 60°C for five hours, then precipitated in methanol and dried to give 22.5 grams of off-white solids. The IV of the polymer was determined to be 0.29 dl/g. The exposure level required for solvent resistance of a 3.7 micron coat was 1980 mJ/cm². Example 16

0.45 CPDA/0.55 BTDA/DMDE

In the same manner as in Example 7, cyclopentane-1,2,3,4- tetracarboxylic dianhydride (CPDA) (5.20 g, 0.025 moles), diaminodurene (9.03 g, 0.055 moles) and BTDA (9.84 g, 0.031 moles) were reacted in NMP (125 ml) and toluene (40 ml) with isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (18.4 g) at 55°C for six hours, then precipitated in methanol and dried to give 8.6 grams of off-white solids. The IV of the polymer was determined to be 0. 17 dl/g .

Example 17

0.45 THNSDA/0.55 BTDA/DMDE

In the same manner as in Example 7, 3,4-dicarboxy-1 ,2,3,4-tetrahydro- 1-naphthalenesuccinic acid dianhydride (THNSDA) (7.43 g, 0.025 moles), BTDA (9.84 g, 0.0305 moles) and diaminodurene (9.03 g, 0.055 moles) were reacted in NMP (125 ml) and toluene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18.5 g) at 60°C for five hours, then precipitated in methanol and dried to give 24.6 grams of off-white solids. The IV of the polymer was determined to be 0.59 dl/g. The exposure level required for solvent resistance of a 5.1 micron coat was 500 mJ/cm².

Example 18

0.45 EPTPA/0,55 BTDA/DMDE In the same manner as in Example 7, ethylenediaminetetraacetic anhydride (EDTDA) (6.34 g, 0.025 moles), BTDA (9.84 g, 0.0305 moles) and diaminodurene (9.03 g, 0.055 moles) were reacted in DMAC (125 ml) and xylene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18 g) at 60°C for five hours, then precipitated in methanol and dried to give 19.5 grams of off-white solids.

Example 19

0.15 TMAC/0 425 MCTC/0.425 BTDA/DMDE

A solution of diaminodurene (9.03 g, 0.055 moles) and isoquinoline catalyst (3.8 g) in NMP (92 g) was prepared under nitrogen and cooled to -15°C. To this solution was added trimellitic anhydride acid chloride (TMAC) (1.74 g, 0.0083 moles) in NMP (33 g). After warming to ambient temperature, MCTC (Epiclon B-4400) (6.18 g, 0.023 moles) and xylene (35 ml) were charged and the solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After cooling to room temperature, BTDA (7.62 g, 0.023 moles) was added and the resulting solution was stirred at ambient temperature. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18.4 g) at 60°C for five hours, then precipitated in methanol and dried to give 22.7 grams of off-white solids. The IV of the polymer was determined to be 0.49 dl/g.

Example 20

Hydrogenation of MCTC to MCHA

To a one gallon pressure vessel were charged 25.0 grams of MCTC (Epiclon B4400), 2.5 grams of 10% Pd on carbon catalyst, and 1.9 liters of DMAC. The vessel was pressurized with 30 psi hydrogen and stirred for four hours at ambient temperature. The reaction product was filtered to remove catalyst and evaporated to dryness, leaving 25.3 grams of the product, 5-(2, 5-diketotetrahydrofuryl)-3-methylcyclohexane-1 ,2-dicarboxylic anhydride. A ¹³C NMR showed that the peaks at 126.5 and 129.8 ppm, characteristic of the carbon-carbon double bond, were absent indicating that the double bond was consumed.

Example 21 0.45 MCHA/0.55 BTDA/DMDE

In the same manner as in Example 7, the reaction product of Example 20 (6.54 g, 0.025 moles), BTDA (9.83 g, 0.0305 moles) and diaminodurene (9.03 g, 0.055 moles) were reacted in NMP (125 ml) and xylene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid was chemically imidized with acetic anhydride (18.4 g) at 50°C for six hours, then precipitated in methanol and dried to give 21.9 grams of beige solids. The exposure level required for solvent resistance of a

5. 1 micron coat was 500 mJ/cm².

Example 22

0.1 MCHA/0.45 6FDA/0.45 BTDA/DMDE

A solution containing 5-(2,5-dioxotetrahydrofuryl)-3-methylcyclo- hexane-1,2-dicarboxylic anhydride (MCHA) (24.74 g, 0.093 moles), 6FDA (185.7 g, 0.418 moles), and diaminodurene (114.4 g, 0.697 moles) in NMP (920 g) and toluene (917 g) was prepared at room temperature under nitrogen. The solution was heated to an azeotropic reflux and water was collected in a Dean-Stark trap. After water condensation had ceased, the toluene was distilled off. After cooling to room temperature, BTDA (137.0 g, 0.425 moles), diaminodurene (38.15 g, 0.232 moles), and NMP (1 ,142 g) were added to form a viscous solution. The resulting polyamic acid was chemically imidized with acetic anhydride (274 g) and pyridine (118 g) at 50°C for three hours, then precipitated in methanol and dried to give 467 grams of off-white solids. The IV of the polymer was determined to be 0.68 dl/g. The exposure level required for solvent resistance of a 5 micron coat was 330 mJ/cm². Example 23

Hvdrogenation of bicvclo[2.2.2]oct-7-ene-2.3.5.6-tetracarboxylic di¬ anhydride (BCOEL

Bicyclo[2.2.2]oct-7-ene-2,3,5,6-tetracarboxylic dianhydride is hydro- genated according to the procedure in U.S. Patent No. 5,412,108, incorporated herein by reference in its entirety, to produce the saturated dianhydride (or tetraacid or diester-diacid synthetic equivalents to the dianhydride, depending on hydrogenation conditions). Thus, BCOE (100 g) is dissolved in a mixture of tetrahydrofuran and water (1 :1 by volume, 1.2 liter) and 5% Rh/carbon catalyst on CP-56 carbon (20 grams) obtained from Englehard Corporation is added. The slurry is placed in a glass lined stainless steel autoclave. The autoclave is purged with nitrogen, then pressurized to approximately 1000 psig with hydrogen. The temperature of the autoclave is raised to 60°C and stirred at 1000 rpm for four hours. The reaction mixture is cooled, the autoclave vented and the contents removed. The slurry is filtered to remove catalyst. The filtrate is dried, yielding as the product bicyclo[2.2.2]octane-2,3,5,6-tetracarboxylic acid.

Example 24 The reaction product produced in Example 23 above is incorporated into a polyimide following the procedure of Example 21. The hydrogenated reaction product (7.16 grams, 0.025 moles), BTDA (9.83 g, 0.0305 moles) and diaminodurene (9.03 g, 0.055 moles) are reacted in NMP (125 ml) and xylene (45 ml) with an isoquinoline catalyst (2.5 ml) to form a viscous solution. The resulting viscous polyamic acid is chemically imidized with acetic anhydride (18.4 g) at 50°C for six hours, then precipitated in methanol and dried to give the polyimide product.

Comparative Example 1 This comparative example demonstrates that the process and product of Example 4 of U.S. 5,322,549 (Hayes) were not suitable for producing a photosensitive polyimide dielectric coating or a method to prepare one. Note: In Hayes' example, APBP is written as "1 ,4-bis(4-aminophenoxy)biphenyl)". This appears to be improper nomenclature since the name, as written, is a chemically impossible structure; according to the structural and CAS indexing this compound is what we refer to as APBP. The procedure of Example 4 of U.S. 5,322,549 was repeated as follows:

A solution of 4,4'-methylene-bis(2,6-diisopropylaniline) (36.66 grams, 0.10 mole), 4,4'-bis(4-aminophenoxy)biphenyl (APBP)(18.64 grams, 0.05 mole), BTDA (36.61 grams, 0.114 mole), and Epiclon B-4400 (10.00 grams, 0.038 mole) was formed in N-methylpyrrolidone (400 grams) and mixed overnight. The solution did not have the viscosity of a high molecular weight polymer after mixing overnight and so was mixed an additional three days. At this point the IV was measured and determined to be 0.25 dl/g. To this reaction solution was added acetic anhydride (61.24 grams, 0.60 mole) and triethylamine (60.72 grams, 0.60 mole). The resulting reaction solution was mixed overnight at room temperature and precipitated with water in a Waring blender. The resulting solid was filtered, washed twice with water and three times with methanol, then air dried for three days. The solids was further dried in a vacuum oven at 125°C for five hours to yield 97.6 grams product. The product was not soluble at 0.5 grams/dl in NMP so an IV was not determined. The product was not soluble in GBL so evaluating the photosensitivity was not possible. Because of the insolubility of the product it was not possible to form a high quality film.

This invention has been described in terms of specific embodiments set forth in detail. It should be understood, however, that these embodiments are presented by way of illustration only, and that the invention is not necessarily limited thereto. Modifications and variations within the spirit and scope of the claims that follow will be readily apparent from this disclosure, as those skilled in the art will appreciate.

Claims

That which is claimed is:

1. A polyimide coating comprising a polyimide comprising at least one recurring unit selected from the group consisting of

and

coated on a substrate.

2. The polyimide coating of Claim 1 wherein the remainder of recurring units are derived from aromatic dianhydrides including at least one dianhydride containing a photosensitizing moiety, and at least one aromatic diamine which is substituted in the two ortho-positions relative to at least one

N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyi.

3. A polyimide coating comprising a polyimide comprising the following recurring units

(I) wherein R¹ and R² comprise aromatic tetravalent radicals which may be the same or different and wherein at least one of said tetravalent radicals contains a photosensitizing moiety; R³ comprises at least one tetravalent aliphatic radical; Y¹ is at least one divalent radical of an aromatic diamine and comprises at least about 1 mole percent of a divalent radical of an aromatic diamine which is substituted in the two ortho-positions relative to at least one N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyi; and Y² may be the same as or different from Y¹ and is at least one divalent radical of an aromatic or aliphatic diamine, coated on a substrate.

4. The polyimide coating of Claim 3 wherein R¹ is

R² may be the same as or different from R¹ and comprises at least one tetravalent aromatic radical;

R³ comprises at least one tetravalent aliphatic radical; Y¹ is

or mixtures thereof; X, X , X , and X , independently are alkyl of 1 to 6 carbon atoms; Z, Z¹, Z², and Z³ independently are hydrogen or alkyl of 1 to 6 carbon atoms; and the mole ratios of m, n, and o range from about 99-1 :0-98:1-99.

5. The polyimide coating of Claim 3 wherein R¹ is

R² is

R is selected from

and

or mixtures thereof; Y¹ is

Y is the same as Y¹ or is a different aromatic or aliphatic diamine; and the mole ratios of m, n, and o range from about 90-10:0-60:10-90.

6. The polyimide coating of Claim 5 wherein Y¹ and Y² are

7. The polyimide coating of Claim 5 wherein m is from about 40 to about 50 mole percent, n is from about 50 to about 40 mole percent, and o is about 10 mole percent.

8. A process for the preparation of a relief image on a carrier which comprises exposing a coating of a radiation-sensitive polymer applied on said carrier wherein the polymer is a polyimide comprising the following recurring units

(1⁾ wherein R¹ and R² comprise aromatic tetravalent radicals which may be the same or different, and wherein at least one of said tetravalent radicals contains a photosensitizing moiety; R³ comprises at least one tetravalent aliphatic radical; Y¹ is at least one divalent radical of an aromatic diamine and comprises at least about 1 mole percent of a divalent radical of an aromatic diamine which is substituted in the two ortho-positions relative to at least one N atom by alkyl, cycloalkyl, alkoxy, alkoxyalkyl, or aralkyi; and Y² may be the same or different from Y¹ and is at least one divalent radical of an aromatic or aliphatic diamine, imagewise to actinic or high-energy radiation through a photomask, and then removing the non-exposed portions with a developer.

9. A process for preparing a polyimide comprising the following recurring units

^

(i) wherein R¹ is

R² may be the same as or different from R¹ and is at least one aromatic tteettrraavvaalleenntt rraaddiiccaall ooff aann aarroommaattiicc ddiiaannhhyyddrriiddee;; RR^'3 comprises at least one tetravalent radical of an aliphatic dianhydride; Y¹ is

or mixtures thereof; X, X¹, X², and X³, independently are alkyl of 1 to 6 carbon atoms; Z, Z¹, Z², and Z³ independently are hydrogen or alkyl of 1 to 6 carbon atoms; and Y² may be the same or different from Y¹ and is at least one divalent radical of an aromatic or aliphatic diamine; which process comprises:

(a) reacting an aliphatic dianhydride monomer

or a precursor therefor with at least one diamine of the formula

H₂N-Y¹-NH;

and, optionally, at least one diamine of the formula

H₂N-Y²-NH₂ to form imide segments using azeotropic dehydration,

(b) incorporating the thermally imidized product of step (a) into a polyamic acid by adding at least one aromatic dianhydride monomer selected from

and diamine to bring the reaction into stoichiometric balance, and

(c) chemically imidizing the polyamic acid to form a polyimide

10 A process according to Claim 9 for preparing a polyimide comprising the following recurring units

(I) wherein R¹ is

R² ιs

R is selected from

and

or mixtures thereof; Y¹ is

Y² is the same as Y¹ or is a different aromatic diamine, which process comprises

(a) reacting an aliphatic dianhydride monomer

or a precursor therefor with at least one diamine of the formula

H₂N-Y¹-NH₂ and, optionally, at least one diamine of the formula

H₂N-Y -NH₂

to form imide segments using azeotropic dehydration;

(b) incorporating the thermally imidized product of step (a) into a polyamic acid by adding at least one aromatic dianhydride monomer selected from

and diamine to bring the reaction into stoichiometric balance; and

(c) chemically imidizing the polyamic acid to form a polyimide.