Production of Electronic Coatings by Spin Coating a Partially Fluorinated Polyamic Acid Composition

Polyimides derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane are useful, inter alia, for the production of electronic coatings on semiconductor wafers, such as polysilicon wafers. One way of forming such coatings on the wafers is to apply a solution of a polyamic acid polymer derived from 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane to the wafer and thereafter bake the coated wafer to cure (imidize) the resin so that the corresponding polyimide is formed. In order to apply solutions of such polyamic acids to the wafers spin coating procedures are used, and in these operations purity, integrity, and uniformity of the resultant coating are essential.

Unfortunately it has been discovered that in humid atmospheres (e.g., about 55% relative humidity or above) solutions of such polyamic acid polymers in some commonly used solvents (e.g., N-methylpyrrolidone) tend to be unstable in the sense that during spin coating operations undesirable precipitate formation occurs on the rotating surface of the wafer. The result is that the coating loses adhesion to the wafer and flies off of it during spin coating.

The need thus exists for solutions of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyamic acid polymers which do not undergo undesirable precipitate formation on rotating wafer surfaces when used in spin coating operations even if conducted under conditions of 55% relative humidity. This invention is deemed to fulfill this need in an effective and efficient manner.

In accordance with this invention there is provided a partially fluorinated polyamic acid polymer composition especially adapted for use in spin coating wafers of semiconductive materials. Such composition comprises a solution of (i) a 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyamic acid polymer having an inherent viscosity in the range of about 0.2 to about 1.5 dL/g--as measured in N-methylpyrrolidone at room temperature (25.degree. C.) at a concentration of 0.5 g/dL--in (ii) a solvent containing at least 10% (preferably at least 15%) by weight of a cycloaliphatic ketone or mixture of cycloaliphatic ketones, such that the solution (a) contains on a weight basis from about 5% to about 40% of such polyamic acid and (b) does not undergo precipitate formation during spin coating in an atmosphere of up to at least about 55% relative humidity. These solutions preferably contain on a weight basis from about 10% to about 25% of such polyamic acid. It is also preferable that the inherent viscosity of the polyamic acid (as measured at a concentration of 0.5 g/dL in N-methylpyrrolidone at room temperature) fall in the range of about 0.3 to about 0.9 dL/g.

Preferably, the polyamic acid is produced in the cycloaliphatic ketone solvent (with or without a co-solvent) although if desired, it may be formed in a different reaction medium, recovered therefrom, and then dissolved in the cycloaliphatic ketone solvent (with or without a co-solvent). Alternatively, the polyamic acid may be formed in the cycloaliphatic ketone solvent and one or more co-solvents may then be added thereto, or the polyamic acid may be formed in the co-solvent(s) and one or more cycloaliphatic ketone solvents may then be added thereto. As is well known, polyamic acids are formed by reacting essentially equimolar proportions of a primary diamine with a tetracarboxylic acid dianhydride. Thus in the practice of this invention it is preferred to produce the solutions by reacting in a reaction solution consisting essentially of one or more cycloaliphatic ketones, an essentially equimolar mixture of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and a 2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane, e.g. 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane, a mixture of 2,2-bis[4-(3-aminophenoxy)phenyl]hexafluoropropane and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane, or most preferably, 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane.

Polyamic acids are converted to polyimides at about 140.degree. C. or above. Thus during the synthesis reaction, the reaction mixture should be kept at a suitably low temperature (usually below about 100.degree. C.) in order to avoid premature formation of excessive quantities of the polyimide. Likewise the solutions of this invention should not be exposed to excessive temperatures until it is desired to convert the polyamic acid-coated article into a polyimide-coated article.

As noted above, the solvent used contains at least 20% by weight of one or more cycloaliphatic ketones, and such ketone(s) may be the sole or the predominant solvent employed in the solution. Typical cycloaliphatic ketones which may used include

cyclohexanone

methylcyclohexanone

2,5-dimethylcyclopentanone

2,6-dimethylcyclohexanone

cycloheptanone

cyclooctanone

4-ethylcyclohexanone

cyclononanone

cyclodecanone

2-cyclopentenone

3,5-dimethyl-2-cyclohexen-1-one

3-ethoxy-2-cyclohexen-1-one

isophorone

1-decalone

2-decalone

and the like, including mixtures of two or more such compounds. Cycloparaffinic ketones are preferred because of the absence of ring unsaturation and their lower cost, cyclohexanone being particularly preferred. Other solvents may be copresent provided they do not, in the concentrations employed, adversely affect the stability and desirable spin coating characteristics of the resultant solution when used in an atmosphere of up to about 55% relative humidity. Generally speaking, other solvents which may be present in appropriate proportions include aromatic hydrocarbons preferably (but not necessarily) having boiling points of at least 110.degree. C. and dipolar aprotic solvents preferably (but not necessarily) having boiling points of at least 150.degree. C.

Examples of such co-solvents include

N,N-dimethylformamide

N,N-dimethylacetamide

N-methylpyrrolidone

dimethylsulfoxide

tetrahydrofuran

diethylene glycol dimethyl ether

triethylene glycol dimethyl ether

dimethoxyethane

acetone

methylethylketone

toluene

xylene

1,2,3,4-tetramethylbenzene

1,2,3,5-tetramethylbenzene

1,2-diethylbenzene

1,3-diethylbenzene

1,4-diethylbenzene

3,5-diethyltoluene

n-butylbenzene

3-propyltoluene

4-propyltoluene

tetrahydronaphthalene

and the like, including mixtures of two or more such solvents. When the cycloaliphatic ketone is a low melting solid at room temperature, it should be mixed with a liquid cycloaliphatic ketone or some other suitable liquid solvent such that the resultant mixture is a liquid at somewhat below room temperature.

Needless to say, the reactants and solvent(s) used in forming the compositions of this invention should have sufficiently high purities to satisfy the requirements of electronic coatings. Thus the solids are preferably recrystallized from highly pure solvents and the liquids are preferably purified by use of distillation or other purification techniques.

In another of its forms, this invention provides a method of forming a coating upon a planar substrate such as a semiconductor wafer. In this method a suitable quantity of a composition of this invention is applied to the central region of the planar surface and the substrate is rotated at a speed sufficient through centrifugal effect to cause the composition to flow outwardly to the perimeter of the surface and in so doing form a substantially uniform liquid coating thereon. Ordinarily rotational speeds in the range of about 1,000 to about 10,000 rpm for periods in the range of about 10 seconds to about 5 minutes are most useful, although departures from either or both of these ranges may be employed where the circumstances warrant or justify such departures. Generally speaking, the higher the rotational speed, the thinner the coating. Once the liquid coating has been formed over the planar surface the coated substrate is heated to an elevated temperature normally in the range of 80.degree. to about 130.degree. C. to dry the coating without destroying its integrity. Thereafter the dried coating is heated to a still higher temperature (e.g., in the range of about 200.degree. C. to about 450.degree. C.) to cure the coating. It has been found that if the dried coated substrate is heated to a temperature of at least about 350.degree. C., the solvent resistance of the coating is improved.

A wide variety of substrates may be coated in this manner, including metals, ceramics, high temperature resistant polymers, intermetallic compounds and compound semiconductors (e.g., GaAs, GaAs.sub.x P.sub.1-x, etc.), and the like. Usually, but not necessarily, the substrate will be in the form of a disc or wafer.

The practice and advantages of this invention are illustrated in the following examples. Examples 1 and 2 typify procedures that may be used to produce the compositions of this invention. Methods by which the compositions of this invention may be utilized in spin coating operations are illustrated in Example 3.

2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyamic acid polymer was produced by adding 21.027 g of 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (98.6% pure) to a stirred solution of 24.887 g of 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in 100 g of N-methylpyrrolidone (NMP) heated to 80.degree. C. Stirring was effected by means of a double spiral agitator, and the system was maintained under a dry nitrogen atmosphere. An additional 8.01 g of NMP was used to rinse the anhydrides into the diamine solution. The reaction mixture was kept at 80.degree. C. for two hours with stirring. The resultant polyamic acid had an inherent viscosity of 0.99 dL/g as measured in NMP at room temperature at a concentration of 0.5 g/dL. Portions of this solution were thoroughly mixed overnight with cyclohexanone to produce polyamic acid solutions in solvent systems composed by weight of 55% NMP-45% cyclohexanone and 68% NMP-32% cyclohexanone. The latter solution was filtered through a 0.2 micron filter using nitrogen pressure.

A 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis[4-(aminophenoxy)phenyl]hexafluoropropane polyamic acid was produced by reaction between 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride and 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane in N-methylpyrrolidone at 70.degree.-76.degree. C. The mixture was stirred by means of a double spiral agitator, and the system was maintained under a dry nitrogen atmosphere. After adding the anhydride to the amine solution, the reaction mixture was kept at 76.degree. C. for two hours with stirring. The resultant polyamic acid had an inherent viscosity of 0.53 dL/g as measured in NMP at room temperature at a concentration of 0.5 g/dL. A portion (60.08 g) of this solution was thoroughly mixed overnight with 14.27 g of cyclohexanone to produce a polyamic acid solution in a solvent system composed by weight of 75% NMP-25% cyoIohexanone. This solution was filtered through a 0.2 micron filter using nitrogen pressure.

Spin coating tests were carried out in order to determine the behavior of different polyamic acid solutions under conditions of controlled relative humidity. The procedure employed in these tests involved use in a room of controllable humidity of a spin coater equipped with a rotatable vacuum chuck for holding the wafer in place in a horizontal position, and a fume exhaust system. Commercially available silicon wafers three inches in diameter were used as the substrates for the coatings. With the wafer held in a horizontal non-rotating position, a quantity of approximately three grams of coating solution was applied to the center of the wafer, and the wafer was then spun at 5000 rpm for one minute. During this time the wafer was subjected to visual observation to determine the characteristics of the coating. Coatings which develop a milky white appearance (precipitate formation) or which spin off pieces of coating are unsatisfactory. Satisfactory coatings show neither such defect; rather, they remain clear, smooth and uniform in appearance.

The results of these tests are summarized in the following table wherein NMP represents N-methylpyrrolidone, CH represents cyclohexanone, No PPT signifies that no precipitate formation occurred during the spin coating operation, and PPT signifies that precipitate formation occurred during the spin coating operation. The percentages shown for the solvent and solids (solids represents the concentration of the polyamic acid in the solvent) are on a weight basis, and the inherent viscosities shown (which are a measure of the molecular weights of the polymeric acid polymers) were measured in N-methylpyrrolidone at room temperature (25.degree. C.) at a polymeric acid concentration of 0.5 g/dL, and are expressed as dL/g.

The coated wafers from Runs 6 and 7 above were cured for 30 minutes at 400.degree. and 350.degree. C., respectively. The polyimide coating of Run 6 was 7.2 microns thick, whereas the thickness of the polyimide coating from Run 7 was 7.5 microns.

While the compositions of this invention are well adapted for use in spin coating applications, they may be used for other purposes, such as in formation of coatings by spray coating or immersion techniques, formation of films by solvent casting procedures, formation of composites by impregnation, and the like.

The foregoing disclosure has been presented for purposes of illustration and not limitation. As can readily be appreciated by those skilled in the art, this invention is susceptible to considerable variation in its practice within the spirit and scope of the ensuing claims.