WO1990012051A1

WO1990012051A1 - Base-soluble polyimide release layers for use in microlithographic processing

Info

Publication number: WO1990012051A1
Application number: PCT/US1990/001671
Authority: WO
Inventors: Terry Brewer; Tony Flaim; James E Lamb, Iii; Gregg A. Barnes
Original assignee: Brewer Science Inc
Current assignee: Brewer Science Inc
Priority date: 1989-03-30
Filing date: 1990-03-30
Publication date: 1990-10-18
Anticipated expiration: 1991-09-30
Also published as: AU5433890A

Abstract

Base-soluble release layer compositions for microlithographic processing, comprising nonamic acid functionalized polyamic acid/imide resins are disclosed. These materials permit concurrent lithographic development of photoresist and release layers. They also afford effective liftoff, by alkaline media, even after high imidization.

Description

BASE-SOLUBLE POLYIMIDE RELEASE LAYERS FOR USE IN MICROLITHOGRAPHIC PROCESSING

BACKGROUND OF THE INVENTION

The present invention generally relates to new and improved polyamic acid/imide microlithographic com¬ positions , their method of manufacture, and particularly their use in a novel concurrent wet-development and im¬ proved lift-off process .

Photoresist compositions are commonly used in microlithographic processing and generally consist of a diazoquinone photosensitizer and a novolak resin binder. Normally, such compositions are coated onto semiconductor substrates ; and, when exposed to light of the proper wavelength, they are chemically altered in their solubility to alkaline developer solutions . Positive- working photoresists are initially insoluble in the alkaline developer, but after exposure to light, the ex¬ posed regions will dissolve or "wet-develop" in alkaline solution forming indented, micron-size line features . Sub- sequently, for many applications, the undissolved portion of the resist must be stripped from the substrate.

Positive-working novolak photoresists, however, are being increasingly used under conditions which render them insoluble in conventional strippers. Ion implanta¬ tion, plasma hardening, deep UV hardening and other high temperature processing conditions produce, for example, crosslinking reactions within the resist. This makes stripper penetration and resist dissolution, which are es¬ sential to removal of the resists, virtually impossible. Oxidative strippers such as hot sulfuric acid- hydrogen peroxide mixtures can be effective against intractable resists, but removal is often slow or incom¬ plete. Moreover, these treatments are restricted to use on unmetallized substrates. Alternatively, removal of intrac¬ table resists is sometimes possible by soaking in hot chlorinated and/or phenolic solvents. However, toxicity and disposal problems associated with these materials are critical drawbacks to their use.

In the past there have been attempts to remove, otherwise intractable, photoresist compositions from me- talized substrates with safe stripper solvents, devoid of the prior art problems. For example, IBM's U.S. Patent 3,873,361 taught that novolak photoresist hardbaked at 210 degrees centigrade could be stripped with the conventional stripper solvent N-methyl pyrrollidone. Although this liftoff process was specifically designed to accommodate the formation and removal of metallic masking layers above the resist, in practice, it simply failed to work because the N-methyl pyrrollidone did not dissolve the hardened photoresist.

Subsequently, IBM and others developed special solvent-soluble, liftoff (release) layers which were sandwiched between the substrate and either glass (Figures 1 and 2), the photoresist (Figure 3), or the metal mask (Figure 4). Disclosures of such release layer technology may be found in the following list of prior art publica¬ tions: L.J. Fried, J. Havas, J.S. Lechaton, J.S. Logan, G.

Paal and P.A. Totta, IBM J. Res. Develop., 26(3),

362 (1982).

B.J. Lin in Introduction to Microlithographv; Theory, Materials, and Processing; ACS Symposium Series Vol. 219, L.F. Thompson, C.G. Willson, and M.J. Bowen, Eds.; American Chemical Society (Washington); p. 287, 1983. J. M. Moran and D. Maydan, J. Vac. Sci. Technol., 16(6) 1620 (1979).

D.M. Tennant, J. Vac. Sci Technol., Bl(2), 494 (1983). J.A. Underhill, V.C. Nguyen, M. Kerbaugh, and D. Sundling in Advances in Resist Technology and Processing II i985 SPIE Vol. 539; Society of Photo-Optical Instrumentation Engineers (Bellingham); p. 83, 1985.

P. Grabbe, E.L. Hu., and R.E. Howard, J. Vac. Sci. Technol, 21 (1), 33 (1982).

K.G. Sachdev, R. W. Kwong, M.R. Gupta, J.S. Chece, and JS. Sachdev, U.S. Patent 4,692,205 to IBM Cor¬ poration (1987).

H.A. Protschika (IBM), European Patent Application 0 257 255 (1987). One such solvent-soluble release layer material is polysulfone. This material has the advantage of enabling the liftoff of metal mask layer by conventional solvent stripping. Although the material also serves to insulate and protect the metal substrate from attack by harsh oxidative strippers, it all the same requires such harsh strippers to liftoff the photoresist layer if hardened.

Polysulfone can serve to liftoff the photoresist material itself as in the fifth step of Figure 3, but not without other drawbacks.

Such polysulfone release layers are insoluble to con¬ ventional alkaline developing solutions. Accordingly, un¬ like the photoresist they are not "wet-developable". Pat¬ terns in the release layer must be made by "dry- development" with, for example, reactive ion etching. In fact, little if any, commercial use has been made of these special release layers, in large part because they must be dry-developed.

In conventional Microlithography, micron feature sizes developed by dry etching are excessively more expen¬ sive than wet etching. Additionally, the dry developable release layer material requires separate plasma etching equipment in addition to that required to etch the photoresist layer and other layers of multilayer microlithographic processing. The addition of even more equipment and more steps becomes such a serious drawback that those in the art have preferred the toxicity, disposal, and other restrictions associated with employing non-conventional strippers, to remove the photoresist layers, rather than deal with a dry-developable release layers.

Some otherwise acceptable release layer materials do not adhere sufficiently to semi-conductor substrates or are incompatible surfaces for applying resist layers, or other organic or inorganic layers, thereto.

Accordingly, a wet-developable release layer of material that could be co-developed concurrently with the photoresist material, without requiring separate plasma etching equipment, and which could be lifted off by immer¬ sion in more mild and less toxic solvents and which would not errode etalized substrates, while providing good adhesion to semiconductor substrates and a compatible sur¬ face for applying resist layers, or other organic or inor¬ ganic layers, would be a surprising advancement in the art fulfilling a long felt need in the industry.

SUMMARY OF THE INVENTION

It is therefore a principle object of the present in¬ vention to provide a new and improved wet-developable, release layer composition for multilayer microlithography which permits, otherwise insoluble, photoresist layers to be lifted off by mild, non-toxic, conventional strippers. It is another principle object of the present inven¬ tion to provide a polyamic acid/imide release layer which remains soluble in alkaline media even after substantial thermal imidization.

It is a further object of the present invention to provide a new and improved method for making wet- developable release layers in multilayer microlithography concurrently wet-developable with the positive-working photoresist.

It is an additional object of the present invention to provide a new and improved microlithographic process for concurrently wet-developing a photoresist layer along with conventional strippers whereby the need for dry- developed release agents, separate plasma processing equipment, and other additional steps and/or expense is negated.

It is also an object of this invention to provide a new and improved material for liftoff of overlying, non- imaged film.

These objects and others are generally fulfilled by a new and improved polyamic acid (ester)/imide polymer com¬ position with regularly interposed nonamic acidic- functional moieties along the polymer backbone which are abnormal to the amic acid structure. These compositions may be employed as a release layer, sandwiched between a semiconductor substrate and a photoresist layer, concur¬ rently wet-developed with the photoresist, thermally baked, and yet remain soluble in alkaline media for ease of liftoff. The attached drawings, the following descriptions of the drawings, the detailed description of the preferred embodiments, and the examples will more fully explain and illustrate the invention.

DESCRIPTION OF THE DRAWINGS

Figures 1 thru 4 illustrates four alternative prior art processes which employ dry-developable release layer concepts. In each process the photoresist layer is wet- developed, while the release layer is dry-developed with separate plasma etching equipment. Additionally, the photoresist layer, if hardened, must be lifted with harsh strippers which are either toxic and have disposal problems, or would be deleterious to the substrate if the photoresist were removed simultaneously with the release layer.

Figure 1 illustrates the six principle steps of the IBM Metal Liftoff Process using dry-processable release layers.

Figure 2 illustrates the six principle steps of a tri-level process using dry-developable planarizing layers (or very thick release layers).

Figure 3 illustrates the five principle steps of the IBM ion Milling Process for metal inter layers using dry- developable release layer systems. Figure 4 illustrates the seven principle steps of the metal-over-release layer ion planarization mask process.

Figure 5 illustrates the four principle steps of the imaging process for concurrently wet-developing the release layer and its adjacent positive-working photoresist.

Figure 6 illustrates a prior art process for applica¬ tion of prior art polyimide coatings starting with a polyamic acid solution, but which polyimide coatings can not be used as release layers for the photoresist.

Figure 7 illustrates a diagram of the polyamic acid chemistry involved in the general manufacturer of such materials.

Figure 8 illustrates the process of the present in¬ vention using wet-developable release layers to assist the removal of high-temperature baked photoresist.

Figure 9 illustrates ion implantation processes using the wet-developable release layer process of the present invention.

Figure 10 illustrates metal liftoff processes using the wet-developable release layer compositions of the present invention.

Figure 11 illustrates processes using the wet- developable release layers of the present invention as a planarizing film. Figure 12 illustrates processes using the wet- developable film of the present invention to remove an epoxy encapsulant.

Figure 13 illustrates the formulas of two polyimide resins useful as release layers in the present invention.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENTS

The release layer compositions of the present inven¬ tion are generally derived from the polyamic acid chemistry depicted at Figure 7 but with critical dif¬ ferences from past polyamic acid/imides .

In the past , such polyimide coatings have been generally offered for use in microlithographic processes because they have good chemical resistance, high tempera¬ ture stability, and good dielectric properties . They have been used as one of a few organic materials which can re¬ place glass dielectrics and passivation coatings in IC devices . In Figure 7 , the letter R equals

CH₃ β -CH₂-, -o-, -s-, -< - , -s- , and CH₃ 8

other bridging groups,

and the letter n represents the number of repeat units in the polymer and is usually greater than 10. These compositions are not normally spin-coated directly from solution in the polyimide form. Instead they are ap¬ plied by spin-coating in the precursor polyamic acid form. The polyamic acid precursor then is heated to ap¬ proximately 170 degrees C to remove the solvents and to partially imidize the film. This allows a controllable etch rate in alkaline developers as the composition is then patterned along with a photoresist layer. After pat¬ terning and stripping the resist, the polyamic acid film is heated to above 200 degrees C to complete the imidiza- tion and remains on the substrate. There is a strong inverse relationship between tem¬ perature and solubility which renders these prior art polyamic acids of little use in release layer technology. That is, prolonged exposure to temperatures above 150 de¬ grees C and substantial i idization reduce the level of solubility in alkaline media to the point that such polyamic acid/imides would normally be of little use as a release layer.

Nevertheless, in accordance with the present inven¬ tion it has been discovered that polyamic acid/imide com¬ positions, in which acidic functional groups are inter¬ posed at regular positions alone polymer backbone, will impart sufficient solubility in alkaline media to convert these materials into useful release layers even after sub¬ stantial imidization. This occurs with no deleterious ef¬ fects on the co-development rate of small feature size patterns in the preferred embodiments of the present in¬ vention.

The acidic functionalized groups may include, for ex¬ ample, carboxylic acids (-C00H), aromatic hydroxyls (aryl-OH), and sulfonic acids (-S0₃H). Typical acid func¬ tionalized polymers may be seen at Figure 13A and Figure 13B. It is particularly preferred that the acidic func¬ tional moieties be attached to the diamine side of the polyamic acid/imide because it is synthetically more con¬ venient to prepare the functionalized diamines than to prepare functionalized dyanhydrides. It is dLnφortant to know that the polyamic acid/imides of the present invention remain sufficiently soluble in spite of the high thermal imidization which would otherwise render prior art polyamic acid/imides unsuitable as release layers.

Diamines with acidic functionalities suitable in con¬ densation reactions for preparing compositions of the present invention are commercially available. Among the preferred group of such diamines is as follows:

3, 5-diaminobenzoic acid (and other isomers such as the 3, -isome ) ,

3,3'-dihydroxy- ,4'-diaminobiphenyl, o-tolidijqLe disulfonic acid,

2,4-diaminophenol,

3-amino-4-hydroxyphenyl sulfone,

3,3'-dicarboxy-4,4'-diaminobiphenyl,

2,4-diamino-6-hydroxypyrimidine,

2,5-diaminobenzenesulfonic acid.

Many dianhydrides can be used to react with the func¬ tionalized diamines. Suitable dianhydrides include the following:

3,3'4,4'- benzophenone tetracarboxylic dianhydide (BTDA) pyromellitic dianhydride (PMDA)

3,3',4,4' - biphenyl tetracarboxylic dianhydride (BPDA) diphenylsulfone -3,3',4,4'-tetracarboxylic dianhydride. Two especially preferred release layer compositions, shown in Figure 13a and 13b, are copolymers of 3,5- diaminobenzoic acid and BTDA and 3,3'-dihydroxy-4,4'- diaminobiphenyl and PMDA.

Materials resembling the hydroxy polyimide shown in Fig. 13b were recently reported by Khanna and Mueller (10) as being suitable bases for high temperature-stable posi¬ tive photoresists. These materials were based on diamines and dianhydrides with fluorinated bridging groups. They are unsuitable for purposes of the present invention be¬ cause they are soluble in photoresist solvents and can be removed when the resist is spincoated. Moreover, they are extremely expensive.

Only a few combinations of the above diamines and dianhydrides are spincoatable in the form of polyimides. Accordingly, it is particularly preferred in the process of this invention to spincoat the materials as polyamic acids and then thermally cure them to the polyimide form before development.

Preferred solvent systems for polyamic acid prepara¬ tion and spincoating include alkyl amides such as N-methylpyrrolidone and dimethylacetamide, methyl sul- foxide, cyclic ketones such as cyclohexanone, and glymes such as 2-methoxyethyl ether. The development rate of the polyimide release films in aqueous base is highly dependent on the polymer struc¬ ture. Generally speaking, the greater the level of acidic functional moieties on the polyimide, the faster the development rate. Some monomer combinations yield polyimides which develop too fast at desired bake tempera¬ tures and can not be patterned to small feature sizes, for example, 3,5 diaminobenzoic acid and PMDA rather than BTDA baked for 30 minutes at 200 C rather than BTDA. In such instance it is preferred that the development rate be reduced by including other diamine components which do not bear acidic functional groups into the polymer structure. High molecular weight aromatic diamines are particularly useful in this regard since they have a large dilution ef¬ fect on the polymer repeat unit. A large number of such diamine materials have been described in the literature, for example, see the general reference "Polyimides: Syn¬ thesis, Characterization, and Applications," Vols. I & II; K.L. Mittal, Ed.; Plenum Press, New York (1984). A few preferred diamines are , 4 ' -oxydianiline, or ODA, (particularly when copolymerized with PMDA) ,

2,2-bis [4-(4-a_π)_inophenoxy)phenyl]propane, or BAPP, bis[4-(4-aminophenoxy)phenyl]sulfone, or BAPPS.

One particularly preferred embodiment of this inven¬ tion comprises terpolymers of 3,5-diaminobenzoic acid/BTDA/BAPPS wherein the mole ratio of 3,5- diaminobenzoic acid to BAPPS is 2:1 to 4:1. Another way to slow the development rate of the release layer films, but without resorting to structural modifications, is through the use of additives. These in¬ clude compatible polymers with low developer solubility and reactive-low-molecular weight (MW) compounds which are capable of crosslinking the release layer polymer.

Multifunctional epoxides compounds are especially ef¬ fective, low MW additives. In a preferred embodiment of the invention it is not necessary to completely imidize (to more than 50%) release films before they are developed when these additives are present. Unlike other uses of these additives, the extra acidity contributed by any un- cyclized amic acids is apparently compensated for by the additive. A related benefit of additive use is that the curing temperature of the film can be reduced. Traditional bisphenol A-type epoxy resins and cycloaliphatic diepoxides are effective additives when used at 1-20 wt. % based on polyamic acid solids.

An alternative means for reducing the imidization re¬ quirements is to modify the polymer by esterifying acid positions of the amic acid group with either an aromatic or aliphatic alcohol. This causes essentially the same change in solubility as imidization. If esterification is employed, the bake requirements may be reduced to as low as 100 C. Other release layer properties such as high tem¬ perature stability and solubility in alkaline media are unaffected. In accordance with the preferred method of this in¬ vention the polyamic acids are prepared as follows: The diamine (s) is (are) charged into a sealable reactor fitted with a heavy stirrer and nitrogen purge. It is dis¬ solved in a portion of the solvent. The dianhydride is then washed into the stirring diamine solution with the balance of the solvent to give a dianhydride/diamine mole ratio in the range 0.700 - 1.100. Ratios in the range 0.85 - 1.00 are preferred. The solution is allowed to stir at ambient temperature for 24 hours to complete the polymerization. The polymer solids level is usually ad¬ justed to 10-25 wt.%.

The polyamic acid solution is diluted to any con¬ venient level needed to obtain a desired film thickness when spincoated. Additives, if necessary, should be dis¬ solved in the polyamic acid solution using vigorous mixing. After formulation, the products are preferably stored under refrigerated conditions to preserve physical and chemical properties. Thereafter, they may be dis¬ tributed and employed as release layers in microlithographic imaging processes.

The basic imaging process for the polyimide release layer compositions is described in Fig. 5. The processes described in Figs. 8-11 are standard schemes which may use release layer composition of the present invention. The unique properties of base-soluble polyimides also makes them applicable to these, device-related processes where intractable layers must be removed. These processes may or may not entail photoimaging. For example, failure analysis of IC devices often involves stripping an intractable epoxy layer (encapsulant) from the surface of a device before electrical tests can be made. Using a high temperature stable polyimide release layer beneath the epoxy coating would greatly simplify this process (See Figure 12). The life of the device would not be jeopardized by the polyimide release film since it has the requisite thermal and electrical properties to remain within the device.

The use of base-soluble polyimides can also be envi¬ sioned in allied industries such as the manufacture of printed circuit boards, electronic displays, sensors, etc. , where patternable films with good chemical and tem¬ perature resistance are required as an integral part of a device or are needed to simplify the fabrications of a device.

Preferably release film thicknesses in the range 500-10,000A give the desired lithographic performance (that is, they can be imaged to feature sizes as small as one micron) and provide rapid release of positive resists. Various combinations of solution solids content, spincoat- ing speeds, and spinning times will give films in this thickness range. Preferred ranges for these parameters are shown below. spinning speed: 1000-7000 RPM spinning time: 10-180 seconds polymer solids level: 2-30 wt.% The release layer materials are useful on all semi¬ conductor substrates including silicon, silicon dioxide, silicon nitride, silicon carbide, glasses, gallium ar¬ senide, aluminum and other metals. Applications of an adhesion promoter, such as hexamethyldisilazane or an or- ganotrialkoxysilane, to the substrate before coating the release film does not deteriorate performance and may be employed if desired for certain types of lithographic quality.

After spincoating, the release film is preferably baked to cause more than 80% imidization of the film un¬ less esterilled polymers or low MW additives are used. Preferred bake temperatures lie in the range 140 - 250 C. The most preferred temperatures (which provides the best lithographic control) is obtained by correlating the structure of the release polymer and the type of solvents used for spincoating. For example, higher bake tempera¬ tures are required for thick films spun from heavy sol¬ vents such as N-methyl-pyrrolidone.

Convection oven baking, infrared track, and hotplate baking give acceptable results. Oven bake times range from 5-120 minutes; hotplate bake times range from 15-300 seconds. Where desired, imidization can also be achieved by chemical techniques including exposure to gaseous reagents and high energy beams.

Application of the positive photoresist, softbaking, exposure and development may follow the procedures recom¬ mended by the manufacturer of the photoresist. The preferred developers are aqueous solutions of sodium or potassium hydroxides, tetramethylammonium hydroxide, choline hydroxide, and other aqueous alkalies. During the development step, the photoresist is etched away first, exposing the release layer film, which is developed concurrently although second in sequence. The time required to develop the release layer film will depend on its thickness, its thermal history and its structure. Preferably 5 to 120 seconds is sufficient. Overdevelopment is preferably avoided because it will con¬ tinue to undercut beneath the resist (Fig. 5). In liftoff processes (Fig. 10), however, a certain degree of undercut is preferred.

A variety of processing steps may occur before the photoresist is lifted (by dissolving the release layer) . These include substrate etching by wet or dry methods, metal deposition, glass deposition, ion implantation and various high temperature processes. The good chemical resistance and excellent high temperature stability of polyimide release films means that they will pass through these steps largely unaffected.

Release of the resist is accomplished by immersing or spraying the specimen with an alkaline solution that dis¬ solves the release layer component. A room temperature photoresist developer or heated developer comprising aqueous alkali can often serve as the release bath. Some¬ times, at higher processing temperatures, flowed resist may cover the exposed edges of the release layer pattern and impede the penetration of the aqueous alkali release bath into the release layer.

In these instances, it is preferred to add to such a release bath an organic solvent to assist penetration such as glycol ether and/or N-methyl-pyrro1idone. In a par¬ ticularly preferred embodiment, a nonaqueous alkaline media is employed comprising organics such as glycol ethers or N-methylpyrrolidone with the nonaqueous alkali ethanolamine. This nonaqueous alkaline component, such as ethanolamine, can be particularly effective to cause dis¬ solution of the .polyimide release films where simple or¬ ganic solvent mixtures axe not effective alone. The immer¬ sion or spray time required to lift the resist varies with the processing conditions. Complete resist removal generally occurs within 1-60 minutes under immersion con¬ ditions.

The processes of the invention have been described with the use of positive photoresists because of their preferred use in the microelectronics industry and the codevelopment objective. In principal, the polyimide release layer materials of this invention can be equally applicable to processes involving electron beam, x-ray, negative and deep UV resists.

These imaging Systems, however, require an organic solvent mixture as the developer rather than aqueous alkali. Since polyimide release films are highly resistant to organic solvents, this does not present a problem, but a two-step development procedure would be needed; first using an organic solvent to develop the resist, then the release layer would be developed using aqueous base.

GLOSSARY OF TERMS

1. positive photoresist - Usually a mixture of an alkali-soluble phenolic resin (novolac) and a photosensi¬ tive dissolution inhibitor. In this form, the mixture can not be dissolved in aqueous alkali to produce an image. Exposure of the resist to UV light causes the photoinhibitor to chemically rearrange forming a car- boxylic acid compound. This compound and the phenolic resin can then be etched away by aqueous base (or developer) to create a positive image. Hence, the term "positive-working" resist.

2. planarization - The ability of a polymer coating to create a level surface when spincoated over irregular topography.

3. strippers - Liquid chemical media used to remove photoresists after processing is finished. Strippers are normally of two types: 1) mixtures of strong aqueous acids or bases with hydrogen peroxide, and 2) organic solvent mixtures which may contain organic bases to speed attack on positive photoresist. 4. developers - For positive resists, generally 1-10% aqueous solution of an alkali metal hydroxide or a tetra- alkylammonium hydroxide. The solutions may also contain buffers and surfactants.

5. wet-developed or wet-processed - Refers to an etching process wherein an aqueous developer is used to pattern a photoresist or release layer film.

6. wet-etching - Any etching process for resist, glass, silicon, etc., which involves a liquid etchant.

7. dry-developed or dry-processed - Refers to an etching process wherein an aqueous developer is used to pattern a photoresist or release layer film or other layer within a masking structure.

8. drv-etching - Any process for resist, glass, silicon, etc. , which uses reactive ions as the active etching species.

9. plasma-developed or plasma-processed - Same as 7, but ion source is nondirectional, i.e., it etches isotropi- cally.

10. rective ion etching or fRIE) - Same as 8, but ion stream is focused so that it etches only in the direction of focus. 11. ion implantation - The use of a high energy ion beam to introduce dopant atoms into semiconductor substrates.

12. ion milling - Similar to 10, but the ion species is concentrated into a high energy beam.

13. glasses and dielectrics - Electrically insulating in¬ organic coatings such as silicone dioxide, silicone car¬ bide, and silicone nitride. Sometimes used as a mask for dry-processing of underlying organic films by oxygen plasma or oxygen RIE. Glasses may be grown at high tem¬ perature, e.g., silicon dioxide coatings at about 1000 C in the presence of water vapor. They may also be produced by chemical vapor deposition (CVD) which involves the in¬ troduction of reactive gases over a substrate at high tem¬ peratures. Still another technique is the application of spin-on-glass coatings (SOG's). SOG's are solutions of or- ganosilicon compounds which form loosely structured glasses when heated to high temperatures.

14. sputtering - A coating process wherein collision of an ionized gas with a metal target causes metal atoms to be transferred from the target onto a substrate.

The following examples and tables are illustrative of the invention. EXAMPLE 1

A variety of wet-developable polyimide release layers were prepared and demonstrated using the following procedures: The materials shown in Table 1 were applied in their polyamic acid form onto these three inch silicon or silicon dioxide wafer substrates by spincoating. Solution solids were adjusted to approximately 6 wt.% to give a 2000-2500A film when spun at 4000 RPM for 60 seconds. The films were then imidized by baking. Bake temperatures were correlated to provide the best lithography at 1-2 micron feature sizes. Positive photoresists (SHIPLEY MICROPOSIT 1470) were spun over the release films at 5000 RPM for 30 seconds and then softbaked at 110 C for 15 minutes on an infrared track. Final resist thicknesses were about 1 micron. The wafers were developed in a room temperature solution of 1:1 (v/v) SHIPLEY Microposit MF-312 and deionized water. (MF-312 is a concentrated aqueous solu¬ tion of tetramethylammonium hydroxide and buffers.) Development time ranged between 5 and 15 seconds to give good quality 2.0 micron geometries concurrently in both the resist and the release layers. The ability to lift the resist after high temperature baking of the release layer-resist composite was used to test release capability. The test specimens were baked at 200 C for 30 minutes in a convection oven after development. This treatment rendered the resist virtually insoluble in con¬ ventional commercial organic strippers, organic solvents TABLE 1

Monomers/ Additive Imidization Release

Substrate Moles Solvent Conditions Conditions Bath Results

(1) Silicon 3.5 diamino- N-Methyl none 210 C for 2 Greater than benzoic/1.0 pyrrolidone 30 minutes 90% release in 5

BTDA/1.0 and diglyme in oven minutes

(2) Silicon Dioxide

(9) Silicon Cycloalipha- 195 C for 30 tic diepoxide minutes in

(Cyracure

6100 by

Union Carbide

10 Wt. 9% of nomer solids

(3) Silicon none 210 C for

(4) Silicon 3.5 diamino none 190 C for benzoic/0.75 30 minutes BAPP/0.25 in oven BTDA/1.0

TABLE 1 Cont'd

Monomers/ Additive Imidization Release Substrate Moles Solvent Conditions Conditions Bath Results

Greater than resist release in 5 minutes

(5) Silicon 190 C for 2 2 minutes on hotplate

(6) Silicon II 190 C for 30 1 minutes in oven

(7) " 3,5 diamino- 200 C for 30 2 benzoic/0.75 minutes in oven r. BAPPS/0.25 BTDA/1.0

(8) " it

(10) " Same as A epoxy 195 C for 30 Sample 1 resin minutes in oven

(Shell

10 wt.% of

% of monomer solids

(11) Silicon 3,5 diamino- Union 175 C for 30

benzoic/0.75 Carbide minutes in oven BAPPS/0.25 Cyracure 6100 BTDA/1.0 Cycloaliphatic diepoxide at

10 wt.% of monomer solids

TABLE 1 Cont'd

Monomers/ Additive Imidization Release

Substrate Moles Solvent Conditions Conditions Bath Results

90% Release

(12) " II 190 C hotplate II II for 2 minutes

(13) " 3.5 diamino- N-Methyl- none 200 C for 2 II benzoic/0.80 pyrrolidone 30 minutes ODA/0.20 and cyclohexa- BTDA/1.0

(14) •' 3.5 diamino- none 50 C for 2 tl benzoic/1.0 30 minutes in

BPDA/1.0 oven ro

(15) " 3.5 diamino- none 200 C at 30 benzoic/0.75 BAPPS/0.25 PMDA /1.0

(16) " 3.3' dihydroxy none -4, 4 '- diani- nobiphenyl/1.0 PMDA /1.0

(17) " Same as sample " none 90% release in 16 but BTDA instead 10 minutes of PMDA

(18) " Same as Same as Same as Same as 70 C No resist removal Sample 1 Sample 1 Sample 1 Sample 1 N-methyl because no alkaline pyrollidone component was present

ADDED EXAMPLES

19. Same criteria and results as Example 7, but polymer prepared in 72/25 diglyme/cyclohexanone. Imidization conditions were 190 °C/s min. on hotplate.

20. Same criteria and results as Example 7, but 1.0 mole PMDA used to make polymer rather than BTDA.

21. Same criteria and results as Example 7, but substrate was silicon dioxide.

22. Same criteria and results as Example 7, but substrate was aluminum.

23. Same criteria and results as Example 7, but imidization conditions were ' 205 °C /2 minutes on a hotplate.

Control wafers with photoresist coated directly over the substrate (no release film was present) showed less than 5% pattern removal within 15 minutes when placed in the release baths. Although the control test are not shown in Table 1 each control used unidentical release bath to its respective release sample. With the release film present, however, more than 90% of the pattern could be lifted within 15 minutes. Two release baths were treated.

The first [BATH 1] was warm developer (60 C); the second

[BATH 2] was a 60/20/20 (v/v/v) mixture of dipropylene glycol methyl ether, N-methylpyrrolidone and ethanolamine heated to 65 C.

Table 1 gives the composition of the release materials and other pertinent conditions which provided the lithographic quality and release results indicated.

What Is Claimed Is:

1. A polyamic acid imide polymer composition, useful as a new and improved wet-developable release-layer in multilayer microlithography, said polymer composi¬ tion comprising; effective amounts of acidic func¬ tional moieties abnormal to the amic acid structure regular positions along the polymer backbone, to effectively impart solubility in alkaline media despite high imidization.

Claims

2. The composition of Claim 1 wherein the acid positions are esterified so as to reduce the degree of im¬ idization needed to provide solubility in alkaline media without degrading high temperature stability.

3. The compositionof Claim 1 wherein the acidic func¬ tional moieties are selected from the group con¬ sisting of carboxylic acid (-COOH), aromatic hydroxyls (aryl-OH), and sulfonic acids (-S0₃H).

4. The composition of Claim 1 wherein the polyamic acid/imide polymer composition comprises condensa¬ tion reaction products of: a) suitable dianhydrides, and b) functionalized diamines containing the acidic functional moieties.

5. The composition of Claim 4 wherein the dianhydrides are selected from the group consisting of: a) pyromellitic dianhydride (PMDA), b) 3 , 3 ' , 4 , ' -benzophenone tetracarboxylic dianhydride

(BTDA) , c) 4,4'-( exafluoroisopropylidene)-bis-(phthalic anhydride) , d) 4, '-oxydiphthalic anhydride, e) sy -3,3', ,4'-biphenyl tetracarboxylic dianhydride (BPDA) and f) diphenylsulfone-3,3' ,4,4'-tetracarboxylic dianhydride 6. The composition of Claim 4 wherein the functionalized diamines are reflected from the group consisting of: a) 3,5-diaminobenzoic acid, or its 3,4-isomer, ) 3,3'-dihydroxy-4,4'-diaminobiphenyl, c) o-tolidine disulfonic acid, d) 2,4-diaminopheno1, e) 3-amino-4-hydroxyphenyl sulfone, f) 3,3'-dicarboxy-4,4'-diaminobiphenyl, e) 2,4-diamino-6-hydroxypyrimidine, and f) 2,5-diaminobenzenesulfonic acid

7. The composition of Claim 4 wherein the dianhydride BTDA is combined with 3,5-diaminobenzoic acid.

8. The composition of Claim 4 wherein the dianhydride PMDA is combined with the diamine 3,3'-dihydroxy- 4,4'-diaminobiphenyl.

9. The composition of Claim 4 further comprising the presence of other aromatid clamines, which do not bear acidic functional moieties, an order to reduce the development rate of the composition.

10. The composition of Claim 9 wherein the aromatic diamines are selected from the group consisting of: a) 4,4'-oxydianiline (ODA), b) 2,2-bis[4-(4-aminophenoxy)phenyl] propane (BAPP) and, c) bis [4-(4-aminophenoxy)phenyl] sulfone (BAPPS) . 11. The composition of Claim 7 further comprising BAPPS at a mole ratio of 3,5-diaminobenzoic acid to BAPPS

OF 2:1 to 4:1.

12 The composition of Claim 1 further comprising the presence of multifunctional epoxides selected from the group consisting of bisphenol A - type epoxy resins and cycloaliphatic diepoxides to slow the development rate without structural modification in the polymer backbone.

13. A method for making wet-developable, polyamic acid/imide microlithographic polymer coatings having improved resistance to thermal insolubiliza- tion in alkaline media comprising: a) reacting in suitable solvent

[i] nonamic acidic functionalized diamines, and

[ii] an effective amount of a suitable dianhydride, to interpose the acidic functional moieties at regular molar equivalent positions along the polymer backbone; b) curing the coating to impart at least 80% imidization; c) photo imaging and developing patterns in the composition with alkaline developer solution at feature sizes as small as 1 micron. wherein the composition remains soluble in alkaline media even after being highly imidized.

14. The method of Claim 13 wherein the reaction product of step (a) is esterified and imidization is less than 50%.

15 The method of Claim 13 wherein the solvent is selected from the group consisting of N-methylpyrrolidone, dimethylacetamide, others a Iky 1 amide s , methyl sulf oxide, cyclohexanone and other cyclic ketones, 2 -methoxyethyl ether and other glymes, and mixtures thereof.

16 The method of Claim 13 wherein the functionalized diamines are selected from the group consisting of: a) 3,5 - diaminobenzoic acid or its 3,4 isomer, b ) 3,3' - dihydroxy - 4,4' - diaminobiphenyl , c) o-tolidine disulfonic acid, d) 2,4 - diaminophenol , e) 3- amino -4 - hydroxyphenyl sulfone, f ) 3,3' - dicarboxy - 4,4'- diaminobiphenyl , g) 2,4 - diamino - 6 - hydroxypyr midine , and h) 2,5 - diaminobenzenesulfonic acid.

17. The method of Claim 13 wherein the dianhydrides are selected from the group consisting of: a) PMDA, b) BTDA, c ) 4 ,4 ' - (hexafluoroisopropylidene) - bis - (phthalic anhydride) , d) 4,4' - oxydiphthalic anhydride, e) BPDA, and f) diphenyl sulfone - 3,3',4,4' - tetracarboxylic dianhydride.

18. The method of Claim 13 wherein the diamine

3,5 - diaminobenzoic acid is reacted with BTDA.

19. The method of Claim 13 wherein the diamine is 3,3'- dihydroxy - 4,4'- diamine and the dianhydride is PMDA.

20. The method of Claim 18 wherein the solvent is a mix¬ ture of N-methylpyrrolidone and diglyme.

21. The method of Claim 13 wherein a substantial amount of the functionalized diamine is replaced with aromatic diamine which does not contain acidic functional moieties. 22. The method of Claim 21 wherein the aromatic diamine is selected from the group consisting of ODA, BAPP, ^• and BAPPS.

23. The method of Claim 22 wherein the reactions product is terpoly er 3,5 - diaminobenzoic acid/BTDA/BAPPS at a mole ratio of 3,5 diaminobenzoic acid to BAPPS of 2:1 to 4:1.

24. The method of Claim 13 further comprising adding mul¬ tifunctional epoxyresins or other polymers to slow the development rate without resorting to struc¬ tural modification.

25. The method of Claim 24 wherein the expoxy resins are either bisphenol A-type epoxy resins or cycloaliphatic diepoxides in amount of from about 1 to 20% by weight of the polyamic acid/imide solids.

26. The method of Claim 24 wherein the copolymer ODA/PMDA is used to slow the development rate.

27. The method of Claim 13 wherein the composition is cured at from about 120 C to about 250 C.

28. The product made by the method of Claim 13.

29. An improved multilayer microlithographic imaging process comprising: a) preparing a releaseable film of acid functional¬ ized polyamic acid/imide resin, b) coating the film onto the surface of a micro¬ electronic substrate, c) imidizing the coating, d) applying a thin positive-photoresist layer to the coated surface, e) sufficiently softbaking the photoresist in order to remove solvents. f) exposing the resist to photoelectromagnetic energy such as light or electron beams, g) concurrently wet-developing photoimage patterns into the photoresist and release layer with aqueous base, h) exposing the codeveloped layers to dry or wet substrate etching processes, ion implantation,

plasma hardening, deep UV hardening, glass or metal sputtering, or other high temperature processing, i) simultaneously lifting off the photoresist and release layers with alkaline media in from about

1 to about 60 minutes; whereby dry-developable release agents, separate release layer imaging equipment, toxic lift-off solvents and extended lift-off procedures are negated despite high temperature processes, photoresist crosslinking, and other conditions which normally render the photoresist and release layers either incapable of codevelopment or unduly resistant to the penetration and dissolution of conventional strippers.

30. The process of Claim 29 wherein a negative photo resist is employed and a two step development.

31. The process of Claim 29 wherein the releaseable film of polyamic acid/imide resin is prepared by con¬ densing in solvent an acid-functionalized diamine and molar equivalents of a suitable dianhydride. 32. The process of Claim 31 further comprising substitut¬ ing an effective amount of another aromatic diamine for a portion of the functionalized diamine, to sufficiently allow the development rate of the polyamic acid/imide resin to pattern small feature sizes as small as 1 micron.

33. The process of Claim 31 wherein the polyamic acid/imide film is prepared with a polymer solids content of from about 10 to 25 wt.%.

34 The process of Claim 29 wherein the polyamic acid/imide film is prepared at a thickness of from about 5Op to about 10,000 angstroms.

35 The process of Claim 29 wherein the coating is ther¬ mally imidized at over 80% at from about 140 to 250 C.

36. The process of Claim 35 wherein the imidization tem¬ perature is correlated as a function of the polymer structure the thickness of the film, and the type of solvent.

37 The process of Claim 29 wherein the aqueous base developer is selected from the group consisting of alkali metal hydroxides, tetramethylammonium hydroxide, choline hydroxide and other aqueous alkalies. 38. The process of Claim 29 wherein either a room tem¬ perature or heated aqueous base developer is used as the alkaline lift-off media.

39 The process of Claim 29 wherein the alkaline liftoff media contains an organic solvent and either an aqueous or nonaqueous alkaline component.

40 The process of Claim 39 wherein the alkaline liftoff media comprises a) nonaqueous ethanolamine as the alkaline com¬ ponent, and b) organic solvent selected from the group consist¬ ing of glycol ethers, N-alkylamides, and mixtures thereof.

41. The process of Claim 29 without exposure, and photoimaging wherein the photoresist layer is re¬ placed by an intractable epoxy encapsulant layer in failure analysis and the release layer improves its liftoff.

42. Articles of Manufacture prepared by the process of Claims 29.

43. Articles of Manufacture prepared by the process of Claim 41.