Electrically conductive polymeric - Patent Review 5198153

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FIELD OF THE INVENTION

This invention relates to electrically conductive polymers, electrically conductive resists, uses thereof and structures fabricated therewith. More particularly, this invention relates to electrically conductive substituted and unsubstituted polyanilines, substituted polyparaphenylenevinylenes, substituted and unsubstituted polythiophenes, polyazines, substituted polyfuranes, substituted polpyrroles, substituted polyselenophene, substituted and unsubstituted poly-p-phenylene sulfides and polyacetylenes formed from soluble precursors and to selectively forming conducting patterns in these materials and the use of these materials as an electrical discharge layer.

BACKGROUND OF THE INVENTION

Electrically conducting organic polymers have been of scientific and technological interest since the late 1970's. These relatively new materials exhibit the electronic and magnetic properties characteristic of metals while retaining the physical and mechanical properties associated with conventional organic polymers. Technological application of these polymers are beginning to emerge. Herein we describe electrically conducting polyparaphenylenevinylenes, polyanilines, polythiophenes, polyfuranes, polypyrroles, polyselenophene, poly-p-phenylene sulfides, polyacetylenes formed from soluble precursors, combinations thereof and blends thereof with other polymers.

Selected regions of films of the materials of the present invention can be made conducting upon the selective exposure to a source of energy. These materials can be used to make a patternable conductive resist material and can be used as an electrostatic discharge layer, or as an electromagnetic interference (EMI) layer on the surface of a substrate. Because of the electrostatic properties of these materials they can be used as an electron beam resist which acts as its own discharge layer. Moreover, since the materials of the present invention can be readily removed from a surface on which they are deposited, they can be used as a removable electrostatic discharge layer disposed on a dielectric surface under electron beam microscopic examination, for example in a scanning electron microscope (SEM). A removable electrostatic discharge layer permits SEM analysis without destroying the sample.

The article entitled "Photoinitiated Doping of Polyacetylene", T. Clarke et al., J.C.P. Chem Comm., 1981, p. 384, describes insoluble polyacetylene selectively doped with proton acids. The polyacetylene is impregnated with one of the group of triarylsulfonium and diaryliodonium which are innert to the polyacetylene but which on irradiation with a uv light undergo a photochemical reaction leading to the doping of the polyacetylene to make it conductive. The impregnated film can be selectively exposed to light through a mask to selectively dope selected regions of the polymer. Polyacetylene is not suitable, as a resist material as are the materials of the present invention, since the polyacetylene is not soluble. For a polymer to be suitable as a resist material it must be capable of having a preselected pattern formed therein, for example on exposure to radiation, with either the exposed or unexposed region being soluble in a solvent while the other of the exposed or unexposed region is insoluble in the solvent.

The article entitled "Photochemically Doped Polypyrrole" of S. Pitchumani et al., J. Chem. Soc., Chem. Commun., 1983, p. 809, describes photochemically doped polypyrrole using as photochemical dopant diphenyliodoniumhexafluoroarsenate. Doping was accomplished by immersion of the polypyrrole substrate in a solution containing the photochemical dopant in methylene dichloride followed by irradiation with a mercury arc. Polypyrrole is not suitable for a photoresist material since the polypyrrole is insoluble.

In both the article of Clarke et al. and the article of Pitchumani et al. an insoluble solid polymer is immersed in a solution containing a dopant material. The dopant material and the solvent are absorbed into the surface of the material. In both cases the solvent in combination with the dopant is exposed to light to render the irradiated part of the polymer electrically conducting. Because of the non-soluble nature of the polymers, the dopants can only be impregnated or absorbed into the surface of the polymer film.

The articles entitled "Polyaniline; Processability From Aqueous Solutions and Effective Water Vapor on Conductivity" to M. Angelopoulous et al., Synthetic Metals, 21 (1987) pp. 21-30, and the article entitled "Polyaniline: Solutions, Films, and Oxidation State" to M. Angelopoulous et al., Mol. Cryst. Liq. Cryst 160-151 (1988), describe a chemically synthesized emeraldine base form of polyaniline which is soluble in various solvents. The emeraldine base is doped by reacting, the emeraldine powder or film with aqueous acid solution for several hours, for example, aqueous acetic acid or aqueous HCl. In contradistinction, according to the present invention where conductive polymeric materials are used as a resist material the dopant reagent and polymer are mixed in a solvent which is thereafter dried to remove the solvents to form a solid solution of the dopant reagent and the polymer. The solid solution is then selectively exposed to energy, for example, electromagnetic radiation, heat or an electron beam, which causes the reagent to decompose to dope those regions of the polymer which are exposed to the energy forming a conductive polymer in exposed regions. In the exposed region the polymer is rendered insoluble and in the unexposed regions the polymer is soluble and can thereby be removed to act as a negative photoresist which is selectively electrically conducting. A resist material of this kind is particularly useful for electron beam lithographic applications since the resist material can be its own discharge layer.

One problem associated with electron beam lithography is charging of the electron beam resist. This is particularly significant in microelectronic applications. In microelectronic applications, a pattern in a dielectric layer or an electrically conducting layer, can be formed by depositing a resist material thereover. A commonly used method of selectively removing the resist material is to selectively expose the resist material to an electron beam. The resist material in the exposed region is either made soluble or insoluble upon exposure to the electron beam radiation. The solubility of the unexposed region is opposite that of the exposed region. Therefore, the exposed or the unexposed region can be removed. The resist material is typically a dielectric. When an electron beam is directed at a dielectric surface, charges from the electron beam accumulate on the surface creating an electric filed which distorts the electron beam on the surface resulting in a loss of precision and displacement errors. To avoid charging the resist, it is common practice in the art to coat the resist, prior to electron beam exposure, with a thin conducting metal layer. Most metals, e.g. Au and Pd, are difficult to remove. In some cases the metal deposition process can degrade the lithographic properties of the resist due to heat and stray radiation during deposition. The polymer discharge layers of the present invention can be deposited by a simple spin coat process, whereas a metal cannot.

According to one aspect of the present invention, a resist material is provided which is on selective exposure to energy, for example electromagnetic radiation, an electron beam or heat, rendered insoluble and at the same time electrically conducting. The exposed regions are insoluble and conducting and the unexposed regions are soluble and nonconducting. When the source of energy generating the pattern is an electron beam, the pattern which is conducting forms an electron discharge path preventing distortion of the writing beam. If the dielectric layer on which the resist is deposited is thin, the discharge path does not have to be grounded. This avoids the requirement of depositing a metal layer to act as a discharge layer.

The polymer materials which are made electrically conductive according to the present invention have additional utility in providing an easily processable and low cost EMI (electromagnetic interference) layer which can provide shielding of electrical components from electrical noise.

The conductive polymer materials of the present invention can also be used as an electrical discharge layer for scanning electron microscopic applications. Typically, when a sample is being analyzed under a scanning electron microscope a thin metal layer is coated onto the sample. A commonly used metal layer is gold or a mixture of gold with other metals. This thin metallic layer acts as an electrical discharge layer to prevent the accumulation of electrical charge on the surface of the sample being examined. When a metallic material is used as an electrical discharge layer it cannot be easily removed, therefore, the sample being examined must be discarded. This is particularly costly in microelectronic applications where it may be desirable to examine a semiconductor chip or semiconductor chip packaging substrate with an electron microscope where the chip or substrate is functional and useful. Depositing a thin metallic layer onto a substrate would render the chip or substrate not usable. By using the electrically conducting polymeric materials according to the present invention, a functional sample can be coated with the polymer, subjected to examination under an electron microscope and thereafter the electrically conductive polymer can be easily removed permitting electron microscopic examination of the functioning part and permitting it to be subsequently used.

It is an object of this invention to provide an electron beam resist material which functions as an electrical discharge layer.

It is another object of this invention to provide a resist material whose solubility and conductivity are dependent upon a dopant species generated by exposure of a dopant precursor to energy.

It is another object of this invention, to provide a method of forming a solid solution of a polymer which is selectively transformed to the conducting state by being selectively exposed to a source of energy.

It is another object of this invention to provide a conductive polymeric electromagnetic interferance layer.

It is another object of this invention, to provide an electrical discharge layer for substrates exposed to electron beam radiation, wherein the discharge layer is removable.

SUMMARY OF THE INVENTION

A broad aspect of this invention is a polymeric material which has selected regions which are electrically conductive and insoluble and wherein non-electrically conductive regions are soluble.

In a more particular aspect of the present invention, the polymeric material has a partially conjugated .pi. system which is extended by the addition of dopants to the polymeric material to provide at least enough .pi. conjugation so that the .pi. conjugated parts of a molecule of the polymer are substantially in contact with the .pi. conjugated parts of an adjacent molecule.

In another more particular aspect of the present invention, the polymer is selected from substituted polyparaphenylenevinylenes, polyazines substituted or unsubstituted polyanilines, substituted polythiophenes, polyazines, substituted or unsubstituted poly-p-phenylene sulfides, substituted polypyrroles, substituted polyselenophene, polyacetylenes formed from soluble precursors, combinations thereof and blends thereof with other polymers.

In another more particular aspect of the present invention, a reagent containing a doping species is contained within the polymer matrix. The doping species is capable of dissociating upon the application of energy, such as electromagnetic radiation, an electron beam and heat. The dissociated species dopes the polymer making it electrically conductive.

In another more particular aspect of the present invention, the dopant species is selected from the group of onium salts, iodonium salts, borate salts, tosylate salts, triflate salts and sulfonyloxyimides.

Another more particular aspect of the present invention, is a method of writing and developing a negative image in a polymer wherein the undeveloped portions of the polymer are electrically conducting.

In another more particular aspect of a method according to the present invention, a solution is formed from a polymer having an extended .pi. conjugated system with a doping species and a solvent. The solution is deposited onto the surface of a substrate, dried to remove the solvent leaving a solid solution of the polymer and the doping species on the surface. The solid solution is selectively exposed to energy, for example, electromagnetic radiation, electron beam radiation or heat, to render the selectively exposed regions electrically conductive. The unexposed regions are thereafter removed in a solvent leaving a pattern of an electrically conductive polymer.

In another more particular aspect of the present invention, a substrate is coated with the electrically conductive polymers according to the present invention, wherein the electrically conductive polymer coating acts as an electrical discharge layer.

In another more particular aspect of the present invention, the polymers with the dopant therein according to the present invention are disposed on the surface of a sample for electron beam microscope examination to function as an electrical discharge layer eliminating distortion from accumulated charge on the electron beam irradiated surface.

These and other objects, features and advantages will be apparent from the following more particular description of the preferred embodiments and the figures appended thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a UV/visible spectra of the emeraldine base form of polyaniline.

FIG. 2 is the UV/visible spectrum of the emeraldine base form of polyaniline containing an onium salt.

FIG. 3 is the UV/visible spectrum of composition of FIG. 2 after exposure to light at about 240 nm.

FIG. 4 shows UV light induced conducting lines in an emeraldine base/onium salt polymer of the present invention.

FIG. 5 shows the pattern shown in FIG. 4 developed.

FIG. 6 shows a low resolution SEM of a dielectric mask.

FIG. 7 shows a low resolution SEM of the mask of FIG. 6 after the mask has been coated with a conductive polyaniline material.

FIG. 8 is a high resolution SEM dielectric mask.

FIG. 9 is a high resolution SEM of the mask of FIG. 8 coated with an electrically conducting polyaniline material.

FIG. 10 is a photograph of a polyaniline polymer containing an onium salt after exposure to an electron beam to form conducting live therein shown as the dark pattern.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention is a polymeric material which can be used as a negative resist material. The polymer is disposed onto a substrate and selectively exposed to a source of energy. The energy renders the exposed regions insoluble as compared to the unexposed regions and the exposed regions are rendered electrically conductive. These materials are useful as negative resists for microelectronic applications, such as patterning a dielectric or metal layer on the surface of a semiconductor chip or semiconductor chip package substrate. The use of these materials as resist material has particular utility in electron beam lithography.

Resist materials are generally polymers which are dielectrics. If an electron beam is used to write a pattern in a dielectric resist material, electrons can accumulate on the surface of the dielectric resist. These electrons create an electric field which results in distortion of the electron beam which is writing the pattern. State of the art electron beam lithography uses electron beams having a cross-sectional size in the order of a micron and less. It does not take much locally accumulated charge to create a distortion in such a small cross-sectional beam. To avoid this problem it is common practice in the art to deposit over a resist material a thin layer of metal. We have discovered a way of avoiding depositing a thin layer of metal onto a resist material to avoid the charging problem. The polymers of the present invention can be used as a direct substitute for the metal layer. The polymer is deposited onto the surface of the electron beam resist and exposed to a source of energy to render it conducting. The conducting polymers act as a discharge layer.

The polymers used to fabricate the resist materials of the present invention contain a partially conjugated .pi. system. A solution is formed of the polymer. To the solution is added a doping species (dopant precursor) which on exposure to energy generates a dopant which dopes the polymer to the conducting state. The addition of the dopant results in an expansion of the extent of the conjugated .pi. system in the individual polymer molecule. It is not necessary to extend the conjugated .pi. system over the full extent of the molecule. It is only necessary to sufficiently extend the .pi. conjugated system of an individual molecule so that after the solvent is removed the .pi. conjugated part of an individual molecule is adjacent to a part of the .pi. conjugated part of an adjacent molecule. In the .pi. conjugated system an electron is essentially delocalized over the entire .pi. conjugated bonds. These electrons are more loosely bond and are available for electrical conduction. When an electric field is applied, and electron can flow along an individual molecule and hop from one molecule to an adjacent molecule in a region where the .pi. conjugated parts of the adjacent molecules overlap.

To form a negative resist according to the present invention a .pi. conjugated system must be soluble when not exposed to the dopant and insoluble when exposed to the dopant. One possible explanation for the change in solubility upon exposure to the dopant is the following. Dopants can be a cationic species. The dopant transfers positive charges to the .pi. conjugated parts of the polymer through protonation, pseudo protonation or oxidation. The polymer now has positive charges on the backbone and then combine with anions to form ion pairs. It is more difficult for a solvent to have enough solvation energy for these ion pairs than for the pristine polymer.

One type of nonconducting polymer useful to practice the present invention is a substituted or unsubstituted polyaniline having the following general formula: ##STR1## wherein each R can be H or any organic or inorganic radical; each R can be the same or different; wherein each R.sup.1 can be H or any organic or inorganic radical, each R.sup.1 can be the same or different; x.gtoreq.1; preferably x.gtoreq.2 and y has a value from 0 to 1. Examples of organic radicals are alkyl or aryl radicals. Examples of inorganic radicals are Si and Ge. This list is exemplary only and not limiting. The most preferred embodiment is emeraldine base form of the polyaniline wherein y has a value of 0.5.

If the polyaniline base is exposed to a cationic species QA, for example a protic acid wherein Q is hydrogen, the nitrogen atoms of the imine part of the polymer become substituted with the Q cation to form an emeraldine salt as shown in the following equation: ##STR2##

When a protic acid HA is used to dope the polyaniline, the nitrogen atoms of the imine part of the polyaniline are protonated. The emeraldine base form is greatly stabilized by resonance effects as is shown in the following sequence of equations: ##STR3##

The charges distribute through the nitrogen atoms and aromatic rings making the imine and amine nitrogens indistinquishable. For the sake of simplicity the above sequence of equations was shown with a protonic acid HA. However, a cationic species represented by Q A can also be used whereas Q is a cation selected from organic and inorganic cations, for example, an alkyl group or a metal, being most preferably H.

The emeraldine base form of polyaniline is soluble in various organic solvents and in various aqueous acid solutions. Examples of organic solvents are dimethylsulfoxide (DMSO), dimthylformamide (DMF) and N-methylpyrrolidinone (NMP). This list is exemplary only and not limiting. Examples of aqueous acid solutions is 80% acetic acid and 60-88% formic acid. This list is exemplary only and not limiting. Substituted polyanilines are soluble in more solvents such as chloroform and methylenechloride.

A powder of the emeraldine base is mixed in a solvent with a radiation sensitive onium salt. Upon exposure to radiation the onium salt produces free acid which protonates the emeraldine base to yield an emeraldine salt. Anything that generates a cationic species upon electromagnetic or electron beam radiation can dope the polyaniline polymer.

Examples of suitable onium salts include aromatic onium salts of Group IV elements discussed in U.S. Pat. No. 4,175,972, disclosure of which is incorporated herein by reference, and aromatic onium salts of Group Va elements discussed in U.S. Pat. No. 4,069,055, disclosure of which is incorporate herein by reference. Aromatic Group IVa onium salts include those represented by the formula:

[(R).sub.a (R.sup.1).sub.b (R.sup.2).sub.c X].sub.d.sup.+ [MQ.sub.e ]-.sup.(e-f)

where R is a monovalent aromatic organic radical, R.sup.1 is a monovalent organic aliphatic radial selected from alkyl, cycloalkyl and substituted alkyl, R.sup.2 is a polyvalent organic radical forming a heterocyclic or fused ring structure selected from aliphatic radicals and aromatic radicals, X is a Group IVa element selected from sulfur, selenium, and tellurium, M is a metal or metalloid, Q is a halogen radical, a is a whole number equal to 0 to 3 inclusive, b is a whole number equal to 0 to 2 inclusive, c is a whole number equal to 0 or 1, where the sum of a+b+c is a value equal to 3 or the valence of X,

d=e-f

f=valence of M and is an integer equal to from 2 to 7 inclusive, e is>f and is an integer having a value up to 8.

Radicals included by R are, for example, C.sub.(6-13) aromatic hydrocarbon radicals such as phenyl, tolyl, naphthyl, anthryl, and such radicals substituted with up to 1 to 4 monovalent radicals such as C.sub.(108) alkyl such as methyl and ethyl, substituted alkyl such as -C.sub.2 H.sub.4 OCH.sub.3, -CH.sub.2 COOC.sub.2 H.sub.5, -CH.sub.2 COCH.sub.3, etc. R.sup.2 radicals include such structures as: ##STR4##

Complex anions included by MQ.sub.e.sup.- (e-f) of Formula I are, for example, Bf.sub.4.sup.-, PF.sub.6.sup.-, SbF.sub.6.sup.-, FeCl.sub.6.sup.-, SbCl.sub.6.sup.-, BiCl.sub.5.sup.-, AlF.sub.6.sup.-3, GaCl.sub.4.sup.-, InF.sub.4.sup.-, TiF.sub.6.sup.-, ZrF.sub.6.sup.-, etc., where M is a transition metal such as Sb, Fe, Sn, Bi, Al, Ga, In, Ti, Zr, Sc, V, Cr, Mn, Cs, rare earth elements such as the lanthanides, for example, Ce, Pr, Nd, etc., actinides, such as Th, Pa, U, Np, etc. and metalloids such as B, P and As.

Groups IVa onium salts included by Formula I are, for example: ##STR5##

Aromatic group Va onium salts include those represented by the Formula:

[(R).sub.a (R.sup.1).sub.b (R.sup.2).sub.c X.sup.1 ].sub.d.sup.+ [MQ.sub.a ].sup.-(e-f)

where R is a monovalent aromatic organic radical selected from carbocyclic radicals and heterocyclic radicals, R.sup.1 is a monovalent organic aliphatic radical selected from alkyl, alkoxy, cycloalkyl and substituted derivatives thereof, R.sup.2 is a polyvalnet organic radical forming an aromatic heterocyclic or fued ring structure with X.sup.1, X.sup.1 is a