GB2189493A - Self-developing resist comprising polymer of aliphatic aldehyde or cyclic ether - Google Patents

Abstract

A self-developing radiation resist has high sensitivity to energetic radiation and is resistant to dry etching. The resist comprises a polymer which is end-capped to render it thermally stable and typically is cross-linked to reduce dry etching. The polymer is an oxygen heteroatom, linear chain organic polymer having haloalkyl substituents and/or a polymer derived from cyclic ethers; both are adapted to depolymerize in the absence of photo-initiators. For example, azide groups may be incorporated in the polymer by inclusion of m-azidobenzaldehyde with a haloaldehyde and aldehyde, or cyclic ether, in the monomer mixture. The ether may be chloro methyl trioxocane or 1,3 dioxolane. Alternatively, an azido cross-linking agent, e.g. bis azido formate, is added to the polymer resist solution before it is dried on a substrate and exposed.

Description

SPECIFICATION Self-developing resist The present invention relates to self-developing resists that may be used, for example, in the manufacture of integrated circuits.

Self-developing resists that do not require solvents to develop the latent image of the resist after exposure, have been proposed for use in the microlithography of semiconductors for the manufacture of integrated circuits. By avoiding the use of solvents which can soften and swell the resist materials immediately adjacent to the exposed area resulting in loss of contrast and resolution, self-developing resists offer better imaging and simpler processing than conventional solvent developed resists.

Self-developing resists based on polyaldehydes are known; see Ito et al., European Pat. Pub. 111,655 and 126,214. Aldehydes undergo reversible ionic polymerization-depolymerization equilibria. Many ofthese aldehyde polymers have ceiling temperatures (Tc) below room temperatures, and polymerization must be carried out at temperatures well below,. If the reaction is heated, the polymer rapidly depolymerizes to monomers. In order to prevent depolymerization, the polymers are end-capped by acylation, alkylation, and other suitable reactions. End-capped polyphthalaldehyde with triarylsulfonium or diphenyl iodonium hexafluoroarsenates as light-sensitive initiators have been proposed as microlithogrpahic resists.The initiator on exposure to light generates a strong acid which cleaves the polyaldehyde chain. This re-establishesthe polymerization equilibrium at a temperature above Tc and the polymer unzips into monomericfragments.

Since a single molecule of acid is capable of liberating many monomers, the sensitivity of the resist is dependent on the number of monomers cleaved by the acid and resolution of the resist image depends upon the distance the acid can diffuse. Ito et al., European Pat. Pub. 102,450, describes such a resist with a copolymer of poly(tert-butyloxycarbonyloxy-al pha-al kylstyrene) and tert-butylesters of carboxylic acid with cationic photoinitiators such as diaryliodonium metal halides and a dye to sensitize the photoinitiator. The monome- ricfragments from the unzipping ofthese polymers are difficult to vaporize and heating of the substrate is required to develop the image.As this is a photoresist system, the minimum feature size is limited bythe wavelength of the visible light. Electron beam or X-ray or other energetic radiation is necessary to produce submicron features.

Hatada et al., European Pat. Pub. 96,895, describes an organic copolymer of at least two aliphatic aldehydes of the formula RCHO, where R is alkyl, haloalkyl, aralkyl or halogenated aralkyl. These copolymers form resists that depolymerize to volatile monomers on exposure to energetic radiation such as electron beam, X-ray, gamma ray or ion beam radiation. Tests with these resists have been limited to only very thin coatings because contrast values are relatively low and tend to decrease as the resist film thickness increases. Thus, it is difficult to get good resist thickness and good image contrast. The polymers used as resist films by Hatada et al. depolymerize without exposure at relatively low temperatures, e.g., many have up to 20 percentweight loss at temperatures as low as 135"C.Such materials would not survive the usual pre-baking and post-baking environments (e.g., 100 to 150 degrees C). Three further difficulties with the aldehyde copolymer resist of Hatada et al. are: (1 ) They form relatively soft, easily damaged coatings; (2) a thin residue of resist remaining in the exposed areas has been observed; and (3) radiolysis of the halogenated aralkyl moiety can result in cross-linking.

It is an objective of the present invention to provide a new family of self-developing resists.

Another objective is to provide a resist that is particularly adapted for use in processes wherein radiation exposure is one of electron beam radiation, ion beam radiation, X-ray radiation, and gamma ray radiation.

Still another objective is to provide a resist in the form of polymer which, when irradiated, provides sharp, well-defined demarcation between that portion of a resist surface which is exposed to radiation and the contiguous portion which is not.

Afurther objective is to provide a resist whose unexposed regions can be cross-linked by visible/ultraviolet rays to enable dry etching by plasma or other means of the exposed image regions.

These and still further objectives are addressed hereinafter.

Summary ofthe invention The present invention provides novel, self-developing, dry-etchable resists from substantially amorphous end-capped homopolymers of halogenated aliphatic aldehydes, homopolymers of cyclic ethers, and copolymers of halogenated aliphatic aldehydes, halogenated cyclic ethers, aliphatic aldehydes, and cyclic ethers, and provides a process for applying a resist image to a substrate. More particularly, this invention provides a process for producing a positive, radiation-impaged pattern on a substrate by coating the substrate with an oxygen heteroatom, linear chain polymer comprising preferably one monomer with an a-haloalkyl substituent, and exposing those desired areas ofthe coating outside the desired pattern to radiation which induces the coatings in these areas to depolymerize to monomericfragments.

In another form of the invention the copolymer resist materials include azido monomers to impact high- resist durability in microlithographic processes using the plasma-etch technology.

In yet anotherform of the invention the resist is based on copolymers of cyclic ethers or aliphatic aldehydes, halogenated aldehydes, halogenated cyclic ethers, cross-linked with photoactive additives such as azido derivatives of 1-12 carbon acids to further increase the durability of the resist image.

Brief description ofthe drawing Figures 1A-l E are flow sheets which schematically illustrate the steps A-E of one aspect of this invention.

Wherever a resist according to the present teaching is used in connection with radiation imaging or plasma etching of a substrate, cross-hatching and shading is used to distinguish layers of a semiconductorwafer with resist thereon, ratherthanto indicate section views.

Detailed description ofthe invention The process of this invention utilizes as self-developing resists, oxygen heteroatom, linear chain polymers containing preferably one halogenated monomer. These oxygen heteroatom, linear chain polymers are amorphous, have cryogenicTc's and will depolymerize (unzip) at room temperature. Tcforthese polymers is in the range between about 15 degrees C and -70 degrees C, and mostly is below 0 degrees. The dep- olymerization reaction is initiated by a radiation-induced dissociative electron capture process, or other types of processes which produce ionic intermediates.

The novel self-developing resist homopolymers are: (i) halogenated aliphatic polymers (A) having the general formula:

where n is a large integer, R1 is a haloalkyl substituent selected from the group-CH2 a Xa R', R' is an alkyl substituentwith 1-6 carbon atoms, X is chlorine orfluorine, and a is 1 or2, excluding from this group F2CHand FCHT; (ii) cyclic ethers (B) having the general formula:: -(-CH2-CH2-CK2-CH2-O-)#-; -(-CH2CH2-O-CH2-O-)#-; -(-CH2CH2CH2CH2-O-CH2-O-)#-); (2) -(-CH2CH2-O-CH2CH2-O-CH2-O-)#; (B) (iii) halogenated derivative of cyclic ethers (C) which are cyclic ethers (B) containing haloalkyl substituent(R1) on any ofthe backbone carbon atom.

One type ofthe novel self-developing resistcopolymer is derived from two kinds of aldehyde monomer having the general formula (A),(D),:

wherexandyaresmall integers, R1 is a haloalkyl described above, and 132 is an alkyl selectedfromthegroup consisting ofthe aliphatic primary, secondary, or tertiary alkyl group comprising of 1-7 carbon atoms. The halogenated monomer is preferably at leastfive mole percent and more preferably less than 50 mole percent of the polymer in order to have a polymer which can be dissolved in suitable solvents for application to the substrates.

The incorporation of the a-halogenated monomer greatly improves the sensitivity of the resist to energetic radiation,the solubility of the polymerfor easy application to the substrate, and the stiffness of the backbone ofthecopolymerchain,thusproviding a tougherfilm.

A second type of the novel self-developing resist is copolymers derived from a halogenated aliphatic aldehyde mentioned above and a cyclic ether such as tetrahydrofuran, dioxolane, dioxepane, trioxocane, and their alkyl and halogenated alkyl derivatives having the general formula: (B')x (B")y (A)x(C)y; (B)X(C)y (4) (C')x(C")y where B' and B" are two different monomers belonging to the family of B monomers and C' and C" belong to the family of C monomers.

The polymers used for the resists according to this invention preferably have a molecularweightabove 10,000 and up to 50,000.

The preferred halogenated monomers are those containing alpha chloro and fluoro substituted alkyls. The bromo and iodo substituted alkyl derivatives tend to be nonvolatile; polymers containing such monomers generally have low stability and tend to cross-link when irradiated with energetic radiations.

One of the possible mechanisms to greatly enhance sensitivity of the novel self-developing resist is the electron capture dissociation of the alpha halogenated side chain upon irradiation as exemplified by poly(trichloroacetaldehyde),

and by poly(dioxolane) ee CH2CH2d3 + eO-CH,O- CH2-CH2-O-CH2-O~ Formation of the carbocation initiates the depolymerization of the low-ceiling temperature resist material.

The incorporation ofthe alpha halo substituted aldehyde monomers into the oxygen heteroatom linear chain polymer according to the processes ofthis invention surprisingly provides effectice chain scission at low-radiation energy input for self-development of the resist. The effectiveness of our novel self-developing resist is described by two quantities: number of main chain scission, Gs 100 electron volts absorbed and number of monomers liberated.

Gm 100 electron volts absorbed (5) In the case there is negligible chain transfer during unzipping by irradiation, then Gm Gs Mnl where my is the number average molecular weight of the polymer. For the prior art polymethylmethacnylate Gs = 1.3 and Gm < Gs. However, for a typical resist according to this invention, e.g., a copolymerof acetaldehyde and chloral, the Gm is in excess of 10,000, i.e., more than 10,000 monomers are liberated for every 100 electron volts of energy absorbed and Gs is greater than 7.

The radiation energy for exposure and self-developmentofthe resist may be provided by energetic radiation such as gamma ray, X-ray, electron beam of DUV (deep ultraviolet) radiation. Exposure and selfdevelopment by ray and electron beam are very effective in vacuum, but DUV exposure should be in an atmosphere with oxygen present to obtain efficient chain scission for self-development Because the chain scission is an intramolecular event without the use of separate light initiated catalyst, the resolution of selfdeveloping resists according to this invention is high. In the absence of radiation scattering, the resolution would be in the order of 60-270 Angstrom, depending upon the particular resist.

The polymer and copolymer resists of th is invention may be used without further modification, orthey may be modified to increase the durability of the resist image. One such modification suitable for the preparation of self-developing resists for plasma-etching processes is also novel. The principle of this invention is the incorporation by copolymer of another monomer having a photosensitive moiety Z. Following the imaging process, the unexposed resist is irradiated with ultraviolet light which is absorbed by the chromophore Z generating an active species leading to cross-linking of the resist. The cross-linked resist has increased stability toward plasma during the etching process. Since there are only a few such cross-links per molecule, they do not affect the subsequent stripping process.An alternative of copolymerization of a monomer with a Z moiety is the introduction of a bifunctional additive with two Z moietieswhich, upon photolysis, can actto cross-linkthe resist.

A particularly good Z moiety is an azide group which is readily photolyzed to form nitrene and insertion into the C-H bond of the resist molecule. For instance, the azide group may be incorporated by the inclusion of 1-10 mole percent of m-azido-benzaldehyde in the halogenated monomerto form a copolymer, or with the aldehyde monomer and haloaldehyde monomer to form a terpolymer, or with a haloaldehyde monomer and cyclic ether monomer to form a terpolymer. The copolymers orterpolymers should be dissolved in a suitable solvent, applied to a substrate and dried to form a resist layer. The resist layer is exposed by electron beam radiation or the like to form the resist image. The radiation causes the depolymerization or unzipping ofthe exposed portion of the resist, which self-develops the resist image.Afterwards the resist is exposed to ultraviolet radiation to cause photolysis of the azide functionality resulting in cross-linking of the resist.

In an alternate method for cross-linking, the resist to provide a self-developing resist suitable for use in plasma-etching processes, an azido cross-linking agent may be added to a polymer or copolymer solution before applying the resist solution to a substrate and drying it on the substrate. One such cross-linking agent is bisazidoformate. Thus, ten percent by weight of bisazidoformate is added to a solution of a copolymer of aldehyde and haloaldehyde. After the resist solution is applied to a substrate, dried and exposed (selfdeveloped), the resist image formed is exposed to ultraviolet radiation in air. Irradiating the resist image breaks the azidoformate into nitrene, extracts hydrogen from the polymer chains and forms amide linkages as cross-links.

In order to illustrate the processes of the invention, the utilization of a self-developing resist for a plasmaetching step in the manufacture of an integrated circuit is schematically illustrated in Figures 1A-1 E. Referring to Figure lA, a substrate consisting of a silicon wafer 10 having a layer of silicon dioxide 12 is shown. This substrate is coated with a layer of cross-linkable resist 14. The resist is a self-developing, cross-linkable resist as described above. The resist is applied as a solution and dried by techniques well-known in the microelectronic manufacturing art. An imaging mask 16 is shown in position over the resist.

In Figure 1 B the substrate is shown after exposure to the resist to electron beam or X-ray radiation. The exposed portions of the resist have depolymerized and evaporated in the exposed regions labeled 18 leaving the resist image 17. Figure 1 B illustrates a desired resulting structure: A structure having a well-defined demarcation (i.e., a right angle) between the upper surface marked lA ofthe resist image 17 and its sidewall 1 B and between the side wall 1 B and the bottom labeled 1 C. Ideally, the angles are right angles between contiguous regions on the substrate.

In Figure 1 C the resist image has been cross-linked by exposure to ultraviolet radiation.

In Figure 1 the silicon wafer 10 is shown after the unwanted portions 11 ofthe silicon dioxide layer 12 have been removed by plasma etching through the resist image.

In Figure 1 E the silicon wafer is shown with the silicon dioxide pattern 1 2A on it after stripping the resist image.

Methods for the polymerization or preparation of the amorphous resist polymers are illustrated in the examples.

Example 1 Acetaldehydewas purified by: (1 ) stirring over sodium carbonate monohydratefor3-4 hours; (2)filtering; (3) stirring over calcium hydride; and (4) adding 0.1 percent N,N'-d-beta-naphthyl-p-phenylene diamine as an antioxidant. These purifying steps were done in a dry argon atmosphere at 0'C. The purificationwascomple- ted by distillation of the acetaldehyde under a dry argon atmosphere.

Trichloroacetaldehyde was dried over phosphorus pentoxideand distilled in a dry argon atmospherefollowing the procedure of Vogl, MacromolecularSynthesis, viol.6, p. 49.

Toluenewas washed with concentrated sulfuric acid until colorless. It was neutralized with sodium bi carbonate; washed with distilled water and dried with calcium chloride. After drying it was filtered, refluxed overcalcium hydride and distilled in an argon stream.

A reaction vessel was cleaned byflame drying undervacuum, and then filled with dry argon. Seven-andeighteenth ml (133 millimoles) ofthe distilled acetaldehyde, 80 ml of the distilled toluene, and 1 3.3 my (133 millimoles) oftrichloroacetaldehyde was placed in the vessel and cooled to -78 . After cooling, 0.53 ml (2.8 millimoles) oftriethylaluminum catalyst was added slowly with stirring. The reaction vessel was flushed with argon and then sealed. The mixture was allowed to polymerizeforforty-eight hours at a temperature of -78"C.

The reaction was stopped and the copolymerwas precipitated by adding the reaction mixture to 500 ml of dry methanol (at a temperature of -780C), filtered and dried in vacuo. The yield was 3 grams.

Copolymer (3 g) was dissolved in 20 ml of freshly dried and distilled chloroform. While stirring at 0 C, 30 ml of phenylisocyanate (100fold excess) and a few drops of dibutyl tin dilaurate were added, the latter as a catalyst. The mixture was heated to 60into 70' for one hour. Afterthe reaction was completed, the mixture was cooled to 250C and poured into an excess of methanol kept atO C. The product was filtered and dried in vacuo.

Thecopolymerwasthermallystable upto 1 600C; there was a 20 percent weight loss at 2200C bythermogravimetric analysis. Itwas extremely sensitive to energetic radiation, having a Gm = 16,000.

The copolymer material was amorphous and its composition was determined by elemental analysis and the molar ration of the acetaldehyde monomertothetrichloroacetaldehyde monomer was 50/50. The copolymerwas completely soluble in chloroform, toluene, xylene, methylethylketone, tetrahydrofuran and N-methyl pyrrolidone.

Example2 Example 1 was repeated except that the mole ratio of the monomers placed in the reactor was 70 percent acetaldehyde and 30 percenttrichloroacetaldehyde. The copolymer obtained which contained 45 percent chloral, was readily soluble in chloroform and other solvents. Resist films of the copolymer had good thermal stability and showed excellentself-development properties when tested for self-development by gamma radiation.

Example 3 Example 1 was repeated except that the mole ratio of the monomers placed in the reactor was 75 percent acetaldehyde and 25 percenttrichloroacetaldehyde. The copolymer contained 20 percent chloral and had a weight-average molecularweight, Mw, of 36,500 and a number-average molecularweight, Mnt of 23,500. It was readily dissolved in solvents. Gm = 2,680 (gamma radiation).

Example 4 Example 1 was repeated exceptthatthe mole ratio of the monomers placed in the reactor was 80 percent acetaldehyde and 20 percenttrichloroacetaldehyde. The copolymer contained 9.5 percent chloral and was readily soluble in common solvents and provided an elastomeric resist film. The resistfilm was selfdeveloping.

Example 5 The copolymer of Example 2 was dissolved in chloroform and 5 to 10 percent by weight of bisazidoformate was added to the solution. The dried composition of the aldehyde copolymer and the azidoformate was tested for self-developing characteristics. Gamma irradiation resulted in 860 monomer scissions/l 00 elec- tron volts which was sensitive to the additive concentration. The solution was applied to substrate and dried to form a resist layer. The resist layer was exposed and self-developed by a scanning electron beam. The resist layer was then exposed to ultraviolet radiation to produce a resist image for a plasma-etch process.

Example 6 Example was repeated except the copolymer of Example 3 was mixed with the bisazidoformate. The number of monomer scissions/100 electron volts of gamma radiation was measured: Gm = 2,200.

Example 7 Preparation of m-azidobenzaldehyde. A reactor was charged with 50 ml of water and 65 ml of 12 molar hydrochloric acid and the solution was cooled to - 1 0'C. Fifteen grams of m-aminobenzaldehydewas added to the reactor and the reactorwas flushed with nitrogen while maintaining the temperature at - 10 C. A solution of 10 grams of sodium nitrite dissolved in 50 ml of water was slowly added to the reactor. After all the sodium nitrite was added, the solution in the reactorwasfiltered and 9 grams of sodium azide dissolved in 50 ml ofwaterwas gradually added to the filtrate.The resulting solution was extracted with ether, dried over magnesium sulfate and the excess solvent removed by vacuum. This produced a yellow liquid, mazidobenzaldehyde. The m-azidobenzaldehyde was further purified on a chromatographic column before use.

Example 3 was repeated except that 2 percent of m-azidobenzaldehyde based on the total moles of acetaldehyde and trichloroacetaldehyde was added to the monomer mixture. Aterpolymerwas formed. Aterpolymer solution was cast onto a substrate and dried using conventional microlithographic techniques. A self-developed image was formed by exposure to electron-beam radiation. After the image was formed, it was exposed to ultraviolet light to cross-linkthe resist.

Example 8 Example 7 was repeated except that 10 mole percent of m-azidobenzaldehyde was added to the monomer mixture. A polymer which was suitable for a self-developing resist was formed and the resist was readily cross-linked by ultraviolet irradiation.

Example 9 Aqueous solution of monochloroacetaldehyde was converted to the cyclictrimer by treatment with a large excess of concentrated sulfuric acid. After repeated crystallization it was pyrolyzed under nitrogen in the presence of a mixture of cupric sulfate and calcium chloride as catalyst to yield highly pure and stable monochloroacetaldehyde. Polymerization was carried out in toluene at -78"C using BF3-Et20 as initiator. Polymer of monochloroacetaldehyde was end-capped as described in Example 1. Resist films of the homopolymer had good mechanical and thermal properties. It had a Gm value of 10,800.

Example 10 The polymer of Example 9 was dissolved in chloroform and 2 percent by weight of bisazidoformatewas added to the solution. Resist films casted from it showed good self-developing sensitivity of Gm = 5,270. It possessed improved plasma-etching resistance over the homopolymeralone.

Example 11 Atoluene solution of monochloroacetaldehyde and m-azidobenzaldehydewas copolymerized as described in Example 8. The copolymer containing 95 percentmonochloroacetaldehyde and 5 percent mazidobenzaldehyde had a number of average molecularweight of 22,500. It was readily dissolved in solvents.

Gm = 8,550.

Example 12 Equimolarquantities of monochloroacetaldehyde and diethylene glycol in benzene was refluxedfor twenty-four hours in the presence of p-toluene sulfonic acid to produce 2-chloromethyl-1,3, 6-trixocane.

This monomer was polymerized as described in Example 1. The homopolymer of 2-chloromethyl trioxocane hadaGmvalueofl2,100.

Example 13 The polymer of Example 12was dissolved in methylene chloride and 5 percent by weight of bisazidoformatewas added to the solution. The solution was spin coated onto a silicon wafer to form selfdeveloping plasma-etchable resistfilm.

Example 14 A methylene chloride solution of 60 mole percent chloromethyl trixocane and 40 mole percent of 1,3dioxolane containing BF3-Et20 initiator was polymerized at -10 Cfortwenty-four hours. Addition of methanol precipitated the copolymer containing 65 percent of 1 3-dioxolane and 35 mole percent ofchloromethyl trioxocane. The copolymerwas solvent casted into mechanically-strong and radiation-sensitive resistfilm.

Further modifications ofthe invention will occurto persons skilled in the art and all such modifications are deemed to be within the scope ofthe invention as defined by the appended claims.

Claims (5)

1. A radiation resist having extremely high sensitivity to energetic radiation, which resist is selfdeveloping and is resistant to dry etching, said energetic radiation including electron beam radiation, ion beam radiation, X-ray radiation, and gamma ray radiation, said resist being in the form of a substantially amorphous end-capped polymer that includes at least one of the group consisting of: (1) an oxygen heteroatom, linear chain organic polymer having c-haloalkyl substituents; and (2) a polymer derived from cyclic ethers which are adapted to depolymerize in the absence of photoinitiators.

2. A radiation resist according to claim 1 whose Gs is greater than five and whose Gm is greaterthan 100, wherein number of main chain scission 100 electron volts absorbed and number of monomers liberated Gum 100 electron volts absorbed

3. A microlithographic process for producing a radiation-imaged pattern and the manufacturing of highintensity integrated circuit (VLSI), metal oxide semiconductor (MOS) and random access memory (RAM) devices comprising: (a) providing the substrate with a coating of an oxygen heteroatom, linear chain polymer of claim 1; and (b) exposing areas of the coating outside the desired pattern to radiation energy sufficientto depolymerize the coating in the areas to form volatile monomers, leaving the unexposed polymerto form an imaged-resist pattern, thereby obviating the necessity of developing latent images.

4. A resist according to claim 1 having a Gm greaterthan 100,wherein number of monomers liberated Gm 100 electron volts absorbed

5. A device in the form of at least one of a high intensity integrated circuit (VLSI), metal oxide semiconductor (MOS), and random access memory (RAM), having circuit elements incorporating a substrate having a microlithographic energetic radiation-developed image pattern that defines the positions of said circuit elements and formed using a self-developing, dry etching-resistant, energetic amorphous end-capped polymer selected from at least one of an oxygen heteroatom, linear chain organic polymer having a -haloalkyl substituents, and a polymer derived from cyclic ethers of photoinitiators, said energetic develop- ing radiation including at least one of electron, ion, X-ray and gamma ray radiation.