water-soluble chromophore containing copolymers for bottom antireflection coating applications in...

Water-Soluble Chromophore Containing Copolymers forBottom Antireflection Coating Applications in Lithography

IAIN MCCULLOCH,1 PING LU,2 MING KANG1

1 Hoechst Research and Technology, 86 Morris Avenue, Summit, New Jersey 07901

2 AZ Electronic Materials, 70 Meister Avenue, Somerville, New Jersey 08876

Received 22 January 1999; accepted 9 April 1999

ABSTRACT: Regulatory concerns in the semiconductor industry from both VOC emis-sions and solvent handling have prompted the desire to introduce water-based formu-lations in spin-coating fabrication processes. As resolution demands for i-line lithogra-phy are driven to finer feature size and architecture, the need for antireflection coatingsas a means of eliminating feature distortions from back reflected light is increasing.Common image irregularities, such as reflective notching and standing wave effects,can be effectively eliminated through application of an antireflection coating of optimalthickness between the resist layer and substrate. This work describes the design anddevelopment of novel side-chain methacrylate copolymers for use in water-based anti-reflection coating formulations. A pendant chromophore is incorporated into the poly-mer structure to allow optimization of the critical coating optical properties, refractiveindex, and absorption coefficient. The polymer solubility parameter and crosslinkingdensity can be tailored by incorporation of appropriate functionality to allow compati-bility with the device fabrication process. Terpolymers were designed, which werewater-soluble, had a thermally activated crosslinking mechanism, and contained aketo-ester azobenzene chromophore that was highly absorbing at i-line wavelength.Optical properties of polymer films were collected by spectroscopic elipsometry andutilized in the design process to identify the optimum chromophore structure. Litho-graphic images, fabricated utilizing these coatings, were of excellent quality, allowingcritical dimension resolutions of less than 0.3 mm. © 1999 John Wiley & Sons, Inc. J ApplPolym Sci 74: 1304–1316, 1999

ANTIREFLECTION COATINGS

Advances in DRAM chip manufacturing placecontinuing pressure on improving resist imageresolution tolerances. This requires exposuresources of smaller wavelength, and both improvedfabrication designs and materials. Resist imagedefects, including both reflective notching andthin film interference effects, are commonly ob-

served as finer resolution features are obtained,caused by back reflected light from substrate to-pography. One way to eliminate these image de-fects is to introduce a bottom antireflection coat-ing (BARC)1–4 between the substrate and resist,as shown in Figure 1. This coating can be appliedby a spin-casting procedure, similar to that usedto apply the resist. In this process, however, thereare significant VOC emissions, and aggressivesolvents are used, leading to environmental andhandling concerns. One solution to this is to applythe BARC coating from a water formulation.

A major design challenge has been to ensurethat there was no dissolution of the postbakedBARC film on exposure to the solvents used in the

Correspondence to: I. McCulloch, (ISP Corporation, Poly-mer Science Department, 1361 Alps Road, Wayne, New Jersey07470).Journal of Applied Polymer Science, Vol. 74, 1304–1316 (1999)© 1999 John Wiley & Sons, Inc. CCC 0021-8995/99/051304-13

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lithography fabrication process. For a water-castfilm, exposure to the resist solvent generally pre-sents little problem due to the large difference insolubility parameters between film and solvent.However, exposure to the aqueous base developercan result in swelling and stripping, unless eitherthe film, during baking, has undergone extensivecrosslinking, or there has been a chemical reac-tion, resulting in a polymer solubility parameterchange. Conversely, a resist solvent-cast film willhave an affinity to the resist formulation dis-pensed onto the BARC film. This can lead to par-titioning or intermixing of the BARC and resistlayers, causing an image defect in processingcalled “foot formation” due to incomplete develop-ment. Either extensive crosslinking or a polarityswitch during baking is needed to minimize theseproblems.

POLYMER DESIGN

Polymers designed for antireflection coating ap-plications require incorporation of discrete func-tionality in the polymer structure in order to ac-commodate a diverse range of performance prop-erties. Side chain copolymers were designed, inwhich one pendant group would contain the ap-propriate chromophore, another would contain acrosslinking functionality, and functional groupstargeted at, enhancing solubility, providing reac-tive sites for crosslinking, and physical proper-ties, would be incorporated throughout the poly-mer. Chromophores utilized in this application

must be highly absorbing at the i-line exposurewavelength and give rise to a high refractive in-dex. This can be accomplished through incorpora-tion of a high density of chromophore, which hasan electronic absorption spectrum maximum atslightly shorter wavelength than 365 nm, thusenhancing refractive index through anomalousdispersion. In order to firstly design the chro-mophore, an understanding of the molecular ori-gins of electronic absorption was necessary. Theprimary electronic transition is of the p–p* class.The intensity and wavelength of this transitionhas been shown to be dependent on, principally, astabilization of the excited state through both pelectron delocalization and intramolecular chargetransfer. Both the presence of conjugated doublebonds or aromaticity, and an asymmetric electrondistribution, (facilitated through functionalgroups, which act as electron donors or acceptors)both enhances the absorption coefficient andshifts the absorption to longer wavelength.Through the use of both donor/acceptor functionalgroups and p orbital conjugation, a molecular en-gineering approach was taken to design a chro-mophore with a large absorption coefficient ati-line wavelength. Chromophores were designed,which had p electron conjugation at an appropri-ate length, and electron donator/acceptor groupsof appropriate strength for absorption at i-line.Although the polymer composition includes themethylol acrylamide monomer, which can un-dergo a self-condensation crosslinking reaction atelevated temperature, enhancement of the filmcrosslinking density is often necessary. This can

Figure 1 Utility of BARCs in elimination of resist image defects caused by reflection.(a) reflective notching, (b) standing wave effects.

BOTTOM ANTIREFLECTION COATING APPLICATIONS 1305

be achieved through the use of external crosslink-ers, such as melamine–formaldehyde resinsadded to the formulation. These resins can un-dergo a condensation reaction with hydroxyl func-tional groups present in either the polymer orresin itself. Other external crosslinking agents,such as bis-epoxies and b-hydroxyalkylamides,can also be used in combination with hydroxyl orcarboxyl functionality present in the polymer.The polymer must also have sufficient solubilityin the appropriate solvent for the formulation.This is facilitated through judicious choice offunctional groups present in the polymer. Hy-droxyl, acrylamide, carboxylate, and sulfonategroups were utilized to enhance the polymer sol-ubility in water. As the exposed lithographic pat-tern is further subjected to a plasma etch, it isnecessary to maximize the polymer film etchrate.5 This can be theoretically determined by theOhnishi Number,6 which relates the etch rate tothe number density of oxygen atoms present inthe polymer. In order to ensure a high etch rate,therefore, oxygen containing functionality, suchas esters and alcohols, should be present in themolecular structure. Finally, device contamina-tion issues prohibit the presence of metallic ionimpurities in the film. The synthetic route, there-fore, must be metal-ion-free, an important consid-eration for the preparation of diazonium salts.

b-KETOESTER METHACRYLATECOPOLYMERS

b-keto ester methacrylate copolymer systems(shown in Fig. 2) were designed and synthesized.This polymer class offers advantages over currentcommercial systems in that the synthetic routecircumvents a graft reaction to an existing poly-mer, through direct copolymerization of the chro-mophore monomer, thereby enabling greater re-producibility and a higher purity product. Addi-tionally, the high chromophore extinctioncoefficient allows sufficient optical absorption toeliminate the requirement of additional free dyein the coating, which is often the source of con-tamination through sublimation. The chro-mophore exhibits p electron delocalization andasymmetry as well as planarization through hy-drogen bonding, which generates a very high ex-tinction coefficient. The optical properties of thecoating are determined by the design of the diazochromophore monomer. The choice of electron-withdrawing functional group on the chro-mophore is designed not only to optimize dyestrength and absorption maximum, but also toinfluence the solubility of the polymer. If thisgroup is either a carboxylic, or sulfonic acid orsalt, then the polymer can be soluble in water;whereas, when the functional group is a carboxy-lic ester, amide, or ketone, then the polymer can

Figure 2 Structure of b-keto ester methacrylate copolymers. Comonomers shown areN-methylol acrylamide and 2-hydroxyethyl methacrylate, but can also include N-vinylpyrrolidone, methacrylic acid, diethyleneglycol monovinylether, and itaconic anhy-dride.

1306 MCCULLOCH, LU, AND KANG

have solubility in common resist solvents, such asethyl lactate, PGME, and PGMEA.

Comonomers are chosen to both further influ-ence the polymer solubility and provide reactivesites for either internal or external crosslinking.They must also be compatible with a solution freeradical copolymerization. Comonomers for poly-merization have included hydroxyethyl methac-rylate, diethylene glycol monovinyl ether, maleicanhydride (followed by hydrolysis), N-vinyl pyr-rolidone, and N-methylolacrylamide. Both the hy-droxyl and carboxylate functionality present inthe comonomer structure are available forcrosslinking reactions with either a melamine–formaldehyde or an epoxy crosslinking resin,whereas the carboxylate functionality is neces-sary for reaction with b-hydroxyalkylamidecrosslinkers. Internal crosslinking can also beachieved through a self-condensation reaction ofthe methylolacrylamide comonomer.

In order to eliminate intermixing and swelling,functional groups, which can undergo either self-crosslinking or can react with an externalcrosslinker, were also incorporated in the copoly-mer structure. Due to shelf life stability consider-ations, the choice of chemistry is restricted tofunctionality, which is inert in aqueous solutionbut can be activated during the thermal bakingprocess. Self-crosslinking can be achievedthrough the condensation reaction of methylol ac-rylamides, which is effective at temperatures over150°C. The efficiency of this reaction is lowered,

however, through the steric effects of the pendantchromophore comonomer, and it is often neces-sary to supplement the self crosslinking groupwith external crosslinking agents. One class ofexternal crosslinker examined was the mel-amine–formaldehyde resins. This class ofcrosslinker can react through a condensation re-action with hydroxyl, carboxylate, or anhydridefunctionality present in the polymer backboneand can also self-condense. Both types of reac-tions give rise to crosslinked thermoset material.At lower pH, this reaction can occur at lowertemperatures, which causes shelf life concerns.Less reactive crosslinking agents, b-hydroxyalky-lamides, were also identified, in which the activehydroxyl group is available for esterification witha carboxylate or anhydride functionality presentin the polymer. This reaction is only efficient forthis polymer system at temperatures of over170°C and is inert at room temperature. The mo-lecular structure of crosslinked BARCs from bothcrosslinking agents is shown in Figure 3(a) and(b).

An optimum polymer was chosen, and hereinreferred to as Aquabar™, as a result of initialcoating and lithographical evaluation, for bothswing curve reduction experiments (Fig. 8), andthe detailed lithographical analysis (Table IV;and Figs. 9 and 10). This was a terpolymer of theb-keto ester methacrylate chromophore, N-meth-ylol acrylamide, and 2-hydroxyethyl methacry-late. The molecular structure and composition is

Figure 3 External crosslinking agents. (a) A melamine-formaldehyde resin structurecrosslinking with the methylol and hydroxy ethyl functionality of a BARC copolymer.(b) A b-hydroxyalkylamide crosslinking with the chromophore carboxylate functional-ity of a BARC copolymer.


shown in Figure 4, where the electron withdraw-ing group (X) was ammonium carboxylate(COO2NH41), and the monomer ratio, as ana-lyzed by 13C-nuclear magnetic resonance (13C-NMR) spectroscopy, was essentially determinedby the monomer feed ratio a : b : c 5 48 : 31 : 21.The synthesis of this particular composition isgiven as an example in the synthetic experimen-tal section.

OPTICAL CHARACTERIZATION

b keto ester chromophore containing copolymershave extremely high absorption coefficients at i-

line wavelength, with absorption maximumsranging from 355 to 410 nm, depending on theelectron-withdrawing group. While the copolymercomposition can be used to tailor absorption coef-ficients, the strength of the electron withdrawinggroup (X) on the chromophore has a large influ-ence on the absorption maximum. For example,incorporation of the nitro group on the chro-mophore, the most effective common electron-withdrawing group, results in an absorption max-imum of about 410 nm, while the less effectivecarboxylic esters give rise to a chromophore ab-sorption maximum of about 355 nm. Neutraliza-tion of the carboxylate and sulfonate acids to formthe ammonium salts had little influence on eitherthe absorption coefficient or absorption maxi-mum. Absorption coefficient data was collectedfrom thin film samples of b-keto ester copolymerswith a range of electron-withdrawing groups, onquartz slides using a standard ultraviolet–visible(UV-vis) spectrophotometer and are shown in Fig-ure 5. Sample thicknesses were made as large aspractically possible in order to minimize any in-terference effects. This was limited, however, bythe power output of the spectrophotometer. As aresult of this analysis, the ammonium carboxy-late electron-withdrawing group was identified ashaving optimum optical characteristics, as well assuitable solubility properties. Subsequent litho-graphical studies utilized a polymer compositioncontaining this chromophore exclusively. Both re-fractive index anomalous dispersion and absorp-tion coefficient (k) can be more accurately deter-mined by spectral elipsometry7; and data col-lected from the Aquabar™ polymer (Fig. 4) is

Figure 5 The electronic absorption spectra of poly-mer films on quartz.

Figure 6 Dispersion curve for the optical constants nand k of Aquabar™.

Figure 4 Molecular structure of Aquabar™.


shown in Figure 6. At i-line wavelength (365 nm),the refractive index was measured to be 1.62 witha corresponding absorption coefficient (k) of 0.32.When the absorption maximum is slightly hypso-chromically shifted from 365 nm, an enhance-ment in refractive index is observed. Both thestrength and width of the absorption peak influ-ence the extent of this refractive index dispersionbehavior. It is necessary to control the refractiveindex of the coating8 to determine the optimalfilm thickness for minimum reflectivity. Thermalstability of the chromophore through the bakingcycles of the lithography process is also necessary.This was studied, and no changes in electronicspectra were observed. Representative data forpolymer samples is collected in Table I.

COATING EVALUATION

To test swelling and/or intermixing of Aquabar™after resist application, the polymer was coatedand softbaked (SB) at elevated temperatures toensure sufficient crosslinking. Both common re-sist casting solvents, for example, ethyl lactateand developer solution (2.38% tetramethyl ammo-nium hydroxide in water), were sprayed onto theAquabar™-coated wafer for 30 s, which simulatestypical solvent–BARC contact during resist fabri-cation process. The Aquabar™ film thicknesschanges before and after solvent exposure weremeasured by an automatic Nano AFT 215A todetermine the extent of swelling/intermixing. Thefilm thickness changes, collected in Table II, wereobserved to be negligible at temperatures of over170°C, when either external crosslinker was usedat 10 wt % of the active polymer.

LITHOGRAPHY

Swing Curve Analysis

Evaluation of the effectiveness of the BARC wascarried out initially by swing curve simulationand analysis. The simulated swing curve ampli-tude of AZt 7908 (an i-line photoresist productsupplied by AZ Electronic Materials, Somerville,NJ) on Aquabar™ baked at 190°C, shown in Fig-ure 7, was calculated from the optical constants nand k, obtained by spectral elipsometry (Fig. 6).From this, the optimum Aquabar™ thickness wasidentified as being in the range of 1900 Å. Thereduction in swing amplitude was experimentallydetermined by plotting the dose-to-clear (DTC) ofeach wafer versus its corresponding resist thick-ness, as shown in Figure 8. AZt 7908 i-line pho-toresist was coated on wafers precoated with 1900Å Aquabar™ film to achieve resist thickness from0.7 to 1.0 mm. The wafers were subsequently pro-cessed using the conditions described in the ex-perimental section. The % swing reduction wascalculated and reported in Table III.

Experimental

The Aquabar™ solution was coated to the tar-geted thickness, by solution spin-coating, followedby a postapply bake at 190°C for 60 s. (For mini-mum swing amplitude, the thickness was typi-cally 1900 Å.) The AZt 7908 i-line photoresist wasapplied onto the Aquabar™ coated wafers fol-lowed by a 90°C soft bake for 60 s to achieve aresist thickness of 0.974 mm. The coated waferswere imagewise exposed with a 0.54 NA NIKONNSR 1755i7B stepper, then a 110°C postexposurebake for 60 s, and single spray-puddle developed

Table I Optical Properties of Polymer Films

CopolymerComposition

bKE–NMA–HEMa

ElectronWithdrawing

GroupAbsorption

Maximum (nm)Absorption Coefficient

(mm21) (365 nm)

40/20/40 NO2 417 9.740/10/50 COOH 357 13.550/10/40 COOC2H5 355 14.440/20/40 NHCOCH3 383 6.840/20/40 COCH3 362 14.340/20/40 SO3

2NH41 357 8.8

40/20/40 COO2NH41 359 7.8

a bKE is the b-keto ester methacrylate monomer; NMA is N-methylol acrylamide; HEM is hydroxyethyl methacrylate.


with AZt 300 MIF developer for 60 s at 23°C.Resist images were analyzed by a Hitachi S-4000SEM.

Etch Property

The Aquabar™ coating etch rate was designed tobe as high as possible to avoid etch bias duringimage pattern transfer. This can be predicted bythe polymer Ohnishi number,6 calculated frommolecular structure. Increasing the number den-sity of oxygen atoms in the polymer, through in-clusion of molecular functionality, such as esters,carboxylates, sulfonates, and even hydroxylgroups, contributes to enhance the Ohnishi num-ber and increase the coating etch rate. Table IVgives the etch rate of Aquabar™ tested in aCHF3/O2 RIE in comparison to a typical DNQ-Novolak photoresist (AZt 7900). The Aquabar™sample measured exhibited high etch selectivitywith a 1.73 high etch rate than typical DNQ-Novolak photoresist.

Discussion

During formulation of the carboxylic-acid-con-taining copolymers, ammonium hydroxide solu-tion was often added for two reasons. Firstly,addition was often necessary to render the poly-mer water-soluble through neutralization of thecarboxylic acid to the more ionic carboxylate salt.In addition, a higher pH was necessary to ensureshelf life stability in water when the formulationalso contained the external crosslinker Cymel. Inacidic conditions, a condensation reaction of theCymel resin methylol groups can occur, leading tocrosslinking and, hence, gelling of the solution.Shelf life stability, which was defined as zerochange in solution viscosity and absorption coef-ficient, was observed for over 3 months when theexternal crosslinker Primid (b-hydroxyalkylam-ides) was used. The esterification reaction neces-sary for crosslinking requires elevated tempera-ture to occur. To obtain sufficiently crosslinkedfilms after processing conditions, typically, thecrosslinking agent was added at about a 10 wt %fraction of the polymer.

Compatibility of the BARC film with resist,resist solution, and developer solution are neces-sary to avoid fabrication failure and image pat-tern defects. As the polymer is water-soluble, thefilm was generally inert to the application of re-sist solution. After exposure, film stripping andimage defects were observed with initial BARCpolymer samples on development. Stripping wascaused by either dissolution of the polymer film indeveloper solution (aqueous base) or swelling,leading to catastrophic adhesion failure, shatter-ing, and cracking. To eliminate this stripping, twocomplimentary approaches were taken. Firstly,the polymer was designed to incorporate a large(50 mol %) quantity of functional groups thatcould either undergo self-crosslinking or would beamenable to crosslinking with an externalcrosslinking agent, which was also added to the

Table II Aquabar™ Film Thickness Change on Solvent Exposure

Solvent SB Temp (°C) FTinitial (Å) FTfinal (Å) DelFT (˚A) Change (%)

Ethyl lactate 170 1943 1934 29 20.5Developer 170 2130 2163 33 1.5Ethyl lactate 190 1979 1964 215 20.7Developer 190 2216 2226 10 0.4Ethyl lactate 210 2001 1991 210 20.5Developer 210 2044 2046 2 0.1

Figure 7 Normalized swing curve amplitude of AZtAquabar™ (bake temperature: 190°C).


formulation. In addition, the solubility parameterof the polymer was optimized by changing molec-ular composition, such that the polymer was onlyjust insoluble in water. Thus, in formulation, asmall amount (less than 10%) of a cosolvent wasrequired for complete dissolution. An alternativeto adjustment of the solubility parameter was touse a fugitive counterion in a sulfonate salt, suchas ammonium. The ammonium ion, on baking,thermally liberates small amounts of ammoniawhich volatilize and diffuse from the film, leavingan insoluble polymer. This approach has the ad-ditional benefit of increasing the formulation pH,thus stabilizing the melamine formaldehyde ex-ternal crosslinker present ensuring thatcrosslinking will only occur during the bakingstage. Both approaches were successful in pre-venting film stripping on development.

Incomplete development defects were also ini-tially observed and traced to the diffusion of alkylamines into the resist film, inhibiting both photo-chemistry and dissolution. The amines were

formed by Hoffman degradation of alkyl ammo-nium counterions,9 present in the polymer duringthe thermal baking step of the BARC. In thiscase, the amines were not sufficiently fugitive tocompletely volatilize and were still present tosome extent on application of the resist solution.More thermally stable counterions, such as phos-phonates which do not degrade, were also usedand solved this problem. The most effective solu-tion is to either use the ammonium ion, whichvolatilizes on baking, or, preferably, to use a non-ionic chromophore. The resist-solvent-solubleBARC formulation generally formed good qualityfilms and did not have the aforementioned strip-ping problems.

The performance of the BARC coatings wasultimately evaluated by the quality and featuresize resolution of the lithographical images of re-

Table III Swing Ratio Reduction of AZt 7908on Aquabar™, Calculated from Figure 7

Sample FT (˚A) Amplitude Reduction (%)

AZt 7908 0 14.1 0.0Aquabar™ 1964 1.0 93.1

Table IV Etch Properties of Aquabar™

SampleWafer

No.Etch Rate(Å/min)a

Etch SelectivityVersus AZt

7900

Aquabar™ 1 1101 1.796Aquabar™ 2 1075 1.754AZt 7900 1 613 1

a Etch conditions are as follows: Etcher, RIE-10N (SAMCOInc.); Flow Rate CHF3/O2, 40/10 (sccm); RF Power, 60W; Etchtime, 60 s.

Figure 8 Swing curve of AZt 7900 with and without Aquabar™.


sist, coated on the BARC. Initial experiments ex-amined the resist image profile and screened forevidence of an image irregularity known as footformation. It was hypothesized that this occurredas a result of film layer intermixing during theapplication of the resist layer. In an effort to elim-inate this defect, the crosslinking density of thebaked BARC film was again increased throughboth the addition of an external crosslinker to thecoating formulation, as well as incorporation ofadditional functional groups on the polymerwhich act as crosslinking sites. This is illustratedby Figure 9, which shows resist images on Aqua-bar™ formulated both with and without externalcrosslinker. In Figure 9(a), sufficient crosslinkingoccurs to prevent interlayer mixing, and the im-age sidewall makes a clean 90° angle with theBARC. However, foot formation can be observedwhen interlayer mixing occurred, due to the ab-sence of external crosslinker and, hence, insuffi-cient crosslinking, as shown in Figure 9(b). Figure10 shows SEM resist image profiles of AZt 7908on Aquabar™ at decreasing line widths. Feature

sizes of 0.3 mm can be readily obtained with goodprocess latitude.

As the film is spun from solution, film qualityon drying, topographical coverage, and coverageuniformity are all important. By designing theBARC material to be a homogeneous amorphouspolymer system, film quality was generally excel-lent; whereas coating behavior and topographycoverage was optimized through synthesis ofpolymer with a number-average molecular weightof at least 50,000 (solution viscosity becomes aproblem at a high molecular weight).

Synthesis

The synthetic route outlined in Scheme 1 wassuccessful for the synthesis of most keto-estermethacrylate polymers. Synthesis of the mono-mer involves firstly formation of the diazoniumsalt of a substituted aniline derivative using themetal ion free reagent t-butyl nitrite. This saltthen undergoes azo coupling at the active meth-ylene site of the acetoacetate monomer. One com-

Figure 9 (a) and (b) SEM micrographs showing a 0.34-mm line width resist imageprofile on (a) sufficiently crosslinked and (b) insufficiently crosslinked Aquabar™ coat-ing.

Figure 10. SEM micrograph showing resolution & DOF of AZ® 7908 on AqueousB.A.R.C.


mon factor in all synthesis was that no metallicspecies be present. Synthesis of the diazoniumsalt and the subsequent azo coupling step wasfound to be very pH-sensitive. In most cases, dia-zotization was carried out in acidic conditions,unless the amine was insoluble, such as sulfanilicacid. In this case, dissolution in base was pre-ferred, and, on addition of the nitrite reagent, thereaction mixture was made acidic. Azo couplingwas routinely carried out in basic conditions inhigh yield. On purification, the resulting chro-mophore containing monomer can be readily co-polymerized by a solution free radical polymeriza-tion technique, with a range of comonomers dis-cussed previously. The polymerization can be

carried out in the formulation solvent, thus elim-inating a processing step in production. Themonomer must be scrupulously pure to allow ahigh-molecular-weight product, and g-butyrolac-tone solvent was identified as a good solvent forboth dissolution of monomer and polymer, as wellas having good chain transfer properties for freeradical polymerization. Polymer molecularweights from 20,000 to 400,000 can be obtained,depending on composition and conditions. Molec-ular weight polydispersity is usually slightly over2, typical for methacrylate free radical polymer-izations; although when the initiator is added inaliquots, the molecular weight profile can often beasymmetrical and even multimodal, leading to

Scheme 1 Two-step synthesis of b-keto ester copolymers via polymerization of chro-mophore monomer.


higher polydispersity. Analysis of the 13C-NMRspin-coupling constants indicated that the mono-mer incorporation within the polymer chain wasrandom.

An alternative scheme (Scheme 2) involvesfirstly solution free radical copolymerization ofmethacryloyloxyethyl acetoacetate monomer. Theprepolymer formed subsequently undergoes azocoupling with the appropriate diazonium salt, inan analogous manner to that previously describedfor the monomer. Utilization of this graft tech-nique allows formation of polymers in which thechromophore contains functional groups withhigh chain transfer constants which normally in-hibit free radical polymerization. When the chro-mophore contained an ammonium sulfonate salt,this route was identified as being preferable. Theprepolymer was not isolated, and the polymeriza-tion solution made basic to facilitate direct azocoupling with the diazonium salt of sulfanilic

acid. Polymers obtained through this route werein both higher yield and molecular weight thanconventional polymerization of the chromophoremonomer. Optimization of initiator, polymeriza-tion solvent, and conditions may, however, pro-vide a route to conventional polymerization.

PREPARATION OF THE DIAZONIUM-SALTOF AMINOBENZOIC ACID (1)

4-aminobenzoic acid (13.75 g, 0.1 mol) was dis-solved in a mixture of concentrated hydrochloricacid (22.3 mL, 0.25 mol) and methanol (150 mL).The flask was then immersed in a bath of crushedice and cooled until the temperature of the solu-tion was below 3°C. The solution became a whitesuspension. Diazotization was then facilitated bythe slow addition of tert-butyl nitrite (11.3 g, 0.11mol) at a temperature below 5°C. The diazonium

Scheme 2 One-pot synthesis of b-keto ester copolymers via polymer graft reaction.


solution was then stirred in an ice water bath forabout 1 h. The product formed as a yellow solu-tion and was not isolated but was used directly inthe following reaction.

PREPARATION OF THE METHACRYLATEMONOMER (2)

2-(methacryloyloxy)ethyl acetoacetate (22.08 g, 0.10mol) and triethylamine (25.3 g, 0.25 mol) were dis-solved in methanol (200 mL), and the solution wasstirred and cooled below 5°C in an ice water bath.To this solution, the cold diazonium salt solution (1),formed as described previously, was then slowlyadded, while the temperature was maintained be-tween 5 to 10°C. The reaction mixture was allowedto stir for 3 h while warming to room temperature,resulting in the formation of the product as a yellowsuspension. This was then filtered, washed withmethanol, and dried under vacuum to yield 32.7 g(90%) of yellow product.

NMR: 1H-NMR (200 MHz, DMSO-d6, d ppm): 7.95(m, 2H, aromatic); 7.50 (m, 2H, aromatic); 6.05 (s,1H,ACHcisHtrans); 5.65 (m, 1H,ACHcisHtrans); 4.35–4.55 (m, 4H, OCO2CH2CH2CO2O); 3.30 [s, 1H,OCOCH(NA)COO]; 2.45 (s, 3H, CH3CA); 1.85 (s,3H, OCOCH3).

PREPARATION OF COPOLYMER (3)

The methacrylate monomer (2) (9.79 g, 0.03 mol)was dissolved in g-butyrolactone solvent (40 mL)and DMF (45 mL). The solution was warmed to65°C while stirred. On complete dissolution, thesolution was degassed by vigorously bubbling ar-gon, via an inlet needle in a sealed rubber sep-tum, through the solution for about 2 h. N-(hy-droxymethyl)acrylamide (3.6 mL, 0.018 mol) and2-hydroxyethyl methacrylate (1.23 mL, 0.012mol) were then injected into the solution throughthe septum, and the polymerization mixture wasfurther degassed for 30 min. An aliquot from asolution of AIBN (0.10 g, 0.65 mmol, 1 mol % totalmonomer) in g-butyrolactone (1 mL) was theninjected, and the solution was degassed furtherfor 30 min. In total, two aliquots were added atintervals of 5 h. Both inlet and outlet needleswere then removed, and the sealed vessel allowedto stir at 65°C for 20 h. This solution was thenprecipitated into a five-fold excess of 2-propanol.The polymer forms as a yellow solid, which was

then collected by filtration and dried [yield, 12 g(85%); Mn (gel permeation chromatography), uni-versal calibration method, 95,000; Tg, 126°C.

NMR: 1H-NMR (200 MHz, DMSO-d6, d ppm): 7.70–8.00 (aromatic); 7.20–7.50 (aromatic); 5.20–5.40(ONHCH2OH); 4.00–4.30 (OCO2CH2CH2CO2),O); 3.80–3.90 (OOCH2CH2OH), O); 3.50–3.60 (OOCH2CH2OH);3.30 [OCOCH(NO)COO); 2.30–2.50 (OCHCONHO,backbone); 1.50–2.00 (OCH2O, backbone); 0.80–1.00(OCH3). 13C-NMR (200 MHz, DMSO-d6, d ppm): 193(ACCOO); 176 (ACCONHO); 174 (OOCOPhO); 165(OCOOOCH2O); 162 (OCOCH3); 146.5 (OCHNANO);132, 131, 125, 124, 116, 114 (aromatic); 63, 62(OCO2CH2CH2CO2O); 66 (OOCH2CH2OH); 61(OCH2OH); 59 (OOCH2CH2OH); 51–54 (OCH2O, back-bone); 45 (ACA, backbone); 38 (OCHCONHO, backbone);23 (OCOCH3); 18 (OCH3).

PREPARATION OF PREPOLYMER (4)

2-(methacryloyloxy)ethyl acetoacetate (8.83 g,0.04 mol) and 2-hydroxyethyl methacrylate (5.21g, 0.04 mol) were dissolved in DMF solvent (140mL). The solution was warmed to 65°C whilestirred. The solution was then degassed by vigor-ously bubbling argon, via an inlet needle in asealed rubber septum, through the solution forabout 0.5 h. An aliquot from a solution of AIBN(0.335 g, 2 mmol, 1 mol % total monomer) in DMF(2 mL) was then injected, and the solution wasdegassed further for 15 min. Both inlet and outletneedles were then removed, and the sealed vesselwas allowed to stir at 65°C for 24 h. A sample ofthe product was isolated by precipitation into2-propanol and analyzed. To the remaining poly-mer solution was added tetramethyl ammoniumhydroxide solution in MeOH (16.8 mL, 0.04 mol),and the mixture was stirred while cooling to be-low 10°C.

NMR: 1H-NMR (200 MHz, DMSO-d6, d ppm): 4.20–4.30(OCO2CH2CH2CO2O); 3.80–3.90 (OOCH2CH2OH),O);3.50 –3.60 (OOCH2CH2OH); 3.30 (OCOCH2COO);2.20 (OCOCH3); 1.60 –1.90 (OCH2O, backbone);0.70 –1.00 (OCH3). 13C-NMR (200 MHz, DMSO-d6, dppm): 201 (CH3COO); 177 (OCCOO); 176 (OCCOO);167 (OCH2COO); 68 (OCO2CH2CH2CO2O); 66(OOCH2CH2OH); 59 (OOCH2CH2OH); 51–54(OCH2O, backbone); 49 (OCOCH2COO); 45 (ACA,backbone); 30 (COCH3); 17–19 (OCH3).

PREPARATION OF COPOLYMER (5)

To the polymer solution (4) prepared in the pre-vious reaction was added sulfanilic acid (7 g, 0.04


mol) and water (20 mL), followed by isobutyl ni-trite (5.1 mL, 0.041 mol). The resulting suspen-sion temperature was maintained below 10°C. Asolution of HCl (37.8 wt % in water) (3.2 mL, 0.04mol) was added to water (10 mL), and the solutionwas slowly added to the reaction mixture, formingthe diazonium salt, which was then transferred toa pressure equalizing dropping funnel. This solu-tion was then added dropwise to the polymer so-lution (4), and the resultant red solution was al-lowed to stir at room temperature for 3 h. Thesolution was then precipitated into 2-propanol (3: 1), allowing the polymer to form as a solid prod-uct [yield, 18.5 g (87%); Tg, 123°C; Mn (GPC,universal calibration method), 64,000].

NMR: 1H-NMR (200 MHz, DMSO-d6, d ppm): 7.80–8.00 (aromatic); 7.10–7.50 (aromatic); 4.20–4.30(OCO2CH2CH2CO2O); 3.80–3.90 (OOCH2CH2OH),O); 3.50–3.60 (OOCH2CH2OH); 3.30 (OCOCH2COO);2.20 (OCOCH3); 1.60–1.90 (OCH2O, backbone); 0.70–1.00 (OCH3).

SUMMARY

A side chain methacrylate copolymer was designedand synthesized for use as water-soluble bottomantireflection coatings in lithography. The polymercomposition was optimized as a result of extensiveoptical and lithographic evaluation. The b-keto es-ter azo benzene chromophore with carboxylatewithdrawing group was identified as having suit-able absorption maximum for i-line wavelength andhad a high absorption coefficient (k). Solubility ofthe BARC copolymer in water was achievedthrough incorporation of highly polar functionalgroups, such as hydroxyl and carboxylate salts, inthe polymer structure. A two-step synthetic route

was designed to produce the polymer in high yield.In the lithographic fabrication process, to preventstripping failure, the polymer film must be inertduring exposure to both organic and aqueous solu-tions. This was achieved by both internal and ex-ternal crosslinking mechanisms, which were ther-mally activated during the baking fabrication steps.External crosslinkers, which were inert at roomtemperature, were identified and incorporated intocoating formulations. Effective crosslinking alsoprevented interlayer mixing between resist andBARC. This ensured good quality resist imageswithout scumming and foot at the resist/BARC in-terface. High CD resolution tolerances, and goodswing curve reductions, were obtained.

REFERENCES

1. Lynch, T.; Paradis, V.; Spak, M.; Moreau, W. SPIEProc 1994, 2195, 225.

2. Schiltz, A.; Schiavone, P. SPIE Proc 1997, 3049,386–396.

3. Eakin, R.; Detweiler, S.; Stagaman, G.; Tesauro,M.; Spak, M.; Dammel, R. SPIE Proc 1997, 3049,397.

4. McCulloch, I.; Dammel, R.; Lu, P. H.; Kang, M.Presented at the 11th International Conferenceof Photopolymers, Society of Plastics Engineers,Brookfield, CT, Oct. 1997.

5. Dammel, R. Semicond Fabtech 1995, 2, 215.6. Kunz, R.; Palmateer, S.; Forte, A.; Allen, R.; Wall-

raff, G.; DiPietro, R.; Hofer, D. SPIE Proc 1994,2724, 365.

7. Dammel, R.; Sagan, J.; Synowicki, R. SPIE Proc1997, 3049, 963.

8. Dammel, R.; Norwood, R. SPIE Proc 1996, 2724,754.

9. Hoffman, A. W. Ann Chem Pharm 1851, 78, 253.


water-soluble chromophore containing copolymers for bottom antireflection coating applications in...

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