Evaluation of siloxane and polyhedral silsesquioxane copolymers for 157 nm lithography

Evaluation of siloxane and polyhedral silsesquioxane copolymersfor 157 nm lithography

V. Bellas,a) E. Tegou, I. Raptis, E. Gogolides, and P. Argitisb)

Institute of Microelectronics, NCSR Demokritos, 15310 Aghia Paraskevi, Greece

H. Iatrou and N. HadjichristidisDepartment of Chemistry, University of Athens, 15771 Zografou, Greece

E. Sarantopoulou and A. C. CefalasInstitute of Physical and Theoretical Chemistry, NHRF, 48 Vassileos Constantinou Avenue,11635 Athens, Greece

~Received 27 June 2002; accepted 14 October 2002!

Siloxane and silsesquioxane copolymers have been synthesized and first evaluated as potentialcomponents of 157 nm resist materials. In block copolymers of dimethylsiloxane andtert-butylmethacrylate negative imaging chemistry dominates at 157 nm, due to the presence of the siloxanecomponent, although positive imaging in aqueous base developers via chemical amplification wasobtained at longer wavelengths~248 nm!. The same behavior is observed in graft copolymers ofdimethylsiloxane andtert-butyl methacrylate. On the other hand, random copolymers ofpolymerizable polyhedral oligomeric silsesquioxane monomers with various acrylates, includingpartially fluorinated, can be used as components of resist formulations that provide positive imagingat 157 nm, aqueous base development, and physicochemical properties which resemble those ofconventional poly~meth!acrylates. Pattern transfer properties depend on the selection of thesilsesquioxane component. Polymers containing 30% or higher w/w ethyl-substituted silsesquioxanecages provide the necessary etch resistance as well as low surface roughness to oxygen plasma at100 nm film thickness, allowing bilayer 157 nm lithography, even without further absorbanceoptimization. © 2002 American Vacuum Society.@DOI: 10.1116/1.1526358#

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I. INTRODUCTION

Lithography at 157 nm, using fluorine excimer lasersthe exposure source, has recently emerged as the mostous candidate to succeed the currently in production 193lithography, and continue the trend for microelectronic dvice miniaturization. The development of suitable photosists with acceptable lithographic performance representsof the major challenges towards the establishment of mapatterning technology at this wavelength. The most critiissue for 157 nm resist development is the accomplishmof adequate transparency, since traditional resist platfoare too opaque to allow imaging in sufficiently thick filmThere are two main classes of polymeric materials thatfulfill the low absorbance criterion. The first class is the flurocarbon polymers, and the second class is polymerscontain high levels of silicon–oxygen bonding. The mateals must additionally satisfy numerous other requiremesuch as adhesion, high resistance to etching by reactivemas, controlled dissolution behavior, and aqueous basevelopment. Therefore, the design of materials suitable157 nm resist applications presents a great challenge.

The severe limitations imposed by the high absorban1

of most organic polymers~hydrocarbon platforms! at 157 nm

a!Also at: Department of Chemistry, University of Athens, 15771 ZografGreece.

b!Electronic mail: [email protected]

2902 J. Vac. Sci. Technol. B 20 „6…, Nov ÕDec 2002 1071-1023Õ200

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has led research efforts mostly to the synthesis of unconvtional polymers with high C–F content, which, however, sneed optimization to the imaging and etch resistance proties. Incorporation of fluorine can dramatically affect thphysicochemical characteristics of the resist, making aqous base dissolution more difficult, and reducing adhesMoreover, fluorination results in reduced etch resistanc2

Several groups are intensively studying fluoropolymersdevelop resists.

On the other hand, siloxanes and silsesquioxanes coffer an alternative route, provided appropriate materiwith high-resolution positive imaging capabilities can be egineered. Silicon atom outgassing risks are reduced in thtypes of materials due to multiple Si—O bonding in a chainor a network.3 Siloxane polymers have been investigatedthe past mainly as negative tone resists for electron beion beam, and ultraviolet exposure,4 and have been recentlshown to have high resolution and low roughness.5–7 Silox-ane copolymers with various architectures have beensporadically investigated as resist materials for many yeVan de Grampelet al.,8 for instance, prepared polysiloxanewith acid-sensitive side groups. However, the main drawbfor the use of these materials in standard lithographic apcations appears to be their low glass transition temperatOn the other hand, siloxane containing block and graftpolymers have been also proposed, but their lithographichavior at 157 nm is still unexplored.9 Among the silsesqui-oxanes, the ladder-type structure has been extensiexamined as a photosensitive material and, recently

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2903 Bellas et al. : Evaluation of siloxane and silsesquioxane 2903

fluorocarbinol-substituted polymer with acid-labile esfunctionalities has been suggested as a promising postone material for 157 nm applications.10 Contrary to the lad-der type, the cage-type structures have not been investigfor 157 nm lithography although a methacrylate-basedlated material bearing cyclopentyl polyhedral oligomesilsesquioxane has been proposed for etch resistancehancement in 193 nm resist compositions.11

In the present article, we report the research efforts ofteam on the synthesis of different siloxane and cage-tsilsesquioxane copolymers, and their evaluation as poss157 nm resist components. Requirements for positive aqous base development are investigated. The results presdemonstrate the possibility of using polyhedral oligomesilsesquioxane based materials as 157 nm resists, at leathe bilayer mode.

II. EXPERIMENT

A. Polymer synthesis

The preparation and characterization of block copolymhas been described in detail elsewhere (Mw;603103 g/mol,Mw/Mn,1.1).12 3-~3,5,7,9,11,13,15-Heptacyclopentylpetacyclo @9.5.1.13,9 .15,15 .17,13# octasiloxane-1-yl! propylmethacrylate ~cp-POSS! and 3-~3,5,7,9,11,13,[email protected],9 .15,15 .17,13# octasiloxane-1-yl! propyl methacrylate~e-POSS! were purchased from Hybrid Plastics and were used as received. The solvents anmonomers were supplied from Aldrich and purified accoing to standard procedures required for conventional radpolymerization. The initiator 2,28-Azobis~isobutyronitrile!@~AIBN ! Fluka# was purified by recrystallization frommethanol. All the reactions were carried out under prepfied nitrogen atmosphere. The poly~dimethylsiloxane!~PDMS! macromonomer was synthesized by BuLi-initiatanionic ring opening polymerization of hexamethycyclotrisiloxane, followed by termination with 3-~methacryl-oxy!propyl#dimethylchlorosilane, as shown in Scheme 1:

(Mw51.03103 g/mol, Mw/Mn51.08).The PDMS macromonomer was copolymerized withtert-

butyl methacrylate to afford graft copolymers PTBMA-g-PDMS. In a similar manner POSS monomers were copomerized with other monomers, such as methacrylic a

JVST B - Microelectronics and Nanometer Structures

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~MA !, methyl methacrylate, tert-butyl methacrylate~TBMA !, itaconic anhydride~IA ! and 2-~trifluoromethyl!acrylic acid ~TFAA!, in various compositions.1H nuclearmagnetic resonance spectra were recorded on a BrukerMHz instrument with CDCl3 as the solvent, at 25 °C. In acases, characteristic resonances were clearly presentd53.75 ppm~–CH2CH2 CH2-cage! and d50.6 ppm~meth-ylene protons adjacent to the POSS cage!. Since the poly-merizations were carried to near completion~.90% yields!,the ratios of the starting monomers in the copolymerssimilar to the feed ratios.11 In a typical experiment, 0.01 gAIBN and 10 g of the monomers were dissolved in 30 gdry oxygen-free tetrahydrofuran. The reaction mixture wthen placed in an oil bath~70 °C! for 48 h. Each polymer wasisolated after precipitation in methanol. The precipitant wdissolved in THF, and the procedure was repeated twFinally the copolymers were dried under vacuum~50 °C! for2 days. Modulated differential scanning calorimetry mesurements were carried out with a 2920 differential scanncalorimeter~TA Instruments!. Gel-permeation chromatography analyses were carried out on a Waters Breeze1515 sliquid chromatograph with a differential refractometer~Wa-ters 2410! as a detector.

B. Lithographic processing

Solutions~4%–6%! of all polymers were prepared witheither methyl isobutyl ketone~MIBK !, or propylene glycolmethyl ether. The photoacid generators used in resist forlations, were either triphenylsulfonium hexafluoroantimonor perfluoroarylsulfonate based.

Ultraviolet-visible ~UV–Vis! spectra were recorded onPerkin–Elmer UV–Vis Lambda 40. Dissolution was montored by an experimental DRM setup equipped with a laemitting at a wavelength of 650 nm~angle of incidencea;5°). For lowresolution near UV and deep UV exposurea Hg–Xe lamp was used equipped with the appropriateters. High-resolution electron-beam~e-beam! exposures wereaccomplished on a 50 keV e-beam machine~Leica, EBPG-3!. 157 nm exposures were performed using a F2 laser sourcewhich was of the fast discharge type and it was descripreviously in detail.13 Absorption spectra in the visible ultraviolet were recorded with a hydrogen lamp operating inlongitudinal stabilized discharge mode that has beenscribed previously.14 At 157 nm the lamp was stabilized tosignal-to-noise ratio better than 2000 due to the high stabof the discharge mode. High resolution 157 nm exposuwere carried out at International Sematech using an ExitMicrostepper.

C. Oxygen plasma etching studies

Thin films ~typical thickness: 150 nm! of POSS copoly-mers were spin casted on top of 350 nm thick hard banovolac. Etch resistance studies of the films were performin an inductively coupled plasma~ICP! etcher~MET! fromAlcatel ~composed of a cylindrical alumina source and a dfusion chamber!. The temperature of the sample was cotrolled at 15 °C by mechanical clamping and helium ba

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side cooling. The etching conditions were: oxygen plas~100 sccm, 10 mTorr!, source power: 600 W and bias volage:2100 V. For comparison purposes etch resistance sies were also performed in a Nextral-Alcatel reactive ietcher. In both systems, etching rates were monitored wlaser interferometry. Surface roughness was measured band after etching in contact mode, with a Topometrics TM2000 atomic force microscope. The total etching time forsurface roughness measurements was equal to the etchfor the novolac endpoint plus 20% overetching. The totime of etching for etch rate and selectivity measureme~novolac underlayer/resist! was as high as 900% overetchin

III. RESULTS AND DISCUSSION

A. Block and graft copolymers containingpoly „dimethylsiloxane …

Poly~dimethylsiloxane! has been used as solvent develoable, negative resist materials in a number of lithograpapplications under near UV, deep UV, e-beam, and ion beexposure. Although at near and deep UV cases, these pmers contain usually vinyl functionalities and external phtoinitiators to allow effective crosslinking, at shorter wavlengths and e-beam exposures this is not necessary. Tpure PDMS (Mw51203103 g/mol! is crosslinked under 50keV e-beam exposure at relatively low doses~;2 mC/cm2)providing good resolution imaging@Fig. 1~a!#. We foundnext that at 157 nm, PDMS also gives negative tone litgraphic images at substantially low doses~above ;18mJ/cm2). The higher theMw , the lower the needed dose.

In an effort to design siloxane based materials that cogive positive behavior under aqueous base developmen157 nm, we first investigated simple copolymer compotions containing acid cleavabletert-butyl groups in additionto the siloxane moieties. To avoid lowTg problems encoun-tered in materials exclusively having siloxane backbone,synthesized either block copolymers PTBMA-b-PDMScopolymers with poly~tert-butyl methacrylate! backbonewhere siloxane moieties were grafted.

The synthesis of multiblock copolymers, wheSi—O—C bonds are present along the backbone has bpatented by Crivello.15 Nevertheless, this methodology provides copolymers with limited stability and high moleculand compositional polydispersity. The synthetic methodogy we applied for the diblock copolymers has the novethat the resulting materials have high degrees of molecand compositional homogeneity, without containiSi—O—C bonds. The absence of such bonds, which areceptible to hydrolysis, ensures the long shelf life of the cpolymers. The molecular characteristics of diblock copomers having a range of compositions are summarizedTable I. All the samples synthesized and tested exhibpoor film formation properties~e.g., pinhole defects, nonunform films!, attributed to phase separation phenomena. Phseparation is expected since the samples exhibited alsodistinct glass transition temperatures (Tg’s!, as revealed byDSC ~see Table I!. The lithographic evaluation experimenshowed positive tone behavior under 248 nm irradiation

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aqueous base development capability. However, aquebase development caused swelling, because the padeprotection of thetert-butyl groups forms an amphiphilicblock copolymer in the exposed regions, which makesdevelopment irregular. It should be noticed that by replacthe developer with MIBK swelling was reduced. On thother hand, when the samples were exposed to e-beam157 nm, it was found that the negative tone behavior donated @Fig. 1~b!#, even when the PDMS is the minoritphase. In these experiments, aqueous base developmepossible only if 248 nm flood exposure follows the imagwise e-beam or 157 nm exposure~negative imaging!.

The graft copolymers~PTBMA-g-PDMS! were synthe-sized as shown in Scheme 1. In the case of resist formtions based on these polymers film inhomogeneity andvelopment problems still exist, although self-assemphenomena were suppressed or reduced by grafting shPDMS chains (Mw;1000 g/mol! in the methacrylic back-bone. These samples exhibit a singleTg , which is an evi-dence that microphase separation does not occur in the bHowever, segregation is expected16 in the surface region, because of the high difference in surface tension of PDM

FIG. 1. ~a! 70–100 nm lines/spaces of PDMS on hard baked novolac athe pattern transfer step~e-beam exposure dose: 7.75mC/cm2, 50 keV! ~b!0.5 mm–0.25mm lines/spaces obtained with the 50/50 PDMS-b-TBMA cpolymer at an e-beam exposure dose of 38mC/cm2, 50 keV. ~See Table I.!


TABLE I. Imaging properties andTg’s of synthesized siloxane and silsesquioxane copolymers.

Polymer component Imaging

Tg ~°C!Class Monomer compositiona 248 nm e-beam 157 nm

Block ~PTBMA-b-PDMS! 10/90 Posit.c Neg.d Neg. 2130 and 8030/70 Posit. Neg. Neg. 2130 and 8050/50 Posit. Neg. Neg. 2130 and 8070/30 Posit. Neg. Neg. 2130 and 80

Graft 80/20 Posit. Neg. Neg. 14~PTBMA-g-PDMS! 60/40 Posit. Neg. Neg. ¯

Random~POSS containing!

POSS monomerb Comonomer~s!

20 ~cp! TBMA Posit. Posit. ¯ 114.330 ~e! TBMA,

MA, IAPosit. Posit. Posit. 142.5

40 ~e! TBMA, MA Posit. Posit. Posit. 145.130 ~e! TBMA,

TFAAPosit. Posit. Posit. ¯

aMonomers are given in % w/w.bThe parentheses show alkyl substitutions in each case. Also, see Sec. II.cPosit. indicates positive.dNeg. indicates negative.

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~20.4 mN/m! from the methacrylic segments. As a result, wsuspect the surface layer to be enriched with hydrophoPDMS, making the development irregular. Nevertheless,surface enrichment could be favorable during the plasetching step. In general, the physicochemical propercould be more effectively controlled through the siloxachain length optimization but negative imaging was alwapredominant either at 157 nm or at e-beam.

B. POSS-containing copolymers

Silsesquioxanes of the ladder type have been alreadyported with improved physicochemical properties compato the siloxanes, positive tone imaging at 157 nm and proising lithographic behavior.10,17 In this investigation we ex-amined a related class of materials that contain polyhesilsesquioxane moieties~cages! and are provided also in thform of polymerizable acrylate monomers~Scheme 2!:

The cage type possesses a precisely defined structure~mono-dispersed silsesquioxane components! whereas our experi


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ence with ladder silsesquioxanes is that the commerciavailable starting materials are unreliable because theirtual molecular formulas are unknown.18 Moreover, cagestructures are free from remaining silanol bonds Si—OH thatare common in commercial ladder polymers and could cacondensation reactions, resulting to limited shelf life as was to negative tone chemistry.

In this investigation, we synthesized a number of acrylcopolymers containing polyhedral silsesquioxanes as shin Scheme 2 and Table I. The effects of copolymer comsition on Tg and imaging properties are shown in TableFirst, it can be seen that with these materials, positive timaging at 157 nm and e-beam is obtained. Negative tbehavior has not been observed even at high doses~doses upto ten times higher than the clearing dose have been exined!. Furthermore, theTg of the resist formulations containing polyhedral silsesquioxanes ranges from 110 °C to 160for the materials tested. An increase of the polyhedral silsquioxane component results in increases of the materialTg .On the other hand, typical lithographic properties, i.film forming capability, development behavior, developstrength, imaging capabilities, depend strongly oncopolymer composition, as for other acrylate-based systas well. Thus, material properties can be tuned by optimizcopolymer composition. Furthermore, copolymerization wpartially fluorinated monomers~i.e., trifluoromethyl acry-lates! is possible as shown in Table I also allowing absotion reduction at 157 nm. Typical contrast curves with a fmulation containing one of the synthesized copolymers, APOSS ~ACrylate POSS, copolymer composition: e-POS30% w/w, comonomers: TBMA, MA, IA! under deep UVexposure are presented in Fig. 2~a! where the effect of post-exposure bake~PEB! on sensitivity is also shown. It shoulbe also noted that this material has excellent film form

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properties. On the other hand, the best imaging resultsobtained with low strength, diluted, aqueous base deveers. At 157 nm, a limited investigation has been carriedfar. Typical sensitivities~clearing dose! under exposure athis wavelength are in the order of 10 mJ/cm2 or less. Acharacteristic contrast curve of AC-POSS is shown in F2~b! whereas high-resolution lithographic results~obtainedwith alt-phase shift mask! are presented in Figs. 3~a! and3~b!. Further 157 nm evaluation studies are under way

FIG. 2. ~a! Sensitivity curve of a POSS-containing copolymer based formlation ~AC-POSS! at different PEB temperatures~copolymer composition:e-POSS 30% w/w, comonomers: TBMA, MA, IA! under 248 nm broadbandexposure. The PEB time is 2 min in all cases.~b! Sensitivity curve of thesame formulation under 157 nm exposure.~PEB:160 °C, 2 min.!


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SEMATECH and the results will be reported in a forthcoming publication.

The choice of the POSS alkyl substituents is crucialresist formulations for many reasons, including transparencompatibility with the polymer matrix,19 etch resistance, surface roughness after plasma treatment, and developmensues.

Therefore, we designed a methacrylate platform wheither cp-POSS or e-POSS was among the pendant groTheir solubility in common organic solvents~methyl isobutylketone, propylene glycol methyl ether! ensures a facile spincoating, providing regular and planar surfaces. The abbances of the e- and cp-POSS containing methacrylatemopolymers at 157 nm were determined to be 3.1mm21 and7.6 mm21, respectively. At the present e-POSS-basedpolymer compositions, the absorbance is considerably hfor single-layer lithographic applications, i.e., the imaged cpolymers have absorbance values in the range 4.5 tomm21. Nevertheless, these materials could be used forlayer lithography as discussed in the following section.

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FIG. 3. ~a! 120 nm semidensed lines~1:1.5, 1:2, 1:3, 1:4! of AC-POSSformulation~initial film thickness: 78 nm, exposure dose: 8.5 mJ/cm2, PAB:160 °C, 2 min, PEB: 160 °C, 2 min!. ~b! Cross section of 100 nm lines oAC-POSS formulation~initial film thickness: 153 nm, exposure dose: 13mJ/cm2, PAB: 160 °C, 2 min, PEB: 160 °C, 2 min!.

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C. Etch resistance studies

The incorporation of various POSS monomers in meacrylate polymer matrices is expected to offer significetch resistance enhancement, if the POSS percentage iresist formulation exceeds a certain value.11 We set out todetermine the POSS content required for sufficient etchsistance. In Fig. 4~a!, laser interference signals acquiredthe ICP reactor for various POSS contents~20–60 wt %!versus oxygen plasma etching time are presented. WhenPOSS content is 20 wt %~i.e., the silicon content is 6.0%!, alarge but not totally satisfactory etch resistance enhancem

FIG. 4. ~a! Laser interference signals acquired for copolymers of variPOSS contents~20–60 wt %! vs oxygen plasma etching time.~b! Laserinterference signals acquired for both e-POSS and cp-POSS containinpolymers vs oxygen plasma etching time.


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is observed compared to the hard baked novolac whichused as a reference~etch selectivity: 9:1!. Further increase inthe POSS content to 30 wt % provides a sufficient increasthe etch resistance. For POSS contents higher than 30 w~i.e., the silicon content is 9.0%!, practically no difference inthe interferograms is observed~selectivity.20:1!, i.e., thethreshold value for the effective etch resistance is 25–wt %. This value is in good agreement with literature datasilicon containing polymers.20

Figure 4~b! shows the laser interference signals acquifor both e-POSS and cp-POSS versus oxygen plasma etctime. The important role of the alkyl groups surrounding tsilicon cage is shown in the presented graphs. Cp-POSShibits an etch resistance which was worse than e-POSSdenced by the interferograms and the remaining thicknafter long etching times. After 800% overetching, cp-POsamples are only 20 nm thick, while e-POSS samples arenm. More important, at only 20% overetching, the surfaroughness of the cp-POSS was;15 nm ~root-mean-squarevalue! while, on the contrary, e-POSS films showed on;0.25 nm surface roughness. This signifies that e-POfilms are appropriate for high-resolution imaging wheresmall roughness is necessary. The reason for this differemight lie in the inherent POSS morphology: It is possibthat during etching, cp-POSS is stripped by the surroundorganic cyclopentyl groups and the larger distance betwthe cp-POSS units compared to the e-POSS units prevthe formation of a smooth oxide surface.

IV. CONCLUSIONS

In siloxane homopolymers, a very effective crosslinkichemistry takes place at 157 nm exposure. This chemidominates also in block and graft siloxane containing pomers allowing only negative tone behavior at this wavlength. On the other hand, acrylate monomers bearing phedral silsesquioxane moieties provide a straightforward wfor designing positive tone aqueous-base developable rmaterials for 157 nm lithography. Effective pattern transwith oxygen plasma for bilayer lithography has been shoin the case of materials containing ethyl-substituted polydral cages. Copolymerization with low absorbing partiafluorinated monomers may possibly allow the engineeringsingle-layer resists as well.

ACKNOWLEDGMENTS

This work has been funded by the European Union IS30143 Project 157-CRISPIES. Kim Dean and Sashi Pfrom International Sematech are kindly acknowledged157 nm exposures.

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Evaluation of siloxane and polyhedral silsesquioxane copolymers for 157 nm lithography

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Transcript of Evaluation of siloxane and polyhedral silsesquioxane copolymers for 157 nm lithography