Durability of nitrided fluorocarbon polymer films for nanoimprint lithography

Durability of nitrided fluorocarbon polymer films for nanoimprint lithographyHan-Hyoung Kim,Se-Geun Park ⁎,El-Hang Lee,Seung-Gol Lee,Beom-Hoan OMicro-photonics Advanced Research Center,Inha University,253Yonghyun-dong,Incheon,402-751,South Koreaa b s t r a c ta r t i c l e i n f o Article history:Received 28June 2010Received in revised form 23March 2011Accepted 23March 2011Available online 5April 2011Keywords:Anti-sticking layerNano-imprint lithography Nano-indentation Nitrided fluorocarbonHydrophobic fluorocarbon polymer films have been used as anti-sticking and releasing coatings for templates in nano-imprint lithography,but have poor durability against repeated high pressure pressing processes.The addition of nitrogen into fluorocarbon-based gas plasma can provide nitrided fluorocarbon polymer films,as con firmed by Fourier-transform infrared spectroscopy.Fluorocarbon and nitrided fluorocarbon films have almost the same 110.0°contact angle with water drops.The nitrided fluorocarbon films are harder and signi ficantly enhance durability as anti-sticking layers.Durability enhancement has been con firmed by fabricating replicas with silicon templates using nitrided fluorocarbon anti-sticking layers.©2011Elsevier B.V.All rights reserved.1.IntroductionNano-imprint lithography is one of the candidates for next generation lithography according to the International Technology Road Map for Semiconductors.This is because it has low cost and high production ef ficiency [1].This method is capable of de fining patterns of up to 10nm by pressing templates onto organic substrates with UV irradiation [2–4].One of the problems in this method occurs when separating the template from the substrate after pressing:the anti-sticking layer is coated on the templates [5].A fluorinated alkyl silane self-assembled monolayer (SAM)[6]or fluorocarbon polymer has been used because of its hydrophobic characteristics [7,8].Unlike SAM,fluorocarbon polymer can be deposited on various substrates,and so has been widely used [9].A common method used to deposit fluorocarbon anti-sticking film is Plasma Enhanced Chemical Vapor Deposition (PECVD)using CF-based gas and Ar plasma.But anti-sticking layers using fluorocarbon polymer suffer more surface damage during repeated processes than SAM layers.Recently,there have been reports that nitrided fluorocarbon films can be obtained in the anisotropic etching process of silicon oxide,by adding nitrogen gas to the plasma [10].In this work,the effect on the durability of nitridation of fluorocarbon polymer films as an anti-sticking layer of the nano-imprint lithography process has been investigated.Especially,there have been measurements of the hardness and elastic modulus with respect to the critical resistance associated with coating damage.2.Experimental partExperiments about fluorocarbon and nitrided fluorocarbon poly-mer films have investigated their properties and applications for nano-imprint lithography.Fluorocarbon and nitrided fluorocarbon films were deposited on silicon wafers in an inductively coupled plasma chamber by C 4F 8/Ar and C 4F 8/N 2,respectively.During both depositions,the chamber pressure was fixed at 0.2Pa and the RF power was 100W.The wafer temperature was maintained at 10°C.The molding process for nano-imprint lithography was realized using thermal embossing equipment (NA-6,Octocut).Silicon wafers were used as templates and were etched with comb patterns using 50–300nm wide fingers in 50nm steps,as shown in Fig.1.The height of the fingers was 250nm.Two kinds of anti-sticking films were coated for comparison.Chemical analysis of the deposited films used Fourier-transform infrared spectroscopy (VERTEX 80V,Bruker).Water contact angles were measured by sessil drop method using a drop shape analysis system (DSA-100,Kruss).The hardness was observed using a nano-indentation system (G200,Agilent),which evaluated the mechanical properties of the thin films.The indenter-penetration for measure-ment is suf ficient when as shallow as a few nanometers [11].A nano-indenter was the main component for the indentation tests used in nano-indentation.Replica patterns were examined with a Scanning Electron Microscope (SEM)at 15kV acceleration voltage.3.Results and discussionFTIR spectroscopy was used to con firm nitridation of fluorocarbon polymer films for analysis of molecular bonding.Fig.2shows C _N bonding near 2151cm −1when the films were deposited in C 4F 8/N 2gas.A stronger peak was observed for 40%N 2and 60%C 4F 8than forThin Solid Films 519(2011)5490–5493⁎Corresponding author.E-mail address:sgpark@inha.ac.kr (S.-G.Park).0040-6090/$–see front matter ©2011Elsevier B.V.All rights reserved.doi:10.1016/j.tsf.2011.03.124Contents lists available at ScienceDirectThin Solid Filmsj o u r n a l h o m e p a g e :w w w.e l s ev i e r.c o m /l o c a t e /ts f60%N 2and 40%C 4F 8.X-ray photo-electron spectroscopy also con-firmed the existence of an N 1s peak near 400.5eV and thus C _N bonding when N 2was added during deposition [10].The hydrophobic property can be observed from the contact angle wettability and adhesion of a water droplet on the substrate.Fig.3shows contact angles of 110.0°and 109.2°for fluorocarbon and nitrided fluorocarbon polymer films,respectively.Error in contact angle measurement can be minimized by keeping environment temperature,measurement time and liquid dose fixed for all measurement.Each sample was measured three times at different points by contact angle measurement,and then the average value was calculated.It can be seen that the contact angle and lowest surface energy are comparable.From this observation,the hydrophobic characteristics are the same for both films.These results mean that nitride fluorocarbon polymers can be used as anti-sticking layers.Coating collapsibility was also measured with a nano-indenter and compared,as shown in ually,the resistivity for the coating collapsibility is proportional to the elastic modulus of the patterned material [12].Therefore,studying the coating damage provides essential information about the correct material properties.But the initial part of data is ignored in the analysis [13].In general,at the very beginning of indentation,usually in the range of 5–10nm,there are a lot of uncertainties for various reasons,including machine limited resolution within this range,indenter tip roundness,errors in the indenter area calibrations,etc.The indentation loading curve has a lower slope in a tensilely stressed state than in a stress-free state and a higher slope in a compressively stressed state.In other words,the indentation load of an unstressed state indented to a givendepthFig.1.Silicon templates with comb patterns.Line widths are varied from 50to 300nm.2000205021002150220022502300C 4F 8(100%)C 4F 8(60 %)+N 2(40 %)C 4F 8(40 %)+N 2(60 %)N 2(100%)ReferenceA b s o r b an c e (a r b . u n i t s )Wavenumber (cm -1)C=NFig.2.FTIR spectra of C _N bonds in polymeric films deposited by different gas com-binations in PECVD deposition.The reference spectrum was taken from bare silicon.Fig.3.Contact angles of water droplets on (a)fluorocarbon and (b)nitrided fluoro-carbon films on silicon substrates.500100015002000100200300400500600700L o a d o n S a m p l e (m N )Displacement into Surface (nm)C-F polymer by C 4F 8/Ar plasma C-F-N polymer by C 4F 8/N 2 plasmaFig. 4.Loading –unloading curves of fluorocarbon and nitrided fluorocarbon films measured by nano-indenter.0200400600800100012001400160020406080100120140160C-F-N polymerC-F polymerE l a s t i c M o d u l u s (G P a )Indentation (nm)Fig.5.Elastic modulus plot of fluorocarbon and nitrided fluorocarbon films.5491H.-H.Kim et al./Thin Solid Films 519(2011)5490–5493will increase and decrease under compressive and tensile stress,respectively.The results shown in Fig.5are measurements of the elastic modulus nano-indentation,and are exhibited for each 5.7,5.9GPa in the fluorocarbon and nitride fluorocarbon surface (20.7nm).As theindentation depth increases,nitrided fluorocarbon was found to have a more increased tendency than fluorocarbon.Fig.6exhibits the hardness,which was found to increase with indentation.The trend exhibited in the two graphs is that the hardness and elastic modulus increase as depth of the surface of fluorocarbon and nitride fluorocarbon thin films increases.This is considered to be due to the fact that a collapsible pattern is better that a pattern.Nitrided fluorocarbon polymer film prepared by plasma deposition has a greater hardness than does fluorocarbon polymer film.Therefore,it is expected that nitrided fluorocarbon polymer films should have higher durability in the imprinting processes.In order to evaluate the performance of nitrided fluorocarbon polymer and fluorocarbon polymer films as anti-sticking layers for the silicon templates shown in Fig.1,replicas were fabricated using nano-imprint lithography.Anti-sticking layers of both films were deposited with different thicknesses of 10,20and 30nm on the templates.A replica was made into poly methyl methacrylate (PMMA)organic polymer at 150°C and 20bar.Fig.7shows SEM images of the replica,in which patterns were smeared with 10nm thick anti-sticking films,as shown in Fig.7(a)and (b).When the film thickness was increased to 20nm,the replica020040060080010001200140016002468H a r d n e s s (G P a )Indentation (nm)C-F polymer C-F-N polymerFig.6.Hardness plot of fluorocarbon and nitrided fluorocarbon films.Fig.7.Replica images of PMMA made by nano-imprint lithography.(a)10nm thick anti-sticking layer of fluorocarbon film,(b)10nm thick anti-sticking layer of nitrided fluorocarbon film,(c)20nm thick anti-sticking layer of fluorocarbon film,(d)20nm thick anti-sticking layer of nitrided fluorocarbon film,(e)30nm thick anti-sticking layer of fluorocarbon film,and (f)30nm thick anti-sticking layer of nitrided fluorocarbon film.5492H.-H.Kim et al./Thin Solid Films 519(2011)5490–5493made using the template with nitrided fluorocarbon polymer shows a clear pattern transfer.However,the film with a fluorocarbon polymer failed,as shown in Fig.7(c)and (d).When the film thickness was increased to 30nm,as shown in Fig.7(e)and (f),both anti-sticking layers show a good pattern transfer.However,when imprinting was repeated as many as 30times,the pattern replica was poor with the fluorocarbon anti-sticking layer,but the PMMA patterns were still very good with the nitrided fluorocarbon anti-sticking layer,as shown in Fig.8(a)and (b),respectively.Therefore,it can be concluded that nitrided fluorocarbon films as anti-sticking layers of silicon templates are very effective for fine pattern de finition and longtime durability.However,30nm nitrided fluorocarbon layer is relatively thick com-pared to the feature size of about 50nm of the Si templates,and thus further study is bene ficial to get thinner layers of less than 10nm with more improved durability grown by other deposition methods.We have shown that the addition of nitrogen in C 4F 8gas enables the growth of nitrided fluorocarbon polymer films with suf ficient elastic modulus and hardness to ensure durable anti-sticking char-acteristics of silicon templates used in nano-imprint lithography.4.ConclusionWe have investigated the properties and application of fluorocar-bon and nitrided fluorocarbon polymer films.The durability of nitrided fluorocarbon layers is better than that of conventional fluorocarbon layer deposited by PECVD.And it has been shown that the contact angle and lowest surface energy are comparable.To use these results,durability enhancement has been con firmed by fabricating replicas with silicon templates using nitrided fluorocarbon anti-sticking layers.AcknowledgmentThis work has been supported by the Inha University through its Basic Research Promotion 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(b)nitrided fluorocarbon film.5493H.-H.Kim et al./Thin Solid Films 519(2011)5490–5493。