Phenyl-bridged polysilsesquioxane positive and negative resist for electron beam lithography This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 Nanotechnology 23 325302 (http://iopscience.iop.org/0957-4484/23/32/325302) Download details: IP Address: 147.162.184.146 The article was downloaded on 24/07/2012 at 10:12 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience
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Phenyl-bridged polysilsesquioxane positive and negative resist for electron beam lithography
This article has been downloaded from IOPscience. Please scroll down to see the full text article.
Phenyl-bridged polysilsesquioxanepositive and negative resist for electronbeam lithography
L Brigo1, V Auzelyte2, K A Lister3, J Brugger2 and G Brusatin1
1 Industrial Engineering Department, University of Padova, Padova, 35131, Italy2 Microsystems Laboratory, Ecole Polytechnique Federale de Lausanne, Lausanne, 1015, Switzerland3 Center of Micronanotechnology, Ecole Polytechnique Federale de Lausanne, Lausanne, 1015,Switzerland
Received 4 May 2012, in final form 2 July 2012Published 23 July 2012Online at stacks.iop.org/Nano/23/325302
AbstractWe present and characterize an organic–inorganic hybrid sol–gel material, phenyl-bridgedpolysilsesquioxane (ph-PSQ), for use as a new high resolution resist for electron beamlithography (EBL). The resist has a unique characteristic as the only positive tone silica-basedresist available for EBL. Exploring the processing parameters has revealed that it is possible toswitch the behaviour from negative to positive tone by application of a post-exposure bake(PEB). Based on the results from micro-FTIR spectroscopy, a description of the toneswitching mechanisms is proposed. The negative tone behaviour is explained by the etch ratedifference between silanol groups and cross-linked silica, present in unexposed and in exposedareas of the films, respectively. In the case of positive tone, after a PEB, the etch ratedifference between a thermally densified cross-linked silica network and cage-like silicastructures allows us to reveal the pattern. Contrast and sensitivity are estimated under differentprocessing conditions, and the significant parameters for line edge roughness minimization arepointed out. Dense patterns down to 25 nm half-pitch and isolated structures down to 30 nmare demonstrated, exploiting the positive tone, and dense patterns down to 60 nm half-pitchare demonstrated in the negative tone. Etching selectivities in fluorinated gases for ph-PSQnanostructures on silicon substrates are 1–9 for the positive tone and 1–12 for the negativetone.
(Some figures may appear in colour only in the online journal)
1. Introduction
Among high resolution nanofabrication techniques, electronbeam lithography (EBL) is still the most suitable direct-writing tool for its excellent resolution and versatility.Engineering new resist materials is essential to EBL, asthe physical, chemical and structural properties of materialsallow the ultimate resolution of advanced EBL tools to beexploited, and to determine the throughput of the fabricationprocess. Moreover, when the resist is directly employed asfinal device material, high resolution nanostructured filmspresenting specific functional or electro-optical properties
provide convenient platforms for diverse nanotechnologyapplications.
Available resists for EBL can be broadly classifiedinto three main classes: organics, inorganics and hybrids.Conventional organic resists consist of single-componentpolymer material or chemically amplified (CA) systems.Two widespread polymer resists are polymethylmethacrylate(PMMA) [1] and ZEP520 [2]. While patterns of a fewnanometres can be formed in PMMA, those with densehigh aspect ratio features tend to collapse or deteriorate dueto swelling. ZEP520 is capable of higher sensitivity, andexhibits better plasma etch resistance than PMMA, and but
suffers from elevated line edge roughness (LER) because ofpolymer aggregates and entanglement effects due to largechain dimensions. CA resists, such as SAL-601 [3] andNEB31 [4], present high sensitivity and good etch resistance,but suffer from limited storage and post-exposure stabilitydue to diffusion of the photoacid/base generator. In orderto overcome the indicated limitations that typically affectmost polymer or CA systems, new molecular resists havebeen developed. Among them, calixarene derivatives [5]are capable of sub-10 nm resolution, while polysubstitutedderivatives of triphenylene allow high aspect ratio structuresetched into silicon [6].
Improvements in performance with respect to commonorganic systems have been obtained employing inorganicresists: silica-based systems like hydrogen silsesquioxane(HSQ) [7], metal oxides deposited via sputtering or chemicalevaporation, and metal alkoxides stabilized with chelatingagents [8]. Inorganic resists are characterized by highercontrast and etch resistance than organics, but suffer frommuch lower sensitivity. HSQ has consistently shown thehighest resolution in EBL, because it is made of smallmolecular units, resulting in low LER. It is a negative tonespin-on resist that has high etch resistance, but low sensitivity,and suffers from short shelf life and storage problems. Theonly other inorganic spin-on silica-based systems reported inthe literature are methoxysilane-based systems, NIMO-P0701and Accuglass 512B [9, 10].
Recently, the development of organic–inorganic hybrid(HOI) resists has allowed the choice of convenient combina-tions for resolution, contrast and sensitivity. Several nanocom-posite resists have been specifically engineered. Norborneneor chloromethylphenyl groups have been embedded into HSQ(NH37 and CN82, respectively) to increase the sensitiv-ity [11]. Specific molecular units have been incorporated intoorganic resists, in order to improve mechanical properties,prevent collapse of high aspect ratio patterns and increaseetch resistance: fullerene C60 in ZEP520; silica particles inthe polymer backbone of ZEP520 as pendant cage groups;silica particles or polyhedral oligomeric silsesquioxane cagesinto polymer matrices or chemically amplified resists [12].Only one CA epoxy–silica HOI resist, synthesized by sol–gelprocessing, is reported in the literature [13].
Given the very limited number of available silica-basedresists, the research and development of alternative systemsis desirable. The choice of an HOI system allows us to takeadvantage of the intrinsic mechanical, thermal and chemicalstability of inorganics, simple processing and functionalityof organics. In particular, a sol–gel process is an attractiveroute for the synthesis of resist materials as it requires lowtemperatures, and relatively low-cost equipment to be used.Up to now, no positive tone HOI systems have been reported.
This paper presents a hybrid organosilicate material,synthesized by the acid catalysed sol–gel process fromphenyl-bridged silsesquioxane (ph-SQ) precursors. Its perfor-mance as a new high resolution resist for EBL at 100 kV isevaluated. Ph-SQ precursors are molecular building blocksthat contain two trifunctional silyl groups connected byan aryl bridge through non-hydrolysable Si–C bonds. The
hybrid network material, resulting from sol–gel hydrolysisand condensation reactions, presents unique mechanicaland structural properties, with respect to common arylsilsesquioxane-type organosilicates. For instance, enhancedtoughness and controlled nanoporosity are conferred by theabsence of pendant carbon substituents coupled with thepresence of the organic bridge between the silicon atoms [14,15]. Such features are particularly useful when patternedsol–gel films directly serve as the final functional material,for instance, for optical or sensing applications.
The chemical composition, structure and functionalityof a bridged polysilsesquioxane sol–gel material have notpreviously been exploited for high resolution patterningwith EBL. The resist material does not need chemicalamplification: reactive species are generated without theaddition of cross-linker compounds, allowing an improvedfilm storage and post-exposure stability.
The phenyl-bridged polysilsesquioxane (ph-PSQ) sol–gelresist is unique as it is the only high resolution positive tonesilica-based EBL resist. In addition to this, we demonstratethat it is possible to switch the resist tone from negative topositive by applying an annealing step. This unique featurecan be applied in numerous cases for pattern manipulationand inverse in complex systems using localized heating.Moreover, ph-PSQ has already been tested as a positive toneresist for photon-based lithographic techniques, such as UVlithography [16] and x-ray lithography [17, 18], and as athermoset resist for soft nanoimprint lithography [19]. Theversatility to be processed with various patterning tools allowsph-PSQ to be used in mix and match processes for directfabrication of devices.
A comprehensive characterization of the resist perfor-mance has been carried out optimizing processing parametersrelative to the different lithographic steps: post-applicationbake (PAB), exposure dose, post-exposure bake (PEB) anddevelopment, starting with films of variable thickness. Theinteraction mechanisms between electron beam and resistfilms are analysed using micro-Fourier transform infrared(micro-FTIR) spectroscopy, and an explanation for the toneswitching behaviour is given. The resist contrast, sensitivity,resolution, LER and etching selectivity are evaluated.
2. Experimental details
The resist was synthesized via a one-pot sol–gel process usingthe 1,4-bis(triethoxysilyl)benzene monomer (96% purity,Sigma-Aldrich) at room temperature. A solution of 1,4-bis(triethoxysilyl)benzene, ethanol (EtOH) and bi-distilledwater was mixed in monomer/H2O = 1/6 molar ratio, usinghydrochloric acid (HCl) 1 M as catalyst. During synthesis,1,4-bis(triethoxysilyl)benzene precursors
(CH3CH2O)3Si–C6H4–Si(OCH2CH3)3
undergo sol–gel polymerization reactions, hydrolysis andcondensation, under acidic conditions. The ethoxy groupsbonded to Si atoms are partially or completely hydrolysed tosilanols
–Si(OCH2CH3)3n(OH)n n = 1–3.
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Nanotechnology 23 (2012) 325302 L Brigo et al
Silanol groups (–Si–OH) subsequently condense with eachother or with ethoxysilanes (–Si–OCH2CH3), liberating wateror alcohol respectively, and form siloxane bonds
–Si–O–Si–.
The Si–C bonds linking two ethoxysilanes to the bridgingbenzene ring are hydrolytically stable. Thus, as hydrolysis andcondensation progress, a three-dimensional SiOx network,incorporating benzene rings as network formers, growsand the solution becomes more viscous. Resist preparationrequires only 1 h. The hybrid sol is filtered by a PTFE syringefilter (mesh opening of 0.2 µm) before deposition.
Films can be deposited by spin coating on silicon or silicasubstrates with no need for primer application. Thickness canbe varied within a few tens to some hundreds of nanometres bytuning the solution concentration in EtOH and/or spin coatingspeed. The solution needs to be deposited immediately afterthe synthesis, but then resist films can be stored for severalweeks. For a characterization of the final composition of theresist films with FTIR spectroscopy, the reader is referredto [16, 17].
EBL tests have been performed on films with variablethickness in the 60–300 nm range, prepared using sols of SiO2concentration in the 15–40 g l−1 range. Films are opticallytransparent, and the refractive index spans from 1.54 to 1.51at 633 nm when thermal treatments at temperatures in the80–500 ◦C range are applied to films.
The EBL was carried out using a Vistec EBPG5000operating at 100 keV. Positive tone films were developed ina buffered hydrofluoric acid (BHF, NH4F(40%):HF(49%) =7:1) aqueous solution of 1:10 volume ratio, 60 nm thick filmsfor 60–80 s and 300 nm thick films for 5–8 min. For negativetone 180–250 nm thick films, BHF aqueous solutions of 1:7or 1:8 volume ratio were used with 20–50 s immersion times.After development, samples were rinsed in distilled water.
A PEB was carried out in an oven at temperatures inthe 200–500 ◦C range for time intervals ranging from 1 to120 min, shorter times for higher temperatures and longertimes for lower temperatures. When no PEB is applied,ph-PSQ acts as a negative tone resist. In the presence of a PEBstep, the tone switches from negative to positive. ph-PSQ filmsdeveloped after the PEB undergo densification and lose about15% of the initial thickness, when compared to films withoutannealing.
Micro-FTIR spectra were collected in the imaging areasof 180 × 180 µm2 written at doses up to 9900 µC cm−2 in60 nm thick films for the positive tone, and in 180 nm thickfilms for the negative.
3. Results
3.1. Tone switching from negative to positive
In order to provide a description of the effects produced onph-PSQ films by electron beam irradiation and an explanationof the tone switching when exposure is followed by the bake,micro-FTIR spectra of unprocessed and processed films atcertain doses were acquired.
Figure 1. Micro-FTIR absorption spectra acquired in theunexposed area and the areas irradiated at 6500 and at9900 µC cm−2 of the ph-PSQ films in the absence of a PEB havingnegative tone. For increasing doses there is a reduction in the phenyland in the –OH band absorption, a decrease in the absorption of theSi–OH group, and a formation of condensed silica structures(1070 cm−1).
Figure 1 displays the absorption spectrum of anunexposed film superimposed on the spectra of filmsirradiated with 6500 and 9900 µC cm−2 doses, when noPEB was applied to the films. For increasing doses, there isa progressive reduction in the absorption of the C=C-bondstretching of aromatics at 1516 cm−1, showing that phenylbridging groups linked to the inorganic silica network aregradually eliminated. A second evident effect is the decreaseof the –OH stretching band absorption in the 3400–3200 cm−1
range. Such a phenomenon could be explained by a promotionof condensation reactions. This effect is confirmed also bythe decrease in the stretching absorption of the Si–OH groupand of slightly polymerized silica species at 1000–900 cm−1,accompanied by a formation of the shoulder at 1070 cm−1
suggesting the appearance of condensed silica structures uponirradiation.
Figure 2 shows the spectra of the films when a PEBat 350 ◦C for 30 min is applied. Such thermal treatmentpromotes cross-linking reactions in unexposed areas: theSi–O–Si asymmetric stretching peak at 1074 cm−1 is clearlypresent at zero dose, while the –OH and Si–OH stretchingbands show a negligible absorption. A rather condensedsilica network is present, especially if compared with theunirradiated films of figure 1 in the absence of a PEB. Afterirradiation at a dose of 9900 µC cm−2, the increase of theabsorption peak at 1160 cm−1, typical of highly symmetricsilica structures (cages) [20, 21], is evident. The Si–O–Sistretching peak moves to lower wavenumbers, from 1074to 1060 cm−1, indicating the formation of a different silicanetwork, made of low-symmetry branched structures [20, 21],although the growth of the 1160 cm−1 peak can influence thisposition.
Taking into account the results from the micro-FTIRanalysis, we propose the following explanation of the toneswitching behaviour. In the first case, BHF etching rate ishigher in unexposed areas due to the presence of the polarand reactive Si–OH groups, compared to a cross-linked silica
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Figure 2. Micro-FTIR absorption spectra acquired in theunexposed area and the area irradiated at 9900 µC cm−2, in bothcases in the presence of a PEB at 350 ◦C for 30 min. When PEB isapplied the tone changes from negative to positive. The Si–O–Sistretching peak at 1074 cm−1 is present at zero dose. Afterirradiation, the Si–O–Si stretching peak moves to lowerwavenumbers, while the peak at 1160 cm−1, typical of highlysymmetric silica structures (cages), increases.
network formed after electron beam exposure. The differencein their etching rate drives the negative tone behaviour. Inthe second case, when a PEB is applied, highly cross-linkedand thermally densified silica species characterize unexposedareas, while cage silica structures are present in exposed areas.The positive tone behaviour is determined by the sufficientdifference in BHF etching rate of the two diverse silicastructures.
3.2. Resist performance
Given the interest in positive tone silica-based resists of highperformance, an optimization of the lithographic process wascarried out predominantly concentrating on the positive tonebehaviour. Processing parameters of the different lithographicsteps were explored, analysing both 60 and 300 nm thickfilms, with the aim to optimize the overall resist performance,namely contrast, sensitivity, LER and resolution.
A positive tone was obtained by applying a PEB in thetemperature range of 200–500 ◦C prior to development. In thecase of lower PEB temperatures, the resist did not develop atall. On the other hand, when treated at temperatures higherthan 500 ◦C for longer than a few minutes, the film wasdegraded. Optical microscope images of 200 × 200 µm2
square exposures, used to define contrast in both positive andnegative ph-PSQ films, are shown in figure 3. The opticallyvisible square definition and developed dose span suggest theperformance differences between positive and negative tones.
Beside the tone switching, ph-PSQ resist performanceis strongly affected by the PEB, which was used to tuneand improve the resist properties. Figures 4(a)–(d) show aset of contrast curves of positive tone 60 nm thick filmsprocessed towards the optimization of the sensitivity andcontrast. The best results were achieved either employinghigher temperatures of 500 ◦C for a few minutes, or lower
Figure 3. The contrast curve test exposures in (a) positive 60 nmthick and (b) negative 180 nm thick ph-PSQ films. The dose isincreasing by steps of 15% from left to right on the bottom line andfrom right to left on the top line. The square size is 200× 200 mm2.
temperatures of 300 ◦C for a few hours. Using these optimalPEB conditions, the sensitivity was improved more thanfourfold compared, for example, to the value obtained witha 60 min PEB at 300 ◦C. The PEB had lower impact onthe contrast, which varies in the range of 1.2–1.8, with theoptimal value after a PEB at 300 ◦C for 120 min. The highertemperature treatment is also advantageous when lower LERis desirable. A sensitivity of 1800 µC cm−2 and a contrast of1.5 were obtained (figure 4(a)) with 60 nm thick films.
A PAB step, longer development time, as well asdevelopment delay after the exposure alter the resistsensitivity and contrast. A PAB at 80 ◦C for 30 min doubledthe sensitivity (figure 4(b)), while the films treated at a higherPAB temperatures of 150 ◦C did not develop at all. Weobserved that the development time is a critical parameter,because after too long immersion times the developer alsostarts to attack unexposed areas. Twenty seconds or 25%of additional developing time shifts the sensitivity to valuestwice as small, at the price of a lower contrast (figure 4(c)).
The development delay time was tested by developingthe films five days after the exposure and PEB. The result isillustrated for 300 nm thick films in figure 4(d). The delayimproves the contrast by 50%, with a corresponding decreasein sensitivity of 80%. The combination of both delayed andlonger development times leads to improved contrast valuesup to higher than 20 and sensitivity of 3200 µC cm−2. Theoptimal PEB for 300 nm resist was at 400 ◦C for 15 min.Such evidence is significant as it guarantees that exposed filmscould be stored and developed with a delay of few days, whilstmaintaining a well controlled fabrication process.
The negative tone resist contrast curves are show infigure 5. The best contrast for 250 nm thick resist films wasobtained when developed in a stronger BHF solution with1:7 concentration, compared to 1:10 for the positive. Also,the shorter development time of 20–30 s was sufficient tofully remove the unexposed resist. The remaining pattern isnot sensitive to the BHF solution concentration, but it startsto lose sensitivity when exposed to it for longer time. Thesensitivity in this case is decreasing, which is the oppositebehaviour to the positive tone. Sensitivity and contrast ofnegative tone 250 nm thick resist reaches 3900 µC cm−2 and1.5, respectively. Table 1 summarizes contrast and sensitivityresults of both positive and negative resist.
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Figure 4. Contrast curves of the positive tone ph-PSQ resist. (a) PEB temperature and time effect on the contrast and sensitivity of 60 nmthick films, for a fixed 60 s development time. (b) PAB effect on the contrast and sensitivity of 60 nm thick films, processed with a 5 minPEB at 350 ◦C and a 60 s development time. (c) Development time impact on the contrast and sensitivity of 60 nm thick films after a PEB at350 ◦C for 5 min. (d) Development delay impact on the contrast and sensitivity of 300 nm thick films presenting a PEB at 400 ◦C for 5 min,and a 5–8 min development time.
Figure 5. Contrast curves of 250 nm thick negative tone ph-PSQfilms developed in 1:7 and 1:8 BHF aqueous solutions for 20 and30 s. Negative tone films are more sensitive to development time,but can be developed at different developer concentrations.
3.3. Nanostructure fabrication
Variable resolution patterns of isolated lines, dense lines andspaces, and arrays of holes were written into both positiveand negative tone films. Examples of positive tone patterns in60 nm thick resist with sizes from 250 to 25 nm are shownin figure 6. The ph-PSQ resist patterns have a good linedefinition and usually are fully developed. The cross sectionsof 100 and 250 nm half-pitch lines, shown in figure 7, havestraight side walls and no residual layer between them.
Table 1. Summary of contrast, sensitivity, and etching selectivityvalues for positive and negative tone ph-PSQ resist films. Thecontrast γ is defined using from 10 to 100% of the contrast curve.
The LER, along with resist contrast and sensitivity,is also strongly affected by the PEB. Figure 8 compares50 nm half-pitch dense line patterns formed using a PEBat 300 ◦C for 5 min and for 120 min. Longer PEB timesignificantly improves LER, at the price of sensitivity andcontrast (figure 4(a)). The LER was evaluated using SuMMITsoftware and yielded LER values of 8.82 nm and 4.28 nmfor 120 min and 5 min PEB time, respectively. The processlatitude for the positive resist in the case of 100 nm line is3%.
The negative tone behaviour of the ph-PSQ resist isshown in figure 9. Dense patterns of lines from 1000 nm downto 60 nm half-pitch in 180 nm thick films were realized. In thiscase, the remaining silica structures are completely inorganiclines of up to 3:1 aspect ratio. The negative tone patterns havea thin residual film after development that can be removed inthe following process by dry etching.
Etching selectivity tests for nanostructures fabricatedon silicon substrates were performed using a standard dryetching process in fluorinated plasma (SF6/C4F8), resulting
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Figure 6. SEM images of dense lines, dots and isolated lines in 60 nm thick positive tone ph-PSQ films, fabricated applying either a PEB at500 ◦C for 2 min or a PEB at 300 ◦C for 120 min: (a) 250 nm and (b) 50 nm half-pitch lines; (c) 30 nm isolated lines; (d) 250 nm and(e) 50 nm half-pitch holes; (f) 25 nm half-pitch holes.
Figure 7. Cross sections of 250 nm (a) and 100 nm (b) half-pitchdense line patterns in 60 nm thick positive tone ph-PSQ films.
in a selectivity of 1:4 or 1:9 for the positive tone either inthe absence or in the presence of a PAB respectively, and of1:12 for the negative tone. Similar etching selectivity has beenreported for HSQ [7].
4. Conclusions and outlook
An organic–inorganic hybrid sol–gel material synthesizedstarting from phenyl-bridged silsesquioxane precursors hasbeen presented as a new high resolution resist for EBLat 100 kV. This resist is characterized by the possibility
to switch the tone behaviour from negative to positive, ina buffered hydrochloric acid-based developer, by applyinga post-exposure bake. Thanks to a careful optimization ofthe processing parameters, dense patterns down to 25 nmhalf-pitch and isolated structures down to 30 nm have beenrealized, exploiting the positive tone. Dense patterns downto 60 nm half-pitch have been obtained in the negative tone.Sensitivity, contrast and line edge roughness of the ph-PSQresist were improved using different PEB conditions. Thecritical performance limiting factor for ph-PSQ is line edgeroughness, after current optimization reaching the value of4.28 nm. The resist does not need chemical amplification,allowing an improved film storage and post-exposure stability.Tests of dry etching resistance showed optimized 1:9selectivity for the positive tone and 1:12 for the negative tonewith respect to silicon substrates. Further optimization of theprocessing conditions could reasonably result in an improvedresist performance.
Acknowledgments
The authors gratefully acknowledge M Vockenhuber fromPaul Scherrer Institut for LER evaluation, the Universityof Padova through the PLATFORMS strategic project‘Plasmonic nano-textured materials and architectures for
Figure 8. LER change in 60 nm thick positive tone ph-PSQ resist in the case of (a) 5 min and (b) 120 min PEB time at 300, respectively.
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Figure 9. Negative tone resist patterns in 180 nm thick films. SEM images of (a) 1000 nm, (b) 200 nm and (c) 50 nm half-pitch dense lines.
enhanced molecular sensing’ STPD089KSC, and EU researchproject Multiplat (FP7-NMP-2008-SMALL-2) for partialsupport.
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