528Chapter 8Acid Catalyzed-H"RSO;-H20人cH2RSO:HROHR3HBUOHCH2+U=UncatalyzedHOsS-RT=Trigger PullA=Acid GenerationFigure 8.25 Proposed mechanism for catalyzed and uncatalyzed AA-3HB;R representsp-CeH4CF3(reprinted from Ref.131 with permission;2009 American Chemical Society).Resolution10040 nm38 nm36 nm32 nm30 nmAAs ImproveE(mJ/cm?)Z=5480Z-ParameterLER (nm)869460No AANo AA4020With AAZ■25000.511.522.5LER (nm)49(a)(b)Figure 8.26 (a)Comparison of EUV L/S images (Berkeley MET)of the OS1 controlresist +70 mM 6AB,SB =90 C/60 s,PEB 90 C/90 s.(b)Z-parameter plot for aresist without AA and with 70 mM of 3HB or 6AB,SB 90 C/60 s,PEB =90 C/90 s(SB soft bake;PEB-post-exposure bake)(reprinted from Ref.133 with permission;2010 The Japan Society of Applied Physics).74×108mJnm3to54×10-8mJ-nm3(for3HB)and25×10-8mJ-nm3(for 6AB)[Fig.8.26(b)].34To study the importance of the acid strength on the performance of AAs,the same body,but two types of triggers and two types of acid precursors.They found that AAs that produce fluorinated acids (3AB and 3HB)yieldresists with higher sensitivity,better LER,and a better Z-parameter relative toAAs that produce nonfluorinated toluene sulfonic acid (Fig.8.27).Theseresults show fairly clear evidence that AAs that produce fluorinated acids givethe best combination of sensitivity,LER,and exposure latitude.Photoresists for EUV Lithography5293AA3HA3HBNo AA7.65.44.93.51.9LER(nm)4.04.74.29.17.9Z-Parameter2626196326Figure 8.27 Comparison of EUV images(Berkeley MET)using 60-nm lines and spacesusing the OS1 control resist 70 mM concentrations of AAs:3AA,3AB,3HA,and 3HB(reprinted from Ref.132 with permission from The Society of Photopolymer Science andTechnology).In another study,Kruger et al.systematically varied the chemicalstructure of 12 AAs and studied their reaction kinetics by using theZ-parameter to compare the lithographic performance of EUV resistsprepared with and without 70 mM concentration of AA (Fig.8.28).132.135Seven of the AAs were capable of showing improved lithographic perfor-mance (lower Z-parameter)versus the control resist.The Z-parameterimproved three-fold with the addition of 3MA.For the most part,resistsprepared with perfluorobenzenesulfonate esters (3HC,3MC or 2MC)gavepoor lithographic performance versus the control.The authors demonstratedthat these AAs have the poorest thermal stabilities and explained that thepoor lithographic performance of these highly fluorinated sulfonates was dueto their poor thermal stability in the resist.A study by Maruyama et al.6 compared the lithographic performance ofa commercial resist with and without an undisclosed acid amplifier.Theyfound improvements in resolution (from 26 to 24 nm)and sensitivity (from30.6 to 14.9 mJ/cm2),but with a degradation in LWR (from 4.7 to 5.5 nm).Overall,the Z-parameter improved from 9.1 to 6.1 10-8.8.7.3 Fluorine-stabilized AAs and olefin triggersOne of the biggest challenges in designing AAs for use in CARs is that theAAs that generate fluorinated acids tend to be unstable.Product analysisexperiments'37 showed that the pathways for the unwanted,noncatalyticdecomposition reactions involved either Sn2 or S1 reactions betweenphenolic nucleophiles in polymer films or in solution and the primary sulfonicester bonds [Fig.8.29(a)].Therefore,a strategy was devised for blocking bothdecomposition pathways by preparing AAs with a trifluoromethyl (-CF3)group alpha to the sulfonic ester.The authors proposed that the CF3 wouldreduce the rate of the SN2 pathway by increasing steric hindrance,and wouldreduce the rate of the Sl pathway by preventing the buildup of positivecharge.Another strategy for creating stable AAs that can release highly530Chapter 8Process conditionsControl,No AAE(mJ/cm2)=22.4LER(nm)=6.9±0.3Thickness 125 nm2HA2MA3HA3MAE(mJ/cm2)18.419.216.715.4LER(nm)5.6±0.35.0±0.15.9±0.34.6±0.27672MB23.119.316.118.6LER(nm)10.4±0.37.3±0.24.8±0.24.6±0.2Z×107(mJnm)31132HC2MC3HC3MC9.39.86.97.0LER (nm)7.5±0.37.0±0.27.7±0.47.0±0.4Z×107(mJ-nm60 nm (L/S)980nm)18Figure 8.28 1 control resist without AA and 12 resists with 70 mM AA exposed to EUVlight.The sensitivity Esize,LER,and Z-parameter of these resists are reported for 50-nmequal lines and spaces or at best resolution(reprinted from Ref.135 with permission;2010American Chemical Society).fluorinated acids is to use olefins as triggers instead of eaving groups.3Previous to the work of Kruger et al.,all AAs used triggers that were leavinggroups,125-12Combining both the olefin trigger and the o-CF3 concept into onetrigger-body design,Kruger et al.produced acid amplifiers AA4 and AA5[Fig.8.29(b)],which produce pentafluorobenzene-sulfonic acid (C6FsSO3H,PFBS)and triflic acid (CF3SO3H,TfOH),respectively.Using earlier body-trigger designs,the authors were able to prepare AAs that generatePFBS;however,without the o-CF3 concept,they were never able to preparestable AAs that make acids any stronger than PFBS acid.As described above,itwas possible to prepare AAs that generate PFBS,but these AAs did not providegood lithographic performance due to problems with thermal stability.Theolefin trigger and o-CF3 concept provides AAs that can generate PFBS that isstable for 29 days at 145 C.Triflic acid is a superacid that is 50x stronger thanperfluorooctane sulfonic acid (PFOS),138.139yet its AA is stable for days at140C.In addition to the enhanced stability of these new AAs(AA4 and AA5),they also work well as acid amplifiers,as demonstrated using F NMR(nuclear magnetic resonance)kinetics [Fig.8.29(b)].