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Anal Chem 2006 78 34523460 Carbon13 Labeled Polymers An Alternative Tracer for Depth Profi mg of Polymer Films and Multilayers Using Secondary Ion Mass Spectrometry S E HartonJ F A Stevie Z Zhu and H Ade Department of Materials Science 8 Engineering Analytical Instrumentation Facility and Department of Physics North Carolina State University Raleigh North Carolina 27695 13C labeling is introduced as a tracer for depth pro ling of polymer lms and multilayers using secondary ion mass spectrometry SIMS Deuterium substitution has traditionally been used in depth pro ling of polymers but can affect the phase behavior of the polymer constituents with reported changes in both bulkphase behavior and surface and interfacial interactions SIMS can provide contrast by examining various functional groups chemical moieties or isotopic labels 13CLabeled PS 13CPS and unlabeled PS lzCPS and PMMAwere synthesized using atomtransfer radical polymerization and assembled in several model thin lm systems Depth pro les were recorded using a Cameca IMS6f magnetic sector mass spectrometer using both 60keV impact energi CsJr and 55keV impact energr 02 primary ion bombardment with detection of negative and positive secondary ions respectively Although complete separation of 12OH from 13C is achieved using both primary ion species 60keV CsJr clearly shows improved detection sensitivity and signaltonoise ratio for detection of 12C 12CH and 13C secondary ions The use of Cs primary ion bombardment results in somewhat anomalous nonmonotonic changes in the 12C 12ClH and 13C secondary ion yields through the PSPMMA interface however it is shown that this behavior is not due to sample charging Through normal ization ofthe 13C secondary ion yield to the total C 12C 13C ion yield the observed effects through the PSPMMA interface can be greatly minimized thereby signi cantly improving analysis of polymer lms and multilayers using SIMS Mass spectra of 13CPS and 12CPS were also analyzed using a PHI TRIFI I timeof ight mass spec trometer with 15keV Gat primary ion bombardment and detection of positive secondary ions The 12C71H7 ion fragment and its 13Cenriched analogues have signi cant secondary ion yields with negligible mass interferences providing an early indication of the potential for future use of this technique for cluster probe depth pro ling of high molecular weight 13Clabeled fragments Advances in onedimensional depth profiling of polymer films have provided a significant driving force for growth in experi mental and theoretical polymer physics over the past 20 years1 Particularly useful for investigations involving thin films lt1 um 3452 Analytical Chemistry Vol 78 Na 10 May 15 2006 these measurements generally focus on probing various physical phenomena related to twodimensional con nement of polymer chains including segregation phenomena2 and diffusion properties near surfaces and heterogeneous interfaces3 Several experimental techniques have been used for depth profiling of polymer lms and multilayers1 including neutron NR Xray 0r resonant Xray re ectometry i5 Rutherford bacliscattering6 forward recoil spec trometry PRES7 nuclear reaction analysis NRA 8 and second ary ion mass spectrometry SlMS9 These techniques are frequently used to probe the concentration of a labeled polymer often deuterium 2H substituted1 as a function of depth to observe various phenomena including reactive coupling at polymer interfaces 12 polymer chain mobility near surfaces and inter faces1314 surface or interfacial segregation1515 and block copoly Corresponding author Email haraldadencsuedu lDepartment of Materials Science amp Engineering lAnalytical Instrumentation Facility Department of Physics 1 Kramer E J Physica B 1991 173 189 2 Jones R A L Kramer E J Rafailovich M H Sokolov J Schwarz S A Phys Rev Left 1989 62 280 3 Zheng X Rafailovich M H Sokolov J Y S Schwarz S A Sauer B B Rubinstein M Phys Rev Left 1997 79 241 4Rne RVI I V I s 39 University Press New York 2000 5 Wang C Araki T Ade H Appl Phys Left 2005 87 214109 6 Composto R J Kramer E J Mater Sci 199126 2815 7 Composto R J Walters R M Genzer J Mater Sci Erg R 2002 38 1 Min 8 Chaturvedi U K Steiner U Zak 0 Kmusch G Schatz G Klein J Appl Phys Left 1990 56 1228 9 Schwarz S A Wilkens B J Pudensi M A A Rafailovich M H Sokolov J Zhao X Zhao W Zheng X Russell T P Jones R A L Mol Phys 1992 76 937 10 Harton S E Stevie F AAde H Vac Sci Techriol A 2006 24 362 11 Kim B J Kang H Char K Katsov K Fredrickson G H Kramer E J Macromolecules 2005 38 6106 12 Harton S E Stevie F A Spontak R J Koga T Rafailovich M H Sokolov J C Ade H Polymer 2005 46 10173 13 Russell T DelineV R Dozier W D Felcher G P Agiawal G Wool R P Mays J W Nature 1993 365 235 14 Zheng X Sauer B B Alsten J G V Schwarz S A Rafailovich M H Sokolov J Rubinstein M Phys Rev Left 1995 74 407 15 HarihaianA Kumar S K Rafailovich M H SokolovJ Zheng X Duong D H Schwarz S A Russell T P Chem Phys 1993 99 656 16 Reynolds B J Ruegg M L Mates T E Radke C J Balsam N P Macromolecules 2005 38 3872 101021lac0601330 CCC 3350 2006 American Chemical Society Published on Web 04082006 M kDa 00 I 02 I 04 I 06 I 08 39 10 2 Volume Fraction H PS Figure 1 Isothermal phase diagram of a symmetric blend both polymers have the same number of monomeric segments of 100 deuterium substituted PS 2HPS and unlabeled PS 1HPS gener ated using parameters from ref 20 assuming a concentration independent nominal meanfield interaction parametery 2 x 10 4 125 OC The solid line is the binodal coexistence curve while the dashed line is the spinodal At high molecular weights 2HPS Mn gt 1120 kDa classical phase separation such as nucleation and growth and spinodal decomposition can occur mer ordering 18 Even though deuterium substitution has led to many advances it can affect the phase behavior of the polymer constituents with reported changes in both bulk phase behavior see Figure 1 1920 and surface and interfacial interactions 15 More recently it was found that deuterium substitution can have a profound effect on the properties of a polymer polymer hetero geneous interface with the observation of diffusioncontrolled segregation of deuteriumlabeled polystyrene ZHPS to a 1H PS 2H PSpolymethyl methacrylate PMMA interface21 Because deuterium labeling can introduce changes in the properties being measured alternative methods of tracer labeling are desired for optimal analysis This paper details the development of one such method While the commonly used techniques NR FRES and NRA rely on deuterium substitution to provide contrast in polymer films SIMS can utilize the contrast provided by various functional groups chemical moieties or isotopic labels9 22 23 This makes SIMS particularly versatile for depth profiling of polymer films and multilayers although this versatility comes at a cost Depth profiling using SIMS is particularly sensitive to matrix effects induced by changing densities or chemical environments 25 which are encountered at polymer surfaces polymer polymer heterogeneous interfaces and polymer inorganic substrate in terfaces SIMS involves bombarding a target film with primary ions such as 02 0 Csf Arf Xef Auf Gaf and 360 and detecting positive or negative secondary ions that are sputtered 17 Coulon G Russell T P Deline V R Green P F Macromolecules 1989 22 2581 18 Anastasiadis S H Russell T P Satija S K Majkrzak C F Chem Phys 1990 92 5677 19 Russell T P Macromolecules 1993 26 5819 20 Budkowski A Steiner U Klein J Schatz G Europhys Lett 1992 18 705 21 Harton S E Stevie F A Ade H Macromolecules 2006 39 1639 22 Harrison C Park M Chaikin P M Register R A Adamson D H Yao N Polymer 1998 39 2733 23 Wilson R G Stevie F A Magee C W Secondary Ion Mass Spectrometry A Practical Handbook for Depth Pro ling and Bulk Impurity Analysis John Wiley amp Sons New York 1989 24 Deline V R Katz W Evans C A Williams P Appl Phys Lett 1978 33 832 25 Wilson R G Lux G E Kirschbaum C L Appl Phys 1993 73 2524 from the surface23 26 27 When the primary ion uence is below the socalled static limit 1012 1013 ionscmZ 28 detection is primarily from the top monolayer 1 nm of the film This is the static SIMS technique which is useful for observing surface composition or twodimensional surface imaging27 For onedimensional depth pro ling dynamic SIMS or even threedimensional imaging29 analysis conditions must be implemented where the primary ion uence is above the static limit thereby causing an erosion of the lm at a controlled rate sputtering rate SR This allows for information to be obtained regarding chemical composition as a function of depth through the sample With dynamic SIMS though the chemical information obtainable is often quite different from that with static SIMS analysis as ioninduced mixing and chemical degradation can significantly alter the chemical struc tures below the surface 30 This often limits profiling of organic species to simple atomic or diatomic species such as H C CH 0 CN and Br9 SIMS instruments have three types of mass analyzers namely quadrupole magnetic sector and timeof ight TOF Quadru pole mass spectrometers are commonly used for depth profiling of various types of samples including metals semiconductors and organics Because of the low primary ion impact energies possible with these instruments lt1 keV subnanometer depth resolutions are possible when depth pro ling highly structured semiconductors and depth resolutions less than 10 nm have been reported for depth profiling of polymer films and multilay ers15 16 These instruments are limited by low mass resolution typical mAm 300 making it impossible to completely separate mass resolve secondary ions with identical nominal masses such as 2H 2014 amu from 1H2 2015 65 amu and 13C 1300 335 amu from 12OH 13007 82 amu23 Quadrupole instru ments can however be used for depth profiling of deuterium labeled polymers as there is negligible mass interference from H when detecting negative ions9 In contrast to quadrupole instruments magnetic sector mass spectrometers have relatively high mass resolving capabilities with a maximum mAm N 2 x 104 although 6 8 x 103 is somewhat of a practical upper limit for maintaining reasonable detection sensitivity These instru ments can mass resolve secondary ion species such as 2H from 1H2 and 13C from 12OH with detection of positive or negative secondary ions10 23 Transmission of secondary ions into the detector is much greater for magnetic sector instruments than with quadrupole instruments but charging is often problematic for the analysis of insulators These problems can be overcome with various active or passive charge neutralization measures that include a conductive coating negative primary ions eg 0 and electron bombardment 33 Timeof ight mass spectrometers 26 Honig R E Appl Phys 1958 29 549 27 Winograd N Anal Chem 2005 77 142A 28 Honig R E Int Mass Spectrom Ion Processes 1985 66 31 29 Jerome J Zhu S Seo Y S Ho M Pernodet N Gambino R Sokolov J Rafailovich M H Zaitsev V Schwarz S DiNardo R Macromolecules 2004 37 6504 30 Postawa Z Czerwinski B Winograd N Garrison B J Phys Chem B 2005 109 11973 31 Chia V K F Mount G R Edgell M J Magee C W Vac Sci Technol B 1999 17 2345 32 Pivovarov A L Stevie F A Griffis D P Appl Surf Sci 2004 231 232 786 33 Migeon H N Schuhmacher M Slodzian G Surf Interface Anal 1990 16 9 Analytical Chemistry Vol 78 No 10 May 15 2006 3453 have seen significant growth in recent years particularly with developments of the socalled cluster probes eg A113 SF5 and C600 34 Their mass range mass resolution and secondary ion transmission generally exceed that for the other analyzers Although their use has been restricted primarily to static SIMS compositional analysis and twodimensional imaging 35 TOF SIMS have seen some use in depth pro ling of tracerlabeled polymers in polymer lms36 and their use in depth profiling of somewhat high molecular weight species or fragments 100 amu has received considerable attention in recent years37 40 Here we demonstrate and detail the use of 13C labeling for depth profiling of polymer films and multilayers PS was synthe sized with 13C labeled styrene monomer 13CPS and analyzed in multilayers containing lms of 13CPS or 13C PS unlabeled PS 12C PS blends Using a Cameca IMS6f magnetic sector mass spectrometer we were able to mass resolve 12C1H from 13C with mAm 3000 Two primary ions were investigated 02 and Cst with detection of positive and negative secondary ions respec tively10 02 and Cs are two commercially available primary ion sources that are commonly used for depth profiling using SIMS23 24 1 41 42 Their use is well known to provide enhanced secondary ion yields when compared to primary ion bombardment using inert gases such as Ar43 46 It was found that Cs provides considerably higher 12C 13C and 12OH detection sensitivity than 02 for the conditions implemented but the interface between PS and PMMA is more susceptible to matrix effects with Cs primary ion bombardment Because a submatrix 13C is analyzed and can be normalized to the total carbon secondary ion yield 12C 13C to convert the secondary ion yield to 13CPS concentra tion a reduction in matrix effects encountered at the PSPMMA interface can be realized Mass spectra of 13CPS and 12CPS were measured using a PHI TRIFT I TOF mass spectrometer static SIMS with Gat primary ion bombardment and detection of positive secondary ions These results demonstrate the potential for future use of TOF SIMS for depth profiling high molecular weight 100 amu fragments of 13C labeled polymers 13C labeling of polymers should thus be a very bene cial approach for analysis using TOF or magnetic sector SIMS and the 13C labeling method presented here should be applicable for analysis of various soft condensed matter systems in fields ranging from polymer science to biology EXPERIMENTAL SECTION Polymer Synthesis 13C PS 12CPS and PMMA were synthe sized using atomtransfer radical polymerization47 The chemical structures of PS and PMMA are shown in Figure 2 The 34 Weibel D Wong 8 Lockyer N Blenkinsopp P Hill R Vickerman J C Anal Chem 2003 75 1754 35 Castner D G Nature 2003 422 129 36 Hu X Zhang W Si M Gelfer M Hsiao B Rafailovich M Sokolov J Zaitsev V Schwarz S Macromolecules 2003 36 823 37 Fuoco E R Gillen G Wijesundara M B J Wallace W E Hanley L Phys Chem B 2001 105 3950 38 Wagner M S Anal Chem 2005 77 911 39 Szakal C Sun S Wucher A Winograd N Appl Surf Sci 2004 231 231 183 40 Mahoney C M Yu J Gardella J A Anal Chem 2005 77 3570 41 Wagner M S Anal Chem 2004 76 1264 42 Deline V K Evans C A Williams P Appl Phys Lett 1978 33 578 43 Storms H A Brown K F Stein J D Anal Chem 1977 49 2023 44 Williams R Lewis R K Evans C A Hanley P R Anal Chem 1977 49 1399 3454 Analytical Chemistry Vol 78 No 10 May 15 2006 PS PMMA Figure 2 Chemical structures for PS and PMMA 0L and 6 are the backbone carbons in PS both of which are 13C substituted in the 13CIabeled styrene monomer polymerization mechanisms and procedures have been described in detail previously48 and will only be brie y outlined here Before the synthesis was performed copper I bromide 98 CuBr SigmaAldrich was purified according to established procedures49 Initiator ligand and solvent were used as received For synthesis of atactic 13C PS 2 mL of unlabeled styrene 99 SigmaAldrich and 1 mL of or 13Clabeled styrene monomer 99 Isotec were put through a neutral activated alumina column Into a 10 mL Schlenk ask 105 mg of CuBr 16 uL of NNN N N penta methyldiethylenetriamine 99 PMDETA SigmaAldrich and monomer were added The ask was sealed with a rubber septum and the solution was bubbled with N2 for 20 min to remove 02 Next 42 uL of 1bromoethylbenzene 97 BrEb SigmaAldrich was injected into the solution the ask was immersed in an oil bath held at 110 OC and its contents were continuously stirred using a magnetic stir bar After 75 h the reaction was stopped 7 5 monomer conversion and the solution was diluted with tetrahydrofuran THF Acros and run through a neutral activated alumina column to remove the CuBr This solution was precipi tated into 1 L of methanol MeOH Fisher filtered and dried overnight at 70 0C For atactic 12CPS synthesis 50 mL of styrene which had been put through a neutral activated alumina column 144 mg of CuBr and 210 mL of PMDETA were added to a 100 mL roundbottom ask The ask was sealed with a rubber septum bubbled with N2 for 25 min and 70 nL of BrEb was injected into the solution The solution and ask was immersed in an oil bath held at 110 0C with continuous stirring After 65 h the reaction was stopped and the solution was diluted with THF and run through a neutral activated alumina column This solution was precipitated into 2 L of MeOH filtered and dried overnight at 70 OC The molecular weights and polydispersities of 12CPS and 13CPS were measured using gel permeation chromatography GPC with a chloroform diluent at 30 0C after calibration with atactic PS standards Polymer Laboratories Using differential scanning calorimetry DSC with a cooling cycle of 10 OC min the in ectionpoint glass transition temperatures Tg39s were determined to be 100 0C for both 12CPS and 13CPS For synthesis of PMMA methyl methacrylate gt 99 Fluka was put through a basic activated alumina column To a 125mL roundbottom ask 30 mL of monomer 30 mL of phenyl ether 99 Acros 43 mg of CuBr and 63 ML of PMDETA were added The ask was sealed with a rubber septum and bubbled with N2 45 Franzreb K Lorincik J Williams P Surf Sci 2004 573 291 46 Krohn V E Appl Phys 1962 33 3523 47 Matyjaszewski K Xia J Chem Rev 2001 101 2921 48 Xia J Matyjaszewski K Macromolecules 1997 30 7697 49 Matyjaszewski K Patten T E Xia J H Am Chem Soc 1997 119 674 Table 1 characteristics of Polymer Utilized Polymer Molecular Weights and Polydispersities As Measured Using GPc 30 C cHtIl and Tg s Measured Using DSc 10 Clmin Cooling Cycle polymer MWkDa MwMn TgOC lacPS 794 120 100 12CPS 737 119 100 PMMA 909 1 26 100 Table 2 PolymerSolvent Solutions Used To Prepare the Six Different Sample Typesa 13CPS solution mass polymer vv vol mL solvent 1 40 mg 13CPS 100 1toluene 2 90 mg 12CPS 0 3toluene 3 6 mg 13CPS 31chloropenmne 111 mg 12C PS 4 115 mg PMMA 0 3toluene 5 15 mg 13CPS 100 2benzene 6 15 mg 12CPS 0 2benzene Volume percent 13CPS is on a solventfree basis ie concentration in the cast im for 30 min The ask was then immersed in an oil bath held at 90 C its content continuously stirred using a magnetic stir bar and 42 ML of ethyl 2bromoisobutyrate 98 SigmaAldrich was injected The reaction was stopped after 4 h and the solution was diluted with THF and run through an activated neutral alumina column The solution was then precipitated into 2 L of hexanes Fisher filtered and dried overnight at 70 C Using DSC the Tg was determined to be 100 C 10 Cmin cooling cycle implying a somewhat random monomeric sequence distribution atactic PMMA50 This was further confirmed using proton nuclear magnetic resonance spectroscopy5152 1H NMR Varian Mercury 400 MHz in deuteriumlabeled chloroform 998 Aldrich which shows 4 isotactic 38 heterotactic and 58 syndiotactic triad distributions mm mr and rr respectively PMMA tacticity is highly sensitive to the choice of polymerization technique ie radical or anionic and conditions ie tempera ture solvent and initiator 52 The molecular weight and polydis persity were measured using GPC with a CHC13 diluent at 30 C after calibration with atactic PS standards and converted to the correct values for atactic PMMA using the universal calibration principle53 with known MarkiHouwink parameters for atactic PS K 49 x 10 3 mLg a 0794 and atactic PMMA K 43 x 10 3 mLg a 08054 The characteristics of the polymers utilized are summarized in Table 1 Sample Preparation Six solutions summarized in Table 2 were prepared for six different sample types First HPLC grade benzene Aldrich was distilled over calcium hydride and stored under N2 Toluene Fisher 1chloropentane SigmaAldrich and HPLC grade hheptane Aldrich were used as received Silicon Table 3 Six Different Sample Types Utilizeda h h h sample solnl nrh methodl soln nr2n method 501113 nrii methodg A 1 180 C 2 125 F 0 B 3 175 C 2 125 F 0 C 4 145 C 3 175 C 2 125 F D 4 145 C 3 175 C 4 145 F E 5 70 C 0 0 F 6 70 C 0 0 a Showing the solutions from Table 2 used for each corresponding layer n 1 2 or 3 the thickness ofthe layer ha as measured using ellipsometry and th me od use to prepare the corresponding layer iiei direct casting C or oating F 100 wafers were cut to 25 cm x 25 cm squares soaked in BakerCleanJTB111 JT Baker for 30 min and subsequently washed with deionized DI water They were then etched in 10 vv aqueous hydro uoric acid washed with DI water and placed in a UViozone oven for 30 min to build a SiOx layer 2 nm on the hydrogenpassivated Si surface Samples prepared for dynamic SIMS depth profiling were all spincast at 3000 rpm samples AiD For samples C and D solution 4 was cast and annealed for 30 min at 125 C Solution 3 was then cast directly onto the PMMA layer as 1chloropentane is a selective solvent for PS over PMMA For samples A7C the bottom layers were annealed at 125 C for 24 h and then the top 12CPS layer was cast onto the Si SiOx substrates scored with a sharp tip oated into DI water and picked up with the bottom layers These polymer film assemblies were then annealed at 80 C for 12 h to remove residual solvent while preventing interdii fusion55 For sample D the bottom layers were annealed at 125 C for 24 h and the top PMMA layer was cast onto a microscope cover glass scored with a sharp tip oated into water and picked up wi i the bottom layers This polymer film assembly was annealed at 125 C for 3 h Samples for TOF SIMS analysis samples E and F were prepared using a procedure that was outlined previously58 13C PS solution 5 and 1ZCPS solution 6 were spincast at 5000 rpm onto individual substrates and annealed on a hot plate which had been cleaned wi i h heptane for 5 min at 100 C to remove residual solvent The samples were then washed wi i h heptane to remove any potential surface contaminants such as poly dirnethylsiloxane All layer thicknesses were measured individu ally using singlewavelength ellipsometry Rudolph Auto El III Properties of all six sample types samples AiF are summarized in Table 3 Secondary Ion Mass Spectrometry All dep i profiles were performed using a Cameca IMS6f magnetic sector mass spec trometer A 20nm nominal Au coating was sputtered onto samples AiD before the SIMS analysis to help charging Even though the Au in the rastered area is removed at the beginning of the analysis the remaining Au surrounding the raster crater provides a conductive pa i for charge removal passive charge neutralization 57 Typical analysis conditions for 02 50 Fuchs K Friedrich C Weese J Macromolecules 1996 29 5893 51 Frisch H L Mallows C L Bovey F A Chem Phys 1966 45 1565 52 Ferguson R C Macromolecules 1969 2 237 53 Dobkowski Z Appl Polyrri Sci 1984 29 2683 54 Brandrup J Immergut E H Grulke E A Eds Polymer Handbook 4th ed Wileyrlnterscienoe Hoboken NJ 1999 Vol 2 55 Agrawal G Wool R P Dozier W D Felcher G P Zhou J Pispas S Mays J W Russell T P Polym Sci Part B Polyrri Phys 1996 34 56 Vanden Eynde X Bertiand P Penelle J Macromolecules 200033 5624 57 McKinley J M Stevie F A Granger C N Renard D Vac Sci Techriol A 200018 273 Analytical Chemistry Vol 78 Na 10 May 15 2006 3455 Table 4 Sputtering Rates 53 Approximated Using the Known Thicknesses of the Various Layers in Samples A D and the Known Sputtering Times and Raster Areasa T S Cs R R HmBiO 02 SRSRSi m3iO CS SRSRSi Si 100 0033 10 0095 10 PS 0046 11 011 14 PMMA 011 32 030 34 a Values are also normalized using measured values for intrinsic Si 100 SRSRSi primary ion bombardment included a 30nA primary current rastered over a 180 um x 180 Mm area with 55 keV impact energy 10 kV primary with 45 kV sample bias and mAm 3000 The angle of incidence for the primary ions was 41 Transport of ions in matter TRIM simulations58 using the SRIM 2003 commercial software package show a penetration depth Rp m 10 nm with a straggle ARI m 5 nm for 02 implantation into PS and PMMA under these conditions Positive secondary ions were detected from a 60Mmdiameter optically gated area positioned in the center of the raster For Csf typical analysis conditions included a 10 nA primary current rastered over a 180 pm x 180 Mm area with 60keV impact energy 5 kV primary with 1 kV sample bias and mAm 2910 The angle of incidence for the primary ions was 27 TRIM simulations show Rp m 17 nm with ARp m 3 nm for Cs implantation into PS and PMMA under these conditions Negative secondary ions were detected from a 60Mmdiameter optically gated area positioned in the center of the raster For sample C charge neutralization for Cs bombardment with detection of negative secondary ions was evaluated using the so called electron cloud method33 Electrons at normal incidence to the sample are provided with a potential just below that of the sample and are present just above the surface In this self compensating method any charging due to ion bombardment of the surface of the sample is compensated by electrons that are drawn from the electron cloud For 5 kV primary ions with a 1 kV sample bias the typical electron coverage area is N125 pm in diameter and an ion beam raster 110 pm x 110 pm 30pm diameter detection area was used for the analysis At least two spots per sample were analyzed for samples A D TOF SIMS analyses static SIMS were performed using a PHI TRIFT I TOF mass spectrometer with 15 keV Ga primary ion energy and detection of positive secondary ions A 600pA primary ion current was used over a 100 mm x 100 Mm detection area with a 7 2 kV extraction voltage Data acquisition time was set to 7 min resulting in a total ion uence of 5 x 1011 ionscm2 per analysis Mass spectra collected for three different spots for samples E and F were analyzed using WinCadence software RESULTS AND DISCUSSION The sputtering rates SR for PS and PMMA were determined for both 55 keV 02 and 60keV Cs bombardment from the known thicknesses of the various layers for systems A C as determined using ellipsometry The rate approximations are summarized in Table 4 The real space depth profiles of sample A using the typical analysis conditions for Cs and 02 primary 3456 Analytical Chemistry Vol 78 N0 10 May 15 2006 Ion Yield Countss Volume Fraction of 13CPS o 39160392603936039460 503916039150392603925039300 Effective Depth nm Effective Depth nm Figure 3 SIMS depth profiles of sample A a b showing 12C solid line 12ClH dashed line and 13C bold line secondary ion yields and c d volume fraction of 13CPS in the 12CPS13CPS bilayer Both a c 60keV Cs and b c 55keV 02 primary ion bombardment with detection of negative and positive secondary ions respectively provide high SN and high detection sensitivity 12ClH is completely mass resolved from 13C for both primary ions Shown in c and d are the midpoint 50 and 16 and 84 intensity lines which are used to semiquantitatively describe the asymmetry of the profiles see Table 5 The solid lines in c and d are a fit using a step function convoluted with a Gaussian and an exponential function see eqs 2 4 resulting in the regression parameters summarized in Table 5 The effective depth was determined by assuming a constant PS sputtering rate throughout the film assembly ion bombardment as described above are shown in Figure 3 In Figure 3a and b the 12OH profile clearly traces the 12C profile but not that of 13C This clearly establishes that 12OH and 13C have been completely mass resolved High 13C detection sensitivity for Cs and 02 bombardment is also revealed via the efficient detection of the 13C background natural abundance in the top 12CPS layer Panels c and d of Figure 3 show the normalized profiles for the experimental volume fraction of 13C PS p as a function of effective depth 2 based on the PS sputtering rates as determined by Y13 Z YNA 602 W 1 where Y13z is the 13C ion yield normalized to the total atomic C 12C 13C ion yield at depth 2 YNA is the normalized 13C ion yield of pure 12CPS natural abundance and Yp is the normalized 13C ion yield of pure 13CPS Under the conditions implemented here for both 02 and Cs bombardment excellent signalto noise ratio S N is observed in the profiles in Figure 3c and d even though there is only 103 13C in the 13CPS relative to 12C 13C as determined using TOF SIMS see below The experimental profiles shown in Figure 3c and d for 60 keV Cs and 55 keV 02 reveal asymmetric profiles through the 12CPS13CPS interface for both ion probes The experimental widths corresponding t0 208416 05016 and 58 Ziegler J F Biersack J P Littmark U The Stopping and Range oflons in Solids Pergamon New York 1985 Table 5 Widths of the Profiles Shown in Figure 1cd Based on 8416 203416 5016 05016 and 8450 03450 Intensity Crossings and Regressed Parameters Jeff and la Eqs 2 4 208416 Hm 05016 Hm 08450 Hm Oeff Hm 4d Hm 2 19 7 12 42 10 Cs 14 5 9 17 9 8450 08450 intensity changes are tabulated in Table 5 The differences between the 05016 and 08450 values are a measure of the asymmetry Furthermore the 8416 depth resolution for 60 keV Cs is better than that with 55keV 02 primary ion bombardment 208416 14 and 19 nm respectively which is consistent with previous observations 59 The pro le line shapes and nearly a factor of 2 difference between 05016 and 08450 observed through the 12CPS13CPS interface clearly demonstrate that these profiles cannot be ac curately represented by a symmetric Gaussian instrument resolu tion convoluted with a sharp intrinsic profile The observed convolution type arises primarily from ioninduced mixing of the 13C and 12C matrix and implantation of the 13C further into the film tailing60 Analogous behavior has been observed for 30Si implants in a 28Si matrix60 A physically meaningful convolution function is required in order to compare SIMS depth profiles to various theoretical models when probing physical phenomena at polymer surfaces and heterogeneous interfaces 21 Here we employ a simpli ed version of a convolution scheme that has been outlined previously 61 It combines a Gaussian with a standard deviation Oeff that accounts for ioninduced mixing and sources of uncorrelated convolution such as sample roughness and intrinsic interfacial width16 17 and an exponential for the observed tailing which has a characteristic decay length id The Gaussian convolu tion function is 02 if 2 lt oooogt lt2 1 1 ex 2u1Zae pl 20eff and the exponential decay is described by Fe 1dexp fd 2 000 3 02 and F 2 are numerically convoluted using the Fourier transform method62 with a Heaviside step function 02 16 where 02 0 oo 21 and 02 1 21 oo 4 to approximate the experimental profiles These convoluted profiles are fit to the data with Oeff and id as regression parameters 59 Harton S E Koga T Stevie F A Araki T Ade H Macromolecules 2005 38 10511 60 Dowsett M G Barlow R D Allen P N Vac Sci Technol B 1994 12 186 61 Allen P N Dowsett M G Surf Interface Anal 1994 21 206 62 Press W H Flannery B P Teukolsky S A Vetterling W T Numerical Recipes in Fortrau 77 The Art of Scienti c Computing 2nd ed Cambridge University Press New York 1992 010 cl 008 0 3 006 A 39 39 oo i o w m0 004 gig Q9 0 E 5 002 g 8 101i C 0 CS 00 so 0 o 05 o OOOWe V I g g 105 E 008 d 39gt 10 LL C GE 006 o 0 665006626965 O 103quot o 04 Mg 00 g 29 gt 002 0 O 000 0 100 ii 3 039003bQ lt j f 2 o 100 200 300 400 50 100 150 200 250 Effective Depth nm Effective Depth nm Figure 4 SIMS depth profiles of sample B a bilayer consisting of 12CPS top and 5 vv 13CPS 12CPS showing a b 12C solid line 12ClH dashed line and 13C bold line secondary ion yields and c d volume fraction of 13CPS as a function of effective depth The use of a c 60keV 10 nA Cs primary ion bombardment provides greatly improved detection sensitivity and SN for depth profiling of 13CPS when compared to b d 55keV 30 nA 02 bombardment Results for 60keV Cs and 55 keV 02 primary ion bombardment are summarized in Table 5 The best fit line shapes are also plotted as solid lines in the graphs shown in Figure 3c and d clearly indicating that the combination of a Gaussian and exponential decay function yields excellent results The effective location of the interface 21 was determined by the fit Although an attempt has been made to put physical meaning into the functionalization of the profiles shown in Figure 3c and d molecularlevel complexi ties inherent to SIMS analysis of polymer f1lms25 63 make further interpretation of the regressed parameters Aeff and id difficult The detection sensitivity and S N were further evaluated using sample B which contains a layer of 5 vv 13CPS 12CPS and a top layer of 100 12CPS Here the 13CPS doped film only contains 40 13C above the naturalabundance background Figure 4 shows the depth profiles analyzed using both 60keV Cs and 55keV 02 primary ion bombardment Panels a and b of Figure 4 show complete mass resolution of 12OH from 13C and a marginal increase in 13C secondary ion yield from the 12CPS top layer to the 13CPS doped layer The normalized profiles in Figure 4c and d show decreased S N compared to the profiles in Figure 3c and d Furthermore 60keV 10 nA Cs provides higher detection sensitivity and S N than 55keV 30 nA 02 for 12C 13C and 12OH It is well known that Cs primary ion bombardment often provides improved detection sensitivity depth resolution and sputtering efficiency see Table 4 all of which are observed here for a variety of detected species and matrixes when compared to 02 under similar conditions25 44 Unfortunately the use of Cs has also been observed to produce somewhat anomalous secondary ion yields through a heterogeneous polymer polymer interface particularly with the PSPMMA interface10 As shown in Figure 5 sample C there is quite a difference in secondary ion yields through the 13CPS12CPS PMMA interface for 60keV Cs Figure 5a that is absent for 55keV 02 Figure 5b primary ion bombardment 63 Delcorte A Bertrand P Garrison B J Phys Chem B 2001 105 9474 Analytical Chemistry Vol 78 No 10 May 15 2006 3457 I I I I i 4 5 10 103 391 u 39 2 Ion Yield Countss 2 10 120 395 1 12 1 1o 02 3 H 100 I I I 39 o 100 200 300 460 Effective Depth nm Figure 5 SIMS depth profiles of sample C using a 60keV 10 nA Cs and b 55keV 30 nA 02 primary ion bombardment The effective depth was determined by assuming a constant PS sputtering rate throughout the film assembly The two vertical lines show the approximate location of both the 12CPS12CPS13CPS 135 nm and 12CPS13CPSPMMA 310 nm interfaces A O l A O 0 A O 01 12C Ion Yield Countss 100 200 300 400 Effective Depth nm Figure 6 Effect of variation of 60keV Cs primary ion current on SIMS depth profile of sample C The nonmonotonic changes in the secondary ion yields at the 12CPS13CPSPMMA interface 310 nm appear to be nearly independent of primary ion current This supports the conclusion that sample charging is not the underlying cause of the behavior observed at the heterogeneous polymer polymer interface The two vertical lines show the approximate location of both the 12CPS12CPS13CPS 135 nm and 12CPS 13CPSPMMA 310 nm interfaces The effective depth was determined by assuming a constant PS sputtering rate throughout the film assembly Even though the anomalous 12C secondary ion yield at the 13C PS 1ZCPS PMMA interface is nearly independent of Cs primary ion current Figure 6 implying that sample charging is not the underlying cause of these nonmonotonic changes in secondary ion yields through the interface sample charging was explicitly ruled out by evaluating 12C secondary ion energy spectra at various locations through the film and through the use of active charge neutralization with the so called electron cloud method33 These energy spectra of a 3 nA Cs primary ion beam rastered over a 110 Mm x 110 um area are shown in Figure 7 along with the approximate depth at which each spectrum was recorded inset No significant shift in the location of the 12C yield maximum is observed and a shift would be expected if sample charging occurred Using Cs primary beam and charge neutralization with the electron cloud method depth profiles were generated and are shown in Figure 8 Again similar to Figures 5 and 6 the secondary ion yield through the PSPMMA interface was nonmonotonic even though the sample has now been adequately charge 3458 Analytical Chemistry Vol 78 No 10 May 15 2006 E 100 E quot 1 2 4 E 3104 3 C 391 i103 o 10 2 2 I z 010 0 100 200 300 400 I O Effective Depth nm N x V 10392 U D N 75 10393 E I 39 I 39 U L 0 2 104 I I I I I I I I 25 O 25 50 75 100 Energy eV Figure 7 12C Secondary ion energy spectra at various depths inset for sample C using 60keV 3 nA Cs primary ion bombard ment No signs of charging are present at any depth in the sample O a n O 100 200 300 400 Effective Depth nm Figure 8 SIMS depth profile analysis of sample C using 60keV Cs ion bombardment 3 nA 110 Mm x 110 Mm raster and the electron cloud method of charge neutralization The two vertical lines show the approximate location of both the 12CPS12CPS13CPS 135 nm and 12CPS13CPSPMMA 310 nm interfaces The nonmonotonic changes in the secondary ion yields at the 12CPS 13CPSPMMA interface are still apparent again proving that sample charging is not the underlying cause of this behavior This also confirms that the electron cloud method of charge neutralization can be implemented without causing excess damage to the polymer film The effective depth was determined by assuming a constant PS sputtering rate throughout the film assembly Ion Yield Countss compensated This is significant for two very important reasons First the anomalous behavior observed with the secondary ion yields at the PSPMMA interface see Figure 5a is clearly not due to sample charging Second this confirms that the electron cloud method of charge neutralization which is only currently used on a magnetic sector SIMS instrument33 can be implemented during analysis of systems involving organic species without causing sample degradation because the electrons do not impact the sample thereby permitting these types of analyses to include a broad range of polymer systems and sample thicknesses In an attempt to gain insight into the underlying mechanism behind the changes in secondary ion yields at the PSPMMA interface a trilayer consisting of PMMA as the top and bottom layers with 5 vv 13CPS 12CPS in the middle layer sample D was assembled to look at the interface while sputtering through an interface from PMMA to PS and PS to PMMA respectively SIMS analysis was performed with a 3 nA 60 keV Cs primary ion bombardment rastered over a 180 um x 180 um area Figure 9 clearly shows somewhat erratic changes in secondary ion yields through both interfaces although the behavior is quite different at each interface almost the inverse of 10x107 10x106g 10x105g 10x104 39 10x103 10x10239 sb 20x10 15x106 10x106 50x105 O 100 200 300 400 Effective Depth nm Figure 9 Characterization of a PMMAlZCPS13CPSPMMA film sample D using 60keV 10 nA Cs primary ion bombardment The effective depth was determined by assuming a constant PS sputtering rate throughout the film assembly The two vertical lines show the approximate location of both the PMMAlZCPS13CPS 75 nm and 12CPS13CPSPMMA 250 nm interfaces Changes in 12C 12C1H and 13C secondary ion yields are apparent through both interfaces a although the behavior is quite different at each interface A plot of the 12C secondary ion yield on a linear scale has also been provided b in order to highlight the changes in secondary ion yields at both PSPMMA interfaces Ion Yield Countss 3 1 C E3 006 O o W ammo 004 9 it O a o 000 3 00 100 150 200 250 300 350 400 Effective Depth nm Figure 10 13CPS SIMS depth profile for Sample C showing that normalization of the 13C ion yield to the total C 12C 13C ion yield see eq 1 can help to alleviate the nonmonotonic behavior at the heterogeneous 12CPS13CPSPMMA interface The 12CPS12CPS 13CPS and 12CPS13CPSPMMA interfaces are located at N135 and 310 nm respectively 0 000 22 Volume Fraction 13CPS each other A plot on a linear scale has also been provided in Figure 9b in order to highlight the changes in secondary ion yields at both PSPMMA interfaces A possible mechanism may be a loss of constant equilibrium primary ion concentration transient sputtering when sputtering through the heterogeneous PS PMMA interfaces which could be caused by the large difference in sputtering rates between PS and PMMA see Table 4 Some difference in sputtering rate and secondary ion yield typically occurs at the onset of dynamic SIMS analysis because the amount of the analysis beam present varies until the implant range of the anlysis beam has been reached23 necessitating the addition of a sacrificial layer to the film or multilayer before the analysis which is composed of a polymer identical to the top layer if accurate profiling of the surface region is required910 Further investigation into yield changes across interfaces will be necessary for a better understanding of this intriguing system Fortunately the method delineated here that relies on 13C labeling might greatly help to alleviate nonmonotonic yield changes as 13C a U 13il ED llm 401 BDJHJBD 01H will ll120 Jul 390 A 25 C 3 n o w 43 Ill Eiii 3 l il I l l I l l ll Ill 1 l J Ilil I I JJJ I lJJJl Jilln O 20 40 EU 80 100 120 L1 gt c m 4 C 93 E A l I I 910 915 920 925 930 935 d 700 000 500 400 300 200 100 Mass amu Figure 1 1 TOF SIMS 15 keV Gaf positive secondary ion mass spectra total counts for a 12CPS sample E and b 13CPS sample F from approximately 10 to 130 amu 015 amu bin and c 12CPS and d 13CPS from approximately 91 to 93 amu 0004 amu bin Natural abundance of 13C in 12CPS was verified with c demonstrating that there are negligible mass interferences for the C7H7 fragment secondary ion yields should be affected nearly identically to 12C matrix ion yields through heterogeneous interfaces Through Analytical Chemistry Vol 78 No 10 May 15 2006 3459 normalization of the 13C secondary ion yield to the total C 12C 13C ion yield see eq 1 changes such as those that occur through the PSPMMA interface can be greatly minimized The 13C SIMS depth pro le from Figure 5a has been normalized in this manner and is shown in Figure 10 There is no readily observable artifact or segregation of 13CPS to the 13CPS212CPSPMMA interface In contrast prior results using an analogous 1HPS22HPSPMMA system have shown strong segregation of 83kDa ZHPS CPS used here is 794 kDa to the heterogeneous interface at 138 C21 Note there is less than a 15 change in the 1H PS22HPS phase diagram shown in Figure 1 from 125 to 138 C2 Therefore the use of 13C labeling can greatly improve the characterization of polymer lms and multilayers using SIMS and improve the physical and theoretical interpretation of various experimentally observed phenomena at polymerpolymer heterogeneous inter faces such as reactive couplinglm and polymer chain mobility14 because the effects of tracer labeling on the properties of the system are greatly reduced Finally with the recent advances in the use of high molecular weight cluster probes for TOF SIMS analysis 34 depth pro ling of high molecular weight fragments 100 amu has become possible37quot This may lead to future use of TOF SIMS for high resolution depth pro ling of 13Clabeled polymers To evaluate this possibility we have looked at the mass spectra of 13CPS and 12C PS samples E and F respectively using a PHI TRIFT I TOF mass spectrometer with 15keV Ga r primary ion bombardment and detection of positive secondary ions as shown in Figure 11 Subtle differences can be found between the spectra of 1ZCPS Figure 11a and 13CPS Figure 11b but particular attention is paid to the intense tropylium ion peak at 9105474 amu 12C71H7 t64 and its isotopically labeled analogues Figure 11c and d From Figure 11c natural abundance of 13C is determined from the peaks at 91 and 92 amu nominal for 1ZCPS thereby demonstrating that there are no signi cant mass interferences for this fragment Quantitative depth pro ling of high molecular weight fragments such as the intense tropylium fragment in PS64 may be possible under highly optimized conditions 39 requiring low primary ion implant depths and ef cient removal of damaged molecular layers CONCLUSIONS The use of 13C labeling as an alternative to deuterium labeling for depth pro ling of polymer lms and multilayers using SIMS has been introduced By the changes in bulkm and surface propertiesz15 of polymers and polymer blends due to isotopic labeling signi cant improvements can now be obtained when probing various physical phenomena at polymer surfaces 64 Affrossman S Hartshome M Jerome R Munro H Pethrick R A Petitjean S Vilar M R Macromolecules 1993 26 5400 3460 Analytical Chemistry Vol 78 Na 10 May 15 2006 and heterogeneous interfaces1 To mass resolve 12C1H 13007 82 amu from 13C 13003 35 amu which requires mAm 3000 a magnetic sector mass spectrometer Cameca IMS6f was used with both 60keV impact energy Cs r and 55keV impact energy 02 primary ions with detection of negative and positive secondary ions respectively Complete mass resolution of 12C1H from 13C was achieved for 60keV Cs r and 55keV 02 r primary ion bombardment in a 1ZCPS 13C PS bilayer lm This type of analysis cannot be performed with a quadrupole mass spectrometer with a typical mAm 30031 It was shown that the convolution of the depth pro les analyzed here have to be described by a combina tion of a Gaussian and an exponential function The parameters derived from a t to this combined convolution function have been summarized in Table 5 for both 60keV Cs r and 55keV 02 thereby permitting future analyses to involve comparison of the SIMS pro les to various theoretical models 1 It has also been shown that 60keV CsJr provides improved detection sensitivity and SN over 55keV 02 primary ion bombardment when depth pro ling 13Clabeled polymers although analysis using Cs r appears to be much more susceptible to changes in secondary ion yields through a heterogeneous polymerpolymer interface such as the PSPMMA interface Sample charging was conclusively ruled out as the underlying cause of this behavior at the PSPMMA interface By using 13C as a tracer these matrix effects could be greatly reduced when depth pro ling 1ZCPS213CPSPMMA mul tilayers thereby proving that the use of this novel technique provides a true tracer for analysis of polymer systems Finally mass spectra of 13CPS and 1ZCPS were analyzed using TOF SIMS 15keV GaJr with detection of positive secondary ions with a PHI TRIFTI mass spectrometer to evaluate the potential use of TOF SIMS for depth pro ling of relatively high molecular weight fragments 100 amu of 13Clabeled polymers Detection of C7H7 r secondary ions having signi cant detection sensitivities and negligible mass interferences was proposed for future investiga tions into quantitative molecular depth pro ling using TOF SIMS ACKNOWLEDGMENT This work was supported by the US Department of Energy DEFG0298ER45737 The authors gratefully acknowledge dis cussions with Prof Bruce Novak regarding the polymer synthesis and Dr Dieter Grif s regarding the SIMS analysis We also thank Prof Alan Tonelli for providing helpful comments on the manu script Received for review January 18 2006 Accepted March 9 06 AC060133O
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