Impact Energy Dependence of SF5+-Induced Damage in Poly(methyl

Anal. Chem. 2004, 76, 1264-1272

Impact Energy Dependence of SF5+-Induced Damage in Poly(methyl methacrylate) Studied Using Time-of-Flight Secondary Ion Mass Spectrometry M. S. Wagner*

Surface and Microanalysis Science Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20889-8371

Ion-induced damage of polymers is a critical factor in the depth profiling of polymer surfaces using polyatomic primary ions. In this study, time-of-flight secondary ion mass spectrometry was used to measure the damage of spin-cast poly(methyl methacrylate) (PMMA) films under 5-keV Cs+ and 2.5-8.75-keV SF5+ bombardment. Under 5-keV Cs+ bombardment, the characteristic PMMA secondary ion intensities decreased rapidly for primary ion doses above 5 × 1013 ions/cm2. The damage profiles of PMMA under SF5+ bombardment contained three distinct regions as a function of SF5+ ion dose: a surface transient, an extended quasi-stabilization of the characteristic PMMA secondary ion intensities, and the decay of these intensities as the silicon substrate was reached. The PMMA film sputtered in a controlled manner for SF5+ ion doses up to 4 × 1014 ions/cm2, with the maximum ion dose limited by the thickness of the PMMA film. Furthermore, the chemistry at the bottom of the sputter crater was significantly less modified by SF5+ bombardment when compared with Cs+ bombardment. The sputter rate was linearly correlated with the SF5+ impact energy while the damage to the PMMA film varied minimally with the SF5+ impact energy. These results were compared with Monte Carlo (SRIM) calculations of the penetration depth and vacancy production for SF5+ at different impact energies. Since the SF5+ impact energy only affected the sputter rate, selection of the appropriate SF5+ impact energy for polymer depth profiling depends solely on the desired sputter rate. Characterization of advanced polymeric materials in three dimensions is critical in their design and development. Secondary ion mass spectroscopy (SIMS) is an excellent technique for spatially resolved analysis of polymers, generating chemical maps of polymer surfaces with lateral resolutions approaching 1 µm.1,2 SIMS has also been used on a limited basis for depth profiling of polymer films, providing elemental information over the depth range of 10 nm to several micrometers.3-17 However, depth * Phone: (301) 975-8542. Fax: (301) 417-1321. E-mail: matthew.wagner@ nist.gov. (1) Adriaens, A.; vanVaeck, L.; Adams, F. Mass Spectrom. Rev. 1999, 18, 48. (2) Pacholski, M. L.; Winograd, N. Chem. Rev. 1999, 99, 2977.

1264 Analytical Chemistry, Vol. 76, No. 5, March 1, 2004

profiling analysis of polymers using monatomic primary ion beams under dynamic SIMS conditions has been limited to examining the monatomic (e.g., H-, C-, O-, F-) and diatomic (e.g., CH-, CN-) secondary ions due to primary ion beam-induced sample damage.18-23 Due to sample damage, molecular information has been unattainable using monatomic primary ion beams for depth profiling. Recent advances in SIMS instrumentation have focused on the use of polyatomic primary ion sources for enhancing the molecular secondary ion yields of organic materials. Several polyatomic ion sources have been utilized, with the first polyatomic primary ion beam used for SIMS measurements being SF6-,24 followed by subsequent experiments using SF5+.25,26 Many other polyatomic (3) Chujo, R. Polym. J. 1991, 23, 367. (4) Stein, J.; Leonard, T. M.; Smith, G. A. J. Appl. Polym. Sci. 1991, 42, 2355. (5) Gray, K. H.; Gould, S.; Leasure, R. M.; Musselman, I. H.; Lee, J. J.; Meyer, T. J.; Linton, R. W. J. Vac. Sci. Technol., A 1992, 10, 2679. (6) Schwarz, S. A.; Wilkens, B. J.; Pudensi, M. A.; Rafailovich, M. H.; Sokolov, J.; Zhao, X.; Zhao, W.; Zheng, X.; Russell, T. P.; Jones, R. A. L. Mol. Phys. 1992, 76, 937. (7) Sauer, G.; Kilo, M.; Hund, M.; Wokaun, A.; Karg, S.; Meier, M.; Reib, W.; Schwoerer, M.; Suzuki, H.; Simmerer, J.; Meyer, H.; Haarer, D. Fresenius J. Anal. Chem. 1995, 353, 642. (8) Pinto, J. R.; Novak, S. W.; Nicholas, M. J. Phys. Chem. B 1999, 103, 8026. (9) Rysz, J.; Ermer, H.; Budkowski, A.; Lekka, M.; Bernasik, A.; Wrobel, S.; Brenn, R.; Lekki, J.; Jedlinski, J. Vacuum 1999, 54, 303. (10) Yokoyama, H.; Kramer, E. J.; Hajduk, D. A.; Bates, F. S. Macromolecules 1999, 32, 3352. (11) Mattsson, J.; Forrest, J. A.; Krozer, A.; Sodervall, U.; Wennerberg, A.; Torell, L. M. Electrochim. Acta 2000, 45, 1453. (12) Chum-Guey, W.; Jiunn-Yih, H.; Shui-Sheng, H. J. Mater. Chem. 2001, 11, 2061. (13) Duan, Y.; Pearce, E. M.; Kwei, T. K.; Hu, X.; Rafailovich, M.; Sokolov, J.; Zhou, K.; Schwarz, S. Macromolecules 2001, 34, 6761. (14) Huang, W. Y.; Matsuoka, S.; Kwei, T. K.; Okamoto, Y.; Hu, X.; Rafailovich, M. H.; Sokolov, J. Macromolecules 2001, 34, 7809. (15) Bulle-Lieuwma, C. W. T.; van Gennip, W. J. H.; van Duren, J. K. J.; Jonkheijm, P.; Janssen, R. A. J.; Niemantsverdriet, J. W. Appl. Surf. Sci. 2003, 203204, 547. (16) Lin, H. C.; Tsai, I. F.; Yang, A. C. M.; Hsu, M. S.; Ling, Y. C. Macromolecules 2003, 36, 2464. (17) Strzhernechny, Y. M.; Schwarz, S. A.; Schachter, J.; Rafailovich, M. H.; Sokolov, J. J. Vac. Sci. Technol., A 2003, 15, 894. (18) Briggs, D.; Hearn, M. J.; Ratner, B. D. Surf. Interface Anal. 1984, 6, 184. (19) Simko, S. J.; Griffis, D. P.; Murray, R. W.; Linton, R. W. Anal. Chem. 1985, 57, 137. (20) Briggs, D.; Hearn, M. J. Vacuum 1986, 36, 1005. (21) Leggett, G. J.; Vickerman, J. C. Appl. Surf. Sci. 1992, 55, 105. (22) Gilmore, I. S.; Seah, M. P. Surf. Interface Anal. 1996, 24, 746. (23) Briggs, D.; Fletcher, I. W. Surf. Interface Anal. 1997, 25, 167. 10.1021/ac035330r Not subject to U.S. Copyright. Publ. 2004 Am. Chem. Soc.

Published on Web 01/22/2004

primary ions have also shown significant improvement in the secondary ion yields of both inorganic and organic species when compared with monatomic primary ions. The origin of this effect is an active area of investigation, with the increase in ionization probability and sputter yield and reduction of fragmentation contributing to the sensitivity enhancement.26,27 Molecular dynamics simulations suggest that the sensitivity improvement is due to an upward (surface-oriented) momentum imparted on large molecular fragments after polyatomic cluster ion bombardment 28,29 and the deposition of much of the damage in the near-surface region.30 However, this effect depends on the substrate, with more massive or more densely packed substrate atoms negating the enhancement of secondary ion yields of organic monolayers under polyatomic primary ion bombardment.31 Under polyatomic primary ion bombardment, some organic surfaces have displayed unique resistance to primary ion beam damage. Initial studies showed that molecular and characteristic fragment secondary ions were obtainable from thin films of small organic molecules (e.g., amino acids, organic salts) and polymers (poly(methyl methacrylate), PMMA) after extended (>1 × 1015 ions/cm2) SF5+ bombardment.26 The distribution of the impact energy of the polyatomic ion to its constituent atoms and high sputter rates were thought to contribute to this observation by removing ion beam-induced damage in the same event that created it. That study was followed by an examination of several different polymers under SF5+ bombardment, concluding that only PMMA and poly(ethylene glycol) continued to produce characteristic secondary ions after extended SF5+ bombardment.32 Comparison of the depth profiles of thin PMMA and poly(vinyl chloride) (PVC) films spin-cast onto silicon wafers indicated that PMMA had a higher sputter rate than PVC under SF5+ bombardment.33 However, that study did not examine the resulting chemistry of the bombarded polymer. Other studies have reported damage cross sections in the static SIMS regime for some polymer surfaces under SF5+ 25,34 and C60+ 35 bombardment. The C60+ study also reported the damage of spin-cast poly(styrene) (PS) films for C60+ ion doses up to 1.3 × 1015 ions/cm2, showing that the characteristic secondary ions for PS were still observed after extended C60+ bombardment.35 The present study complements previous studies by systematically investigating the effect of primary ion impact energy on the (24) Appelhans, A. D.; Delmore, J. E. Anal. Chem. 1989, 61, 1087. (25) Kotter, F.; Benninghoven, A. Appl. Surf. Sci. 1998, 133, 47. (26) Gillen, G.; Roberson, S. Rapid Commun. Mass Spectrom. 1998, 12, 1303. (27) Fuoco, E. R., Gillen, G.; Wijesundara, M. B. J.; Wallace, W. E.; Hanley, L. J. Phys. Chem. B 2001, 105, 3950. (28) Townes, J. A.; White, A. K.; Wiggins, E. N.; Krantzman, K. D.; Garrison, B. J.; Winograd, N. J. Phys. Chem. A 1999, 103, 4587. (29) Nguyen, T. C.; Ward, D. W.; Townes, J. A.; White, A. K.; Krantzman, K. D.; Garrison, B. J. J. Phys. Chem. B 2000, 104, 8221. (30) Postawa, Z.; Czerwinski, B.; Szewczyk, M.; Smiley, E. J.; Winograd, N.; Garrison, B. J. Anal. Chem. 2003, 75, 4402. (31) Krantzman, K. D.; Fenno, R.; Delcorte, A.; Garrison, B. J. Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 202, 201. (32) Brox, O.; Hellweg, S.; Benninghoven, A. In Dynamic SIMS of Polymer Films; Benninghoven, A., Bertrand, P., Migeon, H. N., Werner, H. W., Eds.; Elsevier: Brussels, 1999; p 777. (33) Norrman, K.; Haugshoj, K. B.; Larsen, N. B. J. Phys. Chem. B 2002, 106, 13114. (34) Stapel, D., Thiemann, M.; Benninghoven, A. Appl. Surf. Sci. 2000, 158, 362. (35) Weibel, D. E.; Wong, S.; Lockyer, N. P.; Blenkinsopp, P.; Hill, R.; Vickerman, J. C. Anal. Chem. 2003, 75, 1754.

damage of a model polymer under extended SF5+ primary ion bombardment. Dual-beam depth profiling36 was used to investigate the damage induced by 2.5-8.75-keV SF5+ and 5-keV Cs+ on thin films of PMMA spin-cast onto silicon wafers. Sputter ion doses in the range necessary to profile through the polymer films and uncover the silicon substrate (typically 1 × 1015-2 × 1015 ions/ cm2 for SF5+) were used. Positive and negative ion TOF-SIMS were used to evaluate the influence of SF5+ impact energy on the sputter rate and damage accumulation in PMMA for the optimization of polymer depth profiling. The damage profiles were used to determine the primary ion-induced chemical reactions that occurred in PMMA under Cs+ and SF5+ bombardment. MATERIALS AND METHODS Sample Preparation. Poly(methyl methacrylate) (atactic, MW ≈ 100 000) was purchased from Polysciences, Inc. (Warrington, PA)37 and was used without further purification. A 2% (weight/ volume) PMMA solution in toluene was spin-cast (2000 rpm for 30 s) onto 1 cm × 1 cm silicon wafer pieces. The silicon wafer pieces were ultrasonically cleaned in methylene chloride, acetone, and methanol prior to polymer deposition. The thickness of the polymer films was measured to be 100.4 ( 0.8 nm (average ( 1 standard deviation, average of measurements made on five representative samples) using a model M-2000 Spectroscopic Ellipsometer (J. A. Woollam Co., Lincoln, NE). Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS). Positive and negative ion static TOF-SIMS spectra were obtained using an Ion-Tof IV (Ion-Tof GmbH, Muenster, Germany) equipped with a 10-keV electron impact Ar+ ion source operated at ∼2 pA of pulsed ion current (100-µs duty cycle) and an impact angle of 45°. Sputtering with SF5+ or Cs+ was performed using an electron impact SF5+ or surface ionization Cs+ ion source operated at ∼2 nA of static ion current and an impact angle of 45°. Damage profiles of the spin-cast polymer films were obtained by alternately collecting static TOF-SIMS spectra using the Ar+ ion beam (rastered over a 200 µm × 200 µm area) with sputtering using the SF5+ or Cs+ ion beam (rastered over a 1000 µm × 1000 µm area). The ion dose for each TOF-SIMS spectrum collected using the Ar+ ion source was maintained below the static limit of 1012 ions/cm2,18,22,23 though the total Ar+ ion dose during a damage profile was as much as 6 × 1013 ions/cm2. A pulsed low-energy electron flood gun was used for charge neutralization during the acquisition of the positive ion static TOF-SIMS spectra. The accumulation of any electron beam-induced damage38 should be minimized by the SF5+ sputtering cycles, though electron beaminduced damage was not specifically characterized in this study. In this TOF-SIMS instrument, the sample is at ground potential, allowing the impact energy of the SF5+ ion beam to vary at a constant incidence angle. Dead-time correction for the multichannel plates in the TOF-MS39 did not significantly change the (36) Iltgen, K.; Bendel, C.; Benninghoven, A.; Niehuis, E. J. Vac. Sci. Technol., A 1997, 15, 460. (37) Certain commercial equipment, instruments or materials are identified in this paper to specify adequately the experimental procedure. Such identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. (38) Gilmore, I. S.; Seah, M. P. Appl. Surf. Sci. 2002, 187, 89. (39) Stephan, T.; Zehnpfenning, J.; Benninghoven, A. J. Vac. Sci. Technol., A 1994, 12, 405.

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Table 1. Major Positive and Negative Ion Fragments of Poly(methyl methacrylate) Observed in the TOF-SIMS Spectra Acquired Using 10-keV Ar+

damage (vacancy production) profile of an energetic ion into matter. SRIM calculations in this study were calculated using 5000 incident ions into 500-Å-thick poly(methyl methacrylate) targets. RESULTS AND DISCUSSION Radiation-induced degradation of PMMA occurs through two main pathways: (1) methyl ester pendant group loss accompanied by main-chain cross-linking or main-chain scission47

-(C5H8O2)n- (s) f methyl ester pendant group loss f main-chain cross-linking + main-chain scission (R1) and (2) chain end-initiated or random depolymerization48

-(C5H8O2)n- (s) f C5H8O2 (g)

observed damage profiles and was not used for the results reported in this study. The characteristic positive and negative secondary ions for PMMA are shown in Table 1, with the proposed structures described previously.40-44 Damage Cross Section Calculation. Damage cross sections for the characteristic PMMA secondary ions shown in Table 1 were calculated using the relationship given by Stapel et al.:45

-ln(Ni/Ni0) ) σiνt

(1)

where σi is the damage cross section for fragment i of the mass spectrum, Ni is the observed secondary ion intensity for this fragment after bombardment for a time of length t under a constant primary ion flux of ν (with the product νt, therefore, being the primary ion dose density in ions/cm2), and Ni0 is the initial secondary ion intensity of fragment i. σi is the slope of a linear fit to - ln(Ni/Ni0) versus νt. In the case of the dual-beam approach to depth profiling, the primary ion dose of the analysis beam was much less than the sputter ion dose, minimizing the impact of the analysis beam on the damage cross-section calculations. Monte Carlo Calculations. Ion ranges in poly(methyl methacrylate) were calculated using the Stopping Range of Ions in Matter (SRIM)46 software. SRIM uses Monte Carlo calculations to estimate the penetration depth, concentration profile, and (40) Hearn, M. J.; Briggs, D. Surf. Interface Anal. 1988, 11, 198. (41) Brinkhuis, R. H. G.; van Ooij, W. J. Surf. Interface Anal. 1988, 11, 214. (42) Lub, J.; Benninghoven, A. Org. Mass Spectrom. 1989, 24, 164. (43) Leeson, A. M., Alexander, M. R.; Short, R. D.; Briggs, D.; Hearn, M. J. Surf. Interface Anal. 1997, 25, 261. (44) Briggs, D.; Fletcher, I. W.; Goncalves, N. M. Surf. Interface Anal. 2000, 29, 303. (45) Stapel, D., Brox, O.; Benninghoven, A. Appl. Surf. Sci. 1999, 140, 156. (46) Ziegler, J. F.; Biersack, J. P. The Stopping and Range of Ions in Matter, v. SRIM-2003.12. More information on this software can be found at http:// www.srim.org.

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(R2)

Pendant group loss is accompanied by oxygen loss from the surface (since all of the oxygen in PMMA is contained in the methyl ester pendant group). Main-chain cross-linking or scission also results in the formation of unsaturated carbons in the PMMA backbone. Reaction 1 is signified by an increase in both the carbon-to-oxygen and carbon-to-hydrogen elemental ratios. Reaction 2, however, is accompanied by evolution of gaseous MMA monomer without changing the overall elemental ratios of carbon to oxygen or carbon to hydrogen. Note, however, that the sputtering of whole monomer units (without extended depolymerization) can also result in the retention of the molecular composition of the PMMA film and a film that is indistinguishable from that produced by reaction 2. Reactions 1 and 2 produce distinctly different behaviors in the bombarded polymer and should provide insight into the damage of PMMA by Cs+ and SF5+. Comparison of Monatomic and Polyatomic Bombardment. Figure 1 shows a comparison of representative positive ion spectra of (a) an undamaged PMMA film and a PMMA film after a (b) 5-keV Cs+ and (c) 5-keV SF5+ primary ion dose of 3.5 × 1014 ions/cm2. The undamaged PMMA spectrum shows all of the characteristic PMMA fragments listed in Table 1. After 5-keV Cs+ bombardment, the Cs+ secondary ion was the dominant peak in the positive ion damage profiles due to implantation of cesium in the sample. The characteristic PMMA secondary ions had disappeared from the spectrum after this primary ion dose. After 5-keV SF5+ bombardment, the total secondary ion intensity was reduced, but all of the characteristic PMMA secondary ions were still present in the spectrum. These spectra suggest that the damage of PMMA under monatomic and polyatomic bombardment was substantially different, opening the possibility of depth profiling the polymer film using polyatomic primary ion bombardment. Figure 2a shows a comparison of the positive ion damage profiles for PMMA thin films under 5-keV Cs+ or SF5+ bombardment. The C4H5O+ (m/z ) 69) and Si+ (m/z ) 28) secondary ion (47) Moore, J. A.; Choi, J. O. In Radiation Effects on Polymers; Clough, R. L., Shalaby, S. W., Eds.; American Chemical Society: Washington, DC, 1991; p156. (48) Fragala, M. E., Compagnini, G., Torrisi, L., Puglisi, O. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 141, 169.

have been covered with cesium. The silicon secondary ion intensity at the beginning of the damage profile was due to trace contamination with silicon dust from the silicon substrate or poly(dimethylsiloxane). This contamination was removed by Cs+ sputtering. In the negative ion mode, the characteristic PMMA secondary ion intensities followed trends similar to those observed in the positive ion mode. The negative ion damage profiles also contained information on the elemental composition and degree of saturation of the surface carbon atoms. A qualitative estimate of the C/O elemental ratio and the degree of saturation of the carbons present on the surface was given by the following negative secondary ion intensity ratios:

C/O )

IC- + ICH- + IC2- + IC2HIO- + IOH-

(2)

and

C-H/C-C )

Figure 1. Comparison of representative positive ion spectra of (a) an undamaged PMMA film and a PMMA film after a (b) 5-keV Cs+ and (c) 5-keV SF5+ primary ion dose of 3.5 × 1014 ions/cm2.

intensities are characteristic of the polymer film and the silicon substrate, respectively. Under 5-keV Cs+ bombardment, the C4H5O+ secondary ion intensity decreased rapidly, dropping below 5 counts/s after a Cs+ dose of 3 × 1014 ions/cm2. Similar trends were observed for the other characteristic PMMA secondary ions listed in Table 1. In addition, the total (0-200 m/z, excluding Cs+ at m/z ) 133) secondary ion intensity followed a trend similar to the characteristic PMMA secondary ion intensities, decreasing rapidly as a function of Cs+ primary ion dose. The Cs+ secondary ion rapidly became the dominant peak in the positive ion damage profiles as the surface was implanted with cesium. The silicon substrate was not reached after a Cs+ primary ion dose of 2.5 × 1015 ions/cm2, though it is possible that the silicon substrate may

IH- + ICH- + IC2HIC- + IC2-

(3)

An increase in the C/O ratio would suggest a loss of oxygen from the surface and a predominance of reaction 1 described above while a stable C/O ratio would suggest a predominance of reaction 2 or the sputtering of whole monomer units. A decrease in the C-H/C-C ratio would suggest the formation of unsaturated hydrocarbons on the surface and a predominance of reation 1 while a stable C-H/C-C ratio would suggest either the predominance of reaction 2 or the sputtering of whole monomer units. Under 5-keV Cs+ bombardment, the C/O ratio rapidly increased, reaching a maximum at ∼1 × 1015 ions/cm2 while the C-H/C-C ratio decreased as a function of Cs+ ion dose (Figure 2b). These ratios both suggested that reaction 1 was predominant for PMMA under Cs+ bombardment. The positive and negative ion damage profiles of PMMA under 5-keV Cs+ bombardment suggested that a damaged surface layer was formed in which the methyl ester pendant group of the PMMA had been removed. The increase in the degree of unsaturation of the surface carbons suggests that the remaining polymer backbone chains were highly cross-linked, resulting in a “carbon black-like” damaged surface layer.49 Additionally, initial studies determining the static limit for SIMS measurements of organic materials showed that ion-induced damage in PMMA resulted in the formation of unsaturated polycyclic aromatic hydrocarbons and loss of methyl ester pendant groups (and, therefore, oxygen) from the polymer surface.20,50,51 Other investigators have also noted the formation of the unsaturated aromatic hydrocarbons and decrease in the total secondary ion intensity as a function of primary ion dose density.21,23,52 Previous studies have also shown that bombardment of PMMA by 200-keV He+ or 400-keV Ar+ resulted in the cross-linking of the polymer film, (49) Netcheva, S.; Bertrand, P. Nucl. Instrum. Methods Phys. Res., Sect. B 1999, 151, 129. (50) Briggs, D.; Wootton, A. B. Surf. Interface Anal. 1982, 4, 109. (51) Briggs, D. Surf. Interface Anal. 1982, 4, 151. (52) Lhoest, J.-B.; Dewez, J.-L.; Bertrand, P. Nucl. Instr. Methods Phys. Res., Sect. B 1995, 105, 322.


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Figure 2. Comparison of the ion beam-induced damage of PMMA by 5-keV Cs+ and SF5+ monitored using (a) positive and (b) negative ion TOF-SIMS. The Si+ (m/z ) 28, diamonds) and C4H5O+ (m/z ) 69, squares) signals in (a) were normalized to the initial C4H5O+ signal for Cs+ and SF5+. Filled symbols are used for Cs+ and open symbols are used for SF5+. In the negative ion mode, the C/O and C-H/C-C secondary ion intensity ratios were used. The arrow in (b) marks the SF5+ ion dose at which the SiO2- (m/z ) 60) reached 50% of its maximum intensity. Both (a) and (b) represent the average of four damage profiles for Cs+ and SF5+.

rendering it partially insoluable in chloroform (which is a solvent for PMMA).53 Since the silicon substrate was not reached during Cs+ bombardment, the sputter rate for Cs+ must have been substantially lower than that of SF5+. The sputter rate of PMMA under 5.5-keV Ar+ bombardment has been shown to decrease as a function of Ar+ primary ion dose,27 and the sputter rate under 5-keV Cs+ bombardment used in this study may have followed the same behavior. These results suggest that loss of the methyl ester pendant groups and main-chain cross-linking (reaction 1 described above) contributed to the accumulation of damage in PMMA under 5-keV Cs+ bombardment. The damage profile of PMMA under 5-keV SF5+ bombardment was significantly different than under 5-keV Cs+ bombardment (Figure 2a) and was similar to previous studies.26,27,32 The damage profile of PMMA under SF5+ bombardment can be broken into three distinct regions, shown in Figure 2a: (i) Instead of rapidly decreasing in intensity to background levels, the characteristic PMMA secondary ion intensities de(53) Licciardello, A.; Fragala, M. E.; Foti, G.; Compagnini, G.; Puglisi, O. Nucl. Instr. Methods Phys. Res., Sect. B 1996, 116, 168.


creased rapidly to 40-50% of their initial intensity ((0-1) × 1014 ions/cm2). (ii) The characteristic PMMA secondary ion intensities then decreased more slowly in a “pseudo steady state”, reaching ∼2030% of their initial intensity (1 × 1014-4 × 1014 ions/cm2). (iii) Finally, the Si+ secondary ion intensity increased while the characteristic PMMA secondary ion intensities decreased to background levels (>4 × 1014 ions/cm2). The positive total secondary ion intensity and the negative characteristic PMMA secondary ion intensities also followed similar trends. The silicon secondary ion intensity at the beginning of the damage profile was due to trace contamination with silicon dust from the silicon substrate or poly(dimethylsiloxane). This contamination was removed by SF5+ sputtering. In the negative ion damage profiles for PMMA under 5-keV SF5+ bombardment (Figure 2b), the C/O ratio did not increase as significantly as under Cs+ bombardment. The maximum C/O ratio under Cs+ bombardment was ∼20 times greater than the C/O ratio when the SiO2- signal reached 50% of its maximum under SF5+ bombardment (marked with an arrow in Figure 2b). Additionally, the degree of saturation of the surface carbon did not decrease as rapidly under SF5+ bombardment as it did under Cs+ bombardment. The accumulation of sulfur (S-, m/z ) 32) and fluorine (F-, m/z ) 19) was noticeable during the sputtering of the polymer film and increased rapidly once the silicon substrate was reached. While the treatment of polymers with SF6 plasmas show the formation of fluorocarbon groups on the surface,54,55 no fluorocarbon secondary ions were observed here in either the positive or the negative ion spectra after extended bombardment with SF5+. If any fluorocarbons were formed on the surface, their concentration was too low to be detected using the TOF-SIMS conditions used in this study. Figure 3a shows the damage cross sections of the characteristic PMMA positive secondary ions under 5-keV Cs+ and SF5+ bombardment. For both Cs+ and SF5+ bombardment, the damage cross section was not correlated with the identity of the secondary ion. Additionally, the damage cross sections under 5-keV Cs+ bombardment were 2-5 times higher than the damage cross sections in regions i or iii under SF5+ bombardment and 10-20 times higher than region ii. The damage cross sections measured in region i of the damage profiles were similar to measurements previously made under static SIMS conditions.25,34,35 However, the change in damage cross section from region i to region ii made depth profiling of PMMA using SF5+ possible. The damage cross section for the positive total secondary ion intensity in region ii was 0.15 ( 0.01 nm2, which was comparable to the damage cross section of the positive characteristic PMMA secondary ions. Therefore, each spectrum in the damage profile was normalized to its total secondary ion intensity to correct for this decrease in total secondary ion intensity across the damage profile. Normalization of the characteristic PMMA secondary ions in this way significantly reduced the damage cross section for the characteristic PMMA secondary ions (Figure 3b). The differences between the unnormalized and normalized damage profiles are currently an active area of investigation. Normalization of the Cs+ damage (54) Leonard, D.; Bertrand, P.; Khairallah-Abdelnour, Y.; Arefi-Khonsari, F.; Amouroux, J. Surf. Interface Anal. 1995, 23, 467. (55) Selli, E.; Mazzone, G.; Oliva, C.; Martini, F.; Riccardi, C.; Barni, R.; Marcandalli, B.; Massafra, M. R. J. Mater. Chem. 2001, 11, 1985.

Figure 4. C4H5O+ (m/z ) 69) secondary ion intensity (normalized to its initial intensity for each damage profile) as a function of SF5+ ion dose for impact energies between 2.5 and 8.75 keV. The SF5+ impact energy is marked on each of the damage profiles. Each damage profile shown represents the average of four damage profiles.

Figure 3. (a) Damage cross sections for the characteristics PMMA positive secondary ions listed in Table 1 under 5-keV Cs+ or SF5+ bombardment. The damage cross sections for each of the three regions of the SF5+ damage profile are reported. Error bars represent (1 standard deviation. (b) The damage cross sections of the characteristic PMMA secondary ions were reduced by normalization to the total secondary ion intensity. Note that 1 nm2 ) 10-14 cm2.

profiles did not result in a similar decrease in the damage cross sections of the characteristic PMMA secondary ions (data not shown). Ion-induced damage of polymer films is a balance between the damage of the polymer film by the incident ion and the removal of damaged material by sputtering.56 The high and constant sputter rate of PMMA under SF5+ bombardment was probably responsible for the low accumulation of damage under SF5+ bombardment. While reaction 1 described above probably happened under SF5+ bombardment, reaction 2 also played an important role by increasing the sputter rate and removing damage as it occurred in the PMMA film. Depolymerization (reaction 2) would also maintain a constant molecular composition at the bottom of the sputter crater, which was observed for the PMMA films under SF5+ bombardment. The maintenance of the molecular ion signals could also suggest that whole monomer units were sputtering by SF5+ (as opposed to extended depolymerization). The decreased damage cross sections of the characteristic PMMA secondary ion intensities and the relatively low change in the C/O and C-H/ C-C negative secondary ion ratios suggested that the PMMA at the bottom of the sputter crater sustained minimal damage during SF5+ bombardment.

Influence of SF5+ Impact Energy. The effect of SF5+ impact energy on the PMMA damage profiles was determined using SF5+ impact energies from 2.5 to 8.75 keV. Figure 4 shows the C4H5O+ secondary ion intensity as a function of SF5+ ion dose and impact energy. The damage profiles at each of the impact energies used contained the same three regions described above and shown in Figure 2a. Additionally, the Si+ secondary ion intensity followed a similar trend as shown in Figure 2a, substantially increasing in intensity once the polymer film had been sputtered through (data not shown). In the negative ion damage profiles, the characteristic PMMA secondary ion intensities followed a trend similar to those shown in Figure 4. In addition, the C/O and C-H/C-C ratios followed trends similar to those shown for 5-keV SF5+ in Figure 2b, with the C/O ratio increasing slightly during SF5+ bombardment and the C-H/C-C ratio following a trend similar to that of the characteristic PMMA secondary ions. Neither the C/O nor C-H/C-C ratios changed as a function of SF5+ impact energy. Figure 5a shows the effect of SF5+ impact energy on the sputter rate of the PMMA film. The overall sputter rate of the film at any given time point in the damage profile (SRoverall) was assumed to be a weighted average of the PMMA (SRPMMA) and silicon sputter rates (SRSi, ≈ 0.1 nm3/ion) as follows:

SRoverall )

( ) ( ISi

I max Si

SRSi + 1 -

ISi I max Si

)

SRPMMA

(4)

where ISi is the Si+ secondary ion intensity at a given time point and I max is the maximum Si+ secondary ion intensity in the Si damage profile. Equation 4 was used to model the changing sputter rate of the system as the sputtered material transitioned from PMMA to silicon over the damage profile. The overall sputter rate was then used to define a depth scale for the damage profiles, and the PMMA sputter rate was selected so that the depth at which the Si+ secondary ion intensity reached 50% of its maximum was 100.4 nm (the thickness of the PMMA film). The sputter rates (in nm3/incident ion or monomers/incident ion) give an average (56) Gillen, G.; Simons, D. S.; Williams, P. Anal. Chem. 1990, 62, 2122.


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Figure 5. (a) Effect of SF5+ impact energy on the sputter rate of PMMA. Also included are the data of Norrman et al. (marked as *) measured at 1-keV SF5+ impact energy.33 Note that 1 nm3 of PMMA contains ∼7 MMA monomer units. The best-fit lines are given for the sputter rate with units of nm3/ion. (b) Width of the PMMA-silicon interface as a function of the SF5+ impact energy. Error bars for (a) and (b) represent (1 standard deviation.

Figure 6. Effect of SF5+ impact energy on the (a) C4H5O+ (m/z ) 69) damage cross section and (b) F- (m/z ) 19) appearance cross section measured in region ii of the damage profiles. The appearance cross section of Cs+ (m/z ) 133) is also shown in (b) for comparison. The inset in (b) shows the F- appearance cross sections only. Error bars for (a) and (b) represent (1 standard deviation. Note that 1 nm2 ) 10-14 cm2.

volume or number of monomers of PMMA removed per primary ion impact. Assumption of a constant sputter rate for PMMA was supported by previous studies, showing that the sputter rate for PMMA was constant under bombardment by 5.5-keV SF5+ in the ion dose range on 1 × 1014-4.5 × 1014 ions/cm2.27 The sputter rate as a function of impact energy fell into two regions; in both regions, the sputter rate was linearly correlated with the SF5+ impact energy. In the low-impact energy region (2.5-3.75 keV), the sputter rate increased slightly with increasing impact energy. However, above 5 keV, the change in sputter rate with SF5+ impact energy significantly increased. Also included in Figure 5a are the data of Norrman et al., showing that the sputter rate of PMMA using 1-keV SF5+ was ∼3 nm3/ion.33 These data followed the trend seen in the low-impact energy region. Figure 5b shows the width of the polymer-silicon interface as a function of SF5+ impact energy. After assigning a depth scale using eq 4, the width of the polymer-silicon interface was determined by the width of the damage profile when the Si+ secondary ion intensity was between 16 and 84% of its maximum value for the damage profile.57 The width of the PMMA-silicon

interface was between 20 and 30 nm and did not change with changing SF5+ impact energy. While increased primary ion impact energy would be expected to increase the interface width due to interlayer mixing, topography development during SF5+ bombardment may have hidden this effect. Experiments monitoring the surface roughness during the damage profile are ongoing. Figure 6a shows that the damage cross section of the C4H5O+ secondary ion in region ii of the damage profiles was very weakly correlated with SF5+ impact energy. The damage cross sections for the other characteristic PMMA secondary ions were similar to that of the C4H5O+ secondary ion, showing minimal change with changing SF5+ impact energy. Figure 6b shows the rate of appearance of the F- (m/z ) 19) secondary ion in the negative ion depth profiles of PMMA as a function of SF5+ impact energy. The “appearance cross section” for F- was calculated using eq 1 in a fashion similar to the damage or “disappearance” cross sections. The appearance cross section of the F- secondary ion also served as an indicator of the removal of damaged polymer during sputtering. As with the damage cross sections, the appearance cross section of the F- secondary ion was weakly correlated with SF5+ impact energy. The magnitude of the appearance cross section of the F- secondary ion was similar to the damage cross sections of the characteristic PMMA positive

(57) Standard Guide for Measuring Widths of Interfaces in Sputter Depth Profiling using SIMS. ASTM Standard Designation E 1438-1491, Reapproved 2001.


Figure 7. Depth of the onset of region ii in the PMMA damage profiles as a function of SF5+ impact energy. Error bars represent (1 standard deviation.

secondary ions. The low accumulation of the primary ion species in the polymer therefore suggested that most of the damage was removed during sputtering. This behavior was unlike bombardment with 5-keV Cs+, in which cesium rapidly accumulated on the surface and was not removed by further sputtering. The appearance cross section for Cs+ was ∼60 times higher than the appearance cross section of F- (Figure 6b), showing that the rate of removal of the Cs+ primary ion by sputtering was much less than the SF5+ constituents. Figure 7 shows the effect of the SF5+ impact energy on the depth into the PMMA film of the onset of region ii of the damage profile. The depth was calculated from the SF5+ ion dose needed to reach region ii and the sputter rate. An increase in the depth of the onset of region ii would allow more damage to accumulate before the pseudo steady state was reached. Previous studies have shown that increased primary ion energy (and penetration depth) for monatomic primary ions increased the width of the surface transient region (region i).56 Under monatomic bombardment, the sputter rate for organic materials decreases as a function of primary ion dose, with the width of the surface transient equal to the depth necessary to reach a steady-state sputter rate. The depth to reach region ii trended positively with increased SF5+ impact energy. Since the sputter rate for PMMA under SF5+ bombardment is constant as a function of primary ion dose 27 (but not as a function of impact energy), the origin of the surface transient in PMMA must be due to other factors, such as damage to the surface, the removal of surface contaminants, or the implantation of primary ion species. The increasing sputter rate as a function of SF5+ impact energy probably contributed to the increase in the depth of the onset of region ii. Comparison of Experimental and Monte Carlo Simulations. The penetration depths and total vacancy production of Cs+ and SF5+ were estimated using SRIM.46 For calculation of the penetration depth and total vacancy production of SF5+, the sulfur and fluorine constituents of SF5+ were treated individually, with the impact energies of the constituents being proportional to their mass fraction in the SF5+ primary ion.26 The total vacancy production for SF5+ was determined by addition of the number of vacancies produced by the sulfur and fluorine constituents of the incident SF5+. The SRIM calculation probably underestimates the total number of vacancies produced by ignoring the spatially

Figure 8. Results from Monte Carlo (SRIM) calculations. (a) Average penetration depths (open symbols) and full width measured at the half-maximum of the concentration profiles (filled symbols) for cesium and the sulfur and fluorine components of SF5+ as a function of primary ion impact energy. (b) Total vacancy production and depth of the peak of the vacancy profile for Cs+ and SF5+ as a function of primary ion impact energy.

and temporally overlapping collision cascades of the constituent atoms of SF5+. Figure 8a shows the average penetration depth of the sulfur and fluorine constituents of SF5+ and Cs+ as a function of primary ion impact energy. The penetration depths of fluorine and sulfur were similar at each SF5+ impact energy and the penetration depth was linearly correlated with the SF5+ impact energy. The penetration depth for 5-keV Cs+ was ∼2.5 times greater than 5-keV SF5+. Figure 8a also shows the full width measured at the half-maximum (fwhm) of the primary ion concentration profiles for Cs+ and SF5+. As with the penetration depth, the fwhm for the concentration profiles of the SF5+ constituents increased with increasing SF5+ impact energy. Furthermore, the fwhm for 5-keV Cs+ was ∼30% higher than the fwhm for 5-keV SF5+. Figure 8b shows the total vacancy production for Cs+ and SF5+ and the depth of the peak of the vacancy profile. The total number of vacancies produced, the depth of the peak of the vacancy profile, and the fwhm of the vacancy profile (not shown) were linearly correlated with the SF5+ impact energy. Changing from SF5+ to Cs+ did not significantly change the total number of vacancies produced; however, the depth and width of the vacancy profile significantly increased. Analytical Chemistry, Vol. 76, No. 5, March 1, 2004

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In the dual-beam depth profiling experiments, sputtering with or SF5+ was alternated with collection of static TOF-SIMS spectra using 10-keV Ar+. Therefore, there is some potential that the collection of these spectra could contribute to the damage of the PMMA film. The depth of material removed between TOFSIMS analyses ranged from 1 to 16 nm, depending on the SF5+ impact energy. The penetration depth of 10-keV Ar+ into PMMA (as calculated by SRIM) was ∼15 nm and the depth of the peak of the vacancy profile was ∼10 nm, suggesting that some damage could have accumulated due to the Ar+ primary ions. However, the Ar+ primary ion dose was much less than the Cs+ or SF5+ primary ion dose, minimizing the influence of Ar+ on the damage profiles. An optimized dual-beam experiment would probably utilize polyatomic sputter and analysis beams, taking advantage of the low damage accumulation and the increase in sensitivity concomitant with these ion sources. In light of the SRIM results, it was surprising that the sputter rate was the only monitored parameter that varied with the SF5+ impact energy. The SRIM results showed that the penetration depth of the primary ion and the damage created (i.e., total number of vacancies) were linearly correlated with the SF5+ impact energy. The onset of region ii trended positively with SF5+ impact energy, suggesting the deeper penetration depth and greater sputter rate at higher SF5+ impact energies influenced the extent of the surface transient. However, the damage cross sections and the appearance cross section for F- were not correlated with the SF5+ impact energy. In addition, the C/O and C-H/C-C negative ion ratios were only correlated with SF5+ primary ion dose, not SF5+ impact energy. The SF5+ impact energy had a minimal effect on the surface chemistry on the bottom of the sputter crater. However, the damage of PMMA may still have been affected by the SF5+ impact energy. The high sputter rate of PMMA under SF5+ bombardment may have removed the bulk of the damaged material, leaving a similar surface chemistry at the bottom of the sputter crater regardless of SF5+ impact energy. The extent of the damage of the sputtered material could be monitored directly by measuring the secondary ions generated during the SF5+ sputtering (using a quadrupole or magnetic sector mass spectrometer, for example). Cs+

CONCLUSIONS Positive and negative ion TOF-SIMS have been used to monitor the surface chemistry of spin-cast PMMA films under Cs+ and SF5+ bombardment. The study has shown the following: (1) Under 5-keV Cs+ primary ion bombardment, the intensities of the characteristic secondary ions rapidly decreased in both the positive and negative ion spectra. In the positive ion spectra, Cs+

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rapidly became the only species observed. Under Cs+ bombardment, the PMMA film lost oxygen and the surface carbons became more unsaturated, suggesting loss of the methyl ester pendant group and possible cross-linking of the polymer chains (reaction 1). (2) In contrast with 5-keV Cs+ bombardment, thin PMMA films spin-cast onto silicon substrates displayed three distinct regions under extended bombardment using SF5+. The intensities of the characteristic secondary ions (i) decreased initially after beginning SF5+ bombardment and then (ii) the damage cross sections of the characteristic secondary ions significantly decreased, resulting in a pseudostabilization of these secondary ion intensities. These damage cross sections were ∼1 order of magnitude less than those for 5-keV Cs+. Finally, (iii) the silicon substrate was reached, characterized by a rapid decrease in the PMMA characteristic secondary ion intensities and an increase in the silicon characteristic secondary ion intensities. The C/O and C-H/C-C ratios were relatively constant under SF5+ bombardment, further suggesting that the native PMMA structure was retained and that reaction 2 or the sputtering of whole monomer units was the primary mode of decomposition. (3) Neither the positive nor the negative ion TOF-SIMS spectra showed any evidence of fluorination of PMMA under SF5+ bombardment, with no fluorocarbon species observed in either the positive or the negative ion TOF-SIMS spectra. Fluorine and sulfur were observed in the negative ion TOF-SIMS spectra after bombardment of the PMMA films with SF5+, and the appearance of these species was an indication of the accumulation of damage in the PMMA film during sputtering. (4) The damage cross sections of the characteristic PMMA secondary ions calculated in region ii of the damage profiles, the appearance cross section of F- secondary ions, and the onset of region ii were minimally correlated with SF5+ impact energy. Any differences in damage due to SF5+ impact energy were sputtered away during SF5+ bombardment, leaving a similar surface chemistry at the bottom of the sputter craters regardless of SF5+ impact energy. These results suggest that sputtering using SF5+ is appropriate for the dual-beam depth profiling of PMMA, with selection of the SF5+ impact energy based solely on the desired sputter rate. Future studies will examine the influence of the polymer chemistry on its stability under extended SF5+ bombardment.

Received for review November 10, 2003. Accepted December 19, 2003. AC035330R

Impact Energy Dependence of SF5+-Induced Damage in Poly(methyl

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