Anal. Chem. 2007, 79, 837-845
Temperature-Controlled Depth Profiling of Poly(methyl methacrylate) Using Cluster Secondary Ion Mass Spectrometry. 2. Investigation of Sputter-Induced Topography, Chemical Damage, and Depolymerization Effects Christine M. Mahoney,* Albert J. Fahey, Greg Gillen, Chang Xu, and James D. Batteas
National Institute of Standards and Technology, 100 Bureau Drive, Mail Stop 8371, Gaithersburg, Maryland 20899-8371
Poly(methyl methacrylate) (PMMA) thin films (∼150 nm) on silicon were bombarded with SF5+ polyatomic primary ion projectiles at -75 °C, 25 °C, and 125 °C. The crater bottoms were then characterized using a combination of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS). AFM results indicated increased sputterinduced topography formation in the order of -75 °C < 125 °C < 25 °C, consistent with earlier SIMS depth profile results which illustrated optimum depth profile characteristics at low temperatures and less favorable characteristics at room temperature.1 XPS results indicated that there was a significant amount of C remaining at the crater bottom at 25 °C, suggesting that there is a large amount of organic material remaining despite the loss in characteristic PMMA secondary ion signal in the SIMS depth profile. Specifically, C/Si ratios increased in the following order: -75 °C < 125 °C < 25 °C, consistent with the trend in topography observed in the AFM results. Highresolution C(1s) spectra of the PMMA film indicated a decrease in the O-CdO component with sputtering at both -75 °C and 25 °C. However, there was very little change in the C(1s) spectra in samples sputtered at 125 °C. This was determined to be a result of ion-induced depolymerization which is expected to occur at higher temperatures in PMMA. Residual gas analysis (RGA) gave results that were consistent with this hypothesis, showing increased amounts of PMMA monomer at higher temperatures. Principle components analysis (PCA) of SIMS spectra showed increased PMMA secondary ion intensities coupled with increased O signal in PMMA sputtered at 125 °C. Conversely, SIMS spectra acquired in the sputtered PMMA at -75 °C, and to a smaller degree at 25 °C, showed increased C signals, decreased O intensities, and the appearance of peaks indicative of polycyclic aromatic hydrocarbons, all consistent with increased chemical damage. Overall, these results indicate that while there is increased damage occurring at -75 °C, there is still a significant improvement in the depth profile characteristics. It is concluded that the enhancement in 10.1021/ac061357+ Not subject to U.S. Copyright. Publ. 2007 Am. Chem. Soc.
Published on Web 12/22/2006
low-temperature depth profiles in PMMA results mainly from the changes in the physical properties of the PMMA at low temperatures, yielding a significant reduction in sputter-induced topgraphy. Over the past decade, cluster secondary ion mass spectrometry (SIMS), employing polyatomic primary ion beams such as SF5+ or C60+, as opposed to their monatomic counterparts (e.g., Ar+, Ga+, or Cs+), has become an increasingly important tool for the surface and in-depth characterization of polymeric materials. It has been used to obtain molecular and fragment information as a function of depth in several organic and polymeric systems including polyethers, polyesters, polymethacrylates, proteins, and fluoropolymers.1-12 In addition to homopolymer systems, polymeric blends, multilayers, and copolymer systems have also been successfully characterized.5,8,11,12 Finally, the ability to monitor the in-depth distribution of small molecules embedded in organic and polymeric matrices has been realized.3,4,12 However, this technology still has several limitations for polymeric depth profiling. For example, some polymers are more amenable to depth profiling with cluster SIMS than others (e.g., poly(lactic acid) performs better than poly(methyl methacrylate) (PMMA)],1,3,6 while still * Author to whom correspondence should be addressed. Tel: 301-975-8515. Fax: 301-417-1321. E-mail:
[email protected]. (1) Mahoney, C. M.; Fahey, A. J.; Gillen, G. Temperature-Controlled Depth Profiling of Poly(methyl methacrylate) Using Cluster Secondary Ion Mass Spectrometry (SIMS): 1. Investigation of Depth Profile Characteristics. Anal. Chem., submitted for publication. (2) Gillen, G.; Roberson, S. Rapid Commun. Mass Spectrom. 1998, 12, 13031312. (3) Mahoney, C. M.; Roberson, S. V.; Gillen, G. Anal. Chem. 2004, 76, 31993207. (4) Mahoney, C. M.; Roberson, S. V.; Gillen, G. Appl. Surf. Sci. 2004, 231232, 174-178. (5) Mahoney, C. M.; Yu, J. X.; Gardella, J. A., Jr. Anal. Chem. 2005, 77, 35703578. (6) Wagner, M. S. Anal. Chem. 2004, 76, 1264-1272. (7) Wagner, M. S. Surf. Interface Anal. 2005, 37 (1), 42-70. (8) Wagner, M. S. Anal. Chem. 2005, 77 (3), 911-922. (9) Brox, O.; Hellweg, S.; Benninghoven, A. Proc. 12th Int. Conf. Second. Ion Mass Spectrom. 2000, 777-780. (10) Sostarecz, A. G.; McQuaw, C. M.; Wucher, A.; Winograd, N. Anal. Chem. 2004, 76, 6651-6658. (11) Chen, J.; Winograd, N. Anal. Chem. 2005, 77 (11), 3651-3659. (12) Mahoney, C. M.; Patwardhan, D. V.; McDermott, M. K. Appl. Surf. Sci. 2006, 19, 6554-6557.
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Figure 1. Positive secondary ion intensities of m/z ) 69 plotted as a function of depth for a PMMA film (∼160 nm) on Si. Depth profiles were acquired at variable temperatures.
others experience extensive beam-induced degradation (e.g., polystyrene and polyethylene) resulting in a complete loss of the characteristic secondary ion signal. More research is required to acquire a better understanding of the fragmentation mechanisms that result from interactions between polyatomic primary ion beams and various polymeric materials. In the current work we investigate the effect of temperature on cluster primary ion bombardment (with SF5+) in polymeric materials, a continuation of earlier work in this series focusing on the SIMS depth profile aspects.1 In this earlier study, it was shown that temperature played an important role in depth profiling of PMMA, where the depth profile characteristics (e.g., interface widths, sputter rates, damage accumulation, and overall secondary ion stability) were found to change dramatically with temperature. In particular, it was determined that the optimum PMMA depth profiles were obtained at low temperatures (-75 °C). Low temperatures have proven beneficial for the in-depth characterization of several other polymeric materials as well, including poly(ethylene-co-vinyl acetate), polyurethanes, and poly(lactide-coglycolide), indicating that these effects most likely result from changes in the overall mechanical properties in polymer systems at low temperatures.12,13 At low temperatures, these materials are typically very brittle and have much stronger inter- and intrachain coupling associated with them. Thus, significant changes in the sputter properties (e.g., sputter-induced topography formation) might be expected. There are cases, however, where the application of low temperatures does not enhance the depth profile characteristics (e.g., polystyrene).14 In these cases it is assumed that the difficulty in sputtering (at room temperature) is not related to the mechanical properties of the material, but rather to extensive beam-induced chemical damage. It is hypothesized therefore that depth profile success in polymeric materials is contingent upon the following: (1) extent of beam-induced chemical damage (fragmentation pathways should preserve the main chain structure), and (2) sputter-induced topography formation. Effects were also observed at high temperatures in PMMA depth profiles, where increases in sputter rates were observed, (13) Unpublished results in our laboratory. (14) Mahoney, C. M.; Fahey, A. J.; Gillen, G.; Xu, C.; Batteas, J. D. Appl. Surf. Sci. 2006, 19, 6502-6505.
838 Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
Figure 2. Atomic force microscopy (AFM) topography images (1 µm × 1 µm area) of crater bottoms at different stages in the depth profile process: (a) model depth profile illustrating different regions in depth profile, where position 1 represents the steady-state region, position 2 represents the interfacial region, and position 3 shows the Si substrate after the polymer has been removed; (b-k) PMMA crater bottoms at different sputtering times and temperatures: (b) PMMA surface/0-s sputtering, Rrms ) ∼0.85 nm, (c) 25 °C/200-s sputtering, Rrms ) ∼5.20 nm, (d) 25 °C/520-s sputtering, Rrms ) ∼15.53 nm, (e) 25 °C/1300-s sputtering, Rrms ) ∼11.37 nm, (f) -75 °C/200-s sputtering, Rrms ) ∼2.58 nm, (g) -75 °C/365-s sputtering, Rrms ) ∼2.66 nm, (h) -75 °C/900-s sputtering, Rrms ) ∼0.274 nm, (i) 125 °C/150-s sputtering, Rrms ) ∼2.65 nm, (j) 125 °C/315-s sputtering, Rrms ) ∼1.00 nm, and (k) 125 °C/900-s sputtering, Rrms ) ∼1.00 nm.
accompanied with overall increased stability in the depth profiles. Wagner et al. suggested that the sputter processes of PMMA involve a competition between two processes: (1) methyl ester pendent-chain loss followed by main-chain scission and crosslinking, and (2) chain-end-initiated or random depolymerization.6 It is suggested here that the reason for the increase in sputter rate at high temperatures is the initiation of ion-induced depolymerization reactions above 25 °C. This effect has already been observed at high temperatures (>115 °C) with 300-keV He+ ions.15 It was also noted that the primary and secondary glass transition temperatures played an important role in the depth profiling of polymeric materials in the temperature range 25-125 °C.1 Overall, the changes in the depth profile characteristics in this range were (15) Fragala, M. E.; Compagnini, G.; Torrisi, L.; Puglisi, O. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 141, 169-173.
Table 1. Atomic Concentrations in Crater Bottoms As Determined by X-ray Photoelectron Spectroscopy (XPS) (error bars based on 5% relative error) atomic concentrations(%)
sample
temp (˚C)
PMMA
25
PMMA
-75
PMMA
125
PLA
25
sputter time (s) 0 200 370 1100 150 400 900 0 150 300 800 0 150 275 750
depth profile region
C
O
1 2 3
73.4 ( 3.6 72.9 ( 3.6 73.2 ( 3.7 60.0 ( 3.0 71.1 ( 3.6 68.2 ( 3.4 18.6 ( 0.9 73.6 ( 3.7 74.4 ( 3.7 73.6 ( 3.7 41.6 ( 2.1
26.6 ( 1.3 26.7 ( 1.3 25.8 ( 1.3 26.2 ( 1.3 28.7 ( 1.4 27.3 ( 1.4 40.7 ( 2.0 26.4 ( 1.3 25.6 ( 1.3 24.1 ( 1.2 30.8 ( 1.5
1 2 3
64.7 ( 3.2 68.8 ( 3.4 16.2 ( 0.8
35.3 ( 1.8 31.2 ( 1.6 36.5 ( 1.8
1 2 3 1 2 3
attributed to a combination of large changes in the physical properties (particularly in the glass transition regions) and the initiation of sputter-induced depolymerization effects. The leaststable PMMA depth profiles were obtained at room temperature (25 °C). Here we investigate in detail the morphological, chemical, and molecular composition of SF5+-sputtered crater bottoms in PMMA films formed at varying temperatures in order to correlate the depth profile characteristics determined in earlier work to actual chemical and physical changes occurring during the sputtering process. Analysis of the sputtered crater bottoms was performed by using a combination of atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and secondary ion mass spectrometry (SIMS) to observe changes as a function of temperature as well as to elucidate sputter mechanisms. In addition, residual gas analysis (RGA) was used to monitor the composition of gases in the analysis chamber. It is expected that if depolymerization reactions are occurring at high temperatures, then this can be readily observed by monitoring the gaseous evolution of PMMA monomer (MMA) during the sputter process.15 EXPERIMENTAL SECTION Sample Preparation. Samples for RGA and static SIMS investigations described in this work were prepared by the following method. Atactic poly(methyl methacrylate) (PMMA) (MW ) 100 K, Tg ) 105 °C) was purchased from Polysciences (Warrington, PA).16 Polymer solutions (2% w/w) were prepared in toluene and spun cast onto Si wafers (SI-Tech Inc., Topsfield. MA) at 2000 rpm for 30 s. The wafers were ultrasonicated in toluene, acetone, and methanol prior to the casting. Film-thickness measurements were made by stylus profilometry (Tencor Instruments R-step 200, Milpitas, CA) with a 10-mg stylus force. Resulting PMMA film thicknesses were ∼100 nm. Samplepreparation methods for AFM and XPS studies are described in the preceding work.1 These films had thicknesses of ∼160 nm. (16) Certain commercial equipment, instruments, or materials are identified in this article 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.
F
Si
1.4 ( 0.1
0.4 ( 0.02 1.0 ( 0.1 12.0 ( 0.6 0.2 ( 0.01 4.5 ( 0.2 39.2 ( 2.0
1.4 ( 0.1
2.2 ( 0.1 26.3 ( 1.3
1.2 ( 0.1
46.1 ( 2.3
0.9 ( 0.05
S
0.9 ( 0.05
Time-of-Flight Secondary Ion Mass Spectrometry (TOFSIMS). TOF-SIMS experiments were performed on an Ion-TOF IV (Mu¨nster, Germany) instrument equipped with Ar+ and SF5+ primary ion beam sources.16 Mass spectra were acquired before and after sputtering with SF5+ polyatomic primary ions at variable sample temperatures. The analysis source used for these experiments was a pulsed 10-keV Ar+ beam, which bombarded the surface at an incident angle of 45° to the surface normal. The target current was ∼2 pA (pulsed) with a raster size of 200 µm × 200 µm. Each spectrum was averaged over a 60-s time period. These conditions resulted in Ar+ ion doses that were well below the static SIMS limit of 1013 ions/cm2. Both positive and negative secondary ions were extracted from the sample into a reflectiontype time-of-flight mass spectrometer. The secondary ions were then detected by a microchannel plate detector after a 10-kV post acceleration. A low-energy electron flood gun was utilized for charge neutralization in the analysis mode. The sputter source was a 5-keV SF5+ polyatomic primary ion source, which bombarded the surface at an incident angle of 45° to the surface normal. The target current was maintained at ∼2 nA (continuous current) throughout the sputter process, with a raster area of 750 µm × 750 µm. All beam currents were measured with a Faraday cup. The sputtering was stopped after 30-60 s, depending on the temperature of the sample (30 s for 80 °C and 125 °C, and 60 sfor 25 °C and 50 °C). Shorter sputter times were used at higher temperatures in order to account for the increased PMMA sputter rates observed at these higher temperatures.1 AFM and XPS were both utilized to characterize the PMMA at different sputtered depths (e.g., before sputtering, sputtered PMMA, and PMMA/Si interfacial region). Corresponding temperature-controlled SIMS depth profiles were obtained under conditions similar to those described elsewhere.1 Principle components analysis (PCA) was used as a tool to reduce the dimensionality of both the positive and negative SIMS spectra acquired before and after sputtering with SF5+. All PCAwas (17) Wagner, M. S.; Castner, D. G. Langmuir 2001, 17, 4649-4660. (18) Jackson, J. E. A Users Guide to Principal Components; John Wiley and Sons: New York, 1991. (19) Wold, S.; Esbensen, K.; Geladi, P. Chemom. Intell. Lab. Syst. 1987, 2, 37.
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Figure 3. High-resolution C(1s) and O(1s) X-ray photoelectron spectra of sputtered crater bottoms formed at varying temperatures: (a) 125 °C, (b) 25 °C, and (c) -75 °C. C(1s) binding energies: (1) 284.8 eV (C-C or C-H), (2) 286.7 eV (C-O), and (3) 288.9 eV (OdC-O). O(1s) binding energies: (4) 532.0 eV (C-O-C), and (5) 533.6 eV (CdO).
done with Matlab software (Mathworks Inc., Natick, MA). Peaks of interest were selected, normalized to the total secondary ion intensities, and finally mean-centered prior to the PCA processing. For more information regarding PCA processing, please see refs 17-19. X-ray Photoelectron Spectroscopy (XPS). X-ray photoelectron spectroscopy experiments were performed on a Physical Electronics (Chanhassen, MN) Quantera scanning X-ray microprobe (SXM) incorporating a monochromatic Al anode operated at 15 kV and 40 W in the standard power mode with dual beam neutralization (ions and electrons) and a hemispherical analyzer. Static point acquisitions were collected from the 1000 µm × 1000 µm crater bottoms.1 High-resolution spectra were obtained and atomic concentrations were determined by integration of the C(1s), O(1s), Si(2p), S(2s), and F(1s) photoelectron peak areas. 840 Analytical Chemistry, Vol. 79, No. 3, February 1, 2007
Atomic Force Microscopy (AFM). Tapping mode atomic force microscopy (AFM) images were collected on a Dimension 3100 system (Vecco Instruments, Santa Barbara, CA) operating under ambient conditions. The cantilevers used in the measurements were MPP-111000 (NanoDevices, Inc., Santa Barbara, CA), which have a spring constant of ∼40 N/m, with a resonance frequency of ∼300 kHz, and nominal tip radius of
