- Email: [email protected]
Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis
Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis q S.V. Baryshev a,b,⇑, A.V. Zinovev a, C.E. Tripa a, I.V. Veryovkin a a b
Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA Euclid TechLabs LLC, 5900 Harper Rd., Solon, OH 44139, USA
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Sputter depth profiling Resonance ionization mass spectrometry Imaging mass spectrometry Scanning electron microscopy Ion implantation Genesis mission
a b s t r a c t We report on a combination of imaging mass spectrometry (MS) and scanning electron microscopy (SEM) developed in a custom designed time-of-flight (TOF) MS instrument with laser post-ionization of sputtered atoms. Elemental (by MS) and topographical (by SEM) mapping of surfaces of heavily contaminated Si collectors from the NASA Genesis sample return mission enabled obtaining much more accurate and detailed depth distribution of the Solar Wind Mg and Ca implanted in these collectors. This is because the cleanest areas were identified by the SEM/MS mapping, and high resolution sputter depth profiling at these locations revealed near-surface (0–15 nm) depth distribution of Mg and Ca, that were used for more accurate fluence calculations of these Solar Wind species. MS imaging was virtually nondestructive at primary ion fluence 1012 cm-2, causing no effect on accuracy and precision of quantitative depth profiling that followed the imaging. We also demonstrate importance of such an approach by directly comparing high resolution depth profiles measured on clean areas versus arbitrarily selected areas. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Imaging mass spectrometry (MS) is a boon to biology, chemistry, and materials science. This is because information on elemental distributions it provides is extremely important for studies of biological tissues [1], development of drugs [2] and improving performance of solid-state electronics [3]. Elemental distribution may be represented in two or three dimensions. For performing such 2D and 3D imaging MS, the choice of primary probes is confined to either laser beam (e.g., laser ablation [4] or laser desorption [5]) or ion beam (sputtering using atomic [3] or cluster projectiles [6]). For 3D imaging, be it organic or inorganic materials, mass spectrometry of ion sputtered species transcends all other approaches due to the smallest depth of origin of the species, a few monolayers only, which provides the best depth resolution [7,8]. Importantly, the high sensitivity of this MS imaging method q The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘‘Argonne’’). Argonne, a US Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357. The US Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ⇑ Corresponding author at: Euclid TechLabs LLC, 5900 Harper Rd., Solon, OH 44139, USA. Tel.: +1 630 252 6213. E-mail address: [email protected] (S.V. Baryshev).
(especially when combined with laser post-ionization of sputtered neutrals) may be capable of quasi nondestructive surface analysis because ion irradiation can be limited to extremely low fluences. High resolution imaging of the actual surface topography is also possible if one detects secondary electrons produced by primary ions, similarly to scanning electron microscopy (SEM) [3]. In many cases though, obtaining high quality topography images using an ion probe may require exposure to high ion fluences, which results in considerable surface damage. Conventional SEM, with a 5–20 keV electron probe, is a true nondestructive method to image topography of solids. In the present paper, the strength of a combination of quasi nondestructive SEM/MS imaging is experimentally demonstrated. As an example, elemental/topographical imaging of the contaminated surface of a Si Solar Wind collector of the NASA Genesis mission was performed. This imaging revealed least contaminated areas on the surface to carry high resolution sputter depth profiling analyses of shallow (<100 nm) trace-concentration (ppb) Solar Wind implants Mg and Ca in order to correctly determine their accumulated fluences. 2. Material and methods Samples used in this study were single crystal Si(0 0 1) provided by NASA Johnson Space Center within Genesis return mission program [9].
http://dx.doi.org/10.1016/j.nimb.2014.02.097 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
2
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Fig. 1. The gentleDB system developed in SARISA [10]. The milling Ar+ ion beam comes from the low energy column and is deflected into the front TOF column by the Bending System optics. Being focused with Lens1, it mills the sample surface perpendicularly. The separate pulsed analysis ion gun (also Ar+) probes the surface. Secondary species are then mass analyzed by the TOF spectrometer. The electron gun images surface topography. The photo-ionizing laser beams are not shown; one can imagine laser beams intercepting neutrals between sample surface and Lens1.
Used in this work, a TOF MS instrument, SARISA, can operate in two modes: secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry with resonance enhanced multiphoton ionization (RIMS). To make the SARISA instrument even more versatile, an FEI 2LE electron gun with Schottky source was installed to probe the surface at 60° incidence angle, while secondary electrons are extracted in the direction normal to the surface and directed toward a dedicated detector as described in Ref. [10]. The FEI 2LE gun has its own high voltage power supply unit operated through dedicated FEI software, and FEI digital raster scan unit operated through RevolutionSEM imaging controller made by 4 pi, Inc. Elemental trace analysis of samples was carried in the RIMS mode, such that sputtered neutral species are efficiently converted into photo-ions by resonantly enhanced multi-photon ionization with tunable Ti-sapphire lasers exactly as reported in Ref. [11]. To characterize both surface and bulk, high resolution depth profiling in the dual-beam approach with normal incidence milling was applied. This dual-beam approach is dubbed as gentleDB, and described in great detail elsewhere [8] (see Fig. 1 and its caption for more details). The gentleDB depth profiling makes use of two independent switchable ion beams: one normally incident beam with high direct current and low energy for ion milling, and another one, striking the target at 60° and pulsed, for TOF MS analysis. Thus, there are three beams in the system (two ion beams and one electron beam). All beams were superimposed and centered with respect to SARISA ion optics center by means of Schwarzschild all-reflecting in vacuum microscope [12,13].
3. Results and discussion NASA Genesis mission samples, made of a number of ultrapure materials [14], have collected the Solar Wind (SW) elements upon their implantation in those materials [15]. These samples pose a serious challenge for quantitative elemental and isotopic analyses with high precision and accuracy. This is because (i) the surface contamination (due to the spacecraft outgassing during the flight and, most importantly, due to its crash landing) is very abundant, and (ii) the SW elements are implanted at ultralow trace level concentrations (sub-ppb) within the first 100 nm under the collectors surface. Ion sputter mass spectrometry has already proved to be an invaluable tool in the Genesis mission by determining isotopic distributions of SW oxygen and nitrogen [16,17]. Quantitative elemental MS analyses of Genesis samples for non-gaseous SW elements are even more challenging because they require the highest
possible depth resolution (to distinguish between the terrestrial surface contamination and the implanted SW) in combination with the highest possible sensitivity (to detect these SW elements at sub-ppb concentrations). For a number of SW elements with high terrestrial abundances, this is possibly the only way to accurately determine the implantation depth profile curve needed for accurate calculation of SW fluencies. In Fig. 2 depth profiles of 24Mg and 40Ca in a Genesis SW Si collector (sample #60428) are demonstrated. The wine and olive curves (solid lines and open circles) represent typical results of high resolution gentleDB RIMS depth profiling analysis of heavily contaminated samples. There are 2 regions. Surface contamination, manifesting itself in high RIMS signal intensity, is followed by 10-fold (for Mg) and 100-fold (for Ca) drop leading to a somewhat short plateau of SW. It is commonly assumed that implanted SW depth profiles have Gaussian-like distribution, similarly to that one expects from ion implantation of semiconductors, or by running TRIM/SRIM calculations; we do see such profiles in calibrated Si standard implants used to calculate fluences in Genesis samples [11,18]. Such assumption is used to compensate the lack of knowledge on near-surface depth distribution, which is hard to measure either due to the heavy surface contamination and/or due to the sample breakdown if back-side depth profiling approach is used [19]. At the same time, it is worth mentioning that there is a high likelihood that real SW distributions strongly deviate from the Gaussian shape due to the initial kinetic energy distributions of SW ions and also due to radiationenhanced diffusion of implanted SW species [20,21], which could be further changed by elevated collector temperatures during the flight. Keeping these facts in mind, it is highly desirable to be able to measure actual SW depth distribution in direct experiments. We combined the advantages of elaborated SEM imaging protocol with trace-sensitive surface-only RIMS elemental imaging/ mapping prior to standard gentleDB depth profiling. First of all, SEM imaging allows avoiding large (such as at the bottom of the sample in Fig. 3) and small (left hand inset in Fig. 3) chipped areas or areas containing visible features/inclusions. Such mechanically damaged areas are not suitable for conducting depth profiling. Then, pulsed analysis ion beam was used (60° inclined gun in Fig. 1) on the pristine surface at ultralow fluence condition, f = 1012 ions/cm2. This fluence is one order of magnitude lower than what is required for analyses in static SIMS regime [22]. Parameter Y [sputtered target atoms/incident primary ions] f [incident primary ions/cm2] must be compared to the areal atomic concentration in Si 1.4 1015 atoms/cm2, where Y is the sputtering yield. Thus, using known Y 5 (Si sputtered by 5 keV argon ions at 60° incidence) and comparing the product of Y f = 5 1012
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
3
Fig. 2. 24Mg and 40Ca depth profiles in a fragment of a Genesis Si collector (#60428). The wine and olive curves (solid lines and open circles) represent typical results without preliminary SEM/RIMS imaging. The curves in red and green (solid lines and open squares) are depth profiles conducted on one of 3 sweet spots marked in Fig. 3. It is seen, in some cases surface abundance of Mg and Ca is even lower than that of SW. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Principle of combined imaging by SEM/RIMS. Black and white (most left) is the topographical view of a Genesis Si coupon (#60428) in secondary electrons contrast obtained by SEM. Two right images introduce maps of 24Mg and 40Ca lateral distribution on the sample surface. The color chart is as follows: blue is the lowest Mg/Ca abundance up to a maximum value colored in red. Places marked by # are the sweet spots analyzed by depth profiling; Fig. 2 represents results of one of them. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
atoms/cm2 to 1.4 1015 atoms/cm2 it follows that RIMS mapping still can be considered as virtually nondestructive [23]. RIMS elemental maps are stitched-together squares 300 300 lm2 (set raster size of the analysis beam), large enough for better relevance to actual depth profiling with analysis area 500 500 lm2. This imaging combination allows one to store three dimensional arrays of chemical and topographical information (X, Y coordinates, and secondary ions and electrons signal intensities). An example of an imaging array is shown in Fig. 3 revealing the least contaminated areas, sweet spots (marked by #), which were used later for acquiring depth profiles shown in Fig. 2 in red and green (solid lines and open squares).
In essence, it appears possible to experimentally measure detailed near-surface, first 10–15 nm, distributions of SW Mg and Ca. As seen in Fig. 2, these elements are distributed in Si matrix almost uniformly in terms of concentration. Only 10-fold decrease is observed in RIMS intensity between the surface and the bulk at 100 nm. Reasonable explanation for such trends is a superposition of three factors. First is proton radiation induced diffusion towards the collector’s surface, discussed in more details in [20]; this is for the first 10 nm. Very slow monotonic decrease in Mg and Ca signals between 10 and 100 nm is due to the sum of the slow (roughly, 0.8 keV per atomic-mass-unit implantation energy) and the fast (roughly, 1.2 keV per atomic-mass-unit implantation energy) SW
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
4
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
components [24], as was shown in Ref. [25]; these are second and third factors. Thus, experimentally unraveled full depth distribution of SW 24Mg and 40Ca allowed to estimate their fluences 2.8 1012 atoms/cm2 and 1.6 1011 atoms/cm2 [25]. 4. Conclusions In this work, the experimental demonstration of a combined SEM/RIMS surface imaging is reported. This approach has been applied to identify the least contaminated areas on surfaces of Genesis Solar Wind collector in quasi nondestructive fashion. SEM/RIMS arrays (X, Y coordinates, and secondary ions and electrons signal intensities) were recorded. These arrays, in turn, enabled precise locating least contaminated areas. On pinned areas, high resolution gentleDB RIMS depth profiling analysis revealed near-surface (0–15 nm) distribution of Mg and Ca SW implants. This enabled more accurate reconstruction of the complete shape of SW depth profile. The experimentally obtained shape helped to improve accuracy and precision of SW fluences calculation through integration of these depth profiles, as well as guided a correct subtraction of surface contamination in Mg and Ca depth profiles measured before SEM/RIMS imaging was introduced. Acknowledgments This work was supported by NASA through grant NNH09AM48I (RIMS hardware and methodology, and system integration), and by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division (SEM hardware and methodology). References [1] P. Sjövall, J. Lausmaa, B. Johansson, Mass spectrometric imaging of lipids in brain tissue, Anal. Chem. 76 (2004) 4271–4278. [2] M.M.T. Blaze, A. Akhmetov, B. Aydin, P.D. Edirisinghe, G. Uygur, L. Hanley, Quantification of antibiotic in biofilm-inhibiting multilayers by 7.87 eV laser desorption postionization MS imaging, Anal. Chem. 84 (2012) 9410–9415. [3] F.A. Stevie, S.W. Downey, S.R. Brown, T.L. Shofner, M.A. Decker, T. Dingle, L. Christman, Nanoscale elemental imaging of semiconductor materials using focused ion beam secondary ion mass spectrometry, J. Vac. Sci. Technol., B 17 (1999) 2476–2482. [4] Y. Cui, J.F. Moore, S. Milasinovic, Y. Liu, R.J. Gordon, L. Hanley, Depth profiling and imaging capabilities of an ultrashort pulse laser ablation time of flight mass spectrometer, Rev. Sci. Instrum. 83 (2012) 093702. [5] Y. Coello, A.D. Jones, T.C. Gunaratne, M. Dantus, Atmospheric pressure femtosecond laser imaging mass spectrometry, Anal. Chem. 82 (2010) 2753– 2758. [6] D. Willingham, A. Kucher, N. Winograd, Strong-field ionization of sputtered molecules for biomolecular imaging, Chem. Phys. Lett. 468 (2009) 264–269. [7] J.C. Vickerman, Molecular imaging and depth profiling by mass spectrometry – SIMS, MALDI or DESI?, Analyst 136 (2011) 2199–2217
[8] S.V. Baryshev, A.V. Zinovev, C.E. Tripa, M.J. Pellin, Q. Peng, J.W. Elam, I.V. Veryovkin, High-resolution secondary ion mass spectrometry depth profiling of nanolayers, Rapid Commun. Mass Spectrom. 26 (2012) 2224–2230. [9] C. Allen, J. Allton, G. Lofgren, K. Righter, R. Zeigler, M. Zolensky, Curating NASA’s extraterrestrial samples, Eos, Trans. Am. Geophys. Union 94 (2013) 253–254. [10] I.V. Veryovkin, C. Emil Tripa, M.J. Pellin, Efficient multiple beam ion optics for quantitative surface analysis: from simulations to a fully operational instrument, Physics Procedia 1 (2008) 379–389. [11] A.V. Zinovev, S.V. Baryshev, C.E. Tripa, I.V. Veryovkin, Laser setup multielement RIMS analysis of Genesis return samples, in: Proceedings of 43rd Lunar and Planetary Science Conference (2012) Abstract #2911, http:// www.lpi.usra.edu/meetings/lpsc2012/pdf/2911.pdf. [12] Z. Ma, R.N. Thompson, K.R. Lykke, M.J. Pellin, A.M. Davis, New instrument for microbeam analysis incorporating submicron imaging and resonance ionization mass spectrometry, Rev. Sci. Instrum. 66 (1995) 3168–3176. [13] I.V. Veryovkin, W.F. Calaway, J.F. Moore, M.J. Pellin, D.S. Burnett, A new timeof-flight instrument for quantitative surface analysis, Nucl. Instr. Meth. Phys. Res., Sect. B 219–220 (2004) 473–479. [14] A.J.G. Jurewicz, D.S. Burnett, R.C. Wiens, T.A. Friedmann, C.C. Hays, R.J. Hohlfelder, K. Nishiizumi, J.A. Stone, D.S. Woolum, R. Becker, A.L. Butterworth, A.J. Campbell, M. Ebihara, I.A. Franchi, V. Heber, C.M. Hohenberg, M. Humayun, K.D. McKeegan, K. McNamara, A. Meshik, R.O. Pepin, D. Schlutter, R. Wieler, Space Sci. Rev. 105 (2003) 535–560. [15] D.S. Burnett, G.S. Team, Cosmochemistry special feature: solar composition from the genesis discovery mission, Proc. Natl. Acad. Sci. 108 (2011) 19147– 19151. [16] K.D. McKeegan, A.P.A. Kallio, V.S. Heber, G. Jarzebinski, P.H. Mao, C.D. Coath, T. Kunihiro, R.C. Wiens, J.E. Nordholt, R.W. Moses, D.B. Reisenfeld, A.J.G. Jurewicz, D.S. Burnett, The oxygen isotopic composition of the sun inferred from captured solar wind, Science 332 (2011) 1528–1532. [17] B. Marty, M. Chaussidon, R.C. Wiens, A.J.G. Jurewicz, D.S. Burnett, A 15N-Poor isotopic composition for the solar system as shown by genesis solar wind samples, Science 332 (2011) 1533–1536. [18] S.V. Baryshev, A.V. Zinovev, C.E. Tripa, R.A. Erck, I.V. Veryovkin, White light interferometry for quantitative surface characterization in ion sputtering experiments, Appl. Surf. Sci. 258 (2012) 6963–6968. [19] V.S. Heber, A.J.G. Jurewicz, P. Janney, M. Wadhwa, K.D. McKeegan, D.S. Burnett, Mg isotopic composition of the Solar Wind by SIMS analysis of Genesis targets, Meteorit. Planet. Sci. 46 (2011) A90–A91. [20] B.V. King, M.J. Pellin, D.S. Burnett, Investigation of radiation enhanced diffusion of magnesium in substrates flown on the NASA genesis mission, Appl. Surf. Sci. 255 (2008) 1455–1457. [21] M.J. Calaway, E.K. Stansbery, L.P. Keller, Genesis capturing the sun: solar wind irradiation at Lagrange 1, Nucl. Instr. Meth. Phys. Res., Sect. B 267 (2009) 1101–1108. [22] J.C. Vickerman, D. Briggs (Eds.), ToF-SIMS: Materials Analysis by Mass Spectrometry, 2nd ed., Manchester and IM Publications, Chichester, SurfaceSpectra, 2013. p. 560. [23] H. Zandvliet, H. Elswijk, E. van Loenen, I. Tsong, Scanning tunneling microscopy and spectroscopy of ion-bombarded Si(1 1 1) and Si(1 0 0) surfaces, Phys. Rev. B 46 (1992) 7581–7587. [24] D.S. Burnett, B.L. Barraclough, R. Bennett, M. Neugebauer, L.P. Oldham, C.N. Sasaki, D. Sevilla, N. Smith, E. Stansbery, D. Sweetnam, R.C. Wiens, Space Sci. Rev. 105 (2003) 509–534. [25] I.V. Veryovkin, S.V. Baryshev, D.S. Burnett, M.J. Pellin, C.E. Tripa, A.V. Zinovev, Dual beam sputter depth profiling of Genesis Solar Wind collectors by RIMS, in: Proceedings of 43rd Lunar and Planetary Science Conference (2012) Abstract #2296, http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2296.pdf.
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis q S.V. Baryshev a,b,⇑, A.V. Zinovev a, C.E. Tripa a, I.V. Veryovkin a a b
Argonne National Laboratory, 9700 S. Cass Ave., Argonne, IL 60439, USA Euclid TechLabs LLC, 5900 Harper Rd., Solon, OH 44139, USA
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Sputter depth profiling Resonance ionization mass spectrometry Imaging mass spectrometry Scanning electron microscopy Ion implantation Genesis mission
a b s t r a c t We report on a combination of imaging mass spectrometry (MS) and scanning electron microscopy (SEM) developed in a custom designed time-of-flight (TOF) MS instrument with laser post-ionization of sputtered atoms. Elemental (by MS) and topographical (by SEM) mapping of surfaces of heavily contaminated Si collectors from the NASA Genesis sample return mission enabled obtaining much more accurate and detailed depth distribution of the Solar Wind Mg and Ca implanted in these collectors. This is because the cleanest areas were identified by the SEM/MS mapping, and high resolution sputter depth profiling at these locations revealed near-surface (0–15 nm) depth distribution of Mg and Ca, that were used for more accurate fluence calculations of these Solar Wind species. MS imaging was virtually nondestructive at primary ion fluence 1012 cm-2, causing no effect on accuracy and precision of quantitative depth profiling that followed the imaging. We also demonstrate importance of such an approach by directly comparing high resolution depth profiles measured on clean areas versus arbitrarily selected areas. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Imaging mass spectrometry (MS) is a boon to biology, chemistry, and materials science. This is because information on elemental distributions it provides is extremely important for studies of biological tissues [1], development of drugs [2] and improving performance of solid-state electronics [3]. Elemental distribution may be represented in two or three dimensions. For performing such 2D and 3D imaging MS, the choice of primary probes is confined to either laser beam (e.g., laser ablation [4] or laser desorption [5]) or ion beam (sputtering using atomic [3] or cluster projectiles [6]). For 3D imaging, be it organic or inorganic materials, mass spectrometry of ion sputtered species transcends all other approaches due to the smallest depth of origin of the species, a few monolayers only, which provides the best depth resolution [7,8]. Importantly, the high sensitivity of this MS imaging method q The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (‘‘Argonne’’). Argonne, a US Department of Energy Office of Science laboratory, is operated under Contract No. DE-AC0206CH11357. The US Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. ⇑ Corresponding author at: Euclid TechLabs LLC, 5900 Harper Rd., Solon, OH 44139, USA. Tel.: +1 630 252 6213. E-mail address: [email protected] (S.V. Baryshev).
(especially when combined with laser post-ionization of sputtered neutrals) may be capable of quasi nondestructive surface analysis because ion irradiation can be limited to extremely low fluences. High resolution imaging of the actual surface topography is also possible if one detects secondary electrons produced by primary ions, similarly to scanning electron microscopy (SEM) [3]. In many cases though, obtaining high quality topography images using an ion probe may require exposure to high ion fluences, which results in considerable surface damage. Conventional SEM, with a 5–20 keV electron probe, is a true nondestructive method to image topography of solids. In the present paper, the strength of a combination of quasi nondestructive SEM/MS imaging is experimentally demonstrated. As an example, elemental/topographical imaging of the contaminated surface of a Si Solar Wind collector of the NASA Genesis mission was performed. This imaging revealed least contaminated areas on the surface to carry high resolution sputter depth profiling analyses of shallow (<100 nm) trace-concentration (ppb) Solar Wind implants Mg and Ca in order to correctly determine their accumulated fluences. 2. Material and methods Samples used in this study were single crystal Si(0 0 1) provided by NASA Johnson Space Center within Genesis return mission program [9].
http://dx.doi.org/10.1016/j.nimb.2014.02.097 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
2
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
Fig. 1. The gentleDB system developed in SARISA [10]. The milling Ar+ ion beam comes from the low energy column and is deflected into the front TOF column by the Bending System optics. Being focused with Lens1, it mills the sample surface perpendicularly. The separate pulsed analysis ion gun (also Ar+) probes the surface. Secondary species are then mass analyzed by the TOF spectrometer. The electron gun images surface topography. The photo-ionizing laser beams are not shown; one can imagine laser beams intercepting neutrals between sample surface and Lens1.
Used in this work, a TOF MS instrument, SARISA, can operate in two modes: secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry with resonance enhanced multiphoton ionization (RIMS). To make the SARISA instrument even more versatile, an FEI 2LE electron gun with Schottky source was installed to probe the surface at 60° incidence angle, while secondary electrons are extracted in the direction normal to the surface and directed toward a dedicated detector as described in Ref. [10]. The FEI 2LE gun has its own high voltage power supply unit operated through dedicated FEI software, and FEI digital raster scan unit operated through RevolutionSEM imaging controller made by 4 pi, Inc. Elemental trace analysis of samples was carried in the RIMS mode, such that sputtered neutral species are efficiently converted into photo-ions by resonantly enhanced multi-photon ionization with tunable Ti-sapphire lasers exactly as reported in Ref. [11]. To characterize both surface and bulk, high resolution depth profiling in the dual-beam approach with normal incidence milling was applied. This dual-beam approach is dubbed as gentleDB, and described in great detail elsewhere [8] (see Fig. 1 and its caption for more details). The gentleDB depth profiling makes use of two independent switchable ion beams: one normally incident beam with high direct current and low energy for ion milling, and another one, striking the target at 60° and pulsed, for TOF MS analysis. Thus, there are three beams in the system (two ion beams and one electron beam). All beams were superimposed and centered with respect to SARISA ion optics center by means of Schwarzschild all-reflecting in vacuum microscope [12,13].
3. Results and discussion NASA Genesis mission samples, made of a number of ultrapure materials [14], have collected the Solar Wind (SW) elements upon their implantation in those materials [15]. These samples pose a serious challenge for quantitative elemental and isotopic analyses with high precision and accuracy. This is because (i) the surface contamination (due to the spacecraft outgassing during the flight and, most importantly, due to its crash landing) is very abundant, and (ii) the SW elements are implanted at ultralow trace level concentrations (sub-ppb) within the first 100 nm under the collectors surface. Ion sputter mass spectrometry has already proved to be an invaluable tool in the Genesis mission by determining isotopic distributions of SW oxygen and nitrogen [16,17]. Quantitative elemental MS analyses of Genesis samples for non-gaseous SW elements are even more challenging because they require the highest
possible depth resolution (to distinguish between the terrestrial surface contamination and the implanted SW) in combination with the highest possible sensitivity (to detect these SW elements at sub-ppb concentrations). For a number of SW elements with high terrestrial abundances, this is possibly the only way to accurately determine the implantation depth profile curve needed for accurate calculation of SW fluencies. In Fig. 2 depth profiles of 24Mg and 40Ca in a Genesis SW Si collector (sample #60428) are demonstrated. The wine and olive curves (solid lines and open circles) represent typical results of high resolution gentleDB RIMS depth profiling analysis of heavily contaminated samples. There are 2 regions. Surface contamination, manifesting itself in high RIMS signal intensity, is followed by 10-fold (for Mg) and 100-fold (for Ca) drop leading to a somewhat short plateau of SW. It is commonly assumed that implanted SW depth profiles have Gaussian-like distribution, similarly to that one expects from ion implantation of semiconductors, or by running TRIM/SRIM calculations; we do see such profiles in calibrated Si standard implants used to calculate fluences in Genesis samples [11,18]. Such assumption is used to compensate the lack of knowledge on near-surface depth distribution, which is hard to measure either due to the heavy surface contamination and/or due to the sample breakdown if back-side depth profiling approach is used [19]. At the same time, it is worth mentioning that there is a high likelihood that real SW distributions strongly deviate from the Gaussian shape due to the initial kinetic energy distributions of SW ions and also due to radiationenhanced diffusion of implanted SW species [20,21], which could be further changed by elevated collector temperatures during the flight. Keeping these facts in mind, it is highly desirable to be able to measure actual SW depth distribution in direct experiments. We combined the advantages of elaborated SEM imaging protocol with trace-sensitive surface-only RIMS elemental imaging/ mapping prior to standard gentleDB depth profiling. First of all, SEM imaging allows avoiding large (such as at the bottom of the sample in Fig. 3) and small (left hand inset in Fig. 3) chipped areas or areas containing visible features/inclusions. Such mechanically damaged areas are not suitable for conducting depth profiling. Then, pulsed analysis ion beam was used (60° inclined gun in Fig. 1) on the pristine surface at ultralow fluence condition, f = 1012 ions/cm2. This fluence is one order of magnitude lower than what is required for analyses in static SIMS regime [22]. Parameter Y [sputtered target atoms/incident primary ions] f [incident primary ions/cm2] must be compared to the areal atomic concentration in Si 1.4 1015 atoms/cm2, where Y is the sputtering yield. Thus, using known Y 5 (Si sputtered by 5 keV argon ions at 60° incidence) and comparing the product of Y f = 5 1012
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
3
Fig. 2. 24Mg and 40Ca depth profiles in a fragment of a Genesis Si collector (#60428). The wine and olive curves (solid lines and open circles) represent typical results without preliminary SEM/RIMS imaging. The curves in red and green (solid lines and open squares) are depth profiles conducted on one of 3 sweet spots marked in Fig. 3. It is seen, in some cases surface abundance of Mg and Ca is even lower than that of SW. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. Principle of combined imaging by SEM/RIMS. Black and white (most left) is the topographical view of a Genesis Si coupon (#60428) in secondary electrons contrast obtained by SEM. Two right images introduce maps of 24Mg and 40Ca lateral distribution on the sample surface. The color chart is as follows: blue is the lowest Mg/Ca abundance up to a maximum value colored in red. Places marked by # are the sweet spots analyzed by depth profiling; Fig. 2 represents results of one of them. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
atoms/cm2 to 1.4 1015 atoms/cm2 it follows that RIMS mapping still can be considered as virtually nondestructive [23]. RIMS elemental maps are stitched-together squares 300 300 lm2 (set raster size of the analysis beam), large enough for better relevance to actual depth profiling with analysis area 500 500 lm2. This imaging combination allows one to store three dimensional arrays of chemical and topographical information (X, Y coordinates, and secondary ions and electrons signal intensities). An example of an imaging array is shown in Fig. 3 revealing the least contaminated areas, sweet spots (marked by #), which were used later for acquiring depth profiles shown in Fig. 2 in red and green (solid lines and open squares).
In essence, it appears possible to experimentally measure detailed near-surface, first 10–15 nm, distributions of SW Mg and Ca. As seen in Fig. 2, these elements are distributed in Si matrix almost uniformly in terms of concentration. Only 10-fold decrease is observed in RIMS intensity between the surface and the bulk at 100 nm. Reasonable explanation for such trends is a superposition of three factors. First is proton radiation induced diffusion towards the collector’s surface, discussed in more details in [20]; this is for the first 10 nm. Very slow monotonic decrease in Mg and Ca signals between 10 and 100 nm is due to the sum of the slow (roughly, 0.8 keV per atomic-mass-unit implantation energy) and the fast (roughly, 1.2 keV per atomic-mass-unit implantation energy) SW
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097
4
S.V. Baryshev et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx
components [24], as was shown in Ref. [25]; these are second and third factors. Thus, experimentally unraveled full depth distribution of SW 24Mg and 40Ca allowed to estimate their fluences 2.8 1012 atoms/cm2 and 1.6 1011 atoms/cm2 [25]. 4. Conclusions In this work, the experimental demonstration of a combined SEM/RIMS surface imaging is reported. This approach has been applied to identify the least contaminated areas on surfaces of Genesis Solar Wind collector in quasi nondestructive fashion. SEM/RIMS arrays (X, Y coordinates, and secondary ions and electrons signal intensities) were recorded. These arrays, in turn, enabled precise locating least contaminated areas. On pinned areas, high resolution gentleDB RIMS depth profiling analysis revealed near-surface (0–15 nm) distribution of Mg and Ca SW implants. This enabled more accurate reconstruction of the complete shape of SW depth profile. The experimentally obtained shape helped to improve accuracy and precision of SW fluences calculation through integration of these depth profiles, as well as guided a correct subtraction of surface contamination in Mg and Ca depth profiles measured before SEM/RIMS imaging was introduced. Acknowledgments This work was supported by NASA through grant NNH09AM48I (RIMS hardware and methodology, and system integration), and by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division (SEM hardware and methodology). References [1] P. Sjövall, J. Lausmaa, B. Johansson, Mass spectrometric imaging of lipids in brain tissue, Anal. Chem. 76 (2004) 4271–4278. [2] M.M.T. Blaze, A. Akhmetov, B. Aydin, P.D. Edirisinghe, G. Uygur, L. Hanley, Quantification of antibiotic in biofilm-inhibiting multilayers by 7.87 eV laser desorption postionization MS imaging, Anal. Chem. 84 (2012) 9410–9415. [3] F.A. Stevie, S.W. Downey, S.R. Brown, T.L. Shofner, M.A. Decker, T. Dingle, L. Christman, Nanoscale elemental imaging of semiconductor materials using focused ion beam secondary ion mass spectrometry, J. Vac. Sci. Technol., B 17 (1999) 2476–2482. [4] Y. Cui, J.F. Moore, S. Milasinovic, Y. Liu, R.J. Gordon, L. Hanley, Depth profiling and imaging capabilities of an ultrashort pulse laser ablation time of flight mass spectrometer, Rev. Sci. Instrum. 83 (2012) 093702. [5] Y. Coello, A.D. Jones, T.C. Gunaratne, M. Dantus, Atmospheric pressure femtosecond laser imaging mass spectrometry, Anal. Chem. 82 (2010) 2753– 2758. [6] D. Willingham, A. Kucher, N. Winograd, Strong-field ionization of sputtered molecules for biomolecular imaging, Chem. Phys. Lett. 468 (2009) 264–269. [7] J.C. Vickerman, Molecular imaging and depth profiling by mass spectrometry – SIMS, MALDI or DESI?, Analyst 136 (2011) 2199–2217
[8] S.V. Baryshev, A.V. Zinovev, C.E. Tripa, M.J. Pellin, Q. Peng, J.W. Elam, I.V. Veryovkin, High-resolution secondary ion mass spectrometry depth profiling of nanolayers, Rapid Commun. Mass Spectrom. 26 (2012) 2224–2230. [9] C. Allen, J. Allton, G. Lofgren, K. Righter, R. Zeigler, M. Zolensky, Curating NASA’s extraterrestrial samples, Eos, Trans. Am. Geophys. Union 94 (2013) 253–254. [10] I.V. Veryovkin, C. Emil Tripa, M.J. Pellin, Efficient multiple beam ion optics for quantitative surface analysis: from simulations to a fully operational instrument, Physics Procedia 1 (2008) 379–389. [11] A.V. Zinovev, S.V. Baryshev, C.E. Tripa, I.V. Veryovkin, Laser setup multielement RIMS analysis of Genesis return samples, in: Proceedings of 43rd Lunar and Planetary Science Conference (2012) Abstract #2911, http:// www.lpi.usra.edu/meetings/lpsc2012/pdf/2911.pdf. [12] Z. Ma, R.N. Thompson, K.R. Lykke, M.J. Pellin, A.M. Davis, New instrument for microbeam analysis incorporating submicron imaging and resonance ionization mass spectrometry, Rev. Sci. Instrum. 66 (1995) 3168–3176. [13] I.V. Veryovkin, W.F. Calaway, J.F. Moore, M.J. Pellin, D.S. Burnett, A new timeof-flight instrument for quantitative surface analysis, Nucl. Instr. Meth. Phys. Res., Sect. B 219–220 (2004) 473–479. [14] A.J.G. Jurewicz, D.S. Burnett, R.C. Wiens, T.A. Friedmann, C.C. Hays, R.J. Hohlfelder, K. Nishiizumi, J.A. Stone, D.S. Woolum, R. Becker, A.L. Butterworth, A.J. Campbell, M. Ebihara, I.A. Franchi, V. Heber, C.M. Hohenberg, M. Humayun, K.D. McKeegan, K. McNamara, A. Meshik, R.O. Pepin, D. Schlutter, R. Wieler, Space Sci. Rev. 105 (2003) 535–560. [15] D.S. Burnett, G.S. Team, Cosmochemistry special feature: solar composition from the genesis discovery mission, Proc. Natl. Acad. Sci. 108 (2011) 19147– 19151. [16] K.D. McKeegan, A.P.A. Kallio, V.S. Heber, G. Jarzebinski, P.H. Mao, C.D. Coath, T. Kunihiro, R.C. Wiens, J.E. Nordholt, R.W. Moses, D.B. Reisenfeld, A.J.G. Jurewicz, D.S. Burnett, The oxygen isotopic composition of the sun inferred from captured solar wind, Science 332 (2011) 1528–1532. [17] B. Marty, M. Chaussidon, R.C. Wiens, A.J.G. Jurewicz, D.S. Burnett, A 15N-Poor isotopic composition for the solar system as shown by genesis solar wind samples, Science 332 (2011) 1533–1536. [18] S.V. Baryshev, A.V. Zinovev, C.E. Tripa, R.A. Erck, I.V. Veryovkin, White light interferometry for quantitative surface characterization in ion sputtering experiments, Appl. Surf. Sci. 258 (2012) 6963–6968. [19] V.S. Heber, A.J.G. Jurewicz, P. Janney, M. Wadhwa, K.D. McKeegan, D.S. Burnett, Mg isotopic composition of the Solar Wind by SIMS analysis of Genesis targets, Meteorit. Planet. Sci. 46 (2011) A90–A91. [20] B.V. King, M.J. Pellin, D.S. Burnett, Investigation of radiation enhanced diffusion of magnesium in substrates flown on the NASA genesis mission, Appl. Surf. Sci. 255 (2008) 1455–1457. [21] M.J. Calaway, E.K. Stansbery, L.P. Keller, Genesis capturing the sun: solar wind irradiation at Lagrange 1, Nucl. Instr. Meth. Phys. Res., Sect. B 267 (2009) 1101–1108. [22] J.C. Vickerman, D. Briggs (Eds.), ToF-SIMS: Materials Analysis by Mass Spectrometry, 2nd ed., Manchester and IM Publications, Chichester, SurfaceSpectra, 2013. p. 560. [23] H. Zandvliet, H. Elswijk, E. van Loenen, I. Tsong, Scanning tunneling microscopy and spectroscopy of ion-bombarded Si(1 1 1) and Si(1 0 0) surfaces, Phys. Rev. B 46 (1992) 7581–7587. [24] D.S. Burnett, B.L. Barraclough, R. Bennett, M. Neugebauer, L.P. Oldham, C.N. Sasaki, D. Sevilla, N. Smith, E. Stansbery, D. Sweetnam, R.C. Wiens, Space Sci. Rev. 105 (2003) 509–534. [25] I.V. Veryovkin, S.V. Baryshev, D.S. Burnett, M.J. Pellin, C.E. Tripa, A.V. Zinovev, Dual beam sputter depth profiling of Genesis Solar Wind collectors by RIMS, in: Proceedings of 43rd Lunar and Planetary Science Conference (2012) Abstract #2296, http://www.lpi.usra.edu/meetings/lpsc2012/pdf/2296.pdf.
Please cite this article in press as: S.V. Baryshev et al., Combination of imaging mass spectrometry and electron microscopy for quasi nondestructive surface analysis, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.097