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SIMS Proceedings PapersReceived: 30 September 2009 Revised: 25 March 2010 Accepted: 26 March 2010 Published online in Wiley Online Library: 22 June 2010

(wileyonlinelibrary.com) DOI 10.1002/sia.3523

RIMS analysis of Ca and Cr in Genesis solar windcollectorsI. V. Veryovkin,a∗ C. E. Tripa,a A. V. Zinovev,a B. V. King,a,b M. J. Pellina

and D. S. Burnettc

RIMS depth profiles have been measured for Cr and Ca in Genesis solar wind collector made from Si and compared to suchmeasurements for ion-implanted Si reference material. The presence of surface contamination has been shown to be a significantfactor influencing the total Ca and Cr fluence measured in the Genesis collectors. A procedure to remove the contaminant signalfrom these depth profiles using the reference material implanted with a minor isotope demonstrated that 36% of the measuredCa fluence in our Genesis sample comes from terrestrial contamination. Copyright c© 2010 John Wiley & Sons, Ltd.

Keywords: RIMS; Genesis; solar wind; silicon; depth profiling; SNMS

Introduction

In the NASA Genesis Mission,[1] a spacecraft collected solar wind(SW) particles into a variety of ultrapure materials, includingsilicon wafers. Collectors were exposed to the solar wind atthe L1 Lagrangian point (0.99 AU) for 27 months (December2001 to April 2004). The goal of the Genesis mission was toobtain a comprehensive set of solar elemental and isotopic abun-dances at significantly higher precision and accuracy levels thanpresently available.[1] However, the SW samples delivered bythe mission present significant challenges for analytical tech-niques, in part due to severe terrestrial contamination of thecollectors on re-entry, and in part due to the low SW impu-rity concentrations (ppm to ppt) which were implanted near(within 50 nm) the surface due to the most probable kineticenergy of SW particles of only about 1 keV/amu. In addition,many SW elements are fast diffusers in Si, segregating towardsthe surface.[2] It then becomes difficult to separate SW impu-rities segregated to the surface from terrestrial contamination.However, the quality of scientific results from the Genesis mis-sion critically depends on the ability of analytical techniquesto accurately make this separation. Ion sputtering-based surfaceanalysis techniques such as SIMS and Secondary Neutral MassSpectrometry (SNMS) are the methods of choice to make thisseparation.

We measure the concentration of metallic elements in Genesiscollectors using Resonance Ionization Mass Spectrometry (RIMS),an ultrasensitive surface analytical method, which is a variant ofSNMS capable of detecting SW in samples with lateral dimensionsof less than a few mm and at concentrations from above one ppmto below one ppt. The details of our instrument for Genesis sampleanalysis have been discussed previously.[3] In this paper, we reportfirst on simultaneous RIMS measurements of Ca and Cr performedon Genesis sample #60 179. We also describe and evaluate the data-processing procedure for separating SW implants of Cr and Ca fromterrestrial surface contamination and, based on our experience,discuss what has to be done to improve accuracy and precision ofquantitative RIMS measurements of SW samples.

Experimental

We use sputter depth profiling (rastered 10 keV Ar+ at 60◦

incidence angle, typical currents between 0.6 and 1 µA) toremove sample material from a rectangular 575 × 1225 µm2

crater whose central region is probed by the same beam(elliptical spot 130 × 260 µm2) in a pulsed regime to allowa TOF MS operation. Sputtered neutral atoms were measured,using Resonantly-Enhanced Multi-Photon Ionization (REMPI) withtunable Ti-Sapphire lasers to create photoions which wereextracted into a TOF MS for analysis. Concentration versus depthprofiles were obtained by a sequence of alternating sessions ofsputtering (raster scanned continuous primary ion beam) and TOFMS analysis (500 ns primary ion beam pulses).

Two types of samples have been depth profiled: ion implants toact as primary standards and Genesis SW samples. The standardswere silicon wafers implanted with 80 keV 52Cr and 200 keV 44Ca atidentical 3 × 1017 at·m−2 fluences. In order to verify the referenceion fluencies by an independent analytical method, a few pieces ofthe same wafer material were exposed to the same implanter ionbeam under the same experimental conditions for longer timessuch that ×100 higher fluencies (3×1019 at·m−2, determined froman ion current integrator data) would be implanted. These higherion fluencies in control samples were measured by RutherfordBackscattering Spectrometry (RBS), which confirmed the expectedvalues of 3 × 1019 at·m−2. Under the assumption that ion currentfluctuations in the implanter were negligible, the RBS-measuredfluencies we divided by a factor of 100 thus yielding the reference

∗ Correspondence to: I. V. Veryovkin, Materials Science Division, ArgonneNational Laboratory, Argonne, IL 60439, USA. E-mail: verigo@anl.gov

a Materials Science Division, Argonne National Lbaoratory, Argonne, IL 60439,USA

b School of Mathematical and Physical Sciences, University of Newcastle,Callaghan 2308, NSW, Australia

c Division ofGeological andPlanetarySciences,CaliforniaInstituteofTechnology,Pasadena, CA 91125, USA

Surf. Interface Anal. 2011, 43, 467–469 Copyright c© 2010 John Wiley & Sons, Ltd.

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ion fluences of 3×1017 at·m−2 to be used for quantification of RIMSmeasurements. Because of a limited resolving power of the ionimplanter mass filter, other isotopes, such as 53Cr and 40Ca, werealso implanted in the standard with the same energies at more thanan order of magnitude lower fluencies. For Ca ion implantation,our RIMS measurements showed that the typical ratio between44Ca and 40Ca fluences was between 12 and 12.5, based on theassumption that implant profiles had Gaussian shapes so that theratio between their peak intensities could be used.

To perform RIMS analyses for Cr and Ca, we have used anew combination of REMPI schemes permitting simultaneousmeasurements of both elements. This new arrangement combined422.79 nm light for the resonance step of Ca with 360.53 nm lightboth resonantly exciting Cr as well as ionizing both elements. Thiscombination allowed simultaneous REMPI of two elements withonly two tunable Ti-Sapphire lasers. To enhance the ionizationfor Ca and Cr by a factor of 4, the above beams were combinedwith a high intensity 355 nm light. All laser beams were nearlycollinear, which allowed a prism retroreflector to be implementedreturning the beams back into the photoionization region, foran extra (about a factor of two) signal enhancement. However,with all these ‘tricks’ implemented, photoionization processes forCr and Ca sputtered atoms did not seem to be saturated. It wastherefore important to develop a measurement protocol takinginto account signal changes due to possible drifts in powers andwavelengths of the lasers during RIMS depth profiling.

We found that, in order to obtain reproducible RIMS results,analyses of a Genesis sample and a reference ion implant must becompleted on the same day, thus avoiding day-to-day variationsin laser photoionization conditions. In order to quantitativelycharacterize the overall signal variation during the 6 h neededfor depth profiling, three additional RIMS measurement sessionson a secondary reference material were introduced. An aluminumalloy, Alcan ALC-A380.2-CAC, was used as the secondary reference,since it contained many elements of interest, including Cr and Ca,as ppm-level impurities. The secondary standard was analyzed byRIMS prior to the implant depth profile, between the implant andGenesis profiles and after the Genesis profile. A parabolic fit to Crand Ca RIMS signals from the Alcan sample versus time, allowedsignal changes during the two depth profiling sessions to beaccounted for and showed that RIMS signals were only changingslowly. The total experimental time to obtain the depth profilesand secondary standard spectra was about 8.5 h. A typical variationof the Ca and Cr RIMS signals during this period was within 15%.

Results

Figure 1 compares sputter depth profiles of 52Cr and 40Ca from theion implant standard and from the Genesis sample #60 179 ob-tained in one RIMS experiment conducted in accordance with theprotocol described above. The maximum signal from the 200 keV40Ca implant corresponds to a depth of about 220 nm (at a primaryion fluence of 8 × 1020 m−2). Peak shapes of the both ion implantsagree well with the results from SRIM simulations. But since theenergy of SW ions is about 1 keV/amu, we would expect both SWimplant profiles to show peaks at a primary ion fluence of about2–3 × 1020 at m−2. But instead, we see for both SW Ca and Cra monotonic decrease in signal with depth making it difficult todistinguish where the surface contamination ends, and where theactual SW implant starts.

A similar picture is observed for ion implant standards but it isless severe because one can clearly see a dip between the surface

Figure 1. Comparison of RIMS sputter depth profiles of naturally mostabundant 52Cr and 40Ca isotopes measured in the Genesis Si SW collectorand in the reference ion implant in Si.

contamination and the implant peak. We have conducted testexperiments with two pieces of the same ion implant standard toevaluate the accuracy of our quantification of the implanted ionfluence, if we exclude the surface contamination from the profileintegral by just ignoring all signals from below the dip. In thiscase, the implant fluences were determined to be 2.73 × 1017 and2.71 × 1017 at m−2 for 52Cr and 44Ca, respectively, which differ by≈10% from the accepted values of 3 × 1017 at m−2.

Unfortunately, this could not be confidently done for the SWdepth profiles shown in Fig. 1, and we had to develop a better pro-cedure for separating surface contamination from the SW implant.To do this, we recognize that the Ca isotope profiles are deter-mined, as a function of primary ion fluence x, by an implant dis-tribution, i(x), as well as any contamination function, c(x), which iscaused by surface contaminants mixed into the bulk during depthprofiling. We write the profiles of the two isotopes with depth as

44Ca (x) = i(x) + c(x)/RC (1)40Ca(x) = i(x)/RI + c(x)

where RC is the isotopic ratio (47.06 for 40Ca/44Ca) and RI is theratio of the isotopic fluxes during implantation. The parameter RI

might vary from one analyzed spot to another, due to artifacts ofthe ion beam implantation, such as superposition of two broadion beam profiles with nonoverlapping (due to mass filtering)maximums. As mentioned above, typical values of this parameterfor the Ca implantation were between 12 and 12.5. Thus, usingexperimentally determined RI parameters, the above equationswere solved simultaneously to obtain c(x), i(x) for the implants.The described procedure should work best for implant standardsenriched with a minor natural isotope, 44Ca in our case. The resultsof using Eqn (1) to calculate c(x) and i(x) are shown in Fig. 2,which indicates for Ca that c(x) peaks at a primary ion fluence of8 × 1018 m−2, or about 2 nm sputtered, then decreases sharplyfor fluences above 7 × 1019 m−2 (20 nm sputtered) to reach abackground at a fluence of 5 × 1020 m−2 (120 nm sputtered).

Using this procedure is justified if one can neglect the surfacecontamination mixed into the bulk of the reference sample by the200 keV implanting ion beam. For our experiments, it is possible.First of all, the surfaces of the reference samples prior to the ionimplantation were as clean as the semiconductor industry can getit, with wafers coming straight from the manufacturer. Besides

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RIMS analysis of Ca and Cr in solar wind collectors

Figure 2. RIMS depth profiles of 44Ca and 40Ca isotopic implants togetherwith the surface contamination signal c(x) and implant distribution functioni(x) found by Eqn1.

this, there are two more points that have to be taken into account:(i) Primary ion fluencies in our RIMS depth profiling experiments,as one can see, Figs 1 and 2 were much higher than those fromthe ion implanter: compare the total of 3 × 1017 m−2 from theimplanter to 5 × 1020 primary ions m−2 needed to profile throughthe region of interest; And (ii) SRIM simulations for 10keV Ar at 60◦

and 200 keV Ca at 7◦ onto a 1-nm-thick film of pure Ca on a Sisubstrate showed that there is about four times more transport ofCa into the Si for the Ar beam compared to the Ca implant beam.

So the total amount of mixing by the Ca implant beam is ex-pected to be more than 100 times less than the mixing due to theAr depth profiling beam.

To summarize, the Ca ion implantation into our reference sam-ples did not redistribute the surface Ca much, compared to thenonirradiated surface. Thus, the contamination function c(x) is ex-pected to be similar for the reference and SW samples, and so, canbe determined from reference implant samples and then appliedto the severely contaminated Genesis samples.

The other notable feature of the profile is the peak at6×1019 m−2 which comes from a change in the profiling protocol.The RIMS spectra were obtained using the ion beam pulsed with aduty cycle of 5×10−7. For lower fluences, the depth profiling beamis not raster scanned, whereas for higher fluences, the unpulsedion beam is rastered over a rectangular region four times the beamdiameter. The rise in the Ca signal at the onset of rastering comesfrom the analysis of Ca atoms transported into the analysis areafrom outside regions of the crater by the initial beam rastering.

We could apply the same procedure to determine c(x) from the52Cr and 53Cr RIMS depth profiles from the Cr implant standard.However, the mass filter for the Cr implant was set to mass 52, themajor Cr isotope. It is then more difficult to accurately determinec(x) and i(x) from the standard. We, therefore, have not includedCr contamination analysis in this paper.

Figure 3 shows depth profiles of 40Ca in the Genesis sample#60 179 together with a corrected profile which is the SW 40Ca pro-file with the above contamination function c(x) subtracted. One cansee that the corrected SW profile exhibits a peak at 2×1020 m−2 cor-responding to a depth of 55 nm. SRIM calculations for 1 keV/amu40Ca indicate that the SW implant should peak at about 44 nm, sothere appears to have been a redistribution of Ca during the Gen-esis irradiation away from the implant peak. Previously, radiationenhanced diffusion of impurities to H-induced damage was iden-tified as a likely cause of Mg transport.[2] This procedure effectively

Figure 3. RIMS depth profiles of 40Ca in Genesis SW collector, as-measuredand corrected for surface contamination using function c(x) determinedfrom Eqn1.

classifies the signal from the top 20 nm of the surface as terrestrialcontamination but also removes the signal from contaminationwhich is mixed by the analysis ion beam into deeper regions.

The comparison between integrals of these two SW 40Ca depthprofiles calculated for fluences above 7 × 1019 m−2 revealed thatthe area under the corrected profile is only about 64% of that for theas-measured SW 40Ca profile. Using these values for quantification,and based on the assumption that the integral for corrected 44Caprofile in the implant standard corresponds to the fluence of3×1017 at m−2, SW fluencies of 40Ca were determined to be (1.51±0.05)×1015 at m−2 and (0.97±0.03)×1015 at m−2 for as-measuredand corrected profiles, respectively. The standard deviation inthese values corresponds to the experimental precision originatingfrom the Poisson counting statistics, with error propagationcalculations applied to take into account all data manipulationssuch as normalizations, multiplications, additions/subtractions,etc. For comparison, the SW 40Ca fluence expected to be in Genesiscollectors after 27 months of collection is 1.29 × 1015 at m−2.

Conclusion

We have measured RIMS depth profiles of implant standards for Crand Ca in Si and compared them with measurements of the sameelements in SW collectors (Genesis #60 179). We have developeda procedure using minor isotope-enriched implant standards toremove the signal due to surface contamination from these depthprofiles and demonstrated that 36% of the measured Ca fluencein our Genesis sample comes from terrestrial contamination.

Acknowledgements

The authors wish to thank Andreas Wucher for helpful discussions.This work is supported by NASA through Grants NNH08AH761 andNNH08ZDA001N, by UChicago Argonne, LLC, under contract No.DE-AC02-06CH11357 and by the University of Newcastle.

References[1] The Genesis Mission, (Ed: C. T. Russell), Kluwer Academic Publishers:

Dordrecht/Boston/London, 2003, (also Space Science Reviews, 2003;105, Nos. 3–4).

[2] B. V. King, M. J. Pellin, D. S. Burnett, Appl. Surf. Sci. 2008, 255, 1455.[3] I. V. Veryovkin, W. F. Calaway, J. F. Moore, M. J. Pellin, D. S. Burnett,

Nucl. Instrum. Methods 2004, B219–B220, 473.

Surf. Interface Anal. 2011, 43, 467–469 Copyright c© 2010 John Wiley & Sons, Ltd. View this article online at wileyonlinelibrary.com