SIMS profiling of laser abalation pits in CVD diamond implanted with oxygen- 18 shows that homogenised 193nm excimer laser beam can successfully ablate a layer a few nm thick, removing surface contamination without signicant loss of implanted sample

Butterworth, A.L.

Chater, R.J.

Franchi, I.A.

McPhail, D.S.

Suhaimi, N.

van Calsteren, P.

English

Open Research Online (Open)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDepth sensitive sampling of implanted species inGenesis Collectors using UV laser ablation and SIMSConference or Workshop ItemHow to cite:Franchi, I.A.; Suhaimi, N.; Chater, R.J.; McPhail, D.S.; van Calsteren, P. and Butterworth, A.L. (2004).Depth sensitive sampling of implanted species in Genesis Collectors using UV laser ablation and SIMS. In: 35th Lunarand Planetary Science Conference, 15-19 Mar 2004, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2004/pdf/1681.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDEPTH SENSITIVE SAMPLING OF IMPLANTED SPECIES IN GENESIS COLLECTORS USING UVLASER ABLATION AND SIMS.  I. A. Franchi1, N. Suhaimi2, R. J. Chater2, D. S. McPhail2, P. van Calsteren3 andA. L. Butterworth1,4 1Planetary and Space Sciences Research Institute, Open University, Milton Keynes MK7 6AA,UK (i.a.franchi@open.ac.uk), 2Dept of Materials, Imperial College, London, SW7 2AZ, UK, Dept Earth Sciences,Open University, Milton Keynes MK7 6AA, UK, 4Space Sciences Laboratory, University of California, BerkeleyCA 94720, USA.Introduction:  The energy spectrum of the solarwind ions collected by the Genesis spacecraft is suchthat they are implanted into the ultra-pure collectormaterials to a depth where there should not be anysignificant diffusion loss.  However, this depth is ofthe order of only 100nm [1] which presents two keyproblems when sampling the implanted solarmaterial.  Firstly, the collector materials generallycontain some level of contamination, which given thelow abundance of the solar material in the collectors,even after ≈2 years of collection, means it isnecessary to ensure that only  the implanted ions inthe collector material are sampled. Second, and evenmore demanding, the analytical techniques have toremove or resolve surface contamination on thesamples.  Possible sources of contamination includeabsorbed atmospheric gases or volatiles degassedfrom other parts of the spacecraft, particulate debrisfrom micrometeorite impacts or particles introducedduring cutting, distribution and analysis.Previously, we have demonstrated that UV laserablation was well suited to sampling the implantedgases from diamond and diamond like material [2].The use of a pulsed 193nm homogenized excimerlaser offers exceptionally uniform power densitiesacross the beam and therefore very flat bottomed pits.Here we examine the practicality of using such alaser system to remove a very shallow layer ofsurface material to eliminate the surfacecontamination without significantly affecting theimplanted sample.  We have used Secondary IonMass Spectrometry (SIMS) to determine thedistribution with depth (depth profiles) of variousspecies in the sample before and after laser ablation.Sample and Technique:  A polished sample ofCVD diamond was irradiated with 1.5x1015 18O ionscm-2 at 36keV.  Prior to irradiation the sample wascoated with a thin film of Au to prevent charging ofthe target.  A series of 215µm diameter shallow pitswere then excavated in the sample by laser ablationusing a New Wave 193 UP excimer laser system. 1, 3and 5 pulses of about 30ns using two different energyoutputs of approximately 2 and 4 Jcm-2 were used togenerate a series of pits.A series of SIMS profiles were then generated in3 of the pits as well as in the surrounding areasunaffected by the laser ablation.  The SIMSinstrument was an Atomika DIDA 6500 retrofittedwith a floating low energy ion gun (FLIG) to allowhigh depth resolution analysis.  The samples wereanalysed with a 5 keV nitrogen beam scanned over anarea 200 µm2 across using electronic gating to collectsecondary ions from the central 25% of the crater.The intensities of the negative secondary ions of 12C,16O, 18O and 35Cl were determined from areasapproximately 50µm across.  16O and 35Cl weremonitored as measures of contamination levels.Results and Discussion: Figure 1 shows thedepth profile from  a pristine area of the diamond.Two clear features are apparent – a clear 18O implantprofile and a large amount of surface contaminationas indicated by the high concentrations of 16O and35Cl.  A TRIM calculation [3] simulating the implantis also shown, scaled to match the SIMS 18O profile.A good match between the calculated and observedprofiles indicates that the TRIM calculation can beused to scale the depth of the SIMS analysis –indicating that  the peak of the implant occurs ataround 45nm.  Using the known implantedabundance of 18O as a calibration the amount of 16Osurface contamination is estimated as 3.3x1015 atomscm-2 - approximately 2 orders of magnitude greaterthan the anticipated solar flux in the concentratortarget!  This areal dose suggests that the surface iscontaminated with a monolayer or two of oxide.  Theapparent distribution of the contaminants, whichdecay linearly (when the secondary ion intensity isplotted on a logarithmic scale) down to depths of  ~40nm are probably due to SIMS beam inducedmixing processes.  The decay lengths of ~ 7.5nm/decade are consistent with the range of nitrogen intodiamond at this SIMS primary beam energy (~12nm).  Figure 2 shows the SIMS profile from thecentre of a pit created with 5 pulses of 2 Jcm-2.  Mostof the 18O implant remains but there is an indicationthat the peak is now nearer to the surface.  The levelof surface contamination has been significantlyreduced, with the amount of 16O surfacecontamination now ≈6x1014 atoms cm-2. This is stilldouble the expected solar flux in the concentratortarget.Figure 3 shows the 18O SIMS profiles fromFigures 1 and 2 linearly re-scaled such that the deeperhalf of the two profiles are best matched,compensating for any differences in the sputter ratebetween the two pits.  It appears that some of theshallowest part of the 18O implant has been lostduring the laser ablation.  Comparing the area of theimplant curve of the profile from the laser pit withthat of the pristine implant indicates thatLunar and Planetary Science XXXV (2004) 1681.pdfapproximately 10% of the implanted 18O has beenlost.  Performing a similar exercise with a deeper pitgenerated with 5 pulses at twice the energy (4 Jcm-2)reveals a 20% loss of the implant.  In this case theamount of surface contamination was greater thanthat seen in the shallower pit but given the uncleanhandling experienced by the sample suchirregularities may be expected.  A shallower pitgenerated with 3 pulses at 2 J cm-2 revealed a loss ofonly 8% of the implant while maintaining acomparable level of surface contamination to thatseen in Figure 2.Conclusions:  This initial study of the use ofhomogenized UV laser ablation as a means ofperforming surface clean up of the Genesisconcentrator target material (CVD diamond) foroxygen isotope analysis has shown that sufficientdepth resolution can be achieved to remove a shallowlayer of surface material containing much of thesurface contamination.  The minimum levels ofsurface 16O at ≈6x1014 atom cm-2 observed in theablation pits is mostly likely absorbed atmosphericoxygen (or moisture) as the samples were exposed toatmosphere for some considerable time betweenablation and SIMS analysis.  The higher levels ofcontamination seen in the pristine area may be relatedto the Au coat.  A more realistic analysis protocolwould of course eliminate atmospheric exposurebetween the surface clean-up ablation and thesubsequent sampling of the solar wind-rich portion ofthe collector.The conditions employed here appear to result inthe loss of a small amount of implanted sample (atleast 8%).  Some of this may be due to someunevenness in the ablation process, possibly relatedto the granularity of the CVD diamond.  However,scope exists to reduce the number of pulses and toreduce the power density of the laser beam to furtherlimit the depth of ablation in this clean up stage.Thermal de-gassing of the samples prior to analysiswould also be expected to help significantly reducethe surface contamination.  The energy of the oxygenions implanted in the Genesis concentrator targetmaterial will in fact be higher than the ionsimplanted in the test sample used here [4], with apeak implant depth of around perhaps 75nm asopposed to the 45nm shown in Figure 1.  This offersbetter scope for resolving surface contamination fromthe solar wind sample, although it should be notedthat there will be a considerable spread in energiesfrom the solar wind which will see some of the tail atshallow levels.References: [1] Burnett D. S. et al (2003) Space Sci.Rev., 105, 509-534. [2] Butterworth A. L. et al. (2003) LPSXXXIV, Abstract #1591. [3] Ziegler J. F. and Biersack J. P.(2003) SRIM.com. [4] Wiens et al. (2003) Space Sci. Rev.,105, 601-625.11010010001041050 20 40 60 80 100 120 140 1600 10 20 30 40 50 60 70 8012C16O18O35ClTRIM CalcCountsSIMS Scan #TRIM Depth (nm)Figure 1 – SIMS profile from pristine area of CVDdiamond.  TRIM profile also shown scaled to matchSIMS profiles (negative secondary ions).11010010001041050 20 40 60 80 100 120 14012C16O18O35ClCountsScanFigure 2 – SIMS profile from centre of laser pitcreated with 5 pulses of 2 Jcm-2.Pristine implantLaser pitCountsScanFigure 3  18O SIMS profiles from pristine area (Fig1)and laser pit (Fig2) re-scaled to match deeper half ofprofile to reveal differences close to surface.Lunar and Planetary Science XXXV (2004) 1681.pdf

Depth sensitive sampling of implanted species in Genesis Collectors using UV laser ablation and SIMS

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Depth sensitive sampling of implanted species in Genesis Collectors using UV laser ablation and SIMS

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