Volume 93, Number 3, May-June 1988

Journal of Research of the National Bureau of Standards

Accuracy in Trace Analysis

Imaging Microanalysis of Materialswith a Finely Focused Heavy Ion Probe

R. Levi-Setti, J. Chahala, and Y. L. Wang

The Enrico Fermi Institute andDepartment of Physics

The University of ChicagoChicago, IL 60637

Scanning Analytical Heavy Ion Microscopyat High Lateral Resolution

Liquid metal ion sources (LMIS) in view of thequasi-point-like geometry of their emitting regionand confined emission cone possess brightness(- 0I6 A cm 'sr-') which is adequate for the real-ization of high current density (- I A cm-'), finelyfocused (>20 rm) probes. A 40 keV scanning ionmicroprobe, which makes use of a Ga LMIS (UC-HRL SIM) is currently employed in our laboratoryto obtain chemical maps of materials in a variety ofinterdisciplinary applications [1]. The instrument iscomposed of a two-lens focusing column, a high-transmission secondary ion energy analyzer andtransport system, and an RF quadrupole mass filterfor secondary ion mass spectrometry (SIMS). inaddition, two-channel electron multiplier detec-tors, overlooking the target region, collect sec-ondary electrons or ions for imaging of the surfacetopography and material contrast of a sample.

Although a lateral resolution of 20 nm has beenattained, sensitivity considerations favor operationat somewhat larger (50-70 nmt) probe size. The an-alytical image resolution is in fact critically depen-dent on the statistics of the mass analyzed signal[2], which in turn is determined by the rate of mate-rial removal by sputtering from the sample surface.Such a rate is proportional to the probe current,which decreases with the square of the probe di-ameter for chromatic-aberration-limited probessuch as those extracted from LMIS. Thus at, e.g.,20 nm probe diameter, only 1-2 pA of probe cur-rent are available, the erosion rate is of the order- 10-' rnonolayers/s and probe-size resolution canonly be attained for elements of high ionizationprobability which will provide the highest signalstatistics over an acceptable recording time (>Icount/pixel in a 1024 x 1024 pixel scan for a 512 sacquisition time for the UC-HRL SIM).

In view of the well known range of ionizationprobabilities (ion fractions) among sputtered atomicspecies, as well as of sputtering yields, the attain-able analytical lateral resolution of finely focusedprobes becomes target-species dependent due tothe above considerations. It also follows that forspecies difficult to ionize, high resolution SIMSimaging microanalysis is altogether precluded, un-less by recourse to postionization techniques [3].

Two examples of applications of the UC-HRLSIM to the study of materials will be illustrated inthe present context.

Imaging Micro-SIMS ofAluminum-Lithium Alloys

SIMS techniques are uniquely suited to the studyof Li because of the intense 'Li' signal emergingfrom fast ion bombardment of Li-containing mate-rials. The high resolution imaging capability of theUC-HRL SIM can be fully exploited in this case, asdemonstrated in preliminary studies of Al-Li alloyscontaining up to 12.7 at. % Li [4,5]. In these impor-tant alloys, the 'Li' signal is detected with a signal-to-noise ratio z 10', and it is feasible to image andidentify grain boundary phases and precipitates inthe <100 nm range of dimensions,

Samples of binary Al-Li alloys were solutiontreated for -10 mins at 570 'C and waterquenched. Some of the samples were then aged togive coarse distributions of either the equilibrium 8(AlLi) phase or a combination of 8 and metastable8'(AlLi). The samples were first mechanically pol-ished and then electropolished to produce a flatsurface for SIMS imaging.

A number of SIMS artifacts originating in thesample preparation process could be identified andsubsequently avoided. These involve Li surface en-richment, surface oxidation and chlorine and car-bon contamination. Sputter-cleaning of the samplesin situ with a 2 kV Ar ion gun was generally neces-sary to obtain clean and artifact-free surfaces. Anexample of the detection of 8 plates in Al-12.7 at. % Li aged 100 hours at 250 'C is shown inthe 'LiP map of figure 1(a). The plates form in reg-ular crystallographic orientations. A 3 4AlLi+ imageis shown in figure l(b) where B' particles (dark in-clusions) are detected, showing distinctive mor-phologies that include dendritic growth andnucleation on dislocations. In the latter case, theAl-10.1 at. % Li sample was aged 4 hours at 290'C.

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Volume 93, Number 3, May-June 1988

Journal of Research of the National Bureau of Standards

Accuracy in Trace Analysis

Toward quantification, a calibration curve hasbeen constructed from 'Li'/ 2 7 Al' measurementsperformed on binary Al-Li alloys of known Li con-centration. A linear relation between the 'Li'/"Al + ratio and the composition of the standard wasobserved, as shown in figure 2.

Imaging Micro-SIMS of SiliconNitride Ceramics

Sintered silicon nitride is an advanced ceramicoften used for high-temperature applications. Thesintering process requires the addition of smallamounts of oxides in a nitrogen overpressure toprevent dissociation above 1700 C. Sinteringagents such as MgO, Y203 and others, present in1-10% weight concentrations, react with the sili-con nitride phases and the secondary phases, whichform along grain boundaries determine the me-chanical properties of the ceramic.

Using the UC-HRL SIM, it has been feasible toobtain detailed mapping of the components of theinterboundary phases in both yttria- and magnesia-doped silicon nitride [6]. Both fractured and pol-ished surfaces of these ceramics can be readilyanalyzed in our microprobe, after coating with athin (- 5 nm) Au layer, which is rapidly sputteredaway from the field of view under investigation.

In the case of Y203-sintered silicon nitride, thedominant interboundary phase is YSiO2N, knownto oxidize to Y2Si2O 7. SIMS mapping of thesephases can be obtained for several break-up frag-ments such as Y+, 0-, SiO2 and SiN-. In additiondifferential resputtering of the implanted Ga+probe ions provides detailed descriptions of thestructure of the ceramic comparable to that whichcan be obtained with backscattered electrons. Anexample of the level of detail and resolution thatcan be attained in the imaging microanalysis of thisceramic is shown in figure 3. Figure 3(a) is a 'Ga+map, figure 3(b) a Y9Y+ map of the same fracturesurface. The silicon nitride crystallites representingthe ceramic matrix are clearly outlined in the Ga'map, while the complementary Y+ map describesthe distribution of the interboundary phase.

Quantification of the composition of the inter-boundary phase by SIMS is complicated by thepresence of matrix effects and local ion yield en-hancements in the presence of bound oxygen.Methods are being developed, based on image pro-cessing techniques, to perform vector scan micro-analysis of either the matrix or the interboundaryphases separately. Aided by suitable standards, it is

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expected that quantification on a microscale maybecome feasible by this approach.

Acknowledgments

This research was supported by the NSF Materi-als Research Laboratory at the University ofChicago, the Allied Signal Engineered MaterialsResearch Center, and the National Science Foun-dation under Grant No. DMR-861 2254.

Figure 1. (a) 7 Li+ SIMS image of 8 plates in Al-12.7 at. % Lialloy. 80 Zm full scale. (b) 34AILi+ SIMS image of dendritic 8particles in Al-10.1 at. % Li. 10 Am full scale.

,l 'i 1i

Figure 2. 7Li+/2 7 AI+ calibration obtained for the UC-HRL SIMfrom homogeneous as-quenched samples of binary Al-Li alloys.

Figure 3. (a) Resputtered 6 9Ga+ SIMS image of fracture sur-face of Y20 3-sintered silicon nitride ceramic, 20 ,um full scale.(b) 8 9Y+ image showing distribution of interboundary phase insame area as (a).

Volume 93, Number 3, May-June 1988

Journal of Research of the National Bureau of Standards

Accuracy in Trace Analysis

References

[I] Levi-Setti, R., Wang, Y. L., and Crow, G., Appl. Surf. Sci.26, 249 (1986).

[2] Levi-Setti, R., Chabala, J., and Wang, Y. L., Scanning Mi-cros. Suppl. 1, 13 (1987).

[3] Wang, Y. L., Levi-Setti, R., and Chabala, J., Scanning Mi-crosc. 1 (1987).

[41 Williams, D. B., Levi-Setti, R., Chabala, J. M., andNewbury, D. E., J. Microsc. (1987) in press.

[51 Williams, D. B., Levi-Setti, R., Chabala, J. M., andNewbury, D. E., to be submitted to J. Microsc.

[6] Chabala, J. M., Levi-Setti, R., Bradley, S. A., and Karasek,K. R., Imaging Microanalysis of Silicon Nitride Ceramicswith a High Resolution Scanning Ion Microprobe. Appl.Surf. Sci. (1987) in press.

Synchrotron Radiation ExcitedFluorescence Micro-Analysis

Using a New Imaging Technique

A. Kndchel, M. Bavdaz1, N. Gurker1 ,P. Ketelsen, W. Petersen, M. H. Salehi,

and T. Dietrich

Universitift HamburgInstitut fur Anorganische und Angewandte Chemie

Martin-Luther-King-Platz 6D-2000 Hamburg, FRG

Introduction

The great importance and large field of applica-tions the x-ray fluorescence analysis (XFA) foundin the last decade are due to its extraordinary qual-ities. It is a very flexible method, it is nondestruc-tive, fast and requires only a very simple samplepreparation. It is not necessary to perform the anal-ysis in vacuum, so samples containing volatile com-ponents pose no problem (e.g., biological andmedical samples).

The penetration depth of x-rays ranges from afew Sgm up to several 100 ,tm and is thus muchgreater than that of electrons (used to excite thesample with scanning electron microscopy). Theanalyzed volume of the sample thus increases and

' Technische Universitat Wien, Institut fur Angewandte undTechnische Physik, Karlsplatz 13, A-1040 Vienna, Austria.

results in a high sensitivity of the method. Addi-tionally, x-rays cause much less radiation damagethan corpuscular beams [1].

Using synchrotron radiation as a very powerfulprimary x-ray source, the sensitivity of the methodis very high and is comparably good for all ele-ments having Z>18 [2,3]. Figure I shows the de-tection limits for synchrotron radiation excitedx-ray fluorescence analysis (SYXFA). A I mg/cm 2

multielement sample was measured for 300 s usinga beam diameter of 0.5 mm. The solid line indicatesthe detection limits measured using a white beam,the dotted line those using a graphite monochro-mator adjusted to eliminate the As-Ka/Pb-Lainterference.

The main advantages of applying synchrotronlight as the primary radiation source in XFA couldbe summarized as follows:

* The high photon flux makes filtering,monochromatizing and masking of the pri-mary beam feasible.

* A very broad continuous white spectrum allows simultaneous multi-elemental analysisfeaturing very good limits of detection (abso-lute > 10-" g, relative >0.1 g/g).

* Synchrotron radiation is linearly polarized inthe storage-ring plane, thus improving thesignal-to-noise ratio significantly.

* The source size is very small and the synchro-tron beam is very well collimated, so a highspatial resolution is achievable (see later).

* Since the properties of synchrotron radiationare calculable, the evaluation of the measureddata becomes easier and more reliable.

We are applying SYXFA to help solving differ-ent analytic problems that hardly could be ap-proached with other methods. For example, weanalyzed cosmic dust particles smaller than 10 pumin diameter to find out their composition. A secondexample is the analysis of the printing ink on veryprecious documents, including the 42-line bible byGutenberg.

X-Ray Imaging

SYXFA becomes even more interesting if onesucceeds in combining spatial resolving analysiswith the high sensitivity of the method.

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