58 SCANNIKG TECHSIQC-ES [Proc. SOC. Analyt. Clzem.

Scanning Techniques The following are summaries of two of the papers presented at an Ordinary Meeting of the

Society held on December 3rd, 1969, and reported in the December, 1969, issue of Proceedings

Electron Probe X-ray Microanalysis : A General Review

(p. 203).

BY MISS P. J. I<ILT.ISG\VORTH (Canzbi4dge Scieizti3c Instruments Limited, Electyoit-Probe Dicisioiz, Cawzbridge CB4 3A W )

THE use of electron-beam excited X-radiation as the basis of a quantitative analytical tech- nique was first proposed by Castaing,l who used a stationary electron beam of about 1pm diameter incident upon the specimen. The limitations of this instrumentatioii were quickly appreciated, in particular the time-consuming operations required to obtain concentration profiles, and led to the development a t the Cavendisli Laboratory, Cambridge, of an instrument incorporating beam-scanning facilities.

1 S FOR N AT I 0 I\; P R E SE STATION-

Present day microanalysers are generally able to present information in one of t h e e forms .

Qualitative distribution images are formed by deflecting the electron beam in a square raster over the surface of the specimen, with the X-ray spectrometer set to detect X-rays from the element under investigation. In this mode the intensity of X-ray signal varies with the position of the electron beam and can be used to modulate the brightness of a cathode-ray tube, scanned in synchronism with the beam, building up an element distribution display.

A comparison of element concentration in neighbouring areas (obtained by such a method) will be purely qualitative, and can usefully be augmented by scanning more slowly across a single line on the specimen, and recording the integrated intensity output as y-deflection curve, either on the cathode-ray tube superimposed upon the distribution image, or on a pen recorder, for more accurate evaluation.

Finally, fully quantitative analysis can be achieved by positioning a stationary electron beam upon the area of interest and recording the X-ray intensity from each of the constituent elements. The intensities are compared with those from standard materials, which are usually pure elements. To a first approximation, the concentration C,4 of element A can then be

T

expressed as CA = 2 speci*nen x 100, where I A specimt.n is the recorded intensity from the speci-

men and I , pure is the recorded intensity from a standard of pure A. IA pure

QUASTITATIVE CORRECTION PROCEDURES-

If a more accurate evaluation is required some consideration must be given to the mech- anism of X-ray production within the specimen and to the interaction of the emergent X-ray beam with the specimen material. This leads to the derivation of a number of correction factors.

The atomic number eflect-This is compounded of two opposing functions, dependent upon the back-scatter coefficient and the electron-stopping power of the specimen. Several methods for determining these effects have been proposed, much of the original work having been carried out by Thomas.2 For practical purposes, this effect can be considered to increase with increasing atomic number difference of the constituent elements.

A n absorption correcfion-This is generally required to compensate for the difference in absorption of the measured X-rays during passage through the specimen, compared with those emitted by the standard. Several correction procedures have been proposed, but i t is now generally accepted that the proposals of Philibert3 provide the best compromise between scientific accuracy and a workable formula for everyday use, when certain parameters are modified in accordance with Heinrich* or Duncumb, Shields-Mason and da C a ~ a . ~ As the mathematical process involved is iterative, it lends itself particularlv to computer evaluation.

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March, 19701 SCANNING TECHNIQUES 50

FLuorescence efects -These arise from excitation of secondary X-radiation and take two forms, the more significant of these being excitation by the characteristic primary X-rays, resulting in enhanced intensity of the detected low energy radiations at the expense of those of high energy. Correction for K and L series radiation has been described by Reed,6 but no satisfactory procedure is yet available for systems involving M series spectra; and for most purposes the minor component of radiation due to the continuum must be ignored.

Until now, it can reasonably be said that the accuracy of quantitative analysis has been limited by correction procedures, rather than instrumentation. Use of the best correction procedures now available, in conjunction with most recent values of the relevant coefficients, indicates that for some elemental systems at least this is no longer the case and further instrumental development will again lead to improved results.

XEW TECHNIQUES I S M1CROA4NALYSIS-

Development now hinges around improved facilities peripheral to the technique. The use of servo-operated controls has already reduced the number of precise setting operations required during a quantitative analysis, and is capable of further extension.

The combination of X-ray spectrometry with transmission electron microscopy allow the identification of particles well below the normal mini1nur-n size, although the analysis is no longer strictly quantitative.

Finally, with the development of detection systems for the wavelength range 1 to 10 nni it is now possible to determine, with a suitable spectrometer, the wavelength shifts and peak shape variations due to valency bonding for the lighter elements. Research on these semi- optical spectra opens up new forms of microanalysis.

REFEREXES 1. Castaing, R., Ph.D. Thesis, University of Paris, 1951. 2 . Thomas, I?. M., Brit J . AppZ. Phys., 1963, 14, 397. 3. Philibert, J . , “X-Ray Optics and X-Ray Rlicroanalysis,” \‘olumr 3, Llcade~nic Press, 1963, p. 379. 1. Heinrich, K. F. J . , “Abstracts of the 2nd National Conference on Electron Probe hIicroanalysis,

5. Duncumb, P., Shields-Mason, P. I<., and da Casa, C., “Proceedings of the Vth International

6.

Boston, Mass.,” Paper 7, 1967.

Congress on S-Ray Optics and Microanalysis,” Springer-I-erlag, Heidelberg, In the press. Reed, S. J . B., Brit. J . AFpl. Phys., 1965, 16, 013.

The Examination of Surfaces by Scanning with Charged-particle Beams

BY T. R. PIERCE (Analytical Sciences Division, Atomic Enevgy Heseavch Establishtneizt, Harwell,

Near Didcot, Bevkshive)

XUCLEAR interactions, induced by charged particles, which are used as the basis for activation- analysis procedures, can be broadly divided into two major categories : those resulting in the production of radioactive isotopes and those producing prompt events that are suitable for measurement. The former normally require particle energies of a t least a few million eV, the latter need lower particle energies, varying from a few hundred lteV to a few MeTr. ,4s charged particles lose energy relatively rapidly on passage through matter, interaction of low-energy charged particles will be restricted to the surface layers of a thick target, and only a limited volume of sample will be irradiated if the diameter of the primary incident particle beam is small. Thus analytical methods based on charged-particle irradiation can be readily adapted to scanning operation, provided that prompt radiation emitted during the irradiation can be measured.’ Compared with established methods of small-volume analysis, such as the electron microprobe, charged-particle microprobe techniques have , until recently, received little attention from the analyst and are therefore a t an early stage of development. Rut their ability to provide analytical methods for the determination of light elements, such as carbon and oxygen, has prompted recent investigation.

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60 S C A S S I S G T E C H N I Q U E S [Yroc. SOC. Analyt. Chem.

Analytical methods have been based on the measurement of three different types of radiation that are emitted as a result of nuclear interaction : these are prompt y-radiation,2 particle groups produced as the result of nuclear reaction3 and particles scattered elastically from the sample during irradiation.* In addition, X-rays are produced during charged- particle irradiation and can be used in positive-ion microprobe techniques,5 but X-ray methods are not considered here.

Prompt y-radiation is produced by the de-excitation of excited nuclear states produced as a result of nuclear reaction. As prompt y-radiation can be excited during stable to stable transitions, it is not necessary for a radioisotope to be produced for analysis to be possible. For example, irradiation of carbon with deuterons of about 1 MeV will result in the reaction 1% (d,p) 13C producing carbon-13 in its first excited state, which then decays promptly with the emission of a 3-09-MeV y-ray, even although both carbon-12 and carbon-13 are stable nuclei. As, by careful choice of both particle and energy, prompt y-lines can be excited from most of the light elements, this method is generally applicable to the determination of light elements in surfaces. When several elements contribute lines to the accumulated y-spectrum, conventional methods of y-ray spectroscopy can be used to distinguish con- tributions from the different elements. Neasurement of particle groups, for example protons emitted from the l2C (d,p) 13C reaction, instead of prompt y-radiation, is sometimes preferable to follow the progress of a nuclear reaction. As particle peak shapes are sensitive to the thickness of the layer from which the particles are produced, the method is well applied to the examina- tion of thin films of light elements present on surfaces. The energy of particles scattered elastically from target nuclei can also be used to provide information about surface com- position. Most sensitive results are obtained when a thin film of a heavy element present on a light element matrix is being examined, as under these conditions the contribution from the thin film is free from any background caused by particles scattered from the substrate.

Any of the three analytical methods summarised above can provide information about a small volume of sample in suitable cases and can be used in scanning techniques, provided that the diameter of the incident particle beam is restricted and that the region of the sample from which the radiation yield is emitted is known.

Particle accelerators, capable of producing particles and energies suitable for analysis, are usually designed for general-purpose operation and little attention has so far been paid to thc possibility of providing small-diameter ion beams suitable for microprobe work. How- ever, it has been shown that by combination of four specially designed quadrupole lenses in a Russian quadruplet arrangement, beam diameters of less than 4 pm can be achieved (J. A. Cookson and I;. D. Pilling, unpublished work). Restriction of beam diameter limits the total beam current available for use, but 6 nA of 3-MeV protons have been obtained with a 3-MeV electrostatic generator. Scanning of the particle beam across the sample can be carried out by moving the position of the sample with respect to the beam either mechanically or by electrostatic beam deflection. In the work carried out within the Analytical Sciences Division, A.E.R.E., scanning is usually discontinuous, the position of the beam on the sample being changed between irradiations. So far, positive-ion microprobe techniques have found their main application in the determination of light elements, such as boron, carbon, nitrogen, oxygen, fluorine, magnesium and aluminium, distributed in surfaces, or for examination of thin films of light elements present on sample surfaces, for example, films containing carbon and oxygen present on metal substrates. However, elastic scattering has been used to investigate thin films of some heavy elements, for example, contaminant films of heavy elements present on the surfaces of alumina insulators and gold films on silicon slices.

REFERENCES 1. 2.

3. 4.

5.

Pierce, T. B., Peck, P. F., and Cuff, D. R. A., Nucl. Inst . Meth., 1969, I , 67. Pierce, T. B., and P?Fk, P. F., i p z Shallis, P. W., Editor, “Proceedings of the S.lC Confcrcncc,

W. Heffer & Sons Ltd., Cambridge, 1965, p. 159. Amsel, G., and Samuel, D., Phys. Chem. Soc., 1967, 23, 1707. Rubin, S., in Kolthoff, I. M., and Elving, P. J., Editors, “Treatise on Alnalytical Chemistry,”

Poole, D. M., and Shaw, J. I,., U.K. _4tomic Energy Authority Repovt, A.E.R.E. R5918, 1968.

Nottingham, 1965,

Part I, Volume 4, Interscience Publishers Inc., New York, p. 2075.

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