DETERMINATION OF CLIFF-LORIMER k FACTORS FOR A HITACHI H700H 200 kv SCANNING TRANSMISSION ELECTRON MICROSCOPE

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1 DETERMINATION OF CLIFF-LORIMER k FACTORS FOR A HITACHI H700H 200 kv SCANNING TRANSMISSION ELECTRON MICROSCOPE E. Metcalfe, J. Broomfield To cite this version: E. Metcalfe, J. Broomfield. DETERMINATION OF CLIFF-LORIMER k FACTORS FOR A HITACHI H700H 200 kv SCANNING TRANSMISSION ELECTRON MICROSCOPE. Journal de Physique Colloques, 1984, 45 (C2), pp.c2-407-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C2, supplgment au n02, Tome 45, fgvrier 1984 page C2-407 DETERMINATION OF CLIFF-LORIMER k FACTORS FOR A HITACHI H700H 200 kv SCANNING TRANSMISSION ELECTRON MICROSCOPE E. Metcalfe and J.P. Broomfield Technology Planning and Research Division, Central Electricity Generating Board, Central Electrfcity Research Laboratorz^es, Kelvin Avenue, Leatherhead, U. K. RQsums - La technique propos6e par Cliff-Lorimer pour la microanalyse X des lames minces demande la connaissance des coefficients k qui relient le rapport des intensitss mesurses au rapport des concentrations. Dans cette contribution on d6termine ces facteurs pour un microscope Slectronique en transmission, en employant des tsmoins min6ralogiques et des alliages homogsnes. On compare les valeurs expsrimentales et les valeurs thsoriques obtenues par application des facteurs k. Abstract - The Cliff-Lorimer ratio technique for thin film X-ray microanalysis requires knowledge of the k factors which relate the measured X-ray intensities to the composition of the specimen. This paper reports the determination of k factors at 200 kv for an analytical transmission electron microscope (Hitachi H700H with a LINK 860 system) using mineral standards and homogeneous alloys. The experimental data are compared with calcuhations of theoretical k factors. 1 - INTRODUCTION A simple ratio technique has been proposed by Cliff and Lorimer /l/ for quantitative X-ray microanalysis of thin films in STEM and is now widely used. For a binary alloy A-B the X-ray peak intensities IA and IB are related to the concentrations CA and CB in weight % by the relationship and where k and kbsi are the ratio factors for chemical systems of A and B respectively W'%& silicon. These factors are independent of specimen thickness if the 'thin film criterion', which assumes no absorption or fluorescence of X-rays in the specimen, is satisfied. To date comprehensive ranges of k factors have been determined experimentally only at 100 and 120 kv, and the spread in the values indicates that it is necessary to measure accurate k factors for specific microscope and detector systems. In, this work we have determined k factors both theoretically (using the method of Goldstein et a1 /2/) and exper?%ntally for our 200 kv microscope. 2 - EXPERImNTAL PROCEDURE Independently analysed silicate minerals were used as standards for determining k values, along with single phase homogenized alloys and metal silicides. The mineral standards were kindly loaned by Cliff and Lorimer, and McGill and Hubbard /3/. Article published online by EDP Sciences and available at

3 C2-408 JOURNAL DE PHYSIQUE SPECIMEN Muscovite Biotite Antigorite Cummingtonite Tremolite Actinolite Adularia Soda feldspar Amelia albite Titanium silicide Chromium silicide Vanadium silicide Pyrite FeNiCr alloys TABLE 1. Specimens used in k factor measurement. FORMULA SOURCE KA12Si3A10 (OH)2 l K(Fe,Mg) A@~~O~~(OH)~ M~~s~~o~?oH) Cliff and Lorimer, Dept.?Si80 (OH) Metallurgy, UMIST, Manchester ~ a ~ ~ g ~ ~ l ~ 0 ~ ~? 6 ~ ) ca (Mg,Fe) ig022 (OH) ~Zsi o ~aals?~8~ McGill and Hubbard, NaA1Si308 Dundee University TiSi2 CrSi2 Goodfellow Metals Cambridge VSi, ) CERL, Leatherhead Table 1 summarizes the samples. Mineral and silicide samples prepared in our laboratory were ground and ultrasonically dispersed in methanol. After settling overnight single drops of dilute suspensate were deposited on carbon films on copper grids. Measurements of k factors were carried out using a Hitachi H700H analytical electron microscope fitted with a Kevex energy dispersive detector with a LINK Systems 860 MCA and computer. The X-ray take-off angle was 68' and the specimens were held in a graphite holder to minimise background X-ray emission. All analyses were carried out at 200 kv at an emission current of 30pA. The counting times were varied to give approximately 10,000 count? for each major peak in the spectrum. Data were collected by raster scanning (600nm) over a thin region or by spot counting with the specimen held in a cold stage at just above liquid nitrogen temperature. When using either of these methods no contamination could be seen on the specimen, and rastering the beam over the support carbon film did not reveal any stray characteristic radiation except,cu arising from the grid. Thickness measurements were made on selected specimens by the contamination spot method, which conservatively overestimates the thickness /4/, in order to ensure that the thin film criterion was satisfied. The data were analysed using the LINK RTS2 program which determines the K +Kg intensity. 3 - THEORETICAL CALCULATION kxsi factors were calculated using the method of Goldstein et a1/2/ from the formula where for each element Q is the ionisation cross section for a given shell, w is the probability of X-ray emission after ionisation, a is the ratio of Ka and K intensit B ies expressed as K /(K +K ), A is the atomic weight andeis the attenuation of the X-ray beam before 8eteztign. E is found by calculating the beam intensity reduction in passing through the Be window, the gold layer and the silicon dead layer in the X-ray detector: where p/p, p and Y are the mass absorption coefficients, density and thickness. The detector parameters used for this calculation were YBe=7.5 pm, Y =0.015 um and Y =0.1 I.im. The values of the ionisation cross section used wereahose of Green and ~sislett /5/. It should be noted that 'a' was set equal to 1 throughout so that measured k factors can be compared with those calculated and used by the LINK RTS2 program which sums Ka and K to give a total K intensity RESULTS AND DISCUSSION Figure 1 shows the k factor measurements plotted as a function of atomic number and they are compared with the theoretical calculations. Also shown are the results at

4 TABLE 2 Comparison of experimental and calculated k values at 200 kv ELEMENT CALCULATED MEASURED 95% CONFIDENCE LIMITS Na Mg A S i S K Ca Ti V Cr Mn Fe N i kv of Wirmark et a Table 2 gives a summary of all the results obtained in this study. Figure 1 shows that, in common with the findings at 100 or 120 kv (e.g. /7/), there is good agreement between experimental and calculated values at high X-ray energies (above 2=14). However, increasing divergence is seen at the low energy end of the spectrum. Such divergence has been discussed in the past in terms of a) contamination on the specimen or on the window of the detector b) thicker Be windows than quoted c) mass volatilization of light elements e.g. Na in albite d) poor profile fitting at the low energy end of the spectrum. In this work the window of the detector was changed immediately prior to measuring kxsi factors. A window of a nominal thickness of 7.5 pm was used and it was checked for cleanliness. In order to minimise the effects of specimen contamination, measurements were made by raster scanning over small areas and selected specimens were re-examined in a liquid nitrogen cooled stage in which no contamination could be observed. No difference in kxs. value, within experimental error, was seen. During raster scanning or using the cold stage no volatilization of Na in albite was observed. The RTS2 program which was used to obtain peak intensities uses stored standard profiles for each element. It is important, particularly at the low energy end of the spectrum, that these profiles are obtained from standards with no peak overlaps. For example the Na profile was obtained from NaCl and the intensities determined using RTS2 agreed well with those obtained using other spectrum processing routines. Large differences are seen by all workers between experimental and calculated k NaSi' k and. values. Invoking an extra absorption of low energy X-rays by a tr!per Bekdaow than normally assumed (7.5 pm), or by a contamination layer, will allow the calculated values for 2<14 to approach the measured values. However, increasing divergence will be seen for the k values for the higher atomic numbers. xsi This is demonstrated in Figure 2 where values of Y of 7.5, 12 and 20 pm are used in the equation for the detector efficiency. For ~$74 the window would have to be 20 pm thick to give good agreement between calculated and measured k values. However the values of k for 2>14 would be considerably reduced (e.g. kfesi=0.878) and well below the ~~~erimentall~ measured values. As we have good agreement for 2>20 this cannot be the reason for the divergence at the low energy end of the spectrum. This indicates that 'a systematic error in the theory at the low energy end of the spectrum is likely to be responsible for the lack of agreement and this reinforces the necessity of measuring k factors for specific analytical systems. 5 - CONCLUSIONS Theoretical calculations and experimental determinations of k factors for thin xsi film analysis in a Hitachi H700H STEM with a LINK energy dispersive system show good agreement for 2>16. Only for lighter elements is it necessary to use experimentally determined factors.

5 JOURNAL DE PHYSIQUE Be W I NOOW=7. 5pm , Be WINDOW=l2 pm Be WINDOW=20 pm- - ATOMIC NUMBER Z ATOMIC NUMBER Z Figure 1 - Comparison of experimental results with calculated k factors at 200 kv. The error bars are 95% confidence limits. A shows the data of Wirmark et a Figure 2 - Calculated k factors allowing for different detector window thicknesses. Experimental results are also shown. 6 - REFERENCES 1. CLIFF G. and LORIMER G. W., J. Microsc. 103 (1975) GOLDSTEIN J. I., COSTLEY J. L., LORIMER G. W. and REED S. J. B., SEMI1977 ed. 0. JOHARI, Chicago, IITRI (1977) McGILL R. J. and HUBBARD F.H., Quantitative Microanalysis with High Spatial Resolution, ed. LORIMER, JACOBS and DOIG (London: Met. Soc.) (1981) PAE D. A., SCOTT V. D. and LOVE G., ibid GREEN M. and COSSLETT V. E., Proc. R?y. Soc. 78 (1961) WIRMARK G., THORVALDSSON T. and NORDEN H., EMAG 83, Inst, Phys.(London) (1983). 7. WOOD J., WILLIAMS D. B. and GOLDSTEIN J. I., Quantitative Microanalysis with High Spatial Resolution, ed. LORIMER, JACOBS and DOIG o on don: Met. Soc.) (1981) 24. AGKNOWLEDGEMENTS The authors wish to thank Dr G.W. Lorimer and Mr G. Cliff of UMIST, Manchester University and Mr R.J. McGill of Dundee University for the provision of mineral samples used in this study. Specimens were also supplied by Dr B. Nath of CERL. This work was carried out at the Central Electricity Research Laboratories and the paper is published by permission of the Central Electricity Generating Board.