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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 HI- TACHI H700H 200 kV SCANNING TRANSMISSION ELECTRON MICROSCOPE. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-407-C2-410. �10.1051/jphyscol:1984292�. �jpa-00224007�
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 k V
SCANNING TRANSMISSION ELECTRON MICROSCOPE
E. Metcalfe and J.P. Broomfield
Technology Planning and Research Division, Central E l e c t r i c i t y Generating Board, Central E l e c t r f c i t y 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 rap- port 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 micro- analysis 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 respect- ively 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 determ- ined 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 http://dx.doi.org/10.1051/jphyscol:1984292
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
K(Fe,Mg) A @ ~ ~ O ~ ~ ( O H ) ~ M ~ ~ s ~ ~ o ~ ? o H )
l
Cliff and Lorimer, Dept.?Si80 (OH) Metallurgy, UMIST, Manchester
~ a ~ ~ g ~ ~ l ~ 0 ~ ~ ? 6 ~ ) ca (Mg,Fe) ig022 (OH)
~ Z s i o
~ a A l S ? ~ 8 ~ McGill and Hubbard,
NaA1Si308 Dundee University
TiSi2
Goodfellow Metals
CrSi2 Cambridge
VSi,
) CERL, Leatherhead
Table 1 summarizes the samples. Mineral and silicide samples prepared in our labor- atory 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 spec- imens 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 ies expressed as K /(K +K ) , A is the atomic weight andeis the attenuation of the B 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.
6 4 - 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
TABLE 2 Comparison of experimental and calculated k values at 200 kV ELEMENT CALCULATED MEASURED 95% CONFIDENCE LIMITS
Na 1.95 3.98 0.30
Mg 1.37 1.92 0.08
A1 1.16 1.29 0.06
S i 1.00 ---- ----
S 1.02 0.93 0.10
K 0.92 0.977 0.08
Ca 0.90 0.915 0.06
Ti 0.98 0.965 0.030
V 1.01 0.965 0.028
Cr 1.01 0.976 0.025
Mn 1.06 1.030 0.06
Fe 1.07 1.057 0.05
N i 1.13 1.102 0.07
200 kV of Wirmark et a1 161. 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 B e k d a o w 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.
This is demonstrated in Figure 2 where values of Y xSi of 7.5, 12 and 20 pm are used in the equation for the detector efficiency. For ~ $ 7 4 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 ~ ~ ~ e r i m e n t a l l ~ 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 film analysis in a Hitachi H700H STEM with a LINK energy dispersive system show good xSi agreement for 2>16. Only for lighter elements is it necessary to use experimentally determined factors.
JOURNAL DE PHYSIQUE
Be W I NOOW=7. 5pm --- 4.31, B e W I N D O W = l 2 pm--- 4. 1 Be WINDOW=20 pm- -
ATOMIC NUMBER Z ATOMIC NUMBER Z
F i g u r e 1 - Comparison o f e x p e r i m e n t a l F i g u r e 2 - C a l c u l a t e d k f a c t o r s a l l o w - r e s u l t s w i t h c a l c u l a t e d k f a c t o r s a t 200 kV. i n g f o r d i f f e r e n t d e t e c t o r window The e r r o r b a r s a r e 95% c o n f i d e n c e l i m i t s . t h i c k n e s s e s . E x p e r i m e n t a l r e s u l t s a r e
A shows t h e d a t a o f Wirmark e t a 1 161. a l s o shown.
6 - REFERENCES
1 . CLIFF G. and LORIMER G. W . , J. M i c r o s c . 103 (1975) 203.
2 . GOLDSTEIN J . I . , COSTLEY J. L . , LORIMER G. W. and REED S. J. B . , SEMI1977 e d . 0. JOHARI, Chicago, IITRI (1977) 315.
3 . McGILL R. J . and HUBBARD F.H., Q u a n t i t a t i v e M i c r o a n a l y s i s w i t h High S p a t i a l R e s o l u t i o n , e d . LORIMER, JACOBS and D O I G (London: Met. S o c . ) (1981) 3 0 . 4 . PAE D. A., SCOTT V. D . and LOVE G . , i b i d 5 7 .
5. GREEN M. and COSSLETT V . E., P r o c . R?y. Soc. 78 (1961) 1206
6. WIRMARK G . , THORVALDSSON T. and NORDEN H., EMAG 8 3 , I n s t , Phys.(London) ( 1 9 8 3 ) . 7. WOOD J . , WILLIAMS D . B. and GOLDSTEIN J . I . , Q u a n t i t a t i v e M i c r o a n a l y s i s w i t h High
S p a t i a l R e s o l u t i o n , e d . LORIMER, JACOBS and DOIG o on don: Met. S o c . ) (1981) 24.
AGKNOWLEDGEMENTS
The a u t h o r s wish t o t h a n k D r G.W. Lorimer and M r G. C l i f f of UMIST, Manchester U n i v e r s i t y and M r R.J. McGill of Dundee U n i v e r s i t y f o r t h e p r o v i s i o n of m i n e r a l samples u s e d i n t h i s s t u d y . Specimens were a l s o s u p p l i e d by D r B. Nath of CERL.
T h i s work was c a r r i e d o u t a t t h e C e n t r a l E l e c t r i c i t y R e s e a r c h L a b o r a t o r i e s and t h e p a p e r i s p u b l i s h e d by p e r m i s s i o n of t h e C e n t r a l E l e c t r i c i t y G e n e r a t i n g Board.
