ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY WITH A THERMALLY STABILIZED FIELD EMISSION GUN

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ANALYTICAL TRANSMISSION ELECTRON

MICROSCOPY WITH A THERMALLY STABILIZED FIELD EMISSION GUN

B. Thomas, J. Bourgeot, P. Chemelle, A. Rîbes

To cite this version:

B. Thomas, J. Bourgeot, P. Chemelle, A. Rîbes. ANALYTICAL TRANSMISSION ELECTRON

MICROSCOPY WITH A THERMALLY STABILIZED FIELD EMISSION GUN. Journal de Physique

Colloques, 1984, 45 (C2), pp.C2-387-C2-388. �10.1051/jphyscol:1984287�. �jpa-00224002�

JOURNAL DE PHYSIQUE

CoIIoque C2, suppl6ment au n02, Tome 45, f6vrier 1984 page C2-387

ANALYTICAL TRANSMISSION ELECTRON MICROSCOPY WITH A THERMALLY STABILIZED F I E L D EMISSION GUN

B. Thomas, J. Bourgeot, P. Chemelle and A. Ribes IRSID, 78105 Saint-Gerrnain-en-kye Cedex, France

R&sumb - On examine les ameliorations qu'apporte i'empioi d'un canon 1 Bmission de champ 1 la microanaiyse en microscople Biectronique en transmission.

Abstract - The improvements in the microanalytic capabilities in transmission electron microscopy are described and discussed.

This paper presents the results of several years experlence of thin specimen microanaiysis using a Philips EM 400T TEM/STEM transmission electron microscope. Over the past two years the instrument, which was originally equipped with a standard tungsten hairpin gun, has been operating with a thermal- ly stabilized field emission gun. The X ray spectrometer is an EDAX 9100/60 system fitted with

30 mmL detector which has an effective in situ resolution of 157 eV and a collection angie of 0.13 Sr.

1 - GUN CHARACTERISTICS

The values of the brightness and the beam current stability estimated from measurements made on our instrument are compared in Table I.

TABLE 1 - Electron beam characteristics for the W-hairpin and thermally stabilized FEG.

2 - OPERATING PERFORMANCE a) Briqhtness

Emitter

W-hairpin FEG

In practice because of the limitations to count ratio imposed b y the pulse rejection electronics of the spectrometer and because of beam broadening in thin foils, the higher brightness of the FEG can be fully exploited only under certain experimental conditions.

Brightness

A.m .sr .V

1 - 2 x 10 4 3-5

10

Current stability

In the case of our spectrometer, iine shift, iine broadening and counting dead time restrict the maximum usable input count rate to about 3000 cps. For a thin foil of stainless steel, the beam currents at 120 kV required to attain this figure in three different foil thickness are given in Table II together with appro- ximate values of the corresponding beam diameter for the FEG and the thermionic gun, and estimates of beam broadening in the foils. It can be seen that in those practical applications of microanaiysis where resolution is of no great concern, the W-hairpin gun can be competitive with the field emitter.

Peak to peak

% relative

<+0,l % t 7 t o + _ 1 5 %

Drift

% reiative/hour

< ^{0.5 %}

<0,5 %

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984287

C2-388 JOURNAL DE PHYSIQUE

TABLE I1 - Estimates of spot sizes and spatial resolution for dlfferent foil thickness and maximum Input count rates.

When the foil thickness is less than about 200 nm however, the FEG enables the analytical spatial reso- lutlon to be Improved but only by a factor of about 2-4 if maximum count rates are maintained. When the latter restriction is removed, further improvement In spatial resolution can be attained b y reducing the beam spot size and for equal values of the spatial resolution the counting rate advantage of the FEG Increases as the foil thickness decreases.

These improvements can be put to practical aevantage in studies of segregation and concentration gra- dients, in the Identification of small particles embedded in foils and for the quantitative analysis of small particles extracted on replicas. As an example of the latter, it has been possible to obtain peak count rates (after background subtraction) of 20-40 cps for the sum of metallic elements present in

nm diameter fcc carbides of the type (MO, V, Ti)C with peak to background ratios between 5:l and 2:l.

These figures confirm that the practical particle size limit is somewhere between 1 at 2 nm.

Foll thickness (nm)

50 100 200

b) Beam current stability.

The magnitude of the beam current fluctuations observed with the field emitter depends on the tip con- ditioning but is typically 3 10 % of the mean current; the frequency range extends from several kHz to a few cHz. Provided that count rates are kept within the usual limits, current fluctuations of this order are not expected to put any undue strain on the pulse processing electronics and this appears to be born out In our experience as no changes in peak resolution or peak shape were observed with a change of the electron source,

Current for 3000 cps. (nA)

1.0 0.5 0.25

b

In the case of composite specimens, since the X ray emissions from dlfferent elements are measured si- multaneously, the usual Polssonian counting statistics are respected between elements. in the case of pure element standards (bulk or thin) It is necessary to extend the counting times over a period suffl- ciently long to even out the very low frequency fluctuations in beam current (several minutes) to reduce the confidence limits of the measurements to a satisfactory level.

Spot size (nm)

C) Specfrometer operatlnq characteristics.

In order to control the operating characteristics of the spectrometer and to provide data for the calcula- tion of kAB values for quantitative analysis, measurements of the X ray emission intensities at 20 kV from a range of pure bulk standards have been carried out at 6 month intervals over the past three years. Examples of the results are given in Table Ill. No long term trend in the relative intensities from the standards has so far been detected.

FEG 4 . 5- 3.5 2.7

Diameter of X ray production cylinder (nm)

FEG W

TABLE 111 - Mean values and dispersion of emlssion intensities from bulk standards referenced on the Tl , K line.

W 33 25 20

10 16 42

43 34 4 6

3 - CONCLUSIONS Line

Mean ratio 9 5 % C L

The use of a field emlssion source improves substantially the microanalytical capabilities of a transmission electron microscope particularly in the thinner regions of thln folls and for small particles extracted on replicas. No adverse effects with regard to spectrometer operating characterlstlcs have been observed.

-- ^*

AIK 1,42

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A@- 0,532

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FeK 0,636 23.0%

SiK 1,71 +1,4%

ZnK 0,293 22.7%

VK 0,913 _+2,0%

NiK 0,460 20.2%

AuL

0,100

t1,0%