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Submitted on 1 Jan 1984
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ENERGY SPREAD MEASUREMENT IN FIELD ELECTRON MICROSCOPY
M. Troyon, Jiang Jia-Ying
To cite this version:
M. Troyon, Jiang Jia-Ying. ENERGY SPREAD MEASUREMENT IN FIELD ELEC- TRON MICROSCOPY. Journal de Physique Colloques, 1984, 45 (C2), pp.C2-155-C2-156.
�10.1051/jphyscol:1984234�. �jpa-00223947�
JOURNAL DE PHYSIQUE
Colloque C2, supplément au n°2, Tome 45, février 1984 page C2-155
E N E R G Y SPREAD M E A S U R E M E N T IN FIELD ELECTRON M I C R O S C O P Y
M. Troyon and Jiang Jia-Ying
Laboratoive de Microscopi.e Electronique U.E.R. Sciences, B.P. 347, 51062 Reims Cedex, France
Rés inné - La dispersion énergétique des cathodes à émission de champ d'orien- tation <100> et <111> a été mesurée dans les conditions de fonctionnement de la microscopie conventionnelle. Les différents paramètres qui l'influencent sont mis en évidence.
Abstract - The energy spread of field emission oriented <100> and <111> tips has been measured in the working conditions of CTEM. The different parameters that influence it are pointed out.
One of the interests of the field emission cathodes is their low energy spread at the emission. We have already shown by an indirect method {1} that in the microscope the beam emitted by a <100> field emission cathode can suffer of a very large energy broadening. We intended to verify very precisely with an energy filter what is, in the microscope, the energy broadening of a field emission illuminating system for specimen illuminating conditions of conventionnal transmission microscopy. For this purpose a very simple retarding energy filter {2} was set in the microscope camera chamber (Figure 1 ) . This way, all the broadening effects occuring from the catho- de to the specimen are taken into account and so our measurements give energy spread value at specimen level. We have verified on a resistor net and by computing trajec- tories inside the filter that its resolution is about 0.05 eV at 15 kV with an input aperture of 200 urn. The curve I = f(E) is obtained with a microcomputer that commands the retarding electrode potential. The curve acquisition is done in several times so as to get rid of beam noise inherent to field emission. Then the microcomputer derives the curve I = f(E) to get the curve N(E) = f(E), N(E) representing the num- ber of electrons of total energy E. The width of this curve at half height gives the energy spread AE.
For an oxygen processed <100> tip there is a strong dependance of total beam current Ij, chiefly for tips of small radius. The extracting potential V0 and factor 6 = E/V gives an indication of radius size. Low V0 and large 6 correspond to small radius.
For example (Fig. 2 ) , with a cathode heated at 1420 °C having g = 4.8 x 10^ cm- 1
(V0 = 1950 v at Ij = 10 y A ) , AE varies from 1.15 eV at I T = 10 yA to 2.6 eV at I T = 100 yA. The same filament at T = 850 °C has AE = 1 eV for I T = 10 yA. With tips of larger radius the values of AE are much smaller : for example with a tip having 8=1.2 x lt^cm"1 (V0 = 3600 v at IT = 10 y A ) , at 1420 °C AE varies from 0.95 eV at IT = 10 yA to 1.6 eV at IT = 100 yA. The same tip at smaller temperature (T = 850 °C) has AE = 0.85 eV at I T = 10 yA.
We have also measured energy spread of a <111> oriented tip. At 850 °C, AE varies from 0.6 eV at 10 yA to 0.8 eV at 100 yA. This kind of filament is much less sensi- ble to I T variation. We suppose that the reason is the current per unit solid angle is lower with this kind of tip (at same total current the <111> oriented tip gives a probe current 3 to 5 times smaller than the <100> tip). This is due to the fact that the <111> tip gives a less angularly confined beam : that means an emission aperture angle a larger.
If we refer to Knauer {3} who gives for the broadening effect the following expres- sion :
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984234
C2-156 JOURNAL DE PHYSIQUE
where R i s t h e t i p r a d i u s , t h e o t h e r s parameters being already defined above, we can see t h a t energy broadening i s not only dependent on t o t a l c u r r e n t and t i p radius but a l s o on emission a p e r t u r e angle. This a e f f e c t i s may be t h e reason explaining t h a t energy spread of < I l l > t i p i s s m a l l e r than
<loo>
t i p a t same t o t a l c u r r e n t but a t lower probe c u r r e n t .11) TROYUN M . , Optik 52 (1979) 401
I21 BUNTING G.D., M a n z e s t e r F i f t h Eur. Cong. Electron Microsc., (1972) 150.
131 KNAUER W., Optik 59 (1981) 335.
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t i p l r s t anode
2nd anode
c o n d e n s e r l e n s E
c r o s s o v e r o b j e c t i v e l e n s
200 p a p e r t u r e E
r e t a r d i n g f i l t e r I I
f a r a d a y cup
PA computer
F i g . 1 : E x p e r i m e n t a l s e t - u p f o r e n e r g y s p r e a d measurement
< l o o > w t i p
< I l l > w t i p 4 -1
O C
10
= 2 . 2 ~ 1 0 cm 850 O CF i g . 2 : Energy s p r e a d v e r s u s t o t a l e m i t t e d c u r r e n t
