hll.l>8^. 811
A LASER HEATED SCHOTTKY
EMISSION GUN
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I
A LASER HEATED SCHOTTKY
EMISSION GUN
FOR ELECTRON MICROSCOPY
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. B. BOEREMA, HOOGLERAAR IN DE AFDELING DER ELEKTRO-TECHNIEK, VOOR EEN COMMISSIE AANGEW^: DOOR HET COLLEGE VAN DEKANEN, T DEDIGEN OP WOENSDAG 7 MEI 1975 TE li
DOOR
KAREL DIEDERICK VAN DER MAST
natuurkundig ingenieur geboren te Haren
Dit proefschrift is goedgekeurd door de promotor Prof. Dr. I r . J. B. Le Poole
En wat hebben wij, dat wij niet ontvangen hebben? (naar 1 Corinthiers 4 vers 7)
D a n k b e t u i g i n g
De ontwikkeling van de nieuwe electronenbron die in deze d i s s e r t a t i e i s beschreven, i s mogelijk geweest dankzij de adviezen en c o n s t r u c -tieve bijdrage van veel anderen.
Graag wil ik daarvoor mijn waardering uitdrukken.
In de e e r s t e plaats denk ik hierbij aan de m e d e w e r k e r s binnen de werkgroep electronenoptica, geleid door prof. dr. ir. J. B. Le Poole: d r . J. E. Barth M . S c , ir. J. Kramer, ir. E. Koets, L. E. M. de Groot, P. E. van der Reijken, J. F. Staneke en de s e c r e t a r e s s e mejuffrouw E. M. Stoutjesdijk.
V e r d e r wil ik graag noemen:
i r . F. H. Groen en J . Sjardijn (werkgroep optica) i r . P. van Zuylen en dr. ir. L. A. Fontijn (TPD)
i r . F. H. Plomp en d r . i r . G. G. P . van Gorkom (N.V. Philips)
Ook de dienstgroepen binnen de afdeling der Technische Natuurkunde hebben een groot aandeel gehad in deze ontwikkeling:
- Van de Instrumentmakerij de heren L. Knoll, J. C. H. von Stein, G. M. C. de Vette, E. P . W. Zapf, A. van Mullem, H. J. G. van Noort, A. van Dam, F. A. M. van Aerde en J. Schreuder.
- Van de Tekenkamer de heren G. v. Eden, C. A. J. Wolfs, A. van Spronsen en A. S. G. de Knegt.
- Van de Pompkamer de heren H. C. Mollevanger en L. W. Lander. - Van de Electronische Dienst ir. B. V i s s e r , W. Jongsma, E. Ruys en
CONTENTS Introduction
1. DEVELOPMENT
1.1 p r a c t i c a l b r i g h t n e s s limits for conventional t r i o d e s o u r c e s
1.2 previous developments 1.3 a new concept
1.4 expectations for the shape of the wire end 1.5 the first experiments
1.6 the cathode t r a n s p o r t s y s t e m
1.7 from electron beam heat,ing to l a s e r heating 1.8 p r e l i m i n a r y r e s u l t s and m e a s u r e m e n t s 2. A 100 keV ELECTRON SOURCE
2.1 general r e q u i r e m e n t s 2.2 the general setup 2.3 the t r a n s p o r t system 2.4 optics 2.5 stabilization 2.6 electronics 2.7 electron optics 3. GUN PROPERTIES 3.1 b r i g h t n e s s m e a s u r e m e n t s 3.2 t h e o r e t i c a l maximum b r i g h t n e s s 3.3 the energy s p r e a d 3.4 b e a m c u r r e n t stability
4. THE ELECTRON SOURCE IN PRACTICE 4.1 operation experiences
4.2 the source used for t r a n s m i s s i o n electron micro 4.3 scanning t r a n s m i s s i o n electron microscopy 4.4 micrographs
SUMMARY SAMENVATTING STELLINGEN
I n t r o d u c t i o n
For a long time many applications in the field of electron optics have been limited by the p r o p e r t i e s of the electron source. This made the electron source a popular r e s e a r c h subject.
In the case of surface scanning electron microscopy (SEM) as well a s t r a n s m i s s i o n scanning electron microscopy (STEM) the b r i g h t n e s s of the source limits the scanning speed. In the case of t r a n s m i s s i o n electron microscopy (TEM) high coherency is d e s i r e d for high r e s o l -ution microscopy and holographic imaging. Low illumination c u r r e n t density causes long exposure t i m e s , which in t e r m puts higher d e -mands on the stability of the microscopes.
For microfabrication p r o c e s s e s or micro-etching it i s d e s i r a b l e to have a high probe c u r r e n t for an economic production speed, etc.
Until now, two new types of electron s o u r c e s have come into use: the field emission source and the lanthanum hexaboride source. In this t h e s i s the development of a new type will be p r e s e n t e d and discussed: the l a s e r heated Schottky emission gun: a gun with a con-tiniously renewed tip, which i s heated by a focussed l a s e r beam. The important c h a r a c t e r i s t i c s and p a r a m e t e r s of the source will be discussed.
The aim of t h i s t h e s i s i s to give the r e a d e r a good understanding of the course of the development, r a t h e r than to p r e s e n t and d i s c u s s only the final device, so an attempt is made to give a description of the r e s e a r c h and development p r o c e s s which led to the p r e s e n t e l e c -tron source.
In t h i s way not only the r e s u l t of this p r o c e s s but also a lot of the accumulated experience i s presented.
DEVELOPMENT
1.1 P r a c t i c a l b r i g h t n e s s l i m i t s f o r c o n v e n t i o n a l t r i o d e s o u r c e s
When a small a r e a AO i s passed by an electron c u r r e n t AI within a s m a l l solid angle Aw, the b r i g h t n e s s B i s e x p r e s s e d by:
B = AOAu) AI 1.1.1 The brightness, derived theoretically from the c u r r e n t density at the surface of the cathode j , the cathode t e m p e r a t u r e T and the a c c e l e r -ating voltage V i s :
B = i -j eV 1.1.2 77 k T + 1
k = 8.61 X lO'^eVK"'' (Boltzmann constant)
This value of the b r i g h t n e s s is at a given j , T and V, a thermodyna-m i c a l invariant describing a property of the electron radiation, and i s therefore completely determined by the p r o p e r t i e s of the source. However, the obtainable current densities in electron p r o b e s a r e also affected by mechanical vibrations, strayfield effects and l e n s -a b e r r -a t i o n s .
Equation 1.1. 2 shows u s , that the only way of i n c r e a s i n g the b r i g h t n e s s at a constant accelerating voltage i s by increasing the factor j / T . This can be done by i n c r e a s i n g the t e m p e r a t u r e of the tungsten cathode. However, t h i s d e c r e a s e s the lifetime of the fila-ment, because with the t e m p e r a t u r e the evaporation r a t e i n c r e a s e s too, and even more rapidly than the electron emission.
B ( T ) in A/cm^ ste
t (T) in hours
Fig. 1. 1. 1
Cathode life time t and brightness B at 100 kV as function of the cathode t e m p e r a t u r e T for a 125 nm tungsten filament.
In o r d e r to get a quantitative idea of the lifetime dependence of conventional filaments on the t e m p e r a t u r e it can be assumed that a d e c r e a s e in filament diameter of 10 fo i s tolerable.
When the diameter of the filament i s a, the evaporation r a t e i s Ve, we find for the lifetime t of filaments:
t = 0.1 a / 2 Ve 1.1.3 Values for the b r i g h t n e s s B and the lifetime t for a 125 fim tungsten
cathode are given in figure 1.1.1.
When, for instance, a minimum lifetime of 10 hours i s r e q u i r e d the cathode t e m p e r a t u r e is about 2900 K, the maximum c u r r e n t density about 7 A cm-2 and the maximum b r i g h t n e s s at 100 kV i s about 9 X 10^ A cm-2 sr-1. Usual cathode t e m p e r a t u r e s a r e about 2800 K. So in fact the lifetime limits the b r i g h t n e s s of conventional triode guns.
1.2 P r e v i o u s d e v e l o p m e n t s
Since 1962 the electron optics group of the department of applied p h y s i c s in Delft has made s e v e r a l efforts to overcome the lifetime
limit mentioned in section 1.1.
The direction of field emission was not chosen p r i m a r i l y because of the high vacuum r e q u i r e m e n t s . Further t h e r e was already a large number of l a b o r a t o r i e s working in t h i s direction. In the opinion of J. B. Le Poole it i s b e t t e r to avoid u l t r a high vacuum when it i s not r e a l l y n e c e s s a r y for the experiments t h e m s e l v e s . Long pumping and baking t i m e s too often mean an undesired i n c r e a s e of the time needed for the experiments, which a c t s a s a d e t e r r e n t . However, at p r e s e n t these r e s t r i c t i o n s have become l e s s important, a s a r e s u l t of recent developments in high vacuum techniques.
Efforts were made to i n c r e a s e the b r i g h t n e s s by heating the cathode m a t e r i a l locally by an electron probe and compensate the evaporation by slowly moving the cathode under the heating probe. E x p e r i m e n t s w e r e done with m a s s i v e cathodes^ and thin tungsten foil cathodes^. The disadvantages of massive cathodes a r e the high energy r e q u i r e d for heating the cathode and the fact that the electron emission a s well as the heating must take place at the same side of the cathode.
These two p r o b l e m s a r e solved by using thin tungsten foil cathodes (see figure 1.2.1). However, a new problem was introduced: the t h e r m a l expansion caused unpredictable shapes of the foil. The foil had a tendency to bulge.
At the s a m e t i m e , energy filter e x p e r i m e n t s w e r e c a r r i e d out in the group. A line source instead of a round source would noticeably d e c r e a s e the r e c o r d i n g t i m e s in these experiments. Consequently, the electron source with the locally heated wire^ came into the
Fig. 1.2.1
Tungsten foil as cathode for electron sources. 1. electron source; 2. magnetic electron lens; 3. tungsten foil; 4 . direction of movement of the foil; 5. accelerating electrode; 6. emitted electrons.
picture. Preliminary unpublished experiments were carried out by J. Kramer and B. E. Bol Raap in 1970.
The tungsten wire (10 nm diameter) was heated by an electron probe. During operation the wire was slowly pulled through. The emitted electrons were accelerated by a cylindrical positive elec-trode placed around the wire, (see figure 1.2.2).
Maximum current densities at the cathode surface were measured to be in the order of 200 A cm- 2.
Because of the very promising results of these measurements a few prototypes were builHjS. The brightness and the energy spread were measured and the results confirmed the theoretical values.
^y/^^^
Fig. 1.2.2
The locally heated wire source.
1. conventional electron source; 2. electrostatic lens; 3. cathode w i r e ; 4. direction of movement; 5. cylindrical electrode; 6. anode; 7. emitted electrons.
For a 100 kV electron source of this type the b r i g h t n e s s would be in the order of 10^ A cm-2 s r - ' . The measured energy spread was about 1.5 eV.
In January 1972 a project was s t a r t e d to build a 100 kV locally heated wire source on a Philips EM 300 microscope.
Initially the gun was designed with the cathode heated by an electron probe. In the final design a focussed laser beam was applied because the laser heated Schottky gun which had been developed in the same time required laser heating. This made the two s o u r c e s interchange-able on the microscope.
The schematic d i a g r a m of the 100 kV wire gun i s given in figure 1.2.3.
Fig. 1.2.3 100 kV wire source.
1. laser beam; 2. vacuum window; 3. focussing lens; 4. m i r r o r ; 5. cathode wire; 6. cylindrical electrode; 7. anode; 8. insulator.
References:
1. J. W. A. Zwart: Afstudeerverslag,Delft University of Technology 1968.
2. P. J. Rus : Kandidaatsverslag, Delft University of Technology 1964.
3. J. B.Le Poole : Dutch Patent Application nr. 7018701.
4. J. B.Le Poole, J. Kramer and K. D. van der Mast: Proceedings of the fifth European Congress on Electron M i c r o -scopy, 1972, p . 8 .
5. K. D. v.d.Mast: Afstudeerverslag, Delft University of Technology 1972.
1.3 A n e w c o n c e p t
In a private discussion between J. Kessler of the University of Arizona, and Le Poole about the locally heated wire gun Kessler suggested heating the end of a tungsten wire and using the molten tungsten sphere at the wire-end a s the cathode.
Later E. Barth of the Delft University of Technology, asked me if the Schottky effect o c c u r r e d in the wire gun.
These two r e m a r k s , combined with the stage of development of the w i r e gun at that time, led to a new concept for an electron source. The Schottky effect
The Schottky effect i s the effective d e c r e a s e of the potential b a r r i e r A$, faced by the electrons in the conducting energy levels when an external field E i s applied at the metal surface (see figure 1.3.1). it) 1 1 1 ixïtungsten'ixv:; vacuum N. Fig. 1. 3. 1
Lowering of the effective potential b a r r i e r $ caused by the external field.
a. z e r o potential; b. image force potential curve; c. potential caused by external field; d. net potential curve.
The magnitude of the effect can be analysed by calculating the poten-t i a l curve based on poten-the image force popoten-tenpoten-tial resulpoten-ting from poten-the apoten-t- at-traction between an electron outside the surface and its image behind the surface. The net potential is now the sum of the z e r o field image force potential and the potential due to the field.
We find for the lowering of the potential b a r r i e r :
A$ = - ƒ - ^ "l 1.3.1 The charge of an electron -e=
V4ir eo
Fm-This i n c r e a s e s the thermionic emission by gain factor Ge which i s
-eA$
Ge = exp k T 1.3.2 Inserting t h i s into the Richardson equation we find for the total cur-r e n t density j at the cathode sucur-rface now:
j = Ge AT^ exp ( " ^ 1.3.3 (A= 60 A cm-2 K while k r e p r e s e n t s the Boltzmann constant).
In figure 1.3.2 the gain factor Ge and the lowering of the poten-t i a l b a r r i e r A* a r e plopoten-tpoten-ted againspoten-t poten-the field spoten-trengpoten-th E for various t e m p e r a t u r e s . For higher t e m p e r a t u r e s the gain factor d e c r e a s e s , but in spite of this effect higher t e m p e r a t u r e s always i n c r e a s e the emission. 1000 100 10 1 -Ge -" ^ 1 1 1 2500K /3OOOK . //35OOK # • / -J L 1 1 1 10' 10° 10=" 10' ,10 E in v / m Fig. 1. 3. 2
a: the lowering of the effective work-function <J> a s a function of the field E.
b : the gain factor Ge caused by the Schottky effect for different t e m -p e r a t u r e s .
An easy calculation on the wire gun showed that the Schottky ef-fect was negligibly s m a l l in that case. Raising the voltage of the cy-lindrical electrode around the wire would i n c r e a s e the effect, but in order to obtain a significant effect the p r e a c c e l e r a t i n g voltage must be chosen so high that the energy dissipation in the cylindrical e l e c -trode would be unacceptably large.
The next n u m e r i c a l example i l l u s t r a t e s this:
A 10 Mm diameter tungsten w i r e in a locally heated w i r e gun would normally give a total emission of 10 mA. The t e m p e r a t u r e i s then
about 3500 K. For a gain factor 10 at that t e m p e r a t u r e we need a field strength of about 4 x lO^ V m"'' at the surface of the cathode w i r e . That means a potential of 18 kV on the electrode with a diam-e t diam-e r of 2 mm. Thdiam-e total dissipation thdiam-en would bdiam-e 1.8 kW.
By using thinner w i r e s the field would i n c r e a s e but they a r e i n p r a c -tical.
The suggestion of Kessler
K e s s l e r suggested using a molten drop at the end of the wire (figure 1.3.3). However, when the wire end melts the melted tung-sten would immediately be pulled to the wire by the surface tension until it i s cooled down below the melting point. So t h i s can never be a stable mode of operation.
Fig. 1.3.3 The suggestion of Kessler.
1. conventional electron source; 2. electron lens; 3. cathode wire with molten end; 4. direction of movement; 5. first anode; 6. emitted e l e c t r o n s .
The new concept
Because the melting point of tungsten d e t e r m i n e s the maximum t e m p e r a t u r e of the cathode, we have to apply the Schottky effect to r e a c h higher b r i g h t n e s s e s . We already saw the impossibility of using t h i s effect in the case of the wire gun. But by using the end of the wire it might become possible because here we can i n c r e a s e the field,which i s somewhat stronger because of the different geometry by d e c r e a s i n g the r a d i u s of the tip. The shape of the wire end must be stable and therefore the operation t e m p e r a t u r e must be below the melting point of tungsten. An advantage of this concept i s that the Schottky effect takes place only at the tip of the wire. Thus the total thermionic emission from the wire end will hardly i n c r e a s e and huge energy dissipations will not occur.
To i n c r e a s e the c u r r e n t density by a factor 10 we need a field of about 4 X 10^ V m-''. So, for a tip with r a d i u s of 1 |um we need a first anode voltage of 5 x 4 x 10^ x 10'^ = 2000 V, which i s r e a s o n -able.
The factor 5 i s used because we do not have a complete sphere but a spherical shape at the end of a cone.
cathode surface i s about 230 A c m ^ . So the b r i g h t n e s s of an e l e c -tron gun at 100 kV used in this mode, including the Schottky effect, would b e :
10 x 2 3 0 10 5
8.61 X 10-5 X 3.5 X 10' = 2.4 X 10^ A cm2 s t e r -T h i s value i s comparable with the b r i g h t n e s s e s measured in field emission s o u r c e s .
: l l l l l l l l WIIIIITTr 6
Fig. 1.3.4 The new concept.
1. conventional source; 2. electron lens; 3. the cathode wire; 4. direction of movement; 5. first anode; 6. second anode; 7. Schottky emission.
1.4. E x p e c t a t i o n s f o r t h e s h a p e of t h e w i r e e n d In order to have an operable cathode the shape of the wire end must be stable. Experimental work has been done elsewhere 1 on changes in shape of field emission t i p s at lower t e m p e r a t u r e s but only at t e m p e r a t u r e s where evaporation plays hardly any r o l e . Although many mechanisms will determine the tip shape, such as evaporation, surface migration, surface tension, the e l e c t r i c a l field and the energy density in the heating probe, a few things a r e predictable about the shape, presupposing that a stable shape e x i s t s . The general shape
The wire has to be t r a n s p o r t e d continuously to compensate the evaporation. Because the wire i s heated only at the end the cathode t e m p e r a t u r e is highest at the wire end. Therefore the closer a wire p a r t comes to the end, the more m a t e r i a l has been evaporated from that w i r e p a r t . This will cause a conical shape at the wire end. See figure 1.4.1.
The curvature of the tip
Because of surface migration the shape of the final tip can change quite rapidly under the p r e s s u r e caused by the surface ten-sion and the e l e c t r o s t a t i c field forces. When the e l e c t r o s t a t i c force
'A focussed laserbeam
Fig. 1.4.1
The general shape of the wire end.
exceeds the force of surface tension, a sharp tip will be pulled out by the electrostatic field force and because of the high field emission
c u r r e n t s this tip will melt and iniate a vacuum a r c . This is called a blow-up. Therefore the minimum curvature of the tip for a stable operation is determined by the first anode potential V.
This limit can easily be calculated: the electrostatic field E at the tip surface with r a d i u s r i s :
kr
k is a constant determined by the geometry of the cone and the first anode. A reasonable value i s 5. The p r e s s u r e P g because of t h i s field i s :
' E 2 " ° k 2 r 2 V"
The p r e s s u r e P g because of the surface tension i s :
1.4.1
1.4.2 For equilibrium applies: Pg = P E and hence
^^ 2 y So finally we find: 1 N" 2 ^ 0 '^ '••nn 1.4.3 1 So ^ 4 r k^ 1.4.4
For tungsten the surface tension constant y i s about 2.5 N m" '. We can now derive the dependence between E^^^ and the first anode voltage V, and find:
4 y k
Knowing the maximum field strength a s a function of the first anode voltage we can calculate the Schottky effect for different t e m -p e r a t u r e s and the c u r r e n t density at the ti-p surface. This i s -plotted in figure 1.4.2. However, later on it was found that the tip curvature limit i s not determined by this equilibrium but by another p r o c e s s . This i s described in section 3.2.
E in V/m r in j in A / c m ^ 200 600 1000 UOO 1800 y k ^ i n V Fig. 1.4.2
r m i n . Enja^ ^"<^ 'he c u r r e n t density j for T= 3500 K as a function of V/k, if P E = P S
-The starting point for these considerations i s the fact that the wire t e m p e r a t u r e i n c r e a s e s towards the wire end. When this i s no longer t r u e , other shapes will occur. Therefore the energy c u r r e n t density in the focussed l a s e r b e a m has to be high enough to satisfy this r e -quirement.
C r y s t a l s t r u c t u r e
Another effect was expected too. Because of the high mobility of the surface atoms at high t e m p e r a t u r e s a c r y s t a l s t r u c t u r e will no
longer exist at the surface and therefore the electron emission will be uniform over the tip surface.
Reference:
1. W. P. Dyke, F. M. Charbonnier, R. W. Strayer, R . L . F l o y d , J. P. Barbour and K. J. Trolan: Journal of Applied Physics, Vol. 31,
1.5 T h e f i r s t e x p e r i m e n t s
The previous thoughts are v e r y promising. They would allow designing an electron gun for v e r y high brightness, without the n e -c e s s i t y of oriented tungsten wire and u l t r a high va-cuum. Therefore, experiments were s t a r t e d to investigate the behaviour of the tip and the engineering p r o b l e m s involved.
The first experiments were c a r r i e d out a s simply a s possible. (See figure 1.5.1).
Fig. 1. 5.1 The first experiments.
1. conventional source; 2. magnetic electron lens; 3. cathode wire; 4. quartz funnel; 5. fluorescent s c r e e n ; 6. first anode; 7. fluorescent
s c r e e n ; 8. optical projection microscope; 9. optical projection screen.
A single lens column provided a 15 keV electron heating probe. The tungsten wire was pushed through a quarz funnel into the probe. A fluorescent electron s c r e e n beyond the probe enabled one to see if the probe i s focussed at the p r o p e r place. To watch the shape of the w i r e end a little optical projection microscope was applied. The hot w i r e end was projected on a little screen. Behind the first anode a little fluorescent s c r e e n was placed to observe the emission pattern from the tip.
The experiments led to the following conclusions:
1. The optical projection of the wire end was v e r y helpful. In spite of its resolving power of about 1 ^ m the shape of the wire end was clearly visible.
An energy c u r r e n t of 1 Watt in a 20-30 |Um diameter probe seamed to be enough to melt the wire end.
3. A problem r o s e when the first anode voltage was switched on. The e l e c t r i c field at the tip pushed the heating electrons away. At anode voltages over a few hundred volts the heating probe was dif-ficult to control.
4. Another difficulty was the cathode wire t r a n s p o r t . Because of the difference in the dynamic and static coefficient of friction the wire has always a tendency to jiggling in the quartz funnel.
F u r t h e r m o r e the movements of the wire end perpendicular to the w i r e axis were not s m a l l enough in this system.
Especially the shortcomings of the wire t r a n s p o r t were the main p r o b l e m in reaching a stable wire end shape. For two r e a s o n s
move-ments of the cathode wire perpendicular to the axis are not allowed: firstly it will get outside the heating probe and cool down, and s e c ondly the electron source has to be geometrically stable. And a s -suming a tip r a d i u s of about one micron the electron source will be even s m a l l e r than that, e.g. in the o r d e r of 200 A. So if we want to use t h i s principle in building a working electron source we have to design a t r a n s p o r t s y s t e m for cathode w i r e s with the following spec-ifications:
1. Because the evaporation has to be compensated the t r a n s p o r t velocity has to be in the o r d e r of 0.01 - 0 . 1 jim s e c " ! during a long time (over 100 hours).
2. T r a n s v e r s e movements of the wire should not exceed a few hun-dred A.
3. It must be small enough to place in a microscope gun chamber. 4. To prevent mechanical vibrations the length of the free end of the
w i r e is limited to some mm.
So the first experiments revealed the necessity of a better t r a n s -p o r t system and that an i n c r e a s e of the rigidity of the heating beam by using higher energy electrons was desirable.
1.6 T h e c a t h o d e t r a n s p o r t s y s t e m
At this stage of the project it became clear that the whole idea of the new electron source could only be r e a l i s e d if a t r a n s p o r t s y s -t e m wi-th -the r e q u i r e m e n -t s men-tioned in sec-tion 1.5 could be con-structed. Therefore a lot of attention was paid to the design of that system.
For the experiments the following s y s t e m was developed: The cathode wire has to be t r a n s p o r t e d v e r y straightly along the w i r e axis. This can be done by clamping the wire on a movable plat-form. Straight movement i s a s s u r e d by a set of p a r a l l e l springs. See figure 1.6.1. The wire axis has to coincide with the direction of movement of the platform.
Z'TTTTTT 6 . '
Fig. 1.6.1 T r a n s p o r t system.
1. cathode wire; 2. movable platform; 3. and 4. pai'allel springs; 5. direction of movement; 6. t r a n s p o r t wire; 7. wire clamp.
The r e q u i r e d velocity i s v e r y low, and must be controlled p r e c i s e l y . This can be done by applying a wire which controls the position of the moving platform by means of its t h e r m a l expansion. This wire i s called the t r a n s p o r t w i r e . In figure 1.6.1 we see that the heating c u r r e n t through the t r a n s p o r t w i r e d e t e r m i n e s the location of the p l a t -form and consequently the velocity i s determined by the variation in t i m e of the heating c u r r e n t . However, the total t r a n s p o r t length i s now limited to the maximum extension of the t r a n s p o r t w i r e .
To overcome this limitation a second clamp i s placed just behind the clamp on the moving platform (see figure 1.6.2). This clamp i s not movable but fixed and normally in open position. When the platform c o m e s to the end of its stroke the fixed clamp will close and hold the cathode wire, the clamp on the platform will open, after which the t r a n s p o r t wire will cool down quickly and the platform r e t u r n s to its s t a r t i n g position.
Fig. 1.6.2
The t r a n s p o r t system with second clamp.
Then the platform clamp will take over the cathode wire again and the fixed clamp will open. The cathode wire can again be t r a n s p o r t e d .
A total t h e r m a l expansion of 0.2 mm is a reasonable value. With a t r a n s p o r t velocity of 0.1 /im s-^ only one t a k e - o v e r every 30 minutes
i s n e c e s s a r y . The clamping planes in figure 1.6.2 are mutually p e r -pendicular. This has a special r e a s o n . The high r e q u i r e m e n t s make it n e c e s s a r y to use v e r y straight tungsten w i r e s . The tungsten w i r e s w e r e straightened by annealing them under tension in vacuum. After the two clamps open en close successively a straight w i r e will be centered automatically along the intersection line of the two clamping p l a n e s . So when the t r a n s p o r t system i s once aligned a new cathode w i r e i s always aligned automatically. The models proved to have a t a k e - o v e r reproducing accuracy of about one micron.
The clamps a r e also controlled by t h e r m a l expansion of thin tungsten w i r e s . Initially the clamping planes were made of s t a i n l e s s steel. Later on, however, gold was used to prevent spot welding of the cathode wire on the clamping planes.
1.7 F r o m e l e c t r o n b e a m h e a t i n g t o l a s e r h e a t i n g In spite of the new cathode wire t r a n s p o r t system and a more rigid electron beam of 30 kV e l e c t r o n s , the behaviour of the heating probe near the field at the wire end remained difficult to control.
Probably the experiments could be done with a 100 kV beam but in p r a c t i c e t h i s would not be v e r y attractive because of the extra high potentials. Therefore, instead of the electron probe, a focussed l a s e r beam, which obviously i s not affected by the e l e c t r i c field, was used for heating the wire end.
Other points to be considered a r e :
1. The whole heating system, consisting of the l a s e r , the l a s e r modulator and the n e c e s s a r y e l e c t r o n i c s a r e working on earth potential, which makes the s y s t e m l e s s complicated for higher voltages.
2. The evaporation of tungsten b e c o m e s more harmful in t h i s case because it i s inevitable that either a m i r r o r or a glass surface will be coated by evaporated tungsten. This makes periodic cleaning or replacing of these surfaces n e c e s s a r y .
3. The l a s e r light will be partially reflected by the tungsten surface. Therefore the energy c u r r e n t density has to be higher than the energy c u r r e n t density which was n e c e s s a r y in the c a s e of the electron probe.
For light incident on a surface:
Pg[j3 i s the absorbed energy, P^ ^ ^ the total energy of the light and €x the s p e c t r a l emissivity.
Ultimately the e x p e r i m e n t s were c a r r i e d out with a Neodymium YAG laser. The combination of size and beam energy makes this l a s e r - t y p e quite suitable for t h i s purpose. The wave-length of t h i s l a s e r light i s 1.06 y^m.
In figure 1.7.1 it can be seen that a reasonable value for e;^^ of tung-sten at 3500 K for X = 1 Mm will be 0.35.
I I I I 1 I I I I I J
a20 A25 0.3O MO 0.50 0.60 0.*0 10 1.5 2 3
" - X in n
Fig. 1.7. 1
The emissivity of tungsten e^ 3s a function of the wave length A at different t e m p e r a t u r e s .
Reference:
1. J. C. de Vos: Physica 20, 690 (1954).
1.8 P r e l i m i n a r y r e s u l t s a n d m e a s u r e m e n t s
Compared to electron beam heating the use of a laser made the heating of the tip very easy indeed.
Several experiments were carried out to obtain data about the be-haviour of the tip. The first experiments already showed the exist-ence of a stable tip shape.
The shape of the tip
Wires were used with diameters of 10, 15, 20, 25 and 30 fim. When the laser beam is well focussed the shape of the wire end be-comes comical with a smooth round tip on top of the cone.
The v a r i o u s d i a m e t e r s of the w i r e s used gave no e s s e n t i a l difference in the shape. The time needed for forming a stable tip shape, s t a r t -ing with a simply cut off wire, i s about 5 minutes. The energy of the l a s e r b e a m used was about 1.5 Watt. The beam was focussed within a 30 pim probe. The shape of a tip i s shown in figure 1.8.1. (Opname Centr. Lab. TNO Delft).
Fig. 1.8. 1
Two surface scanning electron micrographs at different magnifications of one tungsten tip in stable shape. The tip diameter is 0.6 Mm, tht wire diameter 10 i^m.
T e m p e r a t u r e and field dependences of the tip shape
Stable tip shapes were made under different c i r c u m s t a n c e s . The t e m p e r a t u r e could be v a r i e d by pushing the wire end l e s s or more into the probe. The total emission from the wire could then be used a s an indication for the tip t e m p e r a t u r e . F u r t h e r m o r e , the field at the tip surface could be changed by means of the first anode voltage. The r e s u l t s were quite amazing: the r a d i u s of curvature of the tip was quite independent of the t e m p e r a t u r e and the first anode voltage. This was not understood until much later and will be explained in section 3.2. This r a d i u s of curvature always remained about 0.3 jum.
Tip behaviour for higher anode voltages
If the first anode voltage exceeds a certain value the tip explodes after a while.
Initially this was ascribed to c l u s t e r s of other m a t e r i a l in the cath-ode wire.
The r e a l causes, however, were found later on. The final tip p r o -truded through the l a s e r probe and cooled down too much. Then the tip dulling p r o c e s s b e c o m e s too slow to compensate the sharpening effect of the evaporation and the field. When the l a s e r b e a m was powerful enough and focussed very well, this effect was eliminated and higher first anode voltages could be reached without the o c c u r -r e n c e of tip explosions.
However, even with this precaution t h e r e still exists a c r i t i c a l maxi-mum first anode voltage. Higher voltages always cause tip explo-sions (See section 3.2).
C u r r e n t density m e a s u r e m e n t s
In these e x p e r i m e n t s the c u r r e n t density was determined by m e a s u r i n g the c u r r e n t through a s m a l l a p e r t u r e behind the tip. (See figure 1.8.2).
1
--Fig. 1.8.2Measurement of the current density j at the tip surface.
The c u r r e n t density j can be calculated from:
2 Trr Of
T h i s measurement i s not exactly c o r r e c t , because of the focussing effect of the field caused by the tip shank and the flat first anode. So the angle a will be a little bit l a r g e r .
In this way, for stable tips, c u r r e n t densities of 6000 A cm-2 were measured. Even higher current densities, up to 30.000 A cm-2, were m e a s u r e d but the t i p s were not stable during those m e a s u r e m e n t s . However, the usability of this type of cathode was proved.
The c r y s t a l s t r u c t u r e dependence of the emission
As was said in section 1.4 no effect of the c r y s t a l s t r u c t u r e was expected. The following experiment proved that to be t r u e :
A fluorescent s c r e e n was placed in front of the tip. By evaporation a field emission tip was made at the wire end. Then the tip was cooled down to r o o m t e m p e r a t u r e and the first anode voltage was switched
on. For a few seconds the field emission pattern of a clean tip b e -came visible on the s c r e e n whereafter this pattern changed into the pattern of a contaminated tip because the vacuum was only about 10"" t o r r .
Then the tip t e m p e r a t u r e was r a i s e d slowly by means of the l a s e r beam. After a while the clean pattern r e a p p e a r e d on the screen.
At v e r y high t e m p e r a t u r e s this pattern disappeared again and the s c r e e n was uniformly illuminated. So at higher t e m p e r a t u r e s (esti-mated to be over 3000 K) no c r y s t a l s t r u c t u r e could be discerned any more in the electron emission pattern. The half angle under which the emission pattern could be observed was about 0.2 radian. P r e l i m i n a r y beam c u r r e n t stabilization
T h e r e a r e two possibilities for controlling the tip t e m p e r a t u r e . F i r s t l y by pushing the wire end l e s s or more into the laser beam, secondly by modulating the laser b e a m intensity.
Because the final objective was an electron source with a stable b e a m current, a few experiments were c a r r i e d out to investigate the possibilities for stabilization of the b e a m current.
The b e a m c u r r e n t will be roughly proportional to the first anode c u r r e n t , while a signal derived from t h i s c u r r e n t i s on the same potential level a s the t r a n s p o r t system. Therefore by pushing the w i r e end l e s s or more into the focussed l a s e r beam the total e m i s -sion can be stabilized. Because the t r a n s p o r t system is a mechanical s y s t e m b a s e d on t h e r m a l expansion it can only be used for stabilizing in the low frequency region. The best r e s u l t s were obtained by inte-grating the signal derived from the first anode c u r r e n t . This signal controls the t r a n s p o r t wire c u r r e n t . See figure 1.8.3.
Fig. 1. 8. 3
Slow beam current stabilization.
1. cathode wire; 2. focussed laser beam; 3. first anode; 4. t r a n s p o r t s y s t e m ; 5. first anode voltage supply; 6. measuring r e s i s t o r for the first anode current; 7. reference voltage supply; 8. integrator and c u r r e n t amplifier.
The beam current can be stabilized in the higher frequency r e -gion by modulating the laser beam. Because the tip is so very small the emission will follow intensity variations in the laser beam with frequencies of 10 Hz. So, modulating the laser can be used for sta-bilizing and modulating the electron beam to very high frequencies. Experiments with a very simple laser modulator (a razor blade on a loudspeaker cone) proved laser modulating to be usable for stabi-lizing the beam current.
Conclusions
The preliminary experiments revealed the new concept to be very useful. The tip shape seems to be stable enough while laser heating is to be preferred to electron beam heating. To investigate the problems involved in constructing an electron gun under normal working conditions based on this principle and to be able to image the virtual electron source a prototype was designed for use in the EM 300 microscope.
2 - A 100 KeV ELECTRON SOURCE 2.1 G e n e r a l r e q u i r e m e n t s
A 100 kV electron source has been built on the available EM 300 microscope. In order to come quickly to r e s u l t s available p a r t s of the EM 300 were used as much a s possible, for instance the cable, the insulator and the gun chamber.
In o r d e r to be able to investigate the usability of the source for the different applications the source has to be adapted to the column in such a way that the microscope r e m a i n s usable in the different modes (TEM as well a s STEM and SEM).
This project was s t a r t e d during the development of the wire gun and therefore the two s o u r c e s were made interchangeable on the m i c r o -scope.
2.2 T h e g e n e r a l s e t u p
The general setup is shown in figure 2.2.1.
The t r a n s p o r t system (1) is placed on a standard EM 300 insulator (2) and built inside the first anode (4). In this first anode two s m a l l
Fig. 2 . 2 . 1 The general set up.
lenses a r e mounted, one s e r v e s a s an objective for the projection microscope, the other i s the main focussing lens for the l a s e r b e a m (5). The position of the focus can be adjusted by a weak lens just out-side the column. This lens (6) i s movable in x, y and z directions. The laser (7) can be modulated by an acousto optical modulating s y s -t e m (8). The modula-tor signal is derived from an insula-ted a p e r -t u r e (9). The a.c.component of the signal i s led to an amplifier (11) by an RC network (10) to control the modulator (8).
The e l e c t r o n i c s at the high voltage level a r e placed in an i n t e r m e d i -ate tank (12) where the EM 300 high voltage (13) i s plugged in. The tank contains the first anode voltage supply (14) and the t r a n s p o r t s y s t e m c u r r e n t supplies and controls.
The difference signal from the measuring r e s i s t o r (15) and the ref-e r ref-e n c ref-e voltagref-e (16) is intref-egratref-ed and amplifiref-ed by a c u r r ref-e n t ampli-fier (17) and heats the t r a n s p o r t w i r e .
The n o r m a l EM 300 gun chamber (18) i s maintained but provided with two windows (19) for the l a s e r b e a m and the projection m i c r o -scope.
The mechanical gun alignment and tilting system was removed and replaced by two p a i r s of deflection coils (20, 21). They a r e mounted in the place of the first condenser lens, which was also removed. Copper-iron-copper strayfield shields a r e placed around the coils. So the insulator i s fixed on the column. A magnetic strayfield shield was placed around the gunchamber. The anode (22) is maintained. The electrostatic accelerating lens, consisting of the first anode and the second anode, i s easily adjustable for the different accelerating voltages because the second anode in the EM 300 i s movable in the z direction.
The projection ocular and the s c r e e n for the projection microscope a r e mounted outside the column.
2.3 T h e t r a n s p o r t s y s t e m
The basic principles for the t r a n s p o r t system mentioned in section 1.6 a r e used again in the construction of the wire t r a n s p o r t system. However, to make it more suitable for the microscope a few changes were made.
The p a r a l l e l springs
With springs of the shape used in the first type, a z-movement of the wire g e n e r a t e s an x-movement (see figure 1.6.1) which i s negligibly small for s m a l l z-movements. But in order to make a s few take o v e r s as possible the stroke has to be i n c r e a s e d and then the cosine effect b e c o m e s considerable. Applying the r a z o r blade
shape as drawn in figure 2.3.1 the cosine effect is eliminated. F u r t h e r m o r e a s i s shown in figure 2.3.1 the s y s t e m i s made sym-m e t r i c around the cathode wire in o r d e r to sym-make an efficient use of the available space on the insulator and to d e c r e a s e t h e r m a l effects.
7
Fig. 2. 3. 1
Model of the t r a n s p o r t system a s used in the microscope. 1-4. razorblade type p a r a l l e l springs; 5. upper clamps on movable platform; 6. fixed clamps; 7. cathode wire.
The two jaws a r e mechanically coupled in order to use only one w i r e , which will be called the take-over w i r e . The t h e r m a l expan-sion of this wire s e r v e s the two jaws in the right order.
This i s explained on the b a s i s of figure 2.3.2.
Fig. 2 . 3 . 2
C IS the take-over w i r e . When the wire expands lever G will turn clockwise.
Because of wire D lever H will do the same and push against jaw B which is normally held open by a small spring. When jaw B i s closed
lever H cannot turn any more so the wire D will be stretched more and pushes more against the thin metal blade E so the upper jaw A, which i s normally closed by a weak spring, will open.
Beside the fact that only one c u r r e n t is needed for both jaws this concept has some other advantages: jaw A can only open when jaw B i s closed so the cathode wire can never fall down into the microscope. Moreover, the two jaws a r e closed together only for a v e r y short t i m e . This prevents the occurance of excessive cathode w i r e kinking when the upper jaws a r e moving in z-direction while both jaws a r e closed.
T h i s construction makes it possible to take over by hand. This can be done by shifting stud I.
Turning lever H counterclockwise will open both jaws together which i s n e c e s s a r y for inserting a new wire.
The whole system i s electrically insulated and mounted inside the first anode and placed on the insulator. See picture 2.3.3.
In t h i s configuration four e l e c t r i c a l connections on high voltage potential are n e c e s s a r y : the cathode potential, the first anode poten-tial, the t r a n s p o r t wire and the take-over wire. Because the n o r m a l EM 300 cable and insulator h a s only t h r e e connections a special p r o -vision has been made to lead the two t r a n s p o r t system c u r r e n t s through the same cable w i r e .
T h i s i s done in the following way:
The two signals for both c u r r e n t s a r e of opposite potential, one p o s i -tive and one nega-tive. A 300 Hz electronic switch leads either one or the other signal to the c u r r e n t amplifier. Two diodes, mounted on the t r a n s p o r t system, separate the two c u r r e n t s again. See figure 2.3.4.
JL
•«"«lüuiiïp^im i—C^ 7 """"UUUlll]'"""!. Fig. 2. 3.4Block diagram of the t r a n s p o r t system c u r r e n t s .
1. signal generator for the take-over wire; 2. for the t r a n s p o r t wire; 3. electronic switch; 4. current amplifier; 5. H.V. cable; 6-7. diodes; 8. t r a n s p o r t wire; 9. take-over wire.
During the experiments it was found that the a.c. current in the t r a n s p o r t wire caused too much magnetic field in the vicinity of the tip. This s t r a y field deflected the electron b e a m at 300 Hz. To avoid t h i s effect the t r a n s p o r t wire was only heated by a.c. c u r r e n t when n e c e s s a r y , which is only the case during the t a k e - o v e r p r o c e s s .
The take-over time has to be adjusted to the delay t i m e s between the wire c u r r e n t s and t e m p e r a t u r e s . A suitable c o m p r o m i s e has been sought between thicker and stronger w i r e s with longer delay t i m e s and thinner and thus more vulnerable w i r e s but with shorter delay t i m e s . Tungsten w i r e of 30-50 Mm diameter i s chosen. The delay t i m e s are then a few seconds.
Tungsten also has the advantege of showing s m a l l creep effects. 2.4 O p t i c s
L a s e r focussing and modulating
For the focussed laser beam t h e r e a r e only a few r e q u i r e m e n t s : The l a s e r b e a m has to be focussed into a s m a l l a r e a of about 20-30 ^ m d i a m e t e r .
The energy current density in the center of the focussed laser beam must be high enough to prevent the tip from penetrating through the beam.
The focus must be adjusted onto the w i r e - e n d and then r e m a i n stable at the same location. In order to have a good location stability the main focussing lens and the t r a n s p o r t system a r e both mounted on the first anode. The consequently short focal length of the main fo-cussing lens d e c r e a s e s its a b e r r a t i o n s too.
The following calculation shows that the optical r e q u i r e m e n t s to the main focussing lens a r e not very high:
Suppose we need a 10 jum diameter focus of the TEMoo mode (actually more modes a r e used) and have a focal length of 20 mm. A TEMoo mode laser beam in or outside the cavity is described by ^
co^(z) = 0)^(1+ ( — 2 ] ' ) 2.4.1
, 2 2
R ( Z ) F Z(1 + [ - ^ 1 ) 2.4.2
2
For r » ^ 0 these equations become'
A
co(z)=M 2.4.3
R(z) = z 2.4.4 R(z) is the curvature radius and 00 i s the r a d i u s of the wave front.
Applying 2.4.3 we find for the wave front diameter in the main focus-sing lens (for X = 1.06 micron) about 2.5 mm.
The location of the l a s e r can be chosen based on mechanical and other considerations. The size of the waist of the laser b e a m can then be adapted by the weak adjusting lens outside the column and other lenses and m i r r o r s when n e c e s s a r y .
A 1.5 Watt TEMoo mode Neodymium YAG laser i s used.
The l a s e r b e a m must be modulated in order to stabilize the b e a m c u r r e n t . Because the tip follows the very high frequencies of the laser beam variations, the modulator should be a s fast as p o s s -ible. The main variation of the laser beam i s a 7 ^ 100 Hz modulation caused by the a.c. c u r r e n t heating the l a s e r pumping lamps. The variation up to 100 kHz a r e in the o r d e r of 2 fo. To avoid power loss the l a s e r beam i s not polarized and therefore an acousto optical l a s e r modulator was chosen. An acousto optical modulator i s b a s e d upon Bragg reflections of the light on acoustical waves in t r a n s p a r -ent material. The waves a r e induced by a piezo c r y s t a l which is mounted on the cell in which this Bragg reflection takes place. Amplitude modulation of these waves causes intensity modulation of the deflected and undeflected beam (see figure 2.4.1).
The c a r r i e r frequency of the modulator used i s 40 MHz. Because the acoustical wave is generated at the surface while the l a s e r b e a m deflection occurs inside the cell there is a constant delay time b e -tween the modulating signal and the modulation of the laser beam. This delay time depends on the group velocity am he distance b e
-^IZIIIIZIIIIZIIIZ~
\l\l\? ^
^===———nnr
'- 2
Fig. 2 . 4 . 1
The acousto optical l a s e r modulator.
1. beam current signal; 2. 40 MHz d r i v e r ; 3. piezo crystal; 4. modu-lator crystal; 5. laser beam; 6. undeflected beam; 7. deflected laser beam.
tween the piezo c r y s t a l and the l a s e r beam. For the modulator used t h i s i s about 2.5 /zs. This means a phaseshift of 90 d e g r e e s for a
100 kHz signal. This effect limits the bandwidth in which the e l e c -tron beam can be stabilized.
The projection microscope
A n e c e s s a r y aid is the projection microscope.
The objective i s mounted in the first anode just like the main focus-sing lens. A projection ocular i s mounted outside the column. By means of two m i r r o r s the hot tip i s projected onto a s c r e e n of frosted g l a s s . The magnification is about 500 t i m e s which i s a suitable mag-nification for observing the shape of the wire end.
Reference:
1. H. Kogelnik: Imaging of optical modes; The Bell System Technical Journal^ March 1965.
2.5 St a b i l i z a t i o n
A stable electron b e a m c u r r e n t i s required. The main effects causing undesired instabilities a r e :
. fluctuations of the laser beam intensity. . movements of the air in the laser b e a m .
. changes in shape of the cathode wire because of evaporation and surface migration.
With a stabilized l a s e r beam intensity still electron c u r r e n t v a r i -ations o c c u r r e d of a few percent. Therefore, beam c u r r e n t feed-back loops are n e c e s s a r y for stabilization.
As was said in section 2.2, two mechanisms are used for stabiliz-ation: the wire t r a n s p o r t for the lower frequencies and modulating the l a s e r b e a m for the high frequencies (feedback loop I and feed-back loop II, see figure 2.5.1). When feedfeed-back loops a r e used to
ö^wmfl
LOOPI (SLOW) U 10 L o o p n (FAST) 11 = i = C UlliVI I 1) > > > I I I I I , , t n Fig. 2. 5. 1 The two feedback loops.1. cathode wire; 2. t r a n s p o r t system; 3. insulator; 4. first anode; 5. first anode voltage supply; 6. measuring r e s i s t o r ; 7. reference voltage; 8. integrator; 9. H.V. supply; 10. second anode; 11. insu-lated a p e r t u r e ; 12. amplifier; 13. 40 MHz d r i v e r ; 14. modulator head.
stabilize a system a good investigation of the existing time constants in the loops i s n e c e s s a r y , otherwise they can cause undesired oscil-lations.
The slow feedback loop (loop I) has the following time constants: - The l o n g t i m e constant of the integration.
- The time constant of the mechanical m a s s spring system of the t r a n s p o r t system, causing a 10 Hz resonance frequency. The t r a n s p o r t system i s a second order mechanical m a s s - s p r i n g system. The length of the free end of the clamped cathode wire in-c r e a s e s by t h e r m a l expansion when its t e m p e r a t u r e i n in-c r e a s e s . This will i n c r e a s e the effect of the t r a n s p o r t system movements or laser intensity variations without causing a large phase shift because the l a s e r energy dissipation in the tip i s quite independent of the tip t e m p e r a t u r e and because t h e r m a l expansion i s a linear effect which depends only on the absorbed power.
So the most dangerous frequency is the 10 Hz resonance frequency of the mechanical m a s s - s p r i n g system. In order to prevent oscilla-tions the open loop amplification at 10 Hz has to be low enough. The integrator already c a u s e s a slope of 20 db per decade in the f r e
-quency r e s p o n s e function of the open loop amplification so only the amplification has to be adjusted to the 10 Hz amplification r e q u i r e -ment.
The time constants in loop II a r e those corresponding to the t h e r m a l lengthening of the final wire end, the t h e r m a l r e s p o n s e of the tip itself and the delay time of the l a s e r modulator.
Because the t h e r m a l r e s p o n s e time of the tip i s v e r y small and the effect of t h e r m a l lengthening of the wire end never gives a large phase shiftj the high frequency amplification only has to be limited because of the modulator delay t i m e . This can be done by adapting the time constant caused by the capacitance C of the insulated a p e r -t u r e and i-ts measuring r e s i s -t o r R (see figure 2.5.1), or ano-ther t i m e constant after the amplifier.
The d. c, r e s p o n s e of the modulator has to be zero, otherwise the location of the tip will change slowly and the modulator will go out of its range.
There is another phenomenon in the low frequency region. Loop I controls the total emission from the entire wire end instead of the b e a m current. The b e a m c u r r e n t c o m e s only from the tip and i s
l e s s than one thousandth of the total emission from the wire end. Therefore, slow shape changes of the tip are not controlled by the t r a n s p o r t system but only by the modulator.
This would plead for a modulator stabilization region extending far into the low frequencies. But then the beam c u r r e n t will follow only v e r y slowly an adjustment of the first anode c u r r e n t . Thus a com-p r o m i s e has to be sought between these two effects. The ocom-pen loocom-p frequency r e s p o n s e functions are drawn in figure 2.5.2.
db 60 20 0 10"^ Xo"^ 10"'' 1 10 \ 10^ 10^ 10' 10^ Hz Fig. 2. 5.2
The open loop amplification plotted against the frequency for loop I and loop II.
2.6 E l e c t r o n i c s
The e l e c t r o n i c s in the intermediate tank need special attention. The tank contains the first anode voltage supply, the c u r r e n t function g e n e r a t o r s for the t r a n s p o r t system, the reference voltage and the m e a s u r i n g r e s i s t o r for the first anode current.
The c u r r e n t function g e n e r a t o r s consist of semiconductor c i r c u i t s which are easily destroyed by high voltage d i s c h a r g e s in the tank and in the microscope and therefore need special protection. The c i r c u i t s a r e protected by leading the discharge c u r r e n t s immediately to the high voltage supply of the microscope in such a way that they cannot enter the electronics.
This can be done with coils and c a p a c i t o r s : the values of these e l -ements a r e chosen in a way that the a.c. c u r r e n t s needed for leading the two t r a n s p o r t s y s t e m c u r r e n t s through one wire just p a s s the f i l t e r s . It i s important to choose v a r i o u s types of capacitors and coils adapted for the various frequency regions. Special c a r e has to be taken to avoid discharge c u r r e n t loops through the walls of the high voltage case inside the tank. The realization i s shown in figure 2.6.1. The protection has proved to be sufficient. 100 kV d i s c h a r g e s won't h a r m the semiconductors.
Fig. 2 . 6 . 1
Protection circuits of the semiconductors in the intermediate tank. 1. cable to E M 300 high voltage tank; 2. cable to the gun; 3. inter-mediate tank (oil filled); 4. protection circuitry; 5. t r a n s p o r t system electronics.
P r o v i s i o n s have been made for reading the first anode voltage, the first anode c u r r e n t and the two t r a n s p o r t system c u r r e n t s . The t r a n s p o r t system c u r r e n t s indicate when a take-over will take place. The t r a n s p o r t system can also be operated manually both in forward and backward direction.
2.7 E l e c t r o n o p t i c s
The dimensions of the electron source
As was said in section 1.8 the tip at the end of the wire has a h e m i s p h e r i c a l shape. The surface of this sphere i s smooth without visible c r y s t a l s t r u c t u r e .
Therefore, the e l e c t r i c a l field at the surface has spherical s y m m e -t r y -too. As a r e s u l -t of -this spherical s y m m e -t r y -the angular moment u m of momenthe elecmomentrons emimomentmomented from momenthe momentip i s consmomentanmoment. Thus momenthe e l e c -t r o n s s e e m -to come from a small v i r -t u a l source wi-th r a d i u s r y a-t the center of the tip sphere.
If the r a d i u s of the tip sphere is rt, the tip t e m p e r a t u r e T and the first anode voltage V, we find for r y :
(see figure 2.7.1)
1 ^ „ n o . -. I / T
g ^ . . t = 9 - 3 x 1 0 - ^ X | / : ^ r t 2.7.1 For instance, If T = 3600 K, V = 1500 V and r^. = 0.3 x 10"^ m.
Then r y - 45 A .
Fig. 2.7.1
The size of the virtual cathode derived from the conservation of mo-mentum: / 2kT An electron leaves the cathode with a velocity V(.: Vc = (/ —^ The velocity Vy of an electron at first anode potential V: Vy = |/ — Conservation of momentum: Vy x ry = V(. x rt
The effect of the conical shank
Because the cathode i s not a sphere inside a s p h e r i c a l first an-ode but a hemisphere on a conical shank in front of the first anan-ode two things a r e different:
1. F u r t h e r away from the tip the s p h e r i c a l s y m m e t r y of the field no longer dominates. Therefore the v i r t u a l cathode i s not in the cen-t e r of cen-the mechanical sphere acen-t cen-the cen-tip bucen-t i s a few cen-tencen-ths of a
millimeter further away from the first anode. This should be con-sidered in designing the accelerating lens. y
2. The e l e c t r i c field close to the tip i s not equal to jr a s it would be in the case of complete spherical s y m m e t r y but i s d e c r e a s e d by a factor k. Calculations on this factor have been done by J. C. Wiesner and T.E. E v e r h a r t ^ . A reasonable value i s about 5. The accelerating lens
In designing the first anode and accelerating lens different a s -p e c t s have to be considered:
1. The effect of the spherical aberration on the effective b r i g h t n e s s in the different modes of use. For all applications a low spherical aberration i s advantageous but the importance of the effect v a r i e s largely from one case to another. (See section 3.1).
2. The Boersch effect, which i n c r e a s e s the energy spread of the beam. According to the theory of Loeffler 2, the generation of an extra energy spread o c c u r s in every c r o s s - o v e r and i s due to electron interactions. Therefore the energy spread generated in v i r t u a l c r o s s - o v e r s i s negligible. In larger c r o s s - o v e r s a larger energy spread i s generated.
These considerations led to the first anode design given in figure 2.7.2. Starting from a basic design of M u n r o ^ the lens was optimized for t h i s system.
The anode lens can be used for v a r i o u s voltages, because the EM 300 anode is movable in axial direction. When distance from the second anode to the first anode is 17 mm, the first anode voltage i s 1500 V, the accelerating voltage i s 60 kV then the focal length f = 4 mm, the
1
Fig. 2 . 7 . 2 The anode lens.
coefficient of spherical aberration Cs = 3 mm and the coefficient of chromatic aberration Cc = 2.5 mm at the low voltage side. The main r e a s o n for the v e r y low Cs and Cc is the short distance from the tip to the first anode.
Alignment of the gun
Mechanical alignment of the source on the microscope column would simultaneously cause a misalignment of the focussed l a s e r b e a m . Therefore, the gun i s aligned electrically by means of a set
of deflection coils. Because the source i s so small, the first con-denser of the microscope i s not n e c e s s a r y and has been removed. In i t s place this set of deflection coils is mounted.
For a proper alignment, the cathode image as well as the a c c e l e r -ating lens have to be aligned on the electron optical instrument. Then the effects of first anode voltage changes a r e partially eliminated too. The changes of the first anode voltage cause a movement of the cath-ode image along the line through the center of the ancath-ode lens, the cathode and the cathode image (see figure 2.7.3).
Fig. 2 . 7 . 3
Movement of the virtual cathode image caused by first anode voltage changes.
1. anode lens (accelerating lens); 2. cathode; 3. virtual cathode images.
If the electron optical axis of the microscope coincides with this line t h i s movement i s reduced to a change in size only.
If one of the s e t s of deflection coils is provided with an extra deflec-tion coil or the provision is made for adding an extra c u r r e n t through one of the coils, independent of the bayonet current, then this situ-ation can be obtained by means of the bayonet in the following way: (see figure 2.7.4)
cathode image can be aligned on the column.
Then the pivot point of the bayonet has to be adjusted onto the cathode image.
Finally the coupled bayonet c u r r e n t s have to be adjusted in a way that the movements of the virtual cathode are eliminated. This adjustment can be found easily by wobbling the first anode voltage a few volts. At the proper adjustment the cathode image no longer moves. / ' //
M / / M i
KI
E2
Fig. 2 . 7 . 4Alignment of the source and anode lens,
a. the virtual image is aligned only; b. the virtual image as well as the anode lens a r e aligned.
1-2. alignment coils.
The stability of the bayonet coil c u r r e n t s
The stability of the deflection coil c u r r e n t s of the bayonet i s i m -portant because variations in these c u r r e n t s cause an instable source
location. The r e q u i r e m e n t s can be calculated in the following way (see figure 2.7.5):
If the distance a between one of the deflection coils and the a c c e l e r -ating lens is small compared to the distance i between the virtual cathode image and the lens the angle ;3 under which the image i s seen from the coil will be nearly equal to the angle a under which the cathode i s seen from the lens. Hence /3 ~ S(fig.2.7. 5)jd is the diameter of the s o u r c e , f is the focal length. f
Fig. 2 . 7 . 5 (see text)
For the focal length we have to take the focal length at the high volt-age side which is about 25 mm while d i s about 100 A. When the maximum deflection angle i s y and we tolerate a variation of 0.25 oi^ we find for the maximum variation AI in the deflection coil c u r r e n t :
^ I = 0 . 2 5 A ^ I O I ! 2.7.2
Ijiax Z iy y
So the requirement for the bayonet c u r r e n t s depends on the maximum angle of deflection needed and hence on the mechanical prealignment accuracy.
In p r a c t i c e a c u r r e n t stability of 5.10"^ was sufficient. However, if t h e r e i s a r e a l c r o s s - o v e r in the neighbourhood of the deflection coils the requirement can become much lower.
References:
1. J. C. Wiesner and T.E. E v e r h a r t : Jl. Appl. Phys. Vol.44, no. 5, May 1973, p. 2140.
2. K.H. Loeffler: Z. angew. Phys. Vol.27, no. 3, 1969, pp. 146-149. 3. E. Munro: Ph. O. Dissertation, University of Cambridge, 1972.
3 - GUN PROPERTIES 3.1 B r i g h t n e s s m e a s u r e m e n t s
In general the u s e r of an electron gun will not be interested in the b r i g h t n e s s but the obtainable probe c u r r e n t . Although it is p o s s -ible to define an effective b r i g h t n e s s related to the obtainable probe c u r r e n t , this would not be v e r y useful because for this type of source it depends so strongly on the mode of application.
The applications can be classified in two ways: a. scanning or non-scanning applications
b . applications in which the source i s magnified or demagnified. Classification a is made because the effect of 50 Hz s t r a y fields or mechanical movements tan i n c r e a s e the effective source diameter. In the case of non-scanning applications the only way to d e c r e a s e these effects i s by increasing the demagnification. But in the case of scanning applications these effects can be eliminated when the frame frequency or the line frequency i s t r i g g e r e d on the 50 Hz main fre-quency. Suppose the line time in a scanning microscope i s synchron-ized with the main frequency. The line along which the specimen should be scanned i s not straight because of the s t r a y fields or mech-anical vibrations. However, when the next line i s disturbed in exactly the same way (same phase and amplitude) t h i s will not have any effect on the resolving power of the scanning microscope (see figure 3.1.1). The only effect it has i s a small image distortion and a small modu-lation of the contrast which will not h a r m in most of the scanning ap-plications.
In figure 3.1.1 STEM m i c r o g r a p h s a r e shown of a Pt shadowed r e p -lica of 0.5 jum gratings. To i n c r e a s e the effect, an extra strayfield i s applied.
The effect of mechanical vibrations can also be reduced because many of the mechanical vibrations a r e related to the main frequency. ( T r a n s f o r m e r s , motors in other r o o m s of the building etc.).
The improvement by synchronization can be v e r y large: when the source s e e m s to move 10 t i m e s its own diameter it will only give a distortion oi 1% on an image of a thousand lines when the scan is synchronized. However, when the scan i s not synchronized the r e -solving power would be reduced by a factor 10.
Classification b. is made for the following reason. When the source is enlarged the spherical aberration of the accelerating lens d e c r e a s e s the effective b r i g h t n e s s but when the source i s demagni-fied the spherical aberration of the same lens b e c o m e s l e s s import-ant and the spherical aberration of the final probe or objective lens will dominate. This can easily be demonstrated on the b a s i s of the fact that the product of the angular and linear magnification is con-stant.
Fig. 3. 1. 1 STEM micrographs,
^
Fig. 3. 1. 2
Domination of the spherical aberration of the objective in the case of de magnification.
1, source; 2. accelerating lens; 3. objective lens; 4. probe.
In figure 3.1.2 Cgi and Cs2 a r e the coefficients of s p h e r i c a l a b e r r a -tion of the a c c e l e r a t i n g lens and the objective lens. Cj^ i s the coef-ficient at the high voltage side of the lens, pi and p2 a r e the diam-e t diam-e r s of thdiam-e abdiam-erration disks.
We find: Csi ^-and: hence: P2 = MC31 Oi\ + C32 al Cs1 «1 C s 2 a g ( 1 + p T -3.1.1 3.1.2 ' S 2 C „ al (1 + M'' Cs2 3.1.3
So in the case of demagnification of the probe the factor M' s m a l l compared to 1.
^ s l
-S2
I S
The b r i g h t n e s s (B A I ) can be determined by m e a s u r i n g the
AOAto
c u r r e n t coming through two a p e r t u r e s while the source is focussed on the first a p e r t u r e . Requirements a r e that the first a p e r t u r e has to be i r r a d i a t e d uniformly and that every point in the second a p e r -t u r e i s i r r a d i a -t e d equally by every poin-t of -the firs-t a p e r -t u r e (see figure 3.1.3).
If the a p e r t u r e s have a separation distance 1 and d i a m e t e r s dj and dg and the current through the a p e r t u r e s is I, we find for the b r i g h t n e s s B:
d l l
J-A—
i l
Fig. 3 . 1 . 3 The b r i g h t n e s s measurement by means of two a p e r t u r e s . I — I — I Fig. 3.1.4Brightness measurement in the microscope.
1. condenser a p e r t u r e ; 2. condenser lens; 3. image of the source; 4. objective lens.
An interesting feature of this m e a s u r e m e n t i s that in t h i s way a too optimistic value can never be measured. If one of the r e q u i r e m e n t s i s not fulfilled the m e a s u r e d b r i g h t n e s s will be too low.
The b r i g h t n e s s of the source on the microscope can be measured in a similar way: by means of condenser II the source i s focussed on the specimen plane. Now the diameter of the source on the specimen plane i s d.,, while the diameter of the condenser II a p e r t u r e is d j . See figure 3.1.4.
To eliminate the source motions related to the main frequency, the source image was scanned by means of the wobbler coils over the specimen plane r e l a t e d to the main frequency. The source diameter in the specimen plane can be calculated from the line width on the s c r e e n and the magnification of the microscope. The c u r r e n t can be m e a s u r e d on the insulated microscope screen. The measured bright-n e s s i s bright-now the t h e o r e t i c a l value dimibright-nished by the effect of the s p h e r i c a l and chromatic a b e r r a t i o n s of the accelerating lens. This effect can be reduced by using a small Cj a p e r t u r e . The C^ a p e r t u r e however, cannot become too s m a l l because of the Airy disk.
So the magnification of the accelerating lens and the C^ a p e r t u r e diameter a r e optimized. The total effect i s calculated to be of the s a m e order a s the source diameter.
In this way the uncorrected b r i g h t n e s s was found to be 2.5 x 10^ A cm"2 sr~i for 60 kV accelerating voltage.
F u r t h e r data: first anode c u r r e n t 3 mA first anode voltage 1400 Volt cathode wire diameter 16 /im
