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Submitted on 1 Jan 1987
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A TIP OSCILLATION PHENOMENON
M. Drechsler, A. Maas
To cite this version:
M. Drechsler, A. Maas. A TIP OSCILLATION PHENOMENON. Journal de Physique Colloques,
1987, 48 (C6), pp.C6-215-C6-218. �10.1051/jphyscol:1987635�. �jpa-00226839�
A TIP OSCILLATION PHENOMENON
M. Drechsler and A. ~ a a s *
CRMC2-CNRS, Campus de Luminy, c a s e 913, 13288 Marseille cedex 09, France
*AG Festkorperoberflachen, Universitat Bonn, 5300 Bonn, F.R.C.
RESUME : Une 4tude des coue stables dea pointes en microscopic
4lectronique 8 transmission montre 1
'
exiGence d'
une oscillation des pointes avec dee amplitudes de-
200 A et dea fr6quencea de-
0.3 Hertz (Cur Au, 9000C). L'oscillation eat expliquee par une interaction de deux forces : (1) une force Blectro-statique caue6e par l'impact dea 6lectrone et annihil6e par une d6charge d86mission de champ et (2) une force 6laatique d'une couche graphitique la- surface de la pointe.
ABSTRACT : A detailed study of stable tip necks by in situ
transmiasion electron microscopy shows an esistence of tip oscillations with amplitudes in the order of 200 A and frequencies in the order of 0.3 Hertz (Cu, Au at
-
9000C). The oscillation is explained by an interaction of twp forces : (1) an electrostatic force caused by electron impact and annihilated by a field emission discharge and (2) an elastic force of a graphitized layer on the tip surface.1. INTRODUCTION
An attempt to study stable necks as described in the foregoing paper /1/
in some detail lead to the finding that a tip can oscillate as briefly mentioned elsewhere /2/. The surprising result stimulated us to study the tip oscillat ion phenomenon. This paper descr ibes the exper imental f indinge as well as a first model for its explanation.
THE TIP OSCILLATION EXPERIMENT
The exper imental condit ions for the appearence and demonstration of a tip oscillation are : (1) a small cone angle tip ( <
-
30) ; (2) a relatively high temperature (- 0.8 Tm < T < Tm ; Tm-
melting point) and (3) an in situ observation or registration (video) of the tip /2//3/. The oscillations are visible in situ in the bright field (fig.
1) or in the dark field mode (TEM) (fig.
2) and somtimea also by SEM. These conditions are fulfilled at present only in the experiments with copper and gold t ips
.
Typical oscillation character iatice are : (1) the tip neck diameter oscillateg with an amplitude in the order of 300 A (fig. 2) ; (2) the tip length oscillates
Fig. 1
-
Bright f i e l d image (TEM) o f a phase of an o s c i l l a t i n g t i p (Cu).
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987635
C6-216 JOURNAL DE PHYSIQUE
0
with an amplitude i n t h e order of 200 A. The o s c i l l a t i o n frequency is i n t h e order of 0.3 Hertz, but f a i r l y i r r e g u l a r . A t i p neck o s c i l l a t i o n is not a r a r e event but a f r e q u e n t l y found phenomenon provided t h e t i p n ~ c k is f i n e and t h e g r a p h i t i z e d s u r f a c e layer is not t o o t h i c k ( <
-
103 A).A l l t h e o s c i l l a t i o n s a r e b e t t e r v i s i b l e on video f ilms than on micrographs ae i n f i g . 2 .
We have found t i p o s c i l l a t i o n s s o f a r on Cu and Au t i p a , but such o s c i l l a t i o n s e x i s t probably a l s o on t i p s of other metals i f adequate exper imental conditions a r e introduced.
Pig. 2
-
Pour phases of a Cu t i p o s c i l l a t ion per iod.TEM dark f i e l d mode.
Br i g h t appears t h e graphi- t i z e d l a y e r . M i ~ i m u m neck diameter
-
200 A.EXPLANATION OF THE TIP OSCILLATIONS
I s it possible t o understand t h e s e t i p o s c i l l a t i o n s ? A f i r s t attempt of such an explarlution i e described i n t h e following. Pig. 3a ehowe schematically t h e i n i t i a l phase of an o s c i l l a t i o n period. A s o l i d metal drop is separated from t h e new m e t a l l i c t i p end but s t i l l hold by a g r a p h i t i z e d layer /1/. The impact of t h e microecope e l e c t r o n s charges t h e anare o r l e s s i s o l a t e d metal drop but not t h e t i p which i e on e a r t h p o t e n t i a l ( f i g . 3b). The charge of t h e drop leads t o an e l e c t r o e t a t i c f o r c e between drop and t i p . This f o r c e seems t o press t h e hollow layer p a r t i n such a way, t h a t t h e neck diameter as w e l l a s t h e t o t a l t i p
t h e t i p o s c i l l a t i o n s . meta t i p end
a)
a) I n i t i a l phase ( a f t e r drop formation)
:
t i p on e a r t
p o t e n t i t r o n s cause :
a p o t e n t i a l d i f f e r e n c e
b) a drop temperature increase
b ) Change of phase r o s t a t i c f o r c e
by e l e c t r o n impact deformation
minimum
C ) Phase o f minimum
neck diameter f i e l d e l e c t r o n
before a f i e l d discharge and f i e l d i o n c u r r e n t between t i p and drop
d ) phase a f t e r f i e l d discharge
disappearence o f t h e e l e c t r o s t a t i c f o r c e t h e e l a s t i c counter-force reproduces phase (a).
C6-218 JOURNAL DE PHYSIQUE
length decreases ( f i g . 3b). The charge c r e a t e s a l s o an e l e c t r i c f i e l d between drop and t i p . Thiu f i e l d ehould increase t o values i n t h e order of t h a t of f i e l d emission ( l o 7 t o 108 V/cm) when t h e drop p o t e n t i a l a r r i v e s values i n t h e order of 102 t o 103 v o l t . The energy of t h e impinging electron8 (30 kV, 100 kV and 3 MV i n our experiments) is by f a r suf f f i c i e n t t o c r e a t e such potent i a l a
.
Cont inious e l e c t r o n impact should lead t o a f o r c e increase or a f u r t h e r decrease of t h e neck diameter and t h e drop-tip d i s t a n c e . The appearence of a drop protrusion which is t y p i c a l f o r a f i e l d protrueion of a f i e l d emitter ( f i g . 2 and 3 ) ) (perhaps a Taylor cone ? ) may be a proof of t h e presence of t h e high f i e l d s t r e n g t h i n t h i s phase. This f i e l d s t r e n g t h may cause a l s o a e t a b l e f i e l d emission c u r r e n t between drop and t i p . The continioualy ihcreasing f i e l d e t r e n g t h should cause another known e f f e c t : t h e change of a e t a b l e f i e l d emission i n t o an unstable emission or a sudden g r e a t increase of t h e c u r r e n t . Thie should occur between phaee c and d of f i g . 3. A d i f f e r e n c e t o t h e known i n s t a b i l i t y must be a l i m i t a t i o n of t h e c u r r e n t due t o t h e l i m i t e d charge of t h e drop. Such drop discharge must lead t o a sudden diaappearence of t h e e l e c t r o s t a t i c f o r c e s o t h a t t h e counter f o r c e of t h e e l a s t i c a l l y deformed layer ( f i g . 3c and 3d) should push t h e drop back t o its i n i t i a l p o s i t i o n s . When t h e i n i t i a l phaee ( f i g . 3a) i e reeetabliehed. t h e eyetem is ready f o r another o s c i l l a t i o n period, e t c .I t is c l e a r t h a t such a model needs t o be confirmed, completed and perhaps corrected. Nevertheless t h e preeented explanation seems t o be t h e only one which is not i n c o n t r a d i c t i o n with one of t h e experimental r e s u l t s obtained eo f a r .
CONCLOS IONS
(1) A t t h e end of metal t i p s v i s u a l i z e d by e l e c t r o n microscopy a formation of s t a b l e necks and s o l i d drops i a found. Such t i p ends become unatable or o s c i l l a t e a t higher temperatures. O s c i l l a t i n g is t h e neck diameter a s w e l l a s t h e t o t a l t i p length.
( 2 ) The o e c i l l a t i o n i e explainable by an e l e c t r o s t a t i c f o r c e produced by t h e impinging microscope e l e c t r o n s . This f o r c e leads t o a drop dieplacement u n t i l an increasing f i e l d emission leads t o a drop discharge combined with a decreaae of t h e e l e c t r o s t a t i c f o r c e s o t h a t an e l a s t i c counterforce brings t h e drop back t o i t s i n i t i a l p o s i t i o n , e t c .
We l i k e t o thank 9. Ramdani and P. Be1 f o r t h e preparation of t h e Cu t i p s .
REP ERENCES
/I/ M. Drechsler, 9. Ramdani, A. Claverie and A. Maas : 34th I n t e r n . P i e l d m i s s i o n Symp.
,
Osaka 1987, Journal de Phye ique./2/ M. Drechsler, S. Ramdani and A. Maas : paper accepted by Surface Science
/3/ M. Drecheler and S. Ramdani : 33rd Intern. F i e l d hniseion Symp., B e r l i n 1986, Journal de Phyeique C7-11 (1986) 177-181
