HAL Id: jpa-00226832
https://hal.archives-ouvertes.fr/jpa-00226832
Submitted on 1 Jan 1987
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
STEADY AND EXPLOSION MODES IN ELECTRON EMISSION FROM LIQUID METAL SOURCE
K. Hata, S. Nishigaki, M. Inoue, T. Noda, H. Tamura
To cite this version:
K. Hata, S. Nishigaki, M. Inoue, T. Noda, H. Tamura. STEADY AND EXPLOSION MODES IN
ELECTRON EMISSION FROM LIQUID METAL SOURCE. Journal de Physique Colloques, 1987,
48 (C6), pp.C6-177-C6-182. �10.1051/jphyscol:1987629�. �jpa-00226832�
JOURNAL D E PHYSIQUE
Colloque C6, suppl6ment au noll, Tome 48, novembre 1987
STEADY A N D EXPLOSION M O D E S IN ELECTRON EMISSION F R O M LIQUID METAL SOURCE
K. Hata, S. Nishigaki, M. Inoue, T. Noda and H. ~arnura*
Toyohashi University of Technology, Tempaku-cho, Toyohashi 440, Japan
*~itachi Keisoku Techno-Research, 882 Ichige, Katsuta 312, Japan
With a liquid metal electron source, we showed a possibility of steady field emission (SFE) at 33th IFES. In comparison with a pulsed explosive emission (EE) observed by Swanson and Schwind, it is important to know causes which determine the mode in the liquid metal electron emission. Through experiments of extracting electrons from a Ga-In-Sn liquid alloy with core tungsten tips of various radii of curvature, it was found that only the EE mode was observed with tungsten tips of apex radii above 10 pm. On the other hand, the SFE mode was obtained in weaker electric fields with those below 10 pm. The volume of liquid metal adhered to the solid tip to form a cone seems to be a main factor to determine the emission mode. In the SFE mode, the emission area estimated by the Fowler-Nordheim plot was heavily influenced by the volume of metal adhered to the tip, and it ranged from 10-lo to 10-l4 cm2 for the tip with a apex radius of 0.2 pm.
1. Introduction
We reported the characteristics of field emission from a Ga-In-Sn liquid alloy in 33th IFES. In the emission current waveform, a stable emission was observed, though the waveform showed superposition of periodic pulses with a frequency dependent on the circuit constants. It was thus doubted whether DC component really exists in the emission current or the repetitive pulses decay through the circuit constants. Moreover, there was a question that the emission area estimated by the Fowler-Mordheim plot was deduced to be several orders of magnitude smaller than that for the solid emitter in spite of the energy spread staying around the same.
In this paper we show existence of the SFE mode and consider the mechanism which determines the mode of emission, SFE or EE.
In addition, we describe experimental results on the relation of effective source size with core tip radius to answer the above question for the emission area.
2. Experimental
The liquid metal electron gun we employed was a needle type already reported l). The tungsten tip with 0.5 mm dia. can be put in and out through a center hole of the liquid metal reservoir by manipulating a micrometer knob from outside of vacuum. This mechanism allows us to adjust the distance from the tip to an extractor, and also to control the volume of liquid metal adhered to the tip surface. The metal was Ga-In-Sn ternary alloy with K.P. of ~ O C , and not heated during electron extraction.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987629
C6-178 JOURNAL DE PHYSIQUE
By means of tips whose apex radii were measured by SEM, we observed the waveform of the emission current and of the radius effects on V-I characteristics. The measurement circuit is shown in figure 1. The resistance of 100 k O connected with the extractor is to protect the oscilloscope.
Throughout the experiment, vacuum pressure was 0.6 to 1 x lo-' Torr and drift in the emission current around 20% an hour, respectively.
3. Results and Discussion 3.1 SFE and EE modes of
electron emission .Jrr
Waveforms of the emission current from the tip with apex radius of 1 p m are shown
in figure 2, where (a) is for Figure 1
current 1 o f 200 p A under Circuit for measurement.
GND.
Figure 2 Wafeforms of electron emission.
(a) SFE current at 200 PA.
Scale: 100 pA/div, vertical and
2 ms/div, horizontal.
applied voltage V of ca. -2 kV, and it obviously shows the SFE mode. On the other hand, (b) shows waveforms of voltage (Ch.1) and current (Ch.2) from the same tip when exploded by raising V to -4 kV. The current waveform ( C h . 2 ) shows periodic spikes when the current pulse height exceeds the sonrce capacity.
This is the EE mode of electron emission 2). Only in this mode photon emission from Ga neutral atoms (417.206 and 403.298 nm) was observed at the tip apex.
Figure 3 S E N images of the tip apex ( a ) Before and ( b ) after the explosion.
Figure 3 shows SEM images of the same tip (a) before and (b) after repeated explosions followed by decontamination by electron bombardment. It was found that the explosion destroyed not only the liquid cone but also the core tungsten tip, and resulted apex radius was ca. 20 pm. The ti.!? radius of around 10 p m or larger never gave SFE but EE mode only. To this fact, we consider that causes which determine the mocie are as follows:
In general, the field strength E at the emitter apex is expressed by the equation
where
a
is the geometrical factor in cm-l and it is constant for the conventional solid emitter. In the liquid emitter, however, R is an implicit function of V. The liquid volume adhered to the tip apex is in proportion to the cube of the apex radius. For a tip of dull apex, the change of B with respect to V is fairly large due to a huge volume of the liquid. In this case, E will rapidly increase and easily exceed the field strength adequate for the SPE mode, resulting only in the EE mode. Whereas for a sharp tip, the liquid volume is so small that change in 13 is slight, causing the SFE mode.3.2 V-I characteristics
We investigated V-I characteristics in dependence on the core tip radius by using tips with ca. 0.2 and 1 pm. Shown in
JOURNAL DE PHYSIQUE
r = lym
0
- 2m '
s ',
"A @"
''
A
8,
A
d,
'
I I I I
Figure 4 Measured dependence o f total emission current on applied voltage and its Fowler-Nordheim plots for t h e apex radii o f 0.2 and 1 pm,
( a ) and ( b ) : apex radius of 0.2 p m , ( c ) and ( d ) : 1 pm.
figure 4 are experimental results in the form of V-I characteristics and Fowler-Nordheim plot. The tip with 0.2 p m apex radius gave (a) and (b), where each group corresponds to the same degree of adhesion. Groups 1 and 2 show three different results in accordance with tip - extractor distance as the parameter, denoting square 0
,
triangle A and circle 0 for the distances of ca. 500, 450 and 400 pm, respectively. For group 3, the distance was set at ca. 100 pm. The volume of adhered metal for group 2 was much less than that for group 1. In figures 4(a) and (b), it is found that threshold voltages greatly differ from each other, even with the same tip radius of around 0.2 ,urn, particularly depending on the adhered amount as seen in comparison of groups 1 and 2.The da-1n-sn alloy we employed has a composition of Ga 72.5 : In 24.5 : Sn 3 in atomic %. Work functions of these elements are as close to each other as 4.2, 4.12 and 4.42 eV, respectively 3 ) . ' The work function of this alloy as a whole was assumed to be 4.2 eV, 'equal to that of Ga. The effective emission area was estimated by the F-N plot adopting the work function of 4.2 eV. It showed a large variation ranging from 10-10 to 10-l4 c,*.
Figures 4 (c) and (d) were obtained with a tip of 1 p m radius. Difference in threshold voltages was relatively small for such an apex radius. The emission area showed a narrow variation ranging from 10-lo to 10-I' cm2.
For the emission area in the order of 10-lo c m 2 , the apex radius is ca. 100 nm, as large as that of the conventional solid emitter, so the energy spread due to Boersch effect 4)is small.
Therefore, the spread E = 0.3 eV, which we obtained l) at the emission current of 17 p A , seems to be reasonable.
4. Conclusion
From the experiment on the field electron emission from liquid metal using Ga-In-Sn ternary alloy, the following were obtained:
1) The SFE mode existed in addition to the known EE. Moreover, the key factor which determined the mode of electron emission was apex radius of the core tip.
2) In the SFE mode, the electron emission area was dependent upon the volume of metal adhered to the tip surface. The smaller the apex radius of the core tip, the stronger was the dependency. The emission area varied from 10-lo to 10-l4 cm2 when the tip with 0.2 p m radius was used.
Concerning l), we tried to explain the electron emission mode qualitatively by using an idea of liquid metal transformation in a high electric field. And 2) made us realized that we had employed the electron gun in the region of narrow energy spread we reported before.
Finally, in our observation of the emission patterns in the SFE mode, though no photographs were taken due to unavailability on our apparatus, one or a few stable bright spots appeared with no symmetry. Phis predicts formation of a stable microprotuberance. It will be supported also ,by the fact that F-N plots in figure 4 fit straight lines. There are still unsolved questions, however, why the electron emission starts at a voltage much less than the threshold based on the Taylor cone, and whether or not the stable liquid microprotuberance is really formed in atomic order, and so on.
C6-182 JOURNAL DE PHYSIQUE
References
1 ) K.Hata, S.Nishigaki, M - W a t a n a b ~ , T.Noda, H - T a m u r a and H.Watanabe: J. de P h y s i q u e
47
Suppl. 11 ( 1 9 8 6 ) C7-375.2 ) L.W.Swanson a n d G.A. Schwind: J. Appl. Phys.
2
( 1 9 7 8 ) 5655.3 ) H.B.Michaelson: J. Appl. Phys.
48
( 1 9 7 7 ) 4729.4 ) H.Boersch: 2 . Phys.
