GB2190786A

GB2190786A - Liquid metal field emission electron source

Info

Publication number: GB2190786A
Application number: GB08612305A
Authority: GB
Inventors: Roy Clampitt; Peter Hanley
Original assignee: OXFORD APPL RES Ltd; Oxford Applied Research Ltd
Current assignee: OXFORD APPL RES Ltd; Oxford Applied Research Ltd
Priority date: 1986-05-21
Filing date: 1986-05-21
Publication date: 1987-11-25

Abstract

Liquid metal wetted to a cathode anchor in the point-in-a-plane geometry is field-formed into a sharp liquid metal point by application of a high electric field, to produce continuous and stable microampere levels of electron current in an ordinary high vacuum environment. The liquid metal may be gallium, tin or indium on a tungsten or molybdenum anchor. The anchor may be a sharply pointed needle or a small bore tube.

Description

SPECIFICATION Liquid metal field emission electron source This invention relates to field emission of electrons from liquid metal cathodes in vacuum.

Positive metal ions can be extracted from the surface of a sharp solid metal, termed an anode, in vacuo by subjecting the anode to a high negative electric field. The phenomenon is known as "field evaporation". By reversing the polarity of the electric field electrons can be extracted from the surface of such a sharp solid point, termed a cathode. This phenomenon is known as "field emission". The ion or electron currents so obtained are generally very low (--nanoamps), the threshold field for electron emission being lower, typically by one order of magnitude, than that for ion emission. Despite that the ion or electron currents from such respective anodes or cathodes are low, these solid emitters find practical application in, for example, electron microscopes, particularly since the sources are of high brightness.In order to sustain stable beams, however, such sharp metal point emitters must be maintained in an ultra-high vacuum environment (typically 10-'0 torr).

Positive metal ions can also be extracted from the surface of sharp liquid metal points in vacuo by application of a high electric field.

The positive ion current so produced is orders of magnitude greater than that from sharp solid metal points, typically several microamperes. This ion source also is of very high brightness, but in contrast to its solid metal anode counterpart, it does not require an ultra-high vacuum environment for stable operation. Ion sources of this type are called liquid metal ion sources. Conventional field evaporation theory does not predict that such high ion currents will be obtained from sharp liquid metal points, nor that stable ion currents will be sustained ordinary high vacuum (10-5-10-5 torr). An example of prior art on liquid metal ion sources is embodied by one of us in British Patent No.1,574,611.

By reversing the polarity of the electric field we have now discovered that microamperes of electrons, (i.e. very high in comparison with conventional field emission), can be obtained continuously and that stable electron currents can be sustained in ordinary high vacuum (10-5-10-6 torr). Conventional field emission theory does not predict that such high electron currents will be obtained from sharp liquid metal points, nor that stable electron currents will be sustainable in ordinary high vacuum. We find, however, that the threshold field for electron emission from a liquid metal point is lower, typically by an order of magnitude, than that for ion emission from a liquid metal point, suggesting that the mechanism of electron emission from liquid metal points is phenomenologically similar to field (electron) emission from solid metal points.We have not directly measured the brightness of this electron source. However, by reference to the body of knowledge on liquid metal ion sources which rely for their operation on a field-forming point and which differ from the mentioned electron sources only by a reversal of the polarity of the point-forming electric field, we can predict that the electron source will be of comparable brightness.

Prior art on electron emission in vacuo from field-deformed liquid metal surfaces teaches that very high pulsed electron currents (hundreds of amperes) only are obtained, by a phenomenon known generally as explosiveelectron emission. One group of research workers, whilst studying pulsed electron emission, observed oscillographically the growth profile of the current pulse. In the temporal region of onset of electron emission, typically in the nanosecond time-scale and immediately prior to the almost-step function surge of current leading to explosive emission, they observed electron emission which was attributed by them to transient field emission. However this work does not teach that continuous and stable currents of electrons will be sustainable in this regime of current growth, without leading on to the pulsed explosive electron emission regime.

A specific embodiment of the invention will now be described: The design, geometry and fabrication techniques required to produce liquid metal electron sources are identical to those of the prior art of liquid metal ion sources. The geometric arrangement of a liquid metal electron emitting cathode source is identical to that of the corresponding liquid metal ion emitting anode source i.e. that of a point-in-a-plane. A feature common to both types of liquid metal sources, and to those other devices of the prior art, such as the mercury-wetted relay, which depend for their operation on electricfield induced extrusion of the liquid metal surface into a sharp point, is the need for a cathode anchor. This may typically be a sharp solid electrode to which the liquid metal is anchored by wetting or equally a fine hollow metal tube filled and likewise wetted to it by the liquid metal.In order to preserve the point-in-a-plane geometry i.e. to provide a high electric field to the liquid metal surface at modest applied voltages, (typically less than 10 kV) the preferred aforementioned embodiment of liquid metal-containing electrodes should be of small diameter, typically less than 0.5 mm. In the case of the fine hollow tube embodiment of the cathode electrode wetted at its periphery by liquid metal, the surface of which assumes the concave shape which characterises a wetted surface deformed by surface tension forces, the application of a sufficient voltage difference between the cathode and an adjacent, typically planar electrode results in the extrusion of the wetted surface into a conical form, the apex of which constitutes the sharp liquid metal point.

In the case of the sharp solid electrode embodiment of the cathode it is generally necessary in the prior art to ensure that the radius of curvature of the point of the electrode is less than, typically, 10 microns in order to avoid the production of more than a single sharp liquid metal point on application of the aforementioned electric field, although it is recognised that for some applications not requiring a single emitting point, such multiple emission points may be a desirable feature.

When the voltage difference, typically 4 kV, is of a polarity, in either of the above embodiments of the cathode, to favour electron emission therefrom, it is observed that electron currents of greater than one microampere can be produced and sustained stably in an ordinary vacuum environment, typically 10-5 torr.

In principle any liquid metal element or alloy would make a suitable cathode material provided that its vapour pressure at the melting point is sufficiently low (10-4 torr) as to avoid vapour discharge or arc formation; and that it wets well and substantially non-corrosively to the cathode holder. Convenient liquid metals are gallium, tin or indium, which all exhibit low melting points, very low vapour pressures at their respective melting-points and which also wet readily and substantially non-corrosively to cathode holders made of, for example, tungsten or molybdenum.

In order to maintain the cathode in a molten state it can be heated by any of the standard procedures, such as resistively, radiatively or by electron bombardment. In one embodiment, the cathode anchoring structure is almost identical to that of a thermionic electron emitting filament used in, for example, scanning electron microscopes. Such a structure comprises a hair-pin filament mounted on a ceramic base. To the apex of this filament a needle-shaped tungsten wire electrode is spotweided, the radius of curvature of which, however, is much larger than that of a thermionically-operated electron emitter and is typically ten microns. The needle-shaped wire electrode and the spotweld juncture of the hair-pin filament are wetted with liquid metal by any of the standard wetting practices such as dipping momentarily, in a high vacuum environment, into a pool of very hot liquid metal, which in the example of indium would typically be 1000"C. In this embodiment of the cathode, the indium metal is maintained in a molten state by resistively heating the tungsten hair-pin filament.

The cathode can be operated as in prior art electron or ion sources in either a diode or triode configuration, the latter possessing a 'Wehnelt' or grid control electrode.

Claims

1. A liquid metal field emission point source of electrons producing continuous and stable microampere levels of electron current in ordinary high vacuum from a sharp liquid metal point formed in a high electric field.

2. A liquid metal field emission point source of electrons as claimed in Claim 1 wherein the liquid metal cathode comprises a solid tungsten needle of ten microns radius wetted by a film of liquid metal.

3. A liquid metal field emission point source of electrons as claimed in Claim 1 wherein the liquid metal cathode comprises a small bore tube filled with and wetted by liquid metal.

4. A liquid metal field emission point source as claimed in any preceding claim wherein the liquid metal cathode is of gallium.

5. A liquid metal field emission point source of electrons as claimed in any preceding claim wherein the cathode structure takes the form of a filament and base as commonly used in scanning electron microscopes.

6. A liquid metal field emission point source of electrons as claimed in claim 5 wherein the electron emitting source forms part of a triode electrode structure.

7. A liquid metal field emission source of electrons substantially as described herein.