JPH0498738A - Liquid metal ion source - Google Patents

Abstract

PURPOSE:To make it possible to perform electric field emission of ion in a low electric field by providing a mechanism irradiating the surface of a needlelike liquid metal electrode with light. CONSTITUTION:A liquid metal electrode 16 is installed, keeping insulation, in a vacuum vessel 14 and applied with a positive bias voltage via power source 12. The electrode 16 is energized and heated by a power source 11 and keeps a melting state of metal. Ion is effectively extracted by being provided with a bias electrode 15, and is prevented to diffuse in to space as much as possible. The kinetic energy of ion is decided by the voltage applied from accelerating power source 13. In the case of vacuum ultraviolet, utilizing the flange of a light introducing window 18a the vacuum vessel 14 is connected to a light source and a spectral system. The ion being radially radiated from the tip of the liquid metal electrode 16 passes through the bias electrode 15, and thereafter enters into a convergent lens system 17 and is formed into beam, and enough ion current can be obtained in a low electric field.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a liquid metal ion source installed in a focused ion beam device, a secondary ion mass spectrometer, and the like.

[Conventional technology]

Published by Nikkan Kogyo Shimbun, 1986, edited by Japan Society for the Promotion of Science,
"Electron/Ion Beam Handbook", 2nd board, No. 22
As described on pages 4 to 229, in a liquid metal ion source, when a positive voltage is applied to a liquid metal electrode kept in a molten state, a negative pressure is generated on the surface of the liquid metal at the tip of the emitter due to a positive electric field. When the surface tension is exceeded, the liquid metal deforms into a conical shape called a tiller cone. The diameter of the tip is sufficiently reduced so that the electric field there becomes several tens of V/
When the particle size becomes about nm, atoms are ionized and desorbed from the surface by the field evaporation process.

In the prior art, liquid metal ion sources have been used as ion sources for focused ion beam devices and secondary ion mass spectrometers, taking advantage of the above-described phenomenon. There are two types of emitters for liquid metal ion sources: capillary type and needle type. In the former case, a low melting point metal is used because it is necessary to send liquid metal from a sample reservoir to the tip of a thin tube with an inner diameter of about 20 μm. A pressurizing mechanism for liquid metal delivery is also required. The latter has a tip diameter of 0.5~] and uses a round solid metal needle as the base of the tiller cone. The material is selected on the condition that it is well wetted by the liquid metal and does not react with it. Tungsten and nickel are often used. The needle type is currently the mainstream because it has a simpler structure and can ionize high-melting point metals.

[Problem to be solved by the invention]

In field evaporation of a liquid metal ion source, unlike field evaporation of a solid, the tip diameter of the tiller cone serving as an emitter automatically decreases as the applied voltage increases. In order to realize an electric field high enough to cause field evaporation, it is necessary to reduce the tip diameter to about several nanometers. The current-voltage characteristics of a needle-type liquid metal ion source depend on the shape of the needle, its surface condition, its positional relationship with the sample reservoir, the characteristics of the process in which the liquid metal sample is supplied to the tiller cone, such as the structure of the sample reservoir, and viscous resistance. Much depends. As the tip diameter decreases, the influence of the characteristics of the sample supply process becomes more pronounced, and the current −
Abnormalities begin to be observed in voltage characteristics. In order to overcome this, attempts have been made to reduce the viscous resistance by shortening the protrusion of the needle from the sample reservoir, cleaning the needle surface, and increasing the cone angle of the needle tip, but the effects are not sufficient. do not have. Therefore, in addition to these measures, we developed a liquid metal that achieves stable electric field evaporation with sufficient strength in a low electric field and with a large tip diameter of the tiller cone, without being noticeably affected by the characteristics of the sample supply process or viscous resistance. An ion source is required.

[Means to solve the problem]

The liquid metal ion source of the present invention applies a positive voltage to the acicular liquid metal electrode to ionize and desorb surface atoms from near the tip of the acicular liquid metal electrode. It is characterized by having a mechanism for irradiating light onto the surface of the device.

[Effect]

The principle of field evaporation is explained by a charge exchange model. FIG. 2 shows an interatomic potential for explaining electric field evaporation of atoms on the surface of a liquid metal electrode. mtmtv is the potential energy, the horizontal axis R is the interatomic distance, ■ in the figure is the ionization potential of the element constituting the liquid metal electrode, φ
is its work function. The electrically neutral potential when surface atoms A3 are separated from surface A of the liquid metal electrode perpendicular to the surface changes slightly due to the polarization of the electron cloud due to the electric field, but basically its shape is do not change. In FIG. 2, U, , F represent the potential of the electronic ground state in the presence of an electric field. Ul represents the potential of the charge exchange state formed when the electrons of the desorbing atoms transfer to the liquid metal electrode. It changes its shape significantly due to Coulomb interaction with the electric field. In FIG. 2, the potential of the charge exchange state in the presence of an electric field is represented by 01'. When the electric field is sufficiently strong, the potential of the charge exchange state intersects the potential of the electronic ground state UfiF near its minimum point. If the electronic correlation between these states is strong, this intersection becomes a pseudo-crossover denoted by C, and the electronic ground state (U a'
) to the charge exchange state (u+F>) occurs. Since the charge exchange state (U + F) is a continuous energy state, the surface atoms A are ionized and desorbed from the surface in the form of A8+.

When the electronic ground state potential (U,''') is photoexcited to the electronic excited state potential (U,'') by irradiation with the light 30 of energy hν, a pseudocrossing (C
) can be moved to a pseudo-crossing (C゛) on the higher energy side. In this case, the slope of the electric field potential (U') that enables field evaporation can be reduced.

Therefore, evaporation can be performed in an even lower electric field.

As a result, the tip diameter of the tiller cone is relatively large,
A liquid metal ion source that enables stable field evaporation while the sample is smoothly supplied to the tip can be realized.

〔Example〕

An example of a liquid metal ion source will be described below.

FIG. 1 shows a schematic diagram of a light irradiation type liquid metal ion source, in which a liquid metal electrode 16 is placed in a vacuum container 14 while maintaining insulation. A positive bias voltage is applied to the liquid metal electrode 16 by the bias power supply 12 . The liquid metal electrode 16 is electrically heated by the heating power source 11 to keep the metal in a molten state. By providing the bias electrode 15, ions can be extracted efficiently and their spatial divergence can be prevented as much as possible. The kinetic energy of the ions is determined by the voltage applied from the acceleration power source 13.

Light is applied to the liquid metal electrode 16 from an oblique direction using the light introduction window 18 . In the case of vacuum ultraviolet light, the flange of the light introduction window 18 is used to connect the vacuum vessel 14 to a vacuum ultraviolet light source and a spectroscopic system (not shown). Ions emitted radially from the tip of the liquid metal electrode 16 are biased by the bias voltage 8i1.
5, it enters a focusing lens system 17 and is converted into a beam.

Gallium was used as the liquid metal sample 20. The sample reservoir 19 is filled with it. Gallium has a melting point of 29° to 78°C, so it melts easily. Therefore, except for ion implantation, it is the most commonly used liquid metal sample in focused ion beam devices. The evaporation electric field of the -valent ions is 15V/nm.
It is known that When electric evaporation was observed using the liquid metal ion source of the present invention while irradiating dye laser light with a wavelength of 403 nm (3.1 eV), the evaporation electric field decreased to 8 V/nm, which is about 52%. Furthermore, abnormalities in current-voltage characteristics, which were thought to be caused by disturbances in the supply of liquid metal sample to the tip of the tiller cone, were significantly reduced.

〔Effect of the invention〕

As explained above, according to this method of performing field evaporation while irradiating light, field emission of ions becomes possible even with a lower electric field than in the conventional method. As a result, a sufficient ionic current can be obtained under the conditions that the tip diameter of the liquid metal electrode is relatively large and the transport of the liquid metal sample to the electrode tip proceeds more smoothly than in the past. Furthermore, since the power supply device used may be of a low voltage type, the device using electric field evaporation becomes simpler and cheaper.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the liquid metal ion source of the present invention, and FIG. 2 is a diagram showing the principle of field evaporation. 11... Heating power supply, 12... Bias power supply, 13.
... Accelerating power source, 14... Vacuum container, 15... Bias electrode, 16... Liquid metal electrode, 17... Focusing lens system, 18... Light introduction window, 19... Sample reservoir, 20・
...Liquid metal sample.

Claims (1)

[Claims] a sample reservoir containing a liquid metal sample to be ionized; a liquid metal electrode protruding from the sample reservoir and connected to a heating power source; a focusing lens system disposed in front of the liquid metal electrode; A liquid metal ion source comprising a bias electrode disposed between a metal electrode and a focusing lens system, the liquid metal ion source comprising a mechanism for irradiating light onto the surface of the liquid metal electrode.