A liquid metal ion source (LMIS) is an

ion source An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines. Electron ionization Electro ...

which uses

metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typi ...

that is heated to the liquid state and used to form an

electrospray The name electrospray is used for an apparatus that employs electricity to disperse a liquid or for the fine aerosol resulting from this process. High voltage is applied to a liquid supplied through an emitter (usually a glass or metallic capilla ...

to form

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

s. An electrospray

Taylor cone A Taylor cone refers to the cone observed in electrospinning, electrospraying and hydrodynamic spray processes from which a jet of charged particles emanates above a threshold voltage. Aside from electrospray ionization in mass spectrometry, th ...

is formed by the application of a strong electric field and

s are produced by field evaporation at the sharp tip of the cone, which has a high electric field. Ions from a LMIS are used in

ion implantation Ion implantation is a low-temperature process by which ions of one element are accelerated into a solid target, thereby changing the physical, chemical, or electrical properties of the target. Ion implantation is used in semiconductor device fa ...

and in

focused ion beam Focused ion beam, also known as FIB, is a technique used particularly in the semiconductor industry, materials science and increasingly in the biological field for site-specific analysis, deposition, and ablation of materials. A FIB setup is a s ...

instruments. Typically

gallium Gallium is a chemical element with the symbol Ga and atomic number 31. Discovered by French chemist Paul-Émile Lecoq de Boisbaudran in 1875, Gallium is in group 13 of the periodic table and is similar to the other metals of the group ( alum ...

is preferred for its low melting point, low

vapor pressure Vapor pressure (or vapour pressure in English-speaking countries other than the US; see spelling differences) or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phase ...

, its relatively unreactive nature, and because the gallium ion is sufficiently heavy for

ion milling An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...

Development

The LMIS technique originated in the development of

colloid thruster A colloid thruster (or "electrospray thruster") is a type of low thrust Electrically powered spacecraft propulsion, electric propulsion rocket engine that uses electrostatic acceleration of charged liquid droplets for propulsion. In a colloid thrus ...

spacecraft A spacecraft is a vehicle or machine designed to spaceflight, fly in outer space. A type of artificial satellite, spacecraft are used for a variety of purposes, including Telecommunications, communications, Earth observation satellite, Earth ...

propulsion system Propulsion is the generation of force by any combination of pushing or pulling to modify the translational motion of an object, which is typically a rigid body (or an articulated rigid body) but may also concern a fluid. The term is derived from ...

s. Research beginning in the early 1960s showed that liquid metal can generate large numbers of ions. By the early 1970s, these results spawned the development of LMIS ion microprobes. Initially, in the development of this technique, the liquid metal was supplied by a

capillary A capillary is a small blood vessel from 5 to 10 micrometres (μm) in diameter. Capillaries are composed of only the tunica intima, consisting of a thin wall of simple squamous endothelial cells. They are the smallest blood vessels in the body: ...

tube. This method can be difficult to control at low emission currents. The technique of "blunt-needle" LMIS was discovered by accident in the early 1970s. For this method a thin-film of liquid metal is allowed to flow to the apex of a sharp needle.

Focused ion beam

Most focused ion beam instruments use a liquid-metal ion sources (LMIS) often with gallium. In a gallium LMIS, gallium metal is placed in contact with a

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isol ...

needle and heated gallium

wets During the 1980s, members of the moderate wing of the British Conservative Party who opposed some of the more hard-line policies of Prime Minister Margaret Thatcher were often referred to by their opponents as "wets". Thatcher coined the usage i ...

the tungsten and flows to the tip of the needle where the opposing forces of surface tension and electric field produce the cusp shaped Taylor cone. The tip radius of this cone is ~2 nm. The electric field at this small tip is typically greater than 1 x 10⁸ V/cm and causes ionization and field emission of the gallium atoms. The ions are then accelerated to an energy of 1–50

keV Kev can refer to: Given name * Kev Adams, French comedian, actor, screenwriter and film producer born Kevin Smadja in 1991 * Kevin Kev Carmody (born 1946), Indigenous Australian singer-songwriter * Kev Coghlan (born 1988), Scottish Grand Prix mot ...

and focused onto the sample with

electrostatic lens An electrostatic lens is a device that assists in the transport of charged particles. For instance, it can guide electrons emitted from a sample to an electron analyzer, analogous to the way an optical lens assists in the transport of light in an op ...

es. LMIS produces a high current density ion beam with a small energy spread and can deliver tens of nanoamperes of current to a sample with a spot size of a few nanometers.

References

{{Reflist Ion source