Liquid-Metal Ion Source (LMIS)
- Practical Electron Microscopy and Database -
- An Online Book -

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A liquid-metal ion source (LMIS), e.g. Ga (gallium) ion source in FIB, can have small emission area and high brightness (e.g. > 106 A/cm·sr), and thus enable small beam diameters and current densities between 2 and 10 A/cm2.

For instance, in focused ion beam (FIB) systems, a reservoir of Ga is normally positioned in contact with a sharp tungsten (W) tip. Figure 2548a shows the schematic illustration of the LMIS and lens system of an FIB. The liquid Ga wets the needle and flows to the W tip. A high extraction electrical field (e.g. >108 V/cm) is applied to pull the liquid Ga into a sharp cone with the radius of 5 - 10 nm. Ions are emitted as a result of field ionization and post-ionization, and then are accelerated down the FIB column. Schematic illustration of the LMIS and lens system of an FIB

Figure 2548a. Schematic illustration of the LMIS and lens system of an FIB.

In FIB, the electrical field between tip and extractor electrode forms a conical surface that is so-called Taylor cone. Figure 2548b shows the Taylor cone shapes of the LMIS in various extractor fields, resulting in different ion beam currents.

Taylor cone shapes of the LMIS in extractor fields

Figure 2548b. Taylor cone shapes of the LMIS in various extractor fields, resulting in ion beam currents of 0 (gray), 5, (blue), 10 (pink), and 20 µA(red), respectively.

Table 2548 lists some materials for liquid metal ion sources (LMISs).

Table 2548. Some materials for liquid metal ion sources.

Alloy
Melting Temperature (°C)
Applications
Ga
29.8
Routine milling, deposition, …
Bi
271
Sputtering; n-type doping in Si

Note that the use of Ga in FIB has many advantages.

 

 

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