Focused ion beam

The most common sources of ions are the so-called liquid metal, in which gallium is used. The melting point of gallium is ~ 30 ° C.

In addition to gallium, gold and iridium are also used in the sources. In a gallium source, the heated metal comes into contact with a tungsten needle. Gallium moistens tungsten, and a large electric field (more than 10 ⁸ V / cm ) causes ionization and emission of gallium ions. Then the ions are accelerated to an energy of 5-50 keV and are focused on the sample using an electrostatic lens . In modern installations, the current reaches tens of nanoamps , which focuses into a spot of several nanometers .

The first FIPs were created in the early 90s. The principle of operation of the FIP is similar to the operation of an electron microscope with a small but significant difference - in FIPs an ion beam is used instead of an electron beam.

Gallium ions after acceleration by an electric field collide with a sample. The kinetic energy of the ions is enough to “knock out” ( English sputtering process ) the atoms of the material from the sample. At low currents, a small amount of material is removed. In modern FIPs, a resolution of about 5 nm is achieved ^{[1] [2]} ). At high currents, an ion beam easily cuts a sample with submicron accuracy.

If the sample is made of a non-conductive material, then ions accumulate on its surface, which repel the ion beam. To avoid this, the accumulated charge is neutralized by the flow of electrons. FIPs of the latest developments have their own image system, so there is no need to use an electron microscope to control the processing process ^[3] .

Unlike an electron microscope, FIP “destroys” a sample. When gallium ions hit the surface of the sample, they “tear out” the atoms that make up the sample. During surface treatment, gallium atoms are also implanted into the depth of the sample by several nanometers. The surface of the sample then comes to an amorphous state.

FIP can process the surface of the sample very thinly - it is possible to remove a layer from the surface to a depth equal to the atomic size, without completely affecting the next layer. The surface roughness of the sample after treatment with an ion beam is less than a micron ^{[4] [5]}

The main fundamental difference between FIP and focused electron beam methods (such as SEM , PEM, and ) is the use of ions instead of electrons, which significantly changes the processes on the surface of the sample under study. The most important characteristics for the consequences of interacting with the sample are:

Ions more electrons

• Since ions are larger than electrons, they cannot penetrate so easily into a single atom of a sample. The interaction mainly includes the outer shell and leads to ionization and destruction of chemical bonds of atoms on the surface.
• The penetration depth of ions is much less than the penetration depth of electrons of the same energy.
• The stopped ion in the material is captured by the matrix.

Ions are heavier than electrons

• Since ions are heavier, they can acquire a greater momentum . So, at the same energy as the electron, the momentum of the ion can exceed the momentum of the electron by 370 times.
• At the same energy, ions move slower than electrons, but this is insignificant compared to the scanning speed and in practice does not matter.
• Magnetic lenses are not as effective as for electrons; therefore, electrostatic lenses are used.

Ions have a positive charge, and electrons have a negative charge.

• This difference has insignificant consequences and affects the polarity of the field controlling and accelerating the ion beam.

Thus, ions have a positive charge, are heavy and slow, while electrons are negatively charged, have a small size and mass, and at the same time have a higher speed. The most important consequence of the above properties is that the ion beam will remove atoms from the surface of the sample. In this case, the position of the beam, the residence time and size can be well controlled. Therefore, it can be used for controlled etching, up to the nanometer scale. ^[6]

• Ion source
• Reflected Electron Diffraction (EBSD)
• Scanning electron microscope
• Scanning Helium Ion Microscope
• Ion beam analysis

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