Ion beam

 ( Plasma Technology And Applications ) 

The ion current density $j$ that can be accelerated using a gridded ion source is limited by the space charge effect, which is described by Child's law: $$j\; \backslash approx\; \backslash frac\{4\backslash epsilon\_0\}\{9\}\; \backslash sqrt\{\backslash frac\{2e\}\{m\}\}\; \backslash frac\{(\backslash Delta\; V)^\{\backslash frac\{3\}\{2\}\}\}\{d^2\},$$ where $\backslash Delta\; V$ is the voltage between the grids, $d$ is the distance between the grids, and $m$ is the ion mass.

The grids are spaced as closely as possible to increase the current density, typically $d\backslash sim\; 1\backslash \; \backslash mathrm\{mm\}$. The ions used have a significant impact on the maximum ion beam current, since $j\; \backslash propto\; m^\{-\{1\}/\{2\}\}$. All else being equal, the maximum ion beam current with krypton is only 69% of the maximum ion current of an argon beam; with xenon the ratio drops to 55%.

Ion beam application, etching, or sputtering, is a technique conceptually similar to sandblasting, but using individual atoms in an ion beam to ablate a target. Reactive ion etching is an important extension that uses chemical reactivity to enhance the physical sputtering effect.

In a typical use in semiconductor manufacturing, a Photomask can selectively expose a layer of photoresist on a substrate made of a semiconductor material, such as a silicon dioxide or gallium arsenide wafer. The wafer is developed, and for a positive photoresist, the exposed portions are removed in a chemical process. The result is a pattern left on the surface areas of the wafer that had been masked from exposure. The wafer is then placed in a vacuum chamber, and exposed to the ion beam. The impact of the ions erodes the target, abrading away the areas not covered by the photoresist.

Focused ion beam (FIB) instruments have numerous applications for characterization of thin-film devices. Using a focused, high-brightness ion beam in a scanned raster pattern, material is removed (sputtered) in precise rectilinear patterns revealing a two-dimensional, or stratigraphic profile of a solid material. The most common application is to verify the integrity of the gate oxide layer in a CMOS transistor. A single excavation site exposes a cross section for analysis using a scanning electron microscope. Dual excavations on either side of a thin lamella bridge are utilized for preparing transmission electron microscope samples. Giannuzzi, Lucille A., Stevie, Fred A. Introduction to Focused Ion Beams: Instrumentation, Theory, Techniques, and Practice, Springer 2005 – 357 pages

Another common use of FIB instruments is for design verification and/or failure analysis of semiconductor devices. Design verification combines selective material removal with gas-assisted material deposition of conductive, dielectric, or insulating materials. Engineering prototype devices may be modified using the ion beam in combination with gas-assisted material deposition in order to rewire an integrated circuit's conductive pathways. The techniques are effectively used to verify the correlation between the CAD design and the actual functional prototype circuit, thereby avoiding the creation of a new mask for the purpose of testing design changes.

Ions beams are also used for analysis purposes in Materials science. For example sputtering techniques can be used for surface analysis or depth profiling by performing secondary ion mass spectrometry. It is also possible to gain information from the spectroscopy of transmitted or backscattered primary ions, e.g. depth profiles can be obtained from Rutherford backscattering (RBS) spectra. In difference to secondary ion spectroscopy scattering based techniques like RBS are often less destructive to the sample.

• Ion source
• Ion thruster
• Ion wind

• Stopping parameters of ion beams in solids calculated by MELF-GOS model
• ISOLDE – Facility dedicated to the production of a large variety of radioactive ion beams located at CERN