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Anion beam is a beam ofions, a type ofcharged particle beam. Ion beams have many uses inelectronics manufacturing (principallyion implantation) and other industries. There are manyion beam sources, some derived from themercury vapor thrusters developed byNASA in the 1960s. The most widely used ion beams are of singly-charged ions.
Ion current density is typically measured in mA/cm2, and ion energy inelectronvolts (eV). The use of eV is convenient for converting between voltage and energy, especially when dealing with singly charged ion beams.[1]
Most commercial applications use two popular types of ion source, gridded and gridless, which differ in current and power characteristics and the ability to control ion trajectories.[1] In both caseselectrons are needed to generate an ion beam. The most common types ofelectron emitter arehot filament and hollowcathode.
In a gridded ion source,DC orRF discharge are used to generate ions, which are then accelerated and decimated using grids and apertures. Here, the DC discharge current or the RF discharge power are used to control the beam current.
The ion current density that can be accelerated using a gridded ion source is limited by thespace charge effect, which is described byChild's law:where is the voltage between the grids, is the distance between the grids, and is the ion mass.
The grids are spaced as closely as possible to increase the current density, typically. The ions used have a significant impact on the maximum ion beam current, since. All else being equal, the maximum ion beam current withkrypton is only 69% of the maximum ion current of anargon beam; withxenon the ratio drops to 55%.[1]
In a gridless ion source, ions are generated by a flow of electrons, without grids. The most common gridless ion source is theend-Hall ion source, with which the discharge current and the gas flow are used to control the beam current.
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Ion beams can be used for material modification (e.g. bysputtering or ion beam etching) and forion beam analysis.
Ion beam application, etching, or sputtering, is a technique conceptually similar tosandblasting, but using individual atoms in an ion beam toablate a target.Reactive ion etching is an important extension that uses chemical reactivity to enhance the physical sputtering effect.
In a typical use insemiconductor manufacturing, amask can selectively expose a layer ofphotoresist on asubstrate made of asemiconductor material, such as asilicon dioxide orgallium arsenidewafer. 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 avacuum 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.[2]
Another common use of FIB instruments is fordesign verification and/orfailure 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 performingsecondary 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 fromRutherford backscattering (RBS) spectra.[2] In difference to secondary ion spectroscopy scattering based techniques like RBS are often less destructive to the sample.
Inradiobiology a broad orfocused ion beam is used to study mechanisms of inter- and intra- cellular communication,signal transduction and DNA damage andrepair.
Ion beams are also used inparticle therapy, most often in the treatment of cancer.
Ion beams produced by ion and plasma thrusters on board a spacecraft can be used to transmit a force to a nearby object (e.g. another spacecraft, an asteroid, etc.) that is irradiated by the beam. This innovative propulsion technique namedIon Beam Shepherd has been shown to be effective in the area of active space debris removal as well as asteroid deflection.
High-energy ion beams produced byparticle accelerators are used inatomic physics,nuclear physics andparticle physics.
Ion beams can theoretically be used to make a weapon, but this has not been demonstrated. Electron beam weapons were tested by the U.S. Navy in the early 20th century[citation needed], but thehose instability effect prevents them from being accurate at a distance of over approximately 30 inches.
