Ion beam

An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. A variety of ion beam sources exists, some derived from the mercury vapor thrusters developed by NASA in the 1960s. The most common ion beams are of singly-charged ions. Ion current density is typically measured in mA/cm^2, and ion energy in eV. The use of eV is convenient for converting between voltage and energy, especially when dealing with singly-charged ion beams, as well as converting between energy and temperature (1 eV = 11600 K). 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. In both cases electrons are needed to generate an ion beam. The most common electron emitters are hot filament and hollow cathode. In a gridded ion source, DC or RF 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 j {displaystyle 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 ≈ 4 ϵ 0 9 2 e m ( Δ V ) 3 2 d 2 {displaystyle japprox {frac {4epsilon _{0}}{9}}{sqrt {frac {2e}{m}}}{frac {(Delta V)^{frac {3}{2}}}{d^{2}}}} , where Δ V {displaystyle Delta V} is the voltage between the grids, d {displaystyle d} is the distance between the grids, and m {displaystyle m} is the ion mass. The grids are placed as closely as possible to increase the current density, typically d ∼ 1   m m {displaystyle dsim 1 mathrm {mm} } . The ions used have a significant impact on the maximum ion beam current, since j ∝ m f r a c − 1 2 {displaystyle jpropto m^{frac-{1}{2}}} . Everything else being equal, the maximum ion beam current with krypton is only 69% the maximum ion current of an argon beam, and with xenon the ratio drops to 55%.