Low Energy Beam Transport (LEBT) Systems
The ion source is only the first component in an accelerator system. Typically the ion source is electrically isolated from ground and “floats” on a voltage of up to several hundred kilovolts (alternatively the source could be grounded and the “target” floating). This voltage gives energy to the ions produced in the ion source. The energetic ions are focused, accelerated, analyzed, and transported to the apparatus that utilizes the ions for a specific purpose. For example, ion implantation is a common method used to alter the properties of semiconductors. Directly implanting the ions into the semiconductors is much more efficient and faster than waiting for the atoms to diffuse into the bulk substrate at elevated temperatures (up to several hundred degrees C). The implantation depth depends on the energy of the ions–the higher the energy, the greater the depth of penetration. Successful ion implantation requires specific properties of the beam. For example the beam ions should have the same energy and have parallel trajectories (within 0.1° or so). The beam exiting the ion source generally has very different properties. Most often the ion source beam is comprised of a variety of different elements in a variety of different ionization states. Therefore some form of low energy beam transport (LEBT) is employed to separate the ions of interest from the remainder. This LEBT is also used to focus the beam ions into the acceleration device.
Accelerator applications require ion beams to be matched into the acceptance of the accelerator. Matching means that the transverse (horizontal and vertical) sizes and convergence angles of the beam are correctly matched to the acceptance of the accelerator. If more than one ion species is contained in the beam, it may also be necessary to eliminate the unwanted ions.
Low energy beam transport (LEBT) systems typically employ a variety of diagnostic instruments to measure different properties of the ion beam. Typical diagnostic instruments include beam profile monitors, species analyzers, emittance scanners, and current measurement devices. LEBTs also employ devices to focus and/or bend the ion beam. Most often these devices are magnetic elements, but electrostatic focusing and bending are also used. Solenoid magnets are effective low energy focusing elements because they have equal focal strength in both horizontal and vertical planes. Quadrupole magnets are also used, but are utilized either in pairs or triplets to provide effective focusing in both transverse planes. Like solenoids, electrostatic cylindrical lenses, often called Einzel lenses, have equal focusing in the transverse planes. However the focal properties of electrostatic fields can be strongly affected by the effective charge density in the beam. Experience has shown that electrostatic focusing becomes ineffective for 20-40 keV proton beams when the beam current exceeds a few milliamperes.
LEBT design requires a thorough understanding of the beam requirements at the end of the system. Usually a number of different LEBT configurations may be employed to produce the same beam properties, however the system design always represents a compromise between flexibility, ease of operation, complexity, and cost. In addition, many LEBTs are located behind shielding walls and therefore need to be remotely controlled.
Experience in LEBT design is important because, in general, the properties of the ion beam are not always well known. For example, the beam is comprised of charged particles (either ions or electrons). The electrostatic repulsion between like charges tends to push all beam particles away from one another. This effect increases with increasing beam current. Left unchecked, the beam simply expands at an increasing rate as the space-charge forces accelerate the beam particles away from each other. At modest vacuum pressures (~10-6 Torr), the beam ionizes residual gas atoms in the vacuum system. The positively charged ions are quickly kicked out of the beam path by electrostatic repulsion with the ions in the beam (alternatively the electrons are kicked out by repulsion from an electron beam). The residual electrons (ions) become trapped in the electrostatic fields of the beam thereby reducing the effective charge in the beam path. These trapped ions (electrons) can neutralize 99% or more of the space charge of the beam. The degree of neutralization depends on the particular LEBT design and the residual vacuum pressure. Space-charge neutralization cannot occur at vacuum pressures less than 10-7 Torr and in systems with electrostatic focusing and/or bending elements. Note also that ionization of residual gas atoms can overload electrostatic focusing elements by producing an electric current from the residual gas. Such currents are particularly troublesome for positively biased electrodes because electrons drawn from the residual gas can overload the power supplies biasing these electrodes.
Typical LEBT systems for ion implanters utilize a pair of dipole magnets. The first is used to separate the desired ion species from the remainder of the beam ions. The second is used to correct aberrations produced in the first and produce the parallel ion trajectories needed for implantation. The unwanted ions are dumped onto a pair of collimator slits downstream of the first magnet. Beam steering, focusing, and additional collimators are often located between the two dipole magnets.
Typical LEBT systems for accelerator applications use solenoid focusing (for high beam current) and electrostatic focusing (for lower beam current). Solenoids are preferred because only one “knob” is needed to adjust the focusing strength. Magnetic quadrupoles have been used, but the tuning of a quadrupole LEBT is complex.