**1. Introduction**

Ion implantation of semiconductors is the largest application of ion beam processing. In integrated circuits, flat-panel displays, and other devices, precise regions are implanted through masks with n-type and p-type dopants and other materials. Additional applications of ion beams exist, typically requiring high currents and fairly precise control of the ion dose. Ribbon beams are also still used in isotope separator systems, where pure isotopes are required for medical and other purposes. This chapter focusses on parallel DC ribbon ion beams, i.e. beams in which the breadth greatly exceeds its thickness; in this work I shall use the term beam breadth to indicate the major dimension, and thickness to indicate the minor dimension, regardless of system orientation in space. This is consistent with the ribbon analogy.

An excellent review of ion beam technology through its first century was given by Freeman [1]. The first isotope separators were developed by E.O. Lawrence based on his experiments in the 37-inch cyclotron at Berkeley, and used for uranium separation in the Manhattan project in the 1940s [2], using 16-inch broad ribbon beams. The ion beam was generated in an ion source immersed in the same uniform magnetic field used to separate the isotopes; there was therefore a strong and uniform magnetic field in the ion source oriented in the direction of the beam breadth. Electrons from a filament, inside a chamber with a single slot-shaped exit, were trapped by this magnetic field in a tight cylindrical region, where they ionized the uranium-containing vapor, from which the ribbon beam was extracted.

In the 1950s Freeman worked on applying aspects of this technology to peaceful purposes. His ion source used a magnetic field generated by a dedicated magnet which was separate from the analyzer magnet used for isotope separation. He ran a straight tungsten filament down the center of the region where it was desired to ionize the ion source vapor, and electrons were confined in complex pattens is a zone surrounding this filament. René Bernas [3] devised a similar ion source, with a coiled filament at one end of a similar arc chamber.

Freeman at Harwell then collaborated with Lintott Engineering in Horsham, UK, represented in the US by High Voltage Engineering in Massachusetts, to develop ion beam systems operating at tens of keV with mass resolving power around 60–100 to create pure beams of dopants for ion implantation [4]. Lintott's early implanters were not known for reliability. Implanters were then commercialized by several companies in the 1970s including Extrion Corp, founded by Rose and others. The first commercially successful implanters tended to use lower currents than 1 mA, and scanned their ion beams across the surface of small silicon wafers; while usable, this scanning introduced variations in the incident angle, which resulted in variations in the range of the implanted ions, since the ions at certain incident directions could undergo planar or axial channeling in the monocrystalline silicon substrates. These systems often used a cold-cathode Penning ion source, producing a cylindrical beam of a few hundred microamps.

In 1988 Peter Rose founded Nova Associates Inc., with Ryding and Wittkower, to develop high-current implanters, starting with the Nova NV10–80. These took full account of the need for space-charge neutralization to transport currents of 10 mA or more of heavy ions at energies from 10 to 80 keV through a mass analyzer. These used a Freeman ion source, generating a ribbon beam about 44 mm tall and 3 mm thick at the ion source exit. Since a space-charge neutralized beam cannot be scanned electrostatically, they used two-dimensional mechanical scanning to uniformly raster the beam across the wafer surface. A batch of wafers was loaded into a circle onto a disk, which was spun to provide one direction of motion, and translated slowly in a radial direction to provide the other.
