Plasma-Immersion Ion Implantation

Abstract

Plasma-immersion ion implantation (PIII) is an emerging technology for the surface engineering of semiconductors, metals, and dielectrics. It is inherently a batch-processable technique that lends itself to the implantation of large numbers of parts simultaneously. It thus offers the possibility of introducing ion implantation into manufacturing processes that have not traditionally been feasible using conventional implantation.In PIII the part to be treated is placed in a vacuum chamber in which is generated a plasma containing the ions of the species to be implanted. The plasma based implantation system does not use the extraction and acceleration methods of conventional mass-analyzing implanters. Instead the sample is (usually) repetitively pulsed at high negative voltages (in the 2–300 kV range) to implant the surface with a flux of energetic plasma ions as shown in Figure 1. When the negative bias is applied to a conducting object immersed in a plasma, electrons are repelled from the surrounding region toward the walls of the vacuum chamber, which is usually held at ground potential. Almost all the applied voltage difference occurs across this region, which is generally known as a sheath or cathode fall region. Ions are accelerated across the sheath, producing an ion flux to the entire exposed surface of the work-piece. Because the plasma surrounds the sample and because the ions are accelerated normal to the sample surfaces, implantation occurs over all surfaces, thereby eliminating the need for elaborate target manipulation or masking systems commonly required for beam line implanters. Ions implanted in the work-piece must be replaced by an incoming flow of ions at the sheath boundary, or the sheath will continue to expand into the surrounding plasma.Plasma densities are kept relatively low, usually between 108 and 1011 ions per cm3. Ions must be replenished near the workpiece by either diffusion or ionization since the workpiece (in effect) behaves like an ion pump. Gaseous discharges with thermionic, radio-frequency, or microwave ionization sources have been successfully used.Surface-enhanced materials are obtained through PIII by producing chemical and microstructural changes that lead to altered electrical properties (e.g., semiconductor applications), and low-friction and superhard surfaces that are wear- and corrosion-resistant. When PIII is limited to gaseous implant species, these unique surface properties are obtained primarily through the formation of nitrides, oxides, and carbides. When applied to semiconductor applications PIII can be used to form amorphous and electrically doped layers. Plasma-immersion ion implantation can also be combined with plasma-deposition techniques to produce coatings such as diamondlike carbon (DLC) having enhanced properties. This latter variation of PIII can be operated in a high ionenergy regime so as to do ion mixing and to form highly adherent films, and in an ion-beam-assisted-deposition (IBAD)-like ion-energy regime to produce good film morphology and structure.