**2. Plasma immersion ion implantation process**

Ion implantation based on linear accelerator technology has been long developed to modify material surfaces. The large implantation areas required by industrial applications and the extended processing times demanded by the treatment have made this implantation modality both expensive and complex, limiting its usefulness. By contrast, PIII technology [Conrad et al. 1987] overcomes many of the linear accelerator shortcomings providing a high ion density in a simple, fast, effective and economical way.

The PIII process has been amply described in several papers [Conrad et al. 1987], [Anders, 2000]. A brief description of it can follow. The sheath that normally surrounds an unbiased conductor (sample) submerged in plasma is characterised by an excess of electrons, no matter that the plasma is initially (*t*=0) in quasi-neutrality. When a voltage pulse, typically a few microseconds long, is applied to the "sample", the sheath is drastically altered and can even vanish momentarily. The comparatively small inertial mass of the electrons allows them to be expelled from the close vicinity of a cathode sheath, or negatively biased "sample", in a very short time. Consequentially, the ion array, or matrix, becomes exposed thanks to the ion's greater inertia. Later on, this charge distribution is enhanced as further electrons are repelled to the point in which the electric field of the biased "sample" is completely shielded. Thus, few centimetres away from the close vicinity of the "sample", the plasma remains unaltered, with the possible exception of the plasma waves created by the bias pulse.

The "sample" bias originates a short distance positive charge gradient and, with it, a potential gradient, namely, the electric field which impulses the ions towards the "sample". Once the ion matrix appears, a steady ion current flow onto the piece, to the extent of the availability of ions in the matrix. As the ions are implanted, the piece emits additional electrons according to its work function and, clearly, to the ion energy. The loss of these electrons extends the sheath by uncovering more ions. The bias pulse width and plasma density are usually adjusted in order, for as many sheath ions as possible, to be implanted into the piece, which is kept, therefore, immerse in the plasma. At the same time, the plasma represents a load to the high voltage pulsed energy supply which bias the work piece. By the end of the few microsecond pulse, the ion matrix is depleted and the system returns to very much the same initial conditions previous to *t*=0.

Conventional beam-line ion implantation has proven to modify significantly the surface properties of different materials. Nevertheless, PIII offers an alternative to conventional beam-line ion implantation. It has shown the advantages of relative simplicity, high ion fluence, the possibility of implanting complex three-dimensional objects, achieving an area treatment independent of the processing time and providing safe low temperature processing. By contrast, PIII is limited by the lack of charge to mass separation, having an implant energy distribution non homogeneous and the generation of X-rays from the production of secondary electrons.
