Thermal diffusion and ion implantation of impurities into silicon.

Ion implantation is a lower temperature process than thermal diffusion in which very precise numbers of impurity atoms are implanted into a wafer from a high energy ion beam. The ions enter the semiconductor with high velocity and dissipate their energy through a series of collisions. In the process, they travel a distance R, called the range, which is larger than the perpendicular distance penetrated below the surface of the wafer before coming to rest. The perpendicular penetration depth, called the projected range R_p, depends on the energy of the incident ion on impact with the wafer (Figure 8). Since the collisions are random, there will be a range of depths over which the ions are distributed varying predominately from R_p-

R _p to R_p+

R_p.

R_p is called the projected straggle. Also, there is some scattering of the ions in the direction normal to the axis of incidence as they penerate into the semiconductor. This is measured by the lateral straggle

Figure 8: Schematic diagram to show the range, the projected range R_p, and the projected (R_p) and lateral R straggle in ion implantation.

A schematic (and greatly simplified) diagram of an ion implanter is shown in Figure 9. Its consists of five main components; (i) the ion source where appropriate ion species are generated; (ii) the analyser magnet which removes or filters out unwanted ions; (iii) the ion beam accelerator column which gives the selected ion species its ebergy; (iv) the horizontal and vertical scanning electtrodes which deflect the ion beam electrostatically and allow it to be scanned, or rastered, across the wafer; (v) the target which holds the wafer. The whole unit is housed inside a large vacuum chamber and wafers are transferred into and from the implanter one at a time.

Figure 9: Schematic diagram of an ion implantation system.