2.4 Ion Implantation and Diffusion

Dopant atoms are deposited on wafer surface and then diffused in at high temperature. The diffusion time and temperature as well as dopant concentration on surface must be controlled to obtain desired dopant profile.


Fig 8. Ion Implantation. Picture on right shows approach of ions toward wafer.


Current approach to doping is ion implantation. Ions of dopant atoms are accelerated to a high velocity in an electric field and impinge on target wafer. The ions penetrate through the oxide layer and enter into silicon. Penetration depths of 500 to 5000 A are easily achieved. Penetration depth depends on size of ion and energy applied. The ions do not penetrate the photoresist layers which are typically 10,000 A thick. By manipulating the acceleration voltage, the average implantation depth can be precisely controlled. The dopant concentration can be carefully controlled by monitoring implantation current. Ion implantation technique enables us to introduce dopants at low temperature and it allows the incorporation of a variety of atoms. Disadvantage being slow speed and cost. During high velocity impact of dopant atoms, the underlying crystal is damaged and a post implantation anneal at 800 C for 30 minutes is often required to remove the created defects, prior to profiling dopant concentration by a diffusion step.