Evidence from SIMS of Doubly-Charged Boron Contamination During Singly Charged-Ion Implantation

Abstract

When a dopant is introduced into a semiconductor material by ion implantation, it is sometimes desirable to accelerate and implant the ion in a multiply-charged state. This has the effect of increasing the energy and range of the ion without increasing the accelerating potential. Most modern ion implanters are of the pre-analysis type. In this design the ions are first accelerated through a modest extraction potential, e.g., 25 keV. This is followed by deflection for mass-to-charge selection in an analyzer magnet, after which the selected ions undergo final acceleration. Charge-exchange reactions between the doubly-charged ions and residual gas have been found to occur between the analyzing magnet and the final acceleration section. These reactions produce singly-charged ions that receive only half of the energy of the doubly-charged ions during final acceleration. For the case of B++ implantation the resulting implant profile shows a shallow-depth shoulder due to B+, the amplitude of which may be greater than 50% of the main peak.