Low‐Energy Contaminant in B++ Implantation and Its Elimination

Abstract

The problem of low‐energy B+ contamination in the implantation of B++ ions using pre‐analysis type ion implanters is examined and the causes of contamination mechanisms are discussed. Experiments were performed on pre‐analysis‐type medium‐current implanters to investigate the effects on B++ beam purity due to the use of solid boron source and liquid boron source in an effort to lower ion source chamber operating pressure, the use of electrostatic ion beam filter to remove low‐energy contaminants from the dissociation of B2+ molecules if any, the use of aluminum waveguide liner to reduce gas desorption, and the effects of improved vacuum after cryopump regeneration. Experimental results indicated that, despite all these improvements, uncontaminated B++ implants cannot be assured for either BF3 gas or solid LiBF4 source. The secondary electrostatic analysis is demonstrated to be a sensitive technique for setup and monitoring of beam purity of doubly charged ion implants, and essentially uncontaminated B++ implants have been achieved with either BF3 gas source or the LiBF4 solid source. This was previously impossible without the refined secondary electrostatic analysis technique.