CONTROL OF BF2 DISSOCIATION IN HIGH-CURRENT ION-IMPLANTATION

High-current BF2+ implants have been studied under various implant conditions, extraction and acceleration voltages, and vacuum conditions to determine the critical controls required to eliminate (or minimize) dissociation of BF2+ during ion implantation. Dissociated BF2+ ions can produce a higher-energy contaminant which extends the junction depth beyond the desired value and destroys devices. In this study, preamorphized Si wafers were used to minimize the channeling tail and optimize the measurement sensitivity. SIMS analysis was performed to determine the size and position of the unwanted energy-contaminant peak (if any). Results are presented and discussed which show that implants between 20 and 60 keV, where the total energy is provided in one stage, have no BF2 dissociation peak, and for implants between 60 and 80 keV, where the extraction voltage is 60 kV, there is no measurable high-energy contaminant. However, if the extraction voltage is lowered to 20 kV for the equivalent implant, there is a discernible energy contaminant. The size of this peak can vary significantly, depending on the implanter design and pumping capability. In a system with extraction energy limited to low values and/or limited post-analysis pumping, this value can be greater than 5%. In this paper it is shown how this value can be minimized. 130 keV BF2 implants are also reviewed and comparisons made between 50 keV BF2+ and 11.2 keV B+ implants. In addition a second energy-contamination peak associated with the stripping of the dissociated B+ ion into B++ has been observed at higher beam-line pressure and is discussed in detail in this paper.