Effect of BF2+ induced Mo++ on long retention time in 0.25 μm DRAM technology

The aim of this work was to establish if molybdenum (Mo) is detrimental to DRAM performance. If so, to determine the limits for the ion source parameters and in turn for the beam currents used in BF₂ ⁺ implants, in order to prevent deleterious levels of Mo occurring. The effect of increased beam current has also been investigated. The implanted element profiles and their diffusion following RTA have been analysed by SIMS, the damage level by XTEM, while MOS test structures were used to analyse relevant electrical parameters and cell retention time. Reliability tests were finally performed on the wafers with the highest Mo level. Mo peak concentration of above 1E19/cc was measured in the wafers implanted with the highest Arc Current (Arc I). The main source parameter responsible for the high Mo⁺⁺ production was found to be the Arc I. Tests performed on wafers implanted at different Arc I, show a deleterious effect of Mo on the retention time of DRAM cells. However, provided an appropriate Arc I limit is used, there is no need for the more expensive and more maintenance dependent tungsten ion source