Using fluorine implantation techniques, researchers at the U.K. University of Southampton and STMicroelectronics have taken a bipolar transistor to a cut-off frequency of 110GHz, twice as fast as the current record.
“By using fluorine implants, the transistor can operate at a higher frequency which means it will be twice as fast as it was before,” said Professor Peter Ashburn, leader of the research team.
The fluorine implant was used to suppress boron diffusion in the base of a double poly silicon bipolar transistor. This means that the base width is narrower with electrons traveling across it faster.
Secondary ion mass spectroscopy and transmission electron microscopy have shown that retention of fluorine in the silicon is maximized when a high-energy fluorine implant is combined with a low thermal budget inert anneal, said the research team.
“TEM images show that a high-energy fluorine implant into germanium pre-amorphised silicon eliminates the end of range defects from the germanium implant and produces a band of dislocation loops deeper in the silicon at the range of the fluorine implant,” said the researchers who worked in collaboration with STMicroelectronics.
The researchers say there are two possible factors that are contributing to the boron diffusion by the fluorine implant. The first is that the elimination of the germanium end of range defects, and the associated interstitial population by the fluorine implant removes a source of transient enhanced diffusion.
The second factor could be that any interstitials released by the dislocation loops at the range of the fluorine implant would be expected to recombine at the vacancy-fluorine clusters before reaching the boron profile.
The research could pave the way for a boost in performance of transistors. “We have already beaten the world record,” commented Ashburn. “We have just improved the performance of silicon to a level which was only previously possible with silicon geranium.”
Ashburn said there was scope to reduce the boron diffusion by a further 50 percent and looking at other materials that could also enable the diffusion.
Silicon Bipolar Transistor Reaches 110GHz
By Alex Mayhew-Smith, Electronics Weekly
