Effects of Nonmelt Laser Annealing on a 5keV Boron Implant in Silicon
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Effects of nonmelt laser annealing on a 5keV boron implant in silicon Susan Earlesa, Mark Lawa, Kevin Jonesa, Rich Brindosa, Somit Talwarb a SWAMP Center, University of Florida b Verdant, San Jose CA Abstract To investigate the effects of ramp rate on the transient enhanced diffusion of boron in silicon, laser thermal processing (LTP) in the nonmelt regime has been investigated. A nonmelt laser anneal has been performed on a 5 keV, 1e15 boron implant. The implant energy of 5keV was chosen to simplify analysis. A rapid thermal anneal (RTA) at 1000oC and furnace anneals at 750 oC were used to show the effect of post annealing on the LTPd samples. Results show the sheet resistance drops by up to a factor of two for samples receiving the nonmelt LTP and the RTA compared with the samples just receiving the RTA. An increase in the hall mobility was also observed for the samples receiving the LTP. The nonmelt LTP was also shown to strongly affect the extended defect density. During post anneals, a higher density of smaller defects evolved in the samples receiving the LTP. Introduction Increasing the ramp up rate during thermal processing has been shown to decrease the transient enhanced diffusion (TED) of boron in silicon. Plots of the ramp up rate versus diffusion length show that the ramp up rate would need to be around 1012 oC /sec to result in a diffusion length of zero, and hence no TED[1,2]. Unfortunately, conventional rapid thermal processing systems have peak ramp up rates of 200-400 oC. However, using a laser for thermal processing results in a ramp up ramp which approaches the 1012 oC /sec that present data suggests is needed for zero TED. Therefore, in order to reduce TED while achieving dopant activation, the following work uses laser thermal processing (LTP) to investigate its effect on boron implanted into silicon. Experimental A 5 keV, 1e15 B+ implant into a CZ grown silicon wafer was processed with a 308nm XeCl excimer laser using a 15ns pulse at energy densities ranging from 0.4 to 0.6 J/cm2. Following the LTP some samples received an RTA for 5sec at 1000°C and some were furnace annealed for 15 min at 750°C. Control samples received the RTA and no LTP. This allows us to observe the effects of the laser preanneal on the boron implant. Al-Si contacts were then evaporated onto square samples which were then annealed at 450oC in nitrogen for 30min to produce ohmic contacts for Hall measurements. These samples were then analyzed using SIMS, Hall Effect, and Plan-view TEM. Results Figure 1 shows the SIMS profiles of the boron as-implanted and following the LTP. As expected, no movement in the boron profile was observed following the LTP when compared to the as-implanted profile. Figure 2 shows the SIMS profiles of the boron asimplanted, following the RTA, and after the LTP and RTA. A comparison of the SIMS
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between the samples receiving just the RTA and the samples receiving the LTP and the RTA shows the junction depth increases from 0.16 to 0.18 µm with very little difference in diffusion for the LTP
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