Laser Thermal Processing of Alternate Dopants in Silicon
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Laser Thermal Processing of Alternate Dopants in Silicon Mark H. Clark, Kevin S. Jones, Michael Rendon1 and Kevin A. Gable, University of Florida, Department of Materials Science and Engineering, 525 NEB #33 Gainesville, FL 326-6130, U.S.A. 1 Motorola Advanced Products R&D Lab, 3501 Ed Bluestein Blvd. MD-K10 Austin, TX 78721, U.S.A. ABSTRACT Traditionally, the choice of dopant has been limited to those species with the highest solid-solubility, however, Laser Thermal Processing (LTP) is not fundamentally limited by solid-solubility. Therefore, it is of interest to evaluate alternate dopants that have previously been excluded due to low solid-solubility. To this end, alternate dopants of 14 N, 121Sb (n-type), 27Al, 70Ga, and 115In (p-type) are compared to conventional dopants of As and B respectively, after LTP and post-LTP thermal processing. Dopants were implanted into silicon wafers of opposite background doping type that had previously been amorphized to a depth of approximately 300 angstroms by a 15 keV 28Si+ implant of 1x1015/cm2 dose. An implant energy of 5 keV was sufficiently low to confine the implanted ions to the amorphous layer, with the exception of B, which required an energy of 2 keV. All species were implanted at doses of 1x1014, 5x1014 and 1.5x1015/cm2. Samples were LTP utilizing a 308 nm, 18 ns laser pulse with a fluence of 0.680 J/cm2. Post-LTP thermal processing of the samples consisted of a 900 oC rapid thermal anneal (RTA) in a nitrogen ambient for a duration ranging from spike to 300 seconds. Measurements of the sheet resistance, mobility and carrier concentration were taken after both LTP, and the post-LTP thermal processing. Experimental results show that LTP of alternate dopants increases the electrically active carrier concentration of Ga, Al and Sb above solid-solubility. Additionally, the amount of deactivation upon postLTP thermal processing depends on the alternate dopant species. INTRODUCTION Variations of Laser Thermal Processing (LTP) have been studied to varying degrees for over thirty years. More recently, LTP has received renewed interest for the unique advantages it offers over conventional ion implantation coupled with rapid thermal annealing (RTA) for fabricating highly activated, ultra-shallow junctions necessary for future technology nodes. Traditionally, electrical activation has been limited by the solidsolubility of the dopant species in the silicon lattice. Therefore, in order to maximize the electrically active concentration, dopant species are chosen with the highest solidsolubility. In contrast, due to the rapid rate of the liquid-phase-epitaxy, LTP makes it possible to incorporate electrically active dopants in excess of solid-solubility, thus resulting in a low sheet resistance 1. Consequently, LTP is not fundamentally limited by solid-solubility and thus allows consideration of unconventional/alternate dopants that have been historically excluded due to low solid-solubility. By utilizing LTP, unconventional dopants may be electrically activated to sufficient concen
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