Low temperature dopant activation for applications in thin film silicon devices

One of the major areas of research for integrated electronic systems is the development of systems on glass or plastic to optimize the performance/cost tradeoff. These new substrate materials impose stringent constraints on electronic device fabrication, including limitations on chemical and thermal processes. Processes that do not use temperatures greater than 900°C have the increased flexibility for application involving new substrate materials. Silicon is a semiconductor material that can have very different conductive properties based on the levels of impurities. A conventional method of adding impurities is ion implantation. When a substrate is implanted, the ions will break up the ordered crystal lattice and induce damage in the substrate. Interstitial impurities cannot contribute to conductivity; therefore thermal activation is critical for device operation. Annealing is a thermal process that serves two purposes; to re-crystallize the substrate, and to electrically activate the dopant ions. The mechanism of dopant activation in silicon under low-temperature (600°C) annealing conditions is re-crystallization. By exploring rapid thermal annealing (RTA) and furnace processing, a physical model of activation is presented for three dopant ions (boron, phosphorus, and arsenic) over a wide dose range. Sheet resistance and spreading resistance profiling (SRP) have been used to characterize the electrical activation of dopants. Secondary ion mass spectroscopy (SIMS) and x-ray diffraction analysis have been used to determine the distribution of the implanted impuries. Results indicate that eighty to ninety percent of the dopant can be activated at the reduced temperature of 600°C; dependent on the dose implanted