Dopant Implantation and Activation in Polycrystalline-SiGe

Abstract

ABSTRACTThe activation of dopants and the evolution of the microstructure of poly-SiGe films during the activation. anneal are of interest for applications such as removable diffusion sources for shallow junctions in CMOS circuit processing, low resistance contact layers and elements for thin film transistors in active matrix flat panel displays. To study these effects, 2300Å poly-SiGe films with Ge content between 20 and 48% were deposited on 600Å poly-Si seed layers by APCVD on thermally oxidized Si substrates. Growth temperatures were between 600 and 700°C. These films were then implanted with B and As at energies of 40 and 80 keV, respectively, at two different doses of 5 × 1013/cm2 and 5 × 1016/cm2. To activate the dopants, the samples were furnace annealed at temperatures from 600 to 1000°C in an N2 ambient. Additional samples were rapidly annealed at temperatures between 800 and 1050°C for times up to 120 seconds. The sheet resistance, measured using a four-point probe, of 5 × 1016 /cm2 arsenic implanted films increased by a factor of two as the Ge content in the film increased from 20 to 48%. The boron doped film, on the other hand, showed no increase in sheet resistance with Ge content. TEM showed that in all cases the grain size increased after anneal. However, the grain sizes of the annealed, arsenic doped samples were on average 5 times larger than those of the boron-doped samples with the same anneal. The sub-grain structure of these films also changed after implantation and annealing. In particular, the twin planes increased in size in boron doped samples and sub-grain twinning virtually disappeared in the arsenic doped samples. In 5 × 1016 /cm2 As implanted samples, precipitates were formed which may indicate that the As solubility limit in the films has been exceeded. These precipitates are believed to be As or Asrich and could be responsible for the sheet resistance increase with Ge fraction, since the maximum solubility for As in Ge is lower than in Si by about a factor of ten.