Absorption of carbon from residual gases during Ti implantation of alloys

Secondary ion mass spectrometry (SIMS) has been used to analyze the surface composition of several steel and Ni-plated substrates implanted with Ti ions (5×1017 Ti/cm2 at 190 keV). SIMS showed clearly the carbonaceous subsurface layer previously reported in Auger and nuclear backscattering analyses of Ti-implanted Fe-based alloys. Moreover, substrates implanted in a vacuum which was backfilled with isotopic 13CO gas showed 20 to 50× higher mass 13/ mass 12 ratios for surface concentrations and integrated subsurface doses than substrates implanted in normal vacuum atmospheres at pressures near 10−6 Torr. Auger analysis confirmed the subsurface C profile and found no O (〈1 at %) below the oxide layer. The experiments provide direct evidence that C atoms can be absorbed from residual gas molecules in a vacuum chamber near room temperatures during implantation of carbide-forming ions. A second possible source of C atoms is also discussed.