Sealed-interface local oxidation technology

A new regime of local oxidation, dubbed SILO for Sealed-Interface Local Oxidation, is explored. In SILO processing, a film of silicon nitride is in intimate contact with the silicon surface. The ubiquitous native oxide is effectively eliminated by using nitrogen ion implantation into silicon or plasma-enhanced nitridation to form a "sealing film" of approximately 100 Å in thickness. The oxidation rate of both types of films is characterized and found to be nearly equivalent. A 100-Å film can mask the growth 0f 7000 Å of oxide in wet oxygen at 950° C. With a sealed interface it is found that the usual "bird's beak" formation is completely suppressed in local oxidation. An approximate theoretical analysis shows that even a very thin interfacial oxide, acting as a lateral diffusion path for the oxidant species, can lead to a significant bird's beak. With a sealed interface using a 90-Å film, the thick-oxide to bare-silicon transition region is chisel shaped, with approximately 45° slopes. The transition region is even more abrupt if a conventional LPCVD nitride film is deposited on the sealing film before patterning. However, for total nitride thicknesses greater than about 300 Å, defects are generated along the pattern edges aligned in [110] directions. Crystal damage generated during oxidation is found to be due to the intrinsic stress in the LPCVD nitride film. Argon-ion implantation into LPCVD nitride is found to be effective in reducing the defect density. A defect-free abrupt profile is produced by combining SILO with a nitride-oxide sandwich.