Rapid thermal oxidation of sub-3-nm gate dielectrics

05/01/1997

Rapid thermal oxidation of sub-3-nm gate dielectrics

Relative to conventional oxides, oxynitrides offer improved resistance to dopant diffusion, increased reliability, and enhanced hot carrier degradation resistance. Oxynitride growth processes are also self-limiting because Si-N bonds cause the lattice to collapse, and this closely packed network retards diffusion. These characteristics make oxynitrides especially attractive for thin gate dielectrics. According to the SIA National Technology Roadmap, 0.10-?m design rules will require gate dielectrics in the 3-nm thickness range.

Researchers Kiran Kumar and co-workers at the University of Texas`s Microelectronics Research Center, Austin, with John K. Lowell of Applied Materials, writing in a recent issue of Applied Physics Letters, report tight control of oxynitride thickness and consistently low leakage currents for oxides in the 2-3 nm thickness range. The researchers fabricated MOS capacitors, first growing a preoxide layer on cubic zirconia substrates by rapid thermal oxidation (RTO) in NO at 1050?C for various times. Next, the preoxide was reoxidized in O2 or N2O to obtain varying thicknesses. Finally, the group deposited the gate polysilicon layer, doped it in a POCl3 furnace, and etched by wet chemistry.

Leakage currents for the D, E, and F processing conditions (see table) were lower than for pure oxides of the same thickness; leakage was entirely due to quantum mechanical tunneling. Oxide integrity degraded, for O2 reoxidation, as NO preoxidation time increased (processing conditions A, B, and C). The researchers attributed this result to growth of a thin oxygen-rich interfacial layer and reduced interface hardness. Using N2O as the reoxidizing ambient eliminated this degradation. - K.D.