UMC Extends Reliability of Gate Dielectrics

While it is clear that high-k dielectrics and metal gates eventually will be needed for ULSI transistors, in the meantime, device manufacturers continue to push present transistor-level schemes using nitrided oxides and polysilicon gates.

Researchers at UMC Corp. (Hsinchu, Taiwan) recently developed a new gate dielectric process that involves remote plasma nitridation of an N₂O-grown oxide, followed by RTA in NO, which delivers oxides with superior interface properties, reduced leakage current and improved reliability relative to other gate dielectrics of similar thickness (~1.4 nm) for the 100 nm device node. The process differs from other nitridation schemes (NO nitrided oxides or remote plasma nitrided [RPN] oxides) in that it combines three steps to optimize performance. A comparison with RPN, NO and reoxidized RPN oxides found superior interface properties, including effective channel mobility, smooth interfaces and good nitrogen concentration near the poly/oxide interface with the N₂ O/RPN/NO process. The oxides also exhibited reduced leakage current and superior reliability, including stress-induced leakage current (SILC), hot carrier injection and negative bias temperature instability.

The UMC researchers found that higher nitrogen concentrations (1.3 × 10¹⁵cm^-2) can be introduced with the N₂O/RPN/NO process than with other schemes. Smoother interfaces were evidenced by TEM inspection and the channel mobility data, because mobility is limited by surface roughness. Gate leakage is the lowest among all techniques and is reduced by 100× relative to NO processing alone at 1 V (W/L = 200/2 µm). The N₂O/RPN/NO process demonstrated the smallest SILC, the longest time to soft breakdown and tighter dielectric breakdown distributions. For both p and n MOSFETs, the N₂O/NPN/NO process yielded better immunity to hot carrier injection degradation.

For more details, see the proceedings from the 2002 IEEE International Reliability Physics Symposium.