Highly Reliable Rapid Thermal Selective Gate Re-Oxidation Process of Advanced Metal Gate Stacks with Tungsten Electrode

Selective rapid thermal oxidation (RTO) is needed to oxidize a tungsten (W) / tungsten nitride (WN) / poly silicon gate structure after gate patterning. Si/SiO₂ and tungsten can coexist in a gas ambient of up to approximately 20% H₂O in H₂ at temperatures which are suitable for RTO (800degC -1100degC) [1]. We present the details of such a selective thermal re-oxidation process for technology nodes beyond 70 nm including optimization to reduce the thermal budget. Optical measurement of spectral reflectivity of the W films is a simple and superior method to ensure selectivity towards tungsten oxidation in comparison to the previous use of sheet resistance measurements. This is supported by secondary ion mass spectroscopy (SIMS) measurements. Steam-based selective oxidation with an external burner system results in a large process window with respect to H₂O concentration as well as thermal budget.