PMA effects on Al/HfO2 high-K PMOS capacitors

A two-step post-metallization annealing (PMA) process has been developed and it has demonstrated its significant benefit for high-k metal gate (HKMG) in reducing post chemical mechanical polish (CMP) Al corrosion defects. After metal gate CMP, samples are firstly treated with N₂O plasma in a 400°C PECVD (plasma enhanced chemical vapor deposition) chamber for surface oxidization, before which the chamber has been cycled with pumping through N₂ and noble gas mixture so as to remove moisture and/or particles on chamber wall. Then, N₂O gas switched off and NH₃/N₂ mixture plasma turned on to nitridize the metal oxides. A multi-variable (e.g. Gas ratio, RF power, plasma treatment time, chamber pressure, etc.) statistical design of experiment (DOE) was performed to optimize processes for growing ultra-thin metal oxynitride (e.g. AlON, TiON, TaON, etc.) on top surface of the metal gate, which will work as the 2nd etch stop layer (ESL) as well. TEM results showed that metal has been dragged upwards desirably and 30~60Å self-aligned insulated film grown upon Al/HfO₂ capacitor, but 50-100Å diffusing layer was also observed unavoidably, which may induce metal loss and lead to a higher electrical resistance. With PMA treatment, PMOS post-CMP corrosion defects have been reduced significantly by ~85% in isolated SRAM (Static Random Access Memory), by ~50% in dense area, and by ~75% at the gate boundary area. The correlation of IOFF v.s IDSAT and IDSAT v.s VDSAT (saturation voltage at sub-threshold regime) demonstrated a comparable performance with that of non-PMA process. In light of the findings from this study, a trade-off between defect reduction and N/O atom diffusion downward metal gate should achieve with further PMA optimization.