Impact of in situ NH₃ preannealing on sub-100 nm tungsten polymetal gate electrode during the sealing nitride formation

We investigated the effects of in situ NH₃ preannealing immediately before sealing nitride deposition on the wordline sheet resistance (R_s) of a tungsten polymetal gate electrode (W/WN_x/poly- Si). While in situ NH₃ preannealing is very effective for the suppression of W-O whisker formation, it was found that excessively long-time NH₃ preannealing could cause an unexpectedly abnormal increase in the wordline R_s of short gate length electrodes, typically < 100 nm. Since X-ray diffraction (XRD) and secondary ion mass spectroscopy (SIMS) analyses reveal that long-time NH₃ preannealing results in the nitridation of tungsten sidewall surfaces, the wordline R_s increase in a small-gate-length sample subjected to long-time NH₃ preannealing is mainly due to tungsten sidewall nitridation, which is localized at the surface region of the tungsten sidewall. In addition, at a long-time NH ₃-preannealed gate electrode, an abnormal inflation of the tungsten sidewall occurred due to a slight increment in lattice spacing by tungsten nitridation.