Referring to FIG. 16D, the composition of the Vt modulation layer 218 (including portions of the capping layer 216 and portions of the sacrificial layers 205) is consisting with that discussed with respect to the FIGS. 5A and 5B. In the present embodiments, the capping layer 216 and the sacrificial layers 205 both include Si1-xGex (0.4<x<1) with the same Ge %. Alternatively, the Ge % in the capping layer 216 may be different from that in the sacrificial layers 205. For example, the Ge % in the capping layer 216 may be greater than that in the sacrificial layers 205. The Ge % is used to fine-tune (or modulate) the Vt since the value of Vt increases with the increasing Ge % (e.g., the value of Vt is proportional to the Ge %). In one example, the tuning range of Vt increases by 2.5 times when the Ge % increases from about 40% to about 100%. In another example, the Vt is fine-tuned (or modulated) in a range of about ?20 mV to about ?50 mV.
At operation 126, referring to FIGS. 17A-20B collectively, the method 100 forms gate stacks 620. Structures and compositions of various components of the gate stacks 620 are consistent with discussion with respect to FIGS. 5A and 5B and will thus be omitted for the discussion of the method 500 below for purposes of simplicity.
