Gate sidewall structures of semiconductor devices and methods of forming the same

A method, comprising:

• depositing a dummy gate material layer over a first fin-shaped active region;
• patterning the dummy gate material layer to form a dummy gate electrode, wherein the dummy gate electrode has a footing feature at an interface between the first fin-shaped active region and the dummy gate electrode;
• oxidizing the footing feature and a sidewall portion of the dummy gate electrode to form a dielectric gate spacer;
• and
• replacing a remaining portion of the dummy gate electrode with a gate structure.

A semiconductor structure, comprising:

• source/drain features coupled to a channel region of a fin;
• a gate structure over a substrate and comprising a first portion disposed directly over the channel region and a second portion immediately adjacent to the first portion;
• a first gate spacer comprising a first portion extending along a sidewall surface of the first portion of the gate structure and a second portion extending along a sidewall surface of the second portion of the gate structure,
• wherein, in a cross-sectional view cut through the second portion of the gate structure without cutting through the channel region, the second portion of the first gate spacer comprises a non-uniform thickness.