FinFET with dummy fins and methods of making the same

A semiconductor structure includes a semiconductor fin protruding from a substrate and oriented lengthwise along a first direction, a dielectric fin disposed over the substrate and oriented lengthwise along a second direction perpendicular to the first direction, where the dielectric fin defines a sidewall of the semiconductor fin along the second direction and where the dielectric fin includes a first dielectric layer disposed over a second dielectric layer that differs from the first dielectric layer in composition, and a metal gate stack disposed over the semiconductor fin and oriented lengthwise along the second direction.