Semiconductor structures and methods of forming the same

A method, comprising:

• forming a first dielectric layer over a first conductive feature;
• forming a second dielectric layer over the first dielectric layer, the first dielectric layer and the second dielectric layer having different compositions;
• forming a patterned mask over the second dielectric layer, the patterned mask having an opening directly over the first conductive feature;
• performing a first etching process to form a trench extending through the first dielectric layer and the second dielectric layer to expose a top surface of the first conductive feature, wherein etchant of the first etching process modifies a portion of the first dielectric layer exposed by the trench;
• performing a second etching process to remove the patterned mask and the modified portion of the first dielectric layer, wherein etchant of the second etching process further reacts with a part of a remaining portion of the first dielectric layer to cause a volume expansion of the remaining portion of the first dielectric layer;
• and
• forming a second conductive feature in the trench.

A semiconductor structure, comprising:

• a gate structure over a channel region;
• a source/drain feature coupled to the channel region;
• a first dielectric layer over the gate structure;
• a second dielectric layer on the first dielectric layer;
• and
• a gate via extending along the first dielectric layer and the second dielectric layer to couple to the gate structure,
• wherein the first dielectric layer comprises a first portion surrounding a part of the gate via and a second portion surrounding the first portion, and a composition of the first portion is different than a composition of the second portion.