Metal silicide for contact and methods of forming same

A method comprising:

• providing a structure comprising a source/drain feature and an interlayer dielectric (ILD) layer disposed over the source/drain feature;
• forming a contact trench in the ILD layer to expose a top surface and a sidewall of the source/drain feature;
• forming a first silicide layer comprising a first horizontal portion on the top surface of the source/drain feature and a first vertical portion on the sidewall of the source/drain feature;
• forming a second silicide layer on the first silicide layer, wherein the second silicide layer comprises a second horizontal portion on the first horizontal portion and a second vertical portion along the first vertical portion;
• and
• forming a metal fill layer in the contact trench,
• wherein the first horizontal portion has a first thickness, the first vertical portion has a second thickness, the second horizontal portion has a third thickness, and the second vertical portion has a fourth thickness,
• wherein the first thickness is greater than the second thickness, and the third thickness is less than the fourth thickness.

A semiconductor structure, comprising:

• a bottom transistor and a top transistor disposed above the bottom transistor, wherein the bottom transistor comprises a bottom source/drain feature and the top transistor comprises a top source/drain feature;
• a first silicide layer disposed on the top source/drain feature and the bottom source/drain feature;
• a second silicide layer disposed on the first silicide layer;
• and
• a metal fill layer disposed on the second silicide layer,
• wherein the first silicide layer and the second silicide layer have different compositions,
• wherein the first silicide layer comprises a first horizontal portion on a top surface of the top source/drain feature and a first vertical portion along a sidewall of the top source/drain feature, and
• wherein the first horizontal portion has a first thickness, and the first vertical portion has a second thickness less than the first thickness.