Method for forming a semiconductor device

A method for forming a semiconductor device, comprising:

• forming, over a substrate, a stacked transistor structure comprising:
• a bottom channel structure and a top channel structure stacked on top of the bottom channel structure, a gate structure extending across the bottom and top channel structures, a first and a second bottom source/drain (S/D) structure on the bottom channel structure, and a first and a second top S/D structure on the top channel structure, wherein the first bottom and the first top S/D structures are formed at a first side of the gate structure, and the second bottom and the second top S/D structures are formed at a second side of the gate structure opposite the first side;
• forming a first and a second bottom S/D contact on the first and the second bottom S/D structures, respectively;
• forming a contact isolation layer capping the first and second bottom S/D contacts to form capped first and second bottom S/D contacts, and covering the capped first and second bottom S/D contacts with an interlayer dielectric layer;
• forming, at the first side of the gate structure, a first contact trench exposing the first top S/D structure over the capped first bottom S/D contact;
• forming, at the second side of the gate structure, a second contact trench exposing the second bottom S/D contact and the second top S/D structure;
• and
• forming a first top S/D contact in the first contact trench, in contact with the first top S/D structure, over the capped first bottom S/D contact, and a second top S/D contact in the second contact trench, in contact with the second top S/D structure and the second bottom S/D contact.