Nanosheet substrate to source/drain isolation

A method of forming a nanosheet device, comprising:

• forming alternating separation layers and nanosheet channel layers on a substrate, and a dummy gate structure on the alternating separation layers and nanosheet channel layers;
• forming a curved recess in the substrate surface adjacent to the nanosheet channel layer stack using a directional etch;
• depositing a protective layer on the curved recess, dummy gate structure, and exposed sidewall surfaces of the nanosheet layer stack;
• removing a portion of the protective layer on the curved recess to form a downward-spiked ridge around the rim of the curved recess;
• extending the curved recess deeper into the substrate to form an extended recess;
• forming a sacrificial layer at the surface of the extended recess in the substrate;
• and
• removing the sacrificial layer from the surface of the extended recess to form an enlarged recess.

A nanosheet device, comprising:

• one or more nanosheet channel layer segments on a support pillar formed by a substrate;
• an active gate structure on the one or more nanosheet channel layer segments;
• and
• an isolation fill layer having a rounded portion adjacent to an upper portion of the support pillar, wherein the isolation fill layer is in an augmented recess and on the sidewalls of the support pillar.