Metal-comprising bottom isolation structures

A method comprising:

• forming a sacrificial base layer over a substrate;
• forming a semiconductor stack over the sacrificial base layer, wherein the semiconductor stack has a first semiconductor layer over the sacrificial base layer, a sacrificial layer over the first semiconductor layer, and a second semiconductor layer over the sacrificial layer;
• before forming an epitaxial source/drain structure, removing the sacrificial base layer to form a gap between the substrate and the first semiconductor layer;
• and
• forming a metal-containing, multilayer dielectric structure in the gap, wherein the forming the metal-containing, multilayer dielectric structure includes performing a deposition process to form a metal-containing dielectric material in the gap and a nitridation treatment process that causes expansion of the metal-containing dielectric material.

A method comprising:

• forming a plurality of channel structures over a substrate;
• forming a gate stack that wraps around the plurality of channel structures;
• and
• forming an isolation structure between a bottom one of the plurality of channel structures and the substrate, wherein the forming the isolation structure includes:
• forming a first metal-containing dielectric layer,
• forming a second metal-containing dielectric layer,
• forming an intermediate dielectric layer between the first metal-containing dielectric layer and the second metal-containing dielectric layer, wherein each of the as-formed first metal-containing dielectric layer, the as-formed second metal-containing dielectric layer, and the as-formed intermediate dielectric layer has an amorphous structure, and
• performing an annealing process to convert the amorphous structure of each of the as-formed first metal-containing dielectric layer, the as-formed second metal-containing dielectric layer, and the as-formed intermediate dielectric layer into a crystalline structure.