Monolithic formation of a set of interconnects below active devices

A method of fabrication, the method comprising:

• providing a substrate, the substrate comprising bulk semiconductor material;
• forming an opening in the bulk semiconductor material patterned for a first wiring structure;
• filling the opening in the bulk semiconductor material with a metal such that the first wiring structure is formed at least partially within the bulk semiconductor material;
• epitaxially growing active semiconductor material over the first wiring structure so that the bulk semiconductor material and the active semiconductor material are in direct contact with each other;
• forming one or more active devices in the active semiconductor material over the first wiring structure;
• and
• forming a second wiring structure over the one or more active devices, wherein the active semiconductor material and the bulk semiconductor material include a same semiconductor material,
• the active semiconductor material is epitaxially grown over the first wiring structure so that the first wiring structure is fully within the same semiconductor material, and
• the active semiconductor material is epitaxially grown over the first wiring structure so that the first wiring structure comprises a patterned layer of the metal fully within the same semiconductor material with air gaps above the metal.