Substrate with a buried conductor under an active region for enhanced thermal conductivity and RF shielding

A semiconductor device is provided, the semiconductor device comprising a substrate having merged cavities in the substrate. An active region is over the merged cavities in the substrate. A thermally conductive layer is in the merged cavities in the substrate, whereby the thermally conductive layer at least partially fills up the merged cavities in the substrate. A first contact pillar connects the thermally conductive layer in the merged cavities in the substrate with a metallization layer above the active region.