An unreleased thermopile IR sensor and method of fabrication is provided which includes a new thermally isolating material and an ultra-thin material based sensor which, in combination, provide excellent sensitivity without requiring a released membrane structure. The sensor is fabricated using a wafer transfer technique in which a substrate assembly comprising the substrate and new thermally isolating material is bonded to a carrier substrate assembly comprising a carrier substrate and the ultra-thin material, followed by removal of the carrier substrate. As such, temperature restrictions of the various materials are overcome.