We show the use as a thermal photosensor of a thermoelectric (TE) microsensor based on ultrathin suspended Si films. The reduced thickness of the structural films enhances the extremely large thermal insulation of the sensing area (~43 µW/K), since phonons scatter in the surfaces, and guarantees a reduced thermal mass (in the µJ/K range). The sensitivity of the device is evaluated by heating with an argon laser (λ = 457 nm) in the range 0⁻10 mW, reaching sensitivities of around 6 × 108 V/(W·m2) in high vacuum conditions and 5 × 107 V/(W·m2) in environments of air at atmospheric pressure. Open circuit voltage measurements with and without light illumination with a 406 nm diode laser operating at 4 mW were conducted at temperature differences up to 50 K between the central hot region and the Si frame. The slight decrease of the Seebeck coefficient is related to the increase of carriers by photogeneration.
