Abstract: A method for forming a semiconductor sensor device comprises providing a substrate (4) and forming a sacrificial layer (18) over the substrate. The sacrificial layer (18) is then patterned and etched to leave a portion (19) on the substrate (4). A first isolation layer (6) is formed over the substrate (4) and portion (19) of the sacrificial layer and a conductive layer (12), which provides a heater for the sensor device, is formed over the first isolation layer (6). The portion (19) of the sacrificial layer is then selectively etched to form a cavity (10) between the first isolation layer (6) and the substrate (4), the cavity (10) providing thermal isolation between the heater and the substrate.