Fabrication and characterization of a piezoresistive humidity sensor with a stress-free package

Abstract. A highly miniaturized piezoresistive humidity sensor has been developed. The starting point of the development was a 1 × 1 mm² piezoresistive pressure sensor chip. As sensing material, a polyimide was used that swells with increasing adsorption of water molecules. To convert the swelling into an electrical signal, a thin layer of the polyimide was deposited onto the bending plate of the pressure sensor. The humidity sensor was characterized in a climate chamber. The measurements show a sensitivity of 0.25 mV per percent relative humidity (%RH) and a non-linearity of 3.1% full scale (FS) in the range of 30–80% RH. A high cross-sensitivity to temperature of around 0.5 mV °C⁻¹ was measured, so temperature compensation is necessary. For stress-free packaging of the sensor chip, a novel packaging technology was developed.