Fabrication and characterization of SRN/SU-8 bimorph cantilevers for temperature sensing

Polymer coated ceramic cantilevers are highly sensitive to temperature changes, due to the large difference in coefficient of thermal expansion of the two materials. Silicon rich nitride (SRN) cantilevers coated with SU-8 for temperature sensing were fabricated and tested. The devices were coated with SU-8 through spin coating before they were released to obtain bimorph cantilevers. Deflection versus temperature measurements were carried out in ambient and nitrogen atmosphere. In ambient atmosphere, two different effects were observed as the temperature was cycled between 20 and 50°C; (i) shrinkage of SU-8 with increasing temperature, due to evaporation of absorbed moisture and (ii) thermal expansion of SRN and SU-8. Separately, these two effects generate bending in opposite direction. The observed overall bending was seen to be a combination of both. In nitrogen atmosphere, the cantilevers showed a unidirectional deflection versus temperature characteristic. Transverse bending of the cantilevers resulted in an increased rigidity of the cantilevers which could explain the ∼50% lower temperature response compared to the theoretical values.