Elimination of stress-induced curvature in microcantilever infrared focal plane arrays

This paper reports an approach to eliminating stress-induced curvature in microcantilever-based infrared focal plane arrays (FPAs). Using a combination of argon ion beam machining and rapid thermal annealing (RTA), we successfully modified curvatures of free-standing SiNx/Al bimaterial FPAs. The SiNx/Al FPAs were fabricated using a surface micromachining technique with polyimide as a sacrificial material. The as-fabricated FPAs were concavely curved because of the imbalanced residual stresses in the two materials. To modify the FPAs curvature, first, Ar ions with energies of 500 eV was used, which sputter etched PECVD SiNx at a rate of 4 nm/min, and 20 min of ion beam machining reduced the FPAs curvature from −1.92 to −0.96 mm−1. Then based on the investigation on the thermomechanical behavior of both the e-beam Al and PECVD SiNx films during the thermal cycling, RTA was proposed to further modify the FPAs curvature. It is found that 5 min of RTA at 375 °C resulted in flat FPAs with acceptable curvatures (<0.10 mm−1).