The Strong Effect of NiCr Adhesion Layers in Surface Micromachined MEMS Sensors

We demonstrate that a very thin NiCr adhesion layer may have a strong effect on the deformation of released structures. We consider MEMS switch sensors that are constructed from suspended perforated disks. The switches are fabricated by depositing a NiCr adhesion layer over a sacrificial layer, followed by deposition of a seed gold layer. This thin seed construction is then thickened by electrodeposition of a second gold layer. The last process step before releasing the structure is wet etching of the adhesion layer. We measured the curvature of many disk structures, that are identical in all geometry and process parameters, except for the level of NiCr overetch. The level of NiCr overetch strongly affects the curvature of the released structures. Much of the residual deformation occurs during packaging, which requires a relatively long period of elevated temperature. We suspect that interdiffusion during packaging determines the residual curvature. This interdiffusion should be different in regions where the adhesion layer has been overetched, which may explain the dependency of disk curvature on the level of NiCr overetch. It is suggested that careful attention should be given to the adhesion layer, which is often neglected as it is very thin. Such attention may help to improve the reliability and functionality of surface-micromachined metallic devices.