Stress gradient modification of the electroplated Ni-diamond nanocomposite for MEMS fabrication

The paper investigates the formation of stress gradient in electroplated Ni-based nanocomposite films and presents a process scheme to effectively reduce the stress by lowering plating current density. The stress gradient is characterized by measuring the radius of curvature from the surface profile of Ni-based micro-cantilever beams. For the case of the Ni-based films plated with the current density from 15.3 down to 0.8 mA/cm ², the stress gradient of the films is formed in the range of -7.13 to -3.23 MPa/μm. According to the cross sectional SEM images, it is found that the Ni-based film plated with 0.8 mA/cm ² can have a uniformly distributed grain size that can result in a lower stress gradient. By lowering the plating current density from 15.3 to 0.8 mA/cm ², about 41% and 21% of the stress gradient reduction can be realized in the Ni and Ni-diamond nanocomposite films, respectively, which can be utilized for MEMS resonator fabrication without having structural warpage. Furthermore, experimental results show that the performance of the Ni-diamond nanocomposite resonator can be enhanced in terms of the increase of resonant frequency and quality factor.