Correlation of Structure and Stress in Electroplated Thick Ni Films
Here we report the evolution of Ni stress in electroplated Ni films and investigate its origins as a function of electroplating time. First, low stress 200-nm-thick Ti / 200-nm thick Ni seed layers are deposited onto Si (100) wafers via electron-beam evaporator.Then, Ni films are electroplated onto the seeded Si (100) wafers in an All-Chloride bath. We observe that Ni grains nucleate prominently in the [111] direction aligning with Si[100]. Using optical curvature measurements, scanning electron microscopy (SEM), and X-ray diffraction (XRD) measurements, we quantify the electroplated Ni stress and investigate its evolution as a function of electroplated Ni thickness, grain size, crystal orientation, crystallite size, lattice spacing, and dislocation density. Particularly, when averaged electroplated Ni thickness was increased from 2.1 um to 5.9 um, Ni tensile stress reduced from 604.8+-137.9 MPa to 339.8+-74.7 MPa as (1) average Ni grain size increased by 40.9+-1.3 % (Ni lattice constant expanded by 0.19+-0.03 % in the [220] in-plane direction and shrunk by 0.22+-0.04 % along normal [111] direction), (2) Ni grain growth along Ni [110] was inhibited and that along Ni [100], [111], and [311] were promoted, (3) Ni crystallite size improved by 26.3+-6.2%, and (4) Ni dislocation density reduced by 42.7+-10.0%. Overall, Ni stress is shown to be strongly correlated with the Ni structure, which is a function of electroplating time, and both can be quantified through optical and structural characterization techniques.