Real-time in situ monitoring of internal stress of the electroplating processes using FBG sensors

To study the internal stress during the electroplating processes, the Fiber Bragg Grating (FBG) sensors were electroplated with Ni, Zn, Cu films, respectively. During the electroplating, the current density was set to be 24 mA/cm2. The changes of internal stress of the metal films can be calculated according to the shift of central wavelength of the FBG sensors, blue shift showing a compressive stress and red shift showing a tensile stress. Results show: (i) the central wavelength of the FBG sensor blue shifted 2.26 nm after electroplating Ni for 420 min, the associated compressive stress in FBG sensor was calculated to be 551 MPa; (ii) the central wavelength of the FBG sensor red shifted 0.29 nm after electroplating Zn for 420 min, the associated tensile stress in FBG sensor was calculated to be 69 MPa; (iii) the central wavelength of the FBG sensor red shifted 0.04 nm after electroplating Cu for 420 min, the associated tensile stress in FBG sensor was calculated to be 10 MPa; (iv) a model characterizing the relationship between the internal stress and the central wavelength shift of the FBG was proposed, the feasibility and associated conclusions are verified by the nanoindentation tests.