Pulse Electroplating of Au Films with Ultra High Strength
Authors: Chen Chun-Yi , Yoshiba Masaharu , Tang Haochun , Chang Tso-Fu Mark , Yamane Daisuke , Machida Katsuyuki , Masu Kazuya , Sone Masato 
Published: February 2020

Au is the most commonly used as contact material for high reliability electrical connectors, circuit boards, and as functional component for micro-electrical-mechanical system (MEMS) devices [1,2]. Electroplating has been applied extensively in fabrication of Au films used for all kinds of devices. Among the electroplating techniques, pulse electroplating is a technique known to control the properties of the deposited films by regulating the pulse amplitude and width. Therefore, morphology, crystal structure and deposition rate of the Au films can be controlled precisely [3].

      In this work, grain refinement and surface smoothening of Au films were achieved by pulse electroplating using non-toxic sulfite-based electrolyte. Micrographs obtained by atomic force microscopy (AFM) and X-ray diffraction (XRD) patterns indicate that the surface of the pulse electroplated Au films possess less defect, lower surface roughness, smaller grain size, and denser texture when compared with the Au films prepared by conventional constant-current electroplating. By XRD analysis and Scherrer equation calculation, the estimated grain size of the Au film prepared by pulse electroplating was 10.5 nm, and was much smaller than the grain size of the Au film prepared by constant-current electroplating, which was 22.8 nm. The grain size achieved by the pulse electroplating is even smaller than previous reports [3,4]. This can be attributed to the higher pulse current density which leads to an increase in the nucleation rate during the on-time period.

     Mechanical strength in micro-scale of the Au films was also evaluated to determine the reliability and lifetime of the components composed of the electroplated Au. After the electroplating, micro-pillars made from the Au films were fabricated by focus ion beam (FIB) milling. Size of the fabricated pillar was fixed at 10 mm ×10 mm×20 mm. The micro-compression test was carried out using a test machine specially designed for micro-sized specimens developed in our laboratory. The pulse electroplated Au micro-pillar acquired a large increase in the compressive strength from 600 to 800 MPa when the grain size approaches 10.4 nm, presumably due to the grain-boundary strengthening known as the Hall-Petch effect [5]. This strength is much higher than that of bulk Au materials, which is only 55~200 MPa. The strength is even higher than that of the micro-pillar fabricated from the constant-current electroplated Au films. These results reveal that the pulse electroplated Au micro-pillar possesses better ductility and malleability than that fabricated by constant-current electroplating and demonstrate that pulse electroplating is a promising technique to fabricated Au functional micro-components especially for the advanced MEMS devices.

Reference:

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