Fine Grained Au Films with Controllable Mechanical Strength By Pulse Plating for Micro-Electrical-Mechanical System Accelerometer
Authors: Chen Chun-Yi , Yoshiba Masaharu , Chang Tso-Fu Mark , Yamane Daisuke , Machida Katsuyuki , Masu Kazuya , Sone Masato 
Published: February 2020

Au materials fabricated by electroplating are commonly used as contact materials for high reliability circuit boards, electrical connectors, relays, and micro- and nano-scale electronic components for many decades because of their high electrical conductivity, chemical stability, corrosion resistance, and ductility. In recent years, Au has become a promising material to be used as the movable structures and proof mass in micro-electrical-mechanical system (MEMS) accelerometer devices, mostly because of the high density (19.3×103 kg/m3 at 298 K), which is about 10 times higher than that of silicon (2.33×103 kg/m3at 298 K) [1]. However, Au is known to be a soft material. The mechanical strength becomes a concern in miniaturization of the MEMS device. Yield stress of bulk Au is reported to be 55~200 MPa [2]. Decreasing grain size of the Au materials is expected to further enhance the mechanical properties according to the Hall-Petch relationship [3].
Pulse electroplating (PE) has been reported to be effective in fabricating Au materials with fine grains, high uniformity, and low porosity [4]. Also, it is possible to control the composition and the film thickness by regulating the pulse amplitude and width. Most importantly, an increase in the nuclei density could be achieved to obtain electroplated films with finer grains. On the other hand, for evaluating mechanical properties of the electroplated films, Vickers micro-hardness test is the most popular method. However, the hardness results are often affected by the substrate. It cannot show the real strength of the electroplated materials, especially for films in mico-/nano-scale. Therefore, it is necessary to evaluate micro-mechanical strength of the Au electroplated films for the practical applications in the miniaturized devices.
Au electrolyte used in this study was a commercially available sulfite-based electrolyte provided by Matex Japan (Matex Gold NCA). The electrolyte contains 50 g/L of Na2SO3, 50 g/L of (NH4)2SO3, and 21.63 g/L of Na3[Au(SO3)2] with pH of 8.0 and 5% sodium gluconate. Cu plates and Pt plates were used as the cathode and anode, respectively. For the PE, the pulse current (Ion) was 10 mA/cm2, and the off-time current (Ioff) was 0 mA/cm2. The on-time (Ton) of the PE varied from 1 to 100 ms, and the off-time (Toff) was 10 ms. The reaction temperature was 40 ºC for the gold electroplating. The Au films prepared by the PE showed less defect, lower surface roughness, finer grain size, and denser texture when compared with the Au films prepared by the conventional constant-current electroplating (CE). Micro-mechanical properties of Au micro-pillars fabricated from the Au films were evaluated by micro-compression test.
Dimensions of the fabricated pillars were 10 μm×10 μm×20 μm. The compression tests were carried out using a test machine specially designed for micro-sized specimens equipped with a flat-ended diamond indenter at a constant displacement of 0.1μm/s. The Au micro-pillars prepared by the PE showed controllable high strength ranged from 500 to 800 MPa, where the grain size was adjusted by the PE parameters. Finest grain size was estimated to be 10.4 nm, and the highest compressive strength was 800 MPa [5]. The compressive strength obtained is much higher than the values reported in other studies [6,7]. The high strength is suggested to be due to the grain-boundary strengthening mechanism. The results demonstrated that the PE method and the sulfite-based electrolyte are promising for applications in miniaturization of the MEMS devices. To the best of our knowledge, this is the also first report on micro-mechanical strength of pure Au materials fabricated by the PE.
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