Structure Stability of Electrodeposited Au-Cu Alloy Micro-Cantilever Evaluated By Long-Term Vibration Test for Applications As Movable Components in MEMS Devices
Authors: Nitta Kyotaro , Tachibana Koichiro , Tang Haochun , Chen Chun-Yi , Chang Tso-Fu Mark , Yamane Daisuke , Konishi Toshifumi , Machida Katsuyuki , Masu Kazuya , Sone Masato 
Published: February 2020

This study reports structure stability of micro-cantilevers composed of Au-Cu alloy for applications as moveable components in micro-electrical-mechanical system (MEMS) accelerometers. Au-based micro-components are developed to replace conventional Si-based components in a MEMS accelerometer to allow further miniaturization of the device [1]. However, the mechanical strength of Au is relatively low when compared with that of Si. The low mechanical strength raises concerns on structure stability of movable micro-components composed of Au-based materials, which could affect reliability and lifetime of the device.
Electroplating is a promising technique in manipulating properties of the metallic materials for applications in MEMS. For instance, grain refinement effect often observed in electroplating can be employed to enhance the mechanical strength by the grain boundary strengthening mechanism [2]. In addition, alloy electroplating is a fascinating technique capable of controlling composition of the metallic material, and alloying the metallic material allows utilization of the solid solution strengthening mechanism to further enhance the mechanical strength [3]. By synergistic effects of the grain boundary strengthening and solid solution strengthening mechanisms, an ultra-high yield stress of 1.15 GPa [3] is reported for electroplated Au-Cu micro-specimens, which is comparable to fracture strength of Si.
In order to apply the high-strength Au-Cu alloy as movable structures in MEMS devices, investigating the vibration fatigue is necessary to confirm the reliability and the lifetime. In this study, long-term vibration tests of micro-cantilevers fabricated by combining lithography and Au-Cu alloy electroplating were conducted to reveal effects of the vibration fatigue on structure stability of the Au-Cu alloy micro-cantilevers. Fabrication process of the micro-cantilevers is illustrated in Fig. 1. The Ti barrier layer and the Au seed layer were deposited by sputtering, and the layer thicknesses were both at 100 nm.
The Au–Cu electrolyte used in this work was a commercially available electrolyte provided by MATEX Co. Japan, which contained 17.3 g/L of X3Au(SO3)2 (X = Na, K), 1.26 g/L of CuSO4, and ethylenediaminetetraacetic acid as the additive with pH at 7.5. The electroplating was carried out at 50 ◦C, and the current density was varied from 0.1 to 3 mA/cm2. A piece of Pt plate was used as the anode. Image of the Au-Cu alloy micro-cantilevers before the vibration test is shown in Fig. 2. Design-length of the micro-cantilever (l) was varied from 50 ~ 1000 μm, and design-width of the micro-cantilever (w) was ranged from 5 ~ 20 μm. Thickness of the Au-Cu alloy layer (t) was from 2.3 ~ 5.0 μm.
The long-term vibration test was carried out under conditions of the cycle number in a range from 103 to 106, the frequency of 10.0 Hz, and the acceleration of 1.0 G (1 G = 9.8 m/s2). The structure stability was determined by evaluating height profiles of top surface of the micro-cantilevers before and after the vibration test by a 3D optical microscope.
Height profiles of a micro-cantilever composed of 97.7 % Au with the length at 1000 μm, the width 20 μm, and the thickness at 3.9 μm before and after the vibration test are shown in Fig. 3. Fluctuations in the height before the vibration test, the 0 cycle result, were mainly results of the surface roughness. After 106 cycles of the vibration test, the micro-cantilever was still intact as observed from the optical microscope. The height profiles did not change much after the vibration. In order to quantify the structure stability, deflection at tip of the micro-cantilever was determined. The tip deflection merely changed 0.37 μm for the 1000 μm long micro-cantilever after 106 cycles of the vibration test. This result confirmed high structure stability in the electroplated Au-Cu alloy micro-cantilever and demonstrated the superiority of this Au-Cu alloy structure in applications as movable components in MEMS devices.
[1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett., 104 (2014) 074102.
[2] H. Tang, C.-Y. Chen, T. Nagoshi, T.-F.M. Chang, D. Yamane, K. Machida, K. Masu, M. Sone, Electrochem. Commun., 72 (2016) 123–130.
[3] H. Tang, C.-Y. Chen, M.Yoshiba, T.Nagoshi, T.-F.M. Chang, D.Yamane, K.Machida, K.Masu, M.Sone, Electrochemical Society, 164 (2017) 244–247

Figure 1