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Electrodepostion of Au-Cu Alloy Micro-Cantilevers and the Young’s Modulus by Resonance Frequency Method
Kyotaro Nitta1,Haochun Tang1,Chun-Yi Chen1,Tso-Fu Mark Chang1,Daisuke Yamane1,Shinichi Iida2,Katsuyuki Machida1,Hiroyuki Ito1,Kazuya Masu1,Masato Sone1
Tokyo Institute of Technology1,NTT Advanced Technology Corporation2
Gold materials are commonly applied in electronic devices because of the superior corrosion resistance, electric conductivity, and chemical stability . Recently, micro-electro-mechanical systems (MEMS) inertial sensors utilizing gold materials in the main components are reported to have 1000 times higher sensitivity than conventional MEMS inertial sensors [2,3], which is mostly because of the high mass density. For application of gold-based materials in next generation MEMS devices, further strengthening is needed to ensure high structure stability. Regarding strengthening of gold materials, grain boundary strengthening and solid solution strengthening mechanisms can be applied simultaneously by alloy electrodeposition process , which the yield stress reached 1.15 GPa when the grain size was refined to 5.3 nm with the copper concentration at 12.3 wt%. On the other hand, Young’s modulus of the high strength Au-Cu alloys is also needed in design of MEMS components, such as the micro-scale spring in MEMS inertial sensors. In this study, Young’s moduli of micro-cantilevers composed of the Au-Cu alloy are measured by a non-destructive resonance frequency method .
The Au-Cu alloy micro-cantilevers were fabricated by electrodeposition and lithography. The Ti adhesion 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 EDTA as the additive with pH at 7.5. The electrodeposition was carried out at 50 °C, and the current density was varied from 0.5 to 2 mA/cm2. A piece of Pt plate was used as the anode. Design-length of the micro-cantilever was varied from 50 ~ 1000 μm, and design-width of the micro-cantilever was ranged from 5 ~ 20 μm. Thickness of the Au-Cu alloy layer was from 2.6 ~ 4.0 μm. Composition of the films was characterized by energy dispersive x-ray equipped in a scanning electron microscope.
Young’s moduli of the Au-Cu alloy micro-cantilevers were calculated from resonance frequencies of the micro-cantilevers. The resonance frequencies were experimentally obtained as shown in the following. First, a voltage pulse (amplitude: 10V, pulse width: 100μs) was applied between the cantilever and a fixed electrode to initiate free vibration mode. Next, a laser doppler vibrometer was used to measure displacements of tip of the cantilever. Finally, the resonant frequency was obtained from a FFT (fast Fourier transform) analyzer.
Surface conditions of Au-Cu alloy micro-cantilevers produced at a current density ranged from 0.5 to 2 mA/cm2 were all uniform and smooth. The Cu content increased from 1.40 to 4.03 wt% as the current density increased from 0.5 to 2 mA/cm2, which is because standard reduction potential of Cu is more negative than that of Au, and an increase in the current density leads to a more negative applied potential. Young’s modulus of a micro-cantilever composed of 98.3 % Au with the length at 700 μm, the width 20 μm, and the thickness at 3.95 μm was 80.9 GPa. The value is close to that of pure gold (79 GPa) and much lower than that of pure copper (117 GPa). On the other hand, no obvious trend was observed when the width changed from 10 to 20.
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