Abstract: Recently, MEMS accelerometers employing gold (Au) components prepared by electrodeposition have been reported to be small in size while retaining high sensitivity due to Au’s high mass density, enabling the detection of body tremors and muscular sounds. However, the low mechanical strength of Au compared to the Si-based materials used in conventional MEMS accelerometers leads to insufficient structural stability for long-term use of the device. To enhance the mechanical strength of Au-based materials, multi-layered metal technology has been proposed. Cantilever-like structures are commonly used in the movable components of MEMS devices. An effective strategy to enhance structural stability is to repeatedly perform electrodeposition of gold, followed by deposition of a material with high mechanical strength, to create a multi-layered structure. According to the Euler–Bernoulli beam theory, the structural stability of a cantilever-like structure can be improved by using materials with a high Young's modulus (E). However, although E is an intrinsic property of materials that should remain constant as the specimen size changes, the E of small specimens is reported to differ as the size changes. The E of a small specimen with a specific geometry is called the effective Young's modulus (E_eff). The E_eff of a micro-cantilever can be determined from its resonance frequency, which can be measured using a laser Doppler vibrometer. This report describes the preparation of various Ti/Au multilayered structures. Firstly, the effective permittivity of complex three-dimensional (3D) Ti/Au multilayered structures was determined from the resonance frequency obtained using a laser Doppler vibrometer. Next, the effects of these structures on long-term stability were studied using vibration tests. Finally, we discuss the relationship between Ti/Au multilayered structures with characteristic E_eff values and long-term structural stability. This could contribute to the design of highly sensitive and stable MEMS accelerometers.

Keywords: Electrodeposition, material evaluation, MEMS, inertial sensor, Young's modulus

Acknowledgements: We appreciate T. Sakai, S. Iida, and T. Konishi at NTT-AT Corp. for their technical support. This work was supported by JST, CREST Grant No. JPMJCR21C5, Japan

Biography: Masato Sone completed his Doctor of Engineering degree at Tokyo Institute of Technology (now Institute of Science Tokyo) at the age of 28. He then worked as a researcher at Nippon Oil Company from 1996 to 2000. He was an assistant professor, then a research associate professor, at the Tokyo University of Agriculture and Technology from 2000 to 2005. He then became an associate professor, and subsequently a professor, at the Institute of Science Tokyo, a position he still holds today. In 2018, he became a visiting professor at National Chiao Tung University in Taiwan. His specialisms are microelectronics, metallurgy, surface finishing, chemical engineering, liquid crystals and polymer science. His recent research has focused on developing a novel electrodeposition process for use in MEMS technology, as well as evaluating the physical properties of electrodeposited materials.