(Invited) Mechanical Property Design of Multilayered Ti/Au Microcomponents by Electrodeposition Toward High-Sensitive MEMS Accelerometers
Authors: Sone Masato , Chang Tso-Fu Mark , Kurioka Tomoyuki , Chen Chun-Yi , Chakraborty Parthojit , Ito Hiroyuki , Machida Katsuyuki , Miyake Yoshihiro 
Published: February 2024

Recently, MEMS accelerometers employing Au components prepared by electrodeposition are reported to have a small size whiling retaining a high sensitivity [1], which is mainly contribute by Au’s high mass density, as shown in Fig. 1. Detections of body tremors and muscular sounds are realized using this highly sensitive MEMS accelerometer [2]. However, low mechanical strength of Au, when compared to Si-based materials commonly used in conventional MEMS accelerometers, leads to insufficient structure stability for the long-term use of the device. To enhance the mechanical strength of Au-based materials, several approaches have been reported; for example, multi-layered metal technology [3], and alloying Au with Cu [4]. On the other hand, effects of such approaches on the long-term structure stability of the Au-based materials with dimensions in micro-scale remain uncleared. Considering the relationship between mechanical properties of metallic materials and size of the specimens evaluated, evaluation of the long-term structure stability of Au-based materials in micro-scare is essential for commercial uses of the Au-based MEMS device.

Cantilever-like structures are commonly used in movable components in MEMS devices [1, 3]. Repeatedly electrodeposition of gold following by deposition of a material with a high mechanical strength to realize a multi-layered structure is an effective strategy to enhance the structural stability [5]. From the Euler-Bernoulli beam theory, the structural stability of a cantilever-like structure is improved by using materials having high Young's modulus (as noted E). Although, the E is an intrinsic property of materials, which should be constant as the specimen size changes, but the E of small-sized specimens is reported to differ as the size changes. The E of a small-sized specimen with a specific geometry is called the effective Young’s modulus (E
eff). The E
eff of a micro-cantilever could be determined from the resonance frequency of the micro-cantilever, and the resonance frequency could be measured by a laser doppler vibrometer [4].

In this report, various Ti/Au multi-layered structures, as shown in Fig. 2, were prepared. Firstly, the E
eff of complex three dimensional (3D) multi-layered Ti/Au structures was determined from the resonance frequency obtained with a laser doppler vibrometer, as shown in Fig. 3. Next, we studied the effects of these various Ti/Au multi-layered structures on the long-term structure stability by vibration tests. Lastly, we discuss the relationship between the Ti/Au multi-layered structures with characteristic E
eff values and long-term structure stability. The discussion could contribute to the material design for highly sensitive and stable MEMS accelerometers.

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Figure 1