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**71**

**Chapter 5**

**Abstract**

**1. Introduction**

*and Katsuyuki Machida*

Electrodeposition of Gold Alloys

Strengthening of electrodeposited gold-based materials is achieved by alloying with copper according to the solid solution strengthening mechanism. Composition of the Au–Cu alloys is affected by the applied current density. The mechanical properties are evaluated by micro-compression tests to evaluate the mechanical properties in microscale to take consideration of the sample size effect for applications as microcomponents in MEMS devices. The yield strength reaches 1.15 GPa for the micropillar fabricated from constant current electrodeposited Au–Cu film, and the film is composed of 30.3 at% Cu with an average grain size of 5.3 nm. The yield strength further increases to 1.50 GPa when pulse current electrodeposition method is applied, and the Cu concentration is 36.9 at% with the average grain size at 4.4 nm.

and the Mechanical Properties

*Haochun Tang, Tso-Fu Mark Chang, Chun-Yi Chen,* 

*Takashi Nagoshi, Daisuke Yamane, Toshifumi Konishi* 

**Keywords:** electrodeposition, gold-based alloys, mechanical property, microcompression test, Hall-Petch relationship, solid solution strengthening

In recent years, microelectromechanical system (MEMS) capacitive accelerometers have been developed and used in a variety of consumer electronics for

medical and health care fields, accurate sensing with sub-1 G detection is necessary to monitor hardly detectable body motions [4, 5]. To detect such low acceleration in a compact sensor module, various types of MEMS accelerometers based on silicon (Si) bulk micromachining have been reported [6, 7]. In order to suppress the thermal-mechanical noise (i.e., Brownian noise (BN) [8]) for the highly sensitive detection, a large proof mass is required. Limited choices of materials for the proof mass and other movable components in a CMOS-MEMS accelerometer have been a major challenge to reduce the BN, which becomes more critical when the parasitic capacitance is reduced in miniaturized devices. Yamane et al. [9–11] propose a miniaturized MEMS accelerometer by using a post-CMOS process with electrodeposited Au in the main components, which enables further size reduction of the proof mass and the device footprint without compromising the sensitivity. With the application of electrodeposited Au in MEMS accelerometers [9–11], a wide range

) [1–3]. For applications in

**1.1 Application of Au materials in MEMS devices**

acceleration detection in a range of 1–5 G (1 G = 9.8 m/s2

## **Chapter 5**
