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Indexed by:期刊论文

Date of Publication:2009-08-01

Journal:CHINESE JOURNAL OF MECHANICAL ENGINEERING

Included Journals:SCIE、EI、CSCD、Scopus

Volume:22

Issue:4

Page Number:557-561

ISSN No.:1000-9345

Key Words:electroplated nickel; micro-tensile testing; Young's modulus; porosity

Abstract:Mechanical properties of structural materials are particularly important for design, performance realization and reliability analysis of microelectromechanical systems (MEMS). Furthermore, accurate database of mechanical properties at the micro scale can provide indispensable reference for establishing MEMS standard. Electroplated nickel film is one of the most favored structural materials used in MEMS, thus its mechanical properties has been studied for many years. However, the measured values show large scatter in Young's modulus of nickel film. Young's modulus and yield stress of electroplated nickel film are measured by using a micro-tensile testing instrument. The tensile load applied on the specimen is measured by a load cell with accuracy 0.25 mN directly, without additional friction. Through measuring the axial stiffness coefficient of the tensile instrument in situ, the tensile strain of the specimen is obtained by using two-serial spring model. The electroplated nickel films were fabricated from sulfamate baths, and the gauge section is 500 mu m long and 10 mu m wide nominally, and thickness range between 25 mu m and 50 mu m. The obtained Young's modulus from tensile testing is 83 +/- 6 GPa for nickel specimens electroplated at current density of 20 mA/cm(2) and it increases to 124 +/- 5 GPa as current density is decreased to 10 mA/cm(2). The phenomena are interpreted in terms of porosity of microstructure. The higher current density produced microstucture with low density and high volume fraction of pores, and the microstructure of high porosity corresponds to a lower modulus. The measured values of Young's modulus are consistent with those of calculated from the exponential empirical formula between Young's modulus and porosity. The micro-tensile testing instrument can also be used for mechanical measurement of other MEMS films.