Annealing Effects on the Microstructure and Mechanical Properties of Liga Nickel for Mems

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ANNEALING EFFECTS ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF LIGA NICKEL FOR MEMS H. S. Cho1, W. G. Babcock2, H. Last3 and K. J. Hemker1 1 Departments of Mechanical Engineering and Materials Science and Engineering, The Johns Hopkins University, Baltimore, MD 21218, USA 2 IIT Research Institute, 201 Mill St., Rome, NY 13440; Formerly with Naval Surface Warfare Center, Indian Head Division, Indian Head, MD 20640 3 Institute for Defense Analyses, Alexandria, VA 22311, USA ABSTRACT The emergence of elevated temperature applications for microelectomechanical systems (MEMS) has resulted in the need for an understanding of the relation between the microstructural stability and the mechanical properties of their structural elements. Hardness and tensile tests and microstructural observations have been conducted with as-deposited and annealed conditions of LIGA-Ni microsamples that were deposited with current densities of 6mA/cm2 and 50mA/cm2. The Young’s modulus was found to be independent of the annealing temperature, but the hardness and yield strength decreased dramatically at temperatures above 400oC. The microsamples deposited at a current density of 6mA/cm2 showed much higher strength and hardness than those deposited at 50mA/cm2. This increased strength has been related to the observation that the microstructure of the 6mA/cm2 microsamples is much finer than was observed for 50mA/cm2. INTRODUCTION Deposition of material into a microfabricated mold is one of the prevalent manufacturing processes for creating the mechanical elements of MEMS devices [1]. LIGA (Lithographie, Galvanoformung and Abformung) is the most widespread process for thick metallic MEMS components. In the LIGA process, the primary template is generated by synchrotron radiation lithography and filled with metals or alloys by electrodepostion. The resultant metal (or alloy) structures can be used for direct use or as a mould insert for the fabrication of secondary plastic templates. The main advantage of the LIGA process over the traditional silicon surface engineering techniques is the fact that high-aspect-ratio 3-D structures can easily be made with high precision [2]. The microstructure and corresponding mechanical properties of the electrodeposited Ni are closely related with process parameters such as bath composition, current density, pH, bath temperature, and agitation operation [3]. High temperature exposure of electrodeposited Ni structures has been shown to alter the pre-existing microstructure and result in softening and lower room temperature strength [4-7]. Jacobson et al. [4] have reported that the yield strength of electrodeposited Ni was significantly decreased by annealing at 500oC for 1 hour. Christenson at al. [5] and Xie et al. [7] have also reported that the yield strength of LIGA-Ni samples decreases considerably when annealed for 1 hour at 600oC and 800oC, respectively. According to Singh et al. [6], the hardness of the electrodeposited Ni samples fabricated under different bath composition and current density decrease