Micro-Compression Characterization of Electrodeposited Nickel Phosphorus Amorphous Alloys for MEMS Application
Authors: Hotta Tokihiro , Chang Tso-Fu Mark , Sone Masato 
Published: December 2020

This study reports mechanical property evaluation of electrodeposited nickel phosphorus amorphous alloys. Electrodeposited nickel materials have excellent properties such as high elastic modulus, high strength, low internal stress, and excellent conductivity, and are expected to be applied to micro-sized machines and micro electro mechanical systems [1]. An example is nickel phosphorous amorphous alloys, which are commonly used as structure materials in microelectromechanical system devices [2].
Electrodeposition is a powerful technique to manipulate properties of metallic materials used in electronic devices. For example, nickel-phosphorus alloys can be prepared by electrodeposition with an electrolyte containing nickel chloride and nickel sulfate as source of the nickel, and sodium hypophosphite as source of the phosphorus [3, 4]. Properties of the electrodeposited nickel phosphorous alloy can be controlled by the bath temperature, current density, pH, sodium hypophosphite concentration, etc.
In order to use the nickel phosphorus amorphous alloy as structure components in a MEMS device, it is necessary to investigate the mechanical properties using specimens having similar size as those used in a MEMS device. Hence, mechanical property characterization of electrodeposited nickel phosphorus amorphous alloys has to be conducted using specimens having dimensions in micrometer scale.
Nickel phosphorus amorphous alloys were electrodeposited on pure copper plates using a commercially available electrolyte provided by MATEX Co. Japan. Thickness of the nickel phosphorous alloy was controlled to be at around 50 μm. Composition of the nickel phosphorus amorphous alloy was determined by energy dispersive X-ray spectroscopy (EDS). Specimens used in the micro-mechanical testing were micro-pillars having a square cross-section. The micro-pillars were fabricated by focus ion beam (FIB) system. Before the FIB process, the nickel phosphorus electrodeposited copper plate was thinned down to roughly 100 μm. Dimensions of the micro-pillar were 5 × 5 × 10 μm3, 8 × 8 × 16 μm3, 15 × 15 × 30 μm3, and 20 × 20 × 40 μm3. Shape of the micro-pillar was observed in a scanning electron microscope (SEM) before and after the micro-compression test to confirm the exact dimension and deformation behavior. Then micro-compression test was conducted using a micro-mechanical testing system developed in our group [1]. The yield strength was determined from the engineering stress-strain curve generated from the micro-compression test. Vickers hardness of the nickel-phosphorus amorphous alloys was also measured to compare with the yield strength obtained from the micro-compression test.
Fig. 1 and 2 show SEM images of the 20 × 20 × 40 μm3 micro-pillar before and after the micro-compression test, respectively. After the compressive deformation, slip deformation, which is often seen in amorphous metals, was observed. Evolution of this slip deformation could be identified from the stress oscillation observed in the stress-strain curve shown in Fig. 3. Similar slip deformation and stress oscillation were also observed in nickel phosphorous amorphous alloy micro-pillars with other sizes. Table 1 shows the 0.2% yield strength and 1% flow strength determined from the stress-strain curves. The 0.2% yield strength fluctuated between 2.01 - 2.08 GPa and the 1% flow strength changed between 2.37 - 2.47 while the one side of the micropillar varied between 10 - 40 μm. Micro-compression test of the nickel phosphorous amorphous alloys revealed that there was no obvious relationship between size of the micro-pillar and the yield strength in the range evaluated in this study. This result was different from the sample size effect commonly observed in metallic materials with high crystallinity [5].

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Figure 1