13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- copper/parylene core/shell nanowire surface fastener was investigated and showed much better electrical properties. The copper nanowire array on the silicon substrate was prepared through template-assistant electro-deposition, which provided the electrical conductive function. The parylene shell was deposited on Copper nanowire array through CVD method, which provided surface compliance to increase contact areas, thereby realizing larger bonding strength. In term of pressing the two nanowire array against each other, high adhesion strength was obtained at the same time due to the von der Waals forces between the interleaved nanowires. The according electricity resistance measurement showed that the electrical resistance of core/shell nanowire array was small. 2. Experiment details. 2.1. Electrodeposition of copper nanowires Polycarbonate (PC) membranes (ISOPORE, Millipore Inc.) containing 0.1 µm diameter pores were used as the templates. The cathode was fabricated through sputtering 50 nm Cr adhesion layer and 100 nm Au seed layer on the substrate. With the help of isolation holders, the PC template was fixed right above the substrate. Copper nanowire arrays were then synthesized by electrodepostion under a constant current of approximately 0.003A. The electro-deposition electrolyte used was a 0.4 M CuSO4⋅5H2O solution, adjusted at pH=2 with sulfuric acid. The electro-deposition was performed at room temperature and without stir. After etching in methylene chloride to remove the PC membrane, the freestanding copper nanowire array on the substrate was obtained. 2.2. Parylene coating As shown in figure 1, parylene-N was deposited on copper nanowire arrays by using a DACS-LAB deposition system. A typical deposition condition used in this work was 160°C for the vaporization of the parylene dimmer precursor, 650°C for the pyrolysis of the dimmer into monomers, and 60mTorr for the vacuum chamber. Through controlling the amount of the loaded precursor, the according thickness of parylene shell could be obtained. 2.3. Testing of parasitic resistance and bonding strength The samples were connected to each other under the preload of 29.4N，and then the parasitic resistance was measured by the four-point probe method (figure 3a) after the preload was completely removed. During the measurement, electrical current from 0 to 20 mA was applied by the current source to the four-point probe measuring circuits and U-I curves were generated. The shear bonding strengths were tested by measuring the maximum forces that the surface fasteners could afford. The weights of a balance ranging from 1g to 1kg were used to fulfill this testing. Fig. 1 Schematic of parylene coating
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