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In recent years, MEMS (microelectromechanical systems) devices are required to have high functionality and small size, especially for MEMS acceleration sensors, and sensitivity of a MEMS acceleration sensor is highly dependent on the Brownian noise [1,2]. The Brown noise is known to be inversely proportional to mass of movable parts in the sensor, and the noise is lowered by increasing the mass [1,2], which implies a number of volume is needed to ensure high sensitivity. However, increasing size of the component is not preferable. Because of this, a design of gold-based MEMS acceleration sensor is proposed. The density of gold is 19.3 g/cm3, and gold has high corrosion resistance and chemical stability. Most importantly, gold electrodeposition process can be easily integrated with current MEMS fabrication process. Mechanical properties of the material are essential informtion for design of MEMS components. For instance, the yield stress is needed to guarantee elastic deformation of the material during operation of the MEMS device. Mechanical properties of metallic materials are known to change with size of the sample used in a mechanical property characterization becomes smaller than micrometer scale, which is known as the sample size effect [3]. Therefore, mechanical property characterization of materials toward MEMS should be conducted using samples having the same dimensions as those used in MEMS. Various micro-mechanical testing methods have been developed to characterize micro-specimens, such as micro-compression test, micro-tensile test, and micro-bending test. Among them, micro-bending test is suggested to be the most suitable method to evaluate materials for applications in MEMS when compared with micro-compression and micro-tensile tests since cantilever-like structure is commonly utilized in movable components, such as the micro-spring in MEMS accelerometers. On the other hand, micro-cantilevers are reported to have the sample geometry effect [4], which the sample size effect is only observed when varying thickness of the micro-cantilever. Also, it has been reported that the sample size effect is dependent on ratio of the sample diameter to the average grain size, and materials prepared by electrodeposition are usually polycrystal. Thus, relationship between the sample geometry effect and the average grain size is investigated in this study. Two gold films were electrodeposited at different current density to change the average grain size. The average grain size was determined by electron backscatter diffraction, and the average grain size of film A and B were 1.2~1.3 and 0.6 μm, respectively. Samples used in the micro-bending test were micro-cantilevers fabricated from the gold films by focus ion beam. Thicknesses of the micro-cantilever were ranged from 6.0 to 13 μm. The micro-bending tests are conducted using a test machine specially designed for micro-specimens developed in our group as shown in Fig. 1. The yield stresses were in a range of 428 to 553MPa, which were all higher than that of the bulk gold (55 to 220 MPa). Table 1 shows the detailed dimensions and yield stress of all samples. Yield stresses were plotted against the log scale of thickness shown in Fig. 2. At first, the sample geometry effect was confirmed in samples fabricated from both film A and B. Then, the yield stresses of samples fabricated from film B were all higher than those of film A, which was a result of the Hall-Petch relationship. In Fig. 2, the slop was steeper for film A indicating the sample geometry effect was more obvious in samples composed of larger grains. [1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, and K. Masu, Appl Phys Lett 104 (2014) 074102 [2] K. Machida, T. Konishi, D. Yamane, H. Toshiyoshi, K. Masu, ECS Trans. 61 (2014) 21-39. [3] J.R. Greer and J.Th.M. De Hosson: Plasticity in small-sized metallic systems: Intrinsic versus extrinsic size effect. Prog. Mater. Sci. 56, 654 (2011). [4] K. Suzuki, “Sample Geometry Effect on Mechanical Property of Electrodeposited Gold Evaluated by Micro-Bending Test", 45th International Conference on Micro & Nano Engineering (MNE2019), Rhodes Greece, Sep. 2019 Figure 1

Nickel [1], cobalt [2], and their alloys have been investigated as important engineering materials because of their unique properties, such as magnetic, heat-conductive and high hardness. Much interest is focused on the application of Ni-Co alloy films in micro-electrical-mechanical systems (MEMS) devices especially their considerable potential in manufacturing of magnetic actuator due to excellent electrical and mechanical properties [3]. Reliability of electronic components is highly dependent on the mechanical property, which is closely related to its average grain size according to grain boundary strengthening mechanism [4]. Nanocrystalline materials can offer enhanced mechanical strength compared with coarse-grained counterparts when device components are scaled to the micro-scale regime with synergistic effect of the sample size effect [5]. Meanwhile, alloying allows utilization of the solid solution strengthening mechanism to further enhance the mechanical strength [6], and Ni-Co forms a solid solution over the whole concentration range, making it easy to control the mechanical and magnetic properties. Electroplating is a promising technique in controlling crystalline properties of Ni-Co alloy films because the morphology, composition, grain size, and deposition rate of the deposited materials can be facilely controlled by varying the electroplating parameters, such as the current density, bath composition, and temperature [7]. Therefore, in this study, the effects of applied current density on average grain size, composition, and micro-mechanical properties of electrodeposited Ni-Co films are evaluated for fabrication of micro-components in electronic devices. The electroplating was carried out at 55 ℃, and the current density was varied from 5 to 20 mA/cm2. A piece of Pt plate was used as the anode. Crystalline structure of the Ni-Co films was characterized by X-ray diffraction (XRD), average grain size was determined using XRD in conjunction with the Scherrer method. Microhardness tests were conducted on a Vicker's hardness tester using a load of 0.025 kg, applied for 15 s. Composition was analysed by energy dispersive X-ray spectroscopy (EDX) system. Furthermore, alloy films are processed into micro-pillars in a size of 10×10×20 μm3with focused ion beam (FIB, FB2100, Hitachi) milling. The optical microscope photograph of Ni-Co alloy micro-pillar fabricated with current density of 18 mA/cm2 is shown in Fig. 1. A high yield stress of 1.65 GPa is determined after the micro-compression test. Vickers hardness as a function of d -1/2 for the alloy specimens is shown in the scatter plot of Fig. 2, where d is average grain size. Microhardness value has a positive linear relationship with d -1/2 approximately, which corresponds well to Hall-Petch relationship. Maximum Vicker's hardness value of 526 Hv was obtained with a current density of 10 mA/cm2, which is much higher than those of pure Ni (306 Hv) and Co (403 Hv) films prepared in this study, indicating the effect of solid solution strengthening. [1] T. Yamamoto, K. Igawa, H.C. Tang, C.Y. Chen, T.F.M. Chang, T. Nagoshi, O. Kudo, R. Maeda, M. Sone, Microelectron. Eng. , 213, 18 (2019). [2] X. Luo, C.Y. Chen, T.F.M. Chang, H. Hosoda, M. Sone, J. Electrochem. Soc. , 162, D423 (2015). [3] M. Duch, J. Esteve, E. Gómez, R. Pérez-Castillejos, E. Vallés, J. Micromech. Microeng. , 12, 400 (2002). [4] C.Y. Chen, M. Yoshiba, T. Nagoshi, T.F.M. Chang, D. Yamane, K. Machida, K. Masu, M. Sone, Electrochem. Commun. , 67, 51 (2016). [5] M. D. Uchic, D. M. Dimiduk, J. N. Florando, W. D. Nix, Sc ience , 305, 986 (2004). [6] H. Tang, T.F.M. Chang, Y.W. Chai, C.Y. Chen, T. Nagoshi, D. Yamane, H. Ito, K. Machida, K. Masu, M. Sone, J. Electrochem. Soc. , 165, D58 (2018). [7] A. Bai, C.C. Hu, Electrochimica Acta , 47, 3447 (2002). Figure 1

This study reports mechanical properties of single-crystal pure gold toward for applications as movable components in micro-electrical-mechanical system (MEMS) inertial sensors requiring high sensitivity. Gold is commonly applied in electronic devices because of the excellent corrosion resistance, chemical stability, and conductivity. Recently, MEMS inertial sensors utilizing gold materials are reported to have low Brownian noise and high sensitivity while keeping the dimensions small by taking advantage of gold’s high mass density [1]. For design of electronic device, mechanical property characterization of the constituent material is essential. In general, mechanical strengths of metallic materials are governed by movement of the dislocation, and the dislocation movement is affected by the crystal orientation, grain boundaries, texture, impurities, etc. In addition to these, mechanical properties of metallic materials are affected by size of the sample used in the evaluation when the sample size is reduced to micro or sub-micro scale, which is known as the sample size effect [2]. This sample size effect would cause mechanical properties of micro-scale metallic materials to differ from those of bulk-size materials. Therefore, in order to make contribution to design of MEMS devices, the mechanical property characterization must be conducted using specimens having dimensions and deformation system alike components used in MEMS. Micro-bending test is suggested to be the most suitable micro-mechanical property characterization method toward MEMS devices since movable components in a MEMS device would experience both compressive and tensile stresses during operation. Regarding the sample size effect, the strength has a power-law relationship with cross-sectional area of the specimen [2]. In bending test, the loading direction is perpendicular to the cantilever’s width direction and parallel to the thickness direction. Because of this, the width and the thickness are expected to affect the sample size effect differently. On the other hand, the dislocation movable is also highly affected by the grain boundary and eventually the sample size effect [3]. Hence, single crystalline specimen is often used to examine the sample size effect to eliminate influences from the grain boundary. In this study, micro-cantilevers composed of single-crystal pure gold were fabricated using focus ion beam (FIB). All micro-cantilevers were ensured to have the loading direction parallel to the [1-10] orientation and the neutral plane parallel to the [110] orientation. Lengths of the micro-cantilevers were fixed at 50 μm. The thicknesses and widths were varied between 5 to 15 μm. The bending test was performed using a machine specially designed for micro-specimens developed in our group as shown in Fig. 1. The strain rate was fixed at 0.125 %/sec. Fig. 2 shows scanning electron microscope (SEM) images of the 50×9.7×10.5 μm3 micro-cantilever before and after the micro-bending test. From these images, slip lines and necking were observed at the base of the cantilever where stress and deformation were concentrated after the bending test. This slip lines indicated the active slip plane according to the Schmid’s law. The engineering stresses, σ, were calculated utilizing the Euler-Bernoulli beam theory [4]. The engineering strains are calculated from ratio of the d/y [5], where d is displacement of tip of the indenter. Fig. 3 shows engineering stress-engineering strain curves of the five micro-cantilevers. Yield stresses of the 6.9, 10.5, and 15.1 μm thick micro-cantilevers were 195, 175, and 128 MPa, respectively. Yield stress of the 4.7, 9.7, and 15.1μm width micro-cantilevers were 177, 175, and 183 MPa, respectively. These yield stresses were all lager than the value of bulk-size pure gold, which were results of the sample size effect. In addition, the yield stress increased as the thickness decreased, whereas it does not change much with the width reduction. This result suggested the sample size effect was only observed when changing the thickness, but not the width. These findings confirmed mechanical properties of micro-cantilevers were affected by the sample geometry, which is named as the sample geometry effect. [1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, and K. Masu, Appl. Phys. Lett. 104 (2014) 074102 [2] J.R. Greer, W.C. Oliver, W.D. Nix, Acta. Mater. 53 (2005) 1821-1830. [3] J.R. Greer and J.Th.M. De Hosson, Prog. Mater. Sci. 56 (2011) 654-724 (2011). [4] K. Asano, T.F.M. Chang, H.C. Tang, T. Nagoshi, C.Y. Chen, D. Yamane, H. Ito, K. Machida, K. Masu, M. Sone, ECS J. Solid State Sci. Technol., 8 (2019) P412-P415. [5] E. Demir, D. Raabe, F. Roters, Acta. Mater. 58 (2010) 1876-1886. Figure 1

Electrodeposited gold is used in movable micro-components of micro-electro-mechanical systems (MEMS) devices to allow further enhancement in the sensitivity and miniaturization of the device [1]. Mechanical strengths of gold are low among materials commonly used as structure materials in electronic devices. Electrodeposition method is advantageous in strengthening metallic materials because of the ability to refine the average grain size to nano-scale, which the mechanical strength increases along with a reduction in the grain size based on the grain boundary strengthening mechanism or the Hall-Petch relationship. Alloying of metallic materials is also an effective strategy to achieve the strengthening according to the solid solution strengthening mechanism. On the other hand, mechanical properties of metallic materials exhibit the sample size effect when dimensions of the specimen are reduced to micro or sub-micro scale. Therefore, mechanical property characterization of materials for MEMS applications should be conducted using specimens having dimensions in micro-scale. Various micro-mechanical testing methods have been developed in our group to characterize micro-specimens, such as micro-compression test [2,3], micro-tensile test [4], and micro-bending test [5]. Among them, micro-bending test is suggested to be the most suitable method to evaluate materials for applications in MEMS when compared with micro-compression and micro-tensile tests since cantilever-like structures are often applied in MEMS components, such as the micro-spring in a MEMS accelerometer. In this study, Au-Cu alloys are prepared by electrodeposition, and micro-mechanical properties of the electrodeposited Au-Cu alloys are characterized by a micro-bending test. Young’s modulus is an important information in design of movable micro-components. A non-destructive Young’s modulus evaluation method, resonance frequency measurement, is conducted using the Au-Cu alloy micro-cantilevers. Also, long-term vibration tests are conducted to reveal structure stability of the Au-Cu alloy micro-cantilevers. The Au-Cu electrolyte used in this work was a commercially available electrolyte provided by MATEX Co. Japan, which contained 17.3 g/L of X3Au(SO3)2 (X = Na, K), 1.26 g/L of CuSO4, and ethylenediaminetetraacetic acid as the additive with pH at 7.5. The electrodeposition was carried out at 50 °C, and the current density was varied from 0.1 to 8 mA/cm2. Specimens used in the micro-bending test were micro-cantilevers fabricated from the Au-Cu alloy films by focused ion beam. Dimensions of the micro-cantilevers fabricated were 10 µm × 10 µm × 50 µm. The micro-bending tests were conducted using a test machine equipped with a spherical diamond indenter at a constant displacement rate of 0.05 µm/s. For the Young’s modulus and long-term structure stability evaluations, the micro-cantilevers were prepared by electrodeposition combined with lithography process. The long-term vibration test was carried out under conditions of the cycle number in a range from 103 to 106, the frequency of 10.0 Hz, and the acceleration of 1.0 G (1 G = 9.8 m/s2). The structure stability was determined by evaluating height profiles of top surface of the micro-cantilevers before and after the vibration test by a 3D optical microscope. The Au-Cu alloy film electrodeposited at 5 mA/cm2 had the finest average grain size among films prepared in this work, which the grain size was 4.76 nm. Bending test of a micro-cantilever prepared from this film shows a high yield stress at 1826 MPa. Gold, copper, and Au-Cu alloys are known to be ductile materials, but brittle fracture was observed during the bending test when the current density used in prepared the film was higher than 5 mA/cm2. From the resonance frequency method, Young’s moduli of the Au-Cu alloy micro-cantilevers were ranged from 68.0 to 79.5 GPa, which were similar to the value of bulk-size specimens and indicated no sample size effect on the Young’s modulus. Regarding the long-term structure stability, after 106 cycles of the vibration test, all the micro-cantilever were still intact, and the height profiles did not change much after the vibration. Results obtained in this study confirmed high mechanical strength and structure stability in the electroplated Au-Cu alloys and demonstrated the advantage in applications as movable components in MEMS devices. [1] D. Yamane et. al. Appl. Phys. Lett. 104 (2014) 074102. [2] C.Y. Chen et. al. Electrochem. Commun. 51 (2016) 51-54. [3] H. Tang et. al. J. Electrochem. Soc. 164 (2017) 244-247 [4] Y. Kihara et. al. Mater. Lett. 153 (2015) 36-39 [5] K. Asano et. al. ECS J. Solid State Sci. Technol. 8 (2019) P412-P415.

Sn/SnO2 composite materials are promising anode materials in lithium ion battery. Electrodeposition with supercritical CO2(sc-CO2) emulsified electrolyte (EP-SCE) is an effective method in controlling morphology (Ni [1], Cu[2], and Co[3]) and crystallinity (ZnO [4]and TiO2 [5]) of electrodeposited materials. Sc-CO2 is CO2 at temperature and pressure above its critical point, which is 304.1 K and 7.38 MPa. CO2 is non-polar, therefore, the solubility of metal salts and electrical conductivity are both very low in sc-CO2. In order to conduct electrochemical reactions, surfactants are used to form emulsified electrolyte. Co-deposition of SnO2 along with electrodeposition of metallic Sn can occur at the same time in a SnCl2 based electrolyte containing NO3 - ions [6]. The EP-SCE is expected to be effective in controlling concentration and properties of the SnO2 in metallic Sn film. In this study, Sn/SnO2 composite films are prepared by electrodeposition with supercritical CO2 (sc-CO2) emulsified electrolyte, and the mechanical properties in micro-scale are evaluated by compression test of micro-pilliars fabricated from the Sn/SnO2 composite film for practical applications. Details of the high pressure electrochemical apparatus can be found in a previous study [1]. The electrolyte is composed of 0.22 M SnCl2, 0.015 M HCl and 0.31 M (NH4)3C6H5O7. The current density applied is varied from 2.8 to 17.3 mA/cm2. For the EP-SCE, the surfactant is C12H25(OCH2CH2)15OH. 0.2 vol.% of the surfactant is used to form the sc-CO2 emulsions. 20 vol.% of CO2 at 343 K and 15 MPa under agitation are used to ensure the formation of a stable emulsion. Electrodeposition without the sc-CO2 emulsions at ambient condition (CONV) was also conducted to be used as a comparison. Surface morphology of the films prepared by EP-SCE were very smooth and compact, and the CONV films were porous. On the other hand, there is not much different in the average grain size between the EP-SCE and CONV films, which was about 50 μm for both the EP-SCE and CONV films. The oxygen content in the EP-SCE films can be up to 28.3 at.% by adjusting the electrodeposition parameters. Micro-pillars with a 20×20 μm2 cross-section and a height at 40 μm were fabricated by focus ion beam from the deposited films. The yield stresses were ranged from 10 to 40 MPa when the current density changed. Acknowledgment: This work was supported by JST CREST Grant Number JPMJCR1433. Reference: [1] Electrochim. Acta 55 (2010) 6469-6475 [2] Thin Solid Films 529 (2013) 25-28 [3] J. Electrochem. Soc. 162 (2015) D423-D426 [4] J. Phys. Chem. C 117 (2013) 25596-25603 [5] Electrochem. Commun. 33 (2013) 68-71 [6] J. Alloys Compd. 403 (2005) 335-340

This study reports structure stability of micro-cantilevers composed of Au-Cu alloy for applications as moveable components in micro-electrical-mechanical system (MEMS) accelerometers. Au-based micro-components are developed to replace conventional Si-based components in a MEMS accelerometer to allow further miniaturization of the device [1]. However, the mechanical strength of Au is relatively low when compared with that of Si. The low mechanical strength raises concerns on structure stability of movable micro-components composed of Au-based materials, which could affect reliability and lifetime of the device. Electroplating is a promising technique in manipulating properties of the metallic materials for applications in MEMS. For instance, grain refinement effect often observed in electroplating can be employed to enhance the mechanical strength by the grain boundary strengthening mechanism [2]. In addition, alloy electroplating is a fascinating technique capable of controlling composition of the metallic material, and alloying the metallic material allows utilization of the solid solution strengthening mechanism to further enhance the mechanical strength [3]. By synergistic effects of the grain boundary strengthening and solid solution strengthening mechanisms, an ultra-high yield stress of 1.15 GPa [3] is reported for electroplated Au-Cu micro-specimens, which is comparable to fracture strength of Si. In order to apply the high-strength Au-Cu alloy as movable structures in MEMS devices, investigating the vibration fatigue is necessary to confirm the reliability and the lifetime. In this study, long-term vibration tests of micro-cantilevers fabricated by combining lithography and Au-Cu alloy electroplating were conducted to reveal effects of the vibration fatigue on structure stability of the Au-Cu alloy micro-cantilevers. Fabrication process of the micro-cantilevers is illustrated in Fig. 1. The Ti barrier layer and the Au seed layer were deposited by sputtering, and the layer thicknesses were both at 100 nm. The Au–Cu electrolyte used in this work was a commercially available electrolyte provided by MATEX Co. Japan, which contained 17.3 g/L of X3Au(SO3)2 (X = Na, K), 1.26 g/L of CuSO4, and ethylenediaminetetraacetic acid as the additive with pH at 7.5. The electroplating was carried out at 50 ◦C, and the current density was varied from 0.1 to 3 mA/cm2. A piece of Pt plate was used as the anode. Image of the Au-Cu alloy micro-cantilevers before the vibration test is shown in Fig. 2. Design-length of the micro-cantilever (l) was varied from 50 ~ 1000 μm, and design-width of the micro-cantilever (w) was ranged from 5 ~ 20 μm. Thickness of the Au-Cu alloy layer (t) was from 2.3 ~ 5.0 μm. The long-term vibration test was carried out under conditions of the cycle number in a range from 103 to 106, the frequency of 10.0 Hz, and the acceleration of 1.0 G (1 G = 9.8 m/s2). The structure stability was determined by evaluating height profiles of top surface of the micro-cantilevers before and after the vibration test by a 3D optical microscope. Height profiles of a micro-cantilever composed of 97.7 % Au with the length at 1000 μm, the width 20 μm, and the thickness at 3.9 μm before and after the vibration test are shown in Fig. 3. Fluctuations in the height before the vibration test, the 0 cycle result, were mainly results of the surface roughness. After 106 cycles of the vibration test, the micro-cantilever was still intact as observed from the optical microscope. The height profiles did not change much after the vibration. In order to quantify the structure stability, deflection at tip of the micro-cantilever was determined. The tip deflection merely changed 0.37 μm for the 1000 μm long micro-cantilever after 106 cycles of the vibration test. This result confirmed high structure stability in the electroplated Au-Cu alloy micro-cantilever and demonstrated the superiority of this Au-Cu alloy structure in applications as movable components in MEMS devices. [1] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett., 104 (2014) 074102. [2] H. Tang, C.-Y. Chen, T. Nagoshi, T.-F.M. Chang, D. Yamane, K. Machida, K. Masu, M. Sone, Electrochem. Commun., 72 (2016) 123–130. [3] H. Tang, C.-Y. Chen, M.Yoshiba, T.Nagoshi, T.-F.M. Chang, D.Yamane, K.Machida, K.Masu, M.Sone, Electrochemical Society, 164 (2017) 244–247 Figure 1

Au materials fabricated by electroplating are commonly used as contact materials for high reliability circuit boards, electrical connectors, relays, and micro- and nano-scale electronic components for many decades because of their high electrical conductivity, chemical stability, corrosion resistance, and ductility. In recent years, Au has become a promising material to be used as the movable structures and proof mass in micro-electrical-mechanical system (MEMS) accelerometer devices, mostly because of the high density (19.3×103 kg/m3 at 298 K), which is about 10 times higher than that of silicon (2.33×103 kg/m3at 298 K) [1]. However, Au is known to be a soft material. The mechanical strength becomes a concern in miniaturization of the MEMS device. Yield stress of bulk Au is reported to be 55~200 MPa [2]. Decreasing grain size of the Au materials is expected to further enhance the mechanical properties according to the Hall-Petch relationship [3]. Pulse electroplating (PE) has been reported to be effective in fabricating Au materials with fine grains, high uniformity, and low porosity [4]. Also, it is possible to control the composition and the film thickness by regulating the pulse amplitude and width. Most importantly, an increase in the nuclei density could be achieved to obtain electroplated films with finer grains. On the other hand, for evaluating mechanical properties of the electroplated films, Vickers micro-hardness test is the most popular method. However, the hardness results are often affected by the substrate. It cannot show the real strength of the electroplated materials, especially for films in mico-/nano-scale. Therefore, it is necessary to evaluate micro-mechanical strength of the Au electroplated films for the practical applications in the miniaturized devices. Au electrolyte used in this study was a commercially available sulfite-based electrolyte provided by Matex Japan (Matex Gold NCA). The electrolyte contains 50 g/L of Na2SO3, 50 g/L of (NH4)2SO3, and 21.63 g/L of Na3[Au(SO3)2] with pH of 8.0 and 5% sodium gluconate. Cu plates and Pt plates were used as the cathode and anode, respectively. For the PE, the pulse current (Ion) was 10 mA/cm2, and the off-time current (Ioff) was 0 mA/cm2. The on-time (Ton) of the PE varied from 1 to 100 ms, and the off-time (Toff) was 10 ms. The reaction temperature was 40 ºC for the gold electroplating. The Au films prepared by the PE showed less defect, lower surface roughness, finer grain size, and denser texture when compared with the Au films prepared by the conventional constant-current electroplating (CE). Micro-mechanical properties of Au micro-pillars fabricated from the Au films were evaluated by micro-compression test. Dimensions of the fabricated pillars were 10 μm×10 μm×20 μm. The compression tests were carried out using a test machine specially designed for micro-sized specimens equipped with a flat-ended diamond indenter at a constant displacement of 0.1μm/s. The Au micro-pillars prepared by the PE showed controllable high strength ranged from 500 to 800 MPa, where the grain size was adjusted by the PE parameters. Finest grain size was estimated to be 10.4 nm, and the highest compressive strength was 800 MPa [5]. The compressive strength obtained is much higher than the values reported in other studies [6,7]. The high strength is suggested to be due to the grain-boundary strengthening mechanism. The results demonstrated that the PE method and the sulfite-based electrolyte are promising for applications in miniaturization of the MEMS devices. To the best of our knowledge, this is the also first report on micro-mechanical strength of pure Au materials fabricated by the PE. References: [1] D. Yamane, T. Konishi, T. Matsushima, K.Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett. 104 (2014) 074102. [2] H.D. Espinosa, B.C. Prorok, B. Peng, J. Mech. Phys. Solid 52 (2004) 667–689. [3] N. J. Petch, J. Iron Steel Inst. 174 (1953) 25–28. [4] J. Horkens, L. T. Romankiw, J. Electrochem. Soc. 124 (1977) 1499–1505. [5] C.Y. Chen, M. Yoshiba, T. Nagoshi, T.F.M. Chang, D. Yamane, K. Machida, K. Masu, M. Sone, Electrochem. Commun., 67 (2016) 51–54. [6] D. Jia, K.T. Ramesh, E. Ma, Acta Mater. 51 (2003) 3495–3509. [7] Z. Gan, Y. He, D. Liu, B. Zhang, L. Shen, Scripta Materialia 87 (2014) 41–44.

According to the International Data Corporation (IDC) report [1], the worldwide wearable devices market will swell to 111.9 million networked devices sold in 2018 projecting the trend and blossom of this potential technology. To fulfill both conductivity and flexibility for wearable device, combining the conductive materials and the flexible fabrics, such as metal and polymer, becomes the greatest challenge. There are many techniques to achieve the combination, such as sputtering and evaporation. Among these, electroless plating is the most potential method owing to its eminent advantages, e.g. low processing temperature, low cost, and simple operation procedures [2]. Electroless plating consists of a pretreatment step to clean and roughen the substrate, a catalyzation step to inlay the catalyst seeds, and a plating step to carry out the metallization on the substrate. A novel catalyzation step with the assist of supercritical carbon dioxide (sc-CO2) was conducted in this study. Distinct from the conventional catalyzation, catalyzing the polymer in sc-CO2 allows exclusion of the pretreatment step and further results a defect-free and smoother surface [3]. With the outstanding self-diffusivity, high compatibility with non-polar materials, and low surface tension characteristics of sc-CO2, organo-metallic catalysts can be embedded into the polymer without damaging the structure. Furthermore, with sc-CO2, the chemical solution used in the catalyzation step can also be withdrawn, which is friendlier to the environment. In previous studies, we have proposed an electroless plating method involving Ni-P plating step on polyimide followed by catalyzation in pure sc-CO2 containing Pd (II) acetylacetonate (Pd(acac)2) [3,4]. Ni-P solution is a common electrolyte for the plating due to the advantages, such as relatively high deposition rate and high corrosion resistivity. To obtain higher mobility and the application of wearable devices, softer silk is chosen as the substrate in this study. Moreover, silk is a common material in clothes. The combination of Ni-P and silk is thus considered to be a potential candidate for the wearable devices. This work, therefore, executed the electroless plating between Ni-P and silk. A piece of the silk textile was immersed in the sc-CO2 fluid containing Pd(acac)2 at 80 oC and 15 MPa. Appropriate amount of the Pd(acac)2 catalyst was used to maintain the concentration at the saturation concentration throughout the catalyzation step. No pretreatment was conducted in this study. The plating step was thereupon being carried out on the catalyzed silks at 70 oC and atmospheric pressure in the acidic Ni-P solution. Various lengths of plating times were performed to inspect the relationship between the plating time and the properties. The surface morphology, cross-section and composition were examined by an optical microscope (OM), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX). The phase and crystal structure were identified by an X-ray diffractometer (XRD). The electrical resistance was measured by the four-point probe method. By conducting the catalyzation in sc-CO2, smooth and uniform coverage of Ni-P coatings were successfully plated on the silk textiles under various plating times. The composition, thickness, and coverage trend increase with plating time. The composites turn out to be conductive as the plating time is extended to 2 mins. Lowest electrical resistance at 1.23 ohm can be achieved with plating time at 4 min. References [1] R.T. Llamas, Worldwide Wearable Computing Device 2014–2018 Forecast Update: December 2014, U.S.A.: IDC Research, Inc. (2014). [2] G.O. Mallory and J.B. Hajdu, "Electroless plating: fundamentals and applications", United Kingdom: Cambridge University Press, (1990). [3] B.H. Woo, M. Sone, A. Shibata, C. Ishiyama, K. Masuda, M. Yamagata, and Y. Higo, Microelectron. Eng. 86 (2009) 1179-1182. [4] B.H. Woo, M. Sone, A. Shibata, C. Ishiyama, K. Masuda, M. Yamagata, and Y. Higo, Surf . Coat . Techno ., 203 (2009) 1971-1978.

Au is the most commonly used as contact material for high reliability electrical connectors, circuit boards, and as functional component for micro-electrical-mechanical system (MEMS) devices [1,2]. Electroplating has been applied extensively in fabrication of Au films used for all kinds of devices. Among the electroplating techniques, pulse electroplating is a technique known to control the properties of the deposited films by regulating the pulse amplitude and width. Therefore, morphology, crystal structure and deposition rate of the Au films can be controlled precisely [3]. In this work, grain refinement and surface smoothening of Au films were achieved by pulse electroplating using non-toxic sulfite-based electrolyte. Micrographs obtained by atomic force microscopy (AFM) and X-ray diffraction (XRD) patterns indicate that the surface of the pulse electroplated Au films possess less defect, lower surface roughness, smaller grain size, and denser texture when compared with the Au films prepared by conventional constant-current electroplating. By XRD analysis and Scherrer equation calculation, the estimated grain size of the Au film prepared by pulse electroplating was 10.5 nm, and was much smaller than the grain size of the Au film prepared by constant-current electroplating, which was 22.8 nm. The grain size achieved by the pulse electroplating is even smaller than previous reports [3,4]. This can be attributed to the higher pulse current density which leads to an increase in the nucleation rate during the on-time period. Mechanical strength in micro-scale of the Au films was also evaluated to determine the reliability and lifetime of the components composed of the electroplated Au. After the electroplating, micro-pillars made from the Au films were fabricated by focus ion beam (FIB) milling. Size of the fabricated pillar was fixed at 10 mm ×10 mm×20 mm. The micro-compression test was carried out using a test machine specially designed for micro-sized specimens developed in our laboratory. The pulse electroplated Au micro-pillar acquired a large increase in the compressive strength from 600 to 800 MPa when the grain size approaches 10.4 nm, presumably due to the grain-boundary strengthening known as the Hall-Petch effect [5]. This strength is much higher than that of bulk Au materials, which is only 55~200 MPa. The strength is even higher than that of the micro-pillar fabricated from the constant-current electroplated Au films. These results reveal that the pulse electroplated Au micro-pillar possesses better ductility and malleability than that fabricated by constant-current electroplating and demonstrate that pulse electroplating is a promising technique to fabricated Au functional micro-components especially for the advanced MEMS devices. Reference: [1] N. Togasaki, Y. Okinaka, T. Homma, T. Osaka, Electrochim. Acta, 51 (2005) 822. [2] D. Yamane, T. Konishi, T. Matsushima, K. Machida, H. Toshiyoshi, K. Masu, Appl. Phys. Lett., 104 (2014) 074102. [3] M. S. Chandrasekar, M. Pushpavanam, Electrochim. Acta, 53 (2008) 3313. [4] Y. Lee, S. K. Ahn, Y. Roh, Surf. Coating Tech., 193 (2005) 137. [5] A.H. Chokshi, A. Rosen, J. Karch, H. Gleiter, Scripta Metallurgica, 23 (1989) 1679.

As an important class of materials with a multitude of potential applications, metal oxide thin films and methods for their preparation have gathered a lot of attention. The synthetic approaches include hydrothermal methods, cathodic deposition, anodic oxidation, sol-gel method, and spray pyrolysis. Among them, cathodic deposition offers a low-cost yet effective process for production of metal oxides with controllable morphology and crystal structure. For a typical cathodic deposition, hydroxyl ions are generated by polarizing the substrate cathodically, which then causes a concentration gradient of hydroxyl ions from the substrate surface to the bulk solution. Since the concentration of hydroxyl ions is locally high at the substrate surface, metal ions initially dissociated in the electrolyte would combine with the hydroxyl ions and subsequently produce metal hydroxides and/or oxides on the surface of the substrate. By employing cathodic deposition, thin films of metal oxides such as TiO2 and SnO2 can be directly deposited on conductive substrates with complicated surface geometry. Such a feasibility of obtaining TiO2 and SnO2 thin films has significant implications in the aspects of photoelectrochemical applications, H2O2 sensor, and anode material for Li-ion batteries. However, the as-deposited TiO2 and SnO2 are usually amorphous and an additional heat treatment process, for example, annealing at 400 °C for 1 hr, is needed to obtain crystalline TiO2 and SnO2. The need of the post-heat treatment limits selection of the substrates in cathodic deposition of the metal oxides, which further hinders the applicability of the products. Therefore, it is practically significant if crystalline metal oxides can be obtained directly from cathodic deposition without the post-heat treatment. On the other hand, the effect of pressure on crystallization of metal oxides deposited from the solution phase is rarely investigated. In a previous study, grain size of the TiO2 cathodically deposited with a supercritical CO2 (sc-CO2) emulsified electrolyte (SCE) was found to be increased with an increase in the pressure, which a post-heat treatment at 400 °C for 1 hr was still needed to have crystalline TiO2 [1]. Supercritical CO2 (sc-CO2) is CO2 at temperature and pressure above its critical point. Sc-CO2is non-polar, therefore, the solubility of metal salts and electrical conductivity are both very low in sc-CO2. In order to conduct electrochemical reactions, a surfactant is used to form emulsified electrolyte. In this study, the effect of the sc-CO2 emulsified electrolyte (SCE) on proproperties, especially the crystallinity, of the TiO2 and SnO2 thin films deposited cathodically will be reported. Details of the high pressure electrochemical apparatus can be found in a previous study [2]. CO2 with a minimum purity of 99.9% was used. The TiO2 electrolyte was composed of 0.47 M NaCl, 25 mM TiCl3, and 75 mM NaNO3. The SnO2 electrolyte was composed of 25 mM SnCl2 and 75 mM NaNO3. For cathodic deposition of both TiO2 and SnO2, 80 vol.% of the aqueous electrolyte was used. For the TiO2, a constant current density and temperaure of 2.5 A/dm2 and 40°C, respectively, were used. For the SnO2, a constant current density and temperaure of 2.5 A/dm2 and 40°C, respectively, were used, and the pressure was varied from atmospheric pressure to 35 MPa. For the SnO2, a constant current density of 1 A/dm2 was used. The tempreaure was varied from 70 to 100°C, and the pressure was vaired from atmospheric pressure to 25 MPa. For both the TiO2 and SnO2 thin films cathodically deposited at elevated pressure, an improvement in the crystallinity with an increase in the pressure was observed from the X-ray diffraction analysis. The high-resolution transmission electron microscopy and selected-area electron diffraction confirmed that the TiO2 deposited at 35 MPa had the anatase crystal structure. The electrochemical impedance spectroscopy also showed an enhancement in the charge carrier dynamics at the TiO2 electrode/electrolyte interface with an increase in the pressure, which is a result of the improved crystallinity. The effect of pressure on crystallinity of the SnO2 was more obvious than that on the TiO2. Grain size of the SnO2 films estimated by the Scherrer equation was found to increase from 3.47 to 14.82 nm with an increase in the pressure from 5 to 25 MPa. References [1] T.F.M. Chang, W.H. Lin, Y.J. Hsu, T. Sato, M. Sone, Electrochem. Commun. 33 (2013) 68-71. [2] T.F. M. Chang, M. Sone, A. Shibata, C. Ishiyama, Y. Higo, Electrochim. Acta, 55 (2010) 6469-6475.

In recent years, rechargeable Li-ion batteries (LIBs) have attracted much attention because of the wide applications not only for portable electronics but also electric vehicles. However, the current LIBs do not meet the requirements of the market, such as high energy density, long cycle life, and low cost. To solve this problem, great efforts have been made to develop new materials for both anode and cathode. For anode materials, SnS is considered as a potential candidate to replace the conventional graphite anode, because it has much higher theoretical capacity (782 mA h g-1). Herein, we report the fabrication of ultrathin single-crystalline SnS nanoflakes by facile hydrothermal method. The SnS nanoflakes possess two-dimensional layer structures which have large surface area and open-edge morphologies, and are expected to enhance the electrochemical performance when compared with their bulk counterparts. Sn commercial foils (99.5% purity, Nilaco Corp) were rinsed with ethanol, and then placed into the cysteine-containing solution. The solution was subsequently transferred to a Teflon-line autoclave. The reaction was conducted at 160 ̊C for 24h, and the aotoclave was cooled down to room temperature. After washing and drying, Sn foil with SnS nanoflakes was directly used as anode of the coin cell without any binder, and Li foil was used as counter electrode. Figure 1 (a) shows surface morphology of the Sn foil. The randomly aligned flake-like structures were observed on the Sn foil. The thickness of the nanoflakes was less than 20 nm, and the width was in the range of micrometer. The HRTEM image and diffraction pattern are given in Figure 1 (b). Clear lattice fringes and the dot diffraction pattern both reveal that the SnS nanoflakes are single crystalline. The lattice fringes with lattice spacing of 3.4 Å and 3.7 Å correspond well to the (012) and (101) planes of SnS, respectively. The cyclic voltammogram (CV) profiles of the SnS nanoflakes on Sn foil are demonstrated in Figure 1 (c). The potential applied is ranged from 0.5 V to 2.0 V to avoid involvement of Sn foil in the reaction. During the first cycle, anodic peaks were observed at ca. 0.72, 0.80, and 1.85 V. Sharp anodic peaks at 0.72 and 0.80 V can be attributed to the electrochemical de-alloying of Sn–Li alloy. Cathodic peaks at ca. 0.68 and 1.4 V were also observed. The cathodic peaks can be attributed to the decomposition of SnS into Sn and the subsequent reaction between Sn and Li ion. The charge-discharge profiles show the initial reversible capacity was 990 mA h g-1, and much higher than the theoretical reversible capacity. This is attributed to synergistic effect of unique internal layer structure and ultra-thin flake structures. In summary, the two-dimensional ultrathin single-crystalline SnS nanoflakes are proved as one of the promising candidate as anode material for the new generation of LIBs. Figure 1. (a) SEM image and (b) HRTEM image and diffraction pattern of SnS nanoflakes. (c) CV profiles of the ultra-thin SnS nanoflakes electrode from 0.5 to 2.0 V with scan rate of 0.1 mVs-1. REFERENCE J. Mater. Chem., 2012, 22, 23091–23097.

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