Fabrication of High-Aspect-Ratio Electrode Arrays for Three - CiteSeerX

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JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 16, NO. 4, AUGUST 2007

Fabrication of High-Aspect-Ratio Electrode Arrays for Three-Dimensional Microbatteries Fardad Chamran, Yuting Yeh, Hong-Seok Min, Bruce Dunn, and Chang-Jin Kim

Abstract—Silicon-micromachining techniques have been combined with conventional material-synthesis methods to develop microelectrodes for 3-D microbatteries. The resulting electrodes feature an organized array of high-aspect-ratio microscale posts fabricated on the current collector to increase their surface area and volume for a given footprint area of the device. The diameter of the posts ranges from a few micrometers to a few hundred micrometers, with aspect ratios as high as 50. The fabrication approach is based on micromolding of the electrode materials and subsequent etching of the mold to release the electrode structures. Deep reactive-ion-etching or photo-assisted anodic etching has been used to form an array of deep holes in the silicon mold. Electroplating or colloidal-processing method has been used to fill the mold with battery-electrode materials. Measurements on electrochemical half-cells indicated that the 3-D electrode arrays, which are composed of vanadium oxide nanorolls or carbon, exhibited much greater energy densities (per-footprint area) than that of the traditional 2-D electrode geometries. The use of electroplating enabled us to fabricate 3-D interdigitated arrays of nickel and zinc; and battery operation was demonstrated. [2006-0293] Index Terms—Batteries, energy storage, microbattery, microelectrodes, microelectromechanical devices, three-dimensional (3-D) battery.

I. I NTRODUCTION

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HREE-DIMENSIONAL (3-D) battery architectures have been proposed as a potential solution to miniaturizing portable power sources [1]–[3]. These architectures are based on developing 3-D electrode arrays of high-aspect-ratio micropost structures [4], [5]. With this configuration, it is possible to maintain a small areal footprint for the battery without compromising power and energy density. These electrodes make use of the out-of-plane dimension (height) in contrast to traditional battery electrodes, which use only the in-plane surface. By

Manuscript received December 31, 2006; revised April 6, 2007. This work was supported in part by the Office of Naval Research under Grant N0001401-1-0757 and in part by the Material Creation Training Program (MCTP), an NSF Integrated Graduate Education and Research Training (IGERT) Program at UCLA. Subject Editor F. Ayazi. F. Chamran was with the Mechanical and Aerospace Engineering Department, University of California, Los Angeles, CA 90095 USA. He is now with Innovative Micro Technology, Santa Barbara, CA 93117 USA (e-mail: [email protected]). Y. Yeh was with the Materials Science and Engineering Department, University of California, Los Angeles, CA 90095 USA. He is now with Kyocera Wireless Inc., San Diego, CA 92121 USA. H.-S. Min and B. Dunn are with the Materials Science and Engineering Department, University of California, Los Angeles, CA 90095 USA. C.-J. Kim is with the Mechanical and Aerospace Engineering Department, University of California, Los Angeles, CA 90095 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/JMEMS.2007.901638

increasing the height of the electrode, it is now possible to provide more active material for the battery without increasing the footprint area of the battery. This change from 2-D to 3-D represents an important paradigm shift in the development of electrochemical power sources. As pointed out by Long et al. [2], powering of autonomous microelectromechanical-system (MEMS)-based sensors with the current 2-D thin-film battery technology requires footprint areas on the order of several square centimeters. In contrast, 3-D batteries potentially offer sufficient energy and power with a much smaller (in square millimeters) areal footprint. It should also be noted that the high surface area of 3-D electrode configurations in combination with short diffusion distances enables the battery to sustain high discharge rates [2]. This paper describes different approaches for fabricating 3-D electrode and battery architectures. To allow integration of MEMS devices and their power source on the same chip, we prefer to use fabrication methods consistent with MEMS technologies. Our general approach in fabricating the 3-D electrode is based on micromolding [4]. The mold is prepared by high-aspect-ratio etching techniques, while the electrodefabrication process depends on the specific battery system, which, in turn, determines the choice of electrode materials. A clear benefit of this approach is that a variety of different electrode materials can be fabricated into post-electrode-array structures. In the research reported here, the mold is made of silicon, and high-aspect-ratio holes are etched through the wafer using two different methods, deep reactive-ion etching (DRIE) and photo-assisted anodic etching. Depending on the battery type, either electroplating (for nickel–zinc batteries) or colloidal processing (for lithium-ion batteries) is used to fill the holes with the desired electrode material. Upon removing the silicon mold by etching, 3-D microelectrode arrays are obtained. Electrochemical measurements on both half-cells and complete battery cells are reported. The architecture of the 3-D electrodes consists of an array of high-aspect-ratio posts placed on current collectors. This configuration is shown schematically in Fig. 1 for two different designs. The design in Fig. 1(a) is an array with a single current collector, while the design in Fig. 1(b) is based on two sets of electrodes on separate current collectors arranged in an interdigitated manner. The former constitutes a half-cell; the latter represents a full battery. II. F ABRICATION OF S ILICON M OLDS The silicon mold features an array of high-aspect-ratio cylindrical holes. Two different etching methods, photo-assisted

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CHAMRAN et al.: FABRICATION OF ELECTRODE ARRAYS FOR THREE-DIMENSIONAL MICROBATTERIES

Fig. 1.

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Schematic of high-aspect-ratio electrode post arrays. (a) Single electrode. (b) Interdigitated electrodes.

anodic etching and DRIE, successfully created holes in silicon wafers with aspect ratios as high as 50 and 10, respectively. A. Photo-Assisted Anodic Etching The mechanism for this type of etching and some of its applications have been reviewed in detail [6]–[8]. High-aspectratio holes can be etched in n-type silicon by anodic etching in hydrofluoric acid (HF) with illumination from the backside. In typical experiments, an n-type (100) silicon wafer with resistivity of 50 Ω · cm was used as the substrate, and 1500-Å thermal silicon dioxide was grown on the surface. The oxide was grown by heating in flowing oxygen at 1000 ◦ C for 16 min. An array of small squares was opened on the oxide using photolithography and reactive-ion etching (RIE). The squares had a variety of dimensions, from 5 to 12 µm on a side, and the array had pitches from 10 to 24 µm. The silicon was then etched in KOH at 80 ◦ C for 15 min to form the inverted pyramids whose sharp tips serve as the nucleation sites for the subsequent anodic etching. After removing the thermal oxide in buffered oxide etchant (BOE; six parts 40% NH4 F and one part 49% HF), the high-aspectratio holes were produced in the silicon by anodic etching. In this process, a platinum sheet served as the cathode, and the sample was illuminated from the backside using a high-power light source (Marubeni America Corporation) operating at 870 nm. The area of silicon exposed to HF was circular in shape with 2-cm diameter. The electrolyte was 4% aqueous HF that contained a few droplets of a wetting agent (Kodak Foto-Flo), which helped remove the hydrogen bubbles generated during the etching. The sample was etched at constant anodic voltage of 4 V, while the current density was kept constant at 6 mA/cm2 by adjusting the illumination intensity. Fig. 2 is a schematic of the Teflon electrochemical cell that was custom-designed and built for the anodic etching described above. A peristaltic pump (Masterflex) circulated the HF through the Teflon tubing and the two inlets and outlets in order to agitate the electrolyte and provide more uniform etching across the sample. The silicon was biased from the backside using a copper ring. To provide ohmic contact between the silicon and the copper, a thin layer of In–Ga eutectic was

Fig. 2. Schematic of custom-built photo-assisted electrochemical cell for anodic etching of silicon.

applied at the contact area. An O-ring was used to seal the sample and prevent leakage of the electrolyte. The setup was tilted 90◦ compared to the image shown in Fig. 2 to facilitate the removal of hydrogen bubbles from the surface. The two power supplies for biasing and illumination used LabVIEW software to provide constant voltage and current during the etching process. Fig. 3 shows the cross section of a sample after being etched for 75 min. The holes are 6 µm in diameter and 80 µm in deep, with 16-µm pitch size. To fabricate molds with deeper holes, the silicon was etched for longer periods of time. Most of the molds required 5 h to etch 250 µm deep into silicon. To make the holes extend through the entire thickness of the wafer, the sample was blank etched from the backside using DRIE. B. Deep Reactive–Ion Etching (DRIE) Another method used to create deep holes in silicon was DRIE, using PlasmaTherm SLR770 ICP and the standard

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Fig. 3. Silicon mold after photo-assisted anodic etching for 75 min. The holes are 6 µm in diameter and 80 µm in depth.

Bosch process. DRIE was used when holes with bigger openings and lower aspect ratios, as compared to the anodic-etching process, were desired. Thick (10 µm) photoresist (AZ 4620) was used as the etching mask, and cylindrical holes of 50–120 µm in diameter were etched through the entire thickness (500 µm) of the wafer. III. F ORMATION OF E LECTRODE P OST S TRUCTURES Two different methods were used to fabricate the electrode arrays, as shown in Fig. 1. Electroplating was used to form the interdigitated electrode array that served as the basis for the nickel–zinc battery. Colloidal processing was used for fabricating 3-D electrode arrays with materials suitable for the lithiumion battery. In both cases, sacrificial etching of the mold was required to obtain the micropost structure. A. Electroplating of Electrode Arrays Nickel and zinc post electrodes were fabricated by electroplating within the holes etched in silicon molds. First, single electrode arrays [Fig. 1(a)] of nickel and zinc were prepared to help establish the electroplating-process conditions and for studying the electrochemical properties of the individual electrodes in a half-cell configuration. Based on this paper, the full nickel–zinc battery was fabricated by electroplating both nickel and zinc electrodes in an interdigitated configuration. 1) Single Electrodes: Nickel or zinc electrodes were fabricated by electroplating the metal post in the silicon mold and subsequently etching the mold. Fig. 4 is a schematic of the fabrication process at a cross section of Fig. 1(a). The highaspect-ratio molds that had been fabricated using anodic etching of silicon were used as the electroplating template. In step 1, 1500 Å of thermal oxide was grown to passivate the silicon during electroplating steps. In step 2, a Ti/Ni (100/1000 Å) seed layer was evaporated by electron beam on the bottom surface of the mold. This process was followed by electroplating 20 µm of nickel. This nickel layer serves as the current collector for the electrode and effectively closes the holes that had been

Fig. 4. Process flow for the fabrication of 3-D nickel or zinc post array electrodes.

etched through the silicon wafer. The nickel layer also serves as the seed layer for the subsequent electroplating. In step 3, nickel or zinc was electroplated inside the mold. To minimize the formation of nickel oxide on the seed layer, which may reduce the adhesion of the electroplated posts, the above steps were carried out immediately in order to minimize exposure to air. A tape was applied on the nickel current collector to prevent it from being electroplated. Commercially available nickel electroplating solution (Technic Inc.) was used for electroplating nickel, Nickel sheet was used as the counter electrode. The zinc electroplating solution was prepared by dissolving zinc sulfate (240 g), ammonium chloride (15 g), aluminum sulfate (30 g), and saccharin (1 g) in 1 L of deionized (DI) water [9]. Zinc sheet was used as the counter electrode during electroplating. Nickel and zinc were electroplated at current densities of 10 and 20 mA/cm2 , respectively. For these plating conditions, the nickel and zinc electroplating rates were determined to be 9 and 15 µm/h, respectively. Extra precaution was taken to prevent


Fig. 5.

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High-aspect-ratio post array electrodes of (a) nickel and (b) zinc.

hydrogen-bubble generation and forming dense electrodes, by keeping the current densities low. In step 4, the mold was etched away to release the post array. The silicon-dioxide layer at the top of the mold was first etched in BOE to expose the silicon, and then, the silicon was etched by XeF2 . In the final processing step, the silicon dioxide was etched from the surface of the posts using BOE, and the sample was rinsed in DI water. Highaspect-ratio nickel and zinc posts are shown in Fig. 5(a) and (b), respectively. The posts are 8 µm in diameter and 200 µm in height. 2) Interdigitated Electrodes: Fabricating interdigitated sets of nickel and zinc electrodes required a different scheme from that used for the single-electrode arrays. A glass wafer was used rather than silicon in order to electrically isolate the nickel and zinc collectors on the substrate. Fig. 6 shows the process flow at the cross section “AA” of Fig. 1(b). First, the current collectors were patterned as interdigitated lines on a glass substrate. In step 1, after photolithography, the glass was etched in BOE for 5 min to recess the area of the current-collector lines. In step 2, Ti/Au (100/1000 Å) current collectors were deposited by electron-beam evaporation and patterned by lift-off. In this way, the metal surface was at the same level as that of the glass surface for subsequent processing. In step 3, the silicon mold and the glass substrate were anodically bonded. The reason for this step is that, if the silicon template is not properly sealed, the metal will electroplate underneath the mold and short-circuit the adjacent current collectors. Anodic bonding was carried out at 450 ◦ C and 1000 V. Prior to this process, 150-nm thermal oxide was grown on the silicon mold for passivation purposes. In step 4, nickel and zinc were electroplated successively, with the current collectors providing the electroplating seed layers. After the nickel electrodes were electroplated by using one of the current collectors, the sample was rinsed in DI water and then zinc electrodes were electroplated using the other current collector. The electroplating conditions were similar to that of the single-electrode-array formation described previously. In step 5, the silicon and thermal oxide were etched away in XeF2 and BOE, respectively, to release the electrode structures. At this stage of the process, the sample consists of an interdigitated

array of nickel and zinc post electrodes, as shown in Fig. 7(a). The posts are 50 µm in diameter and 400 µm in height. The footprint area covered by electrodes is 5 × 5 mm2 . To prepare the electrodes for a functional nickel–zinc battery, nickel hydroxide was electrochemically deposited on each post of the nickel-electrode array (step 6 in Fig. 6). This deposition technique is based on electro-reduction of nickel nitrate on nickel metal [10]. A solution of 1M aqueous nickel nitrate at 85 ◦ C was used as the electrolyte. To provide a uniform electric-field distribution and conformally coat the nickel posts, the zinc post array was used as the counter electrode. The deposition of 1 µm of nickel hydroxide required 80 s at a constant current density of 20 mA/cm2 . Fig. 7(b) shows the conformal deposition of nickel hydroxide on the nickel posts. The nickel cathode is in discharged mode after the nickel hydroxide deposition. The nickel hydroxide will transform to nickel oxyhydroxide after some charge–discharge cycling to form a fully charged battery [4]. B. Colloidal Processing of Electrode Arrays This approach benefits from the fact that the colloidal processing of materials has been a topic of considerable interest over the past decade [11]. Two different colloidal systems, one based on transition metal oxide and one based on carbon, were formed into post-array structures. The electrode-fabrication process involves forming a colloidal suspension of the electrode material in a suitable solvent, forced infiltration of the suspension in the silicon mold (Fig. 8), followed by etching of the mold to release the post structure. Vanadium oxide nanorolls (VONR) and mesocarbon microbeads (MCMB) were used as the electrochemically active components in the colloidal electrode materials. VONR and MCMB are well known for their lithium-ion intercalation properties and can be used as the positive and negative electrodes, respectively, for the lithium-ion battery [12], [13]. The MCMB is commercially available, while the VONR was synthesized by collaborators, as previously reported the study in [12]. The positive electrode consisted of the electrochemically active constituent, VONR powder

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The filled mold was then dried in an oven at 200 ◦ C for 3 h to remove the residual solvent and to melt the PVDF binder. Silver epoxy (EP21TDC/S, MasterBond) was spread on one side of the mold. The epoxy functions as the current collector by electrically connecting all the post electrodes and also providing mechanical stability by holding the rods in place after etching the mold. The silicon was etched in XeF2 to release the postarray structure. A similar process was implemented to fabricate the carbon-post structures using the MCMB. The carbon posts were composed of 90 wt% of MCMB and 10 wt% of PVDF binder. SEM images of the VONR and carbon-post arrays are shown in Fig. 9(a) and (b). IV. E LECTROCHEMICAL P ROPERTIES OF 3-D E LECTRODES

Fig. 6. Process flow for fabricating interdigitated nickel and zinc post array electrodes.

(80 wt%), which was mixed with carbon black (Ketjenblack, Akzo Nobel) (10 wt%) and polyvinylidene fluoride (PVDF; Degussa) (10 wt%). The carbon black was added to increase the conductivity of the mixture, and the PVDF functioned as a binder. The mixed powder, typically 5 mg, was dispersed in 50 ml of propylene carbonate (PC; anhydrous, Aldrich) which served as the solvent. To obtain homogeneity, the suspension was sonicated for 30 min followed by stirring for 12 h. The solution was poured into a stainless-steel syringe that was driven by a high-pressure syringe pump (PHD 2200, Harvard Apparatus). The syringe was connected to a syringefilter holder that contained the perforated silicon mold with a 0.2-µm nylon filter at the outlet. As the solvent was removed by forced filtration, the suspended particles consolidated in the silicon mold.

Electrochemical properties of 3-D electrodes were determined for two different configurations: as a half-cell and as a full battery. The half cell was prepared by colloidal processing and involved the use of electrode materials commonly used in lithium-ion batteries. The interdigitated configuration enabled us to characterize the behavior of a full 3-D nickel–zinc battery. The electrochemical properties of 3-D electrode arrays of VONR and carbon (MCMB) were determined using a conventional three-electrode arrangement for half-cell experiments [14]. The VONR or MCMB array served as the working electrode, with lithium-metal foils as reference and counter electrodes. The electrolyte used in these studies was 1M LiClO4 in a 1 : 1 (volume ratio) mixture of ethylene carbonate and dimethylcarbonate. The half-cells were characterized by cyclic voltammetry and galvanostatic charge–discharge experiments, which were carried out using an EG&G 273 potentiostat/galvanostat and an Arbin BT4 battery-testing system. For the VONR samples, the potential range of interest is between 3.3 and 2.2 V versus Li/Li+ , while the MCMB samples were examined over the potential range between 2 and 10 mV versus Li/Li+ . All experiments were carried out in an argon-filled glove box. The electrochemical properties of the VONR 3-D electrode arrays are in good agreement with the results published previously for 2-D conventional electrodes [12]. The oxidation and reduction peaks around 3.0 V [Fig. 10(a)] indicate the reversible intercalation/deintercalation of lithium. It can be seen from the overlapping C–V curves that the 3-D microelectrode arrays can uptake, store, and release lithium repeatedly, making it suitable as a positive electrode for a secondary lithium-ion battery. The similar peak positions for intercalation and deintercalation of the 3-D microelectrode arrays compared to that of a 2-D electrode [12] indicate that fundamental electrochemical properties of the VONR remain unchanged despite the processing steps involved in fabricating the micropost structure. Fig. 10(b) shows the discharge characteristics for the 3-D VONR electrode at a current density of 2 mA/cm2 . If we consider the footprint area of 10 mm2 , we find that the energy density of the 3-D electrode array is approximately 15 J/cm2 , a fivefold increase in areal energy density compared to that of thin-film batteries. The fundamental lithium-intercalation properties of the 3-D MCMB array are also in good agreement with those of conventional electrodes. Although not shown here, the cyclic


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Fig. 7. (a) Interdigitated post array electrodes of nickel and zinc on their own current collectors. (b) Nickel post electrode conformally coated with nickel hydroxide.

Fig. 8. Schematic of colloidal process using forced infiltration of the suspension in the silicon mold.

voltammograms indicate that the basic intercalation/deintercalation response for MCMB is unaffected by the processing involved in fabricating 3-D electrodes. The advantage of the 3-D configuration, that of having higher areal capacity (in milliampere hours per square millimeter) is shown in Fig. 11. To calculate the areal capacity, it is necessary to estimate the “weight” of the active electrode material loaded. Given the volume of the electrode, the weight of the electrode was derived by assuming an electrode porosity of 20% and a density of 2 g/cm3 . While it is important to note that the gravimetric

energy density for the 3-D and conventional 2-D electrodes are comparable (∼240 mAh/g), the 3-D electrode leads to an areal capacity some ten times greater than that of the 2-D electrode. These results establish that the 3-D configuration enables us to incorporate more active material, and thus produce higher capacity, without increasing the footprint area of the battery. The use of electroplating to fabricate the interdigitated structure enables us to test the behavior of a nickel–zinc battery consisting of nickel hydroxide and zinc as the cathode and anode, respectively, and KOH as the electrolyte. A 6M KOH electrolyte was added to the interdigitated structure after deposition of the nickel-hydroxide layer (Fig. 6). The battery was then charged and discharged under low-current conditions (10 µA). The use of low current at the inception of battery testing is a standard process used to increase capacity. Ordinarily, current would be increased after approximately ten cycles, however, in this paper, this was not done because the KOH severely etched the Zn electrode after a few cycles. Fig. 12 displays the charge–discharge curves of the battery for the first six cycles. It can be seen that the battery shows proper response, and its capacity increases during each cycle, as expected for the gradual transformation of the nickel hydroxide to nickel oxyhydroxide. The capacity reaches 2.5 µAh/cm2 on the sixth cycle. Thereafter, etching of the Zn electrode became increasingly apparent, and the cycling was stopped before the battery was fully charged. A theoretical capacity of 32 µA h/cm2 was calculated for this design. To achieve higher capacities, forming a denser array of nickel–zinc interdigitated electrodes and a thicker layer of nickel hydroxide are essential. Moreover, to

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Fig. 9. SEM images of (a) VONR and (b) carbon post array electrodes.

Fig. 10. Electrochemical properties of 3-D electrode arrays that is composed of VONRs. (a) Cyclic voltammetry experiments indicating the reversible intercalation of lithium. (b) Galvanostatic discharge curves.

and also the use of a low-solubility-zinc electrolyte to reduce the corrosion of the zinc anode [15], [16]. V. C ONCLUSION

Fig. 11. Comparison of calculated and observed values of lithium areal capacity for MCMB electrodes having different configurations. 3-D arrays and conventional 2-D thick films.

improve the life of the battery, zinc needs to survive in the alkaline electrolyte for longer periods of time. In future work, this will require electroplating a zinc alloy instead of pure zinc

A series of microfabrication procedures were used to construct 3-D electrode arrays that are the central features in 3-D battery architectures. Electrode fabrication is based on etching high-aspect-ratio holes in silicon molds and then using either electroplating or colloidal processing to fill the holes, with the electrode material determined by the battery system. The electrode arrays reported in this paper include materials of interest for lithium-ion batteries, VONRs and carbon, and the electrode materials used in nickel–zinc batteries. Electrochemical measurements indicated that the 3-D electrode arrays used in lithium-ion batteries exhibited much greater energy densities (surface-area normalized) than that of traditional 2-D electrode geometries. The use of electroplating enabled us to fabricate and test a 3-D battery architecture based on interdigitated arrays of nickel and zinc. The results showed a functional battery for the first few cycles before the zinc electrodes were severely etched in the KOH electrolyte. To improve the energy densities, the design of the electrodes should be optimized to provide a higher volume of the active electrode materials (i.e., denser array of electrodes and thicker nickel-hydroxide coating).


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Fig. 12. Charge–discharge curve for the interdigitated 3-D Ni–Zn microbattery.

ACKNOWLEDGMENT The authors would like to thank Prof. S. Tolbert for providing the VONR for the Li-ion battery.

R EFERENCES [1] R. W. Hart, H. S. White, B. Dunn, and D. R. Rolison, “3-D microbatteries,” Electrochem. Commun., vol. 5, no. 2, pp. 120–123, Feb. 2003. [2] J. W. Long, B. Dunn, D. Rolison, and H. White, “Three-dimensional battery architectures,” Chem. Rev., vol. 104, no. 10, pp. 4463–4492, Oct. 2004. [3] M. Nathan, D. Golodnitsky, V. Yufit, E. Strauss, T. Ripenbein, I. Shechtman, S. Menkin, and E. Peled, “Three-dimensional thin-film Li-ion microbatteries for autonomous MEMS,” J. Microelectromech. Syst., vol. 14, no. 5, pp. 879–885, Oct. 2005. [4] F. Chamran, H.-S. Min, B. Dunn, and C.-J. Kim, “Three-dimensional Nickel–Zinc microbatteries,” in Proc. IEEE Int. Conf. MEMS, Istanbul, Turkey, Jan. 2006, pp. 950–953. [5] C. Wang, L. Taherabadi, G. Jia, M. Madou, Y. Yeh, and B. Dunn, “C-MEMS for the manufacture of 3D microbatteries,” Electrochem. Solid-State Lett., vol. 7, no. 11, pp. A435–A438, 2004. [6] V. Lehmann and H. Foll, “Formation mechanism and properties of electrochemically etched trenches in N-type silicon,” J. Electrochem. Soc., vol. 137, no. 2, pp. 653–659, 1990. [7] V. Lehmann, “The physics of macropore formation in low doped N-type silicon,” J. Electrochem. Soc., vol. 140, no. 10, pp. 2836–2843, 1993. [8] H. Foll, M. Christophersen, J. Carstensen, and G. Hasse, “Formation and application of porous silicon,” Mater. Sci. Eng., R, vol. 280, pp. 1–49, 2002. [9] F. A. Lowenheim, Modern Electroplating, 2nd ed. New York: Wiley, 1963. [10] G. H. A. Therese and P. V. Kamath, “Electrochemical synthesis of metal oxide and hydroxides,” Chem. Mater., vol. 12, no. 5, pp. 1195–1204, 2000. [11] J. A. Lewis, “Colloidal processing of ceramics,” J. Amer. Ceram. Soc., vol. 83, no. 10, pp. 2341–2359, Oct. 2000. [12] D. Sun, C. W. Kwon, G. Baure, E. Richman, J. MacLean, B. Dunn, and S. H. Tolbert, “The relationship between nanoscale structure and electrochemical properties of vanadium oxide nanorolls,” Adv. Funct. Mater., vol. 14, no. 12, pp. 1197–1204, 2004. [13] J. Yao, G. X. Wang, J.-H. Ahn, H. K. Liu, and S. X. Dou, “Electrochemical studies of graphitized mesocarbon microbeads as an anode in lithium-ion cells,” J. Power Sources, vol. 114, no. 2, pp. 292–297, Mar. 2003. [14] A. J. Bard and L. R. Faulkner, Electrochemical Methods-Fundamentals and Applications, 2nd ed. New York: Wiley, 2001. [15] F. R. McLarnon and E. J. Cairns, “The secondary alkaline zinc electrode,” J. Electrochem. Soc., vol. 138, no. 2, pp. 645–664, Feb. 1991. [16] T. C. Adler, F. R. McLarnon, and E. J. Cairns, “Low-zinc-solubility electrolyte for use in zinc/nickel oxide cells,” J. Electrochem. Soc., vol. 140, no. 2, pp. 289–294, 1993.

Fardad Chamran received the B.S. and M.S. degrees in mechanical engineering from Sharif University of Technology, Tehran, Iran, in 1996 and 1998, respectively, and the Ph.D. degree in mechanical engineering from the University of California, Los Angeles (UCLA), in 2006. From 1997 to 2000, before pursuing his studies at UCLA, he was a Project Engineer for Sazeh Consultants and PacFab, Inc. He is currently a Process Development Engineer at Innovative Micro Technology, Santa Barbara, CA.

Yuting Yeh received the B.S. degree in materials science from National Tsing Hua University, Hsinchu, Taiwan, R.O.C., in 1999, and the M.S. and Ph.D. degrees in materials science and engineering from the University of California, Los Angeles, in 2003 and 2007, respectively. He is currently a Battery Engineer at Kyocera Wireless Inc., San Diego, CA. His research focuses on the development of novel Li-ion batteries for nextgeneration applications.

Hong-Seok Min received the B.S. and M.S. degrees in materials science and engineering from Seoul National University, Seoul, Korea, in 2000 and 2002, respectively, and the Ph.D. degree in materials science and engineering from the University of California, Los Angeles (UCLA) in 2007. He has been developing 3-D architectures for electrochemical power sources in the Materials Science and Engineering Department, UCLA. His research focuses on design, fabrication, and characterization in 3-D microbatteries for micro power sources.

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Bruce Dunn received the B.S. degree in materials science from Rutgers University, New Brunswick, NJ, in 1970, and the M.S. and Ph.D. degrees in materials science and engineering from the University of California, Los Angeles (UCLA), in 1972 and 1974, respectively. He was a Staff Scientist at the General Electric Corporate Research and Development Center before joining the UCLA faculty in 1980. He currently holds the Nippon Sheet Glass Chair in the Materials Science and Engineering Department, UCLA. His research interests concern the synthesis of ceramics and inorganic materials and the characterization of their electrical, electrochemical, and optical properties. A continuing theme in his research is the use of sol-gel methods to synthesize materials, which incorporate specific dopants and are capable of developing unique microstructures and properties. The areas currently being studied in his group include biosensors, intercalation compounds, aerogels, and organic/inorganic hybrid materials.

Chang-Jin (CJ) Kim received the B.S. degree from Seoul National University, Seoul, Korea, in 1981, the M.S. degree from Iowa State University, Ames, along with the Graduate Research Excellence Award, in 1985, and the Ph.D. degree in mechanical engineering from the University of California, Berkeley, in 1991. Since joining the faculty at the University of California, Los Angeles (UCLA), in 1993, he has developed several microelectromechanical-system (MEMS) courses and established a MEMS Ph.D. major field in the Mechanical and Aerospace Engineering Department. Directing the Micro and Nano Manufacturing Laboratory, he is also an IRG Leader for the NASA-supported Institute for Cell Mimetic Space Exploration and a founding member of the California NanoSystems Institute at UCLA. His research is in MEMS and nanotechnology, including design and fabrication of micro/nano structures, actuators, and systems, with a focus on the use of surface tension. Prof. Kim was the recipient of the 1995 TRW Outstanding Young Teacher Award, the 1997 NSF CAREER Award, and the 2002 ALA Achievement Award. He has served on numerous Technical Program Committees, including Transducers and the IEEE MEMS Conference, and on the U.S. Army Science Board as Consultant. He is currently serving as Chair of the Devices and Systems Committee of the American Society of Mechanical Engineers (ASME) Nanotechnology Institute, as a Subject Editor for the JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, on the Editorial Advisory Board for the IEEJ Transactions on Electrical and Electronic Engineering, and on the National Academies Panel on Benchmarking the Research Competitiveness of the U.S. in Mechanical Engineering.