preparation and characterization of three-dimensional micro-electrode for micro-supercapacitor based...

SCIENCE CHINA Technological Sciences

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*Corresponding author (email: [email protected])

• RESEARCH PAPER • July 2012 Vol.55 No.7: 2013–2018

doi: 10.1007/s11431-012-4869-7

Preparation and characterization of three-dimensional micro-electrode for micro-supercapacitor based on inductively

coupled plasma reactive etching technology

WEN ChunMing1,2*, WEN ZhiYu1,2, YOU Zheng3 & WANG XiaoFeng3

1 Key Laboratory of Fundamental Science on Micro/Nano-Device and System Technology, Chongqing University, Chongqing 400030, China; 2 Microsystem Research Center, Chongqing University, Chongqing 400030, China;

3 Department of Precision Instruments and Mechanics, Tsinghua University, Beijing 100084, China

Received October 27, 2011; accepted March 26, 2012; published online May 28, 2012

The capacity of supercapacitor charge storage depends on the size of the electrode surface area and the active material on the electrodes. To enhance the charge storage capacity with a reduced volume, silicon is used as the electrode material, and three-dimensional electrode structure is prepared to increase the electrode surface area on the footprint area by inductively coupled plasma reactive etching (ICP) techniques. The anodic constant current deposition method is employed to deposit manganese oxide on the electrode surface as the electroactive material. For comparison, samples without slot are prepared with a two-dimensional electrode. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) are used to characterize the surface morphology of the electrode structure and the deposited electroactive material. Electrochemical prop-erties of the electrode are characterized by the cyclic voltammetry (CV) and the constant current charge-discharge method. Experimental results show that our approach can effectively increase the electrode surface area with more electroactive sub-stances, and hence can increase storage capacity of the micro-supercapacitor.

Micro Electro Mechanical System (MEMS), micro-supercapacitors, three-dimensional electrode, inductively coupled plasma reactive etching, manganese oxide, silicon

Citation: Wen C M, Wen Z Y, You Z, et al. Preparation and characterization of three-dimensional micro-electrode for micro-supercapacitor based on induc-tively coupled plasma reactive etching technology. Sci China Tech Sci, 2012, 55: 20132018, doi: 10.1007/s11431-012-4869-7

1 Introduction

Supercapacitor is the worldwide research focus with great potential applications [1–4], due to its high efficiency of charge and discharge, large capacity, long life, fast charging, high current discharge, easy to use, long-term use of main- tenance-free and so on. Traditional supercapacitors are bulky and inconvenient for applications with low power consumption requiring small volume. Using traditional su-percapacitor will also greatly increase the total size and

weight, which is not suitable for the portable equipment or concealing applications. Because of its low charge and dis-charge efficiency, slow charge and discharge, low power density, traditional micro-batteries are not suitable for many real applications. There is a great need for miniaturization of supercapacitors. In order to fabricate the micro-energy storage devices with small volume and medium energy storage level, the development of supercapacitor with Micro Electro Mechanical System (MEMS) technology is becom-ing the research focus.

The current preparation methods of micro-supercapacitors include printing [5–7], electrodeposition [8–11], cata-lytic reduction [12, 13], sputtering and so on [14–17].

2014 Wen C M, et al. Sci China Tech Sci July (2012) Vol.55 No.7

Printing electrodes are prepared with the special ink. The height of the electrode is only several μm [5, 6], the surface area is small with small mount of active material, which leads to the specific capacitance of 2.1 mF cm2. Catalytic reduction method requires high temperature [12], while it is hard to integrate multiple devices on the same silicon. Liu et al. [14] prepared RuO2 electrode by sputtering. The elec-trodes surface area is small, the active electrode material is limited, and the specific capacitance of the footprint is 40.7 mF cm2.

Manganese widely exists in nature, and therefore man-ganese oxide is cheap and has little adverse effects on the environment. Due to its better electrochemical performance, manganese oxide is a good alternative material for the preparation of the electrochemical capacitor electrode. The capacitance performance of manganese oxide thin film is better than other status [18−20], therefore the present re-searches focus on preparing manganese oxide thin film. The major methods include sol-gel [21, 22], thermal decomposi-tion [23, 24], oxidation-reduction [25], electrodeposition, etc. [26−29]. Electrodeposited electrodes can be prepared by simple equipment at low temperature, with low cost, and the electrodes can be directly formed by deposit manganese oxide on the three-dimensional micro-structure.

In this paper, we propose a method to increase the elec-trode surface area by inductively coupled plasma reactive etch-ing (ICP) technique. As active material, manganese oxide is directly deposited on the electrode structure to form the three-dimensional electrode with electrodeposition tech-nique. With this method electrode with demanded complex shape can be directly produced. This electrode preparation process does not require high temperatures. It also can en-hance the capacitor’s charge storage capacity without in-creasing its volume, and has strong practical significance to the system desiring for small size but high capacitance.

2 Experiment

2.1 Preparation of three-dimensional micro-electrode structure

Supercapacitor charge storage capacity depends on the elec-trode surface area and the active material on the electrode. For a given electroactive material, it is an effective way to enhance storage capacity by increasing the capacitor elec-trode surface area. We utilized the advantage of MEMS technology to fabricate three-dimensional electrode struc-ture on the same footprint to increase the electrode surface area [30], thus enhancing the charge storage capacity.

The ICP technique is an important technology in the integrated circuit (IC) and MEMS manufacturing. It can etch grooves into the materials such as semiconductor (such as silicon), semiconductor oxides (such as SiO2), photoresist, metal and so on, with especially some high aspect ratio

trenches. It is one of the dry etching methods. Utilizing the composed mixture of gases with ionized plasma and without ionization neutral gas molecules formed by specific gas in the environment of high-voltage and low-pressure environment, it can precisely remove the surface material for certain thickness without affecting trench sidewall material by impact and chemical reactions under role of the external electromagnetic field. It is a processing method which can achieve high etching rate (about 10 μm min1),high etching selectivity, high aspect ratio, and the steep sidewall. It also can carve through 500 μm substrate [31, 32]. In this paper, microelectrode was fabricated with ICP process. Silicon wafer was cleaned and then baked in the oven for 10 min. 2000 Å (Germany FHR MS 100X6-L) of aluminum was sputtered as the etch mask and AZ1500 photoresist was used for coating. After exposure and developing, aluminum was removed to form the ICP etching window (UK STS LPXICPASE-SR). Etching process parameters are as foll- ows: chamber temperature is 40°C, wafer temperature 25°C, RF power 650 W, voltage 320 V, the total etching time 100 min. Etching process of the SF6 flow 103 sccm, O2 flow 15 sccm, DC bias voltage 128 V, power 20 W, cavity pressure 20 mtorr. Deposition process of C4F8 flow is 85 sccm, cavity pressure is 40 mtorr.

After the formation of the three-dimensional structure by etching the silicon, the titanium (Ti) tungsten (W) alloy (40 nm) was sputtered as an intermediate layer to enhance the bonding force between the collector and silicon, and 120 nm sputtered gold (Au) as the current collector.

2.2 Electroactive material deposited

A certain amount of manganese sulfate was accurately weighted using electronic balance (FA2204B), which was dissolved in deionized water to prepare the electro-deposition solution with a concentration of 0.2 mol L1. Platinum electrode was used as cathode and the three-dimensional structure of silicon was used as the anode. The versastat 3 (Princeton applied research, USA) was used for electrodeposition with a constant deposition current density of 6 mA cm2 and the deposition time of 33 s.

The preparation process of three-dimensional and elec-troactive material deposition are given in Figure 1. The processing procedure is the same for the two-dimensional without the structure for the comparison experiments, but without procedures (b), (c) and (d) in Figure 1.

2.3 Electrochemical performance test

A certain amount of sodium sulfate was accurately weighted with the electronic balance (FA2204B) and then dissolved in deionized water to form the solution with concentration of 0.4 mol L1. This electrolyte solution was used to test the electrochemical performance. The classic three-electrode system and electrochemical workstation (versastat v3) were

Wen C M, et al. Sci China Tech Sci July (2012) Vol.55 No.7 2015

Figure 1 3D electrode microstructure processing flowchart. (a) Substrate preparation; (b) preparation of mask layers; (c) etching pattern transfer; (d) ICP etching to form three-dimensional structure; (e) sputtering intermedi-ate layer and collector; (f) electrodeposition.

used to test the electrochemical characteristics of the three- dimensional structure electrode by cyclic voltammetry and constant current charge-discharge.

3 Results and discussion

3.1 Three-dimensional microelectrode structure sur-face morphology and electroactive material composition characterization

The characteristics of the electrode surface topography and material composition were tested with SEM (TESCAN VEGA II LMU) and Energy Dispersive EDS (OXFORD INCA 350X). The results are shown in Figures 2 and 3.

The surfaces of the three-dimensional microelectrode structure fabricated by the ICP etching are smooth, without cracks, burrs, etc. The shape of microelectrode is exactly the same as the mask, and therefore we can produce the desired electrodes with complex shape by the ICP technology. Be-cause three-dimensional electrode structure is formed by downwards etching with the ICP technology, this can in-crease the electrode surface area without increasing the ca-

Figure 2 SEM image of 3D micro-electrode structure.

Figure 3 EDS image of material deposited on the surface electrode.

pacitor volume, which has great importance for the small size of MEMS systems.

The spectrum of the electrode surface material is shown in Figure 3. As it can be seen from the figure, there are manganese and oxygen peaks in the electrode surface de-posits spectrum, indicating that the electrode surface-active substances contain manganese and oxygen, are also con-sistent with literature report [33, 34]. It can be concluded that the deposited electroactive material is manganese ox-ide.

3.2 Three-dimensional micro-electrochemical charac-terization

3.2.1 Cyclic voltammetry (CV) characteristics

The curves of the three-dimensional electrode cyclic volt-ammetry are shown in Figure 4, where there is a broadening in the oxidation-reduction peaks at different scan rates. The absence of the clear oxidation-reduction peaks, the square shape of the curve with a good symmetry, and the indication of an ideal capacitive characteristic indicate the facts of a


larger surface area of the electrode, no clear electrode po-larization during the scanning process, and a smooth pro-cess of storing and discharging. The CV curves of two- dimensional electrode are shown in Figure 5, which shows a clear reduction peak which may be due to a smaller elec-trode surface. The electrode polarization phenomenon is more serious in the scanning process, which appeared in reduction peak.

At the lower scan rate, most of the active material can react with proton of the electrolyte which enters the body of manganese oxide. With the increasing scan rate, because of the mass transfer problem, the ability of the proton getting into the material body of manganese oxide is reduced. Part of the active substance in the internal cannot participate in the reaction, leading to a decrease in the utilization of active material in the electrode. With a further increase of the scan rate, mass transfer problem becomes more serious. The proton cannot get into the bulk manganese oxide, and hence the oxidation-reduction reaction only occurs within the ac-tive material on the electrode surface, leading to a rapid decrease in the electrode capacitance, as indicated in the distortion of the curve and the linear increase of the en-closed area of the CV curve with the increase of the scan rate.

Figure 4 CV curves of 3D electrode.

Figure 5 CV curves of 2D electrode.

3.2.2 Constant current charge-discharge characteristic

Electrochemical workstation versastat v3 was utilized to test the electrode charge-discharge characteristic, as shown in Figures 6 and 7. Figure 6 gives the discharge time curves of the three-dimensional microelectrode at different discharge densities. The figure shows a better symmetry of the charge and discharge curves, indicating a better performance of the electrode charge- discharge. However, the discharge time decreased rapidly at high-density discharge, indicating the poor performance at the high power charge and discharge. At the same time, in the initial stages of discharge, voltage dropped more rapidly, indicating the larger internal re-sistance of the electrode. Figure 7 is a charge-discharge curve of two-dimensional electrode without structure. The discharge curves have the same symmetry with the three- dimensional electrode charge-discharge curve, however, the discharge time is much shorter than the three-dimensional electrode for the same discharge density.

The electrode structure of the unit area capacitance is calculated by the following formula:

d d d

d / d ,d d d

Q T T TC Q V I I

T V V V

(1)

Figure 6 3D micro-electrode discharge time curves at different discharge densities.

Figure 7 2D micro-electrode discharge time curves at different discharge densities.

Wen C M, et al. Sci China Tech Sci July (2012) Vol.55 No.7 2017

/ ,Cs C S (2)

where C is the electrode capacitance, I is the charge or dis-charge current, T is the discharge time, V is the discharge potential difference, S is the electrode footprint, and Cs is the specific capacitance of electrode.

The comparing charts of two kinds of electrode specific capacitance of the electrode footprint are shown in Figure 8. At different charge and discharge densities, three-dimensional electrode specific capacitance at the electrode foot-print differs from the two-dimensional electrode specific capacitance. The specific capacitance of three-dimensional electrode is always larger than the two-dimensional elec-trode and the difference becomes greater at the low dis-charge density. The evidence shows a clear increase in the electrode surface area with the fabricated three-dimensional structure electrodes on the basis of the two-dimensional plane, which enhances the charge storage capacity. This becomes much clear at a low charge and discharge density. The value of specific capacitance is more than five times. The reason causing this phenomenon is that a part of the manganese oxide, especially at the bottom of the structure, cannot participate in the oxidation-reduction reactions. As charge-discharge current increases, more and more electro-active materials cannot store and release charge, which lead to the decrease in the electrode specific capacitance.

4 Conclusions

In this paper, with the direct electrochemical deposition method, we use the ICP technique to fabricate three- dimensional structure to increase electrode surface area, and hence to enhance the charge storage capacity of the elec-trode to form three-dimensional electrode. The deposition current density is 6 mA cm-2 and the deposition time is 33 s. At a discharge density of 1 mA cm2, three-dimensional electrode specific capacitance is 33.49 mF cm2, and two- dimensional electrode specific capacitance is 4.85 mF cm2.

Figure 8 Comparison chart of 3D and 2D electrode specific capacitances.

The three-dimensional electrode unit footprint specific ca-pacitance is clearly greater than the two-dimensional elec-trode. Because the ICP etching is a downwards etching process of forming three-dimensional electrode structure, it can increase the electrode surface area without increasing volume. Since there is no requirement for high temperature, the electroactive substances can be directly deposited on the structure surface, so can produce complex shapes three- dimensional electrode according to the practical demand. This is of great importance for the desired small size system such as MEMS. Methods used in this paper can be used to fabricate capacitors integrated with electronic devices on the same silicon wafer, thus reducing the external cables and system size, increasing system reliability and stability, decreasing the cost and enhancing the overall system per-formance.

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preparation and characterization of three-dimensional micro-electrode for micro-supercapacitor based...

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