Localized electrochemical micromachining with gap control

Sensors and Actuators A 108 (2003) 144–148

Localized electrochemical micromachining with gap control Li Yong a,∗ , Zheng Yunfei a , Yang Guang a , Peng Liangqiang b a

Department of Precision Instruments and Mechanology, Tsinghua University, Beijing, PR China b Institute of High Energy Science, Chinese Science Academy, Beijing, PR China Received 29 July 2002; received in revised form 3 June 2003; accepted 5 June 2003

Abstract An approach to electrochemical micromachining is presented in which side-insulated electrode, micro gap control between the cathode and anode, and the pulsed current are synthetically utilized. An experimental set-up for electrochemical micromachining is constructed, which has machining process detection and gap control functions; also a pulsed power supply and a control computer are involved in. Microelectrodes are manufactured by micro electro-discharge machining (EDM) and side-insulated by chemical vapor deposition (CVD). A micro gap control strategy is proposed based on the fundamental experimental behavior of electrochemical machining current with the gap variance. Machining experiments on micro hole drilling, scanning machining layer-by-layer, and micro electrochemical deposition are carried out. Preliminary experimental results show the feasibility of electrochemical micromachining and its potential capability for better machining accuracy and smaller machining size. © 2003 Elsevier B.V. All rights reserved. Keywords: Electrochemical micromachining; Side-insulated electrode; Gap control; Pulsed current

1. Introduction Electrochemical machining (ECM) includes two reverse aspects: electrolyzing for metal removal and electroforming for material deposition. In electrolyzing process, metallic materials (even extremely hard alloys or poor-cutting materials) can be machined with fine surface roughness. In electroforming process, metallic films are easily electroplated and even three-dimensional shapes can be selectively defined. In ECM process, materials are removed or deposited with the transferring of ions based on the anodic dissolution mechanism, so that high precision is achievable and it has the feasibility of micromachining. In recent years, numerous research and development activities have shown that electrochemical micromachining is a promising technique with flexibilities in micro electromechanical systems (MEMS) and some advanced manufacturing areas. Compared with micro EDM, electrochemical micromachining has advantages of high removal rate and no tool wear in process. Also, it is cost effective and requires no special equipment in comparison with competing technologies such as focused ion beam (FIB) machining and laser beam machining, etc. [1]. ∗ Corresponding author. Tel.: +86-10-62772212; fax: +86-10-62784691. E-mail address: [email protected] (L. Yong).

0924-4247/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0924-4247(03)00371-6

For high-precision machining, Philips Center for Manufacturing Technology formerly developed an ECM method by means of side-insulated tool, which formed tooth profiles with width of 0.37 mm and height of 0.77 mm, and groove patterns of 0.9 mm width on thin metal foils [2]. IBM developed electrochemical micro-drilling process by using double-sided through-mask, and fabricated array holes on 125 ␮m thick stainless steel sheet [3]. Moreover, a novel electrochemical etching of single crystal silicon in hydrofluoric acid with one mask was presented, in which pore diameter or trench width is controlled by adjusting the light intensity supplied with the illumination from the back side of silicon substrate. Free standing beams with height of 40 ␮m, width of 2 ␮m and length of 250 ␮m were fabricated [4]. By combining electrochemical etching and deposition with scanning tunneling microscope (STM), a line pattern with 200–300 nm in width and 100 nm in depth was etched, and a bump pattern with 300 nm in diameter, and 200 nm in height was deposited [5]. By an integration of electrochemical etching and electroplating, nickel gears with diameter of 600 ␮m and thickness of 100 ␮m, diameter of 1700 ␮m and thickness of 30 ␮m were fabricated [6]. In LIGA process, electroplating plays an important role in forming metallic microstructures. Alternatively a process for the fabrication of electroplated microstructures using photosensitive polyimide molds was developed which uses ordinary optical masks and ultraviolet light exposure [7].

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For batch fabrication of metallic microstructures, sacrificial layer technique was introduced to multi-layers electrochemical deposition so that 3-D electroplated microstructures are fabricated [8,9]. In direct writing way, a 3-D microfabrication by localized electrochemical deposition was developed, which can potentially produce submicrometer feature sizes [10]. By reviewing the above mentioned research and development achievements, we observed that the factors evidently influencing electrochemical micromachining may come down to electrolyte and its flow, and current distribution between electrodes, even though ECM process has a variety of behaviors. From the standpoint of machining technique, we focused our attention on micro gap control between electrodes, and partly insulated tool electrode for localizing machining current, for electrochemical micromachining. In this paper, an approach to electrochemical micromachining is presented in which side-insulated electrode, micro gap control between the cathode and anode, as well as the pulsed current are synthetically utilized. An experimental set-up for electrochemical micromachining is constructed and a micro gap control strategy is proposed. Machining experiments on micro hole drilling, scanning machining layer-by-layer, and micro electrochemical deposition are described.

2. Electrochemical micromachining set-up The developed electrochemical micromachining set-up is shown in Fig. 1. It comprises following components: a micro feeding mechanism with resolution of 0.1 ␮m, a precise XY stage with resolution of 1 ␮m, a power supply for micro ECM, a machining process detecting module, and a control computer. The power supply offers the voltage and current to the electrodes (tool and workpiece). The machining process detecting module samples the signals (machining voltage and

Fig. 1. Micro ECM set-up.

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Fig. 2. Pulsed power supply.

current) that reflect the machining status and then inputs them into the computer, in which signals are processed and appropriate decisions are worked out to drive the micro feeding mechanism and the XY stage. Therefore, the electrode gap can be controlled automatically according to the situation of machining. The more detailed configuration of pulsed power supply is depicted in Fig. 2. It consists of a pulse generator, voltage and power amplifiers, feedback circuit (sample resistance, signal converting and current amplifier circuit), and A/D and I/O interface circuits. Through the output enabling port, the output can be switched on or off at any moment easily by computer control. This function can help to improve the accuracy and efficiency in micro ECM process by cutting off the output current during the backward and forward motion duration, to keep machining occurs only within micro gap range.

3. Fundamental behavior and process control Based on the fundamental experiments carried out on the developed micro ECM set-up, a typical relationship between machining current and electrode gap is drawn as in Fig. 3. This curve was drawn under the following conditions: 10% NaClO3 solution, 5 V dc voltage, the Cu tool electrode (cathode) with a diameter of 200 ␮m, and stainless steel as workpiece (anode). When tool electrode touches workpiece surface, machining current jumps up, which can be used to detect the

Fig. 3. Current behavior with electrode gap.

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Fig. 5. Micro holes.

Fig. 4. Flow chart of gap control.

position of workpiece. Then, let the tool electrode move away from the workpiece surface; there is a sudden drop in current around 10 ␮m gap width. So, we can control the electrode gap within a few 10 ␮m by utilizing the sudden current variance signal. Fig. 4 shows the flow chart of gap control in micro ECM process for micro hole drilling. At first, tool electrode is positioned to form the gap between tool and workpiece. The machining current is sampled during feeding of the tool towards the workpiece. The status of machining is judged by computer via the current sample. It shows short circuit when detecting the current jump-up. Then, a decision is made to withdraw the tool electrode to maintain the tiny gap. When the tool is moved backwards, it stops at the place several micrometers away from the jump-up place for a while to wait for removal, and is then moved forward. Then, the gap is maintained in a tiny range continuously.

4. Machining experiments Following experiments including micro hole drilling, scanning machining and micro electrochemical deposition are carried out, by the use of machining set-up and the localized machining techniques. In electrochemical metal removal processes, microelectrodes are manufactured by wire electro-discharge grinding method used in micro electro-discharge machining (micro EDM) [11]. And in electrochemical deposition, Pt–Ir alloy probe tool is prepared by electrochemical removal machining itself [12].

There are inevitably machining errors between the final workpiece and the tool electrode, due to the dispersive current in electrolyte. Micro hole drilling seems to be a simple process, but it requires adequate consideration about process control and machining techniques. Side-insulated electrode, micro gap control between the cathode and anode, and pulsed current are synthetically utilized. The side-insulation of electrode and micro gap control contribute directly to localized machining. The pulsed current put on the cathode and anode has a function to agitate electrolyte, so as to promote the electrochemical reaction. Fig. 5a shows a micro hole machined electrochemically in 10% NaClO3 solution under dc current. A side-insulated electrode with diameter of 302 ␮m is used. The electrode material is tungsten and a thin SiC insulating layer is coated on it via chemical vapor deposition (CVD). The machining gap is controlled in the range of 15–20 ␮m and the hole diameter about 420 ␮m is obtained on stainless steel plate with depth 200 ␮m. In micromachining gap, the electrolyte is easily boiled by the high concentration of the machining current. And the dregs produced during machining process may adhere on the surface of workpiece and tool electrode, which makes machining difficult to continue. However, if pulse voltage is used, these problems can be eliminated. The temperature of the electrolyte is cooled down and the dregs are swept off during the pulse-off duration. Fig. 5b shows a micro hole machined electrochemically in 10% NaClO3 solution by using a pulse voltage with a pulse duration of 0.5 ms and an interval of 0.5 ms. A copper electrode is side-insulated with Lucite and then used in experiments with diameter of 180 ␮m. The hole diameter about 220 ␮m is obtained on stainless steel plate with depth 300 ␮m through machining gap control. With comparison to dc current, the enlarged part of hole diameter compared to the electrode is much reduced by means of pulse one. 4.2. Scanning machining

4.1. Micro hole drilling Ordinarily, electrochemical machined final workpiece (anode) shape is not a direct inverse of the tool (cathode).

For 3-D microstructures or microstructures with high aspect ratio, the machining method by scanning layer-by-layer is tried in ECM process. In this case, the machining gap is

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Fig. 6. Scanning machining.

fixed within a tiny but suitable range. Microstructures can be machined through scanning movements between the anode and the cathode as shown in Fig. 6. During machining process, the electrode is fed step-by-step in Z-direction, and the workpiece is moved for scanning in XY plane between each step. A microstructure scan machined by direct writing micro ECM is shown in Fig. 7. The machining gap between the anode and cathode is limited within 10–15 ␮m. The microelectrode used is also coated with SiC via CVD. The cantilever beam obtained has length of 3.6 mm, width of 150 ␮m, and thickness of 300 ␮m. Because the tool electrode with diameter of 300 ␮m is used, the machined grooves are not very narrow. But the consistency of the cantilever beam retained shows the controllability in shape and dimensional accuracy. 4.3. Electrochemical deposition In electrochemical deposition, the synthetic utilization of side-insulated electrode, micro gap control and pulsed current is also considered as an effective technique for microfabrication. But the apparent different point is that in

Fig. 8. Electrochemical deposited micro pole.

the closed loop control of micro gap, the tool electrode is moved off the depositing region according to its deposition rate. During deposition, pulse current can promote the diffusion of metal ions in solution to make the concentration of ions uniform. Thus, the generation of hydrogen bubbles may be decreased and compact structures can be deposited. The insulation of probe tool used in electrochemical deposition is much easier than that in removal process. Because the tool electrode is always placed above the workpieces, the insulating film is permitted to be far thicker, with only a micro tip bared. Fig. 8 shows a micro copper pole deposited in electrolyte (250 g/l CuSO4 ·5H2 O + 75 g/l H2 SO4 ) through machining gap controlling. In the deposition process, an insulated tool electrode (Pt 90%–Ir 10% alloy) with about 100 ␮m radius of the tip bared and a pulse voltage (Vp–p = 3 V; frequency of current = 1 kHz; ton /toff = 2) are used. In observation, the growing speed of the pole reaches 5 ␮m/s. During the deposition process, some hydrogen bubbles are still observed, which made the surface of the structure rough. Further investigations, such as the addition of the electrolyte, are undergoing.

5. Conclusion

Fig. 7. Scan machined microstructure.

In electrochemical machining, dispersive current distribution restricts its high-precision machining or micromachining. An effective way for electrochemical micromachining is localizing the electric field by partly insulation. Therefore, an approach to electrochemical micromachining has been made by utilizing side-insulated electrode, micro gap control, and pulsed current synthetically. An experimental set-up is firstly constructed, which has machining process detection and gap control functions, in addition to pulsed power supply. The current between anode

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and cathode can be switched on or off in time for further machining localization. Based on the fundamental experimental behavior of electrochemical machining current with the gap variance, a micro gap control strategy via following the current jump-up was presented, which constrained the machining gap within 10 and 20 ␮m. Machining experiments on micro hole drilling, scanning machining layer-by-layer, and micro electrochemical deposition were carried out. Preliminary experimental results showed feasibility of the localized electrochemical micromachining. In seeking for better machining accuracy and smaller machining size, it is further needed to do more research work on electrolyte, electrode’s insulation and systematic control of machining process.

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Acknowledgements The authors would like to thank Prof. J.G. Zhu, Prof. B. Yang, and Dr. Y.L. Shao of Institute of Nuclear Energy, Tsinghua University, for their assistance in preparing side-insulated electrodes by CVD.

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Biographies Li Yong received his BSME degree from Harbin Institute of Technology, China, in 1982 and received the MS and PhD degrees from Niigata University, Japan, in 1987 and 1991, respectively. In 1992, he joined Department of Precision Instruments and Mechanology, Tsinghua University, China, and has been working on the areas of MEMS and mechatronics. He is currently interested in the research and development of micromachining and microflow control systems, including silicon process, micro EDM, micro ECM, micro valve micro pump and micro thruster, etc. Zheng Yunfei received his BS and MS degrees in mechanical engineering from Tsinghua University, China, in 1999 and 2002, respectively. His main research topic is micromachining including micro ECM and micro EDM, etc. He is currently a senior associate engineer in Semiconductor Manufacturing International Corporation (SMIC), China. Yang Guang received his BS degree in mechanical engineering from Tsinghua University, China, in 2002. He is currently working toward the PhD degree in electrical engineering at Louisiana Tech University, USA, where his research is focused on microstructure analysis and microfabrication on polymers.