MICRO-ELECTRO-DISCHARGE MACHINING BY MEMS ACTUATORS WITH PLANAR ELECTRODES MICROFABRICATED ON THE WORK SURFACES

Chakravarty Reddy Alla Chaitanya and Kenichi Takahata* The University of British Columbia, Vancouver, Canada

*Corresponding author: 2332 Main Mall Vancouver, B.C. V6T 1Z4 Canada; Tel: +1-604-827-4241; Fax: +1-604-822-5949; E-mail: takahata@ece.ubc.ca

ABSTRACT This paper reports a MEMS-based micro-electro-

discharge machining technique that is enabled by the actuation of micromachined planar electrodes defined on the surfaces of the workpiece. The 18-µm-thick copper electrodes suspended with the anchors are formed on a stainless-steel wafer/workpiece. A DC voltage of 80-140 V is applied between the electrode and the workpiece through a resistance-capacitance circuit that controls the pulse energy and timing of spark discharges. The suspended electrode is electrostatically actuated towards the wafer, resulting in a breakdown, or spark discharge. This instantly lowers the gap voltage, releasing the electrode, and the capacitor is charged up through the resistor. Sequential pulses are produced through the self-regulated discharging-charging cycle. Micromachining of the stainless-steel wafer is demonstrated using an electrode with the area of 1.6×1.03 mm2, achieving removal depth of 20 µm with 100 V that provides the electrode’s displacement of ~30 µm. The pulse formation is also implemented using only the parasitic/built-in capacitance between the electrode and the workpiece. A dynamic characteristic of the built-in capacitance is experimentally analyzed.

1. INTRODUCTION Micro-electro-discharge machining (µEDM) is a

non-contact micromachining technique that can be used to cut any type of electrically conductive materials. The process involves thermal erosion induced by miniaturized spark discharge pulses generated between a microscopic electrode tip and the workpiece in a dielectric fluid. The technique is capable of producing real three-dimensional microstructures while achieving the smallest size of 5 µm with submicron tolerance [1]. These attractive features have been leveraged for producing micro components as well as prototyping various bulk-metal-based MEMS [2, 3]. However, the throughput is inherently low because the technique is essentially a serial process that uses a single electrode tip in conjunction with numerical control (NC) of the tip and the workpiece, producing structures individually. Batch-mode µEDM that uses arrays of high-aspect-ratio microelectrodes has been demonstrated to achieve high parallelism/throughput of the process [4]. In this approach, the arrays were fabricated using a LIGA process [5] and were advanced into the workpiece using the vertical NC stage in an µEDM apparatus. It was observed that the throughput was substantially improved over the traditional process due to the use of the arrays, but the drawback is the high costs incurred in the LIGA process. In addition, the process still requires the NC capability for vertical positioning of the arrays.

In general, non-traditional micromachining techniques such as EDM, mechanical, and electrochemical machining that use single-tip microtools are performed with ultra-precision NC apparatus, which incur high costs of ownership, and the energy and space required to operate such machines are as large as those for macro-scale machining apparatus and do not scale well with the size of the objects to be machined. There have been some efforts for the realization of “micro factory” to address such issue. A micro lathe was developed as part of the concept [6]. The basic approach was an extension of precision engineering, i.e., the miniaturization of conventional NC systems. The use of MEMS may be one way to implement selected micromachining processes. This paper describes an µEDM method where planar electrodes are microfabricated directly on the surfaces of the work material and actuated for controlled generation of discharge pulses towards high-throughput, low-cost, and high-precision processing that does not require NC machines.

2. DEVICE PRINCIPLE The developed µEDM method uses planar electrodes

that are suspended by the anchors through the tethers above the surfaces of the conductive workpiece with a relatively large gap as shown in Fig. 1a. A resistance-capacitance (RC) pulse generation/timing circuit, which is a typical configuration for µEDM to achieve reduced parasitic capacitance in the circuit [1], is coupled between the device

Figure 1: (a: upper) Cross sectional view of MEMS-based µEDM and its process steps; (b: left) dynamic behavior of discharge voltage and current corresponding to the steps.

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and a DC voltage source (80-140 V) so that the electrode serves as a cathode whereas the workpiece is an anode. The workpiece with the electrode is immersed in dielectric EDM oil. In addition to the external capacitance Ce as part of the RC circuit, as indicated in Fig. 1a, there is a parasitic/built-in capacitance, Cb, between the electrode-anchor and the workpiece, which contributes to an increase in the total capacitance of the system.

Figure 1 illustrates the mechanical and electrical behaviors of the device in the machining process. Initially, the device is disconnected from the power supply and the RC circuit (Ce is fully charged) hence the electrode is stationary (step-1). Upon the connection to the circuit, Cb is also charged up to the supply voltage, electrostatically driving the electrode towards the workpiece. With properly designed structures at a selected voltage, the phenomenon known as “pull-in” takes place when the restoring spring force through the tethers can no longer balance the electrostatic force with the decrease of the gap spacing [7]. This results in a breakdown before the electrode physically touches the surface of the workpiece, producing a spark current due to a discharge from the capacitors (step-2). The thermal impact generated by the spark removes the material and leaves a crater-like shape on the surface. The discharge lowers the voltage at the capacitors, i.e., between the electrode and the workpiece, releasing the electrode (step-3). Simultaneously, the capacitors are charged through the resistor, restoring the voltage at the gap and inducing the electrostatic actuation again. This sequence of pull-in and release of the electrode is used to achieve self-regulated generation of discharge pulses that etch the material by repeating the unit removal by a single discharge. The mechanism with the large separation at the static state is also intended to automatically prevent irregular continuous arcing as it lowers the gap voltage, releasing the electrode back to the original position and physically terminating the arc. The wide gap also promotes easier flushing of byproducts produced during the machining.

3. DEVICE DESIGN AND FABRICATION To demonstrate the concept of the new µEDM method

with simple construction and fabrication of the devices, the electrodes were selected to be single-layer structures without particular features underneath (as illustrated in Fig. 1a) in this initial effort. The electrode-tether structures were constructed by copper that has been used as an electrode material for µEDM [4]. Various planar electrodes were designed and fabricated with sizes ranging from hundred’s of microns to millimeters for vertical and torsional actuation. A sample layout of the electrode structure designed for the torsional actuation is shown in Fig. 2. This particular design consists of a 1.6×1.03-mm2 electrode, 1.4×0.45-mm2 tethers, and 2.5×2.5-mm2 anchors. As described later, a Cu layer with 18-µm thickness was used to fabricate the suspended electrode-tether structures, and 30×30-µm2 holes are provided in the structures to enhance the sacrificial etching for releasing during the fabrication process.

A pull-in voltage, VPI, for torsional actuation in the similar configuration can be described by [8, 9]:

WLKdVPI 3

383.0ε

= ;

−−= 4

43

12136.3

316

ab

abab

lGK (1)

where d is the original separation between the electrode and the workpiece, ε is the permittivity of the EDM oil, L is the length of the electrode, W is the width of the electrode, K is the spring constant, G is the shear modulus of elasticity, 2a is the width of the tether, 2b is the thickness of the tether and l is the length of the tether. (The lateral dimensions are noted in Fig. 2.) This expression and the one for vertical actuation in [7] were used to design the device structures. For example, using the equation (1) and the constants (G=45 GPa for Cu and ε=1.59×10-11 F/m for kerosene-based EDM oil), the structure in Fig. 2 was designed to be pulled in when the gap, d, is 30 µm or smaller with the applied voltage of 120 V.

Figure 3 shows the cross-sectional view of the fabrication process for the device, which uses stock Cu foil similar to the process reported in [10]. In this effort, stainless steel was selected as work material and served as the substrate. First, a stainless-steel wafer is thoroughly cleaned and degreased with acetone. A layer of hexamethyldisilazane (HMDS) adhesion promoter is spun on the wafer. A thick photoresist (SPR220, Rohm and Haas Co.) is then double coated on the wafer to form a sacrificial layer with the thickness of 30-40 µm, which is followed by a soft baking process on a hot plate C90° for 5 min (step-1). Next, a 1-µm-thick photoresist (S1813, Rohm and Haas Co.) is spun on the sacrificial layer (step-2), and then the Cu foil with 18-µm thickness is laminated on the S1813 that serves

Figure 3: Fabrication process flow.

Figure 2: A sample design layout of the µEDM device.

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as an adhesive layer between the Cu foil and the sacrificial layer. The S1813 resist is soft baked on a hot plate C90° for 10 min to solidify the layer, making the Cu foil rigidly fixed down to the substrate through the sacrificial layer (step-3). Then, a 5-µm-thick layer of SPR220 is spun on the Cu foil and patterned using a mylar mask with the layout of the devices (step-4). The developed SPR220 is used as a mask for wet etching of Cu in a commercially available ferric chloride solution (CE-100, Transene Co., Inc.) (step-5). Finally, timed etching of the sacrificial layer is performed in acetone, providing suspended electrodes while leaving the anchors attached to the substrate as they are designed to have the area much larger than that of the electrodes and tethers (step-6). As noted earlier, the holes created in the electrode-tether structures promote undercutting during the sacrificial etching process. Figure 4 shows the fabricated devices after the sacrificial etching.

4. EXPERIMENTAL RESULTS AND ANALYSIS

Figure 5 shows a set-up used for µEDM tests. The fabricated device was placed in an ultrasonic bath filled with the EDM oil. An RC circuit with a 20-KΩ resistor and a 100-pF capacitor was connected to the device and the DC voltage source as shown in Fig. 5. The electrical discharge pulses were monitored using a current probe (CT-1, Tektronix, Inc.) inserted between the capacitor and the electrode. A laser displacement gauge (LK-G82, Keyence Co.) that provided the spot size of 70 µm with the resolution and response speed of 0.2 µm and 20 µs, respectively, was used to measure the vertical actuation of the electrode in situ. An µEDM process produces byproducts that include the particles removed from the workpiece and carbon residues produced from the EDM oil. The removal of the byproducts is important to prevent irregular arcing and stabilizing the process. This was achieved by applying ultrasonic waves during µEDM, i.e., machining was performed while continuously “cleaning” the workpiece in the ultrasonic bath.

The sequential pulses of micro spark discharge were successfully generated using the electrostatic actuation mechanism (Fig. 6). The peak current, pulse duration, and charging time constant were approximately measured to be 2.5 A, 50 ns, and 1 µs, respectively, in the set-up used. Figure 7 shows the stainless-steel workpiece machined using the electrode in Fig. 4b with 100 V for 10 min. The removed depth was measured to be up to 20 µm. Figure 8 shows a result of the in-situ measurement of the displacement of an electrode with the initial gap separation of ~40 µm using the laser gauge. For a clear visibility of the actuation effect, the

Figure 4: (a: left) An optical image of the fabricated devices on a stainless-steel substrate; (b: right) an SEM image of the device with the layout shown in Fig. 2.

Figure 7: An SEM image of the µEDMed stainless-steel substrate (electrode removed after machining) with inset of close-up.

Figure 8:Vertical displacement of electrode measured while switching on and off the EDM process.

Figure 5:A set-up for µEDM tests.

Figure 6: (a: upper left) Measured current pulses of micro spark discharge; (b: upper right) single pulse close-up; (c: lower) an optical image of micro sparks captured through electrode’s holes.

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temporal measurement was performed while switching the machining voltage (of 100 V), i.e., EDMing on and off manually. The result indicates the vertical actuation of 30-33 µm at the measurement point, suggesting the breakdown gap of 7-10 µm or less.

The discharge energy of a single pulse is given by CV2/2 [1], where C is the total capacitance that is the sum of the external capacitance (Ce) and the built-in capacitance (Cb) in the set-up shown in Fig. 5, and V is the applied voltage. The energy determines the degree of the thermal impact and the size of the resultant crater-like shape hence the roughness and quality of the machined surfaces. In order to reduce the discharge energy for finer machining along with a simple circuit configuration, µEDM has also been demonstrated by using only Cb to form the RC circuit without Ce. For this implementation, it is important to evaluate the value of Cb, however, the direct measurement is difficult because Cb is a dynamic parameter as the actuation of the electrode modulates it, and Cb at the timing of a breakdown, Cbb, is the value of interest. The time constant, τ=RCbb, in a charging cycle can be used to indirectly estimate Cbb. The constant τ was determined by probing the voltage at the electrode and measuring the charging time. Figure 9 plots τ measured in the dynamic mode (i.e., while EDM is on) for various devices with different electrode-tether areas as well as Cbb calculated from the results. The static value, Cb, measured directly by probing the devices is also plotted in Fig. 9 for comparison. The result indicates approximate linear dependence of both Cb and Cbb (or τ) on the area of the device. Figure 9 also suggests, as predicted, that Cbb is substantially increased from Cb, modulating the discharge energy.

5. DISCUSSION In the electrostatic actuation method, the machinable

depth is limited as the machining enlarges the gap spacing between the electrode and the workpiece, lowering the electrostatic force to pull in the electrode. Higher voltages can be used to increase the force while compromising the surface roughness and quality in the machined structures as they increase the discharge energy as well. To extend the machinable depth, applying external force to the electrodes

towards the workpiece instead of using the electrostatic actuation to sustain the machining process is an option that requires an NC stage with an appropriate fixture. Another potential solution for these constraints is to decouple the voltage for the actuation and the discharge pulse formation by having separate electrodes at the cost of simplicity of the device construction and fabrication process.

6. CONCLUSION A new technique for µEDM using the electrostatic

actuation of the planar electrodes microfabricated on the workpiece has been studied. Laminated 18-µm-thick Cu foil was patterned to construct the planar electrodes suspended with the anchors, which were actuated with the machining voltage of 80-140 V for well-controlled pulse formation of micro spark discharges. Micromachining of stainless steel with removal depth of 20 µm was demonstrated using the 1.6×1.03 mm2 electrode and the voltage of 100 V. The built-in capacitance of the electrode structure was exploited to form the pulse generation circuitry and experimentally characterized both in dynamic and static modes. The developed method offers an opportunity to eliminate the need for high-precision NC machines for µEDM thereby achieving dramatic improvements of the process in not only the cost reduction but also the machinable area by introducing large-area patterning techniques for the device fabrication.

ACKNOWLEDGMENT We would like to thank NSERC for their financial

support to this research.

REFERENCES [1] T. Masaki, K. Kawata, T. Masuzawa, “Micro Electro-Discharge Machining and its Applications,” Proc. IEEE MEMS, 1990, pp. 21-26. [2] L.L. Chu, K. Takahata, P. Selvaganapathy, Y.B. Gianchandani, J.L. Shohet, “A Micromachined Kelvin Probe with Integrated Actuator for Microfluidic and Solid-State Applications,” J. MEMS, 14(4), 2005, pp. 691-698. [3] K. Takahata, Y.B. Gianchandani, “Bulk-Metal-Based MEMS Fabricated by Micro-Electro-Discharge Machining,” Proc. IEEE Canadian Conf. Electr. Comput. Eng. (CCECE), 2007, pp. 1-4. [4] K. Takahata, Y.B. Gianchandani, “Batch Mode Micro-Electro- Discharge Machining,” J. MEMS, 11(2), 2002, pp.102-110. [5] H. Guckel, “High-Aspect-Ratio Micromachining via Deep X-Ray Lithography,” Proc. IEEE, 86(8), 1998, pp. 1586-1593. [6] Y. Okazaki, N. Mishima, K. Ashida, “Microfactory - Concept, History, and Developments,” J. Manuf. Sci. Eng., 126(4), 2004, pp. 837-844. [7] S. Pamidighantam, R. Puers, K. Baert, H.A.C. Tilmans, “Pull-in Voltage Analysis of Electrostatically Actuated Beam Structures with Fixed-Fixed and Fixed-Free End Conditions,” J. Micromech. Microeng., 12, 2002, pp. 458-464. [8] J. Cheng, J. Zhe, X. Wu, “Analytical and finite element model pull-in study of rigid and deformable electrostatic microactuators,” J. Micromech. Microeng., 14, 2004, pp. 57-68. [9] W.C. Young, Roark’s Formulas for Stress and Strain, 7th ed., McGraw-Hill, 2002. [10] F. Zhang, A.K. Prasad, S.G. Advani, “Investigation of a Copper Etching Technique to Fabricate Metallic Gas Diffusion Media,” J. Micromech. Microeng., 16, 2006, pp. N23-N27.

Figure 9: Built-in capacitance in static (Cb) and dynamic (Cbb) modes vs. device area.

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