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LOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 18(2008)105009(7pp) doi:10.1088/0960-1317/8/10/105009 MEDM: MEMS-enabled micro-electro-diScharge machining Chakravarty Reddy alla Chaitanya and Kenichi Takahata Department of Electrical and Computer Engineering, The University of British Columbia, 2332 Main Mall uver BC V6T 1Z4 Canada E-mail: allac@ece. ubc. ca and takahata @ece. ubc.ca Received 17 July 2008, in final form 7 August 2008 Published 5 September 2008 Online at stacks. iop. org/JMM/18/105009 Abstract This paper reports a photolithography compatible micro-electro-discharge machining technique that is performed with microelectrode actuators driven by hydrodynamic force. The movable planar electrodes suspended by the anchors are microfabricated directly on the orkpiece. The electrode structures with fixed-fixed and cantilever configurations are defined by patterning 18 um thick copper foil laminated on the workpiece through an intermediate photoresist layer and released by sacrificial etching of the resist layer. All the patterning and sacrificial etching steps are performed using dry-film photoresists towards achieving high calability of the machining technique to large-area applications. The parasitic capacitance of the electrode structure is used to form a resistance-capacitance circuit for the generation of pulsed spark discharge between the electrode and the workpiece. The suspended electrodes are actuated towards the workpiece using the downflow of dielectric machining fluid, initiating and sustaining the machining process. Micromachining of stainless steel is experimentally demonstrated with a machining voltage of 90 V and continuous flow of the fluid at a velocity of 3. 4-3.9 s, providing a removal depth of 20 um with an average surface roughness of 520 nm. The experimental results of the electrode actuation are shown to agree well with the theoretical estimations (Some figures in this article are in colour only in the electronic version) 1. Introduction LIGA, however, incurs high costs in electrode fabrication There have been some efforts that attempt to address the cost Micro-electro-discharge machining (REDM) is a non-contact effectiveness issue in electrode fabrication, at the expense of micromachining technique that can be used to cut any compatibility with photolithography-based methods [6, 7]. In type of electrically conductive material. The technique is addition, the batch-mode method still requires an NC stage for capable of producing real three-dimensional microstructures advancing the arrays into the material while achieving the smallest size of 5 um with submicron It has recently been shown that uEDM can be lerance [1]. These attractive features have been leveraged implemented using electrodes that are microfabricated for producing micro mechanical components as well as directly on the surfaces of the workpiece using standard for prototyping various micro-electro-mechanical systems photolithography and etching processes [8]. This method MEMS) and devices [2, 3. However, the throughput is exploits the machining voltage to electrostatically actuate inherently low because the traditional technique is essentially movable microelectrodes, eliminating the need for NC a serial process that uses a single electrode tip together machines from the machining process. This approach is with numerical control (NC) of the tip and the workpiece, suitable for selected electrode structures and applications producing structures individually. Batch-mode HEDM for relatively shallow machining due to the limitation of that uses microelectrode arrays fabricated by a deep x-ray the electrostatic actuation range. This paper reports a new lithography (LIGA)process [4] was demonstrated to achieve MEMS-based micro-EDM(MEDM) method where planar high parallelism/throughput of the process [5]. The use of electrodes microfabricated on a workpiece are actuated with 0960-131708/10500907530.00 @2008 IOP Publishing Ltd Printed in the UK

IOP PUBLISHING JOURNAL OF MICROMECHANICS AND MICROENGINEERING J. Micromech. Microeng. 18 (2008) 105009 (7pp) doi:10.1088/0960-1317/18/10/105009 M3 EDM: MEMS-enabled micro-electro-discharge machining Chakravarty Reddy Alla Chaitanya and Kenichi Takahata Department of Electrical and Computer Engineering, The University of British Columbia, 2332 Main Mall, Vancouver, BC V6T 1Z4, Canada E-mail: allac@ece.ubc.ca and takahata@ece.ubc.ca Received 17 July 2008, in final form 7 August 2008 Published 5 September 2008 Online at stacks.iop.org/JMM/18/105009 Abstract This paper reports a photolithography compatible micro-electro-discharge machining technique that is performed with microelectrode actuators driven by hydrodynamic force. The movable planar electrodes suspended by the anchors are microfabricated directly on the workpiece. The electrode structures with fixed–fixed and cantilever configurations are defined by patterning 18 μm thick copper foil laminated on the workpiece through an intermediate photoresist layer and released by sacrificial etching of the resist layer. All the patterning and sacrificial etching steps are performed using dry-film photoresists towards achieving high scalability of the machining technique to large-area applications. The parasitic capacitance of the electrode structure is used to form a resistance–capacitance circuit for the generation of pulsed spark discharge between the electrode and the workpiece. The suspended electrodes are actuated towards the workpiece using the downflow of dielectric machining fluid, initiating and sustaining the machining process. Micromachining of stainless steel is experimentally demonstrated with a machining voltage of 90 V and continuous flow of the fluid at a velocity of 3.4–3.9 m s−1 , providing a removal depth of 20 μm with an average surface roughness of 520 nm. The experimental results of the electrode actuation are shown to agree well with the theoretical estimations. (Some figures in this article are in colour only in the electronic version) 1. Introduction Micro-electro-discharge machining (μEDM) is a non-contact micromachining technique that can be used to cut any type of electrically conductive material. 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 mechanical components as well as for prototyping various micro-electro-mechanical systems (MEMS) and devices [2, 3]. However, the throughput is inherently low because the traditional technique is essentially a serial process that uses a single electrode tip together with numerical control (NC) of the tip and the workpiece, producing structures individually. Batch-mode μEDM that uses microelectrode arrays fabricated by a deep x-ray lithography (LIGA) process [4] was demonstrated to achieve high parallelism/throughput of the process [5]. The use of LIGA, however, incurs high costs in electrode fabrication. There have been some efforts that attempt to address the costeffectiveness issue in electrode fabrication, at the expense of compatibility with photolithography-based methods [6, 7]. In addition, the batch-mode method still requires an NC stage for advancing the arrays into the material. It has recently been shown that μEDM can be implemented using electrodes that are microfabricated directly on the surfaces of the workpiece using standard photolithography and etching processes [8]. This method exploits the machining voltage to electrostatically actuate movable microelectrodes, eliminating the need for NC machines from the machining process. This approach is suitable for selected electrode structures and applications for relatively shallow machining due to the limitation of the electrostatic actuation range. This paper reports a new MEMS-based micro-EDM (M3 EDM) method where planar electrodes microfabricated on a workpiece are actuated with 0960-1317/08/105009+07$30.00 1 © 2008 IOP Publishing Ltd Printed in the UK

J. Micromech. Microeng. 18(2008)105009 CR Alla Chaitanya and K Takahata removal of the material by pulses of the spark discharge while Dielectric anchor/spacer/Electrode feeding the electrode into the material. The tolerance and surface quality of uEDM depend on the energy of the single electric discharge, Epsc, which is expressed as [1] EDM fluid flow where C is the capacitance of the rC circuit and V is the electrode machining voltage. If the electrode is supported by a spring L】 so as to be movable in the vertical direction, with a proper Spark discharge structural design and certain operating conditions, it can be displaced towards the workpiece with electrostatic forc …--.…八 produced by the machining voltage to reach the breakdown gap and generate a discharge current. The'pull-in mechanism can optionally be used to obtain a relatively large displacement Figure l Cross sectional view of MEMS-based uEDM and its [8, 9]. The repeated cycle of attraction of the electrode, process steps. breakdown and voltage drop, release of the electrode and recovery of the voltage maintains the pulse generation and an external force for controlled generation of discharge pulses. automatically advances the electrode in a self-regulated The method aims to enable the machining process without manner while removing the material. However, as the the constraint attributed to the electrostatic actuation, thereby material is removed, it reaches a state where the electrostatic achieving high applicability of the technique. The movable force is no longer large enough to pull the electrode down electrodes are microfabricated using dry-film photoresist towards the machining surface and sustain the breakdown processes,targeting the potential application of uEDM to gap against the restoring force through the spring, limiting yery-large-area and /or non-planar samples with high precision further machining. The force can be increased by raising the and throughput at low cost voltage V; however, this results in larger discharge energy This paper is organized as follows. Section 2 describes Epsc, leading to the degradation of tolerance and surface the machining principle and method. The design of the roughness in the machined structures. The application of force movable electrode devices is discussed in conjunction with to the electrode externally can be a solution to this constraint the actuation mechanisms in section 3. Section 4 presents In addition, this approach permits one to make the original details of the fabrication processes for the devices. The results gap spacing larger beyond the range that the pull-in method of experimental characterization for the fabricated devices and available, which, in fact, is required for the sacrificial removal the demonstration of uEDM with the devices are reported in of the dry-film photoresist described in the following section section 5, followed by a discussion as well as analysis of the The larger original gap also promotes easier flushing of the experimental results in section 6. Section 7 concludes the byproducts, which contains the particles removed from the overall effort workpiece and carbon residues produced from the EDM fluid during the process, when the electrodes are in a resting state 2. Machining principle There are various potential methods for applying extemal force to the electrode for the actuation towards the workpiece The developed MEDM method uses planar electrodes that are surfaces. Some micromachined actuators have been reported suspended by the anchors above the surfaces of the conductive to use extemally assisted actuation mechanisms [10-12. For workpiece with a relatively large gap in a dielectric liquid HEDM application, one of the simplest and most effective (figure 1). A resistance-capacitance(RC)pulse generation/ methods would be the use of downflow of the EDM fluid timing circuit [1, 5] is coupled between the device and a in which the electrodes and workpiece are immersed. The de voltage source (80-100 V) so that the electrode serves flow rate, i.e., hydrodynamic pressure can be controlled as a cathode whereas the workpiece is an anode. The so that the electrodes are externally displaced to induce a parasitic/built-in capacitance that is present between the repeated cycle of pulse generation, implementing the EDM electrode-anchor structure and the workpiece is leveraged to process. Furthermore, the downflow is expected to enhance the form the rc circuit as shown in figure 1[8]. The voltage removal of the bypro cts from the applied between the workpiece and the suspended electrode proces auses a breakdown when the gap separation between them reaches the threshold distance(typically a few microns in 3. Electrode design and actuation mechanism HEDM). The breakdown leads to a spark current and an instant voltage drop due to a discharge from the capacitor. To experimentally demonstrate the machining method, various The spark current causes a thermal impact that removes the test electrodes were designed and fabricated for fixed-fixed- material, leaving a crater-like shape at the breakdown point on and cantilever-type configurations with sizes ranging from the workpiece surface. The voltage is restored with time as the hundreds of microns to millimeters. Sample designs ar capacitor is recharged through the resistor of the circuit for the shown in figure 2. For a feasibility study, this effort utilized next breakdown. The machining is implemented by repeated simple construction and fabrication for the electrodes with

J. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata Figure 1. Cross sectional view of MEMS-based μEDM and its process steps. an external force for controlled generation of discharge pulses. The method aims to enable the machining process without the constraint attributed to the electrostatic actuation, thereby achieving high applicability of the technique. The movable electrodes are microfabricated using dry-film photoresist processes, targeting the potential application of μEDM to very-large-area and/or non-planar samples with high precision and throughput at low cost. This paper is organized as follows. Section 2 describes the machining principle and method. The design of the movable electrode devices is discussed in conjunction with the actuation mechanisms in section 3. Section 4 presents details of the fabrication processes for the devices. The results of experimental characterization for the fabricated devices and the demonstration of μEDM with the devices are reported in section 5, followed by a discussion as well as analysis of the experimental results in section 6. Section 7 concludes the overall effort. 2. Machining principle The developed M3 EDM method uses planar electrodes that are suspended by the anchors above the surfaces of the conductive workpiece with a relatively large gap in a dielectric liquid (figure 1). A resistance–capacitance (RC) pulse generation/ timing circuit [1, 5] is coupled between the device and a dc voltage source (80–100 V) so that the electrode serves as a cathode whereas the workpiece is an anode. The parasitic/built-in capacitance that is present between the electrode–anchor structure and the workpiece is leveraged to form the RC circuit as shown in figure 1 [8]. The voltage applied between the workpiece and the suspended electrode causes a breakdown when the gap separation between them reaches the threshold distance (typically a few microns in μEDM). The breakdown leads to a spark current and an instant voltage drop due to a discharge from the capacitor. The spark current causes a thermal impact that removes the material, leaving a crater-like shape at the breakdown point on the workpiece surface. The voltage is restored with time as the capacitor is recharged through the resistor of the circuit for the next breakdown. The machining is implemented by repeated removal of the material by pulses of the spark discharge while feeding the electrode into the material. The tolerance and surface quality of μEDM depend on the energy of the single electric discharge, EDSC, which is expressed as [1] EDSC = 1 2CV 2 (1) where C is the capacitance of the RC circuit and V is the machining voltage. If the electrode is supported by a spring so as to be movable in the vertical direction, with a proper structural design and certain operating conditions, it can be displaced towards the workpiece with electrostatic force produced by the machining voltage to reach the breakdown gap and generate a discharge current. The ‘pull-in’ mechanism can optionally be used to obtain a relatively large displacement [8, 9]. The repeated cycle of attraction of the electrode, breakdown and voltage drop, release of the electrode and recovery of the voltage maintains the pulse generation and automatically advances the electrode in a self-regulated manner while removing the material. However, as the material is removed, it reaches a state where the electrostatic force is no longer large enough to pull the electrode down towards the machining surface and sustain the breakdown gap against the restoring force through the spring, limiting further machining. The force can be increased by raising the voltage V; however, this results in larger discharge energy EDSC, leading to the degradation of tolerance and surface roughness in the machined structures. The application of force to the electrode externally can be a solution to this constraint. In addition, this approach permits one to make the original gap spacing larger beyond the range that the pull-in method is available, which, in fact, is required for the sacrificial removal of the dry-film photoresist described in the following section. The larger original gap also promotes easier flushing of the byproducts, which contains the particles removed from the workpiece and carbon residues produced from the EDM fluid during the process, when the electrodes are in a resting state. There are various potential methods for applying external force to the electrode for the actuation towards the workpiece surfaces. Some micromachined actuators have been reported to use externally assisted actuation mechanisms [10–12]. For μEDM application, one of the simplest and most effective methods would be the use of downflow of the EDM fluid in which the electrodes and workpiece are immersed. The flow rate, i.e., hydrodynamic pressure can be controlled so that the electrodes are externally displaced to induce a repeated cycle of pulse generation, implementing the EDM process. Furthermore, the downflow is expected to enhance the removal of the byproducts from the machining area during the process. 3. Electrode design and actuation mechanisms To experimentally demonstrate the machining method, various test electrodes were designed and fabricated for fixed–fixedand cantilever-type configurations with sizes ranging from hundreds of microns to millimeters. Sample designs are shown in figure 2. For a feasibility study, this effort utilized simple construction and fabrication for the electrodes with 2

J. Micromech. Microeng 18 (2008)105009 CR Alla Chaitanya and K Takahata as the main component of typical EDM fuids, E=120 GH 10000 for copper, and the intrinsic stress assumed to be negligible), Hole: 30x30 the pull-in voltages for the 70 um gap are calculated to be approximately 730 V and 420 V for the fixed-fixed and cantilever electrodes in figure 2, respectively. In either case the pull-in voltage is far greater than a typical range of 上=5000 AEDM voltage(60-110 v)[1]. The external actuation using hydrodynamic force can be an effective means to address the Anchor Electrode limitation not only in the feed depth but also in initiating (b) electrodes and the workpiece are large 4. Fabrication 2050 (Unit: um) Figure 3 illustrates a cross-sectional view of the fabrication 2500 process for the suspended planar electrodes. As described earlier, this effort explores the use of dry film photoresists for Figure 2. Sample designs of the pEDM devices with(a) fixed-fixed and (b) cantilever configuratie all the lithography steps. The lamination of the dry films is commonly performed using a hot-roll laminator(XRL-120 single-layer structures without particular features underneath of 1.3 cm s. Type-304 stainless steel in the form of a 3 (as illustrated in figure 1). The electrode structures are formed wafer was selected as the work material and served as the using stock copper foil with 18 um thickness as the substrate for fabrication in this effort material, providing a relatively thick yet uniform layer of Two processes, shown as(a)and(b)in figure 3,were copper with no residual stress.(Copper has been used as developed for the device fabrication. In process(a),the an electrode material for HEDM [5, 7, 8]). All the anchors devices are fabricated directly on the substrate, whereas in used in the devices have the common dimensions of 2.5 x process(b), they are formed and supplied on a piece of dry 2.5 mm. The maximum vertical deflection of a fixed-fixed film that can be laminated and released on a selected surface of or cantilever electrode, y, with uniformly applied pressure, p, the workpiece. The latter approach can potentially be useful due to the fuid flow can be described by [13] for processing workpieces that have non-planar surfaces to be machined or very large dimensions that are not compatible Cel (2)with standard photolithography tools where E is Young's modulus of the electrode material. l is the For process(a), a negative photoresist(PM240, DuPont length of the electrode, I is the moment of inertia given by Co., DE, USA)with 35 um thickness is first laminated twice I=wh/12. where w and h are the width and thickness of on a thoroughly cleaned wafer to form a sacrificial layer with the electrode, respectively, and a is a constant that depends a total thickness of 70 um(step al). This thickness or greater on the electrode configuration(384 for fixed-fixed and8 was observed to be required for proper release of the designed for cantilever). As discussed in the next section, the gap electrodes performed at the last step of the process. Next, this study is 70 um that corresponds to the thickness of the layer with ick copper foil is laminated on the sacrificial same laminator(step a2). Then, a 15 um thick dry-film sacrificial layer necessary for the electrode release. negative photoresist(SF306, Macdermid Co., CO, USA) This separation requires high voltages to pull the electrodes laminated on the copper foil(step a3)and patterned using down to the breakdown position. The pull-in voltage, Vpl, for a mylar mask with the layout of the devices and a standard the suspended electrodes can be described by 19 mask aligner(step a4). The SF306 photoresist is developed in an alkaline aqueous developer, which is then used as a (3) mask for wet etching of copper in a ferric chloride solution 27EA (step a5). Finally, to release the electrodes, timed etching of the where g is the initial gap separation, A is the area of the sacrificial resist is performed in the developer for 2.5 hat room capacitive electrode and E is the permittivity of the EDM fluid. temperature without agitation(step a6). The 30 x 30 um2 The parameter K is the effective stiffness of the suspended perforations defined in the electrodes(figure 2) promote the structures, which is defined for the fixed-fixed structure as undercutting during the sacrificial etching process, while KFF= 2kP/[(k1/4)-tanh(k! /4)] where k= vP/(EI) leaving the resist to be the spacers at the anchors that have no nd P is the axial force created by the combination of the holes. The stream of the developer is used for the last 5 min intrinsic stress in the suspended structure and the nonlinear of the sacrificial etch to flush the resist residues, followed by stress that arises due to the deflection of the structure. The cleaning in acetone. Figure 4 shows the electrode devices effective stiffness for the cantilever can be represented as fabricated by this process. The built-in capacitances of the KcL 2Ewh /(31). Using equation (3)with relevant fabricated fixed-fixed and cantilever electrodes in figure 2 ar constants(h= 18 um, 8=1.59 x 10- m- for kerosene measured in air to be 7.2 pF and 3. 4 pF, respectively

J. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata (a) (b) Figure 2. Sample designs of the μEDM devices with (a) fixed–fixed and (b) cantilever configurations. single-layer structures without particular features underneath (as illustrated in figure 1). The electrode structures are formed using stock copper foil with 18 μm thickness as the original material, providing a relatively thick yet uniform layer of copper with no residual stress. (Copper has been used as an electrode material for μEDM [5, 7, 8]). All the anchors used in the devices have the common dimensions of 2.5 × 2.5 mm2 . The maximum vertical deflection of a fixed–fixed or cantilever electrode, y, with uniformly applied pressure, p, due to the fluid flow can be described by [13] y = pwl4 αEI (2) where E is Young’s modulus of the electrode material, l is the length of the electrode, I is the moment of inertia given by I = wh3/12, where w and h are the width and thickness of the electrode, respectively, and α is a constant that depends on the electrode configuration (384 for fixed–fixed and 8 for cantilever). As discussed in the next section, the gap separation between the electrodes and the substrate used in this study is 70 μm that corresponds to the thickness of the dry-film sacrificial layer necessary for the electrode release. This separation requires high voltages to pull the electrodes down to the breakdown position. The pull-in voltage, VPI, for the suspended electrodes can be described by [9] VPI = 8Kg3 27εA (3) where g is the initial gap separation, A is the area of the capacitive electrode and ε is the permittivity of the EDM fluid. The parameter K is the effective stiffness of the suspended structures, which is defined for the fixed–fixed structure as KFF = 2kP/[(kl/4) − tanh(kl/4)] where k = √P/(EI) and P is the axial force created by the combination of the intrinsic stress in the suspended structure and the nonlinear stress that arises due to the deflection of the structure. The effective stiffness for the cantilever can be represented as KCL = 2Ewh3/(3l 3). Using equation (3) with relevant constants (h = 18 μm, ε = 1.59 × 10−11 F m−1 for kerosene as the main component of typical EDM fluids, E = 120 GPa for copper, and the intrinsic stress assumed to be negligible), the pull-in voltages for the 70 μm gap are calculated to be approximately 730 V and 420 V for the fixed–fixed and cantilever electrodes in figure 2, respectively. In either case, the pull-in voltage is far greater than a typical range of μEDM voltage (60–110 V) [1]. The external actuation using hydrodynamic force can be an effective means to address the limitation not only in the feed depth but also in initiating the breakdown even when the gap separations between the electrodes and the workpiece are large. 4. Fabrication Figure 3 illustrates a cross-sectional view of the fabrication process for the suspended planar electrodes. As described earlier, this effort explores the use of dry film photoresists for all the lithography steps. The lamination of the dry films is commonly performed using a hot-roll laminator (XRL-120, Western Magnum Co., CA, USA) at 120 ◦C and a feed speed of 1.3 cm s−1 . Type-304 stainless steel in the form of a 3 wafer was selected as the work material and served as the substrate for fabrication in this effort. Two processes, shown as (a) and (b) in figure 3, were developed for the device fabrication. In process (a), the devices are fabricated directly on the substrate, whereas in process (b), they are formed and supplied on a piece of dry film that can be laminated and released on a selected surface of the workpiece. The latter approach can potentially be useful for processing workpieces that have non-planar surfaces to be machined or very large dimensions that are not compatible with standard photolithography tools. For process (a), a negative photoresist (PM240, DuPont Co., DE, USA) with 35 μm thickness is first laminated twice on a thoroughly cleaned wafer to form a sacrificial layer with a total thickness of 70 μm (step a1). This thickness or greater was observed to be required for proper release of the designed electrodes performed at the last step of the process. Next, the 18 μm thick copper foil is laminated on the sacrificial layer with the same laminator (step a2). Then, a 15 μm thick negative photoresist (SF306, Macdermid Co., CO, USA) is laminated on the copper foil (step a3) and patterned using a mylar mask with the layout of the devices and a standard mask aligner (step a4). The SF306 photoresist is developed in an alkaline aqueous developer, which is then used as a mask for wet etching of copper in a ferric chloride solution (step a5). Finally, to release the electrodes, timed etching of the sacrificial resist is performed in the developer for 2.5 h at room temperature without agitation (step a6). The 30 × 30 μm2 perforations defined in the electrodes (figure 2) promote the undercutting during the sacrificial etching process, while leaving the resist to be the spacers at the anchors that have no holes. The stream of the developer is used for the last 5 min of the sacrificial etch to flush the resist residues, followed by cleaning in acetone. Figure 4 shows the electrode devices fabricated by this process. The built-in capacitances of the fabricated fixed–fixed and cantilever electrodes in figure 2 are measured in air to be 7.2 pF and 3.4 pF, respectively. 3

J. Micromech. Microeng. 18(2008)105009 CALla anya and K Ta (1)Laminate double Hot-roll laminator lAminate double sist films for 70- esist films for 70- um-thick sacrificial/ anchor layer anchor layer on 18μ m-thick Cu (2) Laminate18μm thick Cu foil (2) Laminate 15-um- other side of Cu foil essex (3)Laminate 1 thick resist 4 UV light (3)Pattern the top ( 4)Pattern the top kpe Mylar Electrode, Anchor (4)Wet etch Cu foil (5)Laminate on the (wEt etch Cu foil workpiece Developer resist in developer and cleaning Figure 3. Two dry-film processes developed for the fabrication of the movable electrode devices on the workpiec tainless steel substrate 3 wafer of Dry-film stainless steel (a)Ar Cu electrodes Patterned copper Figure 5. A6 6 cm- piece of sacrif m photoresist with patterned electrode devices(the 3"wa eath the resist film was placed for dimensional comparisor (a)了 AnchoElectrode layer during the etching process(step b4). Figure 5 shows a piece of sacrificial dry film with arrays of patterned copper electrodes prior to the lamination on a workpiece. After the completion of the electrode fabrication, the protective film of sacrificial resist is removed and laminated on the sample to be machined(step b5). Sacrificial etch is performed in a similar c 4. A SEM image of (a)a fixed-fixed electrode and (b)a manner to that described in process(a) er electrode both with the layouts shown in figure 2, and (c an optical image of the fabricated devices. 5. Experimental results For process(b), the copper foil is laminated with a double- Figure 6 shows a set-up used for uEDM tests as well as layer sacrificial film of PM240 photoresist on one side and with characterization of the electrode structures. The substrate SF306 photoresist on the other side of the foil ( steps bl and b2). with the fabricated devices was placed in an ultrasonic bath The SF306 resist is patterned (step b3)and used as a mask for filled with low-viscosity dielectric EDM oil(EDM 185 wet etching of copper while the protective film of PM240 Commonwealth Oil Co., ON, Canada). A 20 KS2 resistor photoresist is kept intact to avoid any damage to the sacrificial was connected between the device and the dc voltage source

J. Micromech. Microeng. 18(2008)105009 CR Alla Chaitanya and K Takahata Oscilloscope Current probe mete ektronix CT-2 20kQ Fluid 50ns EDM fluid Copper electrode Ultrasonic (b)Perforation Figure 6. A set-up used for the characterization of electrode actuation and uEDM tests Electrode Figure 8.(a) Measured pulses of disch Decreasing velocity 90 V with an inset of single pulse clos of spark light captured through the el g16 contact of the electrode to the substrate. The resist at the anchors were measured to show no detectable in thickness after immersing the devices in the EDM 2-3 days, suggesting that the swelling effect due 12 absorption of oil from their sidewalls is negligible With the application of machining voltage and the injection of the fluid to the electrodes, sequential pulses of micro spark discharge were successfully generated and Fluid flow velocity(m/s) sustained at flow velocities of 3.9 m- and 3. 4 ms-I for the Figure 7. Built-in capacitance versus fluid flow velocity measured fixed-fixed and cantilever electrodes, respectively(figure 8) with a fabricated device with the design shown in figure 2(a). The typical peak current and pulse duration were measured to be 2-3 A and 50 ns, respectively, in the set-up used. Figure 9(a)shows the stainless-steel workpiece machined to form the RC circuit with built-in capacitance, as shown figure 6. The electrical discharge pulses were monitored using the cantilever device(figure 2(b)at 90V for about 15 min. The pattern of the cylindrical structures in the sing an ac current probe, which has a minimal loading on machined area corresponds to that of the holes of the the discharge circuit. A variable-speed motor pump was used electrodes. The machined structures were characterized pressure to the electrodes for their actuation. The ultrasonic using a WykoTM NT1100 optical profiler(figure 9(b).The measurement indicates a removal depth of 20 um and an rave was applied to the fluid bath during the process to assist average surface roughness of 520 nm in the machined areas in the dispersion of the byproducts. As described earlier with Figure 9(c) shows an optical image at one of the holes in equation (D), the discharge energy Epsc, or machining quality the electrode. The image was taken after machining but depends on the built-in capacitance of the device, which is a before removing the electrode structure from the workpiece dynamic parameter as it is partially determined by the movable indicating a discharge gap of about 10 um between the electrode(in addition to the fixed anchors ). The behavior of machined cylindrical structure and the perimeter of the the capacitance was characterized using an HP 4275A LCR electrode hole meter with varying How rate of the EDM fluid, as shown in Figure 7 shows the built-in capacitance of the fixed-fixec 6. Analysis and electrode structure versus the flow rate in both increasing and decreasing directions, measured while applying no voltage to consistency with theoretical estimations, The pressure measured in air, the increased static capacitance was expected onto the electrode immersed in a fluid by a flow of the due to operation in a liquid ambient. The plot in figure 7 that is injected from a circular nozzle perpendicular to the indicates a nonlinear rise of the capacitance, i.e., decrease of electrode plane can be represented by [14 the capacitive gap with the flow velocity as well as a highly elastic behavior of the electrode structure during the actuation. The capacitance was observed to become immeasurable at a where H is the normal distance between the exit of the nozzle flow velocity of 5. ms and greater, indicating the physical and the electrode, d is the diameter of the nozzle, p is the

J. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata Figure 6. A set-up used for the characterization of electrode actuation and μEDM tests. Figure 7. Built-in capacitance versus fluid flow velocity measured with a fabricated device with the design shown in figure 2(a). to form the RC circuit with built-in capacitance, as shown in figure 6. The electrical discharge pulses were monitored using an ac current probe, which has a minimal loading on the discharge circuit. A variable-speed motor pump was used to inject the EDM oil at a controlled rate to apply fluidic pressure to the electrodes for their actuation. The ultrasonic wave was applied to the fluid bath during the process to assist in the dispersion of the byproducts. As described earlier with equation (1), the discharge energy EDSC, or machining quality depends on the built-in capacitance of the device, which is a dynamic parameter as it is partially determined by the movable electrode (in addition to the fixed anchors). The behavior of the capacitance was characterized using an HP 4275A LCR meter with varying flow rate of the EDM fluid, as shown in figure 6. Figure 7 shows the built-in capacitance of the fixed–fixed electrode structure versus the flow rate, in both increasing and decreasing directions, measured while applying no voltage to the electrode. Compared to the 7.2 pF built-in capacitance measured in air, the increased static capacitance was expected due to operation in a liquid ambient. The plot in figure 7 indicates a nonlinear rise of the capacitance, i.e., decrease of the capacitive gap with the flow velocity as well as a highly elastic behavior of the electrode structure during the actuation. The capacitance was observed to become immeasurable at a flow velocity of ∼5.4 m s−1 and greater, indicating the physical /A (a) (b) Figure 8. (a) Measured pulses of discharge current at a voltage of 90 V with an inset of single pulse close-up, and (b) an optical image of spark light captured through the electrode’s holes. contact of the electrode to the substrate. The resist spacers at the anchors were measured to show no detectable change in thickness after immersing the devices in the EDM oil for 2–3 days, suggesting that the swelling effect due to the absorption of oil from their sidewalls is negligible. With the application of machining voltage and the injection of the fluid to the electrodes, sequential pulses of micro spark discharge were successfully generated and sustained at flow velocities of 3.9 m s−1 and 3.4 m s−1 for the fixed–fixed and cantilever electrodes, respectively (figure 8). The typical peak current and pulse duration were measured to be 2–3 A and 50 ns, respectively, in the set-up used. Figure 9(a) shows the stainless-steel workpiece machined using the cantilever device (figure 2(b)) at 90 V for about 15 min. The pattern of the cylindrical structures in the machined area corresponds to that of the holes of the electrodes. The machined structures were characterized using a WykoTM NT1100 optical profiler (figure 9(b)). The measurement indicates a removal depth of ∼20 μm and an average surface roughness of 520 nm in the machined areas. Figure 9(c) shows an optical image at one of the holes in the electrode. The image was taken after machining but before removing the electrode structure from the workpiece, indicating a discharge gap of about 10 μm between the machined cylindrical structure and the perimeter of the electrode hole. 6. Analysis and discussion It is worth evaluating the measurement results and their consistency with theoretical estimations. The pressure applied onto the electrode immersed in a fluid by a flow of the fluid that is injected from a circular nozzle perpendicular to the electrode plane can be represented by [14] p = 50 (H/d)2 ρv2 2 (4) where H is the normal distance between the exit of the nozzle and the electrode, d is the diameter of the nozzle, ρ is the 5

J. Micromech Microeng. 18(2008)105009 CALla anya and K Ta Original surfaces Material: Stainless steel 6,8 Top surface of copper electrode Figure 9. Micromachined result obtained with a cantilever electrode: (a) a SEM image and (b) optically measured geometry of the machined structures (electrode removed after machining);(c)a top view at one of the holes of an electrode that was stuck to the workpiece ining, showing a circular surface of the workpiece through the hole with a discharge gap of 10 um. the same equations, the flow velocities that were required Cantilever to initiate the breakdown are estimated to produce 40 um displacement as seen in figure 10, leaving 30 um gap spacing. Fixed-fixed This is approximately consistent with the estimated pull-in gap of 25 um at 90 V for the cantilever electrode obtained using Measured velocities(3.4 equation (3).(For the fixed-fixed, this gap is estimated to be for fixed-fixed required to somewhat smaller as it is stiffened by the nonlinear stress due initiate pulses to the forced deflection by flow already. The machining process/system as well as the device easured construction will need some improvement and optimization short circuit for increased performance and practicality of the process. It was observed that some of the electrodes were stuck to the substrates while implementing the process, preventing further Fluid flow velocity, v(m/s) machining(figure 9(c)). This is likely caused by local welding 10. Theoretical deflections versus fluid flow velocity for the short-circuited to the substrates). The result can be partially trodes in figure 2 calculated using the experimental ons of the fluidic set-up used (figure 6) due to open-loop control of fluid flow in the set-up used in this study, which could force the electrodes to physically touch the density of the fluid and v is the velocity of the fluid flow substrate. In addition, it was visually observed that the carbon at the nozzle exit. Figure 10 plots the deflections of the residues tended to adhere to the electrodes and remained sample electrodes with varying flow velocity of the EDM fluid around the machining space. This can lead to two deleterious obtained by equations(2)and (4)with the relevant constants phenomena. One is the generation of secondary discharges associated with the experimental set-up used in this study through the carbon particles, which can cause excess material (H=19 mm, d=1.3 mm, and p=796 kg m-3for the EDM removal. The result of the relatively large discharge gap of fluid used). The hydrodynamic pressure p is assumed to be 10 um(figure 9(c)may be related to this effect. The other is uniform over the electrodes for this approximated estimation. the generation of irregular arcing between the electrode and the The plot indicates the calculated displacement of 75 um for workpiece that produces a significant amount of heat [15]. This he fixed-fixed electrode with the flow velocity of 5.4 m s- may have contributed to both the local welding, possibly along at which the short circuit was observed, which matches well with the physical contact of the electrode to the workpiece, and with the actual gap separation(70 um) between the electrode the excess removal effect. These issues will be addressed by and the workpiece under the assumption. In addition, with using feedback control for the fluid flow in synchronization

J. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata (a) (c) (b) Figure 9. Micromachined result obtained with a cantilever electrode: (a) a SEM image and (b) optically measured geometry of the machined structures (electrode removed after machining); (c) a top view at one of the holes of an electrode that was stuck to the workpiece during the machining, showing a circular surface of the workpiece through the hole with a discharge gap of ∼10 μm. Figure 10. Theoretical deflections versus fluid flow velocity for the two electrodes in figure 2 calculated using the experimental conditions of the fluidic set-up used (figure 6). density of the fluid and v is the velocity of the fluid flow at the nozzle exit. Figure 10 plots the deflections of the sample electrodes with varying flow velocity of the EDM fluid obtained by equations (2) and (4) with the relevant constants associated with the experimental set-up used in this study (H = 19 mm, d = 1.3 mm, and ρ = 796 kg m−3 for the EDM fluid used). The hydrodynamic pressure p is assumed to be uniform over the electrodes for this approximated estimation. The plot indicates the calculated displacement of 75 μm for the fixed–fixed electrode with the flow velocity of 5.4 m s−1 at which the short circuit was observed, which matches well with the actual gap separation (70 μm) between the electrode and the workpiece under the assumption. In addition, with the same equations, the flow velocities that were required to initiate the breakdown are estimated to produce ∼40 μm displacement as seen in figure 10, leaving 30 μm gap spacing. This is approximately consistent with the estimated pull-in gap of 25 μm at 90 V for the cantilever electrode obtained using equation (3). (For the fixed–fixed, this gap is estimated to be somewhat smaller as it is stiffened by the nonlinear stress due to the forced deflection by flow already.) The machining process/system as well as the device construction will need some improvement and optimization for increased performance and practicality of the process. It was observed that some of the electrodes were stuck to the substrates while implementing the process, preventing further machining (figure 9(c)). This is likely caused by local welding of the electrodes to the substrates (as they were measured to be short-circuited to the substrates). The result can be partially due to open-loop control of fluid flow in the set-up used in this study, which could force the electrodes to physically touch the substrate. In addition, it was visually observed that the carbon residues tended to adhere to the electrodes and remained around the machining space. This can lead to two deleterious phenomena. One is the generation of secondary discharges through the carbon particles, which can cause excess material removal. The result of the relatively large discharge gap of 10 μm (figure 9(c)) may be related to this effect. The other is the generation of irregular arcing between the electrode and the workpiece that produces a significant amount of heat [15]. This may have contributed to both the local welding, possibly along with the physical contact of the electrode to the workpiece, and the excess removal effect. These issues will be addressed by using feedback control for the fluid flow in synchronization 6

J. Micromech. Microeng 18(2008)105009 CR Alla Chaitanya and K Takahata with the discharge pulse generation as well as enh University of British Columbia for assisting in the fabrication dispersion of the carbon byproducts(by optimizing process rave application, use of flushing steps, etc)and / or the carbon production using pure water as a dielectric fiuid for References the eDM process [16] The electrodes need to be multi-layer constructions that [1] Masaki T, Kawata K and Masuzawa T 1990 Micro incorporate custom features onto the bottom of suspended electro-discharge machining and its applications Proc. plates(corresponding to the single-layer electrodes used in IEEE Conf Micro Electro Mechanical Systems pp 21-6 this effort). The addition of the microstructures at the bottom [2] Takahata K and Gianchandani Y B 2007 Bulk-metal-based MEMS fabricated by micro-electro-discharge machining of the support can vary the electrostatic force depending on Canadian Conf on Electrical and Computer engineering the geometry of the custom features. The use of external force Vancouver: Canada, 22-26 April) pp 1 for electrode actuation would contribute to minimizing such [3] Reynaerts D, Meeusen w, Song X, Van Brussel H dependences of the MEDM method on application-associated Reyntjens S, De Bruyker D and Puers R 2000 Integrating actors. The microfabrication of the suspended multi-layer lectro-discharge machining and photolithography: work in progress J. Micromech Micoeng. 10 189-95 structures can potentially be approached using similar dry-[4) Guckel H 1998 High-aspect-ratio micromachining via deep Im processes with additional patterning and electroplating x-ray lithography Proc. IEEE 86 1586-9 steps; the sacrificial etching of the dry-film resist will need to [5] Takahata K and Gianchandani Y B 2002 Batch mode be optimized accordingly micro-electro-discharge machining IEEE J Microelectromech. Syst. 11 102-10 [6 Liao Y, Chen S, Lin C and Chuang T 2005 Fabrication of high 7. Conclusions aspect ratio microstructure arrays by micro reverse wire-EDMJ. Micromech Microeng. 15 1547-55 A MEMS-based uEDM method using hydrodynamic force [7 YisM, Park MS, Lee Ys and Chu cN2008 Fabrication of a for the actuation of microelectrodes fabricated on a workpiece stainless steel shadow mask using batch mode micro-EDM Technol. 14 411-7 has been studied. The dry-film photoresist processes with [8] Alla Chaitanya CR and Takahata K 2008 Micro-electro- its sacrificial etching were developed for the fabrication discharge machining by MEMS actuators wit of suspended electrode structures of copper with 18 um electrodes microfabricated on the work surfaces IEEE Int thickness. The fabricated devices were successfully utilized, Conf on Micro Electro Mechanical Systems 2008) as both movable electrodes and capacitive elements of the pp375-8 Ise generation circuitry, to produce pulsed micro sparks [9] Pamidighantam S, Puers R, Baert K and Tilmans H A C 2002 Pull-in voltage analysis of electrostatically actuated beam with the electrodes driven by controlled flow of the EDM structures with fixed-fixed and fixed-free end conditions fluid. Micromachining of stainless steel was experimentally (10) Judy J W, Muller R S and Zappe H H 1995 Magnetic J. Micromech. Microeng 12 458-64 demonstrated using the devices, achieving a removal depth of 20 um with a machining voltage of 90 V. The results obtained microactuation of polysilicon flexure structures IEEEJ. suggest that HEDM can potentially serve as a non-NC,[11 Nguyen N T, Troung T Q, Wong K K, Ho S S and Low CLN arge-area batch processing technique with high applicability 004 Micro check valves for integration into polymeric enabled by the external actuation approach. Theoretical microfluidic devices J. Micromech Microeng. 14 69-75 nalysis of the measured results obtained with the fabricated [12] Lee s w, Kim D J, Ahn Y and Chai Y G 2006 Simpl devices revealed that the behavior of the electrodes actuated by structured polydimethylsiloxane microvalve actuated by external air pressure J. Mech. Sci. 220 1283-8 fluidic flow could be described well with the analytical models [13 Young WC2002 Roark's Formulas for Stress and Strain used (New York: McGraw-Hill) [14] Rajaratnam N 1976 Turbulent Jets(New York: Elsevier) Acknowledgments [15] Jameson E C 2001 Electric Discharge Machining(Dearborn, MI: Society of Manufacturing Engineers) The authors would like to thank the NSERC for their financial 6] Lin CT. Chow H M, Yang L D and Chen Y F 2007 Feasibility study of micro-slit EDM machining using pure water Inf. J. support to this research and Ms Vijayalakshmi Sridhar at the Adv Manuf Technol. 34 104-

J. Micromech. Microeng. 18 (2008) 105009 C R Alla Chaitanya and K Takahata with the discharge pulse generation as well as enhancing the dispersion of the carbon byproducts (by optimizing ultrasonic wave application, use of flushing steps, etc) and/or minimize the carbon production using pure water as a dielectric fluid for the EDM process [16]. The electrodes need to be multi-layer constructions that incorporate custom features onto the bottom of suspended plates (corresponding to the single-layer electrodes used in this effort). The addition of the microstructures at the bottom of the support can vary the electrostatic force depending on the geometry of the custom features. The use of external force for electrode actuation would contribute to minimizing such dependences of the M3 EDM method on application-associated factors. The microfabrication of the suspended multi-layer structures can potentially be approached using similar dry- film processes with additional patterning and electroplating steps; the sacrificial etching of the dry-film resist will need to be optimized accordingly. 7. Conclusions A MEMS-based μEDM method using hydrodynamic force for the actuation of microelectrodes fabricated on a workpiece has been studied. The dry-film photoresist processes with its sacrificial etching were developed for the fabrication of suspended electrode structures of copper with 18 μm thickness. The fabricated devices were successfully utilized, as both movable electrodes and capacitive elements of the pulse generation circuitry, to produce pulsed micro sparks with the electrodes driven by controlled flow of the EDM fluid. Micromachining of stainless steel was experimentally demonstrated using the devices, achieving a removal depth of 20 μm with a machining voltage of 90 V. The results obtained suggest that μEDM can potentially serve as a non-NC, large-area batch processing technique with high applicability enabled by the external actuation approach. Theoretical analysis of the measured results obtained with the fabricated devices revealed that the behavior of the electrodes actuated by fluidic flow could be described well with the analytical models used. Acknowledgments The authors would like to thank the NSERC for their financial support to this research and Ms Vijayalakshmi Sridhar at the University of British Columbia for assisting in the fabrication process. References [1] Masaki T, Kawata K and Masuzawa T 1990 Micro electro-discharge machining and its applications Proc. IEEE Conf. Micro Electro Mechanical Systems pp 21–6 [2] Takahata K and Gianchandani Y B 2007 Bulk-metal-based MEMS fabricated by micro-electro-discharge machining Canadian Conf. on Electrical and Computer Engineering (Vancouver, Canada, 22–26 April) pp 1–4 [3] Reynaerts D, Meeusen W, Song X, Van Brussel H, Reyntjens S, De Bruyker D and Puers R 2000 Integrating electro-discharge machining and photolithography: work in progress J. Micromech. Microeng. 10 189–95 [4] Guckel H 1998 High-aspect-ratio micromachining via deep x-ray lithography Proc. IEEE 86 1586–93 [5] Takahata K and Gianchandani Y B 2002 Batch mode micro-electro-discharge machining IEEE J. Microelectromech. Syst. 11 102–10 [6] Liao Y, Chen S, Lin C and Chuang T 2005 Fabrication of high aspect ratio microstructure arrays by micro reverse wire-EDM J. Micromech. Microeng. 15 1547–55 [7] Yi S M, Park M S, Lee Y S and Chu C N 2008 Fabrication of a stainless steel shadow mask using batch mode micro-EDM Microsyst. Technol. 14 411–7 [8] Alla Chaitanya C R and Takahata K 2008 Micro-electrodischarge machining by MEMS actuators with planar electrodes microfabricated on the work surfaces IEEE Int. Conf. on Micro Electro Mechanical Systems (MEMS 2008) pp 375–8 [9] Pamidighantam S, Puers R, Baert K and Tilmans H A C 2002 Pull-in voltage analysis of electrostatically actuated beam structures with fixed-fixed and fixed-free end conditions J. Micromech. Microeng. 12 458–64 [10] Judy J W, Muller R S and Zappe H H 1995 Magnetic microactuation of polysilicon flexure structures IEEE J. Microelectromech. Syst. 4 162–9 [11] Nguyen N T, Troung T Q, Wong K K, Ho S S and Low C L N 2004 Micro check valves for integration into polymeric microfluidic devices J. Micromech. Microeng. 14 69–75 [12] Lee S W, Kim D J, Ahn Y and Chai Y G 2006 Simple structured polydimethylsiloxane microvalve actuated by external air pressure J. Mech. Sci. 220 1283–8 [13] Young W C 2002 Roark’s Formulas for Stress and Strain (New York: McGraw-Hill) [14] Rajaratnam N 1976 Turbulent Jets (New York: Elsevier) [15] Jameson E C 2001 Electric Discharge Machining (Dearborn, MI: Society of Manufacturing Engineers) [16] Lin C T, Chow H M, Yang L D and Chen Y F 2007 Feasibility study of micro-slit EDM machining using pure water Int. J. Adv. Manuf. Technol. 34 104–10 7

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