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, respectivelyJ. 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