CA2649289A1 - Micro-electro-discharge machining methods and apparatus - Google Patents

Abstract

A micro-electric-discharge machining method may be applied to fabricating structures from hard materials. One or more electrodes may be fabricated over a surface of a workpiece. The electrodes may be supported by resiliently-deformable members in such a manner that the electrodes can be advanced toward the workpiece by applying a potential difference between the electrode and workpiece until a discharge occurs.

Description

MICRO-ELECTRO-DISCHARGE MACHINING METHODS AND APPARATUS
Technical Field [0001] The invention relates to machining and, in particular to electro-discharge machining, also known as EDM.

Back rg ound [0002] Electro-discharge machining (EDM) is a technology that may be applied to machining electrically-conductive materials. In EDM, an electrode immersed in a dielectric fluid is brought near to a surface of a workpiece. An electrical potential is applied between the electrode and the workpiece. The electrical potential results in the release of energy by way of an electrical discharge between the electrode and the workpiece. The discharge causes some material to be removed from the workpiece.

[0003] EDM techniques can be used to machine hard materials that are otherwise difficult to machine. EDM techniques can be very precise. A modern EDM machine typically has a numerically controlled (NC) position controller connected to control the position of an electrode relative to a workpiece. The electrode may be advanced to cut into the workpiece.

[0004] EDM techniques may be applied on a small scale. Micro-electro-discharge machining ( EDM) involves erosion of a workpiece by small spark discharge pulses generated between a microscopic electrode tip and a workpiece in a dielectric fluid.
EDM is described in:
= T. Masaki, K. Kawata, T. Masuzawa, Micro Electro-Discharge Machining and its Applications, Proc. IEEE MEMS, 1990, pp. 21-26;
= 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; and, 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.

[0005] Some EDM techniques that are described in the literature involve a serial process that uses a single electrode tip in conjunction with numerical control (NC) of the tip and the workpiece to produce features of structure individually. The throughput of such techniques is inherently low. Batch-mode EDM that uses arrays of high-aspect-ratio microelectrodes may achieve high parallelism and increased throughput.

[0006] A batch-mode EDM process is described in: K. Takahata, Y.B.
Gianchandani, Batch Mode Micro-Electro- Discharge Machining J. MEMS, 11(2), 2002, pp.102-110.
This paper describes arrays of EDM electrodes fabricated using a LIGA
process. The arrays of electrodes were advanced into the workpiece using the vertical NC
stage in an EDM apparatus. Making electrode arrays by the LIGA process is undesirably expensive.
Gianchandani et al. US 6586699 discloses a EDM process using semiconductor electrodes.

[0007] Gianchandani et al. US 6624377 discloses EDM apparatus and methods which involve an array of electrodes formed on a substrate.

[0008] Masaki et al. US 6809285 discloses an EDM apparatus having a vibrator that changes a relative distance between a tool electrode and a workpiece at a prescribed frequency.

[0009] There is a need for EDM techniques that are practical and more cost-effective in certain applications. There is a particular need for such techniques that may be applied effectively to machine small-scale features.

Summary of the Invention [0010] This invention has a number of aspects. These aspects may be applied individually or together.

[0011] One aspect of the invention provides EDM methods which involve fabricating one or more EDM electrodes on a surface of a workpiece, for example, using etching or other lithographic techniques. The electrodes may have shapes defined by a mask. In some embodiments, large numbers of electrodes are fabricated on the workpiece at the same time.

[0012] Another aspect of the invention provides EDM methods in which an EDM
electrode is supported over a workpiece surface by a resiliently deformable mechanical member. The EDM electrode is caused to advance toward the workpiece surface at least in part by applying an electrical potential between the EDM electrode and the workpiece surface. The advance of the electrode causes deformation of the resiliently deformable mechanical member. The electrical potential is reduced upon the occurrence of electrical discharge between the electrode and the workpiece surface. The reduction of the electrical potential allows a restoring force exerted by the resiliently deformable mechanical member to draw the electrode away from the workpiece surface toward its original position.

[0013] Another aspect of the invention provides a structure comprising a workpiece having a surface to be machined and one or more EDM electrodes formed in a layer attached to the surface. In embodiments, the EDM electrodes are supported over the surface by one or more resiliently-deformable mechanical members. In embodiments, a plurality of EDM electrodes are provided on the workpiece surface.

[0014] Another aspect of the invention provides a structure comprising one or more EDM
electrodes formed in a layer that can be pressed against or attached to the surface of a workpiece. Pads or other electrical connection points are provided for the application of a potential difference between the EDM electrodes and the workpiece. In embodiments, the EDM electrodes are supported over the surface by one or more resiliently-deformable mechanical members. In embodiments, a plurality of EDM electrodes are provided on the workpiece surface.

[0015] Another aspect of the invention provides apparatus for performing EDM
that comprises at least one EDM electrode supported by a resiliently deformable mechanical member and an electrical circuit connected to apply an electrical potential between the EDM electrode and an adjacent workpiece surface. In embodiments, the apparatus comprises a spacer that spaces the EDM electrode a predetermined distance above the workpiece surface when the spacer is against the workpiece surface and no electrical potential difference exists between the EDM electrode and the workpiece surface.

[0016] Another aspect of the invention provides apparatus for performing EDM
that comprises at least one EDM electrode supported by a resiliently deformable mechanical member and a fluid outlet located to apply a force to the EDM electrode by causing fluid to flow against the EDM electrode. In embodiments fluid flow at the fluid outlet is controlled by a controller which synchronises the fluid flow to an EDM cycle such that a fluid flow pattern is synchronized to the occurrence of EDM discharges. The controller may control both the fluid flow and an electrical circuit connected to apply an electrical potential between the EDM electrode and an adjacent workpiece surface.

[0017] Another aspect of the invention provides a self-adhesive layer having defined therein one or more EDM electrodes supported by resiliently deformable mechanical members. The self-adhesive layer may be affixed to a surface of a workpiece, immersed in a suitable EDM fluid and applied as discussed herein to perform EDM on the workpiece surface. The self-adhesive layer may be removed after the EDM is completed.

[0018] Further aspects of the invention and features of specific embodiments of the invention are described below.

Brief Description of the Drawings [0019] The accompanying drawings illustrate non-limiting embodiments of the invention.

[0020] Figure 1 is a schematic diagram of apparatus according to an example embodiment of the invention.

[0021] Figures 2A through 2D illustrate operation of an EDM apparatus according to an example embodiment.

[0022] Figures 3A through 3F illustrate steps in a method which involves fabricating EDM apparatus on a surface of a workpiece.

[0023] Figures 4A through 4E illustrate example configurations for EDM
electrode structures.

[0024] Figure 5 is a scanning electron microscope image showing a prototype EDM
electrode structure.

[0025] Figure 6 is a scanning electron microscope image showing a magnified view of a cavity formed by EDM under an EDM electrode like that of Figure 5 with an inset showing a magnified view of a portion of the cavity.

[0026] Figure 7 is a microphotograph of an array of prototype EDM electrodes on a workpiece surface.

[0027] Figure 8 is a plan view of a design for a prototype EDM electrode assembly showing some dimensions.

100281 Figure 9 is a plot showing electrical current as a function of time in a circuit driving a prototype EDM electrode.

[0029] Figure 9A is a plot showing results of capacitance measurements for a number of prototype EDM electrode structures.

[0030] Figure 10 is a schematic cross section through an EDM electrode assembly comprising an EDM electrode with projecting features.

100311 Figure 11 is a schematic view of a EDM apparatus including fluid outlets for applying forces to EDM electrodes.

[0032] Figures 12 and 13 are flow charts illustrating methods according to embodiments of the invention.

Description [0033] Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention.
Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense.

[0034] Figure 1 shows EDM apparatus 10 according to an example embodiment of the invention. Apparatus 10 comprises a vessel 12 which contains a suitable dielectric fluid 14. Fluid 14 may comprise, for example, an oil marketed for use in EDM
applications Fluid 14. A workpiece 15 having a surface 16 which it is desired to machine is located such that at least the portion of surface 16 which it is desirable to machine is covered by fluid 14.

[0035] An EDM electrode 18 is supported over surface 16 by resiliently-deformable mechanical elements 20 which couple EDM electrode 18 to one or more anchors 19.
Electrically-insulating spacers 22 are located between anchors 19 and surface 16. In some embodiments, anchors 19 are attached to surface 16 by spacers 22.

[0036] A circuit 24 cooperates with EDM electrode 18 to produce current pulses. circuit 24 may comprise a resistance-capacitance (RC) circuit for example. In the illustrated embodiment, a power supply 25 has a positive terminal (anode) 26A electrically connected to workpiece 15 and a negative terminal (cathode) 26B electrically connected to EDM electrode 18. Although the negative terminal of the power source is typically connected to the EDM electrode 18 so that the discharge of electrons occurs from EDM
electrode 18 to workpiece 15, for workpieces of some types of material it may be preferable to connect the negative terminal to the workpiece and the positive terminal to EDM electrode 18. In some embodiments, power supply 25 has an output voltage of at least about 50 volts. In some cases the output voltage may exceed 80 volts.
For example, the output voltage of power supply 25 may be in the range of about 80 to about 140 volts.
[0037] In some embodiments, circuit 24 comprises active components such as transistors.
For example, transistor-based pulse generation circuits (such as those circuits provided in some commercially-available EDM machines) may be used to provide the required potential difference between electrodes and workpieces in embodiments of the invention.
[0038] In the illustrated embodiment, a resistor 28 is located in a current path between power supply cathode 26B and EDM electrode 18. In the illustrated embodiment, the electrical connection to EDM electrode 18 is made by way of anchors 19 and mechanical members 20.

[0039] Capacitance 29 exists between EDM electrode 18 and workpiece 15.
Capacitance 29 may be provided by capacitive coupling between anode- and cathode-sides of the circuit (which may be parasitic capacitance). A capacitor may be coupled between the anode- and cathode- sides of the circuit to increase capacitance 29.

[0040] In some embodiments, one or more ultrasonic transducers 27 are provided.

Ultrasonic transducer 27 may ultrasonically agitate fluid 14 during operation of apparatus 10. In some embodiments, vessel 12 is a vessel of an ultrasonic cleaner which includes one or more ultrasonic transducers 27 and circuitry to drive transducers 27.

[00411 In some embodiments, multiple EDM electrodes are applied to a workpiece at the same time. The EDM electrodes may be operated to machine different areas of the workpiece. In some such embodiments a separate RC circuit is provided to supply electrical current to each EDM electrode. This can be advantageous because it permits each EDM electrode to operate essentially independently of other EDM
electrodes. The RC circuits may be powered by a common power supply 25.

[0042] In some cases where multiple EDM electrodes are provided to machine different parts of a workpiece surface, the operation of different ones of the EDM
electrodes may be controlled separately to permit each part of the workpiece surface to be machined to a desired depth. In some embodiments, a standard array of EDM electrodes may be provided on a workpiece surface and a desired pattern may be machined into the workpiece surface by selectively operating some of the EDM electrodes and not others.
[0043] Figures 2A through 2D illustrate the use of apparatus 10 to machine a cavity in surface 16 of substrate 15. As soon as an electrical potential is applied between EDM

electrode 18 and workpiece 15, an attractive force 30 begins to pull EDM
electrode 18 toward surface 16. With properly designed structures, at a selected voltage, the attractive force overcomes the restoring forces exerted by members 20. This force deforms resiliently-deformable, members 20 as shown in Figure 2B. This phenomenon may be called "pull-in". Pull-in is described, for example, in 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.

[0044] As EDM electrode 18 approaches surface 16 the strength of the electric field between EDM electrode and surface 16 increases. Eventually electrical discharges 32 occur. The electrical discharges erode workpiece 15.

[0045] Electrical discharges 32 cause a decrease in the potential difference between EDM
electrode 18 and workpiece 15. This, in turn, reduces the electrostatic force of attraction between EDM electrode 18 and workpiece 16. At this point, the restoring forces resulting from the deformation of members 20 pull EDM electrode 18 away from workpiece 15 toward the position of Figure 2A.

[0046] After discharges 32 the electrical potential difference between EDM
electrode 18 and workpiece 15 recovers over time (where the electrical circuit of Figure 1 is used to apply the potential difference, the time taken depends upon factors such as the values of resistor 28, capacitance 29). When the potential difference has built up sufficiently for the electrostatic force between EDM electrode 18 and substrate 16 to start to pull EDM

electrode 18 toward workpiece 15 the cycle repeats. In some embodiments, the time taken for the potential difference to build up is significantly less than the time that it would take for members 20 to pull EDM electrode 18 back to its initial position. In such embodiments, EDM electrode 18 vibrates near the distance above surface 16 of workpiece 15 for which the voltage of power supply 25 can cause discharges 32.

[0047] The rate at which the cycle repeats is dependent on a range of factors including:

= mechanical properties of EDM electrode 18 and the members 20 that couple EDM
electrode 18 to anchors 19;

= the mass of EDM electrode 18;
= the configuration of electrode 18 (e.g. the size, shape of electrode 18 as well as the number, size and arrangement of holes or other apertures in electrode 18);

= electrical properties of fluid 14;

= fluidic properties (e.g. viscosity) of fluid 14;
= the voltage of power supply 25;
= electrical properties of the circuit that establishes electrostatic attraction between EDM electrode 18 and workpiece 15 (such as values of capacitance 29 and resistor 28 when the circuit of Figure 1 is used).

In some embodiments, the frequency of the movements of EDM electrode 18 toward and away from workpiece 16 is at least 100 kHz. In some embodiments the frequency of the movements of EDM electrode 18 toward and away from workpiece 16 is at least 1 MHz.
[0048] After a large number of cycles, a cavity 35 having a shape corresponding to that of EDM electrode 18 is eroded into surface 16 of workpiece 15 as shown in Figure 2D.

EDM electrode 18 and associated structures may be removed from workpiece 15 after cavity 35 has been formed. The process illustrated in figures 2A to 2D can be seen to provide self-regulated generation of discharges that erode the workpiece material.

[0049] There are many possible variations in the construction and operation of apparatus 10. Figures 3A through 3F illustrate one embodiment in which an EDM electrode is fabricated on a workpiece by an etching technique. In some embodiments, the etching technique is a lithographic technique of a type applicable to the manufacture of micro-electro-mechanical systems (MEMS).

[0050] Figure 3A shows a workpiece 15 to which a sacrificial layer 40 has been applied.
Sacrificial layer 40 may, for example, comprise a suitable resist. In this embodiment, sacrificial layer 40 acts as a spacer and defines the initial spacing between an EDM
electrode, as fabricated, and the surface 16 of the workpiece 15 on which the EDM

electrode is fabricated. Sacrificial layer 40 is applied by spin coating in some embodiments. In an example embodiment, sacrificial layer 40 comprises a layer of photoresist about 30 to 40 m in thickness applied by spin-coating and then soft baked.
[0051] In making a prototype embodiment, a stainless-steel wafer workpiece was thoroughly cleaned and degreased with acetone. A layer of hexamethyldisilazane (HMDS) adhesion promoter was spun on the wafer. A thick photoresist (SPR220 available from Rohm and Haas Co.) was then double coated on the workpiece to form a sacrificial layer. The photoresist was soft baked by heating it at 90 C for 5 minutes on a hotplate.

[0052] Figure 3B shows the application of a layer 42 of an adhesive agent. In some embodiments, layer 42 comprises a layer of a resist. In an example embodiment, layer 42 comprises a layer of photoresist about 1 m thick applied by spin coating. In making the prototype embodiment, a 1 m-thick layer of S 1813 photoresist available from Rohm and Haas Co. was spun onto the sacrificial layer 40.

[0053] In Figure 3C, a metal layer 44 is adhered to sacrificial layer 40 by way of adhesive layer 42. Metal layer 44 may, for example, comprise a layer of copper, tungsten, a tungsten alloy (e.g. copper/tungsten), or another material having properties acceptable for use as an EDM electrode. In an example embodiment, metal layer 44 comprises a copper layer having a thickness of about 18 rn that is laminated onto layer 42 and the structure is soft-baked. In making the prototype embodiment an 18 m-thick layer of copper foil was laminated onto the adhesion layer 42 of S1813 resist and the workpiece was soft baked on a hot plate at 90 C for 10 minutes to solidify adhesion layer 42.
After soft baking the copper foil was firmly affixed to the workpiece by way of the sacrificial layer 40.

[0054] In Figure 3D, a patterned layer 45 of resist is applied to metal layer 44. Patterned layer 45 may, for example, comprise:
= a layer of photoresist patterned by exposure through a mask to actinic radiation such as ultraviolet light;
= a layer of resist patterned by way of an electron beam, laser or the like;

= a layer of resist applied in a pattern by ink-jetting, silk screening or the like.
In an example embodiment, patterned layer 45 comprises a layer of photo-resist approximately 5 m thick applied by spin coating, patterned by exposure to ultraviolet light through a mask and then processed to remove either exposed or unexposed areas of the photoresist. The resist of patterned layer 45 may be a positive- or negative-working resist.

[0055] In making the prototype embodiment, a 5- m-thick layer of SPR220 resist was spun onto the copper foil, patterned using a Mylar mask which defined the layout of an array of devices and developed.

[0056] In Figure 3E the portions of metal layer 44 that are not protected by patterned mask 45 are etched away. This may be done, for example, by performing wet etching using a suitable etching agent. The etching of metal layer 44 defines portions of metal layer 44 that will become EDM electrodes 46 supports 48 and members which connect EDM electrode portions 46 to anchor portions 48, all having desired geometries.

[0057] In making the prototype embodiment the developed SPR220 was used as a mask for wet etching of the copper foil in a commercially available ferric chloride solution (CE-100 available from Transene Co., Inc.).

[0058] Figure 3F shows the completed structure after portions of sacrificial layer 40 which underlie EDM electrodes 46 have been removed. Removal of selected regions of sacrificial layer 40 may be achieved by rinsing with a suitable solvent, for example. In making the prototype embodiment, the sacrificial layer underlying EDM
electrodes and resiliently-deformable members previously defined in the copper foil was removed by timed etching of the sacrificial layer in acetone. Holes in the EDM electrodes and resiliently-deformable members helped to promote removal of the sacrificial layer from under these structures. This left the EDM electrodes and resiliently deformable members suspended over the workpiece. The anchors remained attached to the workpiece due to their larger areas.

[0059] The method illustrated in Figures 3A to 3F results in one or more resiliently-mounted EDM electrodes being formed on workpiece 15. The use of a patterning process (such as suitable lithography techniques) to pattern EDM electrodes directly on the workpiece permits placing an array of EDM electrodes in desired locations relative to the workpiece and to one another to the accuracy of which the patterning process is capable.
In some embodiments, dry-film photoresists are used in the patterning of EDM
electrodes and associated structures.

[0060] In some embodiments, electrical conductors for applying electrical potential to a plurality of EDM electrodes are patterned in the same metallic layer from which EDM
electrodes are fabricated.

[0061] EDM electrodes and the resiliently-deformable members that support the EDM
electrodes may have any of a wide range of geometries. Some example geometries are illustrated in Figures 4A to 4D. Figure 4A shows a structure 50 in which an EDM
electrode portion 52 is suspended between anchors 51 by members 53. Members 53 can twist in response to torsional forces and/or bend and stretch to allow EDM
electrode portion 52 to move toward an underlying surface under applied electrostatic forces.

[0062] In the illustrated embodiment, EDM electrode portion 52 comprises a pointed tip 54 that will be closest to an underlying surface if EDM members 53 twist. In the illustrated embodiment, holes 55 are provided in members 53 and EDM electrode portion 52. Holes 55 permit a solvent or other process to remove an underlying sacrificial layer 40 (see Figures 3E and 3F) during fabrication. Anchors 51 are not penetrated by apertures and so sacrificial layer 40 remains in place below anchors 51 bonding anchors 51 to the underlying workpiece.

[0063] Figure 4B shows an alternative structure 60 in which an EDM electrode portion 64 is freely suspended over an underlying substrate by way of sinuous elements extending between anchors 61 and EDM electrode portion 64. EDM electrode portion 64 is penetrated by apertures 65 which are useful in removing a sacrificial layer from underneath EDM electrode portion 64 during fabrication.

[0064] Figure 4C shows an alternative structure 70 which provides a plurality of EDM
electrode portions 72 (four EDM electrode portions are illustrated but more or fewer may be provided) suspended between anchors 71 by resiliently flexible members 73.
In some embodiments, gaps 76 between adjacent EDM electrode portions 72 are small enough that a single cavity is formed by the concerted action of EDM electrode portions 72.
[0065] Figure 4D shows a structure 80 comprising an assemblage of EDM
electrode areas 82 of different shapes and configurations supported above a surface by resiliently-deformable members.

[0066] Figure 4E shows another structure 84 comprising an EDM electrode area coupled to anchors 87 by resiliently flexible members 86.

[0067] It is not mandatory that an EDM electrode be fabricated on the surface of a workpiece. In some embodiments of the invention, EDM electrode structures as described herein are fabricated separately from the workpiece and then applied to the workpiece. A
separately-fabricated EDM electrode assembly may be laminated to a workpiece by way of a suitable adhesive on anchor areas or held to a workpiece surface by pressing the EDM electrode assembly against the surface.

[0068] Figure 5 is a scanning electron microscope image of a prototype EDM
electrode 100 spaced apart from the surface 101 of a stainless steel workpiece 102.
Figure 6 is a scanning electron microscope view of a cavity 103 made in the stainless steel workpiece 102 by EDM using an electrode like that in Figure 5. Holes 104 in the electrode 100 of Figure 5 are large enough that portions of workpiece 102 underlying holes 104 are not eroded by EDM and remain as an array of projections 105. Figure 7 is a micro photograph showing an array of prototype EDM electrodes formed on a substrate.

[0069] In the prototype embodiment:

= the EDM electrode 100 had an area of 1.6 mm x 1.03 mm, = resiliently deformable members 103 had dimensions of 1.4X0.45 mm;
= anchors were 2.5 mm x 2.5 mm;

= EDM electrode 100, resiliently deformable members 103 and the top surfaces of anchors 106 were patterned from a layer of copper having a thickness of 18 m;

= holes 104 were 30 m in diameter;
= power supply voltage was 100V;

= the resistance of resistor 28 (see Fig. 1) was 20 kQ;

= vertical displacement of electrode 100 toward surface 101 was -30 m;
= the depth of cavity 103 after 10 minutes of operation was 20 m;

= capacitance was provided by the parasitic/built-in capacitance between electrode 100 and workpiece 102 and a 100 pF capacitor.

[0070] A pull-in voltage, VPi, for torsional actuation of an EDM electrode having the general configuration of the prototype shown in Figure 5 can be expressed as:

0.83Kd3 1 L3W ( ) VPI
=
where:
d is the original separation between the electrode and the workpiece surface;

E is the permittivity of fluid 14 (typically EDM oil - for kerosene-based EDM
oil, E is about 1.59X 10-" F/m);
L is the length of the electrode (see Figure 8);
W is the width of the electrode (see Figure 8); and, K is the spring constant of torsional members that couple the electrode to anchor points.
For the illustrated embodiment, an estimate of K is given by:

K= Gab3 16 - 3.36b 1- b 4 (2) l 3 a 12a where:
G is the shear modulus of elasticity of the material from which the resiliently deformable (in this case torsional) members are made (for copper, G is approximately 45 GPa );
2a is the width of the resiliently deformable members (see Figure 8);
2b is the thickness of the deformable members; and, 1 is the length of the deformable member (see Figure 8). The prototype structure shown in Figure 5 was designed to be pulled in when the gap, d, is 30 m or smaller by an applied voltage of 120 V.

[0071] Figure 9 shows current as a function of time for a prototype EDM
electrode.
Applying a voltage of 100 V resulted in sequential pulses of micro spark discharge. The peak current, pulse duration, and charging time constant were measured to be approximately 2.5 A, 50 ns, and 1 s, respectively.

[0072] Other factors being equal, the accuracy of EDM processes can be improved by reducing the energy of discharges. Increasing the discharge energy tends to cause rougher machined surfaces. The discharge energy of a single pulse is given by CVZ/2, where C is the total capacitance and V is the applied voltage. One way to reduce the discharge energy for finer machining is to design EDM apparatus to have a low capacitance. This may be done by using a low-value capacitor in RC circuit 24 or not providing a separate capacitor.

[0073] Figure 9A shows the results of measurements of the capacitive coupling between a prototype electrode structure and workpiece. The capacitive coupling (i.e.
parasitic capacitance) is a dynamic parameter because it is affected by the spacing between the EDM electrode and workpiece, which varies during operation. The value of interest is Cbb, the capacitance at the time a discharge is initiated. Cbb can be monitored indirectly by measuring the time constant i=RCbb, in a charging cycle since R is known.
Figure 9A
plots i measured while EDM is on for various prototype devices with different electrode-tether areas as well as Cbb calculated from the results (points to which line 109A

is fitted). The values Cb measured directly by probing the prototype devices with EDM
off is also plotted in Figure 9A for comparison (points to which line 109B is fitted).
Figure 9A shows an approximately linear dependence of both Cb and Cbb on the area of a device.

[0074] As described above, EDM electrodes may be made as planar structures.
Such structures may be made with single-layer fabrication techniques. An EDM
electrode as described herein may have projections or other shaping on its side facing the workpiece.
Such projections or other shaping may be provided by way of a multi-layer fabrication process, for example.

-1g-[0075] Figure 10 illustrates an EDM electrode 110 comprising projecting shapes 111 on its surface 112 facing a workpiece 113. EDM electrode 110 is supported by resiliently deformable members indicated schematically by springs 115 which connect to anchors 116. Operation of EDM electrode 110 as described above erodes cavities 117 corresponding to projecting shapes 111.

[0076] Some embodiments provide one or more mechanisms in addition to electrostatic force to urge EDM electrodes toward a workpiece surface. Such additional mechanisms may be applied to facilitate increasing the depth of cavities formed by EDM
and/or to permitting operation under reduced voltages (thereby reducing discharge energy and the roughness of the machined surfaces).

[0077] Figure 11 shows an example apparatus 120 in which a number of EDM
electrodes 121 are supported over a surface of a workpiece 122. A fluid outlet 123 is positioned over each EDM electrode 121. Fluid (which may be EDM oil) from a fluid supply can exit from a fluid outlet 123 when a corresponding valve 125 is open. Fluid emitted from a fluid outlet 123 impinges against the corresponding EDM electrode 121 and pushes the EDM electrode toward workpiece 122. Valves 125 are controlled by a control system 127. In some embodiments, valves 125 are opened in a manner that is synchronized with the detection of discharges between EDM electrodes 121 and workpiece 122 and/or with a waveform generated by EDM power supply 128 applying electrical power to EDM electrodes 121. In such embodiments fluid outlets 123 may emit pulses of fluid toward EDM electrodes 121. In some embodiments, fluid outlets comprise inkjet-type nozzles.

[0078] In some embodiments, fluid outlets 123 may be operated to direct streams of fluid more-or-less continuously onto EDM electrodes 121 thereby applying continuous forces to EDM electrodes 121. The forces urge EDM electrodes 121 toward workpiece 122 and thereby help electrostatic forces to move EDM electrodes 121 into positions where electrical discharges can occur. In such embodiments, the forces resulting from the action of fluid on EDM electrodes 121 should not be so large as to hold EDM
electrodes 121 in contact with the workpiece.

[0079] Where fluid forces are not synchronized with the motion of any individual EDM
electrode 121 a single fluid outlet 123 may be provided to direct fluid onto a plurality of EDM electrodes 121.

[0080] A range of other mechanisms may be provided to assist in bringing EDM
electrodes toward a surface of a workpiece. These include:

= Pieces of magnetic material may be provided on EDM electrodes 121. A
magnetic field may be applied so that the magnetic material experiences a force directed toward the workpiece. The magnetic field may be provided by a permanent magnet or an electromagnet. In some embodiments, the magnetic field is provided by an electromagnet that is controlled to apply force to the magnetic material in synchronization with the generation of discharges.

= A mechanical member may be applied to move an EDM electrode 121 toward a workpiece. The mechanical member may be actuated in any suitable manner.
[0081] Figures 12 and 13 are flow charts that illustrate methods according to embodiments of the invention. Figure 12 illustrates a method 130 for machining a surface of a workpiece. The method involves applying an array of EDM
electrodes to the workpiece (block 132). The EDM electrodes are attached to the workpiece and are movable toward a surface the workpiece. Block 132 may optionally include forming on the workpiece (but electrically insulated from the workpiece) electrical conductors for electrical current to the EDM electrodes. In block 134 an EDM potential is applied between the electrodes and the workpiece. The EDM potential may be applied by connecting the EDM electrodes to a DC power supply by means of RC circuits or other suitable circuits, for example. Block 34 is continued until cavities are made in the workpiece surface by the EDM electrodes. In block 136 the EDM electrodes are removed from the workpiece.

[0082] Figure 13 illustrates a method 140 for operating an EDM electrode to machine away material from a surface of a workpiece. In block 142 an electrical potential is applied between an EDM electrode and an adjacent workpiece. In block 144 forces applied to the EDM electrode at least in part by the electrical potential are allowed to move the EDM electrode toward the workpiece. Motion of the EDM electrode causes deformation (e.g. bending, stretching, and/or twisting) of resiliently deformable members supporting the EDM electrode. Block 144 continues until an electrical discharge occurs between the EDM electrode and the workpiece in block 146.

[0083] In block 148 the resiliently deformable members urge the electrode away from the workpiece. In block 149 the electrical potential difference between the EDM
electrode and the workpiece is allowed to recover. Blocks 144 to 149 are repeated until the desired machining is completed.

[0084] Embodiments as described herein may offer one or more advantages over other EDM techniques. For example, = In the absence of forces, the EDM electrode may have a relatively large separation from the workpiece surface. This automatically prevents irregular continuous arcing. Discharge automatically reduces the gap voltage and hence reduces the electrostatic forces pulling the electrode toward the workpiece. When resiliently deformable members pull the EDM electrode away from the workpiece any arcing is terminated.

= The wide gap between the EDM electrode and the workpiece promotes easier flushing of byproducts produced during the machining.

= Accurate EDM patterning can be performed over a large area in a relatively short time.
= Precision NC positioning systems are not required for performing the EDM
erosion of the workpiece.
= Methods and apparatus as described herein may be applied to large area micromachining of non-planar samples (using lamination/bonding of electrode arrays).

[0085] Apparatus and methods as described herein have a wide range of applications.
Non-limiting examples of such applications include:
= making molds for patterning small features on the surfaces of plastic or glass;
= patterning surfaces of medical implants or other medical devices;

= forming fluid passages and other components of microfluidic systems;
= forming nozzles for inkjet printing, fuel, or the like;

= marking surfaces with desired shapes, patterns, indicia;

= forming cavities under selected portions of MEMs devices;
= etc.

[0086] Where a component (e.g. a circuit, controller, assembly, device, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a "means") should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention.

[0087] As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example:

= In some applications an EDM electrode could be supported on a cantilever. It is not mandatory that there be multiple resiliently deformable members supporting an EDM electrode.
= In some embodiments, material for an EDM electrode is deposited by a process such as sputtering, vacuum evaporation, plating (e.g. electroplating) , or the like.
= Any suitable processes may be applied to make EDM electrodes and associated structures as described herein.
= In some embodiments, masking a layer to lay out EDM electrodes and/or other parts is performed by processes involving one or more of laser ablation of a resist, exposing a photo resist by direct writing with a laser, electron beam or the like, directly depositing a resist in a desired pattern using suitable printing methods or the like.
= In some embodiments EDM electrodes are provided in a self-adhesive layer.
One or more EDM electrodes supported by resiliently deformable mechanical members are defined in the layer. The self-adhesive layer may be affixed to a clean surface of a workpiece, immersed in a suitable EDM fluid and applied as discussed herein to perform EDM on the workpiece surface. The self-adhesive layer may be removed after the EDM is completed. The self-adhesive layer may have EDM electrodes arranged in a pattern suitable to mark, shape or pattern the workpiece surface in a desired manner. The self-adhesive layer may be provided with protective release sheets on one or both of its faces. The release sheets may be removed prior to or during application of the self-adhesive layer to a workpiece.

[0088] Aspects of the invention include, without limitation:

1. Apparatus comprising any new, useful and inventive feature, combination of features or sub-combination of features as described or depicted herein.

2. Methods comprising any new, useful and inventive step, act, combination of steps and/or acts or sub-combination of steps and/or acts as described herein.

Claims (17)

1. An EDM method comprising: fabricating one or more EDM electrodes on a surface of a workpiece; and applying a potential difference between one or more electrode and the workpiece.

2. A method according to claim 1 wherein fabricating comprises using etching or other lithographic techniques.

3. A method according to claim 1 wherein the electrodes have shapes defined by a mask.

4. An EDM method comprising supporting an EDM electrode over a workpiece surface by a resiliently deformable mechanical member; and causing the EDM
electrode to advance toward the workpiece surface at least in part by applying an electrical potential between the EDM electrode and the workpiece surface;
wherein the advance of the electrode causes deformation of the resiliently deformable mechanical member, the electrical potential is reduced upon the occurrence of electrical discharge between the electrode and the workpiece surface, and the reduction of the electrical potential allows a restoring force exerted by the resiliently deformable mechanical member to draw the electrode away from the workpiece surface toward its original position.

5. Apparatus comprising a workpiece having a surface to be machined and one or more EDM electrodes formed in a layer attached to the surface.

6. Apparatus according to claim 5 wherein the EDM electrodes are supported over the surface by one or more resiliently-deformable mechanical members.

7. Apparatus according to claim 6 wherein a plurality of EDM electrodes are provided on the workpiece surface.

8. Apparatus comprising one or more EDM electrodes formed in a layer that can be pressed against or attached to the surface of a workpiece.

9. Apparatus according to claim 8 comprising pads or other electrical connection points for the application of a potential difference between the EDM
electrodes and the workpiece.

10. Apparatus according to claim 9 wherein the EDM electrodes are supported over the surface by one or more resiliently-deformable mechanical members.

11. Apparatus according to claim 9 wherein a plurality of EDM electrodes are provided on the workpiece surface.

12. Apparatus for performing EDM comprising at least one EDM electrode supported by a resiliently deformable mechanical member and an electrical circuit connected to apply an electrical potential between the EDM electrode and an adjacent workpiece surface.

13. Apparatus according to claim 12 wherein the apparatus comprises a spacer that spaces the EDM electrode a predetermined distance above the workpiece surface when the spacer is against the workpiece surface and no electrical potential difference exists between the EDM electrode and the workpiece surface.

14. Apparatus for performing EDM comprising at least one EDM electrode supported by a resiliently deformable mechanical member and a fluid outlet located to apply a force to the EDM electrode by causing fluid to flow against the EDM
electrode.

15. Apparatus according to claim 14 comprising a controller configured so that fluid flow at the fluid outlet is controlled by a controller which synchronises the fluid flow to an EDM cycle such that a fluid flow pattern is synchronized to the occurrence of EDM discharges.

16. Apparatus according to claim 15 wherein the controller is configured to control both the fluid flow and an electrical circuit connected to apply an electrical potential between the EDM electrode and an adjacent workpiece surface.

17. A self-adhesive layer having defined therein one or more EDM electrodes supported by resiliently deformable mechanical members wherein the self-adhesive layer may be affixed to a surface of a workpiece, immersed in a suitable EDM fluid and applied to perform EDM on the workpiece surface.