Micro and Precision Engineering Research Group - Micro electro-discharge machining

The micro-EDM process

Currently the majority of microcomponents are fabricated by micro-electronic production technologies. Although these techniques produce dimensionally accurate structures, they lack the ability to elaborate the 'third dimension' and are mostly bound to silicon as substrate material. The major challenge for the future is the development and production of real threedimensional microstructures. Micro-EDM (Micro Electro Discharge Machining) is a flexible machining technique offering the possibility to produce freeform microstructures in metals and in doped silicon. EDM or spark erosion is an electrothermal machining process which applies the erosive effect of discharges, created between a tool electrode and a workpiece electrode, to remove workpiece material. Both electrodes are emerged in a dielectric fluid which cools the electrodes and removes the debris out of the sparking gap.

Figure 1: AGIE Compact 1.

Figure 2: Rotating chuck and ceramic guide.

The Department of Mechanical Engineering, division PMA, possesses an AGIE Compact 1 die sinking EDM-machine (figure 1). This machine is modified to enhance its capabilities to machine microcomponents . In the micro-EDM case, the tool electrode is a tungsten wire with a diameter of 150 µm clamped in a rotating chuck and guided by a ceramic guide (figure 2). To shape the tool electrode a WEDG (Wire Electro Discharge Grinding) unit is developed, which is sketched in figure 3. This technology allows the machining of small and complex shaped tool electrodes on the machine itself with a high and repeatable accuracy. PMA built up a remarkable experience on the machining of doped silicon. Studies are performed on methods to smooth machined surfaces, on wear compensation of the tool electrode, on tensile strength of machined components and much more.

Figure 3: WEDG.

Machining of threedimensional structures

Micro-EDM has the ability to machine complex micromechanical structures in all kinds of electrically conductive materials. In the PMA laboratory doped silicon and steel are investigated as workpiece materials because of their good mechanical properties. In particular silicon machining is focused on because of its use in the well established photolithographic techniques. In the design and manufacturing of a uni-axial inclinometer (see below) the compatibility of both technologies is described. Hereunder a few photos illustrate machined microstructures; a silicon bevel gear, a silicon S-shaped microbeam, a steel micropropeller with an outer diameter of 1 mm and a silicon finger of a force controllable microgripper.

Figure 4: Si bevel gear.	Figure 5: Si S-shaped beam.	Figure 6: Steel micropropeller.	Figure 7: Microgripper jaw with force sensor.

Design and manufacturing of a bi-axial inclinometer

This work emphasises the threedimensional machining capabilities of micro-EDM. A bi-axial inclinometer is designed, produced and tested. The sensor is based on a symmetrical structure which consists out of a mass suspended on a thin beam (figure 8). When the inclinometer is rotated, the beam will bend under the influence of the weight of the mass. This deflection is measured optically by determining the position of a light (LED) spot on a 2-dimensional PSD surface (figure 9). The benefit of using micro-EDM is that the mechanical structure of the sensor is monolitic; the housing, the microbeam and the mass are made of one piece of steel. This avoids unwanted and unpredictable misalignment errors during assembly of microcomponents. On top of that is the fragile microbeam well protected in the surrounding housing, not only during manipulation but also while using the sensor in severe conditions.

Figure 8: Principle of the bi-axial inclinometer.

Figure 9: Detailed design of the inclinometer.

The outer dimensions of the inclinometer are 9x9x8 mm (figure 10). Figure 11 shows measurement results of one axis of the inclinometer. The sensitivity of this axis is 9.35 mV/°. The fabrication of this symmetrical sensor proves that micro-EDM offers the ability of threedimensional machining of miniature mechanical devices.

Figure 10: Bi-axial inclinometer prototype.

Figure 11: Test results.

Design and fabrication of an uni-axial inclinometer combining photolithographic techniques and micro-EDM

The aim of this research is the integration of micro-EDM and photolithography in one single microsystem. The case study of an inclinometer is used to verify the possibilities of such an integration. The design consists of a seismic mass connected to the frame by a flexible suspension. In this sense, the design is quite similar to other inertial sensors. It should therefore be emphasised that this sensor is only a 'vehicle' to study the above mentioned process integration. The proposed sensor measures 10x10x2 mm3 and can measure inclinations ranging from -60 to 60 degrees relative to the vertical, with a specified bandwidth of 10 Hz. The suspension is hyperstatic and consists of two double-folded leaf springs, as shown on figure 12.

Figure 12: Seismic mass of the uni-axial inclinometer.

Figure 13: Process steps.

The beams have a width of 60 µm, with a smallest section (see detail on figure 12) of 45 µm wide. This means that EDM can offer at least an aspect ratio of 15 for this type of structure. The readout of the inclinometer is based on a capacitive sensing principle. An important issue is the inter process alignment. The lithographic technique aligns successive masks on optical marks on the substrate. These optical marks are patterned onto the wafer. The EDM technique aligns by means of electrical contact between electrode and substrate. The alignment is based on retrieving the position and the orientation of the structured wafer by touching several points of the structure's geometry. For the moment, an alignment accuracy of 5 µm can be achieved. The final process outline is shown in figure 13. As can be seen from this figure, the wafer is first machined by EDM for creation of the alignment holes. Next, photolithographic processing is used for machining the capacitor gap in the silicon and for depositing the metal layer for the dummy capacitor plate on the seismic mass. Subsequently the seismic mass is released by micro-EDM. Finally, (flip-chip) bonding the glass plate, carrying the opposing capacitor plates, completes the sensor. A special lay-out facilitates the bonding of the interconnection wires. This research is in collaboration with the Department of Electrical Engineering (ESAT/MICAS).  Related info at ESAT/MICAS

Machining of micromoulds

Micromoulding is a promising and spreading technology to reproduce a diversity of microcomponents such as turbines, optical benches, fluidic components, capillary analysis systems and so on. Presently, these micromoulds are machined by processes such as LIGA, bulk etching and e-beam lithography all offering poor threedimensional products. In contrast, micro-EDM is a flexible machining technique offering the possibility to produce freeform microstructures in metals and doped silicon. Consequently, micro-EDM is well suited for prototyping as it is much cheaper for the production of single freeform micromoulds. With the aim of examining the mouldability and demouldability of micro-EDMed structures, basic shapes are machined. These basic shapes consists out of spherical, cylindrical and rectangular structures machined in die steel (ORVAR). Figure 14 shows a spherical mould with a diameter of 1 mm. Figure 15 shows a PMMA replication of this mould. To combine the examined basic structures, a temple shaped mould is machined (figure 16). The structure consists out of a rectangular base and a sphere on top of it. At the crest of the temple there is a cylindrical feature. In front of the temple the abbreviation of our division 'PMA' is written with a tool electrode of 30 µm diameter. The temple is surrounded by 4 pillars with a diameter of 40 µm and a height of 400 µm. The machining took place in a single set-up and without operator interventions.

Figure 14: Spherical mould.	Figure 15: PMMA replication.
Figure 16: Temple shaped mould.

It can be concluded that micro-EDM is a proper and flexible technology to machine freeform threedimensional micromoulds. The usage of the WEDG unit makes it viable to flexibly produce shaped electrodes for the machining of small and accurate features. Basic structures such as spheres, cylinders and pockets were machined in die steel. The microstructures were moulded using a hot-embossing and injection moulding set-up. Although there were no tapered walls, the PMMA structures could be demoulded. By and large micro-EDM is a well suited and economical production technique to machine complex threedimensional micromoulds.

A design environment for mechanical microstructures based on the micro-EDM technology

The last decennia the micro-electronic industry has enormously been expanding due to the sophisticated and user-friendly CAE-tools for the design of integrated circuits. Currently, new design environments are developed for the increasing demand for micro-scaled electromechanical systems for example micro-inertial sensors, micropumps, micro-electromechanical switches and so on. The aim of this work is the development of a design environment based on micro-EDM as main process technology. The microparts can be parametrically designed in a user friendly environment using predefined features (figure17). Besides these predefined features the designer can create his own user defined features which are automatically checked on their producability (figure18). The final design is entirely checked on design flaws before the CAE-system automatically generates the machining code for AGIE Compact 1 machine. To check the mechanical characteristics of the microsystem FEM-based simulation can be performed. Using this environment the design of micromechanical structures will be fast and straightforward. On top of that, there is no need for specialised knowledge to design complex threedimensional structures.

Figure 17: Parametric design using predefined features.

Figure 18: User-defined feature.