Micro-EDM: a flexible and rapid production
technology for 3D freeform microstructures !
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
Figure 10: Bi-axial inclinometer prototype.
Figure 11: Test results.
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
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.