COMMERCIAL MICRO MANUFACTURING
in this issue
electro-chemical machining
focus on Switzerland
micro cutting
mems packaging
THE MAGAZINE FOR MICRO, HIGH-PRECISION
AND MEMS MANUFACTURERS
12
12
ECM | ARTICLE
An Art of Fine Surface Tuning:
ELECTRO-CHEMICAL MICRO MACHINING
WORDS | M. DISERENS AND
P.-F. CHAUVY, MICROPAT SA
Cleanroom UV lithography
issued from the semiconductor wafer based industry has
made possible the realisation of extremely precise micro
structures. However, we live in a nonflat world and it is
clearly not mostly built in silicon or glass. This provocative
introduction summarises micropat’s credo that there is still
a bottleneck in the industrial integration of micro with
macro technologies.
This article presents recent developments in electrochemical micro-machining (ECMM), which can offer a direct
combination of extreme precision micro patterning with
standard mechanical machining, in order to manufacture
micro structured bulk metallic workpieces.
Electro-chemical Micro Machining
Electro-chemical processes for shaping and surface structuring of
metals bring together a variety of techniques such as
electropolishing (for deburring and surface finishing), electrochemical machining where the shape of a cathode is reproduced
on the anode by anodic dissolution and electro-chemical micro
machining which is an evolution of chemical milling [1]. In the next
paragraphs we will deal with a particular development of this
later technique, namely “through-mask electro-chemical micromachining using laser lithography”
Process Description
The first step of the process is the coating of the samples with a
thin (10 μm) polymeric coating. A uniform thickness deposition
can be obtained on complex shape objects by electrophoresis.
This protective layer is then locally exposed to a focused laser
beam. A machine vision imaging setup and motorised stages
allow for the precise positioning of the laser irradiation sites.
Clean and very reproducible ablation of the film is obtained
using a short pulse UV laser. The result of this lithographic
process is a precisely micro structured protective coating, with
well-defined openings to the underlying metal surface.
The samples are then immersed into an electrolyte. A power
supply is connected to provide an electric tension between the
processed piece and an inert counter electrode. When optimised
voltage profiles and hydrodynamic conditions are applied, the
unprotected areas of the workpiece are neatly attacked and a
smooth and shiny engraving is achieved. Precise dimensioning of
the engraved patterns is obtained by monitoring the electric
charge flown through the system. This control is very precise and,
unlike for chemical etching, is independent of the aging and
temperature of the bath.
<< Figure 1: Micro fluidic circuit on stainless steel
with 60 μm wide channels
a) General view showing micro-macro assembly
b) 60 μm wide 25 μm deep curved channel
c) High density 60 μm wide channel array
d) Smooth transitions between different sections. >>
After accomplishment of the electrochemical dissolution, the
polymeric coating is removed with a stripper.
Technical Achievements
So far convincing results have been obtained on most stainless
steels (austenitics), titanium and its alloys (e.g. Ti6AlV4), shape
memory Nitinol, CoCr. Recently we have been able to micro
pattern some tool steels such as DIN 1.2343, which is a widely
used material in the polymer injection moulding industry.
Round cavities or cylindrical channels can be precisely etched
with diameters in the range of 30 to 300 μm. Positioning and
dimensional accuracy can be as low as 1 μm and this on areas as
large as 150x150 mm2.
The technology is well adapted for processing both flat
substrates and more complex 3D-shape objects.
Technology Advantages and Application Fields
An advantage of ECMM in comparison to laser machining,
chemical etching, electric discharge machining or micro milling is
the incomparable smoothness of the obtained features. The
interior of the engraving appears shiny to the eyes, residual
Continued on page 12
10 | commercial micro manufacturing international Vol 5 No.6
ECM | ARTICLE
BELOW: << Figure 3:
Simulated dissolution
profiles. >>
LEFT: << Figure 4:
Titanium dental implant
micro structured with 27,000
hemispherical cavities
(50 μm in diameter). >>
<< Figure 2:
a) Decorative engravings on a bulk steel block (hot
working steel DIN 1.2343, 15x20x30 mm), maximum
depth of micro pattern is 25 μm
b) Close up on a decorative micro pattern
c) Schematic of the corresponding depth profile >>
Continued from page 10
roughness Ra have been measured below 25 nm (observe the
mirror-like patch on the left side of figure 1a). The
electropolished surfaces exhibit high reflectivity. It permits to
create reflective optically variable images, for security marking, as
well as decorative elements including optical illusions (observe
the suggested depth perception of figure 2a). Manufacturing of
optical elements is also an interesting possibility.
a)
Precision cold forming is not necessarily the perfect solution in all
cases, but as an engineering process it deserves greater
consideration as part of manufacturers’ portfolios of production
options. More importantly, at a time of rising raw material costs
precision cold forming can provide a valuable method of
protecting profit margins.
From a metallurgical point of view, the technique leaves the
material properties unaltered: no oxidation, no darkening, neither
loss of corrosion resistance is induced. The metallic surface is left
in a passive state without residual stresses.
ECMM processing reach semiconductor manufacturing quality
standards in terms of precision and reproducibility, while
overcoming limitations of the latter on a few points: no need of
cleanroom, great versatility in design (no mask fabrication, the
laser path is read from a CAD file) and compatibility with complex
macro geometries.
This last point is of primary importance as it allows the micro
patterning of solid 3D object such as biomedical implants (see
figure 4). Direct micro structuring of prosthesis may be
challenging in terms of production speed, but in some niche
domains, such as cardiac stents, the technique has been retained
for the creation of micro reservoirs for drug eluting devices.
A limitation of the technique is the quasi-isotropic shape of the
attack. No high aspect ratio features can be obtained and an
important underetch occurs. ECMM is therefore intrinsically not
well adapted for the micro drilling of through-holes. In
electropolishing conditions, the removal of the metallic ions from
the surface is mass transport limited. This purely diffusive process
follows the Laplace equation and experimental etch profiles
closely match numerical simulations (see figure 3).[2]
In terms of productivity ECMM can be industrially competitive.
The sequential laser lithography step duration depends on the
complexity of the pattern design, but it is rather fast since only a
very thin polymer layer has to be ablated. The electrodissolution
can also be time consuming, but many structures can be
processed in parallel. Dissolution of 20 biomedical implants each
bearing thousands of 60 μm diameter cavities can be achieved in
20 min.
For the aforementioned reasons, ECMM is of particular interest
for the fabrication of micro fluidic circuits (see figure 1). This
direct machining methodology is well suited for titanium high
pressure micro fluidics (closed channels are obtained by diffusion
bonding of a cover plate). Alternatively ECMM can be
implemented for the realisation of circuits on steel master plates,
which after optional additional mechanical machining can be
mirrored by nickel electroforming to obtain submasters, which
are then used for large-scale production of polymeric replicates.
Conclusions
The use of modern electrochemical methods allows very precise
micro patterning of metallic surfaces. The original ECMM
technique developed by micropat SA combines the ease of use
of laser micro machining with the surface finish of an
electropolishing process. It can be used in a large variety of
applications such as: micro fluidics, high quality aesthetic
engraving (watch industry, security marking), biomedical
applications (favourable topography for bone remodelling or
drug eluting reservoirs) and more generally all kind of replication
tools.
References:
[1] D. Landolt et al., Electrochim. Acta, 48, 3185 (2003).
[2] C. Madore et al., J. Electrochem. Soc., 146, 2526 (1999).
12 | commercial micro manufacturing international Vol 5 No.6