RIKEN Review No.43(J anuary, 2002): Focused on 2nd International Symposium on LaserPrecision Microfabrication (LPM 2001)
Laser-assisted chemical micromachining of
metals and alloys
Andreas Stephen,∗Thorsten Lilienkamp, Simeon Metev, and Gerd Sepold
BIAS-Bremen Institute of Applied Beam Technology, Germany
Three different laser-assisted processes are described and herewith produced components in the fields of
micromechanics are presented. First, excimer laser projection technology is modified in order to produce
large-area microstructures. It enables the transfer of structures having dimensions much larger than the
laser beam section by synchronized scanning of the mask and the substrate. This technology is used to
manufacture master structures in polymers for injection molding inserts. The insert tool itself is produced
by electroforming of the master leading to a metallic copy. In this way an insert tool is performed for the
production of microfluidic components. Its structured area is 2cm2at a resolution better than 3
laser-assisted wet chemical etching using a cw-Nd:YAG laser is described. The principle of this micromachining
method is based on a local thermal activation of chemical etching reactions on the surface of the material.
The direct processing of the workpiece resulted in high accuracy microstructuring with smooth surfaces and
without any debris or thermal influence on the material properties. Among others one example in the field
of applications in micromechanics is the fabrication of superelastic micro-grippers prepared by cutting of
temperature sensitive shape memory alloys. The achieved sidewall angle is about 3 degrees and the surface
roughness less than 0.4
processes leads to complex shaped microstructures in metallic parts. Thereby, additional microstructures of
specific shape, e.g. v-shaped grooves, are machined by laser-assisted wet chemical etching into metallic inserts
produced by electroforming of excimer laser machined masters. They are used for hot embossing tools enabling
the production of special housings which can be hermetically sealed by ultrasonic welding.
?m for machined 200
?m thick foils. Third, a combination of the two afore mentioned
The market for precision metallic microparts is continuously
growing and penetrating into new areas of application e.g.
molding inserts used for hot embossing of low-cost microflu-
idic devices in the field of biomedical analysis methods or
microtools made of superelastic alloys for medical applica-
The differences in size and shape of microstructures contained
in a single mold is remarkable for many novel applications.
Often, positive as well as negative structures are required.
Negative ones can be machined by processes leading to ma-
terial removal. However, when machining positive structures
by such processes the volume of the material to be removed is
much bigger than the volume of the structures itself and thus
not efficient with respect to processing time. Those metallic
structures can be efficiently fabricated by micromachining of
polymer substrates leading to a negative master which can
be afterwards converted into a positive metallic form by elec-
Furthermore, many machining processes negatively influence
specific material properties.
alloys, e.g. by electrical discharge machining (EDM), often
leads to a reduction or totally loss of the superelastic prop-
erties caused by high temperatures of the treatment and/or
to insufficient surface qualities for many applications.
Machining of shape memory
Lasers can be efficiently applied for micromachining of metals
as well as polymers with high resolution and quality.1)Most
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laser-induced processes for direct micromachining of metals
without using masking techniques are based on material re-
moval via evaporation by high intensity pulsed laser irradi-
ation. Consequently, redeposition of evaporated or resolid-
ificated material as well as mechanical and thermal loading
of the structured part can negatively influence the functional
properties of laser-produced microparts.
Such problems can be efficiently avoided by using for material
removal the process of laser activation of chemical etching
reactions at the interface between the solid and a reactive
fluid.2)Laser-assisted wet chemical etching of metals can be
achieved in different solutions of acids or bases using cw-lasers
in the visible or near infrared region as e.g. the Nd:YAG
laser operating at 1064nm.3)The interaction of a focussed
laser beam with the material surface being covered with a
liquid etchant leads to an enhanced thermochemical etching
reaction and thus to a material removal from the irradiated
In contrast to metals, precise and clean machining of poly-
mers is possible by laser photoablation (photochemical dry
etching) due to the specific material properties as low ther-
mal conductivity and high photochemical sensitivity. For an
effective micromachining a high absorption of the material
as well as short pulses and high repetition rates are neces-
sary. Therefore, excimer lasers are most suitable for machin-
ing polymers, e.g. a KrF-excimer laser operating at 248nm.
The aim of this paper is to present some applications and
machining results for laser-assisted photo- and thermochem-
ical etching. Furthermore, a combination of both processes
linked by an intermediate processing step of electroplating to
Fig. 1. Setup for laser-assisted photochemical etching.
machine complex shaped microstructures is demonstrated.
Laser-assisted photochemical etching of polymers
For large-area microstructuring of polymer substrates, the
laser beam of a KrF-excimer laser operating at a wavelength
of 248nm was homogenized by an array of microlenses in-
plane with a mask, which was imaged in a projection scale
of 5:1 onto the substrate. The mask and the substrate were
synchronously translated perpendicular to the laser beam us-
ing two computer-controlled xy-translation stages, as shown
schematically in Fig. 1.
Laser-assisted thermochemical etching of metals
A detailed description of the experimental set-up used for
laser-assisted thermochemical etching of metals can be found
elsewhere.4)In brief, the beam of a cw-Nd:YAG laser operat-
ing at a wavelength of 1064nm in its fundamental Gaussian
mode at a maximum power of 16W or alternatively a sec-
ond one operating at multi-mode conditions with a maxi-
mum power of 600W was focussed to an estimated focal
spot diameter of about 20µm respectively 200µm on the
metal surface, for instance electroplated nickel or superelas-
tic nickel-titanium, immersed in a liquid etchant consisting of
5.6M H3PO4and 1.5M H2SO4. Using a computer-controlled
xy-translation stage samples were moved under the focussed
beam at speeds ranging from 10µm/s to 100µm/s.
To perform laser-assisted jet-chemical etching a special liq-
uid phase etching cell is integrated. The cell consists of two
parts, a co-axial nozzle assembly and a basin, which are con-
nected to each other by elastomer bellows. The nozzle can
be adjusted laterally and in height with respect to the laser
beam focus. The etch liquid enters the nozzle tip in such a
way that a swirl is given the liquid flow and is injected co-
axial to the laser beam directly into the irradiated area. The
basin holds the workpiece which is submerged into the etch-
ing liquid. The basin is mounted onto computer-controlled
xyz-stages allowing a relative movement of the nozzle over a
100 × 100mm2area at a resolution of 0.1µm to position the
workpiece with respect to the laser beam.
The machined microstructures were investigated by scanning
electron microscope tests (SEM, Zeiss DSM 920). Addition-
ally, the roughness of the treated surface was determined by
a stylus profilometer (Dektak II ST).
Machining results and applications
Laser-assisted photochemical etching of polymers
The typical wavelengths of excimer lasers provide photon en-
ergies up to 6.4eV which is sufficient for direct non-thermal
cracking of polymer bonds leading to ablative photochemi-
cal decomposition (cold ablation). However, this process is
characterized by a combination of photochemical and pho-
tothermal interactions. Due to the typical pulse duration
of several ns which suppresses thermal conduction and the
low penetration depth of less than 1µm the pulse energy is
absorbed by a very small material volume. The laser in com-
bination with a demagnifying mask projection system allows
the generation of complex miniturised structures.
The size of the micromachined area by excimer laser pho-
toablation in a conventional static projection scheme is lim-
ited due to the maximum available laser power as well as
the maximum size which can be imaged by the objective lens
with high resolution.With the developed dynamical pro-
jection scheme a sequently machining of small parts of the
structure leads to a large-area microstructure of several cm2
only limited by the travel range of the used stages. The syn-
chronous scanning of the mask and the substrate in opposite
direction and perpendicular to the laser beam results in a
lateral resolution < 3µm.
Microstructuring polymers was performed at repetition rates
of 5Hz and energy densities of approximately 1J/cm2at the
surface of the sample. To avoid redeposition of ablated mate-
rial the sample was flushed with compressed air while process-
ing. Figure 2 shows a part of a 10×20mm2sized microstruc-
ture in polycarbonate achieved by using the technology of
synchronous scanning. The depth of the structure is 60µm
and the angle of the walls approximately 15 degrees. Due to
the scanning process a residual waviness of less than 0.5µm
An application for the manufactured master structure in
polymer is to fabricate inserts for hot embossing or injec-
tion molding of low-cost microfluidic components in poly-
mers. The insert tool itself can be produced by electroforming
or metal injection molding (MIM) of the master leading to
a metallic copy. Figure 3 shows an insert tool made of steel
produced by MIM of the master shown in Fig. 2.
Fig. 2. Part of a large-area master in polycarbonate.
Fig. 3.Sintered part in steel made by MIM of the master (MIM by
Laser-assisted thermochemical etching of metals
At room temperature many metals are protected against cor-
rosion by a thin native oxide layer on the surface and behave
in many aggressive media like a noble metal. Especially in
phosphoric acid negligible corrosion rates < 10−8µm/s at
room temperature for e.g. titanium can be observed.
increase of the temperature results in a shift of the chemi-
cal equilibrium towards the formation of soluble metal ions
and hydrogen. Time resolved measurements of the electrical
potential difference against an electrochemical reference elec-
trode identify two main stages of the etching process: first
dissolution of the passivation layer occurs, followed by disso-
lution of the metal.
Localized heating of the passivated metal by focussed laser
radiation results in analogy to thermal corrosion to a local-
ized dissolution of the passivation layer followed by chemical
etching of the metal. The temporal evolution of the elec-
trical potential under focussed laser irradiation shows strong
similarities to the thermal process. After the end of laser
Fig. 4. Dependence of etch rate on flow rate and laser power.
irradiation a sudden increase of the electrical potential re-
veals an immediate interruption of the etching reaction due
to repassivation of the metal surface. In particular, upon
laser irradiation a temperature much higher than the boiling
point of the liquid can be reached on the metal surface. At
such high temperatures etch rates several orders of magni-
tude higher than at the boiling temperature of the etchant
were measured. The measured exponential dependence of
the laser-induced etch rate on laser power also supports the
thermal nature of the process.
The process of laser-assisted wet chemical etching of metals
differs substantially from processing in gaseous media due to
the ionic nature of the reactants and the electrical conduc-
tivity of the substrate. Laser-assisted wet chemical etching
benefits from the wide range of available chemical reactions
and the high density of reactants in the liquid. Since the
density resp. concentration of reactants is influenced by the
reaction itself and boiling of the etchant at high temperatures
(well below the melting point of the metal) leads to formation
of bubbles a fast exchange of the arising reaction products is
very essential to avoid saturation effects of the etch rate. In
addition, the ionic nature of reactants offers the possibility
for an electrochemical enhancement of the reaction.
Laser-assisted jet-chemical etching
A fast exchange of reactans can be achieved by laser-assisted
jet-chemical etching. It leads to an improvement of process-
ing speed as well as quality due to an efficient mass trans-
port and cooling of the workpiece by a direct injection of
the etchant into the laser-irradiated area. Additionally, ho-
mogeneous flow rates all over the workpiece can be achieved
enabling large-area processing. Figure 4 summarizes the de-
pendence of etch rate on flow rate and laser power for nickel-
titanium. It shows that etch rates up to 80000µm3/s can
be achieved for laser powers up to 7W and a flow rate of
2m/s. Compared with this, the etch rate at a flow rate of
20m/s is only half as much. This is due to a higher cooling
effect by the liquid jet-stream thus leading to lower thermal
activation of the metal resulting in weaker etching reactions.
The corresponding shape fidelities are up to 12µm for a flow
rate of 2m/s and 3µm for 20m/s. To achieve such a low
shape fidelity for a flow rate of 2m/s the laser power is re-
duced to 3W leading to an etch rate of only 10000µm3/s.
Thus, an increase of the processing speed at equal qualities
can be achieved by simultaneously increasing the laser power
Fig. 5.Dependence of etch rate on applied voltage.
and the flow rate.
Laser-assisted jet-electrochemical etching
Electrochemical enhancement can be performed by applying
an electrical field. The range of voltages leading to laser-
induced electrical currents is approximately limited between
the cathodic hydrogen formation and the flade potential.
Laser-induced currents in this region which are due to etching
reactions are caused by thermal activation of the anodic dis-
solution and/or laser-induced breakthrough of the passivation
layer. The corresponding etch rates are shown in Fig. 5 for
nickel-titanium using a laser power of 5W and a flow rate of
10m/s. The measured voltage without applying an electrical
field is −0.15V and leads to an etch rate of 25000µm3/s and a
shape fidelity of 10µm. Compared to this, voltages of −0.3V
or 0V lead to etch rates of 35000µm3/s resp. 15000µm3/s
and shape fidelities of 10µm resp. 2µm. Thus, the process-
ing speed or quality can be increased by electrochemical en-
Cutting of foils
An application for laser-assisted thermochemical etching is
cutting of metal foils, e.g. to fabricate microtools made of
superelastic alloys for medical applications. In this case it
is very essential to achieve smooth surfaces inside the cut,
nearly perpendicular side walls and to keep the treatment
temperature below the transition temperature of the alloy.
Otherwise, the specific properties of the superelastic mate-
rial will be reduced and rather fragile geometries can not be
In Fig. 6 the dependence of the depth of etched grooves in a
200µm thick foil on the laser power is represented at different
scanning speeds. In the parameter range investigated leading
to high accuracy of the treatment an approximately linear de-
pendence of etched depth on laser power is observed. Because
of the lateral heat diffusion from the zone of laser action, an
aspect ratio of approximately 1 was achieved.
One medical application is a micro-gripper made of supere-
lastic nickel-titanium alloy which was fabricated by laser-
assisted jet-chemical etching. The process was specifically
optimised to form the micro-gripper structure from 200µm
thick foils without thermal influence on the materials prop-
erties and low surface roughness.
part, Fig. 7 shows the tip of the micro-gripper. The treat-
As the most essential
Fig. 6.Dependence of etch depth on laser power and speed.
Fig. 7.Tip of a micro-gripper made of superelastic NiTi alloy (De-
sign by Bartels Mikrotechnik GmbH).
ment resulted in high machining qualities with surface rough-
nesses Ra of approximately 0.3µm and cutting angles of 3
degrees. The competing technologies like laser cutting using
a Ti:Sapphire femtosecond laser or electrical discharge ma-
chining (EDM) show deficits referring to this. Furthermore,
these results can be applied for other new products or the
enhancement of the quality of existing products.
Structuring with defined shape
The realizable structures are not limited in size because laser-
assisted thermochemical etching is a direct machining process
without masking techniques. Thus, the microstructure is gen-
erated successively by scanning the laser beam which however
results in high processing times. An advantage is the possibil-
ity to generate structures with defined shape as for instance
Grooves in metals can be generated by moving the workpiece
perpendicular to the laser beam. Due to the thermal acti-
vation of chemical etching reactions the width and depth of
the grooves are determined by the temperature distribution
on the surface and the duration of the temperature rise.5)
Therefore, the shape of the groove reflects the intensity dis-
Fig. 8. Predetermined overlaps for a v-shaped groove.
tribution of the incident laser beam. Because of the lateral
heat diffusion from the zone of laser action high aspect ratios
can not be realised by single scanning of the groove. The
width as well as the depth simultaneously increase with in-
creasing laser power. Higher aspect ratios were realized by
multiple scanning of the laser beam along the same groove.
The groove width is almost independent on the number of
scans as the temperature increase is confined to the bottom
of the groove. This leads to a continuously increasing depth
and, in consequence, to higher aspect ratios. By this method
aspect ratios higher than 10 corresponding to side wall angles
less than 5◦were obtained.5)
An expansion of this method to achieve defined shapes is the
multiple scanning along the groove axis with simultaneous
lateral shift of every scan. By controlling the overlaps be-
tween the laser irradiated areas the etched depth and hence
the structure’s shape can be determined.6)Figure 8 shows
e.g. the calculated overlaps for a v-shaped groove with defined
side angle in dependence on the distance from its center.
Using these parameters a 150µm deep and 200µm broad v-
shaped groove with a radius of the tip of about 5µm was
fabricated at a scanning speed of 10µm/s and a laser power
of 8W. Figure 9 shows a SEM micrograph of this groove.
The measured surface roughness Rainside the laser irradiated
area is less than 1µm. The overall production time is half an
hour for 2mm length.
Using this processing strategy a wide range of precise struc-
tures with different shapes can be directly micromachined
with a processing speed of approximately 104µm3/s using a
laser system with high resolution. Much higher processing
speeds in the range of 106µm3/s however at lower resolution
can be achieved using high power lasers. Since the etchant
in this case is strongly heated and formation of bubbles oc-
cur due to the high incident laser power an efficient injection
and cooling of the etchant is necessary to machine with high
accuracy and reproducibility as well as to avoid chemical re-
actions outside the laser irradiated area. Figure 10 shows a
cross section of a 250µm wide and 200µm deep groove ma-
chined with a laser power of 140W using the technology of
laser-assisted jet-chemical etching at a flow rate of 10m/s and
additional cooling of the etchant down to 0◦C before it enters
the nozzle of the jet.
Fig. 9.V-shaped groove in nickel machined by multiple scanning.
Fig. 10. Cross section of a groove in nickel machined with a laser
power of 140W.
Furthermore, a defined shape of the structures can be
achieved by forming the intensity distribution of the laser
beam on the sample’s surface by a projection scheme using a
mask of special shape. The main advantage of this method is
that the microstructure can be machined with a single scan
resulting in a high processing speed. For example, Fig. 11
shows the cross section of a groove achieved by using a right-
angled triangle with a length of the hypotenuse of 20mm
projected with a ratio of 100:1 onto the sample.
Combination of the laser-assisted processes
The principle of the processing route used for the combined
method is schematically shown in Fig. 12. It consists of two
laser-assisted processing steps in combination with an inter-
mediate step of electroplating: First, a polymer substrate was
microstructured by excimer laser photoablation in a dynam-
ical projection scheme. This step offers the possibility of a
fast and low-cost fabrication of large-area master structures.
Second, by electroforming of nickel the polymeric master is
converted into a metallic form. Third, additional structures
were micromachined by laser-assisted thermochemical etch-