Microchannels Fabricated by Laser: From the Nanosecond to the
Femtosecond Pulse Duration
María Aymerich
1
, Javier R. Vázquez de Aldana
2
, David Canteli
3
, Carlos Molpeceres
3
and M. Teresa Flores-Arias
1a
1
Photonics4Life Research Group, Departamento de Física Aplicada, Facultade de Físicas,
Universidade de Santiago de Compostela, 15782, Santiago de Compostela, Spain
2
Aplicaciones del Láser y de la Fotónica Research Group, Departamento de Fïsica Aplicada, Facultad de Ciencias,
Universidad de Salamanca, 37008, Salamanca, Spain
3
Centro Láser UPM, Universidad Politécnica de Madrid, Crta. de Valencia km 3.7, 28031, Madrid, Spain
maite.flores@usc.es
Keywords: Pulsed Laser, Laser Ablation, Laser Microstructuring, Microchannels, Laser-matter Interaction.
Abstract: Laser technologies are used to fabricate microchannels over glass substrates. A comparison among the results
obtained when these microchannels are fabricated with pulsed lasers in the three more important regime in
terms of pulse duration is made. A roadmap for obtaining a similar device when different pulsed lasers are
used is presented. The control on the surface roughness, a very important parameter to be taken into account
when biological applications are the possible ones, is also analysed.
1 INTRODUCTION
The last decades the fabrication of microchannels has
presented a huge interest. There are several
techniques used to fabricate these channels like
embossing; injection moulding or other
thermoforming techniques (Tsao, 2009); lithography
techniques (Fu, 2006); electron beam lithography
(Mali, 2006); photolithography (Stroock, 2002) and
soft-lithography (Xia, 1998) among others. Each of
these techniques is more suitable and it is chosen
depending on the substrate where the microchannels
want to be fabricated. Regarding the substrates, the
more commonly materials used for channel
fabrication are polymers (Xu, 2000), silicon
(Dwivedi, 2000) and glasses. The last one presents
numerous advantages such us their hardness, it is
chemically robust and has good electrical and thermal
properties. It has a low autofluorescence and is
transparent in the range of optical inspections
(Carlen, 2009).
Regarding the fabrication techniques, the laser
has outstand when using glass substrates, due to the
speediness of the process, the no contact nature,
versatility, precision and so on (Liao, 2012).
Although laser ablation is more common using
a
https://orcid.org/0000-0002-8036-9654
ultraviolet wavelengths when processing transparent
material in the visible range, IR range has also been
used for this aim (Nieto, 2014), (Nieto, 2010)
Physical phenomena involved in the ablation of
dielectric materials are different depending on the
laser pulse duration. (Liu, 1997), (Chichkov, 1996),
(Weck, 2008)
There are a huge number of application fields of
the glass microchannels, being one of them the
microfluidic studies. This field aroused a big interest
due to its numerous and promising biomedical
applications (Whitesides, 2006), (Sackmann, 2014).
Drug delivery (Metz, 2004), lab-on-a-chip analysis
(Stone, 2004), cell trapping or imitation of vessels for
in-vitro bioassays (Aymerich, 2017). The advantages
of using microchannel is, mainly, the small volumes
of liquid that are needed for the different analysis.
In this work, we present a roadmap for fabricating
microchannels on glass substrates by laser ablation
using pulsed laser in three pulse duration regimes
(nanosecond, picosecond and femtosecond). All the
lasers have their fundamental wavelength in the
infrared spectral range. In particular, it will be
analysed the different irradiances and parameters
should be used when microchannels with a fixed ratio
wants to be fabricated. These microchannels will be
characterised and compared.
Section 2 presents materials and methods. Main
results and discussions are presented in Section 3.
Section 4 summarizes the conclusions.
2 MATERIALS AND METHODS
The lasers used in the development of this works are,
a Rofin Nd:YVO
4
laser, a Lumera Super Rapid-HE
(Nd:YVO
4
) and one Spitfire from Spectra Physics.
The Rofin laser operates in Q-switch regime with 20
ns pulse duration and 1064 nm fundamental
wavelength. The laser setup was combined with a
galvanometer system that addressed the output beam
on the target. The lens used for focusing the laser over
the substrate is a flat field lens with a 100mm focal
length, that allows to operate with the same energy in
an area of 80x80 mm
2
. The Lumera Super Rapid-HE
(Nd:YVO
4
) has a fixed pulse duration of 12 ps and a
fundamental wavelength of 1064 nm. The system had
a motorized table and a processing head with a fix
lens providing a beam radius in focus of 16 µm. The
Spitfire of Spectra Physics emits pulses in the
femtosecond regime, with a fixed pulse duration of
100 fs and it works at its central wavelength of 800
nm. In this case, the beam is focused with an
achromatic lens of 200 mm. For fabricating
microchannels with this laser a computer-controlled
3-axes motorized stage is used. Beam radii for this
system was around 11 µm.
The substrate for fabricating the microchannels
are chosen to be a cheap soda-lime glass obtained
from a local supplier. The chemical composition of
both surfaces of the material is shown in table 1.
Table 1: Chemical composition of the used glass in both
sides of the piece.
Element Weight (%)
O 50.25
Si 33.06
Na 9.08
Ca 4.87
Mg 2.19
Al 0.54
Element Weight (%)
O 48.97
Si 32.34
Na 9.14
Ca 4.91
Mg 2.24
Al 0.49
Sn 1.90
Silicon dioxide (SiO
2
) is the main component in
both surfaces of soda-lime glass, followed by sodium
oxide (Na
2
O) and calcium oxide (CaO). However due
to the process used in the fabrication of these glasses,
in one side of the piece appears tin that play a key role
in laser ablation of the material (Nieto, 2014), (Nieto,
2015).
3 RESULTS
As a first step, we procced to determine the
parameters for each laser system that assure the
homogeneous ablation of the soda-lime substrate.
This is a very important step in order to be sure that
the microchannel fabricated will be homogeneous in
its final form.
We found that in the case of working with the
nanosecond laser we need a pulse energy of 700 µJ, a
repetition rate of 10kHz, a scan speed of the
galvanometer system of 50 mm/s and a pulse
overlapping of 73%. In the case of the laser with
pulses in the picosecond range, the pulse energy was
80 µJ pulse energy, the repetition rate 10 kHz, the
scan speed 20 mm/s and the pulse overlapping 96%.
Finally, for the laser with pulses in the femtosecond
regime, the pulse energy needed is 40 µJ, the
repetition rate is 1 kHz, the scan speed of the 3-axes
motorized stage is 0.6 mm/s and the pulse
overlapping 97%. Scanning speed and repetition rate
were determined to assure the overlapping needed
between two consecutive pulses for obtaining a
homogeneous channel.
All these parameters allow us to determine the
threshold mean fluences. As it is well known there is
several ways to define the threshold mean fluences.
Authors determine it experimentally as the minimum
fluence value that generates ablation in the material
with no overlapped pulses. For the nanosecond case,
it was found to be 138 J/cm
2
, 49 J/cm
2
for the
picosecond regime and 5 J/cm
2
for the femtosecond
one. In order to assure the quality and homogeneity
of the final microchannels, we decide to use an energy
per pulse equal to the double of the threshold energy.
In this way we assure the quality of the final device.
It can be observed that the value of the fluence needed
for ablating the material decrease as the pulse
duration decreases, as predicted in the literature. (Liu,
1997), (Chichkov, 1996), (Weck, 2008).
For comparing the channels obtained with the
three different lasers we propose the fabrication of a
channels with and aspect ratio 2:1 diameter:depth.
We carried out a previous study of the number of laser
scans we need to make, in order to achieve channels
with this ratio. As expected, it is different for each
laser. In particular nine scans for the nanosecond
laser, one laser scan for the picosencond laser and one
laser scan for the femtosecond one are used. For these
numbers of laser scans, we obtain a channel of 8.7 µm
depth and 23.9µm diameter fabricated with the
nanosecond laser; a microchannel 8.4 µm depth x
17.8 µm diameter with the picosecond laser and a
channel with 10.1 µm depth and 20.7 µm diameter in
the femtosecond regime.
Figure 1: 3D confocal images of the microchannels
fabricated with: a) nanosecond, b) picosecond and c)
femtosecond pulse durations.
Figure 1 shows confocal images of channels
fabricated with the different pulsed lasers. We can see
that the channels fabricated with the three lasers have
the same aspect ratio. However, the surface roughness
of the channel was not equal at all. That is due to the
difference in the processes involved in the ablation
process when the three different temporal regimes are
considered. For the nanosecond case, the material is
expelled by heat deposition, melted and vaporized. In
the femtosecond pulse regime, the nature of the
process is more explosive, inducing a direct
vaporization of the material. In this case almost nor
thermal effect nor melting mechanics occur, so the
final channel has a high roughness value in contrast
with the smooth surface obtained in the channels
fabricated with the nanosecond pulses. In the
picosecond situation, a combination of both effects
occurs, resulting in a process where part of the
material is directly vaporized but also has a
significant transfer of heat to the lattice and,
therefore, melted material may appear.
Arithmetical mean roughness of the surface (Sa)
was measured according to ISO 25178. Values for the
channels fabricated with the nanosecond pulse
duration were 178.7 nm, 1028.3 nm for the
picosecond and 1016.3 nm for the femtosecond
situation. As it can be observed, the surface roughness
of the picosecond and femtosecond channels is higher
than the ones manufactured with nanosecond pulse
duration (see Figure 2). This is due to the more
explosive nature of ablation with ultra-short pulses,
where material is directly vaporised.
Figure 2: SEM images of the surface of the microchannels
fabricated with a) nanosecond, b) picosecond and c)
femtosecond pulse duration.
a
b
c
4 CONCLUSIONS
In this work, we presented a roadmap for fabricating
microchannels with a same aspect ratio by direct laser
writing techniques. In particular, there were used
pulsed lasers working at the nanosecond, picosecond
and femtosecond regime. The microchannels were
fabricated on a soda-lima glass substrate. They were
characterized in terms of height and width as well as
in terms of their vale of the surface roughness. Laser
direct writing is shown as a fast, accurate, versatile
and non-contact technique for the manufacturing of
microchannels over soda-lime glass, advantageous
material due to its robustness and competitive cost. It
has been shown that channel with the same aspect
ratio can be obtained with lasers working in the three
temporal regimes. However, the roughness obtained
are very different due to the physical mechanism
involved in each ablative process. Depending on the
application the roughness of the wall of the channel
can be more appropriated. In particular a higher
roughness is more suitable for applications in the
biological area since the cell attachment is higher as
the roughness increases.
ACKNOWLEDGEMENTS
This work has been supported under contracts
RTI2018-097063-B-100, AEI/FEDER, UE,
FIS2017-87970-R Ministerio de Economía y
Competitividad, ED431E 2018/08, Xunta de
Galicia/FEDER, SA046U16, Junta de Castilla y León
and FIS2015-71933-REDT Ministerio de
Competitividad, Spain.
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