Fabrication of microchannels on soda-lime glass substrates
with a Nd:YVO
, Tamara Delgado, María Teresa Flores-Arias
Microoptics and GRIN Optics Group, Applied Physics Department, Faculty of Physics, University of Santiago de Compostela, Santiago de Compostela,
Received 29 November 2013
Received in revised form
28 May 2014
Accepted 4 June 2014
A new method for fabricating microchannels for microﬂuidic applications on soda-lime glass has been
developed. It consists of a combination of a laser direct write technique for fabricating the microchannels
and a thermal treatment for reshaping and/or improving the morphological qualities of the generated
microchannels. The proposed technique allows us to obtain microchannels with a minimum diameter of
8mm and 1.5 mm of depth. A decrease of two orders of magnitude of the average roughness generated
after the laser ablation, reaching values of the orderof the unprocessed glass, has been obtained thanks to
the thermal treatment. The use of pulsed nanosecond lasers for the laser direct write presents the
beneﬁts of using lasers commonly implemented in the industry for laser processing of materials. This fact
makes the technique presented highly competitive compared with others used for glass microstructuring.
&2014 Elsevier Ltd. All rights reserved.
Microﬂuidics is a quickly developing engineering science for
targeting transportation and handling a small volume of liquids in
an increasing number of applications such as biomedical diagnos-
tic, micro fuel cells and microelectronics cooling [1–4]. These
applications demand transparent materials that allow high resolu-
tion imaging, ﬂuorescence microscopy, and also to analyze para-
meters such as laminar ﬂow, mass transport driven by diffusion
rather than turbulence and constant removal of waste products
[5–7]. Glass materials are commonly used due to the beneﬁcial
optical properties, their surface stability and solvent compatibility,
as well as due to their straightforward and well-known fabrication
techniques [8,9]. Glass also overcomes many limitations of poly-
mers because of their mechanical durability, reusability, low auto-
ﬂuorescence and smooth surface. The high cost involved in glass
processing and the material itself limit their usage as disposable
devices, so high quality devices made of glass and disposable low-
cost devices made of plastic are the two routes of challenging
demands to be solved by microﬂuidics.
A huge variety of methods exist for the fabrication of micro-
ﬂuidic devices, the choice among them depends on the size and
shape of the required features as well as on the materials to be
treated. Embossing, injection molding, and similar thermoforming
techniques, while providing excellent throughput and cost, are
ineffective for glass . Lithography techniques require advanced
facilities and numerous processing steps. A big number of
researchers have demonstrated the fabrication of channels in glass
using electron beam lithography, photolithography and, wet and
dry etching [11–14]. These techniques provide high-quality micro-
ﬂuidic systems, with the inconvenience of the required sophisti-
cated equipment located in clean rooms and the production of
toxic waste, so signiﬁcant challenges still remain for fabricating
low-cost and reliable microchannels in glass. In general, in a
microﬂuidic channel the surface roughness is mostly caused by
an inaccurate mask and its imprecise alignment in the fabrication
process. These are common problems that are difﬁcult and
expensive to avoid. In this context, laser micromachining, because
of its non-contact nature, offers several advantages for fabricating
microchannels, including the capability to form complex shapes
with minimal mechanical and thermal deformation. Fabrication of
microchannels on glass by laser ablation techniques has been
investigated and reported using CO
, UV and ultra short pulse
lasers [15–17]. Literature shows that the fabrication of microchan-
nels is more efﬁcient with pulsed laser instead of CW laser [2,18].
Regarding the pulsed laser, although femtoseconds lasers give
better results than nanoseconds lasers, in terms of surface quality
and accuracy of the elements fabricated on glasses, debris deposi-
tion and so on , the advantages of using nanosecond IR lasers
comes from the difference in cost between femto and nanosecond
lasers, and therefore, the higher possibilities to industrially imple-
ment this technique.
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/optlaseng
Optics and Lasers in Engineering
0143-8166/&2014 Elsevier Ltd. All rights reserved.
E-mail address: email@example.com (D. Nieto).
Optics and Lasers in Engineering 63 (2014) 11–18
One of the main aims of this paper is to overcome the still
signiﬁcant challenges for fabricating low-cost and reliable micro-
ﬂuidic modules in transparent media using nanosecond lasers
with comparable quality of those obtained with femtosecond
lasers. In the present work, a fast, simple but reliable process is
reported for the fabrication of the microchannels using a Nd:YVO
nanosecond IR laser. The fabrication method combines a laser
direct-write technique with a thermal treatment that reduces the
damage created on the glass during the laser ablation and change
the roughness (Ra) improving the morphological quality of the
microchannels. These two steps make possible to fabricate micro-
channels with comparable characteristics and quality as those
obtained by other methods , preserving the advantages of the
laser direct-write technique (ﬂexibility in terms of design, fast
prototyping, low-cost, non-contaminantion of the sample, etc).
In Section 2, the experimental procedure for fabricating micro-
channels is presented. Section 3 analyzes the thermal treatment
and morphological characterization of microchannels as function
of temperature. Section 4 contains some conclusions and remarks.
2. Microchannels fabrication
We present a direct-write technique for fabricating microﬂui-
dics microchannels based on the ablation of a soda-lime glass
substrate with a laser of circular Gaussian beam proﬁle followed
by a thermal treatment. The thermal treatment reduces the
damage created during laser ablation, which lead to an improve-
ment of morphological quality of microchannels generated by
laser. The laser setup consists of a Q-Switch Nd:YVO
operating at 1064 nm combined with a galvanometer system for
addressing the output laser beam (Fig. 1). A ﬂat-ﬁeld lens, of effective
focal length 100 mm, provides a uniform irradiance distribution on
glass substrate over a working area of 80 80 mm
optimize the fabrication process, a previous study was made by
focusing one laser shot on the glass substrate. The mark obtained
was analyzed with a confocal microscope. This initial step let us
determine the laser parameters used for the laser direct-write
process. The morphology of the microchannels in terms of shape
and roughness was analyzed with a confocal microscope SENSO-
FAR 2300 Plm.
For fabricating the microchannels in soda-lime substrate, we
used the following laser parameters: power 8 W, repetition rate
10 kHz, wavelength 1064 nm and pulse width 20 ns. The ablation
threshold (112 J/cm
) was deﬁned as the minimal energy required
for fabricating a homogeneous microchannel in the glass substrate
. We study the inﬂuence of pulse overlapping. It is a crucial
parameter to reach a good ﬁnal microchannel. For achieving
uniform microchannels, we analyze the pulse overlap, Od deﬁned
is the galvo scanner speed (mm/s), 2
is the focused spot
diameter and fis the laser repetition rate.
We have analyzed experimentally the uniformity of the micro-
channels obtained at different scan speed for getting the optimal
pulses overlapping (see Fig. 2). We have used the same laser
parameters (power: 8 W, repetition rate: 10 kHz, pulse width:
20 ns and pulse energy: 0.8 mJ) for fabricating the microchannels
varying only the scan speed from 10 mm/s to 140 mm/s.
In Fig. 2 we can see that the best results were obtained for the
channels fabricated between 40 mm/s and 80 mm/s (Fig. 2dande).
Fig. 1. Laser setup for fabricating the microchannels.
Fig. 2. Confocal images of one microchannel obtained by laser ablation of glass at 10 kHz, 8 W for scan speeds values of (a) 140 mm/s, (b) 120 mm/s, (c) 100 mm/s,
(d) 80 mm/s, (e) 40 mm/s and (f) 10 mm/s.
D. Nieto et al. / Optics and Lasers in Engineering 63 (2014) 11–1812
At low scan speed, the high overlapping of the pulses delivers too
much energy at the surface, which leads to a distortion on the
microchannels (Fig. 2f). For scan speeds higher than 40 mm/s, it can
be appreciated the interaction of each pulse with the substrate
(Fig. 2a–c), which also increases the average roughness and does
not allow the formation of the microchannels. Fig. 3 shows the pulse
overlap, obtained using Eq. (1), and the average roughness measured
at different scan speeds. In terms of pulse overlap (Fig. 3a), values in
the range of 60–80% are needed for fabricating microchannels with
good qualities. For scan speed values in the interval of 40–80 mm/s,
values of roughness around 55 nm are obtained (Fig. 3b).
Fig. 4 shows a microchannel fabricated using the optimal pulse
overlapping obtained from the study presented above and laser
parameters of 8 W, 10 kHz and scan speed 50 mm/s. The resulting
channel exhibits a surface roughness of 54 nm, diameter of 8 mm
and height of 3 mm.
In order to fabricate different microchannel conﬁgurations, a
study of the evolution of depth and diameter with the number of
laserpassesoverthesameplacewasdone.Fig. 5 shows the evolution
of depth and diameter versus the number of laser passes.
It is evident from Fig. 5 that the channel aspect ratio
varies with the number of passes. The diameter dreaches its
saturation value after 5 laser passes, increasing just around
200 nm per laser pass and varying only 1
m after 5 passes, above
these values it is maintained almost constant. In contrast, the
height of the channel hincreases as the number of passes
increases, reaching a saturation value after 6 passes. An increase
m per laser pass is achieved, up to a value of 12
behavior can be related with the non-evacuation of the debris
generated during laser ablation from the bottom of the channel as
well as the lack of focus as the depth of the microchannel
increases. On the other hand, the diameter of the microchannel,
which is maintained almost constant with the number of passes, is
related with the diameter of the laser beam.
The greatest challenge to be overcome using a laser direct-
write technique for fabricating microchannels is to obtain good
quality junctions, since the propelled material of the subsequent
channels is deposited on the existing microchannel, which dete-
riorates the quality of the microchannel. Since the debris gener-
ated change the Ra,crucial for obtaining good micro-optical and
micro-ﬂuidics elements, a study of debris deposition for a micro-
channel with a depth of 4.5
m was undertaken. The laser ﬂuence
) was chosen to be higher than the ablation threshold of
glass, which ensures us to have energy enough to propel the debris
far away from the ablated region. The results are shown in Fig. 6.
In Fig. 6a, we can see how the propelled material, resulting
from the ablation process, is deposited at the edge of the micro-
channel. From this image we extract a transversal proﬁle (Fig. 6b),
which lets us analyze the debris deposition at different distances
from the center of the microchannel. In Fig. 6b, square 1 shows the
portion of the surface measured for determining the average
roughness over the glass with debris near the microchannel that
is the standard deviation of the debris ﬁlm height. The average
roughness is estimated to be 12.3 nm in the region between 7 and
12 mm from the center of the microchannel. Square 2 shows the
portion of the surface measured for determining the average
roughness over the glass with debris, far away from the micro-
channel. The average roughness is estimated to be 12.3 nm in the
region deﬁned by a distance from 12 to 20 mm from the center of
the microchannel. Square 3 shows the portion of the surface
measured for determining the average roughness over the glass
without debris. The roughness is estimated to be 6.2 nm for a
distance greater than 20 mm. The line across the proﬁle shows the
difference in depth of the glass surface and the area where debris
was deposited after ablation. From these data we can obtain
information about the quantity of debris removed and the deposi-
tion when we fabricate shallow and deep channels. In addition, at
high ﬂuencies, where a huge quantity of material is generated
during laser ablation, it was appreciated the need of using a
cleaning process after laser exposure, that involves a chemical
etching process using Hydroﬂuoric acid (HF). Hydroﬂuoric acid is
an etchant which attacks glasses at signiﬁcant high etch rate .
Commercial soda-lime glass used in this work is composed of SiO
O (12.7%), CaO (8.6%), MgO (4.1%) and small amounts
(0.14%) and Al
(0.1%). Etching soda-lime glass in a HF
solution forms insoluble products which are believed to be mainly
The mechanism for eliminating the debris depends on the
concentration of the acid, on the etching time and on the
temperature of the process. However, HF is not only a strong
corrosive, but also highly toxic towards higher concentrations, so
the etching process was performed at 10% HF concentration, which
Scan Speed (mm/s)
56.16 54.4 62.14
Roughness (Ra) (nm)
Scan Speed (mm/s)
Pulse overlap (O
Fig. 3. (a) Evolution of pulse overlapping and (b) roughness versus scan speed in the range of 10–140 mm/s.
Fig. 4. Initial microchannel fabricated by laser ablation.
D. Nieto et al. / Optics and Lasers in Engineering 63 (2014) 11–18 13
in terms of security, reduces considerably percentages of toxic
vapor that contaminate the work space. Fig. 7 shows a SEM top
view image of one microchannel before (Fig. 7a) and after (Fig. 7b)
In Fig. 7a debris deposited at the top of the microchannels
during the laser ablation can be observed. In Fig. 7b we can see
how the debris was successfully eliminated after chemical etching.
The elimination of debris will improve the quality of the micro-
channel, while if maintained at the edge of the microchannel, they
would be mixed with the glass material during the thermal
treatment resulting in bad quality of the ﬁnal microchannel.
3. Thermal treatment
After fabricating the initial structures and eliminating debris
around the channel using HF acid. A thermal treatment was
applied in an oven for improving its morphological properties.
The samples were reﬂowed in a Heraeus muﬂa furnace for 2 h at
temperatures between 620 1C and 670 1C (steps of 10 1C). The
working range has been chosen to be higher than glass transition
temperature of soda-lime glass (Tg ¼564 1C). Fig. 8 shows the
initial shape of the etched glass pattern by laser ablation and the
shape obtained after the material displacement with the thermal
This displacement of material and the consequent accumula-
tion in the bottom leads to a reduction in height. The diameter is
increased due to the material reﬂowed from the top of the edges
of the microchannels to the bottom of the crater. This effect
allows both, the thermal reﬂow and the ﬁlling of the irregular
structure of the crater leading to an improvement on the
morphological qualities. Since the viscosity of glass material is
strongly temperature dependent, different thermal reﬂow tem-
peratures in the range of 620–670 1Cweretestedtostudythe
inﬂuence of temperature on surface curve change. The heights
of the microchannels obtained at different temperatures are
in the range of 200–2.5 mm, and the diameter in the range of
8–25 mm(seeFigs. 9 and 10). Fig. 9 shows the topographic
proﬁle of the fabricated microchannels at different tested
Fig. 6. (a) Confocal image of a channel of height 4.5 μm. Enlarged pictures at the top, shows SEM images of glass (1) without debris and (2) with debris deposited at the edge
of the microchannel. (b) Half transverse proﬁle of a channel with debris deposition at different distances from the center of the microchannel represented by different
regions using numbered squares: square 1 (from 7 to 12 mm), square 2 (from 12 mmto20mm) and square 3 (distance greater than 20 mm).
Fig. 5. (a) Evolution of depth and (b) diameter with the number of laser passes.
D. Nieto et al. / Optics and Lasers in Engineering 63 (2014) 11–1814
As it can be appreciated in Fig. 9, for a temperature of 620 1C,
there was almost no change in the surface shape. For a reﬂow
temperature higher than 670 1C, the initial shape surface proﬁle
becomes ﬂat so microchannels obtained by laser direct-write tend
to disappear. Fig. 10 shows a 3D confocal image of microﬂuidics
microchannels fabricated at different reﬂow temperatures.
The speciﬁcﬂow resistance for each microchannel is strongly
dependent on the geometry and on the roughness. The surface
roughness on the wall of the channel increases by decreasing the
ﬂow rate, since the change of hydraulic resistance is proportional
to the change of surface roughness, for determining the quality of
the microchannels fabricated at different thermal reﬂow tempera-
tures, the roughness was determined at the bottom of the
channels. The roughness average of the glass surface before laser
ablation was 3.68 nm, after laser ablation it increases till 640 nm.
Table 1, shows the evolution of roughness for temperatures
between 620 1C and 670 1C, taken at steps of 10 1C. Since the
purpose of thermal treatment in the case of microchannels is to
reduce the roughness generated during laser ablation, it is impor-
tant to ﬁnd a compromise between shape modiﬁcation and
roughness reduction. Ideally, the shape should be maintained
while the thermal reﬂow should reduce the surface roughness.
By applying the thermal treatment at 620 1C (for 2 h) we were
able to obtain high quality microchannels maintaining the initial
shape and reducing the roughness. At 670 1C the roughness
obtained was similar to the unprocessed glass but the channel
shape changes considerably. In terms of roughness, the thermal
treatment permits us to obtain values of roughness in the range of
unprocessed glass. The roughness average (Ra) at the bottom of
the microchannels before thermal treatment was Ra¼640 nm and
after thermal treatment at 670 1C was Ra¼7.35 nm, this is in the
order of the unprocessed soda-lime glass surface (3.68 nm), which
means that the quality of surface is much better after thermal
treatment. The best result was obtained for 630 1C, at this
temperature the initial shape was maintained almost constant,
and the roughness achieved a value of 57.15 nm, good enough for
microﬂuidics applications on glass [19,20].
After obtaining the best parameters for fabricating optimal
microchannels, in order to study the microﬂuidic capabilities of
our technique, we focused on fabricating microchannels with
different conﬁgurations trying to solve the main problems related
with the laser ablation of glass, in particular for creating uniform
and free of debris channels. Next, attention is turned to the
analysis of the intersection of two microchannels. When the laser
passes twice over the same position, as at the intersection of two
microchannels, the excess of energy on the same point creates a
deeper structure, thus, affecting an accurate ﬂuid ﬂow (Fig. 11a).
This is a crucial point to be overcome for obtaining a good
intersection that becomes in a nice structure for microﬂuidic
For improving these intersections, an important parameter to
take into account is the number of laser pulses delivered at the
beginning and the end of each individual channel. Due to the
characteristic of the Q-switch Nd:YVO
laser used in this work,
when the shutter is opened for the ﬁrst time in order to liberate
the energy stored in the laser cavity, the ﬁrst emitted pulse is more
Fig. 7. (a) SEM image of the microchannel top surface before chemical etching and, (b) SEM image of the microchannel top surface after 10 min in 10% HF aqueous solution.
Fig. 8. Material displacement mechanisms for fabricating microchannels.
Fig. 9. Cross-sectional proﬁle of the microchannels at different thermal reﬂow
temperature. Six different results at different reﬂow temperatures are shown:
T1¼620 1C, T2¼630 1C, T3¼640 1C, T4¼650 1C, T5¼660 1C and T6¼670 1C.
D. Nieto et al. / Optics and Lasers in Engineering 63 (2014) 11–18 15
intense than the following. Usually, this can be corrected with the
software of the laser, which lets us modify and control the ﬁrst
pulses that reach the sample. In the speciﬁc software of the Roﬁn
Powerline E this can be modiﬁed using a control named “Limit”.
Fig. 11 shows the intersection of two microchannels for different
values of this limit.
As it can be appreciated in Fig. 11a, selecting a high value of the
limit (200), too much energy is delivered at the beginning of the
channel, so a deeper structure is created at the intersection of two
channels. In Fig. 11c we select a low value of the limit (50) which
turns in a bad intersection where not all the material is removed
between both channels. Fig. 11bshowstheintersectionobtained
with the optimal value of the limit (100) for the laser parameter used.
Fig. 12 shows different microchannel conﬁgurations fabricated
using the optimal laser parameters obtained for soda-lime glass
substrate (limit 100, power of 8 W, rep. rate 10 kHz and scan speed
100 mm/s). Special attention has been taken on creating uniform
and free of debris channels with very good deﬁnition at intersec-
tions and curves.
A hybrid method for fabricating microﬂuidic microchannels on
soda-lime glass has been developed. It consists of a combination of
Fig. 10. Confocal images of microchannels obtained after thermal treatment at different temperatures.
Comparison of roughness evolution with thermal
Temperature (1C) Ra (nm)
D. Nieto et al. / Optics and Lasers in Engineering 63 (2014) 11–1816
the laser direct-write technique for fabricating the promoting
glass structures and a thermal treatment for reshaping and/or
improving the morphological qualities of the generated micro-
channels. The obtained microchannels have a minimum dia-
meter of 8 mmanddepthof1.5mm. A decrease of the roughness
average generated after laser ablation, of two orders of magni-
tude reaching values of the order of the unprocessed glass, has
been obtained thanks to the thermal treatment applied. The
main advantage of using pulsed nanoseconds lasers for the laser
direct write, includes the beneﬁts of using lasers commonly
implemented for laser processing of materials applications,
which makes the technique presented in this work highly
competitive compared to other techniques commonly used on
This work has been supported by the Consellería de Cultura,
Xunta de Galicia/FEDER, under the Contract EM2012/019.
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