Fabrication of Microfluidic Channels by Femtosecond Laser

Micromachining and Application in Optofluidics

João M. Maia

1,2

, Vítor A. Amorim

1,2

, D. Alexandre

2,3

and P. V. S. Marques

1,2

Department of Physics and Astronomy of Faculty of Sciences, University of Porto,

Rua do Campo Alegre 687, Porto, Portugal

CAP – Centre for Applied Photonics, INESC TEC, Rua Dr. Roberto Frias, Porto, Portugal

Department of Physics of School of Science and Technology, University of Trás-os-Montes e Alto Douro,

Quinta de Prados 5000-801, Vila Real, Portugal

Keywords: Optofluidics, Femtosecond Laser Direct Writing, Micromachining, Chemical Etching, Fused Silica.

Abstract: Micromachining with femtosecond laser can be exploited to fabricate optical components and microfluidic

channels in fused silica, due to internal modification of the glass properties that is induced by the laser

beam. In this paper, we refer to the formation of microfluidic channels, where an optimization of the

fabrication procedure was conducted by examining etch rate and surface roughness as a function of the

irradiation conditions. Microfluidic channels with high and uniform aspect ratio and with smooth sidewalls

were obtained, and such structures were successfully integrated with optical components. The obtained

results set the foundations towards the development of new optofluidic devices.

1 INTRODUCTION

The concept of lab-on-a-chip (LOC) revolves around

the idea of miniaturizing a complete laboratory into

a hand-sized chip, which allows preparation,

transport, reaction and analysis of reagents, without

loss of efficiency and measurement accuracy

(Dittrich, 1996).

These devices integrate multiple systems, such as

microfluidic channels (for fluid movement),

mechanical elements (valves and mixers for control

of reagent flow and reaction (Xu, 2013, Liu, 2013))

and optical elements (mirrors, lenses and

waveguides for in situ analysis (Hwang, 2009)).

Although conventional fabrication techniques,

such as photolithography and soft lithography,

enabled production of microfluidic devices (Vos,

2007, Wu, 2004), some problems remain. First, both

techniques are planar, thus hindering fabrication of

three-dimensional (3D) structures and limiting

integration of components and device

functionalities. Second, integration of microfluidic

systems with optical layers for on-chip optical

detection has been proven to be demanding and

complex.

In order to overcome these issues, femtosecond

laser micromachining has been adopted for the

production of LOC devices. This non-contact

technology relies on the laser ability to internally

modify the sample properties, due to a non-linear

absorption process (Osellame, 2012). The

modification is confined to the focal volume, hence

submicron resolutions are attainable. Given the

ultrashort duration of the laser pulse, the

modification is not a result of thermal diffusion, thus

formation of heat-affected zones is minimized

(Sugioka, 2014). In the case of picosecond pulses

the resulting structures are formed due to cumulative

thermal effects, which result in erratic and irregular

structures (Corbari, 2013).

For ultrashort pulses, the laser-matter interaction

can be described qualitatively in three steps: (i)

generation of free electron plasma, (ii) followed by

energy relaxation and (iii) modification of the

material properties. When fused silica is exposed to

a femtosecond laser beam, two important structural

modifications may occur, depending on the pulse

energy. For low energies, there is a smooth

modification of the refractive index that can be

exploited to produce optical waveguides, Bragg

grating waveguides, directional couplers, among

other devices (Zeil, 2013, Davis, 1996). For

106

Maia J., Amorim V., Alexandre D. and Marques P.

Fabrication of Microﬂuidic Channels by Femtosecond Laser Micromachining and Application in Optoﬂuidics.

DOI: 10.5220/0006167401060113

In Proceedings of the 5th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2017), pages 106-113

ISBN: 978-989-758-223-3

moderate energies, birefringent sub-wavelength

nanogratings are formed. These structures are more

selective to an etching reaction, which allows

fabrication of microfluidic channels.

Although these two effects may be combined to

produce optofluidic devices (Bellini, 2010,

Applegate, 2006), some problems regarding reactant

detection are still unsolved. As the size of the

microfluidic systems decrease, so does detection

volume, usually in the range of 10

-9

to 10

-12

litres,

and surface-to-volume ratio, hence surface forces

(surface tension, van der Waals and surface

roughness) become dominant (Stone, 2004).

Therefore, the fabrication procedure of microfluidic

channels needs to be optimized, in order to form

highly sensitive optofluidic systems.

Figure 1: Illustration of the FLICE fabrication technique in

fused silica (Said, 2004).

2 MICROFLUIDIC CHANNEL

FABRICATION PROCEDURE

The microfluidic channels were produced using the

FLICE technique (Femtosecond Laser Irradiation

followed by Chemical Etching), which is depicted in

figure 1.

To machine fused silica, we use the 2

harmonic

(515 nm) of a fiber-amplified laser from Amplitude

Systèmes. The laser has a pulse duration of 250 fs at

repetition rate of 500 kHz and maximum average

energy of 1.1 μJ. A schematic of the laser direct

writing unit is depicted in figure 2.

The first block is a power control unit, which is

followed by a beam expander system formed by two

planar-convex lenses. This expander increases the

beam spot size so that it equals the objective lens

entrance aperture. The beam polarization is

determined by a half-wave plate.

The laser beam is focused inside the sample by

an aspherical objective lens with 40x amplification

and numerical aperture equal to 0.55, which prevents

spherical aberrations.

Figure 2: Laser direct writing unit.

The objective is mounted in a piezoelectric stage

(PI P-725) that moves along Z, while the sample is

mounted in two air-bearing stages (Aerotech

ABL10100-LN) that move along the plane XY. This

way we can write 3D structures, by defining the

design geometry in a software program that controls

stage position, speed and pulse energy.

The channels were written by focusing the beam

inside the glass and by stacking multiple scans side

by side, starting from below (in order to avoid beam

propagation through areas already exposed). This

strategy allows design of different cross-section

geometries to be obtained.

After writing, the sample edges were polished to

enable cross-section characterization by microscopy.

Then, the etching reaction was performed by

immersion of the sample in a 10% hydrofluoric acid

(HF) solution. The reaction was performed in an

ultrasonic bath (Branson 2510 Ultrasonic Cleaner) at

frequency of 40 kHz to facilitate debris removal.

After etching the samples were rinsed in

isopropanol and then in deionized water. The

samples were characterized by direct observation at

an optical microscope and by performing SEM

microscopy.

3 OPTIMIZATION OF

FABRICATION PARAMETERS

The production of microfluidic channels was

optimized by studying two main parameters: etch

rate and surface roughness. The etch rate is

associated with the etching selectivity that should be

high, and with the aspect ratio (length/cross-section)

which should be uniform. The channel should also

present smooth sidewalls (low surface roughness), to

enable light coupling and to minimize optical losses

due to scattering.

Fabrication of Microﬂuidic Channels by Femtosecond Laser Micromachining and Application in Optoﬂuidics

107

Multiple channels were fabricated while varying

beam polarization (parallel or perpendicular to the

scanning direction), pulse energy (60 nJ to 300 nJ),

scanning speed (100 μm/s to 500 μm/s), scanning

depth (50 μm to 150 μm) and scan separation (1 μm

to 15 μm).

3.1 Etch Rate Calibration

We started by analyzing the etch rate dependence on

the polarization angle. The results, shown in

figure 3, indicate a strong dependence on the

polarization angle, which results from the

nanogratings orientation.

Figure 3: Etch rate vs polarization angle of single-scan

channels fabricated 150 μm below surface at 500 μm/s and

with 80 nJ pulse energy. The figure inset shows the

nanograting orientation for writing with (A) parallel

polarization and (B) perpendicular polarization, alongside

the light propagation direction (k), the writing direction

(S), and the laser electric field vector (E) (Taylor, 2008).

For perpendicular polarization (90°), the etch rate

is maximum, because the nanogratings align with

the channel axis, enabling fast HF diffusion. For

parallel polarization (0° and 180°), the planes are

normal to the writing direction, hence the HF acid

encounters alternate layers of nanogratings and

pristine material, which reduce the etch rate. To take

advantage of a maximum etch rate, we then decided

to write the channels with perpendicular

polarization.

Analyzing the influence of pulse energy and

scanning speed, figure 4, we observed that, within

the tested range, the etch rate stays uniform. We also

determined that at pulse energies lower than 60 nJ,

there is no formation of nanogratings, and hence

there is no etching selectivity. For scanning speeds

higher than 500 μm/s, we expect the etch rate to start

decreasing, due to a lower contrast between

nanogratings and pristine volumes.

Figure 4: Etch rate as a function of the pulse energy and

scanning speed for channels fabricated with perpendicular

polarization and 150 μm below the silica surface.

The etch rate also depends on the separation

between scans. As shown in figure 5, the etch rate

starts increasing as the separation between scans

decreases, as expected.

Figure 5: Etch rate of channels as function of the

separation between scans along Y and Z. The channels

were fabricated with perpendicular polarization, 150 μm

below surface at 500 μm/s and with pulse energy of 80nJ.

For small overlap, the scans do not interact with

each other and the etch rate is similar to the one

obtained for single-scan channels. However,

decreasing the separation results in an increase of

the stress field around the laser-affected zones

(Champion, 2013), which modifies the irradiated

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

108

volume density, and consequently increases the etch

rate (Agarwal, 1997).

3.2 Tapering Effect

For long etching reactions we also observed that, for

writing with perpendicular polarization, the etch rate

decreases over time, figure 6. This phenomenon

occurs because the HF acid cannot reach the

channel’s end as easily.

Figure 6: Etch rate as a function of the etching reaction

time for channels written with perpendicular and parallel

polarization. The channels written with perpendicular

polarization were fabricated 150 μm below surface at

500 μm/s and with pulse energy of 80nJ. The channels

written with parallel polarization were fabricated 150 μm

below surface at 300 μm/s and with pulse energy of

300 nJ. For perpendicular polarization the etch rate

decreases over time, while for parallel polarization it

remains constant.

Alongside this effect, it also occurs lateral

etching, where the HF acid etches the pristine

material surrounding the channel. These two features

contribute to the formation of tapered channels,

figure 7, and affect significantly the obtained aspect

ratio.

Figure 7: Top view image of a tapered single-scan channel

fabricated with perpendicular polarization at a depth of

50 μm, with pulse energy of 60 nJ and scanning speed of

500 μm/s.

In order to prevent this issue, several solutions

can be adopted: (i) use of KOH at 80°C as an etchant

solution (Kiyama, 2009), (ii) design of a structure

that compensates and balances the etching reaction

(Vishnubhatla, 2009), and (iii) fabrication of vertical

holes connecting the channel to the sample surface

(Ho, 2013). Due to simplicity, we used the latter

technique, whose configuration is depicted in

figure 8.

Figure 8: Fabrication layout of vertical holes (Ho, 2013).

By fabricating holes 200 μm apart from each

other, we obtained channels with uniform aspect

ratio, as shown in figure 9, and with length over one

centimetre.

Figure 9: Top view images of microfluidic channels

fabricated buried in fused silica at different times of

etching reaction.

Figure 9 also reveals the time evolution of the

etching reaction. After 30 minutes, we observe that

the vertical holes have been fully etched, and that

the HF acid diffuses preferentially along the channel

axis, due to the favourable nanogratings alignment.

After 60 minutes, the entire channel has been etched,

but its dark colour reveals that some glass fibers are

still present. After 90 minutes the process is

completed.

Fabrication of Microﬂuidic Channels by Femtosecond Laser Micromachining and Application in Optoﬂuidics

109

3.3 Surface Roughness

Regarding surface roughness, it depends mainly on

two factors: stress field distribution and scan

separation.

The formation of laser-affected zones is

associated with the generation of stress fields around

the irradiated volume. The stress fields induce

birefringence, which can be measured by

transmitting light through the sample.

It is reported that pulse energy, scan separation

and beam polarization affect the accumulated stress

(Champion, 2013). For high pulse energies and for

small separations the accumulated stress increases,

and at extreme conditions (pulse energy above

300 nJ and/or separation of tenths of nanometres)

the stress can be released. This relaxation process

generates cracks that propagate over a few microns

into the surroundings, figure 10.

Figure 10: Cross-section image of a microfluidic channel

presenting cracks. The channel was fabricated with

parallel polarization, 50 μm below surface at 200 μm/s and

with 300nJ pulse energy.

On the other hand, if the laser-affected zones are

separated by tenths of microns, we observe that,

after etching, these volumes do not merge together,

figure 11. By decreasing the scan separation, the

volumes start merging, but the sidewalls are still

strongly corrugated. Therefore, it is necessary to

continue decreasing the scan separation to obtain

smoother sidewalls, while avoiding, at the same

time, crack formation.

Figure 11: Cross-section image of microfluidic channels

fabricated with different scan separations along Y and Z:

1 μm and 15 μm (left) and 1 μm and 7 μm (right),

respectively. Both channels were fabricated with

perpendicular polarization, 150 μm below surface at

500 μm/s and with 80 nJ pulse energy.

3.4 Optimum Conditions

From this study, we found that the optimum

irradiation conditions to obtain long channels with

uniform aspect ratio and smooth sidewalls are: beam

polarization perpendicular to the scanning direction

(to obtain maximum etching selectivity), scanning

speed of 500 μm/s, pulse energy between 60 nJ and

80 nJ (to minimize stress accumulation), scan

separation along Y of 1 μm to 2 μm and along Z of

1 μm to 4 μm and scanning depth between 50 μm

and 150 μm.

Under these conditions, we obtained a maximum

etching selectivity of 140:1.

The surface roughness along Y varied between

100 nm and 200 nm. This value was obtained by

producing the channels at the silica surface and by

measuring the roughness using a profilometer

(Dektak XT from Bruker with stylus with 2 μm

radius and with 3 mg applied force). Regarding

surface roughness along Z we estimate it to be of

tenths of nanometres, given that similar values were

obtained by other groups who produced channels

under identical conditions (Ho, 2013). In addition, it

is expected that the surface roughness is lower along

Z than along Y due to the parallel alignment of the

nanogratings with the channel sidewalls.

Figure 12: Cross-section of a microfluidic channel

fabricated under optimum conditions.

4 ON-CHIP OPTICAL

DETECTION

4.1 Principle of Operation

The results presented in the previous section indicate

that the obtained channels may be integrated with

PHOTOPTICS 2017 - 5th International Conference on Photonics, Optics and Laser Technology

110

optical components for production of optofluidic

devices. In this paper, we present a possible on-chip

optical detection scheme that enables measurement

of refractive index and temperature of the fluid

circulating in the channel.

Figure 13: Optical detection scheme layout.

The proposed scheme, whose layout is shown in

figure 13, consists in placing a Bragg grating

waveguide (BGW1) a few microns from the

microfluidic channel. According to the Bragg law,

equation (1), light being reflected by the BGW

depends on the medium effective index (n





2Λn



(1)

If the channel is a few microns from the BGW,

the medium effective index will depend on the fluid

present in the channel. Therefore, by changing the

fluid properties (concentration, temperature) or by

changing the fluid, we obtain a different Bragg

wavelength (λ



) and are able to sense the fluid

properties. A second BGW, which is surrounded

only by pristine, is fabricated with a different

modulation period (Λ) and is designed to serve as

reference, allowing to exclude thermal effects.

4.2 Proof of Concept

We started by fabricating microfluidic channels with

BGWs at increasing distances from the channel.

The channels were 1.1 cm long and 100 μm

below the glass surface (distance between silica

surface and top of the channel), and were fabricated

with perpendicular polarization, at 500 μm/s with

60 nJ pulse energy and with scan separation along Y

and Z equal to 1 μm and 2 μm, respectively. The

Bragg grating waveguides were produced with

parallel polarization, 125 μm below surface at

400 μm/s and with 250 nJ pulse energy. Two

gratings were produced, one to sense the channel

and designed to operate at 1550 nm and another to

operate at 1558 nm and to serve as a reference. After

writing, the glass facets were polished to improve

light coupling. The etching reaction lasted 90

minutes.

After the devices were fabricated, we observed at

an optical microscope that some of the BGWs had

been etched, figure 14, suggesting that a higher

control of the etching reaction is needed.

Figure 14: Top view of the optofluidic device.

Regarding the remaining devices, we

characterized them, by measuring the Bragg

wavelength when the channels were filled with air.

The measurements were performed without

polarization control. The results, shown in table 1,

indicate that the wavelength shift decreases as the

BGW-channel separation increases. Thus, we

conclude that to achieve higher sensitivities the gap

should be, at maximum, 5 μm.

Table 1: Wavelength shift for increasing separation

between BGW and microfluidic channel.

Separation

BGW-channel

3 μm 4 μm 5 μm 7 μ

Wavelength shift

(±0.05nm)

0.30 0.23 0.21 0.06

For the gap of 3 μm, we then filled the channel

with water and were able to measure a different

Bragg wavelength, figure 15.

The results obtained highlight one of the

advantages of the FLICE technique: it can be used to

integrate monolithically optical layers with

microfluidic channels.

Fabrication of Microﬂuidic Channels by Femtosecond Laser Micromachining and Application in Optoﬂuidics

111

Figure 15: Optical spectrum of a BGW placed 3 μm from

the microfluidic channel, when the channel is filled with

air (red) and with water (blue). An initial spectrum

(black), taken before the etching reaction, is also included.

The inset picture shows that different Bragg wavelengths

are obtained for air and water.

5 CONCLUSIONS

In this project, we employed the FLICE technique to

study the fabrication process of microfluidic

channels. From this study, we were able to produce

centimetre long channels, with uniform aspect ratio

and with crack-free and smooth sidewalls.

This optimization enabled monolithic integration

of the channels with optical components. By

producing Bragg grating waveguides a few microns

from the microfluidic channels, we were able to

detect the fluid through evanescent coupling.

These results, although preliminary, are

important for the development of high-quality

optofluidic devices.

ACKNOWLEDGEMENTS

Project "NanoSTIMA: Macro-to-Nano Human

Sensing: Towards Integrated Multimodal Health

Monitoring and Analytics/NORTE-01-0145-

FEDER-000016" is financed by the North

Portugal Regional Operational Programme (NORTE

2020), under the PORTUGAL 2020 Partnership

Agreement, and through the European Regional

Development Fund (ERDF).

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