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Journal of Manufacturing Processes 35 (2018) 99–106
Contents lists available at ScienceDirect
Journal of Manufacturing Processes
journal homepage: www.elsevier.com/locate/manpro
Near-IR nanosecond laser direct writing of multi-depth microchannel
branching networks on silicon
T
⁎
Dong Hyuck Kama, Jedo Kimb, , Jyoti Mazumderc
a
Advanced Welding & Joining R&BD Group, Korea Institute of Industrial Technology, Incheon 31056, Republic of Korea
Department of Mechanical and Systems Design Engineering, Hongik University, 94 Wausan-ro, Mapo-gu, Seoul 04066, Republic of Korea
c
Center for Lasers and Plasmas for Advanced Manufacturing, University of Michigan, Ann Arbor, MI, 48109, USA
b
A R T I C LE I N FO
A B S T R A C T
Keywords:
Near-IR
Nanosecond laser
Laser direct writing
Microchannel fabrication
Multi-depth channel
Chemical etching
Multi-depth microchannel network is fabricated on silicon using near-IR nanosecond laser direct writing followed by chemical etching. An 11-level branching network, for which the depth ranges from 20 to 200 μm, is
designed and fabricated to be used as a mold for PDMS replica. The bifurcation of the microchannels is designed
according to Murray’s law so that the total cost function is minimized. The detailed fabrication procedure and
parameters are presented, and method of roughness control using both laser processing parameters and etching
parameters are discussed. The efficient manufacturing of such microchannels with minimal roughness can pave
new roads for realizing microdevices with multi-depth microchannels. Such devices have proven use in environmental/biomedical applications such as artificial lungs.
1. Introduction
Microchannel devices are a promising alternative to the current
inefficient gas exchange membranes used for artificial lungs [1–9]. The
inefficiencies of current gas exchange membranes are caused by the
non-physiological features of the gas exchanger module, which is
composed of bundles of loosely-packed microporous hollow fiber
membranes [10]. In order to enhance the gas exchange performance of
artificial lungs while maintaining biocompatibility, microchannels are
considered as artificial capillaries allowing intimate contact with the
blood flow via a gas permeable membrane [11–13]. The short diffusion
path and the large surface to volume ratio of microchannels result in
efficient gas exchange thus allowing devices to be smaller in size [14].
In the fabrication of such microchannel networks, a wide range of microchannel size with reasonable resolution is necessary. Indeed, it is
possible to build multi-depth microchannel networks using well-known
lithography and etching techniques; however, the mask based lithographic technique is inefficient in terms of cost and time, and it is
difficult to lithographically vary the depth of multi-depth structures.
Laser direct writing, on the other hand, is a promising alternative
method which can be used to easily fabricate the multi-depth microchannels, especially because it does not require any masks or a clean
room environment [15–19]. A variety of lasers can be used for fabrication of microfluidic structures onto silicon: from nanosecond lasers to
ultra-short (femtosecond or picoseconds) lasers as well as from IR lasers
⁎
to ultraviolet (UV) lasers. Our group recently showed that multi-depth
microchannel networks could be successfully fabricated using femtosecond laser direct writing onto silicon [20]. Although femtosecond
lasers or UV lasers are currently the best options for micromachining,
since they result in minimal peripheral thermal damage with low debris
formation [21–30], successful manufacturing of the networks using
conventional near-IR nanosecond lasers can be of high impact due to
wide availability and low cost of nanosecond laser systems.
In this paper, the method for fabricating multi-depth microchannels
by a combination of laser direct writing using near-IR nanosecond laser
and acid etching is presented and discussed. We first design the multidepth microchannels according to Murray’s law so that the total cost
function is minimized. A near-IR nanosecond laser is used to engrave
the multi-depth channels onto silicon which is then chemically etched
to achieve the desired precision. The fabricated microchannel network
is then repeatedly used as a mold to create polydimethylsiloxane
(PDMS) replicas. We present detailed fabrication procedure of an 11
level multi-depth microchannel and we discuss, in detail, the parameters required for minimizing the roughness of the channels.
2. Design
The branching structure is designed according to Murray’s law
which states that the radius of the parent vessel (r0) and the radius of
the daughter vessel (r1 and r2), and the branching angles (θ, ø) have
Corresponding author.
E-mail address: jedokim@hongik.ac.kr (J. Kim).
https://doi.org/10.1016/j.jmapro.2018.07.023
Received 16 November 2017; Received in revised form 26 July 2018; Accepted 26 July 2018
1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Fig. 2. (a) Experimental system schematic: polarizer (P), a half-wave plate (1/
2), a quarter-wave plate (1/4), focal lens (FL), and mirror (M). (b) Temporal
profile after modulation: 100 μs pulses consist of 11 pulses with 200 ns pulse
width.
cos θ = cos ϕ = 3 4/2 .
Fig. 1. (a) Schematic of a 4 bifurcated multi-depth microchannel network (b)
Side view of blood flow and gas transfer in the microchannel. Gas exchange
occurs through the thin PDMS membrane on top of microchannel.
The ratio of the length to the depth is designed to be approximately
constant for the entire length of each branch for a respective generation
so that all paths from the inlet to the outlet of the network have the
same length resulting in the same pressure drop. A cross-sectional view
of the multi-depth microchannel structure is presented in Fig. 1(b). The
top of the channel is to be sealed with a gas permeable PDMS membrane where gas diffusion occurs between the blood flowing inside of
the microchannel and the surroundings via the partial pressure difference between the gas-side and the blood-side.
following relationships:
r03 = r13 + r23,
cos θ =
r04 + r14 − r24
,
2r02 r12
cos φ =
r04 + r24 − r14
.
2r02 r22
(1)
(2)
3. Fabrication
(3)
3.1. Laser setup
Such law is necessary so that the cost function, that is, the same of
the energy cost of the blood in a vessel and the energy cost of pumping
blood through the vessel is minimized. Using such relationship, a multidepth microchannel network, as illustrated in Fig. 1(a), can be designed
where the channels become shallower and narrower at each bifurcation. The figure shows a 4 bifurcation sample resulting in 5 different
channels with different depth and since the aspect ratio is approximately 10:1, when the depth is a the width of the channel becomes 10a.
To apply Murray’s law to rectangular cross-sections, hydraulic diameter (Dh ≡ 4 × area/perimeter) is used, instead of the radius. Here,
the flow into the channel can be assumed to be two dimensional, and it
is proportional to the hydraulic diameter since the channels have high
aspect ratios. Therefore, it is reasonable to express Murray’s law in
terms of depth, for example, symmetric bifurcation with the same
daughter depth (d1 = d2), and parent depth (d0), Murray’s law can be
reduced to:
d 0 = 3 2d1 = 3 2d2 ,
(5)
The schematic of the laser machining setup is shown in Fig. 2(a). A
Q-switched Nd: YAG laser (TRW DP-11) operating at 1064 nm is used as
the nanosecond ablation laser. The laser was set to a repetition rate of
500 Hz and a pulse width of 100 μs (duty cycle of 5%). The 100 μs
pulses were modulated to produce a series of 200 ns pulses with 10 μs
interval as shown in Fig. 2(b). The repetition rate of 500 Hz was chosen
since this repetition rate was the highest possible (for high fabrication
efficiency) while maintaining the beam shape shown in the figure.
When the repetition was modulated to a higher value using the Qswitch, the rectangular pulsed beam shape began to deform resulting in
inconsistent laser ablation. The average power of the incident beam on
the silicon sample was controlled using a combination of a half-wave
plate and a polarizer. A quarter-wave plate and a polarizer were used to
circularly polarize the beam at the target. At focus (far-field), the beam
shape at the center with a symmetric diffraction is near-Gaussian, and a
pin-hole was used to filter the diffraction spots surrounding the bright
central lobe. Using 100 mm focal length objective, the focal spot size of
(4)
100
Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Fig. 4. Processing of microchannel with laser parallel scan: the pitch (Δd), the
scan speed (v), focal position (z), and the number of scanned layers (N).
3.3. Laser direct writing
In laser processing of microchannels, each layer is scanned parallel
to each other with a pitch (Δd) of 20 μm as illustrated in Fig. 4. To
control the depth of the laser ablation, processing parameters, such as
the average power (P) / the focal position (z) / the scan speed (v) / the
number of scanned layers (N), are varied. Note that z is defined as the
length of the focal point below the sample surface in mms. In the
process, it was found that the average power and the focal position
determine the spatial beam profile, and the scan speed determines the
distance between the pulses.
The roughness of the fabricated channel can also be controlled by
the above-mentioned processing parameters. Since the geometry of a
crater created by a single pulse is determined by the average power and
the focal position, one can control this to determine the geometry which
minimizes the roughness. However, reducing the average power and
the focal length results in increased number of scanned layer to obtain
same channel depth. For the current low repetition rate of 500 Hz, any
increase in the number of scanned layer significantly increases the
fabrication time, and therefore, it is not used in this study. For the
power and focal range used here, the speed ranging from 8 to 16 mm/s
provides the required surface roughness.
Fig. 3. Fabrication procedure of the multi-depth microchannel branching network using near-IR nanosecond laser direct writing and chemical etching.
full width 1/e2 maximum (FW1/e2M) was measured to be approximately 30 μm in diameter. Programmable X-Y-Z stages were used to
control the position and the motion of laser beam spots.
3.4. Chemical etching
3.2. Fabrication procedure
Together with laser processing parameters, the etching time using
the solution of HF and HNO3 is also a critical parameter to determine
the depth and the surface roughness of microchannels as illustrated in
Fig. 5(a). Initial difference in the depth of the laser ablation remains
constant since the etching is applied at the same rate. Note that the
etching rate of the width of the channels is approximately twice that of
the depth as etching occurs in the two opposite wall directions of a
channel simultaneously as illustrated in Fig. 5(b). The etching time is
The step-by-step microchannel fabrication procedure is illustrated
in Fig. 3. First, a commercially available N-type silicon wafer oriented
in the < 100 > direction with a 3000 Å nitride coating is micromachined using the nanosecond laser which is to be used as a mold. As
a post-processing method, chemical wet etching with a solution (49%
HF and 69% HNO3 with a ratio of 10:1) is used to smooth the roughness
of the laser ablated surface [31]. The nitride layer is resistive to this
etchant, so the laser-ablated area is selectively etched. The actual
branching networks are created out of PDMS using the laser machined
silicon structures as molds. To replicate PDMS structures from the laser
machined silicon master, Sylgard 184 pre-polymer base and Sylgard
184 curing agent are mixed at a 10:1 ratio in weight. The mixture is put
into a vacuum chamber for 3 h to remove any air bubbles generated in
the mixing process. The surface of the master mold is salinized in a
vacuum chamber before the PDMS pre-polymer liquid is poured. Then
the pre-polymer liquid cured for 3 h at 60 °C and then peeled off from
the silicon master mold. To get the same positive pattern on PDMS, the
negative patterned PDMS replica is used as a mold and the procedure to
make the PDMS replica from the silicon structure is repeated as shown
in Fig. 3. Finally, to make the closed structure a 40 μm thick PDMS film
is placed on top of the PDMS branching network and plasma oxidized
for bonding and to make the channel surface hydrophilic.
Fig. 5. (a) Fabrication of multi-depth structure onto a silicon wafer with laser
ablation followed by acid etching. (b) Etching occurs in the two opposite wall
directions of a channel, whereas etching in depth occurs on the bottom.
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Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Fig. 6. SEM images of the nanosecond laser machined microchannels with various scan speed and the dependence of channel depth and the surface roughness w.r.t.
the scan speed. Operating parameters: 200 ns pulse width, 1 W average power, single pass, 0.5 mm focal position, 30 μm focal spot size, and 40 s chemical etching.
Between average laser power of 0.5 and 0.7 W, the incident beam starts
to penetrate into silicon surface and thermal damage occurs in the irradiated silicon leading to increased surface roughness. For average
laser power over ∼2 W, the redeposit of molten material onto the ablated surface becomes noticeable resulting in significant increase in the
surface roughness.
Focal position, z, is also a parameter that affects the channel depth
and surface roughness. Fig. 8 shows the SEM images of the microchannel, dependence of depth, and dependence of surface roughness
w.r.t z. The figure shows that defocusing attenuates the beam intensity
on the target resulting in shallower channel depths with the correspondingly smoother surface. However, it was found that as the beam
shifts out of the focal range, intensity profile becomes distorted resulting in irregular beam shape and intensity distribution. Therefore,
the focal position control did not show much merit in controlling the
channel depth and the roughness compared to v control and P control.
To evaluate combined effects of the processing parameters, dependent variables (channel depth and surface roughness) are measured for
factorial combinations of main independent variables (scan speed,
average power, and focal position). The scan speed is varied from 4 to
40 mm/s for 15 different combinations of the average power, and the
focal position as given in Table 1 and the results are presented in Fig. 9.
The corresponding roughness is shown for different channel depth
using various combinations of v, P, and z. The variation of P is indicated
by the shape of points, and the variation of z are indicated by the
contrast. The multiple points represent various values of v. As mentioned above, the figure shows that combinations of various P and v are
the most effective means of controlling the microchannel depth and the
roughness. While the effect of the scan speed on channel depth is found
to be negligible, the surface roughness of channel can be minimized
with appropriate choice of scan speeds. For the power and focal position used here, the speed ranging from 8 to 16 mm/s was found to
provide acceptable surface roughness. In the figure, the dotted line is
drawn along the points, which have the minimum average roughness
(Ramin) for each depth. From this, it can be seen that the surface
roughness increase is unavoidable as one increases the channel depth.
Fig. 10 shows surface roughness measurements as a function of
selected as the time it takes for a channel having zero ablation depth
(only the nitride layer removed) to reach the minimal channel depth in
the designed network. For example, if the minimal required depth of
the channel in the sample is 10 μm, the most shallow channel is laser
machined so that only the nitride layer is removed while other areas are
laser machined using predetermined parameters to the corresponding
depths (10 μm short of the desired depth). After all the laser machining
is done, the etching time is chosen so that the etchant etches exactly
10 μm resulting in the multi-depth microchannel sample. As can be seen
in the next section, when the desired depth and the roughness is out of
range for a single path nanosecond laser machining, multiple passes and
multiple numbers of etching can be used for desired channel depth and
surface roughness.
4. Results and discussion
Fig. 6 shows the SEM images, variation of depth, and average surface roughness variation w.r.t the scan speed, v of the fabricated microchannels. It is shown in the figure that the channel depth variation
after a single laser pass is small while the surface quality variation is
large. At low v, solidified molten material and splashed debris in the
craters, created by the overlapped laser pulses, resulting in high surface
roughness. The figure shows that appropriate choice of v is important in
fabricating a smooth surface because the scan speed in pulsed laser
ablation determines the extent of the overlap of the craters.
Fig. 7 shows the SEM images of the microchannel, the dependence
of the channel depth, and variation of surface roughness w.r.t the
average power, P. The channel depth linearly increases with increasing
P indicating that the average power can be an effective parameter in
controlling the depth of the microchannel. However, the surface
roughness increases with increasing P. The higher pulse energy results
in deeper penetration into the silicon sample, but the stronger thermal
reaction of irradiated silicon causes increased roughness. Abrupt
changes in roughness near P of 0.7 and 2.25 W are observed in the
figure. At the power just under the ablation threshold of silicon (0.7 W),
only the surface nitride layer is removed, and the microchannel is
created only by chemical etching resulting in ultra-smooth surfaces.
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Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Fig. 7. SEM images of the nanosecond laser machined microchannels with various average power and the dependence of channel depth and the surface roughness
w.r.t. the average power. Operating parameters: 200 ns pulse width, 8 mm/s scan speed, single pass, 0.5 mm focal position, 30 μm focal spot size, and 40 s chemical
etching.
channel surface gets smoother, and thus the acceptable machining
range with a reasonable surface roughness expands. At the same time,
however, the minimum channel depth increases with etching time: the
minimum depths corresponding to 20, 30 and 40 s etching are ∼20,
∼30 and ∼50 μm respectively for the current experimental setup. Note
channel depth, fabricated using the optimized parameters (shown in
Fig. 9), with different acid-etching times: 20, 30 and 40 s. The data
shown here is an average value of 3 independently fabricated samples.
The depth of the channel is varied by manipulating the average power
while v = 8 mm/s is kept constant. As the etching time increases, the
Fig. 8. SEM images of the nanosecond laser machined microchannels with various focal position and the dependence of channel depth and the surface roughness
w.r.t. the focal position. Operating parameters: 200 ns pulse width, 1 W average power, 8 mm/s scan speed, single pass, 30 μm focal spot size, and 40 s chemical
etching.
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Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
summarized in Table 2. We find that when the surface roughness is to
be maintained below ∼3.5 μm Ra in all the channels, the 11-level
network must be created with two alternating rounds of laser ablation
and chemical etching. As can be seen in Fig. 10, Ra ≤ 3.5 can be
achieved in channel depths from ∼20 to ∼80 μm after 20 s chemical
etching time. However, to achieve the same roughness level for larger
channels, the etching time must be increased. For example, channel
depths from 50 to 200 μm (1 st – 7th levels) are first laser ablated, and
then chemically etched for 20 s. After alignment of this silicon sample,
the rest of channels (8th – 11th levels) are created and then chemically
etched for another 20 s. In summary, channels with depths of
50–200 μm experience a total 40 s of etching time while the channels
with a depth of less than 50 μm experience only 20 s etching time as
summarized in Table 2. The SEM images and the channel depth measurements of the multi-depth channels (which shows 1st-2nd level bifurcation and 2nd-3rd level bifurcation) are shown in Fig. 11(b). The
figures show that after each bifurcation, the depth can be designed to be
shallower and shallower according to Murry’s law. Note that since
different laser processing parameters are required for the different levels, there exists an overlap region (indicated by the dark inverted
triangular shape) where the channel depth is larger than the designed
value. However, such area is reduced as the channel size decreases and
can be further minimized by careful design of the beam path prior to
the fabrication processes.
Comparison of the microchannel depth vs. surface roughness value
for the samples fabricated using nano- and femtosecond laser, respectively, is shown in Fig. 12. The figure shows that because of the significantly small heat affected zone associated with femtosecond laser,
significantly lower values of surface roughness at large channel depth is
expected. This can also be seen from the SEM images of the channel
bottom surface where at the same channel depth of 150 μm, lower
surface roughness is shown for the sample fabricated using femtosecond
laser. Nevertheless, approximately equal surface roughness is possible
for channel depth smaller than 125 μm for both nano- and femtosecond
laser fabricated samples. Such result shows that although the versatility
of the nanosecond laser is limited compared to that of femtosecond
laser, it can be used to fabricate quality microchannels for various uses.
Table 1
The ranges of the laser parameter (Average power, P, focal position, z, and
number of scan passes, N) use to find an optimized microchannel fabrication
conditions.
Parameter
Sampling points
Average power, P (W)
Focal position, z (mm)
Scan speed, v (mm/s)
Number of scan passes, N
0.5, 1.0, 1.5, 2.0, and 2.5
0.0 (at focus), 0.5, and 1.0
4, 8, 12, …, 40
1
Fig. 9. Average surface roughness of the nanosecond laser ablated microchannels w.r.t. the channel depth. The depth and the surface roughness is
measured after 33 s chemical etching.
5. Conclusions
In this study, a method of fabricating multi-depth microchannel
networks with the laser direct writing followed by acid etching is developed. The nanosecond pulses have been employed successfully in the
laser process by optimizing processing conditions. It has been challenging for the nanosecond ablation to be used in micromachining due to
the surface roughness caused by thermal effects including melting,
boiling, and fracture. This study shows that optimal combinations of
processing parameters can minimize the surface roughness while
channel depth is varied. The method provides the ability to realize
artificial vascular network simulating the mammalian respiratory blood
vessel tree obeying Murray’s law. The utility of the laser technique as a
tool for rapid prototyping of artificial vasculatures is demonstrated by
fabricating multi-depth branching structures.
In addition to the artificial lung development, the simplicity and
flexibility of the laser technique are beneficial to the developments of
MEMS, lab-on-a-chip systems, and other biotechnology applications.
The maskless laser process will reduce time and cost compared to the
conventional photolithography based technique in the device development. The capability of the laser technique to fabricate three-dimensional multi-level structures offers more design options than conventional microfabrication such as photolithography and etching.
Fig. 10. Average roughness, Ra vs. ablation depth after 20, 30, and 40 s chemical etching. The samples are prepared with the same processing parameters
except for the etching time as post-processing.
that the surface roughness is observed to decrease near the minimum
depth. This is because the crater size created around the threshold
fluence is not enough to create a proper overlap of the craters at the
scan speed of 8 mm/s. Thus, the surface quality around the minimum
depth can be improved by decreasing the scan speed which will result
in sufficient overlap of between each crater.
The 11-level branching network, for which the depth range is 20 to
200 μm engraved onto a silicon wafer with laser machining and acid
etching is shown in Fig. 11(a). The depth of each generation of the
networks and corresponding combinations of processing parameters are
104
Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Fig. 11. (a) The 11-level branching network fabricated using a combination of near-IR nanosecond laser direct writing and chemical etching on < 100 > silicon
wafer. The gas exchange device made out of PDMS using the fabricated silicon branching network as a mold is also shown. (b) SEM images of the 1st-2nd level and 2nd3rd level bifurcations of the multi-depth microchannel fabricated using laser direct writing on silicon.
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Journal of Manufacturing Processes 35 (2018) 99–106
D.H. Kam et al.
Table 2
The depths of each generation of 11-level branching network and the corresponding processing parameters. The scan speed is maintained as v = 8 mm/s
throughout the fabrication of the channels.
Level
st
1
2nd
3rd
4th
5th
6th
7th
8th
9th
10th
11th
Depth
(μm)
202
160
127
101
80
64
50
40
32
25
20
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Parameters
P (W)
z (mm)
N
Etching time (s)
1.5
1.79
2.95
0.86
1.84
0.56
0.25
1.25
0.76
0.33
0.25
0
0
2
1
2
1
1
2
2
1
1
2
1
1
1
1
1
1
1
1
1
1
40
40
40
40
40
40
40
20
20
20
20
Fig. 12. Comparison of the surface roughness vs. channel depth for nano- and
femtosecond laser ablated microchannel samples. Scan speed of 8 mm/s is used
for the samples fabricated using nanosecond laser while two different scan
speeds (160 and 480 mm/s) are used for the samples fabricated using femtosecond laser.
Acknowledgments
This research work was supported by the Industry Core Technology
Development Program funded by the Ministry of Trade, Industry & Energy
(Korea) and by the Basic Science Research Program through the National
Research Foundation of Korea (NRF) funded by the Ministry of Education
(No. 2015R1A6A1A03031833, No. 2016R1D1A1B03935743, No.
2016R1D1A1B03935743).
References
[1] Potkay JA. The promise of microfluidic artificial lungs. Lab Chip 2014;14:4122–38.
[2] Potkay JA, Magnetta M, Vinson A, Cmolik B. Bio-inspired, efficient, artificial lung
employing air as the ventilating gas. Lab Chip 2011;11:2901–9.
[3] Bellan LM, Pearsall M, Cropek DM, Langer R. A 3D interconnected microchannel
network formed in gelatin by sacrificial shellac microfibers. Adv Mater
106
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