V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
459–464
WETTABILITY STUDIES ON
FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON
SURFACES
[TUDIJ OMAKANJA POVR[INE N-TIPA SILICIJA
TEKSTURIRANE S FEMTOSEKUNDNIM LASERJEM
Vipparla Srikanth
1,2
, G. L. Samuel
1*
, Wei Dongbin
2
1
Indian Institute of Technology Madras, Chennai, India
2
University of Technology Sydney, Australia
Prejem rokopisa – received: 2023-03-28; sprejem za objavo – accepted for publication: 2023-08-16
doi:10.17222/mit.2023.838
This present study examines the wetting behavior of N-type silicon surfaces that have been textured using a femtosecond laser.
By employing three different patterns, i.e., square pillars, micro dimples, and circumferential grooves, and manipulating key
femtosecond laser parameters such as laser power (ranging from8Wto12W)andrepetitions (ranging from 40 to 60), the
wettability properties of the silicon surfaces are modified. The wettability properties of the surface were evaluated by measuring
the contact angle by the sessile-drop method using distilled deionized water as a testing liquid. The textured surfaces displayed
various wettability characteristics, varying from hydrophilic to hydrophobic. The hydrophobic behavior was observed on sur-
faces with a peak laser power of 12 W, 60 repetitions, and the lowest pitch of 160 μm. For the square pillar and micro-dimple
textures, contact angles of 146° and 120°, respectively, were measured. Conversely, the circumferential grooves exhibited hydro-
philic behavior with a contact angle of 20°. These results were achieved at laser powers of 10 W and 8 W, higher pitch values,
and increased repetitions. The contact angle decreased with an increase in pitch and a decrease in repetitions and laser power.
Based on the experimental findings, it can be concluded that the wettability of silicon surfaces can be controlled for specific ap-
plications using a single-step laser ablation technique. The desired wettability characteristics can be achieved by carefully ad-
justing the key femtosecond-laser parameters and geometrical features.
Keywords: wettability, texturing, femtosecond laser, hydrophobic, hydrophilic, silicon
Povzetek: avtorji v pri~ujo~em ~lanku opisujejo {tudijo oziroma preiskavo omakanja povr{ine N-tipa silicija, ki so ga pred tem
teksturirali s femtosekundnim (1 fs = 1·10
–15
s) laserjem. Z uporabo femtosekundnega laserja z mo~jo od8Wdo12Win40do
60 ponovitvami so izdelali tri razli~ne vzorce (kvadratne stebri~ke, mikronske jamice in kro`ne brazde oz. utore) in s tem
spremenili oziroma modificirali omo~ljivost povr{ine izbranega silicija. Omakanje povr{ine so ovrednotili z merjenjem
kontaktnega kota z metodo sesilne kapljice nastale na povr{ini vzorca. Pri tem so kot preizkusno kapljevino uporabili destilirano
deionizirano vodo. Medsebojne primerjave teksturiranih povr{in vzorcev so pokazale razli~no omakanje in sicer od hidrofilnega
do hidrofobnega. Hidrofobno obna{anje kapljevine so zaznali na povr{inah z vr{no mo~jo laserja 12 W in 60 ponovitvami ter
najmanj{em koraku 160 μm. Pri teksturi s kvadratnimi stebri~ki so avtorji izmerili kontaktni kot oziroma kot omakanja 146° in
pri teksturi z mikro jamicami 120°. Obratno pa so na teksturi s kro`nimi brazdami opazili hidrofilno obna{anje s kontaktnim
kotom 20° pri mo~i laserja 10 W in 8 W, vi{ji globini jamic in ve~jem {tevilu ponovitev. Ugotovili so, da se kontaktni kot
zmanj{uje z nara{~anjem globine teksture in z zmanj{evanjem mo~i laserja ter {tevila ponovitev. Avtorji ~lanka na osnovi
eksperimentalnih ugotovitev zaklju~ujejo, da se omakanje silicijeve povr{ine za specifi~ne aplikacije lahko kontrolira s pomo~jo
tehnike eno-stopenjske laserske ablacije. @eljene karakteristike omo~ljivosti povr{ine se lahko dose`e s skrbno nastavitvijo
klju~nih parametrov femtosekundnega laserja in geometrijskih zna~ilnosti.
Klju~ne besede: omo~ljivost, teksturiranje, femtosekundni laser, hidrofobno in hidrofilno obna{anje povr{in, polprevodni{ki
silicij
1 INTRODUCTION
Silicon surfaces with unique wettability properties
have a wide range of applications in intelligent sensors,
microdisplays, gyroscopes, and miniaturized thermal
radiation testers across various fields, such as sensing,
biomedical, military, aerospace, and intelligent commu-
nications.
1
Normally, N-type silicon surfaces exhibit
hydrophilicity with a contact angle of approximately
70–78°. However, their wettability properties can be
modified by introducing physical structures or applying
chemical coatings. Among these approaches, fabricating
physical structures is preferred over chemical coatings as
it preserves the surface chemistry and offers extended
durability.
2
Various manufacturing methods can be used
to obtain surface microstructures, including conventional
photolithography, sandblasting, electrochemical tech-
niques, powder spraying, and electron spinning. Among
them, femtosecond-laser ablation is an efficient and
rapid method for increasing surface roughness and creat-
ing micro-patterns.
3–7
Laser ablation allows for precise
control in generating stable, three-dimensional (3D) sur-
face structures, thereby enhancing superhydrophobic
characteristics and potentially addressing the tribological
issues in micro/nano-electromechanical (M/NEM) de-
vices.
1
Zhu et al. investigated the generation of cross-pat-
Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464 459
UDK 546.28:621.791.725 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 57(5)459(2023)
*Corresponding author's e-mail:
samuelgl@iitm.ac.in (G. L. Samuel)

terned periodic structures on silicon surfaces through la-
ser irradiation.
8
Wang et al. utilized pulsed-laser
irradiation to fabricate textures on silicon surfaces, dem-
onstrating that a combination of melting and surface
evaporation could produce bump-like structures that can
influence the contact angle.
9
Yang et al. combined laser
texturing with silanization to create superhydrophobic
surfaces on silicon materials. According to the Wenzel
model, the intrinsic wettability of a solid surface is en-
hanced by increasing the surface roughness, meaning a
hydrophilic surface becomes even more hydrophilic with
an increase in average surface roughness.
1
Femtosecond-
and nanosecond-laser ablation have been employed to
generate various texture shapes such as lines, grids, and
spots, as well as nanoscale laser-induced periodic surface
structures (LIPSSs) and nanopillars with roughness dis-
tributions formed by overlapping LIPSS with micro-
columns. By adjusting laser process parameters such as
power and repetition rate, the wettability properties of
surfaces can be significantly modified.
10,11
Considering
the widespread use of N-type silicon wafers in different
fields based on their wettability properties, it is essential
to establish a comprehensive study that explores the ef-
fect of femtosecond-laser parameters and pattern shapes
on the wettability of silicon surfaces. In this study, a sin-
gle-step laser ablation process is employed to modify the
wettability properties of the surface without altering its
chemical composition, resulting in stable and chemically
pure surfaces. Three geometrical shapes, namely square
pillars, micro dimples, and circumferential grooves, are
fabricated by varying the pitch, i.e., the distance between
consecutive structures and laser operating parameters
such as power and repetitions. The wettability of N-type
silicon surfaces is observed to vary from near super-
hydrophobic for high laser power and repetitions in the
case of square pillar structures to near super hydrophilic
for low power and repetitions in the case of circumfer-
ential grooves.
2 EXPERIMENTAL
The experiments are performed on a commercially
available, N-type silicon wafer with a thickness of
0.5 mm. Samples were cleaned in an ultrasonic bath for
15 min using acetone before subjecting it to laser irradia-
tion. A diode-pumped femtosecond-laser system (SAT-
SUMA HP2, Amplitude Systems) with a central wave-
length of 1030 nm, pulse duration of 350 fs, and pulse
repetition rate of 2 kHz to 2 MHz was used for the sur-
face treatment. The laser beam was directed to a set of
x-y axes galvo scanning mirrors and then focused on the
surface by a 250-mm focal length lens to a spot size of
120 μm, incident perpendicularly to the sample surface.
The laser scanning was performed as per the pro-
V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
460 Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464
Figure 1: Topography of femtosecond-laser-textured surfaces: a) square pillars, b) height profile of square pillars, c) micro-dimples, d) height
profile of micro dimples, e) circumferential grooves, f) height profile of circumferential grooves.

grammed path for the three patterns. As shown in Ta-
ble 1, three levels for each parameter, i.e., laser power
(12 W, 10 W, 8 W), pitch (160 μm, 190 μm, 220 μm),
and repetitions (40, 50, 60), are considered. The micro-
textures’ topography and height profiles, as shown in
Figure 1, were obtained by a 3D microscope, Olympus
DSX 1000.
Table 1: The laser Processing Parameters considered for the experi-
ments
Laser Power (W) Pitch (μm) Repetitions
8 160 40
10 190 50
12 220 60
The morphology of the laser-treated surfaces and
XPS spectra indicating the chemical composition were
analyzed by Inspect F50 Field Emission gun-based
High-Resolution Scanning Electron Microscope
(FESEM), presented in Figures 2 and 7. The contact an-
gle, which characterizes the wettability of a surface, is
evaluated by the sessile drop test using 5 μL of distilled
deionized water as a testing medium at room tempera-
ture. The textured samples were first cleaned in an ultra-
sonic water bath, then the static contact angle on the sur-
face was measured using a goniometer.
3 RESULTS AND DISCUSSION
3.1 Analysis of Surface Morphology
Figure 2 presents SEM images depicting laser-ab-
lated surfaces with microns and nanostructures, individu-
ally or in combination. In Figure 2a, the structure exhib-
its a hierarchical arrangement of micro and nano features
reminiscent of a lotus leaf, generating a roughness-in-
duced hydrophobic effect. Notably, Figures 2b, 2f, and
2j illustrate that applying a laser power of 12 W leads to
a concentration of higher fluence laser beams on the sur-
face, resulting in greater material removal than surfaces
treated with laser powers of 10 W and 8 W. This discrep-
ancy is particularly evident in the micro-dimples de-
V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464 461
Figure 2: SEM Images of laser-treated surfaces: a) micro dimple at 12 W, b) outer layer of the micro dimple at 12 W, c) micro dimple at 10 W,
d) micro dimple at 8 W, e) square Pillar at 12 W, f) outer layer of the square pillar at 12 W, g) Square pillar at 10 W, h) square pillar at 8 W, i) cir-
cumferential groove at 12 W, j) outer layer of circumferential groove at 12 W, k) circumferential groove at 10 W, l) circumferential groove at 8 W

picted in Figures 2b and 2c. Furthermore, when operat-
ing at 60 repetitions, the laser beam traverses the same
path more frequently. Consequently, the bare silicon sur-
face is exposed to laser treatment for an extended dura-
tion compared to 50 and 40 repetitions, leading to in-
creased material irradiation. As the pitch decreases, the
gap between the micro textures diminishes, resulting in
the overlap of heat-affected zones.
3.2 Analysis of wettability of surfaces
Surface wettability can be characterized by measur-
ing the contact angle, which allows for classifying sur-
faces as hydrophobic or hydrophilic. Figure 3 illustrates
some of the contact angles observed on laser-textured
surfaces. Asymmetry in the contact angle can arise due
to surface heterogeneity, i.e., non-uniform surfaces with
variation in chemical composition and roughness, sur-
face contamination, i.e., presence of contaminants such
as dust particles, oils, or residues, evaporation effects,
i.e., evaporation of the liquid droplet occurs over time,
and surface tension gradient. i.e., the surface tension of
the liquid droplet may not be perfectly uniform across its
interface. Figure 3a shows a nearly super-hydrophobic
surface with a contact angle of 146°. This particular sur-
face was achieved through a square pillar texture, utiliz-
ing a peak power of 12 W, 60 repetitions, and a pitch of
160 μm. Conversely, a nearly super-hydrophilic surface
with a contact angle of 20° was observed on a circumfer-
ential groove pattern fabricated at a laser power of 8 W,
40 repetitions, and a pitch of 220 μm. A wide range of
contact angles, from hydrophilic to hydrophobic, can be
obtained using various combinations of laser parameters
and pitch, as illustrated in Figure 4. It is worth noting
that the laser power and repetitions directly influence the
contact angle. Figure 5 demonstrates that the contact an-
gle usually increases with increased laser power and rep-
etitions for all three patterns square pillars, micro-dim-
ples, and circumferential grooves. Higher laser powers
generally result in more significant changes to the sur-
face topography, as they can lead to more pronounced
surface roughness due to increased material removal or
melting, subsequently contributing to the higher contact
angle.
Figure 6 demonstrates the impacts of pitch and repe-
titions on the contact angle. The contact angle decreases
as the pitch increases for all three patterns. This trend
can be attributed to the widening gap between consecu-
tive microtextures as the pitch increases. Consequently,
V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
462 Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464
Figure 5: Influence of laser power and repetitions on the contact angle: a) square pillar at pitch 190 μm, b) micro dimples at pitch 190 μm, c) cir-
cumferential groove at pitch 190 μm
Figure 3: Contact angles of textured silicon surfaces: a) Square pillar
at laser power 12 W, 60 repetitions, pitch 160 μm, b) Micro dimple at
laser power 12 W, 60 repetitions, pitch 160 μm, c) Circumferential
groove at laser power 10 W, 60 repetitions, pitch 160 μm, d) Micro
dimples at 8 W, 40 repetitions, pitch 190 μm e) Square pillar at 10 W,
60 repetitions, pitch 190 μm f) Circumferential groove at laser power
8 W, 40 repetitions, pitch 220 μm
Figure 4: Spectrum of contact angles for the textured N-Type silicon
surfaces

This reduction in surface roughness contributes to the
overall decrease in the contact angle.
3.3 Analysis of Surface Chemical Composition
The XPS spectra of the laser-treated surfaces and a
polished sample are depicted in Figure 7. The two spec-
tra present peaks corresponding to silicon (Si) and oxy-
gen (O) constituents. After laser treatment, the oxygen
peak increases, which may result in the formation of sili-
con oxide (SiO
2
), which is hydrophilic in nature. How-
ever, the thickness of the oxide layer influences the sur-
face’s wettability, a very thin oxide layer may be too
rough or uneven to promote good wetting. At the same
time, a very thick layer may be too smooth and prevent
liquid from penetrating the surface.
8
4 CONCLUSIONS
A novel, single-step laser-processing technique, re-
quiring no additional post-processing steps, has been de-
veloped to fabricate a wide range of contact angles on
N-type silicon, ranging from nearly super hydrophobic to
hydrophilic. The study examines the influence of laser
processing parameters, such as laser power and repeti-
tions, and geometric parameters, such as texture shape
V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464 463
Figure 6: Influence of pitch and repetitions on the contact angle: a) square pillar at laser power 8 W, b) micro-dimples at laser power 8 W,
c) circumferential grooves at laser power 8 W
Figure 7: XPS Spectra indicating the chemical composition of the laser interacted zone and the non-interacted zone for square pillar texture at la-
ser power 12 W, 60 repetitions, and pitch 160 μm

and pitch, on the wettability properties of N-type silicon.
When utilizing square pillar and micro-dimple textures at
a high peak power of 12 W and 60 repetitions, both tex-
tures effectively increase the contact angle of the silicon
surface. This enhancement is particularly pronounced in
the case of the square pillar texture, which covers a
larger laser-processed area compared to the other two
textures. Notably, the pitch of the texture exhibits an in-
verse relationship with the contact angle, while laser
power and repetitions exert a direct influence. The find-
ings of this study demonstrate the feasibility of creating
both hydrophobic and hydrophilic stable surfaces on
N-type silicon using femtosecond-laser processing. This
advancement has significant implications for various in-
dustries, including sensing, biomedical and military ap-
plications, where precise control over surface wettability
is crucial.
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V. SRIKANTH et al.: WETTABILITY STUDIES ON FEMTOSECOND-LASER-TEXTURED N-TYPE SILICON SURFACES
464 Materiali in tehnologije / Materials and technology 57 (2023) 5, 459–464