Direct writing of continuous and discontinuous sub-wavelength periodic surfacestructures on single-crystalline silicon using femtosecond laserRajamudili Kuladeep, Chakradhar Sahoo, and Desai Narayana Rao

Citation: Applied Physics Letters 104, 222103 (2014); doi: 10.1063/1.4881556 View online: http://dx.doi.org/10.1063/1.4881556 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/104/22?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Plasmonic formation mechanism of periodic 100-nm-structures upon femtosecond laser irradiation of silicon inwater J. Appl. Phys. 116, 074902 (2014); 10.1063/1.4887808 Polarization dependent formation of femtosecond laser-induced periodic surface structures near steppedfeatures Appl. Phys. Lett. 104, 231117 (2014); 10.1063/1.4882998 The origins of pressure-induced phase transformations during the surface texturing of silicon using femtosecondlaser irradiation J. Appl. Phys. 112, 083518 (2012); 10.1063/1.4759140 Pulse number dependence of laser-induced periodic surface structures for femtosecond laser irradiation ofsilicon J. Appl. Phys. 108, 034903 (2010); 10.1063/1.3456501 On the role of surface plasmon polaritons in the formation of laser-induced periodic surface structures uponirradiation of silicon by femtosecond-laser pulses J. Appl. Phys. 106, 104910 (2009); 10.1063/1.3261734

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Direct writing of continuous and discontinuous sub-wavelength periodicsurface structures on single-crystalline silicon using femtosecond laser

Rajamudili Kuladeep, Chakradhar Sahoo, and Desai Narayana Raoa)

School of Physics, University of Hyderabad, Hyderabad 500046, India

(Received 17 April 2014; accepted 22 May 2014; published online 4 June 2014)

Laser-induced ripples or uniform arrays of continuous near sub-wavelength or discontinuous deep

sub-wavelength structures are formed on single-crystalline silicon (Si) by femtosecond (fs) laser

direct writing technique. Laser irradiation was performed on Si wafers at normal incidence in air

and by immersing them in dimethyl sulfoxide using linearly polarized Ti:sapphire fs laser pulses of

�110 fs pulse duration and �800 nm wavelength. Morphology studies of laser written surfaces

reveal that sub-wavelength features are oriented perpendicular to laser polarization, while their

morphology and spatial periodicity depend on the surrounding dielectric medium. The formation

mechanism of the sub-wavelength features is explained by interference of incident laser with

surface plasmon polaritons. This work proves the feasibility of fs laser direct writing technique

for the fabrication of sub-wavelength features, which could help in fabrication of advanced

electro-optic devices. VC 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4881556]

Easy and industrially viable nanofabrication techniques

that consistently produce well-defined features much smaller

than the wavelength of the visible light and that can even

facilitate production of features smaller than the diffraction

limit are in great demand. These techniques help in the fabri-

cation of exotic nanodevices encompassing electrical, opti-

cal, and mechanical properties of various materials. In recent

years, direct feature writing using femtosecond (fs) laser has

been touted as an excellent fabrication technique owing to its

ease of operation and more importantly owing to the out-

come which in turn facilitates the possibility of advance

applications.1–3 One amongst such applications is Si based

electronic devices the fabrication of which often demand a

variety of small surface features that can be easily obtained

by fs laser direct writing technique.

Laser-induced periodic surface structures (LIPSS) on the

surfaces of a variety of materials including semiconductors,4–6

metals,7,8 and dielectrics (including transparent materials)9–12

are reported in the literature. Formation of two distinct types

of LIPSS exhibiting low spatial frequency LIPSS (LSFL) and

high spatial frequency LIPSS (HSFL) is possible. LSFL spa-

tial period is close to the irradiation wavelength, and their for-

mation is well-explained by the theory of optical interference

between the incident laser and a surface-electromagnetic

wave created during the irradiation.13 In contrast, HSFL have

spatial periods significantly smaller than the irradiation wave-

length. The formation of these sub-wavelength patterns is not

well-explained and still under discussion. Nonetheless, several

authors have suggested possible mechanisms, such as second

harmonic generation,14 excitation of surface plasmon polari-

tons (SPPs),15 and coulomb explosion,10 which lead to the for-

mation of HSFL.

In this context, fabrication of continuous LSFL at air-Si

interface and discontinuous HSFL at DMSO (dimethyl

sulfoxide)-Si interface by laser direct writing technique

using near infrared fs laser pulses assumes great importance.

This is exactly the theme of the present Letter which dis-

cusses about fs laser induced LSFL and HSFL on Si.

Dependence of the parameters like laser fluence, number of

applied pulses per unit area, laser polarization, and surround-

ing dielectric medium on the morphology, orientation, and

periodicity of LIPSS on the Si surface is also discussed.

Fabrication of LIPSS on Si surface was carried out

by focusing fs laser beam in air and DMSO (by placing Si

wafer in a dish containing DMSO, typical level of DMSO

above Si surface is �2 mm). Commercially available p-type

(10–20 X�cm) (1 1 1) Si wafers have been used in the experi-

ments. The Si wafers were etched with 8% HF for 5 min to

remove the native oxide layer on the surface. The laser source

utilized in direct writing experiments was a Ti:sapphire

oscillator-amplifier system (Spectra Physics, Spitfire) operat-

ing at a wavelength of �800 nm and delivering �1 mJ output

energy pulses at a repetition rate of 1 kHz. Duration of each

laser pulse was about �110 fs. The fs laser pulses are linearly

polarized. Neutral density filters and the combination of half

wave plate and a polarizer were utilized to adjust the irradia-

tion fluence. The laser beam was incident normal to the Si sur-

face and focused by a microscopic objective lens with a

numerical aperture of 0.25, and writing was performed trans-

verse to the laser propagation direction. In order to account

for the reflection losses of the lens, all the energy/fluence

measurements were performed after the lens. Computer con-

trolled translational stages (Newport, USA) were arranged to

translate the sample in x, y, and z directions. Scanning of the

sample was done in both along and normal to the laser polar-

ization direction. Si wafer was rinsed for 5 min in acetone af-

ter irradiation, in an ultrasonic cleaner in order to remove any

debris formed during the ablation process. Detailed characteri-

zation of the morphological changes of the Si surface after

laser irradiation was performed by Zeiss ultra55 ultra high re-

solution field emission scanning electron microscope

(FESEM) operated at an accelerating voltage of 5 kV. Surface

topography has been mapped using atomic force microscopy

a)Author to whom correspondence should be addressed. Electronic addresses:

dnrsp@uohyd.ernet.in and dnr_laserlab@yahoo.com

0003-6951/2014/104(22)/222103/4/$30.00 VC 2014 AIP Publishing LLC104, 222103-1

APPLIED PHYSICS LETTERS 104, 222103 (2014)

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module in NTEGRA Aura model (NT-MDT, Russia) scan-

ning probe microscope. The mapping was carried out in

semi-contact mode under ambient conditions. Doped silicon

cantilever (NSG01 from NT-MDT) was used as the probe.

Probe height and radius of curvature are �14–16 lm and

�10 nm, respectively. Scan rate was 0.5 Hz.

Figure 1 shows FESEM images of the features directly

written on the Si surface in air by linearly polarized fs laser

pulses at laser fluences of 0.3 J/cm2 and 0.41 J/cm2 at different

writing speeds. Below the threshold fluence of 0.2 J/cm2, fea-

ture formation was not observed, which is consistent with the

threshold damage fluence reported in literature.16 The writing

speeds are annotated in Figure 1. Direction of the laser polar-

ization and the direction of sample translation are shown by

dashed arrow and solid arrow in Figure 1(a), respectively.

Features in Figures 1(a), 1(c), 1(e), 1(g), and 1(i) are fabricated

with laser fluence of 0.41 J/cm2, while Figures 1(b), 1(d), 1(f),

and 1(h) are fabricated with 0.3 J/cm2, with different writing

speeds, respectively. Inside the features fabricated with fluence

of 0.41 J/cm2 at lower writing speeds of 50 and 100 lm/s,

some aperiodic structures are formed at the center of the visi-

ble laser modified region, while long continuous LSFL are

formed near the edges of the fabricated microstructure on the

both sides as shown in Figures 1(a) and 1(c). The observed

aperiodic structure area is having longer line width in the fea-

ture written with 50 lm/s compared with 100 lm/s. But in the

features fabricated with the same laser fluence of 0.41 J/cm2 at

higher writing speeds of 200 and 300 lm/s, continuous and

periodic ripple patterns are observed all over the laser modified

region, in which few of the ripples are having branches

(Y shape) as shown in Figures 1(e) and 1(g). Regular and con-

tinuous LSFL are observed in the features fabricated with

lower laser fluence of 0.3 J/cm2 at lower scanning speeds of 50

and 100 lm/s as shown in Figures 1(b) and 1(d), respectively.

At higher writing speeds, anisotropy in the continuity of ripple

formation arises irrespective of laser fluence. Ripple pattern

becomes discontinuous and less ordered for both the laser flu-

ences of 0.41 J/cm2 and 0.3 J/cm2 at respective writing speeds

of 400 and 200 lm/s as shown in Figures 1(i) and 1(f) and

with further higher writing speeds at both the laser fluences we

did not observe any formation of subwavelength features as

shown in Figure 1(h), which is due to the decrease of the pulse

number per unit area.17 As the laser beam is translated, the per-

iodic ripples continue uninterrupted for hundreds of micro-

meters. In general, the investigated LSFL are oriented nearly

perpendicular to the electric field vector of the laser, independ-

ent of the direction of the sample movement relative to the

polarization of the laser beam. If the laser polarization direc-

tion is rotated by 90� using a half-wave plate in the optical

path, the direction of the LSFL also flipped by 90�. Figure 1(j)

shows magnified FESEM image of laser induced periodic rip-

ples of Figure 1(d). The average period of the ripples along the

laser polarization direction is 580 6 30 nm, which is about 1.4

times smaller than the free-space wavelength of the laser and

ripples are 430 6 20 nm wide.

Surface topography maps of the features on Si surface

written with laser fluence of 0.3 J/cm2 at 100 lm/s writing

speed are shown in Figures 2(a) and 2(b). The root mean square

FIG. 1. FESEM images showing the formation of LSFL on Si surface in air

after irradiation with fs laser pulses. (a), (c), (e), (g), and (i) are written with

laser fluence of 0.41 J/cm2; (b), (d), (f), and (h) are written with laser fluence

of 0.3 J/cm2. Figure 1(j) shows magnified FESEM image of laser induced

periodic features of Figure 1(d). The writing speeds are labeled on the

images, laser polarization and writing directions are indicated by the dotted

and solid arrows, respectively, in Figure 1(a).

FIG. 2. Surface topography of fs-laser

irradiated Si surface. (a) Two-

dimensional and (b) three-dimensional

view of the sub-wavelength structures

fabricated with laser fluence of

0.3 J/cm2 at 100 lm/s writing speed.

222103-2 Kuladeep, Sahoo, and Narayana Rao Appl. Phys. Lett. 104, 222103 (2014)

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(RMS) roughness of the fs-laser irradiated surface is �50 nm,

and the mean value of maximum height of sub-wavelength

structures is found to be�215 nm.

Features obtained by fs laser direct writing at DMSO-Si

interface are shown in Figure 3. As shown in Figure 3(a),

discontinuous HSFL are formed on Si with linearly polarized

fs pulses with laser fluence of 0.36 J/cm2. The average period

and width of these ripples along the laser polarization direc-

tion are �120 and �80 nm, respectively, which is about 6.7

and 10 times smaller, respectively, than the free-space wave-

length of the laser. In this case also, formation of ripples is

observed to be always perpendicular to the laser polarization

irrespective of the sample scanning direction. Interestingly,

with circular polarized light, circulate nanofeatures are

observed on the whole irradiated area as shown in Fig. 3(b).

The average feature size is found to be �50 nm. Similar to

air-Si interface, anisotropy of the formation of LIPSS on

DMSO-Si interface also dependent on laser fluence and writ-

ing speed (pulse number per unit area). Formation of these

sub-wavelength features on Si is explained in terms of the

excitation of SPPs on the surface layer. The discussion is

presented in the next paragraph.

From the above experimental results, it can be conclude

that the morphology, orientation, and spatial periodicity of the

fabricated LIPSS on Si surface is dependent on various pa-

rameters like laser polarization, laser fluence, applied pulse

number per unit area, and surrounding dielectric medium.

Formation of sub-wavelength features on metals, dielec-

trics and semiconductors has been well explained by the in-

terference of laser light with SPPs.13 When irradiated with

ultrashort laser pulses with damage threshold fluence, semi-

conductor or dielectric (or even metal) surface will form a thin

layer in which incident laser pulse will predominantly pro-

duces a high density of free electrons Ne; the optical properties

of this excited material (thin layer) should be determined by

abundant hot electrons rather than the intrinsic properties of

solid material. This excited material layer including Ne works

as a thin metal layer and supports the formation of SPPs which

in turn undergo interference with the laser light leading to the

formation of sub-wavelength features.4 SPPs can be excited

when the familiar dispersion relation

ks ¼ ke0 þ ed

e0ed

� �1=2

is satisfied,18 where k and ks are the wavelengths of incident

laser and surface plasmons, e0 is the real part of the dielectric

constant of the laser-excited material, and ed is the dielectric

constant of the dielectric material.

Spatial period (K) of the ripples formed by the interfer-

ence of laser with SPPs is expressed as

K ¼ kkks

6sin h:

In the present experiments, laser is incident normally (i.e.,

h¼ 0�) on the silicon surface, therefore, the spatial period

can be expressed as

K ¼ ks ¼ ke0 þ ed

e0ed

� �1=2

:

In summary, dependence of parameters like laser flu-

ence, pulse number per unit area, laser polarization, and sur-

rounding dielectric medium on the fs laser surface pattern is

revealed in material processing. The experimental results

show the formation of continuous LSFL or discontinuous

HSFL by irradiation of Si wafers at normal incidence in air

or by immersing them in DMSO using linearly polarized

Ti:Sapphire fs laser pulses. Formation of sub-wavelength

structures is only possible within rather narrow range of laser

fluences which are oriented perpendicular to laser polariza-

tion, while their surface morphology and periodicity depends

on the surrounding dielectric medium. This work proves that

the ultra-fast laser inscription technique is simple, efficient,

universal, and environmental friendly, which might attract

remarkable interest that leads to the applications in the fields

like nanoelectronics and nanophotonics.

R. Kuladeep acknowledges University Grants Commission

(UGC), India, for financial assistance through JRF and SRF.

D. Narayana Rao acknowledges financial support from

Department of Science and Technology (DST) India through a

Project Nos. SR/S2/LOP-17/1012 and DST-ITPAR Phase III.

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FIG. 3. FESEM images of fabricated

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DMSO (a) with linear polarized light

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222103-4 Kuladeep, Sahoo, and Narayana Rao Appl. Phys. Lett. 104, 222103 (2014)

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