M. Zamfirescu, M. Ulmeanu, F. Jipa, O. Cretu, A. Moldovan, G. Epurescu, M. Dinescu, R. DabuNational Institute for Laser Plasma and Radiation Physics, Atomistilor 409, 077125 Magurele, Bucharest, Romania

E-mail: marian.zamfirescu@inflpr.ro

Applications of LIPSS

Femtosecond Laser Induced Periodic Surface Structures on ZnO Thin Films

AcknowledgmentsThis work was supported by the National Research Programe CEEX-ET 5848.

• Nanogratings generated by LIPSS

Laser Induced Periodic Surface Structures (LIPSS) were obtained on ZnO thin films deposited on sapphire substrate. The structures were obtained by scanning the sample surface with a focalised femtosecond laser beam in air. The period and depth of the created structures were measured by AFM and SEM. The dependence of the morphology of the periodic structures on the experimental conditions such as laser fluence, laser polarisation and laser scanning speed, were investigated. Such microprocessing method suggests a possible technique to produce nanogratings, micropolarizers, or nanopatterned surfaces for micro-sensors.

Scan in X direction

Scan in Y direction

Scan in Z direction

E

E

E

1 m

Scaning speed 0.01 mm/sLaser fluence 0.45 J/cm2

XYZ - translation stages

Sample

Video camera

Microscope objective

100X, NA 0.5

Dichroic mirror

LASER beam 775 nm, 200 fs

Experimental Set-up

FEMTOSECOND LASER - CPA system - pulse duration 200 fs - repetition rate 2 KHz - wavelength 775 nm - pulse energy attenuated in the range of 1-200 nJ

TRANSLATION STAGES - XYZ – steppers - resolution 100 nm - travel 4 mm - max speed 2 mm/s

FOCUSING LENS - NIR microscope objective - numerical aperture 0.5 - magnification 100X - focalized laser spot 3 m

When the laser fluence is kept near the ablation threshold, by scanning the sample periodic structures are induced on the sample surface. SEM images reveal ripples with 150 nm spacing, much below the laser wavelength. Clear and uniform grooves are created in the direction perpendicular to the laser polarization.

500 nm

1 m

VISUALIZATION - CCD camera 752x582 pixels - field of view 40x60 m - optical resolution < 1 m

Polarization dependence of LIPSS AFM characterization of laser processed ZnO film

Dependence of the ripples depth and period on the experimental conditions

• Micropolarizers

• Enhanced surface sensitivity of micro-sensors

Scanning speed 0.1 mm/sScanning offset 0.5 µm200 x 500 µm2 scanning areaLaser fluence 0.34 J/cm2

Nanograting obtained by multiple scan of ZnO film

At laser fluence 1.1 J/cm2 and scanning speed ≤ 0.1 mm/s the ZnO is completely ablated. Since the sapphire substrate has a higher ablation threshold, a quite selective ablation process can be expected. Therefore, the thickness of ZnO film can be estimated from AFM measurements: Depth = 140 nm.

Depth = 140 nm

Period = 190 nm

Scanning speed 0.1 mm/sLaser fluence 0.57 J/cm2

0.2 0.3 0.4 0.5 0.6 0.740

60

80

100

120

140

160

Rip

ple

's d

ep

th (

nm

)

Laser Fluence (J/cm2)

0.2 0.3 0.4 0.5 0.6 0.7

140

160

180

200

220

Rip

ple'

s pe

riod

(nm

)

Laser Fluence (J/cm2)0.02 0.04 0.06 0.08 0.10

130

140

150

160

170

180

190

Rip

ple'

s pe

riod

(nm

)

Scanning speed (mm/s)

Laser Fluence = 0.45 J/cm2

0.00 0.02 0.04 0.06 0.08 0.1040

50

60

70

80

90

100

Laser Fluence = 0.45 J/cm2

Rip

ple

's d

ep

th (

nm

)

Scanning speed (mm/s)

Conclusions

Laser processed ZnO film

The investigated samples are ZnO thin films (about 150 nm thickness) deposited on sapphire substrate by RF plasma assisted pulse laser deposition (PLD).

The samples were irradiated by a focalized femtosecond laser beam with energy density from the ablation threshold of ZnO - 0.28 J/cm2 , up to 0.6 J/cm2 , below the ablation threshold of sapphire, in order to have a selective ablation process.

The surface of the samples was scanned by laser with scanning speed from 5 to 100 µm/s. The scanning direction was changed in XYZ in order to observe the polarization effect on ripples orientation.

0.005 0.01 0.05 0.1

0.45

0.28

0.57

1.10

Scaning speed (mm/s)

Laser

flu

en

ce (

J/cm

2)

30 µm

• The period of the laser induced structures is in the range of 130 to 200 nm depending on laser energy and scanning speed.

• The depth of the created grooves is from 50 to 140 nm, depending on the laser fluence and the scanning speed, and is comparable with the thickness of the film. The width of the grooves is about 50 nm, suggesting a high aspect ration from 1:1 up to 3:1.

• Periodically structures perpendicular to the laser polarization were created in ZnO films under laser irradiation above the ablation threshold.

• When proper experimental conditions are chosen, continuous structures over few microns without cracks or bifurcations occur.

• The sample was multiple scanned along the polarization direction in order to obtain periodic structures over a large area of few hundreds of µm2.

• A nanograting with period of 150 nm, more than 100 nm depth and total surface of 200x500 µm2 was created on ZnO.

Scanning speed 0.1 mm/sLaser fluence 0.45 J/cm2

Depth = 83 nm

Period = 176 nm

Scanning speed 0.1 mm/sLaser fluence 1.1 J/cm2

2

0

22

331

2

phB

e

B

e

e

kkNe

km

T

The ripples formation can be explained as the interference between the incident light field and the electric field of the electron plasma wave in material [1].

Ne - electron plasma density, related to the number of pulses.

Te – electron plasma temperature, related to the laser pulse energy.

References[1] Y. Shimotsuma, P.G. Kazansky, L. Qiu, K. Hirao, Phys. Rev. Lett. 91, 247405 (2003).[2] O. Varlamova, F. Costache, M. Ratzke and J. Reif, Appl. Surf. Sci. 253, 7932 (2007).[3] R. Wagner, J. Gottmann, A. Horn and E. W. Kreutz , Appl. Surf. Sci. 252, 8576 (2006).

Keywords: femtosecond laser processing, ripple, nanogratings, ZnO