Femtosecond laser interaction with silicon under water confinement

doi:10.1016/j.tsf.2004.04.043

Thin Solid Films

Volume 467, Issues 1–2, 22 November 2004, Pages 334-341

https://doi.org/10.1016/j.tsf.2004.04.043 Get rights and content

Abstract

Femtosecond laser interaction with silicon was investigated in water and in air, with 130-fs laser pulses at 800 nm wavelength. Under water confinement, higher modification thresholds, lower ablation depths and similar incubation factors were found in comparison to the dry experiment. Morphological features of the laser-induced cavities also differed. In contrast to air experiments, debris redeposition was negligible, while the ablated material remained suspended in the water layer phase. Underwater cavities obtained at high fluences and high number of pulses per spot showed anomalous profiles, consistent with a strong spatial deformation of the laser beam coupled into the target. Ripples formed at the edges of the modified area showed varying spacings: ∼100 and ∼700 nm for water and air experiments, respectively. Differences to the air experiment were related to a complex combination of fluence-dependent non-linear effects occurring in the water layer and to pulse-number-dependent shielding effects induced by cavitation bubbles and suspended ablated material.

Introduction

Material laser processing in the presence of water has been often preferred to the otherwise more common dry treatment for specific technological applications [1], [2]. Higher plasma pressure and longer duration of the shock waves are advantageous for laser shock processing, where changes in the material structure and stress state result in improved surface hardness, fatigue strength, and corrosion resistance of the material. Generations of bubbles as well as water explosion have been successfully employed in steam laser cleaning for the removal of particles from surfaces. In most cases, water convection and bubble motion contribute to the removal of debris redeposition, resulting in cleaner and more precise laser machining, cutting and welding. The high heat capacity of water provides a better heat sink, cooling effectively heat sensitive substrates and the ejected material. Formation of nanoparticles by laser ablation of solids in liquids has been achieved thanks to the confinement effects on vaporised material within the liquid layer.

In our investigations, the use of a water layer during material laser processing had a different motivation: to allow the coupling of electrochemical techniques for in-situ monitoring of laser machining on differently conducting multilayers. Electrochemical monitoring was demonstrated for nanosecond laser ablation of aluminium oxide/aluminium substrates, where changes in the electrochemical potential [3] or the measurement of a current transient [4] yielded on-line information about the ablation depth, the nature of the ablated material, and provided a quantitative method to determine the shock-affected zone in the insulating coating.

Electrochemical monitoring was then applied to laser ablation of the same Al₂O₃/Al substrate in the femtosecond time regime. However, despite working at laser fluences above the ablation threshold of Al₂O₃, the oxide layer could not be ablated. The result was attributed to the formation of a shielding plasma within the water layer, which hindered the effective irradiation of the specimen surface.

In the light of these results, underwater femtosecond experiments were performed on other materials. The present work reports on laser ablation of silicon in water contact. The behaviour of this material in dry femtosecond laser ablation is well known [5], therefore providing a good reference for underwater irradiation. This semiconductor exhibits a lower ablation threshold than water, so that water-related plasma shielding should become less probable (F_Si=0.26 J cm⁻² at λ=800 nm and τ=130 fs [5], F_H₂O=0.58–1.11 J cm⁻² at λ=580 nm and τ=100 fs [6], [7]). The aim of the present study is to assess differences between femtosecond laser ablation of silicon in air and in water contact, both quantitatively (modification thresholds, ablation depths, incubation coefficient) and qualitatively (morphology of the cavity, presence of the often observed columnar structures, formation of ripples).

Section snippets

Experimental

Femtosecond laser experiments were carried out using a commercial Ti:sapphire oscillator and regenerative amplifier system (Spitfire, Spectra Physics). The chirped-pulse amplified laser arrangement delivered 130-fs pulses of linearly polarized light, at a central wavelength of 800 nm. The pulse duration was measured using a scanning autocorrelator (APE Berlin, PulseScope). The energy output was continuously controlled by a zero-order half-wave plate combined to a linear polarizer. Repetition

Results and discussion

Femtosecond laser experiments were performed for various energy values E of the laser pulses (3 μJ≤E≤250 μJ) and number of pulses per spot (N=1, 10, 100, 1000, 10000). F_th denotes the modification threshold fluence. This value is determined later and given in Table 1.

Irradiation of silicon under water confinement was characterized by specific features. For low E or N, a reddish light was observed at the silicon/water interface, with intermittent visible spots along the beam path in the water

Conclusions

The effects of femtosecond laser irradiation of silicon under water and in air environment were investigated. Both quantitative and qualitative differences were found. Compared to air experiments, laser-induced underwater cavities showed: (i) smaller ablation depths, (ii) higher modification thresholds, (iii) similar incubation coefficients, (iv) confinement of the ablated material within the water phase, resulting in the formation of a milky suspension and in negligible redeposition of the

Acknowledgements

The authors would like to thank Dr. I. Dörfel (BAM Berlin, Lab. V.11), Mr. H. Mädebach (BAM VIII.22), Mrs. B. Strauß and Mrs. S. Benemann (BAM VIII.23) for the TEM, light microscopic, and SEM investigations, respectively.

References (23)

A. Kruusing
Opt. Lasers Eng.
(2004)
A. Kruusing
Opt. Lasers Eng.
(2004)
W. Kautek et al.
Electrochim. Acta
(2003)
T. Tsuji et al.
Appl. Surf. Sci.
(2003)
A. Brodeur et al.
Opt. Commun.
(1996)
S.C. Rae
Opt. Commun.
(1994)
J. Reif et al.
Appl. Surf. Sci.
(2002)
P. Rudolph et al.
Thin Solid Films
(2004)
A. Cortona et al.
Phys. Chem., Chem. Phys.
(2001)
J. Bonse et al.
Appl. Phys., A
(2002)

D.X. Hammer et al.

IEEE J. Quantum Electron.

(1996)

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Femtosecond laser interaction with silicon under water confinement

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

Opt. Lasers Eng.

Opt. Lasers Eng.

Electrochim. Acta

Appl. Surf. Sci.

Opt. Commun.

Opt. Commun.

Appl. Surf. Sci.

Thin Solid Films

Phys. Chem., Chem. Phys.

Appl. Phys., A

IEEE J. Quantum Electron.