Cent. Eur. J. Phys. • 6(2) • 2008 • 327-331
DOI: 10.2478/s11534-008-0026-0
Central European Journal of Physics
Impact of the laser wavelength and fluence on the
ablation rate of aluminium
Research Article
Mihai Stafe12∗ , Ionut Vladoiu1 , Ion M. Popescu1
1 Department of Physics, University “Politehnica” of Bucharest, Spl. Independendei 313, 060042 Bucharest, Romania
2 Present address: Department of Experimental Physics, University of Szeged, Dom ter 9, H-6720 Szeged, Hungary
Received 29 Jun 2007; accepted 3 December 2007
Abstract:
The dependence of the ablation rate of aluminium on the fluence of nanosecond laser pulses with
wavelengths of 532 nm and respectively 1064 nm is investigated in atmospheric air. The fluence of the
pulses is varied by changing the diameter of the irradiated area at the target surface, and the wavelength
is varied by using the fundamental and the second harmonic of a Q-switched Nd-YAG laser system. The
results indicate an approximately logarithmic increase of the ablation rate with the fluence for ablation
rates smaller than ∼6 µm/pulse at 532 nm, and 0.3 µm/pulse at 1064 nm wavelength. The significantly
smaller ablation rate at 1064 nm is due to the small optical absorptivity, the strong oxidation of the
aluminium target, and to the strong attenuation of the pulses into the plasma plume at this wavelength.
A jump of the ablation rate is observed at the fluence threshold value, which is ∼50 J/cm2 for the second
harmonic, and ∼15 J/cm2 for the fundamental pulses. Further increasing the fluence leads to a steep
increase of the ablation rate at both wavelengths, the increase of the ablation rate being approximately
exponential in the case of visible pulses. The jump of the ablation rate at the threshold fluence value is
due to the transition from a normal vaporization regime to a phase explosion regime, and to the change
of the dimensionality of the hydrodynamics of the plasma-plume.
PACS (2008): 52.38.Mf, 42.62.-b, 81.05.Bx, 79.20.Ds
Keywords:
ablation rate • nanosecond laser pulses • metal
© Versita Warsaw and Springer-Verlag Berlin Heidelberg.
1.
Introduction
The efficiency of material removal under the action of
short, intense laser pulses is described by the ablation
rate, which gives the maximum thickness of the layer ablated during irradiation with a laser pulse. Understanding
and controlling the ablation rate is essential for micro∗
E-mail: [email protected]
patterning and pulsed laser-deposition in electronics, optoelectronics, and micromechanics, where the ablation rate
is a key parameter [1–7].
The ablation rate is strongly influenced by the thermal
and optical properties of the processed material (e.g., mass
density, surface reflectivity, optical absorptivity, thermal
conductance) [1–6, 8, 9], ambient conditions [2, 9, 10], and
by the characteristics of the processing laser beam (e.g.,
pulse duration, number of pulses, energy, fluence, wavelength) [1, 2, 5, 6, 11–18]. Due to their being cost effective, there is an ongoing effort to develop new techniques
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Impact of the laser wavelength and fluence on the ablation rate of aluminium
based on nanosecond laser ablation, and to determine the
optimum parameters of the nanosecond laser-pulses that
allow one to obtain high quality micropatterns on different metallic or non-metallic materials [8–10, 19–22]. The
experiments indicate that the ablation rate increases logarithmically with the fluence of the nanosecond pulses in
the case of semiconductors and dielectrics [2, 5, 16], while
for values of the fluence higher than a threshold there
is a steep increase of the ablation rate [16, 19–21]. The
wavelength of the laser has a significant influence on the
ablation rate: the shorter the wavelength, the higher the
ablation rate. The dependence of the ablation rate on
the laser wavelength is mainly due to the superposition
of three inter-related phenomena: the decrease of the intrinsic absorptivity of dielectrics and semiconductors, the
increase of the surface reflectivity, and the enhancement
of the absorptivity of the laser-produced plasma with increasing the laser wavelength.
Here, we investigate the dependence of the ablation rate
of aluminium on the fluence of nanosecond laser pulses in
ambient atmosphere conditions, using nanosecond pulses
at 1064 nm and 532 nm wavelengths. The fluence at the
surface of the metallic target surface is varied by changing
the diameter of the laser beam in the focal plane of a lens.
We demonstrate that the ablation rate is approximately
one order of magnitude higher for the visible pulses, and
the threshold fluence that marks a jump of the ablation
rate is three-fold higher in the case of pulses at 532 nm
as compared to the 1064-nm wavelength pulses.
2.
Experiments
The experimental setup consists of a Q-switched Nd-YAG
laser system, a focusing lens and an aluminium target
placed perpendicular to the laser beam. The experiments
were carried out in ambient atmosphere conditions. The
laser system works in the TEM00 mode and generates
fundamental pulses at 1064 nm wavelength which can
be frequency doubled (532 nm wavelength) by sending
them through a second harmonic generator module. The
laser pulses are characterized by a duration of 4.5 ns,
repetition rate of 10 Hz, and energies of 360 mJ/pulse
and 180mJ/pulse for the fundamental and second harmonic
pulses, respectively.
The laser pulses are focused at normal incidence on a 1
mm-thick aluminium plate by a focusing lens system. The
target is first placed in the focal plane of the convergent
lens (f/10, f=10 cm) giving a diameter of the irradiated
area of 1 mm and 0.4 mm for the fundamental and second
harmonic pulses, respectively. Consequently, the fluence
at the target surface was approximately 20 J/cm2 and 100
J/cm2 for the two wavelengths.
To vary the fluence of the laser beam we varied the diameter of the irradiated area by moving the target away
from the focal plane. To avoid the effect of air breakdown
that would lead to the waste of laser energy in heating
the plasma plume ignited in front of the sample, the aluminium target is translated axially toward the incoming
laser beam. The translation of the target is done with micrometric resolution by using a mechanical stage on which
the sample is fixed. The sample is translated with increments (along the axial direction) of 5 mm and 2 mm for
pulses at 1064 nm and 532 nm wavelengths, respectively.
The increment steps for translating the sample were chosen such that the relative variation of the diameter of the
irradiated area from one position to the other is approximately the same at the two wavelengths. Thus, the diameter of the irradiated area increases from its minimum
value in the focal plane up to 2.5 mm and 1.6 mm for the
two wavelengths, while the corresponding fluence decays
from its maximum value in the focal plane down to 6 J/cm2
in both cases. Because the diameter of the crater is higher
than its depth (the aspect ratio is < 1), the ablation can
be considered as one-dimensional and, consequently, the
diameter of the irradiated area could be approximated by
the diameter of the crater that is drilled into the aluminium
target [9, 22]. The fluence was then determined by dividing
the energy of the pulse by the irradiated area [9].
The dependence of the ablation rate on the laser fluence
was derived from the depth of the craters that are drilled
in multiple-pulse regime into the aluminium target at each
particular position along its axial path. Because the ablation rate usually depends on the number of consecutive
laser pulses [2, 14], we determined the number of laser
pulses for drilling a crater such that the ablation rate is
approximately constant for each successive laser pulse.
Our measurements indicate that 200 fundamental pulses
and 10 second harmonic pulses ensure an approximately
constant ablation rate and, moreover, enable us to obtain
a crater sufficiently deep to allow measurement of the ablation rate with a relative error of maximum 5%. The ablation rate was then calculated by dividing the depth of the
crater depth by the number of laser pulses that irradiated
the target for drilling the crater. The depths and the diameters of the craters were measured using a metallographic
microscope with micrometric resolution.
3.
Results and discussion
The dependence of the ablation rate (∆h) of aluminium on
the fluence (F ) of the fundamental and second harmonic
pulses is depicted in Figs. 1, 2, 3, 4.
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Mihai Stafe, Ionut Vladoiu, Ion M. Popescu
whereas for the for the pulses at 1064 nm wavelength, the
fitting curve is
∆h = 0.1 ln[1.05 F (J/cm2 )] (µm).
Figure 1.
Ablation rate of aluminium in open air as a function of
laser fluence of a 4.5 ns laser pulse. The laser wavelength is 532 nm.
For the case of the second harmonic pulses, the increase
of the fluence to a value of ∼50 J/cm2 leads to an approximately logarithmic increase of the ablation rate to a
maximum of ∼6 µm/pulse (Fig. 1). In a similar manner, for
the fundamental laser pulses, the increase of the fluence to
∼15 J/cm2 leads to an approximately logarithmic increase
of the ablation rate to a maximum of ∼0.3 µm/pulse (Fig.
3). In the case of the pulses at 532 nm wavelength, the
fitting curve is described by the equation
The maximum ablation rate is approximately one order of
magnitude higher when the target is irradiated with second harmonic pulses instead of fundamental pulses. The
much higher efficiency of the ablation in the case of second
harmonic pulses is likely due to the superposition of three
phenomena. First, the increase of the optical absorptivity
of aluminium upon decreasing the wavelength [2, 5] leads
to strong localized heating and, consequently, to a high
ablation rate. Second, since the surface of the target surface appears to be covered with a very compact slag-layer
after irradiation with infrared pulses, the oxidation of the
surface during the interval between the laser pulses could
lead to a further decay of the intrinsic absorptivity of the
target for laser pulses at 1064 nm wavelength as compared
to those at 532 nm [2, 9]. Third, the dependence of the absorption coefficient of plasma (αIB ) on the wavelength (λ)
αIB ≈ λ3
Ablation rate of aluminium in open air as a function of
laser fluence of a 4.5 ns laser pulse. The laser wavelength is 532 nm. The dashed lines indicate the fitting
curves for the ablation rate vs fluence for values of the
fluence smaller than the threshold.
∆h = 1.1 ln[5.85 F (J/cm2 )] (µm)
(1)
(3)
implies that the absorption of the laser beam into the ignited plasma plume via the inverse-Bremstrahlung effect
is significantly stronger in the case of longer wavelengths
[2, 5, 11, 23]. As a consequence, the plasma produced by
the leading edge of an infrared pulse becomes much more
absorbing and efficiently reflective for the remaining pulse
as compared to a visible pulse. This prevents the infrared
pulse from reaching the target surface and producing further ablation shortly after the onset of the pulse.
A jump of the ablation rate is observed for a threshold
value of the fluence Fth , which is ∼50 J/cm2 for the second harmonic, and ∼15 J/cm2 for the fundamental pulses.
Further increase of the fluence above the threshold value
leads to a steep increase of the ablation rate at both wavelengths (Figs. 2, 4). In the case of visible pulses, the steep
increase can be approximated by an exponential function
(Fig. 2):
∆h = 4.1 exp[0.007 F (J/cm2 )] (µm).
Figure 2.
(2)
(4)
The results above on the existence of a threshold fluence
and the steep increase of the ablation rate upon increasing the fluence above the threshold value are consistent
with previous work on metallic and non-metallic materials
in different irradiation conditions [16, 19–21]. There, ablation was performed in vacuum conditions, and the fluence
was varied via the pulse energy. Values of the threshold
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Impact of the laser wavelength and fluence on the ablation rate of aluminium
Figure 3.
Ablation rate of aluminium in open air as a function of
laser fluence of a 4.5 ns laser pulse. The laser wavelength is 1064 nm.
fluence were reported in the range of 5 − 25 J/cm2 , and it
was suggested that the existence of a threshold fluence is
mainly due to phase explosion [16, 19–21]. For the particular case assessed here, the jump of the ablation rate is
likely due to the following two causes. First, because the
increase of the ablation rate is accompanied by a significant amount of bright sparks that emerge into the plume,
the jump of the ablation rate is likely related to the transition from a normal vaporization regime of the irradiated
sample (which is characterized by a small mass-removal
rate) to a phase-explosion regime (which is characterized
by ejection of small particulates and melt droplets and an
enhanced mass-removal rate). The ejection of droplets is
enhanced by the recoil pressure exerted by the plasma
plume onto the melted surface [24], such that the infrared
pulses (which heat the plasma stronger) could start the
ejection process at a lower threshold fluence (∼15 J/cm2 )
as compared to the visible pulses (∼50 J/cm2 ). The second cause contributing to the jump in the ablation rate
consists in the change of the dimensionality of the hydrodynamics of the plasma plume from one-dimensional
to three-dimensional. In the one-dimensional regime the
expansion of the plasma-plume is mainly axial, being characterized by high density and strong absorptivity. In constrast, in the three-dimensional regime the plasma plume
expands in both axial and radial directions, and is characterized by low density and weak absorptivity [5]. The
jump appears when the diameter of the laser beam (central
Airy disk) becomes the same order of magnitude with the
hydrodynamic length of the laser-produced plasma. This
relationship between the beam diameter and the hydrodynamic length of the plasma at threshold is demonstrated
as follows. For an usual plasma temperature of 105 K [11],
Figure 4.
Ablation rate of aluminium in open air as a function of
laser fluence of a 4.5 ns laser pulse. The laser wavelength is 1064 nm. The dashed lines indicate the fitting
curves for the ablation rate vs fluence for values of the
fluence smaller than the threshold. The dotted line indicates the steep increase of the ablation rate with fluence
for values of the fluence above the threshold.
the plasma hydrodynamic length [2, 11],
lh = vp τ
(5)
is ∼50 µm, which is similar to the diameter of the laser
beam corresponding to the threshold fluences Fth of either visible or infrared pulses. In equation (5), vp =
p
γ kB Tp /M is the expansion velocity of the plasma plume
considered as an ideal gas with adiabatic coefficient
γ = 5/3, temperature Tp and the atomic mass M, τ is
the pulse duration, and kB is the Boltzmann constant.
4.
Conclusion
The impact of the fluence and wavelength of the nanosecond laser pulses on the ablation rate of aluminium was
investigated in ambient atmosphere conditions. We varied the fluence of the laser pulses by changing the diameter of the irradiated area at the target surface, and
the wavelength by using the fundamental and the second
harmonic beams of a Q-switched Nd-YAG laser system.
The results indicate that the ablation rate has an approximately logarithmic increase with the fluence for ablation
rates smaller than ∼6 µm/pulse and ∼0.3 µm/pulse when
using pulses at 532 nm and 1064 nm wavelengths, respectively. The much higher efficiency of the ablation in the
case of second harmonic pulses as compared to the fundamental pulses is due to the superposition of the following
three effects: the increase of the optical absorptivity of
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Mihai Stafe, Ionut Vladoiu, Ion M. Popescu
aluminium with decreasing wavelength, the weaker oxidation of the surface of the target when irradiated at short
wavelengths, and the decay of the plasma absorption via
the inverse-Bremstrahlung effect at short wavelengths.
The increase of the fluence to a threshold value of ∼50
J/cm2 for the second harmonic pulses, and ∼15 J/cm2 for
the fundamental pulses, leads to a jump of the ablation
rate. Further increase of the fluence above the threshold value leads to a steep increase of the ablation rate at
both wavelengths. In the case of visible pulses, the steep
increase of the ablation rate with the fluence above the
threshold fluence can be described by an exponential function. The jump of the ablation rate at the threshold fluence
is likely due to the superposition of two effects. First, the
appearance of a significant number of bright sparks (particulates and melted material) within the plasma plume indicates a transition from the normal vaporization to phase
explosion with increasing the fluence of the pulses above
the threshold value. Second, at the threshold fluence the
hydrodynamic length of the plasma becomes similar to the
diameter of the laser beam (central Airy disk), and the dimensionality of the hydrodynamics of the plasma plume
changes from one-dimensional to three-dimensional.
References
[1] J.F. Ready, Effects of high-power laser radiation (Academic Press, New York – London, 1971)
[2] D. Bauerle, Laser processing and chemistry
(Springer-Verlag, Berlin – Heidelberg – New
York, 2000)
[3] I.M. Popescu et. al., Aplicatii ale laserilor, (Ed.
Tehnica, Bucuresti, 1979, in Romanian)
[4] D. Bauerle, Proceedings of an International Conference Laser Processing and Diagnostics, University of
Linz (Springer-Verlag, Berlin Heidelberg, 1984)
[5] M. von Allmen, A. Blatter, Laser-Beam Interactions
with Materials (Springer-Verlag, Berlin, 1995)
[6] I. Ursu, I. N. Mihailescu, A. M. Prokhorov, V. I.
Konov, Interactiunea radiatiei laser cu metalele (Editura Academiei R.S.R., Bucuresti, 1986, in Romanian)
[7] P. Simon, J. Ihlemann, Appl. Phys. A 63, 505 (1996)
[8] B.N. Chichkov et al., Appl. Phys. A 63, 109 (1996)
[9] A.E. Wynne, B.C. Stuart, Appl. Phys. A 76, 373 (2003)
[10] S.M. Klimentov et al., Physics of Wave Phenomena
15, 1 (2007)
[11] S. Amoruso, R. Bruzzese, N. Spinelli, R. Velotta, J.
Phys. B 32, 131 (1999)
[12] B.W. Rottke, J. Ihlemann, H. Schmidt, A. Scholl, Appl.
Phys. A 60, 13 (1995)
[13] A. Bogaerts, Z. Chen, Spectrochimica Acta B 60, 1280
(2005)
[14] M. Stafe, C. Negutu, I.M. Popescu, Shock Waves 14,
123 (2005)
[15] M. Stafe, C. Negutu, I.M. Popescu, Appl. Surf. Sci.
253, 6353 (2007)
[16] N.M. Bulgakova, A.V. Bulgakov, Appl. Phys. A 73, 199
(2001)
[17] B. Garrison, T. Itina, L. Zhigilei, Phys. Rev. E 68,
041501 (2003)
[18] E.G. Gamaly, A.V. Rode, A. Perrone, A. Zocco, Appl.
Phys. A 73, 143 (2001)
[19] C. Porneala, D.A. Willisa, Appl. Phys. Lett. 89, 211121
(2006)
[20] J.M. Fishburn, M.J. Withford, D.W. Coutts, J.A. Piper,
Appl. Optics 43, 6473 (2004)
[21] M.R.H. Knowles, G. Rutterford, D. Karnakis, A. Ferguson, The International Journal of Advanced Manufacturing Technology 33, 95 (2007)
[22] A. Semerok et al., Appl. Surf. Sci. 138–139, 311 (1999)
[23] S. Amoruso, M. Armenante, V. Berardi, R. Bruzzese,
N. Spinelli, Appl. Phys. A 65, 265 (1997)
[24] Q. Lu, Phys. Rev. E 67, 016410 (2003)
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