- Email: [email protected]
Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers
Applied Surface Science 278 (2013) 255–258
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers Pierre Lorenz ∗ , Martin Ehrhardt, Klaus Zimmer Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, 04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 15 June 2012 Received in revised form 12 November 2012 Accepted 13 November 2012 Available online 23 November 2012 Keywords: Laser etching Fused silica Fs laser Absorber layer LIFE
a b s t r a c t Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation ( = 780 nm, tp = 150 fs, f = 1 kHz). The etching process of fused silica is studied in dependence on the laser fluence, the laser pulse numbers as well as metal absorber layer thicknesses. Especially the influence of the varied parameters on the surface morphology, the etching depth, and the surface modification is presented and discussed. The etched trenches were analyzed with white light interferometry. A fluence window from 0.5 to 2.5 J/cm2 for the LIFE process of fused silica with a single laser pulse using a metal layer with a thickness from 5 to 50 nm was found. The measured maximum etching depth of approximately 100 nm suggests that the LIFE process is appropriated for precision machining. The attempt to simulate the fs laser LIFE process by a thermodynamic model was moderate successfully and exhibits significant differences within the experimental results. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The surface structuring of dielectrics by laser irradiation has an outstanding potential for fast, easy and nm-precision fabrication of free-formed dielectric surfaces, e.g. for micro-optical applications. However, the precise machining of transparent materials with a well-defined etching depth and a low surface roughness is a challenge for laser processing. The dielectric materials can be structured by direct ablation using VUV, ps, and fs lasers [1,2] and by excimer laser radiation at high laser fluences [3–5]. However, the direct ablation method tends to an increased surface roughness. For example, the direct ablation process of fused silica using fs laser radiation was manifold studied [3,6–9]. Further options for precise patterning offer methods using specific surface modifications for enhancing the low absorption of dielectrics. Examples for rear side etching techniques are laser-induced back side wet etching (LIBWE), laser etching at a surface-adsorbed layer (LESAL), and laser-induced back side dry etching (LIBDE) [10–13]. However, for such applications like, e.g. glass fibre structuring the front side patterning is favourable. Thus, laser-induced front side etching (LIFE) was developed that is making use of thin
∗ Corresponding author. Tel.: +49 0341 235 3291; fax: +49 0341 235 2584. E-mail address: [email protected] (P. Lorenz). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.047
adsorbing layers to absorb the laser radiation near the front side surface and enables a high surface quality of the etching region especially with materials possessing a high absorption coefficient. The fs laser–solid interaction can be estimated by a thermodynamic model under the assumption of a two-temperature model whereat the thermodynamic approach is discussed for homogeneous bulk materials by, e.g. Chichkov et al. [14]. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation. 2. Experimental set-up Fused silica (FS) was selected for this study due to the significance for optical applications. The polished surface of the samples was coated by magnetron-sputtering with different thick layers of chromium (z = 5 nm, 20 nm and 50 nm). The metal-layer-covered fused silica surface exhibits a surface roughness of <2 nm rms. The chromium-covered surfaces were irradiated with fs laser radiation. As laser source, a Ti:sapphire laser with a wavelength of 780 nm, a pulse duration of 150 fs, a repetition rate of 1 kHz, and a nearly Gaussian profile is focussed onto the sample surface by a lens with a focal length of 60 mm. The focussed laser beam on the sample surface exhibits a Gaussian radius of (12 ± 1) m. At 780 nm laser wavelength the chromium layer exhibits a reflectivity R of 0.57 (for bulk materials) and an absorption coefficient ˛ of 5.55 × 105 cm−1 (˛−1 ≈ 18 nm) [15].
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P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
specified at the position of the maximum of the laser fluence (or in other words: in the middle of the Gaussian laser beam profile) whereat the shown etching depth was calculated by the arithmetic average of the etching depth of several single ablation holes. Furthermore, the surface morphology was also measured by WLI images at selected laser fluences which are summarized in Fig. 2. The etching depth behaviour can be separated into three different parts:
z = 5 nm
100
etching depth d [nm]
50 0
z = 20 nm
100 50 0
z = 50 nm
100 50
I
II III
0 0
1
laser fluence
2
3
[J/cm ]
Fig. 1. Etching depth in fused silica with fs laser from WLI measurements; dependence on the laser power ˚ at the pulse number N = 1 for different metal layer thickness (z = 5 nm, 20 nm, 50 nm). The etching depth–laser fluence dependency was fitted by a linear function d = ım (˚ − ˚th ).
The laser fluences as well as pulse numbers per area were varied. The etching depth, the surface roughness as well as the surface morphology was measured by white light interferometry (WLI). 3. Results The chromium/fused silica system was irradiated with different laser fluences ˚ and different chromium layer thicknesses z. The etching depths are summarized in Fig. 1. The etching depth was
- At the low fluence regime (part I) ˚ < 0.5 J/cm2 (for one laser pulse (N = 1)), the laser irradiation tends to a modification and partial removal of the metal layer but no etching of the fused silica can be detected. - The moderate fluence 0.5 < ˚ < 2.6 J/cm2 irradiation (part II) tends to a well-defined removal of the fused silica with a low surface roughness whereat the etching profile is given by the nearly Gaussian laser profile (see Fig. 2). The presented waviness of the measured etching trenches is induced by the partial overlap of the single pulses. Furthermore, for all used metal layer thicknesses the measured etching depth in fused silica is linearly to the laser fluences. The etching depth–laser fluence dependence can be analytically described and fitted by: d = ım · (˚ − ˚th )
(1)
with ım : slope coefficient and ˚th : ablation threshold. These first results disclose that the plot parameters are independent on the metal layer thicknesses for metal layer from 5 to 50 nm. A slope coefficient ım of (30 ± 5) nm/(J/cm2 ) and an ablation threshold ˚th of (0.4 ± 0.2) J/cm2 were calculated. The part II is the process window for the LIFE process. - The high laser fluence irradiation (˚ > 2.6 J/cm2 ) tends to undefined etching regions with a high surface roughness and a high ablation rate due to the direct ablation process of the fused silica [16], whereat the surface roughness increases with increasing laser fluences.
Fig. 2. WLI measurements: surface morphology of the fused silica etching region with high (˚ = 12 J/cm2 ) and moderate laser fluences (˚ = 1.76 J/cm2 ) at fs laser radiation ( = 780 nm; 1 pulse; tp = 150 fs), ((i) inverted 3D etching profile).
P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
Furthermore, the direct ablation process exhibits a significant incubation effect, the direct ablation threshold ˚DA presents a disth tinct pulse number dependency; it is [16]: ˚DA (N) = 2.6 J/cm2 · N −0.39 th
(2)
The experimental found values agree with published results [17]. This effect tends to a reduction of the laser fluence range for the LIFE process window for a pulse overlap of more than 1. Already with the fourth pulse, the laser fluence range for the LIFE process is reduced by more than 50%. 4. Discussion The etching depth was measured by WLI and a linear behaviour of the etching depth–laser fluence dependency and an independency of the etching depth on the metal layer thickness were found. For ns laser radiation, a linear etching depth–laser fluence dependency was found, too [16,18]. However, the nanosecond LIFE process presented a distinct and complex etching depth–layer thickness dependency [16,18] whereas the femtosecond LIFE process exhibits no measurable etching depth–layer thickness dependency. Furthermore, the ultrashort (fs) laser pulse–solid interaction exhibits distinct incubation [16] in contrast to the ns laser pulse–solid interaction. The ns LIFE process can be described by a simple thermodynamic model quantitatively well [18]. For the fs LIFE process, a twotemperature model [14] must be regarded. Under the assumption that the laser-induced excitation of the electrons and the subsequent relaxation is much faster than the thermal diffusion, the temperature distribution in the metal layer induced by the laser radiation can be separately treated from the thermal diffusion. In approximation, the temperature distribution in the metal layer (a few picoseconds after the laser shot (the electron relaxation time)) can be described by [14]: Ti ≈ Te2 (L )
Ce 2Ci
≈
Fa ˛ −˛z e Ci
(3)
with Ti : lattice temperature, Te : electron temperature, L : laser pulse duration, Ce × T : electron heat capacity, Ci : lattice heat capacity Fa : absorbed laser fluence and ˛: absorption coefficient, z: depth. In the second step, the thermal diffusion in the Cr/SiO2 system can be calculated by solving the heat equation for different laser fluences and metal layer thicknesses under the assumption of the fs laser imprinted temperature profile as initial condition. Furthermore, under the assumption that the etching depth is defined by the maximum depth zmax where the temperature achieved the evaporation temperature Teva of 2503 K, this very simple estimation works very well at the ns laser simulation [18]. In summary, the resulting etching depth d dependence on the laser fluence ˚ and the metal layer thickness z can be estimated by an analytical equation. It is [16]:
linear etching depth–laser fluence dependency in agreement with the thermodynamic estimation. However, the predicted etching depth–metal layer thickness dependency cannot be found experimentally. Therefore, the secondary processes must be discussed for the explanation of the ablation process. The most expectable process seems to be the interaction of the laser-induced plasma of the laser plume with the fused silica after the laser pulse which is finally causing phase transition including evaporation. The melting depth zm can be estimated from the energy balance of the laser pulse (˚q : part of the laser fluence which induces the heating of the material) and the melting enthalpy according to zm ≈
˚q · H
(5)
with a density = 2648 kg/m3 and a melting enthalpy H ≈ 2.1 J/kg [19]. The comparison of the measured single pulse etching depths of ∼20 nm at 1 J/cm2 with the estimated melting depth considering the full pulse energy density and inducing the materials melting of 18 m shows a clear disagreement. In consequence, this cannot be the dominating mechanism. Hence, the etching mechanism is probably a laser-induced plasma-thermal etching mechanism after the ultrashort laser pulse interaction with the metal film. However, the evaluation of the processes involved in surface heating by laser-induced plasma is very complex and depends on very different parameters.At the fs LIFE process, the ablation process is most likely more dominated by the plasma formation and the plasma–solid interaction than the ns LIFE process due to the shorter pulse length. A further extension of the theoretical model of LIFE with fs laser radiation by plasma formation as well as plasma–solid interaction will be necessary. 5. Conclusion Beside the LIFE process with ns laser radiation [16,18] also the usage of fs laser radiation allows the fabrication of well-defined, nm-precision etching trenches with the LIFE process. The etching depth behaviour can be separated into three different parts: a low (only modification of the metal layer, no damage of FS), a moderate (well-defined removal of FS, etching depth linear to laser fluence) and a higher laser fluence regime (direct ablation process, increased surface roughness). The moderate laser fluence regime is the process window for the LIFE process and etching depths up to 100 nm can be achieved. The theoretical estimation can describe the behaviour of the etching depth–laser fluence dependency well. However, the theoretical predicted etching depth–layer thickness dependency was not found experimentally. A further extension of the theoretical model by plasma formation as well as plasma–solid interaction will be necessary. Acknowledgement
d(˚, z) ≈ ı0 · (˚ − (˚0th + A˚ · z)) · (1 − exp(−˛ı · z)) ·(1 − R∞ () · [1 − () · exp(−2 · ˛() · z)])
257
(4)
with ı0 : slope coefficient, ˚0th : ablation threshold for a metal layer with a thickness of zero, A˚ : slope coefficient of the ablation threshold, ˛ı : exponent parameter, R∞ : bulk reflectivity, : function dependent on the complex refractive index of the chromium and fused silica [18] and ˛: absorption coefficient of the metal layer.The thermodynamic approach for the quantitative description of the ns laser radiation interaction works well [18]. However, the thermodynamic results for the fs laser radiation interaction show that the ablation process cannot be explained only due to a thermodynamic approach. The measurements presented a
We appreciate the support by the DFG under grant ZI 660/9-1. References [1] P.R. Herman, R.S. Marjoribanks, A. Oettl, K.P. Chen, I. Konovalov, S. Ness, Applied Surface Science 154–155 (2000) 577–586. [2] M. Lenzner, J. Krüger, W. Kautek, F. Krausz, Applied Physics A 68 (3) (1999) 369–371. [3] J. Ihlemann, B. Wolff, P. Simon, Applied Physics A 54 (4) (1992) 363–368. [4] J. Ihlemann, B. Wolff-Rottke, Applied Surface Science 106 (1996) 282–286. [5] J. Ihlemann, Applied Surface Science 54 (1992) 193–200. [6] D.M. Krol, Journal of Non-Crystalline Solids 354 (2008) 416–424. [7] J. Gottmann, D. Wortmann, M. Hörstmann-Jungemann, Applied Surface Science 255 (2009) 5641–5646.
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P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
[8] M. Dubov, V. Mezentsev, I. Bennion, D.N. Nikogosyan, Measurement Science and Technology 18 (2007) L15–L17. [9] J. Ihlemann, A. Scholl, H. Schmidt, B. Woltf-Rottke, Applied Physics A 60 (1995) 411–417. [10] R. Böhme, K. Zimmer, D. Ruthe, B. Rauschenbach, Journal of Laser Micro/Nanoengineering 1 (2006) 190. [11] K. Zimmer, R. Böhme, C. Vass, B. Hopp, Applied Surface Science 255 (2009) 9617. [12] M. Ehrhardt, G. Raciukaitis, P. Gecys, K. Zimmer, Applied Physics A 101 (2010) 399. [13] R. Böhme, Laser-induced backside wet etching of glasses and crystals, Dissertation. urn:nbn:de:gbv:3-0000011953.
[14] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. Tünnermann, Applied Physics A 63 (2) (1996) 109–115. [15] Landolt-Börnstein Database Springer Materials, Springer, Berlin/Heidelberg/ New York, 2010 www.springermaterials.com [16] P. Lorenz, M. Ehrhardt, K. Zimmer, Conference Proceeding, Proceedings of the SPIE, vol. 8243, 2012, p. 82430Y. [17] B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchick, B.W. Shore, M.D. Perry, Physical Review B 53 (4) (1996) 1749–1761. [18] P. Lorenz, M. Ehrhardt, A. Wehrmann, K. Zimmer, Applied Surface Science 258 (2012) 9138. [19] N.G. Ainslie, J.D. Mackenzie, D. Turnbull, Journal of Physical Chemistry 65 (10) (1961) 1718–1724.
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Laser-induced front side etching of fused silica with femtosecond laser radiation using thin metal layers Pierre Lorenz ∗ , Martin Ehrhardt, Klaus Zimmer Leibniz-Institut für Oberflächenmodifizierung e. V., Permoserstr. 15, 04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 15 June 2012 Received in revised form 12 November 2012 Accepted 13 November 2012 Available online 23 November 2012 Keywords: Laser etching Fused silica Fs laser Absorber layer LIFE
a b s t r a c t Laser-induced front side etching (LIFE) is a method for laser etching of transparent materials using thin absorber layers. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation ( = 780 nm, tp = 150 fs, f = 1 kHz). The etching process of fused silica is studied in dependence on the laser fluence, the laser pulse numbers as well as metal absorber layer thicknesses. Especially the influence of the varied parameters on the surface morphology, the etching depth, and the surface modification is presented and discussed. The etched trenches were analyzed with white light interferometry. A fluence window from 0.5 to 2.5 J/cm2 for the LIFE process of fused silica with a single laser pulse using a metal layer with a thickness from 5 to 50 nm was found. The measured maximum etching depth of approximately 100 nm suggests that the LIFE process is appropriated for precision machining. The attempt to simulate the fs laser LIFE process by a thermodynamic model was moderate successfully and exhibits significant differences within the experimental results. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The surface structuring of dielectrics by laser irradiation has an outstanding potential for fast, easy and nm-precision fabrication of free-formed dielectric surfaces, e.g. for micro-optical applications. However, the precise machining of transparent materials with a well-defined etching depth and a low surface roughness is a challenge for laser processing. The dielectric materials can be structured by direct ablation using VUV, ps, and fs lasers [1,2] and by excimer laser radiation at high laser fluences [3–5]. However, the direct ablation method tends to an increased surface roughness. For example, the direct ablation process of fused silica using fs laser radiation was manifold studied [3,6–9]. Further options for precise patterning offer methods using specific surface modifications for enhancing the low absorption of dielectrics. Examples for rear side etching techniques are laser-induced back side wet etching (LIBWE), laser etching at a surface-adsorbed layer (LESAL), and laser-induced back side dry etching (LIBDE) [10–13]. However, for such applications like, e.g. glass fibre structuring the front side patterning is favourable. Thus, laser-induced front side etching (LIFE) was developed that is making use of thin
∗ Corresponding author. Tel.: +49 0341 235 3291; fax: +49 0341 235 2584. E-mail address: [email protected] (P. Lorenz). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.11.047
adsorbing layers to absorb the laser radiation near the front side surface and enables a high surface quality of the etching region especially with materials possessing a high absorption coefficient. The fs laser–solid interaction can be estimated by a thermodynamic model under the assumption of a two-temperature model whereat the thermodynamic approach is discussed for homogeneous bulk materials by, e.g. Chichkov et al. [14]. In this study, chromium layers were applied as absorber for etching trenches in fused silica with femtosecond Ti:sapphire laser radiation. 2. Experimental set-up Fused silica (FS) was selected for this study due to the significance for optical applications. The polished surface of the samples was coated by magnetron-sputtering with different thick layers of chromium (z = 5 nm, 20 nm and 50 nm). The metal-layer-covered fused silica surface exhibits a surface roughness of <2 nm rms. The chromium-covered surfaces were irradiated with fs laser radiation. As laser source, a Ti:sapphire laser with a wavelength of 780 nm, a pulse duration of 150 fs, a repetition rate of 1 kHz, and a nearly Gaussian profile is focussed onto the sample surface by a lens with a focal length of 60 mm. The focussed laser beam on the sample surface exhibits a Gaussian radius of (12 ± 1) m. At 780 nm laser wavelength the chromium layer exhibits a reflectivity R of 0.57 (for bulk materials) and an absorption coefficient ˛ of 5.55 × 105 cm−1 (˛−1 ≈ 18 nm) [15].
256
P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
specified at the position of the maximum of the laser fluence (or in other words: in the middle of the Gaussian laser beam profile) whereat the shown etching depth was calculated by the arithmetic average of the etching depth of several single ablation holes. Furthermore, the surface morphology was also measured by WLI images at selected laser fluences which are summarized in Fig. 2. The etching depth behaviour can be separated into three different parts:
z = 5 nm
100
etching depth d [nm]
50 0
z = 20 nm
100 50 0
z = 50 nm
100 50
I
II III
0 0
1
laser fluence
2
3
[J/cm ]
Fig. 1. Etching depth in fused silica with fs laser from WLI measurements; dependence on the laser power ˚ at the pulse number N = 1 for different metal layer thickness (z = 5 nm, 20 nm, 50 nm). The etching depth–laser fluence dependency was fitted by a linear function d = ım (˚ − ˚th ).
The laser fluences as well as pulse numbers per area were varied. The etching depth, the surface roughness as well as the surface morphology was measured by white light interferometry (WLI). 3. Results The chromium/fused silica system was irradiated with different laser fluences ˚ and different chromium layer thicknesses z. The etching depths are summarized in Fig. 1. The etching depth was
- At the low fluence regime (part I) ˚ < 0.5 J/cm2 (for one laser pulse (N = 1)), the laser irradiation tends to a modification and partial removal of the metal layer but no etching of the fused silica can be detected. - The moderate fluence 0.5 < ˚ < 2.6 J/cm2 irradiation (part II) tends to a well-defined removal of the fused silica with a low surface roughness whereat the etching profile is given by the nearly Gaussian laser profile (see Fig. 2). The presented waviness of the measured etching trenches is induced by the partial overlap of the single pulses. Furthermore, for all used metal layer thicknesses the measured etching depth in fused silica is linearly to the laser fluences. The etching depth–laser fluence dependence can be analytically described and fitted by: d = ım · (˚ − ˚th )
(1)
with ım : slope coefficient and ˚th : ablation threshold. These first results disclose that the plot parameters are independent on the metal layer thicknesses for metal layer from 5 to 50 nm. A slope coefficient ım of (30 ± 5) nm/(J/cm2 ) and an ablation threshold ˚th of (0.4 ± 0.2) J/cm2 were calculated. The part II is the process window for the LIFE process. - The high laser fluence irradiation (˚ > 2.6 J/cm2 ) tends to undefined etching regions with a high surface roughness and a high ablation rate due to the direct ablation process of the fused silica [16], whereat the surface roughness increases with increasing laser fluences.
Fig. 2. WLI measurements: surface morphology of the fused silica etching region with high (˚ = 12 J/cm2 ) and moderate laser fluences (˚ = 1.76 J/cm2 ) at fs laser radiation ( = 780 nm; 1 pulse; tp = 150 fs), ((i) inverted 3D etching profile).
P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
Furthermore, the direct ablation process exhibits a significant incubation effect, the direct ablation threshold ˚DA presents a disth tinct pulse number dependency; it is [16]: ˚DA (N) = 2.6 J/cm2 · N −0.39 th
(2)
The experimental found values agree with published results [17]. This effect tends to a reduction of the laser fluence range for the LIFE process window for a pulse overlap of more than 1. Already with the fourth pulse, the laser fluence range for the LIFE process is reduced by more than 50%. 4. Discussion The etching depth was measured by WLI and a linear behaviour of the etching depth–laser fluence dependency and an independency of the etching depth on the metal layer thickness were found. For ns laser radiation, a linear etching depth–laser fluence dependency was found, too [16,18]. However, the nanosecond LIFE process presented a distinct and complex etching depth–layer thickness dependency [16,18] whereas the femtosecond LIFE process exhibits no measurable etching depth–layer thickness dependency. Furthermore, the ultrashort (fs) laser pulse–solid interaction exhibits distinct incubation [16] in contrast to the ns laser pulse–solid interaction. The ns LIFE process can be described by a simple thermodynamic model quantitatively well [18]. For the fs LIFE process, a twotemperature model [14] must be regarded. Under the assumption that the laser-induced excitation of the electrons and the subsequent relaxation is much faster than the thermal diffusion, the temperature distribution in the metal layer induced by the laser radiation can be separately treated from the thermal diffusion. In approximation, the temperature distribution in the metal layer (a few picoseconds after the laser shot (the electron relaxation time)) can be described by [14]: Ti ≈ Te2 (L )
Ce 2Ci
≈
Fa ˛ −˛z e Ci
(3)
with Ti : lattice temperature, Te : electron temperature, L : laser pulse duration, Ce × T : electron heat capacity, Ci : lattice heat capacity Fa : absorbed laser fluence and ˛: absorption coefficient, z: depth. In the second step, the thermal diffusion in the Cr/SiO2 system can be calculated by solving the heat equation for different laser fluences and metal layer thicknesses under the assumption of the fs laser imprinted temperature profile as initial condition. Furthermore, under the assumption that the etching depth is defined by the maximum depth zmax where the temperature achieved the evaporation temperature Teva of 2503 K, this very simple estimation works very well at the ns laser simulation [18]. In summary, the resulting etching depth d dependence on the laser fluence ˚ and the metal layer thickness z can be estimated by an analytical equation. It is [16]:
linear etching depth–laser fluence dependency in agreement with the thermodynamic estimation. However, the predicted etching depth–metal layer thickness dependency cannot be found experimentally. Therefore, the secondary processes must be discussed for the explanation of the ablation process. The most expectable process seems to be the interaction of the laser-induced plasma of the laser plume with the fused silica after the laser pulse which is finally causing phase transition including evaporation. The melting depth zm can be estimated from the energy balance of the laser pulse (˚q : part of the laser fluence which induces the heating of the material) and the melting enthalpy according to zm ≈
˚q · H
(5)
with a density = 2648 kg/m3 and a melting enthalpy H ≈ 2.1 J/kg [19]. The comparison of the measured single pulse etching depths of ∼20 nm at 1 J/cm2 with the estimated melting depth considering the full pulse energy density and inducing the materials melting of 18 m shows a clear disagreement. In consequence, this cannot be the dominating mechanism. Hence, the etching mechanism is probably a laser-induced plasma-thermal etching mechanism after the ultrashort laser pulse interaction with the metal film. However, the evaluation of the processes involved in surface heating by laser-induced plasma is very complex and depends on very different parameters.At the fs LIFE process, the ablation process is most likely more dominated by the plasma formation and the plasma–solid interaction than the ns LIFE process due to the shorter pulse length. A further extension of the theoretical model of LIFE with fs laser radiation by plasma formation as well as plasma–solid interaction will be necessary. 5. Conclusion Beside the LIFE process with ns laser radiation [16,18] also the usage of fs laser radiation allows the fabrication of well-defined, nm-precision etching trenches with the LIFE process. The etching depth behaviour can be separated into three different parts: a low (only modification of the metal layer, no damage of FS), a moderate (well-defined removal of FS, etching depth linear to laser fluence) and a higher laser fluence regime (direct ablation process, increased surface roughness). The moderate laser fluence regime is the process window for the LIFE process and etching depths up to 100 nm can be achieved. The theoretical estimation can describe the behaviour of the etching depth–laser fluence dependency well. However, the theoretical predicted etching depth–layer thickness dependency was not found experimentally. A further extension of the theoretical model by plasma formation as well as plasma–solid interaction will be necessary. Acknowledgement
d(˚, z) ≈ ı0 · (˚ − (˚0th + A˚ · z)) · (1 − exp(−˛ı · z)) ·(1 − R∞ () · [1 − () · exp(−2 · ˛() · z)])
257
(4)
with ı0 : slope coefficient, ˚0th : ablation threshold for a metal layer with a thickness of zero, A˚ : slope coefficient of the ablation threshold, ˛ı : exponent parameter, R∞ : bulk reflectivity, : function dependent on the complex refractive index of the chromium and fused silica [18] and ˛: absorption coefficient of the metal layer.The thermodynamic approach for the quantitative description of the ns laser radiation interaction works well [18]. However, the thermodynamic results for the fs laser radiation interaction show that the ablation process cannot be explained only due to a thermodynamic approach. The measurements presented a
We appreciate the support by the DFG under grant ZI 660/9-1. References [1] P.R. Herman, R.S. Marjoribanks, A. Oettl, K.P. Chen, I. Konovalov, S. Ness, Applied Surface Science 154–155 (2000) 577–586. [2] M. Lenzner, J. Krüger, W. Kautek, F. Krausz, Applied Physics A 68 (3) (1999) 369–371. [3] J. Ihlemann, B. Wolff, P. Simon, Applied Physics A 54 (4) (1992) 363–368. [4] J. Ihlemann, B. Wolff-Rottke, Applied Surface Science 106 (1996) 282–286. [5] J. Ihlemann, Applied Surface Science 54 (1992) 193–200. [6] D.M. Krol, Journal of Non-Crystalline Solids 354 (2008) 416–424. [7] J. Gottmann, D. Wortmann, M. Hörstmann-Jungemann, Applied Surface Science 255 (2009) 5641–5646.
258
P. Lorenz et al. / Applied Surface Science 278 (2013) 255–258
[8] M. Dubov, V. Mezentsev, I. Bennion, D.N. Nikogosyan, Measurement Science and Technology 18 (2007) L15–L17. [9] J. Ihlemann, A. Scholl, H. Schmidt, B. Woltf-Rottke, Applied Physics A 60 (1995) 411–417. [10] R. Böhme, K. Zimmer, D. Ruthe, B. Rauschenbach, Journal of Laser Micro/Nanoengineering 1 (2006) 190. [11] K. Zimmer, R. Böhme, C. Vass, B. Hopp, Applied Surface Science 255 (2009) 9617. [12] M. Ehrhardt, G. Raciukaitis, P. Gecys, K. Zimmer, Applied Physics A 101 (2010) 399. [13] R. Böhme, Laser-induced backside wet etching of glasses and crystals, Dissertation. urn:nbn:de:gbv:3-0000011953.
[14] B.N. Chichkov, C. Momma, S. Nolte, F. von Alvensleben, A. Tünnermann, Applied Physics A 63 (2) (1996) 109–115. [15] Landolt-Börnstein Database Springer Materials, Springer, Berlin/Heidelberg/ New York, 2010 www.springermaterials.com [16] P. Lorenz, M. Ehrhardt, K. Zimmer, Conference Proceeding, Proceedings of the SPIE, vol. 8243, 2012, p. 82430Y. [17] B.C. Stuart, M.D. Feit, S. Herman, A.M. Rubenchick, B.W. Shore, M.D. Perry, Physical Review B 53 (4) (1996) 1749–1761. [18] P. Lorenz, M. Ehrhardt, A. Wehrmann, K. Zimmer, Applied Surface Science 258 (2012) 9138. [19] N.G. Ainslie, J.D. Mackenzie, D. Turnbull, Journal of Physical Chemistry 65 (10) (1961) 1718–1724.