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Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique
Applied Surface Science 253 (2007) 8059–8063 www.elsevier.com/locate/apsusc
Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique Cs. Vass a,*, K. Osvay a, M. Csete a, B. Hopp b b
a Department of Optics and Quantum Electronics, University of Szeged, H-6720 Szeged, Do´m te´r 9, Hungary Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Do´m te´r 9, Hungary
Available online 25 February 2007
Abstract A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: l = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2u = 288) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methylmethacrylate c = 1.85 mol/dm3). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180–200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser induced backside wet etching; Two-beam interference; Grating fabrication
1. Introduction The laser-induced backside wet etching (LIBWE) technique [1] is an intensively studied, promising procedure for micromachining of transparent materials. The most UV transparent materials (fused silica, quartz crystal, pyrex, CaF2, BaF2, sapphire and other glass types [1–4]) can be micro-machined by this method. The LIBWE has some important advantages, which makes it more beneficial than the direct ablative machining method, as low threshold fluence, the fine etch depth control, the debris-free etched holes, and the large etched area. The mechanism of the LIBWE is considerably complex; this process has already been detailed in several studies. According to these, the material removal from the transparent target surface is induced by the high temperature (softening or melting the target) [5], the mechanical attack of the liquid vapor bubble and jet [6–9], and the chemical modification of the thin contacting layer of the transparent material (carbon contaminate from the
* Corresponding author. E-mail address: [email protected] (C. Vass). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.087
liquid) [10] being responsible for the ablation-like material removal. One of the most important questions from applications’ point of view is whether this machining method is suited for efficient and competitive production of micro- and submicrometer structures into transparent targets. There are only a few studies which reported gratings with micrometer size period produced by LIBWE technique. Zimmer et al. [11] and Bo¨hme et al. [12] produced 787 and 760 nm surface relief grating by mask projection in fused silica transparent target. Ding et al. also presented their results with similar structure dimension (grating period: 750 nm) [9]. Moreover, Ihlemann et al. produced 830 nm fused silica grating using F2 laser ablation and mask projection principle [13]. Since the cost and complication of a usual VUV laser system cannot be widely afforded, the LIBWE technique could become a popular tool in many laboratories. On the other hand, the projection technique (which is common in the above referred excellent studies) limits the effectivity of this method. In this paper, we combined the two-beam interference method with the LIBWE technique eliminating the mask projection. This new method makes possible to produce fused silica grating with a period smaller than that previously
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Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
reported [9]. During our experiments we fabricated fused silica grating relieves with 550 nm period, which are the smallest LIBWE generated gratings at present. The dependence of the grating parameters on the most important etching features are also presented. 2. Experimental The transparent targets were fused silica plates with a thickness of 1 mm (Suprasil II, Heraeus). Naphthalene dissolved in methyl-methacrylate with c = 1.85 mol/dm3 concentration (saturated solution) was used as liquid absorbent. In our experiments basically a Q-switched, frequency-doubled Nd:YAG laser (pulse duration: FWHM = 10 ns, repetition rate: 10 Hz) was used in combination of the traditional two-beam interference method. In order to produce as uniform structure as possible, the profile of the laser beam was smoothed in two steps. First, the second harmonic beam at 532 nm was spatially
Fig. 1. Scheme of the experimental set-up.
Fig. 2. AFM images about the produced grating relief. The dimension of each image is 10 mm 10 mm. In the right top corners of the images are denoted the maximum value of the Z range.
Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
filtered. Then, following the external fourth harmonic generation stage consisting of a CLBO crystal, the ultimate Supergaussian-like spatial profile of the 266 nm pulses were formed in a second spatial filter just prior to the beam splitter. The s-polarized laser beam was split into two parts, which were interfered at a certain incident angle on the backside surface of the fused silica plate contacting with the liquid absorber (Fig. 1). The interference fringe separation ( p) determines the period of the grating [14]:
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etched depth was measured by a Dektak 8 profilometer operated with a tip having 2.5 mm radius. 3. Results and discussion
where l is the wavelength in air and u is the incident angle of the laser beam at the boundary of air and the fused silica plate. The u was 148 in our experiments resulted 550 nm interference fringe separation. The numbers of laser pulses were varied from 50 to 1800 (50, 100, 300, 600, 1200, 1800, respectively), while the applied fluences were adjusted from 285 to 680 mJ/cm2 (285, 330, 420, 510, 590, 680, respectively) by a rotating halfwave plate. The diameter of irradiated area was 0.85 mm. The morphology of the gratings was studied by a PSIA XE100 atomic force microscope (AFM) in non contact mode. The
The LIBWE threshold fluence for fused silica was measured and found to be 285 mJ/cm2, since this value was the minimum fluence, which caused etching during our experiments. The AFM images about the produced gratings can be seen in Fig. 2. The maximum values of the Z range are denoted in the right top corner of the images (see the explanation in the rightbottom corner of the full image). The first qualitative analysis of the images shows that the quality of the gratings decreases due to the increasing of pulse number and/or the laser fluence. They become wavy with much longer wavelength than the grating period, and the grating grooves are ‘broken’ at the highest pulse numbers and fluences. One of our aims was to characterize these relieves for the correct quantitative analysis. One of the most important grating parameters is the modulation depth, which found to be in the range of 50– 200 nm. The results are represented in Fig. 3a and b. The maximum of the modulation depths in the function of the number of pulses is at around 100 pulses in the most cases. This
Fig. 3. The modulation depth vs. number of laser pulses (a) and laser fluence (b).
Fig. 4. The Ra parameter of the low-pass filtered images vs. number of laser pulses (a) and laser fluence (b).
p¼
l ; 2 sin u
(1)
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Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
value is important in the future applications, where the high modulation depth is an essential requirement. The dependence of the modulation depth on the fluence shows apparent irregularities: one extreme and a local minimum at 330 and 510 mJ/cm2, respectively. The first can be probably originated from the change of the etching mechanism due to probably the beginning of the melting of a thin layer of the fused silica surface [6]. The low frequency waves of the grating can be characterized by the roughness parameter (Ra) of the surface without the grating structures. Therefore, the AFM images were low-pass Fourier-filtered to remove the grating (cutoff below 1430 nm wavelength), and after that the Ra parameter was measured. In Fig. 4a and b can be seen the roughness parameter of the base surface of the gratings. The quality differences between the gratings also can bee seen qualitatively on the AFM images. The Ra of the filtered images increases with increasing laser fluence, which shows agreement with the earlier results about the etched surface roughness without grating at the low fluence region [4,7]. The etch depth measurements help to understand the reason of the rough surface in the case of high fluence and large number of pulses. The etch depth changes from 18 nm to 30–40 mm (Fig. 5a and b). The increase of the thickness of the
Fig. 6. An AFM image (a) and a cross section (b) about our best quality 550 nm period fused silica grating (produced at F = 330 mJ/cm2; 50 pulses).
removed material (etched depth) – using higher pulse number and/or higher laser fluence – enhances the original unevenness (low frequency roughness) of the fused silica surface, which can be seen with the comparison of Figs. 4 and 5. The braking of the grating lines (see the AFM images at right-bottom corner of Fig. 2) probably can be attributed to the partially molten target surface and grating lines. The observable rounded-end line pieces also indicate the melting of the grating lines. The final question is obvious: which are the best etching parameters for the production of best quality grating? The etch depth and the low frequency roughness should be minimal in this case. In our experiment, these conditions are realized in the case of 50 pulses and 330 mJ/cm2 (etched depth = 18 nm, Ra = 2.1 nm), providing a modulation depth of 120 nm. Although this value is not of the maximal, but at higher modulation depth the quality of the grating is degraded. The maximum modulation depth of the gratings was 200 nm, which was achieved using 100 pulses at 420 mJ/cm2. Fig. 6 shows an AFM image and a cross section of the nearly faultless grating relief surface. Our gratings can be used as UV transmission gratings. In our test carried out with the 266 nm laser beam, the first and the second order of the diffracted beams could be seen at approximately 29–308 and 758, respectively. 4. Conclusions
Fig. 5. The etched depth of the holes vs. number of laser pulses (a) and laser fluence (b).
We introduced a new method to produce submicrometer gratings in fused silica surface: a traditional two-beam interference arrangement was combined with the technique of laser induced backside wet etching. High quality fused silica grating relieves having 550 nm period were fabricated, which are the smallest LIBWE generated gratings at present. These results reveal that our combined method is well suited for
Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
fabrication of surface relief grating in fused silica and presumably other transparent dielectrics. The optimal etching parameters were established for both the best grating quality and the highest modulation depth. These gratings may become attractive to further applications, since they were also proved to operate as UV transmission gratings. Acknowledgements The authors gratefully acknowledge the financial support of Hungarian scientific Research Foundation OTKA (TS 049872) and the Hungarian Ministry for Culture and Education (NKFP 3A/071/2004). B. Hopp is indebted for his MTA Bolyai scholarship. References [1] J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68 (1999) 111–113. [2] K. Zimmer, A. Braun, R. Bo¨hme, Appl. Surf. Sci. 208–209 (2003) 199– 204.
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[3] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Microelectron. Eng. 67–68 (2003) 438–444. [4] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Thin Solid Films 453–454 (2004) 31–35. [5] Cs. Vass, B. Hopp, T. Smausz, F. Igna´cz, Thin Solid Films 453–454 (2004) 121–126. [6] Cs. Vass, T. Smausz, B. Hopp, J. Phys. D: Appl. Phys. 37 (2004) 2449– 2454. [7] X. Ding, Y. Kawaguchi, H. Niino, A. Yabe, Appl. Phys. A 75 (6) (2002) 641–645. [8] H. Niino, Y. Yasui, X. Ding, A. Narazaki, T. Sato, Y. Kawaguchi, A. Yabe, J. Photochem. Photobiol. A: Chem. 158 (2003) 179. [9] X. Ding, Y. Kawaguchi, T. Sato, A. Narazaki, R. Kurosaki, H. Niino, J. Photochem. Photobiol. A: Chem. 166 (2004) 129–133. [10] R. Bo¨hme, D. Spemann, K. Zimmer, Thin Solid Films 453–454 (2004) 127–132. [11] K. Zimmer, R. Bo¨hme, A. Braun, B. Rauschenbach, F. Bigl, Appl. Phys. A 74 (2002) 453–456. [12] R. Bo¨hme, J. Zajadacz, K. Zimmer, B. Rauschenbach, Appl. Phys. A 80 (2005) 433–438. [13] J. Ihlemann, S. Mu¨ller, S. Puschmann, D. Scha¨fer, M. Wei, J. Li, P.R. Herman, Appl. Phys. A 76 (2003) 751–753. [14] H.M. Phillips, D.L. Callahan, R. Sauerbrey, G. Szabo´, Z. Bor, Appl. Phys. Lett. 58 (24) (1991) 2761–2763.
Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique Cs. Vass a,*, K. Osvay a, M. Csete a, B. Hopp b b
a Department of Optics and Quantum Electronics, University of Szeged, H-6720 Szeged, Do´m te´r 9, Hungary Research Group on Laser Physics of the Hungarian Academy of Sciences, H-6720 Szeged, Do´m te´r 9, Hungary
Available online 25 February 2007
Abstract A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: l = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2u = 288) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methylmethacrylate c = 1.85 mol/dm3). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180–200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials. # 2007 Elsevier B.V. All rights reserved. Keywords: Laser induced backside wet etching; Two-beam interference; Grating fabrication
1. Introduction The laser-induced backside wet etching (LIBWE) technique [1] is an intensively studied, promising procedure for micromachining of transparent materials. The most UV transparent materials (fused silica, quartz crystal, pyrex, CaF2, BaF2, sapphire and other glass types [1–4]) can be micro-machined by this method. The LIBWE has some important advantages, which makes it more beneficial than the direct ablative machining method, as low threshold fluence, the fine etch depth control, the debris-free etched holes, and the large etched area. The mechanism of the LIBWE is considerably complex; this process has already been detailed in several studies. According to these, the material removal from the transparent target surface is induced by the high temperature (softening or melting the target) [5], the mechanical attack of the liquid vapor bubble and jet [6–9], and the chemical modification of the thin contacting layer of the transparent material (carbon contaminate from the
* Corresponding author. E-mail address: [email protected] (C. Vass). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.02.087
liquid) [10] being responsible for the ablation-like material removal. One of the most important questions from applications’ point of view is whether this machining method is suited for efficient and competitive production of micro- and submicrometer structures into transparent targets. There are only a few studies which reported gratings with micrometer size period produced by LIBWE technique. Zimmer et al. [11] and Bo¨hme et al. [12] produced 787 and 760 nm surface relief grating by mask projection in fused silica transparent target. Ding et al. also presented their results with similar structure dimension (grating period: 750 nm) [9]. Moreover, Ihlemann et al. produced 830 nm fused silica grating using F2 laser ablation and mask projection principle [13]. Since the cost and complication of a usual VUV laser system cannot be widely afforded, the LIBWE technique could become a popular tool in many laboratories. On the other hand, the projection technique (which is common in the above referred excellent studies) limits the effectivity of this method. In this paper, we combined the two-beam interference method with the LIBWE technique eliminating the mask projection. This new method makes possible to produce fused silica grating with a period smaller than that previously
8060
Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
reported [9]. During our experiments we fabricated fused silica grating relieves with 550 nm period, which are the smallest LIBWE generated gratings at present. The dependence of the grating parameters on the most important etching features are also presented. 2. Experimental The transparent targets were fused silica plates with a thickness of 1 mm (Suprasil II, Heraeus). Naphthalene dissolved in methyl-methacrylate with c = 1.85 mol/dm3 concentration (saturated solution) was used as liquid absorbent. In our experiments basically a Q-switched, frequency-doubled Nd:YAG laser (pulse duration: FWHM = 10 ns, repetition rate: 10 Hz) was used in combination of the traditional two-beam interference method. In order to produce as uniform structure as possible, the profile of the laser beam was smoothed in two steps. First, the second harmonic beam at 532 nm was spatially
Fig. 1. Scheme of the experimental set-up.
Fig. 2. AFM images about the produced grating relief. The dimension of each image is 10 mm 10 mm. In the right top corners of the images are denoted the maximum value of the Z range.
Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
filtered. Then, following the external fourth harmonic generation stage consisting of a CLBO crystal, the ultimate Supergaussian-like spatial profile of the 266 nm pulses were formed in a second spatial filter just prior to the beam splitter. The s-polarized laser beam was split into two parts, which were interfered at a certain incident angle on the backside surface of the fused silica plate contacting with the liquid absorber (Fig. 1). The interference fringe separation ( p) determines the period of the grating [14]:
8061
etched depth was measured by a Dektak 8 profilometer operated with a tip having 2.5 mm radius. 3. Results and discussion
where l is the wavelength in air and u is the incident angle of the laser beam at the boundary of air and the fused silica plate. The u was 148 in our experiments resulted 550 nm interference fringe separation. The numbers of laser pulses were varied from 50 to 1800 (50, 100, 300, 600, 1200, 1800, respectively), while the applied fluences were adjusted from 285 to 680 mJ/cm2 (285, 330, 420, 510, 590, 680, respectively) by a rotating halfwave plate. The diameter of irradiated area was 0.85 mm. The morphology of the gratings was studied by a PSIA XE100 atomic force microscope (AFM) in non contact mode. The
The LIBWE threshold fluence for fused silica was measured and found to be 285 mJ/cm2, since this value was the minimum fluence, which caused etching during our experiments. The AFM images about the produced gratings can be seen in Fig. 2. The maximum values of the Z range are denoted in the right top corner of the images (see the explanation in the rightbottom corner of the full image). The first qualitative analysis of the images shows that the quality of the gratings decreases due to the increasing of pulse number and/or the laser fluence. They become wavy with much longer wavelength than the grating period, and the grating grooves are ‘broken’ at the highest pulse numbers and fluences. One of our aims was to characterize these relieves for the correct quantitative analysis. One of the most important grating parameters is the modulation depth, which found to be in the range of 50– 200 nm. The results are represented in Fig. 3a and b. The maximum of the modulation depths in the function of the number of pulses is at around 100 pulses in the most cases. This
Fig. 3. The modulation depth vs. number of laser pulses (a) and laser fluence (b).
Fig. 4. The Ra parameter of the low-pass filtered images vs. number of laser pulses (a) and laser fluence (b).
p¼
l ; 2 sin u
(1)
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Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
value is important in the future applications, where the high modulation depth is an essential requirement. The dependence of the modulation depth on the fluence shows apparent irregularities: one extreme and a local minimum at 330 and 510 mJ/cm2, respectively. The first can be probably originated from the change of the etching mechanism due to probably the beginning of the melting of a thin layer of the fused silica surface [6]. The low frequency waves of the grating can be characterized by the roughness parameter (Ra) of the surface without the grating structures. Therefore, the AFM images were low-pass Fourier-filtered to remove the grating (cutoff below 1430 nm wavelength), and after that the Ra parameter was measured. In Fig. 4a and b can be seen the roughness parameter of the base surface of the gratings. The quality differences between the gratings also can bee seen qualitatively on the AFM images. The Ra of the filtered images increases with increasing laser fluence, which shows agreement with the earlier results about the etched surface roughness without grating at the low fluence region [4,7]. The etch depth measurements help to understand the reason of the rough surface in the case of high fluence and large number of pulses. The etch depth changes from 18 nm to 30–40 mm (Fig. 5a and b). The increase of the thickness of the
Fig. 6. An AFM image (a) and a cross section (b) about our best quality 550 nm period fused silica grating (produced at F = 330 mJ/cm2; 50 pulses).
removed material (etched depth) – using higher pulse number and/or higher laser fluence – enhances the original unevenness (low frequency roughness) of the fused silica surface, which can be seen with the comparison of Figs. 4 and 5. The braking of the grating lines (see the AFM images at right-bottom corner of Fig. 2) probably can be attributed to the partially molten target surface and grating lines. The observable rounded-end line pieces also indicate the melting of the grating lines. The final question is obvious: which are the best etching parameters for the production of best quality grating? The etch depth and the low frequency roughness should be minimal in this case. In our experiment, these conditions are realized in the case of 50 pulses and 330 mJ/cm2 (etched depth = 18 nm, Ra = 2.1 nm), providing a modulation depth of 120 nm. Although this value is not of the maximal, but at higher modulation depth the quality of the grating is degraded. The maximum modulation depth of the gratings was 200 nm, which was achieved using 100 pulses at 420 mJ/cm2. Fig. 6 shows an AFM image and a cross section of the nearly faultless grating relief surface. Our gratings can be used as UV transmission gratings. In our test carried out with the 266 nm laser beam, the first and the second order of the diffracted beams could be seen at approximately 29–308 and 758, respectively. 4. Conclusions
Fig. 5. The etched depth of the holes vs. number of laser pulses (a) and laser fluence (b).
We introduced a new method to produce submicrometer gratings in fused silica surface: a traditional two-beam interference arrangement was combined with the technique of laser induced backside wet etching. High quality fused silica grating relieves having 550 nm period were fabricated, which are the smallest LIBWE generated gratings at present. These results reveal that our combined method is well suited for
Cs. Vass et al. / Applied Surface Science 253 (2007) 8059–8063
fabrication of surface relief grating in fused silica and presumably other transparent dielectrics. The optimal etching parameters were established for both the best grating quality and the highest modulation depth. These gratings may become attractive to further applications, since they were also proved to operate as UV transmission gratings. Acknowledgements The authors gratefully acknowledge the financial support of Hungarian scientific Research Foundation OTKA (TS 049872) and the Hungarian Ministry for Culture and Education (NKFP 3A/071/2004). B. Hopp is indebted for his MTA Bolyai scholarship. References [1] J. Wang, H. Niino, A. Yabe, Appl. Phys. A 68 (1999) 111–113. [2] K. Zimmer, A. Braun, R. Bo¨hme, Appl. Surf. Sci. 208–209 (2003) 199– 204.
8063
[3] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Microelectron. Eng. 67–68 (2003) 438–444. [4] G. Kopitkovas, T. Lippert, C. David, A. Wokaun, J. Gobrecht, Thin Solid Films 453–454 (2004) 31–35. [5] Cs. Vass, B. Hopp, T. Smausz, F. Igna´cz, Thin Solid Films 453–454 (2004) 121–126. [6] Cs. Vass, T. Smausz, B. Hopp, J. Phys. D: Appl. Phys. 37 (2004) 2449– 2454. [7] X. Ding, Y. Kawaguchi, H. Niino, A. Yabe, Appl. Phys. A 75 (6) (2002) 641–645. [8] H. Niino, Y. Yasui, X. Ding, A. Narazaki, T. Sato, Y. Kawaguchi, A. Yabe, J. Photochem. Photobiol. A: Chem. 158 (2003) 179. [9] X. Ding, Y. Kawaguchi, T. Sato, A. Narazaki, R. Kurosaki, H. Niino, J. Photochem. Photobiol. A: Chem. 166 (2004) 129–133. [10] R. Bo¨hme, D. Spemann, K. Zimmer, Thin Solid Films 453–454 (2004) 127–132. [11] K. Zimmer, R. Bo¨hme, A. Braun, B. Rauschenbach, F. Bigl, Appl. Phys. A 74 (2002) 453–456. [12] R. Bo¨hme, J. Zajadacz, K. Zimmer, B. Rauschenbach, Appl. Phys. A 80 (2005) 433–438. [13] J. Ihlemann, S. Mu¨ller, S. Puschmann, D. Scha¨fer, M. Wei, J. Li, P.R. Herman, Appl. Phys. A 76 (2003) 751–753. [14] H.M. Phillips, D.L. Callahan, R. Sauerbrey, G. Szabo´, Z. Bor, Appl. Phys. Lett. 58 (24) (1991) 2761–2763.