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Investigation of the two-photon polymerisation of a Zr-based inorganic–organic hybrid material system
Applied Surface Science 252 (2006) 4845–4849 www.elsevier.com/locate/apsusc
Investigation of the two-photon polymerisation of a Zr-based inorganic–organic hybrid material system B. Bhuian a, R.J. Winfield a,*, S. O’Brien a, G.M. Crean a,b b
a Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland Department of Microelectronic Engineering, University College Cork, Cork, Ireland
Received 3 May 2005; accepted 24 July 2005 Available online 28 October 2005
Abstract Two-photon polymerisation of photo-sensitive materials allows the fabrication of three dimensional micro- and nano-structures for photonic, electronic and micro-system applications. However the usable process window and the applicability of this fabrication technique is significantly determined by the properties of the photo-sensitive material employed. In this study investigation of a custom inorganic–organic hybrid system, cross-linked by a two-photon induced process, is described. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This enabled dry films to be used in the polymerisation process. A structural, optical and mechanical analysis of the optimised sol–gel material is presented. A Ti:sapphire laser with 80 MHz repetition rate, 100 fs pulse duration and 795 nm is used. The influence of both material system and laser processing parameters including: laser power, photo-initiator concentration and zirconium loading, on achievable micro-structure and size is presented. # 2005 Elsevier B.V. All rights reserved. Keywords: Two-photon polymerisation (2PP); Material system; Sol–gel
1. Introduction Two-photon polymerisation (2PP) is a versatile technique for the fabrication of novel and complex 3D structures with submicron resolution including photonic crystals [1], coils [2], gear wheels [3] and other miscellaneous 3D artefacts [4]. The technique relies on a tightly focused single laser beam having sufficient intensity and energy to initiate photo-polymerisation in a suitable material. The two-photon absorption transition results in localisation of the polymerisation process to the focal volume of the optical delivery system. Objects can therefore be built up in three dimensions using a single beam and a suitable XYZ stage system. In addition there is no laser absorption in the material as the initiator does not absorb at the laser wavelength. However to date most 2PP has been performed using liquid resins that require containment. Furthermore some resins have oxygen inhibited curing making processing in a closed cell difficult.
* Corresponding author. Tel.: +353 21 4904377; fax: +353 21 4270271. E-mail address: [email protected] (R.J. Winfield). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.094
There is significant interest in developing enhanced materials in order to expand the usable fabrication process window and/or the quality of the final manufactured part. Materials systems based on acrylates, epoxies and inorganic– organic hybrid materials have been developed [1,4,5]. In addition novel photo-initiators that have high two-photon efficiencies when excited by lasers in the near IR have also been demonstrated [6]. However acrylate and epoxy resins are not amenable to homogenous incorporation of inorganic components for enhancement of thermo-mechanical stability, etc. In situ reactions to produce nanoparticles within a polymer host have been demonstrated but evidence of inhomogeneity has also been observed [7]. These organic resins can also suffer from the containment and curing problems mentioned earlier. Inorganic–organic hybrids have been shown to be suitable for 2PP. Their properties can be modified by the incorporation of dopants and other functionalised groups. This enables the formation of a material with both organic and inorganic chemical-functionality and hence tailored material properties, e.g. polymeric optical transparency in conjunction with enhanced mechanical strength due to the inorganic component. The materials are typically produced by a sol–gel process and
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B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
have found other photonic applications such as in the fabrication of multimode waveguides [8]. Hybrid material systems containing silicon have also been demonstrated [9] but again the 2PP process took place in a liquid cell. The use of hybrid materials containing zirconium opens up new possibilities. The addition of zirconium is known to strengthen the system, modify the refractive index and improve the mechanical stability. The formation of significant amount of Si–O–Zr resulting from the reaction of silanol groups with Zr– OR groups has been demonstrated, and seems to be kinetically favoured when zirconium alkoxide is added to a pre-hydrolyzed solution of a silicon alkoxide [10]. Consequently, due to higher rates of condensation, deposited materials dry quickly to form solid films. This material therefore offers significant potential advantages for 2PP over previous resins since it does not require a reaction cell. In addition the dry film would be immobile under stage movement. In this paper we describe the synthesis and performance of such a Zr-based inorganic–organic hybrid system, cross-linked by the two-photon induced process. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This makes the cured material potentially stronger than conventional acrylate or epoxy resins. The photo-initiator used was 4,40 -bis(diethylamino)benzophenone (Sigma–Aldrich) chosen for its strong single photon absorption in the 320–420 nm region. The advantage of using a longer wavelength photo-initiator is that it should work more efficiently with the 795 nm laser employed. We present a structural, optical and mechanical analysis of the optimised sol–gel material polymerised by a Ti:sapphire laser (wavelength 795 nm, 80 MHz repetition rate and 100 fs pulse duration). The mechanical properties are described as a function of percentage zirconium. The influence of laser power, focusing system and photo-initiator concentration on the 2PP laser patterned structures are also described.
Guassian profile to an Airy pattern as the beam is truncated. Since the 2PP process is non-linear, a better resolution can be achieved from the Airy pattern. However this is at the expense of efficiency. For this reason a truncation factor of 3 is used in the polymerisation studies. Resins were prepared by hydrolysis of an organo-silane precursor to which a chelated zirconium alkoxide was added. Water was then added to promote condensation reactions between hydrolyzed precursors and a photo-initiator species was added (followed by filtration) immediately prior to deposition. A process schematic is shown in Fig. 2. Firstly, methacryloxypropyl trimethoxysilane was hydrolyzed with dilute HCl. In a separate reaction vessel methacrylic acid and zirconium isopropoxide were combined in a molar ratio of 4:1 and were stirred for 30 min. Chelated zirconium isopropoxide was then added to the hydrolyzed silane precursor. A small amount of water was then added to each mixture which was then stirred for 30 min. The 2 wt.% of the photo-initiator 4,40 -bis(diethylamino) benzophenone was then added to each mixture, followed by filtration through a 2 mm filter, immediately prior to use. In order to study the effect of photo-initiator content on polymerisation behaviour a series of samples was prepared: hydrolyzed methacryloxypropyl trimethoxysilane and chelated zirconium isopropoxide were added together to form a mixture containing 5 mol% Zr, as above. This mixture was then divided in to portions to which a small amount of water was then added, followed by stirring for 30 min. The photo-initiator 4,40 bis(diethylamino)benzophenone was added, followed by filtration through a 2 mm filter, immediately prior to deposition. The photo-initiator contents were 0.66, 1.33, 2.0, 2.66 and 3.33 wt.%. In order to study the effect of zirconium content on polymerisation behaviour, another series of samples with increasing zirconium concentration was prepared. 1, 5 and 10 mol% chelated Zr (a control sample without added Zr was also prepared) was added to portions of the hydrolyzed silane precursor. A small amount of water was then added to each mixture, followed by stirring for 30 min. The photo-initiator
2. Experimental The experimental 2PP fabrication system used a Ti:sapphire femtosecond laser, wavelength 795 nm, pulse repetition rate 80 MHz and pulse length 100 ns. The beam was delivered into a 1.25 numerical aperture (NA) immersion focusing objective via a variable beam expander as shown in Fig. 1. The resin was spun onto 120 mm thick glass substrates and mounted inverted onto an XYZ stage system. The polymerisation process was observed via a CCD camera. As the laser was designed to operate at a constant average power, reflective neutral density filters were used to control the power delivered to the resin. The unpolymerised material was dissolved in isopropyl alcohol. The transmission of lasers with Gaussian beam profiles through optical systems with limiting apertures has been studied by several authors [11]. It can be shown that the focal spot diameter through a focusing lens system changes from a
Fig. 1. Two photon polymerisation system.
B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
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Fig. 3 shows the 2PP laser patterned line dimensions written at various velocities as a function of photo-initiator concentration (1.33, 2.0, 2.66 and 3.33 wt.%). The laser average power was 54 mW in each case. The zirconium content of the resin was 5 mol%. It is seen that the polymerisation efficiency
increases as a function of photo-initiator concentration. In this experiment the laser dosage is proportional to the reciprocal of the scan speed. This would suggest that structures can be written faster with increased photo-initiator content. Fig. 4 shows similar laser patterned line measurements as a function of zirconium content (1, 5 and 10 mol%). It is assumed that all Zr added to the formulation during sol–gel preparation is present in the resin used for 2PP. This is based on the observation that the Zr source (Zr-isopropoxide) is stabilized by chelation with methacrylic acid prior to mixing with the silicon component. In addition, there is no evidence of Zr precipitation in either the liquid sol–gel or dried resin material, as shown by preliminary SEM examination. Therefore, the Zr must remain in the resin, since neither Zr nor Si are volatile under the experimental conditions used for 2PP. Again the average laser power was 54 mW in each case and the photo-initiator concentration was 2 wt.%. The polymerised volume does appear to change with the different zirconium loadings. This may be related to the change in absorption at 398 nm (this wavelength corresponds to two laser photons) as seen in Fig. 5b. The transmission spectra for the uncured resins used in this investigation are shown in Fig. 5a and b as a function of varying photo-initiator (zirconium content fixed at 5 mol%) and varying zirconium content (photo-initiator content fixed at 2 wt.%). The data was normalised for a film thickness of 7 mm. The photo-initiator was designed for use in the UV region and therefore has a strong absorption in the 320–420 nm region. The samples show little absorption at the laser wavelength (795 nm) and so no laser attenuation would be expected. It is interesting to note that the effect of adding zirconium to the resin is to introduce an absorption feature in the region 450– 500 nm. This feature is not seen if the resin has either no photoinitiator or no zirconium. This absorption feature increases with zirconium concentration.
Fig. 3. Characteristics of the Zr-loaded polymer with increasing photo-initiator concentration (zirconium concentration 5 mol%).
Fig. 4. Characteristics of the Zr-loaded polymer with increasing zirconium concentration (photo-initiator concentration 2 wt.%).
Fig. 2. Sol–gel preparation flow chart.
4,40 -bis(diethylamino)benzophenone was added, followed by filtration through a 2 mm filter, immediately prior to deposition. Spectral measurements were made on a Shimadzu UV-2410 PC UV-visible spectrophotometer. The samples were spincoated onto quartz and the film thickness measured using a Nanometric NanoSpec 3000 interference film instrument. Roughness measurements were made using a Zygo Corporation NewView 5000 Zygo interferometer using a 50 microscope objective, 1.8 zoom setting and 320 280 pixels camera mode. The Young’s modulus and hardness of the polymerised material was determined using a MTS NanoIndenter nano microindenter. 3. Results
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B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
Fig. 7. Woodpile structure with 2.5 mm period.
Fig. 5. Transmission spectra of the zirconium loaded resins with: (a) constant Zr concentration of 5 mol%, (b) constant photo-initiator concentration of 2 wt.%.
In order to measure roughness a thin film sample was prepared with the beam power through the objective set at 90 mW and a scan speed of 100 mm s1. The resin had a Zr content of 5 mol% and a photo-initiator concentration of 2 wt.%. A vertical pillar was built up by repeated traversing of the substrate to a height of 150 mm as shown in Fig. 6. The width was 60 mm and thickness 4 mm. The pillar was then laid horizontal to give a 150 mm 60 mm surface for analysis. The sampling area was 10 mm 10 mm. An average over three areas gave an Ra roughness value of 14 nm. This is consistent with values obtained using a polyurethane acrylate resin where values in the range 4–11 nm were measured [12]. It is known that increasing the zirconium content of an inorganic–organic hybrid system such as this increases the hardness of the material. Thin film samples of resins with zirconium content 0, 1, 5 and 10 mol% were prepared, using the pillar writing technique. Values of Young’s modulus measured
Fig. 6. Pillar structure.
were in the range 1.5–6.0 GPa and hardness 90–530 MPa as measured by the microindenter. The structures written by the laser were subsequently immersed in solvent and after this stage the structures were observed to contract. Linear shrinkage measurements were made on the top surface of a woodpile structure (as shown in Fig. 7) written in the zirconium loaded sol–gel. The top surface was chosen as it is not anchored to any other structure (e.g. the substrate). Fig. 8 shows the results made by comparing the overall width of the top surface with the design value (50 mm). As can be seen in Fig. 8, shrinkages of 10–16% were measured. Although no published quantified data was found, it is known that shrinkage occurs in similar systems [10].
4. Discussion A novel Zr:Si inorganic–organic hybrid polymer system has been synthesised by a sol–gel process. This resin has been demonstrated to be suitable for a two-photon polymerisation process. Zirconium loadings of up to 10% have been used without the need for nanoparticle fillers. An efficient photoinitiator was used to cross-link the polymer system. The sol–gel material can be used as a dry film precursor and does not require a containment cell. This is particularly useful considering that high numerical aperture objectives generally require index matching fluid and this system can easily be exposed inverted through the substrate.
Fig. 8. Shrinkage in Zr-loaded sol–gel.
B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
The surface quality of the processed layers is fully suitable for photonic applications. High aspect ratio pillar structures and woodpile lattice structures have been demonstrated. This work opens a new processing window for dry processing of functional films with significant potential for fabrication of 3D structures on complex substrates. Acknowledgements The authors would like to thank the help of Graham Cross, Trinity College Dublin for the micro-hardness measurements and financial support from the Higher Educational Authority (Ireland) Programme for Research in Third Level Institutes – EcoElectronic project.
[2] [3] [4]
[5] [6]
[7] [8] [9]
References [1] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCord-Maughon, J.
[10] [11] [12]
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Qin, H. Ro¨ckel, M. Rumi, X.-Li. Wu, S.R. Marder, J.W. Perry, Nature 398 (1999) 51. M. Miwa, S. Juodkazis, T. Kawakami, S. Matsuo, H. Misawa, Appl. Phys. A 73 (2001) 561. H.-B. Sun, T. Kawakami, Y. Xu, J.-Y. Ye, S. Matsuo, H. Misawa, M. Miwa, R. Kaneko, Opt. Lett. (2000) 1110. J. Serbin, A. Egbert, A. Ostendorf, B.N. Chichkov, R. Houbertz, G. Domann, J. Schulz, C. Cronauer, L. Frohlich, M. Popall, Opt. Lett. (2003) 301. T.-W. Lee, O. Mitrofanov, C.A. White, J.W.P. Hsu, Mater. Res. Soc. Symp. Proc. 776 (2003) 143. S.M. Kuebler, K.L. Braun, W. Zhou, J.K. Cammack, T. Yu, C.K. Ober, S.R. Marder, J.W. Perry, J. Photochem. Photobiol. A 158 (2003) 163. K. Kaneko, H.-B. Sun, X.-M. Duan, S. Kawata, Appl. Phys. Lett. 83 (2003) 426. S. O’Brien, G. Redmond, G.M. Crean, Proc. SPIE 5731 (2005) 39. R. Houbertz, L. Frohlich, M. Popall, U. Streppel, P. Dannberg, A. Brauer, J. Serbin, B.N. Chichkov, Adv. Eng. Mater. 5 (2003) 551. L. Delattre, F. Babonneau, Chem. Mater. 9 (1997) 2385. H. Urey, Appl. Opt. 43 (2004) 620. K. Takada, H.-B. Sun, S. Kawata, Appl. Phys. Lett. 86 (2005) 071122.
Investigation of the two-photon polymerisation of a Zr-based inorganic–organic hybrid material system B. Bhuian a, R.J. Winfield a,*, S. O’Brien a, G.M. Crean a,b b
a Tyndall National Institute, Lee Maltings, Prospect Row, Cork, Ireland Department of Microelectronic Engineering, University College Cork, Cork, Ireland
Received 3 May 2005; accepted 24 July 2005 Available online 28 October 2005
Abstract Two-photon polymerisation of photo-sensitive materials allows the fabrication of three dimensional micro- and nano-structures for photonic, electronic and micro-system applications. However the usable process window and the applicability of this fabrication technique is significantly determined by the properties of the photo-sensitive material employed. In this study investigation of a custom inorganic–organic hybrid system, cross-linked by a two-photon induced process, is described. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This enabled dry films to be used in the polymerisation process. A structural, optical and mechanical analysis of the optimised sol–gel material is presented. A Ti:sapphire laser with 80 MHz repetition rate, 100 fs pulse duration and 795 nm is used. The influence of both material system and laser processing parameters including: laser power, photo-initiator concentration and zirconium loading, on achievable micro-structure and size is presented. # 2005 Elsevier B.V. All rights reserved. Keywords: Two-photon polymerisation (2PP); Material system; Sol–gel
1. Introduction Two-photon polymerisation (2PP) is a versatile technique for the fabrication of novel and complex 3D structures with submicron resolution including photonic crystals [1], coils [2], gear wheels [3] and other miscellaneous 3D artefacts [4]. The technique relies on a tightly focused single laser beam having sufficient intensity and energy to initiate photo-polymerisation in a suitable material. The two-photon absorption transition results in localisation of the polymerisation process to the focal volume of the optical delivery system. Objects can therefore be built up in three dimensions using a single beam and a suitable XYZ stage system. In addition there is no laser absorption in the material as the initiator does not absorb at the laser wavelength. However to date most 2PP has been performed using liquid resins that require containment. Furthermore some resins have oxygen inhibited curing making processing in a closed cell difficult.
* Corresponding author. Tel.: +353 21 4904377; fax: +353 21 4270271. E-mail address: [email protected] (R.J. Winfield). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.094
There is significant interest in developing enhanced materials in order to expand the usable fabrication process window and/or the quality of the final manufactured part. Materials systems based on acrylates, epoxies and inorganic– organic hybrid materials have been developed [1,4,5]. In addition novel photo-initiators that have high two-photon efficiencies when excited by lasers in the near IR have also been demonstrated [6]. However acrylate and epoxy resins are not amenable to homogenous incorporation of inorganic components for enhancement of thermo-mechanical stability, etc. In situ reactions to produce nanoparticles within a polymer host have been demonstrated but evidence of inhomogeneity has also been observed [7]. These organic resins can also suffer from the containment and curing problems mentioned earlier. Inorganic–organic hybrids have been shown to be suitable for 2PP. Their properties can be modified by the incorporation of dopants and other functionalised groups. This enables the formation of a material with both organic and inorganic chemical-functionality and hence tailored material properties, e.g. polymeric optical transparency in conjunction with enhanced mechanical strength due to the inorganic component. The materials are typically produced by a sol–gel process and
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B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
have found other photonic applications such as in the fabrication of multimode waveguides [8]. Hybrid material systems containing silicon have also been demonstrated [9] but again the 2PP process took place in a liquid cell. The use of hybrid materials containing zirconium opens up new possibilities. The addition of zirconium is known to strengthen the system, modify the refractive index and improve the mechanical stability. The formation of significant amount of Si–O–Zr resulting from the reaction of silanol groups with Zr– OR groups has been demonstrated, and seems to be kinetically favoured when zirconium alkoxide is added to a pre-hydrolyzed solution of a silicon alkoxide [10]. Consequently, due to higher rates of condensation, deposited materials dry quickly to form solid films. This material therefore offers significant potential advantages for 2PP over previous resins since it does not require a reaction cell. In addition the dry film would be immobile under stage movement. In this paper we describe the synthesis and performance of such a Zr-based inorganic–organic hybrid system, cross-linked by the two-photon induced process. The material was produced by sol–gel synthesis using a silicon alkoxide species that also possessed methacrylate functionality. Stabilized zirconium alkoxide precursors were added to the precursor solution in order to reduce drying times and impart enhanced mechanical stability to deposited films. This makes the cured material potentially stronger than conventional acrylate or epoxy resins. The photo-initiator used was 4,40 -bis(diethylamino)benzophenone (Sigma–Aldrich) chosen for its strong single photon absorption in the 320–420 nm region. The advantage of using a longer wavelength photo-initiator is that it should work more efficiently with the 795 nm laser employed. We present a structural, optical and mechanical analysis of the optimised sol–gel material polymerised by a Ti:sapphire laser (wavelength 795 nm, 80 MHz repetition rate and 100 fs pulse duration). The mechanical properties are described as a function of percentage zirconium. The influence of laser power, focusing system and photo-initiator concentration on the 2PP laser patterned structures are also described.
Guassian profile to an Airy pattern as the beam is truncated. Since the 2PP process is non-linear, a better resolution can be achieved from the Airy pattern. However this is at the expense of efficiency. For this reason a truncation factor of 3 is used in the polymerisation studies. Resins were prepared by hydrolysis of an organo-silane precursor to which a chelated zirconium alkoxide was added. Water was then added to promote condensation reactions between hydrolyzed precursors and a photo-initiator species was added (followed by filtration) immediately prior to deposition. A process schematic is shown in Fig. 2. Firstly, methacryloxypropyl trimethoxysilane was hydrolyzed with dilute HCl. In a separate reaction vessel methacrylic acid and zirconium isopropoxide were combined in a molar ratio of 4:1 and were stirred for 30 min. Chelated zirconium isopropoxide was then added to the hydrolyzed silane precursor. A small amount of water was then added to each mixture which was then stirred for 30 min. The 2 wt.% of the photo-initiator 4,40 -bis(diethylamino) benzophenone was then added to each mixture, followed by filtration through a 2 mm filter, immediately prior to use. In order to study the effect of photo-initiator content on polymerisation behaviour a series of samples was prepared: hydrolyzed methacryloxypropyl trimethoxysilane and chelated zirconium isopropoxide were added together to form a mixture containing 5 mol% Zr, as above. This mixture was then divided in to portions to which a small amount of water was then added, followed by stirring for 30 min. The photo-initiator 4,40 bis(diethylamino)benzophenone was added, followed by filtration through a 2 mm filter, immediately prior to deposition. The photo-initiator contents were 0.66, 1.33, 2.0, 2.66 and 3.33 wt.%. In order to study the effect of zirconium content on polymerisation behaviour, another series of samples with increasing zirconium concentration was prepared. 1, 5 and 10 mol% chelated Zr (a control sample without added Zr was also prepared) was added to portions of the hydrolyzed silane precursor. A small amount of water was then added to each mixture, followed by stirring for 30 min. The photo-initiator
2. Experimental The experimental 2PP fabrication system used a Ti:sapphire femtosecond laser, wavelength 795 nm, pulse repetition rate 80 MHz and pulse length 100 ns. The beam was delivered into a 1.25 numerical aperture (NA) immersion focusing objective via a variable beam expander as shown in Fig. 1. The resin was spun onto 120 mm thick glass substrates and mounted inverted onto an XYZ stage system. The polymerisation process was observed via a CCD camera. As the laser was designed to operate at a constant average power, reflective neutral density filters were used to control the power delivered to the resin. The unpolymerised material was dissolved in isopropyl alcohol. The transmission of lasers with Gaussian beam profiles through optical systems with limiting apertures has been studied by several authors [11]. It can be shown that the focal spot diameter through a focusing lens system changes from a
Fig. 1. Two photon polymerisation system.
B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
4847
Fig. 3 shows the 2PP laser patterned line dimensions written at various velocities as a function of photo-initiator concentration (1.33, 2.0, 2.66 and 3.33 wt.%). The laser average power was 54 mW in each case. The zirconium content of the resin was 5 mol%. It is seen that the polymerisation efficiency
increases as a function of photo-initiator concentration. In this experiment the laser dosage is proportional to the reciprocal of the scan speed. This would suggest that structures can be written faster with increased photo-initiator content. Fig. 4 shows similar laser patterned line measurements as a function of zirconium content (1, 5 and 10 mol%). It is assumed that all Zr added to the formulation during sol–gel preparation is present in the resin used for 2PP. This is based on the observation that the Zr source (Zr-isopropoxide) is stabilized by chelation with methacrylic acid prior to mixing with the silicon component. In addition, there is no evidence of Zr precipitation in either the liquid sol–gel or dried resin material, as shown by preliminary SEM examination. Therefore, the Zr must remain in the resin, since neither Zr nor Si are volatile under the experimental conditions used for 2PP. Again the average laser power was 54 mW in each case and the photo-initiator concentration was 2 wt.%. The polymerised volume does appear to change with the different zirconium loadings. This may be related to the change in absorption at 398 nm (this wavelength corresponds to two laser photons) as seen in Fig. 5b. The transmission spectra for the uncured resins used in this investigation are shown in Fig. 5a and b as a function of varying photo-initiator (zirconium content fixed at 5 mol%) and varying zirconium content (photo-initiator content fixed at 2 wt.%). The data was normalised for a film thickness of 7 mm. The photo-initiator was designed for use in the UV region and therefore has a strong absorption in the 320–420 nm region. The samples show little absorption at the laser wavelength (795 nm) and so no laser attenuation would be expected. It is interesting to note that the effect of adding zirconium to the resin is to introduce an absorption feature in the region 450– 500 nm. This feature is not seen if the resin has either no photoinitiator or no zirconium. This absorption feature increases with zirconium concentration.
Fig. 3. Characteristics of the Zr-loaded polymer with increasing photo-initiator concentration (zirconium concentration 5 mol%).
Fig. 4. Characteristics of the Zr-loaded polymer with increasing zirconium concentration (photo-initiator concentration 2 wt.%).
Fig. 2. Sol–gel preparation flow chart.
4,40 -bis(diethylamino)benzophenone was added, followed by filtration through a 2 mm filter, immediately prior to deposition. Spectral measurements were made on a Shimadzu UV-2410 PC UV-visible spectrophotometer. The samples were spincoated onto quartz and the film thickness measured using a Nanometric NanoSpec 3000 interference film instrument. Roughness measurements were made using a Zygo Corporation NewView 5000 Zygo interferometer using a 50 microscope objective, 1.8 zoom setting and 320 280 pixels camera mode. The Young’s modulus and hardness of the polymerised material was determined using a MTS NanoIndenter nano microindenter. 3. Results
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B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
Fig. 7. Woodpile structure with 2.5 mm period.
Fig. 5. Transmission spectra of the zirconium loaded resins with: (a) constant Zr concentration of 5 mol%, (b) constant photo-initiator concentration of 2 wt.%.
In order to measure roughness a thin film sample was prepared with the beam power through the objective set at 90 mW and a scan speed of 100 mm s1. The resin had a Zr content of 5 mol% and a photo-initiator concentration of 2 wt.%. A vertical pillar was built up by repeated traversing of the substrate to a height of 150 mm as shown in Fig. 6. The width was 60 mm and thickness 4 mm. The pillar was then laid horizontal to give a 150 mm 60 mm surface for analysis. The sampling area was 10 mm 10 mm. An average over three areas gave an Ra roughness value of 14 nm. This is consistent with values obtained using a polyurethane acrylate resin where values in the range 4–11 nm were measured [12]. It is known that increasing the zirconium content of an inorganic–organic hybrid system such as this increases the hardness of the material. Thin film samples of resins with zirconium content 0, 1, 5 and 10 mol% were prepared, using the pillar writing technique. Values of Young’s modulus measured
Fig. 6. Pillar structure.
were in the range 1.5–6.0 GPa and hardness 90–530 MPa as measured by the microindenter. The structures written by the laser were subsequently immersed in solvent and after this stage the structures were observed to contract. Linear shrinkage measurements were made on the top surface of a woodpile structure (as shown in Fig. 7) written in the zirconium loaded sol–gel. The top surface was chosen as it is not anchored to any other structure (e.g. the substrate). Fig. 8 shows the results made by comparing the overall width of the top surface with the design value (50 mm). As can be seen in Fig. 8, shrinkages of 10–16% were measured. Although no published quantified data was found, it is known that shrinkage occurs in similar systems [10].
4. Discussion A novel Zr:Si inorganic–organic hybrid polymer system has been synthesised by a sol–gel process. This resin has been demonstrated to be suitable for a two-photon polymerisation process. Zirconium loadings of up to 10% have been used without the need for nanoparticle fillers. An efficient photoinitiator was used to cross-link the polymer system. The sol–gel material can be used as a dry film precursor and does not require a containment cell. This is particularly useful considering that high numerical aperture objectives generally require index matching fluid and this system can easily be exposed inverted through the substrate.
Fig. 8. Shrinkage in Zr-loaded sol–gel.
B. Bhuian et al. / Applied Surface Science 252 (2006) 4845–4849
The surface quality of the processed layers is fully suitable for photonic applications. High aspect ratio pillar structures and woodpile lattice structures have been demonstrated. This work opens a new processing window for dry processing of functional films with significant potential for fabrication of 3D structures on complex substrates. Acknowledgements The authors would like to thank the help of Graham Cross, Trinity College Dublin for the micro-hardness measurements and financial support from the Higher Educational Authority (Ireland) Programme for Research in Third Level Institutes – EcoElectronic project.
[2] [3] [4]
[5] [6]
[7] [8] [9]
References [1] B.H. Cumpston, S.P. Ananthavel, S. Barlow, D.L. Dyer, J.E. Ehrlich, L.L. Erskine, A.A. Heikal, S.M. Kuebler, I.-Y.S. Lee, D. McCord-Maughon, J.
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