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We report the 3D structuring and metallization of a zirconium-based organic-inorganic photosensitive material doped with metal-binding tertiary amine moieties. The influence of the amine group content on the material structurability, the structure resolution and the metallization efficiency was examined. The silver plating technique was also optimized by investigating the metal seeding and reduction parameters. We have compared the silver plating vs. the metal bath techniques and we have found that while the metal bath technique gives good quality metal coatings, ohmic conductivity can only be obtained by silver plating.
©2011 Optical Society of America
Introduction
There has been a lot of interest recently in the design and fabrication of three-dimensional (3D) metallic nanostructures for use in several applications including micro-electro mechanical systems (MEMS), plasmonics, metamaterials, and biosensors [1–5]. However, the fabrication of complex, high resolution, fully 3D metallic structures is still a challenging task, since traditional lithographic techniques, involving the layer-by-layer structuring of metallic structures, can only allow the deposition of a few layers, or the creation of high-aspect ratio two-dimensional (2D) structures.
Ultrafast lasers have been proposed as a potential technology for the fabrication of 3D metallic micro/nanostructures employing one of the following three approaches: multiphoton reduction of metal ions, direct laser writing into a positive-tone photoresist followed by electrochemical deposition of metal, and direct laser writing of negative photoresists and subsequent metallization of the 3D nanostructures using electroless plating.
The first approach, multiphoton reduction of metal ions, is the only method that can create pure metallic, fully 3D nanostructures [6–9]; however, it is limited, by the reduced transparency of the metal ion solutions at the laser wavelengths commonly used (500-800 nm). To overcome this problem, dilute metal-ion solutions are used, which result in 3D structures lacking mechanical stability and integrity.
The second method, direct laser writing (DLW) of a positive-tone photoresist followed by electrochemical deposition of metal, has been demonstrated only recently [2,8]. DLW by multi-photon polymerization allows the fabrication of arbitrarily-shaped, fully three-dimensional dielectric nanostructures with sub-100 nm resolution; in most cases negative photoresists are used to create dielectric 3D structures [3]. However, recently the use of a positive resist was reported to create hollow structures, which are subsequently filled by gold using electrochemical deposition. The structures fabricated were of excellent quality, with a uniform metal deposition and high mechanical stability provided by the hollow frames, resulting in very high resolution gold structures. However, due to the deposition of the gold through the structure holes, this technique can only be used to fabricate a limited range of geometries, and it is fundamentally a 2.5 dimensions fabrication technique.
The third method, direct laser writing of negative photoresists and subsequent metallization of the 3D nanostructures using electroless plating, is the one that has attracted the most attention to date [9–11]. The term electroless plating was originally adopted to describe a method of plating metallic substrates without the benefit of an external source of electric current. In general it is characterised by the selective reduction of metal ions only at the surface of a catalytic substrate immersed into an aqueous solution of metal ions, with continued deposition on the substrate through the catalytic action of the deposit itself [12,13]. In this case, again two different approaches have been followed; the first uses lithographic materials such as the negative photoresist SU8, or acrylate materials to fabricate the 3D structures. However, in order to activate the surface for electroless plating, additional processing for surface functionalization is required [14] and hence the quality, structural integrity and resolution of the structures depend on the photosensitive material and the subsequent surface processing step. The resolution and structure integrity can be very good, however, as the density of the metal binding sites on the structure cannot be controlled, the metallization quality can vary. This is addressed by the second approach, which uses a photosensitive material doped with the metal binding sites [15]. In this case, the density and distribution of the binding sites can be controlled and high quality metallized structures are obtained, however, research to date has not produced any materials suitable for practical applications.
In this work, we report the structuring and metalization of a zirconium-containing organic-inorganic photosensitive material doped with tertiary amine metal-binding moieties. Hybrid materials provide a lot of possibilities for the development of photosensitive materials. Their preparation, modification and processing is straightforward and, in combination with their high optical quality, post-processing chemical and electrochemical inertness, good mechanical and chemical stability, they have found several applications in photonics and biomedical devices when structured by DLW [16–22]. The condensation of a silicon alkoxide with a zirconium alkoxide has been shown to enhance the material’s mechanical stability and allow the modification of its refractive index [23–25]. In addition, hybrid materials provide the possibility of the incorporation of various functional groups using a guest–host or a side-chain–main-chain strategy. One example is the incorporation of a nonlinear optical chromophore to produce an electro-optically active sol-gel. In this study, we build on our previous work on the structuring of a silicon-zirconium hybrid material processing pendant photopolymerizable methacrylate moieties [24], by adding a 2-(dimethylamino) ethyl methacrylate (DMAEMA) monomer capable of participating in the photopolymerization step to incorporate covalently bound metal-binding groups in the 3D structures. We examine the influence of the DMAEMA content on the material structurability, structure resolution and metallized structure quality. We optimize the silver plating technique by investigating the metal seeding and reduction parameters. Finally, the silver plating vs. the metal bath techniques were compared in terms of both the quality of the metal coating and the ohmic conductivity of the structures.
Experimental
Materials and Methods
All the chemicals used in this work were obtained from Sigma-Aldrich, and were used without further purification.
The material used for the fabrication of the 3D structures is an organic-inorganic hybrid composite, produced by the addition of methacryloxypropyl trimethoxysilane (MAPTMS) to zirconium propoxide (ZPO, 70% in propanol). 2-(dimethylamino)ethyl methacrylate (DMAEMA) was added to provide the metal-binding moieties. MAPTMS and DMAEMA were used as the organic photopolymerizable monomers, while ZPO and the alkoxysilane groups of MAPTMS served as the inorganic network forming moieties. 4,4-bis(diethylamino) benzophenone (BIS) was used as a photoinitiator.
MAPTMS was first hydrolyzed using HCl solution (0.1 M) at a 1:0.1 ratio. After 5 minutes, ZPO was slowly added to the hydrolyzed MAPTMS at a 3:7 molar ratio. After stirring for 15 minutes, DMAEMA was added. The (MAPTMS + ZPO):DMAEMA molar ratio was varied between 5:5 and 9.5:0.5. Finally, the photoinitiator, at a 1% w/w concentration was added to the mixture. After stirring for a further 15 minutes, the composite were filtered using a 0.22 µm syringe filter.
The samples were prepared by drop-casting onto 100 micron-thick silanized glass substrates, and the resultant films were dried on a hotplate at 50 °C for 10 minutes before the photopolymerization. The heating process led to the condensation of the alkoxide groups and the formation of the inorganic matrix. Next, the organic moieties attached to the inorganic backbone were polymerized using two photon polymerization, resulting in the formation of irreversible and fully saturated aliphatic C-C covalent bonds that further increase the connectivity of the material [24].
After the completion of the component build process by DLW, the samples were developed for 20 minutes in a 50:50 solution of 1-propanol:isopropanol, and were further rinsed with isopropanol.
3D structure fabrication
The procedure for the fabrication of three-dimensional microstructures by DLW has been described extensively by many groups [17,21,24,25]. In this set-up, a Ti:Sapphire femtosecond laser oscillator was employed (800 nm, 75MHz, with integrated dispersive mirrors that pre-compensate the beam delivery and focusing optics to achieve sub-20fs duration pulses into the sample, Femtolasers Fusion). The photopolymerized structure was generated using an x-y galvanometric mirror digital scanner (Scanlabs Hurryscan II), controlled by SAMLight (SCAPS) software. The scanner was adapted to accommodate a high numerical aperture focusing microscope objective lens (40x, N.A. = 0.95, and 100x, N.A. = 1.4, Zeiss, Plan Apochromat). To achieve better focusing, the laser beam was expanded 5x using a telescope lens arrangement. Movement on the z-axis and large-scale movement on the x-y plane was achieved with a three-axes linear encoder stage (PI). The beam was controlled using a mechanical shutter (Uniblitz), while the beam intensity was adjusted by a motorized attenuator (Altechna). The stages, the shutter, and the attenuator were computer-controlled using the 3DPoli software [26]. A CCD camera was mounted behind a dichroic mirror for online monitoring of the polymerization process. The structures were fabricated in a layer-by-layer fashion with the last layer on the surface of the coverslip.
Metallization
For the metallization of the 3D structures, two different protocols were investigated, and were first tested on the spin-coated films and large 3D structures fabricated by DLW. The metallization process comprised three main steps: first, the treatment of the surface with a metal ion precursor which allowed to binding of the metal species onto the 3D structures, next a metal reduction step to form the metal nanoparticle seeds on the nanostructures and finally a silver bath/plating process to obtain the metal coated structures. These protocols are described in detail below.
Metallization Protocol 1 (MP1)
The material pre-treatment for electroless silver plating affects the early stages of the plating process, and plays an important role in the successful metal plating. The surface of the hybrid organic-inorganic structures prepared in this work were pre-activated with metallic seed particles before the plating process. For this reason, the samples were first treated with a 5.5 x10−4 M aqueous gold chloride trihydrate (HAuCl4.3H2O) solution for 35 mins and then washed twice in deionized water for 10 mins. During this step, the amine groups became protonated and positively charged and thus bound the anionic metal species ([AuCl4]‾) via electrostatic interactions [27].
Next, the structures were immersed in a 6,6 M aqueous NaBH4 solution for 35 mins in order to reduce the gold ions and form the seed gold nanoparticles. The immersion time in the NaBH4 solution was determined by the change in the colour of the material from yellow to dark red, attributed to the light refraction by the metal particles. Finally, the film was rinsed with copious amount of de-ionized water and dried in air. The presence of the gold nanoparticles was confirmed by XRD.
For the silvering bath process, the method reported by Kuebler et al. was followed [14]. In general, the silvering bath is highly unstable and decomposes as soon as the silvering process starts. Silvering occurs via the adhesion of silver particles, formed in the solution, to a pre-activated substrate. To control decomposition, the fine silver particles were stabilized in the solution using a colloidal dispersion of arabic gum. The solution prepared comprized 12 mL of a 33 wt % prefiltered aqueous gum arabic solution, 2 mL of an aqueous citrate buffer (1.5 M citric acid and 0.5 M trisodium citrate, pH 3.5), 3 mL of a 37 mM aqueous silver lactate solution, and 3 mL of a 0.52 M aqueous hydroquinone solution.
The above solutions were mixed at room temperature and were stirred thourougly until a homogeneous solution was obtained. Next, the gold nanoparticle activated structures were immersed into the silver bath for various time periods without agitation. After the silver plating process the samples were removed from the bath and were rinsed several times with copious amount of de-ionized water in order to remove any materials that were not attached to the structures. Finally, the silver-coated structures were dried in air at room temperature before being measured.
Metallization Protocol 2 (MP2)
The second procedure used for the metalization of the 3D microstructures was a modification of the method reported by Kawata et al. [15]. For seeding, the samples were first immersed in a 0.05 mol/L AgNO3 solution at room temperature for 38 hours, and after thorough rinsing with double distilled water, they were subsequently dipped into an aqueous sodium borohydride solution (6,6 M) to reduce the silver ions and form silver nanoparticles; this process was found to be sufficient to fully cover the film surface with densely packed silver nanoparticles and allowed the selective silver plating of the structures in the next step. The amount of silver plated onto the structures was found to be proportional to the immersion time in the AgNO3 solution and the sodium borohydride solution, and the films turned out to be “mirror-like” only for the longer metal seed and reduction times.
For the silver plating a 0.2 M AgNO3 aqueous solution was mixed with 5.6% NH3 and 1.9 M glucose (C6H12O6 > 98%) as a reducing agent, at a volumetric ratio of 5:3:8. The samples were immersed in the solution for less than 5 minutes because after this time the solution started to degrade and extensive silver reduction was observed in solution, besides the silver particles deposited on the nanostructures. The plating process was repeated by immersing the sample in a fresh silver plating solution. Finally, the samples were extensively rinsed with acetone and de-ionized water and were dried in air at room temperature before being analyzed.
In order to improve the stability of the silver bath and extend the silver plating process, an alternative method was also employed which involved the use of a 33 wt% colloidal dispersion of arabic gum added at a 1:10 volume ratio in the above silver plating solution. The samples were immersed in the arabic gum containing silver plating solution for 18 minutes after which time the colour of the silver bath started changing again, signifying the degradation of the solution. Finally, the samples were removed for the silver bath and were washed thoroughly in acetone and double distilled water before being measured.
X-ray Diffraction
Drop-casted thin films onto 100 micron-thick glass substrates were prepared and dried at 50 °C for 10 mins before being polymerized. They were subsequently metalized as described above. A Rigaku RINT 2000 Series wide angle diffractometer was used to determine the crystallinity of the metal coating. Measurements were made from 2θ = 5° to 120° in steps of 2θ = 0.02° using the following operating conditions: I = 178 mA and V = 40 kV.
SEM - FESEM
For the Scanning Electron Microscopy (SEM) measurements a JEOL JSM-6390LV microscope was used, whereas high resolution SEM images were obtained on a Field Emission JEOL JSM 7000F SEM. The samples were gold sputtered and the observation was made at 15 kVolts.
Conductivity measurements
The equipment used for analyzing the current-voltage characteristics of the metallized structures was a Tektronix 370 curve tracer. Voltage was applied through needles deposited on the samples and the resistance value was calculated by the reciprocal of the slope of the line of the V-I curve. The graphs were linear fits to the plots acquired by the curve tracer at low currents.
Results and Discussion
Effect of the DMAEMA content of the material
The influence of the DMAEMA content of the material on its structurability, structure resolution and metallization quality of the microstructures was examined. The results are detailed in the following paragraphs.
Structurability and resolution
Different concentrations ranging from 5 to 60 mole% of the metal-binding monomer DMAEMA were added in the zirconium-containing organic-inorganic photosensitive material in order to investigate the structurability of the final material. 3D structures we fabricated using the process described above. SEM images of the fabricated structures are shown in Fig. 1 .
Fig. 1 Details of 3D structures fabricated using DMAEMA (a) 5%, (b) 10% (c) 30% and (d) 50%. (b) insert: a view of the full structure.
We found that it is possible to produce 3D structures using up to 60% DMAEMA in the hybrid composite, however as seen in Fig. 1(a), the highest resolution is achieved for 5% DMAEMA. As the amount of DMAEMA increases, the material’s stiffness reduces due to the increase of the organic content of the material, and the structures lose mainly their mechanical stability, but also their resolution (Fig. 1(d)). However, at the high organic loadings, the increased elasticity of the material allows the fabrication of larger structures without cracking. For this reason, the 30% DMAEMA composite was chosen for small, high resolution structures (Fig. 1(c)), whereas the 50% DMAEMA composite was selected for larger structures, when the structure resolution is not the main issue (Fig. 2 ).
Metallization quality
In order to investigate the metallization quality vs. the material composition, structures were fabricated by DLW using materials with different DMAEMA contents. Figure 3 shows metalated woodpile structures made from materials with different DMAEMA content after 25 minutes silver plating. Figure 3(a) shows a woodpile structure fabricated using a material with 5% DMAEMA. As discussed earlier, this composition gave the highest resolution structures. However, after metallization, it was clear that this low DMAEMA content did not provide enough metal binding sites on the surface of the structures, and instead of a uniform metal coverage, the structures were covered by scattered nanoparticles which grew bigger as the plating time increased. This is more obvious when films instead of structures were used for the metalization process. Similar results were obtained for the structures fabricated using 10% DMAEMA (Fig. 3(b)). On the other hand, as the percentage of DMAEMA increased to 50%, there were sufficient binding sites to provide uniform metal coverage during the plating process however, the quality of the structures deteriorated, because the organic content increased and the mechanical stability of the structures decreased (Fig. 3(d)). As a compromise of the structure resolution and the metallization efficiency the 30% DMAEMA composite was chosen for small, high resolution and high metalization quality structures (Fig. 3(c)).
Fig. 3 Metalated 3D structures fabricated using a composite material containing (a) 5%, (b) 10%, (c) 30% and (d) 50% DMAEMA.
Influence of the seeding parameters on the metallization quality
Figure 4 shows the XRD pattern obtained from a film that was seeded with the gold nanoparticles. The broad peak at around 20° is assigned to the hybrid material. The appearance of the peaks at 38°, 44°, 64°, 78° and 82° (corresponding to the (111), (200), (220), (311) and (222) planes, respectively) are characteristic of the crystalline structure of the gold particles and verify the formation of the gold seeds on the film.
The film was subsequently covered with silver using electroless plating. Because silver and gold have the same crystallographic planes the peaks attributed to the silver coating (data not shown) overlap with those of the gold nanoparticles and thus it is difficult to assess the silver plating on the films by XRD. Conductivity measurements, discussed below, were used to assess the silver coating quality. In order to obtain a smooth metallized surface, it is important to control both the size and density of the seed nanoparticles and for this the seeding process was varied by immersion of the films in 0.05 mol/L AgNO3 (aq) solution at room temperature for a variation of time between 24 and 38 hrs. The most densely packed Ag nanoparticles on the structures' surface were observed at the maximum time of immersion, determining the seeding time at 38h for the following experiments. The DMAEMA content of the material (discussed above) and the silver reduction time were also varied and their effect on the density and size of the silver particles were investigated.
Figure 5 shows the influence of reduction time on the size and density of the seed nanoparticles. The immersion time in the reducing agent solution was varied between 2 and 12 hrs and afterwards the samples were plated with silver. As seen in Fig. 5(a) after 2 hrs of reduction the silver particles on the film surface are sparsely dispersed, while upon increasing the reduction time the particles start to grow larger in size and form a more dense metal coating on the surface, until they start forming big clusters for the very long reduction times (Fig. 5(e)).
Fig. 5 Effect of reduction time (a) 2 hrs, (b) 6 hrs, (c) 8 hrs, (d) 10 hrs and (e) 12 hrs on particle size.
Conductivity Measurements
As mentioned above, the optimum DMAEMA content for small size and high resolution structures is 30%, while for larger structures, for which the material flexibility is important and resolution is not so important, a 50% DMAEMA-containing material was used. The conductivity of the metalated structures for both DMAEMA concentrations was investigated, using the MP1 and the two variations of MP2 (with and without arabic gum) metallization protocols described earlier, and structures similar to the ones shown in Fig. 3. It was found that the structures metallized using MP1, did not exhibit conductivity, while the ones metallized using MP2 in the presence of arabic gum were found to have diode conductivity when they were seeded for 38 hrs and reduced for more than 8 hrs. Moreover, structures metallized using MP2 without arabic gum were measured to have ohmic conductivity.
Silver plating is selective and occurs only on the pre-treated areas of the material. The metal binding step as well as the reduction of the metal ions influences the silver layer quality and thickness. Thus, different seeding and reduction times using MP2 were investigated. The results for a 50% film are shown in Table 1 . It is clear that the structures that have been seeded and reduced for longer times exhibit lower resistance
Table 1. Effect of Seeding and Reduction Time on Film Resistance (samples containing 50% DMAEMA)
Next, using the optimum seeding and reduction parameters for each material, the resistance of the structures for the two different DMAEMA compositions, 30% and 50%, was measured. The results are shown in Table 2 . The ohmic response of typical structures for both DMAEMA concentrations is shown in Fig. 6 .
Table 2. Effect of the DMAEMA Content on the Resistance of the Structures
These results suggest that the resistance increases with the decrease in DMAEMA content and this is attributed to the lower density of metal binding sites which results in smaller and less dense nanoparticles during the seeding process, and thus in a less uniform silver film after plating.
Conclusions
We have investigated the 3D structuring and metallization of a novel metal-binding organic-inorganic hybrid material. We have examined the influence of the concentration of metal binding moieties on the 3D structure resolution, the material structurability and the metallization quality and we have optimized the silver plating technique by investigating the metal seeding and reduction parameters. We have compared three different metal plating protocols, and we have found that only one of them provides metallated structures exhibiting ohmic conductivity.
Acknowledgments
The work of A.G. was supported by the EU Marie Curie Fellowship Program: FASTQUAST (PITN-GA-2008-214962). The work of C.R. was supported by the Marie Curie Transfer of Knowledge project NOLIMBA (MTKD-CT-2005-029194). We are grateful to Mrs. Aleka Manousaki and Ms Maria Kayambaki for expert technical assistance with SEM and the conductivity measurements, respectively.
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