SU8-Silver Photosensitive Nanocomposite

ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9 SU8-Silver Photosensitive Nanocomposite** By SØbastien Jiguet,* Arnaud Bertsch, Heinrich Hofmann, and Philippe Renaud Composite materials offer a combination of properties and a diversity of applications, which cannot be obtained with metals, ceramics or polymers alone. In particular, the insertion of a conductive phase (metallic or inorganic powders or conductive polymers) in an insulating polymer matrix, can result in an enhancement of its electrical conductivity. Various studies have been focused on such electrically conductive polymer composition (ECPCs), which do not have the disadvantages of the pure metal (high density, low chemical resistance, complex manufacturing process). The powder loaded polymer becomes a functional material with specific properties, and can be used for electrical or electromagnetic applications, etc.[1,2] The case of ECPCs based on epoxy resins, are of particular interest to understand the properties of binary composites (morphology, electrical and thermal conductivity), which are obtained in most cases after a heat treatment to crosslink the ECPCs materials. Their main application is conductive adhesives used in the electronic field for connecting and bonding, but it can also be used for manufacturing sensors.[3±5] In this paper we report a new conductive photosensitive composite material, allowing the direct manufacture of electrically conductive micro-components. It is based on a blend containing silver particles embedded in SU-8, an epoxy photopolymer essentially used for the fabrication of high aspect ratio structures by UV-LIGA. SU-8 components are useful for MEMS, fluidic and packaging applications,[6,7] but also as masters for micro-injection-molding (Fig. 1). Nanocomposite characterizations: Formulations containing up to 40 vol.% of silver filler were produced to evaluate the effect of the insertion of silver particles in the photopolymer matrix, both on the photo-polymerization phenomenon and on electrical conductivity of the resulting photo-patterned structures. ± [*] S. Jiguet, Dr. A. Bertsch, Prof. P. Renaud Swiss Federal Institute of Technology EPFL-STI-IMM-LMIS4, Lausanne, Switzerland E-mail: [email protected] Prof. H. Hofmann Swiss Federal Institute of Technology EPFL-STI-IMX-LTP, Lausanne, Switzerland [**] This work was supported by the Swiss National Science Foundation. We would like to thank the Interdisciplinary Center of Electron Microscopy for their contribution to the sample preparations and observations. DOI: 10.1002/adem.200400068  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 719 COMMUNICATIONS [5] P. B. Messersmith, E. P. Giannelis, J. Polym. Sci. A ± Polym. Chem. 1995, 33, 1047. [6] C. W. Nah, H. J. Ryu, W. D. Kim, S. S. Choi, Polym. Adv. Tech. 2002, 13, 649. [7] R. Krishnamoorti, R. A. Vaia, E. P. Giannelis, Chem. Mater. 1996, 8, 1728. [8] X. Kornmann, L. A. Berglund, J. Sterte, Polym. Eng. & Sci. 1998, 38, 1351. [9] R. A. Vaia, E. P. Giannelis, Macromolecules 1997, 30, 7990. [10] E. Manias, A. Touny, L. Wu, K. Strawhecker, B. Lu, T. C. Chung, Chem. Mater. 2001, 13, 3516. [11] S. T. Lim, Y. H. Hyun, H. J. Choi, M. S. Jhon, Chem. Mater. 2002, 14, 1839. [12] N. Ogata, S. Kawakage, T. Ogihara, Polymer 1997, 38, 5115. [13] R. A. Vaia, B. B. Sauer, O. K. Tse, E. P. Giannelis, J. Polym Sci. B ± Polym Phys. 1997, 35, 59. [14] J. H. Wu, M. M.Lerner, Chem. Mater. 1993, 5, 835. [15] J. H. Chang, D. K. Park, J. Polym. Sci. B ± Polym. Phys. 2001, 39, 2581. [16] N. Ogata, S. Kawakage, T. Ogihara, J. Appl. Polym. Sci. 1997, 66, 573. [17] C. J. G. 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Mooney, J. Colloid Sci. 1951, 6, 162. [29] R. G. Larson, The Structure and Rheology of Complex Fluids, OUP, New York, 1999. [30] J. C. Halpin, J. L. Kardos, Poly. Eng. & Sci. 1976, 16, 344. [31] M. van Es, F. Xiqiao, J. van Turnhout, E. van der Giessen, in Specialty Polymer Additives, Principles and Applications (Eds.: S. Al-Malaika, A. W. Golovoy) Blackwell Science, 2001, Chapter 21. COMMUNICATIONS Jiguet et al./SU8-Silver Photosensitive Nanocomposite Fig. 1. Multilevel piece of watch in SU8 (Courtesy of Mimotec SA). Fig. 3. Reflectance spectrum of SU8-silver composite material in the UV-Visible range for different silver powder loads. Photopolymerization: The use of silver particles as conductive filler is related to the low absorption of the silver nanoparticles at the irradiation wavelength in comparison to other metallic fillers as gold, copper, or aluminum. However, the absorbance of the resulting composite material is strongly dependant of the silver content as shown in Figures 2 and 3. Increasing the silver particle volume fraction results in a decrease of the polymerized thickness, for an irradiation dose of 900 mJ.cm±2. Above the percolation threshold determined at around 6 vol.% (see next section), the polymerized layers have a thickness of less than 20 micrometers, whereas for pure SU-8 polymer, layers of more than 500 micrometers in thickness can be obtained for the same dose.[8] The polymerized thickness, obtained with the composite resist we studied, is of course smaller than the height of microcomponents made of pure SU8, but still bigger than the height that can be obtained with many photoresists. The reduction of the polymerized thickness of the SU8-silver composite resist compared to pure SU8 is related to the absorption of light by the silver nanoparticles and to the high reflectance of the medium as shown in Figure 3. Fig. 2. Evolution of the cure depth versus the silver powder load in the SU8-silver photosensitive composite material. The incident energy is 900 mJ cm±2. 720  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim The reflectance measurements have been obtained using an integration sphere, which allows taking into account all reflections on the sample independently of its thickness. The intrinsic optical properties of the silver particles are not the only cause of these absorption and reflection phenomena: the size and shape of the silver particles also contribute to enhance them. According to several effective medium optical models (Maxwell-Garnett, Bruggeman or Bergam) the influence of the filler properties, such as volume fraction, aspect ratio, and size, affects significantly the optical properties of the composite. Because of the wide range of particle shapes and sizes, as presented in Figure 4, it is difficult to predict the optical behavior of the composite and to make a comparison between theory and measurements. The size and shape of the silver particles and their distribution in the SU8 matrix can be seen from the TEM pictures presented in Figures 4 and 5. The silver nanoparticles tend to agglomerate; this can be related to their different affinity with the solvent and SU8 monomer, but the formulation process and substrate coating step may also induce this particle agglomeration. The effect of the light absorption of the silver particles can be deduced from the TEM image presented in Figure 5: The silver particles act like a mask, preventing the light to crosslink the SU8 monomer deep under the composite-substrate interface. However, some polymerized zones can be observed behind the silver particles, resulting from light scattering. As irradiation is done in backside mode, the surface of the sample is rough, as a consequence of the inhomogeneous penetration of light in the material. Electrical conductivity: The addition of conductive fillers into the insulating SU8 polymer matrix in the form of silver powder results in a conductive composite material. The physical properties of the SU8-silver composite are governed by the properties of the clusters that are formed when contacts are established between the dispersed filler. When the largest cluster has the same size as the dimension of the sample, also called infinite cluster, the percolation occurs. This phenomenon can be characterized by observing the dielectric proper- http://www.aem-journal.de ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9 Jiguet et al./SU8-Silver Photosensitive Nanocomposite a) b) b) Fig. 4. TEM images of the silver powder illustrating the variety of shape and size. ties of the composite, which evolves from insulating characteristics to conductive when increasing the filler content. Figure 6 shows the evolution of the electrical conductivity of the patterned composite components as a function of the loading in silver particles. A drastic change in conductivity appears at the percolation threshold, which can be observed for silver volume fractions between 5 and 6 vol.%. Using a geometric model based on a regular array on which spherical particles are distributed statistically, simple binary composites show a percolation threshold at higher volume fraction, around 16 vol.%.[9,10] The characteristics of the silver powder used in our experiment (the particle size distribution as described in Tab. 1, the shape and the aspect ratio of the particles and aggregates as shown in Fig. 4) are related to the very low powder content at the percolation threshold we measured.[11±13] For the same volume fraction, percolation can oc- ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9 Fig. 5. (a)TEM image of the SU8-silver composite (microcut); (b) illustrates the polymerization propagation: crosslinked SU8 by direct irradiation (zone1), by scattering phenomenon (zone 2), and non polymerized SU8 (zone 3), removed after the development step. http://www.aem-journal.de  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 721 COMMUNICATIONS cur when using elongated particles, even if it is not the case if spherical particles were used. The percolation threshold can be determined more precisely by impedance spectrometry by observing the change from capacitive to conductive behavior of the composite as a function of its powder content. A conductive composite has a similar behavior as a resistance, and shows a constant impedance for a wide range of frequencies. For an insulating composite, the electrical behavior can be compared to the one of a capacitor, in that case, its complex impedance changes significantly with the frequency. Figure 7 shows the variations of the complex impedance of the composite with the frequency for different powder loadings close to the percolation threshold. The switch between capacitive and conductive behavior a) COMMUNICATIONS Jiguet et al./SU8-Silver Photosensitive Nanocomposite Fig. 6. Evolution of the electrical conductivity of the SU8-silver composite versus silver filler volume fraction, before and after thermal treatement. Fig. 8. Evolution of the weight loss of each component of the composite during an isotherm at 120 C. Table 1. Physical characteristics of the silver powder PUAG119 from METALOR. Figure 8 illustrates that the composite weight loss during the thermal treatment comes essentially from the SU8 polymer, and is probably related to the evaporation of volatile organic compounds (remaining solvents) trapped in the polymer matrix. The small weight loss coming from the silver powder can be explained by the removal of the particle surface coating (surfactant). The comparison between impedance spectrometry and TGA characterizations (Fig. 9) shows similar evolutions. When increasing the temperature, the composite impedance evolves in a similar way as the weight loss of the composite, which can be linked to a rearrangement of the particles in the polymer matrix due to the epoxy resin shrinkage.[3,5,15] Structures and resolution: To use this new electrically conductive composite material with photosensitive properties for microfabrication, resolution tests have been performed. Structures in SU8-silver composite have been produced by UVLIGA in back side and in front side modes. The back side mode has been preferred because of the better adhesion between the composite microstructures and the substrate. Figures 10 and 11 show structures patterned in backside mode. The ones obtained without direct contact between the mask and the resist have a lateral resolution typically of around 30 lm (Fig. 11a), whereas the integration of the mask on the substrate, as explained previously, improves significantly the resolution, which becomes better than 5 lm (Fig. 11b). Conclusion A new electrically conductive photoresist has been developed. It is based on silver nanoparticles embedded in SU8, a photosensitive negative-tone resist. This composite material becomes electrically conductive, when a sufficient quantity of powder is added in the SU8 resin, defining the percolation threshold. It happens for a low silver volume fraction (around 6 vol.%), which is economically interesting. Depending on the silver volume fraction, the composite structures show a wide range of electrical conductivities. A soft thermal treatment can further enhance the conductivity of one order of magni- Characteristics Results Density 2.5 Specific area 1.84 m2.g-1 Size distribution at 10 % 0.2 lm Size distribution at 50 % 1.5 lm Size distribution at 90 % 2.5 lm g.cm-3 defines the percolation threshold,[14] and can be observed around a 6 vol.% silver particle loading. The application of a soft thermal treatment (an isotherm at 120 C) to the SU8-silver composite structure induces an enhancement of the electrical conductivity of the conductive composite of more than one order of magnitude as shown in Figure 6. Thermogravimetric analyses have been carried out with a DSC-TGA Mettler apparatus in order to explain this change. Fig. 7. Complex impedance characterization of SU8-silver composites by impedance spectrometry. 722  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.aem-journal.de ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9 Jiguet et al./SU8-Silver Photosensitive Nanocomposite COMMUNICATIONS Fig. 9. Comparison between the impedance evolution and the weight loss of a SU8-silver composite during an isotherm at 120 C. (a) tude. The SU8-silver structures made by UV-LIGA in back side mode and with the integrated mask, show a resolution better than 5 lm. Experimental The composite photoresist material is prepared by mixing homogeneously the photosensitive SU-8 resin (from Gerstel SA) and the silver powder (PUAG119 from METALOR). The physical properties of the powder are presented in tab. 1; the mean cluster size of the silver powder is of 1.5 lm, but the primary particle size is in the nanometer range. The formulation viscosity is adapted by adding gamma-butyrolactone (from Fluka) as solvent of the photopolymer, to allow a suitable use of the blend without sedimentation. However, the resulting formulations have a rheological behavior, which do not allow spin-coating for spreading them on wafers, and a scraper has been used for the deposition of the layers of composite resist on the wafer. The SU8-silver composite structures have been manufactured by the UV-LIGA process. Once the photosensitive composite material is spread on the surface of a wafer, a pre-ex- Fig. 10. Test structures in composite: a 5 mm diameter wheel. ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9 (b) Fig. 11. SEM images of 8 lm thick structures containing 10 vol.% of silver powder (resolution test). posure bake of a few minutes on a hotplate at 95 C is started to evaporate the solvent. The polymerization is begun by a UV exposure, done through a mask with a Karl Suess Mask Aligner, either in front side or backside mode. It is followed by a post-exposure bake of 15 min on a hotplate at 95 C. The development of the structures is done with the appropriate developer. Irradiation made in backside mode allows a better adherence of the final patterned composite on the substrate, but results in a loss of resolution because the mask is not in contact with the resist. To have both a good adherence of the patterned structures on the wafer and a good resolution, the mask can be defined directly on the substrate.[16] This is obtained by depositing a layer of 1.5 lm of amorphous silicon on the surface of a quartz wafer. This layer is patterned by dry etching using a photopatterned positive photoresist layer as mask. http://www.aem-journal.de  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 723 COMMUNICATIONS The thickness of the obtained structures in SU8-silver composite was measured by a surface profiler (Alpha-Step 500, TENCOR) and their electrical resistivity was characterized by resistivity meter four-point measurements (FPP-5000, VEECO). Impedance spectrometry was also employed in a fourpoint configuration to characterize the electrical behavior of the SU8-silver composite; Two LCR-meters were used alternatively to sweep a large frequency range, from 100 Hz to 10 MHz (HP4284A was used for the frequencies between 20 Hz and 1 MHz and HP 4285A for the frequencies between 75 kHz and 30 MHz). ± [1] S. K. Bhattacharya, Metal-filled polymers: properties and applications (Eds: S. K. Bhattacharya), Marcel Dekker, INC., New-York 1986. [2] V. E. Gul', Structure and properties of conducting polymer composites, (Eds: V. E. Gul'), VSP BV, The Netherlands 1996 [3] D. Lu, Q. K. Tong, IEEE Transactions on electronics Packaging Manufacturing 2000, 22, 228. [4] M. Sun, Microelectronics Journal 2001, 32, 197. [5] R. Strumpler, J. Appl. Phys. 1996, 80, 6091. [6] H. Lorenz, M. Despont, J. Micromech. Microeng. 1997, 7, 121. [7] M. O. Heuschkel, L. J. GuØrin, Sensors Actuators B 1998, 48, 356. [8] H. Lorenz, M. Despont, Sensors Actuators A 1998, 64, 33. [9] S. Kirkpatrick, Review of Modern Physics 1973, 45, 574. [10] R. Zallen, The Physics of Amorphous Solids, Vol. 11 (Eds: R. Zallen), Wiley, New- York 1983. [11] A. Y. Dovzhenko, P. V. Zhirkov, Phys. Lett. A 1995, 204, 247. [12] I. Balberg,N. Binenbaum, Phys. Rev. B 1983, 28, 3799. [13] F. Carmona, Physica A 1989, 157, 461. [14] R. Biller, J. Physics 1985, 18, 989. [15] G. R. Rushau, S. Yoshikawa, J. Appl. Phys. 1992, 72, 953. [16] M. C. Peterman, P. Huie, J. Micromech. Microeng. 2003, 13, 380. ______________________ Polyamide-6/Silica Nanocomposites** By Monserrat García,* Javier García-Turiel, Ben Norder, Francisco Chavez, Bart J. Kooi, Werner E. van Zyl, Henk Verweij, and Dave H. A. Blank Growing research interest is focused on hybrid materials consisting of polymers containing nanoscale inorganic fillers,[1] because of the enhanced conductivity,[2,3] improved mechanical properties (e.g. stiffness and toughness),[4,5] and selective separation[6] that could result. Mechanical properties of filled polymers are mainly influenced and characterized by degree of dispersion, amount of filler added, crystallinity, ± [*] Dr. M. García, Prof. D. H. A. Blank Inorganic Materials Science and MESA+ Research Institute, Faculty of Science and Technology University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands E-mail: [email protected] J. García-Turiel Van't Hoff Institute for Molecular Sciences University of Amsterdam, Nieuwe Achtergracht 166 ITS, 1018 WV Amsterdam, The Netherlands B. Norder Faculty of Applied Sciences University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands Dr. F. Chµvez Department of Chemical Engineering Princeton University, NJ 08544, Princeton, USA. Dr. B. Kooi: Department of Applied Physics, Materials Science Group University of Groningen 9747 AG Groningen, The Netherlands. Dr. W. E. van Zyl Department of Chemistry and Biochemistry Rand Afrikaans University P.O. Box 524, Auckland Park, 2006, South Africa Prof. H. Verweij Department of Materials Science and Engineering, The Ohio State University Columbus, OH 43210±1178, USA [**] The author thanks Prof. M.H. Wagner and Prof. O. Manero for their valuable feedback and Wim Vis and Tom Nijmeijer for the sample preparation. This work was performed with financial support from the Netherlands Organization for Scientific Research (NWO-PPM). 724  2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/adem.200400059 ADVANCED ENGINEERING MATERIALS 2004, 6, No. 9