A new method for direct patterning of Poly(dimethylsiloxane) (PDMS) microstructures is developed by taking advantage of photorefractive effect in a functionalized substrate. Here we show that when a x-cut Iron doped Lithium Niobate (LN) crystal is exposed to appropriate structured laser light, a charge density pattern builds-up in the crystal and a space charge field arise that is able to induce self-patterning of the PDMS liquid film deposited on its surface via the dielectrophoretic effects. Proper heating treatment allows to achieve polymeric linking process creating a solid and stable PDMS microstructures. The self-patterned structures replicate the illuminating light pattern. We show that 1D and 2D patterning of PDMS gratings can be achieved. This new soft-lithographic approach can pave the way for realizing PDMS micro-structures with high degree of flexibility that avoids the need of moulds fabrication.
©2010 Optical Society of America
PDMS is a polymeric compound extensively employed in latest years in the fields of micro/nano fluidics [1,2], lab-on-chip (LOC)  and micro/nanoelectromechanical (MEMS/NEMS) devices . Among its distinctive features significant roles are played by moulding handiness and the biocompatibility. The PDMS prototyping is always accomplished by soft-lithography , a technique based on the fabrication and/or replication of predefined structure using stamps, molds, or photomasks. In Ref . PDMS patterning is obtained by pouring it onto a substrate with established patterns and removing the one in excess with a blade. A. Pawlowski et al. used a similar method to create a PDMS mask employed to fabricate structures in glass by powder blasting . Different techniques are based on both wet chemical and dry (plasma) etching  “Decal Transfer Microlithography” was implemented where patterned PDMS is added to a substrate . Several applications of this technique are reported in literature [10,11]. Another approach for direct patterning of thin PDMS films is the “bond-detach” method proposed for nanolithography patterning . Besides previous techniques, a range of point wise methods exist to perform surface nano-patterning, such as the dip pen writing . Great interest in PDMS microstructure patterning is due to the high transparency of PDMS in visible light that motivates the integration of optical component in microfluidic devices made of the same material. Many PDMS LOC devices made of microoptical components  such as 2D lenses , prisms , and waveguides  have been demonstrated. In particular, for waveguides, core and cladding are both obtained with PDMS mixture where the refractive index difference between the two parts is realized diluting the precursor mixture with hexane. Different structures, such as one-dimensional gratings, are realized by a three steps method based on holographic interferometry and micro-molding process . Thanks to all its features, PDMS devices are in the last years largely employed in biology, for example, for stretching and aligning DNA molecules . Recently, it has been demonstrated that an array of PDMS microbumps with a periodic structure can be obtained by a different driving mechanism based on pyroelectric effect [20,21] on the surface of periodically poled LN crystal .
In this paper we propose a completely new method for direct patterning of PDMS relief structures by laser light onto a functionalized substrate of LN crystal. This method will be reported, by the authors, as Light Induced Patterning (LIP). The process is a full field technique that, conversely to the point wise ones, allows PDMS patterning on the whole crystal surface with a single exposure to the suitable laser light. LN is a photorefractive material and when exposed to spatially modulated laser light it produces inside the crystal a space-charge field [23,24]. Such space-charge distribution generates an electric field gradients patterns on its surface. When the sample is covered by a thin liquid film of PDMS, the electric field generated on its surface, is responsible for a rearrangement of the PDMS film due to dielectrophoretic forces . The liquid PDMS follows the geometric pattern of laser light illumination pattern. Recently it has been demonstrated that charged and uncharged particles can be respectively trapped by electrophoretic and dielectrophoretic forces on the surface by photorefractive effect on LN crystal [26,27]. However here we show, for the first time, that liquid polymer, i.e. PDMS, can be patterned by dielectrophoretic forces generated by photorefractive effect driven by laser illumination. Indeed, thanks to LN and photorefractive effect it is possible to obtain PDMS patterning without external voltage supplies. Moreover, while the light source generates the PDMS structure, an appropriate thermal treatment applied to the crystal, induces the cross-linking of the PDMS, leading o a stable and reliable PDMS microstructure. Illustration of the method to fabricate such PDMS micro-structures and their accurate characterization by interferometric technique is reported in this paper. The PDMS microstructures could find application in microfluidic and bio-photonics lab-on-chip devices.
2. Device fabrication and operation principles
We employed as substrate an x-cut Fe2+/3+ doped LN crystal whose dimensions were 1cm × 1cm in y and z axes while thickness (x-axis) was 500μm, the iron dopant level is 0.05% weight. The covering substance was a dielectric liquid: PDMS. LN crystal wasn’t coated with intermediate matter. The fabrication process was divided in three stages: (i) PDMS spinning on the substrate, (ii) PDMS self-patterning under the effect of electric field generated on the substrate by laser irradiation and (iii) PDMS curing by suitable heating process. In order to obtain a thin and continuous PDMS film with a uniform thickness onto the LN substrate we used a spin coating process. A small amount of liquid PDMS (about 30ml) was dispensed on the substrate and spin-coated for 2 minutes at 6000 rpm (please note that was the maximum velocity supported by our spin-coater) [see Fig. 1(a) ]. A particular care is necessary for PDMS preparation and spinning so this first fabrication step was performed in aerated place and far away heating sources to avoid PDMS getting dirty and solidification. After the spinning, the sample was positioned in the optical set-up shown in Fig. 1(b) in order to achieve PDMS patterning. We employed an Argon Ion laser at wavelength of 514nm. The laser beam was made linearly polarized by means of a Glan-Taylor polarizer and then expanded in order to get a beam size larger than the whole LN surface. We realized two different arrangements by using a linear grating and a 2D optical target with radial pattern, respectively. Such amplitude gratings were inserted in the light optical path for projecting onto the sample a structured illumination laser light. The final aim was to fabricate a PDMS structure replicating the projected spatial light distribution. A linear light distribution was obtained by means of an 100μm amplitude grating (AG), inserted in the setup as depicted in Fig. 1(b), The AG was imaged by a lens (L) whose focal length is 25.4mm. The LN substrate, previously covered with a liquid PDMS film as described above, was positioned in the conjugate plane of the grating, in horizontal position as shown in Fig. 1.On the LN sample, the replication of the AG spatial intensity distribution, is obtained [see Fig. 1(b)]. Laser power on the sample was about 1W and its polarization is parallel to the c-axis [z direction in Fig. 1(a)]. The illumination was able to excite the charge carriers inside the crystal. The space-charge field  generated inside the material modulate the refractive index via electro-optic effect thus forming a phase grating inside the crystal, that is a replica of the AG grating inserted in the optical set-up, as occurs usually in photorefractive materials. Almost at the same time the space-charge field , generated inside the material, induced, on the upper surface, dielectrophoretic forces that were able to pattern the liquid PDMS film, according to the light pattern distribution. The strength of the space charge field is around 2kV/mm . In general, the force exerted on a dielectric immersed in an electric field is given by the Kelvin Polarization Force Density,Equation (1) can then be written as follows:Eq. (2). Due to such force an hydrodynamic pressure causes trapping and gathering up of PDMS in stripes to form a grating with the same period of the phase grating written inside the crystal. An heating plate (not shown in Fig. 1) is positioned about 2mm above the sample in order to solidify the PDMS. The plate temperature variation is controlled by software and it’s driven from 25°C, with a rate of 0.1°C per second, up to 100°C and it remains constant at 100°C for 20 minutes. A schematic picture and an image of the final device are showed in Fig. 1(c), 1(d), respectively. Two dimensional patterning is accomplished by means of the same optical setup but substituting the 100μm linear grating with the two dimensional one.
3. Results and discussion
3.1. One dimensional patterning
PDMS grating obtained after curing process is showed in Fig. 2(a) , 2(b) where two optical microscope images under bright field with different magnification are presented.Solidified PDMS is gathered up in stripes; each stripe coincides with the grating structure present inside the LN substrate. At lower magnification, a large field of view is imaged and an expert eye can easily detect, in the grating imaged lines, the presence of distortions, typically produced by the spherical aberration of the imaging lens. Moreover some defects are visible in Fig. 2(a) created by the presence of some contamination of the PDMS by dust particles.
In order to characterize the PDMS microstructures, two different measurements were performed. One in reflection geometry by a confocal microscope in order to evaluate the thickness of the PDMS stripes. The second was in transmission geometry, by a digital holographic microscope set-up . To perform the measurement in reflection configuration the full PDMS microstructure was previously covered by a thin reflective aluminium layer (about 200 nm in thickness). In Fig. 3 are shown the results obtained by a confocal microscope. Two dimensional image of 5 PDMS grating stripes is showed in Fig. 3(a), the red points correspond to the higher part of the structured PDMS. The profile is approximately half sinusoidal as reported in profile line along the central portion of the sample [see Fig. 3(b)]. The mean depth value of the PDMS structure results to be of 2μm. Pseudo three-dimensional view of the microstructure is presented in Fig. 3(c).The second analysis is an interferometric measurement carried out in transmission geometry by means of a Digital Holography (DH) setup based on a Mach-Zehnder interferometer (see Fig. 4 ) to characterize, in microscopic configuration, a small portion of the PDMS structure. The employed laser wavelenght is 532nm.The laser beam is first spilt in two beams by a beam-splitter (PBS) and properly expanded (object-beam and reference-beam). The object-beam impinge on the PDMS grating and then is collected by a 10 × microscope objective (represented by lens L2, in Fig. 4, in order to make intelligible the set-up drawing). A LN focussed image is obtained on the image-plane. The reference-beam, reflected by the mirror (M) and a second beam-splitter (BS), impinge directly on a Charge Coupled Device (CCD) camera sensor-plane, which is at a distance “d” from the image-plane. On the CCD plane, an interference-pattern (hologram) is generated by the optical interference between the object-beam and the reference-beam. The resulting image is recorded and numerically processed to calculate the complex optical wavefield on the image plane, i.e. the plane conjugate to the grating one by the MO .The interference pattern recorded by the CCD contains information about the amplitude as well as the phase of the transmitted wavefront, in this way from the numerical back propagation of the complex wavefield it is possible to evaluate both intensity and phase distribution in each plane between the hologram plane and the image plane. Phase retardation introduced by the sample grating in the object beam is calculated just after the sample, the wrapped phase map (phase bonded between –π and π) for a PDMS stripe is depicted in Fig. 5(a) . The unwrapped phase distribution is calculated by means of a conventional unwrapping routine. Two-dimensional image and pseudo three-dimensional view of the continuous phase distribution are presented in Fig. 5(b), 5(c), respectively. Figure 5(d) is a plot of the measured phase retardation showed in Fig. 5(c) along a single line. The DH measured phase difference is about 30rad, in accordance with the calculated value (33rad), taking in account the grating depth, measured by the confocal microscope (2μm), the PDMS refractive index (1.4) and the laser wavelength (532nm), we used for the DH measurement. We reported comparative results carried out by both interferometric and confocal microscopes as complementary and independent investigation tools. DH is a non-invasive technique and measurements are carried out in transmission configuration while the confocal microscopy needs aluminium covering and is performed in reflection mode. In our experimental conditions we were able to prepare PDMS liquid film with thickness no thinner then 1μm due to the limited speed of our spin coater. We believe that thinner layer could lead to obtain pattern with smaller periods for this specific PDMS liquid.
3.2. Two dimensional patterning
In order to prove the capability to pattern a more complex geometry we demonstrate the possibility to carry out PDMS microstructure with radial channels. We used a 2D grating having a radial symmetry. A picture of such a target is shown in Fig. 6(a) . The fabrication process was extended from the centre to the edges of the Fe:LN crystal sample involving all the crystal surface. According to the intrinsic preferential direction of the photorefractive effect, as can be noted in Fig. 6(b), the refractive index change, due to photorefractive effect, is absent in some portions of the crystal while it has a very good uniformity inside an angle of 140 degree around the Y axis. That implies the two dimensional photorefractive grating has an axial symmetry around the Z axis.Since the PDMS liquid layer self-patterning follows the geometry of the internal phase grating, i.e. the presence of electric field, it is demonstrate that good channels structures can be achieved in correspondence of the photorefractive grating. [see Fig. 6(c), 6(d)]. Even if the results shown in Fig. 6 prove the possibility to fabricate 2D structures, it should be noted that limitations lie on the geometries achievable by the proposed LIP approach, i.e. structures having their main direction along the z-axis. However this drawback can be overcome planning accurate design that allows for complex structures of any shape. However such aspect will be a subject of future investigation. An example of complex geometry is shown. A target with word “HIGH” is used [Fig. 7(a) ] and the corresponding PDMS replica is shown in Fig. 7(b). In this case the target is rotated compared to the LN sample and, even though PDMS doesn’t follow exactly the induced structure, it’s able to bound itself also in y and z directions.
We demonstrated a simple but novel surface-charge lithographic process for fabricating PDMS microstructures having 1D and 2D geometries, by LIP approach. The process is based on a direct laser induced self-patterning of PDMS by using, as functionalized material, a Fe2+/3+ doped LN crystal and exploiting its intrinsic photorefractive properties. Linear periodic and radial arrays of micro-structures have been realized. The proposed procedure is a quick and chip alternative to conventional PDMS patterning methods which usually involve several and sophisticated fabrication steps. Our LIP approach, does not require the fabrication of moulds, typically needed in soft-lithography. It has a high degree of flexibility in fabricating distinct geometric patterns by a proper light distribution design, which could be simply obtained by a Spatial Light Modulator (SLM) . The possibility to pattern the polymeric matter in different geometries, depending on the structured light pattern, could candidates this method for the fabrication of devices to be employed in many application fields, from microfluidic to biology, such as cell-patterning and lab-on-chip experiments. LIP structures themselves could be utilized as moulds in soft-lithographic processes. Further developments, to increase device spatial resolution, are under investigation by preparing thinner starting layers, by using different liquid compounds, such as PS (polystyrene), and alternative depositing techniques, in order to perform light induced patterning with lower periodicity.
This work is supported by “Intesa di programma MIUR/CNR per il Mezzogiorno”. Authors would like to thank Prof. Eusebio Bernabeu and Prof. Luis Miguel Sanchez, “Universidad Complutense - Departamento de Optica” of Madrid, for helpful discussions and suggestions and for the performing confocal microscope measurements.
References and links
1. B. H. Jo, L. M. Van Lerberghe, K. M. Motsegood, and D. J. Beebe, “Three-dimensional micro-channel fabrication in polydimethylsiloxane (PDMS) elastomer,” J. Microelectromech. Syst. 9(1), 76–81 (2000). [CrossRef]
3. Q. Kou, I. Yesilyurt, V. Studer, M. Belotti, E. Cambril, and Y. Chen, “On-chip optical components and microfluidic systems,” Microelectron. Eng. 73–74, 876–880 (2004). [CrossRef]
4. T. Sulchek, R. Hsieh, J. D. Adams, S. C. Minne, C. F. Quate, and D. M. Adderton, “High-speed atomic force microscopy in liquid,” Rev. Sci. Instrum. 71(5), 2097–2099 (2000). [CrossRef]
6. K. S. Ryu, X. Wang, K. Shaikh, and C. Liu, “A method for precision patterning of silicone elastomer and its applications,” J. Microelectromech. Syst. 13(4), 568–575 (2004). [CrossRef]
7. A. Pawlowski, A. Sayah, and M. A. M. Gijs, “Precision poly-(dimethyl siloxane) masking technology for high-resolution powder blasting,” J. Microelectromech. Syst. 14(3), 619–624 (2005). [CrossRef]
8. J. Garra, T. Long, J. Currie, T. Schneider, R. White, and M. Paranjape, “Dry etching of polydimethylsiloxane for microfluidic systems,” J. Vac. Sci. Technol. A 20(3), 975 (2002). [CrossRef]
10. W. R. Childs and R. G. Nuzzo, “Patterning of Thin-Film Microstructures on Non-Planar Substrate Surfaces Using Decal Transfer Lithography,” Adv. Mater. 16(15), 1323–1327 (2004). [CrossRef]
11. W. R. Childs and R. G. Nuzzo, “Large-area patterning of coinage-metal thin films using decal transfer lithography,” Langmuir 21(1), 195–202 (2005). [CrossRef]
12. A. L. Thangawng, M. A. Swartz, M. R. Glucksberg, and R. S. Ruoff, “Bond-detach lithography: a method for micro/nanolithography by precision PDMS patterning,” Small 3(1), 132–138 (2007). [CrossRef] [PubMed]
13. F. Stellacci, “Towards Industrial-Scale Molecular Nanolithography,” Adv. Funct. Mater. 16(1), 15–16 (2006). [CrossRef]
14. Q. Kou, I. Yesilyurt, V. Studer, M. Belotti, E. Cambril, and Y. Chen, “On-chip optical components and microfluidic systems,” Microelectron. Eng. 73–74, 876–880 (2004). [CrossRef]
15. S. Camou, H. Fujita, and T. Fujii, “PDMS 2D optical lens integrated with microfluidic channels: principle and characterization,” Lab Chip 3(1), 40–45 (2003). [CrossRef]
16. A. Llobera, R. Wilke, and S. Büttgenbach, “Poly(dimethylsiloxane) hollow Abbe prism with microlenses for detection based on absorption and refractive index shift,” Lab Chip 4(1), 24–27 (2004). [CrossRef] [PubMed]
18. W. C. Chuang, C. K. Chao, and C.-T. Ho, “Fabrication of high-resolution periodical structure on polymer waveguides using a replication process,” Opt. Express 15(14), 8649–8659 (2007). [CrossRef] [PubMed]
19. P. Björk, S. Holmström, and O. Inganäs, “Soft lithographic printing of patterns of stretched DNA and DNA/electronic polymer wires by surface-energy modification and transfer,” Small 2(8-9), 1068–1074 (2006). [CrossRef] [PubMed]
21. P. Ferraro, S. Grilli, L. Miccio, and V. Vespini, “Wettability patterning of lithium niobate substrate by modulating pyroelectric effect to form microarray of sessile droplets,” Appl. Phys. Lett. 92(21), 213107 (2008). [CrossRef]
22. S. Grilli, M. Paturzo, L. Miccio, and P. Ferraro, “In situ investigation of periodic poling in congruent LiNbO3 by quantitative interference microscopy,” Meas. Sci. Technol. 19(7), 074008 (2008). [CrossRef]
23. F. Argullo-Lopez, G. F. Calvo, and M. Carrascosa, “Fundamentals of Photorefractive Phenomena” in Photorefractive materials and their applications I, P. Gunter and J. P. Huignard eds. (Springer 2006) pp 43–82.
24. K. Buse, “Light-induced charge transport processes in photorefractive crystals I: Models and experimental methods,” Appl. Phys. B 64(3), 273–291 (1997). [CrossRef]
26. X. Zhang, J. Wang, B. Tang, X. Tan, R. A. Rupp, L. Pan, Y. Kong, Q. Sun, and J. Xu, “Optical trapping and manipulation of metallic micro/nanoparticles via photorefractive crystals,” Opt. Express 17(12), 9981–9988 (2009). [CrossRef] [PubMed]
27. H. A. Eggert, F. Y. Kuhnert, K. Buse, J. R. Adleman, and D. Psaltis, “Trapping of dielectric particles with light induced space-charge fields,” Appl. Phys. Lett. 90(24), 241909 (2007). [CrossRef]
28. M. Luennemann, U. Hartwig, and K. Buse, “Improvements of sensitivity and refractive-index changes in photorefractive iron-doped lithium niobate crystals by application of extremely large external electric fields,” J. Opt. Soc. Am. B 20(8), 1643–1648 (2003). [CrossRef]
29. P. Ferraro, S. De Nicola, A. Finizio, G. Pierattini, and G. Coppola, “Recovering image resolution in reconstructing digital off-axis holograms by Fresnel-transform method,” Appl. Phys. Lett. 85(14), 2709–2711 (2004). [CrossRef]
30. L. Miccio, D. Alfieri, S. Grilli, P. Ferraro, A. Finizio, L. De Petrocellis, and S. D. Nicola, “Direct full compensation of the aberrations in quantitative phase microscopy of thin objects by a single digital hologram,” Appl. Phys. Lett. 90(4), 041104 (2007). [CrossRef]
31. N. J. Jenness, K. D. Wulff, M. S. Johannes, M. J. Padgett, D. G. Cole, and R. L. Clark, “Three-dimensional parallel holographic micropatterning using a spatial light modulator,” Opt. Express 16(20), 15942–15948 (2008). [CrossRef] [PubMed]