Effects of Friction Coefficient and Cohesion between a Mold and a Polymer Resist during Demolding Process in Hot Embossing

Transcription

1 Journal of Photopolymer Science and Technology Volume 29, Number 1 (2016) C 2016SPST Effects of Friction Coefficient and Cohesion between a Mold and a Polymer Resist during Demolding Process in Hot Embossing Rui Zhang 1,2, Qing Wang 1,2, Xu Zheng 1,2, Lijun Ma 1,2 and Jintao Zhang 1,2 1 Shandong University of Science and Technology, College of Civil Engineering and Architecture, Institute of Nano Engineering, 2 Shandong Provincial Key Laboratory of Civil Engineering Disaster Prevention and Mitigation 579 Qianwangang Road, Qingdao, Shandong , China qwang@sdust.edu.cn Pattern fidelity in hot embossing is influenced to a large extent by defects induced with mold releasing from polymer resist. It is necessary to investigate contact properties of interface between the mold and the polymer resist during demolding process. In this paper, influences of friction coefficient and cohesion on deformation in the polymer resist during the demolding are studied via finite element method. An optimization finite element model with nickel mold/polymer resist structure was constructed, considering both mechanical properties of the polymer resist and optimal element number. Simulation results show that there exist the optimum friction coefficient and the optimum cohesion allowing the deformation of the polymer resist to be minimal. These optimum contact properties can be applied in guiding selections of material and improving quality of embossed patterns. Keywords: Hot embossing, demolding, contact properties, deformation, finite element method 1. Introduction In recent years, the increasing applications of polymer-based devices in bio-, chemical- and optical-mems (micro-electro mechanical system) have resulted in mature polymer fabrication technology [1]. Hot embossing is one of the most promising techniques to fabricate micro- and nano-scale patterns with high throughput and at low cost [2]. Compared with other techniques, hot embossing can replicate delicate microstructures with high aspect ratios and small distortions [3]. Along with the development of hot embossing technologies, large-scale parallel replication and automation can be achieved for industrial production. Thus, hot embossing is then not only popular in the laboratory but also attractive to industry [4]. Hot embossing consists of four sequential steps: preheating, molding, cooling and demolding. First, a thermoplastic polymer resist is heated up over its glass transition temperature (T g ). Second, a mold with desired structures is pressed into the polymer resist. Then, the mold/resist assembly is cooled down below the T g after complete filling. Finally, the mold is separated from the resist during demolding and the fine pattern on the mold is transferred to the resist [5]. In the demolding process, microstructure deformations and even fracture defects are often induced because of the adhesion between the mold and the resist [6]. The deformations and defects will lead to quality reduction of the replica and contamination of the mold. Therefore, optimizing the demolding process plays an important role in eliminating pattern defects, improving embossing quality, and prolonging service life of the mold [7]. The extensive studies have been performed both experimentally and numerically on the demolding process [8]. In respect of improving demolding technique, the reduction in defects via strengthening adhesion between the resist and the underlay was investigated experimentally by Satoshi et al. [9]. Computer models and simulation tools for the hot embossing process were developed by Worgull et al. [10,11]. Song et al. simulated the demolding process and demonstrated influences of demolding rate and demolding angle on inner stress of the resist [12]. In respect of improving the mold design, Mekaru et al. observed that increasing sidewalls incline angle of the mold could decrease demolding defects [13]. Jena et al. compared different mold materials and the epoxy mold was recommended due to its lowest surface roughness [14]. Received March 30, 2016 Accepted May 11,

2 Furthermore, in terms of dealing with the contact interface, Reedy et al. employed a friction surface interaction model to study effects of interfacial strength on demolding [15]. Chan-Park et al. proved that the lowering of interfacial strength reduced the maximum stress in polymer, and proposed that the interfacial strength could be modified by putting a release coating on the nickel (Ni) mold [16,17]. Guo et al. showed that using polytetrafluoroethylene (PTFE) films as a release agent on the Ni mold could reduce shear forces and minimize the area risking high stress of damage [18]. Different methods of placing PTFE films were explored by Jaszewski et al. [19]. Tian et al. proved that Ni-PTFE composite mold had lower friction coefficient and longer lifetime than the Ni mold [20]. The Ni-PTFE molds were fabricated by Zhang et al. and high fidelity replicas were produced using the novel mold [21]. Although previous research efforts have been made, the mechanical response of the polymer resist during demolding under various conditions of contact properties in hot embossing has not been fully understood. An in-depth and systematic study on the contact properties is exigent for determining material selections that lead to small deformation in the polymer resist and thus the success of demolding. In present study, we investigated the demolding process using numerical simulation based on an optimization model to explain the influences of friction coefficient and cohesion on the deformation in the polymer resist. An optimum friction coefficient and an optimum cohesion were obtained through comparing deformation-demolding displacement curves, and the materials for mold and resist were proposed based on these optima. 2. Simulation Method Geometry Model Demolding process was analyzed by the finite element method using ANSYS A two-dimensional (2D) model with Ni mold/pmma resist structure and boundary conditions adopted for simulation is shown in Fig. 1. The height (H) and the width (W) of the embossed nanostructure were 100 nm. The contact between the mold and the resist was defined to be slip-allowed, indicating the mold separation from the resist was allowed. Displacement of 100 nm normal to the mold was applied on the top surface of the model. The bottom surface of the model was fixed. Horizontal displacement on the right edge was confined and a symmetric boundary was applied on the centerline in order to save the calculation time Material Properties The polymer resist (PMMA) is much easier to deform than the mold (Ni), as a result, an accurate presentation of mechanical properties of polymer resist is crucial for the demolding simulation. Due to the assumption of incompressibility and isotropy made for PMMA, a Mooney-Rivlin model was used to accommodate the properties of PMMA [22]. The Mooney-Rivlin model was simply used to study the characteristics of stress and deformation of polymer resist in the elastic range, without considering the viscoelastic and plastic effects. In this simulation, the Mooney-Rivlin model parameters, C 10 and C 01, were included as PMMA properties considering the actual deviatoric deformation. Fig. 1. Two-dimensional geometry model and boundary conditions adopted for simulation. They are derived from the following approximate relations [23]: 6(C01 C 10) E, C C, (1) 10 4 C10 E, 1 C01 E, (2) where E refers to Young s modulus of PMMA. Other material properties of the mold and the resist are summarized in Table Finite Element Model Before the demolding process, the curing process and cooling down of the resist can lead to the distribution of residual stress in the resist, and the existence of residual stress will affect the normal force on interface in the demolding process. However, the demolding process was separated from the preceding processes in this simulation. It was assumed that the demolding process begun as an ideal state without considering the residual stress in the 40

3 Structure Material Table 1. Material properties of the mold and the resist. Density (kg m -3 ) Young s modulus (MPa) Poisson s ratio Mold Ni Mooney-Rivlin constants (MPa) Resist PMMA C 10 =440/C 01 =110 Table 2. Friction coefficient of Ni-PTFE with different content of PTFE PTFE content (g/l) Friction Coefficient resist. The plane strain assumption was adopted to simplify the simulation as the perpendicular deformation of polymer resist to the mold surface was negligible during demolding. A 2D four-nodal linear structural element PLANE 42 was used to represent the Ni mold, and a 2D four-nodal hyperelastic structural element PLANE 182 was employed to represent the PMMA resist. A contact element of CONTA 172 was applied for the interface between the Ni mold and the PMMA resist, representing the slip-allowed boundary [12]. Moreover, the refinement mesh number is decisive for enhancing the precision of subsequent finite element calculation. In this simulation, the mesh number of 2726 was found to be the optimum, and the mesh number meant the number of elements. 3. Results and Discussions Influence of the friction coefficient Previous studies have demonstrated that the mold of Ni-PTFE composite material has smaller friction force than that of the Ni mold [18]. It was found that tribological properties of polymer blends differ from those of monomer [24]. Table 2 lists the friction coefficient (μ) of Ni-PTFE with different content of PTFE [21]. It can be seen that increment of PTFE contributes to lowing the friction coefficient. In our study, an optimum content of PTFE that allows the deformation in the polymer resist during the demolding process to be minimal, has been ascertained using the optimization model. Based on the simulated results, the influence of the friction coefficient on stress and deformation in resist was analyzed. Fig. 2. Distribution of von Mises stress for the friction coefficient (μ) of 0.40, 0.26, and 0.18 at the demolding displacement of 45 nm and 65 nm, respectively. 41

4 Fig. 3. Influence of the friction coefficient (μ) on the deformation at the top corner node A during the whole demolding process. Figure 2 shows the stress distribution for the demolding process under various conditions of friction coefficient. It can be found that stress and deformation at the top of the pattern are influenced remarkably by the friction coefficient. As can be seen, in the case of demolding displacement is 45 nm, depressed deformation occurs at the top of the pattern. As friction coefficient decreases, both the deformation and the maximum stress decrease. However, in the case of demolding displacement is 65 nm, raised deformation appears at the top of the pattern. As friction coefficient decreases, differing from the prior case, both the deformation and the maximum stress increase. In particular, when friction coefficient equals 0.18, stress concentrates at the top edge of the pattern in both cases. Figure 3 shows the influence of friction coefficient on the deformation at the top corner node during the whole demolding process. As can be seen in Fig. 3, in the initial stage of demolding, the lowering of the friction coefficient decreases the deformation and the minimum deformation occurs when friction coefficient (μ) is lowest (μ=0.18). However, in the late period of demolding, the minimum friction coefficient (μ=0.18) results in the maximum value of deformation. Comparatively, friction coefficient of 0.20 causes the least fluctuation, which indicates the node deformation is minimized. From the table of friction coefficient of Ni-PTFE corresponding to the different contents of PTFE in Table 2, the PTFE content can be optimized to 15 g/l when the friction coefficient is chosen to be 0.20 for a preferable demolding Influence of the cohesion Adhesion between the mold and the polymer resist plays an important role during the demolding process [18]. Herein, the adhesion is referred to molecular attraction exerted between the surfaces of mold and resist in contact. In this section, cohesion as a molecular attraction by which the particles of resist are united throughout the mass, is studied for selecting an optimum value. The demolding process with different cohesions was simulated and the influence of cohesion on stress and deformation in resist was discussed. Figure 4 shows the stress distribution for the demolding process under various conditions of cohesion. Fig. 4. Distribution of von Mises stress for cohesions (COHE) of 0.1 MPa, 10 MPa, and 1000 MPa at the demolding displacement of 45 nm and 75 nm, respectively. 42

5 demolding displacement reaches 70 nm. Afterwards, the deformation decreases with increasing cohesion. Then the optimum cohesion is selected to be 100 MPa, and corresponding material for polymer resist can be used. Finally, these optimum contact properties can be applied in guiding the material selections. The optimization of materials not only facilitates demolding with less deformation in the polymer resist, but also benefits hot embossing with high fidelity of patterns. Acknowledgments This work was supported by Taishan Scholarship Project of Shandong Province, China (No. tshw ). Fig. 5. Influence of cohesions (COHE) on the deformation at the top corner node A during the whole demolding process. As can be observed, when demolding displacement is 45nm, depressed deformation occurs at the top of the pattern. As the cohesion increases, neither the deformation nor the maximum stress is changed. However, when demolding displacement is 75 nm, raised deformation appears at the top of the pattern. As the cohesion increases from 0.1 MPa to 1000 MPa, accompanied with a decrease in the raised deformation, the maximum stress is reduced approximately by half. For further investigation, the influence of cohesion on the deformation at the top corner node A during the whole demolding process is presented in Fig. 5. These results show that deformation induced by mold separation from the resist does not depend on the cohesion before the demolding displacement reaches 70 nm. Afterwards, the deformation gradually decreases with the increase of the cohesion. When the cohesion is 100 MPa, the deformation at the top of the resist is minimized. Therefore, the optimum cohesion of 100 MPa can facilitate demolding with less pattern defects. 4. Conclusion The influences of friction coefficient and cohesion on the stress and deformation of the polymer resist during demolding have been investigated. The simulated deformation-demolding displacement curves reveal that there exist the optimum friction coefficient and the optimum cohesion allowing the deformation to be minimal. Firstly, with increasing friction coefficient, the maximum stress and the deformation increase at the beginning of demolding and decrease in the late demolding. Then the optimum friction coefficient is selected to be 0.20, and the optimum PTFE content in Ni-PTFE mold is ascertained to be 15 g/l. Secondly, the deformation does not depend on the cohesion before the References 1. O. Rötting, W. Röpke, H. Becker, and C. Cärtner, Microsyst. Technol., 8 (2002) H. Becker and U. Heim, Sensor. Actuat. A-Phys., 83 (2000) M. Worgull and M. Heckele, Microsyst. Technol., 10 (2004) M. Heckele and W. K. Schomburg, J. Micromech. Microeng., 14 (2004) R1. 5. Y. Hirai, S. Yoshida, and N. Takagi, J. Vac. Sci. Technol. B, 21 (2003) T. Shiotsu, K. Uemura, T. Tochino, S. Ooi, Y. Onishi, M. Yasuda, H. Kawata, T. Kobayashi, and Y. Hirai, Microelectron. Eng., 123 (2014) T. Kitagawa, N. Nakamura, H. Kawata, and Y. Hirai, Microelectron. Eng., 123 (2014) T. Shiotsu, N. Nishikura, M. Yasuda, H. Kawata, and Y. Hirai, J. Vac. Sci. Technol. B, 31 (2013) 06FB S. Takei, T. Ogawa, R. Deschner, and C. G. Willson, Microelectron. Eng., 116 (2014) M. Worgull, J. -F. Hétu, K. K. Kabanemi, and M. Heckele, Microsyst. Technol., 12 (2006) M. Worgull, J. -F. Hétu, K. K. Kabanemi, and M. Heckele, Microsyst. Technol., 14 (2008) Z. Song, J. Choi, B. H. You, J. Lee, and S. Park, J. Vac. Sci. Technol. B, 26 (2008) H. Mekaru, O. Koizumi, A. Ueno, and M. Takahashi, Microsyst. Technol., 16 (2010) R. K. Jena, C. Y. Yue, Y. C. Lam, P. S. Tang, and A. Gupta, Sensor. Actuat. B, 163 (2012) E. D. Reedy Jr. and J. V. Cox, J. Eng. Mater. Technol. ASME, 135 (2013) M. B. Chan-Park, Y. C. Lam, P. Laulia, and S. C. Joshi, Langmuir, 21 (2005) M. B. Chan-Park, Y. Yan, W. K. Neo, W. Zhou, J. Zhang, and C. Y. Yue, Langmuir, 19 (2003) Y. Guo, G. Liu, Y. Xiong, and Y. Tian, J. Micromech. Microeng., 17 (2007) 9. 43

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