Studies of Polymer Deformation and Recovery in Hot Embossing

STUDIES OF POLYMER DEFORMATION AND RECOVERY IN HOT EMBOSSING

X. C. Shan1*，Y. C. Liu1, H. J. Lu1, Z. F. Wang1 and Y. C. Lam2

1 Singapore Institute of Manufacturing Technology (SIMTech), 71 Nanyang Drive, Singapore 638075 2 Nanyang Technological University, 50 Nanyang Ave, Singapore 639798

• E-mail (X.C. Shan): xcshan@simtech.A-star.edu.sg

ABSTRACT

In large area micro hot embossing, process temperature plays a critical role in both the local fidelity of microstructure formation and global uniformity. Micro embossing at the lowest temperature with acceptable fidelity can improve the global flatness after demolding. This paper focuses on polymer deformation and recovery in micro embossing when the process temperatures are below its glass transition temperature (Tg). In this study, PMMA (Polymethyl Methacrylate) substrates (Tg =105 °C) were employed as the process material, and the process temperature ranged from 25 °C to Tg. It was found that at temperature below Tg-55 °C, significant recovery occurred after processing, but it was still possible to form permanent structures with sufficiently high loading stress. With an increase in temperature, plastic deformation increased and was the dominate polymer deformation for permanent cavities formation. However, the formation of protrusive structures was not complete since there was little polymer flow. The polymer will lose its storage modulus at a raised temperature and microstructures could be formed with high fidelity. A compromise between the local fidelity and global flatness has to be reached in micro hot embossing. Keywords: viscoelastic, recovery, indentation, embossing

1. INTRODUCTION

An amorphous polymer such as PMMA (Polymethyl Methacrylate) behaves as a viscoelecstic solid at raised temperature. At a temperature far above its Tg, the polymer behaves more like a viscous liquid and flows relatively easily into the corners of an embossing mold. As such, microstructures with high fidelity can be obtained [1, 2]. In large area hot embossing, the process temperature, which has a direct effect on the visco-elastic behavior of a polymer, plays a critical role to both the local fidelity and global uniformity of microstructure formation. To improve the fidelity of embossed microstructures, high process temperature (as high as Tg +60°C) are preferred. High temperature embossing, however, has its disadvantages, such as difficulty in demolding and significant residual thermal stress due to the different coefficients of thermal expansion between the mold and the polymer. As a result, the embossed devices will suffer from global warpage or distortion due to mechanical and residual thermal stresses [3]. However, with low temperature hot embossing, the global flatness of an embossed substrate could well be improved significantly. Hence, it is important to perform micro embossing at a process temperature as low as possible, but with acceptable fidelity, especially for large area and high aspect embossing.
This paper focuses on the microstructure formation, polymer deformation and recovery in micro hot embossing at temperature lower than a polymer’s Tg. Both an indentation system [4] and a micro embosser were used to investigate the relationship of microstructure formation versus process temperature and applied stress.
Indentations conducted from 25°C to 50°C (Tg - 55°C) indicated that PMMA behaved like a visco-elastic solid, with viscoelesticity dominated the formation of microstructures with large recovery after demolding. From 85°C (Tg -20 °C) to near Tg, plastic deformation dominated the structure formation, and permanent cavities could be formed on PMMA substrates with decreased recovery with an increase in temperature. However, profile formation of protrusive structures over this temperature range was not complete since there was little polymer flow. It is also found that plastic deformation still contributed significantly in micro embossing even above Tg. With increasing process temperature, microstructures could be formed under lower loading. Considering the local fidelity and global flatness of embossed substrates, micro hot embossing at low process temperature, but with acceptable fidelity, should be preferred.

2. MECHANICS OF POLYMER DEFORMATION

2.1. Viscoelasticity of Polymers

An amorphous polymer like PMMA exhibits highly temperature-dependent viscoelasticity, which determines ©EDA Publishing/DTIP 2007 ISBN: 978-2-35500-000-3

X. C. Shan，Y. C. Liu, H. J. Lu, Z. F. Wang and Y. C. Lam Studies of Polymer Deformation and Recovery in Hot Embossing

the elastic and permanent deformation of the polymer. Viscoelasticity, which is the response of the polymer to an applied stress, contains both an elastic and a viscous components [5]. The total deformation of the polymer under a mechanical stress will contain

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