Fabrication and application of silicon-reinforced PDMS masters
Introduction
Polydimethylsiloxane (PDMS) is a biocompatible [1], ultra-violet transparent [2], and gas-permeable elastomer [3] that can withstand a wide temperature range (−100 to 100 °C). PDMS is easy to process and has been widely applied in the micromachining field [1], [2], [3], [4], [5], [6], [7], [8]. In particular, it has been used as the master material in soft lithography for pattern transfer [7], [8]. PDMS is soft and flexible, enabling it to have intimate contact with substrates and consequently making good pattern transfer to those substrates. On the other hand, due to residual stress induced in the fabrication process, PDMS may have large residual deformations that lead to a misalignment problem, i.e., the PDMS patterns are not generated at the designed locations. To solve the misalignment problem, it is necessary to reduce the residual deformations of PDMS by increasing the stiffness.
Particles or fibers are often used to reinforce a material [9]. In those applications, the average stiffness of the reinforced material has increased due to the contribution of the reinforcements. Using a similar idea, we developed a method in [10] to reinforce PDMS using stiffer SU-8 (a negative photoresist) particles. With the addition of SU-8 particles, the global residual strain of the reinforced PDMS was reduced from 5% to 1%. In order to reduce also the local deformations of PDMS and overcome the non-uniform distribution problem of SU-8 particles in PDMS, we proposed to reinforce a PDMS master using an SU-8 truss structure [11], and the corresponding PDMS residual deformation was reduced to 0.58% on average. In [12], we further explored the reinforcement of a PDMS master using an oxide-coated silicon (Si) plate. The Si plate is stiffer and has a smaller thermal expansion coefficient than SU-8, and could reduce the residual deformations of PDMS masters to approximately zero in principle. However, we faced a critical obstacle (i.e., a peeling-off problem) in generating a Si-reinforced PDMS master using a conventional molding process [12]. PDMS is not photo-definable (i.e., not a photoresist). Its patterning is usually made by a conventional molding method [4], [5], [6], in which PDMS is first coated on and then peeled off from a rigid mold. When a Si plate of a large size is needed and embedded in the PDMS, the reinforced PDMS master becomes rigid and is difficult to peel off due to limited flexibility of Si. That is, it is difficult to separate two rigid structures of large sizes without any damages. For example, when the PDMS layer is about 246 μm thick, only Si plates as large as about one-fifth of a 4 in Si wafer may be used to reinforce this PDMS layer due to the peeling-off problem [12]. This presents an obstacle when a larger pattern area is needed in a reinforced PDMS master. In this work, a new molding process is developed to overcome this obstacle. A flexible SU-8 mold is adopted in the new molding process. The SU-8 mold is first placed on and then peeled off from a rigid PDMS master. In such a way, a Si-reinforced PDMS master of a size as large as a 4 in wafer can be produced. Meanwhile, a new way of releasing large SU-8 structures is presented.
It was indicated in Ref. [7] that many non-polar solvents (e.g., toluene and dichloromethane) cannot be used because they can swell the pure PDMS master. In the preliminary test, we found that chloroform (a non-polar solvent) also swells the pure PDMS master, and that the swelling deformation can be much reduced if a Si-reinforced PDMS master is used in the pattern transfer. Therefore, in this work we apply the reinforced PDMS master to pattern poly-3-hexylthiophene (P3HT) (Riek company), a conducting polymer that is usually dissolved using chloroform.
The outline of this paper is as follows: in Section 2, the design and reinforcing principle of a Si-reinforced PDMS master are introduced. In Section 3, the new molding process and corresponding experimental results are presented and discussed. In Section 4, the reinforced PDMS masters are applied to pattern P3HT. Finally, in Section 5, this work is summarized and concluded.
Section snippets
Design of a Si-reinforced PDMS master
Schematic of a Si-reinforced PDMS master is shown in Fig. 1. The master consists of a microstructure-formed PDMS layer and a 4 in SiO2-coated Si wafer. Due to the contribution of the Si plate, both residual and deflecting deformations of the PDMS master are expected to be reduced. On the other hand, the PDMS surface in the reinforced master still maintains its high flexibility along the vertical direction and thus has intimate contact with a substrate when the master is employed for pattern
The new molding process of generating a Si-reinforced PDMS master of a 4 in wafer size
This new molding process includes four steps (Fig. 3):
- (1)
fabricate an SU-8 structure (mold) on a transparency using a standard ultra-violet lithographic process (spin-coating, soft-baking, exposure, development, and post-baking),
- (2)
spin-coat liquid PDMS (ratio between PDMS and its curing agent is 10:1) on a 4 in oxide-coated Si wafer; the thickness of the PDMS layer can be controlled by adjusting the quantity of the liquid PDMS, spin time and speed,
- (3)
place the transparency on the PDMS layer with the
The patterning approach
Whitesides et al. [7] indicated that many non-polar solvents (e.g., toluene and dichloromethane) cannot be used because they can swell the PDMS master. We found that the chloroform (a non-polar solvent) also swells the PDMS master. Fig. 8 shows the comparison of pure PDMS and reinforced PDMS under the effects of chloroform. Before getting exposed to chloroform, both of them were flat (Fig. 8a). After contacting the solvent, the pure PDMS membrane buckled up due to the swelling deformation.
Summary and conclusion
A new molding process was presented to generate a Si-reinforced PDMS master of a large wafer size (i.e., 4 in) using a flexible SU-8 mold. Different from a conventional molding process, which has a flexible PDMS and a rigid mold, the new method has a rigid PDMS substrate and a flexible mold. Meanwhile, it was found that an SU-8 structure could be easily released when a transparency served as the substrate in fabricating this SU-8 structure, which provides a simple way to obtain free-standing,
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