Reinforcement of a PDMS master using an oxide-coated silicon plate

Abstract

In this work, a new method was developed to increase the stiffness of Polydimethylsiloxane (PDMS) masters using oxide-coated silicon plates, aimed at reducing the residual and deflecting deformations of the PDMS masters for proper pattern transfer. Using this method, these two types of deformations in the reinforced PDMS master have been reduced.

Introduction

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). It is not photo-definable (i.e. not a photoresist), and is usually patterned by a molding process [4], [5], [6]. 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 make good pattern transfer to those substrates. On the other hand, due to its large expansion coefficient, PDMS may have large residual deformation after it is patterned via a molding process. The residual deformations of PDMS masters may cause misalignment problems along horizontal directions when they are employed to transfer patterns on pre-defined features on the substrates. Furthermore, due to its low stiffness, PDMS may have pairing and deflecting deformations when it is used to transfer patterns [7], [8]. Paring deformations mean that the long structures in PDMS masters tend to stick together under their own weight, leading to the failure of pattern transfer. Meanwhile, in order to have a good pattern transfer from a PDMS master to a substrate, a pressure is applied on the PDMS master to make it have intimate contact with the substrate. Under this pressure, the surfaces at the bottom of the convex PDMS features may have large deflections, making those surfaces come into contact with the substrate (which are so-called sagging deformations) and causing undesirable printing. Even small deflections of those surfaces may cause problems. For example, cross-section changes in concave PDMS patterns may lead to generation of improper patterns in a microcapillary molding method of soft lithography [9]. Therefore, it is necessary to reduce these three types of deformations (i.e. residual, pairing, and deflecting deformations) in a PDMS master for transferring patterns to substrates in a reliable manner.

In the three types of deformations, the pairing problem appears relatively easy to solve. When the ratio between the feature height and the feature separation in the PDMS master is smaller than 0.5, the two neighboring structures should not have contact with each other. PDMS structures are the replica of the corresponding structures in the mold. Therefore, if the concave features in the mold are designed to have a low aspect ratio, then the pairing deformations of the PDMS structures can be avoided. However, the aspect ratios of voids cannot be too low. For example, voids of low aspect ratio (<0.2) are susceptible to sagging deformations [10]. Thus, it seems that, to avoid the pairing deformations, a good aspect ratio of recessed PDMS patterns is between 0.2 and 0.5. In this work, we focus on reducing residual and deflecting deformations of a PDMS master by reducing its thermal expansion coefficient and increasing its stiffness, respectively.

The stiffness of a material can be increased by adding, for example, stiffer particles or fibers inside [11]. The average stiffness of the resulting composite material increases due to the contribution of the reinforcements. Likewise, if those particles or fibers have smaller thermal expansion coefficients than this material, then the average thermal expansion coefficient of the resulting composite material is reduced. Accordingly, the residual and deflecting deformations of the composite material should be smaller than those of a pure material. Using a similar idea, we added SU-8 particles in a PDMS master [12]. SU-8 is a negative photoresist. It has a higher stiffness and a lower thermal expansion coefficient than PDMS. With the addition of SU-8 particles, average residual strain of the reinforced PDMS was reduced from 5 to 1%.

Nevertheless, there exist two problems associated with the particle and fiber reinforcement approaches. First, the local stiffness and thermal expansion coefficient of the material between two neighboring particles or fibers still remain the same such that large local residual and deflecting deformations may still exist. Second, particles or fibers may not uniformly distribute in the composite material. The composite material is usually made by first adding particles or fibers in a liquid solution of the material, then stirring the solution to make particles or fibers uniformly suspended inside the solution, and finally solidifying the solution. Particles or fibers may sink down inside the solution after the stirring process, causing the non-uniform distribution problem [12], [13]. To avoid these two obstacles in the particle and fiber reinforcement approaches, we chose a plate, instead of particles and fibers, to reinforce PDMS. The plate was embedded throughout the PDMS, allowing for a better control of both global and local deformations of the PDMS.

The outline of this work is as follows. In Section 2, the design of a reinforced master is presented and reduction of its deformations is discussed. In Section 3, the fabrication of the reinforced master is introduced. In 4 Reduction of deflection, 5 Summary, residual and deflecting deformations in the pure and reinforced PDMS masters are compared based on experimental and numerical results, respectively. Finally, in Section 6, this work is summarized.