D. Koh, A. Wang, P. Schneider, B. Bosinski, K.W. Oh
University at Buffalo,
Keywords: PDMS, mixing ratio, curing agent, flexible electrodes
Summary:This paper reports a new method of transferring various metal layers to PDMS by creating strong bonding between the metal and PDMS without any chemical treatment or adhesive layer. It is a very simple and cost-efficient fabrication process for creating flexible electrodes. Many studies have tried to create flexible electrodes by transferring a thin copper pattern onto PDMS [1-2]. Conventional methods use sacrificial or anti-adhesive layers, for transferring copper onto PDMS because the adhesion force between PDMS and copper is very weak. These conventional methods are complicated and expensive. Our proposed method allows the transfer of various metals, including copper, to PDMS by creating strong bonding between the metal and PDMS. Without strong bonding between PDMS and metal, the metal is easily removed and the quality is poor. In this paper, copper, nickel, and silver were tested. As shown in Figure 1, the metal transfer process is simple, pour uncured PDMS of 5:1 mixing ratio (pre-base polymer : curing agent) on metal surface (we used copper as an example), and bake PDMS at 150˚C for 18 minutes. Since each metal has different condition for transfer, various metal transfer is possible by controlling the PDMS mixing ratio and baking time. The process in Figure 1 can generate strong bonding between metal and PDMS because of metal oxidation and PDMS thermal aging caused by heating the metal and uncured PDMS as shown in Figure 2 [3-5]. Oxidized metal atoms are bonded to the curing agent, which is diffused to the surface by thermal aging. In order to support this explanation, copper metals were transferred to PDMS with different mixing ratios (Figure 3) then PDMS with a 5:1 mixing ratio was baked on two copper plates (Figure 4). Only one of them was spin-coated with curing agent (Figure 4 a)). These show that an increase of the curing agent in PDMS mixture enhances the bonding force between metal and PDMS. Next, the transfer of patterned metal layers (copper, nickel and silver) of 50nm thickness was tested as shown in Figure 5. The stability of the flexible electrode was tested by measuring changes in the resistance of the transferred copper electrode when the electrode is deformed. Two flexible electrodes (50mm × 15mm × 1 mm) were prepared with 400nm thick copper, then the resistance change of the electrode while stretching and bending were tested and the results are shown in Fig. 6. The resistance of the copper electrode increased as it was stretched or bent. The resistance increased from 5.16Ω to 759.2Ω by stretching and from 2.84Ω to 41.29Ω by bending. The resistance of the electrode is increased at a maximum of 147 times, but this is still acceptable in many applications. The proposed technique is simple and low cost fabrication method which also allows thin flexible electrodes by creating strong bonding between electrodes and PDMS. Additionally, this method can be used to create electrodes or magnetic structure integrated microfluidic devices.