K. Li, J.A. Hernández-Castro, T. Veres
National Research Council of Canada,
Keywords: micro/nano fabrication, micro/nano polymer membrane, UV curing, nanoimprinting, nan hot-embossing
Summary:Micro- and nano-porous membranes have a wide range of applications including plasmonics, data storage, and energy devices, as well as biomedical devices. Most of them use Si, SiN, or highly periodic anodic alumina membranes. They are mechanically stable and offer the advantage of maintaining the membrane’s shape against external forces that arise during the handling process, but they are fragile and brittle. Alternatively, flexible polymeric membranes are relatively less expensive to fabricate and offer several advantages, e.g. conformal wetting and easy peel-off without significant damage and distortion. They are getting more and more attractive for biological applications. However it is non-trivial to make polymeric membranes with regular, straight, open through pores because it is challenging to obtain ‘freestanding’ and ‘residual-layer-free’ structures in the fabrication process, especially as pore sizes get smaller. To enrich the libraries of fabrication methods of polymeric membrane with micro/nano pores in large aspect ratio and over large surface areas, here we present a simple, yet robust method that could be scaled up for mass production to eventually bring down the fabrication cost. The membranes are replicated from an intermediate template with micro- or nano-pillars by using the spontaneous capillary flow (SCF) method. The polymerization is done either by UV or thermal curing. The membrane is separated from the template under liquid condition, without the need of any external mechanical force, which helps to effectively avoid permanently distorting or breaking the membrane during the process. For most cases, polyvinyl alcohol (PVA) is proposed to be used as the intermediate template because PVA is a highly hydrophilic polymer with good thermally stability, it has a high hydrolysis, and is dissolvable in water, which has negligible effects on the polymeric membrane. In this case, the separation of membrane from template is carried out under water. For certain specific group of polymers which swell, but without permanent damage effect, due to polar solvents such, the separation of the membrane from the intermediate template is done under methanol, and the template could be made from a type of cyclic olefin copolymer, which can be fabricated rapidly and cost effectively by hot-embossing. A group of membranes of different materials, including EBECRYL (radical UV polymerisation), UVACURE (cationic UV polymerisation) UV lacquer, PFPE urethane methacrylate UV resin (MD700), optical adhesive UV resin with high refractive index (NOA84), medical adhesive UV resin (1161-M), as well as thermally curable polydimethylsiloxane (PDMS) have been successfully fabricated. The size of the pore in the membrane achieved so far ranges from 100 µm down to 200 nm. The thickness of the membranes varies from 10 µm up to 100 µm. As high as 16.7 of aspect ratio (the thickness of the membrane to the diameter of the pore) has been achieved in a membrane with thickness of 100 µm while the diameter of a pore is approximately 6 µm. Uniform open-through hole membranes with hole size of 15 µm and thickness of 30 µm over an area of 44mmx44 mm have also been achieved.