Rolled-up nanoporous membranes by nanoimprint lithography and strain engineering

It is extremely challenging to enable nanoscale patterning in three dimensional (3D) curved geometries using conventional nanolithographic approaches. In this paper, we describe a highly parallel approach that combines nanoimprint lithography (NIL) and thin film bilayer strain engineering to spontaneously roll-up nanopatterned membranes into curved geometries. Specifically, we first patterned a silicon nitride (Si₃N₄) / silicon (Si) bilayer using nanoimprint lithography followed by plasma etching to create well defined pores. The diameter of the pores was further reduced by physical vapor deposition of platinum to sizes as small as 50 nm. After patterning, the bilayers were released from the substrate by etching an underlying SiO₂ sacrificial layer. Based on the high deposition stress values for low pressure chemical vapor deposition (LPCVD) deposited Si₃N₄ and Si, we varied the thickness of the bilayer to realize rolled-up tubes with different radii of curvature; these curvature values were in good agreement with a finite element analysis model (FEM). The assembled nanoporous tubes had well defined pores along their curved interface and can be applied for drug delivery, separations and ion-sensing devices. We highlight biocompatibility of the devices by encapsulating β-TC-6 islet cells of relevance to cell encapsulation therapy for diabetes. More broadly, we believe that this approach of combining NIL with strain engineering processes could be utilized to create a range of precisely nanopatterned curved structures in a highly parallel manner.