Molecular transport through nanoporous silicon nitride membranes produced from self-assembling block copolymers

Abstract

To achieve fast and selective molecular filtration, membrane materials must ideally exhibit a thin porous skin and a high density of pores with a narrow size distribution. Here, we report the fabrication of nanoporous silicon nitride membranes (NSiMs) at the full wafer scale using a versatile process combining block copolymer (BCP) self-assembly and conventional photolithography/etching techniques. In our method, self-assembled BCP micelles are used as templates for creating sub-100 nm nanopores in a thin low-stress silicon nitride layer, which is then released from the underlying silicon wafer by etching. The process yields 100 nm thick free-standing NSiMs of various lateral dimensions (up to a few mm(2)). We show that the membranes exhibit a high pore density, while still retaining excellent mechanical strength. Permeation experiments reveal that the molecular transport rate across NSiMs is up to 16-fold faster than that of commercial polymeric membranes. Moreover, using dextran molecules of various molecular weights, we also demonstrate that size-based separation can be achieved with a very good selectivity. These new silicon nanosieves offer a relevant technological alternative to commercially available ultra- and microfiltration membranes for conducting high resolution biomolecular separations at small scales.

Block Co-Polymers for Nanolithography: Rapid Microwave Annealing for Pattern Formation on Substrates

“…Firstly, the use of plasma etching to pattern transfer lithographically defined polymer topographies to the substrate is necessitated by the relatively poor etch resistance of polymers [8][9][10]. Whilst the majority of BCP work focusses on pattern formation at silicon substrates, it is becoming clear that the BCP pattern will be required to be transferred to a hard-mask layer before the device layer is patterned.…”

“…Si3N4 is a well-used hard mask and is investigated in detail here. There are reports of the fabrication of ultrathin Si3N4 membrane by BCP lithography [8,9], however, directed self-assembly of BCPs has yet to be demonstrated on Si3N4 substrates. Secondly, there is a clear need to develop pattern formation techniques that are commensurate with the high wafer throughput used in the semiconductor industry [10,11].…”

“…Two issues of key interest are the time taken to induce long range order in the BCP thin films and substrate surfaces (other than simple silicon termination) which are routinely used in conventional UV-photolithography. Hard masks such as silicon nitride (Si3N4) are used in substrate stacks that allow pattern transfer from low-dimension, thin polymer patterns to the substrate with high fidelity and aspect ratio [7][8][9].…”

Fabrication techniques enabling ultrathin nanostructured membranes for separations

“…Additionally, most ultrafiltration membranes are orders of magnitude thicker than the pore diameter resulting in low permeability . Improvements in nanofabrication and recent interest in nanomanufacturing offer promise in making significant improvements in ultrafiltration . The ideal molecular or protein sieve is a membrane with a uniform distribution of pores in an ultrathin film, while fabricated and supported in a manner that is scalable and robust enough for practice use.…”

“…Precise size exclusion and permeability are two important characteristics that a membrane must exhibit in order to be successfully applied in chemical and biological processes . The ideal nanofabrication method should allow controlling feature size and distribution while providing a large area of fabrication to ensure mass manufacture feasibility . Fabrication techniques can be divided into two categories according to their overall approach being top‐down or bottom‐up; Fig.…”