Direct Release of Sombrero‐Shaped Magnetite Nanoparticles via Nanoimprint Lithography

DOI: 10.1002/admi.201400511 remanence over a wide range of particles sizes from tens of nanometers to micrometers. Zhang et al. used ethylene tetrafl uoroethylene (ETFE) to create a “working stamp” instead of using a “master” silicon mold for imprinting and demonstrated improved process in terms of defects prevention and direct release. Specifi cally, this process prevented signifi cant defects being induced from the silicon mold as well as the degradation, with use, of the silicon mold itself. [ 22 ] Thus, they were able to achieve direct release of high-quality disk-shaped magnetic nanoparticles in solution using an ETFE “working stamp,” bilayer resist lift-off and silicon etching process specifi cally designed for the release of the nanoparticle. [ 22 ] This nanoimprint lithography process, however, was only developed for the release of metal magnetic nanoparticles rather than the more biologically relevant magnetite nanostructures. Here, we present an optimized fabrication process for complex, sombrero-shaped, magnetite nanoparticles using nano imprint lithography via an ETFE mold. We modifi ed the nanoimprint lithography process to fabricate magnetite nanoparticles with a diameter of ≈500 nm directly on Si wafer and released them through a dry etching procedure. By using the combination of the ETFE mold, defect free nanoimprint, bilayer lift-off, and dry etching release, we fabricated magnetite nanoparticles with controlled size, shape, structure, and defects. To demonstrate this process, Fe 3 O 4 /Ti multilayer, sombreroshaped SAF nanoparticles, potentially useful for molecular imaging and detection, [ 23 ] were fabricated and characterized using scanning electron microscopy (SEM) and a quantum design physical property measurement system (PPMS). The reproducible fabrication of magnetite nanoparticles was carried through several steps: replication of ETFE, fabrication of bilayer resist, nano-imprinting, lift-off, and release in solution ( Figure 1 ). We followed the recipe used by Zhang et al., but adjusted it for magnetite lift-off. [ 22 ] To replicate ETFE mold from silicon master mold, we cleaned and ultra-sonicated a sheet of ETFE (DuPont Tefzel) with acetone and isopropyl alcohol for 5 min, blow-dried with nitrogen, and placed onto a clean Siwafer. The Si master mold with 350 nm holes in a hexagonal lattice, from Lightsmyth Technologies, was placed on the ETFE sheet, then imprinted using a Nanonex NX-B 100 compact thermal nanoimprinter, at 250 °C and 450 psi for 2 min. Next, we spin-coated a bilayer-resist, consisting of LOR 1A (MicroChem) and NXR-1025 (Nanonex), and soft-baked each layer at 200 °C and 150 °C on a hot plate, respectively. After the bilayer-resist was ready, we patterned it with the replicated ETFE mold using Nanonex NX-B100 imprinter ( Figure 2 A). Large area hole arrays (350 nm) of the top layer were obtained. Next, we performed anisotropic oxygen plasma reactive ion etching (RIE) to remove the imprint resist residue, then we Magnetic nanoparticles exhibit high potential for biological and biomedical applications, [ 1–4 ] ranging from magnetic particle imaging (MPI), [ 5 ] to direct drug delivery and hyperthermia [ 6 ]