Available Technology

Large Ultrathin Free-Standing Polymer Films or Polyelectrolyte Enabled Liftoff (PEEL)

Ultra-thin polymer films are films with thicknesses below 100 nm. The thinnest freestanding films found in open literature are 20 nm thick and typically have diameters of less than 100 um. Polymer films of this thickness are typically made by spin coating or dip coating a dilute solution of the polymer onto a substrate. The substrate must be specially prepared to release the film: it can be freshly cleaved mica, or it can be a more typical substrate such as clean glass or silicon prepared with a release layer. This layer is typically a sacrificial layer that is dissolved when the film is removed from the substrate, such as sputtered salt or soap layers or similar release agents. The films are removed from the substrate by immersing the substrate into water, which separates the film from the substrate, and the film will float on the water surface. From here, the films are transferred to the holder, commonly a grid with small (typically sub-micron to 100 um) openings that define the free-standing area of film. The current method has several disadvantages, most of which are connected to the substrate. The substrate preparation can introduce roughness, especially in the case of sputtered liftoff layers, which can be on the order of several nanometers, which leads to film non-uniformity and becomes more severe as the film thickness is reduced below 20 nm. Sacrificial liftoff layers can contaminate the film and decrease the strength of the film. At thicknesses lower than 30 nm, the liftoff of the film from the substrate becomes impossible for some preparations. A second drawback is the inability to produce large free-standing films. This inability is related to the shape of the holder, the liftoff technique itself, and the properties of the polymer that is used to produce the thin film. Large films will often tear when lifted out of the water, and in some cases they tear while drying.
Patent Abstract: 
LLNL’s Polyelectrolyte Enabled Liftoff (PEEL) process makes changes to the substrate preparation, the holder and liftoff technique, and suggests modifications to the material itself to enable the preparation of large ultrathin free-standing films. PEEL enables ultrathin films by chemically modifying the deposition substrate and decreasing the interfacial energy so that even thin films with small strain energies will delaminate.
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