Large-area three-dimensional optical imaging and sub-diffraction photolithography with polydimethylsiloxane (PDMS) nanotip array
Date of Issue2014
School of Materials Science and Engineering
With the challenge of obtaining high resolution three-dimensional images of nanostructures over large areas as well as fabricating these nanostructures in a low-cost manner, developing cost-effective strategies which enable high-resolution, large-area nanostructures imaging as well as fabrication is a long standing goal in nanotechnology community. In this thesis, a pristine or modified polydimethylsiloxane (PDMS) tip array consisting of thousands of nanotips was utilized to address the above challenge. The first part of the dissertation explored a new optical microscopy which enables the measurement of topography and chemical properties of sample surfaces by taking advantage of the interaction between a transparent and elastomeric tip array and underlying surfaces. Since the reflected light intensity at the apexes of nanotips changed significantly when the soft probes contact and separate with underlying sample surfaces, the exact contact and separation positions in the vertical direction can be precisely determined and therefore used for measuring the feature height. This imaging method has never been reported before. One remarkable advantage of parallel scanning optical microscopy (PSOM) is the multiple tip nature. As hundreds of nanotips scan at different places on the underlying surface simultaneously, the image covering the areas of square millimeter scale could be obtained in just one run with sub-diffraction vertical resolution. Currently, the feature height down to 35 nm can be measured by PSOM, which has broken the diffraction limit in the vertical direction. Three-dimensional topographical image covering the surface area of 0.15 mm2 was acquired by using 91 tips parallel scanning. The adhesive force between tips and chemically modified surfaces during the separation process can be detected. Based on this, the hydrophilic and hydrophobic surfaces can be differentiated by this tip array. This potentially enables the probes to map the spatial distribution of different functional groups on the surfaces. The application of low-cost PDMS tips as the scanning probes and utilization of white light intensity change as the feedback impart the cost-effective and simple characteristics to PSOM. The second part of the thesis explored near-field photolithography approaches allowing for nanoscale sub-diffraction nanostructure fabrication over large areas on surfaces in a low-cost manner. In this work, pristine and metal-coated PDMS nanostructures were used as the photomasks to produce wafer-scale sub-diffraction nanostructures via near-field photolithography. The PDMS based photomasks gain the merits of low-cost, easy fabrication, ease-of-use, repeatedly usage. Since the elastomeric PDMS nanostructures can contact with underlying surfaces intimately, which enables the incident light to expose underlying photoresist in the optical near-field, the diffraction limit has been circumvented and therefore sub-diffraction nanostructures can be produced. The following three types of near-field photolithography strategies were developed in this work. Firstly, wafer-scale sub-100 nm near-field photolithography strategy with metal-coated elastomeric masks was developed. The incident light was strictly allowed to pass from the nanoscopic apertures at the apexes of tips to expose underlying photoresist, producing sub-100 nm features over wafer-scale areas based on common mask aligner patterning platform. Secondly, a centimeter-scale sub-100 nm near-field photolithography strategy with light leaking photomasks was developed. By using electron-beam evaporation to evaporate metals towards the PDMS relief nanostructures of vertical side walls with controlled evaporation direction, two-side or one-side nanoscopic apertures at the side walls were produced straightforwardly. Through the apertures, the incident passed to expose underlying photoresist at nanoscale areas. This facile near-field photolithography strategy bypassed the complicated procedures of creating nanoscopic apertures after metal-coating, while possessed the capability of producing sub-100 nm features with arbitrary shapes. Thirdly, wafer-scale sub-100 nm near-field photolithography by using V-shape transparent and elastomeric nanotip array as photomasks was developed. Rather than utilize opaque metal layer coating to fabricate the photomasks, herein, the V-shape total reflective PDMS nanostructures were used as the light intensity modulator. Only the photoresist at the nanoscale contact areas between the apexes of tips and surfaces was allowed to be exposed completely, generating sub-100 nm nanopatterns over wafer-scale areas. At last, a facile method was developed to synthesize large-area single sub-10 nm nanoparticle array in-situ by polymer pen lithography (PPL). Herein, small molecules such as ethylene glycol (EG) or glycerol were utilized to facilitate the delivery of nanoparticle precursors to the substrates in polymer pen lithography. Subsequently, large-area ordered single nanoparticle arrays including sub-10 nm Ag nanoparticle, 30 nm Au nanoparticle and 80 nm Fe2O3 nanoparticle have been synthesized in-situ with controllable size and pitches.