Browsing by Subject "Nanolithography"

Ferroelectric thin films (FETFs) offer a promising new platform for advancing liquid-phase interfacial sensing devices. FETFs are capable of expressing surface charge densities that are an order of magnitude higher than those of traditional charged surfaces in liquid environments (e.g., common oxides, self-assembled monolayers, or electrets). Furthermore, the switchable polarization state of FETFs enables patterning of charge-heterogeneous surfaces whose charge patterns persist over a range of environmental conditions. Integration of FETFs into liquid-phase interfacial sensing devices, however, requires the fabrication of films with nanometer-scale surface roughness, high remnant polarization values, and interfacial stability during prolonged exposure. The objectives of my research were to i) fabricate ferroelectric ultra-smooth lead zirconium titanate (US-PZT) thin films with nanometer-scale surface roughness, ii) establish the interfacial stability of these films after prolonged exposure to aqueous environments, iii) measure the interfacial forces as a function of film polarization and ionic strength, iv) calculate the surface potential of the US-PZT surface using electric double layer (EDL) theory, and v) demonstrate the guided deposition of charged colloidal particles onto locally polarized US-PZT thin films from solution.

I demonstrate the use of ferroelectric US-PZT thin films to manipulate EDL interaction forces in aqueous environments. My work conclusively shows that the polarization state of US-PZT controls EDL formation and can be used to induce the guided deposition of charged colloidal particles in solution.

I present a robust fabrication scheme for making ferroelectric US-PZT thin films from a sol-gel precursor. By optimizing critical thermal processing steps I am able to minimize the in-plane stress of the film and reliably produce US-PZT thin films on the wafer-scale with mean surface roughness values of only 2.4 nm over a 25 μm2 area. I then establish US-PZT film stability in water by measuring changes in film topography, crystallinity, surface chemistry, and electrical properties as a function of exposure duration. My results show that fabrication of crack-free US-PZT thin film is critical for long-term film fidelity in aqueous environments. Furthermore, I found no change in film topography or bulk composition with increasing exposure duration. Prolonged exposure to aqueous environments, however, gradually oxidizes the surface of the US-PZT wich results in a decrease in film resistivity and polarization saturation. Next, I used colloidal probe force microscopy (CPFM) to measure the EDL interaction force as a function of separation distance between polarized US-PZT thin films and a clean borosilicate probe. CPFM measurements were performed on oppositely polarized US-PZT thin films, which expressed either a positive or negative surface charge, and over a range of ionic strengths. The inner-Helmholtz plane (IHP) potential of the US-PZT was determined by fitting the CPFM force-separation data to number of EDL models, including; an analytical EDL model using a constant potential boundary condition with a Stern layer, a charge regulation EDL model, and a numerical EDL model using the non-linear Poisson-Boltzmann equation. Each model provides good agreement with the experimentally measured and predict high IHP surface potential for the polarized US-PZT thin films in solution. Finally, I demonstrate the use of polarized US-PZT to induce the guided deposition of positively or negatively charged colloidal particles from aqueous environments. I explore the effects of ionic strength, particle size, surface roughness, and pH on particle deposition.

Overall, this work demonstrates, for the first time, that FETFs can be used as a platform to manipulate colloidal particles in aqueous environments. The experimental results demonstrate that the surface charge of the FETF is reduced by charge shielding and perform similarly to traditional, charged surfaces in aqueous environments.

During the past two decades, soft lithographic techniques that circumvent the limitations of photolithography have emerged as important tools for the transfer of patterns with sub-micron dimensions. Among these techniques, microcontact printing (uCP) has shown special promise. In uCP, an elastomeric stamp is first inked with surface-reactive molecules and placed in contact with an ink-reactive surface, resulting in pattern transfer in the form of self-assembled monolayers in regions of conformal contact. The resolution in uCP is ultimately limited to the diffusion of ink and the elastomechanical properties of the bulk stamping material.

One way to improve resolution is to eliminate diffusion by using inkless methods for pattern transfer. Inkless catalytic-uCP uses a chemical reaction between a stamp-immobilized catalyst and surface bearing cognate substrate to transfer pattern in the areas of conformal contact. By using pre-assembled cognate surfaces, the approach extends the range of surfaces readily amenable to patterning while obviating diffusive resolution limits imposed by traditional uCP.

In this thesis, we report two methods using inkless catalytic uCP: biocatalytic-uCP utilizes an immobilized enzyme as a catalyst whereas catalytic-uCP utilizes an immobilized small molecule as a catalyst, such as an acid or base. Both catalytic techniques demonstrate pattern transfer at the microscale while using unconventional, acrylate-based stamp materials. Previous results produced with catalytic-uCP have shown pattern transfer with sub-50 nm edge resolution. In this demonstration of catalytic-uCP, we use the technique to demonstrate a bi-layered patterning technique for H-terminated silicon, the foremost material in semi-conductor fabrication. This technique simultaneously protects the underlying silicon surface from degradation while a highly-reactive organic overlayer remains patternable by acidic-functionalized PU stamps. Lines bearing widths as small as 150 nm were reproduced on the reactive SAM overlayer, which would not be possible without circumvention of diffusion. Before and after patterning, no oxidation of the underlying silicon was observed, preserving desired electronic properties throughout the whole process. This bi-patterning technique could be extended to other technologically-relevant surfaces for further application in organic-based electronic devices and other related technologies.