Thesis (Ph.D.) - University of St Andrews, January 2007.

Thesis (Ph. D.)--University of Texas at Austin, 2008. / Vita. Includes bibliographical references.

Thesis (M.S.)--University of North Texas, May, 2008. / Title from title page display. Includes bibliographical references.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 168-176). Available also in a digital version from Dissertation Abstracts.

Three projects are described in this thesis that combine microfabrication techniques with optical micromanipulation. The aim of these projects is to use expertise in microlithography and optical tweezing to create new tools for Lab-on-Chip devices. The first project looks at the creation of microgears that can be moved using an optical force. The microgears include one dimensional photonic crystal that creates birefringence. This allows the transfer of angular momentum from a circularly polarised light beam to the microgear, making them spin. The microgears are simulated, fabricated and tested. Possible biological applications are suggested. The second project looks at creating microchannels to perform micromanipulation experiments in. Different methods of fabricating the microfluidic channels are compared, and the resulting chambers are used to find the maximum flow rate an optical sorting experiment can be performed at. The third project involves using a thin photoconductive layer to allow the optical control of an electrical force called dielectrophoresis. This light induced dielectrophoresis (LIDEP) allows similar control to optical tweezing but requires less irradiance than optical tweezing, allowing control over a larger area with the same input optical power. A LIDEP device is created and experiments to measure the electrical trap size that is created with a given optical spot size are performed. These three projects show different microfabrication techniques, and highlight how well suited they are for use in optical manipulation and microfluidic experiments. As the size of objects that can be optically manipulated matches well with the size of objects that can be created with microfabrication, it seems likely that many more interesting applications will develop.

The speed of microelectronic devices is controlled by the size of the transistor gate. In order to create faster devices, the size of this transistor gate must shrink. Microlithography is the method used to define patterns in semiconductor devices, and it is optimized periodically to create smaller features. It is a subtractive process that relies on the selective removal of sections of a photosensitive polymeric film called photoresist. This photoresist is exposed to patterned ultraviolet radiation that changes the local solubility of the film and allows for the creation of relief patterns in the resist using a developing solvent. Decreasing the wavelength of the light used to expose the patterns is the primary method for decreasing the minimum feature size that can be printed by this process. There are a number of challenges associated with decreasing the exposure wavelength for conventional lithographic processes. First of all, the polymeric films must be transparent at the exposure wavelength in order to allow light to propagate through the entire thickness of the film. Secondly, there is a limit in the thickness of the photoresist films that can be used. This thickness limits the etch resistance of the film. In fact, the issues concerning etch resistance and transparency are generally in opposition. This makes designing photoresist platforms for future lithographic applications very difficult. Therefore, to overcome these limitations, we are developing an unconventional approach to microlithography. In our approach, entitled Surface Monolayer Initiated Polymerization, polymer structures are formed on a surface by polymerizing a monomer in a patterned fashion using a self-assembled monolayer that can be locally activated to initiate the reaction. This process has been demonstrated by creating patterned polystyrene films on native silicon dioxide surfaces. In these initial studies, it took more than one day to create features. This is unacceptable for a lithographic application. The kinetics of all the processes involved in making these patterned layers is described. Along with these rate constants, means of optimizing these rates are also presented. Additionally, the patterns grown in these initial studies exhibited poor uniformity. Methods of optimizing the patterns formed are also presented.

This thesis presents the development of a novel atomic force microscope (AFM) based nanofabrication technique termed as thermochemical nanolithography (TCNL). TCNL uses a resistively heated AFM cantilever to thermally activate chemical reactions on a surface with nanometer resolution. This technique can be used for fabrication of functional nanostructures that are appealing for various applications in nanofluidics, nanoelectronics, nanophotonics, and biosensing devices. This thesis research is focused on three main objectives. The first objective is to study the fundamentals of TCNL writing aspects. We have conducted a systematic study of the heat transfer mechanism using finite element analysis modeling, Raman spectroscopy, and local glass transition measurement. In addition, based on thermal kinetics analysis, we have identified several key factors to achieve high resolution fabrication of nanostructures during the TCNL writing process. The second objective is to demonstrate the use of TCNL on a variety of systems and thermochemical reactions. We show that TCNL can be employed to (1) modify the wettability of a polymer surface at the nanoscale, (2) fabricate nanoscale templates on polymer films for assembling nano-objects, such as proteins and DNA, (3) fabricate conjugated polymer semiconducting nanowires, and (4) reduce graphene oxide with nanometer resolution. The last objective is to characterize the TCNL nanostructures using AFM based methods, such as friction force microscopy, phase imaging, electric force microscopy, and conductive AFM. We show that they are useful for in situ characterization of nanostructures, which is particularly challenging for conventional macroscopic analytical tools, such as Raman spectroscopy, IR spectroscopy, and fluorescence microscopy.

Step and flash imprint lithography (SFIL) was developed in 1999 at The University of Texas at Austin as a high resolution, cost-effective alternative to photolithography for nanoscale patterning. Unlike current projection steppers, which are resolution limited by diffraction phenomena, SFIL tools have demonstrated patterning capability down to 20 nm, a resolution currently unattainable using traditional lithographic techniques. The combination of high resolution and low cost of ownership make SFIL a strong candidate for future semiconductor integrated circuit manufacturing. For SFIL to be viable as a high volume process, there are numerous technical issues that need to be resolved. Reverse-tone step and flash imprint lithography (SFIL-R) is a reverse tone variant of SFIL that requires the successful application of a planarizing topcoat over topography through spincoating. Photopolymerizable nonvolatile fluids are ideal topcoat materials because they planarize better than volatile fluids during spincoating and can continue to level after spincoating. Fluid mechanics analyses indicate that complete planarization using capillary force is slow. Therefore, defining the acceptable or critical degree of planarization (DOP[subscript crit]) becomes necessary. Finite difference simulation of the spincoat and post-spin leveling processes was used to determine the planarization time for various topographic and material property combinations. A new material, Si-14, was designed to have ideal planarization characteristics and satisfy SFIL-R processing requirements and was used to validate the models through profilometry and interferometry experiments. During spincoating, minimizing the spin speed generates more planar films, however, this increases the spin time. To rectify this problem, a 2-stage spincoating process -- a first step with high spin speeds to achieve the target thickness quickly and a second step with low spin speeds to improve planarization -- was proposed and experimentally demonstrated. An alternative planarization technique is to generate a reverse-conformal film coating through Marangoni-driven flow. The SFIL process requires the clean separation of a quartz template from a polymer, and the force required to create this separation must be minimized to prevent the generation of defects. Fracture mechanics analyses show that control of the polymer modulus and interfacial fracture energy is the key to minimizing the separation force. Adjusting the crosslinker concentration in the imprint formulation reduces the modulus but has no significant impact on the fracture energy. On the other hand, adding surfactants to the imprint formulation reduces both the modulus and fracture energy. The fracture energy is further decreased by using a nonreactive, liquid surfactant versus a surfactant that reacts with the polymer matrix. Angleresolved X-ray photoelectron spectroscopy (XPS) results indicate that surfactant migration is more effective with a fluorinated surface treatment compared to an untreated quartz or organic surface. However, the fluorinated surface treatment that drives the migration process degrades over multiple imprints. Based on these results, it was concluded that the use of fluorinated surfactants must be accompanied by a surface treatment that is both stable and of a similar energy or polarity to induce migration and to lower the adhesive strength. Mixed-mode fracture affects the separation force, especially if shear stresses are present. Overfilling the templatesubstrate gap causes large amounts of shear stresses during separation; however, this phenomenon can be prevented by controlling the surface energies of the imprint template and substrate. / text

published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Not available / text