Thesis (Ph.D.)--Chemical Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Henderson, Cliff; Committee Member: Beckham, Haskell; Committee Member: Hess, Dennis; Committee Member: Koros, William; Committee Member: Tolbert, Laren.

Advanced Methods, Materials, and Devices for Microfluidics Celesta E. White 217 Pages Directed by Dr. Clifford L. Henderson Microfluidics is a rapidly growing research area that has the potential to influence a variety of industries from clinical diagnostics to drug discovery. Unlike the microelectronics industry, where the current emphasis is on reducing the size of transistors, the field of microfluidics is focusing on making more complex systems of channels with more sophisticated fluid-handling capabilities, rather than reducing the size of the channels. While lab-on-a-chip devices have shown commercial success in a variety of biological applications such as electrophoretic separations and DNA sequencing, there has not been a significant amount of progress made in other potential impact areas for microfluidics such as clinical diagnostics, portable sensors, and microchemical reactors. These applications can benefit greatly from miniaturization, but advancement in these and many other areas has been limited by the inability or extreme difficulty in fabricating devices with complex fluidic networks interfaced with a variety of active and passive electrical and mechanical components. Several techniques exist for the fabrication of microfluidic devices, but these methods have significant limitations, and alternative fabrication approaches are currently desperately needed. One such method that shows promise for its ability to integrate the desired high levels of functionality utilizes thermally sacrificial materials as place holders. An encapsulating overcoat material provides structural stability and becomes the microchannel walls when the sacrificial material is removed from the channel through thermal decomposition. Disadvantages of this method, however, include numerous processing steps required for sacrificial layer patterning and elevated temperatures needed for the decomposition of initial sacrificial materials. These limitations keep this method from becoming an economical alternative for microfluidic device fabrication. The materials needed for this method to reach its full potential as a valid fabrication technology for m-TAS are not currently available, and it was a major focus of this work to develop and characterize new sacrificial materials, particularly photosensitive polycarbonate systems. In addition to the development of new sacrificial polymers, the framework for a working microfluidic device was developed to show that this concept will indeed provide significant advancements in the development of future generations of microfluidic systems. Finally, novel fabrication methods for microfluidics through combined imprinting and photopatterning of photosensitive sacrificial materials was demonstrated.

To enable fast and reliable processors, advances must be made in the interconnections on the printed circuit board and in the interconnections from the chip to the printed circuit board. Processing techniques have been demonstrated to fabricate a copper-clad encapsulated air dielectric layer to enable low loss off-chip electrical signal lines using sacrificial polymers and the three dimensional patterning capabilities of imprint lithography. The inclusion of an air gap can eliminate the dielectric loss allowing the signal to propagate over longer lengths. Additionally, the low dielectric constant of air lowers the loss contributions from the conductor and increases the signal propagation velocity reducing delay. The metal shielding could minimize the crosstalk noise and radiation losses that are significant at high frequencies. The three dimensional patterning capabilities of imprint lithography fabricated curved structures and rounded terminations which can reduce reflections at discontinuities. Furthermore, imprint lithography also created planarized surfaces which simplified the buildup process. Since imprint lithography, only uses temperature and pressure to make a pattern it is an inexpensive and simple process advancement. The metal-clad encapsulated air dielectric structures were fabricated in a comparable number of registration steps to traditional transmission lines. Implementation of all copper chip to substrate interconnects would provide high conductivity electrical connections, resistance to electromigration while avoiding formation of brittle intermetallics. High aspect ratio polymer molds for copper electroplating interconnects could enable improved integrated circuit electrical performance. The properties of a new aqueous base develop, negative-tone photosensitive polynorbornene polymer have been characterized to develop mechanically compliant all copper connections between the chip and printed circuit board. High aspect ratio features of 7:1 (height:width) were produced in 70 ìm thick films in a single coat with straight side-wall profiles and high fidelity. The polymer films studied had a contrast of 11.6 and a low absorption coefficient. To evaluate the polymer's suitability to microelectronics applications, epoxy cross-linking reactions were studied as a function of processing condition through Fourier transform infrared spectroscopy, nano-indentation, and dielectric measurements. The fully cross-linked films had an elastic modulus of 2.9 GPa and hardness of 0.18 GPa which can improve the mechanical compliance of the copper interconnections. A photo-imprint lithography process was developed to improve the photo-patterning of the polynorbornene polymer for high aspect ratio hollow structures. A shallow photo-imprint stamp was developed to physically displace material in the polymer core. Since the imprint stamp displaces material in the area of the feature, the effective film thickness is reduced compared to the bulk film. The reduction in film height reduced the effects of scattering in the core and also facilitated transport of developer within the core. The photo-imprint lithography process resulted in high aspect ratio hollow core pillars that exceeded optical resolution capabilities for comparable feature sizes.

Polymers play an integral part in the semiconductor electronic industry. Due to the expanding diversity of a polymer s structural design and the resulting properties, different polymers serve as different components in the makeup and fabrication of the electronic package. The limiting factor in computer processing speed shifts from the transistors gate delay to the interconnect delay below a circuit line width of 1.8 μm for interlayer dielectrics. Silicon dioxide has been used as the insulating layer between metal lines for many computer chip generations. Low dielectric constant polymers will need to supplant silicon dioxide as interlayer dielectrics in order to develop reliable circuits for future generations. Along with serving as interlayer dielectrics, low dielectric constant polymers are also incorporated in first and second level electronic packaging. Deposition and patterning of these polymers can be significantly reduced by using photodefinable polymers. Most photodefinable polymers are in a precursor form for exposure and development in order to dissolve in industrial developers. Once developed, the polymer precursors are cured to produce the final polymer structure. This temperature is as high as 350 oC for many polymers. Thermal curing sets limitations on the use of the polymer in the electronics industry because of either the unwanted stress produced or the incompatibility of other electronic components that do not survive the thermal cure. In addition to a low dielectric constant and photodefinability, many other properties are needed for successful implementation. Polymers must be soluble in organic solvents in order to spin coat films. Water absorption increases the dielectric constant of the patterned films and can lead to various adhesion problems and cause delamination of the film. Mismatches between the coefficients of thermal expansion in adjacent layers can produce residual film stresses which leads to warping of the substrate or interfacial delamination. The glass transition temperature must be high because the thermal expansion is greatly increased when the glass transition temperature is exceeded. A high Young s modulus is also required to withstand external forces from thermal, electrical, and packaging stresses. The goal of this research was to develop novel, low dielectric, photodefinable polymers that can be processed at low temperatures. All polymers discussed will contain one of two monomers with hexafluoroalcohol (HFA) functional groups. Fluorine provides many properties that are advantageous for low dielectric applications whereas alcohols absorb water and increase the dielectric constant. Characterization of the polymers show the effect the fluorine has on the alcohol s high water absorption. All polymers will be synthesized by condensation polymerization of a diamine with a dianhydride or diacid chloride. All other polymers will contain a novel HFA diamine. A new thermoplastic polymer structure based on the cyclization of an HFA situated ortho to an amide linkage produces a benzoxazine ring in the polymer backbone. Cyclization to form polybenzoxazines occurs at temperatures considerably lower than that needed to form polyimides. The lowest processing temperatures are achieved with protection of the HFA that can be cleaved with a photoacid generator.

A challenging application space exists for high-aspect-ratio, high-fidelity dielectrics in micro-electro-mechanical system (MEMS), microelectronic, and photonic applications. Photosensitive polymers are widely used in these fields because they are relatively easy to process and pattern, and have good mechanical properties. Photosensitive polynorbornene (PNB)-based dielectrics have been shown to have high sensitivity, excellent photodefinition properties, and high mechanical strength making them suitable for MEMS, microelectronic packaging, and photonic applications. PNB-based dielectrics can be functionalized with epoxide, carboxylic acid, or fluorinated alcohol groups. Epoxy or carboxylic acid groups can be used to provide cross-linkable sites, resulting in improved chemical and thermal properties while fluorinated alcohol groups can provide solubility in aqueous base. The focus of this study has been on the epoxy-based cross-linking of ultraviolet and electron beam (e-beam) sensitive negative-tone PNB-based dielectrics. The impact of multifunctional epoxy-based additives on the cross-linking, photolithographic properties, and adhesion properties of the photosensitive PNB dielectric was investigated. High aspect ratio features of 13:1 (height:width) were produced in 40 µm thick films (a single coat) with straight side-wall profiles and high fidelity. Contrast values as high as 33.4 were obtained at doses below 15 mJ/cm2. To evaluate the polymer's suitability to MEMS and microelectronics applications, epoxy cross-linking reactions were studied as a function of processing condition through Fourier transform infrared spectroscopy (FTIR), nanoindentation, swelling and dielectric measurements. The fully cross-linked films had an elastic modulus of 2.9 GPa and hardness of 0.18 GPa which can improve the mechanical compliance of the packaging device. To explore the feasibility of the PNB dielectric as a highly sensitive e-beam resist for nano scale fabrication, the e-beam initiated reaction between PNB cross-linking sites and the multifunctional epoxy cross-linkers was investigated. In this study, the interaction of an e-beam with the PNB mixture and its compounds was investigated. The contrast, photodefinability, and e-beam activation of the components in the PNB formulations were studied. The PNB polymer had very high e-beam sensitivity and contrast. It was shown that the addition of a photoacid generator (PAG) to the polymer-epoxy mixture enhanced the contrast and sensitivity. Formulations with the additional cross-linker showed improved contrast, sensitivity, and substrate adhesion. 100 nm structures with 13.5 nm line edge roughness (LER) were fabricated. The influence of the developing time, the developer concentration, PEB, and film thickness on the contrast and sensitivity were studied. Structures with contrast values as high as approximately 8 were fabricated at doses as low as 0.38 µC/cm2. The acid-catalyzed epoxy ring opening reaction of the PNB dielectric was studied using FTIR spectroscopy. The photo and thermal acid generation initiated epoxy ring opening reactions and subsequent cross-linking of polymer. Additionally, polymer properties were characterized as a function of processing conditions for this polymer system. It was shown that thermal cure conditions have a substantial impact on the mechanical and electrical properties of the polymer. The rate and ultimate conversion of the epoxy ring opening reaction increased with increasing cure temperature, resulting in a higher degree of cross-linking at cure temperatures above 140°C. Degradation reactions occurred at temperatures above 160°C, indicating loss of epoxide cross-linking groups and linkages. These hypotheses were supported by electrical and mechanical property studies. It was shown that curing the PNB polymer at 160°C for 1 h after develop resulted in full epoxy ring opening and highest cross-link density. This sample showed lower dielectric constant (3.9), residual stress (20 MPa), and solvent swelling (3.1%). Variable frequency microwave (VFM) processing of the PNB dielectric was studied to investigate the rapid curing of the polymer at lower temperatures. The FTIR results showed that the microwave reaction rates were higher at each isothermal cure temperature compared to convective heating, indicating that the rapid VFM curing of PNB at low temperatures is feasible. The PNB film was fully cross-linked after 15 min VFM cure at the low temperature of 150˚C. The shortest time to fully cure the polymer was found to be 5 min at 160°C. Also, the feasibility of rapid VFM curing of PNB in air was studied. All samples VFM-cured (140˚C-180˚C) in air showed no signs of oxidation. The electrical and mechanical properties of VFM-cured films were characterized and compared with thermally cured films to determine the effectiveness of the VFM processing. VFM-cured samples showed higher degree of cross-linking than thermally-cured samples, which was congruent with the FTIR results. Improved or equivalent properties were obtained for VFM-cured samples at shorter cure cycles and lower cure temperatures compared to thermally-cured films. The PNB dielectric was also used as an overcoat material to make micro and nano fluidic channels. In this work, incorporation of advanced micro/nano fluidics with high-sensitivity photonic sensors was demonstrated. 500 nm to 50 µm channels were fabricated by thermal decomposition of epoxy-based PNB polymers. Microdisks with quality factors of over 106 were presented in complementary metal-xide-semiconductor (CMOS) compatible SiN on oxide technology. These ultra-high quality factor SiN resonators were demonstrated in the visible range for the first time. The fluidic structures were interfaced with photonics for index and florescence sensing. This study was a collaboration with Dr. Ehsan Shahhosseini from the Photonics Group at Georgia Tech.