Patterning and cross-linking of functionalized polynorbornene polymers
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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.