Browsing by Subject "Polyimide"

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Characterization and optimization of laser-induced graphene (LIG) based capacitive pressure sensors for aerospace applications
(2024-05) Valivi Reddy, Jahnavi ; Sirohi, Jayant; Mahalingam, Raghav
With growing interest in the applications of Laser-Induced Graphene (LIG) sensors for their affordability, ease of production, and flexibility, this study delves into their potential application in monitoring airframe surface pressures to enhance aircraft performance and aerodynamic efficiency. A capacitive pressure sensor was developed using 1” square LIG electrodes, formed by irradiating a CO₂ laser on a Kapton polyimide film. The sensor’s behavior was studied across a range of frequencies to identify the optimal operational frequency that maximizes its sensitivity to pressure, revealing that the optimal frequencies at an applied AC voltage of 1V were under 2 kHz, where it exhibited a capacitance of 92pF. The sensor’s performance under varying laser parameters, environmental conditions, and contact stimuli was analyzed, and calibration methods were explored to enhance accuracy and reliability. Pressure testing within the pressure range of 0.2-1.7 atm revealed a quadratic relationship between the sensor’s impedance and applied pressure. However, the sensor proved highly susceptible to external stimuli and operational conditions, emphasizing the importance of calibration for accurate measurements in real-world applications.
Design and development of base-catalyzed materials for microelectronics applications
(2016-08) Dick, Andrew R.; Willson, C. G. (C. Grant), 1939-; Ellison, Christopher J.; Freeman, Benny; Mack, Chris; Ho, Paul S
Most lithographic processes in the microelectronics industry rely on the use of processes catalyzed by photochemically generated acids. The generation of organic bases photochemically is much less common, but allows for design of new resolution enhancement techniques and packaging materials. The microelectronics industry has been able to continue its path toward smaller transistors for several decades, but recently 157 nm and EUV lithography processes have faced delays. Alternative strategies such as double patterning are now required to keep the pace of scaling and they greatly increase manufacturing costs. This dissertation discusses a resolution enhancement technique termed pitchdivision designed to extend 193 nm lithography. This process depends on addition of a photobase generator (PBG) to commercial photoresists that enables printing of both positive and negative features, effectively doubling resolution. Using PBGs that require two separate photochemical events to generate base allows for improved image quality over standard PBGs. The use of PBGs in photosensitive polyimide packaging materials is also detailed. In packaging of integrated circuits, there is a need for an insulating material having low dielectric constant that provides support for the wires connecting the silicon chip to the circuit board. Aromatic polyimides meet many of the integration requirements, and can be patterned using PBGs in a base-catalyzed process. However, the UV absorbance of such materials is too high for thick films. The fluorinated polyimide pyromellitic dianhydride-co-2,2’-bis(trifluoromethyl)benzidine (PMDA-TFMB) was therefore auditioned for this use. PMDA-TFMB was printed using 365 nm lithography using near-UV PBGs and achieved resolution as small as 2.5 μm. This material was found to have a dielectric constant around 3.0, and a coefficient of thermal expansion of 6 ppm/K. Further work on the system sought to improve both material properties and lithographic patterning. The use of alternative monomers was explored. New PBGs capable of producing stronger amidine bases were also synthesized and used to cure PMDA-TFMB. Finally, the discovery of new catalysts for low temperature curing of polyimides is described. These materials include organic and inorganic salts that allow for the complete curing of polyimides below 200°C. The material properties of films cured with these catalysts are described.
Directly-patternable benzocyclobutene dielectric materials
(2016-06-24) Hayes, Colin O'Mara; Willson, C. G. (C. Grant), 1939-; Ellison, Christopher; Dong, Guangbin; Martin, Stephen F.; Mack, Christopher
Silicon Valley, America’s bastion of innovation is named for the tiny pieces finely patterned silicon transistors that are the “brains” of all modern computational devices. However, without the ability to protect these valuable chips and translate their electrical signals to other hardware, silicon transistors are useless to the world. The demand for these microelectronic packages to perform better, cost less and be made in high yield is perhaps the next great challenge in the microelectronics industry. This dissertation covers several approaches to next generation packaging materials. In chapter two, the design, synthesis and characterization of long wavelength photobase generators for patterning polyimides is discussed. The rest of the dissertation focuses understanding and applying benzocyclobutene (BCB) thermosets to a packaging application. BCB materials have been combined with polyhedralsilsesquioxane (POSS), ring opening metathesis polymerization (ROMP) norbornenes, and addition polymer norbornenes to produce several original resins with interesting and attractive properties. New chemistry has been employed to understand the relationship of the BCB electrocyclic ring opening and Diels Alder crosslink.
Impact of humidity and polymer blending on the gas transport properties of polybenzimidazoles
(2019-07-17) Moon, Joshua David; Freeman, B. D. (Benny D.); Paul, Donald R.; Sanchez, Isaac C; Riffle, Judy S; Lynd, Nathaniel A
Polybenzimidazoles (PBIs) are attractive polymers for gas separation membranes due to their high chemical and thermal stability and rigid, size-selective molecular structures. Opportunities exist for using PBIs for high temperature H₂/CO₂ separation, among other separations, where significant amounts of water are often present. However, PBIs are uniquely hydrophilic glassy polymers, and the impact of humidity on PBI gas transport properties is not well understood. Highly sorbing penetrants like water are often considered to affect molecular transport in polymers through phenomena such as competitive sorption, antiplasticization, and plasticization, but greater fundamental understanding is needed to relate these phenomena to other key concepts in polymer transport like free volume. Additionally, opportunities exist to improve low PBI gas permeabilities through material modification. This study investigates fundamentals of water sorption, dilation, and diffusion in PBIs to develop a systematic understanding of how water uptake affects molecular transport in hydrophilic glassy polymers. Water vapor sorption and swelling in PBIs were experimentally measured, which enabled direct evaluation of polymer free volume changes arising from water uptake. Gas transport properties were measured across a range of humidities using a custom experimental apparatus and correlated with humidity-induced free volume changes. This analysis enabled unique insight into the tradeoff between competitive sorption, antiplasticization, and plasticization effects of water sorption on PBI transport properties. Similar analysis could be used to investigate fundamentals of mixed penetrant sorption and diffusion in other polymers. Finally, a method of improving PBI gas separation properties by blending PBIs with a more permeable polymer was investigated. Commercial PBI was blended with an ortho-functional polyimide capable of undergoing thermal rearrangement at high temperatures. Films of PBI blended with a small fraction of polyimide exhibited matrix-droplet morphologies that enabled synergistic combination of PBI and polyimide gas separation properties. Heat treatment caused thermal rearrangement of the polyimide phase, increasing blend H₂ permeabilities, while also increasing structural order in the PBI phase, increasing blend H₂/CO₂ selectivities. The net result of heat treatment was simultaneous improvement in both H₂ permeability and H₂/CO₂ selectivity at ambient temperatures, surpassing the 2008 H₂/CO₂ upper bound
Photocrosslinkable nonlinear optical polymers and directly-patternable polyimide dielectrics
(2014-08) Bell, William Kenneth, III; Willson, C. G. (C. Grant), 1939-; Ellison, Christopher; Anslyn, Eric; Ho, Paul; Keatinge-Clay, Adrian; Rose, Michael
The development of high-efficiency nonlinear optical (NLO) polymers has opened up many opportunities in the field of electro-optics. However, current NLO polymers do not meet stability requirements for semiconductor integration. In an effort to improve this, we examined the effects of crosslinking following electric field poling. A series of photocrosslinkable polymers bearing side chain chromophores was synthesized, poled and evaluated on the basis of the thermal stability of Second Harmonic Generation. Photoinitiation allowed for control of the onset of curing. Crosslinking was monitored by FTIR and optimal conversion was achieved by applying a slow temperature ramp during exposure. The ultimate stability of the poled polymers was directly related to the number of crosslinking substituents attached to the chromophore pendant group. With two reactive groups per chromophore significant SHG was retained at temperatures beyond the polymer Tg. In integrated circuit packaging there is a need for directly-patternable polymers of low dielectric constant. Bridging the gap between the high-value silicon chip and circuit board is a substrate comprising alternating layers of metal conductor and polymer dielectric. PMDA-ODA, an aromatic polyimide, meets many of the requirements for integration and can be patterned using a photobase generator (PBG). Due to absorbance by the PMDA-ODA precursor, this PBG must have activity at visible wavelengths. Several oxime urethanes were synthesized and evaluated as candidate long wavelength PBG. These compounds exhibit clean photochemistry and high visible light sensitivity. Unfortunately, carbamate thermal stability is insufficient for patterning PMDA-ODA. For improved material properties, PMDA-TFMB, a fluorinated polyimide, was also evaluated. Importantly, the polymer precursor is sufficiently transparent to employ thermally-stable near-UV photobases. With photobase, 2.5 micron features were resolved in PMDA-TFMB. An ancillary benefit of this methodology is reduced cure temperature (~200 °C), a traditional drawback of polyimides. This material demonstrates a dielectric constant near 3 and a thermal expansion coefficient (CTE) of approximately 6 ppm/°C in-plane. Through-plane thermal expansion is somewhat problematic, with a CTE of approximately 160 ppm/°C, and will likely require a nanoparticle composite strategy. However, this combination of material and lithographic properties make PMDA-TFMB a promising candidate for this application.