Design and development of base-catalyzed materials for microelectronics applications
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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.