APPLIED PHOTOCHEMISTRY FOR MULTICOLOR PHOTOLITHOGRAPHY
Falvey, Daniel E
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Photolithography is the most mature technique for nanomanufacturing. Traditional photolithography generates patterns using a single wavelength of light. Recently, there has been considerable interest in the development of new photoinitiators that can improve the patterning process by incorporating multiple wavelengths of light. This text will focus specifically on organic photoinitiators. Chapter 1 will begin with an introduction to organic photochemistry and the description of laser flash photolysis, a time-resolved UV-Vis spectroscopy technique used for characterizing short-lived intermediates formed as a result of photolysis. Then, it will provide a description of photolithography and explore the limitations of current techniques. Finally, it will describe multicolor photolithography and the characteristics of a multicolor photoinitiator. Chapters 2-4 will explore various organic compounds that have been investigated as potential multicolor photoinitiators. Chapter 2 will focus on the photodecomposition of α-diketones into radicals that can be used to initiate polymerization of sytrenic monomers. Laser flash photolysis, product analysis, and computational modeling will be used to demonstrate that this decomposition occurs through a Norrish type I mechanism where higher excited states are populated via triplet-triplet annihilation. Chapter 3 will explore dithioesters and trithiocarbonates that can be used as initiators for reversible addition-fragmentation chain transfer (RAFT) polymerization. Dithioesters and trithiocarbonates have a long history in the literature but their potential as photoinitiators has not been explored in depth. In chapter 3, computational modeling is used to investigate the excited states of various RAFT agents. Chapter 4 will focus on 2-methoxy-9,10-dioxo-9,10-dihydroanthracen-1-yl 4-methylbenzenesulfonate, a multicolor photoacid generator (PAG). When irradiated with 355 nm light, this PAG releases p-toluenesulfonic acid which can be used to initiate cationic polymerization. The addition of 532 nm light accelerates the acid release, making the PAG a two-color photoinitiator. Multicolor PAGs provide improved resolution over one-color systems which makes them useful for photolithography and nanomanufacturing.