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- Development of Molecular Glasses for Solvent-Free Photolithography utilizing Combinatorial Vapor Deposition (2007)
- This thesis is concerned with solvent-free film preparation of low molecular weight, amorphous, photosensitive resists by physical vapor deposition (PVD), their subsequent UV exposure through a mask and their development to produce features in the micro- and nanometer range. In the frame of the thesis towards a solvent-free lithography, the combinatorial PVD techniques developed in our group allow the exact preparation of gradients or sectors on the same substrate as well as the coevaporation of several substances. Whilst evaporation, different masks can be positioned between the sources and just below the substrate so that the material deposit only within defined sectors of the substrate. By doing so, a so-called sector library with various sectors with different compositions on the same substrate and in one sequence without breaking the vacuum or opening the PVD chamber can be created. In the thesis, two types of low molecular weight, negative tone photoresists were investigated: 1.) Research was carried out on molecular glass photoresists which consist of several components. One of these components, a photoacid generator (PAG), generates acid molecules when exposed to suitable wavelengths. In a post-exposure bake, the formed protons catalytically initiate reactions between the other resist components (chemical amplification). Investigations and characterizations (DSC, TGA, HPLC) were performed on the suitability of various low molecular weight chemically amplified resist (CAR) materials for film deposition via PVD as well as subsequent photolithography processes. Three ternary systems were identified to accomplish the requirements (vapor depositability, formation of glassy films, photostructurability). Each of them consists of a molecule with several phenolic groups, an acid labile crosslinker, and a PAG. By utilizing the combinatorial mask technique of the PVD facility, sector libraries were prepared to optimize the composition of each of the CARs. The exposure dose was also combinatorially optimized by irradiating each of the created sectors with different doses of 365 nm UV light. Additionally, development conditions like dilution and time were optimized. Interestingly, it was found that two resists can be developed with pure water which is a step towards more environmentally friendly processes. With the employed equipment, features with a resolution of 400 nm could be realized. HPLC analysis was utilized to determine the thermal stability of investigated substances and the compositions in the prepared sectors. 2.) Newly synthesized, self-reactive coumarin derivatives were explored in regard to their suitability for an all-dry photolithography process. When exposed to wavelengths above 300 nm, many coumarin derivatives undergo a [2+2] cycloaddition. Depending on the number of functional coumarin groups, dimers, chains, or networks can be generated by this photoreaction. The ten coumarin esters presented in this thesis were, except for one, newly synthesized and characterized by NMR spectroscopy, HRMS, elemental analysis, DSC, and TGA. They were synthesized in respect of requirements regarding photoresists and PVD, e.g. glass-forming properties, and thermal stability. Several of the synthesized esters could be evaporated without decomposition. Two esters, 1,3-dibenzoic-5-tert-butylcoumarinester und 3,5-di-tert-butylbenzoiccoumarinester, produced the desired amorphous and thus transparent films. Since the monomers preferably reform when the irradiated product is exposed to wavelengths below 300 nm, irradiation only above this critical wavelength was employed. This circumstance lead to long exposure times of 50 min because of the small overlap between the absorption spectra of the coumarin derivatives and that of the lamp. The successful employment of a sensitizer which could be coevaporated with 3,5-di-tert-butylbenzoiccoumarinester reduced the necessary exposure dose significantly from 50 to 5 min. Since the above described coumarin derivative monomers evaporate in high vacuum, a dry development through the removal of the unexposed monomers by thermal treatment under high vacuum in a self-made facility was also investigated. One of the two vapor-depositable and glass-like monomers, monofunctional 3,5-di-tert-butylbenzoiccoumarinester, could be removed completely by thermal treatment under high vacuum. By doing so, an all-dry photolithography with a low molecular weight coumarin ester could be established. It was shown that a dimerization is sufficient to create an ample difference in melting points for a dry development. In summary, this thesis proved the concept of PVD for the solvent-free preparation of low molecular weight, glassy photoresist films. The application of a newly synthesized coumarin derivative as a negative tone, low molecular weight photoresist allowed an all-dry photolithography, and was realized for the first time for this substance class.