title: Investigations on the amorphous state and high-resolution patterns prepared by scanning probe lithography
Creators: name: Krohn, Felix
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: amorphous materials
subject: molecular glasses
subject: scanning probe lithography
subject: dielectric spectroscopy
subject: glass transition
subject: 540

description: This thesis focuses on the synthesis, fundamental characterization, and partly the application of amorphous low molecular weight materials and polymers. Non-polymeric glass formers, so-called molecular glasses, have gained increasing attention in the last decades. Their application ranges from thin-film processing for micro-manufacturing over organic electronics to pharmaceuticals. However, there is still a lack of understanding concerning fundamental properties of this material class. In particular, detailed investigations on the influence of presence and substitution position of dipolar substituents on T_g, glass-forming ability, and the dynamics of such glasses are missing. Furthermore, specific features in the behavior of asymmetric binary mixtures have recently been discovered. Further investigations of these specific features require tailor-made synthesis of novel molecular glasses to form stable mixtures with low molecular weight additives. While molecular glasses are used in many applications including lithography, they fail to offer specific requirements for resist materials in thermal scanning probe lithography. Thus, the use of polymers for this technique is inevitable. So far only one polymer has been established as resist material, therefore the development of further possible resist materials is of high interest. In this context, the introduction gives an overview about general properties of glass-forming materials, such as glass transition and relaxation behavior. The focus is on (non-polymeric) organic glasses and an overview of design principles yielding stable, high-T_g molecular glasses is presented. The focus then shifts to dynamics as observed in organic glasses and a literature overview is presented dealing with main and secondary relaxation phenomena as observed in organic glasses. As specific features are observed in dynamics of binary mixtures of such glass formers, literature studies addressing these phenomena are presented in more detail and current debates about this topic are discussed. The last part of the introduction gives an overview of the evolution of micro- and nano-lithography, briefly presenting the principles of UV-lithography and focusing on alternative lithographic techniques. State-of-the-art in thermal scanning probe lithography is presented including strategies to improve pattern transfer as well as recent developments of applications going beyond classical lithography for microelectronics. In total, this cumulative thesis consists of four additional chapters, two of which were published as two peer-reviewed full papers. The other two chapters are presented in a manuscript form and intended for submission as full papers shortly. An overview of these topics is presented along with a summary of the major results in the synopsis. The first chapter focuses on the influence of dipole containing nitrile groups and their respective substitution position on T_g and glass-forming ability of molecular glasses based on 9,9’-spirobi[9H]fluorene. The introduction of such highly interacting substituents raises T_g as a function of molar mass by a much higher extent than other substituents previously published in literature. The findings of this topic provide new insight in thermal properties of molecular glasses and may be beneficial for the application of molecular glasses in several fields of research and application. The second chapter features selected compounds studied in the first chapter and focuses on the influence of dipole containing nitrile groups on dielectric relaxation phenomena of these compounds. A clear difference is observed between nitrile groups that have a fixed orientation with respect to the stiff core and others that can change their orientation. The relaxation spectra are compared to those of other well-investigated glass formers. For compounds with a fixed nitrile group, a secondary process with features that have not been reported before is observed. In the third chapter a new asymmetric, binary system of non-polymeric glass formers with a high T_g-contrast of more than 200 K showing neither demixing nor crystallization over the entire temperature and concentration range is developed. Using dielectric spectroscopy, dynamic scanning calorimetry, and dynamic mechanical analysis, a conclusive picture of the dynamics of the investigated system is shown. The relaxation of the high-T_g component shows a plasticizer-effect upon mixing with moderate broadening of the process. The dynamics of the additive at low respective concentrations show a broad distribution of relaxation times and an Arrhenius-like evolution with temperature resembling that of materials in confined space. The obtained results support the interpretation that the concentration-dependent T_g of the α_2-process shows a maximum at intermediate additive concentrations, a currently highly debated issue. The fourth chapter focuses on poly(olefin sulfone)s as new resist materials for thermal scanning probe lithography. These polymers feature a low ceiling temperature and are therefore ideal candidates for this technique. In this context, high-resolution lithography with a resolution down to 10 nm is demonstrated. The obtained patterns are stable towards acidic conditions and thus offer new possibilities for the application of this technique. Sequential infiltration synthesis is used to obtain an etch-resistant hard-mask usable for a subsequent pattern transfer.