Proceedings Article | 25 March 2019
KEYWORDS: Polymers, Luminescence, Lithography, Optical lithography, Molecules, Life sciences, Image processing, Photoresist processing, Stochastic processes, Statistical modeling
Polymeric resist materials are a critical part of the lithographic patterning process. Understanding their behavior, particularly at extreme dimensions becomes challenging: the presence of a rigid substrate or free surface, chemical interactions, and stochastic fluctuations can all play a role. Continuum models for polymer material behavior at small length scales not only become inaccurate, but also fail to capture the statistical variations that must be understood in order to determine what are the ultimate limits to producing defect-free structures. Experimental measurement of nanoscale heterogeneities in polymer properties is difficult. Many of the techniques that possess the required spatial resolution use energetic beams of electrons that rapidly damage soft materials, while x-ray or neutron scattering methods provide only ensemble average measurements. Individual fluorescent molecules, however, can yield a significant amount of information about their local environment. Measurements of the fluorescence lifetimes of suitably engineered fluorophores have long been used in the life sciences to probe local pH, and oxygen, or Ca2+ concentration. Lifetimes may also be sensitive to viscosity, giving information about local molecular configurations at nanosecond timescales. Measurements of fluorophore orientation, and rotational mobility, can indicate local molecular ordering and mobility, respectively.
While the use of single-molecule fluorescence imaging methods in the life sciences has progressed rapidly, its use in materials science has been slower to develop, with only a handful of studies related to lithographic materials. One principal reason for this is that, in materials, fluorophore orientation is often fixed. Single-molecule images therefore have a complex, orientation-dependent structure, that, if not correctly accounted for, can lead to large errors in determining their position and orientation. Reducing the positional inaccuracies to the few-nanometer or better level, requires more sophisticated approaches to fitting single-molecule images and novel imaging hardware. With these approaches, both the location and orientation of individual fluorophores can be determined accurately. This information, when combined with single molecule lifetime measurements can, in principle, provide nanometer scale on the structure and dynamics of polymeric materials. I will discuss our progress in making accurate and precise measurements of fluorophore position and orientation in materials to enable high-resolution imaging, our development of a straightforward approach to determine how localization uncertainty and fluorophore labeling density together limit our ability to resolve nanoscale structures, how lithographic patterning enables us to partially overcome that limit, and how single-molecule orientation measurements can provide information on deformation in polymers at the 10 nm length scale. Finally, I will speculate on how measurement of single-molecule fluorescence lifetimes might provide information on local polymer heterogeneity at various stages of the image formation process in lithographic material systems.
