Even though instrumentation for electron beam lithography (EBL) has progressed immensely since it was first introduced almost 50 years ago, enabling beam spot sizes below 5 nm for certain systems, its lithographic resolution limits are still bound by the primary and secondary electron scattering processes that occur when a specific resist is exposed. As the feature sizes become smaller and resists designed with higher sensitivities, these stochastic processes play an increasing role in the resulting line edge roughness (LER) thus leading to an effect known as shot noise. Unfortunately, unraveling the impact of these processes from the impact of the development step is partly hindered by our inability to measure the 3D profile of the latent image from resists directly after exposure. Furthermore, given the recent rise of chemically-amplified resists (CARs) used for the next generation extreme ultraviolet lithography (EUV), it has become even more critical to find ways to characterize and investigate the shot noise effect.
In this work, we tackle this challenge by applying the resonant soft x-ray scattering (RSoXS) technique in a grazing incidence configuration to extract the cross-sectional profile of resists that have already been patterned, but have yet to be developed (i.e., latent image). We find that the difference in chemistry induced by the chain scission process in exposed PMMA and CAR resists is enough to produce enough scattering contrast at certain X-ray energies near the absorption edge of carbon in order to provide a latent image profile of the pattern with sub-nanometer resolution. In this paper, we will compare the latent image profiles extracted from this RSoXS data to the profiles obtained after development, as well as expand on the nature of this chemical contrast mechanism. We will show how this scattering data may be interpreted and the information used to shed light on the nature of the resolution limit of a specific combination of resist and exposure plus development conditions. Finally, we will elaborate on the impact of the measurement itself on the resulting pattern morphology as well as how similar insights might be gained across other types of resists.
1. Hans C. Pfeiffer, } "Direct write electron beam lithography: a historical overview", Proc. SPIE 7823, Photomask Technology 2010, 782316 (24 September 2010); doi: 10.1117/12.868477