The interaction of extreme ultraviolet (EUV) light with matter is a critical step in EUV lithographic processes, and optimization of the optical material properties of all elements in the lithographic chain (from optical coatings and pellicles to photoresists) is crucial to harnessing the full power of EUV lithography. To optimize these materials, accurate measurements of EUV absorption and reflection are needed to extract the corresponding actinic optical properties and structural parameters. Here, we report on actinic EUV metrology-based absorption and reflection measurements enabled by coherent table-top EUV sources based on high-harmonic generation. We demonstrate the capabilities and flexibility of our setup with measurements on crystalline films, photoresist systems, and carbon nanotube membranes and provide extracted optical parameters, absorption kinetics, and 2D transmission maps, respectively. These results showcase the power of lab-based actinic inspection methods based on compact, coherent EUV sources for providing crucial data for material optimization and lithographic simulation.
Extreme ultraviolet (EUV) lithography (92 eV) has recently entered logic and memory high-volume manufacturing to ensure the continuation of Moore’s Law into advanced technology nodes (sub 5 nm). In parallel to advancements in the lithographic system, the development of suitable photoresists plays an equally important role in pushing the boundaries of EUV lithography. Fundamental work on well-established chemically amplified resists (CAR) for EUV as well as the upcoming resists based on metal-organic materials have indicated that the lithographic mechanism is largely governed by electron mediated chemistry. In a simplified model, the electrons emitted upon ionization of the material generate further secondary electrons, which interact with the resist components and induce a solubility switch driven by electron and radiation chemistry. To develop a better performing resist, it is of utmost importance to understand the photoelectron kinetic energy spectrum, secondary electrons and their generation efficiency, and the electron mean free path in the photoemission process. In this work, we use photoemission spectroscopy with a table-top, coherent, 92 eV photon source to shed light on the chemistry driven by photon exposure. The valence band photoelectron spectrum (PES) of an environmentally stable chemically amplified photoresist (ESCAP), as well as a model material for an open-source metal oxide (OSMO) resist were measured using our tabletop EUV photoemission setup. We report the evolution of the PES as a function of exposure dose; capturing chemical changes.
Science stands on three legs: hypothesis, experiment, and simulation. This holds true for researching extreme ultraviolet (EUV) exposure of photoresist. Hypothesis: For resist exposure as patterns get smaller and closer together, approaching molecular units in width and resist-height, the molecular dynamics will limit the working resolution of the resist due to the formation of printing defects. Without taking proper consideration of these dynamics, the single-patterning lithography roadmap may end prematurely. Experimentally we are developing methods for sub-picosecond tracking of photoionization-induced processes. Using ultrashort pulses of light to excite and probe new materials with techniques that show the interactive dynamics of electronic and nuclear motion at the very limits of light-speed. This certainly holds true for exposing photoresists with EUV where ultrafast photoreactions induce chemical change via multiple pathways such as high-energy ionization fragmentation, recombination, and multispecies combination that ideally end in low-energy electron transfer reactions, analogous to lower energy photoreaction (but with a charge). In the nonideal case, these reaction processes lead to incompatible byproducts of the radiolysis that lead to types of stochastic defects. To do ultrafast studies we must build a foundation of knowledge using atomistic simulation to interpret transient molecular dynamic processes. Before we can do this, we need to learn how to simulate various spectral modalities to provide a starting point. In this work, we examine X-ray Photoelectron Spectroscopy of a model resist and use atomistic simulation to interpret the reactant-product composition of the spectral samples.
The dissociative photoionization of tert-butyl methyl methacrylate, a monomer unit found in many ESCAP resists, was investigated in a gas phase photoelectron photoion coincidence experiment employing extreme ultraviolet (EUV) synchrotron radiation at 13.5 nm. It was found that the interaction of EUV photons with the molecules leads almost exclusively to dissociation. However, the ionization can also directly deprotect the ester function, thus inducing the solubility switch wanted in a resist film. These results serve as a building block to reconstruct the full picture of the mechanism in widely used chemically amplified resist thin films, provide a knob to tailor more performant resist materials, and will aid interpreting advanced ultrafast time-resolved experiments.
The interaction of EUV light with matter is a critical step in EUV lithographic processes and optimization of the optical material parameters of photoresists and reflector/absorber stacks is crucial to harness the full power of EUV lithography. To optimize these materials, accurate measurements of EUV absorption and reflection are needed to extract the corresponding actinic optical properties and structural parameters. Here, we report on two endstations within imec’s AttoLab that enable actinic EUV absorption and reflection measurements. We commission these tools with measurements on model thin film and photoresist systems and provide extracted optical parameters as well as absorption kinetics, respectively. These results showcase the power of these tools for providing crucial data for material optimization and lithographic simulation.
Imec’s AttoLab is the first industrial laboratory capable of watching the ultrafast dynamics of photoresists following 13.5 nm, EUV exposure, and for emulating high-numerical-aperture (high-NA) exposure on 300-mm wafers using two-beam EUV interference. The two respective beamlines are powered by a laser-based high-harmonic generation EUV source. Its capabilities have recently been proven by imaging 20 nm pitch lines and spaces using Lloyd’s Mirror interference lithography. In parallel, time-averaged and time-resolved techniques for studying the ultrafast dynamics of photoresists after EUV exposure, coherent diffractive imaging to study resist interfaces, and more sophisticated interference lithography techniques for printing sub-22 nm pitches on full 300-mm wafers are being developed. Taking advantage of the bright and short EUV pulses now available at imec, we will be able to contribute to a smooth transition towards next generation high-NA lithography.
Recently, imec has installed and commissioned an industrial, ultrafast EUV materials characterization and lithography lab, imec’s AttoLab, with a primary aim to explore limits of photoresist performance and their associated ultrafast chemistries. Here, we demonstrate, for the first time, the use of a table-top, high-harmonic EUV system (KM Labs, XUUS4) to perform interference lithography of sub-22-nm pitch patterns in an Inpria MOx resist via a Lloyd’s mirror interference lithography (IL) tool. Analysis of SEM images enables us to identify potential sources of image blur, which we attribute to out-of-sync vibrations, flare, spectral purity, and laser stability. Nevertheless, these results confirm the ability of table-top, high-harmonic EUV sources to print lithographic patterns below a 22-nm pitch. In future work, we plan to investigate sub-20-nm patterning in different resist formulations, as well as expand the lithographic capabilities in AttoLab to perform IL on full 300-mm wafers.
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