Pellicles play a pivotal role as ultrathin, freestanding protective membranes within extreme Ultraviolet (EUV) lithography machines of ASML, safeguarding reticles against particle deposition. These ASML scanners subject these pellicles to challenging conditions, including a complex mix of EUV photons, hydrogen radicals, and plasma, at elevated temperatures which can induce chemical alterations and various forms of degradation, including dewetting and blistering of layers. Yet, the distinct impact of these conditions on pellicle lifetime remains underexplored.
In this research, we meticulously investigated the individual effects of hydrogen radicals and vacuum EUV light on materials resembling EUV pellicles, while systematically varying the temperature. Our study concentrated on 50 nm thick semi-amorphous SiN thin films.
Our findings reveal that exposure of pellicles to hydrogen radicals only did not induce any discernible chemical, structural, or morphological changes in the SiN thin film. In stark contrast, vacuum EUV photons triggered both chemical and morphological alterations, even at relatively low temperatures of 50°C.
A parallelism is reported between reticle lifetime experiments undertaken on TNO’s EBL2 platform and wafer printing on the ASML NXE EUV scanner installed at imec. EBL2 mimics reticle impact due to exposure of ten thousand wafers in NXE representative conditions in less than a day. In-situ X-ray Photoelectron Spectroscopy (XPS) has shown that a local high-dose EUV exposure removes surface carbon and reduces ruthenium oxide to ruthenium. These effects not only happen at the directly exposed location, but equally centimeters away. Repeating XPS after a period of reticle storage outside of the vacuum, revealed regrowth of such contamination layer and re-oxidation of ruthenium. This learning based on EBL2 explains a small but significant trend noticed in critical dimension measurement results on wafer through a batch of wafers exposed on NXE, depending on the prior storage conditions of the reticle. During first exposures following reticle entry into vacuum reticle storage effects become gradually undone. Both storage-induced mask contamination effects are shown to build-up beyond one month. Local effects of the high-dose EUV exposure remain measurable by EUV reflectometry after several weeks of storage in air.
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