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In the concept phase of EUV lithography much thought was spent on engineering the best multilayer mirror to achieve the optimal combination of wavelength and reflectivity.
The EUV mask started its career in lithography as a commodity, benefitting from the mirror technology with an opaque layer carrying the pattern, for which Tantalum was a straightforward choice. The low NA of 0.25 and the technology nodes for 90nm pitch at the start of the EUV era, allowed the mask industry to drive the fabrication of the Ta-based EUV mask to perfection.
As the understanding of the EUVL grew, its different components – e.g., resist, scanner optics, source - are being tailored to create the perfect aerial image and robust resist pattern. Since recent years, the attention is moving more and more to the EUV mask. Mask specific challenges involve mask deficiency induced stochastic failures, the anamorphicity of high-NA EUVL and mask 3D effects. Pellicle development and mask lifetime understanding are well progressing to control mask deficiency impact on wafer, and mask data prep software is being made aware to handle different mask reduction factors.
Mask 3D effects, as a common denominator for inherent pitch- and orientation-dependent wafer observations, are identified to limit wafer performance for current and future technology nodes of 32nm pitch and below. Initial solutions are proposed by source-mask optimization, where the illumination and mask design are compensating these wafer effects, which are in fact due to the choice of the mask materials.
At imec we started several years ago to target the mask material as the parameter to control the mask 3D effects on wafer. Our strategy towards a material down-selection for a mask technology change evolves on two levels, demonstrating improvements from lithographic perspective and validating compatibility with essential mask absorber requirements. Through simulations we mapped EUV n&k regions with their expected imaging benefit. Our simulation approach and material properties knowledge directed our engineering and characterization of novel absorber films. Various patterning strategies are investigated to ultimately validate the experimental imaging behavior to the simulation predictions. In this effort imec actively drives collaboration with suppliers, stakeholders, and institutes to explore the EUV mask space.
The choice of EUV SPIE committee to have EUV mask as a keynote topic this year emphasizes that this mask material change is no longer a research matter, but a key enabler to fully exploit EUVL now and in the future.
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