EUV-LET 2.0: A compact exposure tool for industrial research at a wavelength of 13.5nm
In this paper the authors present the EUV laboratory exposure tool in a new configuration for in-band operation at a wavelength of 13.5 nm, named EUV-LET 2.0. Compared to the previous configuration for a working wavelength of 10.9 nm, which was aimed at cost-efficient general nanostructuring, the main applications of the EUV-LET 2.0 can be found in the industrial research, development and quality control of EUV photoresists. The tool is capable of photoresist characterization in terms of sensitivity, contrast, outgassing behavior and resolution. Additionally, the exposure tool can be utilized for in-band transmission measurements of photoresists and filters. The EUV-LET 2.0 uses the xenon plasma emission from a compact discharge-produced plasma (DPP) EUV source, filtered to in-band EUV at 13.5 nm with 4 % bandwidth (at full-width at half maximum) by an out-of-band filter (niobium/silicon nitride) and a multilayer mirror (molybdenum/silicon). High-resolution phase-shifting transmission masks define the features and are positioned precisely to the resist-coated wafer. An EUV sensitive charge-coupled device (CCD) camera supports beam alignment and diffraction efficiency measurements. For high-resolution patterning the (achromatic) Talbot lithography is used, which is perfectly suited for partially coherent radiation of a defined bandwidth, as provided by a plasma-based radiation source. The paper addresses the general tool configuration including spectral characterization of the DPP EUV source before and after multilayer reflection, diffraction efficiency measurements of a realized phase-shifting mask, contrast curve measurements and exemplary exposures performed with a single contact hole mask. Different exposure results are achieved by varying the mask to wafer distance and by controlled movement of the wafer stage during an exposure. In this way both, de-magnified contact hole arrays as well as lines and spaces arrays with 35 nm half-pitch are obtained.