Key components for the EUV mask infrastructure include actinic AIMS and pattern inspection, which are crucial for the introduction of EUV lithography into HVM. The usage of pellicles further substantiates the need for actinic light sources. EUV light sources with a high availability and brightness are required to increase the throughput for AIMS and pattern inspection systems.
The first bounce EUV collection optics are subject to harsh debris fluxes in the form of ions, neutrals and droplet fragments comprising the lifetime of the light source. Enhancing the debris mitigation reduces the reflectivity decay and therefore improves the light source cost-of-ownership (CoO).
Key to optimize the debris mitigation strategy is the assessment and quantification of the detrimental plasma debris. In the present work, the high kinetic energy particles including ions and neutrals generated from the laser irradiated droplet target are resolved spatially and temporally for an intermediate pressure regime. The implications of the ion and neutral flux on the collection optics are discussed in this work. By providing fresh targets in the form of micro-meter sized droplets to the droplet irradiation position, a certain variability of the droplet position with respect to the laser focal area is inherent. By actively changing the droplet position with respect to the laser focal area with a control system the influence on ion and EUV propagation direction is studied in this work.
Finally, the long term LPP source operation is assessed. The debris mitigation system is enhanced employing a three-layer strategy demonstrating an increased source cleanliness for a GI collector configuration. Results from a sample exposure test for EUV reflection degradation of the first collector optics and the impact on the CoO will be presented.
The spectral emission properties of a droplet-based laser-produced plasma are investigated in the vacuum ultraviolet (VUV) range. Measurements are performed with a spectrograph that operates from 30 to 180 nm with a spectral resolution of 0.1 nm. The emission spectra are recorded for different metal droplet targets, namely tin, indium, and gallium. Measurements were performed at different pressure levels of the background gas. Several characteristic emission lines are observed. The spectra are also calibrated in intensity in terms of spectral radiance to allow absolute emission power estimations from the light source in the VUV region. The presented experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the out-of-band radiation emission of a tin-based extreme ultraviolet (EUV) source. By tuning the type of fuel, the laser energies, and the background gas, the laser-produced plasma light source shows good capabilities to be operated as a light source that covers a spectral emission range from the EUV to the sub-200 nm range.
In this work, the spectral emission proprieties of a droplet-based laser-produced plasma are investigated in the VUV range. These studies are performed with a spectrograph operating from 30 nm to 180 nm at a spectral resolution of 0.1 nm. The emission spectra are recorded for different droplet-based metal fuels such as tin, indium and gallium in the presence of different background gas pressure levels. The experimental results are relevant for alternative light sources that would be needed for future wafer inspection tools. In addition, the experimental results help to determine the Out- Of-Band (OOB) radiation emission of the EUV source. By tuning the type of fuel, the laser energies and the background gas, the LPP light source shows good capabilities to be operated as a tunable light source that covers a spectral emission range from the EUV to the sub-200 nm range.
EUV sources with high brightness and stability are required for actinic photomask inspection. High availability and cleanliness after IF are additional stringent requirements. EUV lithography is only production ready, if these tools are available with HVM specifications. At the Laboratory for Energy Conversion, ETH Zurich, droplet-based EUV LPP sources have been designed, developed and tested at the system level for the last 8 years and has been commercialized by Adlyte AG. The most advanced facility, namely ALPS II, has been operated as a prototype source for hundreds of hours. In the present work, the EUV plasma is imaged with the help of a pinhole camera. The dimension of the plasma in the direction of the laser axis and the direction of the train equal 60 μm and 70 μm, respectively. The plasma is also imaged using an ICCD with an exposure time of 5 ns. The observed droplet plasma has a characteristic kidney shape. The ICCD is a valuable diagnostic as inspection tools require high pulse-to-pulse reproducibility that cannot be assessed to the full extend using a EUV pinhole camera. Various collector configurations, using either NI or GI, have been integrated into the source. The measurements of the emission characteristics at IF for a GI collector configuration reveal a Gaussian spot shape at IF and a pulse-to-pulse stability of 6.8 % (σ), which matches previous stabilities at the source level. The debris mitigation system employs a three layer strategy between the plasma and IF. Introducing a high momentum flow as a first layer of debris mitigation, the load of tin spots on the collector could be reduced by a factor of 9, hence a significant increase of source life-time is obtained. A quantification by Adlyte of IF cleanliness after 24 hours source operation revealed no relevant contamination with respect to the requirements for Blank Inspection Cleanliness after IF.
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