High-power EUV lithography: spectral purity and imaging performance

With the introduction of the NXE:3400B scanner, ASML has brought extreme ultraviolet lithography (EUV) to high-volume manufacturing (HVM). The high-EUV power of >200 W being realized with this system satisfies the throughput requirements of HVM, but also requires reconsideration of the imaging aspects of spectral purity, both from the details of the EUV emission spectrum and from the deep-ultraviolet (DUV) emission. We present simulation and experimental results for the spectral purity of high-power EUV systems and the imaging impact of this, both for the case of with and without a pellicle. Also, possible controls for spectral purity will be discussed, and an innovative method will be described to measure imaging impact of varying conversion efficiency (CE) and DUV. It will be shown that CE optimization toward higher source power leads to reduction in relative DUV content, and the small deltas in EUV source spectrum for higher power do not influence imaging. It will also be shown that resulting variations in DUV do not affect imaging performance significantly, provided that a suitable reticle black border is used. In summary, spectral purity performance is found to enable current and upcoming nodes of EUV lithography and to not be a bottleneck for further increasing power of EUV systems to well above 250 W.