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Increasing source power levels to meet higher throughput targets, in combination with tightened overlay requirements, drives the need to mitigate reticle heating and enhance performance. Improvements to the EUV scanner will prevent or compensate for overlay impact resulting from reticle heating; however, improving reticle blank thermal properties also plays a role in improving overlay and scanner performance. Reticle heating is a significant contributor to the overlay budget. EUV reticle blanks are made of a Low Thermal Expansion Material (LTEM) which is relatively insensitive to temperature changes. This makes these reticles suitable to use in EUV scanners such as the NXE:4000F, where strong temperature gradients (clamp cooling water temperature) are experienced between the image field and unexposed reticle areas. Reticle blanks have an average zero crossing temperature (ZCT), or temperature of the minimal reticle deformation, of 22 °C. The rate of the thermal expansion as a function of temperature (ZCT slope) is uniform across the reticle and can vary from 1.0 to 1.85 ppb/K2. Local ZCT variations in the reticle, inherent to the manufacturing process, can also create non-uniform thermal expansion, degrading overlay. Reticle heating experiments have been performed to investigate these effects. Mask substrates were fabricated from two different LTEM manufacturing processes with different ZCT slopes and levels of ZCT non-uniformity to isolate the deformation of the reticles and the effects of LTEM thermal properties on overlay. The study shapes our understanding of how reticle heating affects overlay through comparison of simulation models with experimental data. This will enable LTEM vendors to optimize materials for future EUV systems. We provide a recommendation for material requirements to achieve minimal reticle heating impact for over 500W systems.
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