Jack W. Frankovich

High-resolution tool for measuring photomask flatness

As lithography wavelengths reduce, the depth of focus decreases rapidly as well, resulting in the need for flatter photomasks with specifications under 0.25 microns. With the introduction of EUV mask technology, the overlay error budget drives the flatness requirements considerably lower to just 50 nanometers or so. This paper describes a new tool that utilizes near normal incidence interferometry to perform flatness measurements on polished 6025 photomasks that are coated or uncoated. Achieving a low measurement uncertainty required a robust optical and mechanical design. Even nanometer level measurement errors due to gravity sag have to be considered. Supporting the substrate during measurement creates deformations due to gravity that must be dealt with for an accurate evaluation of the flatness. Two improvements to the recently introduced Corning Tropel UltraFlat Mask System that first minimize and then remove the remaining gravity sag errors in photomask flatness measurements will also be discussed.