Photomask registration and overlay metrology by means of 193 nm optics

This paper reports on the current status of PROVETM - a new photomask registration and overlay metrology system currently under development at Carl Zeiss. The scope of the project is to design and build a photomask pattern placement metrology tool which is serving the 32 nm node. Performance specifications of the tool are actually driven by double exposure/ double patterning approaches which will help to extend the 193 nm lithography platforms, while keeping the semiconductor industry conform to ITRS roadmap requirements. A secondary requirement of pattern placement metrology tools is the CD measurement option for design features of interest. Combining both registration and CD measurement reduces the number of process operations a photomask has to encounter during manufacture. Optical design considerations are discussed, which led to the tool being designed for 193 nm illumination corresponding to at-wavelength metrology for most current and future photomask applications. The concept enables registration and CD metrology by transmitted or reflected light. The short wavelength together with a NA of 0.6 also provides sufficient resolution even at working distances compatible with the use of pellicles, hence enabling the tool for qualification of final, production ready masks. Imaging simulations with a rigorous Maxwell solver prove our chosen optical concept to be adequate for the various mask types (e.g. COG, MoSi, EUV) commonly used today and presumably in the future. The open concept does enable a higher NA for future, pellicle free applications.