In-die actinic metrology on photomasks for low k1 lithography

Abstract

New lithography techniques like Double Patterning, Computational Lithography and Source Mask Optimization will be used to drive immersion lithography at 193nm to its limits. The photomask will become more and more a critical optical element in the scanner beam path. Precise image transfer of the circuit features into the resist will be key for the mask manufacture and its qualification. The extremely high MEEF values in low k1 lithography dramatically amplify small process variations on the mask features to the wafer print. Complex mask features using sophisticated OPC and assist features require mask metrology under scanner conditions which measured the optical performance of the mask. Double patterning technology tightens the registration and CDU specification of the patterns at the same time. Especially, overlay becomes more and more critical and must be ensured on every die. In-die registration and CD metrology on arbitrary features at scanner wavelength can measure the mask performance precisely and ensure correct print results and high yield in the wafer fab. Moreover even a complete set of phase shift measurements, CD and registration measurements in the die features can help to ensure that mask manufacture and its qualification provide indeed the largest process window for wafer printing. It is key for higher yield and better performance. In this paper an overview about several actinic in-die metrology techniques will be given. Focus will be on application of in-die CD measurements using the Zeiss WLCD tool as well as in-die registration measurements using the Zeiss Prove tool will be shown and discussed.