Phame(R) - high resolution off-axis phase shift measurements on 45nm node features

Abstract:
The extension of optical lithography to 45nm and beyond goes along with an increased mask complexity and tighter specifications. Both attenuated and alternating phase shift masks (PSMs) require precise control of the phase shift as a function of both pitch and target sizes. Simulations show that the phase shift in the image plane of a microlithography scanner is strongly impacted by numerical aperture (NA), mask pitch, 3D mask effects, and polarization, especially if the feature sizes come close to the imaging wavelength. Carl Zeiss SMS has developed a new phase metrology system that overcomes the limitations of currently existing tools. The new optical metrology tool – Phame(R) – enables the industry to perform in-die phase measurements on alternating PSM (altPSM), attenuated PSM (attPSM), and CPL (chromeless phase lithography) masks down to 120nm half pitch at the mask. The optical beam path of the new metrology system allows actinic phase measurements of 193nm photomasks with a mask side NA up to 0.4, which is 1.6NA scanner equivalent at the wafer. This enables full compatibility to future 193nm immersion scanners down to the 32nm node. Off-axis phase measurement is realized by applying consecutive measurements of single source points according to the scanner relevant illumination settings. Phame(R) measures the scanner equivalent phase and amplitude in the image plane for each coherent source point. For off-axis phase shift extraction Zeiss has developed a new concept called high resolution phase. This high resolution phase is sensitive to the diffraction spectrum and to mask phase errors. In this paper we will explain the off-axis high resolution concept in detail. First measurements have been performed on attPSM with 45nm node test features. The results show strong deviations of the high resolution phase shift depending on the pitch. Isolated features combined with dense features have been investigated. The measurement results will be presented in the paper.