The Study of Attenuated PSM Structure for Extreme Ultra VioletLithography with Minimized Mask Shadowing Effect

摘要

In this paper, we suggest an optimal attenuated phase shift mask (PSM) structure for extreme ultra violet lithography(EUVL) to minimize mask shadowing effect without loss of image contrast. The attenuated PSM proposed in this studyis based on Fabry-Perot structure that consists of tantalum nitride (TaN) attenuator, Al_2O_3spacer, and molybdenum (Mo)phase shifter. Deep ultra violet (DUV) reflectivity can be lowered down to 5% at 257nm for higher efficiency in DUVinspection process through the optimal thickness combination of TaN and Al_2O_3. Since the thickness variation of Modose not affect the DUV reflectivity, the phase shift effect can be controlled by Mo thickness only. As a result,attenuated PSM with phase shift of 180±6° and absorber reflectivity of 9.5% could be obtained. The total thickness ofabsorber stack is only 52nm. The analysis of aerial image was performed using SOLID-EUV simulation tool. Theattenuated PSM showed steeper edge profile and higher image contrast compared to binary mask. Imaging propertiesincluding horizontal-vertical (H-V) critical dimension (CD) bias and pattern shift depending on both pattern size andprocess condition were compared to the binary mask using aerial image simulation. Attenuated PSM showed less H-VCD bias compared to that of binary mask. The 32nm dense pattern shows larger H-V CD bias than 45nm one due tolarger shadowing of smaller pattern size. Especially, 32nm dense pattern at binary mask has very large H-V CD bias. TheH-V CD bias was also affected by the change of focus. However, the H-V bias variation with defocus was below 1nmwithin the process latitude. We also obtained the result that the pattern shift is less sensitive than H-V CD bias to theoptical property of absorber in EUVL.