Overview of stitching for high NA: imaging and overlay experimental and simulation results

Abstract

An increased interest to stitching for High NA EUVL is observed; this is driven by expected higher demand of larger size chips for various applications. In the past a recommendation was published [1] to have 1-5 um band where no critical structures of a High NA layer would be allowed. In [2], we have introduced new insights on at-resolution stitching. In this publication, we present new experimental results obtained on NXE:3400B scanner. In the past we showed NXE feasibility results of vertical lines and contact holes stitching at relaxed resolution (40-48 nm pitch) in a single wafer location. In this study we evaluate stitching behavior through slit at more aggressive resolutions (P36 and P24 lines / spaces).We provide an overview of interactions in the stitching area such as aerial image interactions, absorber reflection, absorber to black border transition, black border vicinity impact and show corresponding experimental and simulations results. We formulate initial requirements for black border edge placement control and show performance of new masks. For stitching with low-n masks, we discuss using sub-resolution gratings to suppress the elevated mask reflectivity. We show rigorous simulations of stitched images, its sensitivity to overlay errors and propose mitigation mechanisms for OPC. Finally, an overview of stitching enablers will be described: from improved reticle black border position accuracy and absorber reflectivity control to mask resolution and OPC requirements.

Evaluation of field stitching optimization for robust manufacturing with high-NA EUVL

“…Thus, stitching will be required to achieve the same exposure field as previous scanner generations. [2,3] Previous studies have shown that the formation of the black border region causes deformation of the multi-layer mirror, which can cause pattern placement errors. This necessitates the use of an image border in the double exposure region.…”

“…These studies have been instrumental in showing the feasibility of stitching but have focused on simple periodic patterns. [2,3] In this study we will simulate the optical transition between the black border and image border, as well as the mechanical stress relaxation caused during the black border etch. We will show that it is necessary to use sub-resolution gratings to minimize the reflectivity of the image border, and we will examine the mask manufacturing tolerances for these gratings.…”

Mask innovations on the eve of high NA EUV lithography

“…In case of low-n mask additional sub-resolution grating on the other mask is needed for background intensity reduction. 12) Additionally, the wafer SEM image and CD plot in Fig. 5 show the transition zone where the wafer CD gradually changes over a range of 50 nm at wafer.…”

“…Imec together with ASML has experimentally demonstrated the stitching feasibility at NA0.33. 11,12) The main interactions in the stitching area are specified in Fig. 5.…”

A study of high NA EUV pattern stitching using rigorous stochastic lithography simulation

“…However, the EUV mask blank size remains at 6"X6" therefore the maximum exposure area on wafer will be reduced by 50%. Complex devices with a die size exceeding this new area limit will need to be formed by "stitching" two separate exposures together [1]. A small overlap region will be required.…”