Printing verification of the improved performance of low-n mask with Sub-Resolution Grating (SRG) in High-NA EUVL

Abstract

The introduction of High NA EUV lithography has led to further improvements in resolution; however, the depth of focus (DoF) has decreased, as it is inversely proportional to the square of NA. While the DoF at a specific pitch can be partially improved through source optimization, the limitations become apparent when dealing with mask layouts containing a wide range of pitches due to mask 3D effect (M3D) induced best focus variation through pitch. In addition, this issue shows a strong dependence on the refractive index of the absorber structure and is most pronounced in a Low-n attenuated phase shifting mask (Low-n mask), as demonstrated by both experimental and simulation results. Low-n masks have recently been widely used in industry, as they align the diffraction phase in specific pitch regions, usually optimized only for dense pitches. As a result, the Low-n mask helps mitigate the fading effect by reducing pole to pole (P2P) offset, thus improving the image contrast at dense pitch regions. However, in the case of three-dimensional masks with a pitch-dependent phase offset, it remains challenging to simultaneously achieve well-aligned best focus and high contrast across all pitches. As a compensatory technique for this issue, we previously proposed sub-resolution grating (SRG) insertion in the pattern area, which effectively mitigates the background intensity particularly for a highly reflective Low-n mask. A reduction of background intensity makes any remaining phase error negligeable and resolves issues about image contrast degradation in isolated pitches as well as side-lobe printing issues. As a result, it aligns the best focus and improves the image contrast across a wide range of pitches. In this paper, we move beyond theoretical and simulation-based validation of the SRG by integrating it into mask patterns with vertical line-space (L/S) pattern pitches varying from 20 nm to 200 nm, and horizontal 2-bar configuration, followed by mask fabrication and wafer exposures with High-NA scanner. The wafer exposure results demonstrate that the SRG effectively mitigates several M3D related issues such as best focus variation through pitch, pitch-dependent contrast loss, Bossung tilt, which are consistent with simulation predictions. We also perform additional simulations based on contours extracted from our fabricated SRG mask layout, to further evaluate its advantages, discuss potential challenges, and explore possible solutions.