Toward practical application of sub-resolution grating for improved process window control in high and hyper NA EUV lithography

Abstract

Background A low-n attenuated phase shift mask (low-n mask) has been widely used in the extreme ultraviolet (EUV) lithography industry as it aligns the diffraction phases between the zeroth and first orders and enhances exposure latitude, as well as boosting throughput. However, the aligned phase is achieved only for a specific pitch, usually the densest pitch. A pitch-dependent phase offset inevitably remains in a mask layout with various pitches. This leads to the challenge of best focus (BF) variation, which is even more pronounced in the case of a low-n mask with a large refractive index difference from vacuum. The BF variation through pitch is more critical at higher numerical aperture (NA), where the available depth of focus is significantly lower than at low NA. In addition, side-lobe printing caused by high reflectivity from the low-n mask is also a concern.

Aim We aim to enhance imaging performance and demonstrate CD-based overlapping process window (oPW) while minimizing BF variation and unwanted side-lobe printing. Then, we aim to provide a potential mask absorber roadmap in hyper NA EUVL from an imaging perspective.

Approach We proposed a sub-resolution grating (SRG) crossing the main features perpendicularly, which not only suppresses unwanted side-lobe printing but also improves the imaging quality with a better BF alignment over a broader pitch range by effectively aligning diffraction phases. Our approach involves a step-by-step validation of unit cell patterns to assess the SRG impact on oPW, beginning with various L/S pitches, followed by 2-bar and 3-bar for both horizontal and vertical orientations. Ultimately, we verify oPW using real logic use cases applicable to high- and hyper NA EUV lithography.

Results Rigorous simulation demonstrates improved oPW for both vertical and horizontal logic metal layers. As for the vertical L/S patterns as the SRGs can cover the entire mask pattern area, all types of low-n masks can be utilized. As for the horizontal L/S patterns, covering the entire mask pattern area with the same orientation of horizontal SRGs may be challenging from the perspective of mask manufacturability. To overcome this constraint, alternative absorber structures could be considered. A higher k absorber gives a better imaging performance with less pronounced BF variation. By combining the advantages of these two, a thin high-k absorber structure with selective SRG insertion demonstrated an oPW higher than our set criteria and showed more than 25% improvement in productivity compared with a Ni-based high-k absorber.

Conclusions The SRG insertion technique, along with proper wavefront optimization, is expected to offer tangible benefits of imaging and enhanced productivity for logic metal layers in high NA and hyper NA.