Anamorphic EUV

Anamorphic high-NA EUV refers to the use of different magnification ratios in the X and Y directions in high-NA EUV lithography optics — specifically 4× reduction in the scanning direction and 8× reduction in the cross-scan direction. This is a fundamental departure from conventional lithography, which uses the same magnification in both directions.

Why Anamorphic?

• Increasing the NA from 0.33 to 0.55 requires collecting light over much wider angles. If the same 4× magnification were maintained in both directions (as in current EUV), the reticle (mask) would need to be impractically large.
• By using 8× reduction in one direction, the mask field size is halved in that dimension, keeping the mask at the standard 6-inch (152 mm) form factor.
• This avoids the enormous cost and complexity of developing new, larger mask infrastructure.

Impact on Mask Design

• Current EUV: 4× magnification in both X and Y. Mask features are 4× larger than wafer features.
• High-NA EUV: 4× in scan direction, 8× in cross-scan direction. Mask features are 4× larger in one direction but 8× larger in the other.
• The mask pattern is therefore stretched in one direction — mask data preparation and OPC (optical proximity correction) must account for this asymmetry.

Consequences

• Halved Field Size: The printable field per exposure is halved in the cross-scan direction (from ~26×33 mm to ~26×16.5 mm). This means more exposures per die for large chips, potentially impacting throughput.
• Stitching: Large dies may need to be split across two or more exposure fields, requiring precise field stitching at the boundaries.
• Mask Making: Mask writing and inspection tools must handle the anamorphic aspect ratio — different resolution requirements in X and Y.
• OPC Asymmetry: Optical proximity effects differ in the two directions due to different magnifications, complicating computational lithography.

Field Size Solutions

• Die Size Management: Many advanced chips already fit within the reduced field.
• Stitching Technology: ASML has developed techniques for stitching adjacent fields with minimal impact on yield.
• Design Co-optimization: Chip architects may adjust floorplans to fit within the smaller field or optimize stitch boundaries.

Anamorphic optics represent a pragmatic engineering compromise in high-NA EUV — trading field size for the ability to use existing mask infrastructure while achieving the resolution improvements needed for sub-2nm nodes.