Mask 3D effects refer to how the physical thickness and topography of mask absorber and phase-shift materials affect the diffraction of light passing through (or reflecting from) the mask, causing deviations from the idealized thin-mask (Kirchhoff) model used in traditional lithography simulation.

Why Mask 3D Effects Matter

• Traditional lithography simulation treats the mask as an infinitely thin plane — light either passes through or is blocked, with no interaction with the mask material's finite thickness.
• In reality, mask absorbers and phase-shift layers have thickness of 50–100 nm (for DUV) or 30–70 nm (for EUV). At feature sizes comparable to the absorber thickness, the 3D structure significantly affects how light diffracts.

Effects of Mask Topography

• Shadowing: Light enters the mask absorber at oblique angles (especially for off-axis illumination and high-NA systems). The absorber sidewalls cast shadows, effectively shifting the apparent feature position.
• Best Focus Shift: The 3D mask structure changes the phase and amplitude of diffracted orders, shifting the best-focus position through-pitch — dense and isolated features focus at different heights.
• Pattern Shift: Features appear to shift laterally depending on illumination angle and absorber profile.
• CD Asymmetry: Left and right feature edges can print at different widths due to asymmetric shadowing effects.
• Pitch-Dependent CD: The mask 3D contribution to CD error varies with feature pitch, complicating process control.

Mask 3D Effects in EUV

• EUV lithography uses reflective masks at an incident angle of 6° off normal. The absorber thickness (~60–70 nm) interacts with the oblique illumination to create significant 3D effects.
• Shadowing in EUV is inherently asymmetric — the absorber shadow falls differently on the left and right sides of features due to the tilted illumination.
• This is a major challenge for EUV patterning, especially at high-NA where the angular range increases further.

Mitigation

• Rigorous EMF Simulation: Use electromagnetic field (Maxwell's equations) simulation of the mask instead of thin-mask approximations. More accurate but computationally expensive.
• Thinner Absorbers: Reducing absorber thickness reduces 3D effects. New materials (high-k absorbers with higher extinction coefficients) achieve the same optical density with thinner films.
• Compensating OPC: Include mask 3D effects in the OPC model to pre-compensate for the distortions.

Mask 3D effects are a dominant source of patterning error in EUV lithography — accurately modeling and compensating for them is essential for achieving the tight CD control required at advanced nodes.