Mechanical deflection of a free-standing pellicle for extreme ultraviolet lithography

• Free-standing EUV pellicle is desirable to avoid non-uniform intensity.
• Nonlinear deflection model is used to predict deflection of EUV pellicle.
• Deflection of pellicle satisfies the desired specification at full scale.
• Trade-off between mechanical and optical properties is highlighted.

In extreme ultraviolet lithography (EUVL), a pellicle is a thin (a few nanometers in scale) protective membrane that can prevent the mask from suffering from defects. However, this thin film can be easily deformed by gravity and other forces. Although the hexagonal-shaped mesh support structure can decrease the stress caused by external pressure, its structural shape can degrade the image quality on the wafer. Therefore, studying the deflection of a free-standing EUV pellicle is needed. We revisited the plate theory and found that a nonlinear deflection term should be added to the deflection equation. The deflection of a 50 nm thick polysilicon pellicle is about 100 μm for a full-scale (100 mm × 100 mm) pellicle. Previously, researchers have tried to include graphene in multi-layer EUV pellicles in order to enhance the mechanical properties of the film. We found that the addition of graphene did not cause any serious deflection problems. This study shows that a free-standing EUV pellicle without mesh support can be used without any noticeable deflection effect on the pattern fidelity.

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