How Photomasks and EUV Pellicles Carry and Protect Chip Patterns in Lithography

How does a photomask carry and project circuit patterns during lithography?

A photomask carries the pattern blueprint that lithography light passes through on its way to the waferCITE:E1. ASML describes the process as light projected through a blueprint of the pattern that will be printed, known as a "mask" or "reticle"CITE:E1. That blueprint is not printed at final size: the pattern on the mask is four times larger than the intended pattern on the chipCITE:E2. Once the pattern is encoded in the light, the lithography system's optics shrink and focus it onto a photosensitive silicon waferCITE:E3. In this sequence, the mask functions as the master copy, the 4× scaling sets the geometric relationship between mask and chip, and the optics perform the final reduction and focusing step onto the waferCITE:E1CITE:E2CITE:E3.

How do EUV photomasks differ optically from conventional masks?

EUV masks reflect light rather than transmitting it, a structural departure from the transmissive mask process described aboveCITE:E7. ASML states that Extreme Ultraviolet Lithography (EUV), a technology entirely unique to ASML, uses light with a wavelength of 13.5 nanometersCITE:E4. Intel's technical documentation specifies that EUV masks must reflect rather than transmit lightCITE:E7. Because the mask-to-wafer projection described in the prior section relies on light passing through the mask, the reflective requirement for EUV marks a distinct optical architecture tied specifically to the 13.5 nm wavelengthCITE:E4CITE:E7.

How does an EUV pellicle prevent contamination and improve yield?

A pellicle is a thin membrane that shields the photomask from contamination during high-volume manufacturingCITE:E5. imec defines the pellicle as the membrane used to protect the mask from contamination during high-volume semiconductor manufacturingCITE:E5. ASML, in a September 14, 2022 article, states that pellicles are used in chipmaking as a way of reducing pattern defectivityCITE:E6. Applied to the reflective EUV mask described above, the pellicle's contamination barrier functions as the mechanism that keeps the mask's reflected pattern free of defect-causing particlesCITE:E5CITE:E6.

What this means

Across the three stages described here, the photomask carries a 4×-scaled blueprint that lithography optics reduce onto the waferCITE:E2CITE:E3; EUV systems replace that transmissive path with a reflective mask operating at 13.5 nmCITE:E4CITE:E7; and the pellicle membrane protects that reflective mask from contamination to reduce pattern defectivityCITE:E5CITE:E6. Each stage depends on the one before it: the reflective requirement exists because of the EUV wavelength, and the pellicle's defectivity-reduction role exists because the reflective mask must remain contamination-free for the 4× pattern transfer to hold.