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Pellicles for EUV-Lithography

Photolithography is one of the main technologies employed in semiconductor manufacturing. In this process, light is used to transfer a pattern on a photomask onto a thin film of a photosensitive resist on the substrate. The non-exposed areas of the resist are typically removed, so that the underlying layer can be either etched or a new layer can be deposited. However, if the photomask is contaminated with particles, these particles are imaged onto the resist causing defects in the integrated circuit leading in the worst case to a complete loss of production. To prevent this, photomasks are protected from particles by a pellicle - a thin transparent film stretched over a frame mounted over the photomask.  The pellicle is far enough away from the mask pattern so that moderate-to-small sized particles that land on the pellicle will be too far out of focus to print on the wafer.

However, as the dimensions of features made using lithography become smaller, physical limitations have become a limiting factor. In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, extreme ultraviolet (EUV) radiation with shorter wavelength has begun to replace UV-light. At EUV-wavelengths, no material and especially conventional pellicles are transparent. The only workable solutions are to use no pellicle risking that even smallest particles reduce the production yield or to use a pellicle of minimal thickness trying to limit the transmission loss of the EUV-light.

Our experiments together with the Fraunhofer Institute for Laser Technology ILT, the Physikalisch-Technische Bundesanstalt PTB (synchrotron Bessy II) and the Paul-Scherrer-Institute (Swiss Light Source) have shown, that CNMs are a promising candidate for use as pellicles in EUV lithography:

  • As a Carbon-based material, CNMs have high transmission for EUV light
  • Free-standing CNMs structurally survived EUV irradiation with a cumulative dose density of approx. 300 kJ/cm2 at 3 Pa partial H2-pressure.

Based on these results we have proposed a roadmap to a full-sized pellicle based on free-standing Carbon Nanomembranes.