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ASML announced that they are going into the pellicle business at 2:15pm on February 25 at the SPIE Advanced Lithography Symposium in San Jose. These are not your garden variety plastic membrane pellicles, mind you, but rather EUV pellicles made out of 50nm thick polysilicon film and stretched on frames that can be attached, removed and reattached on EUV masks. Dr. Carmen Zoldesi of ASML revealed at a special, almost unannounced paper in the last session of the EUVL symposium that ASML had succeeded in building practical full-field pellicles for EUV masks. She even brought one with her, which resulted in a crowd of hopeful lithographers lining up to take cell-phone pictures of something that seemed impossible one year before.

At the 2014 SPIE Advanced Lithography Symposium, Anthony Yen of TSMC stunned the community by announcing that pellicles for EUV masks would be an “operational necessity” to avoid repeating defects from particles that fall onto bare masks during exposure. Since the 13.5nm radiation is rapidly absorbed by solid materials, the pellicle would have to be very thin, large enough cover a full field mask and robust enough to survive. At the time, the best candidates were ~1cm2 sized films of polysilicon, which absorbed >15% of the incident radiation. Still, if fall-on defects did cause defects, impacting chip yield, EUVL adoption would be delayed until they stopped.

Enter Zoldesi and her team at ASML. In the following year, they learned how to make large uniform – and very thin – free-standing films of polysilicon and to coat them with even thinner layers of material that improves heat dissipation, attach them to frames that in turn could be attached and removed from EUV masks without damaging anything. Having the pellicle removable and remountable is important because EUVL reticle manufacturers and users would want to inspect the mask using electron-beam, high-NA DUV or actinic tools, according to Dan Smith of ASML.

The prototype structure presented on February 25 seems to meet the requirements. It covers a full mask field, transmits 85% of the EUV (90% in the future) does not distort the image and survives EUV scanner operation — including thermal cycling and reticle exchanges from atmospheric pressure to vacuum and back. Smith stated that ASML means to make the pellicles available to EUVL customers as well as machines for attaching mounting posts to EUVL masks and other machines to mount and de-mount the pellicles on those mask plates. If ASML is successful in commercializing EUV pellicles it will have removed an important barrier to EUVL adoption. Other challenges await.