Extreme Ultra-Violet Lithography (EUVL) keeps hurting my brain. Just when I can understand how it could be used in profitable commercial high-volume manufacturing (HVM) I hear something that seriously strains my brain. First it was the mirrors and mask in vacuum, then it was the resist and pellicle, then it was the source power and availability, and in each case scientists and engineers did amazing work and showed a way to HVM. Now we hear that EUVL might require fabs to park work-in-progress (WIP) lots of wafers behind a single critical tool with an idealistic 80% availability on a good day, and lots of downtime bad days. Horrors!
For “5nm-node” designs the maximum allowable edge placement-error (EPE) in patterning overlay is only 2nm. While the physics of ~13.5nm wavelength EUVL means that aberration in the reflecting mirrors appears as up to 3nm variation in the fidelity of projected patterns. This variation can be measured and compensated for at the physical mask level, but then each mask would only be good for one specific exposure tool. John Sturtevant—SPIE Fellow, and director of RET product development in the Design to Silicon Division at Mentor Graphics—briefly discussed this on February 26th during Nikon LithoVision held just before SPIE Advanced Lithography.
Sturtevant explained that the Zernike coefficients for EUV are inherently almost 1 order-of-magnitude higher than for DUV at 193nm wavelength, as detailed in the SemiMD article “Edge Placement Error Control in Multi-Patterning.” How the inherent physical sources of aberration must be tightened to avoid image distortion and contrast loss as they scale with wavelength was discussed by by Fenger et al. in 2013 in the article “Extreme ultraviolet lithography resist-based aberration metrology” (doi:10.1117/1.JMM.12.4.043001).
Somewhere around 40nm is the limit on the smallest half-pitch feature that can be formed with a single-exposure of 193-nm wavelength laser light using water immersion (193i) lithography. While multiple-patterning (MP) is needed to achieve tighter half-pitches, smaller features at the same pitch can be formed using technology extensions of 193i. “Chemistry is key player in lithography process,” is the title of a short video presentation by Dow Electronic Materials corporate fellow Peter Trefonas now hosted on the SPIE website (DOI: 10.1117/2.201608.02).
Trefonas as been working on chemistries for lithography for decades, including photoresists, antireflectant coatings, underlayers, developers, ancillary products, and environmentally safer green products. He is an inventor on 61 US patents, has over 25 additional published active U.S. patent applications, is an author of 99 journal and technical publications, and is a recipient of the 2014 ACS Heroes of Chemistry Award and the 2013 SPIE C. Grant Willson Best Paper Award in Patterning Materials and Processes. Now a Senior Member of SPIE, he earned his Ph.D. in inorganic chemistry with Prof. Robert West at the University of Wisconsin-Madison in 1985.
Trefonas explains how traditional Chemically-Amplified (CA) resists are engineered with Photo-Acid Generators (PAG) to balance the properties for advanced lithography. However, in recent years the ~40-nm half-pitch resolution limit has been extended with chemistries to shrink contact holes, smooth line-width roughness, and to do frequency-multiplication using Directed Self-Assembly (DSA). All of these resolution extension technologies rely upon chemistry to create the final desired pattern fidelity.