EUVL Masks may need to be Tool-Specific
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).