stepper | Ed's Threads

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).

—E.K.

On October 14, 2015, ASML Holding N.V. (ASML) published its 2015 third-quarter results: Q3 net sales of €1.55 billion with gross margin of 45.4% (in line with guidance), and guided Q4 2015 net sales at approximately €1.4 billion and a gross margin of around 45%. Due to mismatched financial analyst expectations, Bloomberg reported that ASML’s stock price dropped ~7% in a single day of trading, despite the company also reporting upgrades to both the TWINSCAN NXT 193nm-immersion (193i) and the NXE Extreme Ultraviolet (EUV) tools. In particular, a new record of 1000 wafer exposures in a single day was set by one EUV tool.

The science of controlling the 13.54nm wavelength electromagnetic radiation that we like to call “Extreme Ultra-Violet” or “EUV” (instead of the colloquial scientific term “soft x-ray”) is inherently challenging. The engineering of EUV Lithography is not just challenging but bordering on inherently impossible: from exploding tin plasma source, to all-reflective lenses that absorb energy, to the trade-offs in mask pattern protection. The team at ASML working on the exposure tool—along with the different specialist organizations still working on improved sources, masks, and resists—deserve the industry’s unwavering admiration for the important work they do every day.

In a prepared statement, ASML President and Chief Executive Officer Peter Wennink said, “We have proven the capability both to expose 1,000 wafers per day and, in a manufacturing readiness test, to expose 15,000 wafers in four weeks. We have also achieved a four-week average availability of more than 70 percent at multiple customer sites. The first shipment of our fourth-generation EUV lithography system, the NXE 3350B, is in progress, with two more expected to ship in Q4.”

Still, progress along desired EUV roadmaps continues to be slow, and the competitive target shifts when the 193i exposure tool gains a 10% throughput improvement to 275 wafer-passes/hour (wph). When the 193i tool gains a 30% overlay improvement, that means double-patterning based on litho-etch-litho-etch (LELE) process flows gain in pattern fidelity. Since ASML provides both technologies, delays in orders for EUV just means more sales of 193i tools.

Let’s play with the numbers here…275 wph x 20 hours x 30 days = 165k wafer-passes/month for the NXT:1980. The NXE:3350B can current handle 15k wafer-passes/month. So even if the tools were equally priced, just based on tool depreciation each EUV exposure today costs >10x that of a 193i exposure, which is why pitch-splitting multi-patterning 193i continues to dominate.

—E.K.

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EUVL Masks may need to be Tool-Specific

EUV Cost at 1000 Daily Exposures