The proceedings from the first joint SPIE Photomask and EUVL conference (2017) are now posted. A few remarks are in order:

The ASML tool progresses, with throughput now around 100 wafers per hour. The resolution, line edge roughness, sensitivity (RLS) trade-off still has not been overcome. Several speakers observed that the EUV scanner is dose limited, and that LER suffers as a result. Additionally, EUV photomask TAT exceeds that for 193i. Samsung reported costs of 8X greater, while the Ebeam Initiative reported EUV mask yields around 64%, compared to 90% or greater for AttPSM (193i). Mask corrections specific for the EUV scanner lead to data volumes 3X greater than for 193i.

Much work is being done to develop more sensitive EUV resists. I think that much of this effort is misplaced; until the LER is greatly improved, printing faster simply produces more failing devices. Far more promising is the fundamental research into resist exposure mechanisms. Noteworthy advances were reported by IMEC and by LBNL.

LBNL reminded us that, even with 100% absorption, 30 mJ/cm² is about 20 EUV photons per nm² . Therefore, the expected 1σ noise is around 4.5 photons per nm² ! This is the root cause of the LER problem, and why the dose must greatly increase.

[For those unfamiliar with the challenges posed by EUVL, a look at Wikipedia will be enlightening.]

 

ASML and the University of Twente have posted a preprint showing significant increase in the IR emissivity of SiN membranes by addition of a 2-4 nm ruthenium film. The improved emissivity is enough to lower the temperature of EUV-exposed membranes from >1500K to ~500K in 10^-4 mbar helium. Thus, uncoated membranes glow white hot and collapse, while coated membranes emit no visible radiation and survive quite well.

Reference: P J van Zwol, et al., “Emissivity of freestanding membranes with thin metal coatings”, arXiv:1511.06111.

Pengcheng Li recently posted two preprints to the arXiv on the topic of proximity effect correction (PEC). In the first, he discusses the adjustment of sidewall angle simultaneously with dimension. In the second, he gives a brief but reasonably complete review of the PEC literature.

Adjustment of sidewall angle has a long and storied history of its own. Especially when using low contrast resist processes, it was common practice to “under-size and over-develop” as a global means of straightening up the wall angles. The method discussed here involves a (one-dimensional) distribution of applied doses, optimized by simulated annealing. Not surprisingly, the resulting dose profiles look much like those resulting from earlier calculations. Mr. Li goes on to compare simulated developed resist profiles with experiment. He finds good qualitative agreement.

References:

1. Pengcheng Li, “Optimization of spatial dose distribution for controlling sidewall shape in electron-beam lithography”, arXiv:1509.04694.
2. Pengcheng Li, “A review of proximity effect correction in electron-beam lithography”, arXiv:1509.05169.