In an email to me David Lam responded in detail as follows:
"The real problem is not the doses per se, but the "shot noise". Shot noise is a stochastic, or statistical, process involving some electrons "spilling over" to the edge of the patterned feature making the edge jagged, leading to "line-edge roughness", or LER. The smaller the feature the more serious the problem, and the problems plagues all lithographic technologies: 193nm Optical, EUV and E-Beam. It's a random process due to probability and there is no real cure. However, the problem can be mitigated. Mitigating processes are practiced at fabs today to reduce the effect of shot noise and improve LER when needed. "
Pretty impressive that the man is still so passionate about technology at age 70. I never heard that before about Lam being the first company founded by an Asian American to go public. That's pretty cool.
ASML rcently indicated it is seeking an additional 1.7B in funding to prepare for EUV and 450mm HVM in 2018. This delay is an opportunity for eBeam lithography to provide complementary 1D line cut support of multi-patterned 193i lithography required to sustain HVM >10nm. Enhanced eBeam performance drawing from Model Based Mask Data Prep (MB-MDP) can be implemented so that discrete structures and patterns can be addressed individually with specific shot tasking providing superior control of dose margin. Called Shot or Dose Modulation, this is a new technique of assigning optimized electron beam energy and current to specific devices or geometries. This can enhance eBeam dose control accuracy over a chip (and wafer) potentially improving LER and minimizing shot noise typically encountered with contact holes and complex geometries. Refinement of this technique might be the best approach to minimizing shot noise.
See my recent blog interview with Dr. David K. Lam
I agree the opportunity for a non-EUV lithography is opening up wider. EUV like immersion has issues with increasing NA. E-beam seems to be the dominant alternative. At the same time, there is a common sentiment that putting too many electrons through a small area (especially directly into an insulated device) cannot have good clean consequences. But that is exactly what is needed for overcoming shot noise, more electrons per sq. nm. But this need might not be necessary for ArF immersion masks which cannot shrink 4x feature sizes below ~160 nm anyway. So I think this plan still works out well for mask writing.
With EVU still a distant reality, other technologies are drawing industry attention. EUV has been delayed so many times that its inclusion at 10nm is still not sure. Post 10nm, the problems for which EUV was envisioned will remain the same. We will need to look for reducing the wavelength to increase NA. I wonder why industry is still betting for EVU and not putting some money and faith to other technologies.