EUV lithography is actually Secondary Electron Lithography

[Fred Chen]

I came across an interesting paper from Berkeley presented at IMPACT (Integrated Modeling Process and Computation for Technology) in April this year, presented as a concise poster:

http://cden.ucsd.edu/workshops/Subm..._Bhattarai_Neureuther_Naulleau_2016.04.15.pdf

Rather than directly causing any molecular excitations like DUV radiation, EUV photons are ionizing and release electrons which eventually lose energy and are responsible for the actual molecular interaction, such as PAG (photoacid generator) activation.

View attachment 18792

Hence, as they slow down, the electrons can travel some accumulated nanometers in distance.

View attachment 18793

In fact, even electrons with energy as low as 15 eV (which have around 2 nm mean free path) in energy can still do lithography by exposing resist:

View attachment 18794

From the stochasticity point of view, you have a nominal EUV photon dose that becomes a rather unpredictable (like Monte Carlo) actual electron dose on the local level, as an uncertain number of electrons are distributed in a given volume of resist on the nanometer scale. It could be a contributing factor for resist blur and line edge roughness.

Apparently, the NILS (Normalized Image Log Slope, a metric for image quality; <2 is considered unsatisfactory) is also affected significantly by the secondary electron effects (indicated by the NILS* curves below), as shown from the June 2016 SPIE newsroom article from IBM Research: Successes and frontiers in extreme UV patterning
| SPIE Homepage: SPIE
View attachment 18869

© 2016 SPIE​

Normalized image log-slope (NILS) lines for different illumination (illum) conditions in 0.33 numerical aperture EUV exposures. Adding secondary electron effects to the imaging (lines labeled NILS*) degrades the contrast and thus the minimum resolution.

S. Bhattarai's 2017 PhD thesis:
Page Not Found | EECS at UC Berkeley

The blur from secondary electrons also worsens with increasing dose due to depletion of trapping species: EUV’s Stochastic Valley of Death

 

[Tanj]

I wonder if the electrons could be directed down by adding a positive electrode function to the underlying wafer, sharpening their direction. And if it makes sense to have a resist which is asymmetric, like a surfactant with a conductive carbon chain core standing up, maybe crosslinks part of the top layer functionality. Just a few atoms thick for the sensitive part, but still capable of being spread uniformly. Having "bulk" resists may not be the only way to build and spread them. Or maybe a base layer non-resist which adheres to the substrate with a surface layer true resist bonded, inherently one molecule thick if it is not self-bonding. Upside down damascene.

 

[Fred Chen]

I wonder if the electrons could be directed down by adding a positive electrode function to the underlying wafer, sharpening their direction. And if it makes sense to have a resist which is asymmetric, like a surfactant with a conductive carbon chain core standing up, maybe crosslinks part of the top layer functionality. Just a few atoms thick for the sensitive part, but still capable of being spread uniformly. Having "bulk" resists may not be the only way to build and spread them. Or maybe a base layer non-resist which adheres to the substrate with a surface layer true resist bonded, inherently one molecule thick if it is not self-bonding. Upside down damascene.

It seems to be the right thinking (e.g., US20150042972A1 - Amplification method for photoresist exposure in semiconductor chip manufacturing
- Google Patents) but the sufficient electric field could be breakdown strength (e.g., Phys. Rev. Lett. 119, 266803 (2017) - Charge Catastrophe and Dielectric Breakdown During Exposure of Organic Thin Films to Low-Energy Electron Radiation).