HafSOx and friends.
The HafSOx (Hf(SO4)y(O)z(OH)4-2y-2z·qH2O) exposure mechanism is by now well established.[1,2] Electrons released by the photo-ionization cascade act to destabilize peroxide linkages within the HafSOx clusters, releasing oxygen and rendering the residue insoluble during development. Unexposed clusters retaining their peroxide are soluble and wash away.
Metal (Hf or Zr) oxide nanoclusters stabilized by a thin layer of organic acid originated in the Ober group at Cornell some years ago.[3,4] This group has extended this approach to zinc oxide and demonstrated patterning down to 14 nm lines.
In the HfO2/methacrylate system, the Chabal group at UT Dallas propose that photo-electron exposure causes the acrylate to evolve CO2 and a propenyl radical which initiates cross-linking to a neighboring ligand. Additionally, the residual hafnium-centered cation can also cross-link.
Chemically amplified resist (CAR).
Studying a model CAR system, Pollentier et al. at imec show that EUV exposure leads not only to the desired acid generation, but also to chain scission and, to lesser extent, radical driven cross-linking. These side reactions have been known for a long time in e-beam lithography.
There is good evidence that acid generation is initiated by an electron trapping mechanism.
CARs are probably limited by acid diffusion to resolution on the order of 10 nm. Enomoto and Kozawa propose instead a negative resist with both triarylsulfonium cations and 2,2,2-trisubstituted acetophenone pendant groups, resulting in dual-action cross-linking.
Attending instead to sensitivity, the addition of alkaline earth salts to CAR improves both sensitivity and LER.
It is important to ensure efficient absorption of the EUV photons, these being ever in short supply. Fallica et al. measure film absorption for a number of tin, zirconium, and hafnium oxy-hydroxy cages with assorted ligands and/or counterions. This paper contains a handy reference chart depicting atomic absorption cross-sections of the elements from Z=1 to Z=86 at λ = 13.5 nm.
Of course, it is not enough for the photons to be absorbed. The photo-electrons must be chemically useful. Measuring electron emission of EUV-irradiated 4-halo-2-methylphenols, one finds that only the 4-iodo material shows appreciable Auger intensity, and this reaches down into the chemically interesting energy region below 10 eV.
For highest resolution and lowest LER, one wants a resist containing small molecules. One further wants to avoid crystallization, which leads unavoidably to severe LER. Therefore, the material should form an amorphous film from spin-casting. One also wants good photon absorption (as above) and subsequent chemical reactions leading to developable species. The Ober group at Cornell show that the 3-methyl-phenyl-modified Zn2(CO2)4 metal building unit is one such material, and demonstrate resolution down to 15 nm with good LER. This appears to me to be a very promising line of research.
 R. T. Frederick, J. M. Amador, S. Goberna-Ferrón, M. Nyman, D. A. Keszler, and G. S. Herman, Mechanistic study of HafSOx extreme ultraviolet inorganic resists, The Journal of Physical Chemistry C 122 (2018), no. 28, 16100–16112, available at https://doi.org/10.1021/acs.jpcc.8b03771.
 R. P. Oleksak, R. E. Ruther, F. Luo, J. M. Amador, S. R. Decker, M. N. Jackson, J. R.Motley, J. K. Stowers, D. W. Johnson, E. L. Garfunkel, D. A. Keszler, and G. S. Herman, Evaluation of thermal and radiation induced chemistries of metal oxo–hydroxo clusters for next-generation nanoscale inorganic resists, ACS Applied Nano Materials 1 (2018), no. 9, 4548–4556, available at https://doi.org/10.1021/acsanm.8b00865.
 M. Trikeriotis, W. J. Bae, E. Schwartz, M. Krysak, N. Lafferty, P. Xie, B. Smith, P. A. Zimmerman, C. K. Ober, and E. P. Giannelis, Development of an inorganic photoresist for DUV, EUV, and electron beam imaging, Advances in Resist Materials and Processing Technology XXVII (2010Mar), 76390E, available at https://dx.doi.org/10.1117/12.846672.
 M. Krysak, M. Trikeriotis, E. Schwartz, N. Lafferty, P. Xie, B. Smith, P. Zimmerman, W. Montgomery, E. Giannelis, and C. K. Ober, Development of an inorganic nanoparticle photoresist for EUV, e-beam, and 193nm lithography, Advances in Resist Materials and Processing Technology XXVIII (2011Mar), 79721C, available at https://dx.doi.org/10.1117/12.879385.
 K. Sakai, H. Xu, V. Kosma, E. P. Giannelis, and C. K. Ober, Progress in metal organic cluster EUV photoresists, Journal of Vacuum Science & Technology B 36 (2018Nov), no. 6, 06J504, available at https://doi.org/10.1116/1.5050942.
 E. C. Mattson, Y. Cabrera, S. M. Rupich, Y. Wang, K. A. Oyekan, T. J. Mustard, M. D. Halls, H. A. Bechtel, M. C. Martin, and Y. J. Chabal, Chemical modification mechanisms in hybrid hafnium oxo-methacrylate nanocluster photoresists for extreme ultraviolet patterning, Chemistry of Materials 30 (2018), no. 17, 6192–6206, available at https://doi.org/10.1021/acs.chemmater.8b03149.
 I. Pollentier, Y. Vesters, A. Rathore, P. Vanelderen, J. Petersen, D. De Simone, and G. Vandenberghe, Unraveling the role of photons and electrons upon their chemical interaction with photoresist during EUV exposure, Advances in Patterning Materials and Processes XXXV (2018Mar), available at https://doi.org/10.1117/12.2299593.
 S. Grzeskowiak, J. Kaminsky, S. Gibbons, A. Narasimhan, R. L. Brainard, and G. Denbeaux, Electron trapping: a mechanism for acid production in extreme ultraviolet photoresists, Journal of Micro/Nanolithography, MEMS, and MOEMS 17 (2018Jul), no. 03, 1, available at https://doi.org/10.1117/1.JMM.17.3.033501.
 S. Enomoto and T. Kozawa, Study of electron-beam and extreme-ultraviolet resist utilizing polarity change and radical crosslinking, Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 36 (2018May), no. 3, 031601, available at https://doi.org/10.1116/1.5023061.
 Y. Vesters, J. Jiang, H. Yamamoto, D. De Simone, T. Kozawa, and S. De Gendt, Sensitizers in extreme ultraviolet chemically amplified resists: mechanism of sensitivity improvement, Journal of Micro/Nanolithography, MEMS, and MOEMS 17 (2018Dec), no. 04, 1, available at https://doi.org/10.1117/1.JMM.17.4.043506.
 R. Fallica, J. Haitjema, L. Wu, S. Castellanos, A. M. Brouwer, and Y. Ekinci, Absorption coefficient of metal-containing photoresists in the extreme ultraviolet, Journal of Micro/Nanolithography, MEMS, and MOEMS 17 (2018May), no. 02, 1, available at https://doi.org/10.1117/1.JMM.17.2.023505.
 O. Kostko, B. Xu, M. Ahmed, D. S. Slaughter, D. F. Ogletree, K. D. Closser, D. G. Prendergast, P. Naulleau, D. L. Olynick, P. D. Ashby, Y. Liu, W. D. Hinsberg, and G. M. Wallraff, Fundamental understanding of chemical processes in extreme ultraviolet resist materials, The Journal of Chemical Physics 149 (2018Oct), no. 15, 154305, available at https://dx.doi.org/10.1063/1.5046521.
 H. Xu, K. Sakai, K. Kasahara, V. Kosma, K. Yang, H. C. Herbol, J. Odent, P. Clancy, E. P. Giannelis, and C. K. Ober, Metal–organic framework-inspired metal-containing clusters for high-resolution patterning, Chemistry of Materials 30 (2018), no. 12, 4124–4133, available at https://doi.org/10.1021/acs.chemmater.8b01573.