Plenary Talks

Harry J. Levinson is currently an independent lithography consultant and Principal Lithographer at HJL Lithography. He is the author of three books: Lithography Process Control, and Principles of Lithography, and Extreme Ultraviolet Lithography. In 2022 he received the SPIE Frits Zernike Award in Microlithography. Levinson is currently Editor-in-Chief of the Journal of Micro/Nanopatterning, Materials and Metrology (JM3).

For decades, resists for lithography consisted of polymers with an admixture of photo-sensitive molecules. These substances were typically soluble in organic solvents, enabling their application to the surfaces of silicon wafers by means of spin coating. After many years of scaling, the sizes of the molecules in resists became a significant fraction of the sizes of the features to be patterned, resulting in difficulties in controlling pattern roughness and defects. As a consequence, entirely new approaches involving smaller molecular building blocks are being adopted for formulating resists, particularly for extreme ultraviolet (EUV) lithography.

The pursuit of resists with small molecular building blocks has also led to resists that are deposited by chemical vapor deposition rather than spin coating, another significant departure from the past. With vacuum deposition, it is possible to modulate the resist’s optical absorption from the top to the bottom of the resist film. This can help to address issues associated with high numerical optics. Alternatives to resists consisting entirely of small molecules are also being considered, where the resist molecules are vertically oriented and therefore still having small lateral extent. Such resist are also conveniently deposited by means of vacuum deposition rather than spin coating.

For EUV lithography, as the sizes of resist molecules have become very small, it also has become necessary to increase the density with which photons are absorbed. In turn, this has necessitated the incorporation of chemical elements into resist materials that are strongly absorbing at EUV wavelengths but may not have been used commonly in resists previously. More photons overall are needed, compelling the development of more powerful EUV light sources.

The sizes of molecules in resists may ultimately limit the minimum dimensions of features in integrated circuits. Because the radiation chemistry of EUV resists is driven by secondary electrons that are produced when energetic EUV photons are absorbed, the distances over which these electrons travel before thermalization may also limit minimum feature sizes produced by EUV lithography.

It should be noted that resist materials for patterning large features are also still evolving. There is considerable R&D activity for finding PFAS-free replacements for currently-used lithographic materials. The patterning of larger features is also expanding to new applications, such as advanced packaging, driving the additional development of new lithographic materials.