The importance of inner-shell electronic structure for enhancing the EUV absorption of photoresist materials

In order to increase computation power and efficiency, the semiconductor industry continually strives to reduce the size of features written using lithographic techniques. The planned switch to a shorter wavelength extreme ultraviolet (EUV) source presents a challenge for the associated photoresists, which in their current manifestation show much poorer photoabsorption cross sections for the same dose. Here we consider the critical role that an inner-shell electronic structure might play in enhancing photoabsorption cross sections, which one can control by the choice of substituent elements in the photoresist. In order to increase the EUV sensitivity of current photoresists, it is critical to consider the inner-shell atomic structure of the elements that compose the materials. We validate this hypothesis using a series of halogenated organic molecules, which all have similar valence structures, but differ in the character of their semi-core and deep valence levels. Using various implementations of time-dependent density functional theory, the absorption cross sections are computed for the model systems of CH₃X, X = H, OH, F, Cl, Br, I, as well as a representative polymer fragment: 2-methyl-phenol and its halogenated analogues. Iodine has a particularly high cross section in the EUV range, which is due to delayed absorption by its 4d electrons. The computational results are compared to standard database values and experimental data when available. Generally we find that the states that dominate the EUV oscillator strength are generated by excitations of deep valence or semi-core electrons, which are primarily atomic-like and relatively insensitive to the specific molecular structure.