Small Angle Neutron Scattering Measurements of Photoacid Diffusion-Reaction Paths in a Chemically Amplified Photoresist

Tengjiao Hu, Ronald L. Jones, Wen-li Wu, Christopher L. Soles, Eric K. Lin

National Institute of Standards and Technology, Gaithersburg, Maryland 20899

Dario L. Goldfarb, Marie Angelopoulos

IBM T.J.Watson Research Center, Yorktown Heights, New York 10598

After photogeneration, acid molecules catalyze multiple reactions and provide the mechanism for chemical amplification, within the photoresist matrix. A central issue in photoresist design is the number of times each acid reacts, and of equal importance, the size and shape of the deprotected region created upon reaction. The number of reaction events provided by a single acid has been estimated previously using NMR, where the number is on the order of 100. NMR (nuclear magnetic resonance), however, cannot provide a spatial map describing the volume of deprotection created by that acid. The size and shape of the volume are central to recent simulations describing line edge roughness formation.

 We present results from small angle neutron scattering (SANS) measurements of the deprotection path in a model photoresist formulation. This is the first experimental data of the deprotection volume due to the reaction-diffusion process from well-dispersed acid molecules; a situation analogous to low contrast regions at an exposed line-edge.  Results are presented from studies of a model photoresist of poly(butoxycarboxy styrene) (PBOCSt) with a photoacid generator of perfluorooctylsulfonate (PFOS). To enhance SANS intensity, the t-BOC protection groups are deuterated to provide a change of deuterium content as deprotection occurs. The photo-acid generator (PAG) loading is < 1 % mass fraction of PFOS in order to disperse acid molecules and the post exposure bake is tuned to provide nearly 30 % deprotection. The data are consistent with a deprotection volume characterized by a random walk. As with all random walks, the average deprotection volume features a diffuse boundary. Using the results of these measurements, along with analogous experiments using infrared spectroscopy, we are able to extract the average deprotection volume size and estimate a diffusion coefficient.  The value of diffusion coefficient falls within the range estimated by other researchers for similar PAG’s. The implications of acid deprotection path shape are discussed in terms of current theories of LER formation.