College Park, Maryland      June 6 - 10 , 2004

M2-B5 (11:45 AM): Photoacid reaction-diffusion path in a chemically amplified photoresist

Tengjiao Hu, Ronald Jones, Eric Lin, Wen-li Wu, Dario Goldfarb, Marie Angelopoulos (NIST Polymer Division)

Photo-generated acid molecules act as a catalyst in the deprotection of chemically amplified photoresists. The initial locations of the acid molecules provide reaction centers and the subsequent movement of these small acid molecules during a heating step defines the physical reaction paths. The spatial evolution of the reaction paths is closely related to the overall shape of the latent image. Acid reaction-diffusion paths near the imaging edges play an important role in the line-edge roughness control, which is approaching to only a couple of nanometers in patterns created from photoresist. A few preliminary works have focused on measuring the acid diffusion coefficient in photoresists. The movement of the interface between acid feeding layer and photoresist layer has been used to indirectly infer the acid diffusion because the results are coupled with a further development process. Ion conductivity measurement overcomes this complication but still assumes a Fickian model for acid diffusion. However, there is no such a proof yet. Identifying the relationship between the evolution of microscopic reaction-diffusion paths to a macroscopic latent image is critical information for industrial applications.

A model photoresist polymer with only side groups deuterated was used in our experiment. Deprotection cleaves the deuterated parts and causes a large contrast change in neutron scattering, which can be followed in a standard small angle neutron scattering experiment. At extremely low acid concentration, the scattering provides the form factor of the diffusion path of a single acid molecule in the photoresist. Our results indicate that the acid molecules diffuse randomly in the photoresist. The overlapping and merging of several deprotection paths was studied by varying reaction time at a constant acid concentration. Our result shows a gradual merging of different deprotection paths as determined from the effective fractal dimensions of the observed deprotection volume. The fractal growth of the deprotection volume is closely related to the distribution of the deprotected monomer units and the subsequent final pattern formation after development.

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