ACID-DIFFUSION EFFECT ON NANOFABRICATION IN CHEMICAL AMPLIFICATION RESIST

Acid-diffusion effect on nanometer pattern fabrication in a chemical amplification resist, SAL601 (Shipley Co.), is investigated with a finely focused electron beam. During postexposure bake (PEB), acid generated by the electron beam exposure diffuses and is assumed to cause pattern size changes. A scanning electron microscope, S-900 (Hitachi), which has a beam diameter of approximately 2 nm at 5 kV, is used to make latent images of nanometer isolated lines in a resist film. After electron beam exposure, the resist films with a thickness of 20 nm are baked in different conditions before development. The measured linewidths are found to be proportional to the square root of the PEB time. According to a simple acid diffusion model, this can be explained by the diffusion of generated acid during PEB. A minimum feature size of a 20 nm isolated line is obtained by adjusting the PEB conditions. It is therefore important to control the PEB conditions to suppress the acid diffusion for the critical dimension control of nanofabrication when using chemical amplification resist systems. Fine edge roughness is also observed in the delineated patterns. This is thought to originate from the distribution of diffused acid or from the random distribution of the base polymer with finite molecular sizes.