Lithography Materials - The origins of image blur in chemically amplified resist processing
The following picture of image blur emerges from our studies of TBOC photoresist, a model system that typifies the imaging chemistry of current CA resists.
In practical application, photoresist exposure is carried out using an optical projection tool that, at its resolution limit, produces an aerial image that is diffuse in comparison to the photomask template (see figure below), a consequence of diffraction and scattering within the tool optics. When the resist film is exposed, acid is formed as a replica of this aerial image, so even prior to post-expose processing there is a significant amount of acid present in nominally unexposed regions. That acid will catalyze deprotection, though at a slower rate than in the center of the exposed region. The net effect of this is that the deprotected image will broaden with increasing post-expose heating time, even in the absence of significant acid diffusion. At large spatial scales (in the TBOC photoresist, at linewidths greater than 50 nm), this is the dominant source of image blur. Independently, gradient-driven (Fickian) diffusion can also play a role. Its contribution to image blur depends on the mobility of the acid and on the spatial scale of the image. Diffusion is the dominant factor at small spatial scales (in the TBOC photoresist, at linewidths less than 50 nm).