This project, “Pitch Division Photolithography at I-line,” seeks to accomplish pitch multiplication by using a traditional 248 nm photoresist polymer in conjunction with a photo-acid generator (PAG) and a photo-base generator (PBG). This formulation can achieve a two-fold improvement in resolution without the need for new equipment or significant changes in processing conditions.
The photoresist matrix used in this work is poly[4-[(tert-butoxycarbonyl)oxy] styrene] (PTBOC), which is employed in combination with a PAG in 248 nm resists. When exposed to light, the PAG decomposes to form acid which, upon post-exposure baking, deprotects multiple pendant groups on the PTBOC to produce hydroxyl groups, thereby changing its solubility. This polymer exhibits another key feature: the dissolution rate with respect to dose has a threshold-like response, meaning that below a threshold dose, the polymer will not appreciably dissolve in a developer containing tetramethyl ammonium hydroxide (TMAH), but above this dose, the dissolution rate increases several orders of magnitude. This behavior becomes vital at feature sizes that approach theoretical resolution limits where the aerial image near the photoresist becomes more sinusoidal.
Because the dissolution rate is controlled by the acid content within the polymer matrix, it is possible to cross this dissolution threshold twice with increasing dose if the acid is somehow quenched at higher doses. A PBG is an easy way to achieve this goal. If a PBG is chosen such that it is decomposes more slowly than the PAG and is incorporated with a stochiometric excess, then this dissolution threshold may be crossed twice. The addition of a PBG generates three different regimes with respect to dose: At low doses, neither the PAG nor PBG will have appreciably decomposed and the resist remains insoluble in aqueous base. At medium doses, enough acid will be generated by the PAG to cross the threshold, with too little PBG decomposition to effectively quench said acid. At high doses, both the PAG and PBG have mostly decomposed and the net acid concentration will be below the dissolution threshold. If the relative rates of the PAG and PBG can be tuned such that these two dissolution thresholds properly match the sinusoid intensity profile, the resolution of patterns can be improved by a factor of two. Dr. Xinyu Gu previously demonstrated the feasibility of such a system for 193 nm tools .
In this work, we report several combinations of PAGs and PBGs that meet the above criteria and show promise for exhibiting pitch-division. In some cases, a photosensitizer was needed to enable the decomposition of the PAG. These combinations were tested by exposing a film to a given dose and then developing in an aqueous solution of TMAH. It was found that the relative dissolution rates closely match the ideals as described above. These combinations are ready for testing with an exposure tool to verify and optimize their function as a pitch division photoresist.
 Gu, X. et al. “Photobase generator enabled pitch division: a progress report,” Proc. SPIE 7972, 79720F (2011).