Design Rule of Thick-Resist Lithography for Sidewall Perpendicularity and Critical Dimension Fidelity

Redistribution Layers (RDLs) are pivotal for fanout packaging, enabling the high-density and multi-function integration. The fabrication of RDL heavily relies on the lithography process of photosensitive dielectrics (polyimide) with the thickness of
$1-30\ \mu \mathrm{m}$
. However, it poses much challenge, particularly regarding line profiles and multilayer. There is an urgent need to investigate the effect of thick-photoresist lithography process on the resist sidewall profile and set the design rule of the process, which contributes to accelerate the process exploration of multilayer RDL. In this study, the optimization of thick-resist lithography process to get better sidewall perpendicularity and critical dimension fidelity has been explored through experiments and simulation for the negative photosensitive polyimide on the silicon substrate. Design rule for PI photolithography is then concluded. The process integration of
$315\ \text{mJ}/\text{cm}^{2}$
exposure dose and 30 s development time can guarantee sidewall closed to 90°. The deviation of top and bottom linewidth from design are both within 3% at the thickness of
$8\ \mu \mathrm{m}$
, for the widths from 8 to
$25 \mu \mathrm{m}$
. The trend of experimental and simulation results is the same, and deviation between experiments and simulation of critical dimensions are less than 10%. It proves that process simulation can be employed to establish additional design rules in future studies.