Maskless Lithography for High-Density Package Redistribution Layers

Heterogeneous integration of chiplets requires high-density fine line/space (L/S) interconnections to enable system scaling. However, as Moore's law for transistors slows down, the dies are disaggregated based on their specific functions thereby increasing the package sizes to accommodate various dies of different functionalities. To enable a high-density redistribution layer (RDL) in the package, this paper studies the fine-line patterning capability of maskless aligner systems. The paper evaluates the smallest achievable feature size on four different thicknesses namely 5
$\mu \mathrm{m}, 7 \mu \mathrm{m}, 25 \mu \mathrm{m}$
and 80
$\mu \mathrm{m}$
of dry film photoresists (PRs). This work also investigates the effect of dose and defocus on photoresists. Two of the photoresists (5 μm and 80
$\mu \mathrm{m})$
are positive tone and the other 2 are negative tone. The finest L/S obtained was 1.5μm with the positive tone 5
$\mu \mathrm{m}$
dry film photoresist, 3
$\mu \mathrm{m}$
using the 7
$\mu \mathrm{m}$
negative tone and 20 μm using the 80 μm positive tone resist. This paper also investigates the throughput of the maskless aligners based on the dose, exposure area, and pattern density.