Low Cost Panel-Based 1-2 Micron RDL Technologies with Lower Resistance than Si BEOL for Large Packages

This paper presents the latest lithographic advances to enable scaling of packaging redistribution layer (RDL) feature size towards 1-2 µm line and space on panel-based glass substrates. Critical dimension (CD) of 1 µm line and space with aspect ratio (AR) of 5 was demonstrated in a newly developed chemically amplified plating photoresist by using i-line contact mask aligners and low numerical aperture (NA) 1x projection steppers. The minimum line and space demonstrated was 0.9 µm with AR of 5.5. Such high AR package RDL has much lower trace resistance compared to silicon back-end-of-line (BEOL) RDL, enabling power-efficient higher bandwidth. The relationship between resolution (R) and depth-of-focus (DOF) for low NA projection stepper is discussed. Low cost, low NA, large field steppers are better suited to large panel and large size package fabrication. A cost-effective mechanical fly-cut process was implemented for planarization of the entire wafer or panel with < 1 µm flatness that ensures the success of 1 µm with AR of 5 lithography by using low cost lithographic tools.