Low Cost One Micron Photolithography Technologies for Large Body Size, Low Resistance Panel-Based RDL

This paper presents the latest advances in photolithography technologies to enable scaling of package redistribution layer (RDL) towards critical dimensions (CD) of 1 μm and below. High-bandwidth memory (HBM) channels require not only fine pitch but also low trace delay RDL. High aspect ratio (AR) traces enable lower delays and the photolithographic advances to achieve such traces are demonstrated on panel-based glass substrates for low cost, high I/O density, high bandwidth and large body size packages. CD of 0.9 μ m line and space with an AR of 5.5 was successfully demonstrated using a low numerical aperture (NA = 0.16) 1× i-line projection stepper tool with a novel chemically amplified plating photoresist. The 1 μm lithography technology is a must for high-density RDL to enable 2.5D interposer and embedded fan-out package substrates. These architectures can achieve I/O densities of 500 IOs/mm/layer. The resistance of the trace with an AR of 5 is 5X lower than a trace with an AR of 1. This technological advance will greatly reduce the signal propagation loss and increase the data rate of the RDL traces. The combination of high density and high data rate will greatly increase the system interconnect bandwidth. Furthermore, low NA stepper with a large exposure area will enable the fabrication of large body size interposers at low cost. This paper analyzes the relationship between trace width resolution (w), depth-of-focus (DOF) and materials to conclude that it is feasible to fill the gap between semiconductor back-end-of-line (BEOL) and package substrate RDL technologies. The final section discusses the issues in process development of lithography for multilayer fine line RDLs.