Miniaturized High-Performance Filters for 5G Small-Cell Applications

This paper demonstrates the first panel-based ultra-miniaturized filters with footprint smaller than half of the free-space wavelength at the operating frequencies of 28 and 39 GHz bands for 5G and mm-wave small-cell applications. The thin-film filters can be utilized either as ultra-thin integrated passive devices (IPDs) or embedded into the module substrates. Two filter types: lowpass and bandpass, with three topologies in total, are modeled, designed and fabricated on precision thin-film build-up layers on glass and traditional laminate cores. The modeling, design and optimization phase included the considerations of fabrication tolerances and testability of the filters. Glass is an ideal core material for mm-wave 5G modules and IPDs since it combines the benefits of ceramics for high frequency electrical performance, laminates for large panel processing and low cost, silicon-like dimensional stability and precision patterning, which is essential for mm-wave circuits. Unlike printing used in ceramics, or subtractive etching used in multilayer organics (MLO), this research utilizes semi-additive patterning (SAP) process to form high precision, multilayer redistribution layers (RDL) to design ultra-compact filter topologies with low insertion loss and improved stopband rejection, due to the close-to-ideal translation of lumped-to-distributed components. The simulated results of bandwidth, in-band insertion loss and out-of-band rejection of filters show excellent correlation with the measured results.