First demonstration of reliable copper-plated 30μm diameter through-package-vias in ultra-thin bare glass interposers

This paper reports the first demonstration of the reliability of through package copper vias (TPV) with 30 μm diameter at 120 μm pitch in ultra-thin 100 μm thick glass to achieve high-density vertical interconnections in 2.5D and 3D interposers and packages. Bare glass with 100 μm thickness was used to demonstrate the via formation, metallization and reliability of small through-package-vias. The reliability concerns at this fine pitch were addressed through modeling and design from first principles, followed by experimental validation with test-vehicle fabrication and reliability characterization. Thermo mechanical reliability of TPV was analyzed through finite element modeling to estimate stresses inside TPV during thermal cycling and provide design guidelines. For experimental validation, test samples with daisy chains of glass TPVs were fabricated and subjected to accelerated thermal cycling tests between -55°C and 125°C to assess the thermomechanical reliability. Resistance of each daisy chain was measured periodically as a method to detect failure initiation. Majority of the TPV chains passed the reliability test without significant change in resistance. TPV daisy chains that showed changes in resistance were cross-sectioned and failure analysis indicated that the early failures of TPV in glass were related to process defects coming from via-hole formation and metallization.