Reliability of Copper Through-Package Vias in Bare Glass Interposers

Abstract

Thermomechanical reliability of copper-plated through-package vias (TPVs) in ultrathin bare glass interposers was investigated through modeling, design, fabrication, reliability characterization, and failure analysis. Finite-element models were developed to analyze stress and strain distribution in TPV structures, and to obtain design guidelines for reliable TPVs. In order to experimentally validate the predictions of simulations, bare glass substrates of 100 μm thickness with vias of 30 μm diameter at 120 μm pitch were metallized using Ti/Cu sputtering, followed by patterning and electroplating. Cu TPV daisy chains were fabricated and subjected to thermal cycling test between -55 °C and 125 °C to assess their thermomechanical reliability. Detailed cross-sectional analysis was also carried out by scanning electron microscope imaging of TPV cross sections. No electrical failures were detected in the Cu TPV chains. Failure analysis revealed copper delamination and crack formation in glass. The experimental reliability results are consistent with the thermomechanical models. Design and process recommendations are provided based on the modeling and experimental results.