Development of 2.5D and 3D IC Fabrication and Assembly Technologies

Abstract

2.5D/3D IC assembly is where the traditional foundry and assembly house model breaks down. The greatly reduced feature sizes of microbumps in combination with very large, thin interposers and ICs present many challenges for assembly. With densely integrated packages and stacked thin dies, warpage at various steps of assembly process can lead to die crack, weak or open interconnection, and delamination. Microbump die with copper pillar and solder cap lacks the flexibility of conventional BGA solder balls in warpage compensation. Low temperature dielectric materials for interposer backside passivation lack chemical, thermal stability and mechanical strength compared to similar materials cured at higher temperatures. Ultra- low stand-off between dies creates new challenge for flux residue removal and underfill processing, adhesion and reliability. In this paper, we present learning and results from building two Invensas 2.5D interposer test vehicles with rectangular (19mmx27mm) and square (24mmx24 mm) silicon interposers. Since the industry is still debating where the MEOL processes will take place, these two test vehicles use both assembly centric and fab centric flows. We conducted comprehensive material and process compatibility studies to meet our goal of process capability and product reliability. We explored three different interposer fabrication/ assembly flows that includes copper-to-copper bonding as well as solder capped copper pillar microbumps for microbump die to interposer connection. We optimized the interposer fabrication process to improve interposer quality and warpage. We instituted 100% bump inspection and tight control on microbump die quality and co-planarity to improve assembly yield. We developed robust soldering and cleaning processes to achieve high soldering process yield. We evaluated several methods of flux clean and developed a procedure to achieve optimal flux residue removal without causing mechanical damage to solder joints. We explored vacuum and pressure curing process for underfill and adopted advance underfill cure technology to eliminate voids in underfill. The impact of these factors on assembly yield and reliability will be presented.