Warpage reduction using dielectric layers stress tuning: From analytical model to 3D integration of large die on ceramic substrate

A mechanical study of silicon interposer bow reduction, from wafer level manufacturing to large die stacking including analytical modeling, is presented in this paper. Indeed, understanding and reducing the warpage of a dissymmetrical substrate is fundamental for assembly yield and interconnects reliability. The target here is a bow less than 50 μm for a 650 mm
²
Si-interposer. This mechanical version of a power computing package demonstrator for high reliability applications integrates three top dies, whose largest is 300 mm
²
, with a 225 μm bump pitch, on a 650 mm
²
Si-interposer with a 450 μm ball pitch, itself reported on a 45 × 45 mm
²
ceramic substrate. Assembly is performed in a mass reflow oven. In this paper, the design, methodology and test vehicle process flow are firstly presented. The analytical model of the interposer bow as a function of temperature, based on the Stoney formula for small strains, is then presented. The model calibration is detailed. In this work, three levers are used to manage the bow: interposer thickness, passivation layers stress tuning, and symmetrical behavior of both faces. These actions can either shift the bow whatever the temperature, or change the bow range over temperature. Finally, the experimental results are shown. After front side manufacturing, the 725 μm thick wafer bow is reduced from 230 μm to 105 μm with optimized compressive dielectric layers. The die bow's range from ambient to assembly temperature is reduced from 50 μm to 15 μm thanks to backside processing for the 200 μm thick die. As regards the 300 μm thick die, the temperature has almost no effect on its bow which is -12 μm. Bow values after manufacturing and slopes during thermal cycles fit very well with the model developed and the assembly requirements. These results enable to fine-tune the bow and predict a very good yield and reliability for the assembly of the electrical demonstrator.