Overlay Analysis and Optimization for High-Accuracy Hybrid D2W Bonding

Le résumé fourni par la source

In this study we analyze the misalignment of dielectric bonded die-to-wafer (D2W) assemblies using a direct D2W bonding approach. Understanding the overlay error contributors is essential to enable high accuracy direct hybrid D2W bonding. Next to the translation and rotation placement error, initial results also show an apparent scaling error. A dedicated overlay test vehicle is designed to enable high resolution overlay measurements. Every single 7.2*7.2mm^2 die contains 25 overlay measurement locations, allowing a more detailed intra-die overlay analysis. Patterned 50 µm thin dies are directly bonded to a patterned target wafer using a BESI ChameoUltraPlus high accuracy (hybrid) D2W bonder. After bonding, Scanning Acoustic Microscopy (SAM) shows good bonding yield with very little voids. Next, an infrared (IR) overlay measurement is performed on 25 points per individual bonded die. This high-resolution overlay measurement confirms a significant linear scaling component of around 33ppm, resulting in a ~150nm scaling error in the die corners, in addition to the die placement error. Finite Element Method (FEM) modeling shows that a dielectric layer induced deformation in the thin top die combined with a die-center mechanical bond initiation is the main cause of the observed scaling error. Further analysis of the overlay error reveals only a very small residual fingerprint. This shows the importance of reducing the scaling error, either by exploring alternative bondheads which cause less die deformation, and/or the introduction of stress compensation layers.

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