Application of Picosecond Acoustic Metrology for Monitoring Metal Films in Advanced Packaging

Abstract

High-performance computing applications such as artificial intelligence, and machine learning require massive amount of parallel processing requiring significant memory bandwidth. High bandwidth memory (HBM) solutions have been optimized to enable such applications.  HBM consists of core DRAM die and a logic buffer die in a vertically stacked 3D structure. Customers are adopting front-end solutions to meet the shrinking process tolerances and to meet the stringent reliability requirements. [1]

Picosecond acoustic metrology is a very well-established technique for in-line measurements [2] and can be applied to meet the challenges of advanced packaging applications. In this paper, we present system-level improvements and specific use cases that have accelerated adoption of the technology in fan-out wafer level packaging (FOWLP) and in the HBM process loop. In the FOWLP process, re-distribution lines (RDL) play a critical role in how I/Os are connected and measuring RDL thickness before and after Cu seed etch in necessary as it impacts line resistance and leakage current. In the TSV process loop, multi-layer metal (Au/Ni/Cu) is used to form pads for bonding to the micropillar [3-4]. Thickness of each of the layers plays a critical role in the reliability performance and hence simultaneous in-line thickness needs to be carefully measured and controlled. Similarly, in the microbump process flow, controlling copper seed/barrier thickness is critical as the relative bump heights will vary as a function of the copper seed thickness variation impacting bump co-planarity [5].