In-situ measurement of thermal expansion in Cu/SiO2 hybrid structures using atomic force microscopy at elevated temperatures

Abstract

Thermal expansion of Cu plays a crucial role in the bonding mechanism of the Cu/SiO2 hybrid joints. In-situ heating atomic force microscopy (AFM) was employed to observe the evolution of surface topographies of Cu/SiO2 hybrid structures at various annealing temperatures. We found that the surface profile of the nanotwinned-Cu (NT-Cu) pad in SiO2 via exhibited a 6-nm recess at room temperature and shifted to a 4-nm protrusion at 200 ℃. The microstructural difference between NT-Cu and regular-Cu vias also influences their thermal expansion behaviors. The regular-Cu via has a larger expansion due to its smaller Young’s modulus. The changes in the surface profiles before and after annealing also demonstrate the plastic deformation of the Cu pads. The conservation of the Cu volume further indicates the migration of Cu atoms through creep mechanism at high temperatures, which results from the plastic deformation. This study provides a more comprehensive understanding with direct evidence of the Cu/SiO2 hybrid bonding mechanism.