The focus of this study is on the effect of the heat treatment during hybrid bonding. It includes a detailed investigation of the Cu microstructure and the Cu-to-Cu bond interface. The bond interface was investigated after heat treatment by scanning electron microscopy and energy backscatter diffraction analysis. The test design has a Cu pad size of 4 μm x 4 μm, at a pitch of 8 μm. Cu pads are enclosed in SiO₂. Die-to-wafer hybrid bonding was performed at coupon-level. After hybrid bonding at room temperature, the bonded samples were stored at elevated temperature. The peak temperature of the heat treatment was set to: 200 °C, 250 °C, 300 °C, and 350 °C. Infrared microscopy images and cross-sections show a high alignment accuracy greater 1 μm at 8 μm pitch. The bond interface of the Cu-to-Cu interconnect is closed after heat treatment above 300 °C. The Cu intergrowth is low, grain boundaries at the bond interface are mostly planar. The texture changes with increasing temperature towards <111> direction and its characteristic <115> twin orientation.