Ultra-Thin Cu Nano-Twinned Films Sputtered On Si Wafers for Low Temperature Hybrid Bonding Of Advanced 3D-IC Packages

Abstract

The nanotwinned structure has been attracting a great deal of attention due to its excellent mechanical and electrical properties. In this study, ultra-thin nanotwinned Cu (nt-Cu) films with a thickness of 1.5 μm were fabricated by DC magnetron sputtering with moderate substrate bias. Within these films, an equiaxed-grain transition layer only 100-400 nm thick formed between the nanotwinned region and Si substrate and was observed by focused ion beam (FIB). The XRD and EBSD analyses indicated that the as-deposited nanotwinned films had a strong (111) preferred orientation. The surface roughness of the films, as measured with an atomic force microscope (AFM), was below 10 nm, which was sufficiently smooth for chip bonding at low temperature with minor pressure. Direct bonding of Si chips sputtered with such ultra-thin nt-Cu films with a (111) preferred orientation has been evidenced to yield a sound interface, and this bonding method can be applied for the manufacturing of 3D-IC packages.