Abstract
The technology trends of next generation electronic packaging are moving toward heterogeneous 3D packaging systems. One of the key processes of 3D packaging system is Cu-to-Cu bonding, which is highly dependent on the planarized, activated, and oxygen-free Cu surface. A two-step plasma treatment is studied to form a Cu surface that does not react with oxygen and improves the Cu bonding interface quality at low bonding temperature (300 °C). In this study, the effects of two-step plasma treatment on both sputtered and electroplated Cu surfaces were evaluated through structural, chemical, and electrical analysis. The Cu bonding interface was studied by scanning acoustic tomography analysis after the thermocompression bonding process. Both sputtered and electroplated Cu thin films had the preferred orientation of (111) plane, but sputtered Cu exhibited larger grains than the electroplated Cu. As a result, the roughness of sputtered Cu was lower, and the resistivity was higher than that of electroplated Cu. Based on X-ray photoelectron spectroscopy analysis, the sputtered Cu formed more copper nitrides and fewer copper oxides than the electroplated Cu. A significant improvement in bonding quality at the Cu bonded interface was observed in sputtered Cu.
Highlights
For the sputtered Cu samples, a 50-nm-thick Ti thin film was deposited in a conventional sputter (Sorona, Inc., PyungTak, Korea, SRA-110) under 5 mtorr working pressure and 2500 W power with 80 sccm Ar gas flow
A ~1-μm-thick Cu thin film was deposited under 5 mtorr working pressure and 2500 W power with 80 sccm Ar gas flow
100-nm-thick Cu as a seed layer was deposited on a Ti-deposited wafer followed by ~1-μm-thick electroplated Cu that was done in a manual electroplater (Sungwon Forming, Ansan, Korea, SW-PM2-R2Q1) with 2ASD current density and 6.48A current. Both the sputtered and electroplated samples were subjected to two-step plasma treatment in a direct current (DC) sputter chamber
Summary
Sci. 2019, 9, 3535 to electromigration, and no brittle intermetallic compound (IMC) formation. A good Cu diffusion bonding requires high bonding temperature above 400 ◦ C, which is not practically suitable for a mass production manufacturing. The research includes coverage of surface activated bonding [4], passivation using a self-assembled monolayer [5], wet cleaning [6,7], metal passivation with Pd, Mg, Ag, or Au [8,9,10,11], Cu (111) crystal plane studies [12], Cu/SiO2 hybrid bonding [3,13], and Cu/polymer hybrid bonding [14]. A number of weaknesses such as Cu surface oxidation, Cu planarization by CMP (chemical mechanical polishing), and complete decomposition of passivated material still exist
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