Novel High-Temperature, High-Power Handling All-Cu Interconnections through Low-Temperature Sintering of Nanocopper Foams
Conference
Shahane, N, Mohan, K, Behera, R et al. (2016). Novel High-Temperature, High-Power Handling All-Cu Interconnections through Low-Temperature Sintering of Nanocopper Foams
. 2016-August 829-836. 10.1109/ECTC.2016.352
Shahane, N, Mohan, K, Behera, R et al. (2016). Novel High-Temperature, High-Power Handling All-Cu Interconnections through Low-Temperature Sintering of Nanocopper Foams
. 2016-August 829-836. 10.1109/ECTC.2016.352
Direct Cu-Cu bonding has been pursued by the semiconductor industry as the next interconnection node, for its superior power-handling capability, thermal stability and reliability as compared to traditional solders. However, manufacturability of Cu interconnections has so far been severely limited by the relatively high modulus of Cu, requiring costly planarization processes to address non-coplanarities and warpage to enable bonding in solid-state. This paper introduces a new class of all-Cu interconnections formed by low-Temperature sintering of low-modulus nanocopper foams. The physical properties of these foams are tunable with their nanostructure and morphology, giving design flexibility. The proposed nanocopper interconnections have the following advantages: 1) fabrication compatible with standard semiconductor infrastructures and processes, 2) sub-20GPa elastic modulus pre-sintering providing tolerance to non-coplanarities and warpage in assembly, 3) densification at temperatures below 250°C to form high-density all-Cu interconnections, with excellent electrical and thermal properties, and 4) wide applicability from ultra-fine pitch to large-Area interconnections for high-power devices. This paper reports the fabrication, characterization and first proof-of-concept demonstration of sintering of such nanocopper interconnections. Nanocopper foams were first synthetized by co-sputtering 2μm Cu25Si75 (at%) thin films on Si(100) substrates, and then electrochemically dealloying the thin-films in hydrofluoric acid under a range of applied voltages. Imaging of the foams by scanning electron microscopy (SEM) confirmed that they consisted of 45nm-sized uniform copper ligaments. Further, native oxides present in the as-synthetized samples were effectively removed with a short etching step in acetic acid, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis, without affecting the foams' nanostructure. Differential scanning calorimetry (DSC) characterization indicated an exothermal peak at 180°C, beyond which coarsening may initiate. Finally, isothermal aging of the nanofoams was carried out in N2 atmosphere at 100 and 300oC range for 180-900sec. No sintering was observed after aging at 150oC, as expected from DSC results, while sintering was confirmed after only 180s at 300oC, with observed densification and shrinkage of the foam, successfully demonstrating the proposed concept. In conclusion, a novel nanocopper foam interconnection technology is demonstrated with the potential to address manufacturability, scalability and cost across consumer, high performance and automotive applications.
