Experimental investigations in the intermetallic and microvoid formation in sub-200 °C Cu–Sn bonding

• Published in: Journal of materials science. Materials in electronics Vol. 30; no. 17; pp. 16427 - 16438
• Main Authors: Kannojia, Harindra Kumar, Dixit, Pradeep
• Format: Journal Article
• Language: English
• Published: New York Springer US 01-09-2019; Springer Nature B.V
• Subjects: Annealing; Bonding; Characterization and Evaluation of Materials; Chemistry and Materials Science; Coalescing; Copper; Interfaces; Intermetallic compounds; Materials Science; Microelectromechanical systems; Optical and Electronic Materials; Scallops; Tin; Void fraction
• ISSN: 0957-4522; 1573-482X
• DOI: 10.1007/s10854-019-02017-1

This paper reports the intermetallic growth and microvoid formation in the Cu–Sn layers, which were annealed at low temperatures (sub-200°C) for durations varying from 120 to 1440 min. A 10 µm thick tin was electrodeposited on copper samples. Both Cu
6
Sn
5
and Cu
3
Sn IMCs were formed and had a non-uniform scalloped shaped profile but with different scallops sizes. Void growth was studied at three different locations, i.e., the Cu–Cu
3
Sn interface, within the Cu
3
Sn, and at the Cu
3
Sn–Cu
6
Sn
5
interface. The void size in these locations increased with increasing annealing durations and temperatures due to the coalescence of nearby voids. The void fraction at the Cu–Cu
3
Sn and Cu
3
Sn–Cu
6
Sn
5
interfaces was observed to decrease, whereas the void fraction within the Cu
3
Sn IMC increased with increasing annealing durations. The largest voids were seen at the Cu–Cu
3
Sn interface, while the highest void fraction was found within the Cu
3
Sn IMC. The overall void size and void fractions for all experimental conditions were always smaller than 3 µm
2
and 1.44 µm
−1
, respectively. The obtained results can be used in the hermetic packaging of MEMS devices performed at sub-200 °C. Processing at these low temperatures result in reduced thermo-mechanical stress and also eliminate the molten tin squeezing-out from the bonding zone, which is a known issue in Cu–Sn solid–liquid inter-diffusion bonding performed at temperature > 232 °C.