Ar/N2 Plasma Induced Metastable CuxNy for Cu-Cu Direct Bonding
Authors: Hu Liangxing , Goh Simon Chun Kiat , Tan Chuan Seng 
Published: December 2020

Metal-metal bonding is a promising semiconductor bonding technique, which has been widely developed to establish vertically electrical connections, provide mechanical support, and form hermetical sealing. In this work, we report Ar/N2 plasma induced metastable CuxNy for Cu-Cu die-die bonding performed in ambient environment. Surface characterization is performed on the Cu surface both before and after plasma exposure, revealing that the plasma induced CuxNy surface remains in “activated” state for up to 6 hours compared to the regular Cu surface. Subsequently, the dies are bonded. The bonding quality is assessed for mechanical bonding strength, hermeticity, and electrical conduction. The results support successful bonding with the desired properties. This reported bonding technology can be applied for high-throughput 3D integration and CMOS-MEMS packaging.

Introduction

Microelectromechanical systems (MEMS) packaging requires both mechanical and electrical connections in addition to hermetical sealing, therefore 3D integration is a promising solution [1]. Metal diffusion (thermocompression) bonding, eutectic alloy bonding, and solid-liquid interdiffusion bonding are popular bonding methods [2]. The latter two bonding methods, which are based on solder materials, may be plagued by inter-metallic compound (IMC) reliability issue. On the other hand, metal diffusion bonding, such as Cu-Cu, is absent of IMC issue. However, Cu is prone to rapid oxidation. To overcome this issue, Cu surface treatments such as specialized chemical mechanical polishing (CMP), chemical treatment, surface assembled monolayer (SAM), and plasma activation, have been reported. Among these methods, plasma activation is favorable as the process does not leave behind any residue. In this work, we report on the time-dependent evolution of the Ar/N2 plasma induced CuxNy layer and its application for Cu-Cu die-die bonding performed in ambient environment.

Cu-Cu Die-Die Bonding

Si wafers were firstly patterned and deposited with 10 nm thickness of Ti as adhesion layer, followed with 100 nm thickness of Cu via electron-beam evaporation. Afterwards, the wafers were diced into small dies with dimensions of 5 mm × 7 mm. Thereafter, the Cu surface of the diced dies was exposed to Ar/N2 plasma. During the plasma process, Ar plasma was firstly applied to remove surface contaminants and copper oxides, N2 plasma was subsequently applied on the Cu surface to form an ultra-thin layer of CuxNy for passivating the Cu surface and achieving low temperature bonding. Finally, the “activated” dies were immediately pre-bonded in ambient environment in clean room (18 ºC and 40% humidity) and were annealed at 300 ºC for a duration of 30 min.

Results and Observation

The evolution of water contact angle (θ) with time variation for both the non-activated and Ar/N2 plasma activated Cu surface is shown in Figure 1. Compared to that of the non-activated Cu surface (~60º), the water contact angle of the Ar/N2 plasma activated Cu surface (~10º) is lower initially, signifying that the activated surface is more hydrophilic. Subsequently, the water contact angle of the Ar/N2 plasma activated Cu surface increases gradually until it is comparable to that of the non-activated Cu surface. The results show the metastability and degradation of CuxNy layer, revealing that the CuxNy layer remains in an “activated” state for up to 6 hours post plasma exposure. In addition, the mechanical bonding strength, hermeticity, and electrical conduction of the bonded dies are shown in Figures 2(a) and 2(b), 2(c), and 2(d), respectively. The results show that the dies are bonded successfully with the desired properties.

Conclusion

In this work, Ar/N2 plasma activated Cu-Cu die-die bonding in ambient environment was conducted. Surface analysis revealed that the plasma induced metastable CuxNy surface is more hydrophilic than the non-activated Cu surface and the plasma activated Cu surface remains in an “activated” state for a duration of 6 hours after plasma exposure. In addition, the mechanical bonding strength, hermeticity, and electric conduction of the bonded dies were determined, the results show that the dies were bonded together successfully. This bonding technology can be applied for high-throughput 3D integration and CMOS-MEMS packaging.

Acknowledgement

Authors acknowledge funding support from A*STAR (A18A4b0055) under the “Nanosystems at the Edge” program.

References

[1] K. N. Chen, C. S. Tan, A. F, and L. R. Reif, “Cu Wafer Bonding for 3D IC Applications”, in Wafer Level 3-D ICs Process Technology, C. S. Tan, R. J. Gutmann, and L. R. Reif, Editors, New York, USA, Springer Science & Business Media, 2008, pp. 117-130.
[2] P. Enquist, "Advanced Direct Bond Technology”, in 3D Integration for VLSI Systems, C. S. Tan, K. N. Chen, and S. J. Koester, Editors, New York, USA, Jenny Stanford Publishing, 2012, pp. 175-204.

Figure 1