Zijian Wu

Low temperature Cu-Cu bonding using copper nanoparticles fabricated by high pressure PVD

Copper nanoparticles (Cu NPs) fabricated by physical vapor deposition (PVD) were introduced in Cu-Cu bonding as surface modification. The bonding structure with Ti adhesive/barrier layer and Cu substrate layer was fabricated on both surfaces first. Loose structure with Cu NPs was then deposited by magnetron sputtering in a high pressure environment. Solid state Cu-Cu bonding process was accomplished at 200°C for 3min under the pressure of 20MPa. Die shear test was carried out and an average bonding strength of 36.75MPa was achieved. The analysis of fracture surface revealed a high-reliability bonding structure. According to cross-sectional observations, a void-free intermediate Cu layer with thickness around 10nm was obtained. These results demonstrated that a reliable low temperature time-saving Cu-Cu bonding was realized by Cu NPs between the bonding pairs. This novel bonding method might be one of the most attractive techniques in the application of ultra-fine pitch 3D integration.

Effects of Current Stress for Low Temperature Cu/Sn/Cu Solid-State-Diffusion Bonding

In this work, current stress was applied to investigate electromigration (EM) effects for Cu/Sn/Cu solid-state-diffusion (SSD) bonding in 3D integration. Full-array bumps with different pitches were fabricated on wafers. Asymmetric structure of Cu/Sn bump and Cu bump was deposited by low-cost electroplating process. With optimized surface pretreatment, wafer-level Cu/Sn/Cu SSD bonding was performed at a low temperature of 200 °C for 30 min under a vacuum of 10-5 mbar. As-bonded wafers were annealed at 200 °C for 60 min under N2 atmosphere and then diced into dies. The bonding interface consisted of Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu, without pure Sn remained and Sn overflow. After further annealing process for bonded dies, the middle layer of Cu6Sn5 has been completely exhausted, and the interface transformed to Cu/Cu3Sn/Cu. The bonding strength has reached above 70 MPa. Subsequently, the three-layer structure was subjected to EM test at 150 °C for 500 hours with current density of 2.0 × 104 A/cm2. No void or crack caused by atom migration appeared at the cathode and the anode. Bonding strength also kept at above 70 MPa, and no electrical degeneration occurred according to resistance measurement. It is concluded that high-quality Cu/Sn/Cu bonding was realized using SSD technology and exhibited an expected anti-electromigration capability.

Solid-State-Diffusion Bonding for Wafer-Level Fine-Pitch Cu/Sn/Cu Interconnect in 3-D Integration

Low-temperature Cu/Sn/Cu solid-state-diffusion (SSD) bonding has been investigated in this paper. Twenty-micrometer fine-pitch bumps with daisy-chain and Kelvin structures were fabricated by high-efficiency and low-cost electroplating process. Before bonding, the bump surface was treated with Ar(5% H2) plasma. Wafer-level bonding was performed with a pressure of 6.7 MPa at 200 °C for 60 min. Microstructure of the as-bonded interface consisted of five layers, i.e., Cu/Cu3Sn/Cu6Sn5/Cu3Sn/Cu, no Sn overflow was observed, and pure Sn was completely consumed during bonding process. After annealing at 200 °C for 60 min under N2 atmosphere, Cu6Sn5 was exhausted, and the average shear strength increased to 11.4 MPa. The resistance measurements were approximate to the theoretical estimation. The bonded performance had no significant change after thermal cycling test. The bonding interface exhibited an expected antielectromigration capability. It is concluded that Cu/Sn/Cu SSD bonding would be one of the potential technologies for 3-D integration.

Low temperature Cu-Cu bonding using Ag nanoparticles by PVD

A novel low temperature wafer-level Cu-Cu bonding method using Ag nanoparticles (NP) was proposed and realized in this paper. A bonding structure consisted of Cu bonding pads, TiW barrier/adhesive layer was firstly fabricated on the silicon wafer. Ag NPs were then deposited by physical vapor deposition (PVD) on Cu pads. The morphology of Ag NPs annealed at different temperature was studied. Bonding process was successfully proceeded at 200°C for 10min under the pressure of 20MPa followed by a post-annealing process at 200°C for 1 hour. Die shear test was carried out after bonding process and the shear strength reached 23.59MPa. Scanning electron microscope (SEM) photos of the fracture interfaces were presented and the energy dispersive X-ray (EDX) analysis of the fracture surfaces showed that cracks occurred at the Ag bonded interfaces and TiW/SiO2 interfaces. The results mentioned above demonstrated that a novel low temperature short time Cu-Cu bonding was realized by Ag NPs fabricated by PVD.

Surface activated bonding method applied in MEMS pressure sensor with TSV structures

As one of the traditional bonding method, thermo-compression bonding has the problem of process incompatibility and mechanical instability due to the high bonding temperature. Surface activated bonding (SAB) method can solve such problems by a very low bonding temperature. Oxide layer and contaminations for the normal surface are removed by specific method such as FAB treatment or plasma treatment. When two activated surfaces are brought into contact, clean surfaces are bonded together by atomic force. In this paper, the design and the fabrication of pressure sensor chip with TSV structures would be given, and the details of the surface activated bonding would be given. The results of the bonding would be carefully studied. The SAB method would be compared with thermo-compression bonding in some aspects such as bonding strength, bonding interface condition, etc.