Time-Dependent Evolution Study of Ar/N2 Plasma-Activated Cu Surface for Enabling Two-Step Cu-Cu Direct Bonding in a Non-Vacuum Environment

In this paper, a two-step copper-copper direct bonding process in a non-vacuum environment is reported. Time-dependent evolution of argon/nitrogen plasma-activated copper surface is carefully studied. A multitude of surface characterizations are performed to investigate the evolution of the copper surface, with and without argon/nitrogen plasma treatment, when it is exposed to the cleanroom ambient for a period of time. The results reveal that a thin layer of copper nitride is formed upon argon/nitrogen plasma activation on copper surface. It is hypothesized that the nitride layer could dampen surface oxidation. This allows the surface to remain in an “activated” state for up to 6 hours. Afterwards, the activated dies are physically bonded at room temperature in cleanroom ambient. Thereafter, the bonded dies are annealed at 300ºC for varying duration, which results in an improvement of the bond strength by a factor of 70 ~ 140 times. A sample bonded after plasma activation and 2-hour cleanroom ambient exposure demonstrates the largest shear strength (~5 MPa). The degradation of copper nitride layer at elevated temperature could aid in maintaining a localized inert environment for the initial diffusion of copper atoms across the interface. This novel bonding technique would be useful for high-throughput three-dimensional wafer bonding and heterogeneous packaging in semiconductor manufacturing.

Synthesis of Copper Nitride Layers by the Pulsed Magnetron Sputtering Method Carried Out under Various Operating Conditions

Copper nitride shows various properties that depend on the structure of the material and is influenced by the change in technical parameters. In the present work, Cu–N layers were synthesized using the pulsed magnetron sputtering method. The synthesis was performed under different operating conditions: direct current (DC) or alternating current (AC) power supply, and various atmospheres: pure Ar and a mixture of Ar + N2. The structural properties of the deposited layers were characterized by X-ray diffraction measurements, and Raman spectroscopy and scanning electron microscopy have been performed. Optical properties were also evaluated. The obtained layers showed tightly packed columnar grain features. The kinetics of the layer growth in the AC mode was lower than that observed in the DC mode, and the layers were thinner and more fine-grained. The copper nitride layers were characterized by the one-phase and two-phase polycrystalline structure of the Cu3N phase with the preferred growth orientation (100). The lattice constant oscillates between 3.808 and 3.815 Å for one-phase and has a value of 3.828 Å for a two-phase structure. Phase composition results were correlated with Raman spectroscopy measurements. Raman spectra exhibited a broad, diffused, and intense signal of Cu3N phase, with Raman shift located at 628–635 cm−1. Studies on optical properties showed that the energy gap ranged from 2.17 to 2.47 eV. The results showed that controlling technical parameters gives a possibility to optimize the structure and phase composition of deposited layers. The reported changes were discussed and attributed to the properties of the material layers and technology method.