Opportunity
Metal–metal bonding is critical in aerospace, automotive, semiconductor, and nano-electronics industries, yet existing methods face severe limitations. Soldering uses environmentally harmful fillers like Sn–Pb alloys that are too large for submicron bonding. Fusion welding requires high temperatures and energy, damaging heat-sensitive substrates such as flexible electronics or ferroelectric materials. Solid-state bonding demands excellent plasticity, thermal stability, and interdiffusion, which are difficult to achieve at micro- or nano-scales. For next-generation 2.5D/3D packaging, there is an urgent need for a copper–copper (Cu–Cu) bonding technique that is efficient, low-temperature, low-pressure, environmentally friendly, and applicable to non-planarized surfaces without complex pre-treatments.
Technology
The invention provides a composite material comprising a copper-based heterogeneous nanostructure of nanocrystalline copper (≈80 vol%) interspersed with copper (I) oxide dispersions (≈20 vol%). The material has an average grain size of ~16 nm, contains twin boundaries and stacking faults, and exhibits a low elastic modulus (~35 GPa) – about one-third of bulk copper – along with high yield strength (~950 MPa) and ductility (~40%). It is prepared as a freestanding nanomembrane (35–50 nm thick, >1 cm lateral size) by depositing Cu onto a dehydrated hydrogel substrate (e.g., PVA/glass) via electron-beam evaporation or ion-beam sputtering, then immersing in water to release the membrane. For bonding, one or more layers of this composite membrane are placed between two copper surfaces (non-planarized) and annealed at only 200–300 °C under 10 MPa pressure in vacuum for 3–300 minutes. The heterogeneous structure promotes rapid grain growth across the bonding interface in oxide-free regions, while oxide-rich regions create tough interfaces that blunt cracks. This combination yields ultra-strong Cu–Cu bonding regardless of copper crystal orientation.
Advantages
- Enables Cu–Cu bonding at low temperature (200–300 °C) and low pressure (10 MPa)
- Achieves very high shear strength up to 73 MPa – 35 times stronger than direct Cu–Cu bonding
- No surface planarization or chemical pre-treatment required
- Environmentally friendly process using only deionized water for membrane release
- Low cost compared to conventional techniques that use harsh chemicals (e.g., NaOH, HCl, H₂SO₄, hydrazine)
- The composite membrane is flexible, conformable, and can be transferred easily
- Excellent mechanical properties: high yield strength, ductility, and elastic strain limit (~2.7%)
- Minimal effect on electrical conductivity of copper despite oxide content
Applications
- Cu–Cu bonding in 2.5D and 3D advanced semiconductor packaging
- Metal–metal joining in micro- and nano-electronics, including flexible devices
- Aerospace and automotive interconnect manufacturing
- Heat-sensitive substrate bonding (e.g., organic electronics, ferroelectric materials)
- Large-area bonding of patterned or unpatterned copper surfaces Replacement for soldering, brazing, and solid-state bonding in submicron scale applications
