Investigation of Cu-to-Cu Bonding Featuring Indium Passivation and a Tin Diffusion Barrier Layer

This study investigates the interfacial reactions among solid copper, tin, and indium. Samples were fabricated by sequentially electroplating 3 μm Cu, 5 μm Sn, and 5 μm In onto a silicon substrate. The results indicate that Sn and In completely react at room temperature, forming β-In<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>Sn and Y-InSn<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf>, Additionally, a thin and stable Cu-In intermetallic layer, Cu(In, Sn)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, is observed between the Cu substrate and the Y phase. The effectiveness of Cu-to-Cu bonding with In passivation and an Sn diffusion barrier is evaluated. The bonding process was performed via thermal compressive bonding (TCB) at 150°C under 2 MPa for 1 minute. The findings reveal that while the Sn passivation layer does not entirely prevent In diffusion into Cu, it significantly slows down intermetallic formation. This passivation approach enables Cu bonding under mild processing conditions, reducing the need for high temperature and pressure.