Low temperature CMOS compatible Cu-Cu thermo-compression bonding with constantan alloy passivation for 3D IC integration

In this paper, we report low temperature wafer level Cu-Cu thermo-compression bonding using an ultra-thin Copper — Nickel based alloy layer, constantan as passivation layer. Major bottlenecks in achieving low temperature and low pressure bonding are surface oxidation and roughness. Constantan alloy on Cu surface has a dual role of preventing Cu surface from oxidation and reduces the surface roughness. Passivation reduces the bonding temperature as it prevents copper surface from oxidation. In this endeavor, constantan alloy passivation thickness was optimized by varying thickness of constantan layer on Cu surface and studied the effect of surface roughness by performing AFM analysis. Simultaneously surface passivation was studied using Secondary Ion Mass Spectroscopy (SIMS) depth profiling. The optimum constantan passivation thickness is found to be 2 nm for achieving wafer level Cu-Cu bonding at temperature as low as 150 ˚C at constant contact force of 4.5 kN (5 bar). Our optimized result yielded a very good bond strength of 152 MPa which compared well with the available literatures.

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