Thermocompression bonding is especially important for low pitch (<10 µm) packaging and wafer connections compared to wire bonding, conventional bumps and BGA (Ball Grid Array) technologies. The lack of solder in thermocompression bonding eliminates potential intermetallics at the bond interface leading to increased bonded area fraction, which translates to better quality bonds and higher reliability. In many cases, thermocompression bonding is achieved at higher temperatures and longer times than would be feasible for many applications. A more fundamental understanding of the bonding process, and therefore a path toward lower temperatures and pressures is addressed by studying the surface (interface) diffusion and bonding pressure separately. Comparing these processes separately and the morphology of the bonded interface helps to address which parameters are most important and whether the thermocompression bonding process is simply a sum of the two components or whether there are synergistic effects.

This study investigated roles of temperature and pressure separately in diffusion and surface topology changes under conditions that were comparable to those used during thermocompression bonding. This was achieved using Power Spectral Density Function (PSDF) analysis which is based on statistical information extracted from AFM scans. Scans were taken before and after annealing at different temperatures and time; and before and after a surface flattening process at constant temperature for different time periods. Next, we investigated Au-Au bonded interfaces. 1.2 µm thick, sputter-deposited Au films of intermediate roughness (3-5 nm) were bonded at low temperatures (150 °C-250 °C) and different pressures (3 - 14 MPa) for 15 min.

The initial peak to valley heights of the unbonded Au films, determined using AFM scans, were 20-30 nm and correlation length was approximately 400 nm. The void morphology was assessed using a combination of plan view and cross section focused ion beam / scanning electron microscopy images. For structures bonded at 150 °C, the void morphology was related to initial roughness in both low and high-pressure cases. While the void length decreased along the interface for all three temperatures at 14 MPa pressure, the void height showed dependence on bonding temperature. Variation in pressure did not affect the results for bonding temperature of 200 °C. Void height increased for bonding at 200 °C and decreased for 250 °C. This decrease in void height was not observed for bonding at 3 MPa and therefore variation in void heights could be attributed to applied pressure at 250 °C. The void lengths for higher pressure samples were approximately half the length of lower pressure samples. These results can be understood with combination of stress-enhanced surface diffusivity due to higher pressure and decreased yield strength and elastic modulus with increasing bonding temperature and pressure.