Large Area Plan-View Transmission Electron Microscopy Sample Preparation for Direct-Bonded Interfaces

Metal-metal bonding, and in particular Au-Au bonding, is frequently used for both MEMs structures and for integration of  combining different device structures, including III-V and silicon integration.1 High bond strength, accompanied with good electrical conductivity and hermeticity are often achieved when grain growth occurs across the metal-metal bond interface.  Usually, this type of grain growth requires elevated temperatures and longer annealing times and has been studied for Cu-Cu bonding, but not for Au-Au bonding.  However, the conditions which most effectively initiate the grain growth across either of this bonded interface have not been clearly elucidated. 

We established that the preferential growth of a particular grain across Au-Au bond interfaces is enabled through an epitaxial transformation.  Au (111) grains on one side of the interface grew into the other side – albeit with nanovoids remaining at the original interface.  The microstructure of the transformed side consisted initially of nanotwinned, columnar Au while the grains that extended across the interface were initially randomly oriented under thermocompression bonding conditions.  High resolution transmission electron microscopy measurements addressed the orientation dependence of the growing grains.  This indicates that microstructure control in metal-metal bond pairs can impact the resultant grain structure and hence other properties.