|M.Sc Student||Adi Karpel|
|Subject||Microstructural Evolution of Gold-Aluminum Wire-Bonds|
|Department||Department of Materials Science and Engineering||Supervisor||Full Professor Kaplan Wayne D.|
|Full Thesis text|
Wire bonding is one of the basic industrial techniques to connect semiconductor devices to the macroscopic circuit arranged on a circuit board. Wire-bonding is usually based on high purity 99.99% (4N) Au wires which are joined to Al pads by the application of ultrasonic and thermal energy. The bond is achieved by the formation of an intermetallic region at the Al-Au interface. During the life-time of the device it is exposed to elevated temperatures that lead to a diffusional process and phase transformation at the bond interface. This results in degradation of the bond by the formation of cracks at the intermetallic region. To date characterization of the bond interface was mostly done using energy dispersive spectroscopy (EDS) mounted on a scanning electron microscope (SEM) of annealed samples, or characterization of model specimens based on Al-Au diffusion couples, under the assumption that the reactions which occur during bonding are in the solid state.
In the present study advanced electron microscopy techniques were applied to understand the failure mechanism of 99.999% (5N) model Au wire-bonds and 4N “real” wire-bonds, by analyzing the microstructure of the joins by scanning/transmission electron microscopy (S/TEM) and EDS. The samples were prepared by the lift-out method in a dual-beam focused ion beam (FIB) system. The microstructure of the as-bonded state and after annealing, for different periods of time at 175°C, was characterized.
From the microstructural analysis it was found that the combined thermal and ultrasonic energies during the wire-bonding process result in a solid-liquid transition immediately at the interface, and cooling results in solidification voids. A simulation of the life-time of the device by thermal annealing at 175°C for up to 100 hours results in a volume increase during intermetallic formation/growth, which results in stress induced cracking, and intergranular fracture due to oxidation of the Al-Au intermetallics. Analysis of annealed 4N doped wire-bonds found that dopant additions, which improve the mechanical properties of the wire, result in reduction of stress-induced crack formations at the Au-intermetallic interface.