Interface Evolution of Au-Au Thermocompression Bonding and Nanotwins
Thermocompression bonded gold-gold (Au-Au) interfaces were investigated using electron microscopy to study the micro- and nanostructure post-bonding. Analysis of scanning electron microscopy (SEM) images of Au-Au interfaces at different bonding temperatures, ranging from 150 °C to 250 °C, produced a relationship between the size, shape and distribution of interfacial voids and the quality of the bond. The lowest bonding temperature, 150 °C, had the highest linear and area fraction of voids, as measured from cross-sectional and plan-view SEM images, respectively. The 250 °C bonded devices had the highest hermetic test yield and the lowest void linear and area fractions of the temperatures measured. Cross sectional images of the voids in the 250 °C sample show that voids appear to exist at the triple boundary between the bonding interface and two grains on one side of the interface.
Transmission electron microscopy (TEM) was used to investigate the growth of grains in Au-Au thermocompression bonded interfaces where one of the Au layers is nanotwinned (ntAu). During bonding, the ntAu was shown to partially “detwin”, resulting in a large grain that extends across the bonding interface, which may be indicative of high quality bonds. TEM images of the interface identified grains in the non-nanotwinned Au layer that either did or did not extend across the interface and electron diffraction was used to determine the orientation of each grain. The grains which had (111) planes oriented within 17° of the (111) plane in the ntAu layer grew across the interface while those with larger orientation angles did not. There was no correlation between cross-interfacial growth and orientation of the (100) plane. Additionally, gallium arsenide (GaAs) nanostubs deposited on silicon (Si) were investigated to study the production of stacking faults, including nanotwins, at different growth temperatures (590 °C, 605 °C and 620 °C) and its possible role as a strain relaxation process in the latticed-mismatched GaAs-Si interface. All nanostubs imaged using TEM showed stacking faults, with the highest area fraction of stacking faults in the 605 °C sample. This sample also was the only temperature to produce imaged nanotwins. The strain the GaAs was measured using the fast Fourier transform (FFT) of the TEM lattice images and was found to be lowest in the 605 °C, leading to a potential correlation between stacking faults and/or nanotwins and strain relaxation.