Synchrotron polychromatic X-ray Laue microdiffraction studies of electromigration in aluminum (copper) interconnects and lead-free solder joints.

摘要

As a powerful tool for studies of structural properties of crystalline materials, synchrotron X-ray microdiffraction technique is applied to investigate the reliability related issues in microelectronic devices, from interconnect lines to Pb-free flip chip solder joints.In this dissertation, I firstly introduced the general electric packaging technology as well as the reliability issues, mainly electromigration, whisker growth, and thermal stress coming from the thermal expansion coefficient mismatch. Then I explained in details the experimental setup of the X-ray microdiffraction beamline in the Advanced Light Source at Lawrence Berkeley National Laboratory, especially the optics for the X-ray beam focusing, the sample stages which bring fast precise micron/submicron resolution realistics, and the detectors used for X-ray fluorescence mapping and X-ray diffraction mapping. Some examples are given to explain how texture, orientation, strain/stress are detected by this technique. Equations are derived in details to show how the full strain/stress tensors are obtained from the Laue diffraction patterns together with necessary assumptions.Electromigration is conducted on Al (Cu) interconnect lines and the diffraction patterns are indexed and analyzed carefully to understand the mechanism of the structural evolution at the early stage of electromigration test before void and/or extrusion formation. Electromigration-induced grain rotation, bending, and polygonization are observed. Theoretical simulation is employed to index the orientations of the activated slip systems, as an indication of the plastic deformation induced by electromigration. The shape of the concave interconnect line is also simulated by fitting the rotation angles of the grains as a function of the distance from the grain barycentre to the centre of the interconnect line.On Pb-free Sn-based solder bumps, electromigration is tested. Steady state is achieved under mild conditions, such as low temperature and low electric current density, and the sample surface remains smooth and flat after electromigration. Transient stress is calculated based on the assumption of pure hydrostatic stress as a reasonable boundary condition. Possible errors of the measurement are discussed. The grain orientation evolution induced by electromigration in Sn solders is also detected. The rotation rate in the bulk of the solder bumps is measured as linear at the early stage of electromigration. The rotation driving force is discussed by calculating the electric resistance as a function of orientation and electromigration time in anisotropic beta-Sn.