Ruchirej Yongsunthon, Doctor of Philosophy, 2002
I have developed the application of Magnetic Force Microscopy (MFM) to perform current imaging with sub-micron spatial resolution. Doing so first required isolating the magnetic interaction in the MFM phase measurement from interfering signal sources and establishing methods of relative quantification that do not depend upon specific tip magnetization. This was accomplished by using a specially designed calibration standard. The accuracy of the MFM peak heights for determining relative current densities for sufficiently large (>3mm) line structures was shown to be well within 10%. The Green’s function vertical propagation of MFM data was also used to show that the MFM response is linear to within 5%. Useful instrumental tip response functions were determined for deconvolution of the MFM data.
Asymmetry in MFM peaks heights was shown to reflect spatial
variation in current density across the line width, known as current crowding.
Test samples were designed to reflect structures of interest in the electromigration
community. MFM signatures of current behavior were empirically correlated
with the defect structures. The crowding was found to be particularly
strong for defect structures that have large spatial derivatives and curvatures.
Finally, a systematic study was performed using MFM signatures to determine the current crowding dependence on defect geometry. For a given type of defect, the degree of current crowding asymptotically vanishes as the defect length increases. For defects of similar lengths, the degree of crowding scales linearly with defect width. There is less crowding near defect structures where the current density changes gradually than in structures where the change is more abrupt. However, there is no consistently measurable difference in current crowding due to sharpness of corners.