PROJECT TITLE :

Understanding image contrast to optimize procedures for focused ion beam contact level circuit editing

ABSTRACT:

Contact level focused ion beam (FIB) circuit editing (CE) is a powerful methodology to realize full thickness back side CEs with high success rates. Tungsten contacts present themselves as dark circles surrounded by bright rings and extended halos in the ion-induced secondary electron (iSE) images from the FIB. Similarly, polysilicide gates appear as gray bars surrounded by a brighter perimeter. The iSE images are compared to electron-induced secondary electron images from a scanning electron microscope. Elemental data from energy dispersive spectroscopy and topographical data from atomic force microscopy (AFM) evince that topography rather than conductivity plays a determinant role in the contrast in the iSE images. A model emerges whereby the Ga+ ion beam erodes the dielectrics more quickly than the W contacts and polysilicide gate liners, leaving the W contacts and polysilicide gate liners with rounded sidewalls protruding from the floor of the FIB trench. The high yield of secondary electrons from the sidewalls of protruding structures dominates image formation and follows the secant law with a modified exponent. Topographical data from the AFM combined with the secant law suffice to simulate the main features of iSE images although a complete interpretation of the polysilicide gates mandates considering the conductivity of the materials as well.