Imaging and analysis of subsurface Cu interconnects by detecting backscattered electrons in the scanning electron microscope

Abstract

Cu-Si O2 -Si Nx interconnects that were located 0.65-2.7-μm below the surface of silicon-integrated circuits were imaged in a scanning electron microscope and a transmission electron microscope with a scanning attachment by detecting backscattered electrons (BSEs) with an incident electron-beam energy (Eo) in the range of 30-400 keV. BSE images could be used to detect voids in subsurface Cu interconnects, even in regions covered with upper level Cu lines or vias. As Eo was increased from 30 to 400 keV, structures could be seen as a result of atomic number (Z) contrast farther below the surface while structures closer to the surface had reduced Z contrast. The subsurface beam diameter was measured from BSE images as a function of Eo and depth below the surface. For all Eo, the subsurface beam diameter initially rapidly increased with Si O2 overlayer thickness but, for 150 keV, a leveling off in the beam spread was seen for depths >1.7 μm. Beam broadening affected whether the TaNTa liners that surrounded the Cu conductors could be seen at the edges of the lines; this contrast was observed only when the subsurface beam diameter was ≤1.5× the liner thickness. The BSE information depth for imaging 0.2-μm -sized voids in subsurface Cu-Si O2 -Si Nx interconnect structures at 30 and 150 keV was estimated to be 0.65 and 3 μm, respectively. © 2005 American Institute of Physics.