High Energy Backscattered Electron Imaging of Subsurface Cu Interconnects

The scanning electron microscope (SEM) is a critical tool for in-line defect characterization, critical dimension measurements and failure analysis of semiconductor devices. As devices continue to shrink, the trend in imaging fine semiconductor structures in the SEM has been to use field emission electron sources at low beam energies, < 5 keV, and to detect secondary electrons (SE). This small spot, low beam current configuration can produce high resolution images of the sample surface with minimal sample charging. However, when higher energy incident electron beams are employed and backscattered electrons (BSEs) are detected, images can be obtained with high topographic or atomic number (Z) contrast and detail below the sample surface can be detected [1-5]. A specific application for BSE subsurface imaging is for the characterization of multi-level Cu interconnects passivated with SiO2 or low dielectric constant amorphous dielectrics [6-22]. Here the low Z surface dielectric does not greatly scatter the electron beam allowing the higher Z, Cu-Ta containing subsurface lines to be imaged with BSEs. Defects such as voids in Cu interconnects or metal extruded out of the interconnect can be imaged with one, topdown BSE image, even if the defect is buried under complex, dielectric surface topography or upper level metal. BSE imaging has potential as a technique that is able to characterize subsurface device structures without having to physically alter the sample by cross-sectioning or delayering though electron beam induced damage of devices has been reported [11-12]. Initial work on BSE subsurface imaging of interconnect structures was published in the 1990’s where both SEMs [6-8, 10-15] and transmission electron microscopes (TEMs) with scanning attachments [6, 9, 17] were used to image Al interconnects passivated with SiO2 or SiNx. Since the image resolution was dependent on the beam spread caused by scattering by sample atoms, higher energy incident electron beams were used to reduce beam broadening and to improve the resolution of the BSE images. Energy filtered BSE imaging has been studied as a microtomography technique where a series of images representing specific BSE energies were collected for a given incident electron beam energy [13-15]. Since BSEs which have lost a certain amount of energy are typically generated from a given depth in the sample, an energy-filtered image series represents two-dimensional slices of the sample at various points below the sample surface. Since interconnects are non-uniform in composition laterally, an energy-filtered image can suffer from contrast inversion where a low atomic number layer that is located deep below the surface can have higher intensity than a high atomic number layer located closer to the surface. Therefore, a given BSE energy image will represent a variety of depths below the surface making the microtomography reconstruction difficult. In recent years, there has been renewed interest in using BSE to image subsurface interconnect structures [18-21]. In this paper, subsurface imaging of multi-level Cu interconnects with BSEs is demonstrated by using transmission electron microscopes with scanning attachments with beam energies of 150 keV to 400 keV. The BSE images were used to detect voids in subsurface Cu interconnects and to characterize beam spread at various depths below the surface.