Manufacturing integrated devices with faster clock speeds requires the fine control of three-dimensional gate structures, including line-edge roughness, sidewall angles, and sidewall structures, as well as the control of line widths. In addition, a way to observe underlying structures in devices with multi-layer interconnects is required. As a way to meet future metrology requirements, we propose the use of high-energy scanning electron micrscopy (SEM), which is better suited to the measurement of 3-D structures and underlying structures than conventional low-energy SEM.
High-energy SEM is shown to reveal subsurface structures that are not detected by low-energy SEM. Firstly, a motched gate structure and a polycide gate with a sidewall spacer are observed with spatial resolutions of a few nanometers. The relationship between the thickness of the upper layer and beam energy at which underlying structures are observable is also investigated. The beam should be energetic enough to pass through the upper layer without being broadened, but weak enough that incident electrons are back-scattered by the underlying structures. We were able to observe line structures at depths of up to 800 nm by using incident beams with energy levels from 50 to 100 keV.