Charged particle beam apparatuses have many functions in a plurality of industrial fields, including, but not limited to, inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems. Thus, there is a high demand for structuring and inspecting specimens within the micrometer and nanometer scale.
Micrometer and nanometer scale process control, inspection or structuring, is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices, such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams, due to their short wavelengths.
For instance in semiconductor manufacturing, the trend concerning a circuit pattern formed on a wafer is moving towards micro-fabrication. Hence, the process monitoring of the pattern formation is of increasing importance. For evaluating a very fine pattern having a Critical Dimension (CD) at a wafer surface, e.g. a pattern of a dimension in the order of several tens of nanometers, a Critical Dimension Scanning Electron Microscope (CD SEM) has been in use. With a focusing lens the CD SEM focuses an electron beam emitted from an electron gun onto the wafer and scans over the wafer surface two-dimensionally using a scanning coil. Secondary electrons generated by the electron beam at the wafer surface are captured by a secondary electron detector, and the signal thus obtained is recorded as an image. The amount of generated electrons varies depending on concaves and convexes on the surface of the wafer. Therefore, the SEM image can be used as an indication of the surface topography of the wafer. For instance, a sudden increase of the secondary electron signal seen at an edge portion of the wafer pattern allows for determining the position of the edge in the SEM image and for measurement of structural dimensions at the wafer surface. However, the apparent beam width (ABW) limits the use of a CD SEM system to line widths above about 10 nm.