Approaches to Quantitative Electron Detection in the Scanning Electron Microscope for Topographic Studies

An electron signal in the scanning electron microscope (SEM) usually consists of contributions caused by different contrast mechanisms. The most common in practice are material and topographic contrasts. Quantification of material contrast is rather a simple matter. A backscattered electron detector placed over a specimen gives mainly material contrast which can be quantified by the use of a multichannel analyser like in the energy-dispersive x-ray spectrometry.In case of topographic contrast two problems arise. One of them is dimensional metrology, especially linewidth measurements in microelectronics, the second is reconstruction of a surface relief. The first problem needs detection conditions at which the results of SEM measurements correspond exactly with real dimensions, the second needs a signal which is related with a known formula to a local surface inclination and a procedure for converting the signal into the surface relief.Experiments in the SEM and Monte-Carlo calculations have shown that results of dimensional measurements depend on an energy of a primary beam, on a type of detected electrons (secondary electrons (SE) or backscattered electrons (BSE)) and on a type of a detector.The use of low primary beam voltages and BSE is advisable, The problem of a poor efficiency of BSE detectors at low primary beam voltages can be overcome by accelerating BSE, after they have passed through a grid rejecting SE, by high voltage applied to a scintillator in a BSE detector.