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Jean Lapointe
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Scanning Electron Microscope (SEM)
Microscopes are invaluable tools in the fabrication of semiconductor devices but standard optical microscopes are unfortunately limited to the observation of structures with dimensions larger than or comparable to the wavelength of light, hundreds of nanometers. Electron microscopes on the other hand use a beam of electrons instead of light for imaging purposes and can images structures as small as a few nanometers. The accelerating voltage used to generate the beam of electrons can be varied and electrical and magnetic fields deflect the beam and can focus it to a spot slightly larger than 1 nm. An image is formed by scanning this beam over the specimen and detecting the electrons emitted by the specimen as a function of the beam position. The resulting image will vary according to the topology and composition of the specimen scanned. The energy of the impinging electrons is an important parameter when imaging with a Scanning Electron Microscope (SEM) as the smallest spot sizes are obtained at higher energies but insulating materials require lower energies to limit “charging effects”. The energy of the detected electrons also contains valuable information that can be obtained with the proper choice of detector.
Two scanning electrons microscopes are available for imaging and the characterization of devices at the Nanofabrication facility at IMS. A JEOL 6400 with accelerating voltages ranging between 1 and 30 kV and a Hitachi S-4700 with accelerating voltages from 500 V to 25 kV. The older generation JEOL SEM has only a secondary electron detector while the more recent Hitachi also has an in-lens detector as well as a back-scattered electron detector, more sensitive to material composition. Depending on the specimen composition, both SEM can image structures as small as a few nanometers.
