Measurement of local thickness by electron energy-loss spectroscopy

We review various methods which can be used to derive the thickness of an electron-microscope specimen from its transmission energy-loss spectrum. We have applied a sum-rule technique to various kinds of specimen, using a variety of electron-optical conditions, and estimate its accuracy to be typically ±10% (or±2nm if larger) over the thickness range 10–150 nm. This method requires no knowledge of the physical or chemical properties of the specimen other than its refractive index (∞ for metal). It involves only a low radiation dose, allowing good lateral resolution (<10 nm) to be achieved even by selected-area techniques.