INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS J. Phys. D: Appl. Phys. 35 (2002) 1433–1437 PII: S0022-3727(02)31331-7 Comparison of experimental and Monte Carlo simulated BSE spectra of multilayered structures and ‘in-depth’ measurements in a SEM* E Rau 1 , H Hoffmeister 2 , R Sennov 1 and H Kohl 2 1 Department of Physics, Moscow State University, Moscow 119899, Russia 2 Physikalisches Institut and Interdisziplin¨ ares Centrum f ¨ ur Elektronenmikroskopie und Mikroanalyse (ICEM), Westf¨ alische Wilhelms Universit¨ at, Wilhelm-Klemm-Straße 10, 48149 M ¨ unster, Germany Received 28 November 2001 Published 31 May 2002 Online at stacks.iop.org/JPhysD/35/1433 Abstract Experimental measurements of the energy distribution of the backscattered electrons (BSE) in a SEM were made with an improved electrostatic toroidal spectrometer and Monte Carlo simulations were used to calculate these BSE spectra. Pure materials, thin films on a substrate and multilayered ‘sandwich’-structures were used to record and calculate the energy spectra. The experimental BSE spectra were compared with the results of Monte Carlo simulations. It is shown that from the BSE spectral curves, the depth and thickness of buried films in multilayered structures or of different subsurface inhomogenities can be estimated. The relation between the penetration depth inside the specimen from which the BSE were reflected and the energy of the BSE is given by the Thomson–Whiddington law. The accuracy for the determination of the depth of subsurface layers is generally about 10–20% using appropriate acceleration voltages. 1. Introduction Modern micro- and nanotechnologies require knowledge of the topographical and compositional properties of objects with fractures or structural inhomogeneities below the surface in the order of several micrometres. In particular, layered objects are typically used in microelectronics, x-ray optics and optoelectronics. In these regions of science and technology, a quantitative and non-destructive determination of the parameters of the three-dimensional objects, e.g. thickness of separate film layers of multilayered objects, is necessary. Several methods have been developed that allow accurate measurements of thickness of films deposited on substrates, e.g. the interference method of two optical rays or the ultrasound vibrometer. These methods, however, have a very poor lateral resolution. Furthermore, some of these methods require optically transparent films. * Dedicated to the memory of Prof. Dr Ludwig Reimer, who has contributed significantly to this work. In some cases, the local measurement of thickness of films deposited on substrates with spatial resolutions of several micrometres is possible by measuring the effective backscattering coefficient of film–substrate combinations and comparing the results with data obtained from bulk samples of the substrate and the film material [1–3]. A variant of this method is to detect the signal at the spectral curve maximum of the spectra of backscattered electrons (BSE) with a subsequent comparison of this signal with that of a reference sample [4] or making calculations over the whole spectrum [5]. The ‘in-depth’ imaging in SEM and quantitative BSE microtomography requires a detailed understanding and correct interpretation of BSE spectra of multilayered structures. In this paper, a new method that is based on the laws of Bethe or Thomson–Whiddington is proposed. The measurements of the energy distributions of BSE were done with the improved toroidal BSE spectrometer adapted to a SEM [6]. Additionally, the values were compared with data obtained from Monte Carlo simulations. 0022-3727/02/121433+05$30.00 © 2002 IOP Publishing Ltd Printed in the UK 1433