© 2005 The Royal Microscopical Society Journal of Microscopy, Vol. 218, Pt 2 May 2005, pp. 180– 184 Received 19 November 2004; accepted 2 February 2005 Blackwell Publishing, Ltd. About the role of boundary conditions on compositional imaging with a scanning electron microscope P. G. MERLI, V. MORANDI & F. CORTICELLI CNR-IMM Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy Key words. Backsattered electrons imaging, boundary conditions, dopant profiling, scanning electron microscpy, secondary electrons imaging. Summary We consider the effects of different boundaries on the visibility of a specimen detail providing a compositional contrast in scanning electron microscopy, operating with backscattered electrons or secondary electrons. An object characterized by a gradual variation in composition, an As-doped region in Si, is investigated. The different boundaries in the cross-sectioned specimen correspond to the absence or presence of a poly-Si layer on top of the implanted region, deposited after the annealing treatment. It is shown that the interpretation model used for image formation is of paramount relevance for under- standing the experimental results, indicating that the boundaries of the doped region are important in hindering or enhancing its visibility. The relevance of experimental parameters such as electron energy and probe dimension is also reported. Introduction In an incoherent sequential imaging process a specimen detail will be visible if its features, having a location defined by the probe, give rise to a number of collected electrons sufficiently different from that of the neighbouring region. More precisely, the visibility of the feature will depend on the difference in the number of electrons counted by the detector when the beam is positioned on it (N A ) or on the neighbouring region (N B ). This difference, which can be simulated with a Monte Carlo code, allows us to compute the contrast: From the contrast it is possible to deduce the threshold cur- rent, i.e. the minimum beam current which must be employed to detect a specific level of contrast, C, between two points in an image for a specific frame time (Goldstein et al., 1981). From the threshold current, instrument characteristics such as gun brightness and aberrations of the objective lens, it is possible to deduce the probe size, which defines the resolution (Merli & Nacucchi, 1993). This quite simple approach is used to explain experimental results concerning the observations with backscattered electrons (BSE) (Merli & Nacucchi, 1993) as well as with for- ward scattered electrons (Merli et al., 2003; Corticelli et al., 2003) in scanning and scanning transmission electron micro- scopy (SEM and STEM) and is based on a straightforward assumption: the physical observation of the phenomenon is represented by the number of detected electrons. Several inter- esting consequences related to this approach, mainly regard- ing the role of the different signal components, the contrast effects related to the electron interaction volume and the energy filtering, have been reported (Merli et al., 1995, 1996, 2001). Arguments connecting the resolution to the radial distribu- tion of the emitted electrons around the beam impact point (Yasuda et al., 1995, 1996) are not able to explain the experi- mental results of compositional images of multilayers and, above all, are not suitable to foresee new issues. Here we consider the effect of the boundary conditions on the visibility of a specimen feature and compare the theoreti- cal results deduced by the two approaches with those obtained experimentally. Materials and methods The specimen detail is represented by a gradual variation in composition: a doped region in a mono-crystalline Si specimen. This was implanted with As at an energy of 20 keV at a dose of 5 × 10 15 atoms cm -2 . The simulated (Lulli et al., 1997) as- implanted dopant profile (Fig. 1) shows a spatial extension of the doped layer of about 40 nm and a maximum concentration of about 5% located at a depth between 15 and 20 nm. The specimen was subsequently annealed at 800 °C for 30 min, a thermal treatment that does not modify the peak position and the spatial extension of the dopant profile. Correspondence to: Dr Vittorio Morandi. Tel.: +39 051 6399144; fax: +39 051 6399216; e-mail: morandi@bo.imm.cnr.it C N N N N A B A B | | ( , ) . = - Max