ISSN 1027-4510, Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2012, Vol. 6, No. 6, pp. 911–917. © Pleiades Publishing, Ltd., 2012. Original Russian Text © E.I. Rau, A.A. Tatarintsev, 2012, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, 2012, No. 11, pp. 47–54. 911 INTRODUCTION In recent years, a comparatively simple and obvi- ous pattern of dielectric-target charging under elec- tron-beam irradiation has been substantially revised, defined more exactly, and partially reconsidered. The electron-induced charging phenomenon turned out to be far more complicated than the fundamental depen- dence between the secondary electron (SE) emission coefficient and the primary-electron energy [1–4]. Accordingly, through the use of a scanning electron microscope (SEM), interpretation of the image con- trast of dielectrics was transformed and modernized from a simple representation based on a change in the total electron emission coefficient σ with the energy of irradiating electrons E 0 [4, 5] to a more adequate and correct concept allowing for both experimental condi- tions and new physical mechanisms of local charging of dielectric targets. It has been recently ascertained that the image contrast is affected by the following factors: the resid- ual atmospheric pressure in the SEM vacuum column, SEM operating distance, and position (below or above the objective lens) of the SE detector [8]. Under cer- tain experimental conditions, the contrast of images of charged dielectric regions can invert even at the same energy Е 0 [7, 8]. Thus, both the effect of the electron- probe charging of dielectrics and the causes of contrast formation are rather complicated and, hence, require further examination and refinement. In this paper, the results of investigations, which disclose new significant features in the mechanism behind the formation of the contrast within images of locally charged dielectrics, neither considered nor taken into account previously, are presented. In par- ticular, tertiary electrons, emitted from the SEM lens surface under the action of the SEs of a sample and accelerated to high energies in the electric field of charges of an irradiated target, are demonstrated to play an important role. It has been found that image- contrast inversion can occur at energies Е 0 smaller than the so-called critical crossover energy Е 2c of pri- mary electrons. In the above energy range, it is assumed that σ > 1. The aforementioned investiga- tions can be regarded as the next step toward a more complete understanding of the mechanism of dielec- tric image formation in SEM and the charging phe- nomenon as a whole. STANDARD MODEL OF LOCAL DIELECTRIC CHARGING BY ELECTRONS WITH MODERATE ENERGIES First of all, the specificity of the dielectric image contrast is determined by the fact that the total elec- tron emission coefficient σ = η + δ depends on the energy of the irradiating (primary) electrons Е 0 . In our estimates of the contrast, for the detected signal pro- portional to σ, it is assumed that the coefficient η of backscattered electrons (BSEs) is defined as (1) where β = 0.4 + 0.065ln(E 0 ) and Z is the atomic num- ber of the target material [9]. ( ) 2.5 0 ( , ) 1 exp 0.0066 , ZE Z -   η =β - β   Contrast within Images of Locally Charged Dielectrics in Scanning Electron Microscopy E. I. Rau a, b and A. A. Tatarintsev a a Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences, Chernogolovka, Moscow oblast, Russia b Faculty of Physics, Moscow State University, Moscow, Russia Received February 12, 2012 Abstract—Investigations of dielectric materials via a scanning electron microscope are accompanied by definite difficulties associated with their charging under the action of electron irradiation. Certain aspects intrinsic to the formation of the contrast within images of locally charged regions of dielectric targets are discussed. Tertiary electrons, which are generated on the surface of the microscope’s lens by the acceler- ated field of charges of true secondary electrons emitted from an irradiated sample, is shown to play a sig- nificant role. The contrast of locally charged dielectric images is demonstrated to invert at small electron- beam energies corresponding to an emission coefficient of greater than unity. The origin of this effect is also explained. DOI: 10.1134/S1027451012110080