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Image Contrast in Energy-Filtered BSE Images at Ultra-Low Accelerating Voltages


Yoichiro Hashimoto , 1,2,3 * Atsushi Muto , 2 Eric Woods , 4 Todd Walters , 4 and David C. Joy 3 , 5 1 Hitachi High Technologies Corp. , 24-14 , Nishi-Shimbashi 1-chome , Minato-ku , Tokyo 105-0003 , Japan 2 Hitachi High Technologies America, Inc. , 10 North Martingale Road , Suite 500 , Schaumburg , IL 60173 3 University of Tennessee , Knoxville , TN 37996-2100 4 Georgia Institute of Technology , 345 Ferst Drive NW , Atlanta , GA 30332 5 Oak Ridge National Laboratory , 1 Bethel Valley Road , Oak Ridge , TN 37831


* yoichiro.hashimoto.rg@hitachi-hightech.com


Abstract: Scanning electron microscopy (SEM) at ultra-low landing energies reveals information at the topmost layer of the specimen surface, but the backscattered electron image contrast for certain specimens can be unusual. For primary electron energies above 1 keV, backscattered electron (BSE) yields from a specimen increase with increasing atomic number, providing a brighter image for heavier elements. However, at an electron beam energy of 0.2 keV, a reversal occurs; the BSE yield is greater for light elements than for heavier elements. The effect has been demonstrated for specimens of Au and Si in an SEM with an energy- fi ltering BSE detector.


Introduction


Morphological and compositional information at the topmost surface of a specimen is important in the fi eld of materials science and engineering because these parameters strongly aff ect material properties. T e scanning electron microscope (SEM) has been one of the best tools to evaluate the surface structure of materials. At higher electron energies the focused beam spot size becomes smaller, allowing better image resolution; thus SEM images are conventionally recorded at electron accelerating voltages from 10 to 30 kV. High accelerating voltages, however, are unsuitable for obtaining information from the topmost surface because primary electrons then penetrate deep inside the specimen, enlarging the specimen-electron interaction volume and diluting the information related to the topmost surface. Imaging at low accelerating voltages, with a beam of ultra-low landing energies (ULE), defi ned as a landing energy below 1 keV, is better suited to obtain topmost surface information because the interaction volume and the sampling depth of primary electrons within the specimen becomes small ( Figure 1 ).


Historically, there have been


diffi culties with SEM imaging at ULE. One specifi c problem has been chromatic-aberration enlargement of the electron beam spot size, which causes a degradation of image resolution. To overcome this diffi culty in the Hitachi SU8200 series SEM, a beam deceleration technique has been applied along with improve- ments in the electron source, lenses, and detectors. Specifi cally, a negative voltage is applied to the specimen to


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decelerate the primary electrons aſt er the fi nal lens and just before the electron beam interacts with the specimen. As a result, this technique reduces beam aberrations and achieves a small spot size at ULE. High-quality images can be obtained even at landing energies less than 100 eV. Deceleration also reduces sample beam damage and electron dose. It is sometimes diffi cult to interpret SEM data at ULE because the signal behavior at ULE does not follow the conven- tional theory used at the landing energies above 1 keV [ 1 ]. Previous studies provide secondary electron (SE) and backscat- tered electron (BSE) yields as a function of specimen atomic number and of the incident beam energy; these reference data help to interpret the material contrast at ULE [ 2 ]. However, the material contrast obtained by an actual instrument does not always follow these earlier results because of the acceptance capability of detectors and the complex phenomena occurring in the specimen chamber: electron scattering at the chamber wall, generating SE3 electrons. In this article, we describe an investigation of material contrast at ULE in an installed SEM and compare the yields obtained to some reference studies.


Materials and Methods Instrument . Figure 2a shows a general view of the Hitachi SU8230 FE-SEM. T e electron source is a cold fi eld emission (CFE) electron gun, which is the best choice for obtaining a small beam spot size at low beam energies. In addition, the CFE


Figure 1 : Monte Carlo simulations of electron scattering of primary beam electrons in the specimen (material=carbon). (a) Accelerating voltage 1 kV, (b) accelerating voltage 15 kV.


doi: 10.1017/S1551929516000390 www.microscopy-today.com • 2016 May


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