Ultramicroscopy 8 (1982) 137 144 137 North-Holland Publishing Company RESOLUTION IN SURFACE SCANNING ELECTRON MICROSCOPY OF BULK SAMPLES A. N. Broers IBM, East Fishkill, Route 52, Hopewell Junction, N. Y. Ultimately resolution in today's surface scanning electron microscopes is determined by electron-sample interactions and not by the capabilities of the electron optical systems. In other words, it is not possible to resolve surface features on bulk samples (i.e., samples that are opaque to electrons) that are separated by as small a distance as might be expected from the minimum beam diameter. Tl~e minimum beam diameter in a standard secondary electron SEM is set by the spherical aberration of the final lens and diffraction to less than 2.0nm, but secondary electrons are excited up to several nanometers away from the incident electrons. As a result, the best point to point resolution for secondary electron images of features on the surface of true bulk samples has been 5nm - 10nm. High energy electrons elastically scattered from a very shallow surface layer are also used for imaging the surface of bulk samples. A shorter focal length final lens is used in this case because the sample can be immersed in the magnetic field. This allows the beam diameter to be reduced to 0.5nm but again interaction phenomena limit resolution. Finite penetration of the electrons before they are scattered apparently limits point to point resolution to about 2nm. Contrast is high,however, and ultimately it may be possible to "see" single atomic surface steps. The secondary electron surface scanning electron microscope electron surface image provides higher resolution and contrast (SEM) 1 has become almost as ubiquitous as the optical micro- scope. SEM images are easy to interpret and sample prepara- tion, particularly for electrically conducting samples, is very simple. The SEM provides higher resolution and greater depth of field than the optical microscope, and a variety of special operating modes provide information about electrical and magnetic properties of the sample that cannot be obtained optically. SEM resolution is much higher than that of the optical micro- scope, but it is not as high as that of the TEM (Transmission Electron Microscope) or of the STEM (Scanning Transmission Electron Microscope). There are two major reasons for this. First, in order to collect secondary electrons efficiently, the sample has to be placed outside the magnetic field of the final lens. This means the microscope must have a final lens with a longer focal length than the lenses used in transmission instru- ments where the thin sample can be immersed in the lens field. Longer focal length means higher aberrations and these aberra- tions, combined with the lower accelerating voltage generally used in SEM's, lead to a minimum beam size up to six times larger. Lower accelerating voltage is used to prevent the elec- tron beam from penetrating completely through surface protru- sions, thus producing secondary electrons from both sides of the protrusions and "flairing" out these regions of the image. The larger beam size is not as important as it might seem, however, because the second problem encountered with the secondary electron surface image is that the size of the area from which secondary electrons are emitted is even larger than the beam. The best point to point resolution measured in a secondary electron SEM is about 5rim, although, as we shall see, the beam diameter can be 2nm. In attempts to avoid these restrictions, methods have been developed that allow surface images to be formed with primary electrons scattered from the sample. In this case a short focal length lens can be used because the sample can be immersed in the lens field. This means the beam can be much smaller, but again electron interaction phenomena apparently limit resolu- tion. The distance below the surface from which electrons are scattered is such that the area of the surface that affects the image is again larger than the beam. Nonetheless, point to point resolution is reduced to about 2nm, The high energy than the secondary electron image, but is generally not as useful because the size of the sample is constrained by the necd to fit it between the lens pole-pieces, and the sample has to be examined at a relatively steep angle. (>45 ° from nor- mal). There is the possibility for a method that completely avoids "interaction volume" limitations. This is to form a scanning image similar to the glancing angle projection image reported by Yagi 2. Here contrast arises from interference between waves excited from the top and bottom of surface steps, and does not depend on the types of excitation phenomena used in the other surface images. This phase-contrast image is restrict- ed to smooth samples, and to viewing at very steep angles. The angle of incidence (88 ° from normal) is so steep that the image is virtually one dimensional, but resolution approaching atomic scale can be obtained. Cowley has investigated this type of image. 3 Electron Optics The diameter of the electron beam in an SEM depends on the accelerating voltage (V(v)), the normalized brightness J3(A/cm2.steradian.V) and axial energy spread 6V(v) of the electron gun, and on the aberrations of the final lens. With electron guns employing field emission or lanthanum hexabor- ide cathodes it is possible to closely approach the theoretical minimum (dlim) set by the final lens spherical aberration coef- ficient Cs(m) and diffraction and still retain adequate current to form an image with acceptable signal to noise ratio (S/N). dlinl = 0.9 Csl/4 )~3/4 (m) - wavelength of the electrons (m) This relationship assumes that chromatic aberration and astig- matism are negligible, For SEM's that use thermal cathodes this is not valid, because Boersch effect 4 typically increases chromatic spread to the point that chromatic aberration is more important than spherical aberration. 5 The degree to which dli m is approached by guns of different brightness and energy spread can be seen in fig. 1. Fig. 1 0304-3991/82/0000 0000/$02.75 © 1982 North-Holland