1997 Surface review and letters  
Introduction. In this paper we report the first preliminary experimental results from a new low voltage electron microscope intended for imaging the surfaces of crystals in UHV systems. Our pointprojection shadow image method uses no lenses and may ultimately be capable of subnanometer resolution. It has much in common with the methods of photoemission "holography" , convergent beam electron microscopy and low voltage point-projection transmission electron microscopy (PPM), all of which involve
more » ... the illumination of a scatterer by a spherical wave with a distant detector. This is the geometry originally proposed by Gabor for in-line holography, and the resulting patterns, which have many interesting properties, have been called shadow images, point-projection images or out-of-focus coherent convergent beam patterns. We first relate the point projection microscope to other methods, then provide a brief review of point projection microscopy in the transmission geometry, before extending this to the reflection case. By comparison with other electron imaging techniques, the PPM is perhaps the simplest of all imaging methods, since no lenses are used. In transmission electron microscopy, a magnetic objective lens is used to image the surface of a thin electron-transparent crystalline sample onto a plane conjugate to the detector screen. In the reflection mode (REM), it is usually the specular Bragg beam, reflected from the surface of a bulk crystal, which passes down the optic axis of this objective lens. In the low energy electron microscope (LEEM), a bulk sample acts as a mirror for electrons of a few volts energy which are imaged by magnetic or electrostatic lenses. Beam splitters and accelerators and decelerators are also used. If the image is slightly out of focus, surface steps will be revealed by a pattern of Fresnel edge fringes. These images are also in-line electron holograms, and methods for removing the Fresnel edge fringes by bringing the image back in to focus are demonstrated below. For the photoemission microscope, the entire surface of the crystal acts as an emitter, and becomes a self-luminous object, which emits electrons excited by incident light or X-rays. Again a magnetic (or more commonly electrostatic) lens is used to image this surface onto the detector screen. In the scanning methods (SEM), a high energy primary beam is focused down to sub-nanometer radius and scanned across the surface of a bulk crystal. Low energy secondary electrons are emitted and collected. Their intensity is used to modulate the intensity of a raster-scanned beam in a display tube, which moves synchronously with the high energy beam inside the microscope. In this way, in UHV scanning instruments, SEM images have been formed of single-atom high surface steps. Similar but more useful images can be obtained by the scanning reflection electron microscopy method (SREM), in which a subnanometer probe is again scanned over a crystal surface at a low angle, but a Bragg reflected beam is used to form the scanning image instead of secondary electrons. Crystallographic contrast is then obtained, sensitive to very small surface strains (eg those around surface steps). Beam voltages of 30-100 kV are needed, together with very short working distance magnetic lenses, to obtain subnanometer probe sizes. In photoemission electron
doi:10.1142/s0218625x97000560 fatcat:olefss7epzey7fhx4pw74sivpe