& +-I zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA .__ __ @ Nuclear Instruments and Methods in Physics Research B I 13 (1996) 88-92 ELSEVIER NIOMI B Beam Interactions with Materials&Atoms Focused high-voltage electron beams in material science W. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG Sigle Mar-Plunck-Institut fr zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Metallforschung, Institut firr Physik, HeisenhergstraJe I, 70569 Stuttgart, Germany zyxwvutsrqponmlkjihgfe Abstract High-voltage electron microscopes (HVEMS) have become well established instruments in material science during the past three decades. Apart from the high penetration strength which permits investigations of comparatively thick specimen foils, HVEMs provide the means for a variety of in situ specimen manipulations such as heating, cooling, deformation and large angle tilting, which are the basis for a wide range of applications. Four examples will be discussed: (1) imaging with zyxwvutsrqp atomic resolution with the Stuttgart JEOL ARM1250 microscope; (2) the detailed study of the displacement process of crystal atoms from their regular lattice sites; (3) electron channelling, which can supply information about the location of specific atoms in crystal lattices; (4) the emission of coherent bremsstruhlung from small crystal areas. 1. Introduction The paper outlines the specific qualities of electron beams used in modem transmission electron microscopes (TEM) and gives some examples for the wide range of possible applications in material science. Special emphasis will be placed on high-voltage electron microscopes (HVEM) with typical maximum accelerating voltages of 0.5 to 3 MeV as compared to 0.1 to 0.4 MeV in conven- tional TEMs. In Section 2 the typical features of modem HVEMs will be presented. The improvement of the point-to-point resolution that can be achieved by using high-energy elec- trons will be demonstrated in Section 3. Section 4 will focus on the study of radiation damage processes. Here, use is made of the ability of high-energy electrons to displace atoms in crystals from their regular lattice sites. In Sections 5 and 6 applications will be presented which are better known from experiments with positively charged particles or with electrons of considerably higher energies: electron channelling and coherent bremsstrahlung. 2. The high-voltage electron microscope Apart from larger dimensions (typically 10 m high, 30 tons) the overall design of HVEMs is the same as that of conventional TEMs. The high voltage is generated by a symmetrical Co&oft-Walton type generator. This genera- tor type operates vibration-free and supplies a highly stable voltage (AU/U < 10m6). This is of paramount importance to achieve atomic resolution (see Section 4). Electrons are emitted from the tip of a heated tungsten or LaB, crystal at typical currents of some PA. After passing the accelerator tube the electron beam is shaped and focused onto the specimen by several condenser lenses. Only electrons close to the optical axis of the microscope are selected leading to typical electron currents through the specimen of only a few nA. However, owing to the focusing action of the lenses the electron current density is extremely high ( lO23 e mP2 s-’ 1. This makes radiation damage experiments or chemical analyses possible within reasonable time limits and helps to keep film exposure times short. In order to avoid strong scattering, the specimen thickness should not exceed some 10 nm for high-resolution applications. For conventional imaging and diffraction studies the thickness may amount to some microns. This is a gain of a factor of about five compared to TEMs operated at lower voltages. In crystalline specimens part of the electron beam undergo Bragg reflection. Thus the diffraction pattern in the back- focal plane of the objective lens consists of the transmitted beam and a regular pattern of Bragg reflections. The objective lens and subsequent projector lenses form a magnified image of the specimen with typical magnifica- tions of 10’ to 106. The image is observed on a fluores- cent screen or recorded on photographic plates or by a CCD camera. The lenses are all of magnetic type. The magnetic flux is concentrated within a cylindrical pole- piece gap leading to a rotational symmetry of the magnetic field with typical field strengths of some tesla. 3. High-resolution electron microscopy The imaging of the crystal structure with atomic reso- lution is called high-resolution transmission electron mi- croscopy (HRTEM). The image is formed by the interfer- 0168-583X/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0168-583X(95)01375-X