Microcleavage transmission electron microscopy applied to the interfacial structure of multUayers and microstructure of smaU particles on a substrate

The preparation, characterization, and study of small struct ures are problems of major current activity. 1,2 Many of the physical properties of microstructures such as small particles, muItilayers, or supedattices are strongly connected to their morphology. Structural characterization is mostly accomplished with the use of diffraction (x ray, neutron or electron) and imaging [transmission (TEM) and scanning (SEM) electron microscopy] techniques. In the past, TEM images of epitaxial structures have been obtained from crosssectional specimens observed at different foil orientations. 3 In those cases the fnal thinning of the sample was carried out by ion bombardment. We present here a novel microcleavage technique applied to the imaging of the structure of small particles and superlattices. We believe this is the first time that (a) transmission electron microscopy has been used to image small particles on a substrate and (b) an interfacial image of a multilayer has been obtained. Microdeavage transmission electron microscopy" (MTEM) is a technique which was developed for the fast imaging of microstructures, allowing immediate feedback to the preparation technique and avoiding long and cumbersome ion milling techniques. In MTEM the over!ayer (superlattice, small particle, etc.)/substrate (usually SO combination is cleaved. A standard microscope grid (300-400 mesh, copper) is rubbed against the fresh cleavage and thus randomly shaped fragments of the substrate and overlayer are collected on the grid. Because the fragments are of microscopic size they adhere quite firmly to the grid possibly due to electrostatic charging. We should stress that because the fragments are of microscopic size they cannot be manipulated and they have to be collected by a random process. In most cases, a short search on the grid, easily accomplished with a goniometric head, reveals a fragment as shown in Fig. 1 for a multilayer. The electron beam can thus be oriented at various angles [Fig. 1 (a) parallel and Fig. 1 (b) at an angle 8] with respect to the mliltilayer planes and images of the overlayer and substrate are produced in this fashion. A variety of small particles (Ag, Au, Cu, Cr, Ni, AI) was produced using electron beam gun evaporation (at 1 C -7 Torr) on a room-temperature single crystal ofSi( 11 I). The rate (typically 2-10 A/s) was controlled in a feedback loop using quartz crystal oscillators. MTEM of a nominally 26A-thick Ag film at various angles with respect to the substrate is shown in Fig. 2. The existence of a distribution of sizes is clear from the pictures. A statistical study of the sizes [for Ag on SIC 1] 1) 1 shows them to have a Gaussian distribution centered at about four times the nominal thickness ( 100 A in this case) and a width of about 20 A. This should be compared to the log normal distribution ordinarily found in small particIes. 2 The exact distribution and detailed shape of the particles depend on. the nature of the overlayer. The most striking feature of this work is that as the sample is tilted closer to the parallel direction (0°) the averaging along the direction of the electron beam produces an image which looks like an almost continuous layer on the substrate. We also have prepared a W Ie multilayer on a SiC IiI) substrate, with the W varying from 5 to 20 A, using sputter deposition. A MTEM picture at two different angles with respect to the substrate is shown in Fig. 3. Note that if the sample is perfectly aligned (0°) with respect to the electron beam [Fig. 3(a)] the image shows clearly wen-formed layers with a minimal apparent roughness. However, an image of the multilayer obtained at a tilt angle of ~ 15° reveals additional structure inside the W component of the multi-