475 CHARACTERIZATION OF SURFACE ROUGHNESS IN THIN FILMS AND SMALL PARTICLES R.SILVA*, J.G. PEREZ-RAMIREZ, D. ROMEU, R. HERRERA, R.PEREZ, AND M. JOSE YACAMAN. Instituto de Fisica, UNAM. Apartado Postal 20-364 Deleg. Alvaro Obreg6n 01000, M6xico D.F. and * Centro Regional de Investigaciones B6sicas y Aplicadas de Bahia Blanca. Bahia Blanca Argentina. ABSTRACT The surface roughness of small metallic particles and thin films is de- termined using TEM weak beam dark field images (WBDF) combined with image processing techniques. It is shown that the well known sensitivity of WBDF images to monoatomic steps on the sample surface can be largely improved by the use of image processing. The optimum conditions for step imaging and for the processing for small particles of Au, Pt and Pd and thin films of Au and Ag are discussed. It is found that under most conditions the surface of the particles and the films is very rough at the atomic level in agreement with the current theories of crystal growth. 1.- INTRODUCTION Among the crystal defects more relevant to determine some properties are the ones that affect the surface. Indeed surface defects can play a key role in many phenomena which involve surface interactions. In particular surface roughness is believed to be connected with properties such as cata- lytic activity, resistance to wear, selective oxidation.. .etc. On the other hand it is definitely established that roughness will change a number of properties such as adsorption properties, and even surface stability(l). Surface roughness however is not easy to study in a quantitative way because it is complex and statistical in nature (2). In fact it has been recently shown that surface roughness introduces a problem of dimensionality greater than two (3). Surface roughness is not a planar (or volumetric) defect and should be described in terms of fractals (4). It is of high interest to study the surface roughness at the atomic level. Recently high resolution TEM has been applied to the study of a number of surfaces (5-8). Particu- larly interesting has been the study of surface reconstruction (5) which can be considered a particular case of surface roughness, in which an ordered array of steps are formed. HREM however is not a routine technique and requires a high performance instrument operating at optimum condition. It is also necessary to perform lengthy calculations for matching images with models. Therefore HREM is not convenient and is difficult to apply for routine work. Alternative techniques are always convenient since they may produce similar information. It is well known on the other hand that several TEM techniques such as weak beam dark field (9-10) and bright field axial images (11-12) can under some conditions be extremely sensitive to monoatom- ic changes in thickness. The very weak contrast variations with thickness can be enhanced by image processing techniques. In addition the handling of the pictures by computer allows a large number of particles to be exam- ined. In the present paper we describe the application of WBDF techniques to the study of small metal particles (in the hundred A size range) and metal films grown on insulator substrates. We will show that those techniques can be an excellent alternative for the study of surface roughness. 2.- EXPERIMENTAL METHODS Small particles were grown by evaporation in a UHV environment (10-9 Mat. Res. Soc. Symp. Proc. Vol. 82. ' 1987 Materials Research Society