The Use of Fresnel Contrast to Study the Initial Stages of The in situ Oxidation of Silicon
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THE USE OF FRESNEL CONTRAST TO STUDY THE INITIAL STAGES OF THE in situ OXIDATION OF SILICON FRANCES M. ROSS, J. MURRAY GIBSON* AND W. M. STOBBS University of Cambridge, Department of Materials Science and Metallurgy, Pembroke Street, Cambridge CB2 3QZ, U.K. *AT&T Bell Laboratories, Murray Hill, NJ 07974 ABSTRACT We describe the analysis of Fresnel contrast seen at a free silicon surface in order to characterise the initial clean surface and the formation in situ of the first atomic layers of oxide. INTRODUCTION The oxidation of silicon, particularly in the initial stages, is still a subject of great interest in spite of much study of both oxide structure and oxidation kinetics in this r6gime. In this paper we describe observations made in the electron microscope of the oxidation of a free silicon surface aiming to determine the structure of the initial oxide layer under controlled conditions of oxide growth. This was achieved by the use of a UHV microscope in which it was possible to heat up the silicon specimen so that crystallographic facets formed, creating an unusual specimen geometry with clean flat areas of silicon surface at a range of orientations. The behaviour of a facet which was parallel to the electron beam was then observed as the bare surface was oxidised by the introduction of controlled amounts of oxygen into the microscope. As well as observing the oxide growth at high resolution, we characterised at lower resolution the Fresnel fringes formed at the specimen edge, which we will show to be highly sensitive to the form of the surface. The in situ observations and preliminary data analysis, which we describe in the next section, demonstrate that interesting changes occur in the nature of the silicon surface which are related, in the final section, to its progressive oxidation. THE in situ EXPERIMENT
The observations were made at 200kV in a JEOL 200CX microscope modified to achieve an ultra high vacuum specimen environment by the use of a helium cooled cryoshield, and to enable heating of the specimen in situ [1]. The specimen itself was a 3mm square piece of silicon of normal (110), dimpled and then chemically thinned to perforation, which had been kept in distilled water until insertion into the microscope to avoid contamination. It was found to be tilted by 4.5±0.3' from the 110 normal with one set of (111) planes remaining vertical (note that it was not possible to tilt the specimen in the microscope). Once in the microscope it was heated to about 930'C using 7.6W of direct resistive power. At this temperature the edges of the specimen faceted onto low-index crystallographic planes with clean areas visible between silicon carbide crystallites which also form at this time (figures 1 and 2). The microscope was aligned and the specimen was then further heated to 890'C for three minutes with the electron beam off to allow silica to desorb from the surface. (SiC can be removed only by higher temperature treatment, which would also remove any thin areas of specimen.) The procedure described here is kn
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