Planar oxidation of strained silicon substrates
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We report here on a series of experiments in which relatively low levels of in-plane bending strain were applied to oxidizing silicon substrates. These were found to result in significant decreases in oxide thickness in the ultrathin oxide regime. Both tensile and compressive bending resulted in roughly the same degree of thickness retardation, although compressive bending typically led to somewhat thinner oxides than did tensile bending. An examination of the experimental data indicate that the principal effect seems to occur in the very early stages of oxidation, with only minor effects on subsequent oxide growth. We hypothesize that the observed oxide thickness retardation is related to straining of the underlying silicon lattice at the oxidation front.
I. INTRODUCTION
It has long been strongly suspected that mechanical stresses generated during the oxidation process have a marked effect on the oxidation behavior of silicon. Because of the industrial importance of silicon oxidation, these effects have been the focus of extensive study. It was not until recently, however, that a direct experimental demonstration that mechanical stress and/or strain can affect the oxidation kinetics of silicon has been given. This was done by Mihalyi, Jaccodine, and Delph,1 who loaded a series of thin silicon strips in so-called fourpoint bending under oxidizing conditions (see Fig. 1). Assuming time-independent mechanical behavior, this has the effect of placing the resulting oxide films on the top and bottom of the strip under constant in-plane uniaxial stress, compressive on top of the strip and tensile on the bottom. The underlying silicon substrate is also strained by the applied bending loads. The results of this preliminary set of experiments indicated that applied compressive bending had a retarding effect upon the growth of the oxide layer, while the effect of tensile bending was ambiguous. The purpose of this paper is to report the results of a much more extensive set of experiments conducted on oxidizing silicon strips using the same loading apparatus. For this set of experiments, enhancements to the experimental procedures have considerably improved the quality of the experimental data obtained from the loading
experiments and, among other things, has allowed us to elucidate much more clearly the relative effects of tensile and compressive bending. II. EXPERIMENTAL PROCEDURES
For this set of experiments, all specimens were hydrogen-terminated following RCA cleaning, a procedure that was not followed in the previous series of experiments. Hydrogen termination effectively halts the growth of native oxide prior to thermal oxidation. This proved to be a crucial step in the specimen preparation process, in that it yielded specimens in uniform condition prior to oxidation. Apart from the inclusion of hydrogen termination, the experimental procedure was that followed by Mihalyi, Jaccodine, and Delph,1 which we recapitulate briefly here. The specimens were fabricated from N-type, 〈100〉 orientation silicon wafers polished o
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