The Effects of Intrinsic In-Plane Stress on the Local Atomic Structure of Thermally Grown SiO 2
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THE EFFECTS OF INTRINSIC IN-PLANE STRESS ON THE LOCAL ATOMIC STRUCTURE OF THERMALLY GROWN S10 2 J.T. FITCH. C.H. BJORKMAN, JJ SUMAKERIS, AND G. LUCOVSKY
Departments of Physics and Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-8202
ABSTRACT The effect of thermal history on the local atomic structure of thermally grown and annealed SiO 2 films is discussed. Thin films of SiO 2 were prepared for study by (1) thermal oxidation of silicon at 850°C and 1050°C in a dry oxygen ambient and, (2) by subsequent rapid thermal annealing at temperatures between 850°C and 1200°C. After thermal processing, the film thickness was reduced incrementally by a wet chemical etchback technique to within 50 A of the growth interface. Relief of intrinsic growth stress by viscous flow was investigated, at each thickness increment, on a local atomic scale by infrared spectroscopy and on a macroscopic scale by beam-bending type stress measurements. Results were analyzed in terms of a Maxwell model. Changes in film thickness and refractive index resulting from viscous flow during annealing were shown to obey a Clausius-Mossotti relationship. Local atomic strain values, calculated from shifts in the IR bond-stretching frequency, showed a linear correlation with measured stress values giving a value for a plane-strain Young's modulus of 4.7 x 1011 dynes/cm 2 . This compares favorably with the literature value of 8.8 x 1011 dynes/cm 2 . INTRODUCTION As the basis for Si metal-oxide-semiconductor technology, Si0 2 is the single most important electronic insulator. High quality interfaces are possible in thermally grown Si0 2 despite the 30% larger Si-Si distance in a-Si0 2 as compared to the single crystal Si substrate.
In
previous
work [1-6]
we have
shown
via infrared
spectroscopy (IR),
ellipsometry, and beam-bending stress measurements that the local atomic structure of thermally grown Si0 2 is quite sensitive to thermal history. Systematic variations in these measurements were examined within the framework of a model for local atomic structure. Growth stress generated at the Si/Si0 2 interface is relieved through changes in the bond angle, 20, at the oxygen atom sites. This paper is a continuation of previous work with emphasis on the non-homogeneous thermal history aspect of oxidation with respect to thickness which is inherent in the growth process. We examine thermal history effects using (1) a wet chemical etching technique to section the films, (2) the Clausius-Mossotti relationship [1,7] between density and refractive index, and (3) an analysis of elastic properties of Si0 2 films using a combination of stress and IR data. EXPERIMENTAL PROCEDURE AND RESULTS Silicon dioxide films for this study were grown on "device quality" silicon wafers at 850°C and 10500C using standard procedures which are described in references 1-5. Rapid thermal annealing (RTA) was performed at temperatures between 850°C and 12000C for 100 sec in an argon ambient with an AG Heatpulse model 210M. After thermal processi
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