Strain Dependent Diffusion in the Dry Thermal Oxidation Process of Crystalline Si
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STRAIN DEPENDENT DIFFUSION IN THE DRY THERMAL OXIDATION PROCESS OF CRYSTALLINE Si
C.H. BJORKMAN and G. LUCOVSKY Departments of Physics and Materials Science & Engineering, North Carolina State University, Raleigh, North Carolina 27695-8202, U.S.A.
ABSTRACT We have investigated the influence of strain on the rate of thermal oxidation of crystalline Si. This was carried out by performing the oxidation in two steps at 0 850 C with an intermediate annealing step. It was observed that an anneal at 1050 0C increased the oxidation rate 20% compared with an anneal at 850 0 C. At the same time, the films annealed at higher temperatures exhibited lower levels of strain, as determined by infrared spectroscopy, in the oxide layer grown before the anneal. These results have been interpreted in terms of a strain dependent diffusion model for the thermal oxidation process.
INTRODUCTION Many models have been proposed for the initial rapid dry oxidation of silicon. Among them are: (i) the viscous flow model of Nicollian and Reisman [1]; and (ii) the strain dependent diffusion model of Doremus [2]. The main difference between the two models is that (i) is interface reaction rate controlled and (ii) is diffusion controlled. Previous work [3-5] has shown that the strain varies across the oxide film thickness and that the thickness-averaged strain is a function of the total film thickness. Therefore, if diffusion is determined by the strain in the film, the thicknessaveraged diffusion coefficient would be thickness dependent as well as temperature dependent. The work presented here shows that the oxidation rate is determined by bulk oxide properties, and that the strain dependent diffusion model is a good candidate for describing the thermal oxidation of Si.
EXPERIMENTAL PROCEDURES AND RESULTS P-type (100) silicon wafers with resistivities in the range of 10-30 Q-cm were thermally oxidized in two steps at 8501C in an dry 02 ambient with an intermediate annealing step. First, 300 A of Si0 2 was grown followed by a furnace anneal in N2 at either 850 0C, 950 0 C or 1050 0 C. The second oxidation step at 850 0 C added approximately 270 A of Si0 2 . The samples were then incrementally etched back and infrared spectroscopy was used to determine the frequency of the dominant SiO 2 absorption band at each thickness. The increases in oxide thickness during the second oxidation were 236 A, Mat. Res. Soc. Symp. Proc. Vol. 239. 01992 Materials Research Society
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300 (n)
(D _0 2000)
0
100U)
(D
C) 0 0-
Anneal: 850 0C Anneal: 950 0C Anneal: 1050 0C
Fig. 1. Increase in oxide thickness during the second oxidation.
268 A and 294 A for the films annealed at 850'C, 9501C and 1050 0C, respectively, as is shown in Fig. 1. Up to 10 A of the thickness difference between the 850 0C and 1050 0C anneals can be explained by the volume expansion that occurs during high temperature annealing. The remaining difference between the films annealed at 8500 C and 1050 0C then corresponds to an increase in the average oxidation rate of about 20%. These res
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