UV-Raman Spectroscopy Study on SiO2/Si Interface

We evaluated stresses and crystal qualities at SiO2/Si interfaces formed in dry O2 an O radical using UV-Raman spectroscopy. The stresses induced by oxidation at 1050C were higher than those at 900C and 1000C. It was also shown that crystal quality of Si and stress at SiO2/Si interface formed in dry O2 depend on the degree of dilution of oxygen and the oxidation time. The SiO2/Si interface formed in O radical exhibits larger compressive stress and better crystal quality as compared with that formed in dry O2. Introduction The oxidation is one of the most important processes using for the fabrication of integrated circuit. It is essential to control and characterize SiO2/Si interface because the increasing impacts of variation in electrical characteristics have become one of the most crucial issues for extremely scaled down LSIs [1]. In this study, we evaluated stress and crystal quality at SiO2/Si interface formed in dry O2 and O radical using UV-Raman spectroscopy. Experiment Si(100) substrates were annealed at 1200C in pure Ar atmosphere for 30min to obtain atomically flat surface [1]. After etching the oxide films in a mixed solution of HCl/HF, the substrates were oxidized in O2/Ar atmosphere at 900C, 1000C and 1050C. The percentage of dilution of O2 with Ar was 10% and 100%. These two oxidation atmosphere are abbreviated hereafter as 10% O2 and 100% O2. Oxide film formed in O radical was fabricated using microwave-excited high-density and lowion-energy Kr/O2 mixture plasma at 400 C. Thicknesses of oxide films formed by dry O2 and O radical were 7 nm. The stresses and crystal quality in Si at SiO2/Si interface were evaluated using UV-Raman spectroscopy excited by Ar ion laser ( =364 nm), whose depth of penetration into the Si substrate was approximately 5 nm. The full-width at half maximum (FWHM) of Raman spectra reflects crystal quality. Wider (narrower) FWHM indicates poorer (better) crystal quality. Raman peak shift shows stress. Lower (higher) peak shift means tensile (compressive) stress. The value of Raman peak shift indicates stress magnitude. Raman spectroscopy system used in this study was described in detail elsewhere [2]. Results and Discussion Fig. 1 shows stress distributions measured at 100 positions over 0.25 x 0.25 cm for the oxide film, which was formed in 10% O2 at 1000 C (red) and 1050C (blue), using UV-Raman spectroscopy. Here, the horizontal axis indicates the Raman peak shift. According to this figure, the compressive stresses were observed at SiO2/Si interface formed in the dry O2 at both 1000 C and 1050C. The Raman peak shifted toward higher wavenumber with increasing oxidation temperature from 1000C to 1050C. However, the approximately similar Raman peak shifts were observed for dry oxidation at 900C and 1000C. It is considered that the viscous flow occurred above the temperature between 1000C and 1050C [3]. In Fig. 2 the dependence of a) FWHM and b) Raman peak shift on Tox (oxidation-temperature) measured for the oxidation in 10% O2 (red) and 100% O2 (blue) are compared with that measured for the oxidation using O radical. In this figure the average of values measured at 100 positions over 0.25 x 0.25 cm is only shown, because the deviations were almost the same in all samples. The observation that FWHM measured for 10% O2 is narrower than that measured for 100% O2 implies better crystal quality in the former case. Raman peak shift was also affected by dilution of oxygen, indicating that longer oxidation time results in a larger compressive stress at SiO2/Si interface. Namely, it was shown that crystal quality and stress at Si substrate surface oxidized in dry O2 depend on dilution of oxygen and oxidation time. For the oxidation using O radical the averaged values in FWHM and Raman peak shift were found to be 2.513 cm and 0.055 cm, respectively. The SiO2/Si interface formed in O radical exhibits larger compressive stress as compared with that formed in dry O2. However, crystal quality at SiO2/Si interface formed in O radical was similar to that formed in dry O2 at 900 C, implying that large compressive stress does not necessarily result in poor crystal quality. References [1] R.Kuroda et al., SSDM, pp. 706-707 (2008). [2]A. Ogura et al., Jpn. J. Appl. Phys. 45. 3007 (2006). [3] E.Kobeda et al., J. Vac. Sci. Technol. B6 (2), Mar/Apr (1988).