Infrared Spectra of Ultra-Thin SiO 2 Grown on Si Surfaces
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ABSTRACT The structure of thin Si02 films thermally grown on Si(100) and Si(111) surfaces has been characterized by using infrared internal reflection and x-ray photoelectron spectroscopy. It is found that the infrared absorption peak due to the LO phonon mode originating from the Si-O-Si stretching vibration shows a considerable red shift in the thickness range below 30A. This red shift is interpreted in terms of the compressive stress near the interface. INTRODUCTION The atomic scale characterization of the SiO 2 /Si interface has become more and more important for the development of advanced MOS devices. The quality of the bulk SiO 2 as well as the nature of the SiO /Si interface which influences the performance and its reliability of MOS devices is thought to be strongly related to the chemical structure of ultra-thin SiO? grown on Si surfaces. The Si0 2/Si interface has been extensively studied by a variety of analytical techniques such as XPS [1,21, Auger electron spectroscopy [3], TEM [4], FT-IR [5-7] and positron annihilation [8]. The local atomic structures of the ultra-thin Si0 2 film and the Si0 2 /Si interface have not been fully understood yet [9]. Namely, the composition of suboxides in the interface [13], the strain in the SiO 2 network near the interface [5-7], microvoids in SiO 2 [8] and the possible existence of a crystalline quartz phase at the interface [4] are still crucial issues to be studied. In this paper, the SiO 2/Si system has been systematically studied as a function of oxide thickness by FT-IR-ATR (Attenuated Total Reflection) and XPS in order to gain a better understanding of the structure of very thin SiO 2 near the interface.
EXPERIMENTAL Czochralski grown, p-type Si(100) and Si(l 11) wafers with a resistivity of -lOQ-cm were cleaned by conventional RCA cleaning steps and dipped into 4.5% HF (pH=-1.1) or 40% NH 4 F (pH-8) to remove the chemical oxide (-10A). Finally, the wafer was rinsed in pure water for lmin to eliminate surface Si-F bonds. The thermal oxidation was carried out at a temperature of 800°C in dry 02 or 1000°C either in dry 02 or in a 2% 0 diluted with N 2 . Depth analysis of the chemical structure of the ?iO2/Si system was performed by FT-IRATR and XPS measurements at each Si0 2-thinning step by dilute HF (0.1-0.5%) etching followed by a pure water rinse for 1min. The average oxide thickness was determined from the integrated absorption intensity o f the Si-O-Si stretching modes (1000-1300cm"1) and the integrated intensity ratio of the chemically shifted Si(2p) spectrum originating from the oxide layer to the metallic Si signal from Si substrate. Fairly good correlation between the ATR oxide thickness and the XPS one was obtained. The oxide thickness was also calibrated by ellipsometry for thick SiO The atomic scale morphology of the wafer surfaces before and after oxidation or dilute HF etching of SiO 2 was evaluated by AFM (Atomic Force Microscope). For the purpose of characterizing the SiO 2 lattice vibrational modes normal to and parallel to the surface, p-polar
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