In Situ Infrared Observation of Hydrogenation, Oxidation, and Adsorption on Silicon Surfaces in Solutions
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INTRODUCTION In Si device fabrication, HF acid is widely used to produce oxide-free hydrogenated Si surfaces [1]. However, it is well known that the surface may become oxidized during the subsequent rinse in water containing dissolved oxygen [2]. The existence of any oxide on the Si surface degrades the electrical properties, when low resistance metal contacts and high dielectric capacitors formed on the surface. Recently, it was reported that thin gate oxides formed out of hydrogenated surfaces were more reliable than that out of chemical oxides formed by conventional cleaning procedures [3]. Precise understanding of the initial oxidation of hydrogenated surfaces is important. It is widely accepted, based on experimental evidence obtained by ex situ techniques, that the oxidation of a hydrogenated Si surface begins with oxygen attaching to the back bond of surface Si atoms when oxidation occurs in pure water containing dissolved oxygen and in a dry oxygen atmosphere
[4-8]. In this paper, we investigated surface oxidation in an H 2 0 2 solution and in ozonized water to examine the affects of the redox potential of each. Both solutions are nearly neutral but are different in redox potential. In addition, oxidants such as H 2 0 2 and 03 are intentionally added to solutions used for surface cleaning processes to dissolve some metallic contaminants and to passivate the surface with chemical oxide. Little of the oxidation chemistry is known regarding solutions with a large redox-potential. The progress of oxidation and the consumption of the Si-hydrides structure were monitored using in situ infrared spectroscopy. We used Si (100) and (11), whose major surface structures are di-hydride and mono-hydride, respectively, and examined the relationship between the oxidation reaction and the surface structure. The adsorption of surfactants in an HF solution is also described in this paper. EXPERIMENT Non-doped, FZ-Si prism-shaped wafers which were polished on both sides were used for Fourier transform infrared attenuated total reflection (FT-IR ATR) analysis. We used both (100) and (111). The size of the ATR prism was 53 x 53 mm, and the thicknesses were 1.0 mm for (100) and 0.5 mm for (111). IR analysis was performed using a Nicolet 740 FT-IR spectrometer 63 Mat. Res. Soc. Symp. Proc. Vol. 448 ©1997 Materials Research Society
and an HgCdTe detector cooled with liquid N 2 . The measurement resolution was 4 cm-]. Polarization experiments were performed using wire grid polarizer. We developed a specially designed chemical cell to examine the Si surface in solution [9]. After mounting a conventionally pre-cleaned sample in the chemical cell with ATR geometry, we introduced a 0.5% HF solution into the cell. The surface oxide was then removed and the surface Si bonds were terminated with hydrogen atoms. An oxidizing solution was then introduced for a specified time. To allow sufficient time for spectrum accumulation (typically 1000 times for 10 min), we exchanged the oxidizing solution with water to pose the oxidation reaction.
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