High Sensitivity FT-IR-Ras for Silicon Surface Study
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HISAYOSHI OHSHIMA%, TSUNEO URISU'*, YOSHIYASU YAMADA***, AND TADASHI HA'TORI**" Structure and function property", PRESTO, JRDC, 5-9-4 Tokodai, Tsukuba 300-26, Japan "Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Research Laboratories, Nippondenso Co., Ltd., 500-1 Komenoki, Nisshin, Aichi 470-01, Japan
ABSTRACT A novel configuration for Fourier-transform infrared reflection absorption spectroscopy (FT-IR-RAS) has been devised to study the nature of silicon wafer surfaces in the wide IR irradiation region (especially below 1300 cm-1) with high sensitivity. The configuration is basically similar to a conventional one of FT-IR-RAS except that a optical-flat mirror is placed on a silicon wafer surface and the IR beam is incident on the back side of the wafer with a grazing angle. The sensitivity of the novel technique was estimated by the observation of the stretching vibration absorption of the Si-H bond (2070-2150 cm-1) on HF/NH 4F-treated Si(111) surfaces and Si-O bond (1000-1250 cm- 1 ) of chemically oxidized layer ( -0.7 nm ) on Si(111) surfaces. The dependence of Si-H absorption peaks intensities on the composition of HF/NH F solutions was clearly observed. tburthermore, Si-O bond peak corresponding to the longitudinal optical phonon was also detected. 1.Introduction Recently, there has been a considerable interest in the silicon surface chemistry on HF solution etching and consequently hydrogen termination1' 2 , native oxidation 3' 4, and micro-roughness5 , because of its importance to the advance in ultra large scale integration (ULSI) technology. Many analytical techniques such as Fourier-transform infrared spectroscopy (FT-IR), Xray photoelectron spectroscopy (XPS), high-resolution electron energy-loss spectroscopy (HREELS) or scanning tunneling microscopy (STM) have been utilized. In particular, FT-IR attenuated total reflection (ATR) spectroscopy with a prism shaped out of the silicon samples has been used by many authors1,6 because of its high sensitivity for detecting stretching vibration absorption of the Si-H bond on its surface. However, the use of the silicon prism limits the application of the technique to monitor the nature of silicon wafer surface in ULSI industry. Furthermore, the detection of IR spectrum in the range below 1300 cm-' is difficult due to the strong absorption of the silicon prism itself. A germanium prism has been employed instead of silicon prism to overcome these problems.4' 7 Comparing with FT-IR-ATR, FT-IR reflection absorption spectroscopy (RAS) is very simple configuration (no prism is required) and has the potential for overcoming the problems. However, the technique is generally difficult to apply to semiconductor surface studies because its sensitivity depends on the free-electron characteristics. A buried metal layer structure"'9 is one method of surmounting the sensitivity loss. In this paper, we demonstrate a novel simple technique of FT-IR-RAS for the study of silicon surfaces in a wide IR spectrum range with submonolayer sensitivity. The sensitivity an
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