Surface and Interface Strains Studied by X-Ray Diffraction
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AND INTERFACE STRAINS BY X-RAY DIFFRACTION
STUDIED
Koichi. AKIMOTO, Takashi EMOTO, and Ayahiko ICHIMIYA Department of Quantum Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-01, Japan
Abstract We have developed a technique of X-ray diffraction in order to measure strain fields near semiconductor surface and interface. The diffraction geometry is using the extremely asymmetric Bragg-case bulk reflection of a small incident angle to the surface and a large angle exiting from the surface. The incident angle of the X-rays is set near critical angle of total reflection by tuning X-ray energy of synchrotron radiation at the Photon Factory, Japan. For thermally grown-silicon oxide/Si(100) interface, the X-ray intensity of the silicon substrate 311 reflection has been measured. From comparison of the full width at half maxima (FWHM) of X-ray rocking curves of various thickness of silicon oxides, it has been revealed that silicon substrate lattice is highly strained in the thin ( less than about 5 nm) silicon oxide/silicon system. In order to know the original silicon surface strain, we have also performed the same kind measurements in the ultra-high vacuum chamber. A clean Si(l 11) 7x7 surface gives sharper X-ray diffraction peak than that of the native oxide/Si(l 11) system. From these measurements, it is concluded that the thin silicon oxide film itself gives strong strain fields to the silicon substrates, which may be the reason of the existence of the structural transition layer at the silicon oxide/Si interface. Introduction For future LSI devices with high density memories, the necessary gate oxide thickness is estimated to be below 5 nm. For this thickness regime, a rapid decrease of the breakdown charge (Q.) of SiO2 films has been observed in constant current time-dependent-dielectricbreakdown (TDDB) measurements [1]. The electronic properties of such thin films are affected by the structure of the SiO, films. The existence of a structural transition layer at the SiOISi interface has been reported by X-ray photoelectron spectroscopy experiments [2] and studies of the etching speed of the SiO layer [3,4]. Even a crystalline phase has been found at the SiOjSi interface by electron microscopy [5,6], and X-ray diffraction techniques [7]. In spite of a number of structural studies of the SiO/Si interface, there are many remaining problems, such as the initial oxidation mechanism [8,9]. Recently, a grazing incidence X-ray diffraction geometry using synchrotron radiation has been widely used in structural studies of surfaces and interfaces [10]. Those experiments were based on kinematical theory. However, when using bulk reflections of a nearly perfect crystal, such as Si or GaAs, dynamical effects must be taken into consideration. For the diffraction geometry of a small incident angle to the crystal surface and a large angle exiting from the surface, dynamical effects have been considered in detail by Kishino and Kohra [11]. A few application works have been done in the field of X-ray topography [ 12]
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