Fundamentals of X-Ray Photoemission Spectroscopy
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Abstract Rapid progress in the development of new electronic materials and the steady maturation of silicon-based technologies has resulted in a host of novel electronic devices in which the active region of the structure is confined to an interface or a surface. The chemical, electronic, and physical characterization of surfaces and interfaces in semiconductors and insulators is of critical importance to manufacturing process control as well as to the fundamental electron physics and materials science which support microelectronic device research.
Introduction X-ray photoemission spectroscopy (XPS) is one of a family of surface characterization probes based on the detailed energy analysis of electrons emitted from a sample surface into ultrahigh vacuum. Its primary importance derives from the low level of radiation damage introduced into the sample by soft x-ray excitation, the extent of chemical b o n d i n g a n d c o m p o s i t i o n a l details obtainable from peak position analysis, and its extended observational depth (20-60 A) which make possible the analysis of subsurface or interface structure. Compared with other surface sensitive electron spectroscopies such as Auger electron spectroscopy (AES) and low energy electron loss spectroscopy (LEELS), the primary limitations of XPS are the poor spatial resolution of the x-ray beam (not less than 100 /im) and the relatively low photon fluence which limits the spectral accumulation time to minutes or hours for reasonable signal-to-noise ratios. Consequently, it is best executed in conjunction with complementary surface spectroscopies such as AES, LEELS, and secondary ion mass spectroscopy (SIMS). Applications of XPS to microelectronic science and technology range from basic materials research to analytical support and process m o n i t o r i n g . Some experiments enabled by XPS in electronic materials research include: • detailed analyses of the chemical and electronic structure of buried interfaces;1,2 • determination of surface stoichiometry, chemistry, etching residues, and Fermi level positions in semiconductors, insulators, and metals; • analysis of interfacial reaction chemistry during growth of heterojunctions and contact formation;
• structural analysis of surface bonding geometries through photoelectron diffraction;3 • direct measurement of valence band offsets in heterojunctions;1-4-5 and • extensive analysis of radiation-damageinduced structural modifications. Electron Spectrometer E-AE
and surface sensitivity. The following paragraphs include a short overview of the fundamentals of XPS and its application to semiconductors and insulators, and a discussion of depth profiling requirements for interface analysis. To illustrate a specific application, this article discusses the analysis of the Si/Si0 2 interface in gate oxide quality, metal-oxide-semiconductor (MOS) structures.
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