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December 2, 2003 Electronic Structure of Thin Film Silicon Oxynitrides Measured Using Soft X-Ray Emission and Absorption SpectroscopiesC. McGuinness1, D. Fu1, J.E. Downes1, K.E. Smith1, G. Hughes2, and J. Roche2 The elementally-resolved valence band electronic structure of a thin film silicon oxynitride gate dielectric used in commercial device fabrication has been measured using soft x-ray emission and absorption spectroscopies. Specifically, the valence and conduction band partial density of states in the interfacial region of both the nitrogen and oxygen states was determined. The element-specific band gap for the O 2p states was measured to be 8.8 eV in the interfacial region, similar to that of pure SiO2. However, the band gap for the N 2p states in the interfacial region was measured to be approximately 5 eV.
The silicon oxynitride samples under investigation in this study were device-grade materials, where the nitration treatment consists of annealing the pre-grown SiO2 in an NH3 environment at elevated temperatures. This treatment results in a non-uniform distribution of nitrogen in the layer, with preferential build-up at both the Si-SiO2 interface and the outer surface. Figure 1 shows the N 2p PDOS for SiOxNy as measured in this
experiment. Also shown are SXE spectra from bulk
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The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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The material properties of SiO2 gate dielectric are crucial to the
behavior of metal-oxide-semiconductor field effect transistors. SiO2
possesses a relatively low dielectric constant, which leads to
increased leakage currents in submicron devices. One favored
replacement dielectric material currently in use is SiOxNy (silicon
oxynitride). While much progress has been made in understanding both
the process of oxynitride film formation and the macroscopic
dielectric and device properties of these films, the basic
electronic structure of the nitrogen-rich interfacial layer remains
poorly understood. We have made the first direct measurement of the
element-specific O 2p and N 2p valence and conduction band partial
densities of states for an ultrathin commercial SiOxNy layer. The
measurements were made using synchrotron radiation-excited soft
x-ray emission (SXE) spectroscopy and soft x-ray absorption (SXA)
spectroscopy on beamline X1B at the NSLS. SXE is a unique probe of
the bulk element-specific electronic structure of solids. SXE
spectra reflect the occupied valence band partial density of states
(PDOS — the valence band density of states resolved into its orbital
angular momentum components). SXE measurements are also
element-specific and are bulk-sensitive, with a sampling depth of
well over 100 nm. Finally, as a “photon in - photon out”
spectroscopy, SXE can be used to make electronic structure
measurements on insulating samples, such as SiOxNy. Thus, SXE is in
many ways a more useful probe of buried interfaces than
photoemission spectroscopy, which, as a “photon in - electron out”
spectroscopy, typically measures the electronic structure of solids
within less than 1 nm of the vacuum-solid interface. Our SXE
measurements were made with a Nordgren-type grating spectrometer,
which is the only one of its kind at the NSLS.
-Si3N4 and gas
phase N2. The ability of SXE and SXA to measure the
elementally-resolved band gap in solids is immediately clear from
Figure 1. The N 2p-derived band gap for SiOxNy is approximately 5 eV,
which is much less than the 8.8 eV for bulk SiO2. Such
elementally-resolved measurements are not possible using
photoemission spectroscopy, which is the standard probe of
solid-state electronic structure.