Analysis of nitride films on silicon substrates by ion beam methods
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S. P. Rangarajan and D. M. Hoffman Department of Chemistry, University of Houston, Houston, Texas 77204-5641
(Received 9 January 1995; accepted 15 August 1995)
The simultaneous determination of light element contamination levels and accurate nitrogen-to-metal ratios in nitride thin films deposited on silicon substrates is demonstrated by using a-particle beam energies in the range 3-4 MeV. In this energy range, significant light element sensitivity enhancements are observed, while the heavy elements show classical Rutherford behavior. The use of resonance scattering at different resonance energies is shown to be the method of choice for analyzing BN films on silicon. Also, a technique is suggested for analyzing very thin films in which an __ aluminum foil substrate and buffer layer are used to enhance sensitivities.
1. INTRODUCTION
The sensitivity for light elements such as B, C, N, and 0 in heavy matrices using standard Rutherford backscattering spectrometry (RBS) is low due to the small cross sections of light elements and the superposition of the substrate “background”, especially when silicon substrates are employed.’ One of the nonclassical techniques used to solve the light element analysis problem is nuclear backscattering spectroscopy (NBS). NBS takes advantage of the increased cross sections available at ion beam energies higher than the Coulomb barrier. When the incident beam energy is close to or higher than the Coulomb barrier of the target atoms, excitations in nuclear levels can be induced to yield nuclear reactions or resonance events. If the incident beam energy reaches a certain level, which corresponds to a long lifetime energy level in the compound nuclei, a strong resonance occurs. The resonance cross section may exceed the Rutherford cross section by a factor of several tens or hundreds. As examples, the ( a ,a ) cross section of I6O at the resonance energy of 3.034 MeV is 22.7 times the Rutherford cross section and the cross section for I2C at the resonance energy of 4.265 MeV is 128 times the Rutherford value. When using a moderate incident beam energy that is higher than the Coulomb barrier for light elements but is still below that for heavy elements, the cross section enhancements provide great improvement in light element sensitivity and, in contrast to nuclear reaction analysis (NRA), information concerning the heavy element content is not lost. Feng et al. have used the NBS technique with an a-particle energy of 5.65 MeV to analyze Si3N4thin films,* and Blanpain et aL3 and Revesz et aL4 have employed resonance energies of 3.034 MeV 31 24
J. Mater. Res., Vol. 10, No. 12, Dec 1995
http://journals.cambridge.org
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and 4.265 MeV for the detection and depth profiling of 0 and C, respectively, in heavy atom matrices. In this paper we demonstrate the use of the NBS technique at optimum energies to simultaneously detect C, N, 0, and silicon in Si3N4 films deposited on silicon. The method is also applicable to other nitrides as well as to carbide, oxide, boride, etc.
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