An NMR Study of Hydrogen in Si0 2 Films on Silicon
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AN NMR STUDY OF HYDROGEN IN Si0 2 FILMS ON SILICON DAVID H. LEVY and K. K. GLEASON, Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139 ABSTRACT We have used solid state nuclear magnetic resonance (NMR) spectroscopy to study both "wet" and "dry" thermally grown films of SiO 2 on silicon substrates. For the 5000 A wet film, grown at 1050 *C we observed a single Lorentzian line of 6 kHz HWHM (half width at half maximum). For the 500 A dry film, we observed a convolution of two lines: a) a Lorentzian of 4 kHz HWHM and b) a Gaussian of 20 kHz HWHM. The hydrogen distributions in both oxides are interpreted as a function of these lines. INTRODUCTION Thin (100 A to 1 gtm) silicon dioxide films are used as dielectric layers in semiconductor devices. As device dimensions decrease, the silicon dioxide thickness must also decrease, leading to a situation where the Si/Si0 2 interface region (generally estimated to be approximately 30 A thick[l]) plays a significant role in the structure and performance of a given device. It is clear that hydrogen plays a major role in the chemistry of this interface[2], whether through reactions with dangling bond defects at the interface[3-5], or through its presence in non-stoichiometric silicon bonding configurations near the interface[1]. Various techniques have thus far been used to investigate hydrogen at the Si/SiO2 interface: internal reflection IR spectroscopy[61, secondary ion mass spectroscopy[7], and nuclear reaction techniques[8]. The latter two techniques both involve subjecting the sample to high levels of particle beam radiation, thus potentially changing bonding configurations in the sample, as well as causing hydrogen to migrate[8]. IR spectroscopy, on the other hand, provides insight into bonding configurations at the expense of quantitative data on the hydrogen since the oscillator strength of a particular bond is dependant upon its environment[9]. We have chosen to adapt solid state nuclear magnetic resonance (NMR) to the study of thermally grown SiO 2 films since it is a technique that uses very low energy radiation (radio frequencies), and is sensitive to hydrogen bonding configurations, as well as hydrogen internuclear spacings. In particular, we feel that since hydrogen is a quick, small diffuser, it may segregate near defects in the oxide, thus providing a microscopic probe to the structure of the Si/Si0 2 interface. Mat. Res. Soc. Symp. Proc. Vol. 159. @1990 Materials Research Society
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EXPERIMENTAL A) Evaluation of NMR Data The NMR resonance line can reveal information on bonding, spacing, and the quantity of nuclei under investigation. The bonding configuration is determined from a shift in the resonance frequency of a nucleus (the chemical shift). Based on comparison to the (C6H5)n-SiH(4-n) - (C6H 5)n-SiOH(4-n) system, we expect a resonance shift of approximately 3 ppm between SiH and SiOH bonding configurations[10]. This shift would be detectable on our spectrometer with the use of a line narrowing pulse sequence[l
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