Chemical Structure of N Atoms Incorporated Into 1 nm-thick SiO 2 /Si as Revealed with the Dissolution and Hydrogenation
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Chemical Structure of N Atoms Incorporated Into 1 nm-thick SiO2/Si as Revealed with the Dissolution and Hydrogenation in Hydrofluoric Acid Naomi Mizuta and Satoru Watanabe Fujitsu Laboratories Ltd. 10-1 Morinosato-Wakamiya, Atsugi 243-0197, Japan ABSTRACT By detecting vibrational frequencies for N–H structures, the chemical states of N atoms at silicon nitride on silicon, SiN/Si, and silicon oxide with nitrogen on silicon, SiO(N)/Si, systems were deduced after being chemically decollated with H atoms in dilute HF solution. Using the fact that the decollation was associated with the dissolution of these films, we obtained their depth profile. The frequency was increased to an extent of 10 cm-1 by the existence of a thin oxide layer between the SiN layer and the Si substrate. The N–H structure bound to the Si substrate was observed at 3390 cm-1 just before the completely removal of the surface layer. We also observed the NH structure of crystal SiN form, whose frequency was unchanged to be 3325 cm-1. The same technique was applied to detect the chemical structure of the N atoms in an ultra-thin SiO(N) with 5% of the N atoms. The frequency peak of the bulk N structure was observed with that of OH structures. The peak was shifted from 3375 cm-1 to 3405 cm-1 at the interface region, suggesting densification of the film in the interface region. Prior to complete removal, we observed the surface state of the N atoms and the H atoms. Then the frequency of the NH was unaffected by the film formed with the various methods. No SiH structure was observed on the resultant film surface during etching. INTRODUCTION Incorporation of N atoms in the ultra-thin gate oxide, SiO(N), for advanced CMOS technology is essential to control impurity penetration through the layer and to improve reliability during high-speed operation1. However little has been known of either the chemical structure at the SiO(N)/Si interface or the depth profile of N atoms in the SiO(N) layer2, 3. It is well known that the Si surface after being removed from the native oxide completely in HF solution is terminated with the H atoms4. This hydrogenated surface can be distinguished in infrared-spectra separately according to the chemical structures and the bound sites5, 6. It was concluded that the surface dangling bonds are terminated with the H atoms, but the bulk structure remains after the HF treatment7, 8. Before applying the idea to the chemical
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structure of N atoms at the SiO(N)/Si interface, we prepared two types of CVD-SiN films on Si substrates with and without a chemical oxide at their interfaces. During dissociating the film, we detected two chemical structures of the N atoms with different N–H stretching frequencies. These result from the fact that the thin oxide is either at the interface of the SiN/Si or not. We then applied this procedure to an ultra-thin oxide/Si system and detected the chemical structure of N atoms. SAMPLE PREPARATION The Si samples were cut 50 x 15 mm with 45 degree bevels on each of the short sides from a 0.
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