Paramagnetic Defects Related to Positive Charges in Silicon Oxynitride Films
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ABSTRACT Electron spin resonance (ESR) spectroscopy is used to explore the origin of positive charges in silicon oxynitride thin films formed in a N20 ambient. A new type of paramagnetic center in as-grown oxynitrides appeared as a single peak in the ESR spectrum with a zero crossing g value of 2.0006. This value suggests that the paramragnetic center is a kind of E' center. We investigated the quantitative behavior of the positive charge center and the paramagnetic center through the density changes induced by vacuum ultraviolet irradiation and hydrogen annealing. Based on the similar behavior of both types of center, we argue that the paramagnetic centers can account for a significant part of the positive charges specific to oxynitrides. Since the spin density increased with the oxynitridation temperature, the paramagnetic center is closely related to nitrogen incorporation processes. INTRODUCTION Nitrogen incorporation into silicon-dioxide (SiO 2) gate dielectrics in MOS devices can be used to improve critical properties of the gate dielectrics; for example, to obtain higher dielectric strength against electrical stresses and a better barrier against dopant diffusion. However, the generation of a fixed positive charge due to nitrogen incorporation is a well known flatband problem in oxynitrides formed from various gas species, such as NH 3, N20, and NO [1,2]. Fixed charges degrade channel mobility in MOS field effect transistors as a result of Coulomb scattering near the interface [3-5]. The origin of the nitrogen induced fixed charge, though has not yet been clarified at the atomic scale. Electron spin resonance (ESR) is an effective method for examining the paramagnetic centers that act as oxide defects [6]. Through extensive ESR studies of radiation induced oxide defects, the bonding structure of oxide fixed-charge sites has been related to a kind of paramagnetic center. One of the most important paramagnetic centers is the E' center, which consists of a pair of Si atoms with an unpaired electron residing on one side leaving the other side positively charged (0 3=Si 'Si-= 03)[6,7]. For the oxynitrides, two types of paramagnetic centers (the N20 center and the E' center) have been observed after vacuum ultra-violet (VUV) irradiation of the sample surfaces [8]. In that report, however, neither of the centers were discussed in connection with the positive charges specific to as-grown oxynitrides. In this study, we have examined the nature of a new type of paramagnetic center specific to silicon oxynitrides. To investigate the relationship between the positive charge center and the paramagnetic center, we compared how the densities of both centers are changed by VUV irradiation and hydrogen annealing (400°C).
263 Mat. Res. Soc. Symp. Proc. Vol. 592 © 2000 Materials Research Society
EXPERIMENT The oxynitride films in our experiments were prepared by thermal nitridation on p-type Si(100) using N20 gas. The 10-nm-thick SiON films were grown in a furnace at 850-1050'C. As a reference, 10-nm-thick SiO 2 films were
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