Nanoscale Evaluation of SiO₂ Density by Stretching Vibrations Measurement using STEM–EELS

SiO₂ is one of the most important dielectric materials used as gate insulators and interlayer films in nanoelectronic devices. Although the density of SiO₂ has been evaluated by the Si-O-Si stretching vibration peak using IR (infrared spectroscopy) [1] so far, there is no established technique to evaluate the density distribution in the nanoscale area. In this work, we develop a technique for nanoscale evaluation of SiO₂ using STEM (scanning transmission electron microscopy) – EELS (electron energy loss spectroscopy). While there are a number of reports on vibrational spectra measured by STEM-EELS [2,3], a method for density evaluation using EELS has not been established yet. We found differences in the stretching vibration peaks reflecting the density between SiO₂ with different atomic structures: α-quartz (2.648 g/cm³) and silica glass (2.2 g/cm³). The dependence of the stretching peak on the thickness of the STEM specimen was investigated in detail by experiment and spectral calculation using a dielectric model.<br/><br/>IR spectra showed that the FWHM (full width at half maximum) of the stretching peak was larger for silica glass than for α-quartz. This corresponds to lower density in the silica glass as a result of greater variation in the Si-O-Si bond angle [3]. We also measured vibrational EELS spectra using STEM with a nanometer resolution and a high energy resolution of 16 meV (FWHM of the zero-loss peak). The FWHM of the stretching peak for silica glass was observed to be 5 meV larger than that for α-quartz, which is consistent with the IR results. On the other hand, simulations based on the Kröger formula [4] showed the experimental stretching peaks reflect both surface scattering and bulk scattering. This simulation suggests the specimen thickness is a key factor when comparing the different samples, because the contribution of surface scattering strongly depends on the specimen thickness. Thus, to establish our evaluation method by STEM–EELS, we investigated in detail the dependence of the stretching peaks on specimen thickness in the range of 30 nm to 100 nm. By comparing the FWHM of the samples with the same specimen thickness, we confirmed that there is a significant difference of more than 4 meV in the stretching peaks between silica glass and α-quartz for all the thicknesses.<br/><br/>We prepared α-quartz locally amorphized by electron irradiation for a longer duration (~50,000 times longer) than the vibrational EELS measurements, and investigated its density using the established evaluation method. As a result, the amorphized α-quartz was found to be denser than silica glass and to have the same density as α-quartz. This suggests that the atomic number of α-quartz is maintained during the amorphization under the low electron current (~10 pA) irradiation, which does not cause significant atomic desorption. These results indicate that vibrational EELS using STEM is a promising technique for local density analysis of SiO₂.<br/><br/>[1] N. Yasuda<i> et al.,</i> Appl. Surf. Sci. 117 (1997) 216.<br/>[2] R.F.Egerton<i> et al.,</i> Microsc. Microanal. 26 (2020) 1117.<br/>[3] K. Venkatraman <i>et al.</i>, Microscopy 67 (2018) i14.<br/>[4] G. Lucovsky <i>et al.,</i> J. Vac. Sci. Technol. B 5 (1987) 530.