Determination of the amount of gas adsorption on SiO₂/Si(100) surfaces to realize precise mass measurement

Abstract

The adsorption isotherms on SiO 2 /Si(100) surfaces were measured using a vacuum mass comparator. Samples with a surface area difference of 816.6 cm 2 were used for the measurement, and a substitution weighing method was adopted to reduce the uncertainty due to the drift and non-linearity of the indication of the mass comparator. We measured adsorption isotherms of water vapour on the SiO 2 /Si(100) surfaces outgassed at a temperature of 500 ˚C and found that dissociative adsorption caused an irreversible increase of 0.028 µg cm −2 with an uncertainty of 0.004 µg cm −2 (k = 1). We also found that the physical adsorption of water molecules on hydroxylated surfaces had a monolayer capacity of 0.004 µg cm −2 with an uncertainty of 0.002 µg cm −2 (k = 1). In addition, the adsorption isotherms for ethanol vapour and n-octane vapour, which were different from water vapour in adsorption properties, were measured and analysed.

Realization of the kilogram by the XRCD method

“…The mass of adsorbed gas (such as water vapour) on the sample surface can be evaluated. Mizushima measured the mass of chemisorbed water on Si wafers by using this method [64]. Adsorption isotherms of water vapour on the Si wafer surface, outgassed at a temperature of 500 °C, were measured, and the chemical adsorption coefficient was estimated to be 0.028 μg cm −2 .…”

“…In the SL calculation, only the CWL has to be taken into account, since the PWL is desorbed in the reflectometer vacuum chamber. The CWL influence on the measured OL thickness is corrected using the data published for chemisorbed water [64].…”

“…The uncertainty of this thickness was estimated using the results of surface analysis from the previous measurement [20]. b Calculated from data reported by Mizushima [64]. c The data for d PWL came from comparison of weighing results of the two spheres in nitrogen gas (at a pressure of ~1200 Pa) and in water vapour (at a pressure of ~1200 Pa) [21].…”

Determination of the Avogadro constant by the XRCD method using a28Si-enriched sphere

“…In the surface characterization by PTB in 2015 and that by NMIJ in 2017, the determined CWL thicknesses are strongly correlated because the thicknesses were estimated using the same literature data [34]. In the surface characterizations by PTB and NMIJ in 2015, the determined thicknesses of the CWL and CL are strongly correlated because PTB and NMIJ used the same thicknesses of these layers to derive the Si core volumes [8].…”

On the Transition from Bulk to Ordered Form of Water: A Theoretical Model to Calculate Adhesion Force Due to Capillary and van der Waals Interaction

“…The n mon is the number of adsorbed molecules in one full monolayer per unit area, r A is the cross-sectional area, and m is the diameter of the adsorbate molecule (for water, r A is 0.77 nm 2 and m is approximately 2.75 Å ) [14,15]. The typical values of C and n mon for different materials have been measured and it was shown that the film thickness is strongly dependent on the hydrophilicity of the adsorbent [16].…”