Quantitative surface characterization of silicon spheres by combined XRF and XPS analysis for the determination of the Avogadro constant

Abstract

For the quantitative surface characterization of a monocrystalline silicon sphere, PTB has constructed and put into operation an analytical instrument, which combines x-ray fluorescence and x-ray photoelectron spectroscopy techniques. The main objective of this novel instrument is the characterization of the oxide layer and unintentional contaminations, e.g. from hydrocarbons. It is equipped with a ball manipulator allowing measurements at each point on the surface of ball-shaped samples with a diameter of about 93.7 mm. Monocrystalline silicon spheres with this diameter allow a realization of the SI base unit of mass.

Investigation of a cleaning procedure for silicon spheres used in the realization and dissemination of the redefined kilogram via combined spectroscopic and gravimetric measurements

“…A five-axis ball manipulator allows each point on a sphere to be accessible for measurements. The instrument is equipped with a load lock chamber that features a vacuum transfer system to facilitate the transport of the sphere under vacuum [10]. The quantification method relies on a combination of XRF and XPS analysis.…”

“…The ratios of the O Ka to RRS (Si) of each sample and the mass deposition of oxygen from the calibration are then used to fit a curve for the determination of the mass deposition of oxygen on the sphere. In [10] a logarithmic function originating from the fundamental parameter approach was fitted as the calibration curve. Here, as the oxide layers in this study are very thin, linear behavior can be assumed and linear regression is applied to the calibration points.…”

“…Furthermore, the peak areas are normalized by means of the photoelectric cross sections [19], the inelastic mean free path of the photoelectrons [20] and the transmission function of the spectrometer [21]. The atomic fraction of each element is then calculated from the normalized peak areas [10]. The binding energy of oxidized silicon is shifted towards higher values, thus allowing silicon bound in oxide to be distinguished from the silicon in the bulk.…”

“…The C1s peak shows three different contributions that can be attributed to certain chemical binding states. This allowed the amount of oxygen and hydrogen bound in the carbonaceous layer to be estimated, which is explained in detail in [10].…”

“…Combined gravimetric measurements and a spectroscopic quantification of the surface layer mass were conducted. A custom instrument for silicon spheres that combines X-ray fluorescence analysis (XRF) and X-ray photoelectron spectroscopy (XPS) was used for the characterization of the surface layer [10,11]. A mass comparator with a load lock system compatible with the XRF/XPS instrument was used for the gravimetric measurements.…”