Critical review of the current status of thickness measurements for ultrathin SiO₂ on Si Part V: Results of a CCQM pilot study

Abstract

Results are reported from a pilot study under the Consultative Committee for Amount of Substance (CCQM) to compare measurements of and resolve any relevant measurement issues in the amount of thermal oxide on (100) and (111) orientation silicon wafer substrates in the thickness range 1.5-8 nm. As a result of the invitation to participate in this activity, 45 sets of measurements have been made in different laboratories using 10 analytical methods: medium -energy ion scattering spectrometry (MEIS), nuclear reaction analysis (NRA), RBS, elastic backscattering spectrometry (EBS), XPS, SIMS, ellipsometry, grazing -incidence x-ray reflectometry (GIXRR), neutron reflectometry and transmission electron microscopy (TEM). The measurements are made on separate sets of 10 carefully prepared samples, all of which have been characterized by a combination of ellipsometry and XPS using carefully established reference conditions and reference parameters.The results have been assessed against the National Physical Laboratory (NPL) data and all show excellent linearity. The data sets correlate with the NPL data with average root-mean-square scatters of 0.15 nm, half being better than 0.1 nm and a few at or better than 0.05 nm. Each set of data allows a relative scaling constant and a zero thickness offset to be determined. Each method has an inherent zero thickness offset between 0 nm and 1 nm and it is these offsets, measured here for the first time, that have caused many problems in the past. There are three basic classes of offset: water and carbonaceous contamination equivalent to ∼1 nm as seen by ellipsometry; adsorbed oxygen mainly from water at an equivalent thickness of 0.5 nm as seen by MEIS, NRA, RBS and possibly GIXRR; and no offset as seen by XPS using the Si 2p peaks. Each technique has a different uncertainty for the scaling constant and consistent results have been achieved. X-ray photoelectron spectroscopy has large uncertainties for the scaling constant but a high precision and critically, if used correctly, has zero offset. Thus, a combination of XPS and the other methods allows the XPS scaling constant to be determined with low uncertainty, traceable via the other methods. The XPS laboratories returning results early were invited to test a new reference procedure. All showed very significant improvements. The reference attenuation lengths thus need scaling by 0.986 ± 0.009 (at an expansion factor of 2), deduced from the data for the other methods. Several other methods have small offsets and, to the extent that these can be shown to be constant or measurable, these methods will also show low uncertainty. Recommendations are provided for parameters for XPS, MEIS, RBS and NRA to improve their accuracy.  Crown

Accurate Determination of Quantity of Material in Thin Films by Rutherford Backscattering Spectrometry

“…A metrology exercise to determine the thickness of various native oxides of Si, sponsored by the CCQM (Consultative Committee on the Quantity of Material, or Amount of Substance), aimed to qualify XPS for this application; it used ellipsometry, RBS, EBS and NRA, as well as TEM, grazing incidence X-ray reflectivity (GI-XRR) and other methods [36]. The authors used the extraordinary precision of ellipsometry to determine the correction for the reference attenuation length for XPS at better than 0.5% (1σ) with a rather complicated protocol.…”

The Avogadro constant: determining the number of atoms in a single-crystal 28 Si sphere

“…Therefore, the oxide thickness determination by X-ray reflectometry on the sphere was replaced by X-ray fluorescence measurements with an excitation energy of 680 eV, where the oxygen K fluorescence intensity from the sphere surface was compared with that from flat samples for which the oxide layer thickness was determined by X-ray reflectometry. The total surface layer was modelled, from top to bottom, as follows: a carbonaceous and an adsorbed water layer, a fictive layer of Cu and Ni silicides and an SiO 2 layer [10]. From this model, the SiO 2 thickness was re-evaluated from the ellipsometric data, showing excellent agreement with the X-ray reflectometry data.…”

Realization of the kilogram by the XRCD method

“…The number of electrons detected in the electron spectro meter is strongly dependent on the take-off angle of the detector, which is related to the orientation of the crystal lattice of the sphere. Hence, for an accurate measurement, a so-called reference geometry must be established [61]. Currently, the reference geometry for direct measurements on the spheres cannot be established.…”