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

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Four different silica modficatioas-tbermally grown SiOz, quartz glass, silicalite a d silica gel-bave been investigated by XPS. Binding energies for Si 2p, Si 2s and 0 1s were obtained by employing different methods of static charge referencing. In any case, a C 1s reference was used as Isual. In another experiment, deposited 20 nm gold particles were used to provide an Au 4f,,, binding energy (BE) reference. The two different charging correction procedures result in essentially identical BE data. Furthermore, the results confirm that different modifications of silica give substantially the same relevant BEs in XPS. Though the investigated samples are characterized by quite different concentrations of silanol groups, no significant effect on the measured BES has been found. On the other hand, the analysed silica samples are observed with quite different Si 2p and 0 1s peaks widths. This effect is discussed in terms of differential charging.A rather extensive compilation of XPS literature BEs of silica modifications is given. I N T R O D U C T I O NRecently, we proposed a new method for static charge referencing in photoelectron spectroscopy (XPS).' This method is based on a special procedure to provide a calibrant metal deposit on a sample surface. In brief, the reference metal deposit consists of well-defined metal particles which were prepared as a colloidal metal dispersion. This dispersion is dropped onto the sample surface. Afterwards, the liquid is removed by evaporation, leaving calibrant metal particles on the sample surface.In this paper we present new results obtained by employing the static charge referencing method described above. At first we checked the suitability of the colloidal gold particles used in our laboratory for static charge referencing in XPS analysis of nonconducting samples. This check has been done by employing an electron spectrometer giving a substantially smaller full width at half-maximum (FWHM) for the Au 4f,/, emission compared to the results presented previously. Afterwards, we investigated various modifications of amorphous SiO, and one crystalline modification. All these samples contain silanol groups, which may complicate the interpretation of the XPS data. As a result, we present static charge-corrected binding energies (BE) of Si 2p, Si 2s and 0 1s photopeaks by employing an Au 4f7/, as well as a C 1s reference. We feel that these data are of interest because SiO, is a t Paper presented at ECASlA 91, Budapest, Hungary.Author to whom correspondence should be addressed.technologically very important material which has often been investigated by means of XPS. However, the scatter of the respective BE data is rather too high (e.g. the Si 2p BE scatters over 1.35 eV; cf. Table 1).Finally, we observed an interesting phenomenon with the FWHM of the observed Si photopeaks which can be understood in terms of the differential charging discussion introduced by Barr.' EXPERIMENTAL Equipment and procedureThree different modifications of amorphous SiO, (silica gel, a thermally oxidized...

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Results from a study conducted between National Metrology Institutes (NMIs) for the measurements of the absolute thicknesses of ultra-thin layers of SiO 2 on Si are reported. These results are from a key comparison and associated pilot study under the auspices of the Consultative Committee for Amount of Substance. 'Amount of substance' may be expressed in many ways, and here the measurand is the thickness of the silicon oxide layers with nominal thicknesses in the range 1.5-8 nm on Si substrates, expressed as the thickness of SiO 2 . Separate samples were provided to each institute in containers that limited the carbonaceous contamination to approximately <0.3 nm. The SiO 2 samples were of ultra-thin on (100) and (111) orientated wafers of Si. The measurements from the laboratories which participated in the study were conducted using ellipsometry, neutron reflectivity, X-ray photoelectron spectroscopy or X-ray reflectivity, guided by the protocol developed in an earlier pilot study. A very minor correction was made in the different samples that each laboratory received. Where appropriate, method offset values attributed to the effects of contaminations, from the earlier pilot study, were subtracted. Values for the key comparison reference values (agreed best values from a Consultative Committee study) and their associated uncertainties for these samples are then made from the weighted means and the expanded weighted standard deviations of the means of these data. These results show a dramatic improvement on previous comparisons, leading to 95% uncertainties in the range 0.09-0.27 nm, equivalent to 0.4-1.0 monolayers over the 1.5-8.0 nm nominal thickness range studied. If the sample-to-sample uncertainty is reduced from its maximum estimate to the most likely value, these uncertainties reduce to 0.05-0.25 nm or ∼1.4% relative standard uncertainties. The best results achieve ∼1% relative standard uncertainty. It is concluded that XPS has now been made fully traceable to the SI, for ultra-thin thermal SiO 2 on Si layers, by calibration using wavelength methods in an approach that may be extended to other material systems.