Silicon lattice comparisons related to the Avogadro project: uniformity of new material and surface preparation effects

New high-quality silicon has been produced by Wacker Siltronics as potential starting material for a precision determination of the Avogadro constant, N_A. An assessment of the uniformity of this material is an essential first step in determining whether this material is of sufficient quality to be used in this project. We have made extensive measurements to determine lattice parameter uniformity of several regions of this new material using the NIST lattice comparator. Measurements from this comparator have been shown to have a relative internal consistency near 1×10^-8. In the course of these measurements we noted a significant dependence of lattice parameter values on surface preparation of the samples. Samples prepared by grinding followed by chemical-mechanical (C-M) polishing show a wider distribution than samples prepared by grinding followed by etching. Surface preparation procedures were altered to include etching after C-M polishing. This unexpected dependence on surface preparation raises the possibility that some of the NIST lattice comparison results presented at CPEM96 may be biased by surface preparation effects. To test this possibility, some of the samples included in our CPEM96 contribution have been etched and remeasured. Preliminary estimates of corrections to some NIST CPEM96 lattice comparison results appear to confirm that bias