Silicon lattice comparisons related to the Avogadro project: Uniformity ofnew material and surface preparation effects

New high-quality silicon has been produced by Wacker Siltronics(1) as poten tial starting material for a precision determination of the Avogadro consta nt, N-A. An assessment of the uniformity of this material is an essential f irst step in determining whether this material is of sufficient quality to be used in this project. We have made extensive measurements to determine l attice parameter uniformity of several regions of this new material using t he NIST lattice comparator. Measurements from this comparator have been sho wn to have a relative internal consistency near 1 x 10(-8). In the course o f 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 mere altered to include etching after c-m polishing. This unexpe cted dependence on surface preparation raises the possibility that some of the NIST lattice comparison results presented at CPEM96 may be biased by su rface preparation effects. To test this possibility, some of the samples in cluded in our CPEM96 contribution have been etched and remeasured. Prelimin ary estimates of corrections to some NIST CPEM96 lattice comparison results appear to confirm that bias.