James Yen, Keith Eberhardt
Joseph Fine, David Simons
The individual units of SRM 2135c,d,e consist of 1.0cm x 2.54cm chips that have 9 alternating thin-film layers of Ni (nickel) and Cr (chromium) on top of a silicon substrate; there are 5 layers of Cr and 4 layers of Ni on each specimen.
The SRM's are certified for total Cr and Ni thicknesses, single element layer to layer uniformity, Ni and Cr bilayer uniformity, and Cr and Ni single layer thicknesses. Only the middle 2.00cm of the chips along the longer axis (2.54cm) are certified as usable. Industry will find them useful for calibrating sputtered depth scales and erosion rates in surface analysis.
The certified values of layer thicknesses have uncertainties that include several components: uncertainty due to the calibration of Inductively Coupled Plasma (ICP) Spectrometry measurements, between-specimen differences (obtained by wide field X-ray Fluorescence Spectrometry (XRF)) and within-specimen differences (obtained by small field XRF). These components were combined using propagation of uncertainty.
There were 2 small field XRF measurements made at each point of a 3x9 lattice on each specimen tested for a total of 54=2x3x9 such measurements per unit. These measurements did not have the appearance of purely random scatter on the surface of the specimens. Instead, the measurements appeared to follow a linear function of where on the chip's long axis the measurements were taken. These linear trends were estimated using least squares regression lines. From the regression lines were calculated the maximum biases from the mean that would be produced from measurements made at either end of the 2.0cm long usable portion. These biases were combined with standard deviations of the measurements through propagation of uncertainty to create the combined within-specimen component of the total uncertainty.
The uncertainties in the certified values take the form of
expanded uncertainties corresponding to 95% confidence
as described in the ISO Guide to the Expression of Uncertainty
Figure 13: The plot shows a schematic coordinate grid of Cr measurements on part of the surface of an individual unit of SRM 2135c. There is a linear trend along the longer axis. The vertical axis is not drawn to scale, as the specimen would look quite flat to the naked eye.
Date created: 7/20/2001