James R. Pekelsky

Practical magnetron sputtering system for the deposition of optical multilayer coatings

A magnetron sputtering system is described in which, at any one time, as many as four different 15-cm x 46-cm rectangular planar magnetron targets can be mounted vertically in the deposition chamber. These can be attached to either dc or rf power supplies for direct or reactive deposition of metal, metal oxide, or nitride films. Typical target materials include Ag, Al, C, Mo, Nb, Ni, Si, W, and Zr. Good uniformity can be obtained on stationary substrates, although better results are possible with oscillating substrates. The refractive indices are given for several useful oxide materials. The materials and thicknesses of the individual layers that comprise an optical multilayer system are entered into a computer that subsequently controls the deposition parameters, the substrate motion, and the deposition time. After a relatively simple calibration process, coatings that consist of between 20 and 60 layers can be produced to within an accuracy of 1% or 2%. A wideband optical monitor is available for checking the performance of the multilayer system during its deposition. Several examples of multilayer coatings that were prepared on this equipment are given.

Report on an international comparison of one-dimensional (1D) grating pitch

This paper reports results of an international comparison of one-dimensional (1D) grating pitch calibration by optical diffraction. Comparison results are analysed and discussed following the recommended guidelines for the analysis of CIPM key comparisons. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by SIM, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

Imaging laser diffractometer for traceable grating pitch calibration

A laser diffractometer for calibrating grating pitch is being developed at the National Research Council (NRC). Based on the METAS design, it features a precision rotary table, the sample grating mounted in Littrow configuration, a collimated spatially filtered laser source and the retroreflected diffraction order sent by a beamsplitter through a focusing lens to the null-position photodetector. The angle of various diffraction orders can be measured in sequence and used with knowledge of the laser wavelength to calculate the pitch of the grating. Instead of the usual quad-photodetector, the NRC system uses a charge-coupled device (CCD) video camera to capture an image of the diffraction pattern. Image processing accurately measures diffraction order centroid coordinates without a systematic error due to background noise or neighbouring pattern artefacts. The image coordinate system is also used to adjust the instrument and grating alignment and to investigate these sources of systematic error. Preliminary results with 1D sample gratings ranging in pitch from 700 nm to 10 µm are presented.

Uncertainty evaluation of the NRC imaging diffractometer

A model equation is developed for a diffractometer operating under the Littrow condition. The effects of grating orientation, grating position, source and detector position and source collimation are included. The model is validated against experimental measurements using the NRC imaging diffractometer. Errors due to the effects of coherence and grating position were found to be important for the small area gratings used in these experiments. An uncertainty budget for the NRC imaging diffractometer is developed and reproducibility measurements are presented.

Evaluation of uncertainty in grating pitch measurement by optical diffraction using Monte Carlo methods

Measurement of grating pitch by optical diffraction is one of the few methods currently available for establishing traceability to the definition of the meter on the nanoscale; therefore, understanding all aspects of the measurement is imperative for accurate dissemination of the SI meter. A method for evaluating the component of measurement uncertainty associated with coherent scattering in the diffractometer instrument is presented. The model equation for grating pitch calibration by optical diffraction is an example where Monte Carlo (MC) methods can vastly simplify evaluation of measurement uncertainty. This paper includes discussion of the practical aspects of implementing MC methods for evaluation of measurement uncertainty in grating pitch calibration by diffraction. Downloadable open-source software is demonstrated.

Uncertainty of gauge block calibration by mechanical comparison: a worked example for gauge blocks of dissimilar materials

The definition of gauge black length,and the lowest- uncertainty gauge block calibrations, derive their link to the definition of the meter through the technique of optical interferometry using calibrated optical wavelengths. Following interferometric calibration of gauge blocks, the technique of mechanical comparison of gauge blocks is largely used for the dissemination of the meter. A measurement scenario for the case of low-uncertainty gauge block calibration in which the working standard and client gauge blocks are made of dissimilar materials is described. The correction for stylus deformation is determined experimentally. A detailed tutorial on evaluating the uncertainties for this gauge block calibration in accordance with the ISO Guide to the expression of uncertainty in measurement is reported, including discussion of degrees of freedom and correlations.

Bootstrap calibration of an autocollimator, index table and sine bar ensemble for angle metrology

An in-situ bootstrap method used at NRC to calibrate an ensemble of instruments for angle metrology traceable to international standards for angle and length is described. No prior knowledge is assumed, beyond nominal values with arbitrary large uncertainty, for the index-table step angles, the autocollimator scale factor, and the sine-bar length. Only the sine-bar displacements are known as calibrated values with uncertainty traceable to the SI unit for length. First, the nominal-length sine-bar is used to check the autocollimator linearity, stability, and estimate a nominal scale factor, thus giving a first-iteration improvement in the uncertainty of autocollimator readings. Then, the index table (and a polygon) are measured by a full-closure method at the polygon intervals, and index steps of one interval are measured by the caliper method, with results expressed using improved autocollimator readings. This provides improved index angles. Finally, the autocollimator beam is aimed obliquely at the sine-bar mirror, and the beam deflects to the index-table mirror, where it retroreflects back to the autocollimator via the sine bar. As the index table is stepped through a sequence of angles (with improved uncertainty), the sine-bar angle is adjusted in opposite rotation to produce a zero reading by the autocollimator, and the required sine-bar displacement recorded. This provides a better estimate of the sine-bar length. The steps can be re-iterated--hence bootstrap--to further improve the calibration of each device, until a limit is reached. In recent years, we have linked the data from the three setups in a single spreadsheet analysis, allowing the calibration variables to be jointly and optimally adjusted with just one data run through the three setups. Results using a Moeller-Wedel Elcomat HR autocollimator, a Moore 1440 index table and the NRC sine-bar interferometer are presented, along with an uncertainty analysis.

Final Report on SIM.L-K1 (SIM.4.2) Regional Comparison. Stage One: Calibration of gauge blocks by optical interferometry

Results of the Stage One portion of the Inter-American System of Metrology (SIM) regional international comparison of gauge block calibration by optical interferometry are presented. In this measurement round-robin, short gauge blocks, six made of steel and six made of tungsten carbide, in the range of nominal length from 2 mm to 100 mm, were calibrated by five national metrology institutes (NMIs) of the SIM region, and one NMI from EUROMET. By employing the technique of optical interferometry, each of the laboratories establishes a direct link to their national primary standard of length through the calibrated laser wavelengths. Results of central length calibration are presented and discussed with regard to vacuum wavelength correction for refractive index of air, phase-change on re.ection and wringing effects. Measurement uncertainty evaluation is also discussed. Main text. To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCL, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

A new instrument for the dimensional characterization of piston-cylinder units

Calculations of the effective area and elastic distortion of piston–cylinder units (PCUs) used to realize fundamental pressure standards require accurate dimensional measurement of the cylindrical geometry of both the piston and the cylinder. A new instrument capable of measuring the diameter and form of PCUs up to 20 mm diameter with target uncertainties of 20 nm is being developed. The first stage of this development is the design and construction of an instrument capable of measuring external diameter using two mutually opposed contact probes mounted on precision translation stages. The positions of the probes are measured continuously using two sub-nanometre displacement measuring laser interferometers. The instrument provides a flexible working platform for future developments.

Two-part study toward lowest uncertainty calibration of ceramic gauge blocks: interferometry and mechanical comparison techniques

Mechanical comparison calibration of gauge blocks using reference standards that are a different material from the client gauge block can be problematic. This two-part study investigates some of the dominant influences identified in the mechanical comparison calibration of ceramic gauge blocks using steel reference standards. The goal is to develop techniques for lowest uncertainty mechanical comparison calibration in the application of two dissimilar gauge block materials. Of primary interest are: correction for differences in mechanical stylus deformation, and length equivalent thermal corrections in the different materials. In our model, mechanical stylus deformation is evaluated using gauge blocks of known length, calibrated by optical interferometry. The optical phase correction applied in this initial interferometric determination of gauge block length is an important first step. In the first part of this study, optical phase correction for ceramic gauge blocks has been determined using similar techniques by three labs, all of them applying the method of stack experiments using the same gauge blocks, and similar platens. One of the platens is of the same material and made by the same manufacturer as the ceramic gauge blocks. Temperature effects dominate the mechanical comparison calibration of dissimilar materials. In the second part of this study, the importance of the approach to length corrections as a result of temperature variation is demonstrated.