Optical Linescale Metrology

The cornerstone of traceability for the SI Unit of length, the meter, is the Linescale Interferometer (LSI) which provides World-class one-dimensional scale calibrations traceable to the SI as verified by international intercomparisons. The Linescale Interferometer (LSI) provides one-dimensional scale calibrations up to one meter in length.   Its performance and accuracy is validated through international comparisons, most recently the “NANO 3: Line Scale Standards” comparison, performed under the auspices of the International Bureau of Weights and Measures (BIPM).  The LSI provides calibrations for customers in the nanomanufacturing industry, such as major metrology tool suppliers.  It is also the traceability link for most of the other metrology instruments within NIST and the NanoMet Program.  The optical linescale metrology project is focused on further increasing the reliability of the system, and further minimizing measurement uncertainties. A current LSI research goal is aimed at reducing its measurement uncertainties from (5.2 nm + 10^-7 * length) to one half that value to obtain higher accuracy measurements specifically to meet those calibration requirements for advanced semiconductor product specifications.

The NanoMet Program will develop two-dimensional scale measurements and standard wafer and photomask artifacts that provide two dimensional accuracy of nominally 20 nm over a 15 cm field.  Currently the program has calibrated photomask grids and future work will be focused on developing a two-dimensional wafer calibration capability and reducing the uncertainty of the photomask grid measurements. Two-dimensional scale measurements are provided by a Nikon 5i wafer and photomask metrology instrument which has been highly characterized by the NanoMet Program. This instrument operates over a 150 mm by 150 mm range. This instrument uses an optical microscope to locate sample fiducials and has laser-interferometer guided motion stages.  For qualification, the scale calibration is traceable to a reference artifact measured on the LSI.  Uniformity and orthogonality of the two axes are characterized through self-calibration methods, e.g., repeated measurements of the same artifact with rotations, translations, and reversal.

• Maintain a linescale calibration capacity that is second-to-none in the world.
• Reduce the measurement uncertainty of the NIST linescale interferometer by 20% or greater to meet the needs posed by those measurements or instruments whose uncertainties are currently limited by the linescale contributions to their  uncertainty statements.
• Develop a wafer grid plate measuring capability using the Nikon 5i and produce a reference standard which will support industry efforts to map wafer positioning systems and optical field distortions through the use of grid based self-calibration methods.