Traceability of sub-nm length measurements

Confirming the performance of precise milling machines, manufacturing disk-drives and assembling semiconductors all rely on accurate sub-nanometre dimension measurements. The highest accuracy measurements of length, or thickness, are made with non-contact techniques and use optical interferometers to determine length or displacement precisely. Capacitive displacement sensors enable measurements in more challenging environments, where air, vibrations and noise make the use of interferometers impractical, or too expensive. Accurate measurements on the sub-nanometre scale require consistent modelling of the measurement system as well as accurate comparisons of differing measurement techniques and a reduction in measurement uncertainties.

The EMRP project Traceability of sub-nm length measurements (subnano) enabled improved traceability to the SI for sub-nanometre range length measurements made by optical interferometers and improved calibration methods for capacitive sensors used for measuring nano-sized component movements.

The project:

• Established a method to model, evaluate and correct systematic errors in interferometer setups. Adequately correcting for systematic errors will make a significant contribution to lowering the measurement uncertainties of current high-end optical interferometers.
• Developed a new calibration procedure for capacitive sensors with corrections for temperature, pressure, humidity, tilt and edge effects. These methods will enable the accurate and traceable calibration of high-end capacitive probe systems.
• Developed improved NMI capabilities for measuring length with reduced uncertainties which were confirmed by a comparison exercise.

This multidisciplinary EMRP project used capacitive sensors, unique x-ray interferometers and the highest accuracy optical interferometers to develop a leading capability for sub-nanometre length measurement in Europe. This project brought together industrial and scientific measurement experts to deliver best practice and precise nano-measurement methods to a multitude of industrial applications reliant on nanometre precision dimension measurements.

Industrial demand was not the only motivation for increasing the accuracy of subnano dimension measurements. The SI redefinition of the kilogram based on atom counting of silicon spheres is dependent on precisely determining the separations of the sphere’s atoms. This could only be achieved using the tools developed in this project for measuring sub-nano distances with great accuracy.