Traceable three-dimensional nanometrology

Miniaturisation of advanced nano-manufacturing techniques and use of complex nano-objects have helped drive demand for improved accuracy in 3D nano-metrology. For example, advanced dimensional nano-metrology has supported the manufacture of nano-biotechnologies and nano-electronics such as organic photovoltaic cells. Accurate dimensional measurement is a crucial aspect of process development and quality control but measurements in this field were unable to be made traceable, so could not be reliably compared.

To meet industrial demand, measurement accuracy below 1 nm was needed. However, the most frequently used industrial tools such as scanning probe microscopy (SPM), Scanning Electron Microscopy (SEM), and scatterometry typically only provided accuracy within 5 nm, due to limitations imposed by complex geometry and modelling errors.

While SPMs in National Metrology Institutes outperformed commercial SPMs for accuracy, practical 3D measurement services could not be provided due to excess noise and insufficient speed/range. Plus, as knowledge of tip probe-sample interactions was limited to measurements of close-to flat surfaces, even high-resolution instruments experienced errors of up to 30 %. Additionally, nanoscale reference standards were available, but reference materials for accurately characterising probe size were inadequate.

Building on the achievements of EMRP project TReND, in this project SPM-based approaches were developed to traceably measure objects in three dimensions.

A bottom-up traceability approach was developed to determine the 3D dimensions of complex nano-structures to uncertainties of less than 1 nm, using a traceably calibrated lattice of crystal silicon as an internal ruler.

Software was developed for modelling probe-sample interactions, in addition to hybrid methods for merging results from different tools or different channels of a single tool, which were applied to characterise reference materials.

Novel standards for evaluating AFM probe size and SEM beam size were also designed and manufactured.

Noise levels for metrological Atomic Force Microscopes (MAFMs) were reduced to 0.1–0.4 nm, while their range was increased to 22 mm and their speed to 1 mm/s.

A good practice guide for dimensional metrology at the nanometer scale in general and for using the developed reference standards and methodologies was published.

These outputs allowed for traceable 3D metrology at the nanometre level, enabling NMIs to offer SI-traceable measurement and calibration services. Results also contributed to the realisation of the lattice parameter of silicon as a secondary length standard in the Mise en Pratique for the metre, revised in May 2019.

The developed 3D nano-metrology methods offer enhanced control of nanoscale morphology, supporting future innovation in applications such as semiconductors, nano-photonics, and microscopy.