Hybrid AFM/SEM metrology for measuring nanoparticle dimensions

Since the 1990s, we have witnessed the development and use of manufactured nanoparticles (NPs) as building blocks in the composition of new materials. Industry is keen to take advantage of the remarkable properties of NPs, which can behave very differently from solid materials of the same chemical composition and crystallographic structure. But the key to nanomaterials lies in the strong dependence of nano-objects' functional properties on their dimensional properties: size, size distribution and shape. Consequently, the industrial challenge lies in the ability of companies involved in nanomaterials to set up a reliable quality control system to reproduce NPs with the same properties as those observed in the laboratory.

Moreover, the dimensional parameters characterizing a nanoparticle population (size, size distribution, shape, agglomeration state) have a recognized impact on the potential toxicity of NPs. Indeed, questions concerning the possible environmental and health risks of nanoparticles arose in the early 2000s, with the increasing presence of nanomaterial-containing products on the market. In particular, studies have been undertaken to understand possible interactions between NPs and living cells. However, the results are often inconsistent and do not appear to be reproducible. One of the main reasons for this is the unreliability of dimensional measurements.

The development of nanometrology, the science of measurement on the nanometer scale, aims to provide answers to questions concerning the reliability, accuracy and comparability of measurements carried out on nanomaterials. But this new form of metrology is still in its infancy. Measurement methods are not yet standardized, and the need for standards and reference materials is pressing. There's no doubt that measuring a parameter characteristic of a nanoparticle remains a real challenge. Against this backdrop, the greatest efforts have been focused on the dimensional metrology of nanomaterials. In most countries, traceability chains are being set up, and numerous techniques are available to characterize the size, size distribution and shape of NPs. However, there is no such thing as the ideal instrument, and operators need to be aware of the advantages and disadvantages of each measuring technique. This is the case, for example, with the AFM and SEM microscopy techniques we focus on in this article: neither technique provides reliable access to sizes in all spatial dimensions, but only in the plane for SEM and along the vertical axis for AFM. In this article, we will detail a so-called "hybrid metrology" approach, combining the strengths of both AFM and SEM techniques to achieve reliable dimensional information in all three spatial dimensions.