Development of a new hybrid approach combining AFM and SEM for the nanoparticle dimensional metrology.

Loïc Crouzier,Laurent Devoille,Sébastien Ducourtieux,Alexandra Delvallée,Christophe Tromas,Nicolas Feltin,Elodie Barruet,Olivier Taché,Christian Ulysse,Guillaume Noircler

doi:10.3762/bjnano.10.150

Abstract

At this time, there is no instrument capable of measuring a nano-object along the three spatial dimensions with a controlled uncertainty. The combination of several instruments is thus necessary to metrologically characterize the dimensional properties of a nano-object. This paper proposes a new approach of hybrid metrology taking advantage of the complementary nature of atomic force microscopy (AFM) and scanning electron microscopy (SEM) techniques for measuring the main characteristic parameters of nanoparticle (NP) dimensions in 3D. The NP area equivalent, the minimal and the maximal Feret diameters are determined by SEM and the NP height is measured by AFM. In this context, a kind of new NP repositioning system consisting of a lithographed silicon substrate has been specifically developed. This device makes it possible to combine AFM and SEM size measurements performed exactly on the same set of NPs. In order to establish the proof-of-concept of this approach and assess the performance of both instruments, measurements were carried out on several samples of spherical silica NP populations ranging from 5 to 110 nm. The spherical nature of silica NPs imposes naturally the equality between their height and their lateral diameters. However, discrepancies between AFM and SEM measurements have been observed, showing significant deviation from sphericity as a function of the nanoparticle size.

Highlights

atomic force microscopy (AFM) or scanning electron microscopy (SEM) are considered to be reference techniques for measuring the size of nanoparticles (NPs) because the measurements are based on a direct observation of the imaged NP population
We propose in this paper the development of a hybrid metrology that allows for the measurement of the characteristic dimensions of a nano-object in 3D, by combining the measurements performed with AFM and SEM
One problem immediately arises: Where is the profile width to be measured to get a reliable measurement of the NP lateral diameter? In a previous study, by comparing the results experimentally obtained by AFM and SEM on nearly spherical silica NP, we have demonstrated that the method most consistent with AFM values was to take the full width at half maximum (FWHM) [19]

Summary

Introduction

AFM (atomic force microscopy) or SEM (scanning electron microscopy) are considered to be reference techniques for measuring the size of nanoparticles (NPs) because the measurements are based on a direct observation of the imaged NP population. This creates a direct link between the NP dimensional measurement and the meter definition in the international system (SI) of units [1]. The interaction force is kept constant during the scanning thanks to a feedback loop that controls the tip–sample distance This mode is not really suitable for NP imaging because the NPs might be displaced by the tip over the sample. Regardless of the used mode, and due to tip convolution, the obtained image is a function of tip shape and tip radius (estimated to be around a dozen of nanometers) [3,4]

Methods

Results

Conclusion