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Electrical and mechanical measurements on nanomaterials

Berkovich tips have been produced on AFM cantilevers for the first time worldwide. These tips significantly extend the application spectrum of PTB's picoindenter

PTBnews 3.2020
25.09.2020
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The novel picoindenter ‘made at PTB’ uses the tip of an atomic force microscope (AFM) as an indenter to characterize nanomaterials dimensionally and mechanically. This indenter has now been considerably enhanced: in the Laboratory for Emerging Nanometrology (LENA), a focused ion beam was used to produce pyramidal Berkovich tips on AFM cantilevers which can also be used in the picoindenter. Compared to conventional, conical AFM tips, such indenters are mechanically more stable and allow fast dynamic measurements in the long run as well as electrical measurements, due to their high conductivity.

Berkovich tip on a silicon AFM cantilever (large image), together with the ideal dimensions of a Berkovich tip (top left) and a lateral view of the manufactured tip (top right). The angular aperture is (143 ± 0.5)°, which is in good agreement with the definition of tips stated in the ISO standard. The effective tip height is approx. 1 μm and is thus sufficient for mechanical measurements close to the surface, e.g. for determining the indentation-depth-dependent electromechanical behavior of nanoscale semiconducting materials.

Nanoindentation is an established procedure to investigate bulk materials: a small tip of a defined geometry is pressed into the material; based on the material's behavior, this allows conclusions as to its properties to be drawn.

To characterize innovative nanomaterials with high aspect ratios, such as columns with diameters in the nanometer range, nanoindentation is, however, not suited due to its limited depth and force resolution. Enhanced force sensitivity can be achieved by using atomic force microscopes (AFMs). AFMs, however, were originally developed to characterize the topography of a sample. In particular when investigating hard materials by means of indentation, the indenter tip tilts, which leads to problems such as high nonlinearities.

In order to fill the gap between nanoindentation devices and AFMs, a novel picoindenter was developed. It exhibits enhanced sensitivity to depth for a total indentation depth of 10 μm. The picoindenter uses AFM tips as an indenter to characterize nanomaterials dimensionally and mechanically. It has already been demonstrated that it is suited for topographic measurements on mono-atomic step height standards and for quantitatively characterizing extremely soft materials with elasticity moduli in the range of a few MPa.

An important prerequisite for comparable nanomechanical measurements is the use of standardized indenters. The corresponding “nanoindentation standard”, ISO 14577, recommends using pyramidal Berkovich tips for small forces and small indentation depths. Such tips have now been produced for the first time on AFM cantilevers, which can also be used in the picoindenter. For this purpose, the focused ion beam of a device from the Laboratory for Emerging Nanometrology (LENA) was used. LENA is operated by TU Braunschweig in collaboration with PTB. This system is equipped with a 5-axle precision table whose large tilt angle range allows very small pyramidal indenter tips to be prepared. In this way, it is possible to manufacture not only trilateral Berkovich tips, but also different geometries such as quadrilateral Vickers tips.

A detailed geometrical characterization of the pyramidal tips manufactured by means of a focused ion beam is currently being carried out using PTB's patent-pending stylus tip test standard. Another objective of this work is to manufacture standardized indenters on AFM diamond tips to be able to characterize also hard, innovative materials (such as gallium nitride) mechanically. These conductive AFM pyramidal tips can be used to electromechanically characterize semiconducting materials from diverse application fields such as energy generation, medicine, biology and environmental technologies.

Contact

Thomas Ahbe
Department 5.1 Surface Metrology
Phone: +49 531 592-5143
thomas.ahbe(at)ptb.de

Scientific publication

Z. Li, S. Gao, U. Brand, K. Hiller, H. Wolf: A MEMS nanoindenter with an integrated AFM cantilever gripper for nanomechanical characterization of compliant materials. Nanotechnology 31, 305502 (2020)