First production of Berkovich tips on AFM cantilevers for the PTB picoindenter

Advances in nanomaterial-based energy harvesting require quantitative nanomechanical and nanoelectrical characterisation of the near-surface properties of novel nanomaterials, especially nanowires and nanopillars. Traditional nanoindentation devices are established as qualified tools for nanomechanical measurements of bulk materials and for nanoelectric measurements in conductive indenters. However, they are not well suited for nanoelectromechanical measurements of nanomaterials with high aspect ratios due to their limited depth and force resolution. In contrast, scanning probe microscopes (AFM) usually have high force sensitivity, but are not well suited for measuring the nano-mechanical properties of hard materials due to their often large non-linearity and an occurring tilt of the indenter during the indentation process.

To bridge the metrological gap between nanoindentation devices and nanomechanical AFMs, a novel MEMS-Picoindenter with a spectral depth sensitivity of 4 pm/Hz^1/2 for a total indentation depth of 10 µm [1] was developed. The picoindenter uses AFM tips as indenters for nanodimensional and nanomechanical characterisation of nanomaterials. It has already been shown to be suitable for nanotopography measurement of monoatomic step heights and for mechanical characterisation of extremely soft materials with elastic moduli in the range of a few MPa [1]. An important prerequisite for comparable nanomechanical measurements is the use of standardised indenters. In the standard for instrumental hardness testing ISO 14577, which is also called the nanoindentation standard in the range of small forces and small indentation depths, pyramid-shaped Berkovich tips are recommended.

Within the EMPIR project NanoWires (19ENG05), such tips have now been successfully produced for the first time by PTB on AFM cantilevers using a focused ion beam in a dual-beam (FIB-SEM) system at the Laboratory for Emerging Nanometrology (LENA) of the TU Braunschweig. Thanks to the large tilting capability of its 5-axis precision stage for sample positioning, this dual-beam system is able to fabricate very small pyramid-shaped indenter tips as 3-sided Berkovich tips. Figure 1 shows one of the produced tips, the world's first Berkovich tip on an AFM cantilever. The opening angle is (143 ± 0.5)°, which agrees well with the tip definition of the ISO standard. The effective tip height h is approx. 1 µm and is thus sufficient for near-surface nanomechanical measurements, e.g. the determination of the penetration depth-dependent nanoelectromechanical behaviour of semiconductor materials [2].

(a) View of the Berkovich tip along the cantilever axis	(b) Enlarged side view of the tip

(c)  3D-view of the Berkovich tip produced by FIB

Figure 1. Modified Berkovich tip produced on an AFM cantilever

Compared to the mechanically not very stable conical AFM tips, such ISO-compatible and robust indenter tips not only allow quantitative static nanomechanical measurements, but also fast and reliable dynamic measurements in the long term. Furthermore, these AFM Berkovich tips can also be used for nanoelectric measurements due to their high electrical conductivity.
Another aim of the work is to produce standardised indenters on AFM tips made of highly doped diamond and tungsten carbide in order to be able to mechanically characterise hard innovative materials such as GaN and ZnO in the future. It is expected that these FIB-manufactured conductive AFM pyramid tips made of a wide variety of semiconductor materials will find numerous applications in the field of nanoelectromechanical characterisation of nanomaterials for energy production, medicine, biology and environmental technologies.

Literature
[1] Z. Li, S. Gao, U. Brand, K. Hiller, H. Wolf 2020 A MEMS nanoindenter with an integrated AFM cantilever gripper for nanomechanical characterization of compliant materials. Nanotechnology 31 305502 (DOI: doi.org/10.1088/1361-6528/ab88ed)
[2] Y.Q. Chen, X.J. Zheng, S.X. Mao and W. Li 2010 Nanoscale mechanical behavior of vanadium doped ZnO piezoelectric nanofiber by nanoindentation technique J. Appl. Phys. 107, 094302