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Mechanical characterization of nanopillars

Accurate correction of indentation results from nanoobjects

PTBnews 3.2018
02.10.2018
Especially interesting for

developers of nano-force sensors

manufacturers of nanostructures for energy generation and manufacturers of LEDs

nanoindentation, materials testing devices

Pillar-like nanoobjects can now be characterized mechanically using the indentation technique. This has been made possible by a novel evaluation method developed at PTB and by a correction procedure for the compressibility of the columns.

Scanning electron microscopic image of a typical GaN pillar after a nanoindentation measurement with a Berkovich indenter

Micro- and nanopillars made of various semiconducting materials are currently being extensively investigated with regard to their properties for energy harvesting, energy storage and light emission (LEDs). When harvesting energy, the mechanical properties (elasticity and hardness) of the flexible columns play an important role. The nanoindentation technique allows the extraction of the mechanical properties of objects in the micro- and nanometer ranges. Here, a fine indenter (usually made of diamond) that has a prescribed pyramidal shape (Berkovich, Vickers) is pressed into a surface. The indentation depth is acquired during the loading and unloading procedures.

For the indentation of micropillars with a relatively high aspect ratio (height to diameter), PTB’s initial measurements revealed a considerably smaller indentation modulus than that of the original material, and an unusual dependence of the modulus on the indentation depth was observed. The initial analyses seem to suggest that this unexpected dependence of the indentation modulus on the indentation depth may be caused by the rigidity of the micropillars, which has not been taken into account so far. The rigidity depends not only on the radius of the columns, but also on their height. The geometrical parameters have been determined by means of scanning force microscopy. Taking the rigidity of the pillars into account, it was revealed that the indentation modulus depends only to a very small degree on the indentation depth and that it is in excellent agreement with the modulus of the original material.

This correction procedure, which was developed at PTB, has been experimentally confirmed on micropillars consisting of crystalline silicon and gallium nitride manufactured by the Institut für Halbleitertechnik (IHT) of TU Braunschweig. The diameter of the columns was measured at IHT by means of scanning electron microscopy, while the height was measured at PTB by means of confocal laser scanning microscopy.

The extended evaluation approach enables a reliable determination of mechanical properties and thus supports the further development of innovative pillarbased micro- and nanostructures.

Contact

Zhi Li
Department 5.1
Surface Metrology
Phone: +49 531 592-5190
Opens window for sending emailzhi.li(at)ptb.de

Scientific publication

Z. Li, S. Gao, F. Pohlenz, U. Brand, L. Koenders, E. Peiner: Determination of the mechanical properties of nano-pillars using the nanoindentation technique. Nanotechnol. Precis. Eng. 3, 182−188 (2014)