Crystalline Si and GaN micro- and nano-pillars as nanomechanical reference artefacts

Quantitative nanomechanical characterization at the nanometer scale is required in the development, optimization, and applications of various novel low-dimensional materials including ultrathin films and nanostructured materials. Adequate measurement uncertainty with typical nanomechanical measurement techniques including nanoindentation and nano-bending tests can be achieved if the equipment and methods in use have been well characterized metrologically.

The procedures described in ISO standards (for example, ISO 14577-2) for characterization and validation of nanomechanical measurement devices are either time-exhausting or cost-expensive. Here, an alternative approach for measuring micro- and nano-pillars made of single-crystalline semiconductor materials is proposed. Reference artifacts of Si and GaN are used, which are suitable for the characterization of contact-based measurement systems.

The pillar-like artifacts based on crystalline Si and GaN are fabricated using microsystem technology in a cooperation with the Laboratory of Emerging Nanometrology (LENA) at the Technical University of Braunschweig. The diameter and height of pillars are varied in the process. The nanomechanical properties of these structures have to be determined and specified. Important parameters are the bending stiffness kb and the axial compressibility k_o.

In addition, the single-crystal micro- and nano-pillars demonstrate the following features:

• size-independent mechanical properties for the materials Si and GaN even in the submicrometer range with known parameters for each crystal direction [1]
• geometrical dimensions can be measured by AFM and SEM with sufficient accuracy
• the lateral pillar distance is sufficiently large, and thus also applicable for measurements with nearly flat tips, e.g. Berkovich and Vickers indenter tips
• low cost due to possible batch fabrication using microtechnology.

Such well-designed and characterized pillar artifacts are already applicable for the calibration of indentation depth and force as well as the fine positioning system of commercial nanomechanical measurement instruments.

Further tasks, such as algorithms for estimating the topography of pyramidal indenters, are under development. In addition, in the case of using these micro- and nano-structures for energy harvesting [2], the measurement of not only the mechanical but also the electrical properties is required. For this purpose, it will be investigated whether this is possible with special AFM measurement modes.

Fig. 1: SEM image of single crystalline Si <100> pillars, produced by the Institute of Semiconductor Technology (IHT) at TU Braunschweig.

Table 1: Dimensions and specifications of proposed single-crystalline reference artifacts for nanodimensional and nanomechanical measurements (bending stiffness k_b, axial compressibility k_o).

References

[1] Z. Li, S. Gao, F. Pohlenz, U. Brand, L. Koenders, und E. Peiner, „Determination of the mechanical properties of nano-pillars using the nanoindentation technique“, Nanotechnol. Precis. Eng., Bd. 3, S. 182–188, 2014.

[2] M. F. Fatahilah, P. Puranto, F. Yu, J. Langfahl-Klabes, A. Felgner, Z. Li, M. Xu, F. Pohlenz, K. Strempel, E. Peiner, U. Brand, A. Waag, and H. Wasisto, „Traceable Nanomechanical Metrology of GaN Micropillar Array“, Adv. Eng. Mater., 2018, 1800353