MEMS Picoindenter with exchangeable AFM cantilever as an indenter for micro- and nanomaterials

Within the framework of the EMPIR project “Metrology for length-scale engineering of materials” (Strength-ABLE), a MEMS picoindenter has been developed, which is designed to bridge the metrological gap between nanoindentation instruments and AFM-based nanomechanical measurement systems. As shown in Fig. 1, the MEMS picoindenter utilizes a bi-directional electrostatic comb-drive transducer for force and displacement sensing with a resolution of 9 nN and 0.2 nm, respectively. The nominal stiffness and the resonance frequency of the MEMS picoindenter amount to 45 N/m and 3.3 kHz, respectively. This satisfies the metrological demands not only for quasi-static material testing, but also for dynamic measurement methods.

One of the outstanding features of this newly designed picoindenter is an AFM cantilever holder to fix AFM nanomechanical probes, as shown in Fig. 1. It allows the use of commercially available AFM probes as indenters for nano-material testing.

Fig. 1 Schematic representation of the MEMS picoindenter with exchangeable AFM cantilever for nanomechanical testing	Fig. 2 Prototype of the MEMS picoindenter

The MEMS picoindenter is produced by bulk micromachining technology at the Center for Microtechnologies at Chemnitz University of Technology [1]. The passive gripper designed for gripping an AFM cantilever is shown in Fig. 2.

Using PTB’s micro-force measurement facility [1], the suspending stiffness of the MEMS picoindenter has been carefully investigated. A stylus with a tip radius of 5 µm is used to probe the MEMS main shaft and to deflect it along its symmetric axis, as shown in Fig. 3(a). One of the typical calibration curves is depicted in Fig. 3(b), which indicates that the nonlinearity of the MEMS picoindenter should be less than 5⋅10^-4 over the whole 300 µN range.

(a) Probing of the MEMS picoindenter with a tactile stylus with a tip radius R = 5 µm	(b) Typical measured force-deflection curve (slope = stiffness)
Fig. 3 Stiffness measurement of the MEMS picoindenter using PTB’s micro-force measurement facility

For the mounting of the AFM cantilevers onto the MEMS picoindenter, an assembly platform especially for micro-cantilevers has been developed. It consists of a microscope with a long working distance for optical inspection of the spatial relationship between the MEMS structures and the cantilever to be fixed. A six-axis motorized micro-positioning stage is used to mount the AFM cantilever onto the MEMS picoindenter and then to break it (see video 1). Once the AFM tip is worn out, it can be removed from the MEMS picoindenter (see video 2).

Video 1. Installation of an AFM cantilever as an indenter in a MEMS picoindenter	Video 2. Dismantling of a used cantilever from the MEMS picoindenter

Within the next few months of this project, the MEMS picoindenter with the diamond AFM tip will be used to determine the mechanical properties of thin films by means of the nanoindentation technique as defined by ISO 14577.

References

[1] U. Brand, S. Gao, Lutz Doering, Zhi Li, Min Xu, Sebastian Buetefisch, Erwin Peiner, Joachim Fruehauf, Karla Hiller, "Smart sensors and calibration standards for high precision metrology“, 2015, Proc. SPIE 9517, 95170V–95170V–10. [2] Uwe Brand, Sai Gao, Wolfgang Engl, Thomas Sulzbach, Stefan W Stahl, Lukas F Milles, Vladimir Nesterov and Zhi Li: "Comparing AFM cantilever stiffness measured using the thermal vibration and the improved thermal vibration methods with that of an SI traceable method based on MEMS", Meas. Sci. Technol. 28 (2017) 034010 (12pp).