The small size of micro- and nano-structures makes tensile testing challenging. In this study we meet this challenge by combined use of a Focussed Ion Beam (FIB) in Dual Beam configuration, an AFM-cantilever, and a micromanipulator which provide the required accuracy and versatility to measure the mechanical properties of nanowires by tensile testing. AFM cantilevers with a big range of force constants principally enable us to measure the tensile behavior of a great variety of materials.

The accelerating pace of technological miniaturization e.g. development of micro-electromechanical systems (MEMS) and nano-electromechanical systems (NEMS) has attracted much attention of researchers during the last decade. Due to the superior mechanical properties of micro- and nano-structures such as nanowires and nanotubes [1], these have been proposed as new building blocks for novel device architectures. In order to improve and optimize their application and fully utilize their special material properties we need to understand the governing deformation mechanisms. Uniaxial tensile tests are the most commonly performed deformation experiments, which provide basic information on the mechanical properties. Tensile testing is generally preferred, because the stress and strain state in the sample is nominally uniform and the interpretation of data is relatively simple compared to compression experiments. Yet, the hurdle for executing tensile tests at the micro- and nanoscale is high as accurate and reliable testing is limited by experimental uncertainties. Herein, we report a method to characterize mechanical properties of micro- and nanoscale objects by tensile testing, solving the main challenges encountered for micro objects. Our method is based on using an AFM cantilever and a micromanipulator with assistance of FIB/SEM. Preparation and FIB/SEM microscopy is quite simple but can extract precise quantitative data. This method not only provides high accuracy but also offers high versatility by using customized load ranges of different AFM cantilevers.

Experimental Challenges of Microtensile Testing

The main challenges, according to reviews [2] [3], are as follows:
1.

Sample harvesting, manipulation, and gripping
A micromanipulator with the assistance of FIB/SEM can locate, attach, transfer, and manipulate micro objects to the desired testing platform. The manipulation process can be achieved with moderate handling damage mainly due to ion beam cutting or ion-induced metal deposition. Our method uses commercially available Dual-Beam-FIB (FEI STRATA 400 Stem) and micromanipulator systems (Omniprobe). The sample gripping for tensile tests can be accomplished by using local electron- or ion-beam induced Pt deposition. The deposited Pt pads are robust enough to hold during testing.

Force/Displacement Measurement

Small-scale tensile tests require the measurement of tiny forces due to the small sample size. AFM cantilevers are used as load sensors in our method, where in principle the optical system of the AFM is replaced by the FIB/SEM. In the AFM the cantilever is sensitive to very small forces, even forces between atoms can be determined due to the optical detection system used there. The measurement is based on cantilever bending [4] under pressure [5] or lateral load [6], so interpretation of the obtained data is challenging in contrast to data obtained from uniaxial mechanical tests. Ruoff et al. was the first, who performed tensile tests on carbon nanotubes using AFM cantilevers [7]. Orso et al. combined the AFM and FIB techniques to measure the tensile strength of biological samples using piezoresistive AFM cantilevers [8], which are expensive and are not as sensitive as normal ones. With our first prototype we measured the yield strength of copper wires without determination of a stress-strain curve [9]. Here we use a normal AFM cantilever as load sensitive sensor to measure the tiny forces leading to deflections of the cantilever. Furthermore, an AFM cantilever based load measurement system offers a high versatility since the load range can be customized to suit the testing need simply by changing the AFM cantilever.

Strain Measurement

In the case of in-situ tensile tests, differential digital image tracking (DDIT) is an ideal method for strain measurement with SEM or FIB used as image sources during testing [3]. This method can achieve a resolution of up to a thousandth of a pixel. You can find details of the DDIT method in Gianola & Eberl and references therein [3]. The DDIT method has been adapted to our needs in order to measure the strain with the FIB as image source.

Experiments

Figure 1 shows an image of the experimental setup inside of a dual beam SEM/FIB chamber and the steps of the tensile testing procedure. An AFM cantilever used as a force sensor is selected depending on the sample's properties and attached to a support (here a simple screw nut). Single crystalline silver wires with their longitudinal axis in the [110] direction are used as test specimens. They have been transferred to a silicon substrate and placed in the FIB chamber (fig. 1A and C1). The micromanipulator is used to harvest and align the specimen (C1), then after cutting to the appropriate length to transfer it to the AFM cantilever (B). The manipulator moves and touches a silver wire, which is welded on the tip of the manipulator by ion-induced-deposition of Pt. By driving up the probe, an individual wire is lifted up from the silicon substrate (fig. 1C1), then transferred to the tip of the cantilever, and finally fixed to it by Pt deposition (fig. 1C2). The wire also can be welded on the side edge of the cantilever, which is easier to handle. In our experiment a force is applied by driving the manipulator to the right, away from the cantilever with a constant speed of 0.1 µm/s, leading to a deflection of the cantilever (fig. 1C3). The deflection can be used to calculate the force exerted on the wire. The probe continues moving until the failure of the wire takes place (fig. 1C4). The whole tensile loading experiment is recorded on movie from side-view using FIB as image source. By analyzing the digital movie, the force and strain information can be obtained for every image by measurement of the deflection of the cantilever and can be calculated by measuring the stretched wire length and its initial length. Finally, the stress-strain curve can be plotted.