1078 Ádám István Hegyi et al.
Figure 1. a: The initial focused ion beam milling step (marked by the grid) to fabricate the “raw” pillar and the hole around it. A large enough hole is necessary for the compression test. For this milling step, a 30-nA ion current is applied. b: The second and final milling step uses a 45° ion direction angle (see Fig. 2). The ion current is 5 nA, and the area removed during this step is marked by the grid. Both pictures are taken from the ion direction.
This increases the effectiveness of the milling by a factor of 2.5–3 comparedwith the commonly used perpendicular beam setup (Ishitani et al., 2004). The 45° milling direction applied in the second phase allows for the fabrication of micropillars anywhere on the flat sample surface. Recall that the Ga ion beam forms an irradiated layer on the micropillar. However, the slightly tilted final polishing step with the reduced 100pA ion current and lowered acceleration voltage (20 kV or 10 kV) significantly diminishes the deteriorating effect of ion implantation in the studied pillar sizes (Greer et al., 2008). To sum up, the most important features of the proposed
milling procedure are as follows: (i) micropillars can be fabricated at any position on the substrate surface, (ii) preparation is touchless, thus damage or predeformation of the pillar can be avoided during the entire production process, (iii) the final pillar is practically taper-free (the inclination of the side surface was always less than ±0.5°), (iv) the method is relatively fast, with an average milling time of<4 h for a 4×4× 12 µm3 pillar (the fabrication time is similar to the average milling times using the annular method for the same pillar size), and (v) detrimental effects related to Ga implantation can be suppressed.
IN SITU DEVICE
An in situ micromechanical test requires that the testing device be placed inside the chamber of the SEM. Such a compression device was developed in our laboratory for implementation in an FEI Quanta 3D SEM (FEI, Hillsboro, OR, USA). A schematic of the device is shown in Figure 3. Two linear ultrasonic motors position the sample in the
X and Y directions. The AE transducer is mounted on the top of this double stage. In the Z direction, two stages are used.
One is a linear step-motor stage for “raw” movement of the compressing tip closer to the sample. The second stage, moun- ted on the linear step-motor stage, is a piezoelectric positioning (PEP) stage with a resolution of ~0.1nm. During the compres- sion test, only this stage is moved. A standard spring mounted on the PEP stage,with high transversal but very low longitudinal stiffness, is used to measure the external force. The elongation e of the spring is measured by a capacitive sensor with 0.1nm resolution. If the PEP stage is moved a distance d, and the capacitive sensor measures an elongation e, then the sample deformation is ε=d−e,and the actingforce is F=ke,where k is the stiffness of the spring. Pillar compression is performed with a flat punch diamond tip. To avoid charging the compressing head in the SEM, a boron-doped tipmust be used. To measure instabilities related to the PLC effect and
dislocation avalanches, a fast feedback controlling systemand a minimum data collection rate of 1kHz are crucial. This is achieved by analogous proportional−integral−derivative-type feedback electronics and a fast 16 bit AD converter. The range and resolution parameters of the device are summarized in Table 1. To achieve the listed resolutions, the thermal and elastic
elongation of the device components must remain negligible during the entire measurement (typically several minutes). For this reason, several additional parts were added to the compression device. The main issue was reducing the heat produced by the stage motors. Therefore, the Quanta 3D SEM is equipped with an environmental stage with a Peltier sample holder to control the sample temperature. The cold point of the Peltier stage is mounted to the bottom of the device, stabilizing the temperature at 15°C. Another issue was the vibrations of the force sensor spring due to the absence of the damping effect of air in the evacuated
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