MEMS Device for Quantitative In Situ Mechanical Testing in Electron Microscope

Xiaodong Wang, Shengcheng Mao, Jianfei Zhang, Zhipeng Li, Qingsong Deng, Jin Ning, Xudong Yang, Li Wang, Yuan Ji, Xiaochen Li, Yinong Liu, Ze Zhang (+1 others)
2017 Micromachines  
In this work, we designed a micro-electromechanical systems (MEMS) device that allows simultaneous direct measurement of mechanical properties during deformation under external stress and characterization of the evolution of nanomaterial microstructure within a transmission electron microscope. This MEMS device makes it easy to establish the correlation between microstructure and mechanical properties of nanomaterials. The device uses piezoresistive sensors to measure the force and displacement
more » ... of nanomaterials qualitatively, e.g., in wire and thin plate forms. The device has a theoretical displacement resolution of 0.19 nm and a force resolution of 2.1 µN. The device has a theoretical displacement range limit of 5.47 µm and a load range limit of 55.0 mN. 2 of 16 relationships at the nano and atomic scales [12, [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] . Some devices allow the observation of the microstructural evolution and simultaneous measurement of stress-strain curves, thus granting us an opportunity to understand the microscopic mechanisms of deformation and to better guide the development of new materials [27, 28] . Techniques currently available for mechanical testing at nanoscale include nano indentation [29], bugling [30], resonance [31], bending [32], and micro-tensile testing [33] using micro/nanosized samples in SEM, TEM, and atomic force microscope (AFM). For TEM in situ analysis, one of the main functional requirements of the technique for structure-property correlation studies is to measure quantitatively, the stress-strain behavior of the sample in TEM whilst having negligible impact on the double-axis tilt of the TEM sample holder. Nano indentation has been adapted in TEM for in situ deformation measurement [34] . Nano indentation has high displacement and force resolutions of 0.03 nm and 0.1 µN [35], respectively, which is commonly used to determine elastic modulus, hardness, and stiffness of the material surface [36] [37] [38] [39] [40] [41] . However, this technique places the displacement and force sensors at the end of the TEM holder, thereby prohibiting the β-axis tilt and partially limiting the orientation capability of the TEM. Recently, MEMS-based devices, integrating actuators, sensors, and signal processing circuits, on a millimeter and even micrometer scales, have been developed at the head of the TEM holder for studying the structure-property relationships at the atomic scale [42] [43] [44] . Thus, the evolution of mechanical parameters and microstructure of materials have been simultaneously obtained. This has granted an opportunity to study directly the influence of microstructure on the mechanical properties of materials and has provided experimental evidences for designing new high performance materials. To measure accurately, the mechanical properties of nanosized materials, the displacement and force sensor resolutions need to be in the order of tens of nanometers and micronewtons, respectively. Displacement and force in electron microscopes can be determined mainly by imaging and capacitance. The imaging method determines the displacement by measuring the displacement difference between two flexible beams in TEM/SEM. The load is then given as the product of the force sensor beam displacement and beam spring constant [45, 46] . By using the imaging method, it is not possible to output the stress-strain curve of the specimen in real time, thus restricting the deformation of materials to occur only at a very low strain rate. In the capacitance method, the displacement and force are measured based on the capacitance variance resulting from the deformation of the sensors located at the roots of the beams. This method has been widely used in harsh environments because of its superior properties, such as small temperature drift, low power consumption, good process compatibility, and direct signal output [47] . Differential capacitance based sensors have high force and displacement resolutions [27, 48, 49] . However, they have comparatively larger sizes, and thus, they can be used only in single-tilt or small angle double-tilt TEM holders, and they are difficult to use for atomic scale microstructural analysis. Piezoresistive sensors fabricated with semiconductor materials have advantages such as easy fabrication, small size, and high sensitivity [50], and thus, they are good candidates for double-tilt TEM holders. The operating principle of piezoresistive sensors is that their resistance varies with external stress/strain. The electric signals of the resistance can be directly read out using a Wheatstone bridge circuit, which is small and can be integrated with the sensor using the MEMS technique. Because of its small size, a piezoresistive sensor can be easily placed on simple structures such as cantilever and clamped beams [49] . To improve their sensitivity, piezoresistors, in many cases, are fabricated on the beam surface perpendicular to the force direction. The force resolution of piezoresistive sensors has been reported to be as small as nanonewtons [51] , thereby giving rise to the possibility of measuring the mechanical properties of nanosized materials. However, no MEMS-based devices with piezoresistive sensors seem to have been developed for structure-property studies at the nano/atomic scale. In this work, we designed a piezoresistive sensor based MEMS device for mechanical deformation of materials with displacement and force resolutions of 7 nm and 2.2 µN, respectively. The device is small and has a potential to be adapted in SEM/TEM for quantitative uniaxial tensile testing of Micromachines 2017, 8, 31 3 of 16 samples with thickness smaller than 100 nm and width of hundreds of nanometers. With this device in TEM/SEM, we can simultaneously study the mechanical properties and evolution of the material microstructure, which provides an opportunity to bridge the mechanical property-microstructure relationship from the micro to the atomic scales. An aluminum thin film sample, with a thickness of 510 nm, was subjected to trial test the device in SEM to validate its effectiveness. Figure 1 shows a schematic of the testing system design. Its operation control is given in Figure 1a . Mechanical Testing System Description of the System The system comprises an actuation system, a MEMS device, two single-system-power suppliers, and two digital multimeters (6 1 /2 digits). The two single-system-power suppliers provide a DC operating voltage for the two piezoresistive sensors. The beams on the device are driven by a piezoceramic actuator with a travel distance of 100 µm and a minimum step of 7 nm. Two digital multimeters were used to collect the sensor output voltages. Micromachines 2017, 8, 31 3 of 16 In this work, we designed a piezoresistive sensor based MEMS device for mechanical deformation of materials with displacement and force resolutions of 7 nm and 2.2 μN, respectively. The device is small and has a potential to be adapted in SEM/TEM for quantitative uniaxial tensile testing of samples with thickness smaller than 100 nm and width of hundreds of nanometers. With this device in TEM/SEM, we can simultaneously study the mechanical properties and evolution of the material microstructure, which provides an opportunity to bridge the mechanical property-microstructure relationship from the micro to the atomic scales. An aluminum thin film sample, with a thickness of 510 nm, was subjected to trial test the device in SEM to validate its effectiveness. Mechanical Testing System Description of the System
doi:10.3390/mi8020031 fatcat:nfdxlqgz6bavpp5svtx6pvor5u