Papers by Author: Xi De Li

Abstract: In recent years with the development of MEMS and NEMS, various micro and nano scale experiments are required. In general, the smaller the sample, the smaller the force is in the measurement. But it is difficult to load and measure such small force. We developed a probe-type loading and force sensor system to measure micro/nano samples. The system employs a semiconductor strain gauge of a cantilever type sensor and a micro manipulator. A highly sensitive, stable sensing cantilever beam made of single crystal silicon is ion implanted to form the P-type resistor (strain sensor). A tungsten probe with 100 nm radius of curvature was attached to the end of the cantilever as the micro loading tip. We constructed the measurement system and investigated its properties, such as linearity, dynamic response and stability. We also employed microspeckle interferometry to calibrate the force sensor. In preliminary experiments, we successfully obtained the force resolution 0.7 μN and applied our probe-type microforce sensor to calibrate an atomic force microscope (AFM) probe beam and test a single silkworm filament.

Abstract: In this paper, a speckle microinterferometric system was employed to study the thermal deformation of the Cu microbridges with different dimensions. The deflections of the microbridges caused by the thermal loading were measured with real-time by the speckle microinterferometric system and the surface temperatures of the bridges were recorded using a digital thermometer. The deformation evaluation after microbridges buckling was also recorded with our testing system. Then, the experimental results were compared with the finite element analysis (FEA).

Abstract: Recently, quantitatively nondestructive testing (QNDT) is becoming an accepted concept in some industries and scientific research areas. The combination of shearing speckle interferometry (SSI) and mechanical models is employed to quantitatively estimate defect characteristic parameters (DCP), such as coordinates, size, embedding depth, etc. However, quantitative calculation of DCP relies on the actual displacement slope in the mechanical models, but the slope in SSI is represented by the difference of displacements between the two neighboring points with a distance, i.e. the shearing amount. This leads to a deviation in calculating DCP. This paper will investigate the deviation of the relative displacement and derivate displacement introduced by the shearographic approximation in cases of two deformation models, one is a thin circular plate and the other a spherical pressure shell under the pressure loading. Two kinds of defects, a cavity and a crack, are embedded in the structures and their deformations are calculated by FEM.

Abstract: In Microelectromechanical System (MEMS), the capillary force plays an important role owing to the strong capillary force often makes the suspended or moving structures in MEMS adhere to contact the substrate, which lead to the failure of the devices. This paper presents experimental and theoretical results that characterize the capillary adhesion of micro-cantilevers by means of capillary actuation. Micro-cantilever beams were loaded at various locations along the freestanding portion of the beam using the capillary force and the deflections of the beams were real-time and in-situ measured with micro speckle interferometry. The mechanical stability and adhesion of micro-cantilever under capillary forces were examined and the deflecting configurations of the beams and the relationship of the magnitude of the capillary force to the size of the specimen were presented.

Abstract: In the present study, a real-time calibration method for micro displacement sensors is introduced, and a calibration system is developed. SPCM, the sequence pulse counting method previously proposed by us, is capable of automatically determining both the larger range displacement and the performance of a sensor, such as the nonlinear error and the displacement sensitivity within a quarter of the light source wavelength. The new calibration system consists of a programmable motor driven platform, an out-of-plane sensitive electronic speckle pattern interferometry (ESPI), and a sequence image acquisition system. The platform is used to provide displacement changes of a moving component and its displacement is measured by the ESPI and calibrated sensor synchronously. The calibration accuracy of the proposed method is in the submicrometer level and the displacement range can be from sub-micrometer to millimeters depending on the storage capacity of the computer and the correlation property of the interferometer. Three capacitance-type displacement sensors have been calibrated successfully, whose displacement ranges are –300μm to 300μm, –30μm to 30μm, and –3μm to 3μm, respectively.

Abstract: A special speckle microinterferometer has been developed to test the mechanical properties of thin films electroplated on the single crystal silicon wafer. A piezo-actuated micro-loading unit is synchronized with the microinterfermeter to measure thin film deflection in bending with an accuracy of sub-micrometers. All of the film specimens were microfabricated to be the type of microbridge samples. They are made of Cu and NiFe, the sizes from 1102.9µm to 213.7µm long, 491.0µm to 9.7µm wide. The corresponding thicknesses are 9.4µm and 7.6µm, respectively. Deflections of the microbradge samples can be measured full-field and real-time by using the microinterferometer and no patterning or marking of the specimen surface is needed. The loading force is directly measured using a miniature load cell. The Young’s moduli are calculated for both material and sample size from the load-deflection curves. Test techniques, procedures and factors which affect on the deflection measurements are briefly presented along with detailed analyzes of the results.

Abstract: Quantitative nondestructive testing (QNDT) is required for the in-service inspection of high-cost structures whose failure could lead tragic consequences. The optical methods are widely used for NDT and NDE. However, most of them base on the qualitative detection of the partial fringes induced by the defects. The defect characteristic parameters (DCPs), such as coordinates and types, are easily obtained, but it is difficult to obtain other parameters, e.g. the defect size and embedded depth due to the non-unique relation between the DCPs and the deformation of detected defects. In this paper, the optical method, design optimization and FEM are combined to accomplish QNDT. Three types of defects are inspected and their DCPs values are obtained quantitatively. Moreover, factors that influence detection accuracy of the DCPs are also discussed.