Xianglu Dai

The geometric phase analysis method based on the local high resolution discrete Fourier transform for deformation measurement

The geometric phase analysis (GPA) method based on the local high resolution discrete Fourier transform (LHR-DFT) for deformation measurement, defined as LHR-DFT GPA, is proposed to improve the measurement accuracy. In the general GPA method, the fundamental frequency of the image plays a crucial role. However, the fast Fourier transform, which is generally employed in the general GPA method, could make it difficult to locate the fundamental frequency accurately when the fundamental frequency is not located at an integer pixel position in the Fourier spectrum. This study focuses on this issue and presents a LHR-DFT algorithm that can locate the fundamental frequency with sub-pixel precision in a specific frequency region for the GPA method. An error analysis is offered and simulation is conducted to verify the effectiveness of the proposed method; both results show that the LHR-DFT algorithm can accurately locate the fundamental frequency and improve the measurement accuracy of the GPA method. Furthermore, typical tensile and bending tests are carried out and the experimental results verify the effectiveness of the proposed method.

Deformation grating fabrication technique based on the solvent-assisted microcontact molding

A deformation grating fabrication technique based on solvent-assisted microcontact molding (SAMIM) is reported in this paper. The fabrication process can be divided into three steps: imprinting a grating on a medium polymer substrate (MPS) by SAMIM, coating a thin metal film on the MPS, and transferring the film to the measured surface. In order to increase the stiffness of the elastic mold without decreasing its conformal contact formation ability, a re-useable, glass-embedded polydimethylsiloxane (PDMS) mold is used. In addition, a characterization method based on the Fourier transform and phase analysis is proposed to check the quality of the fabricated grating. Verified by experiment, the proposed fabrication technique can fabricate a high-frequency large-area grating on different specimens, which can be a qualified deformation sensor for the moiré method.

A novel method to fabricate micro-gratings applied for deformation measurement around a crack in a thin film

The grating fabrication technique has been of great concern in the use of the grating based deformation measurement method. Although various methods have been developed to effectively fabricate gratings, few are able to fabricate gratings directly onto thin films. In this paper, a novel grating fabricating method using focused ion beam deposition is proposed. The advantage of this method is that it causes smaller damage to the surface of the specimen compared with other methods and is especially suitable for deformation measurement of thin films at micro-scales. The fabricated micro-gratings that have been produced by this method have been used to measure the deformation around a crack in an aluminum film supported on a PDMS substrate. The deformation during the crack propagation has been measured with the fabricated micro-grating and the results indicate that this method is feasible to measure the deformation of thin films at micro-scales. It should also be noted that this method can be used to fabricate gratings on various substrates at micro-scales. The successful results indicate that this method is reliable and suitable for deformation measurement at micro-scales.

Fabrication of a DIC sensor for in-plane deformation measurement

A thin epoxy film with a micro speckle pattern was developed as a deformation sensor, and replicated on the surface of a sample. In order to improve the toughness of the thin epoxy film, a flexibilizer was added into the epoxy compound and the mechanical property of the thin epoxy film was analyzed. The measurement results show that the toughness of the thin film has been improved greatly, which enlarges the range of application of deformation measurement. Finally, the feasibility of application of the thin film digital image correlation (DIC) sensor was verified using a standard tensile experiment. And the interface deformation of the signal lap joint was measured by using fabrication and replication of the micro speckle pattern and micro digital image correlation. The failure mode of a single lap joint at micro-scale lengths was determined by analyzing the captured images of the micro speckle pattern.

Residual stress measurement in thin films using a slitting method with geometric phase analysis under a dual beam (FIB/SEM) system

Stress generated during thin film deposition is a critical issue for many applications. In general, the possible origins of the residual stress include intrinsic and extrinsic stresses. Since high residual stresses can cause detrimental effects on the film, such as delamination and wrinkle, it is of great importance to quantify the residual stress for the optimal design and the evaluation of its mechanical behavior. In this study, a method combining focused ion beam (FIB) milling and geometric phase analysis (GPA) is developed to assess the residual stress of thin films. The procedures of the residual stress measurement using this method include grating fabrication and slot milling by FIB, high-resolution scanning electron microscope (SEM) imaging of the grating before and after stress relaxation, and deformation analysis by GPA. The residual stress can be inferred from the released deformation using the reference displacements of the finite element model. As an application, this method was utilized to measure the residual stress in a TiAlSiN film, and the measured result is in good agreement with that obtained by the curvature method. In order to analyze the measurement error, the influence factors of Ga+ bombardment and the deposited platinum layer on the stress calculation are also discussed in detail.

High-accuracy magnification calibration for a microscope based on an improved discrete Fourier transform

Abstract. Microscopes are widely applied in characterizing feature sizes at the micro-/nanoscale, and magnification calibration plays a key role in achieving precise measurements. However, it is difficult to obtain accurate results by using the general magnification calibration method if comparing the displayed size of a test-piece under microscope and its original one. In this study, a high-accuracy and automatic magnification calibration method that could be applied to different types of microscopes is proposed. A standard grating is employed as the reference, and a high-resolution discrete Fourier transform is used to analyze the images captured under various magnifications in this method. With utilization of the high-order harmonic component in the Fourier spectrum, the proposed method is capable of performing the calibration over a wide range of magnifications while maintaining identical precision. The relative error of the proposed method can be theoretically limited to 0.01%; moreover, the image noise can be tolerated. Furthermore, the validation and extensive adaptability of this method are demonstrated by calibrating the magnification of a scanning electron microscope and an optical microscope.

Study on the high temperature deformation measurement using the mark shearing technique

The mark shearing technique has been successfully utilized for room-temperature strain measurement in solid mechanics research. However, when the method is applied to deformation measurement at high temperature, some new issues will occur. From our experience, we have found that an inevitable thermal radiation will emit from the oven in the measurement and may lead to a distortion of the measuring system. As a result, an undesired measurement error will arise; furthermore, the oxidation resistance marker is crucial to obtain an ideal result. In order to solve the abovementioned issues, the mark shearing techniques for high-temperature deformation measurement are studied in this work, and a novel method for fabricating the marker pattern is proposed, which can be utilized to fabricate a deformation carrier for high temperature on a different specimen surface. In combination with this marker with our self-developed mark shearing system, the high-temperature (1000 °C) mechanical properties of the high-temperature nickel alloy (GH4037) was measured.In order to characterize the thermal deformation caused by the oven during the high temperature experiment, the surface temperature distribution of the mark shearing system is measured by an infrared camera. On the basis of this result, the corresponding thermal deformation is analyzed by the finite element analysis (FEM) method. From the analysis result, we can conclude that the virtual strain of the system incurred by thermal radiation cannot be ignored for a long-time measurement.

Fabrication of high-density moiré gratings

Optical metrology is widely used to characterize surface deformations due to the noncontact, full-field, and highly accurate nature of the measurements. In combination with advanced microscopes, optical metrology methods have recently been extended into the microand nanoscales. In measurements that use the moiré (beat pattern produced between gratings of initially equal spacing) method,1–3 gratings act as sensors that record the original deformation of a specimen. Precise preparation of the gratings is therefore required to ensure accurate measurements. Existing technologies for grating fabrication do not meet all the necessary requirements (i.e., production of high-frequency and large-area gratings at low cost) simultaneously. Ordinary photolithography, due to its diffraction limitation, can generally only produce gratings with hundreds of lines per millimeter. Holographic lithography can be used to make gratings with frequencies of up to 4800 lines per millimeter, but there are strict requirements on the optical system and the alignment techniques. Electron beam lithography and focused ion beam lithography can produce fine gratings with up to 10,000 lines per millimeter, but the grating area is very small (micrometer scale).4, 5 We have developed a new moiré grating fabrication method that is based on the nanoimprint lithography (NIL) technique.6 NIL creates nanoscale patterns through mechanical deformation and has low costs, high throughput, and high resolution. This process has already been used in several microelectronic engineering, optical engineering, and bioengineering applications. Our technique can be used to produce gratings with large areas and high frequencies on different specimens, and at fast rates (see Figure 1).7–9 The preparation of the stamp that is used for imprinting is a key issue in our method. When imprinting is conducted at low pressure, a full-contact condition (between the specimen and Figure 1. (a) Grating fabricated on a traditional (millimeter-scale dimensions) specimen (1200 lines per millimeter, cross type).7 (b) Grating fabricated on a thin (<1 m) film specimen (1200 lines per millimeter, cross type).8

High-frequency deformation grating fabrication techniques and applications

The rapid development of micro-electronics and micro-nano material engineering make it an urgent task to characterize the mechanical properties of micro-device and micro-nano material accurately. Due to the advantages of high precision, high sensitivity and full field measurement, moiré method has been applied in the micro-deformation measurement widely. Since the grating is the indispensable deformation sensor of moiré method, how to fabricate high frequency grating with high quality is the key problem to solve for moiré method. In this paper, some fabrication techniques developed recently with their applications will be summarized, including holographic photolithography, electron beam lithography (EBL), focused ion beam (FIB) and nano-imprint lithography(NIL), aiming to popularize the applications of moiré method in the micro-deformation measurement and provide some valuable guidelines on how to choose a proper fabrication technique.