Minjin Tang

Study of moiré grating fabrication on metal samples using nanoimprint lithography

A moiré grating is a basic optical component used in various moiré methods for deformation measurement. In this study, nanoimprint lithography (NIL) was proposed to produce high frequency moiré gratings on metal samples. A new type of NIL mold and a hot embossing system were developed to overcome the poor flatness and roughness of metal samples. This three-layer mold based on nickel grating was unbreakable, and the self-developed hot embossing system used a bellows cylinder to satisfy the parallelism requirement of grating fabrication on metal samples. In order to generate high quality moiré patterns, the grating profile of the mold was optimized. Then, 1200-3000 lines/mm frequency gratings were successfully fabricated on the different materials such as SiO2, aluminum and stainless steel. In order to evaluate the quality of the replication, the distortion in the fabricated SiO2 grating was analyzed by an inverse moiré method. As an application, the replicated grating on the aluminum sample in combination with the moiré interferometry was used to measure the tensile deformation of the sample. The successful experimental results demonstrate the feasibility and reliability of nanoimprint lithography to produce gratings on metal samples.

Phase-shifting laser scanning confocal microscopy moiré method and its applications

The laser scanning confocal microscopy (LSCM) moire method is an effective method for full-field displacement and strain measurement in the micrometer scale. In this study, the phase-shifting LSCM moire method is proposed for automatic processing of the moire fringe. A combined phase-shifting and loading system for LSCM was developed, which could offer a tensile load on the plate type sample with a maximum load range of up to 500 N. Using the PZT (piezoelectric ceramic) phase shifter in the loading system, the moire fringe phase was shifted and measured. Full field deformation was calculated according to the phase data. With the combination of LSCM and the self-developed loading system, tensile deformation of the aluminum sample was measured by the phase-shifting LSCM moire technique. Full-field deformations under the different loads and elastic modulus of the sample were obtained. Experimental results demonstrated the feasibility of this technique, and the accuracy for deformation measurement is clearly improved.

Micro-scale delaminating and buckling of thin film on soft substrate

In this paper, a simple process is suggested to estimate the interfacial toughness of the material system ?aluminum film/soft PDMS substrate?. The specimen, i.e. the aluminum film deposited on the soft polydimethylsiloxane (PDMS) substrate, is subject to a tensile load, and delaminating and buckling of aluminum film are observed in the perpendicular direction to the tensile strain. With the aid of the buckling blisters, the interfacial toughness of the material system is estimated. Large deformation is considered during the buckling of the thin film, and the interfacial toughness is deduced from a fracture theory. Besides, the evolution from one single blister to three blisters and then four blisters is observed in situ under microscope. This simplified method has potential applications to flexible electronics in which interfacial toughness of the metal film/soft substrate must be well controlled.

A measuring system for mechanical characterization of thin films based on a compact in situ micro-tensile tester and SEM moiré method

A measuring system for mechanical characterization of thin films based on a compact in situ micro-tensile tester and scanning electron microscope (SEM) moire method is proposed. The load is exerted by the tensile tester and the full field strain is measured by SEM moire method. The configuration of the tensile tester and the principle of SEM moire method are introduced. In the tensile tester, a lever structure is designed to amplify the displacement imposed by lead–zirconate–titanate (PZT) actuator. The SEM moire method is applied to measure the strain of the thin film, including both the average strain in the gage section and the local strain distribution at a specific region. As an application, the measuring system is applied to characterize the mechanical property of the free-standing aluminum thin film. The experimental results demonstrate the feasibility of the system and its good application potential for mechanical behavior analysis of film-like materials.

Optimum design of processing condition and experimental investigation of grating fabrication with hot embossing lithography

The cross-section profiles of polymer deformation in the hot embossing lithography process were studied by finite element method for various temperature, time and pressure. In order to successfully fabricate high-frequency grating lines, an optimal imprint condition was selected and the related experiments were carried out. The fabricated gratings were illuminated by the SEM image and AFM analysis, which agree well with the simulated results. Therefore, the finite element methods are helpful for a better comprehension of the polymer flow phenomena governing the pattern definition and the design of optimum processing conditions for successful grating fabrication.

A new method for the characterization of micro-/nano-periodic structures based on microscopic Moiré fringes

Linewidth and opening ratio (ratio of linewidth to period) are important parameters in characterizing micro-/nano-periodic and quasi-periodic structures. Periodic structures are conventionally characterized by the direct observation of specimens under a microscope. However, the field of view is relatively small, and only certain details can be acquired under a microscope. Moreover, the non-uniformity of the linewidth in quasi-periodic structures cannot be detected. This paper proposes a new characterization method for determining the linewidth and opening ratio of periodic structures based on Moiré fringe analysis. This method has the advantage of full-field characterization of the linewidth of micro-/nano-structures over a larger area than that afforded by direct observation. To validate the method, the linewidth of scanning electron microscope (SEM) scan lines was first calibrated with a standard grating. Next, a microperiodic structure with known geometry was characterized using this calibrated SEM system. The results indicate that the proposed method is simple and effective, indicating a potential approach for the characterization of gratings over large areas. This technique can be extended to various high-power scanning microscopes to characterize micro-/nano-structures.

Reconstruction of planar periodic structures based on Fourier analysis of moiré patterns

In recent years, inverse moire methods have been developed to reconstruct micro/nano-scale planar periodic structures with a larger field of view than those constructed using conventional methods. In these methods, moire fringes generated by superposition of the periodic structure and a reference grating are analyzed to reconstruct the periodic structure. There are two approaches to inverse moire methods: the fringe-centerlines method and the phase-shifting method. The former has lower accuracy and is difficult to automate, while the latter requires at least three moire images with complicated processing. A reconstruction method for planar periodic structures using Fourier analysis is proposed. This method can be used to characterize the micro/nano periodic structure from a single microscope moire pattern. At the same time, when combined with a linewidth characterization method, the period and linewidth of the microstructure can be obtained simultaneously. As practical examples, the period and linewidth of a scanning electron microscopy raster are calibrated. Then the microstructures of a micro-electroformed grating and a butterfly wing are reconstructed using the calibrated system. The proposed method provides a tool for the characterization of large area micro/nano periodic structures. Further, this is a promising approach to detect defects in periodic structures.

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