Digital moiré fringe measurement method for alignment in imprint lithography

Accurate layer-to-layer alignment, which is of prime importance for the fabrication of multilayer nanostructures in integrated circuits, is one of the main obstacles for imprint lithography. Current alignment measurement techniques commonly involve an image detection process for coarse alignment followed by a grating interference process for fine alignment. Though this kind of two-level alignment system is reasonable for measurement, when it is used in real imprint lithography, it is inconvenient because of the existence of a complex loading system that needs space for alignment. In this study, we propose a fine alignment method using only image detection using grating images and digital moiré fringe technology. In this method, though the gratings are also selected as alignment marks for accurate measurement, they do not interfere with the physics. The grating images captured from the template and wafer are used to measure angular displacement and to form parallel digital moiré fringes. The relative linear displacement between the template and wafer is determined by detecting the spatial phase of parallel digital moiré fringes. Owing to the magnification effect of digital moiré fringes, this method is capable of generating accurate measurements. According to the experimental results, this digital moiré fringe technique is accurate to less than 10 nm. In addition, without a complex grating interference system, this method has the advantage of being easy to operate.     

Thermal-insensitive moiré interferometry

An optical arrangement has been devised that will permit the measurement of one component of the relative displacement, and therefore the corresponding strain, in a surface at nonambient temperatures by use of coherent optics. The method is based on the technique of moiré interferometry with illumination at grazing incidence. A demonstration of principle is described, with the beams passing through the flame of a blowtorch.     

Application of the moiré deflectometry on divergent laser beam to the measurement of the angle of arrival fluctuations and the refractive index structure constant in the turbulent atmosphere

When a slightly divergent laser beam passes through a turbulent ground level atmosphere and strikes a linear grating, fluctuating self-images are formed at Talbot distances. By superimposing a similar grating on one of the self-images, even for the case of parallel gratings' lines, fluctuating moiré fringes are formed owing to the beam divergence. Recording the successive moiré patterns by a CCD camera and feeding them to a computer, after filtering the higher spatial frequencies, produces highly magnified fluctuations of the laser beam. Using moiré fringe fluctuations we have calculated the fluctuations of the angle of arrival and the atmospheric refractive index structure constant. The implementation of the technique is straightforward, a telescope is not required, fluctuations can be magnified more than ten times, and the precision of the technique is similar to that reported in our previous work.     

Scanning moiré and phase shifting with time delay and integration imaging

Scanning moiré is generated by undersampling of a phase-modulated grating pattern. In projection profilometry the scanning moiré pattern represents equal height and depth contours on a test object. By use of time delay and integration (TDI) imaging, it is possible to generate an on-line scanning moiré pattern from the complete periphery of a rotating cylindrical object. For automated phase and profile unwrapping from scanning moiré fringes, phase-shifting interferometry techniques are most desirable. However, lack of spatial information in the undersampled scanning moiré fringes introduces serious errors in phase unwrapping. We report a method that uses oversupply of data to balance the effect of undersampling. This oversupply is achieved with a TDI feature that permits programmable image magnification in the scanning direction.     

Projection-type moiré topography with frequency modulation technique using liquid crystal digital gratings

A projection-type moiré topography that uses a frequency modulation technique with two liquid crystal digital gratings (LCDGs) is proposed. In this method, moiré contours are filtered to remove the image of the original grating from the moiré contours by moving the image created by the LCDGs. The frequency modulation technique can be used to analyze the fringe of step heights or other separate areas by modulating the fringe interval. The experimental results showed the possibility of measuring three-dimensional shapes and step heights.     

Distinct moiré textures of in-plane electric polarizations for distinguishing moiré origins in homobilayers

In binary compound 2D insulators/semiconductors such as hexagonal boron nitride(h BN), the different electron affinities of atoms can give rise to out-of-plane electric polarizations across inversion asymmetric van der Waals interface of near 0° interlayer twisting. Here we show that at a general stacking order where sliding breaks 2π/3-rotational symmetry, the interfacial charge redistribution also leads to an in-plane electric polarization, with a magnitude comparable to that of the out-of-pla...     

A moiré micro strain gauge

We have built a new strain gauge based on the moiré technique. This strain gauge mainly consists of two frames that can move with respect to each other. Displacements are recorded by using the moiré technique. We use a pair of similar gratings attached to the frames. The gratings are installed in parallel without physical contact and their lines making a small angle with one another. A moiré pattern is formed due to superimposing of the gratings. A diode laser light passes through the moiré pattern and a narrow slit, and hits on a light sensor. In response to external stress, one of the gratings is displaced and, as a result, the moiré fringes move in front of the slit. Due to the fringes movements, the light intensity on the detector varies and is recorded as voltage. The voltage output can be used to measure the strain. This instrument can detect displacements of the order of micron. In this paper we show the experimental results of our instrument.     

Quantitative measurement of displacement and strain by the numerical moirémethod

The numerical moirémethod with sensitivity as high as 0.03 nm has been presented.A quantitative displacement and strain analysis program has been proposed by using this method.It is applied to au edge dislocation and a stacking fault in aluminum.The measured strain of edge dislocation is compared with theoretical prediction given by Peierls-Nabarro dislocation model.The displacement of stacking fault is also obtained.     

Integrating moiré and emission tomography to visualize and diagnose high-temperature flow fields

In this Letter, moiré and emission tomography are integrated to visualize and diagnose high-temperature flow fields, and a jet flame is chosen as a practical example for experiment. The refractive index and intensity distributions are simultaneously obtained by moiré and emission tomography, respectively. Based on the intensity distribution, the structure of the jet flame is well visualized, so that the spatial distribution of species composition can be considered in the temperature reconstruction process. Finally, the refractive index and intensity distributions are matched, and a partition model is adopted to reconstruct the temperature distribution of the jet flame.     

3D deformation and shape measurement using phase-stepping DSPI

A new optical arrangement of phase-stepping digital speckle pattern interferom-etry is presented.The system can be used to measure 3D deformation and shape of a curvesurface simultaneously.The phase at each pixel can be determined by appling phase-steppingtechniques.This not only provides high accurate measurement results,but also permits auto-matic analysis of the experimental data.A speckled object reference beam is used,and itmakes the system has the advantage that the measurements are less sensitive to vibration andwhole body motion.     

Periodic overlayers and moiré patterns: theoretical studies of geometric properties

Single crystal surfaces with periodic overlayers, such as graphene on hexagonal metal substrates, are found to exhibit, apart from their intrinsic periodicity, additional long-range order expressed by approximate surface lattices with large lattice constants. This phenomenon can be described as geometrically analogous to lateral interference effects resulting in periodic moiré patterns which are characterized by two-dimensional moiré lattices. Here we discuss in detail the mathematical formalism determining such moiré patterns based on concepts of two-dimensional Fourier transformation including coincidence lattices. The formalism provides simple relations that allow one to calculate possible moiré lattice vectors in their dependence on rotation angles α and scaling factors p1,p2 for periodic (p1 × p2)Rα overlayers on substrate surfaces described by general Bravais lattices. Specific emphasis will be given to hexagonal lattices where experimental data are available.     

Image hiding based on near-optimal moiré gratings

Image hiding based on time-averaged fringes produced by non-harmonic oscillations and near-optimal moiré gratings is presented in this paper. The secret image is embedded into the background moiré grating. Phase matching and initial stochastic phase scrambling algorithms are used to encrypt the image. The decoding of the image is completely visual. The embedded secret image appears when the encrypted image is oscillated in a predefined direction, according to a predefined law of motion. No secret is leaked when the encrypted image is oscillated harmonically at any amplitude of oscillation. The criterion of the optimality of a moiré grating serves as a fitness function for evolutionary algorithms which are used to identify a near-optimal moiré grating for image hiding applications. Numerical experiments are used to illustrate the functionality of the method.     

Ultra low-level tritium measurement using electrolytic enrichment and LSC†

We describe an advanced methodology for low-level tritium measurement in regard to calibration, electrolytic tritium enrichment, liquid scintillation counting (LSC) measurement, and prevention of sample contamination. Details are given on enrichment parameters and electrode processes for optimisation of enrichment reproducibility and on optimisation of LSC stability. Intercomparison results demonstrate high accuracy of the tritium measurement system. The use of accurate tritium data for groundwater dating in the southern hemisphere is demonstrated with data from several groundwater systems of New Zealand.     

Focal length measurement based on Hartmann–Shack principle

A novel means of focal length measurement is proposed based upon Hartmann–Shack principle of wavefront detection. Mathematical approaches of focal length measurement are deduced without the necessity of knowing the position of lens principle plane. In experiment, laser sources with wavelength of 633 nm and 780 nm are separately used and focal length of three pieces of doublets is measured under two wavelengths mentioned above. As were shown by the results, this method of focal length measurement could not only accurately measure lens focal length, but also evaluate chromatic aberration of them. Since the intensity of light is detected by CCD, optical characteristics of lens beyond the range of visible light are also measurable when response range of CCD allows. This new means of focal length measurement is characterized by high accuracy, fast measuring and easy to setup, thereby, it is applicable in wide fields of area.     

Application of moiré technique to the measurement of the atmospheric turbulence parameters related to the angle of arrival fluctuations

There are several methods for measuring ground-level atmospheric turbulence parameters, such as the refractive index profile and its fluctuations, correlations of the fluctuations in space and time, and the atmospheric refractive-index structure constant. These methods are based mainly on the measurement of fluctuations in intensity and location of an image formed by light propagating in the turbulent atmosphere or the fluctuations in impinging points of narrow light beams traversing the ground-level atmosphere. Exploiting the moiré technique, we suggest a high-precision approach for determining fluctuaions in the angle of arrival. When a low-frequency grating (carrier grating) is installed at a suitable distance from a telescope, its image, practically, forms on the focal plane of the telescope objective. Superimposing a physical grating (probe grating) of the same pitch as the image grating on the image forms the moiré pattern. The atmospheric turbulence distorts the image grating. Processing the fluctuations of successive moiré fringes can yield the mentioned parameters across a rather large cross section of the atmosphere with high accuracy, because of the moiré technique's magnifying character and access to a large volume of data, and does so in a comparatively simple and reliable manner.     

Trapping and transporting aerosols with a single optical bottle beam generated by moiré techniques

We demonstrate optical trapping and manipulation of aerosols with an optical bottle beam generated by the moiré techniques. We observe stable trapping and back-and-forth transportation of a variety of absorbing carbon particles suspended in air, ranging from clusters of nanosized buckminsterfullerene C?? to micrometer-sized carbon powders.     

Metal-phthalocyanine array on the moiré pattern of a graphene sheet

Iron-phthalocyanine (FePc) molecules have been adsorbed on a graphene sheet prepared on the Ir(111) surface. The FePc molecules are flat-lying on graphene, as determined by near-edge X-ray absorption fine-structure, constituting a sub-nanometer thick molecular array at the single-layer coverage. The flat FePc single-layer presents a weak interaction of the organic macrocycle with the graphene surface and Ir subsurface substrate. Further FePc deposition on top of the first flat single-layer determines a three-dimensional island growth with varying molecular orientation.     

Electronic properties of monolayer copper selenide with one-dimensional moiré patterns

Strain engineering is a vital way to manipulate the electronic properties of two-dimensional (2D) materials. As a typical representative of transition metal mono-chalcogenides (TMMs), a honeycomb CuSe monolayer features with one-dimensional (1D) moiré patterns owing to the uniaxial strain along one of three equivalent orientations of Cu(111) substrates. Here, by combining low-temperature scanning tunneling microscopy/spectroscopy (STM/S) experiments and density functional theory (DFT) calculations, we systematically investigate the electronic properties of the strained CuSe monolayer on the Cu(111) substrate. Our results show the semiconducting feature of CuSe monolayer with a band gap of 1.28 eV and the 1D periodical modulation of electronic properties by the 1D moiré patterns. Except for the uniaxially strained CuSe monolayer, we observed domain boundary and line defects in the CuSe monolayer, where the biaxial-strain and strain-free conditions can be investigated respectively. STS measurements for the three different strain regions show that the first peak in conduction band will move downward with the increasing strain. DFT calculations based on the three CuSe atomic models with different strain inside reproduced the peak movement. The present findings not only enrich the fundamental comprehension toward the influence of strain on electronic properties at 2D limit, but also offer the benchmark for the development of 2D semiconductor materials.     

Indentation measurements on the eardrum with automated projection moiré profilometry

Computer modeling of middle ear mechanics is an important tool to investigate its complex behavior, but correct mechanical and elastic parameters are needed to obtain realistic simulations. A possible way to determine eardrum elasticity in situ is the use of point indentation measurements. The eardrum is, however, a small fragile membrane, so a non-contacting high-resolution technique is needed to measure the shape change caused by point indentation. We have developed a projection moiré interferometer combined with an indentation actuator and a high-resolution force sensor. The apparatus applies deformations up to 1 mm with a resolution of $1\mathrm{\mu }\mathrm{m}$, while the indentation force is measured with a resolution better than 1 mN. The moiré setup delivers height data on $512\times 512$ points through phase-shifting, with a height resolution of $15\mathrm{\mu }\mathrm{m}$. Shape recordings are made on a rabbit eardrum at different indentation distances, and indentation force is recorded simultaneously.