With the wide application of micro-nano structures in various fields, micro-nano technology has been developed continuously. Among them, micro-nano surface morphology detection technology has become a research hotspot, because the surface geometry and morphology of micro-nano devices determine the functional level and service life of devices. With the increasing complexity of micro nano device surface morphology and the increasing sharp edges, the detection technologies are facing severe challenges. Among various detection methods, structured illumination microscopy (SIM) has attracted many research interests due to the characteristics of high accuracy, strong adaptability and high efficiency. The existing SIM are mainly based on phase-shift technique, Hilbert transform technique and global Fourier transform technique. However, for complex-surface and steep-edge measurements, it is difficult for traditional SIM to achieve both high accuracy and high efficiency. In this paper, a method based on SIM which combines vertical scan and phase-shift is proposed. In this proposed measurement system, vertical scanning of the object is synchronized with the switching of the phase-shifted fringe pattern and only one fringe pattern needs to be projected, which enables a point-to-point processing defined as local Fourier transform method in this paper to be utilized to extract the modulation information which will reserve high-frequency information of the image so it can be applied to both smooth and rough surfaces. Simulation and experiment are carried out to demonstrate that the proposed method can successfully realize fast and accurate detection of complex-surface and steep-edge.
Significance: Fourier ptychography microscopy (FPM) is a computational optical imaging technology that employs angularly varying illuminations and a phase retrieval algorithm to achieve a wide field of view and high-resolution imaging simultaneously. In the FPM, LED position error will reduce the quality of the reconstructed high-resolution image. To correct the LED positions, current methods consider each of the LED positions as independent and use an optimization algorithm to get each of the positions. When the positional misalignment is large or the search position falls into a local optimal value, the current methods may lack stability and accuracy.
Aim: We improve the model of the LED position and propose an accurate and stable two-step correction scheme (tcFPM) to calibrate the LED position error.
Approach: The improved LED positions model combines the overall offset, which represents the relative deviation of the LED array and the optical axis, with the slight deviation of each LED’s independent position. In the tcFPM, the overall offset of the LED array is corrected at first, which obtains an approximate value of the overall offset of the LED array. Then the position of each LED is precisely adjusted, which obtains the slight offset of each LED.
Results: This LED position error model is more in line with the actual situation. The simulation and experimental results show that the method has high accuracy in correcting the LED position. Furthermore, the reconstruction process of tcFPM is more stable and significantly improves the quality of the reconstruction results, which is compared with some LED position error correction methods.
Conclusions: An LED position error correction technology is proposed, which has a stable iterative process and improves the reconstruction accuracy of complex amplitude.
Maskless photolithography was proposed to achieve the conventional and low-cost micro and nano fabrication, the pivotal of such technology was the application of digital micro-mirrors devices (DMD). Based on maskless photolithography, we designed a specific bifocal compound eyes (BCE) which made up of an array of two superimposed microlens. However, during our experiments, we found the existence of nonlinear relationships among gray levels, exposure intensity and development depths. To precise control the surface profiles, we did several tests and interpolations were used on the data we gathered. Finally, we ascertained the development depth of each grayscale, a gray mask was designed and filled to 1024*768 to fit the size of DMD.
The structured illumination microscopy fills an unmet need for minimally invasive tools to image micrometer and nanoscale structure. However, conventional structured illumination super-resolution methods require precise phase shift of the illumination pattern, which leads to an inefficient image reconstruction process and a complicated imaging system. To solve these problems, we propose a novel image reconstruction method using only two image to acquire the frequencies beyond the diffraction limit. This method has an efficient image post-processing, which obtains extended frequencies without accurate estimation of phase-shifting. The related simulations and experiments are carried out to demonstrate the feasibility of this method.
The microscopy assisted by dielectric microspheres has emerged as an effective way to improve the imaging resolution of conventional optical microscope. The photonic nanojet(PNJ) is simulated by software, and super-resolution imaging experiments are carried out successfully. The results show that the silica(SiO2) microsphere with low refractive index can visualize a grating containing 139-nm-width lines with an interspacing of 139 nm under visible light illumination when it is partially immersed in alcohol. However, the barium titanate glass(BTG) microspheres with high refractive index can achieve super-resolution imaging only fully immersed in alcohol.
Effective measurement of the surface and thickness variation of thin films are important to achieve a special function and better performance for a coated optical device. In this research, we propose a new incoherent technique named Fourier transform-based structured illumination microscopy (FTSIM) to detect the surface topography and thickness distribution. In this technique, a sinusoidal fringe pattern produced by digital micro-mirror devices (DMD) is projected onto the sample. The modulation estimation which depends on the surface and thickness of thin films is obtained by two-dimensional Fourier transform algorithm. Further, separating the reflected signals from the film boundaries, the surface finish of the film, as well as a film thickness map, can be achieved at the same time. With this method, only one pattern is required to determine the modulation value of a whole field. The measurement system is relatively simple and only an ordinary objective is enough to achieve imaging of the sample. Both theory and experiments are conducted in detail to demonstrate that the availability of this method.
Three-dimensional measurement and inspection is an area with growing needs and interests in many domains, such as integrated circuits (IC), medical cure, and chemistry. Among the methods, broadband light interferometry is widely utilized due to its large measurement range, noncontact and high precision. In this paper, we propose a spatial modulation depth-based method to retrieve the surface topography through analyzing the characteristics of both frequency and spatial domains in the interferogram. Due to the characteristics of spatial modulation depth, the technique could effectively suppress the negative influences caused by light fluctuations and external disturbance. Both theory and experiments are elaborated to confirm that the proposed method can greatly improve the measurement stability and sensitivity with high precision. This technique can achieve a superior robustness with the potential to be applied in online topography measurement.
The application of dual-grating moiré fringe is introduced in lithography alignment. Minute angular displacement between the two alignment marks will lead to the tilt of moiré fringe, which has an influence in high-accuracy alignment. Proposed is a novel measurement method of angular displacement based on the phase of moiré fringe. The relationship between phase of moiré fringe and angular displacement is analyzed. Both simulation and experiment indicate that an angular displacement can be determined to achieve a linear displacement measurement with error of nanometer level.
Photon sieve is a new nano-scale imaging aperture. When the diameter of pinholes of photon sieve is less than the
wavelength of incident light, scaler diffraction theory is no longer valid for this condition. So vector theory must be used.
The finite difference time domain (FDTD) is an effectual tool of numerical calculation and analysis of light field. We put
forward researching high NA photon sieve with FDTD method. First we analyze the imaging properties of a single
pinhole of photon sieve with vector diffraction theory and then introduce the principle and realization of FDTD. At last
FDTD method is used to the numerical simulation of a pinhole. The simulation results are aslo compared to the results
using scaler diffraction theory. It shows that scaler diffraction theory is not valid. The necessity and feasibility of using
FDTD method to analyze and design high NA photon sieve is proved.
The relative position of wafer and mask can be calculated by information of Moiré fringe during alignment, a maskless
lithography alignment method based on circular gratings Moiré fringes phase-shifting technique is proposed in this paper.
Circular grating Moiré fringes have characteristics of measuring simultaneously angular displacement and line
displacement. Location information of wafer in alignment can be real-time reflected in spatial phase of Moiré fringes. A
digital micromirror device controlled by a computer is used to generate phase-shifting grating labels, and phase-shifting
Moiré fringes will be formed by superposition of phase-shifting grating labels with grating label on the wafer. The
position information of wafer can be abstained by phase analysis using Fourier transform method combined with
phase-shifting technique,and gives feedback to the displacement stage to realize alignment. The theory basis of this
method is emphatically introduced. Also, application of this method in maskless lithography alignment is analyzed in
detail. Simulation results show that this method is high in accuracy, simple in operation and simple in algorithm. It
provides a feasible method for lithography alignment technique.
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