Optical Resolution Enhancement Research Articles

UV Photonic-Integrated-Circuits-Based Structured Illumination Microscopy With a Field of View Larger than $100\,\mu \text{m}^{2}$

Photonic integrated circuits (PICs) are expected to add practicality and new functionalities in a considerable number of optical applications, including optical microscopy. Here, we present a PIC design allowing a far-field structured UV illumination pattern with a fringe period as low as 370 nm, a fringe visibility of 0.83 over field of views of more than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$150\,\mu m\times 200\,\mu m$</tex-math></inline-formula> and a radiant intensity as large as 0.49 mW. The circuits include single mode waveguides with propagation losses of 3 dB/cm at a wavelength <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\lambda =360\,nm$</tex-math></inline-formula> , two diffraction gratings for beam shaping out of the chip plane, a beam splitter and a phase shifter. Using fluorescent gratings with pitches longer or shorter than the wavelength, as control objects, and a collecting lens with a numerical aperture of 0.5, the current PICs enable to highlight experimentally the Moiré pattern at the heart of the optical resolution enhancement and to achieve a doubling of the optical resolution in the direction of the illumination.

Enhancement of optical resolution in three-dimensional refractive-index tomograms of biological samples by employing micromirror-embedded coverslips.

Optical diffraction tomography (ODT) enables the reconstruction of the three-dimensional (3D) refractive-index (RI) distribution of a biological cell, which provides invaluable information for cellular and subcellular structures in a non-invasive manner. However, ODT suffers from an inferior axial resolution, due to the limited accessible angles imposed by the numerical aperture of the objective lens. In this study, we propose and experimentally demonstrate an approach to enhance the 3D reconstruction performance in ODT. By employing trapezoidal micromirrors, side scattered signals from the sample are measured for various side plane-wave-illumination angles. By combining the side scattered fields with the forward scattered fields, the axial resolution and 3D image quality of ODT are improved, without changing optical instruments. The feasibility and applicability of the proposed method are demonstrated by reconstructing the 3D RI distribution of a red blood cell and HeLa cells in hydrogel. We also present systematic analyses of the improved 3D imaging performance using numerical simulations and experimental measurements for the 3D transfer function, a point object, and a microsphere. The analyses demonstrate an improved axial resolution of 0.31 μm, 4.8 times smaller than that of the conventional method. The proposed method enables the non-invasive and accurate 3D imaging of 3D cultured cells, which is crucial for cell biology studies.

Improvement of imaging characteristics by pupil apodization in a laser scanning system for electro-photography

It is well known that imaging characteristics such as beam spot size and focal depth can be improved by applying apodization of amplitude and phase at the entrance pupil of an optical system. As an optical system for the electro-photography continuously requires highly resolved dot image and extended focal length to obtain more delicate expression with adequate production stability. Advantages from apodization technique can improve system performance and supply high degree of reliability of the optical system. However, issues on reduction in absolute optical power and on side-lobe of beam spot profile have been technical barrier for its useful implementation to imaging optics for commercial devices. Thorough this paper, theoretical feasibility of application of a pupil apodization to a laser scanning optical unit is mainly discussed in the aspects of optical power reduction and side-lobe of spot profile, as well as enhancement of optical resolution and focal depth. In addition, fundamental experimental results on beam spot profile through focal region are followed to uphold feasibility analysis.

Theoretical assessment of optical resolution enhancement and background fluorescence reduction by three-dimensional nonlinear structured illumination microscopy using stimulated emission depletion

Three-dimensional structured illumination microscopy (SIM) enlarges frequency cutoff laterally and axially by a factor of two, compared with conventional microscopy. However, its optical resolution is still fundamentally limited. It is necessary to introduce nonlinearity to enlarge frequency cutoff further. We propose three-dimensional nonlinear structured illumination microscopy based on stimulated emission depletion (STED) effect, which has a structured excitation pattern and a structured STED pattern, and both three-dimensional illumination patterns have the same lateral pitch and orientation. Theoretical analysis showed that nonlinearity induced by STED effect, which causes harmonics and contributes to enlarging frequency cutoff, depends on the phase difference between two structured illuminations and that the phase difference of π is the most efficient to increase nonlinearity. We also found that undesirable background fluorescence, which degenerates the contrast of structured pattern and limits the ability of SIM, can be reduced by our method. These results revealed that optical resolution improvement and background fluorescence reduction would be compatible. The feasibility study showed that our method will be realized with commercially available laser, having 3.5 times larger frequency cutoff compared with conventional microscopy.

The research of micro–nano laser patterning system based on digital micromirror device

This article presents the properties of the micro–nano sacle laser patterning system based on digital micromirror device. The introduction of the excellent spatial light modulator—digital micromirror device—provides a possibility for the patterning system to combine high resolution and accuracy with short writing time. The optical characteristics of the digital micromirror device–input imaging system are analyzed. An overlapping exposure method is proposed to obtain optical resolution enhancement and high fidelity of the input pattern. Based on this method, an optical synthetic-aperture effect is realized without increasing the system complexity or decreasing the process efficiency.

Optimal frequency coverages and parsings for imaging interferometric lithography

Imaging interferometric lithography (IIL) is an optical resolution enhancement technique, based on wavelength-division multiplexing, that combines off-axis illumination with multiple exposures and pupil filtering. In prior experiments, IIL was shown to be capable of improving the resolution of optics to subwavelength regimes but only in isolated parameter settings. The frequency parsing scheme plays a critical role in the resolution of aerial images from IIL and a platform for optimizing these frequency coverage parameters is currently lacking. We present three approaches to the optimization of IIL frequency coverage parameters. First, the exhaustive search approach and, then, the dynamic programming and greedy versions are presented. For unobstructed lenses and large numerical apertures, these approaches converge to frequency coverages with illumination points along the vertical and horizontal axes, while a lens with an obscured center results in a tilted frequency coverage. Two particular frequency parsing strategies are analyzed from the perspective of the quadratic term in the aerial intensity. Simulation results comparing the off-axis exposures with and without the on-axis exposure are presented. The effect of the quadratic terms on the exposure latitude is shown to have a positive effect in the dense edge areas and a negative effect on isolated edges.

Quantitative optical microscope with enhanced resolution using a pixelated liquid crystal spatial light modulator.

This paper presents a brief account of a novel optical microscope, which combines the advantages of two well-known techniques, namely phase contrast and phase stepping, to provide high contrast imaging and precision measurements. The inclusion of a programmable liquid crystal spatial light modulator provides for the phase stepping required, while also allowing flexibility for future improvements. The results shown reveal an important aspect of the system to facilitate quantitative sample measurements, with an enhancement of optical resolution compared with conventional optical imaging systems.

CARL–advantages of thin-film imaging for leading-edge lithography

Upcoming nodes in semiconductor lithography will surely require more complex imaging processes due to demanding critical dimensions. It is still questionable, if standard single layer resist processing and optical resolution enhancement techniques will meet future lithography requirements. Dry developable bilayer resist systems offer high resolution capability and wide focus windows due to the thin imaging photoresist layer that is applied on top of a thick light-absorbing and planarizing bottom resist. The CARL bilayer resist process with its unique “chemical biasing” step allows to grow lines or shrink contact holes after exposure, thus allowing wider process windows. Furthermore, the thin-film shrink concept significantly extends the natural resolution limit compared to standard single layer resist process, which is an attractive approach for future and NGL technologies.

Sub-0.1-µm-Pattern Fabrication Using a 193-nm Top Surface Imaging (TSI) Process

This paper presents a 193-nm top surface imaging (TSI) process for the sub-0.10-µm-device rule. The process achieved a 0.07-µm contact hole, 0.04-µm isolated line, and 0.06-µm space pattern without the need for any optical resolution enhancement technique. An exposure latitude of ±10% was obtained for the process with a 0.10-µm contact hole. We can obtain a sufficient depth of focus (DOF) by using an attenuated phase-shifting mask. We resolved a 0.085-µm line-and-space pattern by using an alternative phase-shifting mask, and obtained a 0.8-µm DOF for a 0.09-µm line-and-space pattern, using an alternative phase-shifting mask. We demonstrated the fabrication of sub-0.10-µm line-and-space binary patterns. Sub-0.10-µm patterns were produced by using the TSI process for 193-nm lithography.

Enhancing a hierarchical, parallel electron beam data conversion processor toward 1 Gbit memory lithography

A new hierarchical, parallel electron beam (EB) data conversion system has been developed to be used in lithography for the fabrication of memories of 1 Gbit or more. The conversion system has been enhanced not only to meet EB writing requirements but also to cover new data processing capabilities required for critical dimension control and for optical resolution enhancement. The conversion system takes full advantage of the hierarchical, parallel technique and generates patterns for various EB lithography systems, including the EX-8 mask writing system, EX-8D wafer direct writing system, and MC-100 defect inspection system. The interface with computer aided design (CAD) systems has been improved by realizing direct input from CADENCE Opus or Edge and from a design rule checker ENMA in addition to CALMA GDSII format. A design guide has been established to guarantee efficient, problem-free hierarchical data conversion. Also, a new data compaction technique has been employed. The system is equipped with functions for increasing EB writing throughput. Moreover, functions for multipass writing and resize writing are available to improve pattern transfer fidelity in the resist development or chromium etching process. In the EB proximity effect correction, Ghost and dose modulation can be selectably used. For alternating phase shifting mask fabrication, a new CAD software to automatically assign shifter phase has been added. When a model 1 Gbit dynamic random access memory pattern with 0.15 μm minimum features designed following the design guide was scaled up for 4× reticle mask making and converted to the resize writing pattern on a workstation with four central processor units, the maximum conversion time per layer was 7 min 17 s, and an average conversion time was about 5 min. The data volume per mask amounted to 456 Mbytes. Using the compaction technique, the data volume was reduced to 1/7.7 at the maximum, and an average data compaction ratio of 4.5 was obtained.

Image gathering and processing for enhanced resolution

Traditional approaches to optical resolution enhancement have involved either the design of appropriate image-formation systems or some type of postprocessing of an image that has already been formed. Results presented in this paper suggest that improved images can be obtained if the image-gathering system is designed specifically to enhance the performance of the image-restoration algorithm to be used. We consider a class of problems in which the total available spatial bandwidth is fixed but the location of this bandwidth along the spatial-frequency axis is to some extent under our control. For example, we might consider either a low-pass system or a bandpass system of the same total bandwidth. We show that system performance can be substantially improved by proper allocation of the available bandwidth in the spatial-frequency domain. The optimum allocation is shown to be a function of the signal-to-noise ratio. We also describe coherent and incoherent optical image-gathering systems that can achieve the desired spatial-frequency passbands.