The recent development of soft x-ray interference lithography in SSRF

This paper introduces the recent progress in methodologies and their related applications based on the soft x-ray interference lithography beamline in the Shanghai synchrotron radiation facility.Dual-beam,multibeam interference lithography and Talbot lithography have been adopted as basic methods in the beamline.To improve the experimental performance,a precise real-time vibration evaluation system has been established;and the lithography stability has been greatly improved.In order to meet the demands for higher resolution and practical application,novel experimental methods have been developed,such as high-order diffraction interference exposure,high-aspect-ratio and large-area stitching exposure,and parallel direct writing achromatic Talbot lithography.As of now,a 25 nm half-pitch pattern has been obtained;and a cm2 exposure area has been achieved in practical samples.The above methods have been applied to extreme ultraviolet photoresist evaluation,photonic crystal and surface plasmonic effect research,and so on.

Developments at SSRF in soft X-ray interference lithography

The soft X-ray interference lithography(XIL) branch beamline at Shanghai Synchrotron Radiation Facility(SSRF) is briefly introduced in this article. It is designed for obtaining 1D(line/space) and 2D(dot/hole)periodic nanostructures by using two or more coherent extreme ultraviolet(EUV) beams from an undulator source. A transmission-diffraction-grating type of interferometer is used at the end station. Initial results reveal high performance of the beamline, with 50 nm half-pitch 1D and 2D patterns from a single exposure area of400 μm× 400 μm. XIL is used in a growing number of areas, such as EUV resist test, surface enhanced Raman scattering(SERS) and color filter plasmonic devices. By using highly coherent EUV beam, broadband coherent diffractive imaging can be performed on the XIL beamline. Well reconstructed pinhole of φ20 μm has been realized.

2-μm single longitudinal mode GaSb-based laterally coupled distributed feedback laser with regrowth-free shallow-etched gratings by interference lithography

We report a type-I Ga Sb-based laterally coupled distributed-feedback（LC-DFB） laser with shallow-etched gratings operating a continuous wave at room temperature without re-growth process. Second-order Bragg gratings are fabricated alongside the ridge waveguide by interference lithography. Index-coupled LC-DFB laser with a cavity of 1500 μm achieves single longitudinal mode continuous-wave operation at 20℃ with side mode suppression ratio（SMSR） as high as 24 dB.The maximum single mode continuous-wave output power is about 10 mW at room temperature（uncoated facet）. A low threshold current density of 230 A/cm^2 is achieved with differential quantum efficiency estimated to be 93 mW/A. The laser shows a good wavelength stability against drive current and working temperature.

The theoretic analysis of maskless surface plasmon resonant interference lithography by prism coupling

The use of an attenuated total reflection-coupling mode of prism coated with metal film to excite the interference of the surface plasmon polaritons （SPPs） was proposed for periodic patterning with a resolution of subwavelength scale. High intensity of electric field can be obtained because of the coupling between SPPs and evanescence under a resonance condition, which can reduce exposure time and improve contrast. In this paper, several critical parameters for maskless surface plasmon resonant lithography are described, and the preliminary simulation based on a finite difference timedomain technique agrees well with the theoretical analysis, which demonstrates this scheme and provides the theoretical basis for further experiments.

Fabrication of 4-Inch Nano Patterned Wafer with High Uniformity by Laser Interference Lithography

We report the fabrication of 4-inch nano patterned wafer by two-beam laser interference lithography and analyze the uniformity in detail. The profile of the dots array with a period of 800 nm divided into five regions is characterized by a scanning electron microscope. The average size in each region ranges from 270 nm to 320 nm,and the deviation is almost 4%, which is approaching the applicable value of 3% in the industrial process. We simulate the two-beam laser interference lithography system with MATLAB software and then calculate the distribution of light intensity around the 4 inch area. The experimental data fit very well with the calculated results. Analysis of the experimental data and calculated data indicates that laser beam quality and space filter play important roles in achieving a periodical nanoscale pattern with high uniformity and large area. There is the potential to obtain more practical applications.

Surface Plasmon Interference Lithography Assisted by a Fabry-Perot Cavity Composed of Subwavelength Metal Grating and Thin Metal Film

A surface plasmon interference lithography assisted by a Fabry-Perot （F-P） cavity composed of subwavelength metal gratings and a thin metal fihn is proposed to fabricate high-quality nanopatterns. The calculated results indicate that uniform straight interference fringes with high contrast and high electric-field intensity are formed in the resist under the F-P cavity. The analyses of spatial frequency spectra illuminate the physical mechanism of the formation for the interference fringes. The influence of the F-P cavity spacing is discussed in detail. Moreover, the error analyses reveal that all parameters except the metal grating period in this scheme can bear large tolerances for the device fabrication.

Quantum interference in laser-assisted photo-ionization excited by a femtosecond x-ray pulse

The photoelectron energy spectra （PESs） excited by monochromatic femtosecond x-ray pulses in the presence of a femtosecond laser are investigated. APES is composed of a set of separate peaks, showing interesting comb-like structures. These structures result from the quantum interferences between photoelectron wave packets generated at different times. The width and the localization of each peak as well as the number of peaks are determined by all the laser and x-ray parameters. Most of peak heights of the PES are higher than the classical predictions.

Bragg-Angle Diffraction in Slant Gratings Fabricated by Single-Beam Interference Lithography

A single-beam interference-lithography scheme is demonstrated for the fabrication of large-area slant gratings, which requires exposure of the photoresist thin film spin-coated on a glass plate with polished side-walls to a single laser beam in the ultraviolet and requires small coherence length of the laser. No additional beam splitting scheme and no adjustments for laser-beam overlapping and for optical path-length balancing are needed. Bragg-angle diffractions are observed as strong optical extinction that is tunable with changing the angle of incidence. This device is important for the design of efficient filters, beam splitters, and photonic devices.

Research on Influence of Electromagnetic Interference on X-Ray Digital Detection

X-ray digital imaging technology has found wide application owing to its advantages of real-time, visualization and rapid imaging. In substations the substantial electromagnetic interference has some influence on the live detection by the X-ray digital imaging technology, hindering the promotion of the technology in the detection of electric equipment. Based on a large number of field tests, the author carded out a series of researches on electromagnetic interference protection measures, image de-noising, and image enhancement algorithms.

Nanofabrication of 50 nm zone plates through e-beam lithography with local proximity effect correction for x-ray imaging

High resolution Fresnel zone plates for nanoscale three-dimensional imaging of materials by both soft and hard x-rays are increasingly needed by the broad applications in nanoscience and nanotechnology.When the outmost zone-width is shrinking down to 50 nm or even below,patterning the zone plates with high aspect ratio by electron beam lithography still remains a challenge because of the proximity effect.The uneven charge distribution in the exposed resist is still frequently observed even after standard proximity effect correction(PEC),because of the large variety in the line width.This work develops a new strategy,nicknamed as local proximity effect correction(LPEC),efficiently modifying the deposited energy over the whole zone plate on the top of proximity effect correction.By this way,50 nm zone plates with the aspect ratio from 4:1 up to 15:1 and the duty cycle close to 0.5 have been fabricated.Their imaging capability in soft(1.3 keV)and hard(9 keV)x-ray,respectively,has been demonstrated in Shanghai Synchrotron Radiation Facility(SSRF)with the resolution of 50 nm.The local proximity effect correction developed in this work should also be generally significant for the generation of zone plates with high resolutions beyond 50 nm.

Fabrication of a 256-bits organic memory by soft x-ray lithography

This paper reports a procedure of soft x-ray lithography for the fabrication of an organic crossbar structure. Electron beam lithography is employed to fabricate the mask for soft x-ray lithography, with direct writing technology to the lithograph positive resist and polymethyl methacrylate on the polyimide film. Then Au is electroplated on the polyimide film. Hard contact mode exposure is used in x-ray lithography to transfer the graph from the mask to the wafer. The 256-bits organic memory is achieved with the critical dimension of 250 nm.

Effects of Prestressing of the Ring Gear in Interference Fit on Flexural Fatigue Strength of Tooth Root

When the gearbox body interference is connected to the ring gear, prestressing occurs in the ring gear, which has a significant impact on the strength and life of the gear. Research on the prestressing of the inner ring gear is in the preliminary stage, and the distribution rule of the prestressing and the influence of each parameter on the interference prestressing have not been derived. In this paper, based on the method of calculating the prestressing of the thick cylinder in interference fit, the ring gear is found to be equivalent to a thick cylinder, and the distribution rule of prestressing of the ring gear in the interference fit is inferred. Then, by modeling and analyzing the gearbox body and ring gear in the interference fit using ABAQUS, the distribution rule of prestressing the ring gear in the interference fit is obtained through a numerical simulation. Finally, the prestressing of the ring gear in the interference fit is measured using X-ray di raction, and the distribution rule of prestressing of the ring gear in the interference fit is obtained through analysis. Compared with the distribution rule of prestressing in theory, numerical simulation, and experiment, the theoretical distribution rule of prestressing is amended through a statistical method, and a more accurate formula of prestressing is obtained. Through the calculation of the stress and bending moment in the dangerous section of the ring gear through prestressing, the formula for checking the tooth root flexural fatigue strength in the interference fit prestressing is inferred. This research proposes a tooth root bending strength conditional formula for the inner ring gear of the interference fit, which serves as a guide for the design and production of the actual interference joint inner ring gear.

Confinement-induced nanocrystal alignment of conjugated polymer by the soft-stamped nanoimprint lithography

Soft-stamped nanoimprint lithography（NIL） is considered as one of the most effective processes of nanoscale patterning because of its low cost and high throughput. In this work, this method is used to emboss the poly（9, 9-dioctylfluorene）film. By reducing the linewidth of the nanogratings on the stamp, the orientations of nanocrystals are confined along the grating vector in the nanoimprint process, where the confinement linewidth is comparable to the geometrical size of the nanocrystal. When the linewidth is about 400 nm, the poly（9, 9-dioctylfluorene）（PFO） nanocrystals could be orderly arranged in the nanogratings, so that both pattern transfer and well-aligned nanocrystal arrangement could be achieved in a single step by the soft-stamped NIL. The relevant mechanism of the nanocrystalline alignment in these nanogratings is fully discussed. The modulation of nanocrystal alignment is of benefit to the charge mobilities and other performances of PFO-based devices for the future applications.

Response of MG63 Osteoblast Cells to Surface Modification of Ti-6Al-4V Implant Alloy by Laser Interference Lithography

The response of human osteoblast-like osteosarcoma cells （MG63） to surface modification of Ti-6Al-4V implant alloy was investigated by Laser Interference Lithography （LIL）. In this work, laser interference lithography was employed to fabricate the microstructures of grooves, dots and dimples onto the surfaces of Ti-6Al-4V samples. Two and three beam LIL systems were developed to carry out the experiments. The laser treatment resulted in the increases of the roughness and the contact angle of water on the implant alloy surfaces. The proliferation of osteoblasts was analyzed by MTT （3-（4,5-dirnethyl- 2-thiazolyl）-2,5-diphenyl-2-H-tetrazolium bromide） assay for the time periods of 4 hours, 2 days, 3 days, and 6 days. The MTT test results demonstrated that the laser treatment surfaces had a positive impact on the proliferation of os- teoblast cells after 24 hours. The alloy surface morphology and the morphological changes of MG63 cells cultured on the laser textured Ti-6Al-4V surface were observed by Scanning Electron Microscope （SEM）. The SEM results indicated that the os- teoblast cells were aligned on grooved surfaces and they were prolonged with the structures. Enzymatic detachment results showed that the 20 μm grooved structures provided the better cell adhesion to the textured Ti-6Al-4V surfaces.

Laser Interference Lithography for Fabrication of Planar Scale Gratings for Optical Metrology

Laser interference lithography is an attractive method for the fabrication of a large-area two-dimensional planar scale grating,which can be employed as a scale for multi-axis optical encoders or a diffractive optical element in many types of optical sensors.Especially,optical configurations such as Lloyd's mirror interferometer based on the division of wavefront method can generate interference fringe fields for the patterning of grating pattern structures at a single exposure in a stable manner.For the fabrication of a two-dimensional scale grating to be used in a planar/surface encoder,an orthogonal two-axis Lloyd's mirror interferometer,which has been realized through innovation to Lloyd’s mirror interferometer,has been developed.In addition,the concept of the patterning of the two-dimensional orthogonal pattern structure at a single exposure has been extended to the non-orthogonal two-axis Lloyd’s mirror interferometer.Furthermore,the optical setup for the non-orthogonal two-axis Lloyd's mirror interferometer has been optimized for the fabrication of a large-area scale grating.In this review article,principles of generating interference fringe fields for the fabrication of a scale grating based on the interference lithography are reviewed,while focusing on the fabrication of a two-dimensional scale grating for planar/surface encoders.Verification of the pitch of the fabricated pattern structures,whose accuracy strongly affects the performance of planar/surface encoders,is also an important task to be addressed.In this paper,major methods for the evaluation of a grating pitch are also reviewed.

Piecewise Linear Weighted Iterative Algorithm for Beam Alignment in Scanning Beam Interference Lithography

To obtain a good interference fringe contrast and high fidelity,an automated beam iterative alignment is achieved in scanning beam interference lithography(SBIL).To solve the problem of alignment failure caused by a large beam angle(or position)overshoot exceeding the detector range while also speeding up the convergence,a weighted iterative algorithm using a weight parameter that is changed linearly piecewise is proposed.The changes in the beam angle and position deviation during the alignment process based on different iterative algorithms are compared by experiment and simulation.The results show that the proposed iterative algorithm can be used to suppress the beam angle(or position)overshoot,avoiding alignment failure caused by over-ranging.In addition,the convergence speed can be effectively increased.The algorithm proposed can optimize the beam alignment process in SBIL.

Flexible Evanescent Wave Interference Lithography System for Sub-half-Wavelength Complex Relief Structures Fabrication

Surface microstructures impart various useful properties to objects,for example,improving optical characteristics,wettability,and sliding properties.It is well known that biomimicking relief structures are effective in making such properties arise and have been studied to be applied to various devices.Furthermore,they are expected to be utilized not only for improving a particular property but also for adding more complex functions on a device's urface by fabricating different multi-functional structures on a single surface in the future.However,to begin with,artificially fabricating such biomimicking special functional relief is difficult.One typical feature of biomimicking surfaces is the dual-scale structure,the smaller one of which is less than 200 nm.Moreover,in the case of realizing the more complex devices,it is necessary to fabricate various forms as changing process conditions dynamically.In this study,we proposed and developed a flexible evanescent wave interference lithography system as a novel fabrication method,which allows us to realize the fabrication of sub-half-wavelength complex relief structures.Firstly,we theoretically analyzed the fundamental behavior of the fabricated structure and found that the proposed concept has the potential to realize one of the target complex structures.Secondly,we developed the proposed system with high process flexibility,in which the number of beams,the azimuth angles,and the polarization can be simply manipulated.Finally,we validated the concept of the designed system by some experiments,where we fabricated dual-scale structures with 840-nm and 190-nm fringe patterns simultaneously.

四激光束干涉光刻制造纳米级孔阵的理论分析

为提供一个在大范围内曝光出深亚微米甚至纳米级周期性密集图形的廉价的方法 ,研究了四激光束干涉光刻的原理 ,分析了干涉曝光的结果 ,并进行了计算机模拟 用现有的光源 ,如4 4 2nm、365nm、2 4 8nm、193nm激光 ,曝光得到的图形的临界尺寸容易做到 180～ 70nm 具有实际上无限制的焦深和容易实现的大视场 适合硅基CCDs。

超精密外差利特罗式光栅干涉仪位移测量系统

开展了扫描干涉光刻机工作台超精密位移测量的实验研究,以提高扫描干涉光刻机的环境鲁棒性。针对扫描干涉光刻机工作台位移测量精度,提出了新型高环境鲁棒性外差利特罗式光栅干涉仪测量系统。介绍了系统测量原理,设计了测量系统,提出了基于Elden公式的系统死程误差建模方法。设计制造了尺寸仅为48mm×48mm×18mm的光栅干涉仪。基于误差模型计算了死程误差,计算结果表明:对于1.52mm死程的光栅干涉仪,宽松的环境波动指标(温度波动为0.01℃、压力梯度为±7.5Pa、相对湿度波动为1.5%、CO2含量波动为±50×10-6)仅引起±0.05nm的死程误差。最后,设计了基于商用双频激光平面镜干涉仪的测量比对系统,开展了光栅干涉仪原理验证实验和测并量稳定性实验。原理验证实验表明:光栅干涉仪原理正确且系统分辨率达0.41nm。测量稳定性实验表明:常规实验室环境下,环境波动引起的死程误差为7.59nm(3σ)@<0.9Hz&1~10Hz,优于同等环境条件下平面镜干涉仪的31.11nm(3σ)@<0.9Hz&1~10Hz。实验结果显示系统具有很高的环境鲁棒性。

激光干涉光刻法制作100nm掩模

介绍了一种利用激光干涉光刻技术得到特征图形,并通过离子束刻蚀将图形转移到铬层上,从而获得掩模的方法。针对掩模透光率以及对干涉图形对比度可能产生影响的两个参数分别进行了数值仿真,从而证明此方法的可行性和参数的优化选择。自搭干涉光刻实验系统,用257 nm的激光光源实现光刻,得到特征尺寸为100 nm的图形,再经过离子束刻蚀,最终得到周期200 nm、线宽100 nm的掩模。