Nanotribological properties of silicon surfaces nanopatterned by laser interference lithography

The problems caused by the adhesive force and friction force become more critical when the size of M/NEMS devices shrinks to micro/nano-scale. The nanotexture-patterned surface is an effective approach to reduce friction force on micro/nano-scale. Laser interference lithography is an attractive method to fabricate micro/nanotextures, which is maskless and allows large area periodical structures to be patterned by a couple of seconds’ exposure in a simple equipment system. We fabricate various nanogrooves with different pitch and space width on silicon wafers by laser interference lithography and chemical etching. We investigate the nanotribological properties of the patterned surfaces by AFM/FFM. We show that friction on the nano/micro-scale is related to the coverage rate of the nanogrooves, which decreases with increase in the space width and decrease in the pitch.     

The three-dimensional photonic crystals coated by gold nanoparticles

We report on the fabrication of metallodielectric photonic crystals by means of interference lithography and subsequent coating by gold nanoparticles. The grating is realized in a SU-8 photoresist using a He-Cd laser of wavelength 442 nm. The use of the wavelength found within the photoresist low absorption band enables fabricating structures that are uniform in depth. Parameters of the photoresist exposure and development for obtaining a porous structure corresponding to an orthorhombic lattice are determined. Coating of photonic crystals by gold nanoparticles is realized by reduction of chloroauric acid by a number of reductants in a water solution. This research shows that the combination of interference lithography and chemical coating by metal is attractive for the fabrication of metallodielectric three-dimensionally periodic microstructures.     

Enhanced surface plasmon interference lithography from cavity resonance in the grating slits

Surface plasmon interference lithography based on grating diffraction has been studied both theoretically and experimentally in recent years. In this paper, we demonstrate that the cavity resonance in the grating slits can improve the subwavelength interference, not only the intensity but also the uniformity of the pattern. Both the typical lithography structure which merely consists of periodic metallic gratings and the modified structure equipped with a reflection layer are studied. The finite element method has been performed to study the interference pattern. Numerical simulations show that the property of the interference pattern is the optimum when cavity resonance happens. This enhancement can be applied to all the lithography structures which are based on the grating diffraction.     

Plasmonic structure generation by laser illumination of silica colloid spheres deposited onto prepatterned polymer-bimetal films

Plasmonic structures are prepared on bimetal films evaporated onto glass substrates applying a multi-step process, and atomic force microscopy is utilized to study the structures after each step. Sub-micrometer gratings are generated on polycarbonate films spin-coated onto silver-gold bimetal layers by interference lithography (IL) applying the fourth harmonics of a Nd:YAG laser. These polymer gratings are used as prepatterned templates in order to deposit silica colloid spheres by spin-coating. It is shown that the conditions of periodic silica sphere-array formation along the template valleys are sufficiently large grating modulation depth, appropriate ratio of silica sphere diameter to grating period, and optimized speed of spinning. The periodic silica sphere arrays are illuminated by a homogeneous KrF excimer laser beam, and periodically arrayed sub-wavelength holes are drilled into bimetal films via colloid sphere lithography (CSL). The characteristic dimensions of the resulted plasmonic structures are defined by the polymer grating period and by the silica colloid sphere diameter. Attenuated total reflection spectroscopy is performed exciting plasmons on different metal-dielectric interfacial structures by the second harmonic of a continuous Nd:YAG laser. The polar and azimuthal angle dependent grating-coupling and scattering effects of the complex periodic structures on the resonance characteristic of plasmons is demonstrated.     

Eliminating the undercut phenomenon in interference lithography for the fabrication of nano-imprint lithography stamp

We show an easy method to eliminate the undercut profile of photoresist by fabricating periodic nano-patterns on a substrate using interference lithography. An undercut phenomenon occurs frequently on the sidewall of photoresist patterns because of the 3-dimensional intensity distribution generated when two beams are merged to make an interferogram. This is mainly caused by the vertical interference between the incident beam and the one reflected from the surface of a substrate, and bottom-anti-reflection-coating (BARC) material is usually used to prevent beams from being reflected onto the substrate. We propose a simple post-process which helps researchers fabricate well-defined patterns without using BARC material. We developed this process to fabricate stamps for nano-imprint lithography at low cost, and show the results of our nano-imprint process which transfers patterns on a stamp directly through thermal resist.     

Laser nano-fabrication of large-area plasmonic structures and surface plasmon resonance tuning by thermal effect

A simple and flexible technique aimed to generate large-area periodic nano-dot array features on metal thin films by laser interference lithography (LIL) has been demonstrated. In this paper, gold nano-dot arrays with a period of ∼450 nm and a dot diameter of ∼100 nm on quartz substrates coated with a gold film of 50 nm thick were fabricated. Multiple enhanced transmission peaks were observed in this patterned film. In addition to the characteristic peak of the gold surface plasmon resonance around 500 nm, multiple shoulder peaks that range from 550 to 700 nm were also observed in the nano-chain array structures. These shoulder peaks disappeared after thermal annealing. It was found that the nano-dots became smaller and well-separated nano-balls under the high temperature annealing process. These nano-structures have potential applications in solar cell, nano-lithography and biosensing.     

大面积金纳米线光栅的制备

利用激光干涉光刻和金纳米颗粒胶体溶液制备了宽度在100 nm以下且总面积达到平方厘米量级的金纳米线光栅结构.制备过程中,首先在表面镀有厚度约为200 nm的铟锡氧化物薄膜的面积为1 cm×1 cm的玻璃基片表面旋涂光刻胶,然后利用紫外激光干涉光刻制备光刻胶纳米光栅结构.有效控制干涉光刻过程中的曝光量、显影时间,获得小占空比的光刻胶光栅.再以光刻胶纳米光栅作为模板,旋涂金纳米颗粒胶体溶液.充分利用金纳米颗粒胶体溶液在光刻胶表面浸润性差的特点,限制旋涂后留存在光刻胶光栅槽中金纳米颗粒的数量,从而达到限制金纳米线宽度的目的.最后在250℃将样品进行退火处理5 min.获得了周期为400 nm且占空比小于1:4的金纳米线光栅结构,其有效面积为1 cm².以波导共振模式与粒子等离子共振模式间耦合作用为特征的光谱学响应特性验证了波导耦合金属光子晶体的成功制备,为小传感体积新型生物传感器的开发提供了性能良好的金属光子晶体芯片.     

Large-area metallic photonic crystal fabrication with interference lithography and dry etching

Large-area one-dimensional periodic metallic structures were fabricated on a waveguide layer with interference lithography and dry etching. As narrow as 115-nm gold nanowires were obtained with a period of 395 nm, covering a homogeneous area as large as 5 × 5 mm². Extinction measurements demonstrate the strong coupling between waveguide mode and gold-particle plasmon resonance. We demonstrate good agreement with a scattering-matrix theory. The dispersion of the metallic photonic crystals was obtained by angle-resolved measurements. This method represents a simple and low-cost way to fabricate large-area metallic photonic crystals.     

Resonance characteristics of localized plasmonic structures with periodic ZnO nano-patterns

Localized plasmonic structures with the periodic ZnO nano-patterns are demonstrated to increase the sensing characteristics of plasmonic sensor. The ZnO nano-patterns with 30 and 50 nm thicknesses are formed on the Au thin film of 50 nm, which have the periodic nano-patterns of 300 nm. Localized plasmonic structures are optimized using the three-dimensional finite-difference time-domain method as a function of incident angle for the width and thickness of the ZnO nano-structures. Localized plasmonic structures with the periodic ZnO nano-holes are fabricated using the double exposure technique by laser interference lithography. The measured resonance angles of 47.5° and 54° are obtained in the localized plasmonic structures with the periodic ZnO nano-patterns of 30 and 50 nm thicknesses, respectively.     

Micro-optical components based on silicon mold technology

Two micro-optical components fabricated by silicon molds are described. One component is micro-cavity in micro-scales and the other is sub-micron grating with a pitch of 200 nm. The feasibility of two methods for the micro-fabrication of the silicon molds is investigated: one method is for making hundreds of micrometer size silicon molds by using conventional photolithography and deep reactive ion etching (Deep-RIE) technique combined with wet etching; and the other is for fabricating sub-micron grating molds by using electron beam lithography and fast atom beam etching (FAB). Sub-wavelength structure is successfully transferred from silicon mold to poly-methyl methacrylate (PMMA) material. The yield and repeatability of the original master are quite good. This technique can also be used to fabricate other micro-scale structures.     

激光全息法制作二、三维光子晶体的模拟计算及禁带分析

激光全息法制作二、三维光子晶体相比于传统半导体微加工及精密机械加工技术具有很多优势，比如通过一次光辐射就可以制作出大体积、均匀的周期性结构，且能更自由、更容易地控制光子晶体结构.提出一种多光束干涉模型，通过设计模型中光束的各项参数，计算分析出二、三维光子晶体的结构.基于平面波展开法，理论计算了fcc结构光子晶体的完全禁带随填充率和介电常数比变化的情况.以上计算结果为后期实验中采用激光全息法制作二、三维光子晶体结构提供了良好的指导方向和理论依据. 关键词： 光子晶体 激光全息 多光束干涉 完全禁带     

基于数字微镜器件亚微米制备技术研究

为了实时、便捷的改变光刻图案以用于微纳光子器件制备,使用数字微镜器件构建了一套无掩模亚微米尺度制备系统.基于阿贝成像原理分析了周期结构在相干光照明下的成像过程,并用数值模拟以及空间滤波实验证明了这个过程.使用此实验系统制作出了周期为900 nm的二维结构以及周期为数十微米的带缺陷结构.实验表明,使用数字微镜器件可以方便的制作出亚微米尺度的图案.     

Laser interference lithography using spray/spin photoresist development method for consistent periodic nanostructures

Generally, a simple immersion method for development of photoresist (PR) has been used to fabricate nanostructures by interference lithography (IL). However, the immersion method has the disadvantage that fabrication is inconsistent, especially for large-area periodic structures. Herein, we introduce the spray/spin PR development (SSPRD) method to fabricate periodic nanostructures using IL. By quantitative analysis and comparison, we characterized the effectiveness of the SSPRD method to develop PR. In our experiments the SSPRD method produced reliable uniform nanostructures, whereas the immersion method showed very poor consistency. In the SSPRD, rotation speed was very important: if it was too low the development speed differed between edges and center; if the rotation speed was too high it caused a distortion of nanostructures by unstable local flow induced by spraying and rotation So, to reduce this distortion, we adopted the puddle developing process; as a result the uniformity and repeatability of developed nanostructures were improved. These results demonstrate that the SSPRD method can be useful for fabrication of consistent periodic nanostructures.     

Color generation in butterfly wings and fabrication of such structures

The wings of the morpho butterfly demonstrate an iridescent blue color over wide viewing angles. The mechanism that generates this blue color is studied. Optical and transmission electron microscopy of the butterfly wings reveal a complex wing structure with as many as 24 layers with periodic structures. The color generation is caused by interference of the multilayer structure as well as diffraction. It is possible to specially design grating structures so that a specific blue color can be generated and observed over wider angles. To demonstrate the grating concept, complex multigratings are designed and fabricated with electron-beam lithography. The light-diffraction properties of these gratings are presented.     

Subwavelength lithography using metallic grating waveguide heterostructure

We present subwavelength periodic gratings achieved by employing a metallic grating waveguide heterostructure (MGWHS). The mask can be designed to make one of its diffraction order (±mth) waves resonate with the surface plasmon wave supported by the MGWHS. With a finite-difference time-domain method, we numerically demonstrate that one-dimensional periodic structure of about 60 nm feature, which is far beyond the diffraction limit, can be patterned with the interference of the 3rd diffraction order waves of the mask at a wavelength of 546 nm. The technique can also be extended to two-dimensional patterns using circularly polarized incidence and for the incidence with an angle θ.     

Fabrication of large-scale periodic silicon nanopillar arrays for 2D nanomold using modified nanosphere lithography

We present a fabrication procedure that can form large-scale periodic silicon nanopillar arrays for 2D nanomold which determines the feature size of nanoimprint lithography, using modified nanosphere lithography. The size of silicon nanopillars can be easily controlled by an etching and oxidation process. The period and density of nanopillar arrays are determined by the initial diameter of polystyrene (PS) spheres. In our experiment, the smallest nanopillar has a full width half maximum (FWHM) of approximately 50 nm, and the density of silicon pillar is ∼10⁹/cm². Using this approach, it is possible to fabricate 2D nanoimprint lithography mask with 50 nm resolution.     

纳米压印多孔硅模板的研究

纳米压印模板通常采用极紫外光刻、聚焦离子束光刻和电子束光刻等传统光刻技术制备,成本较高.寻找一种简单、低成本的纳米压印模板制备方法以提升纳米压印光刻技术的应用成为研究的重点与难点.本文以多孔氧化铝为母模板,采用纳米压印光刻技术对纳米多孔硅模板的制备进行了研究.在硅基表面成功制备出纳米多孔阵列结构,孔间距为350—560 nm,孔径在170—480 nm,孔深为200 nm.在激发波长为514 nm时,拉曼光谱的测试结果表明,相对于单面抛光的硅片,纳米多孔结构的硅模板拉曼光强有了约12倍左右的提升,对提升硅基光电器件的应用具有重要的意义.最后,利用多孔硅模板作为纳米压印母模板,通过热压印技术,成功制备出了聚合物纳米柱软模板.     

Subwavelength structures for broadband antireflection application

The subwavelength structures are designed and fabricated for broadband antireflection application. Under target of zero reflectivity, the parameters of periodic 2-D continuous conical structures are analyzed by the finite-difference time-domain (FDTD) method. The corresponding conical structures are obtained with spatial period of 350 nm and structure height of 300 nm, respectively. The 2-D continuous conical structured surface is fabricated by micro-replication process combining with the originated structure fabrication realized by interference lithography, Ni mold electroplation and replication by using UV imprinting into plastics. The average reflectances of the simulation and replicated polymer prototype are about 0.50% and 0.54% within the spectral ranges of 400-650 nm, respectively. In a word, the subwavelength structured surface with low reflection is developed and proved to be highly consistent with the simulation results.     

T-shaped gate AlGaN/GaN HEMTs fabricated by femtosecond laser lithography without ablation

Femtosecond laser as a maskless lithography technique is able to fabricate structures far smaller than the diffraction limit to a value within sub-micrometer resolution. We present the femtosecond laser lithography without ablation on the positive photoresist is applied in fabricating T-shaped gate AlGaN/GaN HEMT. The feature sizes of femtosecond laser lithography were determined by the incident laser power, the scan speed of the laser focus, the number of scan times, and the substrate materials. T-shaped gate with the smallest gate length 204?nm could be fabricated by dielectric-defined process using femtosecond laser lithography. The fabricated AlGaN/GaN HEMT with 380?nm T-gate exhibits a maximum drain current density of 500?mA/mm and a maximum peak extrinsic transconductance of 173?mS/mm.     

Blue laser lithography for making antireflective submicron structures on silicon

The application of blue laser lithography for creating antireflective submicron structures on a crystalline silicon substrate was evaluated. The assembled blue laser lithography system was obtained by modifying a commercial blue laser optical pickup head and consisting of a 405-nm-wavelength blue laser and a 0.85-numerical-aperture objective lens. Si substrates were patterned with submicron column patterns of various periods and aspect ratios by blue laser lithography using a sputtered Ge-Sb-Sn-O layer as a resist. The reflectance of the patterned Si substrate decreased to 3% on average in the 300–1000 nm wavelength range, with a low sensitivity to the angle of incident light. Such patterned substrates showed potential for application in crystalline Si solar cells.