Fabrication of 50 nm period gratings with multilevel interference lithography

We have developed a multilevel interference lithography process to fabricate 50 nm period gratings using light with a 351.1 nm wavelength. In this process multiple grating levels patterned by interference lithography are overlaid and spatial-phase aligned to a common reference grating using interferometry. Each grating level is patterned with offset phase shifts and etched into a single layer to achieve spatial-frequency multiplication. The effect of the multilayer periodic structure on interference lithography is examined to optimize the fabrication process. This process presents a general scheme for overlaying periodic structures and can be used to fabricate more complex periodic structures.     

Tribological behavior of micro/nano-patterned surfaces in contact with AFM colloidal probe

In effort to investigate the influence of the micro/nano-patterning or surface texturing on the nanotribological properties of patterned surfaces, the patterned polydimethylsiloxane (PDMS) surfaces with pillars were fabricated by replica molding technique. The surface morphologies of patterned PDMS surfaces with varying pillar sizes and spacing between pillars were characterized by atomic force microscope (AFM) and scanning electron microscope (SEM). The AFM/FFM was used to acquire the friction force images of micro/nano-patterned surfaces using a colloidal probe. A difference in friction force produced a contrast on the friction force images when the colloidal probe slid over different regions of the patterned polymer surfaces. The average friction force of patterned surface was related to the spacing between the pillars and their size. It decreased with the decreasing of spacing between the pillars and the increasing of pillar size. A reduction in friction force was attributed to the reduced area of contact between patterned surface and colloidal probe. Additionally, the average friction force increased with increasing applied load and sliding velocity.     

Parallel and serial generated patterned DLC surfaces for creating three-dimensional polymeric stamp structures

This study demonstrates pattern generation on a highly durable and flat diamond-like-carbon (DLC) film with micro/nano-scale resolution using the Atomic Force Microscope (AFM). Parallel processing (masked lithography) and serial local probe processing (maskless lithography) have both been utilized to produce a range of structure shapes at different length scales. The AFM is operated in the electrical conductivity mode which induces oxidation on the DLC surface. The technique offers features with structure depths as small as 20 nm (serial processing) and pattern replication of many centimeters (parallel processing). Moreover parallel processed structures may be further modified via serial patterning using the same instrumentation. As a result, complex shapes can be produced with a depth being controlled by the DLC film thickness and/or by the bias voltage parameters. The patterned DLC structures can be used as a template for fabrication of 3 dimensional polymeric structures.     

Fabrication of CoPt magnetic nanodot arrays by electrodeposition process

We attempted to fabricate patterned media using the electrochemical deposition process along with nanopatterned substrates prepared by the electron beam lithography (EBL), UV nanoimprint lithography (UV-NIL), and spin-on-glass nanoimprint lithography (SOG-NIL) approaches. CoPt was electrodeposited into the nanopatterned substrates and chemical mechanical polishing was carried out to planarize the surface. It was clarified that CoPt nanodot arrays were successfully deposited into the patterned nanopores fabricated by UV-NIL and SOG-NIL as well as by EBL with high area selectivity and uniformity. The density of the CoPt nanodot arrays deposited into the nanopores fabricated by EBL was equal up to an areal recording density of 250 Gbit/in².     

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.     

Optimization of 1D photonic crystals to minimize the reflectance of silicon solar cells

This paper presents a process to easily fabricate photonic crystals (PCs) on silicon to increase the efficiency of solar cells by reducing the sunlight reflection in the front surface of the cell. The process, based on laser interference lithography (LIL) and reactive ion etching (RIE), allows creating nanostructures over large areas with different shapes and dimensions. The reflectance of the resulting surface depends on the height, pitch, width and shape of the created PC. In this work, these parameters have been optimized by computer simulation and the best PC so far found has been fabricated on silicon. We obtain a normal reflectance under 10% in the spectral region between 500 and 900 nm without any other material employed as antireflecting coating.     

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.     

高性能三维纳米结构SERS芯片设计、批量制备与痕量汞离子检测

表面增强拉曼散射（SERS）是一种无损、高灵敏、快速检测痕量重金属离子的光谱技术。通过调控和优化纳米结构图案和尺寸可显著增强局域表面等离子体共振（LSPR）与表面等离子体激元（SPP）的耦合以提升电磁场强度,是获得高性能SERS芯片的重要新途径。提出一种用于检测痕量汞离子的新型金属/介质三维周期纳米结构高性能SERS芯片。利用新型应力分化式双层模板纳米压印方法实现了大面积高均一纳米结构SERS芯片的低成本、批量制备。该芯片成功用于痕量汞离子的高灵敏快速检测。采用有限元方法对压印过程界面微区应力进行模拟,通过调控压印模板纵向结构和横向尺寸对模板进行设计。模拟结果表明,纵向有台阶结构的双层模板图案区域呈现高、低两个应力分区,其中,高应力区占图案~72%的面积,其应力均匀性比单层模板提升17%;低应力区分布在图案边缘~28%的区域,可有效减小脱模切应力。当模板横向尺寸从15 mm缩减至7 mm,界面应力整体提升1～2个数量级,将显著提高压印成功率。使用不同横向尺寸的单、双层模板进行压印实验结果表明,尺寸为7 mm的压力分化式双层模板实现了大面积高均一纳米结构的高质量制备,这与模拟结果高度一致。在压印胶纳米结构上构筑金纳米颗粒得到金属/介质三维周期纳米结构SERS芯片。此芯片对罗丹明6G分子的检测极限为2.08×10-12 mol·L-1,增强因子达3×108,检测均一性RSD为8.07%。该芯片对汞离子的探测限浓度仅为10 ppt(5.0×10-11 mol·L-1),浓度线性工作范围为5.0×10-11～5.0×10-5 mol·L-1,跨度达6个数量级,呈现良好的线性关系(R2=0.966),在目前汞离子检测技术中具有显著优势。提出一种通用的高灵敏快速检测痕量物质的SERS芯片设计和制备方法。这种基于光学原理芯片“结构设计-批量制备-实际应用”的研究范式将连接芯片设计和批量制备两个关键点,推动其实际应用。     

纳米沟槽表面黏着接触过程的分子动力学模拟研究

采用大规模分子动力学方法研究了刚性球型探头与具有不同纳米沟槽基体表面的黏着接触过程,探讨了表面沟槽结构对载荷-位移曲线、接触引力和拉离力以及材料转移的影响规律.研究结果表明:在相同的压入深度下,与原子级光滑表面的黏着接触过程相比,刚性探头与具有纳米沟槽结构基体表面的接触压力较小,接触加载过程中的引力作用范围较大,并伴随载荷的多次跳跃,且接触引力和拉离力均有减小;当沟槽深度相同时,随着沟槽宽度的增大,接触引力和拉离力逐渐减小,当沟槽宽度逐渐接近探头与光滑表面的接触直径时,接触引力和拉离力又逐渐增大,趋于接近探头与光滑表面的接触过程;当沟槽宽度相同时,随着沟槽深度的增大,接触引力相对减小,拉离力变化不大.     

Fabrication of Nanoimprint Stamp Using Interference Lithography

Interference lithography is used to fabricate a nanoimprint stamp, which is a key step for nanoimprint lithography. A layer of chromium in thickness of about 20 nm is deposited on the newly cleaned fused silica substrate by thermal evaporation, and a layer of positive resist in thickness of 150nm is spun on the chromium layer. Some patterns, including lines, holes and pillars, are observed on the photoresist film by exposing the resist to interference patterns and they are then transferred to the chromium layer by wet etching. Fused silica stamps are fabricated by reactive ion etching with CHF3/O2 as etchants using the chromium layer as etch mask. An atomic force microscope is used to analyse the pattern transfer in each step. The results show that regular hole patterns of fused silica, with average full width 143nm at half maximum （FWHM）, average hole depth of 76nm and spacing of 450nm, have been fabricated. The exposure method is fast, inexpensive and applicable for fabrication of nanoimprint stamps with large areas.     

Direct-writing organic three-dimensional nanofibrous structure

Direct-writing technology based on Near-Field Electrospinning (NFES) was used to fabricate an organic three-dimensional nanofibrous circle on the patterned silicon substrate. In NFES, straight jet without splitting and chaotic motion was utilized to direct-write orderly nanofiber. When the collector movement speed was lower than electrospinning rate, the relaxed nanofiber would be lead into the pendulum motion by the electrical field force and Coulomb repulsion force from the residual charges on the collector. When the relative air humidity is lower than 35%, individual nanofiber with larger elastic resistance would reveal a good self-assembly performance. Owing to the guidance of the electric field force at the edge of the micro-pattern, a nanofiber was deposited layer by layer to format a 3D nanofibrous circle on the top surface of the micro-pattern. The dimension scale of 3D nanofibrous circle was smaller than 30 μm. With the help of a microscope, a 3D nanofibrous circle can be deposited precisely on the strip micro-pattern with width of 4 μm. Furthermore, a 3D nanofibrous circle in different shapes can be obtained by using special micro-patterns. This organic three-dimensional nanofibrous circle has created a new aspect for the fabrication of organic micro/nanosystems.     

Antireflective properties of AZO subwavelength gratings patterned by holographic lithography

We fabricate the aluminum-doped zinc oxide (AZO) subwavelength gratings (SWG) on Si and glass substrates by holographic lithography and sequent CH₄/H₂/Ar reactive ion etching process. The etch selectivity of AZO over photoresist mask as well as the nano-scale shape is optimized for better antireflection performance. To analyze the antireflective properties of AZO SWG surface, the optical reflectivity is measured and then calculated together with a rigorous coupled-wave analysis. The reflectance spectrum can be considerably changed by incorporating the SWG into AZO film. As the SWG height of AZO on Si substrate increases, the magnitude of interference oscillations in the reflectance spectrum tends to be reduced with the larger difference between its maxima. The use of optimized SWG can significantly reduce the surface reflection of AZO film at the desired wavelengths. The measured reflectance data of AZO SWG are reasonably consistent with the simulation results. No considerable change in transmission characteristics is observed for AZO SWG structures.     

Large-area micro/nanostructures fabrication in quartz by laser interference lithography and dry etching

Laser interference lithography (LIL) has the capability to fabricate large-area microstructures on the photoresist with only a couple of minutes’ exposure and development. In this study, LIL was adopted to fabricate micro/nanostructures in quartz by combining the following dry-etching process either reactive ion etching (RIE) or inductively coupled plasma (ICP). A layer of gold film was coated on the quartz to act as a hard mask during the dry-etching process. A microhole array in quartz with a thin gold film covered on the surface was fabricated when choosing RIE. Each hole in the microhole array was surrounded with gold nanoparticle capped silica (Au/SiO₂) cones when using ICP instead of RIE. This is due to the thin gold film that serves as the mask for creating the surface roughness required for creating the silica cone structure.     

DLC nano-dot surfaces for tribological applications in MEMS devices

With the invention of miniaturized devices like micro-electro-mechanical systems (MEMS), tribological studies at micro/nano-scale have gained importance. These studies are directed towards understanding the interactions between surfaces at micro/nano-scales, under relative motion. In MEMS devices, the critical forces, namely adhesion and friction restrict the smooth operation of the elements that are in relative motion. These miniaturized devices are traditionally made from silicon (Si), whose tribological properties are not good. In this paper, we present a short investigation of nano- and micro-tribological properties of diamond-like carbon (DLC) nano-dot surfaces. The investigation was undertaken to evaluate the potential of these surfaces for their possible application to the miniaturized devices. The tribological evaluation of the DLC nano-dot surfaces was done in comparison with bare Si (1 0 0) surfaces and DLC coated silicon surfaces. A commercial atomic force microscope (AFM) was used to measure adhesion and friction properties of the test materials at the nano-scale, whereas a custom-built micro-tribotester was used to measure their micro-friction property. Results showed that the DLC nano-dot surfaces exhibited superior tribological properties with the lowest values of adhesion force, and friction force both at the nano- and micro-scales, when compared to the bare Si (1 0 0) surfaces and DLC coated silicon surfaces. In addition, the DLC nano-dot surfaces showed no observable wear at the micro-scale, unlike the other two test materials. The superior tribological performance of the DLC nano-dot surfaces is attributed to their hydrophobic nature and the reduced area of contact projected by them.     

Effect of etching parameters on antireflection properties of Si subwavelength grating structures for solar cell applications

We fabricate the aluminum-doped zinc oxide (AZO) subwavelength gratings (SWG) on Si and glass substrates by holographic lithography and sequent CH₄/H₂/Ar reactive ion etching process. The etch selectivity of AZO over photoresist mask as well as the nano-scale shape is optimized for better antireflection performance. To analyze the antireflective properties of AZO SWG surface, the optical reflectivity is measured and then calculated together with a rigorous coupled-wave analysis. The reflectance spectrum can be considerably changed by incorporating the SWG into AZO film. As the SWG height of AZO on Si substrate increases, the magnitude of interference oscillations in the reflectance spectrum tends to be reduced with the larger difference between its maxima. The use of optimized SWG can significantly reduce the surface reflection of AZO film at the desired wavelengths. The measured reflectance data of AZO SWG are reasonably consistent with the simulation results. No considerable change in transmission characteristics is observed for AZO SWG structures.     

Substrateless micrometric metal mesh for mid-infrared plasmonic sensors

We report on the fabrication and optical properties of thin metal films periodically patterned with square hole arrays of 2 micron pitch, which behave as substrateless plasmonic devices at mid-infrared frequencies. Large (3×3 mm²) meshes were fabricated by metallizing a patterned silicon nitride membrane. The mid-infrared spectra display resonant absorption lines with a Q-factor up to 22 in both transmission and reflection, due to the interaction of the radiation with surface plasmon modes on both faces of the film, allowed by substrate removal. The devices can be used to fabricate surface plasmon-based chemical sensors employing mid-infrared radiation.     

Purcell effect of GaAs quantum dots by photonic crystal microcavities

We fabricate photonic crystal slab microcavities embedded with GaAs quantum dots by electron beam lithography and droplet epitaxy. The Purcell effect of exciton emission of the quantum dots is confirmed by the micro photoluminescence measurement. The resonance wavelengths, widths, and polarization are consistent with numerical simulation results.     

基于DMD微光刻的导光板模板的制作方法

提出了一种基于数字微反射镜(DMD)微光刻的导光模板的制作方法.导光板的网点单元图形由DMD输入,经过缩微光学成像系统缩微后,在光刻胶干板上逐单元网点曝光,再经过显影、微电铸得到导光板模板,在PC薄板材上用微纳米压印制成导光板,厚度仅为0.381 mm.采用自行研制的SVGwriting-DMD激光直写系统,图形的最小分辨率为2 μm,DMD微光刻法无需掩膜版,可实现不同形状、大小、微结构的单元网点图形及网点的排布,便于大幅面的导光板模板的制作.     

Fabrication of nanochannels via near-field electrospinning

A simple and low-cost method is suggested to fabricate nanochannels via Near-Field Electrospinning (NFES). In this process, orderly and patterned nanofibers direct-written by NFES are used as sacrificial templates. Well-defined nanochannels are available after the removal process of both sacrificial fibers and material coating over the fibers. The sacrificial fiber, controlled by NFES, dominates the channel geometry. The channel width ranges from 133 nm to 13.54?μm while the applied voltage increases from 1.2 kV to 2.5 kV. Complicated wave-shape and grid pattern channels are presented under a corresponding movement of substrate. This method integrates electrospinning with conventional MEMS fabrication technology and has a potential in micro/nano manufacturing.     

Ge quantum dot molecules and crystals: Preparation and properties

Templated self-organization has been used to prepare two-dimensional arrays as well as three-dimensional quantum dot crystals (QDC) containing Ge dots in a Si host crystal. Si(1 0 0) substrates have been patterned with two-dimensional hole gratings using extreme ultra-violet interference lithography (EUV-IL) and reactive ion etching. The EUV-IL was realized by multiple beam diffraction using Cr gratings on SiN_x membranes fabricated by e-beam lithography. Si/Ge overgrowth was performed by molecular beam epitaxy. The impact of the microscopic shape and size of the prepattern using the mask design and the EUV-IL exposure dose as parameters on the Ge dot nucleation has been studied with atomic force microscopy, transmission electron microscopy and photoluminescence measurements. Adjusting the growth parameters in multiple layer deposition the initial two-dimensional configuration was transferred into three-dimensional QDC.