Electrostatic force-assisted nanoimprint lithography (EFAN)

We present and demonstrate a novel imprint method, electrostatic force-assisted nanoimprint lithography (EFAN), where a voltage applied between a mold and a substrate generates an electrostatic force that presses the mold into a resist on the substrate. We have successfully used EFAN to pattern nanostructures in a photocurable resist spin-coated on a wafer, with high fidelity and excellent uniformity over the entire substrate, in ambient atmosphere without using a vacuum chamber. In initial tests without any process optimization, 100 nm half-pitch gratings with a residual layer thickness of 22+/-5 nm were imprinted across a 100 mm diameter wafer in about 2 s. Furthermore, numerical calculations show that the field magnitude experienced by the dielectric layers on the substrate is much less than their breakdown limit. Hence, EFAN is well suited for step-and-repeat nanoimprint lithography, and its simple operation can simplify and speed up multilayer alignment process.     

Improvement of the non-uniform resist patterns in the thermal nanoimprint process using Si stamp with nanoscale rod patterns

To acquire the uniform resist patterns in thermal nanoimprint lithography (TH-NIL), the major considerations include control of the resist, stamp and substrate resist under the imprint condition. Examples of these factors are management of the imprinting pressure, imprinting temperature and releasing temperature. Non-uniform patterns of thermal imprinted resist appear after TH-NIL according to the pattern size, substrate size and resist thickness. Particularly, the hole-shaped patterns with a diameter of 100 nm and a height of 100 nm on a 4 inch Si wafer after TH-NIL were deformed under tension to the maximum strain 70%. The experimental results showed that uniform nano-patterns can be acquired by minimizing the thermal mismatch while nanoimprinting through using a pair of Si stamp and Si substrate, thinning the resist thickness and separating the stamp at a relatively high temperature.     

Anisotropic remastering for reducing feature sizes on UV nanoimprint lithography replica molds

We present an approach that uses existing nanoimprint molds and reduces the size of the resulting features significantly via a remastering process utilizing the anisotropic etchant tetramethylammonium hydroxide and a mold casting step. Inverted pyramidal structures and V-grooves were imprinted using these 2.5-dimensional (2.5D) replica molds. Pattern transfer into silicon (Si) substrates was established with an intermediate silicon nitride (SiN(x)) layer that can be etched with a much larger selectivity against the imprint resist than the Si substrate. The 2.5D resist profiles are thus transferred back into binary structures in the SiN(x) layer and subsequently into the Si substrate. A substantial size reduction of the diameter of pits from 91 to 33?nm and the width of lines from 600 to 142?nm was achieved.     

Imprinted laminate wafer-level packaging for SAW ID-tags and SAW delay line sensors

We have developed a wafer-level packaging solution for surface acoustic wave devices using imprinted dry film resist (DFR). The packaging process involves the preparation of an imprinted dry film resist that is aligned and laminated to the device wafer and requires one additional lithography step to define the package outline. Two commercial dry film solutions, SU-8 and TMMF, have been evaluated. Compared with traditional ceramic packages, no detectable RF parasitics are introduced by this packaging process. At the same time, the miniature package dimensions allow for wafer-level probing. The packaging process has the great advantage that the cavity formation does not require any sacrificial layer and no liquids, and therefore prevents contamination or stiction of the packaged device. This non-hermetic packaging process is ideal for passive antenna modules using polymer technology for low-cost SAW identification (ID)-tags or lidding in low-temperature cofired ceramic (LTCC) antenna substrates for high-performance wireless sensors. This technique is also applicable to SAW filters and duplexers for module integration in cellular phones using flip-chip mounting and hermetic overcoating.     

Finite element method simulation of the molding process for thermal nano-imprint lithography

We made a numerical study on the deformation of a viscoelastic polymethyl methacrylene (PMMA) resist when a rigid SiO2 stamp with a rectangular line pattern is imprinted into the PMMA resist for thermal nano-imprint lithography (NIL). The stress distribution in the polymer resist during the molding process is calculated by a finite element method (FEM). Our simulation results reveal that the asymmetric von Mises stress is distributed over the polymer around the external line, which seems to be due to the squeezing flow under the flat space. The stress seems to be concentrated at the sidewall close to the centerline of the whole structure. Our simulation also reveals that a micro gap is formed between the replicated structure and the outer wall of the mold.     

High-sensitivity and wide-directivity ultrasound detection using high Q polymer microring resonators

Small size ultrahigh Q polymer microrings working at near visible wavelength have been experimentally demonstrated as ultralow noise ultrasound detectors with wide directivity at high frequencies (>20 MHz). By combining a resist reflow and a low bias continuous etching and passivation process in mold fabrication, imprinted polymer microrings with drastically improved sidewall smoothness were obtained. An ultralow noise-equivalent pressure of 21.4 Pa over 1-75 MHz range has been achieved using a fabricated detector of 60 μm diameter. The device's wide acceptance angle with high sensitivity considerably benefits ultrasound-related imaging.     

亚波长分辨光刻介质特性研究

利用365nm波段光源、扫描电镜、台阶仪、原子力显微镜和椭偏仪,研究了亚波长分辨光刻介质X AR-N 7700/30型光刻胶的显影速度、对比度、薄膜厚度和折射率等化学、物理特性参数。光刻胶未曝光部分显影速度为23.15nm/s,曝光部分为1.85nm/s,光刻胶的对比度高达2.5,稀释至30%的质量浓度时,光刻胶可旋涂成45nm厚的薄膜。     

Efficient methods of nanoimprint stamp cleaning based on imprint self-cleaning effect

Nanoimprint lithography (NIL) is a nonconventional lithographic technique that promises low-cost, high-throughput patterning of structures with sub-10 nm resolution. Contamination of nanoimprint stamps is one of the key obstacles to industrialize the NIL technology. Here, we report two efficient approaches for removal of typical contamination of particles and residual resist from stamps: thermal and ultraviolet (UV) imprinting cleaning-both based on the self-cleaning effect of imprinting process. The contaminated stamps were imprinted onto polymer substrates and after demolding, they were treated with an organic solvent. The images of the stamp before and after the cleaning processes show that the two cleaning approaches can effectively remove contamination from stamps without destroying the stamp structures. The contact angles of the stamp before and after the cleaning processes indicate that the cleaning methods do not significantly degrade the anti-sticking layer. The cleaning processes reported in this work could also be used for substrate cleaning.     

Nanoimprint glass-like carbon molds fabricated with ECR oxygen ion beams using polysiloxane oxide mask

We have investigated the nanofabrication for glass-like carbon molds with electron cyclotron resonance oxygen ion beam etching technologies using polysiloxane [-R₂SiO-]_n as an electron beam mask and a room-temperature imprint resist material. The maximum etching selectivity of polysiloxane film against glass-like carbon was 27, which was obtained with ion energy of 400 eV. It was found that the optimum etching time to fabricate dots of 500 nm in height was 5 min, which was explored according to the computer simulation. The glass-like carbon molds with square pole and cylinder dots were fabricated with 500 nm in width and diameter, respectively. The optimum imprinting pressure and its depth obtained after the press for 5 min were 0.5 MPa and 0.5 μm, respectively. We carried out the room-temperature nanoimprint lithography process using glass-like carbon molds. The resulting width of imprinted polysiloxane patterns was obtained in good agreement with that of the mold.     

Study of a chemically amplified resist for X-ray lithography by Fourier transform infrared spectroscopy

Future applications of microelectromechanical systems (MEMS) require lithographic performance of very high aspect ratio. Chemically amplified resists (CARs) such as the negative tone commercial SU-8 provide critical advantages in sensitivity, resolution, and process efficiency in deep ultraviolet, electron-beam, and X-ray lithographies (XRLs), which result in a very high aspect ratio. In this investigation, an SU-8 resist was characterized and optimized for X-ray lithographic applications by studying the cross-linking process of the resist under different conditions of resist thickness and X-ray exposure dose. The exposure dose of soft X-ray (SXR) irradiation at the average weighted wavelength of 1.20 nm from a plasma focus device ranges from 100 to 1600 mJ/cm(2) on the resist surface. Resist thickness varies from 3.5 to 15 mum. The cross-linking process of the resist during post-exposure bake (PEB) was accurately monitored using Fourier transform infrared (FT-IR) spectroscopy. The infrared absorption peaks at 862, 914, 972, and 1128 cm(-1) in the spectrum of the SU-8 resist were found to be useful indicators for the completion of cross-linking in the resist. Results of the experiments showed that the cross-linking of SU-8 was optimized at the exposure dose of 800 mJ/cm(2) for resist thicknesses of 3.5, 9.5, and 15 microm. PEB temperature was set at 95 degrees C and time at 3 min. The resist thickness was measured using interference patterns in the FT-IR spectra of the resist. Test structures with an aspect ratio 3:1 on 10 microm thick SU-8 resist film were obtained using scanning electron microscopy (SEM).     

Precise slit-width control of niobium apertures for superconducting LEDs

We introduce a novel three-step procedure for precise niobium (Nb)-etching on the nanometer-scale, including the design of high contrast resist patterning and sacrifice layer formation under high radio frequency (RF) power. We present the results of precise slit fabrication using this technique and discuss its application for the production of superconducting devices, such as superconductor-semiconductor-superconductor (S-Sm-S) Josephson junctions. For the reactive ion etching (RIE) of Nb, we selected CF(4) as etchant gas and a positive tone resist to form the etching mask. We found that the combination of resist usage and RIE process allows for etching of thicker Nb layers when utilizing the opposite dependence of the etching rate (ER) on the CF(4) pressure in the case of Nb as compared to the resist. Precise slit-width control of 80 and 200 nm thick Nb apertures was performed with three kinds of ER control, for the resist, the Nb, and the underlying layer. S-Sm-S Josephson junctions were fabricated with lengths as small as 80 nm, which can be considered clean and short and thus exhibit critical currents as high as 50 μA. Moreover, possible further applications, such as for apertures of superconducting light emitting diodes (SC LEDs), are addressed.     

Organic–Inorganic Nanocomposites: Unique Resists for Nanolithography

Ultra large‐scale integration (ULSI) should lead to 100 nm production circuits by 2006 as predicted by the Semiconductor Industry Association (SIA). For sub‐100 nm lithography it is desirable to synthesize higher performance and higher contrast resists. An optimum combination of high contrast necessary for sub‐100 nm resolution, high sensitivity for high throughput can be achieved by carefully engineering organic–inorganic nanocomposites, acting as optimum resists for a given lithographic process. This review outlines emerging approaches towards the achievement of these goals. A section also highlights selected state‐of‐the‐art organic resists. Nanocomposite resists for sub‐100 nm features have included the incorporation of fullerene C₆₀ in a commercial resist ZEP520 (see Figure). Alternatively, nanoscale silica particles were incorporated in the polymer backbone as covalently bonded pendant clusters. The dispersion of 8–10 nm silica particles in a chemically amplified resist has also been reported. In all these approaches, a higher softening temperature (T_g) and increased rigidity, due to increased density of the film resulted. Higher etch resistance as well as increased mechanical properties and also enhanced resist performance for nanometer pattern fabrication have been obtained in these nanocomposites. Alternative approaches to conventional lithography, based on self‐assembled nanostructures, incorporating inorganic features as well as nanoimprinting via silicon polymer precursors, are also discussed.     

Improved performance of polarization-stable VCSELs by monolithic sub-wavelength gratings produced by soft nano-imprint lithography

We present a novel method for fabricating polarization-stable oxide-confined single-mode GaAs based vertical cavity surface emitting lasers (VCSELs) emitting at 850 nm using a new soft-lithography nano-imprint technique. A monolithic surface grating is etched in the output mirror of the laser cavity using a directly imprinted silica-based sol-gel imprint resist as an etch mask. The opto-electronic performance of these devices is compared to VCSELs fabricated by state-of-the-art electron-beam lithography. The lasers made using the soft nano-imprint technique show single-mode TM lasing at a threshold and laser slope similar to that of devices made by e-beam lithography. The soft nano-imprint technique also enables the fabrication of gratings with sub-wavelength pitch, which avoids diffraction losses in the laser cavity. The resulting single-mode VCSEL devices exhibit 29% enhanced efficiency compared to devices equipped with diffractive gratings.     

An optimal high contrast e-beam lithography process for the patterning of dense fin networks

There are many difficulties to overcome towards the integration of 10 nm CMOS technology. One such major challenge is to keep a tight control of the leakage current of devices while increasing the current drive at a reduced supply voltage. In this context, multi-gated structures, which are used to control the transport in ultra-thin channel (e.g. FinFET), are a promising solution. A critical step during the fabrication process of a FinFET is the patterning of dense, high aspect ratio fins. High demand is therefore placed on e-beam lithography techniques to obtain narrow, sharp, densely packed resist lines. This paper presents a detailed study on the optimum e-beam exposure process using a negative tone e-beam resist, namely Hydrogen Silsesquioxane (HSQ). The impact of the pre-exposure bake temperature, of the Tetramethyl Ammonium Hydroxide (TMAH) concentration in development solution and of development time has been investigated. The standard process uses 2.38% TMAH as a developer, samples being pre-baked on a hotplate at a temperature between 150 and 220 °C for 2 min. By using a lower pre-bake temperature of 90 °C and a more concentrated TMAH solution dosed at 25%, a seven-fold improvement of contrast can be obtained in terms of contrast values. Cross sectional SEM views show fin networks with a pitch ranging from 40 nm to 200 nm. The line profiles are steep and an excellent uniformity is obtained across the whole network, even for lines located at the edge. Dense patterns are presented with lines as narrow as 15 nm and with a 25 nm space.     

应用于纳米制造的新型电子束抗蚀剂Calixarene的工艺研究

为了满足电子束光刻（EBL）对高分辨率、性能优秀抗蚀剂的需求,研究了将Calixarene衍生物作为电子束抗蚀剂在胶液配制、电子束曝光及显影等工艺过程中的相关技术.其中电子束曝光实验在JEOL JBX-5000LS系统上进行.实验结果表明,在入射电子能量50 keV、束流50 pA的条件下,Calixarene可以方便地形成50 nm的单线、50nm等线宽与间距的图形结构.通过与常用电子束抗蚀剂的对比,总结了Calixarene在电子束光刻性能上的优缺点,并分析了其成因.作为一种新型的高分辨率电子束光刻抗蚀剂,Calixarene有望应用在纳米结构制造、纳米尺寸器件及电路的研制等领域.     

Molecularly imprinted Au nanoparticles composites on Au surfaces for the surface plasmon resonance detection of pentaerythritol tetranitrate, nitroglycerin, and ethylene glycol dinitrate

Molecularly imprinted Au nanoparticles (NPs) composites are generated on Au-coated glass surfaces. The imprinting process involves the electropolymerization of thioaniline-functionalized Au NPs (3.5 nm) on a thioaniline monolayer-modified Au surface in the presence of a carboxylic acid, acting as a template analogue for the respective explosive. The exclusion of the imprinting template from the Au NPs matrix yields the respective imprinted composites. The binding of the analyte explosives to the Au NPs matrixes is probed by surface plasmon resonance spectroscopy, SPR, where the electronic coupling between the localized plasmon of the Au NPs and the surface plasmon wave leads to the amplification of the SPR responses originating from the dielectric changes of the matrixes upon binding of the different explosive materials. The resulting imprinted matrixes reveal high affinities and selectivity toward the imprinted explosives. Using citric acid as an imprinting template, Au NPs matrixes for the specific analysis of pentaerythritol tetranitrate (PETN) or of nitroglycerin (NG) were prepared, leading to detection limits of 200 fM and 20 pM, respectively. Similarly, using maleic acid or fumaric acid as imprinting templates, high-affinity sensing composites for ethylene glycol dinitrate (EGDN) were synthesized, leading to a detection limit of 400 fM for both matrixes.     

Sub-20 nm laser ablation for lithographic dry development

Pattern collapse of small or high aspect ratio lines during traditional wet development is a major challenge for miniaturization in nanolithography. Here we report on a new dry process which combines high resolution resist exposure with selective laser ablation to achieve high resolution with high aspect ratios. Using a low power 532?nm laser, we dry develop a normally negative tone methyl acetoxy calix(6)arene in positive tone to reveal sub-20?nm half-pitch features in a ～100?nm film at aspect ratios unattainable with conventional development with ablation time of 1-2?s per laser pixel (～600?nm diameter spot). We also demonstrate superior negative tone wet development by combining electron beam exposure with subsequent laser exposure at a non-ablative threshold that requires far less electron beam exposure doses than traditional wet development.     

纳米光子结构软膜紫外光固化纳米压印

紫外光固化纳米压印是实现纳米结构批量复制的一种新技术．其特点是低成本和高分辨，而且可以达到极高的套刻精度．为了得到大面积图案的均匀复制，可用聚二甲基硅氧烷（PDMS）制备透光的压印软模板．其母版图案可由高分辨率电子柬曝光和反应离子刻蚀的方法在硅片基底上获得，然后用浇注的方法将图案转移到PDMS上．本实验特别发展了紫外光固化纳米压印适用于软膜压印的双层膜图型转移技术．该双层膜由廉价的光胶和聚甲基丙烯酸甲脂（PMMA）构成．对光胶层的压印可用普通的光学曝光仪实现．然后再将图案用反应离子刻蚀的方法转移到PMMA层中．为了证明方案的可行性，在两种不同材料的半导体基片上压印了三角晶格的光子晶体和准晶结构的图案，并用剥离的方法将它们转移到金属薄膜上，最后成功地进行了硅片刻蚀实验．相信这一纳米制做方法对大面积纳米光子结构和光学集成芯片的制造是普遍适用的．     

Modification of spontaneous emission of (CdSe)ZnS nanocrystals embedded in nanoimprinted photonic crystals

Highly luminescent (CdSe)ZnS nanocrystals, with band edge emission in the red region of the visible spectrum, were successfully synthesized and incorporated in a resist, namely mr-NIL 6000. The nanocomposite material was imprinted by using conventional nanoimprint lithography (NIL) process. We report on the fabrication and characterization of nanoimprinted photonic crystals in this new functional material. Experiments showed good imprint properties of the NC/polymer based material and that the surface nanostructuration improves the light extraction efficiency by over 2 compared to a nanoimprinted unpatterned surface.     

Using direct nanoimprinting of ferroelectric films to prepare devices exhibiting bi-directionally tunable surface plasmon resonances

In this paper, we describe an imprint method for the fabrication of bi-directionally tunable surface plasmon resonance (SPR) filters. A periodic metal/ferroelectric film stack exhibiting SPR phenomena was directly imprinted using a sharp mold without the need for a polymer-based resist. Both the refractive index of the surrounding lead zirconate titanate (PZT) films and the period of the textured PZT/metal/PZT structure were dependent upon both the absolute value and sign of the applied potential. The SPR wavelength of the PZT/gold/PZT-based tunable filter varied over a range of greater than 100?nm when applying potentials ranging from 0 to -15?V. This imprinting method has great potential for use in the fabrication of tunable optical filters without the need for complicated processes or specific materials.