Fabricating nanoscale device features using the 2-step NERIME nanolithography process

The 2-step negative resist image by dry etching (2-step NERIME) focused ion beam (FIB) top surface imaging (TSI) process is a novel nanolithography technique for creating nanometer scale resist features using conventional DNQ/novolak resists. The 2-step NERIME process combines the advantages of FIB lithography and TSI processing, delivering high aspect ratio nanometer-scale resist critical dimensions (CDs). Previous work has reported 90 nm resist CDs over substrate topography using the 2-step NERIME process, and 80 nm etched features masked using resist patterned by the 2-step NERIME process. In this work we further extend the 2-step NERIME process, and demonstrate its potential as a low-cost and convenient nanolithography option for proof-of-concept nanoscale fabrication, through the creation of sub-100 nm titanium-nitride (TiN) device features.     

Nanostructure patterning

Research at the US Naval Research Laboratory (NRL) in the lithography, patterning, and fabrication of structures of nanometer scale dimensions is reviewed. Electron-beam (e-beam) lithography at high (50 keV) and low (5 eV) energies on resist systems developed at NRL is described. The low energy lithography takes advantage of the spatially confined e-beam available in a scanning tunneling microscope type probe. This instrument allows an in situ exposure and characterization of an e-beam sensitive material. A novel approach for implementing dose correction for proximity effects in e-beam lithography is presented using an error measure based on the physical realities of lithography. The compositional disordering of GaAs-Ga_xAl_1-xAs heterostructures as a technique for pattern replication is described. Introducing silicon into the heterostructure (by implantation or diffusion) enhances the aluminum and gallium interdiffusion which can be used for patterning. A complete bibliography of recently published results is included     

运用精确电压衬度像技术实现精确定位的原位电子束纳米刻蚀：制造具有悬挂纳米线结构的纳米器件

本文演示了运用精确电压衬度像技术实现原位电子束纳米刻蚀技术的精确定位,并运用该技术制作成具有悬挂结构的纳米开关。通过运用精确电压衬度像定位技术,能够很好地控制偏转电极的定位,误差可减少到大约10nm。通过该技术,不用通过任何刻蚀过程只运用一次电子束纳米刻蚀,便可实现将分散的纳米线夹在两个电阻层中间形成悬挂结构。在原位电子束刻蚀的整个过程中,无需移动样品台从而消除了样品台的移动误差。因此,整个过程中不需要高精确的激光台和定位标记,从而简化了传统的电子束纳米刻蚀工艺。通过该方法制作的纳米开关随着施加电压的改变很好地实现了闭合和断开的状态。这种简化的过程提供了一种简单、低成本、快速的通过改装过的场发射扫描电子显微镜（FESEM）来制作纳米线悬挂结构的方法,并可运用该技术进一步制造多层结构和特殊的纳米器件。     

Fabrication of 70 nm narrow metal nanowire structure on flexible PET film by nanoimprint lithography

As a promising substrate for various kinds of devices, polyethylene-terephthalate (PET) film has many advantages in terms of transparency, flexibility, chemical stability, thermal resistance, mechanical strength and low fabrication cost. In order to build actual device structure on PET substrate, micro to nanometer scale patterns of functional material have to be formed. In this work, 70 nm sized resist patterns with near zero residual layer were made on PET film, using nanoimprint lithography process, based on ‘partial filling effect’. After brief oxygen plasma treatment and e-beam evaporation of functional materials such as Cr metal, resist patterns were lifted-off with acetone solution and 70 nm sized Cr nanowire structure was uniformly formed on flexible PET substrate.     

Metallic nanogaps with access windows for liquid based systems

A new method has been established for the reproducible fabrication of high quality, metallic nanogaps on silicon chips suitable for liquid based nanometer scale devices. Realization of μm structures connected to nanogaps with gap sizes down to 30 nm has been achieved by a combination of an optical and an electron-beam (e-beam) lithography step using an optimised adhesion layer/metallic layer combination (Ti/Pt/Au—three layer combination) and an adopted two layer e-beam resist. The quality of the interconnects between optically and e-beam lithographically defined structures and the surface roughness of the gold nanogaps have been improved by a controlled temperature treatment. With this method the production of a variety of different gap shapes could be demonstrated. Specifically the lithographic structures have been successfully covered by a protection layer, except of a 200 nm×400 nm size access window located on top of the nanogaps, making it suitable for applications in liquid environment such as molecular and/or electrochemical metal deposition.     

Electron beam lithography in nanoscale fabrication: recent development

Miniaturization is the central theme in modern fabrication technology. Many of the components used in modern products are getting smaller and smaller. In this paper, the recent development of the electron beam lithography technique is reviewed with an emphasis on fabricating devices at the nanometer scale. Because of its very short wavelength and reasonable energy density characteristics, e-beam lithography has the ability to fabricate patterns having nanometer feature sizes. As a result, many nanoscale devices have been successfully fabricated by this technique. Following an introduction of this technique, recent developments in processing, tooling, resist, and pattern controlling are separately examined and discussed. Examples of nanodevices made by several different e-beam lithographic schemes are given, to illustrate the versatility and advancement of the e-beam lithography technique. Finally, future trends in this technique are discussed.     

Self-assembled metal/molecule/semiconductor nanostructures for electronic device and contact applications

We report a fabrication approach in which we combine self-assembled metal/molecule nanostructures with chemically stable semiconductor surface layers. The resulting structures have well controlled dimensions and geometries (∼4 nm Au nanoclusters) provided by the chemical self-assembly and have stable, low-resistance interfaces realized by the chemically stable semiconductor cap layer (low-temperature grown GaAs passivated by the organic tether molecules). Scanning tunneling microscope imaging and current-voltage spectroscopy of nanocontacts ton-GaAs fabricated using this approach indicate high quality, ohmic nanocontacts having a specific contact resistance of ∼1 × 10⁻⁷Ω·cm² and a maximum current density of ∼1×10⁷ A/cm², both comparable to those observed in large area contacts. Uniform 2-D arrays of these nanocontact structures have been fabricated and characterized as potential cells for nanoelectronic device applications.     

The use of refractory metal and electron-beam sintering to reduce contact resistance for VLSI

Low contact resistance for metal on silicon is particularly important for VLSI where contact dimensions become ≤1 µm. This paper reports on e-beam sintering of refractory metal contacts on implanted n⁺- and p⁺-silicon layers. With this technique, we have obtained contact resistivities as low as 1.5 × 10^-7Ω. cm²and 1.2 × 10^-7Ω . cm²for n⁺and p⁺contacts, respectively. These values are the lowest contact resistivities which have been achieved experimentally to date. We found no measurable metal-silicon interdiffusion when e-beam sintering was used. Electron-beam-induced MOS damage, including neutral traps, can be removed by a forming gas anneal.     

Generation and characterization of polymeric tridimensional microstructures for micromachine application

In this work, we use the thick layer of polymethylmethacrylate polymer, for micromachining development. In the development of the structures, a three layer process is used. In a silicon wafer is deposited the thick layer spin coating. Over this layer is deposited a thin layer of silicon. The third layer is 1.5 μm of e-beam resist deposited by spin coating. After the deposition of the layers, we perform the e-beam lithography in the top layer resist. This pattern is transferred by plasma etching for the silicon layer. The resolution limits of this process is the resolution of the electron resist and is increased to 0.25 μm (nanometric resolution), using an electron beam spot size of 50 nm and dry development.     

Electron-beam fabrication of GaAs low-noise MESFET's using a new trilayer resist technique

A LO/HI/LO resist system has been developed to produce sub-half-micrometer T-shaped cross section metal lines using e-beam lithography. The system provides T-shaped resist cavities with undercut profiles. T-shaped metal lines as narrow as 0.15 µm have been produced. GaAs MESFET's with 0.25-µm T-shaped Ti/Pt/Au gates have also been fabricated on MBE wafers using this resist technique. Measured end-to, end 0.25-µm gate resistance was 80 ω/mm, dc transconductance gmas high as 300 mS/mm was observed. At 18 GHz, a noise figure as low as 1.4 dB with an associated gain of 7.9 dB has also been measured. This is the lowest noise figure ever reported for conventional GaAs MESFET's at this frequency. These superior results are mainly attributed to the high-quality MBE material and the advanced T-gate fabrication technique employing e-beam lithography.     

Nanometer fabrication in mercury cadmium telluride by electron cyclotron resonance microwave plasma reactive ion etching

It has been recently reported (J.R. Meyer, F.J. Bartoli, C.A. Hoffman, and L.R. Ram-Mohan,Phys. Rev. Lett. 64, 1963 [1990]) that novel electronic and optical effects are anticipated in nanometer scale features of narrow band gap semiconductors such as mercury cadmium telluride (MCT). These efforts could lead to the creation of non-linear optical switches, high efficiency infrared lasers, and unique nanoelectronic devices. This work reports on the first realization of MCT nanostructures through the application of e-beam lithography and reactive ion etching with an electron cyclotron resonance (ECR) microwave plasma source. It is shown that the low energy ions produced by an ECR system can etch MCT with good selectivity over an e-beam resist mask and with high resolution. Using these fabrication methods, 40–70 nm features with aspect ratios of 3–5∶1 and sidewall angles greater than 88° have been demonstrated. Qualitative investigations of some of the etch mechanisms of this technique are made, and results suggest a desorption limited process. ^*Code 6675,^†Code 5613,^‡Code 6864     

Comparison of quartz and silicon as a master mold substrate for patterned media UV-NIL replica process

Nanoimprint lithography (NIL) is a promising candidate technology to fabricate patterned media for the next generation hard disk drives (HDD). The requirement of pattern pitch for the HDD or discrete-track recording (DTR) media will be as small as from 40 to 50 nm by 2011 or 2012. However not only to create such fine pitch but also long e-beam writing time such as 1 week with conventional high resolution resist ZEP520A are critical. This paper addresses the fabrication processes to combine silicon substrate and a new chemically amplified resist (CAR) for the master molds of this NIL. The e-beam writing speed with this new CAR was achieved over 3-times faster while 50 nm fine DTR patterns were demonstrated with rotary stage e-beam writer. Furthermore, the replication with J-FIL from the master mold into quartz working mold was also demonstrated.     

Dependence of the Resistance of the Negative e-Beam Resist HSQ Versus the Dose in the RIE and Wet Etching Processes

This article studied the resistance of the negative electron resist based on hydrogen-silsesquioxane (HSQ) depending on the dose of exposure in the process of Reactive Ion Etching (RIE). These studies showed the strong dependence of resistance on irradiation dose (in case of full development of the e-beam resist) even after annealing the resist 30 minutes 400°C in air. Selectivity up to 14 was obtained in the process of reactive ion etching of silicon in a mixture of gases SF₆: C₄F₈. These results can be used to manufacturing of silicon nanoscale structures. It was shown that the resistance to wet etching in a 5% solution of hydrofluoric acid (HF) is also determined by irradiation dose. Additionally, taking into account the obtained results, silicon nanowires of width 10 nm with an aspect ratio of 1: 10 was manufactured.     

Simulation of multiple etch fronts

We developed a series of algorithms, based on the string model, that simulate the inhomogeneous etching process of polymeric materials, which exhibit swelling during dissolution by an organic solvent, used in IC process fabrication. The swelling creates a gel layer between the solid (polymer) and the liquid (solvent) phases. Usually, simulations based on the string development model assume an abrupt solid-liquid interface. Our algorithms are capable of simulating two interrelated etch fronts, and may easily be extended to simulate n interrelated etch fronts. The speed with which the points of the gel-solid boundary advance depends on the distance of each point from the gel-liquid boundary. Considering the two boundaries as plane curves, one must deal with the problem of what we define as the “distance” of every point of the first curve from the second. The algorithm developed can deal with any pair of non-intersecting curves in two dimensions. Two pairs of curves were used to test the algorithm: the first with curves of a simple form, and the second with curves of a complicated form. Next we simulated the development process of an e-beam resist that exhibits swelling. The results were perfect for the minimum distance subalgorithm, and they predicted a longer development time for the e-beam resist compared with that required for a non-swelling resist.     

Fabrication of large scale arrays of metallic nanodots by means of high resolution e-beam lithography

We present the fabrication of large nanodot arrays with very small single dot sizes made of different metals and on different substrates using e-beam lithography and lift-off techniques. Nanodot arrays have a high potential for bioanalytical applications aiming towards single molecule detection. In addition, they can be used as well defined models of heterogeneous catalysts. For this purpose an independent control of the dot size and the distance between the dots is necessary. This is a limitation for many fabrication techniques, which can be overcome by e-beam lithography. However, aiming for the dot size of few tens of nanometers, we observed a strong influence of the beam focusing and astigmatism on the quality of the fabricated nanodot arrays. A significant improvement, i.e. reduction of defect density as well as better control of the dot size, was achieved by modification of the fine focus and stigmation system of the used e-beam writer. Using lift-off technique, we successfully fabricated platinum dot arrays on glassy carbon with target dot diameter ranging from 25 to 35 nm. By applying additional annealing step, we could fabricate nanodot arrays made of gold on SiO₂ with very small dot sizes down to 6 nm and pitch of 100 nm. Furthermore, the generation of large area arrays of nanosized pillars was demonstrated using the same exposure strategy in a negative tone HSQ resist.     

纳米级高压扫描电子束曝光技术

通过简单添加一些附件,将一台带有扫描附件的商用透射电子显微镜改造为一台高压扫描电子束曝光机,以研究高压下高分辨率、高深宽比抗蚀剂图形曝光及邻近效应的影响。重点介绍了如何获得一个高分辨率的电子光学系统,并利用此系统初步进行了曝光实验,在120nm厚的PMMA胶上获得了53nm线宽的抗蚀剂图形,表明此装置可用于纳米图形的制作。     

Transparent and Reusable Nanostencil Lithography for Organic–Inorganic Hybrid Perovskite Nanodevices

Organic—inorganic hybrid perovskites have attracted considerable attention for developing novel optoelectronic devices owing to their excellent photoresponses. However, conventional nanolithography of hybrid perovskites remains a challenge because they undergo severe damage in standard lithographic solvents, which prohibits device miniaturization and integration. In this study, a novel transparent stencil nanolithography (t-SL) technique is developed based on focused ion beam (FIB)-assisted polyethylene terephthalate (PET) direct patterning. The proposed t-SL enables ultrahigh lithography resolution down to 100 nm and accurate stencil mask alignment. Moreover, the stencil mask can be reused more than ten times, which is cost-effective for device fabrication. By applying this lithographic technique to hybrid perovskites, a high-performance 2D hybrid perovskite heterostructure photodetector is fabricated. The responsivity and detectivity of the proposed heterostructure photodetector can reach up to 28.3 A W⁻¹ and 1.5 × 10¹³ Jones, respectively. This t-SL nanolithography technique based on FIB-assisted PET direct patterning can effectively support the miniaturization and integration of hybrid-perovskite-based electronic devices.     

A process for the fabrication of large areas of high resolution,high aspect ratio silicon structures using a negative tone Novolak based e-beam resist

The non-chemically amplified negative resist ARN 7520 produced by ALLRESIST GmbH shows excellent suitability for fabricating stamps for nanoimprinting with a pitch resolution as high as 70 nm and dense periodical structures using e-beam lithography and dry etching. Due to its chemical formulation, the resist does not swell in the developer, has good sensitivity and contrast. The adhesion of such class of resists is provided by silane containing promoters used before the spin-coating. However, for the lower exposure doses, the bonding of the nanostructures to the surface is still insufficient. Instead of the promoter, we evaporate 3–5 nm Ti layer before the resist spin-coating. This strongly improves the resist adhesion in a wide range of exposure doses, suppresses the influence of the substrate conductivity on the electron beam lithography parameters and also improves the structure profile during dry etching. Reducing ion voltage from 400 to 200 V midway through dry etching also helps to keep the structure walls more vertical. Silicon stamps with lines and spacings of 70–100 nm periods and an area of 3 × 3 mm² have been successfully fabricated.     

Nanostructure fabrication using electron beam and its applicationto nanometer devices

Nanofabrication developed by using electron beam (EB) are described. Ten-nm structures of organic positive and negative resist patterns have been achieved by using a commercially available EB lithography system with energy of 30-50 keV. The self-developing properties of an AlF₃-doped LiF inorganic resist have been studied for sub-10-nm lithography. By optimizing the inorganic resist film quality, 5-nm linewidth patterns with 60-nm periodicity were directly delineated under a 30-keV EB. Moreover, EB-induced deposition is described as an interesting method for nanofabrication. An novel approach for nanolithography using de Broglie wave has been developed. Line and dot patterns with 100-nm periodicity were exposed on a PMMA resist by EB holography with a thermal field-emitter gun and an electron biprism. This technique allows us to produce nanoscale periodic patterns simultaneously. Furthermore, the possibility of nanostructure fabrication by atomic-beam holography has been demonstrated by using a laser-trap technique and a computer-generated hologram made by EB lithography. As applications of EB nanolithography to nanodevices, a 40-nm-gate NMOS Si device and a high-temperature-operation single-electron transistor (SET) are described     

Shape control of polymer reflow structures fabricated by nanoimprint lithography

Nanoimprinted resist pre-forms were modified using thermal reflow. This post-processing of binary structures enabled us to generate lens-like 3-D structures with different shapes by time- and surface chemistry controlled spreading. The method was extended to feature dimensions down to 100 nm. Surfactant coated line nanostructures were found to be limited by a maximum aspect ratio for imprinted pre-forms. This powerful post-processing method can be used either directly as post-processing step in production or for the fabrication of 3-D stamp copies with the desired shapes.