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Molybdenum disulfide (MoS2) is traditionally grown at a high temperature and subsequently patterned to study its electronic properties or make devices. This method imposes severe limitations on the...

Direct patterning of thermoelectric metal chalcogenides can be challenging and is normally constrained to certain geometries and sizes. Here we report the synthesis, characterization, and direct writing of sub-10 nm wide bismuth sulfide (Bi2S3) using a single-source, spin-coatable, and electron-beam-sensitive bismuth(III) ethylxanthate precursor. In order to increase the intrinsically low carrier concentration of pristine Bi2S3, we developed a self-doping methodology in which sulfur vacancies are manipulated by tuning the temperature during vacuum annealing, to produce an electron-rich thermoelectric material. We report a room-temperature electrical conductivity of 6 S m–1 and a Seebeck coefficient of −21.41 μV K–1 for a directly patterned, substoichiometric Bi2S3 thin film. We expect that our demonstration of directly writable thermoelectric films, with further optimization of structure and morphology, can be useful for on-chip applications.

Tailoring writability and obtaining better signal-to-noise ratio performance by tuning the magnetic and microstructural properties of FePt media is of great interest in the bid to achieve high areal densities for next generation hard disk drives (HDDs). Conventional ways to tune FePt media are primarily by either the insertion of additional layers, or by inclusion of segregants such as B, C, Ni, SiO2 etc. Here we describe an approach that involves modifying growth kinetics by N inclusion, via plasma treatment in FePt bilayers (consisting of a hard and a soft FePt layer). The soft FePt layer aids in obtaining easy writability, while nitrogen plasma treatment of the interface of the two FePt layers facilitates suitable tailoring of the microstructure. This approach contributed to lowering of grain dimension as well as reduction in domain sizes in the FePt bilayers, but with the unwanted side-effects of reduction in squareness and ordering. However, we further propose a method to alleviate these concerns by the deposition of the soft FePt layer in partial nitrogen atmosphere, that restored the squareness and ordering while retaining the smaller domains obtained by nitrogen plasma treatment. Thus, the proposed approach provides a potential direction towards meeting the mutually conflicting requirements of easy writability and better signal-to-noise ratio performance in FePt media. ASTAR (Agency for Sci., Tech. and Research, S’pore) Accepted version

This work presents a procedure for large-area patterning of designed plasmon resonators that are much smaller than possible with conventional lithography techniques. Fused Colloidal Nanopatterning combines directed self-assembly and controlled fusing of spherical colloidal nanoparticles. The two-step approach first patterns a surface covered with hydrogen silsesquioxane, an electron beam resist, forming traps into which the colloidal gold nanoparticles self-assemble. Second, the patterned nanoparticles are controllably fused to form plasmon resonators of any 2D designed shape. The heights and widths of the plasmon resonators are determined by the diameter of the nanoparticle building blocks, which can be well below 10 nm. By performing the fusing step with UV ozone and heat exposure, we demonstrate that the process is easily scalable to cover large areas on silicon wafers with designed gold nanostructures. The procedure neither requires adhesion layers nor a lift-off process, making it ideally suited for plasmonics, in comparison with regular electron beam lithography. We use monochromated electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy and boundary element method simulations to demonstrate that the designed plasmon resonators are directly tunable via the pattern design. We foresee future expansion of this approach for applications such as plasmon-enhanced photocatalysis and for large-scale patterning where chemical, optical, or confinement properties require sub-10 nm metal lines.

Moth's eye inspired multiscale ommatidial arrays offer multifunctional properties of great significance in optoelectronic devices. However, a major challenge remains in fabricating these arrays on large-area substrates using a simple and scalable technique. Here we present the fabrication of these multiscale ommatidial arrays over large areas by a distinct approach called sacrificial layer mediated nanoimprinting, which involves nanoimprinting aided by a sacrificial layer. The fabricated arrays exhibited excellent pattern uniformity over the entire patterned area. Optimum dimensions of the multiscale ommatidial arrays determined by the finite-difference time domain simulations served as the design parameters for replicating the arrays on glass. A broadband suppression of reflectance to a minimum of ∼1.4% and omnidirectional antireflection for highly oblique angles of incidence up to 70° were achieved. In addition, superhydrophobicity and superior antifogging characteristics enabled the retention of optical properties even in wet and humid conditions, suggesting reliable optical performance in practical outdoor conditions. We anticipate that these properties could potentially enhance the performance of optoelectronic devices and minimize the influence of in-service conditions. Additionally, as our technique is solely nanoimprinting-based, it may enable scalable and high-throughput fabrication of multiscale ommatidial arrays.

Although step-and-flash imprint lithography, or S-FIL, has brought about tremendous advancement in wafer-scale fabrication of sub-100 nm features of photopolymerizable organic and organo-silicon-based resists, it has not been successful in direct patterning of inorganic materials such as oxides because of the difficulties associated with resist formulation and its dispensing. In this paper, we demonstrate the proof-of-concept S-FIL of titanium dioxide (TiO2) carried by an acrylate-based formulation containing an allyl-functionalized titanium complex. The prepolymer formulation contains 48 wt % metal precursor, but it exhibits low enough viscosity (∼5 mPa·s) to be dispensed by an automatic dispensing system, adheres and spreads well on the substrate, is insensitive to pattern density variations, and rapidly polymerizes when exposed to broadband UV radiation to give a yield close to 95%. Five fields, each measuring 1 cm × 1 cm, consisting of 100 nm gratings were successively imprinted. Heat-treatment of the...

The fabrication of very narrow metal lines by the lift-off technique, especially below sub-10 nm, is challenging due to thinner resist requirements in order to achieve the lithographic resolution. At such small length scales, when the grain size becomes comparable with the line-width, the built-in stress in the metal film can cause a break to occur at a grain boundary. Moreover, the line-width roughness (LWR) from the patterned resist can result in deposited metal lines with a very high LWR, leading to an adverse change in device characteristics. Here a new approach that is not based on the lift-off technique but rather on low temperature hydrogen reduction ofelectron-beam patterned metal naphthenates is demonstrated. This not only enables the fabrication of sub-10 nm metal lines of good integrity, but also of low LWR, below the limit of 3.2 nm discussed in the International Technology Roadmap for Semiconductors. Using this method, sub-10 nm nickel wires are obtained by reducing patterned nickel naphthenate lines in a hydrogen-rich atmosphere at 500°C for 1 h. The LWR (i.e., 3 σ LWR ) of these nickel nanolines was found to be 2.9 nm. The technique is general and is likely to be suitable for fabrication of nanostructures of most commonly used metals (and their alloys), such as iron, cobalt, nickel, copper, tungsten, molybdenum, and so on, from their respective metal―organic compounds.

L10 FePtC granular media are being studied as potential future magnetic recording media and are set to be used in conjunction with heat assisted magnetic recording (HAMR) to enable recording at write fields within the range of current day recording heads. Media structures based on a FePtC storage layer and a capping layer can alleviate the switching field distribution (SFD) requirements of HAMR and reduce the noise originating from the writing process. However, the current designs suffer from SFD issues due to high temperature writing. To overcome this problem, we study a CoPt/FePtC exchange coupled composite structure, where FePtC serves as the storage layer and CoPt (with higher Curie temperature, Tc) as the capping layer. CoPt remains ferromagnetic at near Tc of FePtC. Consequently, the counter exchange energy from CoPt would reduce the noise resulting from the adjacent grain interactions during the writing process. CoPt/FePtC bilayer samples with different thicknesses of CoPt were investigated. Our studies found that CoPt forms a continuous layer at a thickness of 6 nm and leads to considerable reduction in the saturation field and its distribution. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Conventional patterning of HfO2 as a gate dielectric is a multistep complicated process that involves deposition of oxide, photolithography, and hard mask etching. In order to simplify the process of HfO2 patterning, the authors have developed photo- and electron beam-sensitive spin-coatable HfO2 resists for direct writing. They were prepared by reacting hafnium tert-butoxide with benzoylacetone in n-butanol and were found to be highly stable in air. Fourier transform infrared studies suggest that exposure to radiation results in the gradual removal of organic material from the resist and the enrichment of the resist with inorganics. This makes the exposed resist insoluble in organic solvents such as ethanol, thereby providing high-resolution negative patterns as small as ∼10nm wide. A silicon-on-insulator field effect transistor has been fabricated using sol-gel-derived HfO2 resist as a high-k gate dielectric and characterized over a range of temperatures. At room temperature a relatively high gate leaka...

Nano-patterning on the order of sub-10 nm is integral to achieve high-density nano-scale devices for various data storage and data processing applications. However, the additional requirement of planarization and unwanted side-effects of physical or chemical etching have so far limited the patterning of sub-10 nm devices. In this work, we have demonstrated the creation of an array of ∼10 nm ferromagnetic islands through selective phase transformation of paramagnetic multilayers by low-energy proton irradiation. Paramagnetic Co3O4/Pd multilayers masked with patterned PMMA (polymethyl methacrylate) were reduced to ferromagnetic Co/Pd by proton irradiation. A clear contrast of the nano-islands was observed using magnetic force microscopy, establishing the formation of ferromagnetic nano-islands with perpendicular magnetic anisotropy. This process provides a way to circumvent the side-effects associated with both conventional nano-scale pattering and high-energy ion irradiation. Therefore, phase transformatio...

A silicon on insulator field effect transistor for cryogenic operation has been fabricated using a sol-gel derived TiO2 electron beam resist as a high-k gate dielectric and characterized over a range of temperatures. The TiO2 dielectric layer allows too large a gate leakage current for good device operation at room temperature, but the leakage current is strongly suppressed at cryogenic temperatures and good transistor characteristics were observed. The temperature dependence of the gate leakage current suggests that Frenkel-Poole and trap-assisted tunneling dominates the conduction in the dielectric layer. The drain current shows peaks at certain frequencies under continuous wave microwave irradiation, which may be caused by the resonance of electrons trapped in defects at the TiO2/SiO2 interface. These resonances offer the possibility to manipulate single electrons for nonclassical information processing.

To explore the full capabilities of proton beam writing (PBW) as a lithographic tool it is important to investigate potential new resist materials. In PBW the interactions of the protons with the resist are comparable to the electron interactions with the resist in electron beam writing. In both techniques the induced secondary electrons will modify the molecular structure of the resist, therefore electron beam resists are potential candidates for PBW. Here we discuss resist properties such as contrast and sensitivity of two new negative resists for PBW. The first resist is a spin-coatable TiO2 resist for which sub 10 nm resolution has been reported using electron beam writing. In PBW smooth side walls have been observed for this resist. Despite a relative low sensitivity of this resist for PBW (8000 nC/mm2) it has potential applications in the area of integrated optical components such as waveguides and gratings because of its high refractive index. WL-7154 is a UV-sensitive negative resist and shows high sensitivity for PBW (4 nC/mm2). This resist could function as a mold for Ni electroplating to fabricate Ni stamps for nanoimprint- and soft-lithography.

Two-photon polymerization of photosensitive materials irradiated by femtosecond laser pulses is now considered as an enabling technology for the fabrication of 3D structures, especially photonic crystals and photonic crystal templates. Depending on the topology and dielectric constant contrast of photonic crystals, their optical properties can be tailored in a desired manner. Since 1987, when the concept of 3D photonic crystal was introduced by Yablonovitch and John, photonic crystals have been a subject of intensive research. In spite of this, the fabrication of photonic crystals with a full 3D (or omni-directional) bandgap in the visible range is still a challenging task. To realize photonic crystals with a full photonic bandgap, 3D structuring of high refractive index materials is required. The high refractive index (ca. 2.4) and high transparency in the visible spectrum makes TiO2 a very promising photonic material. For many applications, it is desirable to have a simple technique for patterning this material in twoand even three dimensions. Conventionally, TiO2 is patterned by sputtering it onto a pre-patterned organic resist followed by lift-off. When thick films of TiO2 or complicated features are desired, the lift-off process does not work well. For 3D fabrication, the most attractive option so far has been to fabricate templates which were later infiltrated with a high refractive index material, followed by the removal of the original template structure. Another approach is to use inorganic–organic photosensitive materials (ormocers) for the fabrication of photonic crystals. In this case, there exists a possibility to skip the replication steps and to fabricate 3D inorganic structures directly. However, the inorganic content in these materials is not very high and the attempts to fabricate in three dimensions have resulted in porous structures. In order to eliminate the issue of making templates and their subsequent infiltration with a high refractive index material, a direct 3D fabrication of structures appears to be an attractive option. As2S3, a chalcogenide glass, was directly patterned in three dimensions by exploiting its photo-induced metastability. On the other hand, examples of direct patterning of oxides are nonexistent. In this paper, we will demonstrate a direct 3D patterning of TiO2 structures using femtosecond laser pulses. To enable such fabrication, a photosensitive sol– gel-based spin-coatable TiO2 resist was developed. The experimental setup used for the patterning is shown in Figure 1 and is similar to that used before. The TiO2 resist is transparent (refractive index = 1.68) to the femtosecond laser radiation of 780 nm wavelength, and therefore, allows focusing of laser pulses tightly into the material volume. In the focal area, where the intensity of femtosecond laser pulses is high enough to initiate multiphoton processes, bond-breaking in the TiO2 resist makes the irradiated regions insoluble in organic solvents such as acetone. This allows the fabrication of any computer-generated 3D structure by direct laser “recording” into the volume of the TiO2 resist. Because of the threshold behavior and nonlinear nature of this bond-breaking proC O M M U N IC A TI O N

L1 0 FePt displays a large magneto-crystalline anisotropy (K u ∼ 7 × 107 ergs/cc), thereby making it a potential candidate for advanced recording technologies such as heat assisted magnetic recording (HAMR) [1, 2]. There have been many studies to tailor the properties of L1 0 FePt for practical application in hard disk drives (HDDs). These include control of grain size by addition of impurities, use of underlayers for epitaxial growth of (001) FePt at reduced temperatures as well as tailoring out-of-plane coercivity [3-6].

We demonstrate a high yield production scheme to fabricate sub-5 nm co-planar metal–insulator–metal junctions. This involves determining the relationship between the actual gap between the metallic junctions for a given designed gap, and the use of weak developers with ultrasonic agitation to process the exposed resist. This results in an improved process to achieve narrow inter-electrode gaps. The gaps were imaged using an AFM equipped with a carbon nanotube tip to achieve a high degree of accuracy in measurement. The smallest gap unambiguously measured was ~ 2 nm. Gaps with ≤ 5 nm spacing were produced with a very high yield of about 75% for a designed inter-electrode distance of 0 nm. The leakage resistance of the gaps was found to be of the order of 1012 Ω. The entire junction structure was designed to be co-planar to better than 1 nm over 1 μ m2.

Satisfying the mutually conflicting requirements of easy switchability and high thermal stability still remains a hindrance to achieving ultra-high areal densities in hard disk drives. Exchange coupled composite media used with proper exchange control layers (ECLs) presents a potential solution to circumvent this hindrance. In this work, we have studied the role of MgO and Ta ECLs of different thicknesses in reducing the switching field of FePt media. MgO ECL was found to be more effective than a Ta ECL. For a 2 nm MgO ECL, the switching field could be reduced by 41% and at the cost of only a limited loss in thermal stability. Furthermore, a very high switching efficiency of 1.9 was obtained using 2 nm MgO ECL. So, with a proper choice of ECL material and thickness, the switching field of FePt media can be substantially reduced while ensuring high thermal stability and a better signal-to-noise ratio, thus potentially paving the way for very high areal density media. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

Flexible palladium-based H2 sensors have a great potential in advanced sensing applications, as they offer advantages such as light weight, space conservation, and mechanical durability. Despite these advantages, the paucity of such sensors is due to the fact that they are difficult to fabricate while maintaining excellent sensing performance. Here, we demonstrate, using direct nanoimprint lithography of palladium, the fabrication of a flexible, durable, and fast responsive H2 sensor that is capable of detecting H2 gas concentration as low as 50 ppm. High resolution and high throughput patterning of palladium gratings over a 2 cm × 1 cm area on a rigid substrate was achieved by heat-treating nanoimprinted palladium benzyl mercaptide at 250 °C for 1 h. The flexible and robust H2 sensing device was fabricated by subsequent transfer nanoimprinting of these gratings into a polycarbonate film at its glass transition temperature. This technique produces flexible H2 sensors with improved durability, sensitivity, and response time in comparison to palladium thin films. At ambient pressure and temperature, the device showed a fast response time of 18 s at a H2 concentration of 3500 ppm. At 50 ppm concentration, the response time was found to be 57 s. The flexibility of the sensor does not appear to compromise its performance.

Three-dimensional hierarchical patterning of metals is of paramount importance in diverse fields involving photonics, controlling surface wettability and wearable electronics. Conventionally, this type of structuring is tedious and usually involves layer-by-layer lithographic patterning. Here, we describe a simple process of direct nanoimprint lithography using palladium benzylthiolate, a versatile metal-organic ink, which not only leads to the formation of hierarchical patterns but also is amenable to layer-by-layer stacking of the metal over large areas. The key to achieving such multi-faceted patterning is hysteretic melting of ink, enabling its shaping. It undergoes transformation to metallic palladium under gentle thermal conditions without affecting the integrity of the hierarchical patterns on micro- as well as nanoscale. A metallic rice leaf structure showing anisotropic wetting behavior and woodpile-like structures were thus fabricated. Furthermore, this method is extendable for transferring imprinted structures to a flexible substrate to make them robust enough to sustain numerous bending cycles.

UV- and E-beam direct patterning processes using photosensitive precursor films are reviewed in this chapter. For the UV patterning process, discussed in the first part, reaction processes of precursors, such as metal alkoxides modified with β-diketones and metal complexes of carboxylic acids, with UV-irradiation are reported. The discussion of multi-component oxide film, as well as simple oxide film preparation, some applications of direct UV-patterning, and limitation of this process completes this section. The second part covers the progress made in the direct sub-10 nm electron beam patterning of metal oxides over the last 30 years, moving from physical deposition to chemical solution deposition of resists. Patterning of inorganic resists began with thermally evaporated metal halides. They were soon taken over by sputtered metal oxide films due to their excellent environmental stability. However, these inorganic materials, both halides and oxides, suffered from very steep electron dose requirement, thus rendering them useless for practical applications. This gave way to highly electron beam-sensitive chemical solution deposited stabilized metal alkoxides and metal naphthenates, with sensitively close to conventional electron beam resists such as poly(methylmethacrylate), PMMA, and calixarene. Furthermore, they show excellent line edge roughness characteristics at sub-10 nm scale, which is currently unmatched by any other electron beam resist. Recent applications of these resists such as etch mask and their suitability as gate oxides closes the chapter.

We demonstrate the uniaxial alignment of a liquid-crystalline conjugated polymer, poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) by means of nanoconfinement during nanoimprinting. The orientation of the conjugated backbones was parallel to the nanolines imprinted into the polymer film. Polarized UV-vis absorption and photoluminescence spectra were measured to quantify the degree of alignment, showing that the polarization ratio and uniaxial molecular order parameter were as high as 66 and 0.97, respectively. The aligned F8BT film was used as the active layer in a PLED, which resulted in polarized electroluminescence with a polarization ratio of 11. Ambipolar PFET in a top-gate configuration with aligned F8BT as the active semiconducting layer showed mobility enhancement when the chains were aligned parallel to the transport direction. Mobility anisotropies for hole and electron transport were 10-15 and 5-7, respectively, for current flow parallel and perpendicular to the alignment direction.

We demonstrate the fabrication of sub-10-nm high aspect ratio electron beam patterning of ZnO at the center as well as corners of the 500μm square main deflection field using a negative tone zinc naphthenate resist. After electron beam exposure, the resist was developed in toluene, resulting in high-resolution patterns as small as 7nm with an aspect ratio of ∼10 in the center as well as at the corners of the main field. The line edge roughness (3σ) of these lines shows roughness with fluctuations of about 2.8nm; thus making it the smallest value of the line edge roughness measured so far for any electron beam resist. Heating the structures at 500°C resulted in reduction of feature sizes to ∼5nm and in the formation of crystalline ZnO. Such sub-10-nm high aspect ratio patterning within the main deflection field is an important step to enable full-wafer-level nanofabrication.