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Focused Electron Beam Micro- and Nanopatterning of Thin Films Derived from Sol–Gels Based on TiO 2 Precursors for Planar Photonics
... Generally, in the case of sol-gel materials, different types of patterning techniques, such as inductively coupled plasma reactive ion etching (ICP-RIE) and electron cyclotron resonance plasma etching (ECR) [27,30], wet-chemical etching [30][31][32], direct photolithography (PL) [33][34][35], electron-beam lithography (EBL) [36,37] or X-ray lithography (XRL) [36,[38][39][40][41][42][43], and other such as XeF 2 chemical etching [7-9, 11, 44], micromolding in capillaries (MIMIC) [45], and NIL [25,[46][47][48][49], can be implemented. ...
... The resulting under-etched microdisc and microgoblet structures successfully served as optical resonators. Recently, we also studied the direct patterning of sol-gel-derived layers based on TiO 2 precursors using an electron beam, obtaining dense periodic structures that may be potentially used for grating couplers [37]. These materials were not activated with organic dyes. ...
Sol–gel materials based on SiO2 and TiO2 precursors are attractive as a new platform for planar photonics. Particularly interesting are those based on organically modified silica (ORMOSIL), which may improve the luminescent properties of organic dyes. However, their microstructurization remains a challenge as it requires optimization of various technological stages. Here, we report the structurization of thin layers based on ORMOSIL precursor and titanium(IV) ethoxide (TET) containing luminescent rhodamine B (RhB) dye. Films were fabricated using sol–gel synthesis and dip-coating method. Depending on the time of annealing performed at 200 °C, layers with different thicknesses (300–760 nm) and refractive indices (RI) (1.51–1.68) were obtained. Combining photolithography and wet-chemical etching processes made it possible to fabricate well-separated sol–gel waveguides and discs of different diameters. The etching time in buffered hydrofluoric acid (BHF) affected the depth of the etched luminescent microstructures. Additionally, it was found that a longer layer’s annealing time increased the etching selectivity of the substrate over the sol–gel layer. This enabled the obtaining of under-etched sol–gel goblet microstructures. Selected samples were investigated using scanning electron microscopy (SEM). UV–Vis photoluminescence measurements showed that long heat treatment also influenced the emission spectrum’s shape. The stability of the films under ambient conditions was established using spectroscopic ellipsometry. It was proven that films heat-treated at 200 °C did not change their properties during storage time of around 2 months. Relatively high RI, luminescent properties, and structurization potential make these microstructures interesting for application in integrated photonic devices, e.g., light amplifiers or sensing systems.
Motivated by the potential of focused-electron-beam-induced deposition (FEBID) in the fabrication of functional gold nanostructures for application in plasmonic and detector technology, we conducted a comprehensive study on [Au(CH 3 ) 2 Cl] 2 as a potential precursor for such depositions. Fundamental electron-induced dissociation processes were studied under single collision conditions, and the composition and morphology of FEBID deposits fabricated in an ultrahigh-vacuum (UHV) chamber were explored on different surfaces and at varied beam currents. In the gas phase, dissociative ionization was found to lead to significant carbon loss from this precursor, and about 50% of the chlorine was on average removed per dissociative ionization incident. On the other hand, in dissociative electron attachment, no chlorine was removed from the parent molecule. Contrary to these observations, FEBID in the UHV setup was found to yield a quantitative loss and desorption of the chlorine from the deposits, an effect that we attribute to electron-induced secondary and tertiary reactions in the deposition process. We find this precursor to be stable at ambient conditions and to have sufficient vapor pressure to be suitable for use in HV instruments. More importantly, in the UHV setup, FEBID with [Au(CH 3 ) 2 Cl] 2 yielded deposits with high gold content, ranging from 45 to 61 atom % depending on the beam current and on the cleanliness of the substrates surface.
In this study, TiO2 thin films formed by dip-coating on an FTO substrate were obtained and characterized using surface, optical and electrochemical techniques. The impact of the dispersant (polyethylene glycol-PEG) on the surface (morphology, wettability, surface energy), optical (band gap and Urbach energy) and electrochemical (charge-transfer resistance, flat band potential) properties were investigated. When PEG was added to the sol–gel solution, the optical gap energy of the resultant films was reduced from 3.25 to 3.12 eV, and the Urbach energy increased from 646 to 709 meV. The dispersant addition in the sol–gel process influences surface features, as evidenced by lower contact-angle values and higher surface energy achieved for a compact film with a homogenous nanoparticle structure and larger crystallinity size. Electrochemical measurements (cycle voltammetry, electrochemical impedance spectroscopy and the Mott–Schottky technique) revealed improved catalytic properties of the TiO2 film, due to a higher insertion/extraction rate of protons into the TiO2 nanostructure, as well as a decrease in charge-transfer resistance from 418 k to 23.4 k and a decrease in flat band potential from 0.055 eV to −0.019 eV. The obtained TiO2 films are a promising alternative for technological applications, due to their advantageous surface, optical and electrochemical features.
Metallic nanopatterns are ubiquitous in applications that exploit the electrical conduction at the nanoscale, including interconnects, electrical nanocontacts, and small gaps between metallic pads. These metallic nanopatterns can be designed to show additional physical properties (optical transparency, plasmonic effects, ferromagnetism, superconductivity, heat evacuation, etc.). For these reasons, an intense search for novel lithography methods using uncomplicated processes represents a key on-going issue in the achievement of metallic nanopatterns with high resolution and high throughput. In this contribution, we introduce a simple methodology for the efficient decomposition of Pd3(OAc)6 spin-coated thin films by means of a focused Ga+ beam, which results in metallic-enriched Pd nanostructures. Remarkably, the usage of a charge dose as low as 30 μC/cm2 is sufficient to fabricate structures with a metallic Pd content above 50% (at.) exhibiting low electrical resistivity (70 μΩ·cm). Binary-collision-approximation simulations provide theoretical support to this experimental finding. Such notable behavior is used to provide three proof-of-concept applications: (i) creation of electrical contacts to nanowires, (ii) fabrication of small (40 nm) gaps between large metallic contact pads, and (iii) fabrication of large-area metallic meshes. The impact across several fields of the direct decomposition of spin-coated organometallic films by focused ion beams is discussed.
The ability to create metallic patterned nanostructures with excellent control of size, shape and spatial orientation is of utmost importance in the construction of next-generation electronic and optical devices as well as in other applications such as (bio)sensors, reactive surfaces for catalysis, etc. Moreover, development of simple, rapid and low-cost fabrication processes of metallic patterned nanostructures is a challenging issue for the incorporation of such devices in real market applications. In this contribution, a direct-write method that results in highly conducting palladium-based nanopatterned structures without the need of applying subsequent curing processes is presented. Spin-coated films of palladium acetate were irradiated with an electron beam to produce palladium nanodeposits (PdNDs) with controlled size, shape and height. The use of different electron doses was investigated and its influence on the PdNDs features determined, namely: (1) thickness of the deposits, (2) atomic percentage of palladium content, (3) oxidation state of palladium in the deposit, (4) morphology of the sample and grain size of the Pd nanocrystals and (5) resistivity. It has been probed that the use of high electron doses, 30000 μC·cm-2 results in the lowest resistivity reported to date for PdNDs, namely 145.1 μΩ·cm, which is only one order of magnitude higher than metallic palladium. This result paves the way for development of simplified lithography processes of nanostructured deposits avoiding subsequent post-treatment steps.
Microring resonators (MRRs) with ultracompact footprints are preferred for enhancing the light-matter interactions to benefit various applications. Here, ultracompact titanium dioxide ( TiO 2 ) MRRs with sub-10-μm radii are experimentally demonstrated. Thanks to the large refractive index of TiO 2 , the quality factors up to ∼ 7.9 × 10 4 and ∼ 4.4 × 10 4 are achieved for TiO 2 MRRs with radii of 10 μm and 6 μm, respectively, which result in large nonlinear power enhancement factors ( > 113 ) and large Purcell factors ( > 56 ). The four-wave mixing (FWM) measurements indicate that, compared to the large MRR, the FWM conversion efficiency of the ultracompact TiO 2 MRRs can be greatly improved (e.g., − 25 dB versus − 31 dB ), a harbinger of significant superiorities. Demonstrations in this work provide more arguments for the TiO 2 waveguides as a promising platform for various on-chip photonic devices.
Titanium dioxide ( T i O 2 ) microring resonators (MRRs) with high quality factors (Qs) are demonstrated by using a new, to the best of our knowledge, bottom-up fabrication method. Pattern platforms with a T-shaped cross section are first defined by etching a thin top layer of silicon nitride and a thick bottom layer of silica and partially undercutting the silica. Then, T i O 2 is deposited on the platforms to form the T i O 2 waveguides and devices. T i O 2 MRRs with different bending radii, waveguide widths, and gaps in the bus waveguide are fabricated and measured. The intrinsic Q ( Q i n t ) is achieved to be ∼ 1.1 × 10 5 at the telecommunication wavelengths, corresponding to a bend waveguide loss of 3.9 dB/cm while the compact MRR with a radius of 10 µm can still sustain a Q i n t of ∼ 10 5 . These results not only unfold the feasibilities of the proposed bottom-up method for fabricating T i O 2 waveguides and MRRs with high Qs and compact footprints but also suggest a new approach for fabricating waveguides in other materials, of which direct etching is not easily accessible.
TiO2 channel waveguides were fabricated using a DC sputter deposition process, followed by photolithography and reactive ion etching. A SiO2 cladding was deposited using evaporation. SEM, TEM and Raman measurements indicate the presence of both an amorphous and a crystalline phase. As the layer thickness increases, poly-crystalline structures start forming. Loss measurements were performed by imaging the scattered light from the top of the channel waveguides and fitting an exponential decay to the intensity profile. Propagation losses of 7.8 ± 0.52 dB/cm at a wavelength of 632.8 nm and 0.68 ± 0.46 dB/cm at a wavelength of 1010 nm were experimentally characterized.
In this contribution, we compare the performance of Focused Electron Beam-induced Deposition (FEBID) and Focused Ion Beam-induced Deposition (FIBID) at room temperature and under cryogenic conditions (the prefix “Cryo” is used here for cryogenic). Under cryogenic conditions, the precursor material condensates on the substrate, forming a layer that is several nm thick. Its subsequent exposure to a focused electron or ion beam and posterior heating to 50 °C reveals the deposit. Due to the extremely low charge dose required, Cryo-FEBID and Cryo-FIBID are found to excel in terms of growth rate, which is typically a few hundred/thousand times higher than room-temperature deposition. Cryo-FIBID using the W(CO)6 precursor has demonstrated the growth of metallic deposits, with resistivity not far from the corresponding deposits grown at room temperature. This paves the way for its application in circuit edit and the fast and direct growth of micro/nano-electrical contacts with decreased ion damage. The last part of the contribution is dedicated to the comparison of these techniques with other charge-based lithography techniques in terms of the charge dose required and process complexity. The comparison indicates that Cryo-FIBID is very competitive and shows great potential for future lithography developments.
Dense integration of photonic integrated circuits demands waveguides simultaneously fulfilling requirements on compactness, low loss, high nonlinearity, and capabilities for mass production. In this work, titanium dioxide waveguides with a thick core of 380 nm exhibiting a compact mode size (0.43 μm²) and a low loss (5.4 ± 1 dB/cm) at telecommunication wavelengths around 1550 nm have been fabricated and measured. A microring resonator having a 50 μm radius has been measured to have a loaded quality factor of 53500. Four-wave mixing experiments reveal a nonlinear parameter for the waveguides of 21-34 W⁻¹ m⁻¹ corresponding to a nonlinear index around 2.3-3.6 x 10⁻¹⁸ m²/W, which results in a wavelength conversion efficiency of −36.2 dB. These performances, together with the potentially simple dispersion engineering to the fabricated waveguides by the post processes, yield a strong promise for the titanium dioxide waveguides applied in photonic integrated circuits, especially for nonlinear implementations.
We demonstrate the transmission of a 10-Gbit/s optical data signal in the 2 µm waveband into titanium dioxide waveguides. Error-free transmissions have been experimentally achieved taking advantage of a 23-dB insertion loss fiber-to-fiber grating-based injection test-bed platform.
Electron-beam-induced deposition of titanium oxide nanopatterns is described. The precursor is titanium tetra-isopropoxide, delivered to the deposition point through a needle and mixed with oxygen at the same point via a flow through a separate needle. The depositions are free of residual carbon and have an EDX determined stoichiometry of TiO2.2. High resolution transmission electron microscopy and Raman spectroscopy studies reveal an amorphous structure of the fabricated titanium oxide. Ellipsometric characterization of the deposited material reveals a refractive index of 2.2-2.4 RIU in the spectral range of 500-1700 nm and a very low extinction coefficient (lower than 10(-6) in the range of 400-1700 nm), which is consistent with high quality titanium oxide. The electrical resistivity of the titanium oxide patterned with this new process is in the range of 10-40 GΩ cm and the measured breakdown field is in the range of 10-70 V μm(-1). The fabricated nanopatterns are important for a variety of applications, including field-effect transistors, memory devices, MEMS, waveguide structures, bio- and chemical sensors.
We fabricate polycrystalline anatase TiO 2 microring resonators with loaded quality factors as high as 25,000 and average losses of 0.58 dB / mm in the telecommunications band. Additionally, we measure a negative thermo-optic coefficient d n / d T of − 4.9 ± 0.5 × 10 − 5 K − 1 . The presented fabrication uses CMOS-compatible lithographic techniques that take advantage of substrate-independent, non-epitaxial growth. These properties make polycrystalline anatase a promising candidate for the implementation of athermal, vertically integrated, CMOS-compatible nanophotonic devices for nonlinear applications.
We present a bi-layer lift-off fabrication approach to create low-loss amorphous titanium dioxide (TiO2) integrated optical waveguides and resonators for visible and near-infrared applications. This approach achieves single-mode waveguide losses as low as 7.5 dB/cm around 633 nm and 1.2 dB/cm around 1550 nm, a factor of 4 improvement over previous reports, without the need to optimize etching conditions. Depositing a secondary 260-nm TiO2 layer can reduce losses further, with the optimized process yielding micro-ring resonators with loaded quality factors as high as 1.5 × 10⁵ around 1550 nm and 1.6×10⁵ around 780 nm. These losses render our TiO2 devices suitable for visible and telecommunications applications; in addition, the simplicity of this lift-off approach is broadly applicable to other novel material platforms, particularly using near-visible wavelengths.
Oxide thin films synthesized using the sol–gel technique have the advantages of low cost, high thickness control, tunable refractive index and silicon technology compatibility, properties that make them potential materials for optoelectronic applications. For very thin films with low porosity, the determination of sorption and optical properties is quite complex because of the small sample size. Thus, there is a need to use especially designed techniques to obtain reliable results. In this work, a comprehensive study on the porosity evolution of SiO2 and TiO2 thin films using X-Ray Reflectometry and Environmental Ellipsometric Porosimetry is presented. For sol–gel SiO2 thin films, it was found that the effective refractive index increases with thermal treatment as the porosity decreases. However, the refractive index of the walls was found constant. For sol–gel TiO2 films, crystallized in anatase phase, both the effective refractive index and the wall refractive index increase with thermal treatment. SiO2 and TiO2 thermal oxides were also characterized for comparison.
Specific sol–gel formulations and protocols have been adapted to the fine patterning of TiO2 films using a very simplified UVA three-step lithography method. Mechanisms occurring during the photo-patterning procedure
and a post-patterning thermal treatment have been studied by Fourier transform infrared spectroscopy, atomic force microscopy,
X-ray diffraction, and ellipsometry. Discussions are proposed to support the influence of the photo-patterning procedure and
post-patterning treatment on the shape and the dimension of so-formed microstructures. Original additional functionalities
arising from such structures are also presented.
Rib channel waveguides were produced with the application of chemical wet etching of the sol-gel derived silica–titania (SiO2–TiO2) waveguide films. The silica–titania films were etched in water solutions of ammonia fluoride. In the etching process, the waveguide films were selectively masked with a photoresist. This paper presents the theoretical analysis of rib channel waveguides as well as the results of experimental research on slab and rib channel waveguides. For slab waveguides the attenuation of 0.15dB/cm was obtained, and for the produced single mode rib channel waveguides, the attenuation of 1.7dB/cm was obtained.
Polystyrene is a negative electron beam resist whose exposure properties can be tuned simply by using different molecular weights (Mw). Most previous studies have used monodisperse polystyrene with a polydispersity index (PDI) of less than 1.1 in order to avoid any uncertainties. Here we show that despite the fact that polystyrene's sensitivity is inversely proportional to its Mw, no noticeable effect of very broad molecular weight distribution on sensitivity, contrast and achievable resolution is observed. It is thus unnecessary to use the costly monodisperse polystyrene for electron beam lithography. Since the polydispersity is unknown for general purpose polystyrene, we simulated a high PDI polystyrene by mixing in a 1:1 weight ratio two polystyrene samples with Mw of 170 and 900 kg mol(-1) for the high Mw range, and 2.5 and 13 kg mol(-1) for the low Mw range. The exposure property of the mixture resembles that of a monodisperse polystyrene with similar number averaged molecular weight [Formula: see text], which indicates that it is [Formula: see text] rather than [Formula: see text] (weight averaged molecular weight) that dominates the exposure properties of polystyrene resist. This also implies that polystyrene of a certain molecular weight can be simulated by a mixture of two polystyrenes having different molecular weights.
In this review hybrid organic–inorganic (HOI) resists as emerging materials alternative to organic polymers for micro and nanolithography are presented and discussed. In particular, results on sol–gel materials belonging to 3-glycidoxypropyltrimethoxysilane based HOI are presented and reviewed, highlighting as various lithographic techniques can be used to pattern their surface and showing examples of micro- and nano-patterned structures achieved with radiation assisted lithography (UV, X-rays and electron beam) or imprint techniques. It will be demonstrated the particular versatility shown by
some of these materials, that in some case can be processed with all the lithographic methods herein considered, without any significant modification of their main composition and synthesis procedure. Moreover, results about the investigation of interaction between radiation and HOI materials and thermal treatment will be discussed, as well as possible synthesis strategies and composition modifica-
tion developed in order to improve efficiency of curing, tailor HOI properties to specific needs (optical properties, resist composition, mechanical stability, etc.) and explore innovative and non conventional patterning techniques. The reported results highlight as these novel materials, thanks to their solution processability and higher performances respect to commercial polymeric resists, allow to use the above mentioned lithographic techniques in a direct patterning process, strongly simplifying conventional technique and reducing their processing time and costs.
Copyright (2006) American Vacuum Society. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Vacuum Society. The above article may be found at (http://dx.doi.org/10.1116/1.2170096).
An optical in situ technique is presented for monitoring the optical thickness of transparent materials during focused-electron-beam (FEB)-induced deposition. Three precursors were used: formic acid [HCOOH], tetramethyl orthosilicate [Si(OCH3)4], and titanium nitrate [Ti(NO3)4]. Two of them led to optically interesting materials for ultraviolet or visible applications. By cofocusing a laser (514 nm) with the scanning FEB in the plane of the sample, we continuously monitored the intensity of the reflected light during deposit growth. Periodic intensity variations due to interference effects as function of deposit thickness were observed and interpreted. Complex-refractive indices at the 514 nm wavelength of the materials deposited from HCOOH, Si(OCH3)4, and Ti(NO3)4 were calculated from the periodic signal and were, respectively, 1.51+i0.055, 1.56+i0.14, and 2.19+i0.013.
We present the fabrication of a refractive microlens in hybrid SiO2/TiO2 sol-gel glass by electron beam lithography. The hybrid sol-gel material has high transmittance (greater than 95%) in the wavelength range from 362nm to 2000nm. Under the electron beam exposure, the polymerized film thickness was as large as 4 µm. A 3D microlens profile can be formed by exposure of the sol-gel film under different electron dosages that leads to different polymerized film thickness after development. As an example, a microlens with a 250 µm diameter and a 2.05 µm sag height was fabricated.
A negative-tone sensitive SiO2/TiO2 organic-inorganic hybrid sol-gel material was synthesized and characterized for fabrication of multilevel micro-optical elements by direct electron-beam lithography. The exposure was carried out by an in-house modified electron-beam writing system using LEO SEM-982 with Elphy Quantum exposure beam-blanking control system at 25keV. The hybrid Sol-Gel material demonstrated a superb sensitivity for doses between 0.22μC/cm² and 0.33μC/cm².
Background : The use of a focused ion beam to decompose a precursor gas and produce a metallic deposit is a widespread nanolithographic technique named focused ion beam induced deposition (FIBID). However, such an approach is unsuitable if the sample under study is sensitive to the somewhat aggressive exposure to the ion beam, which induces the effects of surface amorphization, local milling, and ion implantation, among others. An alternative strategy is that of focused electron beam induced deposition (FEBID), which makes use of a focused electron beam
instead, and in general yields deposits with much lower metallic content than their FIBID counterparts.
Methods : In this work, we optimize the deposition of tungsten-carbon (W-C) nanowires by FEBID to be used as electrical contacts by assessing the impact of the deposition parameters during growth, evaluating their chemical composition, and investigating their electrical response.
Results : Under the optimized irradiation conditions, the samples exhibit a metallic content high enough for them to be utilized for this purpose, showing a room-temperature resistivity of 550 μΩ cm and maintaining their conducting properties down to 2 K. The lateral resolution of such FEBID W-C metallic nanowires is 45 nm.
Conclusions : The presented optimized procedure may prove a valuable tool for the fabrication of contacts on samples where the FIBID approach is not advised
The high-energy electrons can induce significant physicochemical changes in ionic liquids (ILs), of which polymerization and solidification are particularly interesting due to potential application in microstructures fabrication using electron beam (EB) patterning. However, the role of allyl and alkyl substituents of cation in these phenomena is not clear. Thus, we investigate the effect of electron dose on microscale polymerization of ammonium- and imidazolium-based ILs with different numbers of polymerizable groups. We show that the minimum electron dose for solidification decreases as the number of allyl substituents in cation is increased. Interestingly, ILs without polymerizable substituents are also solidified by EB irradiation. Consequently, we suggest that IL monomers undergo two mechanisms during EB exposure: free radical chain-growth polymerization via allyl groups and cross-linking between monomer/oligomer/polymer radicals. We also study the applicability of IL thin films for EB lithography in terms of sensitivity and contrast of resist material. Moreover, we demonstrate that suspended polymer structures, potentially attractive as photonic components, can be fabricated out of ILs using controlled-acceleration-voltage electron beam lithography (CAV-EBL). This method enables us to examine the effect of electron energy on the solidification depth in IL layer and, as a consequence, estimate the electron range for this material.
The performance of TiO2 waveguides fabricated by a novel resist free electron beam lithography technique is reported. The precursor films were first spin coated on substrates followed by patterning and heating under the electron beam. Subsequent dissolution of the pattern in a suitable solvent and annealing leads to the formation of rectangular TiO2 waveguides. Dimensional control of the waveguides is achieved during the lithography step. Phase and morphology studies confirm the high quality of the waveguides. The TiO2 films crystallize in the anatase structure with transmission >80% and refractive index of 1.95–2.00 in the visible-NIR region. Optical field confinement in the TiO2 waveguides was investigated by orthogonal illumination followed by scanning the waveguide structure with a self-customized optical microscopy system. The optical losses are relatively low as evidenced by the intensity of the light beams propagating through the waveguides. It is further reduced by coupling the light from one waveguide into another waveguide located at close proximity. It is demonstrated that this technique reduces the number of steps while yielding waveguides with performance comparable to any other fabrication technique.
Planar photonic components can be fabricated with high resolution by electron beam patterning of polymer thin films on solid substrates such as silicon or glass. However, polymer films are normally formed by spin-coating of lithographic resists containing not only polymers but also volatile solvents, which is a serious environmental and health issue. Therefore, we investigate a new type of material for planar structures fabrication, i.e. room-temperature ionic liquids (RTILs) with polymerizable allyl group, which are electron beam curable, solvent-free and thus potentially interesting for processing of materials with weak resistance to solvents. We fabricate planar polymer microstructures by electron beam patterning of RTIL thin films in vacuum, which is possible due to negligible volatility of ionic liquids. Three different polymerizable ionic liquids, i.e. [Allmim][Cl] (1-allyl-3-methylimidazolium chloride), [Allmim][NTf2] (1-allyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide) and [Allmmim][NTf2] (1-allyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide), are compared in terms of quality of fabricated microstructures. We demonstrate that a shape of the more viscous RTIL with Cl- anion is less distorted during electron beam activated polymerization than a shape of the less viscous RTILs with large NTf2- anion. Furthermore, the surface tension of NTf2-based ionic liquid decreases significantly with temperature as compared to Cl-based ionic liquid. Thus, we suggest that the thermocapillary effect, that is the Marangoni flow caused by temperature gradient, might be responsible for the differences in the shape of the RTIL-derived microstructures. Also, we analyze the chemistry of electron beam activated polymerization of RTIL by the use of Fourier-transform infrared spectroscopy (FTIR), concluding that because of C=C bonds disappearance the free radical polymerization is a probable reaction mechanism. Finally, we show that polymerized microstructures are potentially attractive as planar photonic components due to good optical properties such as high refractive index.
The feature size of patterns obtained by electron beam lithography (EBL) depends critically on resist properties, beam parameters, development process, and instrument limitations. Frozen layers of simple organic molecules such as n-alkanes behave as negative tone resists for EBL. With the unique advantage of an in-situ thermal treatment replacing chemical development, the entire lithographic process can be performed within a single instrument, thus removing the influence of chemical developers on the feature size. By using an environmental transmission electron microscope, we can also minimize the influence of instrumental limitations and explore the fundamental link between resist characteristics and feature size. Our results reveal that the onset dose of organic ice resists correlates with the inverse molecular weight, and that in the thermal development the role of change in solubility of polymers is mirrored in a shift in the solid/vapour critical temperature of organic ices. With a 0.4 pA beam current we obtained 4.5 nm, 5.5 nm, 8.5 nm, lines with frozen octane, undecane, and tetradecane, respectively, consistent with the predictions of a model we developed that links beam profile and feature size. The knowledge acquired on the response of small organic molecules to electron irradiation, combined with the flexibility and operational advantages of using them as qualified EBL resists, provide us with new opportunities for the design and production of nanodevices, and broadens the reach of EBL especially towards biological applications.
Titanium dioxide (TiO2) is a well-known biocompatible material with important applications in many fields of biomedicine. Here we show our results on the development of a new TiO2 based sol-gel system that can act as negative tone resist for both ultraviolet (UV) and electron beam (EB) lithographies. The system allows the direct patterning of high resolution structures (down to 40 nm). Moreover, it allows the preparation of nanopatterned nanoporous TiO2 simply by changing the conditions of resist development. The presented system can be used as functional tool for the systematic investigation of cell behavior with respect to a complex topographic texture of the substrate (cell guidance). TiO2 structures of different dimensions, geometries and nanoporosity, whose size ranges from a few hundred to a few tens nanometer, are obtained after UV and/or EB lithography. The adhesion of different cell lines, i.e., human osteosarcoma (HOS) and neuroblastoma (N2A), on the patterns is assessed and our results show that cells have a preference for TiO2 patterns with respect to glass.
The authors demonstrate a unique low cost direct process to print 2D, submicron size, and high refractive index nano-pillars using a direct maskless photolithography process. A well collimated i-line source emitting at 365 nm wavelength illuminates a mono layer of silica microspheres of 1 µm diameter deposited on a photosensitive TiO2-based sol-gel layer. No etching process is needed since this layer is directly UV photo patternable like a negative photo-resist. Furthermore this thin layer offers interesting optical properties (high refractive index) and good mechanical and chemical stability and thus can be directly used as a functional microstructure (for PV, sensors applications for example). The paper describes the modeling of the electric field distribution below the spheres during the illumination process, the photochemistry of the TiO2 sol-gel layer process, and preliminary results of TiO2 nano-pillars of 200 nm diameters fabricated on a 3 inches substrate.
Conventional methods for electron beam patterning of TiO2 are based on sputtering and lift-off. This poses significant problems in producing high aspect ratio and stoichiometric structures, especially in the sub-100 nm size range. We describe an alternative approach of preparing spin-coatable TiO2 resists by chemically reacting titanium n-butoxide with benzoylacetone in methyl alcohol. They have an electron beam sensitivity of ∼35 mC cm -2 and are >10^7 times more sensitive to an electron beam than sputtered TiOx and crystalline TiO2 films. Fourier transform infrared studies suggest that exposure to an electron beam results in the gradual removal of organic material from the resist. This makes the exposed resist insoluble in organic solvents such as acetone, thereby providing high-resolution negative patterns as small as 8 nm wide. Such negative patterns can be written with a pitch as close as 30 nm.
In this letter, we have fabricated a thin TiO2 core and electro-optic (EO) polymer cladding derived rib-waveguide and demonstrated its optical modulation at 1550 nm with a VπL figure of merit of 3.3 V cm. We utilized the high refractive index TiO2 to confine a large fraction of light in the relatively low refractive index EO polymer layer. Furthermore, this TiO2 core enables the modulator to be constituted without a transparent top cladding and also can increase the EO activity to a much higher level. As a result, the waveguide has an enhanced in-device r33, corresponding to a VπL of 1.65 V cm in a push-pull Mach-Zehnder interferometer structure.
The chemical modification of titanium alkoxides by alkoxy- and aminoalcohols, ß-diketones, ß-ketoesters, carboxylic and phosphonic acids, and related compounds, and the hydrolysis behavior of the organically modified precursors are reviewed. Special focus is put on structural aspects and on the possibility to introduce functional organic groups. Such precursors have a high potential for innovative materials syntheses because they permit control of the precursor reactivity in sol–gel processes and the preparation of titania-based inorganic–organic hybrid materials. Coordination, solvation, aggregation and redistribution equilibria play an important role in the chemistry of the modified titanium alkoxides, and organic side reactions have to be taken into account.
We report on the formation of titania (TiO2) colloids via the hydrolysis and condensation of alkoxides under a large excess of water. This process is characterized by a rapid precipitation of large aggregates, followed by a slow peptization (deaggregation) induced by the presence of nitric acid. We find that the hydrolysis temperature and the length of the alkoxy group have a minor effect on the size of the peptized colloid. In contrast, the particle size is sensitive to the peptization temperature and exhibits a minimum at 50|SDC. The presence of alcohols inhibits peptization and results in both larger colloids and longer peptization treatments. The smallest size (~20 nm in diameter) is obtained when no alcohol is added to the reaction mixture. The results suggest that the formation of TiO2 nanoparticles is controlled by colloidal interactions, whereas chemical factors (the rate of hydrolysis and condensation) have a secondary role.
The effects of UV-irradiation on the properties of ZrO2 and TiO2 gel films prepared from corresponding metal-butoxides modified with acetylacetone (AcAc) or benzoylacetone (BzAc) have been studied. It was found that the chelate bonds of -diketones remaining in the gel films were dissociated by the UV-irradiation. The UV-irradiation also changed the properties of the gel films such as solubility; the solubility in acidic solutions was decreased for ZrO2 gel films modified with AcAc and TiO2 gel films modified with BzAc became insoluble in alcohol. Based on these findings, a new fine-patterning process has been established, which enables us to make fine-patterns of ZrO2 and TiO2 films on a variety of substrates.
A novel technique has been developed to fabricate surface-relief gratings using photosensitive gel films. Gel films which were derived from metal-alkoxides chemically modified with β-diketones were photosensitive owing to remaining chelate rings; the UV-irradiation of these gel films substantially decreased their solubility in acidic aqueous solutions or organic solvents. Leaching of the gel films after UV-irradiation through a mask and subsequent heat-treatment gave negative patterns of the mask used. Fine-patterns of PLZT films, as well as Al2O3, ZrO2, and TiO2 films, were successfully formed by the above process. Utilizing this process, surface-relief gratings were fabricated on silica glass and Si substrates by the irradiation of the modified gel films with a XeF excimer laser through a phase mask. Very uniform diffraction gratings with a pitch of about 1.0 μm were produced for Al2O3, ZrO2, and TiO2 films, along with PLZT films. The morphology and diffraction efficiency of the gratings obtained were examined in terms of fabrication conditions.
We have investigated the lithographic generation of TiO(x) nanostructures on Si(100) via electron-beam-induced deposition (EBID) of titanium tetraisopropoxide (TTIP) in ultra-high vacuum (UHV) by scanning electron microscopy (SEM) and local Auger electron spectroscopy (AES). In addition, the fabricated nanostructures were also characterized ex situ via atomic force microscopy (AFM) under ambient conditions. In EBID, a highly focused electron beam is used to locally decompose precursor molecules and thereby to generate a deposit. A drawback of this nanofabrication technique is the unintended deposition of material in the vicinity of the impact position of the primary electron beam due to so-called proximity effects. Herein, we present a post-treatment procedure to deplete the unintended deposits by moderate sputtering after the deposition process. Moreover, we were able to observe the formation of pure titanium oxide nanocrystals (<100 nm) in situ upon heating the sample in a well-defined oxygen atmosphere. While the nanocrystal growth for the as-deposited structures also occurs in the surroundings of the irradiated area due to proximity effects, it is limited to the pre-defined regions, if the sample was sputtered before heating the sample under oxygen atmosphere. The described two-step post-treatment procedure after EBID presents a new pathway for the fabrication of clean localized nanostructures.
Titanium dioxide thin films have been synthesized by sol-gel spin coating technique on glass and silicon substrates with and without surfactant polyethylene glycol (PEG). XRD and SEM results confirm the presence of nano-crystalline (anatase) phase at an annealing temperature of 300 degrees C. The influence of surfactant and annealing temperature on optical properties of TiO(2) thin films has been studied. Optical constants and film thickness were estimated by Swanepoel's (envelope) method and by ellipsometric measurements in the visible spectral range. The optical transmittance and reflectance were found to decrease with an increase in PEG percentage. Refractive index of the films decreased and film thickness increased with the increase in percentage of surfactant. The refractive index of the un-doped TiO(2) films was estimated at different annealing temperatures and it has increased with the increasing annealing temperature. The optical band gap of pure TiO(2) films was estimated by Tauc's method at different annealing temperature.