Search

Photodetectors based on colloidal quantum dots (CQDs) and single layer graphene (SLG) have shown high responsivity due to the synergy of strong light absorption from CQDs and high mobility from SLG. However, it is still challenging to achieve high-density and small-footprint devices on a chip to meet the demand for their integration into electronic devices. Even though there are numerous approaches to pattern the chemically fragile CQD films, usually they require non-conventional approaches such as stamping and surface modification that may be non-compatible with semiconductor processing. In this study, we show that conventional lithography and dry etching can be used to pattern QD active films by employing a graphene monolayer passivation/protective layer that protects the surface ligands of CQDs. This protective layer avoids damage induced by lithography process solvents that deteriorate the carrier mobility of CQDs and therefore the photoresponse. Herein we report patterning of CQDs using conventional UV photolithography, achieving reproducible five-micron length PbS CQDs/SLG photodetectors with a responsivity of 108 A W−1. We have also fabricated thirty-six PbS CQDs/SLG photodetectors on a single chip to establish micron size photodetectors. This process offers an approach to pattern QDs with conventional UV lithography and dry etching semiconductor technology to facilitate their integration into current semiconductor commercial technology.

The seeking of alternative energy options is imperative in current times. Direct alcohol fuel cells (DAFCs) are attractive alternative energy conversion devices where the alcohol oxidation reaction...

Graphene oxide (GO) is promising nanomaterial due to its surface versatility by substitution of functional groups and controllable d-spacing. However, most common methods to prepare GO are based on chemical exfoliation by strong oxidation agents forming flake dispersions that are used to form thin films of aggregated flakes. However, because of their discontinuous and amorphous morphology, such GO films have poor thermal, electronic and mechanical properties. Herein, we report a novel and quick method to obtain large and continuous GO multilayers by directly converting a chemical vapor deposited multilayer graphene (CVD-MG) into graphene oxide (CVD-MGO). Our oxidation procedure uses a CVD-MG grown on Nickel (CVD-MG/Ni), combining a gentle acid mixture with the mechanical support of the nickel foil to oxidize the CVD-MG without ripping the basal planes. XPS, EDX, and Raman spectra prove the complete conversion of MG to CVD-MGO. XRD also shows higher crystallinity for CVD-MGO than for Flake-GO film. By reducing the continuous CVD-MGO, we obtain conductive reduced-GO (CVD-rGO), which reaches 80% of the original CVD-MG conductance. Our directly conversion of CVD-MG to CVD-MGO shows great promise for improved transport properties of GO and enhanced performance for filtering and molecule separation.

Quantum dots (QDs) offer several advantages in optoelectronics such as easy solution processing, strong light absorption and size tunable direct bandgap. However, their major limitation is their poor film mobility and short diffusion length (250 nm). This has restricted the thickness of QD film to ∼200-300 nm due to the restriction that the diffusion length imposes on film thickness in order to keep efficient charge collection. Such thin films result in a significant decrease in quantum efficiency for λ700 nm in QDs photodetector and photovoltaic devices, causing a reduced photoresponsivity and a poor absorption towards the near-infrared part of the sunlight spectrum. Herein, we demonstrate 1 μm thick QDs photodetectors with intercalated graphene charge collectors that avoid the significant drop of quantum efficiency towards λ700 nm observed in most QD optoelectronic devices. The 1 μm thick intercalated QD films ensure strong light absorption while keeping efficient charge extraction with a quantum efficiency of 90%-70% from λ = 600 nm to 950 nm using intercalated graphene layers as charge collectors with interspacing distance of 100 nm. We demonstrate that the effect of graphene on light absorption is minimal. We achieve a time-modulation response of1 s. We demonstrate that this technology can be implemented on flexible PET substrates, showing 70% of the original performance after 1000 times bending test. This system provides a novel approach towards high-performance photodetection and high conversion photovoltaic efficiency with quantum dots and on flexible substrates.

Photodetectors based on colloidal quantum dots (CQDs) and single layer graphene (SLG) have shown high responsivity due to the synergy of strong light absorption from CQDs and high mobility from SLG. However, it is still challenging to achieve high-density and small-footprint devices on a chip to meet the demand for their integration into electronic devices. Even though there are numerous approaches to pattern the chemically fragile CQD films, usually they require non-conventional approaches such as stamping and surface modification that may be non-compatible with semiconductor processing. In this study, we show that conventional lithography and dry etching can be used to pattern QD active films by employing a graphene monolayer passivation/protective layer that protects the surface ligands of CQDs. This protective layer avoids damage induced by lithography process solvents that deteriorate the carrier mobility of CQDs and therefore the photoresponse. Herein we report patterning of CQDs using conventional UV photolithography, achieving reproducible five-micron length PbS CQDs/SLG photodetectors with a responsivity of 10

Acoustic metamaterials with negative index of refraction offer great potential for improving resolution and signal transmission in biomedical ultrasound. Their potential has remained unrealized, however, for the lack of a fabrication technology that can create metamaterials compatible with biomedical imaging and operating at frequencies near 1 MHz. The authors report the simulation, fabrication, and characterization of integrated Helmholtz and membrane resonators with characteristic lengths less than 1 mm, produced by silicon-based clean-room microfabrication. Their work finally realizes a negative-index acoustic metamaterial that can be applied in ultrasound imaging and therapeutics.

International audience; A major challenge for negative-index acoustic metamaterials is increasing their operational frequency to the MHz range in water for applications such as biomedical ultrasound. Herein, a novel technology to realize acoustic metamaterials in water using microstructured silicon chips as unit cells that incorporate silicon nitride membranes and Helmholtz resonators with dimensions below 100 μm fabricated using clean-room microfabrication technology is presented. The silicon chip unit-cells are then assembled to form periodic structures that result in a negative-index metamaterial. Finite-element method (FEM) simulations of the metamaterial show a negative-index branch in the dispersion relation in the 0.25–0.35 MHz range. The metamaterial is characterized experimentally using laser-doppler vibrometry, showing opposite phase and group velocities, a signature of negative-index materials, and is in close agreement with FEM simulations. The experimental measurements also show that the magnitude of phase and group velocities increase as the frequency increases within the negative-index band, confirming the negative-index behavior of the material. Acoustic indices from –1 to –5 are reached with respect to water in the 0.25–0.35 MHz range. The use of silicon technology microfabrication to produce acoustic metamaterials for operation in water opens a new road to reach frequencies relevant for biomedical ultrasound applications.

A novel layer-by-layer three-dimensional (3D) architecture allowing one to expand device fabrication in the vertical direction and integrating functional nanomaterials is presented by emulating civil engineering. The architecture uses SU-8 pillars as structural columns, which support multiple horizontal suspended thin films. The films then serve as platforms for the integration of nanomaterials and nanodevices. Multiple graphene layers suspended across SU-8 pillars with precise control on their vertical spacing are demonstrated. In addition to graphene, silicon nitride films that offer high strength yield and thickness control are also presented. Metallic microstructures, plasmonic nanostructures, semiconducting quantum dots, and monolayer graphene on the suspended films are achieved to prove the capability of integrating functional nanomaterials. This work provides the potential to integrate highly compact micro/nanoscale devices at different vertical levels with high surface density, which allows for more capabilities and functionalities in a single device.

Quantum dots (QDs) have extraordinary strong light absorption and size tunable bandgap. However, QD films are typically limited to ~200-300 nm due to their poor charge mobility. This severely limits the quantum efficiency of QD devices for λ

Lead sulfide quantum dots (QDs) have been widely used for various optoelectronic devices due to their high absorption coefficient and tunable bandgap. However, the low mobility of QD films results in poor charge collection and device performance. By combining QDs with graphene into hybrid graphene/QD photodetectors, photocarriers from QDs are transferred to graphene, improving charge collection and transport, drastically increasing the photoresponsivity. Herein, we carry a systematic analysis on how critical tuning parameters such as QD size and QD film thickness affect responsivity, spectral response, and time response. We report the absorption coefficient, refractive index (n, k), penetration depth, and energy bandgap of PbS QDs of different sizes. We study systematically how the photocurrent, photoresponsivity, time response, and power density dependence vary with QD size in hybrid Gr/QD. The bandgap of lead sulfide quantum dots was size-tuned between 0.86 and 1.39 eV. The time response shows that subsecond modulation can be achieved for different QD sizes with a responsivity up to 107 A/W at power densities of 10-5 mW/cm2. We also studied how the performance of the photodetectors is affected by the thickness, discussing the limitations on the thickness by the compromise between light absorption and charge collection. We describe how the optical response shifts toward the infrared as QD films get thicker. Time responses below 1 s are obtained for graphene/QD devices with thickness from 150 nm to 1 μm. This systematic study provides important guidelines to design hybrid graphene/QD photodetectors and tune their spectral response and performance.

In the present study, we present the design, manufacture and analytical performance of a graphene sensor (SLG/SiO2/Si) for the in situ detection of HNO3 as a substitute measure of NO in aqueous conditions. Our electrochemical and FET studies demonstrate that the CVD graphene, which is applied as a sensing platform, can respond rapidly to NO metabolites in physiological environments with a micromolar sensitivity. These results are promising in the development of a sensor that allows real-time monitoring of the physiological levels of NO in cultured cells and plasma.

Herein, we present an electrochemical functionalization strategy for high quality single-layer and multilayer chemical vapor deposited (CVD) graphene directly on a Si/SiO2 chip facilitating electronic interfacing. This method avoids oxidation and tearing of graphene basal planes. We demonstrate effective functionalization by D-(+)-biotin (Bio), 4-(phenyldiazenyl)-aniline (Dz), and gallic acid (Gall) using cyclic voltammetry. Raman spectroscopy and XPS are used to demonstrate effective functionalization. In order to evaluate the effect of the electrochemical functionalization on graphene properties, DC electrical conductivity, XPS, mobility, and carrier density analysis are presented. We show that this functionalization strategy does not degrade graphene mobility (103 cm2 V-1s-1). After functionalization we observe a rise in Fermi level of ∼0.06 eV. In addition, we prove sensing capabilities with a CVD graphene monolayer on the biotin/avidin system by electrical resistance measurements and electrochemical impedance spectroscopy reaching a detection of 2.5 ng ml-1. This paper demonstrates an effective strategy to functionalize high quality CVD graphene on a chip compatible with an electronic interface readout.

Detection of heavy metals such as Pb+2 is critical due to their high toxicity as even trace amounts of them pose a serious detrimental risk to human health. Pb2+ is one of the major toxic and persistent pollutants generated from industry and commonly found in soil, drinking water, and aquatic environments. Due to its high-mobility and one-atom thickness, graphene (Gr) based materials have shown great potential for chemical sensors of heavy metals. Recently, a novel conductive reduced-GO obtained by chemical vapor deposition (CVD-rMGO) showed improved layering structure and conductivity over conventional rGO based on chemically exfoliated flakes. Herein, utilizing this novel rGO obtained from chemical vapor deposition, we showed improved Pb2+ detection using both electrochemical and conductivity sensing. For electrochemical sensing, a CVD-rMGO film is used as working electrode and cyclic voltammetry is used to detect Pb+2 ions accumulated on the CVD-rMGO, obtaining a sensitivity of 4.6 nA nM−1cm−2 and a calculated limit of detection of 0.21 nM. For electrical sensing, the drain current through a CVD-rMGO was monitored as the film as exposed to different concentrations of Pb+2, reaching an estimated limit of detection of 0.101 nM. This work shows that high-quality reduced graphene oxide produced by chemical vapor deposition can serve as a basis enable high-sensitivity detectors of Pb2+.

Understanding the behavior of water-immersed membranes in the megahertz range is critical to develop novel acoustic metamaterials compatible with biomedical ultrasound applications. Herein, we study the influence of water on the resonance frequency and quality factor near the megahertz range of silicon nitride membranes fully immersed in water using laser Doppler vibrometry. The resonance frequency of silicon nitride membranes significantly decreases in water compared to air. For a 40 µm wide membrane, the resonance frequency is reduced from 11.2 MHz in air to 1.24 MHz after immersion in water, which is confirmed by finite element method simulations. Our results indicate that the water mass loading plays a major role in the resonance frequency reduction, with a ratio of water mass to membrane mass of m w a t e r / m m e m b r a n e ∼ 102 and NAVMI factors of Γ∼1.3. We attribute the main losses to acoustic radiation with small contributions from viscous damping. We estimate that silicon nitride membranes with widths below 50 µm are required to build negative metamaterials operating above 1 MHz. The large NAVMI factors suggest strong coupling between membrane motion and acoustic waves in water, which is important to develop metamaterials able to manipulate acoustic fields.

We present a review on stencil lithography and focus on the particular interest and challenges when applying it to the scalable fabrication of nm-size devices. We first describe the basic technique with its main advantages and challenges. Then we review the key results of stencil lithography in nanoscale patterning for the direct deposition of complex materials and the patterning on non-conventional substrates. In particular we discuss the blurring, which is a major limitation to achieving high pattern resolution in stencil lithography. Successful strategies to reduce the blurring based on etching processes and novel stencil designs are presented. Moreover, the paper presents the use of stencil masks beyond deposition, namely for localized etching and ion implantation processes. We also review the current state in research of dynamic stencil lithography, which is based on displacing the stencils with respect to the substrate during deposition. The paper concludes by presenting a broad range of applications for stencil lithography that benefit from the resistless fabrication. The fabricated nano-systems range from plasmonics and gratings for solar cells, electrical contacts for 2-D materials, transistor fabrication on planar and non-planar substrates, to magnetic nanostructures, plasmonic biosensing, cell and protein patterning, flexible devices, as well as the fabrication of NIL stamps and mass sensors integrated with CMOS. (C) 2014 Elsevier B.V. All rights reserved.

Ultrasound is strongly attenuated and aberrated by the skull, principally due to the complex structure and acoustic impedance mismatch of skull which generates strong scattering. Analytical models and finite element methods have shown that negative index metamaterials matching skull properties can enable the transmission of US which enhances bidirectional transmission of ultrasonic signal for imaging. However, it is difficult to realize double-negative metamaterials in near-megahertz range experimentally because of the limitation of fabrication and characterization methods. Here we show the experimental realization of waterborne double negative acoustic metamaterials by coupling micrometer-size membrane-based negative density metamaterials with Helmholtz resonator-based negative modulus materials. Numerical simulation and experimental verification consistently exhibit the double negative behavior between 230 kHz and 410 kHz. We demonstrated the feasibility of creating ultrasonic-range metamaterials for non-invasive ultrasound imaging and neuron stimulation.

Gas vesicles (GVs) are remarkably stable nano-sized gas-filled protein shells proven effective in ultrasonic imaging. The many potential benefits of GVs arise from their strong gas equilibrium at a submicron size as produced by bacteria or algae, producing significant contrast in ultrasound imaging. The actual vibration behavior of GVs, including buckling and collapse, is poorly understood since the GVs are too small for observation methods of sufficient speed to produce details of the GV deformation during exposure to ultrasound. Traditional optical or acoustic microscopy methods are, in any case, not useful, and ex-situtransmission electron microscopy produces useful images but without sufficient time resolution. We propose to instead use laser Doppler vibrometry (LDV) to observe the vibration behavior of GVs. Employing interferometry, LDV offers a far better spatiotemporal resolution. While the typical GV is smaller than 1 μm, an agglomeration of GVs may be used with the LDV to produce a measurable displacement response from a controlled, acoustically delivered pressure. In this talk, we report the fundamental and first harmonic resonance frequencies of GVs and vibration to buckling and collapse at the clinically relevant frequency of 6.5 MHz. We also compare these results with predictions from classic theories of bubble and particle oscillations and finite difference-based computations.

Charge collection is critical in any photodetector or photovoltaic device. Novel materials such as quantum dots (QDs) have extraordinary light absorption properties, but their poor mobility and short diffusion length limit efficient charge collection using conventional top/bottom contacts. In this work, a novel architecture based on multiple intercalated chemical vapor deposition graphene monolayers distributed in an orderly manner inside a QD film is studied. The intercalated graphene layers ensure that at any point in the absorbing material, photocarriers will be efficiently collected and transported. The devices with intercalated graphene layers have superior quantum efficiency over single-bottom graphene/QD devices, overcoming the known restriction that the diffusion length imposes on film thickness. QD film with increased thickness shows efficient charge collection over the entire λ ≈ 500-1000 nm spectrum. This architecture could be applied to boost the performance of other low-cost materials with poor mobility, allowing efficient collection for films thicker than their diffusion length.

In order to keep up with the advances in nano-fabrication, alternative, cost-efficient lithography techniques need to be implemented. Two of the most promising are nanoimprint lithography (NIL) and stencil lithography. We explore here the possibility of fabricating the stamp using stencil lithography, which has the potential for a cost reduction in some fabrication facilities. We show that the stamps reproduce the membrane aperture patterns within ±10 nm and we validate such stamps by using them to fabricate metallic nanowires down to 100 nm in size.

We fabricated amorphous silicon n-i-p solar cells with two types of nanopatterned back reflectors using stencil lithography. One reflector type has a plasmonic grating that is embedded in the ZnO layer; the other one has a metallic grating patterned on top of the Ag layer. From comparing the short-circuit current densities of the two device types, we conclude that light trapping through grating coupling is more efficient than coupling of light through the excitation of localized surface plasmons. The back reflectors were patterned with dot arrays by evaporation of Ag through millimeter-size stencil membranes. The stencils themselves were patterned by wafer-scale nanosphere lithography. The dot arrays have a periodicity of 428 nm and efficiently scatter light in the near-infrared wavelength range. Both back reflectors types lead to the same morphology for the silicon films. This allows us a fair comparison of the two light coupling mechanisms. We found a 14% and 19% short-circuit current density enhancement for the plasmonic and for the metallic grating, respectively. The external quantum efficiency gains between 550 and 650 nm show similar guided modes resonances for both device types, but the excitation is stronger for the device with the metallic grating.

This work reports on a considerable resolution improvement of micro/nanostencil lithography when applied on full-wafer scale by using compliant membranes to reduce gap-induced pattern blurring. Silicon nitride (SiN) membranes are mechanically decoupled from a rigid silicon (Si) frame by means of four compliant, protruding cantilevers. When pressing the stencil into contact with a surface to be patterned, the membranes thus adapt to the surface independently and reduce the gap between the membrane and the substrate even over large, uneven surfaces. Finite element modeling (FEM) simulations show that compliant membranes can deflect vertically 40 mm which is a typical maximal non-planarity observed in standard Si wafers, due to polishing. Microapertures in the stencil membrane are defined by UV lithography and nanoapertures, down to 200 nm in diameter, using focused ion beam (FIB). A thin aluminium (Al) layer is deposited through both compliant and non-compliant membranes on a Si wafer, for comparison. The blurring in the case of compliant membranes is up to 95% reduced on full-wafer scale compared to standard (non-compliant) membranes.

The development of nanoscale lithographic methods on polymer materials is a key requirement to improve the spatial resolution and performance of flexible devices. Here, we report the fabrication of metallic nanostructures down to 20 and 50 nm in size on polymer materials such as polyimide, parylene, SU-8, and PDMS substrates without any resist processing using stencil lithography. Metallic nanodot array analysis of their localized surface plasmon spectra is included. We demonstrate plasmon resonance detection of biotin and streptavidin using a PDMS flexible film with gold nanodots. We also demonstrate the fabrication of metallic nanowires on polyimide substrates with their electrical characteristics showing an ohmic behavior. These results demonstrate high-resolution nanopatterning and device nanofabrication capability of stencil lithography on polymer and flexible substrates.

The optical transparency and high electron mobility of graphene make it an attractive material for photovoltaics. We present a field-effect solar cell using graphene to form a tunable junction barrier with an Earth-abundant and low cost zinc phosphide (Zn_3P_2) thin-film light absorber. Adding a semitransparent top electrostatic gate allows for tuning of the graphene Fermi level and hence the energy barrier at the graphene-Zn_3P_2 junction, going from an ohmic contact at negative gate voltages to a rectifying barrier at positive gate voltages. We perform current and capacitance measurements at different gate voltages in order to demonstrate the control of the energy barrier and depletion width in the zinc phosphide. Our photovoltaic measurements show that the efficiency conversion is increased 2-fold when we increase the gate voltage and the junction barrier to maximize the photovoltaic response. At an optimal gate voltage of +2 V, we obtain an open-circuit voltage of V_(oc) = 0.53 V and an efficiency of 1.9% under AM 1.5 1-sun solar illumination. This work demonstrates that the field effect can be used to modulate and optimize the response of photovoltaic devices incorporating grapheme.

We present a novel photovoltaic cell architecture based on the electric field effect that controls carrier concentration in semiconductors using screening-engineered nanostructured electrodes. The device operates in inversion mode, with a top gate that forms a depletion layer and a p-n junction, and with nanostructured electrodes that collect the photocurrent across the junction. This architecture does not require any doping process or a heterojunction, opening an alternative path to fabricate cells on hard-to-dope materials such as oxides or phosphides. As a proof of concept, we present a field effect solar cell made of Si. To demonstrate the potential of this configuration for alternative materials, we also present a field-effect solar cell made of cuprous oxide, which has a favorable band gap but that is difficult to dope. We control the behavior of the devices with the gate voltage that forms an inversion layer and hence a rectifying p-n junction.

Photovoltaics (PV) are a promising source of clean renewable energy, but current technologies face a cost-to-efficiency trade-off that has slowed widespread implementation. We have developed a PV architecture-screening-engineered field-effect photovoltaics (SFPV)-that in principle enables fabrication of low-cost, high efficiency PV from virtually any semiconductor, including the promising but hard-to-dope metal oxides, sulfides, and phosphides. Prototype SFPV devices have been constructed and are found to operate successfully in accord with model predictions.

This paper reports the feasibility of the fabrication of high aspect ratio structures on substrates via dry etching through a stencil mask placed onto the sample. It demonstrates the possibility to use standard equipment and processes with this novel masking technique, which allows the patterning of fragile and pre-structured surfaces, and avoids the use of resist or additional coating of the sample, reducing costs and processing time. Aspect ratios as high as 13:1 and pattern transfer with a gap of 100 ?m are demonstrated.

In this contribution we describe the application of Ink-Jet printing and Stencil Lithography in bionanotechnology. Both techniques are alternative patterning methods that can be used for the fabrication of biocompatible micro- and nanostructures out of the costly and restricted clean room environment. The applications presented in this contribution are 1) the cell patterning using Au dot arrays deposited on PDMS,by stencil lithography, 2) the fabrication of biosensors based on localized surface plasmon resonance in Au nanodots deposited by stencil lithography and 3) the printing of cells and biomolecules by InkJet printing.

We demonstrate cuprous oxide (Cu2O) based metal insulator semiconductor Schottky (MIS-Schottky) solar cells with efficiency exceeding 3%. A unique direct growth technique is employed in the fabrication, and hexagonal boron nitride (h-BN) serves simultaneously as a passivation and insulation layer on the active Cu2O layer. The devices are the most efficient of any Cu2O based MIS-Schottky solar cells reported to date.

The standard lithographic techniques to fabricate electronic components involve the use of polymers, baking steps and chemicals. This typically restricts their application to flat substrates made up of standard materials. Stencil lithography has been proposed as a stable alternative to the standard lithographic techniques. In this paper, we demonstrate the completely resistless all-through-stencil fabrication of electronic components, by performing all essential fabrication steps—implantation, etching and metallization—using stencil lithography. This is performed on a planar substrate as well as on pre-patterned 3D substrates, thus showing the potential of this technique for applications in the field of accelerometers, pressure, gas and radiation sensors.

Several copy number-altered regions (CNAs) have been identified in the genome of cervical cancer, notably, amplifications of 3q and 5p. However, the contribution of copy-number alterations to cervical carcinogenesis is unresolved because genome-wide there exists a lack of correlation between copy-number alterations and gene expression. In this study, we investigated whether CNAs in the cell lines CaLo, CaSki, HeLa, and SiHa were associated with changes in gene expression. On average, 19.2% of the cell-line genomes had CNAs. However, only 2.4% comprised minimal recurrent regions (MRRs) common to all the cell lines. Whereas 3q had limited common gains (13%), 5p was entirely duplicated recurrently. Genome-wide, only 15.6% of genes located in CNAs changed gene expression; in contrast, the rate in MRRs was up to 3 times this. Chr 5p was confirmed entirely amplified by FISH; however, maximum 33.5% of the explored genes in 5p were deregulated. In 3q, this rate was 13.4%. Even in 3q26, which had 5 MRRs and 38.7% recurrently gained SNPs, the rate was only 15.1%. Interestingly, up to 19% of deregulated genes in 5p and 73% in 3q26 were downregulated, suggesting additional factors were involved in gene repression. The deregulated genes in 3q and 5p occurred in clusters, suggesting local chromatin factors may also influence gene expression. In regions amplified discontinuously, downregulated genes increased steadily as the number of amplified SNPs increased (p

In this work, nanosphere lithography was integrated with standard microfabrication for the wafer-scale fabrication of silicon nitride (SiN) membranes with arrays of submicrometer holes. A monolayer of polystyrene (PS) beads with a mean diameter of 428 or 535 nm was spin coated onto the front side of a (100)-silicon wafer double-side coated with 100 nm of low-stress SiN. The size of the deposited PS beads was reduced by oxygen plasma reactive ion etching. This allowed to tune the hole size in the released SiN membrane while maintaining the hole array periodicity. Using the size-reduced PS beads as a lift-off template in a standard nanosphere lithography lift-off procedure, a 20 nm thick chromium hole etch mask was realized. This hole mask was patterned by UV photolithography, thus allowing for the local dry-etching of holes into the SiN layer. The holey areas were released from the backside in a combined Si dry-and wet-etch process. During the final wet etch, the wafer front side was protected with a KOH-resistant polymeric coating (ProTEK (R)). In this way, holey SiN membranes with side lengths ranging from 400 mu m up to 2.4 mm were fabricated. Preliminary application specific experiments show the membranes' suitability for microfiltration and stencil applications. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3554404]

A quantitative analysis of blurring and its dependence on the stencil-substrate gap and the deposition parameters in stencil lithography, a high resolution shadow mask technique, is presented. The blurring is manifested in two ways: first, the structure directly deposited on the substrate is larger than the stencil aperture due to geometrical factors, and second, a halo of material is formed surrounding the deposited structure, presumably due to surface diffusion. The blurring is studied as a function of the gap using dedicated stencils that allow a controlled variation of the gap. Our results show a linear relationship between the gap and the blurring of the directly deposited structure. In our configuration, with a material source of approximately 5 mm and a source-substrate distance of 1 m, we find that a gap size of approximately 10 microm enlarges the directly deposited structures by approximately 50 nm. The measured halo varies from 0.2 to 3 microm in width depending on the gap, the stencil aperture size and other deposition parameters. We also show that the blurring can be reduced by decreasing the nominal deposition thickness, the deposition rate and the substrate temperature.

The hydrogen sensing characteristics of a single nanotrench fabricated by focused ion beam milling (FIB) in an evaporated palladium microwire are presented. In situ atomic force microscopy (AFM) measurements proved that, in the presence of H(2), the trench closes and electrically connects the initially separated parts of the wire due to the increase in volume of the material. Therewith, an electrical current can be switched through the wire. With experiments under various H(2) concentrations and a mathematical model, we describe the closing mechanism of the trench with respect to various parameters, including the substrate material, film thickness, trench size and wire dimensions. Results have been compared with those from equivalent continuous wires. Thin SiO(2) and polyimide (PI) layers on silicon were used to study the effect of substrate elasticity. Sufficient lateral expansion of Pd to close trenches of up to 70 nm in width has only been observed on PI, which we attribute to its advantageous elastic properties. The scale of the response times allowed the observation of two superposing effects: the chemical conversion of Pd to PdH(x) and the mechanical closing of the trench.

We propose a new process for forming parallel nanobridge patterns by nanostencil lithography. In this process, a low-stress silicon nitride stencil with parallel nanobridge structures is fabricated by a new edge patterning technique where those nanobridges are formed simultaneously via sidewall features using the conventional photolithography and anisotropic dry etching process. After forming primary Cr patterns on the oxidized Si substrate by depositing Cr through the edge-patterned stencil, those patterns are transferred onto the underlying Si layer in a reversed manner, leading to the formation of parallel Cr nanobridge patterns on the Si substrate. Using this process, we have successfully produced 85 nm-wide parallel Cr nanobridge patterns from a stencil with 115 nm-wide nanobridge structures that was fabricated by conventional microlithography. As there is no need for advanced lithography techniques in preparing the nanobridge stencil, the combination of the edge patterning and reverse nanostencil process provides a cost-effective tool for the massive fabrication of parallel nanobridge arrays at the 100 nm scale.

We propose a new process by which patterns produced by nanostencil lithography can be reversed, so that the final pattern on the substrate has the same contrast (filled or empty) as that of the stencil. In this process, the stencil pattern is first formed on an intermediate sacrificial layer, and then transferred onto the underlying substrate in a reverse manner. Using this process, we can form various pattern structures that cannot be produced by the normal stencil process, such as an array of pores or multiple parallel bridges. Because a bridge in the stencil is transferred also as a bridge on the substrate, we can not only avoid the widening of a narrow bridge pattern by the stress-induced bending of the membrane, but also reduce the width of the bridge even further using the pattern blurring. Using SiO2 as an intermediate layer, we have fabricated various reversed Cr patterns on Si, including an array of 800nm circular pores and a 100-nm-wide and 150-nm-long nanobridge.

Resistless processes to realize the pattern transfer of your designs into the substrate present some advantages, as the reduction in fabrication steps. Stencil Lithography (SL) is one of the most used resistless processes and, up to now, it has mainly been used to perform local selective deposition of materials. Here, the local etching of different substrates through a stencil hard mask is presented. The compatibility with different etching conditions, the scalability of the technique and the main challenges are described. Minimum feature dimensions of 500 nm in polysilicon and 200 nm in LS-SiN is presented.

We report the fabrication and characterization of nickel Hall sensors integrated on SU-8 cantilevers. Sensors having active area down to 1 x 1 mum 2 show a magnetic field resolution better than 1 /n muT / Hz 1/2 in the thermal noise frequency range. The fabricated devices have been tested for magnetic imaging using the scanning Hall probe microscopy technique.

Aluminum and gold nanowires were fabricated using 100 mm stencil wafers containing nanoslits fabricated with a focused ion beam. The stencils were aligned and the nanowires deposited on a substrate with predefined electrical pads. The morphology and resistivity of the wires were studied. Nanowires down to 70 nm wide and 5 mum long have been achieved showing a resistivity of 10 microOmegacm for Al and 5 microOmegacm for Au and maximum current density of approximately 10(8) A/cm(2). This proves the capability of stencil lithography for the fabrication of metallic nanowires on a full wafer scale.