Thin-film Metal Electrodes Research Articles

AlN-based BAW resonators with CNT electrodes for gravimetric biosensing

Solidly mounted resonators (SMRs) with a top carbon nanotubes (CNTs) surface coating that doubles as an electrode and as a sensing layer have been fabricated. The influence of the CNTs on the frequency response of the resonators was studied by direct comparison to identical devices with a top metallic electrode. It was found that the CNTs introduced significantly less mass load on the resonators and these devices exhibited a greater quality factor, Q (>2000, compared to ∼1000 for devices with metal electrodes), which increases the gravimetric sensitivity of the devices by allowing the tracking of smaller frequency shifts. Protein solutions with different concentrations were loaded on the top of the resonators and their responses to mass-load from physically adsorbed coatings were investigated. Results show that resonators using CNTs as the top electrode exhibited a higher frequency change for a given load (∼0.25 MHz cm 2 ng −1) compared to that of a metal thin film electrode (∼0.14 MHz cm 2 ng −1), due to the lower mass of the CNT electrodes and their higher active surface area compared to that of a thin film metal electrode. It is therefore concluded that the use of CNT electrodes on resonators for their use as gravimetric biosensors is a significant improvement over metallic electrodes that are normally employed.

Design, Development and Testing of Quartz Tuning Fork Temperature Sensor

In this paper, a low cost quartz tuning fork temperature sensor adopting H-shaped tuning fork resonator to address miniaturization, high resolution and high stability has been designed, developed and tested. The quartz tuning temperature sensor is designed vibrating in flexural mode with a new thermo-sensitive cut. The quartz tuning fork temperature sensor consists of two prongs connected at one end of crystalline quartz plate with thin-film metal electrodes deposited on the faces, which is used to produce vibration in response to alternating voltages and detecting the resonance frequency in the meantime. When an external temperature is change, there is a shift in its natural frequency. Based on this principle, a resonant thermometer is designed. Finite element method is used to analyze the vibratory modes and optimize the structure. The whole structure is 500μm thickness, the length of tuning fork arm is 3076μm and the width of tuning fork arm is 600um, the frequency of tuning fork is about 37kHz with a sensitivity of rough 85 ppm/°C. The experimental results shown that a temperature accuracy of 0.01 °C and a resolution of 0.005 °C within temperature range from 0 °C to 100 °C. All these research are helpful to design satisfactory performance of the sensor for temperature measurement.

Robust Carbon‐Nanotube‐Based Nano‐electromechanical Devices: Understanding and Eliminating Prevalent Failure Modes Using Alternative Electrode Materials

The failure modes common to widely pursued carbon-nanotube-based nano-electromechanical systems are investigated. A fundamental understanding of the underlying mechanisms for failure and their relation to the device design space is developed through complementary in situ electromechanical characterization and dynamic multiphysics models. It is then found that the facile replacement of commonly used metal thin-film electrodes with diamondlike carbon structures leads to a dramatic improvement in reliability. This enables experimental demonstration of numerous actuation cycles without failure, and reliable application to volatile memory operations.

Thin and flexible bio-batteries made of electrospun cellulose-based membranes

The present work proposes the development of a bio-battery composed by an ultrathin monolithic structure of an electrospun cellulose acetate membrane, over which was deposited metallic thin film electrodes by thermal evaporation on both surfaces. The electrochemical characterization of the bio-batteries was performed under simulated body fluids like sweat and blood plasma [salt solution – 0.9% (w/w) NaCl]. Reversible electrochemical reactions were detected through the cellulose acetate structure. Thus, a stable electrochemical behavior was achieved for a bio-battery with silver and aluminum thin films as electrodes. This device exhibits the ability to supply a power density higher than 3 μW cm −2. Finally, a bio-battery prototype was tested on a sweated skin, demonstrating the potential of applicability of this bio-device as a micropower source.

Deposited dielectrics on metal thin films using silicon and glass substrates for hot electron-induced electrochemiluminescence

Electrochemiluminescence by tunnel emission of hot electrons into aqueous solution is a sensitive method for detection of luminophores e.g. rare-earth chelates, which may be used as labels in bioassays. Electrons are injected into solution from an insulating film-coated working electrode, working against a platinum counter electrode. Conductive silicon electrodes with various tunnel dielectric materials e.g. thermal oxide have been used in previous work. In this paper we explore the use of metal thin film electrodes on silicon and glass substrates, using tunneling dielectrics of aluminum oxide and silicon dioxide made by the low-temperature processes of atomic layer deposition or plasma-enhanced chemical vapor deposition.

Liquid State Polydimethylsiloxane (PDMS) Wrinkle Formation Process for Various Applications.

AbstractHere we report an epoch-making simple fabrication for wrinkle formation. The present wrinkle formation process is a solution for controlling the area, shape and direction of wrinkle area by forming wrinkles on the liquid state polydimethylsiloxane directly exposed to sputtered metal particles in the low vacuum plasma chamber in various vacuum states and deposition conditions. Also the process allows us to make extremely flexible metal thin film electrode with approved adhesion. These bring us possibilities of actual electrical and biological applications.

Metal−Molecule−Metal Junctions via PFPE Assisted Nanotransfer Printing (nTP) onto Self-Assembled Monolayers

We report a simple method to form metal-molecule-metal junctions (MMMs) via nanotransfer printing with low surface energy perfluoropolyether (PFPE) based stamps. Transfer printing is demonstrated onto thermally deposited metal thin film electrodes where the root-mean-squared roughness of these films is controlled by the deposition process and varies by 40% or more. Transfer of Au and Co thin films is reported onto Au/SAM and Co/SAM electrodes in well-ordered, 200 nm MMM arrays; furthermore, nickel nanotransfer printing is shown for the first time in the construction of 200 nm arrays of Ni/SAM/Ni junctions. The nanotransfer printing we report is reproducible in high fidelity which is required in the application of practical molecular electronic devices. Current-voltage characteristics are presented.

Piezoelectric acoustic boundary waves in the structure of multilayer thin films/electrode/piezoelectric substrates

The propagation characteristics and temperature coefficients of frequency characteristics of boundary waves, which concentrate the acoustic energy in the interface between the thin film-metal electrode and piezoelectric substrate using the structure of multithin film layers/metal electrode/piezoelectric substrates, are described. The new boundary waves are obtained by using higher velocity thin film as the top surface than internal ones of metal electrode and thin film. The materials with structures of AlN, Al2O3/SiO2/Al, Cu, Ag, Au/rotated Y-X LiNO3, and LiTaO3 are applied. The range of existence of boundary wave for thicknesses of thin films, electrodes on Y-X LiNbO3, LiTaO3, and temperature coefficient of frequency are shown. Experimental results of surface acoustic wave resonators show a good frequency response of admittance at 500 MHz range.

Surface plasmon coupled electroluminescent emission

Besides directly emitting light, electroluminescence in organic light emitting diodes (OLEDs) can excite surface plasmons (SPs) on the metal electrodes of the device. By designing a microcavity OLED with thin film metal electrodes, we can directly probe a SP mode by leaky wave extraction with a high refractive index glass prism coupler. Additional angle and polarization resolved reflection measurements on the OLED multilayer structure together with transfer matrix calculations allow us to unequivocally characterize the electroluminescent-driven SP mode.

Role of Molecular Oxygen in Photocatalytic Oxidative Decomposition of Acetic Acid by Metal Oxide Particulate Suspensions and Thin Film Electrodes

The role of molecular oxygen (O2) in the photocatalytic decomposition of a model compound, acetic acid, was studied using suspensions and electrodes of metal oxide particles, titanium(IV) oxide (TiO2) or tungsten(VI) oxide (WO3). Results for photocurrent efficiency in photoelectrochemical measurements revealed that reaction of O2 with radicals liberated by positive holes increases the overall rate of photocatalytic oxidative decomposition of acetic acid.

Field-effect control of electro-osmotic flow in microfluidic networks

This paper describes microfluidic networks based on glass microchannels with integrated insulated gate electrodes that are used to modify the zeta-potential at specific locations on the wall surface of the microchannels. This modification is used to control direction and size of the electro-osmotic flow (EOF) in the microchannels. A new microfabrication process was developed that compared to earlier work in our institute [R.B.M. Schasfoort, S. Schlautmann, J. Hendrikse, A. van den Berg, Science 286 (1999) 942] leads to mechanically more robust structures and better flow visualization in the gate region. In the new process sequence, the microfluidic chips were fabricated using deposition on a Pyrex glass substrate of a thin-film metal electrode, which was covered with a thin insulating film of silicon dioxide, followed by chemical mechanical polishing prior to bonding to a second glass plate to form closed microchannels. Electrical breakdown of the metal–insulator structures was measured and occurred at 9.6 ± 0.3 MV/cm. Experiments using fluorescent beads to visualize the flow patterns in the microchannel showed that the EOF is linearly dependent on the applied gate voltage. The average EOF could be stopped completely for longitudinal fields of 150 V/cm by applying a gate field of 1.7 MV/cm.

Ag/AgCl microelectrodes with improved stability for microfluidics

A method for fabricating Ag/AgCl planar microelectrodes for microfluidic applications is presented. Micro-reference electrodes enable accurate potentiometric measurements with miniaturized chemical sensors, but such electrodes often exhibit very limited lifetimes. Our goal is to construct Ag/AgCl microelectrodes reliably with improved potential stability that are compatible with surface mounted microfluidic channels. Electrodes with geometric surface areas greater than or equal to 100 μm 2 were fabricated individually and in an array format by electroplating silver, greater than 1 μm thickness, onto photolithographically patterned thin-film metal electrodes. The surface of the electroplated silver was chemically oxidized to silver chloride to form Ag/AgCl micro-reference electrodes. Characterization results showed that Ag/AgCl microelectrodes produced by this fabrication method exhibit increased stability compared with many devices previously reported. Electrochemical impedance spectroscopy allowed device specific parameters to be extracted from an equivalent circuit model, and these parameters were used to describe the performance of the microelectrodes in a microfluidic channel. Thus, stable Ag/AgCl microelectrodes, fabricated with a combination of photolithographic techniques and electroplating, were demonstrated to have utility for electrochemical analysis within microfluidic systems.

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on asmooth crystal cleavage plane with precisely predetermined spacing. Our method does notrequire any high-resolution nanolithography tools, all lateral patterning being based onconventional optical lithography. Using molecular beam epitaxy we embedded athin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide(AlGaAs) layers with monolayer precision. By cleaving the substrate an atomicallyflat surface is obtained exposing the AlGaAs–GaAs sandwich structure. Afterselectively etching the GaAs layer, the remaining AlGaAs layers are used as asupport for deposited thin film metal electrodes. We characterized these coplanarelectrodes by atomic force microscopy and scanning electron microscopy; thisrevealed clean, symmetric and macroscopically flat surfaces with a maximumcorrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAslayer the measured electrode distance was 22.5 nm with a maximum deviation ofless than 2.1 nm. To demonstrate the electrical functionality of our device wepositioned single colloidal gold nanoparticles between the electrodes by an alternatingvoltage trapping method; this resulted in a drop of electrical resistance from∼11 G Ω to∼1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objectslike molecules or more complex aggregates on flat surfaces and the investigation of theirelectrical properties in a freely suspended configuration.

Profiling and modeling of dc nitrogen microplasmas

This article explores electric current and field distributions in dc microplasmas, which have distinctive characteristics that are not evident at larger dimensions. These microplasmas, which are powered by coplanar thin-film metal electrodes with 400-μm minimum separations on a glass substrate, are potentially useful for microsystems in both sensing and microfabrication contexts. Experiments in N2 ambient show that electron current favors electrode separations of 4 mm at 1.2 Torr, reducing to 0.4 mm at 10 Torr. The glow region is confined directly above the cathode, and within 200–500 μm of its lateral edge. Voltage gradients of 100 kV/m exist in this glow region at 1.2 Torr, increasing to 500 kV/m at 6 Torr, far in excess of those observed in larger plasmas. Numerical simulations indicate that the microplasmas are highly nonquasineutral, with a large ion density proximate to the cathode, responsible for a dense space-charge region, and the strong electric fields in the glow region. It is responsible for the bulk of the ionization and has a bimodal electron energy distribution function, with a local peak at 420 eV.

Polymer-based thermo-optic waveguide beam deflector with novel dual folded-thin-strip heating electrodes

A new design scheme for a polymeric waveguide thermo- optic beam deflector is presented and experimentally demonstrated. The refractive indices of the waveguide materials can be thermally tuned by applying electric current through a metal thin film electrode deposited and patterned using standard lithography on top of the waveguide. In doing so, the geometry of the electrode determines the performance and reliability of the beam deflector. We form a pair of heating electrodes sitting opposite to each other like interlocked sawteeth that have a folded-thin-strip substructure, and hence demonstrate an advanced beam deflector that has biangular sweeping capability, low power con- sumption, less wavefront distortion, and high reliability. A full sweep angle of 56.5 mrad (3.24 deg) and 138 switching capability at a 1550 nm wavelength are attained with an average power consumption of 247 mW per switching between adjacent resolvable spots. Switching from the zero-bias spot to the first resolvable spot exhibits a response time of 2 ms in both rising and falling. © 2003 Society of Photo-Optical Instru-

Electric field pulse assisted covalent immobilization and hybridization of DNA in the nanosecond time scale

AbstractA single, square, voltage pulse is used to accelerate both the immobilization kinetics of DNA molecules on a functionalized thin film silicon dioxide surface and the hybridization of complementary DNA strands to immobilized DNA molecules. The voltage pulse is applied to an integrated thin-film metal electrode beneath the functionalized surface. The duration and magnitude of the voltage pulse are compatible with silicon microelectronics circuits. During immobilization, covalent thiol bonding to the functionalized surface occurs during a single pulse lasting only 100 ns. Hybridization to the immobilized complementary strand occurs during 100 μs pulses.

Prototyping disposable electrophoresis microchips with electrochemical detection using rapid marker masking and laminar flow etching.

Two novel methods are described for the fabrication of components for microchip capillary electrophoresis with electrochemical detection (microchip CEEC) on glass substrates. First, rapid marker masking is introduced as a completely nonphotolithographic method of patterning and fabricating integrated thin-film metal electrodes onto a glass substrate. The process involves applying the pattern directly onto the metal layer with a permanent marker that masks the ensuing chemical etch. The method is characterized, and the performance of the resulting electrode is evaluated using catecholamines. The response compares well with photolithographically defined electrodes and exhibits detection limits of 648 nM and 1.02 microM for dopamine and catechol, respectively. Second, laminar flow etching is introduced as a partially nonphotolithographic method of replicating channel networks onto glass substrates. The replication process involves applying a poly(dimethylsiloxane) (PDMS) mold of the channel network onto a slide coated with a sacrificial metal layer and then pulling solutions of metal etchants through the channels to transfer the pattern onto the sacrificial layer. The method is tested, and prototype channel networks are shown. These methods serve to overcome the time and cost involved in fabricating glass-based microchips, thereby making the goal of a disposable high performance lab-on-a-chip more attainable.

Development of a molecular diagnosis assay based on electrohybridization at plastic electrodes and subsequent PCR

Technologies enabling specific recognition of medically relevant nucleic acid sequences will play a pivotal role in future medical diagnosis. Whereas many approaches to molecular diagnosis systems include DNA microarrays on chips and fluorometric detection, the basis of our approach is the use of inexpensive components like plastic or metal thin film electrodes with low multiplexing and an electrochemical detection unit. To increase the sensitivity, PCR can be used as an intermediate step. For selective enrichment, specific nucleic acid probes were covalently attached at their 5′-ends to conducting polycarbonate/carbon fiber electrodes. Complementary oligonucleotides were enriched at the electrodes by cyclic inversion of an electrochemical potential, transferred into a PCR vial and thermally or electrochemically desorbed. The analysis of the PCR product shows the efficiency and selectivity of the electrochemical enrichment. Hybridization of DNA was shown by electrochemical methods, in this work especially by differential pulse voltammetry (DPV) using the single strand specific hybridization redox indicator osmium(VIII)-tetroxide, and potentiometric stripping analysis (PSA). This combination of experimental methods is the basis for a molecular diagnosis system including a disposable nucleic acid modified working electrode for specific enrichment, detection and quantification, and an optional capillary PCR module for fast amplification.

Powder-blasting technology as an alternative tool for microfabricationof capillary electrophoresis chips with integrated conductivity sensors

The fabrication and characterization of a microfluidic device forcapillary electrophoresis applications is presented. The device consists of aglass chip which contains a single separation channel as well as an integratedconductivity detection cell. In contrast to most microfluidic glass devicesthe channels are not wet etched in HF but machined by the newly developedmicro powder-blasting technique which allows the creation of microstructuresbelow 100 µm, and additionally makes parallel hole machining at very lowcosts outside the cleanroom environment possible [1, 2]. The integration ofthe conductivity detector was achieved by leading two thin-film metalelectrodes inside the separation channel. For rapid sample injection the chipis mounted inside an autosampler-based capillary electrophoresis platform. Thedetection electrodes for conductivity detection are read out by lock-inamplifier electronics. First measurements show the successful separation ofvarious ions in the sub-millimeter range.

Detection of metal ions in aqueous solution by voltohmmetry.

A new electrochemical detection principle is described for the trace analysis of dissolved species which can be deposited at polycrystalline thin-film metal electrodes and which change the surface resistance of the electrode. Because the latter parameter is measured in dependence on the applied electrode potential this method is called voltohmmetry. The preparation of the required thin-film electrodes and the experimental set-up is introduced and discussed. Typical voltohmmetric experiments are illustrated by measurements of Tl+/Tl at polycrystalline gold electrodes with a thickness of 15 nm. The analytical capabilities of this new approach are discussed. It is already possible to determine heavy metals such as Tl+, Pb2+ or Cd2+ in the range of a few microgram/L by surface resistance-potential measurements at thin-film electrodes with a simple cyclic technique. Further developments of voltohmmetry are envisaged.