Metal Contact Pads Research Articles

Facile Fabrication of Multilayer Stretchable Electronics via a Two-mode Mechanical Cutting Process.

A time- and cost-effective fabrication methodology via a two-mode mechanical cutting process for multilayer stretchable electronics has been developed without using the conventional photolithography-based processes. A commercially available vinyl cutter is used for defining complex patterns on designated material layers by adjusting the applied force and the depth of the cutting blade. Two distinct modes of mechanical cutting can be achieved and employed to establish the basic fabrication procedures for common features in stretchable electronics, such as the metal interconnects, contact pads, and openings by the "tunnel cut" mode, and the flexible overall structure by the "through cut" mode. Three robust and resilient stretchable systems have been demonstrated, including a water-resistant, omnidirectionally stretchable supercapacitor array, a stretchable mesh applicable in sweat extraction and sensing, and a skin-mountable human breathing monitoring patch. Results show excellent electronic performances of these devices made of multilayer functional materials after repetitive large deformations.

A Micromachined Freestanding Electrochemical Sensor for Measuring Dissolved Oxygen

A freestanding, miniaturized electrochemical oxygen sensor for biomedical implantation has been microfabricated and characterized. The sensor is fabricated on a flexible and biocompatible polyimide substrate using lithographic techniques and singulated with an excimer laser. The sensor consists of a round-shaped planar sensing portion (diameter: 2.4 mm and thickness: $180~\mu \text{m}$ ) and metal lines and contact pads for connection with external analytical tools. The sensing portion is comprised of three electrodes (working, counter, and reference electrodes), an electrolyte reservoir filled with liquid electrolyte, and a semipermeable membrane for oxygen introduction. The electrodes are patterned using a single layer of gold without the adhesion layer to eliminate any multilayer metallic structure in the sensing region. Both polydimethylsiloxane and fluorinated ethylene propylene semipermeable membranes are characterized. The fabricated sensor demonstrated an oxygen response of −14 nA/atm.% and good linearity (correlation coefficient exceeding 0.99). This device can potentially be incorporated into physiological applications, including monitoring oxygen tension in biomedical-related applications over semi-chronic time periods. [2019-0004]

Piezoresistive strain sensor based on monolayer molybdenum disulfide continuous film deposited by chemical vapor deposition

In this paper, a centimeter-scale monolayer molybdenum disulfide (MoS2) film deposition method has been developed through a simple low-pressure chemical vapor deposition (LPCVD) growth system. The growth pressure dependence on film quality is investigated in this LPCVD system. The layer nature, electrical characteristic of the as-grown MoS2 films indicate that high quality films have been achieved. In addition, a hydrofluoric acid treated SiO2/Si substrate is used to improve the quality of the MoS2 films. Piezoresistive strain sensor based on the monolayer MoS2 film elements is fabricated by directly patterning metal contact pads on MoS2 films through a silicon stencil mask. A gauge factor of 104 ± 26 under compressive strain is obtained by using a four-point bending method, which may inspire new possibilities for two-dimensional (2D) material-based microsystems and electronics.

Intertwined Design: A Novel Lithographic Method to Realize Area Efficient High-Voltage SiC BJTs and Darlington Transistors

A novel lithographic method called intertwined design is demonstrated for high-power SiC devices to improve the area usage and current drive with more uniform current distribution along the device. The higher current drive is achieved by employing the inactive area underneath the base metal contact pads; more uniform current distribution is obtained by the center-base design; whereas the hexagon and square cell geometries result in >15% higher current density at lower on-resistance compared with the conventional finger design. For the first time, we have experimentally presented the intertwined design to marry these advantages and realize a high-efficient SiC power device. Center-base high-voltage 4H–SiC BJTs and Darlington pairs with different square and hexagon cell geometries are fabricated and compared with conventional designs to prove the ability of the intertwined design. The method can widely be used for large-area high-voltage BJTs as well as for integrated devices.

Impact of Residual Hardmask Wires on the Performance of Film-Profile-Engineered ZnO Thin-Film Transistors With Discrete Bottom Gates

Root cause for the anomalous degradation in the ON-current of film-profile-engineered ZnO thin-film transistors with discrete bottom gates, a new scheme proposed in our previous work, is investigated. Our findings indicate that the deposited source/drain (S/D) metal contact pads are disconnected owing to two TiN wires hung over the S/D regions, which are unintentionally formed during the fabrication of devices. The disconnected S/D metal contacts cause an increase in the S/D series resistance, and thus, the ON-current is degraded. Several ways for addressing this issue are proposed in this letter, including the simple thinning of gate electrode. As the undesirable TiN wires are eliminated, the devices demonstrate enhanced field-effect mobility and uniformity in performance.

Improved performance mechanism of III–V compound triple-junction solar cell using hybrid electrode structure

To improve the conversion efficiency of InGaP/InGaAs/Ge compound triple-junction solar cells, the hybrid electrode structure consisting of the metal contact pads and the transparent indium–tin-oxide (ITO) film was designed. The metal contact pad was used to form good ohmic contact. The current extraction capability of the resulting solar cells was enhanced by reducing the interval of the metal contact pads. In addition, transparent ITO films were used to replace the conventional bus-bar metal electrodes to avoid shadow loss. To study the improvement mechanisms of the conversion efficiency, the designed electrodes with various intervals among the metal contact pads were fabricated on solar cells, and the performances were evaluated for comparison.

Mechanisms of (NH4)2Sx-treated III-V compound triple-junction solar cells incorporating with hybrid electrode

A hybrid electrode composed of the metal contact pads and the transparent indium-tin-oxide film was incorporated with the (NH4)2Sx-treated InGaP/InGaAs/Ge triple-junction solar cells. The hybrid electrode structure was used to reduce the metal shadow area. The passivation function enabled by the (NH4)2Sx surface treatment was used to enhance the photoluminescence intensity and carrier lifetime of the (NH4)2Sx-treated n-type AlInP window layer. The conversion efficiency of (NH4)2Sx-treated solar cells with the hybrid electrode structure was improved up to 35.73% due to an increase in the absorption of the incident light along with the surface passivation.

UV-assisted alcohol sensing using SnO2 functionalized GaN nanowire devices

Abstract A chemiresistor type sensor for selective alcohol sensing has been realized from gallium nitride (GaN) nanowires (NWs) functionalized with sputter-deposited tin dioxide (SnO2) nanoparticles. Two-terminal devices were fabricated using standard microfabrication techniques with the individual NWs air-bridged between the two metal contact pads. Through a combination of X-ray diffraction (XRD), electron-backscatter-diffraction (EBSD), and TEM/STEM techniques, we confirmed the presence of rutile SnO2 nanocrystals on the GaN surface. A change in device current is observed when the device is exposed to alcohol vapors (methanol, ethanol, propanol, and butanol) at room temperature under 215–400 nm UV illumination with 3.75 mW/m2 intensity at 365 nm wavelength. The sensor reproducibly responded to a wide range of alcohol vapor concentrations, from 5000 μmol/mol (ppm) down to 1 μmol/mol (ppm) in air. Notably, the devices show low sensitivity to acetone and hexane, which allows them to selectively detect the primary alcohol vapors mixed with these two common volatile organic compounds (VOCs). The sensor response was not observed without UV excitation. From the experimental results we found a relationship between the response towards the alcohol vapors and the length of carbon chain in the molecule: the chemiresistive response decreases with the increasing carbon chain from methanol to n-butanol. In addition, we observed that the isomeric branching in i-propanol and i-butanol caused reduced response as compared to n-propanol and n-butanol, respectively. We have qualitatively explained the sensor operation by employing a mechanism, which includes oxidation of analyte molecules on the SnO2 surface, leading to enhanced photoconductivity in GaN nanowire.

Oxide-Confined Formation of Germanium Nanowire Heterostructures for High-Performance Transistors

Over the past several years, the formation of nanowire heterostructures via a solid-state reaction between a semiconductor nanowire and metal contact pads has attracted great interest. This is owing to its ready application in nanowire field-effect transistors (FETs) with a well-controlled channel length using a facile rapid thermal annealing process. We report the effect of oxide confinement on the formation of Ge nanowire heterostructures via a controlled reaction between a vapor-liquid-solid-grown, single-crystalline Ge nanowire and Ni pads. In contrast to the previous formation of Ni(2)Ge/Ge/Ni(2)Ge nanowire heterostructures, a segment of high-quality epitaxial NiGe was formed between Ni(2)Ge and Ge with the confinement of Al(2)O(3) during annealing. Significantly, back-gate FETs based on this Ni(2)Ge/NiGe/Ge/NiGe/Ni(2)Ge heterostructure demonstrated a high-performance p-type transistor behavior, showing a large on/off ratio of more than 10(5) and a high normalized transconductance of 2.4 μS/μm. The field-effect hole mobility was extracted to be 210 cm(2)/(V s). Temperature-dependent I-V measurements further confirmed that NiGe has an ideal ohmic contact to p-type Ge with a small Schottky barrier height of 0.11 eV. Moreover, the hysteresis during gate bias sweeping was significantly reduced after Al(2)O(3) passivation, and our Ω-gate Ge nanowire FETs using Al(2)O(3) as the top-gate dielectric showed an enhanced subthreshold swing and transconductance. Therefore, we conclude that the Al(2)O(3) layer can effectively passivate the Ge surface and also serve as a good gate dielectric in Ge top-gate FETs. Our innovative approach provides another freedom to control the growth of nanowire heterostructure and to further achieve high-performance nanowire transistors.

Catalyst-Free Growth of ZnO Nanowires Based on Topographical Confinement and Preferential Chemisorption and Their Use for Room Temperature CO Detection

Two novel approaches for patterned growth of ZnO nanowires (NWs) based on the selective deposition of zinc acetate (ZA) solution precursors are presented and compared. The first is using the topographical confinement within a photoresist pattern on Si/SiO2 substrates (type I), and the second is using preferential chemisorption on self-assembled monolayer modified Au electrodes on Si/SiO2 substrates (type II). In both approaches, the ZnO seeds from the ZA solution form crystallites without severe defects over a large area via annealing at 350 °C. These seed layers were used to grow ZnO NWs via a catalyst-free vapor-phase deposition method using temperatures of up to 900 °C. The presented method is effective for realizing NW growth on both Si and Au electrodes by preserving the electrode configuration even at such high temperatures used for NW growth, which is a novelty and crucial for future sensor applications. As a result NWs connected through metal contact pads or electrodes have been realized in very simple and effective way. As a test of principle the resulting configurations were used to demonstrate a highly sensitive room temperature CO sensor. A CO concentration as low as 120 ppb was detected using both types of sensors. The type II sensor exhibited enhanced sensing properties compared to that of type I.

Micro-four-point probe Hall effect measurement method

We report a new microscale Hall effect measurement method for characterization of semiconductor thin films without need for conventional Hall effect geometries and metal contact pads. We derive the electrostatic potential resulting from current flow in a conductive filamentary sheet with insulating barriers and with a magnetic field applied normal to the plane of the sheet. Based on this potential, analytical expressions for the measured four-point resistance in presence of a magnetic field are derived for several simple sample geometries. We show how the sheet resistance and Hall effect contributions may be separated using dual configuration measurements. The method differs from conventional van der Pauw measurements since the probe pins are placed in the interior of the sample region, not just on the perimeter. We experimentally verify the method by micro-four-point probe measurements on ultrashallow junctions in silicon and germanium. On a cleaved silicon ultrashallow junction sample we determine carrier mobility, sheet carrier density, and sheet resistance from micro-four-point probe measurements under various experimental conditions, and show with these conditions reproducibility within less than 1.5%.

Investigation of Novel Superconducting Hot Electron Bolometer Geometries Fabricated With Ultraviolet Lithography

While the superconducting community has developed various techniques for the fabrication of nanoscale hot electron bolometers (HEB), the basic structure of an HEB microbridge has been fairly consistent. A typical HEB used in a cryogenic terahertz mixer consists of a thin strip of superconductor contacted on opposite ends by small metal contact pads in a planar configuration. Both the physics and typical fabrication requirements for HEBs inspire this traditional construction. In this paper, however, we demonstrate that alternative HEB geometries are possible, which can in some cases improve the ease of fabrication, alter device characteristics, or alleviate certain device limitations. New HEB structures and associated fabrication processes are presented, building on previous work using the SSNaPS method of nanoscale patterning. These include very short niobium nitride HEBs with self-aligned bridge etch masks, and sidewall-patterned niobium HEB microbridges.

Size-reduced silicon nanowires: Fabrication and electrical characterization

Silicon nanowires of diameters down to 100 nm and typical lengths of 1–3 μm have been fabricated in silicon-on-insulator material by electron beam lithography and plasma etching. They were subsequently size-reduced by photoelectrochemical etching resulting in wire widths down to 10 nm. To enable accurate control of the photoelectrochemical size-reduction, a micro-electrochemical cell was developed, enabling single nanowires to be exposed to the etching solution while being illuminated by a laser or a lamp. The arrangement allows contact leads to be extended to metal contact pads located outside the cell, which can be connected by probes, allowing in situ electrical characterization of a nanowire during etching. In this paper, we describe the experimental setup, the fabrication method and show examples of achieved wire widths together with some preliminary results from the electrical characterization.

Magnetic field dependence of d.c. critical current for large samples of textured bulk YBaCuO

Large samples of textured YBaCuO were prepared at the Texas Center for Superconductivity using a continuous process. These were cut into a rectangular shape with typical dimensions 4.0 cm × 0.5 cm × 0.25 cm. The large physical size of these samples and their ability to carry high currents particularly in zero magnetic field required low resistance contacts to avoid Joule heating at the sample current - electrode interface. Accordingly, low resistance metal contact pads for current and voltage electrodes were applied at NIST Boulder, using a sputter etch, sputter deposition, thermal evaporation and oxygen anneal procedure. Transport critical current density measurements at 77 K, and in magnetic fields up to 6 T were performed at NRL. A holder was designed to hold the samples securely in such a way that would allow for contraction during cool-down and to support them against the anticipated large Lorenz forces. The highest critical current measured with no external field was 393 A. This corresponds to a critical current density of 1871 A cm −2. At 1 T, these values decreased to 229 A or 1090 A cm −2. These critical current values, found in material that was fabricated using a technique capable of being scaled up for larger sizes, demonstrate a potential usefulness in large scale applications.

Effect of Oxygen Annealing on Low-Resistivity Contactsfor High-Tc Superconductors

ABSTRACTA method for making low resistivity contacts to high-Tc superconductors has been developed, consisting of depositing noble metal contact pads (silver or gold) on a clean superconductor surface at low temperatures (<150°C). After annealing the silver contact pads in oxygen at intermediate temperatures (<500°C) for one hour, contact resistivities less than 2 × 10'° Ω-cm2 at 76 K are obtained, about six orders of magnitude less than for indium-solder contacts. Before annealing, the contact resistivities are still very low, in the 10-6 to 10-5 Ω-cm range at 76 K, which would be useful when contacts with low fabrication temperatures are required. The voltage-current characteristics of the contacts are strongly nonlinear after annealing, having a superconducting transition character. This is ascribed to the critical current of the superconducting material being exceeded at the contact interface. External connections to the contact pads have been made using both solder and thermosonic wire-bonding techniques.

Laser Photoetching of Polymers

AbstractLaser photoetching of polymers has been shown to be a useful method of producing via holes or patterns and is of potential importance in microelectronics fabrication. The maskless, self-developing nature of the process also offers unique opportunities in the preparation of packaging assemblies. Etching of via holes at any discretionary site for multichip interconnect, for example, provides the circuit designer with additional design flexibility. The photoetching rate is dependent on the absorption coefficient of the polymer (at the laser wavelength) and the irradiating fluence. Further, there appears to be an optimum absorption coefficient to achieve the maximum photoetch rate for a given fluence level. To laser drill holes for interconnects, it is desirable to irradiate at the highest energy (increased rate) possible without damaging underlying metal contact pads.

Non-Destructive Characterization of Semiconductors Using Organic Thin Films

ABSTRACTRectifying junctions prepared by vacuum deposition of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) and related compounds on both p- and n-type inorganic semiconducting wafers are used for their non-destructive evaluation. By evaporation of metal contact pads onto the organic layer, we can probe many of the fundamental -bulk and surface properties of the semiconductor.