Reliable Contacts Research Articles

PLD deposition of tungsten carbide contact for diamond photodiodes. Influence of process conditions on electronic and chemical aspects

Tungsten carbide, WC, contacts behave as very reliable Schottky contacts for opto-electronic diamond devices. Diamond is characterized by superior properties in high-power, high frequency and high-temperature applications, provided that thermally stable electrode contacts will be realized. Ohmic contacts can be easily achieved by using carbide-forming metals, while is difficult to get stable Schottky contacts at elevated temperatures, due to the interface reaction and/or inter-diffusion between metals and diamond. Novel type of contacts, made of tungsten carbide, WC, seem to be the best solution, for their excellent thermal stability, high melting point, oxidation and radiation resistance and good electrical conductivity. Our research was aimed at using pulsed laser deposition for WC thin film deposition, optimizing experimental parameters, to obtain a final device characterized by excellent electronic properties, as a detector for radiation in deep UV or as X-ray dosimeter. We deposited our films by laser ablation from a target of pure WC, using different reaction conditions (i.e., substrate heating, vacuum or reactive atmosphere (CH4/Ar), RF plasma activated), to optimize both the stoichiometry of the film and its structure. Trying to obtain a material with the best electronic response, we used also two sources of laser radiation for target ablation, i.e., nano-second pulsed excimer laser ArF, and ultra-short fs Ti:Sapphire laser. The structure and chemical aspects have been evaluated by Raman and X-ray photoelectron spectroscopy (XPS), while the dosimeter photodiode response has been tested by the I–V measurements, under soft X-ray irradiation.

Electrophoretic Deposition of Carbon Nanotubes for Interconnections in Microelectronics

ABSTRACTCarbon nanotubes (CNTs) have been considered as a promising interconnect material to replace the solder bump used in the flip chip package because of their special electrical, mechanical and thermal properties, which may promote both the performance and reliability of the flip chip packaging. In this paper, electrophoretic deposition (EPD) of CNTs on substrates has been demonstrated for the interconnect application. EPD is a simple, low cost and high throughput process that is capable to produce densely packed film with good homogeneity at low temperature. By altering the electric fields and deposition time during the EPD process, the thickness of the CNTs film could be controlled. In this study, multi-walled carbon nanotubes (MWCNTs) were successfully coated on the various substrates using the EPD method. A highly uniform CNTs microstructure film with thickness over 5 µm was achieved. In addition, the selective depositions of CNTs on the pre-defined bond pads to form CNTs bumps were also accomplished. By employing typical flip-chip bonding technique, high density CNTs bumps were aligned to form a test chip/host substrate interconnects. The electrical conductivity of the CNTs interconnects was carried out using four-point probe measurement. Reliable electrical contacts with linear relationship in the current-voltage (I-V) characteristic suggesting ohmic behaviour were attained. The overall resistances extracted were also relatively low. These superior electrical properties have demonstrated that the CNTs bumps deposited using EPD method is a viable way to serve as an alternative to current metal solder interconnects material such as Sn-Pb alloys. Hence, it offers a promising interconnect application in the quest for device miniaturization in microelectronic industry.

Fabrication of Self-Aligned Graphene FETs with Low Fringing Capacitance and Series Resistance

Graphene FETs with top-gate and buried-gate structure has been studied. The buried-gate structure shows less fringing capacitance and more reliable contacts. High-performance graphene transistors with self-aligned buried gates have been fabricated. The graphene transistor shows field-effect mobility of over 6,000 cm2/V · s according to the transconductance measurement. The contact resistance and intrinsic mobility have been extracted from both curve fitting and transfer length measurement, and the two results agree well. This result paves the way of high-quality graphene transistor technology for the RF application.

An Investigation of Electrical Contacts for Higher Manganese Silicide

Five metals with large work functions including Co, Ni, Cr, Ti, and Mo and two silicides including MnSi and TiSi2 were examined to determine the best contact material for the thermoelectric material higher manganese silicide (HMS). Three-layer structures of HMS/contact/HMS were prepared in a sintering process. The contact resistance was measured versus temperature. The structures were subjected to x-ray diffraction and energy-dispersive x-ray spectroscopy examination. Thermal stability of the structures was determined by heating the samples to 700°C for different time intervals. The pure metals failed to make reliable contacts due to poor mechanical and chemical stability at high temperatures. In contrast, the metal silicides (MnSi and TiSi2) showed superior chemical and mechanical stability after the thermal stability test. The observed contact resistance of MnSi and TiSi2 was within the range of practical interest (10−5 Ω cm2 to 10−4 Ω cm2) over the entire range of investigated temperatures (20°C to 700°C). The best properties were found for the nanograined MnSi, for which the resistance of the contact was as low as 10−6 Ω cm2.

Contacting nanowires and nanotubes with atomic precision for electronic transport

Making contacts to nanostructures with atomic precision is an important process in the bottom-up fabrication and characterization of electronic nanodevices. Existing contacting techniques use top-down lithography and chemical etching, but lack atomic precision and introduce the possibility of contamination. Here, we report that a field-induced emission process can be used to make local contacts onto individual nanowires and nanotubes with atomic spatial precision. The gold nano-islands are deposited onto nanostructures precisely by using a scanning tunneling microscope tip, which provides a clean and controllable method to ensure both electrically conductive and mechanically reliable contacts. To demonstrate the wide applicability of the technique, nano-contacts are fabricated on silicide atomic wires, carbon nanotubes, and copper nanowires. The electrical transport measurements are performed in situ by utilizing the nanocontacts to bridge the nanostructures to the transport probes.

Nanobelt–carbon nanotube cross-junction solar cells

Nanostructures such as carbon nanotubes (CNTs) and semiconducting nanowires are promising candidates for developing next-generation photovoltaics. Here, we report solar cells using individual single-walled or double-walled CNTs and CdSe nanobelts arranged in simple cross-junction configurations. The CNT and CdSe nanobelts form reliable line contacts at their intersections, resulting in efficient heterojunction solar cells with power conversion efficiencies up to 1.87% and stable performance in air over long periods. Both semiconducting and metallic CNTs can form solar cells with CdSe nanobelts, with similar open-circuit voltages but different short-circuit current densities. We can integrate multiple CNTs in parallel with a single nanobelt to construct an array of cross-junction solar cells simultaneously, with scaled current output, indicating the possibility of parallel device connection and large-scale production. Our results show the potential of utilizing one-dimensional nanostructures to design and fabricate high performance photovoltaic devices with well-defined and scalable structures.

Highly Reliable Ohmic Contacts to N-Polar n-Type GaN for High-Power Vertical Light-Emitting Diodes

We report on the formation of highly reliable Ti/Al-based Ohmic contacts to N-polar n-GaN for vertical light-emitting diodes via laser-annealing. All as-deposited samples are Ohmic with specific contact resistances of 1.1 - 4.3 × 10-4 Ω cm2. After annealing at 250°C, unlike the untreated sample, the laser-annealed samples remain Ohmic with specific contact resistances of 2.6- 3.9 × 10-4 Ω cm2. The laser-annealed samples remain electrically stable up to 60 min at 300°C. Laser-annealing causes the formation of interfacial TiN/β -AlN phases with rock salt structure. Based on X-ray photoemission spectroscopy and scanning transmission electron microscopy results, possible Ohmic mechanisms are described.

Indium as an efficient ohmic contact to N-face n-GaN of GaN-based vertical light-emitting diodes

We propose indium (In), a low work function and nitride-forming element, as an efficient ohmic contact layer to N-face n-GaN. While conventional Al-based ohmic contacts show severe degradation after annealing at 300 °C, In-based ohmic contacts display considerable improvement in contact resistivity. The annealing-induced enhancement of ohmic behavior in In-based contacts is attributed to the formation of an InN interfacial layer, which is supported by x-ray photoemission spectroscopy measurements. These results suggest that In is of particular importance for application as reliable ohmic contacts to n-GaN of GaN-based vertical light-emitting diodes.

Metal-Molecule-Silicon Junctions Produced by Flip Chip Lamination of Dithiols: Effect of Molecular Length and Backbone

The integration of organic molecules with silicon is increasingly being studied for potential as hybrid electronic devices. Creating dense and highly ordered organic monolayers on silicon with reliable metal contacts still remains a challenge. A novel technique, flip chip lamination (FCL), has been developed to create uniform metal-molecule-semiconductor junctions. FCL uses nanotransfer printing to covalently attach self-assembled monolayers to a hydrogen-passivated Si(111) surface. Several dithiol molecules were studied to explore the role of molecular length and chemical structure on the physical and electronic properties of the molecules. The effects of the FCL process on the chemical and physical properties of the imbedded molecular layer were interrogated with polarized-backside reflectance absorption infrared spectroscopy. Electrical measurements were also performed to characterize device structure and to offer better insight into the mechanisms at play in the electronic transport.

Control of junction resistances in molecular electronic devices fabricated by FIB

A major hurdle to realize molecular electronic devices (MEDs) is to make reliable electrical contacts to a single or a few molecules. Our nano-contact platform with a gap size of less than 25nm with resistances above 1000TΩ was built using combined techniques of photolithography, electron beam lithography and focused ion beam milling. In this study, we have used gold nanoparticles (AuNPs) to bridge the nanoelectrode gaps by dielectrophoretic trapping and thus obtain electrical contacts. The electrodes and/or the nanoparticles were functionalised with 1–2nm long alkane-thiol molecules so that the electronic structure of these molecules determines the properties of the electrical junction. Molecules were introduced both by functionalising the nanogap and the nanoparticles and the results of both functionalisation protocols are compared. Here, we show the nanogap–nanoparticle bridge set-up containing metal–molecule junctions that can be used as a base for the development of molecular electronics containing only a few molecules under ambient conditions. Current–voltage (I–V) characterization of alkanethiol/gold junction showed non-linear response where mean geometric resistance of four different junctions could be tuned from 20GΩ to 20TΩ. The results from the measurements on 1-alkanethiol in such devices is a first step to demonstrate that this platform has the potential to obtain stable electronic devices having relatively small numbers of molecules with reliable metal molecule junction by combing top-down and bottom-up approaches.

The Shear Flow Processing of Controlled DNA Tethering and Stretching for Organic Molecular Electronics

DNA has been recently explored as a powerful tool for developing molecular scaffolds for making reproducible and reliable metal contacts to single organic semiconducting molecules. A critical step in the process of exploiting DNA-organic molecule-DNA (DOD) array structures is the controlled tethering and stretching of DNA molecules. Here we report the development of reproducible surface chemistry for tethering DNA molecules at tunable density and demonstrate shear flow processing as a rationally controlled approach for stretching/aligning DNA molecules of various lengths. Through enzymatic cleavage of λ-phage DNA to yield a series of DNA chains of various lengths from 17.3 μm down to 4.2 μm, we have investigated the flow/extension behavior of these tethered DNA molecules under different flow strengths in the flow-gradient plane. We compared Brownian dynamic simulations for the flow dynamics of tethered λ-DNA in shear, and found our flow-gradient plane experimental results matched well with our bead-spring simulations. The shear flow processing demonstrated in our studies represents a controllable approach for tethering and stretching DNA molecules of various lengths. Together with further metallization of DNA chains within DOD structures, this bottom-up approach can potentially enable efficient and reliable fabrication of large-scale nanoelectronic devices based on single organic molecules, therefore opening opportunities in both fundamental understanding of charge transport at the single molecular level and many exciting applications for ever-shrinking molecular circuits.

Novel split-tip proximal probe for fabrication of nanometer-textured, in-plane oriented polymer films

Novel fabrication schemes are required to deposit nanoscale materials that contain molecules oriented in the plane of the surface. The breakage of in-plane symmetry allows devices to be fabricated in this plane, enabling molecular electronics to follow this successful paradigm of semiconductor devices. The authors discuss here the fabrication of a unique split-tip optical nanoprobe that can be used to both orient molecules on a surface with a strong, localized electric field and deposit them with nanoscale resolution. Ultraviolet light injected through the probe into the region of aligned molecules causes the deposition. The production of the split-tip probe is significantly different than that of the related near-field scanning optical microscope (NSOM) probe, since the stresses in the metal layer must be held by the metal film–silica interface rather than within the film as it encircles the silica of a NSOM probe. Mounting of the probe to ensure reliable electrical contacts is also described.

Fabricating nanowire devices on diverse substrates by simple transfer-printing methods

The fabrication of nanowire (NW) devices on diverse substrates is necessary for applications such as flexible electronics, conformable sensors, and transparent solar cells. Although NWs have been fabricated on plastic and glass by lithographic methods, the choice of device substrates is severely limited by the lithographic process temperature and substrate properties. Here we report three new transfer-printing methods for fabricating NW devices on diverse substrates including polydimethylsiloxane, Petri dishes, Kapton tapes, thermal release tapes, and many types of adhesive tapes. These transfer-printing methods rely on the differences in adhesion to transfer NWs, metal films, and devices from weakly adhesive donor substrates to more strongly adhesive receiver substrates. Electrical characterization of fabricated NW devices shows that reliable ohmic contacts are formed between NWs and electrodes. Moreover, we demonstrated that Si NW devices fabricated by the transfer-printing methods are robust piezoresistive stress sensors and temperature sensors with reliable performance.

“For us women, working is an unfulfilled dream”: Womens' wage work and food security

abstract This article reviews the findings from two small community studies in South Africa on the experience of poor households in respect of the economic downturn, employment, and food security, with special emphasis on gender dynamics. This study was implemented by the Human Sciences Research Council and funded by Oxfam International, and sought to explore the human, social and personal experiences of the economic downturn. The study interviewed 29 households in Swedenville, a recently formed shack settlement outside of the Pretoria/Johannesburg city region that mainly comprises rural in-migrants, and in Bergpoort, a rural sending community in Mpumalanga. Most of the shack households and many of the rural households reported not being able to afford preferred foods or to eat three meals a day. The female-headed households without male partners were unable to send workers to the labour market and survived on social grants alone; they clustered at the bottom of both communities' income distributions and suffered the greatest food insecurity. High levels of migration left migrant households isolated and vulnerable, dependent on their own resources, with few reliable contacts in communities made up of strangers. Shack households especially reported surviving on part-time or insecure employment of the male partner only, while the women remained out of the labour force; women's wage employment was low in the rural community as well. Interviews with the women point to a concealed gender struggle over women's access to wage work. To protect the male lead role in domestic relations, younger rural-born men in particular may discourage their previously employed wives or partners from remaining in the labour market. Although women's wage contribution could make these impoverished households viable and food secure, isolated and vulnerable women may acquiesce in order to avoid the greater food security risk of becoming female household heads.

Controlled Fabrication of Nanostructure Material Based Chemical Sensors

AbstractThe use of nanotechnology based materials for chemical sensing has been of great interest since nanocrystalline materials have been shown to offer improved sensor sensitivity, stability, and response time. Several groups are successfully integrating nanostructures such as nanowires into operational sensors. The typical procedure may include random placement (e.g., dispersion, with fine-line patterning techniques used to create functional sensors) or time consuming precise fabrication (e.g., mechanical placement using an atomic force microscope or laser tweezer techniques). Dielectrophoresis has also been utilized, however it can be challenging to achieve good electrical contact of the nanostructures to the underlying electrodes. In this paper we report on a sensor platform that incorporates nanorods in a controlled, efficient, and effective manner. Semiconducting SnO2 nanorods are used as the sensing element for detection of hydrogen (H2) and propylene (C3H6) up to 600oC. Using a novel approach of combining dielectrophoresis with standard microfabrication processing techniques, we have achieved reproducible, time-efficient fabrication of gas sensors with reliable contacts to the SnO2 nanorods used for the detection of gases. The sensor layout is designed to assist in the alignment of the nanorods by selectively enhancing the electric field strength and allowing for the quick production of sensor arrays. The SnO2 nanorods are produced using a thermal evaporation-condensation approach. After growth, nanorods are separated from the resulting material using gravimetric separation. The rods vary in length from 3μm to greater than 10μm, with diameters ranging from 50 to 300nm. Dielectrophoresis is used to align multiple nanorods between electrodes. A second layer of metal is incorporated using standard microfabrication methods immediately after alignment to bury the ends of the rods making contact with the underlying electrodes within another layer of metal. Electrical contact was verified during testing by the response to H2 and C3H6 gases at a range of temperatures. Testing was performed on a stage with temperature control and probes were used for electrical contact. Gas flows into the testing chamber at a flow rate of 4000sccm. Sensor response of normalized current shift, |Igas-Iair|/Iair, was measured at a constant voltage bias. Sensors showed response to both H2 and C3H6. Detection of H2 was achieved at 100oC and response levels improved approximately 12000-fold at 600oC. Detection of C3H6 started at 100oC and improved approximately 10000-fold at 600oC. Detection of at least 200ppm for both gases was achieved at 600oC. Using this novel microfabrication approach, semiconducting SnO2 nanorods integrated into a microsensor platform have been demonstrated and sensing response showed dramatic increases at higher temperatures.

Copper−Metal Deposition on Self Assembled Monolayer for Making Top Contacts in Molecular Electronic Devices

Molecular electronics is an attractive option for low-cost devices because it involves highly uniform self-assembly of molecules with a variety of possible functional groups. However, the potential of molecular electronics can only be turned into practical applications if reliable contacts can be established without damaging the organic layer or contaminating its interfaces. Here, a method is described to prepare tightly packed carboxyl-terminated alkyl self-assembled monolayers (SAMs) that are covalently attached to silicon surfaces and to deposit thin metallic copper top contact electrodes without damage to this layer. This method is based on a two-step procedure for SAM preparation and the implementation of atomic layer deposition (ALD) using copper di-sec-butylacetamidinate [Cu(sBu-amd)](2). In situ and ex situ infrared spectroscopy (IRS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements are used to characterize the chemical modification of the Si/SAM interface, the perturbation of the SAM layer itself, and the metal homogeneity and interaction with the SAM headgroups. This work shows that (i) carboxyl-terminated alkyl monolayers can be prepared with the same high density and quality as those achieved for less versatile methyl-terminated alkyl monolayers, as evidenced by electrical properties that are not dominated by interface defects; (ii) Cu is deposited with ALD, forming a bidentate complexation between the Cu and the COOH groups during the first half cycle of the ALD reaction; and (iii) the Si/SAM interface remains chemically intact after metal deposition. The nondamaging thin Cu film deposited by ALD protects the SAM layer, making it possible to deposit a thicker metal top contact leading ultimately to a controlled preparation of molecular electronic devices.

Depression Outcome Among a Biracial Sample of Depressed Urban Elders

There are a paucity of long-term studies from the United States concerning predictors of outcome among depressed older community adults. This article examines predictors of depression in a biracial sample of older persons in Brooklyn, NY. The authors conducted a naturalistic study of 110 persons aged 55 years and older living in randomly selected block groups who had a Center for Epidemiologic Studies-Depression (CES-D) score of > or = 8 at baseline. Persons were reassessed on an average of 3 years later. Their mean age was 69 years, 52% were women, and 35% were whites, and 65% were blacks, among whom 71% were African Caribbeans. Using George's Social Antecedent Model of Depression, the authors examined the impact of 13 predictor variables on two outcome measures: presence of either subsyndromal or syndromal depression (CES-D score > or = 8) and presence of syndromal depression (CES-D score > or = 16). To control for design effects, the authors used SUDAAN for the data analysis. On follow-up, 82% and 88% of subsyndromally and syndromally depressed persons at baseline, respectively, were depressed (CES-D > or = 8). In logistic regression, baseline depressive symptoms, baseline anxiety symptoms, greater increase in anxiety symptoms during the follow-up period, and higher locus of control were predictors of any level of depression. These four variables along with greater paranoid ideation and/or psychoses and more reliable social contacts were significant predictors of syndromal depression on follow-up. There were no inter- or intraracial differences in outcome. Depressed community elders in Brooklyn have highly unfavorable outcomes. Preventive strategies that target at-risk persons-i.e., especially those with baseline subsyndromal depression, greater anxiety symptoms, and more paranoid ideation and/or psychoses-may reduce the development of severe or persistent depression.

Bending-induced conductance increase in individual semiconductor nanowires and nanobelts

Reliable ohmic contacts were established in order to study the strain sensitivity of nanowires and nanobelts. Significant conductance increases of up to 113% were observed on bending individual ZnO nanowires or CdS nanobelts. This bending strain-induced conductance enhancement was confirmed by a variety of bending measurements, such as using different manipulating tips (silicon, glass or tungsten) to bend the nanowires or nanobelts, and is explained by bending-induced effective tensile strain based on the principle of the piezoresistance effect.

Carbon nanotubes for nanoscale low temperature flip chip connections

In this work we demonstrate a new approach for ultra fine flip chip interconnections based on carbon nanotubes as a wiring material. In contrast to other works we show patterned growth of multi walled CNTs on substrates with pre-structured bond pads including a complete metallization system for electrical characterization. Furthermore, we succeeded achieving a reliable flip chip connection between CNT-covered contact pads and metal pads at temperatures lower than 200 °C. Our goal is a reversible electrical and mechanical chip assembly with CNT bumps. For bonding experiments and electrical characterization a test structure with a damascene metallization including a layer stack of TiN/Cu/TiN was prepared. For CNT growth a thin nickel catalyst layer was selectively deposited with sputtering and a lift-off technique on the contact pads. The CNTs were grown by thermal CVD with ethylene as carbon source. CNT growth parameters like catalyst thickness, gas composition, growth time and temperature were optimized to get dense CNT growth. The metal bumps of the counter chip consist of electroless deposited Ni. With the selected layout we can obtain daisy chain and four-point measurements for lossless determination of single contact resistance. We have obtained reliable electrical contacts with relatively small resistance reaching values as low as 2.2 Ω. As CNT-quality is strongly dependent on the growth temperature we observed a strong change in resistivity of the flip chip connection as the growth temperature was varied. Reliability tests showed long time stability under thermal stress proving a reliable electrical contact between the contact pads. There is an appropriate potential for further optimization of the CNT bump resistance and applying this technology for IC-devices.

Negative Temperature Coefficient Resistance (NTCR) Ceramic Thermistors: An Industrial Perspective

Monitoring and control of temperature is of paramount importance in every part of our daily life. Temperature sensors are ubiquitous not only in domestic and industrial activities but also in laboratory and medical procedures. An assortment of temperature sensors is commercially available for such purposes. They range from metallic thermocouples to resistive temperature detectors and semiconductive ceramics, showing a negative temperature coefficient of resistance (NTCR). NTCR ceramic sensors occupy a respected market position, because they afford the best sensitivity and accuracy at the lowest price. Despite the enormous commercial success of NTCR thermistors, this area of advanced functional ceramics has not been recently reviewed. Nearly 100 years elapsed between the first report of NTCR behavior and the fabrication of NTCR devices. The manufacture of the first NTCR ceramic thermistors was problematic, as often the devices suffered from poor stability and nonreproducibility. Before NTCR ceramics could be seriously considered for mass production of thermistors, it was necessary to devote a large amount of R&D effort to study the nature of their semiconductivity and understand the influence of impurities/dopants and heat treatments on their electrical characteristics, particularly in their time dependence resistivity (aging). Simultaneously, from a technological viewpoint it was important to develop methods enabling reliable and permanent electrical contacts, and design suitable housing for ceramics, in order to preserve their electrical properties under conditions of variable oxygen partial pressure and humidity. These topics are reviewed in this article from an industrial perspective. Examples of common applications of NTCR thermistors and future challenges are also outlined.