Showing papers on "Contact resistance published in 2005"

Nanoscale memory elements based on solid-state electrolytes

Abstract: We report on the fabrication and characterization of nanoscale memory elements based on solid electrolytes. When combined with silver, chalcogenide glasses such as Se-rich Ge-Se are good solid electrolytes, exhibiting high Ag ion mobility and availability. By placing an anode that has oxidizable Ag and an inert cathode (e.g., Ni) in contact with a thin layer of such a material, a device is formed that has an intrinsically high resistance, but which can be switched to a low-resistance state at small voltage via reduction of the silver ions. An opposite bias will return the device to a high-resistance state, and this reversible switching effect is the basis of programmable metallization cell technology. In this paper, electron beam lithography was used to make sub-100-nm openings in polymethylmethacrylate layers used as the dielectric between the device electrodes. The solid electrolyte film was formed in these via-holes so that their small diameter defined the active switching area between the electrodes. The Ag-Ge-Se electrolyte was created by the photodiffusion, with or without thermal assistance, of an Ag layer into the Ge-Se base glass. Combined thermal and photodiffusion leads to a nanophase separated material with a dispersed Ag ion-rich material with an average crystallite size of 7.5 nm in a glassy insulating Ge-rich continuous phase. The nanoscale devices write at an applied bias as low as 0.2 V, erase by -0.5 V, and fall from over 10/sup 7/ /spl Omega/ to a low-resistance state (e.g., 10/sup 4/ /spl Omega/ for a 10-/spl mu/A programming current) in less than 100 ns. Cycling appears excellent with projected endurance well beyond 10/sup 11/ cycles.

Analysis of the parasitic S/D resistance in multiple-gate FETs

Abstract: The multiple-gate field-effect transistor (FET) is a promising device architecture for the 45-nm CMOS technology node. These nonplanar devices suffer from a high parasitic resistance due to the narrow width of their source/drain (S/D) regions. We analyze the parasitic S/D resistance behavior of the multiple-gate FETs using a novel, S/D geometry-based analytical model, which is validated using three-dimensional device simulations and experimental results. The model predicts limits to parasitic S/D resistance scaling, which reveal that the contact resistance between the S/D silicide and Si-fin dominates the parasitic S/D resistance behavior of multiple-gate FETs. It is shown that the selective epitaxial growth of Si on S/D regions alone may be insufficient to meet the semiconductor roadmap target for parasitic S/D resistance at the 45-nm CMOS technology node.

Four decades of research on thermal contact, gap, and joint resistance in microelectronics

Abstract: The Keynote Paper reviews and highlights over 40 years of research on solutions for steady-state and transient thermal constriction and spreading resistances, and thermomechanical models for contact, gap and joint resistances of joints formed by conforming rough surfaces, nonconforming smooth surfaces, and nonconforming rough surfaces. Microgap and macrogap thermal resistance and conductance models are reviewed, and important relations and correlation equations are presented. Contact microhardness, determined by Vickers indenters, are correlated and incorporated into the contact model for conforming rough surfaces. Microhardness parameters are correlated with Brinell hardness values. Elastoplastic contact models for joints formed by smooth sphere-smooth flat and conforming rough surfaces are presented. A simple thermomechanical model for microgaps occupied by oil, grease, grease filled with solid particles, and phase change materials such as paraffins is reviewed, and good agreement with recently published data is noted.

Contact-controlled sensing properties of flowerlike ZnO nanostructures

Abstract: Contact-controlled sensing is realized from flowerlike ZnO structures composed of rods. The rods are around 150 nm in diameter and up to a few micrometers in length. When they are exposed to air, a depletion region formed below the surface of the rods arising from the adsorption of oxygen. Surface depletion does not greatly reduce either the carrier density or the mobility in the rods but significantly modify the potential barrier of the contacts between the rods. Both the large diameter and the long length result in a low sensor resistance down to 104kΩ in air at 300 °C. The exponential increase of the tunneling rate with the thickness and height of the contact barrier leads to a high sensitivity up to 14.6 for 100 ppm ethanol. These results indicate that contact-controlled sensing can be used to fabricate high-performance sensors with both high sensitivity and low resistance.

High-performance flexible zinc tin oxide field-effect transistors

Abstract: Flexible transistors were fabricated by sputter deposition of zinc tin oxide (ZTO) onto plasma-enhanced chemical vapor deposition gate dielectrics formed on flexible polyimide substrates with a blanket aluminum gate electrode. The flexible transistors exhibited high on-currents of 1mA, on/off ratios of 106, subthreshold voltage slopes of 1.6V/decade, turn-on voltages of −17V, and mobilities of 14cm2V−1s−1. Capacitance measurements indicate that the threshold voltage and subthreshold slope are primarily influenced by residual doping in the ZTO rather than by defects at the semiconductor/dielectric interface, and are useful for assessing contact resistance.

Fundamental studies of Au contacts in MEMS RF switches

Abstract: Microelectromechanical systems (MEMS) radio frequency (RF) switches hold great promise in a myriad of commercial, aerospace, and military applications. However, there is little understanding of the factors determining the performance and reliability of these devices. Fundamental studies of hot-switched gold (Au) contacts were conducted using a micro/nanoadhesion apparatus as a switch simulator. Experiments were conducted in a well defined air environment under precisely controlled operating conditions. Fundamental properties were connected to performance with an emphasis on the effects of contact force and electric current on contact resistance (R), microadhesion, and reliability/durability. Electric current had the most profound effect on switch performance. Observations at low current (1–10 μA) include: (1) slightly higher R; (2) asperity creep; (3) high adhesion after rapid switching; (4) switch bouncing; and (5) reasonable durability. Conversely, observations at high current (1–10 mA) include: (1) slightly lower R; (2) melting; (3) no measurable adhesion; (4) less propensity for switch bouncing; (5) necking of contacts; and (6) poor reliability and durability due to switch shorting. Low current behavior was dominated by the propensity to form smooth surface contacts by hammering, which led to high van der Waals force. High current behavior was dominated by the formation of Au nanowires that bridge the contact during separation. Data suggest the presence of an adventitious film containing carbon and oxygen. Aging of the contacts in air was found to reduce adhesion.

Effect of nanoscale heating on electrical transport in RF MEMS switch contacts

Abstract: This paper explores contact heating in microelectromechanical systems (MEMS) switches with contact spot sizes less than 100 nm in diameter. Experiments are conducted to demonstrate that contact heating causes a drop in contact resistance. However, existing theory is shown to over-predict heating for MEMS switch contacts because it does not consider ballistic transport of electrons in the contact. Therefore, we extend the theory and develop a predictive model that shows excellent agreement with the experimental results. It is also observed that mechanical cycling causes an increase in contact resistance. We identify this effect as related to the build-up of an insulating film and demonstrate operational conditions to prevent an increase in contact resistance. The improved understanding of contact behavior gained through our modeling and experiments allows switch performance to be improved.

Measuring the specific contact resistance of contacts to semiconductor nanowires

Abstract: Ohmic contacts to semiconductor nanowires are essential components of many new nanoscale electronic devices. Equations for extracting specific contact resistance (or contact resistivity) from several different test structures have been developed by modeling the metal/semiconductor contact as a transmission line, leading to the development of equations analogous to those used for planar contacts. The advantages and disadvantages of various test structures are discussed. To fabricate test structures using a convenient four-point approach, silicon nanowires have been aligned using field-assisted assembly and contacts fabricated. Finally, specific contact resistances near 5 × 10−4 Ω cm2 have been measured for Ti/Au contacts to p-type Si nanowires with diameters of 78 and 104 nm.

Dielectrophoretic manipulation and electrical characterization of gold nanowires

Abstract: Gold nanowires have been synthesized by template electrodeposition with an alternating field in porous aluminium oxide membranes. The use of dielectrophoresis to manipulate nanowires between electrodes has been investigated. Assembly was optimized by modelling the dielectrophoretic force, on the basis of the interaction between an alternating applied field and the induced dipole moment of gold nanowires. Results confirm the expected assembly characteristics, but also demonstrate how an understanding of the assembly process, and electrical behaviour of the electrode, is necessary for successfully assembling nanowires bridging the electrode gap. Electrical characterization of nanowires assembled between electrodes has been conducted to determine the transport properties of these systems. Single nanowires displayed an Ohmic response with 35 Ω resistance values predominantly due to contact resistance between the electrode and nanowire. By optimizing the assembly and cleaning procedures the contribution of the contact resistance to the measured resistance has been reduced below those reported in the literature.

Electrical properties of ZnO nanowire field effect transistors characterized with scanning probes

Abstract: Single ZnO nanowires are configured as field effect transistors and their electrical properties are characterized using scanning probe microscopy (SPM). Scanning surface potential microscopy is used to map the electric potential distribution on the nanowire. Potential drop along the nanowire and at the contacts are resolved, and contact resistances are estimated. Furthermore, conductive SPM tip is used as a local gate to manipulate the electrical property. The local change of electron density induced by a negatively biased tip significantly affects the current transport through the nanowire.

Investigation of a Duplex Stainless Steel as Polymer Electrolyte Membrane Fuel Cell Bipolar Plate Material

Abstract: Duplex 2205 stainless steel was investigated in a simulated polymer electrolyte membrane fuel cell (PEMFC) environment; stable passive films formed quickly under these conditions. The interfacial contact resistance (ICR) values for 2205 steel with the air-formed film were identical to those for 349 and lower than those for AISI446 steel. The ICR values for the passive film formed in a simulated PEMFC anode environment were lower than those for the passive film formed in the cathode environment, but both were higher than those for air-formed film. X-ray photoelectron spectroscopy analysis revealed that the passive films were mainly Cr 2 O 3 while the air-formed film was composed of Fe-oxides and Cr 2 O 3 . An estimate of the thickness of the surface films was made.

The role of Al on Ohmic contact formation on n-type GaN and AlGaN∕GaN

Abstract: A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred to AlGaN∕GaN structures.

A high-performance short-channel bottom-contact OTFT and its application to AM-TN-LCD

Abstract: We have demonstrated an organic thin-film-transistor (OTFT)-driven active-matrix twisted-nematic liquid crystal display (AM-TN-LCD) on a glass substrate, with a resolution of 160/spl times/120 pixels, 79 ppi. Substrate temperature was kept below the plastic-compatible temperature of 180/spl deg/C throughout the fabrication process. In order to realize an OTFT-driven display with fine resolution, we employed short-channel bottom-contact (BC) pentacene OTFTs. It has been known that their drivability is limited by contact resistance at source/drain (S/D). We found that the S/D contact resistance was markedly reduced when the thickness of the nonohmic Ti adhesion layer for ohmic Au S/D electrodes was reduced less than /spl sim/3 nm. We elucidate that this 3 nm corresponds to the thickness of the accumulating layer in a pentacene channel. When we use a self-assembled monolayer of mercapto-silane-coupling agent as the adhesion layer, the contact resistance becomes negligibly small and BC OTFTs scalable below 10 /spl mu/m were obtained. In addition to this OTFT-cell technology, we developed a low-damage pentacene patterning technique for integration of OTFTs and introduced low-temperature panel assembly process to suppress thermal-stress degradation of pentacene OTFTs, which are the key technologies to achieve OTFT-driven AM-TN-LCD.

Study on ALD In2S3/Cu(In,Ga)Se2 interface formation

Abstract: The solar cell technology based on Cu(In,Ga)Se2 (CIGS) thin-films provides a promising route to cost competitive solar electricity. The standard device structure is ZnO:Al/ZnO/CdS/CIGS/Mo films on a glass substrate, where the first three layers are n-type semiconductors with wide bandgaps, forming a pn-junction with the p-type CIGS absorber layer; the Mo layer serves as a back contact. This thesis deals with analysis of the generation and recombination of electron-hole pairs throughout the device. These processes determine the maximum output power: generation limits the current; recombination limits the voltage.The generation is calculated with an optical model based on complex refractive indices determined for the individual layers. The main features of the optical response of the solar cell can be reproduced with a specular model neglecting scattering. A model including ideally Lambertian scattering at the front and back surface of the CIGS absorber layer is introduced to investigate the possibility to maintain a high current generation with thin absorber layers. The result highlights the relatively poor optical performance of the Mo back contact. TiN and ZrN are explored as alternatives, and improved optical performance is experimentally demonstrated for both materials.The recombination analysis emphasizes that, in general, more than one recombination path of comparable magnitude are operative in parallel. For cells with absorber bandgap increasing from 1.0 eV (CuInSe2) to 1.7 eV (CuGaSe2), a relative increase of interface recombination is found. When these cells are subject to accelerated ageing, degradation is smallest for intermediate bandgaps; an explanation involving different sensitivity to decreased absorber band bending and activation of grain boundaries is suggested. The optical gain with ZrN back contacts is counteracted by increased back contact recombination and contact resistance, but an intermediate layer of MoSe2 is shown to alleviate these problems, allowing for an overall improved efficiency.

Laterally moving bistable MEMS DC switch for biomedical applications

Abstract: In this paper, we have designed a bistable microelectromechanical switch for an implantable lead electrode multiplexer application. Fabrication is based on a single mask process. State changes require an 18 V pulse to the actuators consuming only 0.2 nJ energy. The switch does not consume any energy in either the ON or the OFF state. Total chip size including bond pads is approximately 1.5 mm /spl times/ 1.5 mm. The initial contact resistance is below 5 /spl Omega/ with a contact force in the order of 10 /spl mu/N. The contact resistance stays consistently below 30 /spl Omega/ for the first 40 000 cycles. Breakdown voltage between the two contact members in OFF state is 300 V. We plan to further investigate applicability of this switch in the biomedical field.

Mechanical force sensors using organic thin-film transistors

Abstract: The pressure dependence of pentacene (C22H14) transistors with solution-processed polyvinylphenol gate dielectric on glass substrates is investigated by applying uniaxial mechanical pressure with a needle. We found that organic thin-film transistors are sensitive to applied pressure inherently. The measurements reveal a reversible current dependence of the transfer characteristics where the drain current is switching between two states. Experimental and simulation results suggest that switch-on voltage and interface resistance are affected. The change takes seconds, hinting at trap states being responsible for the effect.

A practical extension of the 3ω method to multilayer structures

Abstract: An improved data reduction method is used to extend the popular 3ω method to general layered geometries. This approach utilizes an unapproximated analytical solution, cast in terms of thermal impedance, to simultaneously measure thermal conductivity, thermal capacity, conductivity anisotropy, and interlayer contact resistance in multilayer planar structures. This method places no restrictions on the number or thickness of individual material layers, and it allows experimental measurements to be taken over a much wider range of frequencies than was previously possible. The search algorithm associated with the model is straightforward, robust, and requires no specialized software to compose. Experimental results are presented for a two-layer borosilicate glass/zeolite structure as well as a Si∕SiO2 structure. In both instances, the algorithm was able to simultaneously extract thermal conductivity and thermal diffusivity values using a single series of 3ω measurements.

Noncontact thermal characterization of multiwall carbon nanotubes

Abstract: In this work, a photothermal experiment is designed and conducted to characterize the thermal transport in carbon nanotubes (CNTs) along the axial direction exclusively. We characterize the thermal contact resistance between CNTs and the substrate. The measured value demonstrates a sound thermal contact between them. Our experimental result reveals a low thermal conductivity of the CNTs along their axial direction. The unique structure of the CNTs characterized in our work indicates that the thermal transport in the axial direction is across the carbon atomic layers. This explains why the measured thermal conductivity is much lower than theoretical predictions, in which the thermal conductivity is along the atomic layer direction. In addition, our transmission electron microscope observation of the CNTs reveals poor structural qualities, which can strongly enhance phonon scattering and reduce the thermal conductivity.

Ohmic contacts to sic

Abstract: In this chapter, the most significant results obtained in the last decade in the field of ohmic contacts to SiC are reviewed First, the basic concepts related to the physics of ohmic contacts and to the contact resistance measurement techniques are briefly reported Then, some aspects concerning the formation of low resistance (10-5-10-6 Ωcm2) ohmic contacts on n-type and for p-type SiC are discussed, focusing on Ni-based and Al/Ti-based contacts Examples of innovative applications on practical devices are also reported, as the simultaneous formation of ohmic contacts on n- and p-type SiC for vertical power MOS devices, obtained adding Al to a standard Ni contact, and a single-metal technology for ohmic and rectifying contacts in MESFETs, using Ti or Ni without post-deposition annealing

Numerical simulations of contact resistance in organic thin-film transistors

Abstract: The origin of the source/drain contact resistance reported in studies of pentacene-based organic thin-film transistors (OTFTs) has been investigated using numerical device simulations. Quantitative agreement with published contact resistance values is obtained, using reasonable values for the physical parameters describing both the semiconductor material and the metal/organic interfaces. In particular, the difference in contact resistance measured in top and bottom contact OTFTs has been reproduced.

An experimental study determines the electrical contact resistance in resistance welding

Abstract: Electrical contact resistance is of critical importance in resistance welding. In this article, the contact resistance is experimentally investigated for welding mild steel, stainless steel, and aluminum to themselves. A parametric study was carried out on a Gleeble® machine, investigating the influence on the contact resistance of interface normal pressure, temperature, and base metal.

Highly efficient vertical laser-liftoff GaN-based light-emitting diodes formed by optimization of the cathode structure

Abstract: Vertical GaN-based light-emitting diodes (LEDs) were fabricated using a laser-liftoff process and the effect of the cathode processing conditions on the properties of the LEDs was investigated. Surface roughening by 10%Cl2∕90%BCl3 plasma etching improved the light emission intensity at an operating current of 20mA; however, the forward operating voltage was increased due to the thin and rough n‐GaN layer. The use of an indium tin oxide (ITO) contact on the roughened n-type GaN surface decreased the forward voltage significantly, by decreasing the spreading resistance of the n-type GaN contact without decreasing the emission intensity. Through the combination of the ITO contact and the surface roughness of the n‐GaN layer, a 100% increase in the extraction efficiency was obtained compared to that of a lateral GaN device, with maintaining a similar forward operating voltage.

A mechanical, electrical, thermal coupled-field simulation of a sphere-plane electrical contact

Abstract: The purpose of this paper is to present a numerical simulation of the behavior of a sphere-plane electrical contact when a high current flows through it. A sequential coupling allows to study the interactions between mechanical, electrical and thermal phenomena occurring under a high current flow (intensity between 20 and 60 kA). The structural deformations and the voltage and temperature distributions are computed with the help of the finite element method via the ANSYS software. The 2D axisymmetric geometry considers a smooth sphere pressed on a smooth plane. The model takes into account the temperature dependency of material properties such as Young's modulus, hardness, electrical and thermal conductivities, specific heat, and coefficient of thermal expansion. The influence of the current intensity, the contact force, and the duration of the current flow on the potential distribution has been studied. As they are coupled, all phenomena are affected by heat generation variations and every phenomenon has an effect on all others and so on. In some particular conditions, analytical calculations make it possible to validate the simulation. Comparisons with experimental results are realized with several contact forces.

Nitride semiconductor light emitting device

Abstract: A nitride semiconductor light emitting element having a laminate S made of a semiconductor crystal layer, wherein the laminate S includes an n-type layer 2, a light emitting layer 3 and a p-type layer 4. The p-type layer 4 has a p-type contact layer 42 to be in contact with the p-side electrode P 2. The p-type contact layer 42 comprises a first contact layer 42 a and a second contact layer 42 b. The first contact layer 42 a is in contact with the p-side electrode P 2 on one surface and in contact with the second contact layer 42 b on the other surface. The first contact layer 42 a is made of Al x1 In y1 Ga z1 N (0

Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts

Abstract: We have investigated Ag-indium tin oxide (ITO) scheme for obtaining high-quality p-type ohmic contacts for GaN-based light-emitting diodes (LEDs). The Ag(1 nm)-ITO(200 nm) contacts exhibit greatly improved electrical characteristics when annealed at temperatures in the range 400/spl deg/C-600/spl deg/C for 1 min in air, yielding specific contact resistances of /spl sim/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. In addition, the contacts give transmittance of about 96% at 460 nm, which is far better than that of the conventionally used oxidized Ni-Au contacts. It is shown that the luminous intensity of blue LEDs fabricated with the Ag-ITO contacts is about three times higher than that of LEDs with oxidized Ni-Au contacts. This result strongly indicates that the Ag-ITO scheme can serve as a highly promising p-type ohmic contact for the realization of high brightness near ultraviolet LEDs.