Silicon Tungsten Research Articles

Extreme nanomechanics: vacuum nanoindentation and nanotribology to 950 °C

Elevated temperature mechanical and tribological properties can be more relevant for practical wear situations than corresponding measurements at room temperature. However, high temperature nanomechanics and nanotribology is highly challenging experimentally. To overcome these challenges the NanoTest*** has been developed with active heating of the indenter and sample with resistive heaters, horizontal loading, patented thermal control method and stage design. By separately actively heating*** and controlling the temperatures of indenter and sample their temperatures can be precisely matched so that there is no heat flow and minimal/no thermal drift during the high temperature indentation,*** and measurements can be performed as reliably as at room temperature. Above 500 °C it is beneficial to use a cubic Boron Nitride indenter with gas purging to limit oxidation of samples. To achieve higher temperatures without indenter or sample oxidation an ultra-low drift high temperature vacuum nanomechanics/tribology system capable of testing to*** much higher temperatures has been recently developed (NanoTest Xtreme). The influence of time-dependent deformation on elevated temperature nanomechanical behaviour is discussed, using published results in Argon on glass-ceramic solid oxide fuel cell seal materials and previously unpublished nanoindentation measurements on single crystal silicon and polycrystalline tungsten using the NanoTest Xtreme in vacuum at temperatures up to 950 °C. Studies of the elevated temperature nano-/micro-tribological*** behaviour of wear-resistant*** nitride-based and MAX-phase coatings are also briefly reviewed.

Characterization of Doped Titanium Oxide Varistor Powder Processed with Latex Binder

In the processing of semiconducting electro ceramic materials the use of appropriate binder can lead to improve characteristics of powder which will reduce density gradient in the green body during compaction. The binder system is anticipated to have an influence on the grain growth and microstructure which in turn affect the electrical performance. To investigate this phenomenon, TiO2 powder doped with tungsten oxide (WO3), silicon oxide (SiO2) and bismuth oxide (Bi2O3) was processed with and without various percent of latex binder. The prepared samples were characterized by evaluating the physical properties like green density, fired density, axial and radial shrinkage, micro hardness and compressive strength. It was observed that TiO2 processed with latex binder exhibited higher green density and less axial and radial shrinkage compared to powder processed without binder. However, the binder level did not have any significant influence on the other characteristics of varistor discs. SEM analysis indicated that the average grain size was also influenced by the percent of binder used and but the higher average grain size was obtained for discs without any binder.

Technological prototype of a silicon-tungsten imaging electromagnetic calorimeter

Highly granular calorimeters and particle flow algorithms are essential tools for achieving an excellent jet energy resolution. This is a key asset for precision physics at future high energy particle colliders aiming at precise physics measurements. The PFA concept is best supported by a silicon tungsten electromagnetic calorimeter. The R&D as carried out by the CALICE collaboration has shown the feasibility of a fine-grained pixelization while assuring high operation stability. Our concept has been selected to compete for the upgrade of the CMS ECAL. The latest version of our detector concept, featuring 1024 channels on 4 dm2 board will be presented including technological choices for an application on an ILC. First thoughts for the CMS ECAL upgrade will be discussed.

Ultra-high purity tungsten and its applications

High purity (>99.999wt.%, 5N) or ultra-high purity (>99.9999wt.%, 6N) tungsten (W) electrodes are vital to the performance and lifetime of high intensity discharge (HID) lamps. On the other hand, alloys based on high purity (>99.99wt.% or 99.995wt.%, 4N or 4N5) W are used as sputtering targets to deposit the films in semiconductor devices. In this study, a highly efficient and cost-saving method for mass producing 6N W powder has been developed. The 5N and 6N W rods and wires for the electrodes of HID lamps have been manufactured by using this powder. In addition, vacuum hot pressing has been employed to prepare 4N5 tungsten–titanium (W–Ti) and 4N tungsten–silicon (W–Si) sputtering targets successfully. The impurities and microstructures of the developed high purity W products have been investigated and are suitable for lighting, semiconductor and photovoltaic applications.

Testing hadronic interaction models using a highly granular silicon–tungsten calorimeter

A detailed study of hadronic interactions is presented using data recorded with the highly granular CALICE silicon–tungsten electromagnetic calorimeter. Approximately 350,000 selected π− events at energies between 2 and 10GeV have been studied. The predictions of several physics models available within the GEANT4 simulation tool kit are compared to this data. A reasonable overall description of the data is observed; the Monte Carlo predictions are within 20% of the data, and for many observables much closer. The largest quantitative discrepancies are found in the longitudinal and transverse distributions of reconstructed energy.

Highly Conformal Amorphous W–Si–N Thin Films by Plasma-Enhanced Atomic Layer Deposition as a Diffusion Barrier for Cu Metallization

Ternary and amorphous tungsten silicon nitride (W–Si–N) thin films were grown by atomic layer deposition (ALD) using a sequential supply of a new fluorine-free, silylamide-based W metallorganic precursor, bis(tert-butylimido)bis(bis(trimethylsilylamido))tungsten(VI) [W(NtBu)2{N(SiMe3)2}2], and H2 plasma at a substrate temperature of 300 °C. Here, W(NtBu)2{N(SiMe3)2}2 was prepared through a metathesis reaction of W(NtBu)2Cl2(py)2 (py = pyridine) with 2 equiv of LiN(SiMe3)2 [Li(btsa)]. The newly proposed ALD system exhibited typical ALD characteristics, such as self-limited film growth and linear dependency of the film growth on the number of ALD cycles, and showed a high growth rate of 0.072 nm/cycle on a thermally grown SiO2 substrate with a nearly zero incubation cycle. Such ideal ALD growth characteristics enabled excellent step coverage of ALD-grown W–Si–N film, ∼100%, onto nanotrenches with a width of 25 nm and an aspect ratio ∼4.5. Rutherford backscattering spectrometry and X-ray photoelectron spectroscopy analysis confirmed that the incorporated Si and W were mostly bonded to N, as in Si–N and W–N chemical bonds. The film kept its amorphous nature until annealing at 800 °C, and crystallization happened at local areas after annealing at a very high temperature of 900 °C. An ultrathin (only ∼4 nm thick) ALD-grown W–Si–N film effectively prevented diffusion of Cu into Si after annealing at a temperature up to 600 °C.

Beam test performance of the SKIROC2 ASIC

Beam tests of the first layers of CALICE silicon tungsten ECAL technological prototype were performed in April and July 2012 using 1–6GeV electron beam at DESY. This paper presents an analysis of the SKIROC2 readout ASIC performance under test beam conditions.

Numerical study and topology optimization of 1D periodic bimaterial phononic crystal plates for bandgaps of low order Lamb waves

The optimum topology of bimaterial phononic crystal (PhCr) plates with one-dimensional (1D) periodicity to attain maximum relative bandgap width of low order Lamb waves is computationally investigated. The evolution of optimized topology with respect to filling fraction of constituents, alternatively stiff scattering inclusion, is explored. The underlying idea is to develop PhCr plate structures with high specific bandgap efficiency at particular filling fraction, or further with multiscale functionality through gradient of optimized PhCr unitcell all over the lattice array. Multiobjective genetic algorithm (GA) is employed in this research in conjunction with finite element method (FEM) for topology optimization of silicon–tungsten PhCr plate unitcells. A specialized FEM model is developed and verified for dispersion analysis of plate waves and calculation of modal response. Modal band structure of regular PhCr plate unitcells with centric scattering layer is studied as a function of aspect ratio and filling fraction. Topology optimization is then carried out for a few aspect ratios, with and without prescribed symmetry, over various filling fractions. The efficiency of obtained solutions is verified as compared to corresponding regular centric PhCr plate unitcells. Moreover, being inspired by the obtained optimum topologies, definite and easy to produce topologies are proposed with enhanced bandgap efficiency as compared to centric unitcells. Finally a few cases are introduced to evaluate the frequency response of finite PhCr plate structures produced by achieved topologies and also to confirm the reliability of calculated modal band structures. Cases made by consecutive unitcells of different filling fraction are examined in order to attest the bandgap efficiency and multiscale functionality of such graded PhCr plate structures.

Controllable film densification and interface flatness for high-performance amorphous indium oxide based thin film transistors

To avoid the problem of air sensitive and wet-etched Zn and/or Ga contained amorphous oxide transistors, we propose an alternative amorphous semiconductor of indium silicon tungsten oxide as the channel material for thin film transistors. In this study, we employ the material to reveal the relation between the active thin film and the transistor performance with aid of x-ray reflectivity study. By adjusting the pre-annealing temperature, we find that the film densification and interface flatness between the film and gate insulator are crucial for achieving controllable high-performance transistors. The material and findings in the study are believed helpful for realizing controllable high-performance stable transistors.

Ignition by Mechanical Sparks: Ignition of Hydrogen/Air Mixtures by Submillimeter-Sized Hot Particles

To broaden the knowledge concerning ignition processes, small, hot, metal/ceramic particles were heated in a quiescent mixture of hydrogen/synthetic air mimicking the behavior of mechanical sparks, for example, from grinding processes. Materials under investigation were silicon nitride, tungsten carbide, and steel types 1.3505, 1.4034, and 1.3541. Their diameters ranged between 300 μm and 800 μm. We measured the particle surface temperature required to achieve ignition of the gas phase. Influences of mixture stoichiometry, particle material, and particle size were investigated.Concerning particle size, it was found that smaller particles require higher temperatures to ignite the gas atmosphere. Also, the particle material had a profound effect on the temperature required to achieve ignition. Mixture stoichiometry was varied between 5%v and 60%v hydrogen, revealing minor influence on the particle temperature at ignition, which was attributed to the high thermal conductivity of hydrogen. Numerical simulations of the ignition process yield surface temperatures of similar magnitude. A minimum gas phase ignition energy density of 0.25 MJ/m3, independent of particle size and gas phase composition, has been derived based on the simulated temperature profiles and by defining an “ignition relevant” volume around the particles.

Test and characterization of a prototype silicon–tungsten electromagnetic calorimeter

New generation high-energy physics experiments demand high precision tracking and accurate measurements of a large number of particles produced in the collisions of elementary particles and heavy-ions. Silicon–tungsten (Si–W) calorimeters provide the most viable technological option to meet the requirements of particle detection in high multiplicity environments. We report a novel Si–W calorimeter design, which is optimized for γ/π0 discrimination up to high momenta. In order to test the feasibility of the calorimeter, a prototype mini-tower was constructed using silicon pad detector arrays and tungsten layers. The performance of the mini-tower was tested using pion and electron beams at the CERN Proton Synchrotron (PS). The experimental results are compared with the results from a detailed GEANT-4 simulation. A linear relationship between the observed energy deposition and simulated response of the mini-tower has been obtained, in line with our expectations.

Analysis of low-dose radiation shield effectiveness of multi-gate polymeric sheets

Computed tomography (CT) uses a high dose of radiation to create images of the body. As patients are exposed to radiation during a CT scan, the use of shielding materials becomes essential in CT scanning. This study was focused on the radiation shielding materials used for patients during a CT scan. In this study, sheets were manufactured to shield the eyes and the thyroid, the most sensitive parts of the body, against radiation exposure during a CT scan. These sheets are manufactured using silicone polymers, barium sulfate (BaSO4) and tungsten, with the aim of making these sheets equally or more effective in radiation shielding and more cost-effective than lead sheets. The use of barium sulfate drew more attention than tungsten due to its higher cost-effectiveness. The barium sulfate sheets were coated to form a multigate structure by applying the maximum charge rate during the agitator and subsequent mixing processes and creating multilayered structures on the surface. To measure radiation shielding effectiveness, the radiation dose was measured around both eyes and the thyroid gland using sheets in three different thicknesses (1, 2 and 3 mm). Among the 1 and 2 mm sheets, the Pb sheets exhibited greater effectiveness in radiation shielding around both eyes, but the W sheets were more effective in radiation shielding around the thyroid gland. In the 3 mm sheets, the Pb sheet also attenuated a higher amount of radiation around both eyes while the W sheet was more effective around the thyroid gland. In conclusion, the sheets made from barium sulfate and tungsten proved highly effective in shielding against low-dose radiation in CT scans without causing ill-health effects, unlike lead.

Optimisation of Process Parameters for Lower Leakage Current in 22 nm n-type MOSFET Device using Taguchi Method

In this article, Taguchi orthogonal array method was used to optimize the process parameters during the design of a 22 nm n-type Metal Oxide Semiconductor Field Effect Transistor (MOSFET) in order to decrease the leakage current (ILEAK) of the device. Titanium dioxide (TiO2) was used as the dielectric layer to replace the traditional silicon dioxide SiO2 and tungsten silicide (WSix) was used as a metal gate to replace polysilicon. The device’s fabrication and electrical characterization were executed using ATHENA and ATLAS modules from Silvaco International. Taguchi’s Power of Three Series L9 orthogonal array was used to optimize the device process parameters and to finally predict the best process parameter combination to obtain the minimum leakage current (ILEAK) using Smaller-the-Better (STB) signal-to-noise ratio (SNR). The optimization resulted in the attainment of the lowest ILEAK mean value of 0.25759 nA/µm which is in accordance to the predicted value given in the International Technology Roadmap for Semiconductors (ITRS) 2011.

Origin of unrealistic blunting during atomistic fracture simulations based on MEAM potentials

Atomistic simulations based on interatomic potentials have frequently failed to correctly reproduce the brittle fracture of materials, showing an unrealistic blunting. We analyse the origin of the unrealistic blunting during atomistic simulations by modified embedded-atom method (MEAM) potentials for experimentally well-known brittle materials such as bcc tungsten and diamond silicon. The radial cut-off which has been thought to give no influence on MEAM calculations is found to have a decisive effect on the crack propagation behaviour. Extending both cut-off distance and truncation range can prevent the unrealistic blunting, reproducing many well-known fracture behaviour which have been difficult to reproduce. The result provides a guideline for future atomistic simulations that focus on various fracture-related phenomena including the failure of metallic-covalent bonding material systems using MEAM potentials.

Effect of finishing and polishing procedures on biofilm adhesion to composite surfaces: An ex vivo study

Introduction: Surface roughness allows plaque accumulation resulting in gingival inflammation, superficial staining and secondary caries. Proper surface finishing and polishing are critical clinical procedures which enhance esthetics and longevity of restorations. This study evaluated adhesion of Streptococcus mutans biofilm on the surface of composite resin discs (nanofilled, Filtek Z350, 3M ESPE, Salt Lake City, UT, USA) after finishing and polishing by different techniques. Methodology: Sixty samples of nanofilled composite resin were prepared in a circular shaped disc- 6 mm × 2 mm and divided randomly in three groups (n = 20) for surface treatments. Control group: composite resin surface in contact with Mylar matrix strips with no finishing or polishing performed, Sof-Lex aluminum oxide disc technique and 30-blade tungsten carbide burs and silicon carbide brushes, Astrobrush. The samples were subjected to biofilm adhesion by inoculation in suitable media. The response variable was the mean CFU/mL present in the Streptococcus mutans biofilms formed on the composite resin surface. Data was statistically analyzed by three-way analysis of variance (ANOVA). Results: The Mean adhesion found in Mylar matrix strip group at 10 dilution was 74.7 ± 3.5, in Sof-Lex group was 147.3 ± 7.0 and in Astrobrush group was 149.4 ± 8.1. This difference in the mean values between the groups was found to be statistically significant (p < 0.01). Conclusion: Mylar matrix strips promoted the least bacterial adhesion, polishing with Sof-Lex aluminium oxide discs provided a smoother surface than Astrobrush and hence less bacterial adhesion than Astrobrush system.

Measurement of hydrogen and helium isotopes flux in galactic cosmic rays with the PAMELA experiment

PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature, with particular focus on the antimatter component. The detector consists of a permanent magnet spectrometer core to provide rigidity and charge sign information, a Time-of-Flight system for velocity and charge information, a Silicon–Tungsten calorimeter and a Neutron detector for lepton/hadron identification. The velocity and rigidity information allow the identification of different isotopes for Z=1 and Z=2 particles in the energy range 100MeV/n to 1GeV/n. In this work we will present the PAMELA results on the H and He isotope fluxes based on the data collected during the 23rd solar minimum from 2006 to 2007. Such fluxes carry relevant information helpful in constraining parameters in galactic cosmic rays propagation models complementary to those obtained from other secondary to primary measurements such as the boron-to-carbon ratio.

Endurance Improvement Technology With Nitrogen Implanted in the Interface of ${\rm WSiO}_{\bf x}$ Resistance Switching Device

Incorporation of nitrogen as an oxygen-confining layer in the resistance switching reaction region is investigated to improve the reliability of resistance random access memory (RRAM). The switching mechanism can be attributed to the formation and rupture of conduction filaments. A compatible WSiON (around 5 nm) layer is introduced at the interface of tungsten silicon oxide (WSiOx) and TiN electrode to prevent the randomly diffusing oxygen ions surpassing the storage region of the WSiON layer. The double-layer WSiOx/WSiON memory structure would enhance the endurance over 100 times so as to better confirm the WSiOx RRAM application of nonvolatile memory.

Construction and testing of a large scale prototype of a silicon tungsten electromagnetic calorimeter for a future lepton collider

The CALICE collaboration is preparing large scale prototypes of highly granular calorimeters for detectors to be operated at a future linear electron positron collider. After several beam campaigns at DESY, CERN and FNAL, the CALICE collaboration has demonstrated the principle of highly granular electromagnetic calorimeters with a first prototype called physics prototype. The next prototype, called technological prototype, addresses the engineering challenges which come along with the realisation of highly granular calorimeters. This prototype will comprise 30 layers where each layer is composed of four 9×9cm2 silicon wafers. The front end electronics is integrated into the detector layers. The size of each pixel is 5×5mm2. This prototype enters its construction phase.We present results of the first layers of the technological prototype obtained during beam test campaigns in spring and summer 2012. According to these results the signal over noise ratio of the detector exceeds the R&D goal of 10:1.

XRD Studies on Residual Stress of the Diamond Films Grown Using Hot Filament Chemical Vapour Deposition Technique on Silicon Nitride and Tungsten Carbide Substrates

This study analyses residual stress measurement using X-Ray diffraction method on ultrafine-polycrystalline diamonds and polycrystalline diamonds films grown using Hot Filament Chemical Vapour Deposition technique (HFCVD) on silicon nitride(Si3N4) and tungsten carbide (WC) substrates in the same chamber at the same time with varied pretreatments prior to HFCVD diamond deposition. Measurements were taken perpendicular to the surface and the measured residual stress states of the diamond films are in compression. Thus, assuming isotropic properties of the film, the diamond films grown have tension residual stress parallel to the surface of the substrate. Residual stress is estimated to have the lowest stress for substrate that has undergone 5g/liter silicon carbide seeding process. Effects of residual stress to adhesion are discussed for both substrates.

Phosphotungstic Acid, Silicotungstic Acid and Silicon Tungsten-Cobalt Heteropoly Acid with Bovine Serum Albumin Interaction

The interactions of bovine serum albumin (BSA) with phosphotungstic heteropoly acid (PW), silicon tungsten heteropoly acid (SiW) and silicon tungsten-cobalt acid (SiWCo) were studied by fluorescence spectroscopy and UV absorption spectroscopy at Tris buffer solution (pH = 7.40). It was found that the fluorescence quenching of PW, SiW and SiWCo with BSA was static and the binding constant, binding site and the thermodynamic parameters were calculated at 298 and 310K. In addition, the conformations of BSA impacted by PW, SiW and SiWCo were researched using synchronous fluorescence. The results showed that PW, SiW and SiWCo all could interact with BSA but they had not changed the conformation of BSA.