Low-damage direct patterning of silicon oxide mask by mechanical processing

To realize the nanofabrication of silicon surfaces using atomic force microscopy (AFM), we investigated the etching of mechanically processed oxide masks using potassium hydroxide (KOH) solution. The dependence of the KOH solution etching rate on the load and scanning density of the mechanical pre-processing was evaluated. Particular load ranges were found to increase the etching rate, and the silicon etching rate also increased with removal of the natural oxide layer by diamond tip sliding. In contrast, the local oxide pattern formed (due to mechanochemical reaction of the silicon) by tip sliding at higher load was found to have higher etching resistance than that of unprocessed areas. The profile changes caused by the etching of the mechanically pre-processed areas with the KOH solution were also investigated. First, protuberances were processed by diamond tip sliding at lower and higher stresses than that of the shearing strength. Mechanical processing at low load and scanning density to remove the natural oxide layer was then performed. The KOH solution selectively etched the low load and scanning density processed area first and then etched the unprocessed silicon area. In contrast, the protuberances pre-processed at higher load were hardly etched. The etching resistance of plastic deformed layers was decreased, and their etching rate was increased because of surface damage induced by the pre-processing. These results show that etching depth can be controlled by controlling the etching time through natural oxide layer removal and mechanochemical oxide layer formation. These oxide layer removal and formation processes can be exploited to realize low-damage mask patterns.     

Microscopic study of electrical properties of CrSi<Subscript>2</Subscript> nanocrystals in silicon

Semiconducting CrSi₂ nanocrystallites (NCs) were grown by reactive deposition epitaxy of Cr onto n -type silicon and covered with a 50-nm epitaxial silicon cap. Two types of samples were investigated: in one of them, the NCs were localized near the deposition depth, and in the other they migrated near the surface. The electrical characteristics were investigated in Schottky junctions by current-voltage and capacitance-voltage measurements. Atomic force microscopy (AFM), conductive AFM and scanning probe capacitance microscopy (SCM) were applied to reveal morphology and local electrical properties. The scanning probe methods yielded specific information, and tapping-mode AFM has shown up to 13-nm-high large-area protrusions not seen in the contact-mode AFM. The electrical interaction of the vibrating scanning tip results in virtual deformation of the surface. SCM has revealed NCs deep below the surface not seen by AFM. The electrically active probe yielded significantly better spatial resolution than AFM. The conductive AFM measurements have shown that the Cr-related point defects near the surface are responsible for the leakage of the macroscopic Schottky junctions, and also that NCs near the surface are sensitive to the mechanical and electrical stress induced by the scanning probe.     

Electric field-induced aligned multi-wall carbon nanotube networks in epoxy composites

CVD-grown multi-wall carbon nanotubes were dispersed as an electrically conductive filler in an epoxy system based on a bisphenol-A resin and an amine hardener. The application of both AC and DC electric fields during nanocomposite curing was used to induce the formation of aligned conductive nanotube networks between the electrodes. The network formation process and resulting network structure were evaluated by in situ optical microscopy and current density measurements as a function of curing time. Parameters such as field strength and nanotube weight fraction were varied. The carbon nanotube agglomeration mechanism was dominated by the electric field-induced forces acting on the nanotubes, which have a negative surface charge after processing in the epoxy. The network structure formed in AC fields was more uniform and more aligned compared to that in DC fields. The specific bulk composite conductivity of fully processed composite samples reflected the differences in the nanotube network structure. Perhaps surprisingly, the network efficiency was not enhanced by this processing method, although the approach does offer the possibility of achieving bulk conductive nanotube-polymer composites with anisotropic electrical properties and a degree of optical transparency.     

Conductive coating on structural ceramics for strain detection utilizing electrical measurements

Strain detection in Al₂O₃ ceramics and glass plates was investigated by coating them with an electrically conducting composite (epoxy resin and needle-like SnO₂(Sb)-coated TiO₂ filler) and by measuring surface resistance during and after loading. By adding more than 6 vol%-filler, the composite became electrically conductive. Surface electrical resistance increased with increasing strain during loading, and the degree of electrical resistance change versus strain was larger when the filler volume fraction was close to the percolation critical volume fraction. In addition, when the specimens were cyclically loaded, residual electrical resistance was observed even after removing load. The value of the residual electrical resistance was dependent on the maximum strain under the stress applied. These results suggest that estimation of maximum strain is possible by measuring resistance of the composite formed on structural ceramics. Based on the results of microfracture observation, the effect of applied stress on the electrical resistance change of electroconductive composites is discussed. ©     

Modification of cation-exchange membrane properties by electro-adsorption of polyethyleneimine

A cation-exchange membrane has been modified by fixation of polyethyleneimine on its surface. This fixation was carried out under an electric field effect, thus it is called electro-adsorption. The polycation formed in an acidic medium migrated toward the membrane and a charged layer was deposited on the surface, and the selectivity towards divalent ions decreased, yielding to the increase of the proton transfer. When the amount of adsorbed PEI increased, the electrical resistance ofthe membrane increased and the transport number of zinc decreased. However, suitable conditions like pH, current density and electro-adsorption time were controlled in the order to obtain membranes with better selectivity and low electrical resistance.     

Scanning tip measurement for identification of point defects

Self-assembled iron-silicide nanostructures were prepared by reactive deposition epitaxy of Fe onto silicon. Capacitance-voltage, current-voltage, and deep level transient spectroscopy (DLTS) were used to measure the electrical properties of Au/silicon Schottky junctions. Spreading resistance and scanning probe capacitance microscopy (SCM) were applied to measure local electrical properties. Using a preamplifier the sensitivity of DLTS was increased satisfactorily to measure transients of the scanning tip semiconductor junction. In the Fe-deposited area, Fe-related defects dominate the surface layer in about 0.5 μm depth. These defects deteriorated the Schottky junction characteristic. Outside the Fe-deposited area, Fe-related defect concentration was identified in a thin layer near the surface. The defect transients in this area were measured both in macroscopic Schottky junctions and by scanning tip DLTS and were detected by bias modulation frequency dependence in SCM.     

The effect of concentration of graphene nanoplatelets on mechanical and electrical properties of reduced graphene oxide papers

Macroscopic, freestanding graphene-based paper-like materials are of interest for use as mechanically strong, stiff, and flexible and electrically conductive materials. Chemically reduced graphene oxide paper shows promise for such applications. In this work, we studied the mechanical and electrical properties of a set of paper materials prepared by filtration of homogeneous colloidal suspensions of hydrazine-reduced graphene oxide with different concentrations. Young’s modulus, fracture strength, and fracture strain of each type of sample was determined by tensile tests. The paper sample prepared from the colloidal suspension with the lowest concentration of reduced graphene oxide platelets had the highest modulus and fracture strength and showed the smoothest surface morphology. The electrical conductivity measured by the four-probe measurement method increased as the concentration was increased.     

Effects of Current Confinement on the Spark Plasma Sintering of Silicon Carbide Ceramics

Despite numerous works have recently reported the densification of silicon carbide (SiC) ceramics using the Spark Plasma Sintering (SPS) technique, the effect of the localized electric current flow near the specimen over the liquid phase sintering process of SiC ceramics and on their microstructural features has not been completely addressed. In the present work, two different SPS setups affecting current flow are selected, one based on the ordinary die/punch setup configuration, and the other employing a BN electrically insulating coating on the inner wall of the die to force the electric current to locally flow through the inner graphite foil in contact with the ceramic compact. The effective electrical resistance and the energy consumed during the SPS runs for both setups, as well as the sintering behavior, microstructure, and mechanical properties of the SPSed materials are analyzed. The BN die coating considerably increases the effective resistance of the system, decreases the power consumption, and accelerates the SiC densification. Besides, ceramic specimens experience significantly higher real temperatures than the set values and, accordingly, coarser microstructures and tougher materials than those for the ordinary setting are produced. The thermoelectrical properties of SiC materials are proposed as fundamental in their SPS process, especially when electrical current is forced through the inner part of the SPS setting around the specimen.     

Conductive-probe atomic force microscopy characterization of silicon nanowire

The electrical conduction properties of lateral and vertical silicon nanowires (SiNWs) were investigated using a conductive-probe atomic force microscopy (AFM). Horizontal SiNWs, which were synthesized by the in-plane solid-liquid-solid technique, are randomly deployed into an undoped hydrogenated amorphous silicon layer. Local current mapping shows that the wires have internal microstructures. The local current-voltage measurements on these horizontal wires reveal a power law behavior indicating several transport regimes based on space-charge limited conduction which can be assisted by traps in the high-bias regime (> 1 V). Vertical phosphorus-doped SiNWs were grown by chemical vapor deposition using a gold catalyst-driving vapor-liquid-solid process on higly n-type silicon substrates. The effect of phosphorus doping on the local contact resistance between the AFM tip and the SiNW was put in evidence, and the SiNWs resistivity was estimated.     

Mechanical and electrical multifunctional poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate)—multiwall carbon nanotube nanocomposites

In this work, nanocomposites of poly(hydroxybutyrate‐co‐hydroxyvalerate) PHBV and multiwalled carbon nanotubes (MWNT) were prepared by melt blending. Mechanical, thermal, morphological, and electrical properties of the prepared PHBV/MWNT nanocomposites were investigated. Differential scanning calorimetry (DSC) and X‐ray diffraction (XRD) results showed MWNT effectively enhanced the crystallization and nucleation of PHBV. Dynamic thermo‐mechanical and static uniaxial mechanical tensile and compressive properties were increased by the addition of MWNT. MWNT observed in the nanocomposites using transmission electron microscopy (TEM) showed dimensions similar to separated nanotubes inferring a good dispersion. The presence of nanotubes in close vicinity with each other formed an interconnecting network that led to the formation of electrically conductive nanocomposites. The electrical resistance of the nanocomposites was reduced with the addition of MWNT. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers     

Boron-doped diamond scanning probe for thermo-mechanical nanolithography

The good thermal conductivity, wear resistance and chemical inertness of a probe material are the essential terms for the processing speed and durability in the probe-based thermo-mechanical lithography. Considering these criteria, diamond can be regarded as the ideal material for a probe. This paper describes the microfabrication, evaluation and application of a boron-doped diamond micro-probe with an integrated resistive heater element. The diamond heater with a pyramidal tip, which is formed at the end of two diamond beams, can be electrically heated by flowing a DC or AC current. The high thermal conductivity of the diamond base supporting the heater element allows a very quick thermal response of 0.45 μs. A hard-wearing sharp diamond tip formed by silicon-lost mold technique shows an excellent durability in contact operation with a sample. Demonstration of thermo-mechanical nanolithography with this heated probe exhibits line patterns with the feature size of 40 nm on a Polymethylmethacrylate (PMMA) film and transferred pits-pattern with the diameter of 230 nm and the pitch of 400 nm onto the silicon substrate.     

Mechanical strain modulation of domain wall currents across LiNbO3 nanosensors

Recently, studies on low-dimensional conducting domain walls (DWs) in insulating ferroelectrics have opened up new research areas that allow information to be mechanically written and electrically read on the nanoscale. Large strains in thin films can change the polarization gradient across the DW region and thus increasing the DW current significantly. This phenomenon can enable the development of high sensitivity mechanical vibration sensors. In this study, the effects of variable uniaxial strain on the structures of 180° conducting DWs in LiNbO₃ (LNO) single-crystal thin films bonded onto Si/SiO₂ substrates were investigated. After the creation of antiparallel domains within each LNO nanosensor integrated at the film surface, strain modulation of DW currents was observed through simple mechanical bending of the film. The DW current increases under application of tensile strain along the axis of polarization, but decreases under application of in-plane compression by a factor of approximately 25. Phase field simulations showed the dramatic change in polarization gradients around the DW regions under the increase in tensile strain, which reduced the band gap. Repetitive band-gap narrowing/broadening with change in local electric field intensity under vibrating mechanical forces can periodically modulate both the carrier density and the DW conduction in the sensors. This finding not only provides the new fundamental physics to enrich the ferroelectric theory, but also paves the way to the near-future development of bending actuators, piezolighters, and micro-/nano-manipulators, etc.     

In situ processing of electrically conducting graphene/SiC nanocomposites

The outstanding electronic and physico-chemical properties of graphene make it an ideal filler in the fabrication of conducting and robust ceramic composites. In this study, a novel single-step approach for processing electrically conducting and well dispersed graphene/SiC nanocomposites is shown. These materials were processed by growing epitaxial graphene with either α- or β-phase SiC ceramics during their densification via spark plasma sintering (SPS). About 4 vol.% of few-layer graphene domains were generated in situ during the SPS process, leading to a conducting graphene network that significantly enhanced the electrical performance of SiC. The in situ graphene SPS growth mechanism arose from the combined action of the electric current, high temperature and partial vacuum. This approach offers unprecedented opportunities for the fast manufacturing of graphene/SiC nanocomposites with superior electrical and mechanical properties, precluding the handling of potentially hazardous nanostructures. This method widens their possible applications, including micro-electromechanical systems, brakes, micro-turbines or micro-rotors.     

Highly stable carbon nanotube field emitters on small metal tips against electrical arcing

Carbon nanotube (CNT) field emitters that exhibit extremely high stability against high-voltage arcing have been demonstrated. The CNT emitters were fabricated on a sharp copper tip substrate that produces a high electric field. A metal mixture composed of silver, copper, and indium micro- and nanoparticles was used as a binder to attach CNTs to the substrate. Due to the strong adhesion of the metal mixture, CNTs were not detached from the substrate even after many intense arcing events. Through electrical conditioning of the as-prepared CNT emitters, vertically standing CNTs with almost the same heights were formed on the substrate surface and most of loosely bound impurities were removed from the substrate. Consequently, no arcing was observed during the normal operation of the CNT emitters and the emission current remained constant even after intentionally inducing arcing at current densities up to 70 mA/cm².     

High erosion resistant Y2O3–carbon electroconductive composite under the fluorocarbon plasma

A new plasma-resistant composite with electrical conductance has been developed for use in the plasma processing equipments of the semiconductor industry. Currently, electrical conductive silicon is widely used as parts for the wafer processing equipments, but its erosion rate is too fast, causing short life time of the parts. In this study, yttrium oxide, which has a high erosion resistance under fluorocarbon plasmas, was used as a matrix material and carbon was added for a conducting phase. The threshold fraction of carbon for the conductance depended on the carbon sources as well as the sintering temperature. The current flow through the carbon phases inside the composite was demonstrated through a scanning probe microscopy technique. The developed composite has an electrical conductivity around 0.01 S/cm with a threshold carbon of 0.6 wt%, at a density of more than 99%; its plasma resistance was about 15 times greater than that of silicon. The developed composite can be used as a substitute for the electrically conductive silicon parts in wafer processing equipments.     

Improving the electrical conductivity of a carbon nanotube/polypropylene composite by vibration during injection-moulding

An isotactic polypropylene/multi-wall carbon nanotube (iPP/MWCNT) composite was prepared by a vibration injection moulding technique. The effect of the vibration field on the electrical conductivity property of samples was investigated. The results show that the electrical conductivities of the samples prepared by vibration injection moulding was far higher than those of samples prepared by conventional injection moulding when the CNT concentration are above 2 wt.% and below 6 wt.%. Besides the electrical conductivity of vibration injection moulded samples are a little higher than those of the compression moulded samples. The higher conductivity was resulted from the MWCNT movement induced by the periodical shear during vibration injection moulding. The agglomerates or individual MWCNT were disentangled, stretched and oriented along the flow direction, resulting in better conducting paths thus greatly increased the electrical conductivity. The electrical conductivity increased with increasing vibration frequency. The difference in the voltage–current relationships among the samples prepared at different vibration frequencies suggests that the mechanism of electrical conductivity of iPP/MWCNT composite changed from a tunnel to an ohmic effect. Compared with conventional injection moulded samples, there was no loss of mechanical properties.     

Diamond tips for automated electrical probing inside a scanning electron microscopy system

Pyramidal tips made from boron doped diamond have become the ultimate choice for electrically measuring semiconductor device structures in electrical atomic force microscopy (AFM). An advanced measurement setup with diamond probing units directly integrated inside a scanning electron microscopy (SEM) system is highly wanted as this allows for accurate tip positioning compared to the optical microscope of a standard AFM and enables also multiple tip measurements. Therefore, we have developed highly conductive in-plane diamond tips with a triangular shape and attached them to Ni cantilevers. We have established a LabVIEW-based setup enabling automated electrical measurements inside a SEM system using stick-and-slip motion nanomanipulators and a parameter analyzer system. To our best knowledge, this paper presents first 1- and 2-tip electrical measurements of microfabricated diamond probes inside a SEM system. Measurements of Si staircase and Ge structures are shown and compared to scanning spreading resistance (SSRM) results. Our work demonstrates that doped diamond tips clearly outperform common tungsten probe needles enabling nanoprobing experiments which were impossible so far. Based on our results, we predict that doped diamond is going to be the standard tip material not only for electrical AFM but also for nanoprobing of semiconductor materials.     

Selective formation of tungsten nanowires

We report on a process for fabricating self-aligned tungsten (W) nanowires with polycrystalline silicon core. Tungsten nanowires as thin as 10 nm were formed by utilizing polysilicon sidewall transfer technology followed by selective deposition of tungsten by chemical vapor deposition (CVD) using WF6 as the precursor. With selective CVD, the process is self-limiting whereby the tungsten formation is confined to the polysilicon regions; hence, the nanowires are formed without the need for lithography or for additional processing. The fabricated tungsten nanowires were observed to be perfectly aligned, showing 100% selectivity to polysilicon and can be made to be electrically isolated from one another. The electrical conductivity of the nanowires was characterized to determine the effect of its physical dimensions. The conductivity for the tungsten nanowires were found to be 40% higher when compared to doped polysilicon nanowires of similar dimensions.     

Mechanical properties and electrical conductivity in a carbon nanotube reinforced silicon nitride composite

The influence of carbon nanotubes (CNTs) addition on basic mechanical, thermal and electrical properties of the multiwall carbon nanotube (MWCNT) reinforced silicon nitride composites has been investigated. Silicon nitride based composites with different amounts (1 or 3 wt%) of carbon nanotubes have been prepared by hot isostatic pressing. The fracture toughness was measured by indentation fracture and indentation strength methods and the thermal shock resistance by indentation method. The hardness values decreased from 16.2 to 10.1 GPa and the fracture toughness slightly decreased by CNTs addition from 6.3 to 5.9 MPa m^1/2. The addition of 1 wt% CNTs enhanced the thermal shock resistance of the composite, however by the increased CNTs addition to 3 wt% the thermal shock resistance decreased. The electrical conductivity was significantly improved by CNTs addition (2 S/m in 3% Si₃N₄/CNT nanocomposite).     

碳纤维增强天然橡胶复合材料性能研究

本文主要通过碳纤维添加量的不同来研究碳纤维用量对NR/CF(橡胶/碳纤维)复合材料的力学性能、导电性、导热性、加工性能和动态力学性能的影响。研究结果表明,NR/CF复合材料在添加3phr碳纤维时,力学性能最好。在添加15phr时,橡胶试样体积电阻率比未添加碳纤维的降低了3个数量级。导热系数比未添加碳纤维的试样最高提高21.8%。随着碳纤维添加量增多,试样抗湿滑性能和滚动阻力都有所上升。