High gradient test at Nextef and high-power long-term operation of devices

The X-band high gradient studies at Nextef, the 100 MW X-Band test station in KEK operated since 2008, is reviewed. Recent high power test results of TD18#2, an 18-cell CLIC prototype structure with HOM damping slots, are given. The measured breakdown rate is ∼10⁻⁵/pulse/m under the operation at 100 MV/m gradient with 250 ns rf pulse width. The rate is higher by one- to two-order compared with that of the T18 structure, which is a disk loaded structure without the HOM slots. The recent performance of the station is also reviewed, especially that of PPM klystrons is discussed. An overview of our on-going programs such as the pulse compression system of Nextef as well as the future plans of X-band study is given.     

Demonstration of the high RF power production feasibility in the CLIC power extraction and transfer structure [PETS]

A fundamental element of the CLIC concept is two-beam acceleration, where RF power is extracted from a high current, low energy drive beam in order to accelerate the low current main beam to high energy [1]. The CLIC Power Extraction and Transfer Structure (PETS) is a passive microwave device in which bunches of the drive beam interact with the constant impedance of the periodically loaded waveguide and excite preferentially the synchronous mode. The RF power produced is collected downstream of the structure by means of the RF power extractor; it is delivered to the main linac using the waveguide network connecting the PETS to the main CLIC accelerating structures [2]. The PETS should produce 135 MW at 240 ns RF pulses at a very low breakdown rate: BDR <10⁻⁷/pulse/m.Over 2010, a thorough high RF power testing program was conducted in order to investigate the ultimate performance and the limiting factors for the PETS operation. The testing program is described and the results are presented.     

Manipulation by dipole probe

A dipole probe is fabricated to manipulate millimeter- to submillimeter-sized objects. A tungsten needle, an alumina tube and a stainless tube are arranged concentrically in order inside the probe. The tip of the stainless tube is ground to form a needle. They are embedded in an epoxy resin and the tip of the probe is shaped hemispherically. The probe has two electrodes, a tungsten needle and a stainless steel needle, inside it. The probe can attract objects by gradient force like a bipolar electrostatic chuck. The attraction force is measured as a function of the applied voltage, and they are compared with those calculated by a 3D FEM. Both the experimental values and the calculated values are proportional to the square of the applied voltage. The determined values are, however, three times greater than those by the calculated values. The difference is ascribed to the incomplete shape of the probe model and the difference of dielectric constants of materials. The probe can attract both conductive gold particles and dielectric foam styrene particles. The probe is placed above the particle and a voltage supplier is turned on. The particle jumps up and adheres at the tip of the probe. The adhesive position is not on the center axis of the probe but the opposite side to the stainless needle against the center of the probe. The distances from the center of the probe are at a range of 0.4–1.05 mm for 20 experiments. The FEM calculation shows that maximum attraction force is for the particle placed at the opposite side to the stainless needle. Release is possible only by turning the voltage supplier off. The particle moves to the bottom of the probe, and falls after 1–2 s. The delay is due to the attenuation period of electrons accumulated at the surface of the probe.     

Observation of nanostructure by scanning near-field optical microscope with small sphere probe

Step and terrace structure has been observed in an area of 1 μm×1 μm on the cleaved surface of KCl-KBr solid-solution single crystal by scanning near-field optical microscope (SNOM) with a small sphere probe of 500 nm diameter. Lateral spatial resolution of the SNOM system was estimated to be 20 nm from the observation of step width and the scanning-step interval. Vertical spatial resolution was estimated to be 5-2 nm from the observation of step height and noise level of photomultiplier tube (PMT). With applying a dielectric dipole radiation model to the probe surface, the reason why such a high spatial resolution was obtained in spite of the 500 nm sphere probe, was understood as the effect of the near-field term appeared in the radiation field equations.     

Growth of smooth 4H-SiC epilayers on 4° off-axis substrates with chloride-based CVD at very high growth rate

4H-SiC epilayers grown on 4° off-axis substrates at high rates usually suffer from step-bunching (very high surface roughness) or of extended triangular defects, both detrimental for device performance.In this study we developed a novel in situ pre-growth surface preparation based on hydrogen chloride (HCl) addition at a temperature higher than that used for the growth. This pre-growth etching procedure minimizes the density of triangular defects which usually occur at low temperatures and simultaneously enables growth at a temperature low enough to reduce step-bunching. Thanks to this surface preparation step, chloride-based chemical vapour deposition (CVD) could be used for rapid epitaxial growth of high quality layers. In this study, layers were grown at rates of 100 μm/h yielding defect free epitaxial layers with very smooth surface (rms value of 8.9 Å on 100 μm × 100 μm area).     

Fractal analysis of side channels for breakdown structures in XLPE cable insulation

The role of frequency in the range 20 through 300 Hz on the breakdown voltage and the breakdown path is studied in cross-linked polyethylene (XLPE) cable insulation using embedded needle. A maximum breakdown voltage of 25 kV is found at 240 Hz, and side channels are observed on the flank of the main channel of the electrical breakdown path. Fractal analysis of the side channel is carried out and it is induced that the frequency dependence of the fractal dimension D of the side channel are similar to that of the electrical trees before breakdown. It is suggested that the space charge can be injected from the needle tip. This leads to partial discharge causing progress of the electrical tree and the breakdown path. Space charge will also result in field-moderating cloud around the needle tip and turn to sidewall charges in the side channels. The frequency dependence of the breakdown voltage of the XLPE with the embedded needle can be clarified based on the fractal analysis of the side channel and the electrical tree of the XLPE insulation.     

n-Type CVD diamond: Epitaxy and doping

Diamond is a semiconductor with superlative properties in terms of operating temperature, breakdown voltage and thermal dissipation for high power electronic applications. The p-type doping is currently controlled but it is not the case for the n-type doping. In spite of the low solubility of substitutional donors bigger than carbon, n-type material can be achieved via impurity doping during the growth of diamond with phosphorus dopant (E_c = 0.6 eV). Theoretical studies also predict arsenic as a shallower dopant (E_c = 0.4 eV). This paper outlines the n-type doping issues with phosphorus and explores the arsenic doping.     

Production of ultrafine atmospheric pressure plasma jet with nano-capillary

The atmospheric pressure plasma jet (APPJ) has been developed to apply to the ultrafine process for future material processing. In this study, we tried to reduce the diameter of APPJs by using micro- or nano-capillary tubes. To measure the existence of the bullet-like charged particles from the capillary tip, the plasma flow generated from capillary tubes with a diameter of about 100 nm to 5 μm were analyzed by the electrostatic probe. The average propagation velocities of the plasmas flow were estimated from the time differences between the probe signal and spiky discharge current signal. The results show that the velocity of plasma jet delivered from 1 μm ? capillary gradually decreased from 11 to 2 km/s due to a collisional drag force with air with an increase of the distance from capillary tips. From the analysis of etching pattern of resist film by the APPJ with a 500 nm diameter capillary, it was confirmed that the depth and width of the treated trench on the resist film were roughly 30 - 50 nm and 500 - 700 nm, respectively.     

Fast phase switching within the bunch train of the PHIN photo-injector at CERN using fiber-optic modulators on the drive laser

The future Compact Linear Collider (CLIC) e⁻/e⁺ collider is based on the two-beam acceleration concept, whereby interleaving electron bunches of the drive beam through a delay loop and combiner rings as well as high peak RF power at 12 GHz are created locally to accelerate a second beam, the main beam. One of the main objectives of the currently operational CLIC Test Facility (CTF3) is to demonstrate beam combination from 1.5 GHz to 12 GHz, which requires satellite-free fast phase-switching of the drive beam with sub-ns speed. The PHIN photo-injector, with the photo-injector laser, provides flexibility in the time structure of the electron bunches produced, by direct manipulation of the laser pulses. A novel fiber modulator-based phase-switching technique allows clean and fast phase-switch at 1.5 GHz. This paper describes the switching system based on fiber-optic modulators, and the measurements carried out on both the laser and the electron beam to verify the scheme.     

Local modification of NaCl thin films on Cu(111) under different bias voltages

The local modification of NaCl thin films on Cu(111) under different bias voltages is investigated using a scanning tunneling microscope (STM) at room temperature. We find that the type of modification of NaCl thin films is dependent on sample bias voltage (V_s). Defects in a triple-layer-thick NaCl film are destabilized and repelled away from the region beneath an STM tip by applying V_s in the range of 0.3 V ≤ |V_s| ≤ 0.5 V. When V_s is larger than + 1.2 V or smaller than − 4.0 V, the removal of NaCl films takes place and a bare Cu surface appears. In this case, the removed NaCl is transferred to the STM tip and can be supplied back to the surface from the STM tip. The redeposition of NaCl enables not only the reformation of single-crystalline NaCl films on a bare Cu surface but also the formation of additional NaCl films on a clean NaCl film surface.     

Fabrication of high breakdown voltage silicon Schottky barrier diodes using various edge termination structures

In this work, the design and fabrication of Au/n-Si Schottky barrier diodes (SBDs) with various edge termination schemes, including a reduced-surface-field-type lateral super-junction, a polycrystalline silicon (poly-Si) floating ring, and a p⁺-poly-Si guard ring, are presented. Experimental results show that the reverse leakage current of the proposed SBDs was reduced and the breakdown voltage increased with an increase of the poly-Si width of the guard ring.It was found that the device and fabrication technology developed in the present study is applicable to the realization of SBDs with a high breakdown voltage (≥ 160 V), a low reverse current density (≤ 5.6 μA/cm²), a low forward voltage drop (≤ 5.6 V @ 1 A/cm²), and an adjustable Schottky barrier height of 0.764 to 0.784 eV.     

STM observation of Ag adatom interactions on the Si(1 1 1)-(7×7) surface

STM imaging of Ag atoms adsorbed on the Si(1 1 1)-(7×7) surface is studied. Appearance of a single Ag adatom on perfect surface is compared with images of adatoms interacting with surface defects and adsorbates. Importance of real-time observation of surface processes for image interpretation is demonstrated on imaging Ag adatoms at various situations. Influence of tunnelling conditions (voltage between a tip and surface) on imaging surface objects is studied and visibility of single Ag adatoms and clusters is discussed.     

Field emission current and vacuum breakdown by a pointed cathode

Vacuum breakdown of very small gap in the range of 30 nm to 2 μm, between a pointed cathode of thin tungsten wire and a plane anode of stainless steel, was experimentally investigated. The experimental setup, which consists of both electrodes and precise positioning mechanisms actuated by piezoelectric devices, was installed in scanning electron microscope (SEM). Breakdown voltage decreases with decrease of gap length and/or radius of curvature at the tip of cathode. Theoretical consideration shows that the evaporation from the anode surface heated by field emission current is a dominant factor of triggering vacuum breakdown.     

Low temperature needle like mullite grain formation in sol-gel precursors coated on SiC porous substrates

Dense mullite coating having thickness in the range of 3 to 5 μm was produced from sol-gel mullite precursor coated on SiC porous substrates at heat treatment temperatures as low as 1300 °C. Mullite formed in the coating layer was characterised by X-ray diffraction. The precursors have an average particle size of 170 nm and the mullite formed in the coating in situ has a grain size of 3-5 μm. Mullite grains formed on the SiC have needle like morphology. The mullite formation has been explained on the basis of reaction between the silica-alumina nano precursor and the needle like morphology has been similar to that formed from a liquid phase. The gas permeation analysis shows that there is considerable difference between gas pressure while using SiC substrate before and after coating and hence clearly indicated reduction in pore size. This particular approach is good since usual mullite formation is at high temperature and is difficult to attain small grain size. Further, in situ formed mullite, in this investigation covers the SiC surface protecting the SiC from oxidation at high temperature.     

Enhanced performance of the AA2050-T8 aluminium alloy following excimer laser surface melting and anodising processes

The potential of laser surface melting (LSM) as a pre-treatment prior to conventional anodising has been evaluated on an AA2050-T8 (Al-Cu-Li) aerospace alloy. A KrF excimer laser was utilized, of wavelength of 248 nm, with variation of the number of pulses received per unit area. After LSM, the specimens were anodised at a constant voltage of 12 V in 0.46 M sulphuric acid for 240 s. Material characterization, in terms of surface morphology, microstructure and phase transformation, was performed using scanning and transmission electron microscopies, interferometry and scanning Kelvin probe force microscopy (SKPFM). The corrosion behaviour was evaluated based on the standard ASTM G34-01 EXCO test, revealing the distinct improvement in performance of the combined laser and anodising treatments.     

A full on-chip, low noise, low power consumption reference generator in monolithic active pixel sensors

The monolithic active pixel sensor (MAPS) is a promising choice to track charged particles in high energy physics experiments, such as the solenoidal tracker at RHIC (STAR). In order to achieve a clean reference voltage and simplify the cable placement, a full on-chip reference generator is presented in this paper. By utilizing a buffer and a series RC network, the proposed circuit can achieve good stability, low power and low noise, without any external components. The output voltage is adjustable to compensate the influence of the fabrication process. The generator has been implemented and fabricated in a standard 0.35 μm CMOS process. Its silicon area is 327 μm×119 μm. The total power dissipation is 677 μW at a supply voltage of 3.3 V. The measured results show that only 5.84% of the total noise in MAPS is induced by the proposed reference generator. The comparison with the other optional circuit based on a current buffer is also presented.     

Using Mather-type plasma focus device for surface modification of AISI304 Steel

A 2.8 kJ plasma focus device with a nitrogen gas filling and a copper anode capsulated by aluminum was used to modify the surface of AISI304 steel substrate, in order to improve its properties. The treatment was carried out using a various number of nitrogen plasma focus shots at a pressure of 0.5 mbar and at two steel sample distances (20 and 40 mm) from the anode. The plasma diagnostics was made using the voltage and current curves recorded by a voltage divider, Rogowskii coil, accompanied with calculations using a five phase radiative Lee model (RADPF5.15a) to determine the temperature and plasma density.The surface hardness of AISI304 steel was increased by ∼175% after plasma treatment and the thickness of the treated layers was about 1-2 μm. Results show that the surface hardness is increased with increasing shot number and decreased with increasing distance from the anode. Changes in surface morphology and the elemental composition were investigated by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX).     

Organic field-effect transistors with thermal-cured polyacrylate gate dielectric films

Chemically and thermally stable, durable, thermal-cured polyacrylates having a hydrophobic nature, which were prepared by mixing polyacrylate having reactive sites and functional-anhydride, exhibit good insulation properties and high breakdown voltage (> 4.0 MV/cm) as a dielectric. Plastic-based organic thin-film transistors with the thermal-acryl dielectric layer showed typical current-voltage characteristics; the field-effect mobility was calculated to be 0.22 cm² V^− 1 s^− 1, while the threshold voltage was approximately − 8 V. It has been found that thin dielectric layers gave higher field-effect mobility.