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Beam size is one of the major characteristics of any electron beam system. Accuracy and resolution of SEM and an EBL system directly depend on beam diameter; it should be monitored and tuned frequently. Currently used methods are not accurate and are operator dependent. A technique is described to determine beam size using an automatic procedure. In the developed method, a specially designed and fabricated test pattern is scanned using an e-beam. A spectra of the signal is analyzed; beam diameter i s automatically determined using a software program. Results of design, fabrication, and analysis of the beam calibration test pattern are presented. © 2008 Elsevier B.V. All rights reserved. PACS: 07.05.Hd; 68.37.Hk Keywords: Electron probe diameter; Software; SEM resolution 1. Introduction The performance of electron-beam lithography (EBL) systems and scanning electron microscopes (SEM) greatly depends on the beam diameter, astigmatism, and the current distribution within the bea m. Consequently, beam size should be monitored and tuned frequently, but the most widely used technique to m easure beam size, edge detection, is not accurate, and more than it is operator dependent. In this method, a beam is scanned across a sharp edge; the resulting SEM signal is differentiated (see Fig. 1a) and this profile is then wide ly believed to represen t the shape of the beam. However, the edge technique involves two major errors: (a) It is a subject to an areaŽ error due to variable region cut by the edge which produce the SEM signal. As a result, for non-rectangular beams, the width of the differentiated SEM signal is smaller than the actual beam size, see Fig. 1b.

3D-investigations of electrically active interfaces in ZnO varistor materials have been performed using electron beam induced current (EBIC) contrast in the scanning electron microscope in combination with the focused ion beam workstation (FIB). The EBIC measurements were performed with spatial resolution better than 100 nm and the depth dependence of the signal was studied by variation of the acceleration voltage. The FIB was used for characterization of the subsurface geometry of electrically active grain boundaries. Experimental EBIC profiles, obtained by scanning the electron beam across grain boundaries, were compared with computer simulations using three different models for the electron–hole generation function. The EBIC contrast has been found to depend strongly on the geometric properties, i.e., the tilt of the grain boundary, and on asymmetries in the depletion region at the boundary. Calculations of the EBIC contrast taking these two effects into account show good agreement with experimental data. The “hot electron effect” close to the electrical breakdown voltage has been experimentally observed. The possibility to study asymmetry in the depletion region and the influence of the applied bias on the electrical properties of individual interfaces using the EBIC technique has been demonstrated.

The electrical properties of two-dimensionally polymerized C60 fullerenes were studied. Fullerene matrices consisting of randomly oriented domains are compared to the highly-oriented rhombohedral phase. The conductivity of the randomly oriented polymers obeys the Arrhenius law and can be described in a multiple trapping model. The oriented phase of polymeric C60 shows a distinct anisotropy in the electrical properties with a metallic-like in-plane conductivity at high temperatures.

An analysis of how the detectivity and lifetime depend on the fabrication process of superconducting antenna-coupled microbolometers has been carried out. The temperature dependences of responsivity and noise equivalent power (NEP) have been estimated in terms of the thermal model. To reveal the main degradation mechanism, 1/f -noise characterization has been used. Monte-Carlo simulation of the annealing procedure of YBa2 Cu3 O7 (YBCO) films for the operating ranges of frequency and temperature has shown that prevailing sources of flicker noise in superconducting microstrips are associated with transitions of oxygen atoms situated close to low-angle boundaries of the film blocks. The magnetron sputtering technique has been optimized to reduce the Hooge parameter for flicker noise to a record-breaking low value for YBCO films of about 10-4 at 93 K. Comparative analysis of chemical, ion and laser etching techniques by low-temperature scanning electron microscopy and magneto-optics allowed the fabrication of microstrips with uniform current distribution characterized by critical current density higher than 106 A cm-2 at 77 K and long-time stability. The process of low-energy ion milling of YBCO films with an Ar+ beam generated in a duopigatron ion source was used to reach a width resolution at the topology edge better than 0.2 µm. The antenna-coupled bolometers fabricated from the superconducting microstrips were used to register microwave radiation at a frequency of 70.3 GHz and temperature of 93 K. It is demonstrated that the developed technology makes possible the fabrication of long-lived YBCO-based antenna microbolometers with electrical NEPe = 1.5 × 10-12 W Hz-1/2 . The calculated response time of the antenna is about 30-150 ns. Further development is associated with fabrication of coupling microbolometers with immersed lens, with predicted optical detectivity D * = (4 × 109 - 4 × 1010 ) cm Hz1/2 W-1 in the wavelength range 100-1000 µm.

Coupling through random superconducting bridges between filaments in a multifilamentary Ag-sheathed Bi2Sr2Ca2Cu3O10+ tape has been investigated by magneto-optical imaging at temperatures from 20 K up to Tc. Magnetic flux distributions have been measured on the surface of an intact tape in the remanent state on applying a strong perpendicular magnetic field. The flux distributions observed at low temperatures reflect the arrangement of individual filaments. At high temperatures, the distribution becomes more similar to that for a uniform monocore tape, indicating that superconducting connections appear between the filaments. To discuss the relative contributions of the intra- and inter-filament currents, a simple model based on the Bean critical state was proposed and applied to analyse the temperature dependent behaviour. The inter-filament coupling, increasing with temperature, reaches at 77 K a point where the currents flowing in large inter-filament loops are roughly equal to the intra-filament currents.

Temperature and magnetic field dependences of the conductivity of YBaCuO films in the transition region are analyzed taking into account spatial inhomogeneity in transition temperature, Tc. (i) An expression for the superconducting contribution to conductivity, \sigma_s(T,H,Tc), of a homogeneous superconductor for H<

Significant progress has been made in the development of III–nitride deep UV light emitting diodes (LEDs) grown on sapphire substrates using AlGaN multiple quantum well (MQW) active regions. This progress was largely based on the advancements integrated in the first reported deep UV LEDs demonstrating sub-milliwatt output power. 1) The key to the success of these devices was based on three technical advancements. First was the use of pulsed atomic layer epitaxy (PALE) to improve the quality of the buffer AlN layer. 2,3) PALE deposited AlxGa1� xN/AlyGa1� yN shortperiod superlattices were also inserted between the buffer AlN and the n-contact AlGaN layer to control the thinfilm stress, thereby mitigating epilayer cracking. 4) Finally, a p-GaN/p-AlGaN heterojunction contact layer was used to improve hole injection. 5) The same technical approaches were subsequently used by Zhang et al. to commercialize deep UV LEDs. 6)

Spatially-resolved studies of a YBaCuO thin film bridge using electron probe microanalysis (EPMA), low-temperature scanning electron microscopy (LTSEM), and magneto-optical flux visualization (MO) have been carried out. Variations in chemical composition along the bridge were measured by EPMA with 3 microns resolution. Using LTSEM the spatial distributions of the critical temperature, Tc, and of the local transition width, dTc, were determined with 5 microns resolution. Distributions of magnetic flux over the bridge in an applied magnetic field have been measured at 15 and 50 K by magneto-optical technique. The critical current density Jc as a function of coordinate along the bridge was extracted from the measured distributions by a new specially developed method. Significant correlations between Jc, Tc, dTc and cation composition have been revealed. It is shown that in low magnetic fields deviation from the stoichiometric composition leads to a decrease in both Tc and Jc. The profile of Jc follows the Tc-profile on large length scales and has an additional fine structure on short scales. The profile of Jc along the bridge normalized to its value at any point is almost independent of temperature.

We report the first ever room temperature (RT) stimulated emission at 214 nm using high quality AlN layers that were grown over patterned sapphire substrates by pulsed lateral epitaxial overgrowth (PLOG) process. The PLOG process yielded fully coalesced layers with total thicknesses in excess of 10 µm resulting in a reduction in the threading dislocation density by several orders. The stimulated emission was achieved at 214 nm under pulsed optical pumping at RT. The RT threshold optical power density was approximately 9 MW/cm2 and the stimulated edge-emission signal was strongly polarized with E∥c.

Local measurements of structural characteristics such as intrinsic microstrain along the c axis of the lattice e=δc/c and its mean square fluctuation 〈e〉, oxygen deficiency x, cation composition, etc. were performed on epitaxial YBa2Cu3O7 films grown on various substrates (MgO, BaSrTiO3/MgO, SrTiO3, LaAlO3, ZrO2/Si, Al2O3). A number of film microstrips were fabricated and the normalized flicker noise intensity (Hooge parameter α) and the resistivity ρ at 300 K were measured at each characterized point. A theoretical model was developed that explains the observed first growth of α with 〈e〉 and the well-known high level of the normal-phase flicker noise in various high temperature superconducting compounds. Comparison of the experimental and simulated dependence of α on 〈e〉, frequency, and temperature permits one to determine numerically the theoretical parameters of the double-well potential with minima located at the chain (O1) and empty (O5) oxygen lattice positions of the CuO plane.

The voltage noise and current inhomogeneities in a thin c oriented YBa 2 Cu 3 O 7− x film on a MgO substrate have been investigated. An unusually sharp peak of the low-frequency noise was 7 K below T c observed with a width of less than 1 K and a strongly nonmonotonical dependence on external magnetic field. The appearance of this peak is found to be related to a single defect which leads to the formation of a weak link in the bottleneck of the current percolation path. This weak link is a latent one; it is undetectable by the standard characterization aechniques and has no effect on the integral transport properties of the film. We were able to reveal it using low-temperature scanning electron microscopy which allowed us to determine the current-density distribution across the weak link with a resolution of μ m. The critical current density of the weak link is found to be j c (77 K) > 5 x 10 6 A/cm 2 which is comparable with the critical current density of high-quality YBaCuO films. Analysis of the experimental dependences in terms of the resistively shunted junction model shows that Abrikosov vortex motion is involved resulting in fluctuations of the phase difference across the weak link leading to a peak of the voltage noise at the temperature at which the current through the weak link is close to its critical current.

Noise properties of c -oriented YBa 2 Cu 3 O 7− x fioms near T c have been investigated both experimentally and theoretically. The films studied are divided into two groups. The resistance noise and transport properties for the films (1) are controlled by the grain boundary junctions. The films (2) are of high structural perfection and have very low flicker noise levels. Their S R ( T )-characteristics near T c are quantitatively described by a novel percolation model. The model takes into account static T c -inhomogeneities of the film and the presence of defects that modulate the local T c . Distribution functions of T c were obtained and defect density of states as well as a scale of T c -relief were estimated. The spatial current distribution is studied by low-temperature SEM. The method is originally used for T c -mapping of the films. A special algorithm for the LTSEM data treatment for inhomogeneous films using the effective medium approach and allowing us to derive local transition curves is proposed.

We investigated the growth of GaN/Al0.20Ga0.80N multiple quantum wells (MQWs) on selective-area-grown a-plane GaN pillars over r-plane sapphire. In contrast to the MQWs grown on planar a-plane GaN templates, these GaN/Al0.20Ga0.8N MQWs on the pillars exhibited pit-free and atomically smooth surface morphology. Their structural quality and their UV emission (at 357 nm) increased with the underlying pillar height. The epitaxy of GaN/AlGaN MQWs on the selective-area-grown pillars is thus a promising and simple approach for fabricating stripe-geometry, high-efficiency, nonpolar UV emitters.

We report a new epitaxy procedure for growing extremely low defect density a-plane GaN films over r-plane sapphire. By combining selective area growth through a SiO2 mask opening to produce high height to width aspect ratio a-plane GaN pillars and lateral epitaxy from their c-plane facets, we obtained fully coalesced a-plane GaN films. The excellent structural, optical and electrical characteristics of these selective area lateral epitaxy (SALE) deposited films make them ideal for high efficiency III-N electronic and optoelectronic devices.

Fully coalesced epitaxial laterally overgrown a-plane GaN films were characterized for their structural and optical quality. The films had a very smooth surface with a root mean square roughness as low as 4.6 ? for a 5 ?m ? 5 ?m atomic force microscope scan area. They exhibited a wing tilt of only 0.27? and optically pumped stimulated emission, which establish their high structural and optical quality. These non-polar films are ideal for fabricating high-efficiency optoelectronic and electronic devices.

Nanopores with conical geometries have been found to rectify ionic current in electrolytes. While nanopores in semiconducting membranes are known to modulate ionic transport through gated modification of pore surface charge, the fabrication of conical nanopores in silicon (Si) has proven challenging. Here, we report the discovery that gold (Au) nanoparticle (NP)-assisted plasma etching results in the formation of conical etch profiles in Si. These conical profiles result due to enhanced Si etch rates in the vicinity of the Au NPs. We show that this process provides a convenient and versatile means to fabricate conical nanopores in Si membranes and crystals with variable pore-diameters and cone-angles. We investigated ionic transport through these pores and observed that rectification ratios could be enhanced by a factor of over 100 by voltage gating alone, and that these pores could function as ionic switches with high on-off ratios of approximately 260. Further, we demonstrate voltage gated control over protein transport, which is of importance in lab-on-a-chip devices and biomolecular separations.

We report on a study to compare the growth of thick AlGaN layers on GaN with different strain-relief interlayers. A set of ten period AlN/AlGaN superlattices was found to be the most efficient approach for the strain relief. The superlattice interlayer not only decreases the tensile strain but also improves the crystal structural quality. Thus, 2-μm-thick, high quality n+-Al0.2Ga0.8N layers can be grown on GaN epilayers without any cracks.

We present a detailed high-pump-current study of self-heating effects in ultraviolet light-emitting diodes (LEDs) grown on sapphire. For deep ultraviolet LEDs on sapphire, our results establish self-heating to be a primary cause of premature power saturation under dc pumping. Even the flip-chip packaged devices undergo a steady-state temperature rise to about 70 °C at a dc pump current of only 50 mA (at 8 V) resulting in a significant decrease in LED output. Temperature rise values estimated from peak emission wavelength shifts and from micro-Raman mapping of the active devices were in good agreement.

We report on an AlN/AlGaN superlattice approach to grow high-Al-content thick n+-AlGaN layers over c-plane sapphire substrates. Insertion of a set of AlN/AlGaN superlattices is shown to significantly reduce the biaxial tensile strain, thereby resulting in 3-μm-thick, crack-free Al0.2Ga0.8N layers. These high-quality, low-sheet-resistive layers are of key importance to avoid current crowding in quaternary AlInGaN multiple-quantum-well deep-ultraviolet light-emitting diodes over sapphire substrates.

Burning of superconducting thin film bridges by large transport currents (up to densities of 2×107 A/cm2) is investigated by magneto-optical imaging of flux distribution and low-temperature scanning electron microscopy providing Tc maps. It is shown that the destruction is preceded by significant penetration of magnetic field inside a weak-pinning region. In bridges containing extended defects magneto-optic investigation is sufficient to locate the incipient burning region. In high-quality bridges free from such defects only a combination of the two techniques will allow prediction of the place of fatal destruction.

Magnetic flux distribution across a high-temperature superconductor strip is measured using magneto-optical imaging at 15 K. Both the current-carrying state and the remanent state after transport current are studied up to the currents 0.97 Ic where Ic is the critical current. To avoid overheating of the sample current pulses with duration 50 ms were employed. The results are compared with predictions of the Bean model for the thin strip geometry. In the current-carrying state, reasonable agreement is found. However, there is a systematic deviation -- the flux penetration is deeper than theoretically predicted. A much better agreement is achieved by accounting for flux creep as shown by our computer simulations. In the remanent state the Bean model fails to explain the experimental results. The results for the currents I < 0.7 Ic can be understood within the framework of our flux creep simulations. However, after the currents I > 0.7 Ic the total flux trapped in a strip is substantially less than predicted by the simulations. Furthermore, it decreases with increasing current. Excessive dissipation of power in the annihilation zone formed in the remanent state is believed to be the source of this unexpected behavior., RevTeX, 11 pages with 10 Postscript figures

Recently the microelectronics devices fabricated on the basis of the topological structures with conic or point elements are applied widely. The traditional method of formation of the structures using an electron beam lithography, a chemical or ion beam (plasma) etching has some disadvantages. For example, the chemical etching of the submicron elements has rather low reproducibility of the geometry profile. It is clearly displayed in the formation of relief in the mesa of the submicron size. The electronic lithography and the polishing ion etching method of the cone formation is complicated due to the problem with the stability of the masking material and the resolution. What is more important the problem of the compatibility with other technological operations of the microelectronics fabrication. The authors offer the simple and reproducible method of fabrication of the submicron structure of a cone type. It bases on the formation of the surface relief by the neutral projectiles sputtering. The present paper reports the possibility of the cone relief fabrication on the wide surface and in the narrow submicron mesa by the argon neutral bombardment.

Inhomogeneity of the properties of YBa2Cu3O7 epitaxial c-oriented films has been investigated. Using a low - temperature SEM the lateral distribution of local critical temperature has been determined with spatial resolution of 2 µm. The technique provides a direct observation of the percolative character of the superconducting transition on a microbridge scale. It is shown for the first time that near the percolation threshold the current channel has a very rough, irregular configuration and the fractal dimensionality of the superconducting cluster is estimated as 1.55. The magnetic field imaging of samples is carried out and a quantitative correlation between Tc-map and the map of critical density of shielding currents is observed. The transition curves R(T) over a wide range of resistances and bias currents are described in terms of the network model. The model takes into account both the spatial distribution of Tc and the presence of random weak links with a wide distribution of their critical currents.

Line and space patterns have been fabricated in Si with periods as small as 1.3μm using a double patterning technique and an optical direct write tool. The pattern density of a single lithography step is shown to be limited by the proximity effect and that gratings with periods smaller than about 2.0μm are beyond the resolution of the 405nm wavelength laser direct write tool employed in these experiments. The lithography-etch–lithography-etch technique, which includes a highly selective reactive ion etch step, provides a robust way to create 4mm long gratings of 1.5μm period. Variations in this two step lithography process limit the practical minimum pitch for long gratings to about 1.4μm.

The authors report on pulsed lateral epitaxial overgrowth of aluminum nitride films on basal plane sapphire substrates. This approach, at temperatures in excess of 1150°C, enhanced the adatom migration, thereby significantly increasing the lateral growth rates. This enabled a full coalescence in wing regions as wide as 4–10μm. Atomic force microscopy and cross-section transmission electron microscopy were used to establish the reduction of threading dislocations in the lateral growth. Cross-sectional monochromatic cathodoluminescence and photoluminescence measurements confirmed the improved optical properties of the laterally overgrown aluminum nitride films.

An original in situ technique was developed to measure the temperature rise and decay of a material under electron-beam (e-beam) irradiation. The technique uses the effect of temperature rise on the electrical conductivity of high-temperature superconductors that are prefabricated as microbridges. This temperature sensor was measured to have a sensitivity greater than two orders of magnitude more than a thin-film thermocouple. A stroboscopic technique ensured nanosecond temporal resolution, and the size of the microbridge allowed for micrometer-scale spatial resolution. These measurement ranges are important for practical e-beam lithography. Experimental results were compared with results simulated by the TEMPTATION (temperature simulation) software, and qualitative agreement of simulated and measured results was found. Factors influencing the accuracy of the temperature measurement are discussed.