11,360 research outputs found
Plasma enhanced chemical vapor deposition of nanocrystalline graphene and device fabrication development
No full textLarge area growth of high quality graphene remains a challenge, and is currently dominated by chemical vapor deposition (CVD) on metal catalyst films. This method requires a transfer of the graphene onto an insulating substrate for electronic applications, and the graphene film quality and performance can vary with the transfer. A more attractive approach is plasma enhanced chemical vapor deposition (PECVD) of graphene and nanocrystalline graphene (NCG) directly on insulating substrates. The aim of this project was to explore the deposition process and microfabrication processes based on these NCG films.A deposition process for nanocrystalline graphene was developed in this work based on parallel-plate PECVD. NCG with thicknesses between 3 and 35nm were deposited directly on wet thermal oxidized silicon wafers with diameter of 150 mm, quartz glass and sapphire glass. High NCG thickness uniformities of 87% over full wafer were achieved. Surface roughness was measured by atomic force microscopy and shows root mean square (RMS) values of less than 0.23nm for 3nm thin films. NCG films deposited on quartz and sapphire show promising performance as transparent conductor with 13kΩ/X sheet resistance at 85% transparency. Furthermore, the suitability of the developed PECVD NCG films for microfabrication was demonstrated. Microfabrication process development was focused on four device types. NCG membranes were fabricated based on through-wafer inductively coupled plasma etching from the back, and consecutive membrane release by HF vapor etching.The fabrication of suspended NCG strips, based on HF vapor release, shows promising results, but was not entirely successful due to insufficient thickness of the sacrificial oxide. Top gated NCG strips are successfully fabricated, and the increased modulation by the top gate is demonstrated. Finally, NCG nanowire fabrication is performed on 150mm wafers. Experiments yielded an increased back gate modulation effect by a reduced NCG thickness, although no nanowire formation was observed. A highly accurate focused ion beam (FIB) prototyping technique was developed and applied to exfoliated graphene in this work. This technique systematically avoids any exposure of the graphene to Ga+-ions through the use of an alignment marker system, achieving alignment accuracies better than 250 nm. Contacts were deposited by FIB- or e-beam-assisted tungsten deposition, and FIB trench milling was used to confine conduction to a narrow channel. A channel passivation method based on e-beam-assisted insulator deposition has been demonstrated, and showed a reduction of ion damage to the graphene. Three fabricated transistor structures were electrically characterized
Performance Evaluation of Distributed-Antenna Communications Systems Using Beam-Hopping
No full textDigital beamforming (DBF) techniques are capable of improving the performance of communications systems significantly. However, if the transmitted signals are conflicted with strong interference, especially, in the direction of the transmitted beams , these directional jamming signals will severely degrade the system performance. In order to efficiently mitigate the interference of the directional jammers, in this contribution a beam-hopping (BH) communications scheme is proposed. In the proposed BH communications scheme, only one pair of the beams is used for transmission and it hops from one to the next according to an assigned BH pattern. In this contribution a range of expressions in terms of the average SINR performance have been derived, when both the uplink and downlink are considered. The average SINR performance of the proposed BH scheme and that of the conventional single-beam (SB) as well as multiple-beam (MB) assisted beam-processing schemes have been investigated. Our analysis and results show that the proposed BH scheme is capable of efficiently combating the directional jamming, with the aid of utilizing the directional gain of the beams generated by both the transmitter and the receiver. Furthermore, the BH scheme is capable of reducing the intercept probability of the communications. Therefore, the proposed BH scheme is suitable for communications, when several distributed antenna arrays are available around a mobile
Free space optical system performance for a Gaussian beam propagating through non Kolmogorov weak turbulence
No full textAtmospheric turbulence has been described for many years by Kolmogorov's power spectral density model because of its simplicity. Unfortunately several experiments have been reported recently that show Kolmogorov theory is sometimes incomplete to describe atmospheric statistics properly, in particular in portions of the troposphere and stratosphere. It is known that free space laser system performance is limited by atmospheric turbulence. In this paper we use a non-Kolmogorov power spectrum which uses a generalized exponent instead of constant standard exponent value 11/3 and a generalized amplitude factor instead of constant value 0.033. Using this spectrum in weak turbulence, we carry out, for a Gaussian beam propagating along a horizontal path, analysis of long term beam spread, scintillation, probability of fade, mean signal to noise ratio and mean bit error rate as variation of the spectrum exponent. Our theoretical results show that for alpha values lower than 11/3 , but not for alpha close to 3 , there is a remarkable increase of scintillation and consequently a major penalty on the system performance. However when alpha assumes values close to 3 or for alpha values higher than 11/3 scintillation decreases leading to an improvement on the system performanc
Stevin Outlet Sluices: Wave impact under a beam
No full textThe Dutch department of Public Works had a problem regarding wave impacts on a beam in the Stevin outlet sluices, located in the Afsluitdijk. Wave impacts on this beam could also cause a peak pressure on the barrier gate, just behind the beam. The numerical program ComFLOW and physical scale experiments were used to predict the wave impacts for different hydraulic conditions (i.e. wave height, wave period and water level). The research questions were: 1. How is the wave load on the northern gates depending on the presence of the military beam? 2. How large is a wave impact load on the bottom of the military beam in the Stevin outlet sluices? 3. How well can the numerical model ComFLOW and physical modelling be used to determine the wave impact on the bottom of the military beam in the Stevin outlet sluices? 2D scaled experiments were performed making use of a model with the (simplified) geometry of the Stevin outlet sluices and regular waves. It was found that the largest wave impacts occurred for water levels equal to the bottom plane of the beam or slightly under it. This happened for the shortest waves in the test domain. The largest pressure measured on the beam was approximately 50 kPa or 35H, with H representing the incident wave height in front of the model. It was also found that the spread in the peak pressures for one single experiment was large. The results of the measured impulse per peak showed far less spread. The effect of wave impacts under the beam was also found on the vertical wall under the beam. The actual pressures however were less and they were decreasing with increasing depth. Besides physical wave impact testing, a few experiments were performed with the beam removed from the model. This resulted in wave simply running up the vertical wall of the model. They did not cause a wave impact. The measurements of both type of experiments, with and without a beam, were compared. This revealed that the total wave impulse on the gate was not affected by the presence of the beam. However the distribution of the pressure within a single wave period was significantly different. In case of a beam, a large impact peak was observed, whereas the other wave only showed a small hump caused by the deflected flow against the vertical wall. When the calculated and measured wave impact results were compared it became clear that ComFLOW underestimated the peak pressures by a factor 2 to 20 for the pressures on the impact plane. The same was done for the peak impulse. This showed that the impulse of the peak on the impact plane were underestimated by a factor 2 at most. These results confirmed that the used grid was too coarse for the program to model the physics correctly. The main conclusions to the research questions were: 1. The presence of the military beam causes a different distribution of the force on the gate within a wave period. The total amount of impulse is more or less the same as for the situation without a beam. With the military beam, a wave impact results in a peak force on the gate. Without the beam, there will be no peak force. 2. The largest measured wave impact pressure is 35H. 3. Both ComFLOW and physical modelling can be used to predict wave impacts for the geometry of Stevin outlet sluices. Much care should be taken when modelling and much attention should go to the input parameters of the program.Hydraulic StructuresHydraulic EngineeringCivil Engineering and Geoscience
Charge-induced pattern displacement in E-beam lithography
No full textElectron beam lithography (EBL) requires conducting substrates to ensure pattern fidelity. However, there is an increasing interest in performing EBL on less well-conducting surfaces or even insulators, usually resulting in seriously distorted pattern formation. To understand the underlying charging phenomena, the authors use Monte Carlo simulations that include models for substrate charging, electron beam-induced current, and electric breakdown. Simulations of electron beam exposure of glass wafers are presented, exposing regular patterns which become distorted due to charge-induced beam deflection. The resulting displacements within the patterns are mapped and compared to experimental displacement maps obtained from patterns in PMMA resist on glass substrates. Displacements up to several hundreds of nanometers were observed at a primary beam energy of 50 keV. Also, various scan strategies were used to write the patterns, in the simulations as well as the experiments, revealing their strong effect on pattern distortion, in shape and in magnitude. A qualitative, in some cases even quantitative, good agreement was found between the simulations and the experiments, providing enough confidence in Monte Carlo simulations to predict charge-induced pattern displacement and shape distortion and to find smart scan strategies to minimize the effects of charging.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.ImPhys/Charged Particle Optic
Synthesis of Multiple Beam Linear Arrays with Uniform Amplitudes
No full textA convex iterative algorithm for the synthesis of uniform amplitude, space-tapered linear phased arrays with simultaneous multiple beam optimization for 5G applications is presented. The performance of the algorithm is demonstrated by the synthesis of two arrays having 16 and 24 elements with 60 and 90 degree scan range, respectively. The effect of phase shifter quantization is also addressed. The results indicate that the space-tapered arrays with multiple beam optimization have improved radiation performance in terms of the side lobe level when compared to both single, broadside, beam optimized space-tapered arrays and uniformly distributed arrays with half wavelength spacing.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Microwave Sensing, Signals & System
Man, squatting on a beam, beside a taut rope passing down a mineshaft, Australia, ca. 1935 [picture] /
No full textPart of the: E.W. Searle collection of photographs.; Title devised by cataloguer based on caption, accompanying information and reference sources.; Also available in an electronic version via the internet at: http://nla.gov.au/nla.pic-vn4655801
Piezoelectric bistable buckled beam energy harvester.
No full textA novel energy harvesting device design is presented to be created via microfabrication techniques. Such devices have countless applications for powering low-current electrical devices, especially wireless sensors or transmitters. This micro-electromechanical system (MEMS) design utilizes the piezoelectric response of a bistable buckled beam to gather electrical energy via ambient vibrations. While many traditional piezoelectric energy harvesters (PEH) consist of simple cantilever beam geometries, this nonlinear design utilizes inertial effects of torsional lever arms to actuate a central buckled beam to snap between its two stable states; such an abrupt strain on the piezoelectric beam potentially produces a significantly increased electrical response over a wider range of excitation frequencies than is possible with simpler linear systems. The geometries of all structural layers of the device are described in detail, in addition to the cleanroom processes needed to create each MEMS device layer. Experimental fabrication process steps and results, performed by the author\u27s work in the University of Louisville\u27s Micro-Nano Technology Center, are described in detail. The most successful, complete microfabrication process flow is given to the best of the author\u27s abilities. Potential improvements and ideas for future work are given in conclusion
On the Bolotin's reduced beam model versus various boundary conditions
No full textThis paper is devoted to the construction of asymptotically correct simplified models of nonlinear beam equations for various boundary conditions. V.V. Bolotin mentioned that in some cases (e.g., if compressed load is near the buckling value), the so-called „nonlinear inertia“ must be taken into account. The effect of nonlinear inertia on the oscillations of the clamped-free beam is investigated in many papers. Bolotin used some physical assumption and did not compare the order of nonlinear terms in original equations. Below we propose our method for deriving those, which we will named “Bolotin's equations“. This approach is based on fractional analysis of original boundary value problems.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Mathematical Physic
Beam displacement and blur caused by fast electron beam deflection
No full textElectrostatic beam blankers are an alternative to photo-emission sources for generating pulsed electron beams for Time-resolved Cathodoluminescence and Ultrafast Electron Microscopy. While the properties of beam blankers have been extensively investigated in the past for applications in lithography, characteristics such as the influence of blanking on imaging resolution have not been fully addressed. We derive general analytical expressions for the spot displacement and loss in resolution induced by deflecting the electron beam in a blanker. In particular, we analyze the sensitivity of both measures to how precise the conjugate focus is aligned in between the deflector plates. We then work out the specific case of a beam blanker driven by a linear voltage ramp as was used in recent studies by others and by us. The result shows that the spot displacement and focus blur can be reduced to the same order as the electron beam probe size, even when using a beam blanker of millimeter or larger scale dimensions. An interesting result is that, by the right choice of the focus position in the deflector, either the spot displacement from the stationary position can be minimized, or the blur can be made zero but not both at the same time. Our results can be used both to characterize existing beam blanker setups and to design novel blankers. This can further develop the field of time-resolved electron microscopy by making it easier to generate pulses with a typical duration of tens of picoseconds in a regular scanning electron microscope at high spatial resolution.ImPhys/Charged Particle Optic
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