Electro-acoustical characterization procedure for cMUTs

A procedure for the electro-acoustical characterization in air of cMUTs is reported. First, the measured input electrical impedance of the transducer is used to calculate the transducer parameters at different bias voltages by fitting it with the Mason model. Then, the single membrane equivalent circuit can be calculated. Second, the cMUT impulse response is obtained through a send-and-receive experiment to be compared with the one predicted by the Mason model. In order to minimize the influence of the emitter in the obtained impulse response an ad hoc broadband piezoelectric transducer centered at the resonant frequency of the cMUT was fabricated. Using this transducer, no deconvolution of the impulse response of the emitter in the cMUT reception pulse is necessary. The procedure is tested for two cMUTs with silicon-rich nitride as structural layer and different membrane diameters (60 and 70 microm).     

PSpice modeling of capacitive microfabricated ultrasonic transducers

Capacitive microfabricated ultrasonic transducers (cMUTs) are the newest and potentially the most promising devices to convert electrical into acoustic signals and vice-versa. These devices are based on the capacitance modulation of a microcondenser which is obtained by microfabrication onto a silicon substrate. The aim of this paper is to describe a PSpice model of the cMUT, based on an analytical distributed model previously reported (IEEE Trans. UFFC 49 (2) (2002) 159-168), which can be used to simulate the performances of a general ultrasound system, either in frequency or time domain. The PSpice model consists of a capacitor with a parallel resistor, which represent the static capacitance and the loss and bias resistances of the transducer, respectively, plus two quadrupoles (GLAPLACE) modeling the mechanical impedance of the membranes and the radiation impedance of the medium. The usefulness of a PSpice model is the possibility to simulate and optimize the cMUT transducers in transmission and reception, along with driving and receiving electronics, in a general ultrasound system. Experimental measurements on a 5 MHz cMUT operating in pulse-echo are in good agreement with model predictions.     

A multiscale model for array of capacitive micromachined ultrasonic transducers

A model is developed for studying the acoustic behavior of a cMUT array. This model is based on separate calculations of the terms describing the behavior of a single cMUT on one hand, and those corresponding to acoustic mutual coupling on the other hand. The terms are combined into an equivalent circuit with matrix terms which displays only one degree of freedom per cell. This approach allows the simulation of several dozen cMUTs considered individually with a very short computer time. A Finite Difference model is used for the simulation of an isolated cell radiating acoustic energy and the determination of its equivalent electromechanical circuit. It is shown for various mutual coupling situations that the coupling between cells can be correctly approximated using a very simple mutual impedance term. The model is compared with experimental results, using a set of different cMUT configurations. Experimental results were obtained with electrical impedancemetry and laser interferometry techniques performed in fluid immersion.     

Air-coupled MUMPs capacitive micromachined ultrasonic transducers with resonant cavities

This work reports performance improvements of air-coupled capacitive micromachined ultrasonic transducers (CMUTs) using resonant cavities. In order to perform this work, we have designed and manufactured a CMUT employing multi-user microelectromechanical systems (MEMS) processes (MUMPs). The transducer was designed using Helmholtz resonator principles. This was characterised by the dimensions of the cavity and several acoustic ports, which had the form of holes in the CMUT plate. The MUMPs process has the advantage of being low cost which allows the manufacture of economic prototypes. In this paper we show the effects of the resonant cavities and acoustic ports in CMUTs using laser Doppler vibrometry and acoustical measurements. We also use Finite Element (FE) simulations in order to support experimental measurements. The results show that it is possible to enhance the output pressure and bandwidth in air by tuning the resonance frequency of the plate (f_p) with that of the Helmholtz resonator (f_H). The experimental measurements show the plate resonance along with an additional resonance in the output pressure spectrum. This appears due to the effect of the new resonant cavities in the transducer. FE simulations show an increase of 11 dB in the output pressure with respect to that of a theoretical vacuum-sealed cavity MUMPs CMUT by properly tuning the transducer. The bandwidth has been also analyzed by calculating the mechanical Q factor of the tuned CMUT. This has been estimated as 4.5 compared with 7.75 for the vacuum-sealed cavity MUMPs CMUT.     

Radiated fields of capacitive micromachined ultrasonic transducers in air

This paper presents a study of the radiated ultrasonic fields of capacitive micromachined ultrasonic transducers (often referred to as cMUTs) in air. These fields were modeled theoretically and then compared to the experimental near-field amplitude variations and directivity patterns of square cMUTs. The good agreement between theory and experiment indicates that the devices can be approximated to plane piston radiators. The fields of multiple elements driven in phase on the same silicon substrate are presented, where again comparison is made to theory. The results indicate that individual elements are unaffected by radiation through the silicon substrate from adjacent devices. It will also be demonstrated that it is possible to use the knowledge of these fields to develop air-coupled ultrasonic surface imaging systems.     

Pt/Ti Electrodes of PZT Thin Films Patterning by Novel Lift-Off Using ZnO as a Sacrificial Layer

We achieve a successful novel lift-off of patterning Pt/Ti electrodes on SiO2/Si substrates by employing ZnO sacrificial layer deposition and patterning, successive uniform Pt/Ti deposition and final lift-off. Then we deposit PZT thin films on the electrodes. Compared with the conventional lift-off processes for the electrodes, this novel process does not need post-annealing, which must be performed after conventional lift-off process. It is demonstrated that the electrodes patterned by the novel lift-off process have stronger adhesion. The electrodes and the PZT films on the electrodes are more compact and smoother than those by the conventional lift-off process.     

Solid-state NMR studies of supercapacitors

Electrochemical double-layer capacitors, or ‘supercapacitors’ are attracting increasing attention as high-power energy storage devices for a wide range of technological applications. These devices store charge through electrostatic interactions between liquid electrolyte ions and the surfaces of porous carbon electrodes. However, many aspects of the fundamental mechanism of supercapacitance are still not well understood, and there is a lack of experimental techniques which are capable of studying working devices. Recently, solid-state NMR has emerged as a powerful tool for studying the local environments and behaviour of electrolyte ions in supercapacitor electrodes. In this Trends article, we review these recent developments and applications. We first discuss the basic principles underlying the mechanism of supercapacitance, as well as the key NMR observables that are relevant to the study of supercapacitor electrodes. We then review some practical aspects of the study of working devices using ex situ and in situ methodologies and explain the key advances that these techniques have allowed on the study of supercapacitor charging mechanisms. NMR experiments have revealed that the pores of the carbon electrodes contain a significant number of electrolyte ions in the absence of any charging potential. This has important implications for the molecular mechanisms of supercapacitance, as charge can be stored by different ion adsorption/desorption processes. Crucially, we show how in situ NMR experiments can be used to quantitatively study and characterise the charging mechanism, with the experiments providing the most detailed picture of charge storage to date, offering the opportunity to design enhanced devices. Finally, an outlook for future directions for solid-state NMR in supercapacitor research is offered.     

Impact of film thickness on threshold voltage of polysilicon TFTs

The threshold voltage is a key parameter in the silicon MOSFETS design and operation. In this paper, we study the factors that contribute to the changes of threshold voltage of thin-film LPCVD polysilicon transistors when varying the thickness of the active layer.The results show that the threshold voltage depends strongly on the film thickness. For high thicknesses, the threshold voltage is shown to be determined by the trapped holes at grain boundaries. The variation of this parameter with film thickness can be attributed to inter-granular trap states density variation in the film.For low thicknesses, a simple electrostatic model of the study structure, associated with a numerical method of solving 2D-Poisson's equations, shows that the changes of threshold voltage of polysilicon TFT depends on grain-boundary properties and charge-coupling between the front and back gates. Based on this consideration, the usual threshold voltage expression is modified. The results so obtained are compared with the available experimental data, which show a satisfactory match thus justifying the validity of the proposed relation.     

Thick aluminium nitride films deposited by room-temperature sputtering for ultrasonic applications

Materials in film form for electromechanical transduction have a number of potential applications in ultrasound. They are presently under investigation in flexural transducers for air-coupled ultrasound and underwater sonar operating at frequencies up to a few megahertz. At higher frequencies, they have the potential to be integrated with electronics for applications of ultrasound requiring high spatial resolution. However, a number of fabrication difficulties have arisen in studies of such films. These include the high temperatures required in many thick and thin film deposition processes, making them incompatible with other stages in transducer fabrication, and difficulties maintaining film quality when thin film--typically sub-1 microm--processes are extended to higher thicknesses. In this paper, we first outline a process which has allowed us to deposit aluminium nitride (AlN) films capable of electromechanical transduction at thicknesses up to more than 5 microm without substrate heating. As an ultrasonic transduction material, AlN has functional disadvantages, particularly a high acoustic velocity and weak electromechanical transduction. However, it also has a number of advantages relating to practicality of fabrication and functionality. These include the ability to be deposited on a variety of amorphous substrates, a very high Curie temperature, low permittivity, and low electrical and mechanical losses. Here, we present experimental results highlighting the transduction capabilities of AlN deposited on aluminium electrodes on glass and lithium niobate. We compare the results with those from standard simulation processes, highlighting the reasons for discrepancies and discussing the implications for incorporation of AlN into standard ultrasonic transducer design processes.     

Effective embedment of activated carbons into the traditional Korean paper ‘Hanji’ and its application to flexible supercapacitors

Demands for flexible electronic equipment such as wearable devices and wrap-round displays have motivated the development of flexible energy storage devices. Although cellulose paper is one of the most promising substrates for flexible devices, its intrinsic limitations, such as poor mechanical durability, hamper its practical use. In this study, we adopted the traditional Korean paper, Hanji, with superior mechanical robustness as a substrate for supercapacitor electrodes. The effective infiltration of activated carbons (ACs) as an electrode material into the dense network of Hanji cellulose fibers was performed by a simultaneous one-pot process of network formation and AC embedment. The fabricated symmetric supercapacitors based on the AC-embedded Hanji electrode exhibited a specific capacitance (C_sp) of 16.0 F/g measured at a scan rate of 10 mV/s with excellent cycle stability, the C_sp retention of 94.5%, over 1000 charge-discharge cycles.     

Laser printing of conformal and multi-level 3D interconnects

A crucial challenge in three-dimensional multi-chip assemblies is to establish electrical connections between discrete devices. Here, we apply laser printing of congruent voxels of silver nanopaste for the fabrication of conformal and 3D multi-level interconnects. By controlling laser fluence, various 3D electrodes including freestanding tabs and side contacts over vertical walls can be directly printed without the need for sacrificial layers, chemical etching or electroplating. The electrical characteristics of the printed interconnects are similar to those currently in use by the semiconductor industry. These results are a promising step forward in the generation of customized interconnects for 3D microelectronics.     

Modelling of the radiated field from multi-element capacitive micromachined ultrasonic transducers

This paper presents results from a theoretical model of the ultrasonic fields radiated by a 3x3 assembly of capacitive micromachined ultrasonic transducer (cMUT) sources on the same silicon substrate. These predictions have, for the first time, been compared directly to the fields measured experimentally using a scanned miniature detector. This work indicates that there is minimal cross-coupling between source elements, and demonstrates that it is possible to predict successfully the field characteristics of various geometries of such cMUT elements, with a view to the development of future imaging systems.     

Gate dielectrics for ultimate CMOS technologies – Limitations and alternative solutions

For nowadays CMOS technologies, the gate oxide thickness has reached a few nanometer range and will be lower than 2 nm for sub-0.1 μ m generations. This scaling of the gate dielectric thickness favors the onset of physical phenomena such as gate polysilicon depletion or quantum effects that limit the MOS device performance in terms of capacitance and leakage current. Moreover, these ultra thin oxide MOS structures are prone to new degradation processes that could reduce their operation lifetime. In this paper, the major limitations raised by the scaling of the gate dielectrics in CMOS technologies are briefly reviewed in terms of MOS capacitance, reliability and new materials issues. More specifically, we first focus on the limitations raised by physical phenomena inherent to MOS capacitors such as polysilicon depletion and quantum effects (carrier confinement and tunneling), impacting their performances. We then address the limitations related to the reliability concerns such as wearout, breakdown, quasi-breakdown, stress-induced leakage current, determining the device lifetime. Finally, the new materials currently envisaged, as replacement solutions in order to overcoming the difficulties due to the gate oxide scaling will be discussed. In particular, the possible solutions based on alternate high permittivity gate dielectrics and metallic gate materials will be emphasized.     

Novel fabrication technologies of planar nano-gap electrodes for single molecule evaluation

Present information technologies use semiconductor devices and magnetic/optical disc. However, they are all foreseen to face fundamental limitations within a decade. Therefore, superseding devices are required for the next paradigm of high performance information technologies. This paper describes prospects for single molecule devices suitable for future information processing technologies, which are expected as the most probable candidate to supersede the present semiconductor devices. The first milestone towards the realization of single molecule devices in future electronics requires quantitative evaluation of single molecule characteristics, which inevitably needs planar nano-gap electrodes between which single molecules are sandwiched, observed their structures and measured their electrical characteristics. Nano-meter electrode pattern fabrication was achieved by electron beam lithography and metal lift-off, while planarization process requires many new technologies including chemical–mechanical polishing (CMP) and wafer bonding. The detailed planarization processing technologies are described in this paper to realize nm-planar nano-scale electrodes.     

Three-dimensional vertical ZnO transistors with suspended top electrodes fabricated by focused ion beam technology

Three-dimensional(3D)vertical architecture transistors represent an important technological pursuit,which have distinct advantages in device integration density,operation speed,and power consumption.However,the fabrication processes of such 3D devices are complex,especially in the interconnection of electrodes.In this paper,we present a novel method which combines suspended electrodes and focused ion beam(FIB)technology to greatly simplify the electrodes interconnection in 3D devices.Based on this method,we fabricate 3D vertical core-double shell structure transistors with ZnO channel and Al₂O₃ gate-oxide both grown by atomic layer deposition.Suspended top electrodes of vertical architecture could be directly connected to planar electrodes by FIB deposited Pt nanowires,which avoid cumbersome steps in the traditional 3D structure fabrication technology.Both single pillar and arrays devices show well behaved transfer characteristics with an Ion/Ioff current ratio greater than 106 and a low threshold voltage around 0 V.The ON-current of the 2×2 pillars vertical channel transistor was 1.2μA at the gate voltage of 3 V and drain voltage of 2 V,which can be also improved by increasing the number of pillars.Our method for fabricating vertical architecture transistors can be promising for device applications with high integration density and low power consumption.     

类桑拿法制备的周期性结构Mo金属催化电极及其在电解水制氢中的应用

采用类桑拿法制备了聚苯乙烯微球模板, 结合双层Mo金属结构, 获得了具有周期性结构的Mo金属催化电极. 通控制氧气对聚苯乙烯球的刻蚀时间, 可有效调制Mo金属催化电极的横、纵向尺寸, 从而获得不同的衬底比表面积. 通过原子力显微镜表面形貌测试、电化学线性扫描、塔菲尔测试以及阻抗谱分析表明: 增大刻蚀时间可有效提高Mo金属催化电极的表面粗糙度和比表面积, 进而降低电荷传输电阻和塔菲尔斜率, 促进催化电极/电解液界面处析氢反应的进行. 采用类桑拿法和双层Mo金属结构制备周期性结构的方法简单, 可大面积化, 同时低温磁控溅射法制备的Mo金属催化电极成本低廉, 温度兼容多种太阳电池器件, 具有形成高效一体化光水解制氢器件的潜力.     

Spin-filter effect and spin-polarized optoelectronic properties in annulene-based molecular spintronic devices

Using Fe, Co or Ni chains as electrodes, we designed several annulene-based molecular spintronic devices and investigated the quantum transport properties based on density functional theory and non-equilibrium Green's function method.Our results show that these devices have outstanding spin-filter capabilities and exhibit giant magnetoresistance effect,and that with Ni chains as electrodes, the device has the best transport properties. Furthermore, we investigated the spinpolarized optoelectronic properties of the device with Ni electrodes and found that the spin-polarized photocurrents can be directly generated by irradiating the device with infrared, visible or ultraviolet light. More importantly, if the magnetization directions of the two electrodes are antiparallel, the photocurrents with different spins are spatially separated, appearing at different electrodes. This phenomenon provides a new way to simultaneously generate two spin currents.     

Modeling of polysilicon diffusion sources during rapid optical annealing

A new model for polysilicon diffusion sources is presented. It considers the following effects: 1) dopant diffusion in grains, in grain boundaries and in the single-crystal silicon substrate, 2) dynamic dopant segregation between grain and grain boundary phases and between the phases of polysilicon and the single-crystal silicon substrate, 3) dynamic dopant activation or clustering in grains and in the single-crystal silicon substrate, 4) dynamic grain growth depending on local grain size and local electron density. These mechanisms with completely different time scales are modeled simultaneously. For the first time this allows the analysis of furnace and rapid optical annealing processes with arbitrary grain growth kinetics even during epitaxial realignment. The advanced model for segregation allows for the effect that dopants in grain boundaries and active dopants in grains as well as in the single-crystal silicon substrate find only a limited number of sites which can be occupied. These limitations are necessary to explain the dopant distributions in polysilicon and in the single-crystal silicon substrate. For the first time the coupling between the concentration of active dopants in grains, between the concentration of dopants in grain boundaries and between the local grain size is shown during doping enhanced grain growth.     

Gigahertz modulators using bulk acousto-optic interactions in thin-film waveguides

When bulk acoustic waves are applied to an optical waveguide, several modulation effects are observed, depending on the type of wave (longitudinal or shear). Longitudinal sound waves frequency-shift the guided light, thus providing a means of modulating light in a wide variety of waveguide materials. Using thin-film mosaic acoustic transducer technology, we have demonstrated such modulation at frequencies in the gigahertz region. By segmenting the acoustic transducer electrodes, the same arrangement can be used for deflecting the light since, with this arrangement, the acoustic field sets up a time-varying grating whose spatial frequency is set by the segment spacing. Theoretical frequency limitations on these devices do not appear to be important until approximately 30 GHz is reached. Thus, they are potentially useful for extremely wide-band data links. Experiments at 1.5 GHz show 30% bandwidth of acoustic modulation using optical heterodyne detection.     

Degradation of II-VI ZnSe-Based Single Quantum Well Light-Emitting Devices

Degradation of ZnSe-based light-emitting devices (light-emitting diodes and diode lasers) are reviewed. These devices quickly degrade (i.e., show a decrease in the amount of light emitted) during continuous operation at room temperature. The best lifetimes are currently only a few hours for cw diode laser operation. Degradation of ZnSe quantum-well devices are shown to correlate with the current density necessary for operation and with the density of preexisting defects. The temperature of the quantum well during operation has been shown to be >250°C. The decrease in light emission correlates with the development of dark spot defects (DSDs) and dark line defects (DLDs) in or near the active quantum-well region of the device. It is shown that stress in the quantum well is not relaxed until late in the degradation process, and then only partially. Instead, the mechanism of degradation is shown to be the injection of point defects due to nonradiative relaxation processes, which ultimately collapse into the DSDs and DLDs. Methods to reduce the degradation of these devices are discussed.