Harmonic surface acoustic waves on gallium nitride thin films

SAW devices operating at the fundamental frequency and the 5th, 7th, 9th, and 11th harmonics have been designed, fabricated, and measured. Devices were fabricated on GaN thin films on sapphire substrates, which were grown via metal organic vapor phase epitaxy (MOVPE). Operating frequencies of 230, 962, 1338, 1720, and 2100 MHz were achieved with devices that had a fundamental wavelength, /spl lambda/(0) = 20 /spl mu/m. Gigahertz operation is realized with relatively large interdigital transducers that do not require complicated submicrometer fabrication techniques. SAW devices fabricated on the GaN/sapphire bilayer have an anisotropic propagation when the wavelength is longer than the GaN film thickness. It is shown that for GaN thin films, where kh(GaN) > 10 (k = 2/spl pi///spl lambda/ and h(GaN) = GaN film thickness), effects of the substrate on the SAW propagation are eliminated. Bulk mode suppression at harmonic operation is also demonstrated.     

Fabrication of nanostructure and formation of nanocrystal for non-volatile memory

In this work, we have demonstrated that the nanocrystal created by combining the self-assembled block copolymer thin film with regular semiconductor processing can be applicable to non-volatile memory device with increased charge storage capacity over planar structures. Self-assembled block copolymer thin film for nanostructures with critical dimensions below photolithographic resolution limits has been used during all experiments. Nanoporous thin film from PS-b-PMMA diblock copolymer thin film with selective removal of PMMA domains was used to fabricate nanostructure and nanocrystal. We have also reported about surface morphologies and electrical properties of the nano-needle structure formed by RIE technique. The details of nanoscale pattern of the very uniform arrays using RIE are presented. We fabricated different surface structure of nanoscale using block copolymer. We also deposited Si-rich SiNx layer using ICP-CVD on the silicon surface of nanostructure. The deposited films were studied after annealing. PL studies demonstrated nanocrystal in Si-rich SiNx film on nanostructure of silicon.     

Direct laser writing of three-dimensional photonic-crystal templates for telecommunications

The past decade has witnessed intensive research efforts related to the design and fabrication of photonic crystals. These periodically structured dielectric materials can represent the optical analogue of semiconductor crystals, and provide a novel platform for the realization of integrated photonics. Despite intensive efforts, inexpensive fabrication techniques for large-scale three-dimensional photonic crystals of high enough quality, with photonic bandgaps at near-infrared frequencies, and built-in functional elements for telecommunication applications, have been elusive. Direct laser writing by multiphoton polymerization of a photoresist has emerged as a technique for the rapid, cheap and flexible fabrication of nanostructures for photonics. In 1999, so-called layer-by-layer or woodpile photonic crystals were fabricated with a fundamental stop band at 3.9 microm wavelength. In 2002, a corresponding 1.9 microm was achieved, but the important face-centred-cubic (f.c.c.) symmetry was abandoned. Importantly, fundamental stop bands or photonic bandgaps at telecommunication wavelengths have not been demonstrated. In this letter, we report the fabrication--through direct laser writing--and detailed characterization of high-quality large-scale f.c.c. layer-by-layer structures, with fundamental stop bands ranging from 1.3 to 1.7 microm.     

Combined AFM nano-machining and reactive ion etching to fabricate high aspect ratio structures

In this paper, a new combined method of sub-micron high aspect ratio structure fabrication is developed which can be used for production of nano imprint template. The process includes atomic force microscope (AFM) scratch nano-machining and reactive ion etching (RIE) fabrication. First, 40 nm aluminum film was deposited on the silicon substrate by magnetron sputtering, and then sub-micron grooves were fabricated on the aluminum film by nano scratch using AFM diamond tip. As aluminum film is a good mask for etching silicon, high aspect ratio structures were finally fabricated by RIE process. The fabricated structures were studied by SEM, which shows that the grooves are about 400 nm in width and 5 microm in depth. To obtain sub-micron scale groove structures on the aluminum film, experiments of nanomachining on aluminum films under various machining conditions were conducted. The depths of the grooves fabricated using different scratch loads were also studied by the AFM. The result shows that the material properties of the film/substrate are elastic-plastic following nearly a bilinear law with isotropic strain hardening. Combined AFM nanomachining and RIE process provides a relative lower cost nano fabrication technique than traditional e-beam lithography, and it has a good prospect in nano imprint template fabrication.     

Polyurea thin film ultrasonic transducers for nondestructive testing and medical imaging

Ultrasonic transducers using polyurea piezoelectric thin film are studied in this paper. Aromatic polyurea thin films, prepared by vapor deposition polymerization, have useful characteristics for use as an ultrasonic transducer. This paper presents the fabrication and experimental evaluation of ultrasonic transducers formed using polyurea films. First, the vapor deposition polymerization process using two monomers is briefly reviewed, and the temperature conditions for higher piezoelectric constants are explored. Second, in order to test the fundamental characteristics of this material as a high-frequency, ultrasonic transducer, a polyurea film of 2.5 microm thickness was deposited on a silicon substrate. In the pulse/echo experiment results, a resonant frequency of about 100 MHz was observed. Third, we fabricated a concave point focus transducer and a cylindrical line focus transducer. To examine the performances of the focus transducers, two-dimensional images of a coin and V(z) curve measurements for an aluminum surface were demonstrated.     

One-dimensional particle transmission in a statistically distributed potential field

In this study, wave functions are used to analyze the transmission of micro-scale particles throughout a sequence of one-dimensional potential barriers. This model can be used to simulate the motion of particles through a crystalline or non-crystalline thin film. Our focus is on the configuration of barriers which can be characterized by their heights, widths and lengths, and on their effects on the transmission probability. As a common phenomenon in crystalline structure, the resonant transmission can be found at classical forbidden energy levels and they form some energy bands. We further investigate the effects of voids and impurities on the transmission of particles by introducing variations to barrier heights. Results indicate that transmission can be weakened or reinforced by the presence of impurities or voids. In order to study more realistic structures, a statistical approach is introduced for us to imitate more variations on the barrier heights. This scheme allows us to study the transmission of particles through amorphous-like thin film. The quantitative results show that resonant energy bands can be significantly reduced by the irregular structures inside an amorphous-like film due to the destructive interference among waves.     

Fabrication of optical gratings by shrinkage of a rubber material

Ordered wavy surface structures generated by deposition of a metal thin film on a pre-stretched PDMS plate were fabricated and its potential application for optical gratings was proposed. The orientation of the generated structures was always perpendicular to the pre-stretched direction and the pitch of the structure could be adjusted ranging from 4.5 μm to 6.8 μm by controlling the strength of the pre-stretched strain and the thickness of the surface metal film. Based on these periodic structures, various optical gratings were demonstrated. With a slight modification of the fabrication scheme, gratings with different orientations can be fabricated on both sides of the PDMS plate, the double-sided gratings, could be fabricated. It is believed the current method has the potential for the fabrication of a large-scale diffractive grating at lower costs.     

Characterization of a mixed sphere and film waveguide at the 1-microm scale by 0.67-microm laser light propagation

A mixed sphere array and film (MSF) waveguide [a mixed system with a polystyrene microsphere array and a poly(methyl methacrytate) thin film waveguide] was fabricated and characterized at a wavelength of 0.67 microm. The attenuation coefficients of the isolated linear thin-film waveguide and the isolated linear microsphere array were measured to be 0.54 dB/microm and 0.98 dB/sphere, respectively. In the MSF waveguide the attenuation coefficients of the thin-film waveguide and the polystyrene microsphere array were 0.61 dB/microm and 1.17 dB/sphere, respectively. A curvilinear MSF waveguide was also fabricated and characterized. Evanescent wave interaction between the thin-film waveguide and the microsphere array was expected.     

Stretchable Thin‐Film Electrodes for Flexible Electronics with High Deformability and Stretchability

Flexible and stretchable electronics represent today's cutting‐edge electronic technologies. As the most‐fundamental component of electronics, the thin‐film electrode remains the research frontier due to its key role in the successful development of flexible and stretchable electronic devices. Stretchability, however, is generally more challenging to achieve than flexibility. Stretchable electronic devices demand, above all else, that the thin‐film electrodes have the capacity to absorb a large level of strain (>>1%) without obvious changes in their electrical performance. This article reviews the progress in strategies for obtaining highly stretchable thin‐film electrodes. Applications of stretchable thin‐film electrodes fabricated via these strategies are described. Some perspectives and challenges in this field are also put forward.     

Investigation of the scaling rules determining the performance of film bulk acoustic resonators operating as mass sensors

Solidly mounted (SMR-type) thin film bulk acoustic resonators operating at 2.2, 4.1, and 8.0 GHz and with lateral extents from 30 to 500 microm were fabricated and their performance as mass sensors was evaluated theoretically as well as experimentally. It was found that increasing the frequency leads to a principally improved performance of these devices. Problems arising for the horizontal as well as the vertical dimension and structure are investigated.     

基于柱状ZnO薄膜的超低阈值电压压敏电阻

利用磁控溅射法在玻璃衬底上制备了基于柱状ZnO薄膜的Al-ZnO-Al三明治结构的超低阈值电压的压敏电阻。XRD和SEM测试结果表明,该压敏电阻中的ZnO薄膜层为结晶性能良好,并且沿ZnO的(002)晶面择优取向生长的柱状薄膜。I-V测试结果表明,这种由柱状ZnO薄膜构成的压敏电阻阈值电压仅3.2 V,为现有压敏电阻中阈值电压最低的压敏电阻。     

Deformation characteristics of an organic thin film transistor

An organic thin film transistor (OTFT) on a flexible substrate with electroplated electrodes has many advantages in the fabrication of low cost sensors, e-paper, smart cards, and flexible displays. In this study, we simulated the mechanical characteristics of an OTFT with various compressive stress conditions using COMSOL. An analysis model, which was limited to channel, source, and drain, was used to investigate deformation and internal stress concentrations. The channel length is 40 microm and the OTFT structure is a top-contact structure. The OTFT was fabricated using pentacene as a semiconducting layer and electroplated Ni as a gate electrode. The deformation characteristics of the fabricated OTFT were predicted in terms of strain and internal stress.     

Micromachined thin film plate acoustic resonators utilizing the lowest order symmetric lamb wave mode

Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology     

GHz frequency ZnO/Si SAW device

The demand for high-frequency low-loss filters generates intensive research on innovative wave guide solutions. In this work, a GHz SAW device based on a ZnO/Si structure was fabricated using classical UV photolithography. The thickness of the piezoelectric thin film was optimized and a specific interdigital transducer structure was used to generate third and fifth harmonic guided waves at 2.5 GHz and 3.5 GHz, respectively, with an aluminum strip larger than 1 micrometer. Different modes have been measured and theoretically identified thanks to an advanced finite-element/boundary-elementbased model. Good agreement is found between theory and experiments. The high-frequency modes have been fully characterized, allowing for accurate design of SAW devices exploiting such modes.     

基于COMSOL的声表面波器件仿真

声表面波器件在通信、传感、射频识别等领域有着广泛的应用.以有限元方法为基础,利用有限元软件COMSOL对声表面波器件进行了仿真.从器件的模型建立入手,按由浅入深的顺序对无电极压电基片、压电基片表面沉积叉指换能器、叉指换能器表面溅射薄膜、薄膜上负载液体的4种结构进行了仿真分析.仿真研究表明:叉指换能器的电极效应会产生正、反特征频率,并且两种频率都随着叉指电极的敷金比与高度增加而向低频偏移;薄膜厚度的增加同样会导致器件频率向低频变化;当器件负载液体用于液体密度检测时,可通过器件频率变化对液体密度的灵敏程度来对薄膜厚度进行优化.其研究结果可以为声表面波器件的设计制作提供依据.     

Progressive failure modelling of woven carbon composite under impact

The design of composite structures or components, subject to extreme loading conditions, such as crash, blast, etc. requires a fundamental understanding of the deterioration mechanism within the composite meso-structure. Existing predictive techniques for the analysis of composite structures and components near and beyond their ultimate strength are either based on simple scalar stress functions, or use very complex damage formulations with many material constants, some of which may be difficult to characterise. This paper presents a simple damage mechanics based progressive failure model for thin woven carbon composites under impact loading. The approach is based on an unconventional thermodynamic maximum energy dissipation approach, which entails controlling damage evolution and hence energy dissipation per second, rather than damage. The method has been implemented into the explicit dynamic finite element code DYNA3D. Numerical simulation results using the proposed model are compared with two experimental impact tests.The analysis methodology proposed in this paper reflects a very simple, but effective technique that can be used to model a wide range of problems from extreme events, such as crash or blast, to birdstrike, when tearing and perforation are major failure mechanisms. As damage is cumulative, the technique allows initial or/and post-impact static loads to be applied to the composite structure or component, thus allowing a cradle-to-grave design methodology.     

Magnetron sputter deposition as visualized by Monte Carlo modeling

The Monte Carlo code SIMTRA, simulating the transport of atoms from the source to the substrate during physical vapor deposition (PVD), is used in several case studies to highlight important issues related to thin film sputter deposition. Atom collisions during gas-phase transport affect the energy distribution and the deposition profile of sputtered atoms. The model is compared with published models for the thermalization of sputtered atoms, and some features of this process are discussed. The vacuum chamber design can be easily implemented in the Monte Carlo code, and this possibility is used to discuss the use of shutters and masks, and the influence of the deposition geometry. The code can also be used to predict the composition when combing different sources, segmented targets, and during combinatorial synthesis of thin films. As the details of the transport are described, the velocity and the density of the gas-phase atoms can be calculated which can assist in the interpretation of several spectroscopic techniques such as laser induced fluorescence. Not only the energy loss of the transported atoms, but also their remaining energy upon arrival at the substrate is important as the incident energy strongly influences thin film growth. To illustrate the latter, the model is also used to study the growth of biaxially aligned thin films. The key parameters influencing the level of alignment can easily be retrieved using SIMTRA.     

A micro force sensor based on a single ZnO belt coated with chromium film

A micro force sensor was fabricated using a single ZnO belt coated with ultra thin Cr film. As a result of the piezoresistive effect of the ultra thin (in nano-scale) Cr film, the bending of the belt led to the change in the resistance of ultra thin Cr film. Based on the mechanics of the materials, the relationship between the deformation and the force was calculated, and a linear relationship between the bending force and the resistance of Cr thin film was deduced at small bending regions. Dielectrophoresis, focused ion beam (FIB) and sputtering were used in the process of the micro force sensor. The experimental results show that the resistance of Cr film is sensitive to the bending force and demonstrate the potential for developing a new class of stable and sensitive nano-sized structures for force sensing.     

Metal-covered photonic bandgap multilayer for infrared hollow waveguides

Infrared hollow waveguides utilizing a dielectric multilayer are examined by use of a photonic bandgap theory. It is shown that, in the waveguide consisting of quarter-wave film stack, the act of covering the dielectric films with a metal layer is effective in the reduction of the number of film layers. To verify the effect of this design, we fabricated a prototype waveguide with three dielectric layers of SiO2/Ta2O5/SiO2 and a silver layer by using a liquid-phase coating technique. From the loss spectrum of the fabricated waveguide, it is confirmed that, as designed, the waveguide shows wideband low-loss property at the wavelength of Nd:YAG laser light 1.06 microm.     

A photoconductor based on heavily Al-doped nano-ZnO thin film and its application to a chaos generator

Nano-ZnO thin film with heavy Al doping was deposited on Si substrate by RF magnetron sputtering, and a photoconductor was fabricated on the nano-ZnO thin film using Au as contact metal, producing with a metal–semiconductor–metal structure. The photoconductor was then applied to Chua's circuit through element replacement. Simulation results demonstrated that a triple-scroll optoelectronic chaotic attractor can be generated in the modified Chua's circuit.