Fabricating HARMS by using megasonic assisted electroforming

Electroforming is a main fabrication method to produce metal microstructures for microelectromechanical systems. Since the mass transport is limited in high aspect ratio microstructures (HARMS), problems such as uneven filling and void formation are often involved in the electroplating of deep molds. In this work, megasonic agitation is employed to improve the electroplating of HARMS, which causes little damage to the resist structure and is thereby advantageous over ultrasonic agitation. Cantilever beams and gaps with a linewidth of 10 μm and an aspect ratio of 25 have been obtained. The main effects of megasonic agitation are analyzed. The results indicate that resonant bubbles in the electrolyte are probably responsible for the mass transport enhancement during the electroforming process.     

Modeling acoustic agitation for enhanced development of PMMA resists

  Although acoustic agitation is known to enhance the development rate of LIGA resists, the operative physical mechanism is not well understood. Analytical and numerical models presented here suggest that the observed enhancement results from steady acoustic streaming and associated polymer fragment transport induced by the high frequency sound waves. The computed steady flow within the feature is torroidal with downflow along the feature walls and upflow in the center. The numerically calculated enhancement of fragment transport is shown to be well approximated by a closed-form expression based on a pair of asymptotic formulas for weak and strong agitation. These transport models are combined with a model of PMMA dissolution kinetics and used to predict development rates for a range of LIGA feature sizes. The results are in good agreement with experimental data from Forschungszentrum Karlsruhe. Received: 10 August 2001/Accepted: 24 September 2001 This work was supported by the Sandia Materials Science Research Foundation and by the Accelerated Strategic Computing Initiative. Sandia is a multiprogram laboratory operated by Sandia Corporation for the United States Department of Energy under contract DE-AC04-94AL85000. This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.     

Comparing methods for the modelling of boundary-driven streaming in acoustofluidic devices

Numerical simulations of acoustic streaming flows can be used not only to explain the complex phenomena observed in acoustofluidic manipulation devices, but also to predict and optimise their performances. In this paper, two numerical methods based on perturbation theory are compared in order to demonstrate their viability and applicability for modelling boundary-driven streaming flows in acoustofluidic systems. It was found that the Reynolds stress method, which predicts the streaming fields from their driving terms, can effectively resolve both the inner and outer streaming fields and can be used to demonstrate the driving mechanisms of a broad range of boundary-driven streaming flows. However, computational efficiency typically limits its useful application to two-dimensional models. We highlight the close relationship between the classical boundary-driven streaming vortices and the rotationality of the Reynolds stress force field. The limiting velocity method, which ignores the acoustic boundary layer and solves the outer streaming fields by applying the ‘limiting velocities’ as boundary conditions, is more computationally efficient and can be used for predicting three-dimensional outer streaming fields and provide insight into their origins, provided that the radius of curvature of the channel surfaces is much greater than the acoustic boundary layer thickness (\(\delta_{v}\)). We also show that for the limiting velocity method to be valid the channel scales must exceed a value of approximately 100 \(\delta_{v}\) (for an error of ~5% on the streaming velocity magnitudes) for the case presented in this paper. Comparisons of these two numerical methods can provide effective guidance for researchers in the field of acoustofluidics on choosing appropriate methods to predict boundary-driven streaming fields in the design of acoustofluidic particle manipulation devices.     

Micromixing via recirculatory flow generated by an oscillatory microplate

Mixing in micro-environment has been explored in a number of studies. This study presents a unique approach of efficient mixing of two heterogeneous streams via two counter-rotating recirculatory streams induced by in-plane resonance of a rectangular microplate actuated via Lorentz force. The generated time-mean flow structure was interrogated for mixing efficacy over a range of excitation voltage, Reynolds number, and Pèclet number, along with numerical analysis to probe the time-mean flow physics. Results show that the recirculatory flow is generated at the plate’s edges due to local flow non-linearity, characteristic of acoustic streaming. The percentage of mixing, at one device length-scale downstream, attains 93% at a low Reynolds number of 0.0037 (based on mean velocity of 0.078 mm/s and channel height of 50 μm) at 8 V excitation. Further characterization via enhanced diffusivity shows a maximum of 80.7-fold increase. Comparison with other active mixers shows the current device achieves mixing in one of the shortest distances. The proposed approach is robust, tunable to attain desired mixing characteristics and essentially independent of the properties of the fluid medium, which should be useful in a number of microfluidic applications requiring fast mixing.     

Using megasonic development of SU-8 to yield ultra-high aspect ratio microstructures with UV lithography

SU-8 resist is commonly used to produce high aspect ratio microstructures in MEMS field. The resist is patterned using either ultraviolet (UV) or X-ray lithography and developed to produce structures ranging from 20 microns to 1.5 millimeters in height. Three processes are currently used to develop SU-8 resist: dip development with or without stirring and dip development with ultrasonic agitation. One difficulty associated with producing high aspect ratio microstructures lies in the development of open and closed fields evenly on the same substrate. To overcome this obstacle, we have applied megasonic agitation to the developer bath, which resulted in faster development rates, uniformed development, and the ability to produce structures with higher aspect ratios. To date, this process has been used to achieve 100:1 aspect ratio open field features and 45:1 intact cylinder arrays using a broadband optical contact aligner.The research work reported in this paper was made possible by support from National Science Foundation under grant ECS-#0104327; Louisiana Board of Regents, National Aeronautics and Space Administration, and the Louisiana Space Consortium under agreement NASA/LEQSF (2001–2005)-LaSPACE, and NASA/LaSPACE under grant NTG5–40115. The authors would also like to thank the Center for Advanced Microstructures and Devices of LSU for the use of the cleanroom facility.     

Optimal 3GPP packet-switched streaming service (PSS) over GPRS networks

3GPP packet-switched streaming service (PSS) is a standardized packet-based mobile streaming service, which is based on IETF RTSP/SDP standards. PSS can be implemented over GPRS networks; however these cannot usually guarantee any data rates or delay bounds, but allow sufficient bandwidth for mobile streaming. GPRS cell reselections pose additional challenges for streaming, since several seconds of data transmission breaks may occur, and data may even be lost. The level of error protection of GPRS is good enough for mobile streaming, if the correct quality of service (QoS) profile is configured. In this paper, we study the effect of different QoS parameters configurations to find optimal values for PSS over GPRS. The paper shows also a method for optimizing cell reselection management at the application layer, in order to provide seamless mobility. Results show that despite all the limitations of a GPRS environment, PSS is feasible with a decent quality of service.

Electrostatically driven synthetic microjet arrays as a propulsion method for micro flight

A propulsion system based on acoustic streaming generated by Helmholtz resonators is presented. High frequency (>60 kHz) electrostatically driven micromachined Helmholtz resonators constitute the basic unit of the system. Microjets produced at the exit of these resonators can be combined to form a distributed propulsion system. A high yield (>85%) fabrication process is introduced for fabrication of individual as well as arrays of resonators. The fabrication results for ten different designs are presented. About 1000 resonators of similar design cover the surface of a 4-in. wafer, effectively converting it to a distributed propulsion system. A number of characterization methods such as monitoring the harmonics of the drive current, laser interferometry, hot-wire anemometry, acoustic spectrum measurement and video particle imaging are used to determine the structural and fluidic behavior of different resonator designs. Collapse and recovery times of the diaphragm in the electrostatic actuator of the resonator are characterized and reduced to less than 10 s by optimizing the perforation design. The occurrence of acoustic streaming in the micron-scale is verified via video particle imaging. The jet streams produced with pulse drive at low frequencies (~1 kHz) are spatially profiled and jet velocities exceeding 1 m/s are measured at the exit of the resonators. It has been verified that the resonance frequencies of the device at 50 and 175 kHz can be closely predicted by modeling.     

High-throughput micromixers based on acoustic streaming induced by surface acoustic wave

Flow characteristics in microfluidic devices is naturally laminar due to the small channel dimensions. Mixing based on molecular diffusion is generally poor. In this article, we report the fabrication and characterization of active surface-acousticwave-driven micromixers which exploit the acoustic streaming effect to significantly improve the mixing efficiency. A side-by-side flow of water and fluorescent dye solution was driven by a syringe pump. Surface wave with a frequency of 13 MHz was launched perpendicular to the flow. The wave was generated by two designs of interdigitated electrodes on LiNbO₃ substrate: parallel electrodes and focusing electrodes. The mixing efficiency was observed to be proportional to the square of the applied voltage. Under the same applied voltage, the focusing type offers a better mixing efficiency. The fabrication of the micromixer is compatible to current technology such as soft lithography and deep reactive ion etching. Despite the high throughput and fast mixing time, the mixer design is simple and could be integrated into any microfluidic platform.     

Objective quality definition of scalable video coding and its application for optimal streaming of FGS-coded videos

With the proliferation of video contents widely distributed over the Internet and the progress of video coding (e.g., H.264/AVC) and transmission technologies, more challenges need to overcome in order to meet the requirements of all users with diverse terminals. Video streaming over IP and wireless becomes a popular issue since the new century.However, there is little work concerning the quantitative analysis on the objective quality of streaming videos. Thus a strict definition of the objective quality and quality variation of scalable video coding (SVC) is required, in order to efficiently transmit video contents over Internet and wireless and reach an attainable subjective quality perception for end-users. Since FGS (fine granularity scalability) video is coded in bit planes, its enhancing layer can be truncated arbitrarily, as a case study of scalable video coding, an objective quality definition for FGS-coded video is introduced in this paper, based on MSE (mean square error) and PSNR (peak signal-to-noise ratio). This definition can also be generalized to any layered scalable coding videos, such as the traditional layered videos in BL + ELs (base layer + enhancing layers) formats or H.264/AVC in BL + CGS (coarse granularity scalability) + FGS structures, and it can be applied to design optimal algorithms for video streaming. Furthermore, It can also be taken as a measure to assess the subjective quality of streaming videos, by incorporating user preferences and terminal capacities.According to this definition, a quality optimal problem of scene in video segments is formulated and solved using the state transfer graph and dynamic programming. The optimal transmission policy is also obtained and compared with a real-time transmission algorithm. Different aggregation levels (segmentation granularity) of video segments for optimal transmission are also examined by experimental data. Simulation results validate our observations.     

Eckart acoustic streaming in a heptagonal chamber by multiple acoustic transducers

A new computational method was developed to simulate a two-dimensional Eckart acoustic streaming field in an ultrasonic heptagonal chamber actuated by multiple acoustic transducers with different associated frequencies and acoustic incident pressures. Simulation was conducted using the superposition of multiple spatial gradients of the Reynolds stresses and the second mean sound pressures at different frequencies. The developed method extends beyond the capabilities of the conventional method that is restricted to uniform frequency and incident pressure. Various acoustic streaming patterns can be feasibly generated by tuning the frequency and incident pressure of each individual transducer. The implementation of multiple acoustic transducers offers flexibility to control acoustic flows in microfluidic devices for various applications. Furthermore, the developed simulation method for acoustic streaming fields provides an optimization tool for the frequency, incident pressure and location of each transducer.     

Development of temperature-control system for liquid droplet using surface Acoustic wave devices

In this paper, we present a liquid-droplet-heating system using a surface acoustic wave (SAW) device. When liquid is placed on a Rayleigh-SAW-propagating surface, a longitudinal wave is radiated into the liquid. If the SAW amplitude increases, the liquid shows non-linear dynamics, such as vibrating, streaming, small droplet flying, and atomizing. This phenomenon is well known as SAW streaming. The liquid temperature is measured during the longitudinal wave radiation and found to increase. First, the mechanism of the liquid-heating effect is discussed on the basis of experimental results. The surface electrical condition is changed to investigate the effect of dielectric heating. The obtained results indicate that the radiated longitudinal wave causes liquid heating and the dielectric heating effect does not. Second, the fundamental properties of the liquid temperature are measured by varying the applied voltage, duty factor, and liquid viscosity. The liquid temperature is found to be proportional to the duty factor and the square of the applied voltage. Therefore, the liquid temperature can be controlled by these applied signals. Also, by using highly viscous solutions, the liquid temperature is increased to more than 100 °C. Moreover, for chemical applications, the possibility of periodic temperature control is tested by varying the duty factor. The obtained results strongly suggest that an efficient thermal cycler is realized. A novel application of the SAW device is proposed on the basis of SAW streaming.     

微流体装置中超声流的研究

应用流体动力学的基本理论,建立起声流场的一维解析模型,对声波在管道内部传播时产生的声场和声流运动进行探讨。然后提出并设计了一种利用PZT激励微管道振动产生超声场的新型微流体装置。该装置为由微管道相连接的两个微腔体,并利用有限元法对超声波激励的微管道和腔体进行瞬态的流场数值分析,得到一阶声压和流场的速度分布结果,最后对数值结果进行处理并讨论了超声波在管道内传播时产生的声流速度以及驱动力的分布情况。     

Electrodeposition of copper into high aspect ratio PCB micro-via using megasonic agitation

This paper presents the use of micro-particle imaging velocimetry (micro-PIV) to analyse fluid flow and hence ion replenishment in PCB micro-via during the electroplating process. The cross section of a PCB via is fabricated in PMMA to allow optical access to the sample. Fluid flow within two 1:1 aspect ratio blind micro-vias, one with straight side walls and the other with tapered side walls were compared. Flow is also analysed in a 1:1 aspect ratio through via. Flow rates measured using micro-PIV are used to validate simulated flow models. The results show that there are increased flow rates within the blind via with tapered side walls. This goes some way to explaining the improved electroplating results obtained in industry when tapered vias are used. Initial experimental results using megasonic streaming to remove bubbles from blind micro-via and promote ion transportation within high aspect ratio PCB micro-via to enhance electrodeposition are also reported.     

Microvias: the next generation of substrates and packages

Shrinking traces and spaces combined with a rising layer count eventually enlarge density, but most fabricators merely choose to shrink via size. Vias (conductive vertical paths or holes in any substrate) were previously drilled mechanically. With the prevalence of lasers, however, the focus has turned to microvias or build-up technologies. Microvias (miniature holes smaller than 150 μm in diameter) enable connections between different layers of a PCB. But how do microvias affect substrate or integrated circuit packaging? The author believes that this innovative technology will revolutionize PCB technology-a point he demonstrates by focusing on how microvias are made and their potential impact on PCB technology. Microvias will dominate the next generation of electronic products because they provide for increased density on a smaller substrate     

Separation of particles using acoustic streaming and radiation forces in an open microfluidic channel

In this study, a method to separate particles, within a small sample, based on size is demonstrated using ultrasonic actuation. This is achieved in a fluid, which has been deposited on a flat surface and is contained by a channel, such that it has a rectangular wetted area. The system utilises acoustic radiation forces (ARFs) and acoustic streaming. The force field generates two types of stable collection locations, a lower one within the liquid suspension medium and an upper one at the liquid–air interface. Acoustic streaming selectively delivers smaller particles from the lower locations to the upper ones. Experimental data demonstrate the ability to separate two sets of polystyrene microparticles, with diameters of 3 and 10 μm, into different stable locations. Methods to reduce migration of larger particles to the free surface are also investigated, thereby maximising the efficiency of the separation. Extraction of one set of 99 % pure particles at the liquid–air interface from the initial particle mixture using a manual pipette is demonstrated here. In addition, computational modelling performed suggests the critical separation size can be tuned by scaling the size of the system to alter which of ARFs and acoustic streaming-induced drag forces is dominant for given particle sizes, therefore presenting an approach to tunable particle separation system based on size.     

An indexing method for wireless broadcast XML data

The paper considers a wireless information system, wherein various pieces of information represented in XML are broadcast via wireless channels, and mobile clients access the broadcast stream using energy-restricted portable devices.In this paper, we propose a wireless XML streaming method designed to provide energy-efficient access to a wireless stream. We construct two hierarchical structures to represent the XML data and their index information, called the XML data tree and XML index tree, respectively. The wireless XML stream is generated by traversing these two structures with some replications. We design three data/index replication strategies (PP, TT, and TP) in the streaming method. We compare the proposed streaming method with a naı¨ve method called the (1, X) method both analytically and experimentally. Also, based on our analysis results, we determine the optimal method of replication.     

Throughput optimization for video streaming proxy servers based on video staging

A video streaming proxy server needs to handle hundreds of simultaneous connections between media servers and clients. Inside, every video arrived at the server and delivered from it follows a specific arrival and delivery schedule. While arrival schedules compete for incoming network bandwidth, delivery schedules compete for outgoing network bandwidth. As a result, a proxy server has to provide sufficient buffer and disk cache for storage, together with memory space, disk space and disk bandwidth. In order to optimize the throughput, a proxy server has to govern the usage of these resources. In this paper, we first analyze the property of a traditional smoothing algorithm and a video staging algorithm. Then we develop, based on the smoothing algorithm, a video staging algorithm for video streaming proxy servers. This algorithm allows us to devise an arrival schedule based on the delivery schedule. Under this arrival and delivery schedule pair, we can achieve a better resource utilization rate gracefully between different parameter sets. It is also interesting to note that the usage of the resources such as network bandwidth, disk bandwidth and memory space becomes interchangeable. It provides the basis for inter-resource scheduling to further improve the throughput of a video streaming proxy server system.

Electrostatically driven synthetic microjet arrays as a propulsion method for micro flight

A novel propulsion method suitable for micromachining is presented that takes advantage of Helmholtz resonance, acoustic streaming, and eventually flow entrainment and thrust augmentation. In this method, an intense acoustic field is created inside the cavity of a Helmholtz resonator. Flow velocities at the resonator throat are amplified by the resonator and create a jet stream due to acoustic streaming. These jets are used to form a propulsion system. In this paper a system hierarchy incorporating the new method is described and the relevant governing equations for the Helmholtz resonator operation and acoustic streaming are derived. These equations can predict various device parameters such as cavity pressure amplitude, exit jet velocity and generated thrust. In a sample embodiment, an electrostatic actuator is used for generation of the initial acoustic field. The relevant design parameters for the actuator are discussed and an equivalent circuit model is synthesized for the device operation. The circuit model can predict the lowest order system resonance frequencies and the small signal energy conversion efficiency. A representative resonator performance is simulated and it is shown that velocities above 16 m/s are expected at jet nozzles. The calculated delivered thrust by this resonator with 0.7 m diaphragm displacement amplitude is 3.3 N at the resonance frequency.     

Algorithms for acoustic localization based on microphone array in service robotics

This paper deals with the development of acoustic source localization algorithms for service robots working in real conditions. One of the main utilizations of these algorithms in a mobile robot is that the robot can localize a human operator and eventually interact with him/herself by means of verbal commands. The location of a speaking operator is detected with a microphone array based algorithm; localization information is passed to a navigation module which sets up a navigation mission using knowledge of the environment map. In fact, the system we have developed aims at integrating acoustic, odometric and collision sensors with the mobile robot control architecture. Good performance with real acoustic data have been obtained using neural network approach with spectral subtraction and a noise robust voice activity detector. The experiments show that the average absolute localization error is about 40 cm at 0 dB and about 10 cm at 10 dB of SNR for the named localization. Experimental results describing mobile robot performance in a talker following task are reported.     

A survey on peer-to-peer video streaming systems

Video-over-IP applications have recently attracted a large number of users on the Internet. Traditional client-server based video streaming solutions incur expensive bandwidth provision cost on the server. Peer-to-Peer (P2P) networking is a new paradigm to build distributed network applications. Recently, several P2P streaming systems have been deployed to provide live and on-demand video streaming services on the Internet at low server cost. In this paper, we provide a survey on the existing P2P solutions for live and on-demand video streaming. Representative P2P streaming systems, including tree, multi-tree and mesh based systems are introduced. We describe the challenges and solutions of providing live and on-demand video streaming in P2P environment. Open research issues on P2P video streaming are also discussed.