Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

一种新的纸基微流开关及其活跃方法

提出了一种新的、基于声表面波的纸基微流开关。通过软光刻技术制作内含两个微孔的聚二甲基硅氧烷(PDMS)微架,其上固定经折叠、长度可变的纸通道。PDMS微架贴附于压电基片之上,并在待连接的两微通道之下方,折叠纸通道最低端离压电基片间距为2 mm。压电基片上采用微电子工艺光刻一对叉指换能器和反射栅。当足够强度的电信号加到叉指换能器对时,激发两相向声表面波,使得压电基片上微流体输运到折叠纸通道,改变其长度,连接其上待连通的两纸基微通道,完成开关功能。对可编程微流器件提供了一种新的编程和开关控制方法。     

Fabrication of the cyclical fluid channel using the surface acoustic wave actuator and continuous fluid pumping in the cyclical fluid channel

A surface acoustic wave (SAW) device has been reported as a micro fluid device such as a pump of a water droplet so far (Renaudin et al. in μTAS, pp 599–601, 2004, 1:551–553, 2005; Sritharan et al. in Appl Phys Lett 88:054102, 2006; Wixforth in Anal Bioanal Chem 379:982–991, 2004; Yamamoto et al. in μTAS, pp 1072–1074, 2005). The SAW device is an interdigital transducer (IDT) fabricated on the piezoelectric substrate only. IDTs are advantageous in terms of integration, miniaturization, free position setting on the substrate and simple fabrication process because of their simple structure. Therefore, the SAW device is easy to apply to integrated chemical system such as lab-on-a-chip. The SAW drives the liquid homogenously by the transmission of surface vibrations of the substrate. Thus, both ends of the channel for pressure loading are not necessary to pump the liquid by using the SAW. The SAW can pump the liquid in both of a closed channel and an opened channel, although continuous flow pumping using an external pump is difficult for no loading pressure in the closed fluid channel. In this paper, we proposed and fabricated the micro fluid devices combined cyclical fluid channel and SAW actuator for liquid pumping. This device is fabricated on a piezoelectric substrate (LiNbO₃) with UV photolithography and wet etching. Structure material of cyclical fluid channel is epoxy photoresist SU-8 100. Then, it is demonstrated to continuous flow pumping and reciprocal flow pumping in the channel. As a result of optimization of a SAW pump’s structural parameter, 32.5, 71.3 and 108.0 mm/s are achieved in the 500, 1,000 and 2,000 μm channel width as a maximum flow velocity.     

Polymeric microneedle fabrication using a microinjection molding technique

Polymeric microneedles fabricated by microinjection molding techniques have been demonstrated using Topas^®COC as the molding plastic material. Open-channel microneedles with cross-sectional area of 100 μm × 100 μm were designed and fabricated on top of a shank of 4.7 mm in length, 0.6 mm in width, and 0.5 mm in depth. The tip of the microneedle has a round shape with a radius of 125 μm as limited by the drill used in fabricating the mold insert. The injection molding parameters including clamping force, shot size, injection velocity, packing pressure, and temperature were characterized in order to achieve best reproducibility. Experimentally, a fabricated microneedle was successfully injected into a chicken leg and a beef liver freshly bought from a local supermarket and about 0.04 μL of liquid was drawn from these tissues immediately. This new technology allows mass production of microneedles at a low cost for potential biomedical applications.     

Fabrication of 1D nanochannels on thermoplastic substrates using microchannel compression

We present a new method to fabricate one-dimensional (1D) nanochannels on a thermoplastic substrate. This method has two main steps. First, a mold with microscale features is used to replicate microchannels on a thermoplastic substrate. Second, the fabricated microchannel is compressed to a 1D nanochannel at a temperature above the glass transition temperature (T_g) of the themoplastics. The effects of compression temperature, compression pressure, retaining time and loading rate on microchannel compression have been studied. Results have shown that a 1D nanochannel of 1–30 μm wide and 200–300 nm deep can be readily fabricated by using this method.     

基于两相向声表面波快速富集悬液中微粒

为提高生化分析灵敏度,提出了一种快速富集悬液中微粒的新方法.它在127.68°旋转Y切割X传播方向的LiNbO3基片上采用微电子工艺制作了2×2叉指换能器阵列,在该叉指换能器阵列的一对对角叉指换能器上同时加经功率放大器放大后的RF信号,以激发两相向声表面波,采用微量进样器将待富集的微流体(微液滴)进样到两相向传播的声路径上,微液滴中的微粒在该两相向的声表面波作用下快速向心富集.淀粉溶液微液滴富集实验结果表明,两相向声表面波作用下,10秒内实现微液滴中淀粉微粒的快速富集.     

Design,fabrication and testing of a piezoelectric energy microgenerator

This article presents the design, fabrication and characterization of a micromachined energy harvester utilizing aluminium nitride (AlN) as a piezoelectric thin film material for energy conversion of random vibrational excitations. The harvester was designed and fabricated using silicon micromachining technology where AlN is sandwiched between two electrodes on top of a silicon cantilever beam which is terminated by a silicon seismic mass. The harvester generates electric power when subjected to mechanical vibrations. The generated electrical response of the device was experimentally evaluated at various acceleration levels. A maximum power of 34.78 μW was obtained for the device with a seismic mass of 5.6 × 5.6 mm² at an acceleration value of 2 g. Various fabricated devices were tested and evaluated in terms of the generated electrical power as well as the resonant frequency.     

Fabrication of micro sloping structures of SU-8 by substrate penetration lithography

In this paper, three-dimensional (3D) micro sloping structures were fabricated by ordinary mask pattern and diffraction phenomenon. Especially, we fabricated the structures with SU-8 negative photoresist and substrate penetration lithography. In this method, exposure is performed arranging in order of a mask, a substrate and the SU-8 resist. There is a gap that is equal to the thickness of the substrate between resist and mask. In narrow slit of mask, resist is less exposed than usual because of Fraunhofer diffraction. The amount of exposure depends on slit width so that the height of SU-8 resist can be controlled. A 173 μm height of structure was obtained in the case of 27 μm width slit and 24.2 μm height of structure was obtained in the case of 7.4 μm width slit. By using this method, high aspect ratio 3D SU-8 structures with smooth sloping were fabricated in the length of 100–300 μm and in the height of 50–200 μm with rectangular triangle mask pattern. In the same way, there is influence of Fresnel diffraction on edge of aperture so that micro taper structures were fabricated. A lot of taper structures were fabricated by the method to make the surface repellency. The contact angle was achieved more than 160° in this study.     

Out-of-plane motion of 260-nm thick guided-mode resonant grating filter controlled by four micro-thermal actuators

We fabricate a wavelength-selective variable-reflection filter driven by four bimorph actuators to realize out-of-plane motion in parallel. The actuators are designed using a simple calculation model for controlling position of the filter anywhere within air gap fabricated in between the filter and silicon substrate. In order to keep a 260-nm thick filter flat, the filter and actuators are stabilized by torsion bars each with a width and length of 3 and 10 μm, respectively. Displacement, flatness and surface roughness of the fabricated filter are measured. As a result, 10.1 μm of the displacement is obtained at 24.7 mW input power using a 70-μm length actuator. Besides, the filter with a flat surface and roughness of 0.08 μm is obtained. Reflectivity of the filter consisting of 800-nm period grating is also measured.     

Driving characteristics of the electrowetting-on-dielectric device using atomic-layer-deposited aluminum oxide as the dielectric

Electrowetting on dielectric (EWOD) is useful in manipulating droplets for digital (droplet-based) microfluidics, but its high driving voltage over several tens of volts has been a barrier to overcome. This article presents the characteristics of EWOD device with aluminum oxide (Al₂O₃, ε _r  ≈ 10) deposited by atomic layer deposition (ALD), for the first time as the high-k dielectric for lowering the EWOD driving voltage substantially. The EWOD device of the single-plate configuration was fabricated by several steps for the control electrode array of 1 mm × 1 mm squares with 50 μm space, the dielectric layer of 1,270 Å thick ALD Al₂O₃, the reference electrode of 20 μm wide line electrode, and the hydrophobic surface treatment by Teflon-AF coating, respectively. We observed the movement of a 2 μl water droplet in an air environment, applying a voltage between one of the control electrodes and the reference electrode in contact with the droplet. The droplet velocity exponentially depending on the applied voltage below 15 V was obtained. The measured threshold voltage to move the droplet was as low as 3 V which is the lowest voltage reported so far in the EWOD researches. This result opens a possibility of manipulating droplets, without any surfactant or oil treatment, at only a few volts by EWOD using ALD Al₂O₃ as the dielectric.     

Oxide bound impact on hot-carrier degradation for gate electrode workfunction engineered (GEWE) silicon nanowire MOSFET

In this present work, we explore the hot carrier fidelity of gate electrode workfunction engineered silicon nanowire (GEWE-SiNW) MOSFET at 300 K using DEVEDIT-3D device editor and ATLAS device simulation software. TCAD simulation shows reduction in the hot carrier reliability of a GEWE SiNW MOSFET in terms of electron temperature, electron velocity and Hot Electron gate current for reflecting its efficacy in high power CMOS applications. Further, a comparative investigation for different values of oxide thickness and high-k has been done to analyze the performance of GEWE-SiNW MOSFET in terms of electrical parameters such as conduction band, DIBL, electric field, electron temperature, electric velocity and gate current. It has been clearly shown that with oxide thickness 0.5 nm the hot-carrier reliability and device performance improves in comparison to oxide thickness 2.5 nm. In addition, with k = 21(HfO₂) device performance in terms of hot-carrier reliability further enhanced due to increased capacitance and thus offer its effectiveness in sub-nm range analog applications.     

Low power highly sensitive platform for gas sensing application

This paper reports a low power miniaturized MEMS based integrated gas sensor with 36.84 % sensitivity (ΔR/R₀) for as low as 4 ppm (NH₃) gas concentration. Micro-heater based gas sensor device presented here consumes very low power (360 °C at 98 mW/mm²) with platinum (Pt) micro-heater. Low powered micro-heater is an essential component of the metal oxide based gas sensors which are portable and battery operated. These micro-heaters usually cover less than 5 % of the gas sensor chip area but they need to be thermally isolated from substrate, to reduce thermal losses. This paper elaborates on design aspects of micro fabricated low power gas sensor which includes ‘membrane design’ below the microheater; the ‘cavity-to-active area ratio’; effect of silicon thickness below the silicon dioxide membrane; etc. using FEM simulations and experimentation. The key issues pertaining to process modules like fragile wafer handling after bulk micro-machining; lift-off of platinum and sensing films for the realization of heater, inter-digitated-electrodes (IDE) and sensing film are dealt with in detail. Low power platinum microheater achieving 700 °C at 267 mW/mm² are fabricated. Temperature calculations are based on experimentally calculated thermal coefficient of resistance (TCR) and IR imaging. Temperature uniformity and localized heating is verified with infrared imaging. Reliability tests of the heater device show their ruggedness and repeatability. Stable heater temperature with standard deviation (σ) of 0.015 obtained during continuous powering for an hour. Cyclic ON–OFF test on the device indicate the ruggedness of the micro-heater. High sensitivity of the device for was observed for ammonia (NH₃), resulting in 40 % response for ~4 ppm gas concentration at 230 °C operating temperature.     

A polymer-metal hybrid flexible mould and application for large area hot roller embossing

In this paper, we report a novel approach to fabricate a low cost, large area and flexible mould and its applications in large area roller embossing. A liquid crystal polymer (LCP) film, which had a high glass transition temperature of 280°C, was clad with copper foils on both sides, was used as a starting material for mould fabrication. The LCP film and the copper foils were 50 and 36 μm thick, respectively. The LCP-Cu flexible mould was obtained through photolithographic patterning and wet etching of the copper foil on top surface of the LCP film. Using this proposed method, a polymer-metal hybrid flexible mould with an area of 150 mm × 150 mm was fabricated. The fabricated mould has a minimum feature size of 25 μm, and has been successfully used to demonstrate large area micro roller embossing. Micro channels, micro dots and micro mixers were embossed on polymeric as well as ceramic green substrates.     

A new inertial piezoelectric rotary actuator based on changing the normal pressure

A new inertial rotary actuator using two piezoelectric stacks as the power converter is proposed. The actuator rotates by regulating the friction force, which is controlled by the normal pressure, and driven by a square-wave or triangular-wave signal. Dynamic and simulation models of the actuator are established to evaluate the performance of the actuator in terms of distance, speed, radial run-out and positioning repeatability. A 40 mm wide and 32 mm high prototype was compared with the simulated model. The results show that the angular speed is proportional to the amplitude and frequency of the driving signal. A 10 μrad angular displacement resolution is obtained by applying a of 15 V_p–p square wave with a frequency of 10 Hz.     

An experimental study on an electro-osmotic flow-based silicon heat spreader

An experimental study has been carried out to estimate the heat transfer improvement offered by a novel electro-osmotic (EO) heat spreader for microprocessor cooling. The proposed design of the elliptical silicon heat spreader can be fabricated at the back surface of the microprocessor as an integral part. Thus, no extra space may be required. The EO heat spreader developed is 0.6 mm³ in volume and it contains a pair of thin gold film electrodes of approximately 1 μm thickness for applying an external electric field that induces electro-osmotic flow. The inner channel surfaces of the heat spreader are electrically insulated with a thin SiO₂ layer to minimize current leakage into the wafer. The EO heat spreader constructed is able to generate a flow rate of 0.2028 μl/min at 2 V/mm. With this heat spreader, the temperature of a heat source may be reduced by up to 4°C without the aid of any external mechanical devices. The heat spreader has the potential to make the temperature uniform, if the heat source is non-uniform in nature.     

MEMS design and fabrication of an electrostatic vibration-to-electricity energy converter

This paper presents a micro electrostatic vibration-to-electricity energy converter based on the micro-electromechanical system. For the 3.3 V supply voltage and 1 cm² chip area constraints, optimal design parameters were found from theoretical calculation and Simulink simulation. In the current design, the output power is 200 μW/cm² for the optimal load of 8 MΩ. The device was fabricated in a silicon-on-insulator wafer. Mechanical and electrical measurements were conducted. Residual particles caused shortage of the variable capacitor and the output power could not be measured. Fabrication processes are being refined to remove the back silicon substrate to eliminate residual particles and parasitic capacitance.     

Micro-PIV measurements of induced-charge electro-osmosis around a metal rod

A cylindrical gold-coated stainless steel rod was positioned at the center of a straight microchannel connecting two fluid reservoirs on either end. The microchannel was filled with 1 mM KCl containing 0.5 μm diameter carboxylate-modified spherical particles. Induced-charge electro-osmotic (ICEO) flow occurred around the metallic rod under a sinusoidal AC electric field applied using two platinum electrodes. The ICEO flows around the metallic rod were measured using micro particle image velocimetry (micro-PIV) technique as functions of the AC electric field strength and frequency. The present study provides experimental data about ICEO flow in the weakly nonlinear limit of thin double layers, in which, the charging dynamics of the double layer cannot be presented analytically. The measured ICEO flow pattern qualitatively agrees with the theoretical results obtained by Squires and Bazant (J Fluid Mech 509:217–252, 2004). Flow around the rod is quadrupolar, driving liquid towards the rod along the electric field and forcing it away from the rod in the direction perpendicular to the imposed electric field. The measured ICEO flow velocity is proportional to the square of the electric field strength, and depends on the applied AC frequency.     

Investigation on force transmission of direct-drive thorax unit with four ultrasonic motors for a flapping microaerial vehicle

We fabricated a trial version of a thorax unit with four ultrasonic motors (USMs) to simulate a dragonfly-scale flapping micro aerial vehicle (MAV). Each wing was directly driven by a two-degree-of-freedom (2-DOF) transmission. An in-house tiny standing-wave USM capable of bidirectional rotation, which weighs just 0.13 g, was employed on trial. The transmission of the thorax unit converts the two USM rotations into strokes and flip motions of the wing. By implementing two 70-mm-long wings, we fabricated a prototype of a 4-DOF MAV and tested its performance. In a lift-compensated situation, upward, forward, and backward movements of the MAV were obtained. The flapping angular velocity was discussed based on quasi-static wing aerodynamics and was accountable for the motor power. Although the power of the USM should be improved, the quick wing drivability, adequate power transmission on the thorax unit, and potential of a 0.2 W motor power in a unidirectional-type USM promise the viability of a direct-drive multi-DOF dragonfly-scale MAV.     

Measurements of microbubble generation process in microchannel using ultra high-speed micro-PTV system

Ultra high-speed micron-resolution particle tracking velocimetry (UH-μPTV) technique has been developed to advance the novel method to generate microbubbles using a T-shaped microchannel. The method can produce microbubbles with 10-μm order diameter by applying the gas pressure of several tens of kilopascal and injecting the deionized water with the speed of a few meters per second. The conventional μPTV was restricted to the velocity measurement of the order of millimeter per second due to a few kilohertz frame rate CMOS camera. On the other hand, the present UH-μPTV technique achieves to measure the liquid velocity of the order of meter per second by combining the bright-field microscopy and the ultra high-speed camera with 1 MHz frame rate. For improving the spatial resolution, the phase sampling method has been introduced and results in 10 velocity vectors in 20 μm × 20 μm area. The validation of the velocity measurement using UH-μPTV has been conducted through the comparison with the theoretical solution, and it has been shown that the proposed technique can capture the velocity vector field higher than 1 m/s. Furthermore, from the 1-μs time-series imaging, the microbubble generation process has been classified into two stages: the intruding stage and the growing stage. It has been shown that the bubble diameter becomes smaller by increasing the liquid velocity with reducing the period of the growing stage. In addition, from the velocity-vector maps, the normal components of velocities to the gas–liquid interface in the intruding stage are compared with those in the growing stage, and it has been observed that the velocity amplitudes in the growing stage are much larger than those in the intruding stage. This fact suggests that the high-speed liquid flow normal to the gas–liquid interface plays an important role in microbubble generation process.     

Micro-ball lens array fabrication in photoresist using PTFE hydrophobic effect

This paper presents a simple method for fabricating a micro-ball lens and its array. The core technology involves the hydrophobic characteristics of polyterafluoroethylene (PTFE) substrate. High contact angle between the melted photoresist pattern and PTFE generates the micro-ball lens and array. The PTFE thin film is spun onto a silicon wafer and oven dried. Photoresist AZ4620 is used to pattern micro-columns with various diameters; 60, 70 and 80 μm. A thermal reflow process is then applied to melt these micro-column patterns into a micro-ball lens array. The achieved micro-ball lens array has a diameter of 98 μm fabricated using 80 μm diameter patterns. This method provides a simple fabrication process and low material cost.