Design and fabrication of microfluidic devices integrated with an open‐ended MEMS probe for single‐cell impedance measurement

When an electrical current with a low frequency is applied to a cell, the current passes through the outside of the cell. Thus, impedance measurements at low frequencies cannot be used to determine the pathological change of the cellular organelle taking place inside the cell. However, increasing the frequency of the electrical current makes the capacitive impedance of the cell decrease, allowing the electrical current to flow through the cell. This study presents the design and fabrication of a microfluidic device integrated with a coplanar waveguide open-ended micro-electro-mechanical-systems (MEMS) probe for the impedance measurement of the single HeLa cell in frequencies between 1 MHz and 1 GHz. The device includes a poly-dimethlysiloxane (PDMS) cover with a microchannel and microstructures to capture the single HeLa cell and a conductor-backed CPW fabricated using a silicon chip and two printed circuit boards (PCB). The effects of the substrate on the characteristic impedance of the conductor-backed coplanar waveguide (CBCPW) structure were investigated under three conditions by utilizing a time-domain reflectometer (TDR). Finally, impedance measurements using the proposed device and a vector network analyzer (VNA) are demonstrated for de-ionized (DI) water, alcohol, PBS, and a single HeLa cell.     

MEMS器件真空封装模型模拟

结合典型的MEMS器件真空封装工艺,应用真空物理的相关理论,建立了MEMS器件真空封装的数学物理模型,确定了其数值模拟算法。据此,对一封装示例进行了计算,获得了真空回流炉内干燥箱及密封腔体真空度的变化情况,实现了MEMS器件真空封装工艺过程的参数化建模与模拟。     

Damage and failure in silicon-glass-metal microfluidic joints for high-pressure MEMS devices

The design, fabrication, and testing of microfluidic joints consisting of Kovar metal tubes attached to silicon using borosilicate glass for high pressure microelectromechanical systems devices are presented. The MIT microrocket, which requires microfluidic joints to sustain pressures of at least 12.7 MPa and temperatures in excess of 700 K, is used to demonstrate the feasibility of the glass sealing methodology. A key concern in such joints is the occurrence of cracks due to residual stresses during fabrication, which can affect the load-carrying capability. To obtain a better understanding of the damage and failure characteristics, a hierarchical approach was taken. First, two types of joint configurations with several glass compositions and geometries were considered at the joint-level. Axial tension and pressure tests were performed, and finite element models were used to obtain the residual stress field and to predict failure loads based on linear elastic fracture mechanics. Subsequently, tests were performed on actual and dummy microrockets to validate the methodology at the device-level. Key observations include the importance of bonding between the Kovar tube and the silicon sidewall, which can help increase joint strength, and the detrimental effects of joint proximity under differential pressure loading and manufacturing defects in multiple joint specimens. In addition to specific experimental and analyses results that allow a physical understanding of the damage and failure mechanisms, another key contribution of this work is the overall insight of the design and analysis of reliable glass-sealed microfluidic packages. This insight will help one make better design and process selections for packages in other high-pressure silicon-based MEMS applications.     

Selection of high strength encapsulant for MEMS devices undergoing high-pressure packaging

Deflection behavior of several encapsulant materials under uniform pressure was studied to determine the best outer encapsulant for MEMS device. Encapsulation is needed to protect movable parts of MEMS devices during high-pressure transfer molded packaging process. The selected outer encapsulant material has to have surface deflection of less than 5 μm under 100 atm vertical loading. Deflection was simulated using Coventorware ver.2005 software and verified with calculation results obtained using shell bending theory. Screening design was used to construct a systematic approach for selecting the best encapsulant material and thickness under uniform pressure up to 100 atm. Materials considered for this study were SMC polyimide, liquid crystal polymer (LCP) carbon fiber and polyphenylene sulfide (PPS) high modulus carbon fiber. It was observed that PPS high modulus carbon fiber has deflection of less than 5 μm for all thickness and pressure variations. LCP carbon fiber is acceptable and SMC polyimide is unsuitable as high strength encapsulant. PPS high modulus carbon fiber is considered the best encapsulation material for MEMS under high-pressure packaging process due to its high strength. The generalized mathematical model and equations developed for predicting deflection of encapsulation under uniform loading could be used to determine the suitability of any candidate material and encapsulation design with similar domed shaped structure.     

Integration of impedance spectroscopy sensors in a digital microfluidic platform

Digital microfluidics combines the advantages of a low consumption of reagents with a high flexibility of processing fluid samples. For applications in life sciences not only the processing but also the characterization of fluids is crucial. In this contribution, a microfluidic platform, combining the actuation principle of electrowetting on dielectrics for droplet manipulations and the sensor principle of impedance spectroscopy for the characterization of the fluid composition and condition, is presented. The fabrication process of the microfluidic platform comprises physical vapor deposition and structuring of the metal electrodes onto a substrate, the deposition of a dielectric isolator and a hydrophobic top coating. The key advantage of this microfluidic chip is the common electric nature of the sensor and the actuation principle. This allows for fabricating digital microfluidic devices with a minimal number of process steps. Multiple measurements on fluids of different composition (including rigid particles) and of different conditions (temperature, sedimentation) were performed and process parameters were monitored online. These sample applications demonstrate the versatile applications of this combined technology.     

MEMS器件热致封装效应的解析建模研究

由封装结构热失配引入的结构变形和应力将对MEMS器件性能产生显著影响,即热致封装效应.基于单元库法思想构建了封装后MEMS器件的解析模型.新建锚区、芯片、封装基板等标准单元,利用节点分析法描述了各结构单元间的热弹性耦合行为,以此作为封装效应评估的基础.利用该模型计算了热致封装效应对常规芯片粘接封装的双端固支梁器件主要性能参数的影响.     

Fabrication and testing of embedded microvalves within PMMA microfluidic devices

The integration of a PDMS membrane within orthogonally placed PMMA microfluidic channels enables the pneumatic actuation of valves within bonded PMMA–PDMS–PMMA multilayer devices. Here, surface functionalization of PMMA substrates via acid catalyzed hydrolysis and air plasma corona treatment were investigated as possible techniques to permanently bond PMMA microfluidic channels to PDMS surfaces. FTIR and water contact angle analysis of functionalized PMMA substrates showed that air plasma corona treatment was most effective in inducing PMMA hydrophilicity. Subsequent fluidic tests showed that air plasma modified and bonded PMMA multilayer devices could withstand fluid leakage at an operational flow rate of 9 μl/min. The pneumatic actuation of the embedded PDMS membrane was observed through optical microscopy and an electrical resistance based technique. PDMS membrane actuation occurred at pneumatic pressures of as low as 10 kPa and complete valving occurred at 14 kPa for ~100 μm by 100 μm channel cross-sections.     

Fabrication of polyimide microfluidic devices by laser ablation based additive manufacturing

Polyimide microfluidic devices (MFDs) have been attached enormous significance because of its excellent organic-solvent inertness, biocompatibility, and thermal stability. In this paper, a novel fabrication method based on the thought of additive manufacturing, which is adding materials layer by layer from bottom to top, was used to construct a multilayer polyimide MFD. The MFD has sophisticated three-dimensional (3D) microchannels with adjustable cross-sectional geometries and high bonding strength, which leads to good reagent mixing performance, large surface-to-volume ratio, and great durability. Starting from a single polyimide film, ultraviolet (UV) laser was utilized to ablate microchannels on the film. Due to the studies over the influence of UV laser on the channel width, the microchannel edge shape is under control, varying from trapezoid to rectangle. From monolayer to multilayer MFDs, thermal bonding with fluorinated ethylene propylene (FEP) nanoparticle dispersion as the adhesive was adopted to stack polyimide films tightly with precise alignment. In this way, microchannels can be connected vertically between layers to form 3D structures. Besides, a homogeneous adhesive interlayer and polyimide-FEP mixing regime were formed, which can provide high bonding strength. Results of computational fluid dynamics simulation of 3D microchannel structures and organic synthesis experiment revealed that our device has great reagent mixing efficiency and promising application prospects in diverse research fields, especially organic chemical and biological studies.

     

Fabrication and evaluation of aluminum nitride based MEMS piezoelectric vibration sensors for large-amplitude vibration applications

Microsystem Technologies - In this work, we designed a novel structure to produce vibration sensors with a linear voltage output that are suitable for large-amplitude vibration applications....     

Flip chip packaging for MEMS microphones

We have developed a microphone package using flip chip technology instead of chip and wire bonding to create smaller MEMS microphones. With this new packaging technology the transducer chip and an ASIC chip are flip chip bonded on a ceramic substrate. The package is sealed by a polymer foil laminated over the chips and by a metal layer. The sound port is on the bottom side in the ceramic substrate. In this paper the packaging technology is explained in detail and results of electro-acoustic characterization and reliability testing are presented. We will also explain the way which has led us from the packaging of Surface Acoustic Wave (SAW) components to the packaging of MEMS microphones.     

MEMS封装技术

介绍了微机电(MEMS)封装技术,包括晶片级封装、单芯片封装和多芯片封装、模块式封装与倒装焊3种很有前景的封装技术。指出了MEMS封装的几个可靠性问题,最后,对MEMS封装的发展趋势作了分析。     

Getter free vacuum packaging for MEMS

Quite a few MEMS devices such as accelerators, gyroscopes, infrared sensors need to work in vacuum environment to enhance their performance. The traditional vacuum packaging methods use getters to increase vacuum maintaining lifetime. However, the getter's characteristics of high temperature activation, powder pollution, large package size limit its use in MEMS vacuum packaging. This study analyzes the factors that influence the vacuum level, and derived a relationship between the balanced vacuum level, the effective leak rate, and vacuum maintaining lifetime. A novel vacuum package design with vacuum buffer cavity was proposed, the vacuum maintaining lifetime could be increased at least 20 times as compared to the ordinary package with the same volume. Reliability experiments were conducted so as to verify that the new package design can achieve reliable vacuum packaging without getters to meet the requirements of device applications for vacuum with about 0.1 Pa in pressure level. This unique package design also provided a complementary way to work with getters to enhance the vacuum package performance and reliability of hermeticity.     

Two-stage polymer embossing of co-planar microfluidic features for microfluidic devices

A two-stage embossing technique for fabricating microchannels for microfluidic devices is presented. A micromachined aluminum mold is used to emboss a polyetherimide (PEI) substrate with a relatively high glass transition temperature (T_g). The embossed PEI is then used as a mold for embossing an amorphous polyethylene terephthalate (APET) substrate with a lower T_g. The resulting APET substrate has the same features as those of the aluminum mold. Successful transfer of features from the aluminum mold to the APET substrate was verified by profilometry, and an application of this method in production of a microfluidic device is presented.     

Integrated electrochemical velocimetry for microfluidic devices

We present a new electrochemical velocimetry approach with direct electrical output that is capable of complete device-level integration. The steady reduction rate of a reversible redox species at an embedded microband working electrode is monitored amperometrically. Only one working electrode of arbitrary width is required; all three electrodes, including counter and reference electrodes, are integrated on-chip for complete miniaturization of the sensor. Experimental results are complemented by a theoretical framework including a full 3D electrochemical model as well as empirical mass transfer correlations and scaling laws. When the sensor is operated in the convective/diffusive transport controlled mode, the output signal becomes a predictable function of velocity in two distinct regimes: (i) in the low velocity regime, the signal is directly proportional to flow rate, and (ii) in the high velocity regime, the signal scales as the cube root of the mean velocity. The proposed velocimetry technique is applicable to all practicable pressure-driven laminar flows in microchannels with known cross-sectional geometry.     

Novel no-moving-part valves for microfluidic devices

This study characterizes and analyzes the performances of micro diffusers/nozzles with five types of enhancement structures and one of conventional micro nozzle/diffuser valve. The pressure drops across the designed micro nozzles/diffusers are found to be increased considerably when the obstacle and fin structure are added. Further, the micro nozzle/diffuser having added circular area reveals the lowest pressure drop, owing to the hydraulic diameter is increased by circular area and lower interface friction. The maximum improvement of the loss coefficient ratio is about 16% for an added 3-fin structure operated at a Reynolds number around 70. Upon this situation, the static rectification efficiency improves 4.43 times than the conventional nozzle/diffuser. Experimental results indicate the performance peaks at a Reynolds number around 70, and an appreciable decline is encountered when the Reynolds number is reduced. It is due to the efficiency ratio of conventional micro nozzle/diffuser significant increases with the Reynolds number.     

Fabrication of layered polydimethylsiloxane/perfluoropolyether microfluidic devices with solvent compatibility and valve functionality

The development of multilayer soft lithography methodology has seen polydimethysiloxane (PDMS) as the preferred material for the fabrication of microfluidic devices. However, the functionality of these PDMS microfluidic chips is often limited by the poor chemical resistance of PDMS to certain solvents. Here, we propose the use of a photocurable perfluoropolyether (PFPE), specifically FOMBLIN^® MD40 PFPE, as a candidate material to provide a solvent-resistant buffer layer to make the device substantially impervious to chemically induced swelling. We first carried out a systematic study of the solvent resistance properties of FOMBLIN^® MD40 PFPE as compared with PDMS. The comparison presented here demonstrates the superiority of FOMBLIN^® MD40 PFPE over PDMS in this regard; moreover, the results permitted to categorize solvents in four different groups depending on their swelling ratio. We then present a step-by-step recipe for a novel fabrication process that uses multilayer lithography to construct a comprehensive solvent-resistant device with fluid and control channels integrated with a valve structure and also permitting easy establishment of outside connections.     

Low cost anodic bonding for MEMS packaging applications

Anodic bonding of Pyrex 7740 glass to bare silicon and oxidized silicon wafer is presented for micro electro mechanical systems (MEMS) device packaging. Experimentally it has been observed that anodic bonding process parameters are varying with different 3D structures. The effects of bonding temperature and voltage are discussed by keeping the temperature constant and varying the voltage. The bonding interface has been studied by scanning electron microscope observations. Effective parameters for MEMS structure such as bonding temperature, voltage has been discussed.     

Digital signal processing methods for impedance microfluidic cytometry

Impedance microfluidic cytometry is a non-invasive, label-free technology that can characterize the dielectric properties of single particles (beads/cells) at high speed. In this paper we show how digital signal processing methods are applied to the impedance signals for noise removal and signal recovery in an impedance microfluidic cytometry. Two methods are used; correlation to identify typical signals from a particle and for a noisier environment, an adaptive filter is used to remove noise. The benefits of adaptive filtering are demonstrated quantitatively from the correlation coefficient and signal-to-noise ratio. Finally, the adaptive filtering method is compared to the Savitzky–Golay filtering method.