Fluidic packaging of microengine and microrocket devices for high-pressure and high-temperature operation

The fluidic packaging of Power MEMS devices such as the MIT microengine and microrocket requires the fabrication of hermetic seals capable of withstanding temperature in the range 20-600/spl deg/C and pressures in the range 100-300 atm. We describe an approach to such packaging by attaching Kovar metal tubes to a silicon device using glass seal technology. Failure due to fracture of the seals is a significant reliability concern in the baseline process: microscopy revealed a large number of voids in the glass, pre-cracks in the glass and silicon, and poor wetting of the glass to silicon. The effects of various processing and materials parameters on these phenomena were examined. A robust procedure, based on the use of metal-coated silicon substrates, was developed to ensure good wetting. The bending strength of single-tube specimens was determined at several temperatures. The dominant failure mode changed from fracture at room temperature to yielding of the glass and Kovar at 600/spl deg/C. The strength in tension at room temperature was analyzed using Weibull statistics; these results indicate a probability of survival of 0.99 at an operational pressure of 125 atm at room temperature for single tubes and a corresponding probability of 0.9 for a packaged device with 11 joints. The residual stresses were analyzed using the method of finite elements and recommendations for the improvement of packaging reliability are suggested.     

Packaging of Microfluidic Devices for Fluid Interconnection Using Thermoplastics

A new packaging method for microfluidic devices is proposed of polymer over-molding to form a fluidic manifold integrated with the device in a single step. The anticipated advantages of the proposed method of packaging are ease of assembly and low part count, making it suitable for low cost and high volume manufacturing. This paper reports the results of a preliminary investigation into this concept. Glass and silicon inserts of 25 times 20 mm in size, used to represent microfluidic devices, were over-molded in an injection molding process with a range of polymers. The inserts were found to survive the molding process intact. The adhesion between overmold and insert was investigated by subjecting the interface between the overmold and insert surface to a hydrostatic pressure of up to 100 lbf/in² (6.9 bar). The durability of the interfacial adhesion to hydrolysis was investigated by immersion in water at 50degC for 24 h before testing. Direct measurements of adhesion strength between polymer and glass were also attempted by tensile tests on lap-jointed samples. The best and most durable adhesion for glass and silicon inserts was found for polyamide (PA) 12, which is a low hygroscopicity PA. The ranking of polymers by their performances in the pressurization tests was consistent with the ranking by the calculated work-of-adhesion values for polymer/glass and polymer/silicon joints.     

Evaluation of thermo-mechanical damage and fatigue life of solar cell solder interconnections

The soldering process of interconnecting crystalline silicon solar cells to form photovoltaic (PV) module is a key manufacturing process. However, during the soldering process, stress is induced in the solar cell solder joints and remains in the joint as residual stress after soldering. Furthermore, during the module service life time, thermo-mechanical degradation of the solder joints occurs due to thermal cycling of the joints which induce stress, creep strain and strain energy. The resultant effect of damage on the solder joint is premature failure, hence shortened fatigue life. This study seeks to determine accumulated thermo-mechanical damage and fatigue life of solder interconnection in solar cell assembly under thermo-mechanical cycling conditions. In this investigation, finite element modelling (FEM) and simulations are carried out in order to determine nonlinear degradation of SnAgCu solder joints. The degradation of the solder material is simulated using Garofalo-Arrhenius creep model. A three dimensional (3D) geometric model is subjected to six accelerated thermal cycles (ATCs) utilising IEC 61215 standard for photovoltaic panels. The results demonstrate that induced stress, strain and strain energy impacts the solder joints during operations. Furthermore, the larger the accumulated creep strain and creep strain energy in the joints, the shorter the fatigue life. This indicates that creep strain and creep strain energy in the solder joints significantly impacts the thermo-mechanical reliability of the assembly joints. Regions of solder joint with critical stress, strain and strain energy values including their distribution are determined. Analysis of results demonstrates that creep energy density is a better parameter than creep strain in predicting interconnection fatigue life. The use of six ATCs yields significant data which enable better understanding of the response of the solder joints to the induced loads. Moreover, information obtained from this study can be used for improved design and better-quality fabrication of solder interconnections in solar cell assembly for enhanced thermo-mechanical reliability.     

Pressure nonlinearity of micromachined piezoresistive pressure sensors with thin diaphragms under high residual stresses

Thermal residual stress plays a significant role in the performance of microelectromechanical system (MEMS) pressure sensor devices. For example, the voltage span and pressure nonlinearity (PNL) on the voltage output of a pressure sensing element can be significantly affected by the residual stresses of passivation films on the silicon diaphragm. The objective of this study is to resolve a pressure nonlinearity problem in terms of silicon nitride residual stress and diaphragm thickness in order to meet the PNL design criteria within ±3% at 25 °C. The curvatures of wafers were measured and the film residual stresses were calculated. Finite element analyses (FEA) were conducted and correlated with the PNL experimental tests. To build a design window for optimization, a central composite design (CCD) method was utilized to significantly reduce the number of FEA runs. It is concluded that the residual stress of PECVD silicon nitride needs to be optimized and controlled in order to reduce the pressure nonlinearity.     

Automated sizing of automotive steering ball joints in parametric CAD environment using expert knowledge and feature-based computer-assisted 3D modelling

Ball joints used in the steering systems of vehicles are exposed to fluctuating loads, which can cause fatal accidents in case of failure. The design of ball joints is an iterative and time-consuming process. Even though the automotive industry is preparing for the era of autonomous self-steering vehicles, parts such as ball joints were not designed using a fully automated parametric design methodology. Recently, parametric design of automotive ball joints based on variable design methodology using knowledge and feature-based computer-assisted-3D modelling methods was studied. However, these studies do not give details of the interactive sizing process within the part and assembly module to determine the final dimensions for avoidance of fatigue failure.This work provides methods and discusses details of the configurable sizing of a ball joint assembly under the boundaries of the developed “parametric design platform”. The platform closes the software gap for the automated reconfiguration and sizing of the ball joint assembly using a three-dimensional (3D) modelling technique. The platform can parametrically change part, material, feature, geometry, assembly and dimension features in a programmable environment. It can also reconfigure the ball joint assembly model considering various structured data conforming to technical standards and reasoning mechanisms with “engineering and geometrical relations” provided in this work, and data gathering along the life cycle of a product. Parameterised 3D solid models and a knowledge base of ball joints are stored in a database, and then an evaluation process within the platform that is capable of sizing ball joints for infinite fatigue-life has been established to verify sizing. It demonstrates the practicability and validity of the automated sizing of a steering ball joint within a configurable design environment and with minimum human expert knowledge and interaction.     

硅--蓝宝石绝压传感器陶瓷密封结构设计与分析

介绍了硅—蓝宝石绝压传感器中氧化铝陶瓷—钛合金的真空密封参考腔小型化结构设计,探讨了异质材料不匹配引起的陶瓷断裂问题,对陶瓷—钛合金封装结构的应力情况进行了计算和分析,通过中间缓冲层设计、减小陶瓷承受的结构残余应力,消除了由陶瓷断裂导致的密封失效;选择热胀系数相近的可伐中间层材料,对比不同厚度缓冲层产生的残余应力,优化中间层结构,采用LTCC加工技术与真空钎焊工艺结合制作了陶瓷—可伐—钛合金密封组件,并通过了高、低温度试验考核:组件密封漏率小于1×10-10 Pa·m3/s,密封可靠,满足绝压传感器使用寿命的要求.     

单片式压电谐振型石英压力-温度传感器设计

提出了一种采用石英力敏谐振器(QFSR)-石英热敏谐振器(QTSR)的单片式压电谐振型石英压力-温度传感器(QPTS),设计了单片式QPTS结构、石英压力传感器的无应力封接方案以及新型压力-伸缩力变换器.单片式QPTS由QFSR和QTSR构成,均采用AT切型,厚度切变模式工作,不同的是QTSR的长边取向与石英X轴的夹角为60°.无应力封接方案使用石英、单晶硅、非晶态SiC、硼硅酸盐玻璃和柯伐合金的组合,并且利用石英化学刻蚀和物理修饰技术以及半导体的新工艺使QFSR和QTSR改性.其中,非晶态SiC层的制作是为了实现应力的缓冲:虽然硅和石英材料的热膨胀系数不匹配,可是二者之间的非晶态SiC层却能够良好地吸收其热应力,成为无应力结构.     

Design and fabrication of a novel microfluidic nanoprobe

The design and fabrication of a novel microfluidic nanoprobe system are presented. The nanoprobe consists of cantilevered ultrasharp volcano-like tips, with microfluidic capabilities consisting of microchannels connected to an on-chip reservoir. The chip possesses additional connection capabilities to a remote reservoir. The fabrication uses standard surface micromachining techniques and materials. Bulk micromachining is employed for chip release. The microchannels are fabricated in silicon nitride by a new methodology, based on edge underetching of a sacrificial layer, bird's beak oxidation for mechanically closing the edges, and deposition of a sealing layer. The design and integration of various elements of the system and their fabrication are discussed. The system is conceived mainly to work as a "nanofountain pen", i.e., a continuously writing upgrade of the dip-pen nanolithography approach. Moreover, the new chip shows a much larger applicability area in fields such as electrochemical nanoprobes, nanoprobe-based etching, build-up tools for nanofabrication, or a probe for materials interactive analysis. Preliminary tests for writing and imaging with the new device were performed. These tests illustrate the capabilities of the new device and demonstrate possible directions for improvement.     

A pneumatically actuated full in-channel microvalve with MOSFET-like function in fluid channel networks

A pneumatically actuated silicon microvalve applicable to integrated microfluidic systems is presented. All the ports of this microvalve are in-channel, and connectable to any surface fluid channels in microfluidic systems. This microvalve controls fluid flow by means of the controlled gap between glass and silicon diaphragm actuated by a control pressure. In addition, the diaphragm is also deformed by the outlet pressure of the microvalve. Due to the feature, this microvalve shows saturation of flow rate like MOSFETs operated at saturation region. The fabricated microvalve device was evaluated focusing on analogous relationship between MOSFET and the microvalve. Fluids such as air and DI-water were well controlled by the control pressure. Fluid starts to flow in the microvalve when the control pressure exceeds its "threshold pressure." Hysteresis due to sticking of diaphragm was not observed in the characteristics. Air flow rate of the microvalve was gradually saturated with the increasing of the outlet pressure as expected. Through the evaluation, analogous relationship between this microvalve and MOSFET has been experimentally demonstrated.     

On eliminating synchronous communication in molecular simulations to improve scalability

Molecular dynamics simulation, as a complementary tool to experimentation, has become an important methodology for the understanding and design of molecular systems as it provides access to properties that are difficult, impossible or prohibitively expensive to obtain experimentally. Many of the available software packages have been parallelized to take advantage of modern massively concurrent processing resources. The challenge in achieving parallel efficiency is commonly attributed to the fact that molecular dynamics algorithms are communication intensive.     

Reliable magnetic reversible assembly of complex microfluidic devices: fabrication, characterization, and biological validation

Current standard procedures for fabrication of microfluidic devices combine polydimethylsiloxane (PDMS) replica molding with subsequent plasma treatment to obtain an irreversible sealing onto a glass/silicon substrate. However, irreversible sealing introduces several limitations to applications and internal accessibility of such devices as well as to the choice of materials for fabrication. In the present work, we describe and characterize a reliable, flexible and cost effective approach to fabricate devices that reversibly adhere to a substrate by taking advantage of magnetic forces. This is shown by implementing a PDMS/iron micropowder layer aligned onto a microfluidic layer and coupled with a histology glass slide, in union with either temporary or continuous use of a permanent magnet. To better represent the complexity of microfluidic devices, a Y-shaped configuration including lower scale parallel channels on each branch has been employed as reference geometry. To correctly evaluate our system, current sealing methods have been reproduced on the reference geometry. Sealing experiments (pressure control, flow control and hydraulic characterization) have been carried out, showing consistent increases in terms of maximum achievable flow rates and pressures, as compared to devices obtained with other available reversible techniques. Moreover, no differences were detected between cells cultured on our magnetic devices as compared to cells cultured on permanently sealed devices. Disassembly of our devices for analyses allowed to stain cells by hematoxylin and eosin and for F-actin, following traditional histological processes and protocols. In conclusion, we present a method allowing reversible sealing of microfluidic devices characterized by compatibility with: (i) complex fluidic layer configurations, (ii) micrometer sized channels, and (iii) optical transparency in the channel regions for flow visualization and inspection.     

Fast and inexpensive method for the fabrication of transparent pressure-resistant microfluidic chips

The recent rise of high-pressure applications in microfluidics has led to the development of different types of pressure-resistant microfluidic chips. For the most part, however, the fabrication methods require clean room facilities, as well as specific equipment and expertise. Furthermore, the resulting microfluidic chips are not always well suited to flow visualization and optical measurements. Herein, we present a method that allows rapid and inexpensive prototyping of optically transparent microfluidic chips that resist pressures of at least 200 bar. The fabrication method is based on UV-curable off-stoichiometry thiol-ene epoxy (OSTE+) polymer, which is chemically bonded to glass. The reliability of the device was verified by pressure tests using CO₂, showing resistance without failure up to at least 200 bar at ambient temperature. The microchips also resisted operation at high pressure for several hours at a temperature of 40 °C. These results show that the polymer structure and the chemical bond with the glass are not affected by high-pressure CO₂. Opportunities for flow visualization are illustrated by high-pressure two-phase flow shadowgraphy experiments. These microfluidic chips are of specific interest for use with supercritical CO₂ and for optical characterization of phase transitions and multiphase flow under near-critical and critical CO₂ conditions.     

Estimating the energy dissipated in a bolted spacecraft at resonance

In this paper a method is established to estimate the energy dissipated in the bolted joints of a satellite structure. The technique is equally applicable to any other bolted structure. A 3D FE model of a detailed bolted joint was created and the relationship between energy dissipated in the joint and the transverse excitation force established using non-linear FE analyses. Experimental tests were carried out on bolted joint specimens to provide data for the FE modelling techniques used. Local joint forces in the satellite were obtained from a FE model of the satellite using a frequency response analysis. These forces were used in conjunction with the joint force–energy relationship to obtain the energy dissipated in the joints. Analysis revealed that, without including joint macro-slip, the energy dissipated in the joints of the current satellite was relatively small.     

Use of a PDMS spring element for ink jet printing applications

Results of the design, microfabrication and testing of a proof-of-concept, diaphragm-type silicone sealing joint are presented. DRIE-etched cavities were filled with a flexible sealing element made of polydimethylsiloxane that supports a silicon piston. A series of sealing joints were produced with variable widths, and the displacement of the piston was measured after applying pressures of up to 1 bar above atmospheric pressure in 0.2 bar increments. Two masks were designed to produce several sets of silicone springs with widths of 2–10.5 μm, each consisting of a 10 μm thick silicon piston that is 2 mm long. Tests performed on the shear spring joints were found to give a displacement of 0.5 μm at 1 bar when the sealing width is 6 μm or more. The sealing joint with a 10 μm width was found to give a displacement of 0.9 μm and an elastic recovery of 88%. The results showed this type of joint in the form of an elastically-deforming seal provides sufficient displacement for propelling liquid droplets as part of a liquid propulsion system.     

A dielectrophoretic chip packaged at wafer level

The paper presents a dielectrophoretic chip, fully enclosed, with bulk silicon electrodes fabricated using wafer-to-wafer bonding techniques and packaged at the wafer level. The silicon electrodes, which are bonded to two glass dies, define in the same time the walls of the microfluidic channel. The device is fabricated from a silicon wafer that is bonded (at wafer level) anodically and using SU8 photoresist between two glass wafers. The first glass die includes drilled holes for inlet/outlet connections while the second glass die assure the electrical connections, through via holes and a metallization layer, between the silicon electrodes and a printing circuit board.     

A silicon rectangular micro-orifice for gas flow measurement at moderate Reynolds numbers: design,fabrication and flow analyses

This work describes a micro-flowmeter for moderate flow rates of gases based on a differential pressure measurement. The micro-flowmeters consist of a microfabricated silicon–glass rectangular micro-orifice plate, with external pressure measurement. We experimentally evaluate the effects of geometrics parameters, Reynolds number and compressibility on the discharge coefficient. The paper examines a series of 13 rectangular micro-orifice sizes, with orifice hydraulic diameters ranging from 115 to 362 µm. The behavior of the discharge coefficient is presented for orifice Reynolds numbers ranging from 200 to 18000. Agreement is shown between the experimental and numerical results of the discharge coefficient. The micro-flowmeters measure moderate flow of air ranging from 1 to 106 mg/s. This demonstration implements a design method of micro-flowmeters that can be used in a broad range of microfluidic applications, such as microreactors and power MEMS.     

A dielectrophoretic chip packaged at wafer level

The paper presents a dielectrophoretic chip, fully enclosed, with bulk silicon electrodes fabricated using wafer-to-wafer bonding techniques and packaged at the wafer level. The silicon electrodes, which are bonded to two glass dies, define in the same time the walls of the microfluidic channel. The device is fabricated from a silicon wafer that is bonded (at wafer level) anodically and using SU8 photoresist between two glass wafers. The first glass die includes drilled holes for inlet/outlet connections while the second glass die assure the electrical connections, through via holes and a metallization layer, between the silicon electrodes and a printing circuit board.

     

A stiffness modeling methodology for simulation-driven design of haptic devices

Efficient development and engineering of high performing interactive devices, such as haptic robots for surgical training benefits from model-based and simulation-driven design. The complexity of the design space and the multi-domain and multi-physics character of the behavior of such a product ask for a systematic methodology for creating and validating compact and computationally efficient simulation models to be used in the design process. Modeling the quasi-static stiffness is an important first step before optimizing the mechanical structure, engineering the control system, and performing hardware in the loop tests. The stiffness depends not only on the stiffness of the links, but also on the contact stiffness in each joint. A fine-granular Finite element method (FEM) model, which is the most straightforward approach, cannot, due to the model size and simulation complexity, efficiently be used to address such tasks. In this work, a new methodology for creating an analytical and compact model of the quasi-static stiffness of a haptic device is proposed, which considers the stiffness of actuation systems, flexible links and passive joints. For the modeling of passive joints, a hertzian contact model is introduced for both spherical and universal joints, and a simply supported beam model for universal joints. The validation process is presented as a systematic guideline to evaluate the stiffness parameters both using parametric FEM modeling and physical experiments. Preloading has been used to consider the clearances and possible assembling errors during manufacturing. A modified JP Merlet kinematic structure is used to exemplify the modeling and validation methodology.     

Six-layer lamination of a new dry film negative-tone photoresist for fabricating complex 3D microfluidic devices

We present a new epoxy-based negative-tone dry film photoresist (DFR) for fabricating multilayer microfluidic devices using a lamination process combined with a standard photolithography technology. As proof-of-concept, a complex 3D-hydrodynamic focusing device was produced via a six-layer lamination process of 33 µm-thick DFR layers. The bonding strength of the new DFR was tested on silicon, glass, and titanium substrates, respectively. A maximum bonding strength of 37 MPa was obtained for the dry film photoresist laminated on glass. No leakage was found, and burst tests proved excellent robustness and sealing reliability of the microchannels.