Showing papers in "Journal of Micromechanics and Microengineering in 2007"

A micro electromagnetic generator for vibration energy harvesting

Abstract: Vibration energy harvesting is receiving a considerable amount of interest as a means for powering wireless sensor nodes This paper presents a small (component volume 01 cm3, practical volume 015 cm3) electromagnetic generator utilizing discrete components and optimized for a low ambient vibration level based upon real application data The generator uses four magnets arranged on an etched cantilever with a wound coil located within the moving magnetic field Magnet size and coil properties were optimized, with the final device producing 46 µW in a resistive load of 4 k? from just 059 m s-2 acceleration levels at its resonant frequency of 52 Hz A voltage of 428 mVrms was obtained from the generator with a 2300 turn coil which has proved sufficient for subsequent rectification and voltage step-up circuitry The generator delivers 30% of the power supplied from the environment to useful electrical power in the load This generator compares very favourably with other demonstrated examples in the literature, both in terms of normalized power density and efficiency

SU-8: a photoresist for high-aspect-ratio and 3D submicron lithography

Abstract: SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

Lab-on-chip technologies: making a microfluidic network and coupling it into a complete microsystem : a review

Abstract: Microfluidics is an emerging field that has given rise to a large number of scientific and technological developments over the last few years. This review reports on the use of various materials, such as silicon, glass and polymers, and their related technologies for the manufacturing of simple microchannels and complex systems. It also presents the main application fields concerned with the different technologies and the most significant results reported by academic and industrial teams. Finally, it demonstrates the advantage of developing approaches for associating polymer technologies for manufacturing of fluidic elements with integration of active or sensitive elements, particularly silicon devices.

Microinjection molding of thermoplastic polymers: a review

Abstract: Microinjection molding (µIM) appears to be one of the most efficient processes for the large-scale production of thermoplastic polymer microparts. The microinjection molding process is not just a scaling down of the conventional injection process; it requires a rethinking of each part of the process. This review proposes a comparative description of each step of the microinjection molding process (µIM) with conventional injection molding (IM). Micromolding machines have been developed since the 1990s and a comparison between the existing ones is made. The techniques used for the realization of mold inserts are presented, such as lithography process (LIGA), laser micromachining and micro electrical discharge machining (µEDM). Regarding the molding step, the variotherm equipment used for the temperature variation is presented and the problems to solve for each molding phase are listed. Throughout this review, the differences existing between µIM and conventional molding are highlighted.

The centrifugal microfluidic Bio-Disk platform

Abstract: This paper reviews the centrifugal 'Bio-Disk' platform which is based on rotationally controlled, multi-scale liquid handling to fully integrate and automate complex analysis and synthesis protocols in the life sciences. The platform offers the crucial ingredients for a rapid development of applications: a coherent library of fluidic unit operations, a device technology for actuation, liquid interfacing and detection as well as a developer toolbox providing experimental testing, rapid prototyping and simulation capabilities. Various applications in the fields of life science, in vitro diagnostics and micro-process engineering are demonstrated.

A passive planar micromixer with obstructions for mixing at low Reynolds numbers

Abstract: Passive mixers rely on the channel geometry to mix fluids. However, many previously reported designs either work efficiently only at moderate to high Reynolds numbers (Re), or require a complex 3D channel geometry that is often difficult to fabricate. In this paper, we report design, simulation, fabrication and characterization of a planar passive microfluidic mixer capable of mixing at low Reynolds numbers. The design incorporates diamond-shaped obstructions within the microchannel to break-up and recombine the flow. Simulation and experimental results of the developed micromixer show excellent mixing performance over a wide range of flow conditions (numerically: 0.01 < Re < 100, experimentally: 0.02 < Re < 10). The micromixer is also characterized by low pressure drop, an important characteristic for integration into complex, cascading microfluidic systems. Due to the simple planar structure of the micromixer, it can be easily realized and integrated with on-chip microfluidic systems, such as micro total analysis systems (μTAS) or lab on a chip (LOC).

Diamond and diamond-like carbon MEMS

Abstract: Diamond and diamond-like carbon (DLC) thin films possess a number of unique and attractive material properties that are unattainable from Si and other materials. These include high values of Young's modulus, hardness, tensile strength and high thermal conductivity, low thermal expansion coefficient combined with low coefficients of friction and good wear resistance. As a consequence, they are finding increasing applications in micro-electro-mechanical systems (MEMS). This paper reviews these distinctive material properties from an engineering design point of view and highlights the applications of diamond and DLC materials in various MEMS devices. Applications of diamond and DLC films in MEMS are in two categories: surface coatings and structural materials. Thin diamond and DLC layers have been used as coatings mainly to improve the wear and friction of micro-components and to reduce stiction between microstructures and their substrates. The high values of the elastic modulus of diamond and DLC have been exploited in the design of high frequency resonators and comb-drives for communication and sensing applications. Chemically modified surfaces and structures of diamond and DLC films have both been utilized as sensor materials for sensing traces of gases, to detect bio-molecules for biological research and disease diagnosis.

Stability of the hydrophilic behavior of oxygen plasma activated SU-8

Abstract: The effect of O2 plasma treatment on surface energy, topography and surface chemistry of the negative photoresist epoxy novolak SU-8 was investigated by contact angle goniometry, atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). Directly after plasma treatment, the surfaces were completely wetted by water with a contact angle between water and the SU-8 surface below 5°. The surface free energy can be increased significantly depending on the plasma dose. The surfaces remained hydrophilic for several months showing a moderate hydrophobic recovery. The surface topography of the plasma treated SU-8 showed a formation of nanoscale aggregates. The rms-roughness of the topography was correlated with the plasma dose. An increased plasma dose induced aggregates of up to 200 nm in size. XPS measurements revealed changes in surface chemistry due to the oxygen plasma process and an increased antimony concentration on the surface.

Rapid prototyping of microfluidic chips in COC

Abstract: We present a novel, cost-efficient process chain for fast tooling and small-lot replication of high-quality, multi-scale microfluidic polymer chips within less than 5 days. The fabrication chain starts with a primary master which is made by well-established cleanroom processes such as DRIE or negative SU-8 resist based surface micromachining. The formation of undercuts in the master which would complicate demolding is carefully avoided. Secondary PDMS masters or epoxy-based masters which are more suitable for common polymer replication schemes such as soft-embossing, hot-embossing or injection molding are subsequently cast from the primary masters. The polymer replica are mainly made of COC and show excellent fidelity with the conventionally micromachined master while displaying no degeneration, even after more than 200 cycles. The use of other polymers such as PMMA is also possible. The process chain further includes surface modification techniques for overall, long-term stable hydrophilic coatings and for local hydrophobic patches as well as a durable sealing based on thermal bonding.

A new type of diabatic flow pattern map for boiling heat transfer in microchannels

Abstract: Flow pattern data and bubble measurements for two small diameter sight glass tubes are used to propose a new type of flow pattern map for evaporating flows in microchannels. Rather than segregating the observations into the traditional flow regimes and an adiabatic map, the new diabatic map classifies flows into three types: (i) the isolated bubble regime, where the bubble generation rate is much larger than the bubble coalescence rate and includes both bubbly and slug flows, (ii) the coalescing bubble regime, where the bubble coalescence rate is much larger than the bubble generation rate and exists up to the end of the coalescence process and (iii) the annular regime, whose extent is limited by the vapor quality at the onset of critical heat flux. This formulation is thought to be more useful for phenomenological modeling of the processes controlling boiling heat transfer and two-phase pressure drops in microchannels, and it also visually defines the feasible operating limit of microchannel heat spreaders at the critical vapor quality corresponding to critical heat flux (CHF). The database covers two refrigerants (R-134a and R-245fa) and two channel diameters (0.509 and 0.790 mm). The micro-evaporator length was varied from 20 to 70 mm, the inlet subcooling from 2 to 15 °C, the mass flux from 200 to 2000 kg m−2 s−1 and heat fluxes up to 597 kW m−2. Three different saturation temperatures were tested: 26, 30 and 35 °C.

Efficient in-droplet separation of magnetic particles for digital microfluidics

Abstract: This paper describes a new efficient in-droplet magnetic particle concentration and separation method, where magnetic particles are concentrated and separated into a split droplet by using a permanent magnet and EWOD (electrowetting on dielectric) droplet manipulation. To evaluate the method, testing devices are fabricated by the micro fabrication technology. First, this method is examined for magnetic particle concentration, showing that over 91% of magnetic particles can be concentrated into a split daughter droplet. Then, separation between magnetic and non-magnetic particles is examined for two different cases of particle mixture, showing in both cases that over 91% of the magnetic particles can be concentrated into split daughter droplets. However, a significant number of the non-magnetic particles (over 35%) co-exist with the magnetic particles in the same daughter droplets. This problem is circumvented by adding a droplet-merging step prior to applying the magnetic field. Finally, over 94% of the total magnetic particles are separated into a one split daughter droplet while 92% of the non-magnetic particles into the other split daughter droplet. This integrated in-droplet separation method may bridge many existing magnetic particle assays to digital microfluidics and extend their application scope.

A micromachined high throughput Coulter counter for bioparticle detection and counting

Abstract: We describe a micromachined Coulter counter with multiple sensing microchannels for quantitative measurement of polymethacrylate particles and pollen. A unique design with sensing microelectrodes in the center of the microchannels is demonstrated. This design creates isolation resistances among channels, and thus circumvents the crosstalk caused by automatic electrical connection among microchannels. When implemented using microfluidic channels, this design is appropriate for the sensing of microscale particles in deionized water or in dilute electrolyte solution. Our design has multiple channels operating in parallel, but integrated with just one sample reservoir and one power source. The results with a four-channel device show that this device is capable of differentiating and counting micro polymethacrylate particles and Juniper pollen rapidly. Moreover, the device throughput is improved significantly in comparison to a single-channel device. The concept can be extended to a large number of sensing channels in a single chip for significant improvement in throughput.

Thermal isolation of microchip reaction chambers for rapid non-contact DNA amplification

Abstract: This paper describes further optimization of a non-contact, infrared-mediated system for microchip DNA amplification via the polymerase chain reaction (PCR). The optimization is focused on heat transfer modeling and subsequent fabrication of thermally isolated reaction chambers in glass devices that are uniquely compatible with non-contact thermal control. With a thermal conductivity approximately an order of magnitude higher than many plastics, glass is not the obvious substrate of choice for rapid thermal cycling in microfluidic chambers, yet it is preferable in terms of optical clarity, solvent compatibility and chemical inertness. Based on predictions of a lumped capacity heat transfer analysis, it is shown here that post-bonding, patterned etching of surrounding glass from microfluidic reaction chambers provides enhancements as high as 3.6- and 7.5-fold in cooling and heating rates, respectively, over control devices with the same chamber designs. These devices are then proven functional for rapid DNA amplification via PCR, in which 25 thermal cycles are completed in only 5 min in thermally isolated PCR chambers of 270 nL volume, representing the fastest static PCR in glass devices reported to date. Amplification of the 500-base pair fragment of ?-DNA was confirmed by capillary gel electrophoresis. In addition to rapid temperature control, the fabrication scheme presented, which is compatible with standard photolithography and wet etching techniques, provides a simple alternative for general thermal management in glass microfluidic devices without metallization.

Three-dimensional hydrodynamic focusing in two-layer polydimethylsiloxane (PDMS) microchannels

Abstract: In this work, we designed and fabricated a three-dimensional hydrodynamic focusing microfluidic device. The device comprises a two-layer PDMS microchannel structure. There are four inlet ports and one outlet port. The fluids are all injected by syringe pumps. A sample flow stream was first vertically constrained into a narrow stream, and then horizontally focused on one small core region from a cross-section perspective, which is useful for cell/particle counting. We showed the numerical and experimental images of the focused stream shape from a cross-section perspective; experimental images were captured using a confocal fluorescence microscope. We also investigated the effect of channel aspect ratio on the vertical focusing effect using CFD simulations. The results showed that the sample flow can be focused successfully in the lower aspect ratio of the main channel (slightly greater than 0.5) in our design. Furthermore, the effect of the Reynolds number on the vertical focusing effect was also investigated. The numerical results showed that the rectangular-like shape of the focused stream from the cross-section perspective was deformed as the Reynolds number was high due to stronger secondary flows produced in the vertical focusing unit. This phenomenon was also demonstrated experimentally. The device only works well at low Reynolds numbers (approximately less than 5). The device can be integrated into an on-chip flow cytometer.

Ink-jet fabrication of electronic components

Abstract: Nanosized metallic particles dispersed in a polymeric matrix have been used conventionally as a paste or ink to print electrically active patterns on different substrates. The potential of ink-jet printing in this field is clearly important but the challenge to date has been how to achieve prints of low volume resistivity from the very low viscosity ink required for ink-jet printing. In this study, ink-jet printing techniques were used to directly deposit metallic conductive patterns to produce wiring boards, antennas, electrodes and so forth. In these methods, aqueous solutions of metal salt and reducing agent were ink-jet printed consecutively onto the substrate, where an immediate chemical reduction transformed the metal cations into very fine metallic particles. The best performing reducing agent for ink-jet metal deposition was found to be ascorbic acid at neutral pH. Using this chemistry, nanosized silver patterns, composed of particles in the size range 10–200 nm, were successfully formed using a standard office ink-jet thermal-head printer. Deposited layers of silver with high electrical conductance up to 1.89 × 10 5 Sm −1 and sheet resistance up to 0.5 � /� were printed whilst higher conductivities might be expected using more appropriate devices. (Some figures in this article are in colour only in the electronic version)

A review of MEMS external-cavity tunable lasers

Abstract: The paper reviews the state-of-the-art of miniaturized tunable lasers constructed by microelectromechanical systems (MEMS) technology, covering various topics of laser configurations, theoretical studies and some design issues, with primary focus on the uniqueness of MEMS tunable lasers in comparison to conventional opto-mechanical counterparts. Further studies have also been presented to investigate the tuning range and stability in order to provide a deep understanding of the specialities of MEMS lasers in the sense of physics. The introduction of MEMS has endowed two special features to tunable lasers. One is that MEMS facilitates external cavities at very short (<100 µm) and even extremely short length (<10 µm), leading to unusual tuning behaviors and different design concerns. The other is that the photolithography of the MEMS process makes it possible to fabricate gratings/mirrors in arbitrary profiles, which may inspire the innovation of new laser configurations that can only be realized by MEMS technology. With further work on integration and packaging, MEMS lasers would be able to deliver their merits of small size, fast tuning speed, wide tuning range and IC integration compatibility, and to broaden their applications to many fields. (Some figures in this article are in colour only in the electronic version)

Performance limits of the three MEMS inertial energy generator transduction types

Abstract: In this paper, trends from the last 10 years of inertial micro-generator literature are investigated and it is shown that, although current generator designs are still operating well below their maximum power, there has been a significant improvement with time. Whilst no clear conclusions could be drawn from reported fabricated devices with respect to preferred transducer technology, this paper presents operating charts for inertial micro-generators which identify optimal operating modes for different frequencies and normalized generator sizes, and allows comparison of the different transduction mechanisms as these parameters vary. It is shown that piezoelectric generators have a wider operating range at low frequency than electromagnetic generators, but as generator dimensions increase, the frequency to which piezoelectric transducers outperform electromagnetic transducers decreases.

Microsystem technologies for implantable applications

Abstract: Microsystem technologies (MST) have become the basis of a large industry. The advantages of MST compared to other technologies provide opportunities for application in implantable biomedical devices. This paper presents a general and broad literature review of MST for implantable applications focused on the technical domain. A classification scheme is introduced to order the examples, basic technological building blocks relevant for implantable applications are described and finally a case study on the role of microsystems for one clinical condition is presented. We observe that the microfabricated parts span a wide range for implantable applications in various clinical areas. There are 94 active and 67 commercial 'end items' out of a total of 142. End item refers to the total concept, of which the microsystem may only be a part. From the 105 active end items 18 (13% of total number of end items) are classified as products. From these 18 products, there are only two for chronic use. The number of active end items in clinical, animal and proto phase for chronic use is 17, 13 and 20, respectively. The average year of first publication of chronic end items that are still in the animal or clinical phase is 1994 (n = 7) and 1993 (n = 11), respectively. The major technology–market combinations are sensors for cardiovascular, drug delivery for drug delivery and electrodes for neurology and ophthalmology. Together these form 51% of all end items. Pressure sensors form the majority of sensors and there is just one product (considered to be an implantable microsystem) in the neurological area. Micro-machined ceramic packages, glass sealed packages and polymer encapsulations are used. Glass to metal seals are used for feedthroughs. Interconnection techniques such as flip chip, wirebonding or conductive epoxy as used in the semiconductor packaging and assembly industry are also used for manufacturing of implantable devices. Coatings are polymers or metal. As an alternative to implantable primary batteries, rechargeable batteries were introduced or concepts in which energy is provided from the outside based on inductive coupling. Long-term developments aiming at autonomous power are, for example, based on electrostatic conversion of mechanical vibrations. Communication with the implantable device is usually done using an inductive link. A large range of materials commonly used in microfabrication are also used for implantable microsystems.

A micromanipulation system with dynamic force-feedback for automatic batch microinjection

Abstract: In this paper, we report the development of a prototype micromanipulation system for automatic batch microinjection of zebrafish embryos. Such automatic batch processing is made possible by (i) the development of a machine vision algorithm to identify the number of embryos in a batch and to locate the centerline of each embryo, (ii) the integration of a piezoresistive micro-force sensor with a micropipette to measure the penetration force of the embryo in real time and (iii) the synthesis of a position control with dynamic force feedback by exploiting the characteristics of the force profile associated with the microinjection process. The effectiveness of this prototype micromanipulation system has been demonstrated in an experiment. The experimental results demonstrate that the technique of position control with dynamic penetration-force feedback is practicable for automatic batch microinjection applications.

Effects of intrinsic hydrophobicity on wettability of polymer replicas of a superhydrophobic lotus leaf

Abstract: The present work suggests a mass-producible and large-scale fabrication method of superhydrophobic polymeric surfaces by means of material processing equipments which can maximize productivity and cost effectiveness. We fabricated two types of polymeric lotus leaf replicas using a nickel mold, i.e. R1 from intrinsically hydrophobic polydimethylsiloxane by means of polymer casting (PC) and R2 from an intrinsically hydrophilic UV-curable photopolymer by means of UV-nanoimprint lithography (UV-NIL). In the case of R1 from PC, although the nano-scaled structures were not well reproduced, the contact angle (CA) was remarkably high and the sliding angle (SA) was also close to that of the original lotus leaf, resulting in a superhydrophobic surface. In contrast to R1, in the case of R2 from UV-NIL, the nano-scaled structures as well as micro-scaled structures were also relatively well reproduced and the CA was increased noticeably by around 99° in comparison to a flat photopolymer. However, unexpectedly, the SA of R2 was much higher than that of R1. This work provides useful tips of polymeric material selection for the industrial mass production of the superhydrophobic polymer surface.

3D microstructuring of Pyrex glass using the electrochemical discharge machining process

Abstract: Electrochemical discharge machining (ECDM) is demonstrated to be a potential process for 3D microstructuring of Pyrex glass. However, the key to widening ECDM micromilling applications lies in how to improve the machining accuracy. To improve the machining quality of the ECDM micromilling process, microgroove machining experiments were conducted in this study. Three factors affecting ECDM micromilling performance—pulse voltage, tool rotational rate and travel rate of tool—were taken up as machining parameters to investigate their influences on machining performance. The results indicate that optimum combinations of both pulse voltage and tool rotational rate will realize better machining accuracy. The feasibility of three-dimensional microstructure machining was demonstrated by layer-by-layer ECDM micromilling machining. Complex structures were made to demonstrate the great potential for the 3D microstructuring of Pyrex glass of the ECDM micromilling process.

Effect of flexural modes on squeeze film damping in MEMS cantilever resonators

Abstract: We present an analytical model that gives the values of squeeze film damping and spring coefficients for MEMS cantilever resonators taking into account the effect of flexural modes of the resonator. We use the exact mode shapes of a 2D cantilever plate to solve for pressure in the squeeze film and then derive the equivalent damping and spring coefficient relations from the back force calculations. The relations thus obtained can be used for any flexural mode of vibration of the resonators. We validate the analytical formulae by comparing the results with numerical simulations carried out using coupled finite element analysis in ANSYS, as well as experimentally measured values from MEMS cantilever resonators of various sizes vibrating in different modes. The analytically predicted values of damping are, in the worst case, within less than 10% of the values obtained experimentally or numerically. We also compare the results with previously reported analytical formulae based on approximate flexural mode shapes and show that the current results give much better estimates of the squeeze film damping. From the analytical model presented here, we find that the squeeze film damping drops by 84% from the first mode to the second mode in a cantilever resonator, thus improving the quality factor by a factor of 6 to 7. This result has significant implications in using cantilever resonators for mass detection where a significant increase in the quality factor is obtained by using a vacuum.

Squeeze-film damping in the free molecular regime: model validation and measurement on a MEMS

Abstract: Squeeze-film damping (SFD) is important in MEMS oscillators because it determines the quality factor of the oscillators. Published models for predicting SFD gave widely different results in the free-molecule regime, where the distance traveled by gas molecules between collisions in free space is much larger than the thickness of the squeezed gas film. The work presented here provides new experimental data for validating SFD models in that regime. The case studied here is where a rigid plate oscillates vertically while staying parallel to the substrate. The test device was an almost rectangular microplate supported by beam springs. The structure was excited by shaking the base. The velocities of numerous points on the plate and of the substrate were measured with a laser Doppler vibrometer and a microscope. An experimental modal analysis curve-fit the multiple frequency response functions to give the damping ratios. The test structure was contained in a vacuum chamber with air pressures controlled to provide a five-order-of-magnitude range of Knudsen numbers. The damping ratios from the measurements are compared with predictions from various published models. The measured damping ratios are close to predictions from models that are based on the Reynolds equation and take into account the inertia of the gas.

Study of the demolding process—implications for thermal stress, adhesion and friction control

Abstract: With the improvements of large-scale parallel replication and automation for hot embossing machines, hot embossing has become not only popular in laboratories but also possible and attractive in industry. Most difficulties in polymer micro-molding are caused by the demolding of molds rather than the filling of them. Due to the lack of accurate analysis tools and simulation tools for demolding, it is difficult to improve the process or give design rules for the molds, which could harm the further applications of hot embossing. This paper gives our studies of the demolding process using LIGA mold inserts. The demolding forces mainly consist of thermal shrinkage stress and adhesive forces. First, a finite elements method (FEM) is applied to analyze thermal stress caused by the shrinkage differences between the mold and polymer using ABAQUS/Standard, and a thermal stress barrier is proposed as an auxiliary structure to protect against the converging stress at the bottom corner of microstructures. Then, regarding the adhesion and friction forces, the nanotribology of PMMA is studied by AFM with nickel and PTFE-coated Si3N4 tips. And based on the measurements, the adhesion and friction forces in a demolding cycle are also simulated by FEM using ABAQUS/Standard. At last Ni-PTFE is recommended as the mold material for achieving a lower surface energy and lower friction force. This work proposes several methods that can optimize the demolding process and introduces some good suggestions for mold tool design.

Temperature compensation of liquid FBAR sensors

Abstract: In this work we demonstrate a practically complete temperature compensation of the second harmonic shear mode in a composite Al/AlN/Al/SiO2 thin film bulk acoustic resonator (FBAR) in the temperature range 25 °C–95 °C. The main advantages of this mode are its higher Q value in liquids as well as its higher frequency and hence higher resolution for sensor applications. For comparative reasons the non-compensated fundamental shear mode is also included in these studies. Both modes have been characterized when operated both in air and in pure water. Properties such as Q value, electromechanical coupling, dissipation and sensitivity are studied. An almost complete temperature compensation of the second harmonic shear mode was observed for an oxide thickness of 1.22 µm for an FBAR consisting of 2 µm thick AlN and 200 nm thick Al electrodes. Thus, the measured temperature coefficient of frequency (TCF) in air for the non-compensated fundamental shear mode (1.25 GHz) varied between −31 and −36 ppm °C−1 over the above temperature range while that of the compensated second harmonic shear mode (1.32 GHz) varied between +2 ppm °C−1 and −2 ppm °C−1 over the same temperature interval. When operated in pure water the former type shows a Q value and coupling coefficient, k2t, around 180 and 2%, respectively, whereas for the second harmonic these are 230 and 1.4%, respectively.

The tool geometrical shape and pulse-off time of pulse voltage effects in a Pyrex glass electrochemical discharge microdrilling process

Abstract: Electrochemical discharge machining (ECDM) is found to be a potential process for the microdrilling of glass wafers. However, some degree of heat-affected zone resulting from the thermal energy during ECDM produces several inherent problems that might be major issues, such as over cut and taper hole. To improve the quality of the ECDM microhole, a flat sidewall–flat front tool electrode is designed to reduce taper phenomena due to the sidewall discharge. In addition, a pulse voltage is applied to improve the heat-affected zone in the rectified dc voltage. The experimental results show that the combination of flat sidewall–flat front tool and pulse voltage conspicuously increases the machining accuracy while retaining the machining rate. This experimental investigation provides a new method that overcomes the disadvantages of conventional ECDM.

Fast prototyping using a dry film photoresist: microfabrication of soft-lithography masters for microfluidic structures

Abstract: Etertec HQ-6100 dry film photoresist was used in this work to fabricate soft-lithography masters applied to microfluidic applications. We demonstrated that the use of this photoresist was a convenient alternative to conventional microfabrication approaches based on DRIE and liquid photoresists for fast-prototyping of microfluidic structures. Our method was at least two times faster than conventional processes and required limited investment for equipments. Finally, this approach was applied to the design and fabrication of microfluidic networks used for gradient generation in bulk solution.

Surface roughness, asperity contact and gold RF MEMS switch behavior

Abstract: Modeling predictions and experimental measurements were obtained to characterize the electro-mechanical response of radio frequency (RF) microelectromechanical (MEM) switches due to variations in surface roughness and finite asperity deformations. Three-dimensional surface roughness profiles were generated, based on a Weierstrass–Mandelbrot fractal representation, to match the measured roughness characteristics of contact bumps of manufactured RF MEMS switches. Contact asperity deformations due to applied contact pressures were then obtained by a creep constitutive formulation. The contact pressure is derived from the interrelated effects of roughness characteristics, material hardening and softening, temperature increases due to Joule heating and contact forces. This modeling framework was used to understand how contact resistance evolves due to changes in the real contact area, the number of asperities in contact, and the temperature and resistivity profiles at the contact points. The numerical predictions were qualitatively consistent with the experimental measurements and observations of how contact resistance evolves as a function of deformation time history. This study provides a framework that is based on integrated modeling and experimental measurements, which can be used in the design of reliable RF MEMS devices with extended life cycles.

An atomistic introduction to anisotropic etching

Abstract: This review-oriented paper presents a simplified model of anisotropic etching of crystalline silicon for the three principal orientations (1 1 1), (1 1 0) and (1 0 0), including their vicinal surfaces. The model combines pit nucleation and step flow with micromasking and diffusion phenomena in order to explain the major morphologic features and their changes with concentration. It also qualitatively explains the orientation and concentration dependence of the etch rate. We conclude that the shallow round pits on (1 0 0) and the elongated zigzag structures on (1 1 0), each of which constitutes the basic morphology of the corresponding surface, are actually the result of the same physical phenomenon, diffusion, disguised by a different underlying symmetry. It is also shown that the formation of hillocks on the two surfaces at different concentrations is a related process. We also describe and support the idea that the rotation of the triangular pits on (1 1 1) is due to a selective blocking mechanism by the etchant cations and explain how the formation of polygonal steps and/or step bunches on miscut (1 1 1) surfaces can occur as a result of diffusion phenomena and not only due to micromasking. Finally, the particular features of Cu as a micromasking agent are explained.

Fabrication techniques of convex corners in a (1?0?0)-silicon wafer using bulk micromachining: a review

Abstract: Silicon bulk micromachining using the wet anisotropic etching process is widely employed for the development of commercial products such as an inkjet printer head, a pressure sensor, accelerometers, infrared sensors, etc using (1 0 0) silicon wafers. In wet anisotropic etching, the resultant shape and size of the microstructures are restricted by crystallographic properties of silicon. If structures such as seismic mass in an accelerometer are required to be created, convex corners will emerge in the etching process. Considerable deformation occurs at convex corners resulting in poor control on the shape and size of the microstructure. Various methods/techniques are developed to overcome the problem of undercutting at convex corners in a (1 0 0) silicon wafer. Here, we have reviewed the fabrication techniques for the realization of convex corners in silicon bulk micromachining technology. The review is restricted to the wet anisotropic etching process which is usually performed in potassium hydroxide solution, ethylenediamine pyrocatechol solution, tetramethylammonium hydroxide, etc. The corner compensation method is the most widely used technique for the fabrication of convex corners. Various types of corner compensating design have been proposed by different research groups. The corner compensation method gives nearly sharp corners. Recently developed techniques, which do not use any compensating design, give perfect convex corners. The limitations and advantages of all the techniques have been discussed. (Some figures in this article are in colour only in the electronic version)