Development of Membrane Micro Embossing (MeME) Process for Self-Supporting Polymer Membrane Microchannel

In this work, we have successfully fabricated self-supporting polymer membrane microchannels with width of 50μm, depth of 50μm, and wall thickness of ~5μm. To fabricate this self-supporting polymer membrane microchannel, we have developed novel micro fabrication process named “membrane micro embossing (MeME)”. In this process, a thin (~μm) thermoplastic polymer membrane set between a master mold and a deformable support substrate is heated to glass transition temperature, and pressed to fit the membrane onto the mold surface. By heat-sealing the embossed membrane with another planar membrane, we can fabricate sealed membrane microchannels. This membrane microchannel surrounded by only thin membrane walls will provide a powerful platform for micro-scale biological and chemical analysis.

Design and Fabrication of a Magnetocaloric Microcooler

Magnetocaloric refrigeration is increasingly being explored as an alternative technology for cooling. This paper presents the design and fabrication of a micromachined magnetocaloric cooler. The cooler consists of fluidic microchannels (in a Si wafer), diffused temperature sensors, and a Gd5(Si2Ge2) magnetocaloric refrigeration element. A magnetic field of 1.5 T is applied using an electromagnet to change the entropy of the magnetocaloric element for different ambient temperature conditions ranging from 258 K to 280 K, and the results are discussed. The tests show a maximum temperature change of 7 K on the magnetocaloric element at 258 K. The experimental results co-relate well with the entropy change of the material.

Electrokinetic Focusing and Dispensing of Particles and Cells on Microfluidic Chips

This work investigated the electrokinetic focusing and dispensing of polystyrene particles and red blood cells on microfluidic chips. Particles or cells were first electrokinetically focused using the merging of focusing streams on the sample stream, and subsequently separated as a result of the focusing. These particles or cells were then selectively dispensed from the focused sample stream using precise application of electrical pulses. The whole process of focusing, separation and dispensing of particles was visualized by a custom-made microscopy system. In particular, the width of the focused fluorescein stream and the accelerated electrophoretic motion of particles and cells were measured in a cross-channel and compared with a proposed analytical model. The electrokinetic manipulation of particles and cells demonstrated in this work can be used for developing integrated lab-on-a-chip devices for studies of cells.

Packaging of In-Plane Thermal Microactuators for BioMEMS Applications

This paper addresses two issues related to in-plane, electro-thermal actuators for BioMEMS applications. First, in order to protect the actuator from biological debris and particulates, a packaging technique using a flip-chip bonded polysilicon cap is demonstrated. The encapsulated actuator transmits motion outside the package via a piston, which moves through a small clearance. The second issue addressed is the reduction in efficiency of the thermal actuator in liquids. By coating the packaged actuator with a thin conformal hydrophobic layer via an atomic layer deposition (ALD) process, the liquid is prevented from entering the encapsulation. This avoids direct contact between the actuator and the surrounding liquid thereby improving its efficiency. The unpackaged and packaged actuators were tested in both air and de-ionized water. Although the packaging resulted in a reduction in the performance of the thermal actuator in air, the actuation efficiency in water was significantly improved due to the isolation of the hot arms from the liquid. This packaging technique is also applicable to other MEMS devices and in-plane actuators such as electrostatic comb drives for engineering as well as biological applications.

Optical and Electrical Methods to Measure the Dynamic Behavior of a MEMS Gyroscope Sensor

A full characterization of the mechanical parameters for vibrating MEMS sensors is required before integrating the mechanical and the electronic part. This is to verify that the main design specifications are fulfilled before sensors are available on the market. The main goal is to accurately establish the well-working devices in the shortest time. In this paper the electrical method based on the measurement of the GND current is used to satisfy this purpose. To check the validity of the achieved results through this method a comparison is done with those obtained through the widely used optical method based on vibration measurements through by means of a Laser Doppler Vibrometer (LDV).

Precise Prediction on Pull-In Instability of a Deformable Micro-Plate Actuated by Distributed Electrostatic Force and Approximate Closed-Form Solutions

This study is dedicated to perform nonlinear asymptotic analysis based on the continuous thin plate model of MEMS capacitive sensor/actuator in order to predict the pull-in voltages/positions more precisely than past works. In these past studies, only discrete models without residual stress were considered. A sensor/actuator is considered in structure of two parallel electrostatically-charged flexible square plates — one thin plate in persistent vibrations to reflect external pressure and another thick plate in relative still as the backplate. The dynamic model in the form of the partial differential equation for the parallel plates is first established based on the balance among plate flexibility, residual stress and electrostatic forces. Assuming harmonic deflection for the vibrating plate clamped on boundaries, Galerkin method is used to decompose the established system p.d.e. into discrete modal equations. Solving the discrete modal equations, plate deflection can be obtained. The pull-in position is next solved from the condition that as the pull-in occurs the electrostatic attraction force on the deflected plate exceeds the elastic restoring force by the deflected plate. It is found from analysis results for some case study that the pull-in position is 1.66 μm with air gap of 3.75 μm. This predicted pull-in position is smaller than the predict position from past works, two-thirds of the gap. In addition to theoretical analysis, experiments are also conducted to verify the correctness of the established model.

Piezoelectric Thin Film AlN Annular Dual Contour Mode Bandpass Filter

The following work presents the analytical, numerical and experimental characterization of a novel piezoelectric Aluminum Nitride MEMS bandpass filter. In contrast to multipole filters employing distinct mechanically or electrically coupled resonator building blocks, the passband of the device in the present work is defined by the proximity of two natural contour modes of vibration in a single annular resonator. The proposed implementation, albeit currently limited to dual-pole filters, results in smaller form factors and reduces device sensitivity to across wafer fabrication tolerances.

An Improved Analytical Model for Deflections of a Circular Multi-Layer Piezoelectric Actuator

Models for simple closed-form analytical solutions for accurately predicting static deflections of circular thin-film piezoelectric microactuators are very useful in design and optimization of a variety of MEMS sensors and actuators utilizing piezoelectric actuators. While closed-form solutions treating actuators with simple geometries such as cantilevers and beams are available, simple analytical models treating circular bending-type actuators commonly used in MEMS applications are generally lacking. This paper presents a closed-form analytical solution for accurately estimating the deflections and the volume displacements of a circular multi-layer piezoelectric actuator under combined voltage and pressure loading. The model for the analytical solution presented in this paper, which is based on classical laminated plate theory, allows for inclusion of multiple layers and non-uniform diameters of various layers in the actuator including bonding and electrode layers, unlike other models previously reported in the literature. The analytical solution presented is validated experimentally as well as through a finite element solution and excellent experiment-model correlation within 1% variation is demonstrated. General guidelines for optimization of circular piezoelectric actuator are also discussed. The utility of the model for design optimization of a multi-layered piezoelectric actuator is demonstrated through a numerical example wherein the dimensions of a test actuator are optimized to improve the displaced volume by three-fold under combined voltage and resisting pressure loads.

Prediction of Laminar Flow in a Microchannel With Transverse Ultrahydrophobic Ribs

One approach recently proposed for reducing the frictional resistance to liquid flow in microchannels is the patterning of micro-ribs and cavities on the channel walls. When treated with a hydrophobic coating, the liquid flowing in the microchannel wets only the surfaces of the ribs, and does not penetrate the cavities, provided the pressure is not too high. The net result is a reduction in the surface contact area between channel walls and the flowing liquid. For micro-ribs and cavities that are aligned normal to the channel axis (principal flow direction), these micro-patterns form a repeating, periodic structure. This paper presents experimental and numerical results of a study exploring the momentum transport in a parallel plate microchannel with such microengineered walls. The liquid-vapor interface (meniscus) in the cavity regions is treated as ideal in the numerical analysis (flat). Two conditions are explored with regard to the cavity region: 1) The liquid flow at the liquid-vapor interface is treated as shear-free (vanishing viscosity in the vapor region), and 2) the liquid flow in the microchannel core and the vapor flow within the cavity are coupled through the velocity and shear stress matching at the interface. Predictions and measurements reveal that significant reductions in the frictional pressure drop can be achieved relative to the classical smooth channel Stokes flow. Reductions in the friction factor are greater as the cavity-to-rib length ratio is increased (increasing shear-free fraction) and as the channel hydraulic diameter is decreased. The results also show that the average friction factor – Reynolds number product exhibits a flow Reynolds dependence. Furthermore, the predictions reveal the impact of the vapor cavity regions on the overall frictional resistance.

Two-Station Embossing Process for Rapid Fabrication of Polymer Microstructures

The hot embossing technique is becoming an increasingly important alternative to silicon-and glass-based microfabrication technologies. The advantage of hot embossing can be mainly attributed to the versatile properties and mass production capability of polymeric materials. However, because of the use of a large mass in thermal cycling, hot embossing is subject to substantially longer cycle times than those in traditional thermoplastic molding processes.1 The longer dwell time at elevated temperatures could further result in degradation of the embossing polymer, especially for thermally sensitive polymers. The problem exacerbates when thick polymer substrates are used. To address this problem, rapid thermal cycling of the tool is needed. One method for rapid thermal cycling is to employ a low-thermal-mass multilayer mold with electrical heating elements installed right beneath the mold surface.2 This method, however, is complex in nature and may be prone to problems caused by mismatching of thermal and mechanical properties between different layers.