Table of contents

A new technique for the fabrication of radio frequency (RF) microelectromechanical systems (MEMS) shunt switches in recessed coplaner waveguide (CPW) configuration on glass substrates is presented. Membranes with low spring constant are used for reducing the pull-in voltage. A layer of silicon dioxide is deposited on glass wafer and is used to form the recess, which partially defines the gap between the membrane and signal line. Positive photoresist S1813 is used as a sacrificial layer and gold as the membrane material. The membranes are released with the help of Pirhana solution and finally rinsed in low surface tension liquid to avoid stiction during release. Switches with 500 µm long two-meander membranes show very high isolation of greater than 40 dB at their resonant frequency of 61 GHz and pull-in voltage less than 15 V, while switches with 700 µm long six-strip membranes show isolation greater than 30 dB at the frequency of 65 GHz and pull-in voltage less than 10 V. Both types of switches show insertion loss less than 0.65 dB up to 65 GHz.

A strain measurement system based on a shear horizontal surface acoustic wave (SH SAW) was developed. The developed system is composed of a SAW microsensor, a printed circuit board (PCB), an adhesive and a strain gauge. When a compression force is applied to the PCB by the strain gauge, the PCB is bent so that external strain energy can be evenly delivered to the microsensor without any detachment of the sensor from the board. When a stretching force is applied to the PCB under the condition that one side of the PCB is fixed and the other side is modulated, the actual length of the SAW delay line between the two interdigital transducers (IDTs) is increased. The increase in the delay line length causes a change in the time for the propagating SAW to reach the output IDT. If strain energy is applied to the piezoelectric substrate, the substrate density is changed, which then changes the propagation velocity of the SAW. Coupling-of-modes modeling was conducted prior to fabrication to determine the optimal device parameters. Depending on the strain, the frequency difference was linearly modulated. The obtained sensitivity for stretching was 17.3 kHz/% for the SH wave mode and split electrode. And the obtained sensitivity for bending was 46.1 kHz/% for the SH wave mode and split electrode. The SH wave showed about 15% higher sensitivity than the Rayleigh wave, and the dog-bone PCB showed about 8% higher sensitivity than the rectangular PCB. The obtained sensitivity was about five times higher than that of existing SAW-based strain sensors.

In this paper, we present a new concept of particle filtration modules for lab-on-a-chip (LOC) devices. The modules are designed as vertical walls that separate fluidic micro channels. In these walls, nano channels that connect the two adjacent micro channels are embedded. Fluid and small particles can penetrate the walls through the embedded nano channels, while particles larger than the nano channels size will be stopped. By keeping the fluid in the surface plane of the LOC, the module can be easily integrated with other LOC modules. To fabricate these modules, we use chemical vapor deposition to deposit nanometer thick sacrificial layers and embed them into the wall structure. Wet chemical enchants are used to remove the sacrificial layers and form the nano channels. This fabrication process can generate 100 nm⁻¹ μm high nano channels with high accuracy and uniformity with well-established micromachining techniques. Two types of modules, surface micromachining design for more flexibility in the choice of substrate material and bulk micromachining design for higher porosity without increasing footprint, are fabricated and successfully tested.

Polymer wafer bonding is a widely used process for fabrication of microfluidic devices. However, best practices for polymer bonds do not achieve sufficient bond strength for many applications. By applying a voltage to a polymer bond in a process called voltage-assisted bonding, bond strength is shown to improve dramatically for two polymers (Cytop™ and poly(methyl methacrylate)). Several experiments were performed to provide a starting point for further exploration of this technique. An optimal voltage range is experimentally observed with a reduction in bonding strength at higher voltages. Additionally, voltage-assisted bonding is shown to reduce void diameter due to bond defects. An electrostatic force model is proposed to explain the improved bond characteristics. This process can be used to improve bond strength for most polymers.

A sensitive, broad-bandwidth piezoelectric microelectromechanical systems (MEMS) transducer based on frequency interleaving of resonant transducers was designed and fabricated. A sputter-deposited piezoelectric zinc oxide (ZnO) thin film on the diaphragm is used to sense and generate acoustic pressure. A high compliance cantilever and spiral-beam-supported diaphragms are designed and built on the edge-released MEMS structure to release initial residual stress and to avoid in-plane tension when bent. Stress compensation has been achieved by adjusting the thickness of each layer of the cantilever and by compensating for the ZnO film's compressive stress with the bimorph structure of the spiral-beam. For a given pressure level and diaphragm size, the maximum strain on the spiral-beam-supported diaphragm is about an order of magnitude larger than that of a rectangular cantilever diaphragm. Also, the acoustic transducer built on the spiral-beam-supported diaphragm has a much higher sensitivity (but with less tolerance on the fabrication process variation and at the cost of lower usable bandwidth) than the one built on a rectangular cantilever diaphragm. By connecting many transducers in parallel, both the sensitivity and acoustic output were improved about 30 times. The interleaving of the transducers increased not only the sensitivity, but also broadened the useable bandwidth.

An integrated optical filter array is demonstrated using simple gray-scale lithography and a subsequent reactive ion etching process. Gray-scale lithography allows three-dimensional structure patterning to form controllable cavity thickness in a Fabry–Perot resonance structure. This approach avoids repeated photolithography and etching processes in conventional filter array fabrications. The filter array is formed by single gray-scale lithography and does not require a repeated alignment process of each filter. The demonstrated filter array is fabricated with silicon dioxide (SiO₂) as a cavity layer and dielectric mirrors of multilayered magnesium fluoride (MgF₂) and zinc selenide (ZnSe). The smallest demonstrated filter size is 10 µm which can be fitted into the size of current CMOS-based photodetectors. However, its ultimate size will be determined by the minimum resolution of gray-scale lithography. This will allow an optical filter array with high resolution and small size which can be directly integrated onto a detector array or CCD for miniaturized spectrometers.

In this paper, we present a variable and self-assembled spherical microlens array (MLA) fabricated by the use of the hydrophilic effect under ultraviolet (UV)/ozone treatment. The optical power and surface roughness of the MLA were further enhanced by applying an external electric field. This method provides a fast, simple and low-cost process because it does not require lithography, heating, or etch-transfer processes. The MLA was made up of negative photoresist SU-8 (n = 1.63) on a glass substrate. Microlenses from 50 to 200 µm diameters with 1–4 min UV/ozone treatment time were successfully fabricated. The optical focusing power of the 100 µm diameter MLA was also improved by using the electric field of 1.7 and 3.4 V µm⁻¹. A 10 µm thick shadow mask was used to define the UV/ozone treatment area to create more hydrophilic surfaces on an SU-8 photoresist base layer on a glass substrate. After hydrophilic zones were created, the glass substrate was immersed into a diluted SU-8 photoresist solution and then removed. Finally, the MLA was formed after applying a parallel electric field followed by an UV curing process. The MLA was kept in spherical shape, while the radius of curvature was changed. Focal lengths from 0.09 to 2.91 mm for a spherical MLA were experimentally demonstrated and investigated.

Since the advent of microelectromechanical systems (MEMS) technology, friction and wear are considered as key factors that determine the lifetime and reliability of MEMS devices that contain contacting interfaces. However, to date, our knowledge of the mechanisms that govern friction and wear in MEMS is insufficient. Therefore, systematically investigating friction and wear at MEMS scale is critical for the commercial success of many potential MEMS devices. Specifically, since many emerging MEMS devices contain more sidewall interfaces, which are topographically and chemically different from in-plane interfaces, studying the friction and wear characteristics of MEMS sidewall surfaces is important. The microinstruments that have been used to date to investigate the friction and wear characteristics of MEMS sidewall surfaces possess several limitations induced either by their design or the structural film used to fabricate them. Therefore, in this paper, we report on a single-crystal-silicon-based microinstrument to study the frictional and wear behavior of MEMS sidewalls, which not only addresses some of the limitations of other microinstruments but is also easy to fabricate. The design, modeling and fabrication of the microinstrument are described in this paper. Additionally, the coefficients of static and dynamic friction of octadecyltrichlorosilane-coated sidewall surfaces as well as sidewall surfaces with only native oxide on them are also reported in this paper.

A microfabricated fluidic chip for sorting red blood cells (RBCs) by size has been designed, fabricated and tested. The performance of the chip has been compared against a flow cytometer using samples from identical populations of cells, and statistically significant (p < 0.0005) differences in the measured cell size distributions were observed. The measurement paradigm reported here differs from previously demonstrated devices such as microfabricated Coulter counters or flow cytometers, in that the analysis is inherently parallel and is thus suitable for high throughput, point-of-care analysis. This study is empirical and semi-quantitative. However, important features of RBC trapping are characterized and indications for improved device design are described.

In the present work, we report preparation and characterization of silicon carbide (SiC) films obtained by RF magnetron sputtering using a SiC ceramic target. The films were deposited in Ar ambient without external substrate heating. The residual stress of the films was measured as a function of sputtering parameters. The stress of the as-deposited films was observed to be compressive for the entire range of sputtering parameters used in the present work. Postdeposition annealing at 400 °C in N₂ ambient was useful in reducing the stress in the films. On sequentially annealing the films at higher temperatures (600 and 800 °C), the nature of the stress changed from low compressive to high tensile. A superhard SiC film with low residual compressive stress (58.7 MPa) was obtained with hardness and Young's modulus values of 49.86 GPa and 363.75 GPa respectively. The x-ray diffraction pattern revealed that the films were either amorphous or nano-crystalline, depending on the deposition parameters and postdeposition annealing temperature. Atomic force microscopy roughness results confirmed good chemical stability of the films in potassium hydroxide and buffered hydrofluoric acid solutions. Several types of micro-structures were fabricated to demonstrate the feasibility and compatibility of these films in MEMS fabrication.

We report on a new method of three-dimensional structuring by means of proton beam writing in p-type gallium arsenide. While up to now vertical features have been created by varying the proton beam energy during irradiation which changes the proton penetration depth and thereby the depth of the material modification, we manufactured 3D structures with a single beam energy but different proton doses supplemented by a subsequent controlled electrochemical etching process. This new approach could simplify 3D structuring in semiconductors and the usage of proton beam writing for the manufacturing of micro electromechanical devices with high aspect ratios and smooth sidewalls.

Polydimethylsiloxane (PDMS) is a popularly used nontoxic and biocompatible material in microfluidic systems, which is relatively cheap and does not break easily like glass. The simple fabrication, optical transparency and elastomeric property make PDMS a handy material to work with. In order to develop different applications of PDMS in microfluidics and bioengineering, it is necessary to modify the PDMS surface nature to improve wetting characteristics, and to have a better control in nonspecific binding of proteins and cells, as well as to increase adhesion. At the moment, the hydrophilic surface modification performance of PDMS is known to recover its hydrophobicity shortly after oxidation modification, which is not stable in the long term (Owen and Smith 1994 J. Adhes. Sci. Technol.8 1063–75). This paper presents a long-term stable hydrophilic surface modification processing of PDMS. The poly(dimethylsiloxane-ethylene oxide polymeric) (PDMS-b-PEO) is used in this project as a surfactant additive to be added into the PDMS base and the curing agent mixture during polymerization and to create hydrophilic PEO-PDMS. The contact angle can be controlled at 21.5–80.9° with the different mixing ratios and the hydrophilicity will remain stable for two months and then slightly varied later. We also investigate the bonding conditions of the modified PDMS to a silicon wafer and a glass wafer. To demonstrate its applications, we designed a device which consists of microchannels on a silicon wafer, and PEO-PDMS is utilized as a cover sheet. The capillary function was investigated under the different contact angles of PED-PDMS and with different aspect ratios of microchannels. All of the processes and testing data are presented in detail. This easy and cost-effective modified PDMS with a good bonding property can be widely used in the capillary device and systems, and microfluidic devices for fluid flow control of the microchannels in biological, chemical, medical applications.

This paper presents a new fabrication method to manufacture alkali reference cells having dimensions larger than standard micromachined cells and smaller than glass-blown ones, for use in compact atomic devices such as vapour-cell atomic clocks or magnetometers. The technology is based on anodic bonding of silicon and relatively thick glass wafers and fills a gap in cell sizes and technologies available up to now: on one side, microfabrication technologies with typical dimensions ≤ 2 mm and on the other side, classical glass-blowing technologies for typical dimensions of about 6–10 mm or larger. The fabrication process is described for cells containing atomic Rb and spectroscopic measurements (optical absorption spectrum and double resonance) are reported. The analysis of the bonding strength of our cells was performed and shows that the first anodic bonding steps exhibit higher bonding strengths than the later ones. The spectroscopic results show a good quality of the cells. From the double-resonance signals, we predict a clock stability of ≈3 × 10⁻¹¹ at 1 s of integration time, which compares well to the performance of compact commercial Rb atomic clocks.

The electro-osmotic flow limit of detection and separation efficiency of glass channels machined using abrasive jet micromachining (AJM) were measured and compared with those for channels machined using conventional wet etching with hydrofluoric acid. It was found that the electro-osmotic mobility in AJM channels was similar to that in wet-etched channels, ∼4 × 10⁻⁴ cm² V⁻¹ s⁻¹ for 20 µm channels, despite a two-decade difference in surface roughness. Similarly, limits of detection measured on the two types of chips were roughly comparable to each other and on the order of 1 nM (injected sample concentration). The separation efficiency calculated from TAMRA dye injections in AJM channels, however, was found to be significantly lower, ∼0.2–0.25 times, than that in wet-etched channels. The effect of surface roughness on the separation efficiency and electro-osmotic mobility in micro-channels is discussed in the context of the literature. Furthermore, experimental data concerning the effect of the AJM process conditions on the surface roughness are presented and discussed with the aim of exploring methods to improve surface quality in AJM. Commercially available self-adhesive elastomeric masks were found to be particularly suitable for rapid prototyping as they provided reasonably high resolution and machining flexibility.

This paper presents a fringe capacitance formula of microstructures. The formula is derived by curve fitting on ANSYS simulation results. Compared with the ANSYS and experimental results, the deviation is within ±2%. The application to determine the pull-in voltage of an electrostatic micro-beam is demonstrated, which agrees very well with the experimental data. The formula presented is very accurate, yields explicit physical meanings and is applicable to common dimension ranges for MEMS devices.

A recent application domain of MEMS technology is in the development of microthrusters for micro-/nanosatellites. Among the various types of MEMS microthruster developed so far, the vaporizing liquid microthruster (VLM) has been widely explored for its capability to produce continuously variable thrust in the micro-Newton (µN) to mili-Newton (mN) range. This paper reports the design and experimental validation of silicon MEMS VLM consisting of a microcavity, inlet channel and converging–diverging (C-D) in-plane exit nozzle integrated in two micromachined bonded chips and sandwiched between two p-diffused microheaters, located at the top and bottom surface of the device. Structural configuration was designed using simple analytical equations to achieve maximum thrust force by controlling the inlet propellant flow and heater power of VLM in an efficient way. In addition, a 3D model using a computational fluid dynamics technique was constructed to simulate the aft section of VLM for the investigation of its aerodynamic behavior. The device fabrication and testing have been briefly described. The fabricated VLM is capable to produce 1 mN thrust using maximum heater power of 3.6 W at a water flow rate of 2.04 mg s⁻¹ using an in-plane C-D exit nozzle of throat area 130 µm × 100 µm. A detailed thrust force measurement was carried out with the variation of input heater power for different mass flow conditions and exit to throat area ratio of the exit nozzle, and the results were interpreted with the theoretical model. The model gives considerable physical insight in the operation of the VLM. Finally, a performance comparison with other published VLM results indicates that the present design can yield comparatively more thrust force with much less input power. A performance comparison with other published VLM results indicates that the present design can achieve improved performance by integrating two heaters with appropriate chamber volume in respect of propellant flow rate and input power for obtaining a supersaturated dry stream.

This paper reports the miniaturization of Bragg-multiplexed pressure sensing membranes for integrated photonic chips. The analysis compares a novel Fabry–Pérot Bragg grating (FPBG) design that integrally spans a thin (54 µm) silica membrane to a recently reported single Bragg grating (BG) design that resides within the membrane. Unlike the single BG, the FPBG maintains spectral integrity as the dimensions of the membrane become sub-millimetre. In addition it is shown that the FPBG structure can also be used for inherent temperature referencing, having a Bragg thermal sensitivtiy of 13.5 pm °C⁻¹, which can be decoupled from pressure effects. For the reported sub-millimetre membrane, pressure resolution was enhanced by a factor of three and spectral bandwidth reduced by over five-fold.

Surface acoustic wave (SAW) excitation of an acoustic field in a trapezoidal glass microfluidic channel for particle manipulation in continuous flow has been demonstrated. A unidirectional interdigital transducer (IDT) on a Y-cut Z-propagation lithium niobate (LiNbO₃) substrate was used to excite a surface acoustic wave at approximately 35 MHz. An SU8 layer was used for adhesive bonding of the superstrate glass layer and the substrate piezoelectric layer. This work extends the use of SAWs for acoustic manipulation to also include glass channels in addition to prior work with mainly poly-di-methyl-siloxane channels. Efficient alignment of 1.9 µm polystyrene particles to narrow nodal regions was successfully demonstrated. In addition, particle alignment with only one IDT active was realized. A finite element method simulation was used to visualize the acoustic field generated in the channel and the possibility of 2D alignment into small nodal regions was demonstrated.

An electrostatic microvalve for pneumatic control of microfluidic devices is presented. The valve consists of several, individually manufactured pieces assembled to form a microvalve. The unique feature is its ability to be integrated with microfluidic systems. The valve was manufactured by depositing a thin chrome layer on poly(methyl methacrylate). A copper foil was used as a flexible membrane. When a voltage was applied between the chrome and the copper foil, the electrostatic force pulled the foil closed against the chrome and stopped the airflow. Parylene C was selected as a dielectric to prevent a short circuit between electrodes. It was determined through testing that a 6 µm parylene layer with a 58 µm cavity depth provided the best combination of a low closing voltage and a high flowrate. These valves worked at pressures up to 40 kPa with an average closing voltage of 680 V, and an average flowrate of 1.05 mL min⁻¹. Tests showed that it may be able to function as a flowrate control valve at pressures greater than 40 kPa. Dielectric charging occurred in the valve. Switching the control voltage polarity with each actuation delayed the onset of dielectric charging. The valve was used to pneumatically control flow in a simplified microfluidic device.

A cavity-silicon-on-insulator (SOI)-based single crystal silicon (SCS) micromechanical resonator has been demonstrated in this paper. The most distinguishing feature of this method is that it solves the restrictions of being released from the sacrificial layer. The resonator structures can be fabricated and released in one step using dry anisotropic etching. The differential drive, single-ended sense configuration is implemented to measure the electrical characterization of the fabricated resonator. The fabricated square plate resonator has been excited in the Lame´ mode at a resonant frequency of 4.126 MHz and exhibits a quality factor (Q) as high as 5.49× 10⁶ at a pressure of 0.05 mbar. This result corresponds to a frequency–Q product of 2.27× 10¹³, which is the highest value demonstrated to date for silicon-based resonators as far as we know. The dependence of Q and resonant frequency on the operating pressure is measured and characterized. The temperature stability of the device is also demonstrated, with the temperature coefficient of resonant frequency less than −20.8 ppm °C⁻¹ in the temperature range from −10 to 60 °C. The high performance of the resonator not only benefits from the superior performance of SCS as a mechanical material, but also the merit of the cavity-SOI structure.

Anisotropic wet chemical etching of quartz is a bulk micromachining process for the fabrication of micro-electro-mechanical systems (MEMS), such as resonators and temperature sensors. Despite the success of the continuous cellular automaton for the simulation of wet etching of silicon, the simulation of the same process for quartz has received little attention—especially from an atomistic perspective—resulting in a lack of accurate modeling tools. This paper analyzes the crystallographic structure of the main surface orientations of quartz and proposes a novel classification of the surface atoms as well as an evolutionary algorithm to determine suitable values for the corresponding atomistic removal rates. Not only does the presented evolutionary continuous cellular automaton reproduce the correct macroscopic etch rate distribution for quartz hemispheres, but it is also capable of performing fast and accurate 3D simulations of MEMS structures. This is shown by several comparisons between simulated and experimental results and, in particular, by a detailed, quantitative comparison for an extensive collection of trench profiles.

This paper proposes solutions for high nonlinearity and structural instability in electrostatically actuated MEMS capacitors. The proposed designs use the flexibility of the moving electrode and nonlinear structural stiffness to control the characteristic capacitance–voltage (C–V) response. The moving plate displacements are selectively constrained by mechanical stoppers to prevent sudden jumps in the capacitance and to eliminate the pull-in. A symmetric double-humped electrode shape is utilized which results in a fairly constant sensitivity in the C–V curve and therefore a linearized response. An analytical and a finite-element coupled-field model are developed to study the behavior of the proposed capacitors and to optimize their design for maximum linearity. The experimental results verify that the designs introduced in this paper improve the linearity of the C–V response and increase the maximum tunability by three times compared to conventional MEMS parallel-plate capacitors. At the same time, they also eliminate the pull-in hysteresis of the response.

This paper describes a technique that uses applied force to dice anodically bonded silicon–glass wafers with high yields. The chips are suspended to the wafer by anchors; when pressure is applied to a chip, stress concentrates at the narrow anchors, which then fracture and release the chip from the wafer. Anchor fracturing has been used to dice crystalline and non-crystalline materials but its application to dicing constructs of various materials has remained challenging because of the disparity with which fractures propagate in different materials and in their interfaces. The technique we present here makes it possible to fracture composite materials (silicon and glass anodically bonded) by eliminating any material interface from the fracturing regions—i.e. the anchors. The approach was tested using two types of anchors fabricated in anodically bonded silicon–glass wafers: in one type, the silicon–glass interface expanded most of the anchor (coincident anchors) but such an interface was inexistent in the other type (non-coincident anchors). The study determined dicing yields—i.e. percentage of chips not damaged by the fracture of the anchors—of ∼40% and 100% for test structures with coincident and non-coincident anchors, respectively. The presence of a silicon–glass interface in the suspending anchors often resulted in fractures propagating away from the anchors, and ultimately in damage to the suspended chips. This technique provides an inexpensive, robust and simple alternative to currently available dicing methods for the glass–silicon wafer pairs frequently used in wafer-level packaging of MEMS.

This paper reports a new technique for masking deep-UV exposure of poly(methyl methacrylate) (PMMA) using a printed wax mask. This technique provides an inexpensive and bulk fabrication method for PMMA structures. The technique involves the direct printing of the mask onto a polymer sheet using a commercial wax printer. The wax layer was then transferred to a PMMA substrate using a thermal laminator, exposed using deep-UV (with a wavelength of 254 nm), developed in an IPA:water solution, and completed by bonding on a PMMA cap layer. A sample microfluidic device fabricated with this method is also presented, with the microchannel as narrow as 50 µm. The whole process is easy to perform without the requirement for any microfabrication facilities.