Table of contents

We present a micro-electro-mechanical system-based experimental technique to measure thermal conductivity of freestanding ultra-thin films of amorphous silicon nitride (Si₃N₄) as a function of mechanical strain. Using a combination of infrared thermal micrography and multi-physics simulation, we measured thermal conductivity of 50 nm thick silicon nitride films to observe it decrease from 2.7 W (m K)⁻¹ at zero strain to 0.34 W (m K)⁻¹ at about 2.4% tensile strain. We propose that such strong strain–thermal conductivity coupling is due to strain effects on fraction–phonon interaction that decreases the dominant hopping mode conduction in the amorphous silicon nitride specimens.

We describe a simple and effective method to fabricate a uniform plastic microlens array (MLA) with high fill-factor over a large area utilizing self-aligned diffuser lithography (SADL). In order to make an intimate contact between the photomask and the positive photoresist during 3D diffuser lithography, which is crucial for obtaining a uniform MLA mold over a large area, we fabricated a self-aligned metal mask directly on top of the positive photoresist, eliminating any air gap between the metal mask and the underlying photoresist. After replication of the developed concave MLA mold onto the poly(dimethylsiloxane) (PDMS), a standard deviation of sag (height) of the MLA was observed by laser scanning confocal lithography. The standard deviation, which indicates uniformity, was reduced by as much as a factor of 6 by applying SADL compared with that obtained from conventional diffuser lithography. Using this method, we fabricated a 7 inch MLA sheet with excellent uniformity. The proposed method can be extensively applied for fabrication of large-size MLA sheets with plastic materials thanks to its simplicity and versatility.

We report on a novel fabrication process based on thermal imprinting for the formation of micron-scale, freestanding, dielectric layers of poly(dimethylsiloxane). This technique is the basis for three-dimensional elastomeric membrane micro-electro-mechanical system applications where the structural material is part of the actuator and the lateral expansion is by vertically applied bias. We have fabricated freestanding smooth defect-free membranes with thicknesses in the range of 0.4–4.8 μm and with diameters of centimeters order of magnitude. A curve was plotted to calibrate the thickness of the elastomer layer to the pressure of the imprint. The adhesion between the polymer and the silicon (Si) chip's surface was reduced by the deposition of a hydrophobic dodecyl-trichlorosilane monolayer on the chips prior to imprinting. The ability to detach the membrane from the chips after imprinting is critical for the production of layers that are freestanding. Additionally, we demonstrate the feasibility of patterning the membranes at the time of imprinting to create freestanding patterned micron-scale membranes. A simple device made up of a freestanding circular membrane with electrodes on the circumference demonstrating the application of the method is presented here. The device's electromechanical characteristics are presented as well.

A band-stop filter using a metamaterial structure (complementary U-shaped split resonator; CUSR) on a silicon substrate with a 13% tuning range is presented for Ka band applications. The metamaterial structure is used as a frequency-selective geometry on a coplanar waveguide (CPW) and tunability is achieved with the help of MEMS bridges. The rejection in the stop band is around 25 dB for the entire tuning range. A low insertion loss of 0.5 dB is obtained in the pass band. A simple electrical model of the proposed device and the design guidelines are presented. The filter is realized by a novel fabrication methodology involving the micromachining of two bonded silicon wafers and initial fabricated results are reported.

This article presents an integrated microfluidic system that is capable of programmably metering, entrapping, coalescing, addressably storing, retrieving and manipulating emulsion droplets. A multilayer, flexible PDMS chip with specially designed fluidic channels dynamically reconfigured by pneumatically actuated diaphragms is utilized to integrate a variety of droplet manipulation schemes. Once droplets are formed, their motions are coordinated by a 2D multiplexing scheme, which exploits the bidirectional movement of diaphragms to implement a random-access microarray. In the prototype demonstration, a PDMS molding and bonding process is used to fabricate the proposed microfluidic system. Emulsion droplets with desired volumes and compositions are produced, addressably stored, manipulated and retrieved from a 4 × 4 array, which employs just 4 (= 2 × log₂4) control inputs for the operation. It has been demonstrated that (1) the integration of droplet manipulation and 2D multiplexing schemes can be achieved readily using bidirectional diaphragm valves, (2) multiplexing of an N × N array could be realized utilizing only 2 × log₂N control inputs and (3) a multifunctional, random-access microarray can be accomplished employing a multilayer PDMS chip. As such, the demonstrated random-access microarray could potentially serve as a platform for continuous tracking and multistep processing of emulsion droplets, which is desired for various biological and chemical applications.

A multidirectional-sensitive inertial microswitch with a polymer–metal composite fixed electrode has been designed and fabricated based on surface micromachining in this work. The microswitch mainly consists of a suspended proof mass as a movable electrode and a T-shaped structure on the substrate with maple leaf-like top and cantilevers around the central cylinder as vertical and lateral fixed electrodes. It can sense the applied shock accelerations from any radial direction in the xoy plane and z-axis. The new vertical composite fixed electrode of the switch is completed by electroplating and electrophoretic deposition, which can realize a flexible contact between the electrodes and reduce the bounces and prolong the contact time. As a result, the stability and reliability of the inertial switch could be greatly improved. The fabricated microswitches have been tested and characterized by a standard dropping hammer system. It is shown that the threshold acceleration of the prototype is generally uniform in different sensitive directions in the xoy plane and z-axis, which is about 70 g. The contact time of the microswitch with the composite fixed electrode is ∼110 µs in the vertical direction, which is longer than that (∼65 µs) without a polymer. The test data are in agreement with dynamic finite-element simulation results.

A surface acoustic wave (SAW)-based gyroscope with an 80 MHz central frequency was fabricated on a 128° YX LiNbO₃ piezoelectric substrate. The fabricated gyroscope is composed of a SAW resonator, metallic dots and a SAW reflective delay line. The SAW resonator, which is activated by a voltage-controlled oscillator, generates a stable standing wave with a large amplitude at an 80 MHz resonant frequency, and the metallic dots induce a Coriolis force and generate a secondary SAW in the direction orthogonal to the propagating standing wave. The SAW reflective delay line is employed to measure the Coriolis effect by analyzing the deviations in the resonant frequency of the SAW reflective delay line. A combined finite element method/boundary element method was utilized to extract the optimal device parameters prior to fabrication. The device was fabricated according to the modeling results and then measured on a rate table. When the device was subjected to an angular rotation, a secondary SAW from the vibrating metallic dots was generated owing to the Coriolis force, resulting in a perturbation of the propagating SAW in the SAW reflective delay line. Depending on the angular velocity, the reflection peak of SAW reflective delay line was changed linearly, and this change was measured by the network analyzer. The measured results matched the modeling results well. The obtained sensitivity was approximately 1.23 deg/(deg/s) in an angular rate range of 0–2000 deg s⁻¹. Good thermal and shock stabilities were observed during the evaluation process proving the shock and heat robustness of the fabricated SAW gyroscope.

Electrohydrodynamic jet (E-jet) printing has emerged as a high-resolution alternative to other forms of direct solution-based fabrication approaches, such as ink-jet printing. This paper discusses the design, integration and operation of a unique E-jet printing platform. The uniqueness lies in the ability to utilize multiple materials in the same overall print-head, thereby enabling increased degrees of heterogeneous integration of different functionalities on a single substrate. By utilizing multiple individual print-heads, with a carrousel indexing among them, increased material flexibility is achieved. The hardware design and system operation for a relatively inexpensive system are developed and presented. Crossover interconnects and multiple fluorescent tagged proteins, demonstrating printed electronics and biological sensing applications, respectively.

The transparent conducting oxide (TCO) film is a significant component in flat panel display, e-paper and touch panel. The tin-doped indium oxide (ITO) material is one of the most popular TCOs. However, ITO has high refractive index, so the phenomenon of high-reflectance limits the wide use of ITO. In this study, the structure and mass production process of new low-reflectance TCO film is verified. Laser interference lithography and the roll-to-roll UV embossing process are used to fabricate subwavelength structures on PET film; then ITO was deposited on structures by roll-to-roll sputtering. When the dimension of structures reaches 300 nm pitch, the optical reflectance and electrical performance of film are reduced to 8.1% at wavelength 550 nm and its transmittance rate is 84.3% at the same wavelength, and the sheet resistance of this film is 50.44 Ω/□. This result indicates that the new TCO proposed in this study is suitable for touch panel and other display applications.

Most popular materials for lenses, such as glass, have high absorption in the infrared range. Due to material restriction, infrared lenses are usually much more expensive. In this paper, we discussed a ubiquitous polymer material, poly(methyl methacrylate) (PMMA), for mid-wavelength infrared (MWIR) applications. PMMA is a low cost material and is widely used in daily life. We examined its optical properties in the mid-infrared range and found that poly(methyl methacrylate) is a highly promising material for MWIR lenses. Besides, liquid PMMA can be formed and solidified easily. Utilizing these characteristics, we proposed a novel way to fabricate PMMA lenses for MWIR range (wavelength from 3.6 to 5 µm). The fabrication process is much easier and less expensive compared with traditional machining processes. We have designed a PMMA Fresnel lens, which has f-number of 1.40, diameter of 10 mm and focal length of 14 mm. We also successfully fabricated the PMMA Fresnel lens using the molding process. Both structure and optical analyses show that the PMMA Fresnel lenses could meet the design parameters.

In the last decade, the development of metamaterials has led to exotic phenomena not shown in nature, including negative refractive index, invisibility cloaking and perfect absorption. To achieve these effects requires creating magnetically resonant subwavelength structures, since naturally occurring magnetism typically occurs at relatively low frequencies. In the far-infrared, or terahertz (THz), region of the electromagnetic spectrum, it is difficult to obtain a strong magnetic response from planar metamaterials at normal incidence. In this paper, multilayer electroplating is used to fabricate three-dimensional (3D) split-ring resonators that stand up out of plane. This enables the maximum coupling to the magnetic response at normal incidence. Characterization using THz time-domain spectroscopy indicates a strong magnetic resonance, and parameter extraction reveals a negative permeability from 1 to 1.3 THz with the minimal value of −2. The successful design, fabrication and characterization of 3D metamaterials provide opportunities to achieve different electromagnetic properties and novel devices in the THz range.

Nanoimprint lithography is emerging as a viable contender for fabrication of large-scale arrays of 5–500 nm features. A fabrication process for the realization of thin nanoporous membranes using thermal nanoimprint lithography is presented. Suspended silicon nitride membranes were fabricated by low-pressure chemical vapor deposition (LPCVD) in conjunction with a potassium hydroxide-based bulk micromachining process. Nanoscale features were imprinted into a commercially available thermoplastic polymer resist using a prefabricated silicon mold. The pattern was reversed and transferred to a thin aluminum oxide layer by means of a novel two-stage lift-off technique. The patterned aluminum oxide was used as an etch mask in a CHF₃/He-based reactive ion etch process to transfer the pattern to silicon nitride. Highly directional etch profiles with near vertical sidewalls and excellent Si₃N₄/Al₂O₃ etch selectivity were observed. One micrometer thick porous membranes with varying dimensions of 250 × 250 µm² to 450 × 450 µm² and a pore diameter of 400 nm have been engineered and evaluated. Results indicate that the membranes have consistent nanopore dimensions and precisely defined porosity, which makes them ideal as gas exchange interfaces in blood oxygenation systems as well as other applications such as dialysis.

A unique hybrid jetting system based on electrohydrodynamic and piezoelectric forces has been designed to verify the control of the drop velocity and to obtain ultrafine droplets with a high jetting frequency. Piezoelectric nozzles have been fabricated using silicon on insulator wafers and Pyrex glass employing a MEMS process and an anodic bonding process. The plate-type electrode and moving stage were used for the printing process. The droplet ejection mechanisms from the nozzle using the hybrid jetting system were captured by a high-speed camera synchronized with a trigger signal. The deformation of the meniscus and the jetting delay time in regard to the high operational firing frequency were investigated. It was found that controlling the droplet velocity without a change in the droplet volume and obtaining a smaller dot (59 µm in diameter) in hybrid printing mode compared with inkjet printing mode (151 µm in diameter) were possible. These results show this system's promising applicability to the fabrication of micro patterning for a wide range of printed electronics applications.

We introduce a simple, reliable and low-cost microfabrication technique utilizing laser ablation of a thin polymer film to pattern polymer nanocomposite at a high resolution. A conductive composite of poly(dimethylsiloxane) (PDMS) and carbon nanotubes (CNTs) was selected due to their wide use in microelectromechanical systems (MEMS) and unique properties including flexibility and piezoresistivity. To pattern nanocomposite, an excimer laser ablated through a thin polyethylene terephthalate film creating mold patterns. PDMS-CNTs nanocomposite was then filled into the mold with excessive amount removed by a smooth-edged tool. Bulk PDMS was poured atop and cured. After debonding devices with relief patterns of polymer nanocomposite could be readily realized. Fabrication conditions were optimized which led to reliable patterning of various microstructures. Detailed surface profiling revealed excellent pattern authenticity and uniformity. Minimal feature size of patterns reached below 20 µm which indicated a significant improvement from prior reports. Moreover, the presented technique required only a software design to rapidly generate new patterns, thereby eliminating costly hardware such as lithography mask, stamp and clean room. Fabrication time and cost could be consequently reduced—ideal for lab prototyping purposes. Sensor examples are discussed to demonstrate versatile applications of polymer nanocomposite in MEMS.

This paper presents a rapid bulk micromachining process named polymer passivation layer for suspended structures etching by using a polymer as a protecting passivation layer at both anisotropic and isotropic etching steps. Without using silicon-dioxide (SiO₂) deposition or boron doping as a protection layer at the releasing step, the proposed method can fabricate suspended single-crystal silicon structures in an inductively coupled plasma reactive ion etching chamber directly, which would simplify the fabrication process and save fabrication time. The current study systematically investigates critical fabrication parameters to verify the feasibility of the proposed method, and discusses the polymer passivation time and removal time of a polymer at the base of a substrate at four different opening gaps of 5, 10, 30 and 50 µm with the 30 µm deep trench to establish suitable recipes for fabricating suspended microstructures. It is also shown that the proposed method can fabricate not only the suspended microstructures with the same thickness, but also suspended microstructures with different thicknesses, as well as in sub-micro scale.

The charging mechanism of electrostatically driven MEMS devices was investigated. This paper shows experimental results of (i) electrostatic discharge (ESD) experiments, (ii) charging mechanism modelling and (iii) Kelvin probe force microscopy tests. It highlighted dielectric failure signature occurred under ESD events and allowed understanding of the underlying breakdown mechanism. A further study of the charging effect in conditions below the breakdown was carried out. A new approach to explore trapping phenomena that take place in thin dielectric used for electrostatic actuation is reported. Indeed a pulse-induced charging (PIC) test procedure aimed at reliability assessment of electrostatically actuated MEMS devices is presented. Based on this method, a procedure for carrying out stress testing was defined and successfully demonstrated on capacitive MEMS switches. In this case, high-voltage pulses were applied as stimulus and the parameter V_capamin, which is directly related to the charging of the insulator layer, was monitored. The PIC stress test results were correlated with conventional cycling stress ones. Finally, temperature-dependent measurements, ranging from 300 up to 355 K, were reported in order to validate the thermal-activated behaviour of the test structures. According to an Arrhenius model, the given reference material showed an activation energy of around 0.77 eV.

An improved scanning probe microscope based contact tester has been designed, constructed and used for cyclic testing of various metal contacts. The tester is designed for contact material evaluation, especially for metal-contact MEMS switch applications, and is capable of simultaneous measurements of contact force and resistance. The tester uses a specially designed silicon force sensor with an integrated contact bump and a mating silicon pillar in order to simulate switch operation. The sensor and the pillar are coated with contact materials, allowing a wide range of contact metals and metal pairs to be evaluated. The testing takes place within a custom-built test chamber in which both plasma and UV-ozone treatments are available for contact cleaning and surface modification. It was found that a dissimilar contact pair of Au–Ru with O₂ plasma cleaning can provide low contact adhesion and low resistance as compared with Au, Ru and Ir contact pairs. Layered structures with a thin layer of Ru on top of Au were also modeled and tested. Ru layers between 50 and 100 nm were effective in reducing adhesion and contact resistance provided the contact force is greater than 300 µN.

This paper presents the fabrication and application of a micro-scale hybrid wicking structure in a flat polymer-based heat pipe heat spreader, which improves the heat transfer performance under high adverse acceleration. The hybrid wicking structure which enhances evaporation and condensation heat transfer under adverse acceleration consists of 100 µm high, 200 µm wide square electroplated copper micro-pillars with 31 µm wide grooves for liquid flow and a woven copper mesh with 51 µm diameter wires and 76 µm spacing. The interior vapor chamber of the heat pipe heat spreader was 30×30×1.0 mm³. The casing of the heat spreader is a 100 µm thick liquid crystal polymer which contains a two-dimensional array of copper-filled vias to reduce the overall thermal resistance. The device performance was assessed under 0–10 g acceleration with 20, 30 and 40 W power input on an evaporator area of 8×8 mm². The effective thermal conductivity of the device was determined to range from 1653 W (m K)⁻¹ at 0 g to 541 W (m K)⁻¹ at 10 g using finite element analysis in conjunction with a copper reference sample. In all cases, the effective thermal conductivity remained higher than that of the copper reference sample. This work illustrates the possibility of fabricating flexible, polymer-based heat pipe heat spreaders compatible with standardized printed circuit board technologies that are capable of efficiently extracting heat at relatively high dynamic acceleration levels.

Through-silicon via (TSV) is an emerging technology for three-dimensional integrated circuit, system-in-packaging and wafer-level packaging applications. Among several available TSV formation methods, Bosch deep reactive ion etching (DRIE) is widely used because it enables the fabrication of TSVs with almost any diameter, from the submicrometer level to hundreds of micrometers. However, the high cost of Bosch DRIE makes it uneconomical for industrial production. We present a novel wafer-level TSV formation approach that is effective and cost-efficient. The proposed method integrates a diode-pumped solid-state ultraviolet nanosecond pulsed laser and rapid wet chemical etching. The former is effective in drilling through 400 µm thick silicon wafers and the latter is used for removing the unwanted heat-affected zone, recast layer and debris left after drilling. Experimental results demonstrate that the combined approach effectively eliminates the unwanted material formed by nanosecond laser pulses. Furthermore, this approach has a significant cost advantage over Bosch DRIE. In summary, the proposed approach affords superior TSV quality, higher TSV throughput and lower cost of process ownership than Bosch DRIE. These advantages could provide the necessary impetus for rapid commercialization of the several high-density fabrication methodologies that depend on TSVs.

Cells regulate their behavior in response to mechanical strains. Cell cultures to study mechanotransuction are typically cm² in area, far too large to monitor single cell response. We have developed an array of dielectric elastomer microactuators as a tool to study mechanotransduction of individual cells. The array consists of 72 100 µm × 200 µm electroactive polymer actuators which expand uniaxially when a voltage is applied. Single cells will be attached on each actuator to study their response to periodic mechanical strains. The device is fabricated by patterning compliant microelectrodes on both sides of a 30 µm thick polydimethylsiloxane membrane, which is bonded to a Pyrex chip with 200 µm wide trenches. Low-energy metal ion implantation is used to make stretchable electrodes and we demonstrate here the successful miniaturization of such ion-implanted electrodes. The top electrode covers the full membrane area, while the bottom electrodes are 100 µm wide parallel lines, perpendicular to the trenches. Applying a voltage between the top and bottom electrodes leads to uniaxial expansion of the membrane at the intersection of the bottom electrodes and the trenches. To characterize the in-plane strain, an array of 4 µm diameter aluminum dots is deposited on each actuator. The position of each dot is tracked, allowing displacement and strain profiles to be measured as a function of voltage. The uniaxial strain reaches 4.7% at 2.9 kV with a 0.2 s response time, sufficient to stimulate most cells with relevant biological strains and frequencies.

This paper reports a novel room-temperature hermetic liquid sealing process where the access ports of liquid-filled cavities are sealed with wire-bonded stud bumps. This process enables liquids to be integrated at the fabrication stage. Evaluation cavities were manufactured and used to investigate the mechanical and hermetic properties of the seals. Measurements on the successfully sealed structures show a helium leak rate of better than 10⁻¹⁰ mbarL s⁻¹, in addition to a zero liquid loss over two months during storage near boiling temperature. The bond strength of the plugs was similar to standard wire bonds on flat surfaces.

This paper proposes an integrated sensor chip for continuous monitoring of a biochemical process. It is composed of a preconcentrator and a thermoelectric biosensor. In the preconcentrator, the concentration of the injected biochemical sample is electrodynamically condensed. Then, in the downstream thermoelectric biosensor, the preconcentrated target molecules react with sequentially injected capture molecules and generate reaction heat. The reaction heat is detected based on the thermoelectric effect, and an integrated split-flow microchannel improves the sensor stability by providing ability to self-compensate thermal noise. These sequential preconcentration and detection processes are performed in completely label-free and continuous conditions and consequently enhance the sensor sensitivity. The performance of the integrated biosensor chip was evaluated at various flow rates and applied voltages. First, in order to verify characteristics of the fabricated preconcentrator, 10 µm -diameter polystyrene (PS) particles were used. The particles were concentrated by applying ac voltage from 0 to 16 V_pp at 3 MHz at various flow rates. In the experimental result, approximately 92.8% of concentration efficiency was achieved at a voltage over 16 V_pp and at a flow rate below 100 µl h⁻¹. The downstream thermoelectric biosensor was characterized by measuring reaction heat of biotin–streptavidin interaction. The preconcentrated streptavidin-coated PS particles flow into the reaction chamber and react with titrated biotin. The measured output voltage was 288.2 µV at a flow rate of 100 µl h⁻¹ without preconcentration. However, by using proposed preconcentrator, an output voltage of 812.3 µV was achieved with a 16 V_pp-applied preconcentration in the same given sample and flow rate. According to these results, the proposed label-free biomolecular preconcentration and detection technique can be applied in continuous and high-throughput biochemical applications.

The local deposition of catalysts is desired in a wide range of catalytic microsystems (microreactors and sensors). In this study, we investigate technologies enabling deposition and patterning of catalyst thin films in a manner compatible with standard micromachining processes. We evaluate and compare deposition techniques based on a combination of a self-assembly, soft-lithography and conventional micromachining. Platinum (Pt) and palladium (Pd) were used as model catalysts, both as a sputtered thin film and as nanoparticles supported on γ-alumina. The thin films were characterized and tested in terms of their catalytic activity based on CO chemisorption measurements, stability and reproducibility.

High-throughput synthesis of carbon nanostructures with reproducible shape and dimensions, at desired locations, has been a key challenge for further exploring carbon nanostructures as functional units in various nanodevices. In this work, carbon structures with dimensions from the 50 nano- to micrometer level have been fabricated by carbonizing a photo-nanoimprint lithography patterned resist polymer (AR-UL-01) at high temperature under inert atmosphere. The resulting carbon nanostructures showed significant vertical shrinkage but minimal loss in the lateral direction. Thermal behavior studies of the resist polymer in the pyrolysis cycle indicated gaseous evolution of various byproducts before the formation of solid carbon. Microstructure, elemental composition and resistivity characterization of the nanostructures produced by this process has shown that the carbon derived from a pyrolyzed nanoimprint resist is very similar to the pyrolyzed photoresist carbon from an SU-8 negative-tone photoresist. This simple approach is valuable as a wafer-level carbon nano-patterning technique.

We introduce a new method for patterning one or multiple types of polymeric microparticles by electrospraying them toward a collecting surface patterned with an array of microelectrodes. By independently programming the electric state of each microelectrode, microparticles can be confined within desired regions with a high patterning contrast. The patterning principle and the role of the floating electrodes are discussed. Copatterning of multiple types of particles on a planar surface and along the vertical direction is demonstrated. With the superior copatterning capacity and the simple configuration, this work is expected to facilitate the development of biosensing, microscale tissue engineering and other microparticle-based total analysis.