Table of contents

We report the design and fabrication of a microheater unit as a key component of an integrated micro gas preconcentrator that has an ultra-small preconcentrator volume (<0.25 µL) and microvalves for fast injection speeds (<1 ms). Monolithic integration of the microvalves into the microheater of the preconcentrator gives rise to challenges in designing the microheater and implementing thermal isolation for low power and energy consumption. A preconcentrator chamber, 3.5 × 1.5 mm² in planform area and 40 µm deep, was built in the device layer of a silicon-on-insulator wafer and filled with an array of microposts with a preconcentrator volume of 0.2–0.25 µL. Different generations of the microheaters and their mating dies were fabricated to show the effects of thermal isolation and thermal mass of the system on the performance of the heater. The microheater assembly with the least thermal mass and most thermal isolation can reach 300 °C in 100 ms with 12.3 W of power and is expected to consume less than 2 J during the operation of each preconcentration cycle.

In order to achieve easy alignment and low-temperature bonding for electronic packaging, cross-aligned strip solder bumps are thermosonically soldered using longitudinal vibration. The viscoplastic behavior and temperature distribution of the solder bump are calculated numerically, and the predicted results show good agreement with experimental results of fluxless thermosonic soldering of the lead-free solder (Sn–3%Ag–0.5%Cu). Melting of solder bumps occurs within 3 s with a perheating temperature of 85 °C, and growth of the intermetallic compound is limited due to the short bonding time. The effects of the bonding time and pressure on the bond quality are analyzed through SEM and bond strength measurement to determine the proper bonding condition of thermosonic soldering.

Tuning of a superconducting microwave resonator, operating at 77 K, is demonstrated using a silicon micro-machined comb-drive actuator. Thermal expansion coefficient differences between the silicon actuator and the MgO substrate are accommodated by the use of stress absorbing springs. A tuning range of 4.6% is demonstrated for a superconducting microwave resonator with a frequency just below 6 GHz.

An integrated micro XY-stage with a 2 × 2 mm² movable table is designed and fabricated for application in nanometer-scale operation and nanometric positioning precision. The device integrates the functions of both actuating and sensing in a monolithic chip and is mainly composed of a silicon-based XY-stage, comb-drive actuator and a displacement sensor, which are developed by using double-sided bulk-micromachining technology. The high-aspect-ratio comb-driven XY-stage is achieved by deep reactive ion etching (DRIE) on both sides of the wafer. The displacement sensor is formed on four vertical sidewall surface piezoresistors with a full Wheatstone bridge circuit, where a novel fabrication process of a vertical sidewall surface piezoresistor is proposed. Comprehensive design and analysis of the comb actuator, the piezoresistive displacement sensor and the XY-stage are given in full detail, and the experimental results verify the design and fabrication of the device. The final realization of the device shows that the sensitivity of the fabricated piezoresistive sensors is better than 1.17 mV µm⁻¹ without amplification, and the linearity is better than 0.814%. Under 28.5 V driving voltage, a ±10 µm single-axis displacement is measured without crosstalk and the resonant frequency is measured at 983 Hz in air.

A MEMS optical coherence tomography (OCT) probe prototype was developed using a unique assembly based on silicon optical bench (SiOB) methodology. The probe is formed by integrating a three-dimensional (3D) scanning micromirror, gradient refractive index (GRIN) lens and optical fiber on SiOB substrates having prefabricated self-aligned slots. The two-axis scanning micromirror is based on electrothermal actuation with required voltage less than 2 V for mechanical deflections up to 17°. The optical probe was enclosed within a biocompatible, transparent and waterproof polycarbonate tube with a view of in vivo diagnostic applications. The diameter of the miniature probe is less than 4 mm and the length of its rigid part is about 25 mm. The probe engineering and proof of concept of the probe were demonstrated by obtaining en face and three-dimensional OCT images of an IR card used as a standard sample.

Cells alter their shape and morphology and interact with their surrounding environment. Mechanical forces developed by cells to their surrounding environments are fundamental to many physiological processes, such as cell growth, division, migration and apoptosis. In this paper, a novel optical Moiré based biomechanol force sensor was developed for cell traction force mapping. We utilized coherent laser beams to illuminate periodic polymeric substrates where isolated cells were cultured. We demonstrated one-dimensional and two-dimensional traction force mapping via optical Moiré for both cardiac myocytes and vascular smooth muscle cells. The magnification effect of the Moiré fringe pattern permits a real time monitoring of the mechanical interaction between isolated cells and their underlying periodic polymeric structures.

Hexagonal network-type silicon microstructures with nanoprotrusions were designed, fabricated, and their surface wettability was characterized. The nanoprotrusions were fabricated by reactive ion etching (RIE). The microstructures were fabricated by deep reactive ion etching (DRIE). For hydrophobicity, plasma polymerized fluorocarbon (PPFC) of a hydrophobic nature was coated onto the surface of the microstructures with nanoprotrusions. Through a comparison of the contact angles between hexagonal network-type microstructures with nanoprotrusions and those without nanoprotrusions, the effect of the nanoprotrusions on the surface wettability was evaluated. The average value of the contact angles measured from microstructures with nanoprotrusions was 18° higher than that measured from microstructures without nanoprotrusions. Nanoprotrusions decrease the contact area between a water droplet and the surface, causing the contact angles to increase considerably. The wetting mode of the nano- and micro-multiscale structured surface changed with the pore ratio of the microstructures. While the water was fully supported by the nanoprotrusions at a low pore ratio, the water fully wetted the nanoprotrusions at a high pore ratio.

Three-dimensional (3D) integration requires through-wafer interconnects, i.e. an integration of electrical connections from one side of the wafer to the other side. In some cases, it involves the lithographic patterning of through-Si via (TSV). For this step, a conformal coating of a resist layer is necessary. In this paper, we present two potential photoresist coating methods for coating a wafer with TSV: spray coating and electrodeposition (ED) of the photoresist. Lithographic patterning inside the TSV is also investigated. Some parameters that influence the pattern definition, such as large gap exposure, resist thickness and via size, are identified and evaluated.

On-chip micro-transformers with a stacked interwinding coil have been developed. The transformer is fabricated using simple and cost-effective MEMS surface micromachining. High-frequency characteristics of the transformer are analyzed by comparing its performances for various coil structures and substrate materials, respectively. The results show that the RF performance of the glass-based transformer is improved compared to that of a silicon-based transformer. An analysis of various coil configuration leads to the conclusion that the metal-to-metal capacitance has a significant influence on the RF characteristics. The process fabrication of the device is simple, highlighting good prospects for future three-dimensional RF-MEMS device application.

We report a prototype protein separator that successfully miniaturizes existing technology for potential use in biocompatible health monitoring implants. The prototype is a liquid chromatography (LC) column (LC mini-column) fabricated on an inexpensive, flexible, biocompatible polydimethylsiloxane (PDMS) enclosure. The LC mini-column separates a mixture of proteins using size exclusion chromatography (SEC) with polydivinylbenzene beads (5–20 µm in diameter with 10 nm pore size). The LC mini-column is smaller than any commercially available LC column by a factor of ∼11 000 and successfully separates denatured and native protein mixtures at ∼71 psi of the applied fluidic pressure. Separated proteins are analyzed using NuPAGE-gel electrophoresis, high-performance liquid chromatography (HPLC) and an automated electrophoresis system. Quantitative HPLC results demonstrate successful separation based on intensity change: within 12 min, the intensity between large and small protein peaks changed by a factor of ∼20. In further evaluation using the automated electrophoresis system, the plate height of the LC mini-column is between 36 µm and 100 µm. The prototype LC mini-column shows the potential for real-time health monitoring in applications that require inexpensive, flexible implant technology that can function effectively under non-laboratory conditions.

This paper presents a method for analyzing the refill process of a piezoelectric inkjet printing head with a high firing frequency for color filter manufacturing. Theoretical and experimental studies on the equivalent length (L_eq) versus jetting characteristics were performed. The new model has shown quantitatively the same result compared with a commercialized simulation code. Also it is identified that the refill time increases with the equivalent liquid length (L_eq) because the viscous force increases. The inkjet printing head has been designed with a lumped model analysis and fabricated with a silicon wafer (1 0 0) by a MEMS process. To investigate how the equivalent length (L_eq) influences the firing frequency, an experiment was conducted using a stroboscope. In the case of colorant ink, it is possible to eject an ink droplet up to 5 kHz with a 40 pl drop volume. On the other hand, the firing frequency calculated with the new model is about 3 kHz under the condition of the equivalent liquid length (L_eq), 250 µm. The difference between the new model and experiment may be a result of a mismatch of initial meniscus position due to the meniscus oscillation. Experimentally the meniscus oscillation is observed through an optical measurement with a visualization apparatus and a transparent nozzle. Hence the efficiency of the new model may be enhanced in a high viscosity range. The methods for increasing the firing frequency are to reduce the equivalent length (L_eq) and to modify the ink property. Because the former tends to decrease a viscous loss and the latter tends to increase a viscous damping, two parameters should be combined adequately within an allowable drop volume.

SU-8 photoresist (PR) is an excellent material for optical applications due to its high refractive index and high transmittance at visible and near-IR wavelengths. This paper reports a simple and novel method of fabricating non-spherical microlens arrays by soft stamping unexposed SU-8 PR. An SU-8 based stamp composed of micro-nozzle arrays and a reservoir structure are first fabricated on a glass substrate using a process of dosage control exposure. The unexposed SU-8 encapsulated in the crosslinked SU-8 shell is used as the 'ink' for the stamping process. The present SU-8 microlens array is then formed by stamping the formed SU-8 structure on a bare glass substrate at a temperature higher than the glass transition temperature (T_g) of the unexposed SU-8 PR. A non-spherical lens with various radii of curvature structure can be formed by controlling the working temperature during the stamping process. Microlens arrays with diameters ranging from 50 to 500 µm are successfully fabricated using this novel process, with the height of the fabricated microlens reaching 200 µm. Experimental investigation results indicate that the formed microlenses have a good surface profile, good uniformity and good optical properties. The measured numerical apertures (NA) of the fabricated microlens range from 0.22 to 0.58. The relationships between the focal length of the fabricated microlens and the lens dimension as well as the process temperature are systematically investigated. The method presented in this study will substantially impact the process for microlens array fabrication.

Here, we describe a dielectrophoretic (DEP) gating technique for preconcentrating and separating biological and non-biological particles in a microfluidic device. The microfluidic devices are surface-micromachined on silicon substrates and are fully encapsulated without substrate bonding procedures. DEP gates in the devices consist of embedded microelectrodes that are coupled to the fluid channels for analyte manipulation with electric fields. We consider several different microelectrode designs such as low and high radius-of-curvature edges, and we detail the time- and frequency-dependent preconcentration of particles. Simulations of the particle motion under the manipulation of DEP forces are found to be in good agreement with the experimental results. Experimental results show that bioparticles such as Penicillium brevicompactum (PBC), T-cells and Escherichia coli (E. coli) undergo positive DEP and are trapped in regions of large electric-field gradient adjacent to the DEP gate. In contrast to our previous demonstration of batch-mode separation of particles with different dielectric properties, here we perform a continuous-mode separation of latex particles and E. coli from a mixture.

Stereolithography (SLA) is a widely used technique for the fabrication of prototypes and small series products. The main advantage of SLA and related solid freeform fabrication (SFF) techniques is their capability to fabricate parts with complex shapes with high resolution. Although the spectrum of available materials has been widened in recent years, there is still a lack of materials which can be processed with SLA on a routine basis. In this work, a micro-SLA (µSLA) system is presented which can shape a number of different photopolymers with resolutions down to 5 µm in the xy-plane and 10 µm in the z-direction. The system is capable of processing various specifically tailored photopolymers which are based on acrylate chemistry. The materials processed for this work range from hybrid sol–gel materials (ORMOCER) to photo-crosslinked elastomers and hydrogels. The elastic moduli of these materials can be tuned over several orders of magnitude and range from 0.1 MPa to 8000 MPa. The reactivity of these monomers is sufficient for achieving writing speeds up to 500 mm s⁻¹ which is comparable to commercial SLA resins. Various test structures are presented which show the suitability of the process for fabricating parts required for applications in micro-mechanical systems as well as for applications in biomedical engineering. Using the presented system, internal channels with a diameter of 50 µm and a length of 1500 µm could be fabricated. It was also possible to manufacture a micro-mechanical system consisting of a fixed axe and a free spinning turbine wheel.

This paper describes a simple and effective method to fabricate oblique or curved 3D microstructures with high uniformity and manifold shapes, utilizing backside 3D diffuser lithography. A negative photoresist spin-coated on the metal-patterned glass substrate is exposed from the backside where an optical diffuser is located and used to randomize the incident ultraviolet (UV) light, which creates the self-aligned curved 3D microstructures. The various 3D microstructures can be obtained simply by adjusting the key process parameters of this method such as the type of diffuser, the UV exposure dose, the opening width of the patterned metal on the glass substrate, etc. Through these parameter studies, a microball lens with a sag of 10 µm to 115 µm, a cylindrical microlens, a 3D planar microlens with circular and biconvex structures and an inverse-trapezoidal microstructure having different sidewall angles of 45°, 51°, 56° and 72° were successfully fabricated. The proposed method can be extensively applied for fabrication of micro-optical components due to its simplicity and versatility.

A micro stagnation-point flow burner was fabricated using low-temperature co-fired ceramic (LTCC) tapes. Methane/oxygen counterflow micro diffusion flames with luminous zones of less than 1 mm in length and 250 µm in width were stabilized in the reaction channel of the burner and analyzed using microscopic imaging spectroscopy. The burner was built with 25 layers of LTCC tapes which were pre-laminated into seven blocks. Integrated sapphire windows and sub-millimeter sized internal channels provide optical accessibility and reactant feeds, respectively. Spatial distributions of CH* and C*₂ species were measured and compared with those obtained from multi-dimensional reacting flow calculations. Results show that the location and size of the diffusion flame may be controlled by flow velocity, or strain rate, just as in larger centimeter scale flames, and that the chemical structure of the micro flames was in agreement with those predicted by the numerical simulations. In this paper, we show that LTCC tape technology may be used to fabricate microburners for continuous flow steady-state homogeneous gaseous combustion and that integrated optical windows may be incorporated into the fabrication process for in situ optical diagnostics. The understanding and analysis of the present reacting flowfield, because of its simplicity, is important to the development of more complicated microreactors and microburners, in which mixing and variable strain rates exist.

Three-dimensional oblique microstructures of SU-8 negative thick photoresists have drawn more attention with the rapid development of inclined UV lithography technology. In this paper, a simple model based on the Fresnel diffraction theory is developed to simulate the inclined UV lithography and predict the profiles of developed oblique SU-8 structures by utilizing the paraxial approximation approach. The reflection, refraction, absorption of the SU-8 photoresists and the reflection at the wafer surface are integrally incorporated in the model to improve the accuracy. The parameters, which have significant influence on the profile quality, have been studied in simulation. Simulation results demonstrate good agreement with the experimental results, where the oblique SU-8 structures were fabricated on glass and silicon wafers. To eliminate reflected UV light at the wafer surface, Ti films were sputtered on the glass wafers, followed by wet oxidation, thus employed as an antireflection layer. By contrast, the silicon wafers were used to fabricate the oblique SU-8 structures with reflected induced patterns. This model is useful to optimize the inclined UV lithography process of SU-8 thick photoresists and improve the efficiency of the design of some micro-electro-mechanical system devices.

This paper discusses a new synthesis approach for electromechanical filters. The structural dynamics aspects are emphasized and joined with an inverse problem methodology to shape the spectrum of a signal passing through the device. Using the inverse problem methodology, the poles and zeros of a multi-degree of freedom structure are assigned, thus shaping the filter's frequency response. Several variants are presented for the same topology where the excitation and sensing locations are chosen according to the desired characteristics of the filter. The decomposition of the spatial behaviour of the device provides the designer with the ability to suppress certain structural modes of vibration thus behaving as a bandstop/bandpass filter for periodic signals. A micro-electromechanical filter was fabricated and a laboratory set-up was constructed for characterization of the micro-electromechanical filter. A comparison between the simulated response of the filter and measured response is shown.

This paper reports the design, fabrication and characterization of high performance miniaturized micro direct methanol fuel cells (microDMFC) functioning at room temperature under a forced low input fuel flow rate (<10 µL min⁻¹) fabricated using silicon microsystems techniques. A room temperature maximum power output of 12.5 mW cm⁻² has been measured at a fuel flow rate of 5.52 µL min⁻¹ for a fuel cell surface area as small as 0.3 cm² (corresponding to a fuel use efficiency of 14.1% at 300 K). At a lower flow rate of 1.38 µL min⁻¹, the fuel use efficiency rises to 20.1% although the power density falls to 4.3 mW cm⁻². The study revealed that improved room temperature cell performances in terms of power density can be achieved at low flow rates (<10 µL min⁻¹) by (i) reducing the fuel cell area and (ii) reducing the microchannel cross-section. The study also revealed that higher fuel use efficiencies are obtained at lower fuel flow rates. Fuel (methanol) for the anode and an oxidant (air) for the cathode are supplied via a compact serpentine network of micron-size microfluidic and gas microchannels; by using silicon microsystems techniques we also render the fuel cell compatible with other silicon technologies such as microelectronics and micro- and nanoelectromechanical systems (MEMS/NEMS).

This paper summarizes the results of the process optimization for SU-8 films with thicknesses ⩽5 µm. The influence of soft-bake conditions, exposure dose and post-exposure-bake parameters on residual film stress, structural stability and lithographic resolution was investigated. Conventionally, the SU-8 is soft-baked after spin coating to remove the solvent. After the exposure, a post-exposure bake at a high temperature T_PEB ⩾ 90 °C is required to cross-link the resist. However, for thin SU-8 films this often results in cracking or delamination due to residual film stress. The approach of the process optimization is to keep a considerable amount of the solvent in the SU-8 before exposure to facilitate photo-acid diffusion and to increase the mobility of the monomers. The experiments demonstrate that a replacement of the soft-bake by a short solvent evaporation time at ambient temperature allows cross-linking of the thin SU-8 films even at a low T_PEB = 50 °C. Fourier-transform infrared spectroscopy is used to confirm the increased cross-linking density. The low thermal stress due to the reduced T_PEB and the improved structural stability result in crack-free structures and solve the issue of delamination. The knowledge of the influence of different processing parameters on the responses allows the design of optimized processes for thin SU-8 films depending on the specific application.

In this paper, we describe a self-priming high performance piezoelectrically actuated check valve diaphragm micropump. The micropump was fabricated from three wafers: two silicon-on-insulator (SOI) wafers and one silicon wafer. A process named 'SOI/SOI wafer bonding and etching back followed by a second wafer bonding' was developed in order to make the core components of this device which included an inlet check valve, an outlet check valve, a diaphragm and a chamber. The movable structures of this device, i.e. the check valves and the diaphragm, were fabricated from the device layers of the two bonded SOI wafers. Taking advantages of SOI wafer technology and etch-stop layers, the vertical parameters of the movable structures were precisely controlled in fabrication. The micropump was self-priming without any pre-filling process. The pumping rate of the micropump was linearly adjustable from 0 to 650l µm min⁻¹ by adjusting frequency. The maximum pumping rate was 860 µl min⁻¹ and the maximum pumping pressure was approximately 10.5 psi. The power consumption of the device was less than 1.2 mW.

This paper presents a torsional micromirror detached from PZT actuators (TMD), whose rotational motion is achieved by push bars in the PZT actuators, detached from the micromirror. The push bar mechanism is intended to reduce the bending, tensile and torsional constraints generated by the conventional bending bar mechanism, where the torsional micromirror is attached to the PZT actuators (TMA). We have designed, fabricated and tested the prototypes of TMDs for single-axis and dual-axis rotations, respectively. The single-axis TMD generates a static rotational angle of 6.1° at 16 V_dc, which is six times larger than that of the single-axis TMA, 0.9°. However, the rotational response curve of TMD shows hysteresis and zero offset due to the static friction from the initial contact force between the cover and the push bar in the PZT actuator. We have shown that 63.2% of the hysteresis is reduced by eliminating the initial contact force of the PZT actuator. The dual-axis TMD generates static rotational angles of 5.5° and 4.7° in the x-axis and y-axis, respectively, at 16 V_dc. The measured resonant frequencies of the dual-axis TMD are 2.1 ± 0.1 kHz in the x-axis and 1.7 ± 0.1 kHz in the y-axis. The dual-axis TMD shows stable operation without severe wear for 21.6 million cycles driven by the 16 V_p-p sinusoidal wave signal at room temperature.

A continuous SF₆/O₂ plasma process at room temperature has been used to etch tapered through-silicon vias using a DRIE-ICP tool. These features (10–100 µm in diameter) are aimed for applications in 3D integration and MEMS packaging. The effects of various process parameters such as O₂ flow rate, platen bias, pressure and substrate temperature on the via profile (depth, slope angle and aspect ratio) development are investigated. The etching mechanism was also studied and x-ray photoelectron spectroscopy (XPS) analysis reveals a SiO_x passivation layer of the order of ∼2 nm on the via sidewall and a substantial temperature dependence. Both tapering and anisotropy of etching depend on this passivation layer formation. Finally, suitable tapered vias with an aspect ratio of ∼5 and a slope angle of ∼83° are obtained by properly balancing the etching regimes. In this condition, a maximum etch rate of 7 µm min⁻¹ is achieved.

This paper describes a new on-chip manipulation method for handling millimeter- and micron-sized objects using oscillating mobile bubbles. It is found that acoustically excited oscillating bubbles can attract and capture neighboring objects. A variety of objects, including hydrophilic glass beads (80 µm), polystyrene beads (100 µm), a fish egg (∼1 mm) and a live water flea (∼1 mm), are successfully captured. The capturing performance is characterized using 80 µm hydrophilic glass particles while varying the acoustic excitation frequency and amplitude. The oscillation amplitude of the bubbles is quantified using high-speed images. At the natural frequencies of the bubbles the capturing range is highest. The capturing range increases as the oscillation amplitude increases. It is also found that while the bubbles are in lateral motion the capturing force is strong enough to hold the captured objects. By integrating acoustic excitation with electrowetting-on-dielectric (EWOD) bubble transportation, it is demonstrated that oscillating mobile bubbles can capture, carry and release neighboring objects on a chip. This new manipulation method may provide an efficient tool for handling millimeter- as well as micron-sized objects such as biological cells.

Blood separation is the first step for subsequent blood tests in clinical diagnosis. Lab-on-a-chip technology provides an automatic, cost-effective and fast solution for a wide variety of blood analyses. The objective of this work is to design a new lab-on-CD microstructure capable of separating blood cells from the whole blood into different reservoirs directly. A CD platform including a microchannel network consisting of a straight main microchannel, a curved microchannel and a branching microchannel has been proposed. The merits of this design are its simple structure, less operating time and high separation efficiency because it utilizes multiple separation mechanisms, for instance, two centrifugal forces and Coriolis force. One centrifugal force is due to the system rotation; the other centrifugal force is due to the curvature of the specifically designed curved channel. In this work, systematical evaluation on the functionality and performance of such a design has been done. Ninety-nine per cent separation efficiency is achieved for diluted blood of 6% hematocrit.

The development of more complex biochips in terms of functionality requires some technological evolutions. A high frequency biological micro-electro-mechanical-system dedicated to biological analysis is a typical case of this requirement for providing compatibility between high frequency propagation and microfluidic circulation. Mixed fabrication technologies using silicon and polymers appear to be a good alternative. We have developed a promising deposition process of an organosilicon polymer by remote afterglow plasma technology, also called plasma polymerized tetramethyldisiloxane (ppTMDS). This technique allows us to obtain high deposition rate values in the range of 160 Å s⁻¹ and a thick layer up to 140 µm without any crack. Moreover, this process is compatible with high throughput microelectronic designs and it is done near room temperature. This last point is very interesting for further development of surface bio-functionalization, for example. We have characterized this siloxane polymer by physico-chemical analysis. The roughness has been optimized, thus allowing the realization of high frequency waveguides. The ppTMDS permittivity presents a low dispersive characteristic and constitutes one of the best low-loss polymers up to 1 THz.

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