We propose and demonstrate Bonding-in-Solution Technique (BiST) for encapsulation of liquid in MEMS devices. Liquid encapsulation enables innovative MEMS devices with various functions, such as hydraulic displacement amplification and scanning mirrors. Interfusion of air bubbles and leakage of the encapsulated liquid must be averted not to deteriorate device performances. Several liquid encapsulation processes have been proposed, such as parylene deposition and polymer thermal bonding. However, they involve vacuum and/or thermal processes and cannot be applied to volatile liquids. In BiST, two structural layers are passively aligned and brought into contact in solution, where the encapsulation cavities are uniformly filled up with the liquid without air bubbles. A UV-curable resin is used as an adhesive that does not require heat or vacuum environment but UV to bond the two layers. The detail processes of BiST are (a)UV-curable adhesive resin is coated onto the bonding surface of a structural layer. The layer contains not only encapsulating cavities but also concave-shaped structures for the following passive alignment. (b) The other layer with convex-shaped structures is brought into contact in solution, when the two layers are passively aligned by matching concave and convex structures. (c) UV light is irradiated and the two layers are permanently bonded while the contact is maintained by the jig. We successfully achieved encapsulation of DI water and glycerin in PDMS and silicon structural layers. No liquid remains in the bonding interface. Since conventional aligners are not applicable to BiST, we experimentally evaluated the accuracy of the passive alignment process in solution that makes use of matching concave and convex structures. We used a PDMS layer with cylinders (concave) and a silicon layer with cavities (convex) to evaluate the alignment in BiST. The height and depth of the cylinders and cavities are designed such that the PDMS cylinders elastically deform when the two layers are brought into contact. The elastic averaging enables the passive alignment of the two layers. We investigated the bonding accuracy with respect to the number of pairs of concave/convex structures and the height of PDMS cylinders while in contact. The bonding accuracy improved as the number of pairs increased while the height of PDMS cylinders did not show a correlation. The alignment accuracy of 5μm in BiST was achieved with 12 pairs of the concave/convex structures. The ultimate goal of our research is to develop innovative MEMS devices with encapsulated liquid, such as a hydraulic displacement amplification mechanism applicable to a tactile display. Glycerin was encapsulated by largely-deformable-PDMS thin membranes and silicon cavities by using BiST. The displacement at the input was successfully amplified at the output associated with the ratio of the cross-sectional areas.

Reinforcement with nanotubes, nanofibers, and nanoparticles is an attractive option for enhancing the properties of micromachined polymeric structures employed in microelectromechanical systems (MEMS). Calculations based on Eshelby-Mori-Tanaka micromechanics predict that the elastic modulus and wave velocity can be increased by over an order of magnitude by reinforcing polymers with aligned, dispersed, single-walled carbon nanotubes (SWNT). Motivated by this prediction, we measured the elastic moduli of polyimide films reinforced with SWNT at volume fractions ranging from 0 to 10%. For dilute composites, the elastic modulus increased with increasing nanotube loading from 2.5 GPa for the neat polymer to 3.5 GPa for a nanocomposite containing 0.5 vol% of SWNT. However, with further increase in the nanotube content, the elastic modulus remained essentially constant even for high loadings of 10 vol% of SWNT. In addition, significantly different elastic moduli were measured for specimens containing the same volume fraction (0.5 vol%) of SWNT produced by two different processes.

Nanoelectromechanical systems (NEMS) based on nanomaterials, especially NEMS resonators made of suspended nanowires, nanotubes or nanosheets have emerged as promising devices for many sensing applications. However, operation of such NEMS resonators in liquids is usually difficult due to strong viscous damping. Here we present our progress in developing NEMS devices based on suspended nanomaterials that can operate in liquids. We present our measurements performed on suspended metallic nanowires driven by AC currents in a magnetic field in liquids.

In this paper, we demonstrate a perfectly-ordered microbowl array with balanced dielectrophoresis (DEP) for a high-throughput single-cell analysis. In order to fabricate well-ordered microbowl array in a large area, we utilized three-dimensional diffuser lithography for photoresist mold and nickel electroplating technique for final microbowl structures on a silicon substrate. Single microbowl has six sharp apexes surrounding the microbowl perimeter. Each microbowl has a diameter of 10 μm, and a height of 9 μm, which can be controllable by patterns on mask and lithography conditions. To investigate feasibility for application to the microbowl array as a single-cell microarray, we used latex beads of 6.4 μm in an average diameter to be captured by dielectrophoretic force. The nickel microbowl array densely packed with a hexagonal geometry played as a bottom electrode, and an ITO-coated glass covered the nickel microbowl array as a top electrode while keeping a uniform gap between two electrodes. After injecting solution containing latex beads through the gap, we applied an AC signal (2 VPP, 1 MHz) between two electrodes to induce high electric field near the sharp apexes of the single microbowl. A negative DEP trap is formed at the center of the single microbowl with balanced DEP force from the six apexes. The experimental result shows that injected latex beads had been successfully and uniformly aligned and trapped at the microbowl array sustained by negative DEP.

Piezoresistors are commonly used in microsystems for transducing force, displacement, pressure and acceleration. Silicon piezoresistors can be fabricated using ion implantation, diffusion or epitaxy and are widely used for their low cost and electronic readout. However, the design of piezoresistive cantilevers is complicated by coupling between design parameters as well as fabrication and application constraints. Here we discuss analytical models and design optimization for piezoresistive cantilevers, and describe several applications ranging from studying electron movement using scanning gate microscopy to measuring the biomechanics of whole organisms.

The effect of film thickness on the electrical resistivity of heavily-nitrogen-doped polycrystalline SiC (poly-SiC) thin films is investigated. The resistivity of poly-SiC thin films decreases by a factor of ˜7 for thickness increasing from 100 nm-thick to 1.78 μm-thick; the resistivity begins to stabilize as thickness approaches 1 μm. The increased resistivity for the thinner films is shown to be related to the grain boundary effect. Secondary ion mass spectrometry indicates a nitrogen concentration of 9×1020 atoms/cm3 in the films. However, Hall measurements reveal that only 45% of the dopants are electrically active in the 100 nm-thick film. The percentage of active dopants rises to 80% when film thickness increases to 680 nm. From the studies of surface roughness and microstructure, it is seen that small grains are formed at the initial stage of deposition, which then grow into larger columnar grains as film thickness increases. The presence of a large density of grain boundaries and limited grain growth for the very thin films contribute to increased electrical resistivity from increased trapped mobile carriers and reduced carrier mobility. The free carrier trapping phenomenon can further be observed in the temperature-dependence of resistance measurement.

As the use of sensor networks has expanded, the demand for robust detectors able to operate in a variety of environments has grown. We present the sensitivity testing of a micro thermal conductivity detector (μTCD) operating in two different modes. The microfabricated device we have designed and tested is composed of a resistive heating element suspended in a micro-channel, which creates excellent thermal isolation and a high heat transfer coefficient between the element and the fluid. The sensitivity of a μTCD integrated into a micro-gas chromatography (GC) system, can be increased by a factor of 10 simply by switching between operation in constant temperature and constant voltage modes. This result agrees with the analytical models and testing data previously reported for macro systems and devices.

Wafer level bonding is an important technology for the manufacturing of numerous Microelectromechanical Systems. In this work the aluminum thermo-compression wafer bonding is characterized. The effects and significance of various bond process parameters and surface treatment methods are reported on the final bond interfaces integrity and strength. Experimental variables include the bonding temperature, bonding time, and bonding atmosphere (forming gas and inert gas). Bonded wafer samples were investigated with scanning acoustic microscopy, scanning electron microscopy, and four point bending test. Interfacial adhesion energy and bond quality were found to be positively correlated with bonding temperature. A bonding temperature of 500 °C or greater is necessary to obtain bond strengths of 8-10 J/m2.

Miniaturized monolithic langasite structures are micromachined using local doping of lan-gasite and wet chemical etching. The diffusion coefficients of niobium, strontium and praseodymium in langasite are determined in order to control the preparation process and to obtain information about the stability of locally doped structures at elevated temperatures. A wet etching process based on phosphoric acid for langasite is developed and used to manufacture microstructured elements like planar and biconvex membranes as well as field emitter diodes. These elements are characterized with respect to their application relevant properties such as resonator quality factor and field emission current at temperatures of 600 °C and above.

A transducer that can act as a highly sensitive and reliable universal sensor capable of detecting and continuously monitoring changes in the physical, chemical and biological domains is a potentially useful scientific tool. The Thin Film Bulk Acoustic Wave Resonator (FBAR) is a microwave device that is becoming increasingly recognised as a universal transduction platform with the added advantage of potential integration into CMOS architecture and array-like formats. This work shows preliminary results on FBAR where a continuous monitoring arrangement demonstrated the capability of FBAR to respond to changes in physical parameters such as temperature and light levels, the work goes on further to show the ability of FBAR to respond to changes in humidity in a gas flow and can have sensitivity increased with the addition of hygroscopic polymers on its surface and finally how FBAR can be adapted to act as a biosensor in the form of an immunosensor with sensitivity some orders of magnitude greater than traditional lower frequency bulk acoustic wave platforms.

Lab-on-a-chip (LOC) is one of the most important microsystem applications with promise for use in microanalysis, drug development, diagnosis of illness and diseases etc. LOC typically consists of two main components: microfluidics and sensors. Integration of microfluidics and sensors on a single chip can greatly enhance the efficiency of biochemical reactions and the sensitivity of detection, increase the reaction/detection speed, and reduce the potential cross-contamination, fabrication time and cost etc. However, the mechanisms generally used for microfluidics and sensors are different, making the integration of the two main components complicated and increases the cost of the systems. A lab-on-a-chip system based on a single surface acoustic wave (SAW) actuation mechanism is proposed. SAW devices were fabricated on nanocrystalline ZnO thin films deposited on Si substrates using sputtering. Coupling of acoustic waves into a liquid induces acoustic streaming and motion of droplets. A streaming velocity up to ˜5cm/s and droplet pumping speeds of ˜1cm/s were obtained. It was also found that a higher order mode wave, the Sezawa wave is more effective in streaming and transportation of microdroplets. The ZnO SAW sensor has been used for prostate antigen/antibody biorecognition systems, demonstrated the feasibility of using a single actuation mechanism for lab-on-a-chip applications.

This paper presents the mechanical characterization of the elastic modulus, hardness and fracture toughness of silicon oxynitride films (SiON) with different oxygen and nitrogen content, subjected to thermal annealing processed at 400 °C and 800 °C. The Fourier-transform infrared (FT-IR) spectroscopy was employed to characterize the SiON films with respect to the absorbance peak in the infrared spectrum. The nanoindentation testing showed that both the elastic modulus and hardness slightly increased after thermal annealing. Finally, the fracture toughness of the SiON films were estimated using Vickers micro-indentation tests and the result revealed that the fracture toughness decreased with increasing rapid thermal annealing (RTA) temperature and nitrogen content. We believe these results benefit microelectromechanical systems (MEMS) in regards to maintaining the structural integrity and improving reliability performance.

Multilayer microcantilevers present in micro-/nano- electromechanical system (MEMS/NEMS) applications serving passive and active structural roles. The application and commercialization of MEMS devices suffer from reliability problems. Appropriate nanocoatings, such as atomic layer deposition (ALD), have been demonstrated to be promising solutions for these reliability problems in MEMS devices. However, the micro/nano- mechanics within and/or between the microcantilevers and nanocoatings are not fully understood, especially when temperature, time, microstructural evolution and material nonlinearities play significant roles in thermomechanical response. The overall goal of this work is to suppress and understand the inelastic deformation and microstructural evolution in multilayer microcantilevers with nanocoatings. Moreover, to better understand the stress relief and Al2O3 suppression mechanism, scanning electron microscopy (SEM) was employed to explore the microstructural evolution.

The Young’s relaxation modulus of Polydimethylsiloxane (PDMS) film specimens was measured by the nanoindenter with a flat punch indenter. In the stress relaxation test, the initial ramp part was carefully considered to develop an accurate viscoelastic contact model. This model was used to fit the load-time data from the experimental tests. The resulting relaxation function was expressed by a general Maxwell equation. In addition, a case study of PDMS micropillar bending tests was performed, and the viscoelastic constitutive law was applied to develop an analytical solution of the reaction force. The results show that the reaction force calculated from the corrected model is generally agreed well with the experimental data.

Aluminum Nitride (AlN) films were grown using Metal Organic Vapor Phase Epitaxy (MOVPE) techniques on Si (111) substrates patterned with SiOx stripes and the vibrational properties of these films were investigated by Fourier transform infrared (FTIR) techniques. The grown films contained a predominantly wurtzite AlN phase in addition to oxidized aluminum and mixed AlN phases. The AlN film on amorphous silicon oxide (SiOx) was prone to corrosion when subjected to wet etching in buffered hydrofluoric acid solution thereby changing the material properties of the AlN film on SiOx. The etching process significantly reduced the oxidized aluminum phase and mixed AlN phases.

Poly-SiGe has quite some potential as structural MEMS layer for CMOS-MEMS integration. However, the contact resistance between SiGe MEMS and top CMOS metal should be low to avoid parasitic effects that would reduce the system performance. In this paper, a new and simple approach is proposed to achieve a low contact resistance between a top CMOS interconnect and a boron doped poly-SiGe MEMS layer deposited at 450 °C. The use of a 20 nm soft sputter etch in combination with a Ti-TiN (5-10 nm) interlayer results in a contact resistivity of 6.2 ± 0.4 × 10-7 Ωcm2 that is lower than previously reported. The uniformity of the contact resistivity across the wafer is also better than the state-of-the-art value.

A simple and low-cost technique is demonstrated to fabricate sub-10 nm planar nanofluidic channels. Native oxide on silicon surface is etched with a multiple hydrofluoric (HF)-etch / SiO2-regrowth process. Shallow Si trenches with 3 nm to 24 nm depths are obtained at an etch rate of 1 nm per HF dip. The trenches are uniform with a surface r.m.s. roughness of 0.4 - 0.6 nm. A low-temperature and low-voltage anodic wafer bonding process is then used to form planar nanofluidic channels. Minimum aspect ratio (depth/width) of the fabricated sub-10 nm nanochannels is around 0.001-0.002.

Microelectromechanical Systems (MEMS) are being extensively investigated as a means of miniaturizing piezoelectric sensors thereby offering higher sensitivity, reduced power consumption, and ability to form compact multi-sensor arrays. Such devices typically employ one or more silicon micromechanical elements (e.g. membranes, cantilever beams and tethered proof masses) driven electromechanically by a polycrystalline piezoelectric film. The use of polycrystalline materials results in inherently less stable and irreproducible device characteristics. For elevated operating temperatures, more robust and refractory materials are also required. In this paper, we describe a MEMS microresonator array capable of operating to temperatures exceeding 600°C enabled by the integration of epitaxially grown piezoelectric AlN films onto single crystal SiC tethered plates. The operation of the microresonators as sensors is illustrated by examining their response to temperature, pressure and chemical analytes.

Conducting polymer materials can be developed as muscle-like actuators for applications in robotics, micro-electro mechanical systems, drug delivery systems etc. These materials are available in a large number of different varieties that can be synthesized and processed in different ways. However, their applications as actuators are limited due to the inability to create conducting polymer materials with robust mechanical properties. Currently most of the dynamic mechanical analysis technologies require the polymer created to be free standing and able to withstand large stresses. This severely limits the development of new materials with potential actuator applications. In this study, a technique to measure the actuation of polymers in the electrochemical deposition environment is described. This allows testing of an electrochemically grown conducting polymer sample on the surface of the deposition electrode itself. Thin polypyrrole films (2 to 20 microns thick) doped with tetraethylammonium hexaflourophosphate were grown on the surface of a glassy carbon electrode. These films were then tested on the surface of the glassy carbon using a custom built electrochemical dynamic mechanical analyzer. A square wave potential (+/- 0.8 V) is applied to the films that results in the actuation of the films. The films are able to generate a changing force of 3 mN of force against a 0.1 N sensor preloaded at 5 mN. The resulting magnitude of the measured force is a function of the film thickness while the change in force due to actuation is approximately constant.

Quantifying the effects of thin metallic coatings on the damping factors of micro- and nanomechanical resonators is important for the design of high-performance devices for sensing and communications. This study presents experimental results for the increase in damping caused by aluminum films coated on cantilevered single-crystal silicon beams. The monolithic silicon beams (100 to 125 microns thick) can operate at the ultimate limits of dissipation established by thermoelastic damping with quality factors ranging from 104 to 105. However, coating these beams with 60 to 100 nm of aluminum can increase the damping by factors of three to five. These results provide guidelines for designing composite micromechanical resonators, and establish the foundation of a new approach for accurate measurement of internal friction in substrate-bonded thin films.