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With miniaturization, better performance and inexpensive devices can be realized with coveted features such as improved reliability, faster response time, cost effectiveness and mass production capability. For miniaturized mechanical structures, viscous damping dominates the dissipation mechanism, which can have an adverse effect on both the frequency response characteristics and the mechanical–thermal noise. For silicon condenser microphones, an optimized viscous damping value has its origin in a well-designed backplate structure. There is an optimum acoustic hole location in the backplate and it is concluded that the location of acoustic holes has more influence on the microphone performance than the number of holes in the backplate. It is also demonstrated experimentally, via a PZT/Si diaphragm-based acoustic device, that its frequency response characteristics can be optimized by attaching a silicon backplate with optimized slots and holes. The bandwidth of the acoustic device is approximately 16 kHz with an estimated A-weighted mechanical–thermal noise of 28.6 dB(A).

In this paper we present the mechanical and microstructural characterization results of through-wafer electroplated copper interconnects. Copper was deposited in very high aspect ratio (~15), narrow (15 µm) through-vias in silicon, which were earlier created by deep reactive ion etching. The two critical mechanical properties, i.e. hardness and modulus of elasticity, and the microstructure of the electroplated copper interconnect were determined by nanoindentation, atomic force microscope and x-ray diffraction techniques. A location-dependent hardness characteristic was shown along the length of electroplated copper interconnects. The modulus and the hardness of copper interconnects at the bottom segment (124 GPa and 1.8 GPa) were found to be higher than those at the top segment (116 GPa and 1.1 GPa). The reason behind these variable hardness values in the copper interconnect was due to the different grain sizes and the microstructure in the electroplated copper. These varying grain sizes were caused by the incremental current densities used during electrodeposition. The thermal strain, generated due to the coefficient of thermal expansion mismatch, was measured by the digital image speckle correlation technique. From the results, the thermal strain in the Y-direction was found to be more dominant than that in the X-direction. The grain sizes and the preferred texture orientation in the electroplated copper were characterized by the x-ray diffraction technique.

In this paper, we report the fabrication of high aspect ratio, highly dense, very fine pitch on-chip copper-pillar-based interconnects for advanced packaging applications. Photoresist molds up to a thickness of 80 µm and having feature sizes as small as 5 µm were fabricated using multi-step coating of the positive tone AZ9260 photoresist. Spin coating and lithography parameters were optimized to achieve smooth and vertical sidewalls. Copper interconnects having an aspect ratio up to 6 and a pitch size of 25 µm were electroplated in the fabricated resist mold. Due to a very small pitch size, the total number of interconnects per cm2 chip area is 160 000, which is much larger than the conventional solder-based interconnects. The electrical resistance of the electroplated copper interconnects was measured by 4-probe kelvin measurement configuration and was found to be in the range of 8–10 mΩ and the corresponding electrical resistivity was calculated as 2.4 µΩ cm. Such low resistive interconnects can carry much larger electrical current without significant electrical loss, which is ideally suitable for next generation packaging applications. X-ray diffraction has shown the presence of the (2 2 0) texture along the length of electroplated copper pillars. Transmission electron microscope reveals the presence of nanoscale copper twins along the length of copper interconnects.

This paper presents a novel approach to fabrication of a silicon microneedle array with porous tips. Dry etching technology with SF6/O2 gas by STS's inductively coupled plasma (ICP) etch tool was used to achieve the pyramidal needle structure. A thin silicon nitride layer was deposited after a thick photoresist layer was coated and reflowed at 120 °C. The silicon nitride layer and residual photoresist on the tips of the pyramidal structures were removed using reactive ion etching (RIE). Electrochemical etching in MeCN/HF was carried out to generate porous silicon on the tips of the microneedles. The fabricated microneedle array has potential applications in drug delivery, since the porous tips can be loaded with a high molecular weight drug. Analytic solutions to the critical loadings of the fabricated microneedle structure are also presented. The variations of the square cross-section were expressed as a function of the axial coordinate to analyze the bending normal stress and critical buckling loading. This analytic method can also be used for other microneedle structures with different cross-sections.

In this paper we present an alternative technology for micromachining gallium arsenide (GaAs) using deep ion implantation. Energetic nitrogen ions at 630 keV and 4 MeV have been used to implant deeply into an n-type GaAs substrate with doses of 2 x 10(14) and I x 10(15) cm(-2). After annealing at 600 degreesC, the nitrogen implanted n-GaAs top layer was converted to semi-insulating GaAs with a thickness of I mum for 630 keV and 2.5 mum for 4 MeV nitrogen ions. A pulsed electrochemical etching process has been developed to selectively remove n-GaAs and to leave the top patterned semi-insulating GaAs layer as a mechanical membrane structure. Various GaAs microstructures, such as cross-bridge, coiled and corrugated membranes, have been successfully fabricated using this micromachining technology.

This paper presents a sandwich-structured MEMS electret-based vibrational energy harvester (e-VEH) that has two opposite-charged electrets integrated into a single electrostatic device. Compared to the conventional two-plate configuration where the maximum charge can only be induced when the movable mass reaches its lowest position, the proposed harvester is capable of creating maximum charge induction at both the highest and the lowest extremes, leading to an enhanced output performance. As a proof of concept, an out-of-plane MEMS e-VEH device with an overall volume of about 0.24 cm3 is designed, modeled, fabricated and characterized. A holistic equivalent circuit model incorporating the mechanical dynamic model and two capacitive circuits has been established to study the charge circulations. With the fabricated prototype, the experimental analysis demonstrates the superior performance of the proposed sandwiched e-VEH: the output voltage increases by 80.9% and 18.6% at an acceleration of 5 m s−2 compared to the top electret alone and bottom electret alone configurations, respectively. The experimental results also confirm the waveform derivation with the increase of excitation, which is in good agreement with the circuit simulation results. The proposed sandwiched e-VEH topology provides an effective and convenient methodology for improving the performance of electrostatic energy harvesting devices.

Masked-maskless etching is a novel bulk micromachining technology for complex silicon microstructures. The convex-corner undercutting of masked-maskless etching is far different from that of normal masked etching. An experimental investigation has been conducted to discover the configuration and topological evolution of the convex-corner undercutting under masked-maskless etching. Based on the experimental results, the compensation criteria for the masked-maskless formed convex-corner undercutting are obtained and the corresponding compensation schemes are provided.

In this paper, a vibration-based MEMS electromagnetic energy harvester (EM-EH) device with two-degree-of-freedom (2DOF) configuration has been presented, modeled and characterized. The proposed 2DOF system comprises a primary subsystem for power generation, and an accessory subsystem for frequency tuning. A lumped parametric 2DOF model is built and examined in respect of energy harvesting capabilities. By controlling the mass ratio and frequency ratio, the first two resonances of primary mass can be tuned close to each other while maintaining comparable magnitudes. The 2DOF configuration is expected to be more adaptive and efficient than the conventional 1DOF structure, which could only operate near its sole resonance. The 2DOF EM-EH chip is fabricated on silicon-on-insulator (SOI) wafer through double-sided deep reactive-ion etching (DRIE). Induction coil is only patterned on the primary mass for energy conversion. With current prototype at an acceleration of 0.12 g, two resonances of 326 and 391 Hz with output voltages of 3.6 and 6.5 mV are obtained respectively, providing good validation for the modeling results. This paper offers new insights of implementing a multimodal MEMS EM-EH device.

This paper demonstrates the different possibilities of micromachining GaAs-based materials for integrated sensors with on-chip integrated circuitry. Special emphasis was laid on the high-temperature applications. The authors report for the first time the microstructuring of GaAs by ion implantation. A GaAs cantilever and Si3N4 diaphragms up to 400 mu m diameter were fabricated using this technology. Using AlGaAs etch-stop layers thin GaAs/AlGaAs membranes can be produced easily. This technique results in excellent thermal isolation. An anemometer with an integrated electronic circuit is presented.

This paper presents the fabrication, modeling and characterization of an out-of-plane electret-based vibrational energy harvester (e-VEH) that has both positive and negative charged electret plates integrated into a single seismic mass system. Strong electrostatic spring-softening effect is induced due to the electric field provided by the double-charged electret plates. An elastic stopper is introduced for reliability concern by limiting the motion of seismic mass and meanwhile serves as a functional element to realize spring-hardening effect. The device has an overall volume of about 0.14 cm3 and is fabricated based on MEMS compatible silicon micromachining technology. When subject to weak excitations, the device exhibits an approximately linear frequency response but changes to a significantly larger broadband when strongly excited due to the combined nonlinear effect from electrostatic force and a mechanical elastic stopper. At a high excitation level of 0.48 g, the experimental results show that the device has 3 dB bandwidths of 3.7 Hz for frequency-up sweep and 2.8 Hz for frequency-down sweep, respectively, which demonstrate a large enhancement compared to the linear response (1.3 Hz). An optimal output power of 0.95 μW is also achieved with a low resonance of 95 Hz. This corresponds to a normalized power density of 37.4 μW cm−3 g−2.

The paper demonstrates an energy harvester using a freestanding piezoelectric diaphragm combined with a proof mass. The diaphragm bearing double-sided spiral electrodes makes use of the d33 piezoelectric effect to realize energy scavenging. The harvester was fabricated by using a MEMS technique. The energy converting performance of the diaphragm was characterized by a shaker system. Proof masses were combined at the center of the diaphragm to tune the resonance of the harvester for the sake of scavenging low frequency vibrational energy. A receptance model was built to explain the vibrational behavior of the combined system. The resonance tuning and energy harvesting performance of the combination system was experimentally verified.

A novel three-dimensional (3D) electret-based micro power generator with multiple vibration modes has been developed, which is capable of converting low-level ambient kinetic energy to electrical energy. The device is based on a rotational symmetrical resonator which consists of a movable disc-shaped seismic mass suspended by three sets of spiral springs. Experimental analysis shows that the proposed generator operates at an out-of-plane direction at mode I of 66 Hz and two in-plane directions at mode II of 75 Hz and mode III of 78.5 Hz with a phase difference of about 90°. A corona localized charging method is also proposed that employs a shadow mask and multiple discharge needles for the production of micro-sized electret array. From tests conducted at an acceleration of 0.05 g, the prototype can generate a maximum power of 4.8 nW, 0.67 nW and 1.2 nW at vibration modes of I, II and III, respectively. These values correspond to the normalized power densities of 16 µW cm−3 g−2, 2.2 µW cm−3 g−2 and 4 µW cm−3 g−2, respectively. The results show that the generator can potentially offer an intriguing alternative for scavenging low-level ambient energy from 3D vibration sources.

This paper reports the characterization and real-time testing of thin-film Pb(Zr0.52Ti0.48) (PZT) micro diaphragm pressure sensor at high temperature (up to 390 °C) and high pressure (up to 105 kPa) conditions. Firstly, the influence of thermal stress on micro diaphragm deformation is investigated theoretically and experimentally. Secondly, the effect of rhombohedral–tetragonal, tetragonal–cubic phase transitions and lattice parameter changes of the PZT composite on the resonance frequency are studied. Thirdly, a good performance of the proposed sensor at low-frequency oscillatory flow generated by a vibrating sphere source (frequency of 35 Hz) in silicone oil bath at different temperatures is reported. The observations lead to the conclusion that despite the fluctuations on resonant frequency with increasing temperature, the proposed PZT sensor can be effectively used in high temperature/pressure applications.

This work demonstrates a nanoelectromechanical pressure sensor based on a nanowire field-effect transistor (NWFET) sensing element. We report that the sensitivity of the pressure sensor can be enhanced up to four times when NWFET operates in subthreshold mode instead of inversion mode. The sensitivity enhancement is attributed to carrier confinement inside the nanowire channel which is obtained by gate bias tuning. In particular, the pressure sensitivity enhances from 0.019 to 0.079 (mA/A)/mmHg by changing the NWFET gate bias from 0.2 V (inversion region) to −0.2 V (subthreshold region). The low-frequency noise characteristics show the significant reduction in drain current noise for NWFET when biased in the subthreshold region, enhancing the signal-to-noise ratio (SNR) from 2 × 106 (inversion region) to 2.4 × 109 (subthreshold region). The result shows that the NWFET-based pressure sensor operates at a low bias with higher piezoresistance and can be used to measure low pressures with a high SNR.

The paper reports the design, fabrication and experimental results of a liquid crystal polymer (LCP) membrane-based pressure sensor for flow rate and flow direction sensing applications. Elaborate experimental testing results demonstrating the sensors' performance as an airflow sensor have been illustrated and validated with theory. MEMS sensors using LCP as a membrane structural material show higher sensitivity and reliability over silicon counterparts. The developed device is highly robust for harsh environment applications such as atmospheric wind flow monitoring and underwater flow sensing. A simple, low-cost and repeatable fabrication scheme has been developed employing low temperatures. The main features of the sensor developed in this work are a LCP membrane with integrated thin film gold piezoresistors deposited on it. The sensor developed demonstrates a good sensitivity of 3.695 mV (ms−1)−1, large operating range (0.1 to >10 ms−1) and good accuracy in measuring airflow with an average error of only 3.6% full-scale in comparison with theory. Various feasible applications of the developed sensor have been demonstrated with experimental results. The sensor was tested for two other applications—in clinical diagnosis for breath rate, breath velocity monitoring, and in underwater applications for object detection by sensing near-field spatial flow pressure.

The vibration modes of a piezoelectric circular microdiaphragm (PCM) are visualized and investigated in this paper. The PCM was previously fabricated by combining sol–gel PZT thin film and MEMS technology (Olfatnia et al 2010 J. Micromech. Microeng. 20 015007). We used a reflection digital holography microscope to visualize different frequency modes. It was found that the degeneracy of the modes with at least one nodal diameter is broken, even though it was expected that these orthogonal modes are degenerated in frequency (Meirovitch 1967 Analytical Methods in Vibrations (New York: Macmillan)). These non-degenerated modes are correlated to the lack of symmetry of the PCM, mainly imposed by the top electrode configuration. The theoretical and experimental measurements of the resonance frequency of different modes show that even though for the first fundamental mode, the diaphragm behaves more like a membrane, in higher modes the stiffness contribution increases, for instance, from 6% in mode (0, 1) to 46% in mode (0, 3). Finite element simulations demonstrate that the frequency shift of the PCM to mass loading increases in higher frequency modes. This shift is almost 8.5 times higher in mode (0, 3) than in mode (0, 1). The impedance characterization of the PCM shows that by applying higher excitation voltages, more vibration modes can be excited. However, these higher voltages induce geometric nonlinearities in the PCM, which in turn increases the resonant frequency of the device.

Localized Ga implantation by means of a focused ion beam and subsequent deep reactive ion etching is used to fabricate microstructures or nanostructures in Si. It is found that the area irradiated with the focused ion beam acts as an etch stop in reactive ion etching. The etch rate ratio between the unimplanted and Ga-implanted area increases to 2.56 with an increasing Ga ion dose up to 1.59 × 1016 ions cm−2. However, the etch rate ratio decreases with further increase in ion dose and the surface of the etched Si microstructures is concave due to the effect of ion sputtering during focused ion irradiation. This technique is promising for the fabrication of 3D nanostructures in Si.