Bulk Micromachining Research Articles

Dual-axis tuning fork vibratory gyroscope with anti-phase mode vibration mechanism

In this paper, a novel micro-machined dual-axis tuning fork gyroscope (DTFG) with an anti-phase mechanism is proposed. The proposed anti-phase mechanism could effectively minimize the undesired lateral motion and ensure the anti-phase resonant mode of the two vibrating frames of DTFG. The gyroscope is fabricated by the high-aspect-ratio silicon-on-insulation bulk micromachining process with a device layer thickness of 45 μm. Furthermore, a CMOS drive/readout ASIC Chip, which is fabricated by a 0.25 μm 1P5M standard CMOS process, is integrated with the fabricated DTFG by direct wire-bonding. The experimental characterizations of DTFG demonstrate that the rate sensitivities of z-axis and x-axis sense modes are 2.2132 mV/DPS and 1.8477 mV/DPS respectively and the associated R2-linearity are 0.9995 and 0.9996.

A novel sandwich capacitive accelerometer with a double-sided 16-beam-mass structure

A novel sandwich capacitive accelerometer with a double-sided, 16-beam-mass structure is presented. In this design, the proof mass is supported by 16 tiny beams distributed uniformly on both sides, which aims to dramatically reduce the cross-axis response. Parameters of the beam-mass structure are analyzed and optimized by analytical modeling and the finite element analysis (FEA) method. The micro-accelerometer is fabricated by bulk micromachining technology, and the proof mass and tiny beams are released by KOH anisotropic wet etching from both sides of the silicon wafer, simultaneously. The resonance frequency and the quality factor of the accelerometer are 4.34kHz and 311, respectively, which are measured in an open-loop system. The measurement results show that the accelerometer has a full-scale (FS) range of 30g, a close-loop sensitivity of 80mV/g, and a nonlinearity of 0.27% of FS. The cross-axis sensitivities are 0.353% (x/z axis) and 0.045% (y/z axis), respectively. The bias stability is 0.63mg for an hour. The accelerometer can withstand high shock of over 10,000g.

A micromachined low-frequency piezoelectric harvester for vibration and wind energy scavenging

To efficiently scavenge ambient vibration energy and wind energy at the same time, a low-frequency piezoelectric harvester was designed, fabricated and tested. A lumped-parameter model of the cantilevered piezoelectric energy harvester with a proof mass was established and the closed-form expressions of voltage and power on a resistance load under base acceleration excitation were derived. After effects of the lengths of the proof mass and electrodes on output power were analyzed, a MEMS harvester was optimally designed. By using aluminum nitride as piezoelectric layer, a MEMS energy harvester was fabricated with bulk micromachining process. Experimental results show that the open-circuit frequency of the MEMS harvester is about 134.8 Hz and the matched resistance is about 410 kΩ. Under the harmonic acceleration excitation of ±0.1 g, the maximum output power is about 1.85 µW, with the normalized power density of about 6.3 mW cm−3 g−2. The critical wind speed of the device is between 12.7 and 13.2 m s−1 when the wind direction is from the proof mass to the fixed end of the cantilever. The maximum output power under 16.3 m s−1 wind is about 2.27 µW.

Fabrication of a flexible penetrating microelectrode array for use on curved surfaces of neural tissues

Conventionally, invasive neural microelectrodes for recording neuronal signals or stimulating the nervous system have been fabricated based on silicon substrate mainly due to well-established manufacturing processes. However, these silicon-based microelectrode devices have an issue of mechanical stability caused by the absence of flexibility when implanted onto curved surfaces of tissues. In this paper, a flexible and penetrating microelectrode array, a hybrid structure composed of silicon and elastomer, was devised and fabricated by bulk micromachining technologies. The structure uses individual silicon needles as independent electrodes in a square array and polydimethysiloxane (PDMS) as a base to support the needles. The dimensions of the electrode array and the needles are adjustable, depending on the number of needles, the pitch between the needles and the targeted penetration depth of the neural tissue. For mechanical characterization, the adhesion between PDMS and silicon was evaluated and the flexibility and integrity of the fabricated structure were investigated through flexural test and insertion test. Also, the electrochemical impedance spectroscopy of the electrodes was measured. The results suggest that the proposed microelectrode array is promising for use in neuronal recording and stimulation over curved surfaces such as cortical surface and peripheral nerves with larger curvatures.

Hierarchical fabrication of TiO2 nanotubes on 3-D microstructures for enhanced dye-sensitized solar cell photoanode for seamless microsystems integration

Surface area plays an important factor in the energy conversion performance of solar cells. It has also emerged as a critical factor in the evolution of high-performance micro-electro-mechanical systems (MEMS) and multifunctional microstructures most of which will benefit from integrated on-chip solar power. Presented here is the hierarchical fabrication of TiO2 nanotubes on non-planar 3-dimensional microstructures for enhanced performance of the photoanode in dye-sensitized solar cells (DSCs). The objective is to increase photoanode performance within a standard 1cm2 lateral footprint area by increasing the vertical surface area through the formation of TiO2 nanotubes on 3-D microstructures. In the interest of the seamless integration of DSCs into MEMS applications, careful attention is given to the processing methods of the 3-D surface enhanced photoanode. Bulk micromachining using wet-etching has been employed to fabricate 3-D microstructures in silicon. Anodization was used to form titania nanotubes within sputtered titanium thin films. Film quality, adhesion, and the formation of the nanotubes are discussed. Nanotubes with outer diameter dimensions of 180nm, inner diameter of 75nm, and heights of 340nm on 15μm2×15μm-deep micro-wells have been fabricated resulting in more than 7times the increase in surface area over planar surfaces and a 20% reduction in surface spectral reflection.

Experimental study of a low-g micromachined electrostatically suspended accelerometer for space applications

In this paper a silicon-based micromachined electrostatically suspended accelerometer (Si-MESA), which employs a free parallelepiped proof mass suspended in six degree of freedom, is designed, fabricated and experimentally tested oriented to space applications. The Si-MESA is a glass/silicon/glass sandwich structure fabricated by bulk micromachining, which mainly consists of two anodic bonding steps, a deep reactive ion etching and a wet etching of Al sacrificial layer. The motion of the proof mass with comb teeth on four outer sides is fully servo-controlled with capacitive position sensing and electrostatic suspension. To facilitate ground test of this low-g Si-MESA against the gravity, the input range in the vertical z axis is set to larger than 2 g, while the range in the lateral x and y axes are as low as on the order of several to tens millesimal of the gravity (mg) to achieve higher resolution for potential space applications. The ground test yields specific performances in the lateral x and y axes as follows: the noise floor of 36.8 μg/Hz1/2 and 0.116 mg/Hz1/2, as well as the resolution of 1.454 × 10?5 g and 1.432 × 10?4 g, respectively. The three-axis Si-MESA features extremely low range, high resolution, and long life-span, and has favorable space application prospects such as drag-free control of micro&small satellite and weak gravity measurement.

A Bulk Micromachined Lead Zinconate Titanate Cantilever Energy Harvester with Inter-Digital IrO<SUB><I>x</I></SUB> Electrodes

A piezoelectric vibration energy harvester with inter-digital IrO(x) electrode was developed by using silicon bulk micromachining technology. Most PZT cantilever based energy harvesters have utilized platinum electrode material. However, the PZT fatigue characteristics and adhesion/delamination problems caused by the platinum electrode might be serious problem in reliability of energy harvester. To address these problems, the iridium oxide was newly applied. The proposed energy harvester was comprised of bulk micromachined silicon cantilever with 800 x 1000 x 20 microm3, which having a silicon supporting membrane, sol-gel-spin coated Pb(Zr52, Ti48)O3 thin film, and sputtered inter-digitally shaped IrO(x) electrodes, and silicon inertial mass with 1000 x 1000 x 500 microm3 to adjust its resonant frequency. The fabricated energy harvester generated 1 microW of electrical power to 470 komega of load resistance and 1.4 V(peak-to-peak) from a vibration of 0.4 g at 1.475 kHz. The corresponding power density was 6.25 mW x cm(-3) x g(-2). As expected, its electrical failure was significantly improved.

Application of triton X-100 surfactant for silicon anisotropic etching in KOH-based solutions

The results of etching of silicon surfaces with different crystallographic orientations in KOH solutions containing a nonionic surfactant Triton X-100 are presented in this paper. The etch rate ratio R(100)/R(110) >1, typical of KOH + IPA and TMAH + Triton X-100 mixtures, is achieved. The surface morphology of Si(hkl) wafers is closely investigated by SEM and AFM. The very low roughness of (110) and its vicinal (hh1) planes is observed and measured. In addition, the relatively smooth (h11) surfaces are obtained in the solution with Triton X-100 surfactant, as compared to the KOH solutions containing alcohols. Due to good smoothness of the studied surfaces, the KOH solution with Triton X-100 seems to be especially interesting for bulk micromachining employing non-standard (hkl) planes. The examples of mesas and trenches fabricated by anisotropic etching in the KOH solution containing Triton X-100 surfactant are presented. Keywords: silicon anisotropic etching;Triton X-100; potassium hydroxide; Si(hkl) surfaces

A micro-tensile testing chip–integrated piezoresistive cells for tension measurement and axial alignment

Mechanical testing of micro/nano-materials is very important for design, performance realization, and reliability analysis of micro/nano-electromechanical systems. Mechanical characterization of micro/nano-materials is rather difficult in the handling and alignment of the specimens, measurement of the displacement, and the load for both the off-chip testing method and the integrated on-chip testing method. In this article, a micro-tensile testing chip–integrated piezoresistive cells and a specimen are developed to solve the problems of tension measurement and axial alignment. Based on the piezoresistive effect, the configuration of the chip, especially the parts of the cantilevers, which carry the piezoresistive cells and the cells, is designed, and the tensile force and non-axis alignment error can be measured simultaneously. The chip is prepared with bulk micromachining of silicon. The performance of the chip is calibrated using an off-chip actuated micro-tensile tester, and the sensitivity coefficient of the piezoresistive cells is equal to 0.017 mV/mN.

Specimen alignment of micro-tensile testing using vernier-groove carrier or specimen with piezoresistive cells

Specimen alignment is critical for the accuracy and the reliability of micro-tensile testing. A vernier-groove carrier fabricated by bulk micromachining of silicon is developed. Through the vernier in the carrier the alignment accuracy can be detected, and the groove and the convex stands guarantee rapid alignment and high repeatability. In order to demonstrate the validity of the carrier, a micro-tensile specimen integrated with piezoresistive cells is designed. The specimen can detect axial alignment precision and tensile force simultaneously. The performances of the carrier and the specimen are tested by using a micro-tensile tester. By comparing the measured results of micro-tensile testing with the carrier and without the carrier, it is confirmed that high alignment precision and high repeatability can be obtained by the carrier. For the specimen, the sensitivity coefficient of the piezoresistive cells for tensile force measurement is equal to 0.017 mV/mN .

Inertial micro‐switch capable of prolonging contact time

A novel inertial micro-switch capable of prolonging contact time and reducing the contact-bouncing effect is proposed. Based on the nonlinear-spring shock stop, both the moveable and the fixed electrodes are made flexible by using a moveable contact point and a cascaded beam, respectively. The switch was successfully fabricated by bulk micromachining process. From the drop hammer test, the switch-on time increased with an increase of the input acceleration and reached a maximum of 335 μs, which is over 30 times larger than that of the traditional switch; and no bounce occurred.

Simple fabrication of an uncooled Al/SiO2 microcantilever IR detector based on bulk micromachining

A simple microfabrication process to make an uncooled aluminum/silicon dioxide bi-material microcantilever infrared (IR) detector using silicon bulk micromachining technology is presented in this work. This detector is based on high banding of the microcantilever due to the large dissimilar in thermal expansion coefficients between the two materials. It consists of a 1 μm SiO2 layer deposited by 200 nm thin Al layer. Since no sacrificial layer is used in this process, complexity related to releasing sacrificial layer is avoided. Moreover Al is protected in Si etchant using dual-doped tetramethyl ammonium hydroxide. The other advantage of this process is that only three masks are used with four photolithography process. Thermal and thermal mechanical behaviors of this structure are obtained using finite element analysis, and the maximum temperature and displacement at the end of cantilever at 100 pW/μm2 absorbed IR power density on top surface are 7.82°K and 1.924 μm, respectively.

Aluminum bulk micromachining through an anodic oxide mask by electrochemical etching in an acetic acid/perchloric acid solution

A well-defined microstructure with microchannels and a microchamber was fabricated on an aluminum plate by four steps of a new aluminum bulk micromachining process: anodizing, laser irradiation, electrochemical etching, and ultrasonication. An aluminum specimen was anodized in an oxalic acid solution to form a porous anodic oxide film. The anodized aluminum specimen was irradiated with a pulsed Nd-YAG laser to locally remove the anodic oxide film, and then the exposed aluminum substrate was selectively dissolved by electrochemical etching in an acetic acid/perchloric acid solution. The anodic oxide film showed good insulating properties as a resist mask during electrochemical etching in the solution. A hemicylindrical microgroove with thin free-standing anodic oxide on the groove was fabricated by electrochemical etching, and the groove showed a smooth surface with a calculated mean roughness of 0.2–0.3μm. The free-standing oxides formed by electrochemical etching were easily removed from the specimen by ultrasonication in an ethanol solution. Microchannels 60μm in diameter and 25μm in depth connected to a microchamber were successfully fabricated on the aluminum.

Experimental Study on Aerodynamics of Microelectromechanical Systems Based Single-Crystal-Silicon Microscale Supersonic Nozzle

In this paper, the design, microfabrication, and direct measurement of the static pressure distribution for the aerodynamics of a single-crystal-silicon microscale supersonic nozzle are described. The microscale supersonic nozzle has a convergent–divergent section and a throat area of 100μm × 300μm. The microscale supersonic nozzle was fabricated by silicon bulk micromachining technology. The degree of the rarefaction of nozzle flow was determined by the Knudsen number (Kn). The operation envelope that determines whether the continuum or rarefied flow assumption is appropriate can be expressed as a function of Kn and related parameters. The effect of nonadiabatic operation on microscale nozzle flow was investigated on the basis of wall heat transfer. These physical correlations were taken into account for the classical Shapiro's equations to analyze the microscale nozzle flow aerodynamics (Shapiro, 1953, The Dynamics and Thermodynamics of Compressible Fluid Flow, Ronald, New York, Chap. 7,8; Greitzer et al., 2006, Internal Flow, Cambridge University, Cambridge, UK, Chap. 2,10). Furthermore, the solutions of Shapiro's equations were compared with the experimental results by the authors and other research institutions in order to demonstrate the validity of the proposed aerodynamics design concept for microscale continuum flow.

Design and modeling of a novel microsensor to detect magnetic fields in two orthogonal directions

In this paper, we present the design and modeling of the electrical–mechanical behavior of a novel microsensor to detect magnetic fields in two orthogonal directions (2D). This microsensor uses a simple silicon resonant structure and a Wheatstone bridge with small p-type piezoresistors (10 × 4 × 1 μm) to improve the microsensor resolution. The resonant structure has two double-clamped silicon beams (1000 × 28 × 5 μm) and an aluminum loop (1 μm thickness). The microsensor design allows important advantages such as small size, compact structure, easy operation and signal processing, and high resolution. In addition, the microsensor design is suitable to fabricate using silicon on insulator (SOI) wafers in a standard bulk micromachining process. An analytical model is developed to predict the first bending resonant frequency of the microsensor structure using Macaulay and Rayleigh methods, as well as the Euler–Bernoulli beam theory. Air and intrinsic damping sources of the microsensor structure are considered for its electrical–mechanical response. The mechanical behavior of the microsensor is studied using finite element models (FEMs). For 10 mA of root mean square (RMS) excitation current and 10 Pa air pressure, this microsensor has a linear electrical response, a fundamental bending resonant frequency of 52,163 Hz, and a high theoretical resolution of 160 pT.

ProTEK PSB coating as an alternative polymeric protection mask for KOH bulk etching of silicon

The utilization of a newly developed photosensitive polymeric coating, ProTEK PSB plays a significant role in realizing simple process steps in the fabrication of MEMS devices using bulk micromachining technology. The photosensitive coating which serves as an alternative to the conventional silicon nitride mask of bulk potassium hydroxide (KOH) etching in devising MEMS devices, particularly in suspended microcantilever structure, is reported in this study. Although the polymeric coating ProTEK PSB acts as an excellent outer protective layer from any pinhole issues, the coating’s lateral etching in the KOH solution is dominant, which results in an undercut problem. Therefore, few investigations have been carried out to identify the most suitable condition for the ProTEK PSB deposition on Si substrate. Initial investigation was done on the effect of Si surface modification on the stability of the ProTEK PSB in KOH etching. It was observed that the surface treatment may reduce the undercut ratio for a short period of KOH etching. However, for the extended hours, the surface treatment is not effective enough to improve the stability of the polymeric coating. Therefore, combinations of ProTEK PSB on three substrates were studied in order to obtain a minimum undercut to etch depth ratio of the polymeric coating in KOH bulk etching. The study showed that the combination of ProTEK PSB patterned on thermal oxide results in the most effective etching condition attributed by minimum undercut ratio. Further investigation was carried out on the effect of the KOH etching concentration on the stability of the ProTEK PSB coating over the long hours of bulk etching process. Three concentrations of KOH etchants, KOH 20 wt%, KOH 45 wt% and KOH with isopropyl alcohol (KOH + IPA) were investigated. The results showed that the stability of the polymeric coating was excellent in KOH 20 wt% concentration with a very minimal undercut ratio of 0.05–0.07. In conclusion, the utilization of the polymeric coating ProTEK PSB serves as an alternative etch mask in KOH wet etching which offers simpler and cheaper device fabrication in bulk micromachining technology.

Deep and vertical silicon bulk micromachining using metal assisted chemical etching

In this paper, a newfound and simple silicon bulk micromachining process based on metal-assisted chemical etching (MaCE) is proposed which opens a whole new field of research in MEMS technology. This method is anisotropic and by controlling the etching parameters, deep vertical etching, relative to substrate surface, can be achieved in micrometer size for 〈1 0 0〉 oriented Si wafer. By utilizing gold as a catalyst and a photoresist layer as the single mask layer for etching, 60 µm deep gyroscope micromachined structures have been fabricated for 2 µm features. The results indicate that MaCE could be the only wet etching method comparable to conventional dry etching recipes in terms of achievable etch rate, aspect ratio, verticality and side wall roughness. It also does not need a vacuum chamber and the other costly instruments associated with dry etching techniques.

Study of Insertion Characteristics of Si Neural Probe with Sharpened Tip for Minimally Invasive Insertion to Brain

We have fabricated various types of Si neural probe for in vivo and in vitro neuronal signal recordings by combining standard photolithography with a bulk micromachining process. To place the probe tip at target areas in the brain precisely, the mechanical properties of the Si neural probes with various tip shapes were carefully investigated under different insertion conditions to brain phantoms. As results, the minimum length and penetration force of the Si neural probe were determined from conditions where the Si neural probe began to penetrate the surface of the brain phantom. To control buckling of the Si neural probe, it is necessary to optimize the insertion rate in accordance with the conditions of the Si neural probe. Although the insertion forces of the Si neural probe with a sharpened tip were smaller than those of the Si neural probe with a normal tip, the effect of the probe tip shape became small with increased insertion speeds.

Creating large out-of-plane displacement electrothermal motion stage by incorporating beams with step features

Realizing out-of-plane actuation in micro-electro-mechanical systems (MEMS) is still a challenging task. In this paper, the design, fabrication methods and experimental results for a MEMS-based out-of-plane motion stage are presented based on bulk micromachining technologies. This stage is electrothermally actuated for out-of-plane motion by incorporating beams with step features. The fabricated motion stage has demonstrated displacements of 85 µm with 0.4 µm (mA)−1 rates and generated up to 11.8 mN forces with stiffness of 138.8 N m−1. These properties obtained from the presented stage are comparable to those for in-plane motion stages, therefore making this out-of-plane stage useful when used in combination with in-plane motion stages.

Dynamic characterization and modelling of a dual-axis beam steering device for performance understanding, optimization and control design

This paper presents a lumped thermal model of a dual-axis laser micromirror device for beam steering in a free-space optical (FSO) communication system, designed for fractionated spacecraft. An FSO communication system provides several advantages, such as larger bandwidth, smaller size and weight of the communication payload and less power consumption. A dual-axis mirror device is designed and realized using microelectromechanical systems technology. The fabrication is based on a double-sided, bulk micromachining process, where the mirror actuates thermally by joints consisting of v-grooves filled with the SU-8 polymer. The size of the device, consisting of a mirror, which is deflectable versus its frame in one direction, and through deflection of the frame in the other, is 15.4 × 10.4 × 0.3 mm3. In order to further characterize and understand the micromirror device, a Simulink state-space model of the actuator is set up using thermal and mechanical properties from a realized actuator. A deviation of less than 2% between the modelled and measured devices was obtained in an actuating temperature range of 20–200 °C. The model of the physical device was examined by evaluating its performance in vacuum, and by changing physical parameters, such as thickness and material composition. By this, design parameters were evaluated for performance gain and usability. For example, the crosstalk between the two actuators deflecting the mirror along its two axes in atmospheric pressure is projected to go down from 97% to 6% when changing the frame material from silicon to silicon dioxide. A feedback control system was also designed around the model in order to examine the possibility to make a robust control system for the physical device. In conclusion, the model of the actuator presented in this paper can be used for further understanding and development of the actuator system.