Comb Fingers Research Articles

Challenges in fabrication of high aspect ratio electrostatic comb-drive microactuator using one-step X-ray lithography

A High Aspect Ratio (HAR) electrostatic comb-drive microactuator of polymethyl-methacrylate (PMMA) is designed to deliver nearly 40 µm uniaxial displacement at 25 V DC. The HAR comb-drive microstructures are fabricated via one-step X-ray lithography (OXL). Polyimide-Au X-ray mask is fabricated and used to pattern 200 μm, 500 μm and 800 μm thick PMMA by deep X-ray lithography (DXRL), followed by drying/release and selective metallization for the development of HAR electrostatic microactuator. The release was optimized with the help of various low-surface-tension liquids. 200 μm and 500 μm thick microstructures were successfully released whereas, 800 μm structures could not be released due to the higher depth and associated capillary force. In addition, non-uniform distribution of Au on the side walls of overlapped region of comb fingers, during metallization, resulted in uneven distribution of electrostatic force followed by short-circuiting of HAR (i.e. 40) microactuator. These are the potential issues in the fabrication of HAR microstructures and devices by OXL and discussed in details in this paper.

Electronic phase shift measurement for the determination of acoustic wave DOA using single MEMS biomimetic sensor

MEMS acoustic sensors have been developed to mimic the highly-accurate sound-locating system of the Ormia ochracea fly, which detects sound wavelengths much larger than its hearing organ. A typical ormia-based MEMS directional sound sensor possesses two coupled wings that vibrate in response to sound according to a superposition of its two main resonant modes, rocking and bending. Vibrations are transduced into electronic signals by interdigitated comb finger capacitors at each wing’s end along with a capacitance measuring circuitry. A sensor designed to exhibit resonant modes closely placed in frequency, enhancing their coupling, was operated with a closed cavity behind the wings. Simultaneous and independent measurements of electronic signals generated at each of the single sensor wings were used to determine incident sound direction of arrival (DOA). DOA was found proportional to the phase shift between them and to the difference over the sum of their amplitudes as well. Single sensor phase shift DOA measurement presented a resolution better than 3° for sound pressure levels of 25 mPa or greater. These results indicate that a single sensor operating in closed-cavity configuration can provide hemispherical unambiguous direction of arrival of sound waves which wavelength is much larger than the sensor size.

Fabrication of MEMS Directional Acoustic Sensors for Underwater Operation.

In this work, microelectromechanical systems (MEMS)-based directional acoustic sensors operating in an underwater environment are explored. The studied sensors consist of a free-standing single wing or two wings pivoted to a substrate. The sensors operate in a narrow frequency band determined by the resonant frequency of the mechanical structure. The electronic readout of the mechanical response is obtained using interdigitated comb finger capacitors attached to the wings. The characteristics of MEMS sensors immersed in silicone oil are simulated using finite element modeling. The performance of the sensors is evaluated both in air and underwater. For underwater testing and operation, the sensors are packaged in a housing containing silicone oil, which was specially developed to present near unity acoustic transmission. The measurements show that the resonant frequency of the sensors obtained in air shifts to a lower frequency when immersed in silicone oil, which is primarily due to the mass loading of the liquid. The peak sensitivity of the MEMS sensors is approximately 6 mV/Pa or −165 dB re 1 V/μPa, and the directional response shows a dipole pattern. The signal-to-noise ratio was found to be about 200 or 23 dB at 1 Pa incident sound pressure. The results show the potential of MEMS sensors to be used in underwater applications for sound source localization.

Quarter-mm2 High Dynamic Range Silicon Capacitive Accelerometer With a 3D Process

A new 3D process is proposed for inertial MEMS sensors where a thin transduction layer based on high-density surface-varying comb fingers, with nano-metric gaps, is patterned below a thick seismic mass layer. The objective is to achieve high-performance in a reduced footprint. An in-plane accelerometer is designed, fabricated with this new 3D process, and tested under acceleration. Initial fabricated devices demonstrate a full-scale range of ±160 g, $7~\mu {g}/ \sqrt {\textit {Hz}}$ Brownian noise floor and more than 4 kHz bandwidth. These figures translate into a dynamic range of more than 150 dB (normalized to 1-Hz bandwidth) with an overall footprint of only ${400}\times {600}\,\,\mu {m}^{{2}}$ .

An Engineering Approach for Modeling and Design of a Diaphragm Based Comb Drive Capacitive Pressure Sensor

The Microelectromechanical (MEMS) diaphragm based comb drive capacitive pressure sensor has two stages, Mechanical (Diaphragm) and Capacitive (Comb Drive). The diaphragm displaces in response to apply pressure and the displacement moves the movable comb structure. The two most common architectures of comb drive are single side comb structure and double fold comb structure. In this paper, an impressive technique for designing and modelling of single side comb structure capacitive pressure sensor is being presented. A mathematical model of the sensor is derived and the model simulates the designed structure with the COMSOL Multiphysics Simulator. The Mechanical Sensitivity, Electrostatic sensitivity and Overall Sensitivity are studied for the designed structure. A comparative study of the mathematic analytical values and simulated output values are examined and are found very much closed to each other. The various parameter like Young’s Modulus, Poisson’s Ratio, Dimensions and Structure of the pressure sensor, number of comb fingers and dimension of the comb finger affecting the sensitivity is widely discussed. Keywords: coupler, fringing effect, electrostatic sensitivity, mechanical sensitivity.

Ultrahigh-quality-factor superconducting microwave resonator on diamond for quantum information processing

We report the development of an ultrahigh-Q aluminum superconducting microwave resonator on diamond. Our lumped-element LC resonator consists of a central inductive wire, interdigitated comb fingers and signal coupling pads. The resonance frequency of the ground-state mode is measured to be 2.80 GHz with a maximum internal quality factor of 1.0 × 106 at 20 mK. Replacing the micron-scale inductive wire with a nanometric one, our resonator will make it possible to detect and manipulate single solid-state qubits, e.g. nitrogen-vacancy centers, using single microwave photons.

Resolving bearing ambiguity with a single bio-inspired direction finding microelectromechanical system acoustic sensor

Microelectromechanical systems (MEMS) directional sound sensors have been developed to mimic the unique hearing mechanism of the parasitoid fly Ormia ochracea. The fly analyzes the superposition of the two natural oscillation modes (rocking and bending) of its coupled eardrums to identify the incident direction of a cricket chirp. The incident sound generates unequal oscillation amplitudes of the eardrums depending on the direction of incidence. The MEMS sensor consisted of two wings connected by a bridge which is attached to a substrate using two torsional legs. For electronic readout of the oscillation amplitude of the wings, a comb finger capacitor was integrated to each wing. The amplitudes of oscillation of the two wings were independently measured to probe the coupling between the rocking and bending modes. The measurement of the sensor response was performed optically, using a scanning laser vibrometer, and electronically using a capacitance to voltage converter. It was found that the oscillation amplitudes of the two wings are strongly affected by the incident direction of sound. In addition, higher wing deflection occurred on the side closest to the incident sound source indicating the ability of the sensor to detect the direction of sound without the right-left ambiguity.

MEMS directional acoustic sensor with charge amplifier based electronic readout

MEMS acoustic sensors based on the operating principle of the Ormia ochracea fly provide a unique way to determine the direction of sound. A typical sensor consists of two wings coupled by a bridge and the entire mechanical structure attached to a substrate using two torsional legs. The sensor mechanical structure has two different vibration modes which can be coupled by placing their resonant frequencies close to each other. The acoustic wave impinging on the sensor induces displacements which are detected using comb finger capacitors at the edges of the wings. Previously, this has been accomplished using the universal capacitive readout MS3110, which is prone to static variations of the comb finger capacitors. In this work, a charge amplifier circuit was developed to independently measure the displacements of the wings under sound excitation. The ability to simultaneously read these displacements is needed to determine the bearing of sound using a single MEMS sensor, in contrast to previously reported works that needed either a calibrated microphone or a second MEMS sensor to determine the incident sound pressure level. Measurements showed that both the phase difference and amplitude difference of oscillating wings were strongly dependent on the incident angle of sound.

Microelectromechanical systems directional acoustic sensor for underwater applications

In this work, a MEMS-based directional acoustic sensor operating underwater is explored. The sensor operates in a narrow frequency band centered at the mechanical resonance. The studied sensor consists of two wings coupled by a bridge which is pivoted to a substrate. Interdigitated comb finger capacitors attached to the wings allow for electronic readout of the mechanical oscillations, which are proportional to the sound direction of incidence. For underwater testing, the sensor was immersed in silicone oil, contained by a urethane housing with near unity acoustic transmission. The characteristics of the MEMS sensor both in air and silicone oil were analyzed using finite element modeling. Performance of the sensor was characterized both in air and underwater. Measured underwater frequency response showed that the resonance frequency of the sensor was shifted to a lower value compared to that of in air. This is primarily due to mass loading from the silicone oil used for immersing the sensor. Peak sensitivity of the sensor was found to be about 6 mV/Pa or –165 dB re 1 V/mPa. The sensor showed good directional response with a dipole pattern. Results show the potential of MEMS sensors for underwater applications to detect the bearing of sound sources.

A two degrees of freedom comb capacitive-type accelerometer with low cross-axis sensitivity

In this paper, we report on a SOI-based comb capacitive-type accelerometer that senses acceleration in two lateral directions. The structure of the accelerometer was designed using a proof mass connected by four folded-beam springs, which are compliant to inertial displacement causing by attached acceleration in the two lateral directions. At the same time, the folded-beam springs enabled to suppress cross-talk causing by mechanical coupling from parasitic vibration modes. The differential capacitor sense structure was employed to eliminate common mode effects. The design of gap between comb fingers was also analyzed to find an optimally sensing comb electrode structure. The design of the accelerometer was carried out using the finite element analysis. The fabrication of the device was based on SOI-micromachining. The characteristics of the accelerometer have been investigated by a fully differential capacitive bridge interface using a sub-fF switched-capacitor integrator circuit. The sensitivities of the accelerometer in the two lateral directions were determined to be 6 and 5.5 fF/g, respectively. The cross-axis sensitivities of the accelerometer were less than 5%, which shows that the accelerometer can be used for measuring precisely acceleration in the two lateral directions. The accelerometer operates linearly in the range of investigated acceleration from 0 to 4g. The proposed accelerometer is expected for low-g applications.

A Silicon Resonant Accelerometer Embedded in An Isolation Frame with Stress Relief Anchor

Bias thermal sensitivity is a significant performance parameter of a silicon resonant accelerometer (SRA) and is normally used to evaluate the degree of engineering practicability. Theoretical analysis demonstrates that temperature-induced stress is the dominant factor that determines the bias temperature drift of the custom-designed SRA. To solve this issue, this paper presents an SRA embedded in an isolation frame with stress insensitive anchor that prevents the resonant beams suffering from the thermal stress along the sense axis and thus improving the bias stability. Moreover, a high sensitivity device is achieved by integrating the vibrating beams with the comb fingers without conventional additional mass design. The experimental results show that the nominal resonant frequency of the SRA is around 93 kHz with the sensitivity and nonlinearity of 223.7 Hz/g and 5.1‰. The thermal sensitivities of the two resonant beams are −27.6 ppm/°C and −28.8 ppm/°C, respectively, which can be considered as the results owing to temperature change of the Young’s modulus without the thermal stress effect. The bias thermal sensitivity and the stability (1σ) after compensation are tested to be approximately 0.7 mg/°C and 1 mg over the temperature range from −40 °C to 60 °C with ±80 g measurement range.

Estimation of Air Damping in Out-of-Plane Comb-Drive Actuators.

The development of new compliant resonant microsystems and the trend towards further miniaturization have recently raised the issue of the accuracy and reliability of computational tools for the estimation of fluid damping. Focusing on electrostatically actuated torsional micro-mirrors, a major dissipation contribution is linked to the constrained flow of air at comb fingers. In the case of large tilting angles of the mirror plate, within a period of oscillation the geometry of the air domain at comb-drives gets largely distorted, and the dissipation mechanism is thereby affected. In this communication, we provide an appraisal of simple analytical solutions to estimate the dissipation in the ideal case of air flow between infinite plates, at atmospheric pressure. The results of numerical simulations are also reported to assess the effect on damping of the finite size of actual geometries.

OPTIMIZATION OF POROUS SILICON CHARACTERISTICS FOR SOLAR BATTERIES

The aim is to form the anti-reflective layers of porous silicon on the solar cell surface in the presence of the current-collecting contact comb at the effective area, as well as the study of the state of front contact system of solar cells after etching on their main characteristics.The analysis of the influence of electrochemical etching on the state of the solar cell front contact system without antireflection coating was carried out on the basis of the dependences of the reflectance spectrum of porous silicon on its thickness and refractive index. In the range of wavelengths of 550-850 nm, a minimal reflection from the porous layer surface for the thickness of porous silicon from 70 to 100 nm was found, and its refractive index ranges from 1.35 to 1.9.The critical time of staying of the solar cell structure in the electrolyte, the excess of which leads to the damage of contact fingers of the front current-collecting contact comb was found. For anodization in an electrolyte C2 H5 OH:HF=1:1, the critical time was 45 s. The reduction of the anodizing time to 10 s at anodic charge density of 0.46 C/cm2 allowed to obtain porous silicon layers without significant damage to the current-collecting comb. By reducing the duration of electrochemical treatment to 3-6 s and increasing the anode current, it was possible to completely prevent the damage of the contact system.The results of study of the main solar cell parameters before and after formation of anti-reflective layer on the basis of porous silicon showed that for mono- and multicrystalline solar cells the increase in photocurrent exceeded 50%, the efficiency increased by 25% and 22% respectively for mono- and multicrystalline samples. At the same time, for all tested samples, the open-circuit voltage decreased by 2.5%, and the fill factor degraded by 10.7% and 18.8% for mono- and multicrystalline solar cells, respectively.

Trapezoidal-shaped electrostatic comb-drive actuator with large displacement and high driving force density

This paper presents a design of a comb finger shape and calculation of a trapezoidal-shaped electrostatic comb-drive actuator (TECA) in order to aim a higher electrostatic force density and larger displacement in comparison with the typical rectangular-shaped electrostatic comb-drive actuator (RECA). Relation between a beam's stiffness and a driving voltage has been examined to predict a pull-in effect occurring in TECA. Micro fabrication and characterization of TECA and RECA systems are performed by using a standard SOI-MEMS technology. Theoretical and experimental results confirm the strong points of TECA's structure (similar to the dimensions of RECA) such as a larger number of movable comb finger arrayed at the same length and larger displacement. At driving voltages of 47.9 and 50 (V), the calculation and measurement displacement of TECA are approximately 2.2 and 1.78 times larger than that of RECA, respectively.

Fabrication of comb structure with vertical sidewalls in Si (110) substrate by wet etching in boiling KOH solution

This paper discusses about the fabrication of comb-structure with vertical sidewall profile by wet chemical etching of Si (110) substrate in boiling KOH solution. Etch rate of the Si (110) substrate in boiling KOH solution is 9 times higher with a 40X reduction of average surface roughness than the etch rate at 75 °C. An etch selectivity of 48:1 for the (110) over (111) plane is observed in the boiling KOH solution (35 wt%). A rhombus shaped comb-finger mask is used to fabricate the comb-structure. Each comb finger has the dimension of length: 500 µm and width: 8 µm with inter-comb spacing of 5 µm. Finally, suspended comb-type MEMS structure is realized by using the dissolved wafer process technique.

A Low-Frequency Dual-Band Operational Microphone Mimicking the Hearing Property of Ormia Ochracea

This paper introduces a directional MEMS microphone designed for hearing aid applications appropriate to low-frequency hearing impairment, inspired by the hearing mechanism of a fly, the female Ormia ochracea . It uses both piezoelectric and capacitive sensing schemes. In order to obtain a high sensitivity at low frequency bands, the presented microphone is designed to have two resonance frequencies below the threshold of low-frequency hearing loss at approximately 2 kHz. One is around 500 Hz and the other is slightly above 2 kHz. The novel dual sensing mechanism allows for optimization of the microphone sensitivity at both frequencies, with a maximum open-circuit (excluding pre-amplification) acoustic response captured via differential piezoelectric sensing at approximately −46 dB (V) ref. 94 dB (SPL) at the resonance frequencies. The corresponding minimum detectable sound pressure level is just below −12 dB. The comb finger capacitive sensing was employed due to a lower electrical response generated from a ground referenced single-ended output by the piezoelectric sensing at the first resonance frequency compared with the second resonance frequency. The capacitive sensing mechanism, connected to a charge amplifier, generates a −28.4 dB (V) ref. 94 dB (SPL) acoustic response when the device is excited at either of the two resonance frequencies. Due to the asymmetric geometry and the $400~\mu \text{m}$ thick substrate, the device is predicted to perform as a bi-directional microphone below 3 kHz, which is shown by the measured directional polar patterns. [2018-0036]

Mode Coupling and Parametric Resonance in Electrostatically Actuated Micromirrors

The main torsional mode of electrostatically actuated micromirrors is known to be dominated by parametric resonance when the actuation is performed via in-plane comb fingers. Here, we show that, for specific geometrical features of the mirror, parametric resonance simultaneously activates a spurious yaw mode. Due to the large torsional rotations, the two modes are nonlinearly coupled, inducing mutual stiffness variations and an unexpected temperature dependence of the main mode. After presenting an experimental evidence of the coupling, we develop and discuss a numerical model capable of capturing the key phenomena and of providing guidelines for a robust design.

Investigation of the use of uniaxial comb-shaped Galfenol patches for a guided wave-based magnetostrictive phased array sensor

This paper investigates the use of uniaxial comb-shaped Fe-Ga alloy (Galfenol) patches in the development of a Magnetostrictive Phased Array Sensor (MPAS) for the Guided Wave (GW) damage inspection technique. The MPAS consists of six highly-textured Galfenol patches with a <100> preferred orientation and a Hexagonal Magnetic Circuit Device (HMCD). The Galfenol patches individually aligned to distinct azimuthal directions were permanently attached to a thin aluminum plate specimen. The detachable HMCD encloses a biasing magnet and six sensing coils with unique directional sensing preferences, equivalent to the specific orientation of the discrete Galfenol patches. The preliminary experimental tests validated that the GW sensing performance and directional sensitivity of the Galfenol-based sensor were significantly improved by the magnetic shape anisotropy effect on the fabrication of uniaxial comb fingers to a Galfenol disc patch. We employed a series of uniaxial comb-shaped Galfenol patches to form an MPAS with a hexagonal sensor configuration, uniformly arranged within a diameter of 1”. The Galfenol MPAS was utilized to identify structural damage simulated by loosening joint bolts used to fasten the plate specimen to a frame structure. We compared the damage detection results of the MPAS with those of a PZT Phased Array Sensor (PPAS) collocated to the back surface of the plate. The directional filtering characteristic of the Galfenol MPAS led to acquiring less complicated GW signals than the PPAS using omnidirectional PZT discs. However, due to the detection limit of the standard hexagonal patterned array, the two array sensors apparently identified only the loosened bolts located along one of the preferred orientations of the array configuration. The use of the fixed number of the Galfenol patches for the MPAS construction constrained the capability of sensing point multiplication of the HMCD by altering its rotational orientation, resulting in such damage detection limitation of the MPAS.

Influence of the side etching effect in DRIE on performance of electrostatic linear comb-drive actuators

Side etching and scalloping are unwanted but unavoidable phenomenon occurring during fabrication process of micro electro-mechanical system devices with the deep reactive ion etching technology. This paper reports the influence of deep reactive ion etching fabrication errors on the geometrical dimension (as a comb finger shape, width of the suspension beam), electrostatic force and displacement of electrostatic linear comb-drive actuators. A number of deep reactive ion etching experiments were carried out in order to find out the optimal etching and passivation time, which allows to achieve vertical and smooth sidewall, as well as acceptable side etching value. Discussion and evaluation of deviation between the theoretically calculated and measured displacements of the electrostatic linear comb-drive actuator were also reported.

A mems variable optical attenuator based on a vertical comb drive with self-elevated stators

Vertical comb drives are critical for electrostatic optical switches and variable optical attenuators (VOAs), but fabricating vertical comb drives requires the formation of two levels of comb fingers and thus suffers from the difficulty of dealing with the comb-finger alignment. This paper presents a vertical comb drive design and its micro-fabrication method that can realize self-aligned two-level comb fingers. The self-aligned vertical comb fingers are enabled by a novel vertically-elevated flat-end (VEFE) bimorph structure. Both the stator and rotor fingers of the vertical comb drive are formed by the same photomask and the same silicon etching step, which automatically ensures accurate alignment of the stator and rotor fingers. The vertical separation between the stator and rotor is created by the VEFE structure. A 1mm-aperture MEMS mirror with the proposed VEFE comb drive has been fabricated using SOI wafers with buried cavities. The mirror rotates 0.94° at 8 Vdc. The resonant frequency is 1.428kHz. The MEMS mirror has been assembled into a VOA module. Measurements show that the VOA achieved a dynamic range of 55dB and an insertion loss of less than 0.4dB and met the telecommunications standards on shock, vibration and long-term stability.