Two-mask Process Research Articles

Micromachined Piezoresistive-sensed Diaphragms for Infrasonic Monitoring

Micromachined, piezoresistive-sensed diaph- ragms are designed and fabricated specifically for infrasound sensing applications. Relatively large diaphragms, up to 6 mm $\times6$ mm, are fabricated to meet the high sensitivity requirements of infrasonic monitoring. A double-stack SOI wafer process is used, resulting in a streamlined two-mask process without the need for doping or ion-implantation. Pressure-sensitivities of diaphragms are studied using a custom static calibration set-up. The piezoresistance of longitudinal and transverse resistors are studied over a large pressure range of ±5,500 Pa so that the linear and nonlinear response regions of diaphragms are observed. Measurements are compared against a nonlinear finite element simulation—shown to accurately predict the sensor’s behavior over a large response range.

Laterally Movable Triple Electrodes Actuator toward Low Voltage and Fast Response RF-MEMS Switches.

A novel actuator toward a low voltage actuation and fast response in RF-MEMS (radio frequency micro-electro-mechanical systems) switches is reported in this paper. The switch is comprised of laterally movable triple electrodes, which are bistable by electrostatic forces applied for not only the on-state, but also the off-state. The bistable triple electrodes enable the implementation of capacitive series and shunt type switches on a single switch, which leads to high isolation in spite of the small gap between the electrodes on the series switch. These features of the actuator are effective for a low voltage and fast response actuation in both the on- and off-state. The structure was designed in RF from a mechanical point of view. The laterally movable electrodes were achieved using a simple, low-cost two-mask process with 2.0 µm thick sputtered aluminum. The characteristics of switching response time and actuation voltage were 5.0 µs and 9.0 V, respectively.

Bio-inspired fluidic thermal angular accelerometer with inherent linear acceleration rejection

This paper reports on the design, simulation, fabrication and characterization of a bio-inspired angular accelerometer. The sensor mimics the semicircular canals in mammalian vestibular systems. The device pairs a fluid-filled microtorus with a thermal detection principle based on thermal convection. The microtorus is intersected by a set of heaters surrounded with temperature detectors on either side, which sense a temperature profile asymmetry upon applied angular acceleration. The device fabrication is based on a two-mask process. Proper arrangement of four resistive temperature sensors in a Wheatstone bridge reduces the impact of heater-induced buoyancy effects. The toroidal microchannel results in inherent geometric linear acceleration insensitivity. The sensor demonstrates a sensitivity of 124 μV/deg/s2 for in-plane angular acceleration, two orders of magnitude suppression of cross-axis angular accelerations, three orders of magnitude suppression of linear accelerations, and a test setup limited dynamic range of ±2000 deg/s2 at 1 Hz.

Effect of residual stress in Pb (Zr0.52Ti0.48) O3 – BiFeO3 multilayer cantilever structure

This paper discussed about the integration issues of Pb (Zr0.52Ti0.48) O3 – BiFeO3 (PZT - BFO) multilayer thin film deposited on silicon substrate for possible application in future micro-electro-mechanical system (MEMS) devices. The PZT - BFO multilayer thin film is deposited on silicon wafer by sol-gel technique. The multilayer film is annealed at 650 °C in air for 60 min. The deposited multilayer film is found to be polycrystalline in nature. The PZT-BFO multilayer exhibited room temperature multiferroic properties (remnant polarization of 37 mC/ cm 2 and remnant magnetization of 3.1 emu / cm 3 ). To fabricate the PZT - BFO multiferroic cantilever structures, a two-mask process flow is developed. Etch rates of the PZT - BFO multilayer (180 nm/ min), ZrO2 buffer layer (35 nm/ min) and SiO2 layer (350 nm/ min) are optimized in CHF3 plasma. The multiferroic cantilever structures are released by isotropic etching of silicon using SF6 plasma. Bending and cracks are observed in the released cantilever structures due to the generation of residual stress in the multilayer thin film. Effect of residual stress on the PZT - BFO cantilever structure is also verified by simulation.

Design, Fabrication, and Characterization of Scandium Aluminum Nitride-Based Piezoelectric Micromachined Ultrasonic Transducers

This paper presents the design, fabrication, and characterization of piezoelectric micromachined ultrasound transducers (PMUTs) based on scandium aluminum nitride (ScxAl1–xN) thin films (x = 15%). ScAlN thin film was prepared with a dual magnetron system and patterned by a reactive ion etching system utilizing chlorine-based chemistry with an etching rate of 160 nm/min. The film was characterized by X-ray diffraction, which indicated a crystalline structure expansion compared with pure AlN and a well-aligned ScAlN film. ScAlN PMUTs were fabricated by a two-mask process based on cavity SOI wafers. ScAlN PMUTs with 50- and 40- $\mu \text{m}$ diameter had a large dynamic displacement sensitivity measured in air of 25 nm/V at 17 MHz and 10 nm/V at 25 MHz, twice that of AlN PMUTs with the same dimensions. The peak displacement as a function of electrode coverage was characterized, with maximum displacement achieved with an electrode radius equal to 70% of the PMUT radius. Electrical impedance measurements indicated that the ScAlN PMUTs had 36% greater electromechanical coupling coefficient ( $\text{k}_{\mathrm {t}}^{2})$ compared with AlN PMUTs. The output pressure of a $7\times7$ ScAlN PMUT array was 0.7 kPa/V at ~1.7 mm away from the array, which is approximately three times greater that of an $8\times8$ AlN PMUT array with the same element geometry and fill factor measured at the same distance. Acoustic spreading loss and PMUT insertion loss from mechanical transmit to receive were characterized with a 15 $\times $ 15 ScAlN PMUT array via hydrophone and laser Doppler vibrometer. [17509-2017]

A-Si:H TFT-Silicon Hybrid Low-Energy X-Ray Detector

Direct conversion crystalline silicon X-ray imagers are used for low-energy X-ray photon (4–20 keV) detection in scientific research applications such as protein crystallography. In this paper, we demonstrate a novel pixel architecture that integrates a crystalline silicon X-ray detector with a thin-film transistor amorphous silicon pixel readout circuit. We describe a simplified two-mask process to fabricate a complete imaging array and present preliminary results that show the fabricated pixel to be sensitive to 5.89-keV photons from a low activity Fe-55 gamma source. This paper presented can expedite the development of high spatial resolution, low cost, direct conversion imagers for X-ray diffraction and crystallography applications.

A dual-mass fully decoupled MEMS gyroscope with wide bandwidth and high linearity

A fully decoupled z-axis MEMS gyroscope with wide bandwidth and high linearity is presented in this paper. The mechanical structure consists of a dual proof mass, which are mechanically coupled together using a pair of double folded springs. The fully decoupled approach is employed to lower the mechanical cross coupling between the drive mode and sense mode. The gyroscope is fabricated using a simple two-mask process based on a silicon-on-insulator (SOI) substrate with a 30-μm-thick device layer. The electrostatic actuation and capacitive sensing gaps of around 1.1-μm provide an aspect ratio of close to 27. The narrow capacitive gap increases the electromechanical coupling and improves the signal to noise ratio (SNR) of the sensor. The fabricated MEMS gyroscope is vacuum packaged in a ceramic chip carrier in order to minimize the Brownian noise floor for achieving a better performance. A off-the-shelf integrated circuit with the excitation and sensing electronics is hybrid connected to the gyroscope. The MEMS gyroscope system demonstrates a bias instability of 9.6deg/h, an angular random walk (ARW) of 0.45deg/h, a bandwidth of greater than 120Hz, and a scale-factor nonlinearity of 770ppm in a full scale range of ±500deg/s at room temperature.

Focusing efficiency evaluation of ultrasonic energy for fabricated Fresnel lens through surface profile estimation and FEA

ABSTRACTHigh focusing efficiency of a multi-level Fresnel lens is desired when it is designed and fabricated. How to estimate the focusing efficiency is an important issue after the Fresnel lens is made. We propose a method to evaluate the focusing efficiency of the fabricated Fresnel lens based on its surface profile. The profile of Fresnel lens fabricated shows how similar to the designed, which has been theoretically predicted a maximal focusing efficiency such as 81% and 40.5% for a four-phase and two-phase level Fresnel lenses, respectively. This method can be applied to evaluate the focusing efficiency of any fabricated surface profile of Fresnel lenses prior to a direct experimental measurement for energy focusing. The focusing efficiency of a four-phase level Fresnel lens designed to operate at a frequency of 100 MHz of an ultrasonic ejector, which was fabricated by a two-mask process using SU-8 photoresist, approximate 60% is illustrated. Finite element analysis was employed to confirm and observe the energy focusing phenomenon of ultrasonic ejector models.

Direct Torsion of Bulk PZT Using Directional Interdigitated Electrodes

We present a bulk unimorph piezoelectric beam, which is directly actuated in a torsional mode. The direct torsion actuation is achieved by Interdigitated Electrodes (IDE), oriented at 45° relative to the beams axis. The IDE are deposited directly on a bulk piezoelectric plate, which serves as both the substrate and as the device layer. Elimination of a substrate layer solves the issues of residual stress and thermal expansion mismatch. In addition, the fact that all electrodes are deposited simultaneously over a single surface, simplifies fabrication to a two-mask process, and facilitates electrical connection to the electrodes. The bulk lead zirconate titanate (PZT) plate is etched using a cheap and simple sand-blasting process. We present proof-of-concept devices, and demonstrate direct torsion deformation of a cantilever beam.

Design and Fabrication of a Long-Arm Comb-Drive Rotary Actuator With Externally Mounted Mirror for Optical Applications

In this paper, we present the design, fabrication, and characterization of a long-arm comb-drive actuator with an externally mounted large mirror for optical applications. The provision for an externally mounted mirror can be useful for many optical applications, which require large and high-quality reflecting surfaces. The angular stretch of the comb fingers at the far end is kept smaller than those near the anchor point. This structural feature allows the design of actuator arm lengths of over 5 mm. The truss of the actuator is designed carefully to support the large movable arm. An analytical model for the electrostatic forces in the actuator was developed, and detailed analyses for side instability and resonance frequency were performed. The complete fabrication process involving bulk micromachining of silicon-on-insulator (SOI) wafer using a two-mask process is described. A procedure for efficient mounting of the mirror utilizing die bonder, vacuum tip, and silver epoxy is demonstrated. Static characterization measurements show that the actuator can achieve a rotational angle of 3° with an applied voltage of 130 V. A resonance frequency of $\sim 246$ Hz for a mirror size of $2~\textrm {mm} \times 100~\mu \textrm {m} \times 1$ mm was determined by optical characterization; this closely matches with the simulation results and the analytical model. [2014-0318]

A high-impedance dual-mode SAW resonator for ultra low power and high data rate FSK modulator

This paper reports on the design and experimental realization of a dual-mode surface acoustic wave (SAW) resonator with two high-impedance resonant peaks for ultra low power binary frequency-shift-keying (FSK) modulator applications. The single-device structure of the dual-mode resonator is believe to be a promising solution to eliminate the loading effect and noise as a result of multiple switches and other parallel connected resonators, and has the potential to reduce the size and complexity of packaging, compared with the multi-resonator multi-frequency approaches. The dual-mode SAW resonator concept is realized by an in-house, two-mask process that delivered symmetric-mode resonators at 404.9MHz with trans-impedance values of 1,100Ω and quality factor of 3134, and anti-symmetric mode resonators at 407.6MHz, with 900Ω trans-impedance and 4014 quality factors. Based on the experimental results, we propose a FSK modulator that integrates this high-transimpedance dual-mode resonator and reaches data rates up to 1.5Mbps. The FSK modulator can be used for low power transmitters in body area network (BAN) and wireless sensor networks (WSN).

Fabrication and Characterization of a Suspended TCD Integrated With a Gas Separation Column

This paper reports the design, fabrication, and performance evaluation of a unique high aspect ratio resistor on glass employed in a micro thermal conductivity detector (μTCD). Finite-element simulations demonstrate significant improvements in power dissipation and warm-up time by suspending the resistors in the microchannels. Microfabrication techniques employed here enable dry release of 170 μm-diameter coil-shaped resistors on Borofloat in a two-mask process. The microfluidic channels on silicon wafers are independently processed using deep reactive ion etching and subsequently anodic bonded to the glass substrate harboring the TCD resistors. Experiments reveal that the suspended resistors need only 13 mW to reach 100 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C in less than 6 ms and could yield 200 ppm minimum detection limit. The completed chip containing a 1-m-long, 100- μm-wide column with embedded TCD resistors occupies an area of 2.5 × 2 cm. The column coated with an OV-1 stationary phase using the static coating method successfully separated a mixture of hydrocarbons, while the TCD resistors connected in a Wheatstone configuration were able to generate the chromatogram verified by that produced by a commercial flame ionization detector.

Transverse Interdigitated Electrode Actuation of Homogeneous Bulk PZT

A new method for achieving transverse bending-mode actuation of piezoelectric devices microfabricated from homogeneous layers of bulk lead zirconate titanate (PZT) is presented. The technique, which employs a set of interdigitated electrodes patterned on a single side of the piezoelectric substrate, takes advantage of an engineered electric field gradient within the PZT that can be optimized by selecting an appropriate electrode gap. Bulk PZT cantilevers have been fabricated by micropowder blasting, allowing the actuation technique to be evaluated experimentally and compared against analytical and finite-element-model results. Optimal poling conditions for the microfabricated cantilevers are reported, together with both quasi-static and resonant behaviors of the devices. The transverse interdigitated electrode topology provides a simple method for realizing high-performance bending-mode piezoelectric microactuators from a single homogeneous layer of bulk PZT using a simple two-mask process.

Effect of Fabrication Parameters on the Characteristics of Fresnel Lens and Piezoelectric Transducers

A two-mask fabrication process of a four-level Fresnel lens was used to evaluate its characteristics through investigating the effect of SU-8 photoresist on the profile of the focusing lens. A two-step deposition of ZnO films was applied to develop a feasible fabrication of a piezoelectric transducer with the structure Al/ZnO/Pt/Ti/SiO2/Si under reasonable conditions, which include deposition pressure of 0.7 Pa and 1.3 Pa, RF power of 100 W and 178 W, and sputtering gas ratio Ar:O2 = 1:3 and 1:1 for first and second step deposition, respectively. Highly c-axis textured ZnO films were successfully obtained through two-step deposition process.

Comb-drive tracking and focusing lens actuators integrated on a silicon-on-insulator wafer

This paper reports integrated tracking (horizontal) and focusing (vertical) actuators using microelectromechanical (MEMS) technology for optical storage. The design, fabrication and characterization of the integrated tracking and focusing actuators are demonstrated. The integrated tracking and focusing lens actuators that consist of springs, a lens holder and comb-drive actuators are implemented to obtain the two-dimensional tuning effect of an objective lens. A two-mask process is used to define the actuators on a silicon-on-insulator (SOI) wafer. The DC displacements and the frequency responses are characterized by applying voltage to the micro lens actuators. Displacements are experimentally characterized with ± 24.6μm in tracking direction and 5.7μm in focusing direction. Moreover, the influences of the cross-axis coupling are measured and evaluated. This MEMS device which has a small form factor provides an excellent response time and size reduction. The minimization and integration of the lens actuators can offer low costs in assembly and expand the application area of the optical head.

Optical tilt sensor with direct intensity-modulated scheme

An optical tilt sensor based on a phase-sensitive diffraction grating with direct intensity-modulated scheme is proposed in this paper. The tilt sensor consists of a coherent light source, an integrated grating, a mechanical sensing part, and an optical signal processing circuit. The mechanical sensing part consists of a bulk proof mass and two cantilevers that can be fabricated with a two-mask process on a silicon-on-insulator substrate. A phase-sensitive diffraction grating, which was formed with the integrated grating and the upper surface of the proof mass, acts as the sensing element for the tilt measurement. Experimental results show this tilt sensor provides the optimal tilt sensitivity of 1.15 V/° and resolution of about 0.0046°, and a measurement range of ±20° in a rotational plane that is perpendicular to the direction of the earth's gravity.

High density vertical silicon NEM switches with CMOS-compatible fabrication

Presented is a resistive, vertically oriented, silicon electromechanical cantilever switch, fabricated with a two-mask top-down CMOS-compatible process. The fabricated switch has a vertical height of approximately 500 nm, a tip width of 35 nm, and an airgap of 30 nm between the cantilever and two lateral gates. Preliminary testing results show an initial pull-in voltage of 17 V with contact held by van der Waals forces even in the absence of actuation voltage. Subsequent switching occurs at 25 V. With low-cost fabrication and high integration density, the vertical nanoelectromechanical (NEM) switch is a promising candidate for memory and computing applications.

Linear High-Resolution BioMEMS Force Sensors With Large Measurement Range

We present a set of displacement-based high-resolution (50 pN) micromechanical force sensors with a large force measurement range (1 μN). Typically, force sensors that have high resolution have a limited force measurement range and vice versa. The force sensors presented here overcome this limitation and, in addition, have a highly linear force-displacement response. The sensors (≈3 mm × 4 mm × 150 m)are composed of a series of flexible beams attached to a rigid probe that deform when subjected to an external force. The force is obtained by optically measuring the displacement of the probe with respect to a fixed reference beam. The force sensors are fabricated using a simple two-mask process that allows for their stiffness to be varied over a wide range. Furthermore, we have developed a novel scheme to avoid capillary forces during the immersion and removal of these sensors from aqueous environments, which makes them highly suited for biological studies. We illustrate the capability and versatility of these sensors by measuring the in vivo force-deformation response of axons in Drosophila melanogaster (fruit fly).

Design and Fabrication of a Planar Three-DOFs MEMS-Based Manipulator

Abstract-This paper presents the design, modeling, and fabrication of a planar three-degrees-of-freedom parallel kinematic manipulator, fabricated with a simple two-mask process in conventional highly doped single-crystalline silicon (SCS) wafers (100). The manipulator's purpose is to provide accurate and stable positioning of a small sample (10 × 20 × 0.2 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ), e.g., within a transmission electron microscope. The manipulator design is based on the principles of exact constraint design, resulting in a high actuation-compliance combined with a relatively high suspension stiffness. A modal analysis shows that the fourth vibration mode frequency is at least a factor 11 higher than the first three actuation-related mode frequencies. The comb-drive actuators are modeled in combination with the shuttle suspensions gaining insight into the side and rotational pull-in stability conditions. The two-mask fabrication process enables high-aspect-ratio structures, combined with electrical trench insulation. Trench insulation allows structures in conventional wafers to be mechanically connected while being electrically insulated from each other. Device characterization shows high linearity of displacement wrt voltage squared over ±10 μm stroke in the xand y-directions and ±2° rotation at a maximum of 50 V driving voltage. Out-of-plane displacement crosstalk due to in-plane actuation in resonance is measured to be less than 20 pm. The hysteresis in SCS, measured using white light interferometry, is shown to be extremely small.

Microcrystalline-Si thin-film transistors formed by using palladium silicided source/drain contact electrode

Microcrystalline-Si thin-film transistors (μC-Si TFTs) formed by using the source/drain contact electrode of self-aligned palladium silicide have been investigated. Both the self-aligned palladium silicided scheme and the previous top-gate staggered structure employ two-mask process steps for fabricating μC-Si TFTs. However, the self-aligned palladium silicided scheme would cause better device characteristics than the top-gate staggered structure, primarily due to more carrier tunneling. For a gate length of 2 μm, as compared to the top-gate staggered scheme, this silicided scheme can result in a 40% improvement of on-state current. In addition, as the gate length is reduced to 1 μm, considerable short-channel effect is caused for both the device schemes.