Standard MEMS Processes Research Articles

Fabrication and DC-Bias Manipulation Frequency Characteristics of AlN-Based Piezoelectric Micromachined Ultrasonic Transducer.

Due to their excellent capabilities to generate and sense ultrasound signals in an efficient and well-controlled way at the microscale, piezoelectric micromechanical ultrasonic transducers (PMUTs) are being widely used in specific systems, such as medical imaging, biometric identification, and acoustic wireless communication systems. The ongoing demand for high-performance and adjustable PMUTs has inspired the idea of manipulating PMUTs by voltage. Here, PMUTs based on AlN thin films protected by a SiO2 layer of 200 nm were fabricated using a standard MEMS process with a resonant frequency of 505.94 kHz, a -6 dB bandwidth (BW) of 6.59 kHz, and an electromechanical coupling coefficient of 0.97%. A modification of 4.08 kHz for the resonant frequency and a bandwidth enlargement of 60.2% could be obtained when a DC bias voltage of -30 to 30 V was applied, corresponding to a maximum resonant frequency sensitivity of 83 Hz/V, which was attributed to the stress on the surface of the piezoelectric film induced by the external DC bias. These findings provide the possibility of receiving ultrasonic signals within a wider frequency range, which will play an important role in underwater three-dimensional imaging and nondestructive testing.

Development of MEMS composite sensor with temperature compensation for tire pressure monitoring system

The pressure and temperature inside the tire is mainly monitored by the tire pressure monitoring system (TPMS). In order to improve the integration of the TPMS system, moreover enhance the sensitivity and temperature-insensitivity of pressure measurement, this paper proposes a microelectromechanical (MEMS) chip-level sensor based on stress-sensitive aluminum–silicon hybrid structures with amplified piezoresistive effect and temperature-dependent aluminum–silicon hybrid structures for hardware and software temperature compensations. Two types of aluminum–silicon hybrid structures are located inside and outside the strained membrane to simultaneously realize the measurement of pressure and temperature. The model of this composite sensor chip is firstly designed and verified for its effectiveness by using finite element numerical simulation, and then it is fabricated based on the standard MEMS process. The experiments indicate that the pressure sensitivity of the sensor is between 0.126 mV/(V·kPa) and 0.151 mV/(V·kPa) during the ambient temperature ranges from −20 °C to 100 °C, while the measurement error, sensitivity and temperature coefficient of temperature-dependent hybrid structures are individually ±0.91 °C, −1.225 mV/(V °C) and −0.150% °C−1. The thermal coefficient of offset (TCO) of pressure measurement can be reduced from −3.553%FS °C−1 to −0.375%FS °C−1 based on the differential output of the proposed sensor. In order to obtain the better performance of temperature compensation, Elman neural network based on ant colony algorithm is applied in the data fusion of differential output to further eliminate the temperature drift error. Based on which, the overall measured error is within 3.45 kPa, which is less than ±1.15%FS. The TCO is −0.017%FS °C−1, and the thermal coefficient of span is −0.020%FS °C−1. The research results may provide a useful reference for the development of the high-performance MEMS composite sensor for the TPMS system.

Compact Micro Thermal Sensor Based on Silicon Thermocouple Junction and Suspended Fluidic Channel

This research proposed a micro biochemical thermal sensor with compact integration capability by combining a novel single thermocouple junction and suspended microfluidic channel. While the thermal sensor was fabricated through standard MEMS processes on the silicon-on-insulator (SOI) wafer, a heavily-doped silicon of high Seebeck coefficient was employed as the highly sensitive material for thermocouple junction. To enable a high sensitivity and fast response detection, a unique structure of air-suspended bridge and a built-on SU-8 polymer microfluidic channel was fabricated to reduce heat loss and sample loading dose for trace amount of heat detection. As high as a sensitivity of 0.27 V/W and rapid thermal response less than 200 ms are achieved in this thermal sensor, which allows for the enzymatic bio-reaction heat detection and laid great potential for portable healthcare applications.

Micropillar based active microfluidic mixer for the detection of glucose concentration

Active microfluidic mixer has attracted remarkable interest towards the applications in the Lab on Chip (LoC) for fast and accurate fluid mixing. In this work, the standard MEMS processes have been successfully implemented to fabricate the active electromagnetic microfluidic mixer incorporating micropillar on flexible membrane for glucose measurement. Three main parts of the mixer, i.e., a planar electromagnetic coil with permanent magnet, a mechanical movable membrane with pillar and microfluidic channels that were fabricated separately and later bonded with corona discharging process to complete the construction of the microfluidic mixer. The mechanical movable membrane and microfluidic channels are made of polymer and fabricated using soft-lithography technique. Analysis results show that the fabricated microfluidic mixer operated using low electrical current of below 0.5 A produced the mixing index above 0.8 within 4 s. Mixture analysis of fluid samples tested at frequencies of 1.1 kHz, 2.3 kHz and 3.7 kHz shows much better mixing performance as compared to its passive mode condition. The functionality of the fabricated mixer was tested by mixing the DNS reagent and glucose solution. The detection of glucose concentration in the microfluidic mixer chip is indicated by comparing the correlation coefficient R2 with the conventional method. The evaluation result shows that the R2 of the mixture produced by the fabricated device shows a comparable value with that of conventional method showing a very promising approach for the diabetic patient diagnosis and food analysis.

Ultrathin single‐crystalline LiNbO 3 film bulk acoustic resonator for 5G communication

This Letter reports a high-performance film bulk acoustic resonator (FBAR) that can be applied in 5G wireless communication. The FBAR has a back-etched free-standing structure using an ultra-thin single-crystalline lithium niobate (LiNbO3) as the piezoelectric film. The thin LiNbO3 was obtained by the smart cut method and FBARs were fabricated by a standard MEMS process. Owing to the superior bulk-like properties of the single-crystal thin film, the fabricated FBARs have a resonant frequency of 5.0 GHz, a quality factor above 1800 and an electromechanical coupling coefficient of 2.66%, demonstrating a promising potential of FBAR filters in 5G and future 6G applications.

Study of Energy Loss Mechanisms in AlN-Based Piezoelectric Length Extensional-Mode Resonators

This paper reports on the investigation of anomalous low quality factors ( ${Q}\text{s}$ ) in AlN thin film-based length extensional (LE)-mode resonators using finite element method (FEM), analytical modeling, and experimental techniques. Different heterostructures of LE resonators having Al/AlN/Si, Al/AlN, AlN/Si, and Si are designed and fabricated using standard MEMS processes. Experimental ${Q}\text{s}$ along with resonances are obtained using electrical and optical readout and are compared with the modeled and analytically obtained ${Q}\text{s}$ . The results show that the thermoelastic damping (TED) in metal electrode, anchor loss, and charge redistribution loss are not the dominant loss mechanisms. With the help of Mason’s network modeling, we investigated the dielectric loss, which is also observed to be negligible for these resonators. An internal mechanical loss, originating from the columnar growth of AlN and observed using Mason’s model, is proved to be the dominant loss mechanism. This observation is confirmed and reinforced by the COMOSL’s FEM analysis of TED in AlN, having columnar growth effect and by experimental results. [2018-0258]

Fabrication of Micromachined Uniform Microtrench Arrays for Silicon based Filtration Membrane

In this paper, we report a uniform microtrench array fabricated on silicon substrate. The proposed system is aimed for the biological molecules separation process via filtration in artificial kidney. The system consists of silicon based membrane having arrays of trench with the dimension of approximately 1 μm. The fabrication of the trenches is following standard silicon MEMS process with additional surface modification of the fabricated trench by using thermal oxidation process. The result shows that trench patterns having size of 1~2 μm have been produced. The thermal oxidation process shows that the trench size could be reduced up to 50%. The study will be beneficial for the development of silicon membrane with uniform microtrench arrays for application in filtration of blood cell from other solutes based on molecule size. When the microtrench is etched through the silicon membrane, a filtration membrane is then formed. The separation of solutes will be better due to the uniform size of microtrench.

Microfluidic chip to interface porous microneedles for ISF collection.

Porous microneedles (MNs) are expected to be applied for diagnostic microfluidic devices such as blood glucose monitoring as they enable a pain-free penetration of human skin and the extraction of interstitial fluids. However, conventional microfluidic systems require additional steps to separate the liquid from a porous structure used for fluid extraction. In this study, we developed a microfluidic system with a hydrodynamically designed interface between a porous MN array and microchannels to enable a direct analysis of liquids extracted by the porous MN array. The microfluidic chip with an interface for the MN array was successfully realized by standard MEMS processes, enabling a liquid flow through the whole microfluidic structure. The porous MN array was fabricated by the salt leaching and molding method, which was integrated with the chip and demonstrated the successful extraction of liquids from an agarose gel-based skin phantom.

A softening laminar electrode for recording single unit activity from the rat hippocampus

Softening neural implants that change their elastic modulus under physiological conditions are promising candidates to mitigate neuroinflammatory response due to the reduced mechanical mismatch between the artificial interface and the brain tissue. Intracortical neural probes have been used to demonstrate the viability of this material engineering approach. In our paper, we present a robust technology of softening neural microelectrode and demonstrate its recording performance in the hippocampus of rat subjects. The 5 mm long, single shank, multi-channel probes are composed of a custom thiol-ene/acrylate thermoset polymer substrate, and were micromachined by standard MEMS processes. A special packaging technique is also developed, which guarantees the stable functionality and longevity of the device, which were tested under in vitro conditions prior to animal studies. The 60 micron thick device was successfully implanted to 4.5 mm deep in the hippocampus without the aid of any insertion shuttle. Spike amplitudes of 84 µV peak-to-peak and signal-to-noise ratio of 6.24 were achieved in acute experiments. Our study demonstrates that softening neural probes may be used to investigate deep layers of the rat brain.

5 GHz laterally‐excited bulk‐wave resonators (XBARs) based on thin platelets of lithium niobate

In a free-standing 400-nm-thick platelet of crystalline ZY-LiNbO 3 , narrow electrodes (500 nm) placed periodically with a pitch of a few microns can eXcite standing shear-wave bulk acoustic resonances (XBARs), by utilising lateral electric fields oriented parallel to the crystalline Y -axis and parallel to the plane of the platelet. The resonance frequency of ~4800 MHz is determined mainly by the platelet thickness and only weakly depends on the electrode width and the pitch. Simulations show quality-factors ( Q ) at resonance and anti-resonance higher than 1000. Measurements of the first fabricated devices show a resonance Q -factor ~300, strong piezoelectric coupling ~25%, (indicated by the large Resonance-antiResonance frequency spacing, ~11%) and an impedance at resonance of a few ohms. The static capacitance of the devices, corresponds to the imaginary part of the impedance ~100 Ω. This device opens the possibility for the development of low-loss, wide band, RF filters in the 3-6 GHz range for 4th and 5th generation (4G/5G) mobile phones. XBARs can be produced using standard optical photolithography and MEMS processes. The 3rd, 5th, 7th, and 9th harmonics were observed, up to 38 GHz, and are also promising for high frequency filter design.

An Artificial Bio-Synapse Based on Ag/a-Si:Ag/a-Si/X Memristors With Different Bottom Electrode X

In this paper, memristors with three different bottom electrodes as p-Si, Ag and ITO have been fabricated successfully. The memristor is designed as Ag/a-Si:Ag/a-Si/X, in which X refers to p-Si, Ag or ITO. The dielectric layers of a-Si:Ag/a-Si are fabricated by co-sputtering and the final device is completed by standard MEMS processes. The I-V curves, voltage sweeps and response currents, short-term memory (STM) to long-term memory (LTM), and the stability of memristors are studied extensively to mimic the synaptic behavior. It is indicated that the bottom electrode of the Ag/a-Si:Ag/a-Si/X memristors has an obviously influence on the performance of the decivce, and it is suggested that an optimized structural design is needed when a memristive layer is already chosen.

Laterally vibrating MEMS resonant vacuum sensor based on cavity-SOI process for evaluation of wide range of sealed cavity pressure

This paper reports a laterally vibrating MEMS resonant vacuum sensor which senses ambient pressure based on the squeeze-film damping effect. The single-anchored double-ended tuning fork structure is proposed to minimize anchor loss and thermoelastic dissipation. The squeeze-film damping gap width is designed to be changeable for the purpose of adjusting the squeeze-film damping effect at different gas pressure. By making the squeeze-film damping dominant and suppressing other energy loss mechanisms, the low pressure end of detectable range is enlarged and as the result a wider detectable pressure range can be achieved. The resonator was fabricated by cavity silicon-on-insulator technique for the purpose of design and fabrication flexibility, and was characterized in a vacuum chamber. The proposed sensor can sense the air pressure at relatively high quality factor from around 60 to 30,000 in the range of 1000–1 Pa. The structure design and fabrication is compatible with standard MEMS processes and provides a path towards the application for the evaluation of the vacuum level of sealed micro-size cavities for wafer level integration.

Low-cost surface micromachined microhotplates for chemiresistive gas sensors

Microhotplate (MHP) based gas sensors have gained significant attention recently due to their small size, low power and feasibility for integration of electronics on the same chip. This study presents the detailed work on design, fabrication and complete characterization of microhotplates based on a standard multi user MEMS process (MUMPs). Suspended-membrane type MHPs were designed using the available layer combinations of MUMPs. FEM simulations were carried out to optimize the heater design by spatially varying the heater current density to achieve uniform temperature distribution over the sensing area. Topography measurements confirmed that the X–Y–Z dimensions of the fabricated MHPs were in accordance with the design. From electro-thermal characterization, the thermal efficiency of the MHPs was evaluated as ~ 10 °C/mW. The suspended membrane showed a homogeneous temperature of ~ 450 °C at 35–40 mW heater power, which was well above the typical operating temperature of chemiresistive gas sensors. The results presented in this paper provide a pathway for realizing cost effective MHPs for gas sensors based on MUMPs.

Design, simulation and fabrication of piezoresistive microcantilevers using standard multi user MEMS process

Microcantilevers are extensively used in Micro Electro Mechanical Systems for a variety of sensing applications. This study presents the modeling, design and simulation of piezoresistive microcantilevers with an embedded heater based on Multi User MEMS Process. Simulations were carried out for the heater, microcantilever and the piezoresistor using a commercially available Finite Element Solver. Electro-thermal analysis of the embedded heater showed >75% temperature uniformity on the microcantilever surface. It was observed that the deflection of the microcantilever tend to become nonlinear with load at elevated temperatures. Piezoresistive simulation showed an increase in sensitivity (ΔI/I), with displacement magnitude of the microcantilever. Fabricated microcantilevers showed that the piezoresistor stayed nearly at ambient temperature up to 5 V heater bias.

Micro thermal diode with glass thermal insulation structure embedded in a vapor chamber

This paper reports a micro thermal diode based on one-way working fluid circulation driven by surface tension force. In forward mode, working fluid evaporates and condenses at a heated and cooled area, respectively, and the condensed liquid returns to the evaporation area due to the wettability difference. By this vapor-liquid phase change mechanism, the overall heat transfer coefficient becomes high. On the other hand, in reverse mode, no continuous evaporation-condensation cycle exists. The conductive heat loss in reverse mode was minimized by an embedded glass thermal isolation structure, which makes overall heat transfer coefficient low. The test device was made by a standard MEMS process combined with glass reflow and gold bump sealing. The overall heat transfer coefficients of 13 300 for forward mode and 4790 for reverse mode were measured. The performance index of the micro thermal diode was about 2.8.

A post-fabrication selective magnetic annealing technique in standard MEMS processes

A selective electrothermal magnetic annealing technique is introduced that provides programming capabilities for mechanical micro-resonators. In the proposed approach, the magnetic properties of resonators can be locally tuned in a post-fabrication batch-compatible process step. A prototype is implemented in a standard microfabrication process, where resonating ferromagnetic elements are suspended on top of a polysilicon resistive heater. The ferromagnetic elements consist of electroplated Nickel (Ni) with minor Iron (Fe) impurities. The electro-thermo-mechanical heating phenomenon is simulated for design purposes. The magnetization of micro-resonators with and without magnetic annealing is measured. The resulting magnetic property enhancement is illustrated by hysteresis (M-H) loop variations.

A High-Temperature Piezoresistive Pressure Sensor with an Integrated Signal-Conditioning Circuit.

This paper focuses on the design and fabrication of a high-temperature piezoresistive pressure sensor with an integrated signal-conditioning circuit, which consists of an encapsulated pressure-sensitive chip, a temperature compensation circuit and a signal-conditioning circuit. A silicon on insulation (SOI) material and a standard MEMS process are used in the pressure-sensitive chip fabrication, and high-temperature electronic components are adopted in the temperature-compensation and signal-conditioning circuits. The entire pressure sensor achieves a hermetic seal and can be operated long-term in the range of −50 °C to 220 °C. Unlike traditional pressure sensor output voltage ranges (in the dozens to hundreds of millivolts), the output voltage of this sensor is from 0 V to 5 V, which can significantly improve the signal-to-noise ratio and measurement accuracy in practical applications of long-term transmission based on experimental verification. Furthermore, because this flexible sensor’s output voltage is adjustable, general follow-up pressure transmitter devices for voltage converters need not be used, which greatly reduces the cost of the test system. Thus, the proposed high-temperature piezoresistive pressure sensor with an integrated signal-conditioning circuit is expected to be highly applicable to pressure measurements in harsh environments.

Design Considerations of High-Current Density Capacitors Micromachined for Si Interposers

Many demanding power system requirements usher motivations for high-density, miniaturized capacitors capable of quickly sourcing large quantities of current. Various efforts achieved high capacitive density (∼500 nF/mm2) at D.C., but many applications promote an interest in transient and high-frequency (HF) characteristics. Thin capacitors (100–640 μm) were micromachined using several industry standard Si MEMS processes resulting in large surface area, high-density capacitive storage devices (7.34 nF/mm2). An evaluation of high-speed (1 GHz), energy storage devices and their respective fabrication technologies has been closely compared by considering each capacitor design's transient ring-down characteristics. Although their capacitive density remains ∼10 nF/mm2, large amounts of current (∼100 A/ns) were sourced while retaining desirable characteristics at high frequencies. These devices have been optimized as embedded passives and demonstrate compatibility with Si interposer technology.

A Random Stimulation Source for Evaluating MEMS Devices using an Exact Solvable Chaotic Oscillator

MEMS devices are nearly ubiquitous, with applications ranging from automobiles to toys, medical equipment to missiles, and cell phones to industrial equipment. At the microscale, fabrication tolerances are significantly less precise than at the scale of traditional machining techniques. This can result in significant differences in the operating characteristics between otherwise identical MEMS devices. A wide bandwidth random excitation source is ideal for evaluating these components, whether used as the forcing function for an electromechanical shaker employed to measure transmissibility, or as a voltage source to evaluate actuator structure resonances and instabilities. An electronic chaotic oscillator provides an ideal wide bandwidth voltage source which is provably random from first principles and may be simply integrated for the aforementioned MEMS testing. This type of system is easily integrated through standard Si MEMS processes and readily lends itself to application as a built-in-self test (BIST) component. These systems guarantee uniform frequency content from D.C. up to 100kHz due to their characteristically random behavior and serve as a strong candidate for providing uniform spectral density to a device under test. The proposed system is a simple, electronic circuit that creates a random, wideband excitation voltage for observing characteristics of MEMS devices. This functionality is achieved by the analog, digital or mixed signal computation of nonlinear differential equations that describe various exactly solvable chaotic systems. By creating Si microsystems which perform these computations, these test sources may be readily fabricated as integrated BIST components for MEMS devices or fabricated separately and integrated by flip chip assembly techniques. Furthermore, by considering the iterated map of this particular category of stimulation source, a direct and easy measurement of the stimulation entropy may be monitored and corrected. This work begins as a theoretical treatment involving the Nonlinear Dynamics of these types of systems including chaotic systems which permit closed form solutions. These systems are described classically through nonlinear differential equations and intuitively through iterated maps. These techniques reveal inherent methods for entropy measurement in these sources which may be implemented and controlled easily using electronic circuits. Subsequently, the simulation, circuit design methodology, circuit simulation, fabrication, testing and hardware verification of these wideband chaotic sources is presented. The development of this work delineates simple, wideband electronic testing circuits which may be fully integrated with MEMS devices using standard Si MEMS processes. The resulting microsystem may be used as the forcing function when measuring transmissibility of MEMS devices, or as a BIST element to evaluate MEMS microstructure characteristics through direct microelectronic fabrication or flip chip integration.

Active gap reduction in comb drive of three axes capacitive micro accelerometer for enhancing sense capacitance and sensitivity

The performance of micro-machined sensors is primarily determined via the sensitivity of sensing electrode to displacement. This paper presents the design, modelling, optimization and fabrication of an active gap reduction mechanism used on a conventional comb drive to enhance the capacitance in a three axes capacitive micro accelerometer. The design parameters of the active gap reduced comb drive (AGRCD) are optimized for best performance. The finite element analysis of the AGRCD is performed for design verification. The modeling and simulation results demonstrated a 534 % increase in sensitivity of the three axes capacitive micro accelerometer. The three axes capacitive micro accelerometer with AGRCD is fabricated using a commercially available standard metal-multi user MEMS processes.