Miniature MEMS Research Articles

Neural network-assisted emission spectra analysis of gases using MEMS sensor: Predicting chemical composition and pressure

The paper explores the potential use of neural networks to predict the chemical composition as well as the total and partial pressure of individual gases in the mixture based on their emission spectra. Excitation/ionization of gas particles is ensured by a miniature MEMS (micro-electro-mechanical) sensor. The impact of measurement conditions on the generated spectra and possible challenges in analysis are addressed. The trained neural networks achieved a mean absolute error of approximately 1.4% in determining the chemical composition of a mixture of methane and hydrogen and 0.8% for a mixture of nitrogen and oxygen. For pressure prediction, spanning several orders of magnitude, a general model predicting pressure for ten different gases achieved a relative mean absolute error of 4.5%. Additionally, a model for gas classification achieved an accuracy of over 99%.

Dual-modality rigid endoscope for photoacoustic imaging and white light videoscopy.

There has been significant interest in the development of miniature photoacoustic imaging probes for a variety of clinical uses, including the in situ assessment of tumors and minimally invasive surgical guidance. Most of the previously implemented probes are either side viewing or operate in the optical-resolution microscopy mode in which the imaging depth is limited to . We describe a forward-viewing photoacoustic probe that operates in tomography mode and simultaneously provides white light video images. We aim to develop a dual-modality endoscope capable of performing high-resolution PAT imaging and traditional white light videoscopy simultaneously in the forward-viewing configuration. We used a Fabry-Pérot ultrasound sensor that operates in the 1500 to 1600nm wavelength range and is transparent in the visible and near infrared region (580 to 1250nm). The FP sensor was optically scanned using a miniature MEMs mirror located at the proximal end of the endoscope, resulting in a system that is sufficiently compact (10mm outer diameter) and lightweight for practical endoscopic use. The imaging performance of the endoscope is evaluated, and dual-mode imaging capability is demonstrated using phantoms and abdominal organs of an ex vivo mouse including spleen, liver, and kidney. The proposed endoscope design offers several advantages including the high acoustic sensitivity and wide detection bandwidth of the FP sensor, dual-mode imaging capability, compact footprint, and an all-optical distal end for improved safety. The dual-mode imaging capability also offers the advantage of correlating the tissue surface morphology with the underlying vascular anatomy. Potential applications include the guidance of laparoscopic surgery and other interventional procedures.

Detection system for a miniature MEMS X-ray source

Detection system for a miniature MEMS X-ray source

Point field emission electron source with a magnetically focused electron beam

This paper presents a field emitter in the form of a silicon tip covered with a layer of carbon nanotubes. The emitted beam is focused with a set of two electrostatic lenses and – which is novelty in such structures – with a magnetic field. The presented approach gave very promising results. The field emitter was able to provide a high emission current (about 50 µA) and a beam with a small and homogeneous spot. Such electron sources are necessary components of many miniature MEMS and nanoelectronics devices. The presented source is dedicated especially for the use in currently developed MEMS X-ray sources and MEMS electron microscopes.

Design of an Einzel Lens with Square Cross-Section

In this paper, the results of modeling and simulation of a microcolumn are presented. The microcolumn is part of a developed miniature MEMS electron microscope equipped with a miniature MEMS high-vacuum micropump. Such an arrangement makes this device the first stand-alone miniature electron-optical device to operate without an external high-vacuum chamber. Before such a device can be fabricated, research on particular elements must be carried out to determine the working principles of the device. The results of the calculations described in this article help us to understand the work of a microcolumn with square holes in the electrodes. The formation of an electron beam spot at the anode is discussed. Further calculations and results show the dependence of the Einzel lens size on the electron beam spot diameter, electron beam current, and microcolumn focusing voltage. The results are used to define the optimal design of the developed MEMS electron microscope.

Miniature MEMS: Novel Key Components Toward Terahertz Reconfigurability

We present an overview of the emerging and enabling miniature MEMS-based components for the terahertz frequencies. The usage of miniature MEMS integrated within the terahertz components is an uprising field of research, which offers reconfigurability, improved performance, and ability to perform unprecedented functions that cannot be achieved using the state-of-the-art semiconductor and optical technologies. In this paper, we discuss the recent progress and future of miniature MEMS-based reconfigurable components and present examples including reconfigurable sources, detectors, metamaterials, and guided-wave components. [2019-0099]

GLOW-DISCHARGE ION SOURCE FOR ON-CHIP INTEGRATED MINIATURE MEMS MASS SPECTROMETER

This work describes a construction, technology, working principle and properties of an ion source dedicated for a miniature MEMS mass spectrometer. It has been examined under what conditions it is possible to ionize gas sample, and to obtain a properly directed ion beam. The influence of such parameters as shapes, dimensions and distances between the electrodes, as well as the value of an applied magnetic and electric field and pressure level on the operation of the instrument have been investigated. The experiment allowed for choosing the optimal parameters, thus it seems that the ion source will fulfil all the requirements necessary for the MEMS mass spectrometer.

Self-calibration of dual, closed surface microphone arrays

With the recent availability of very inexpensive, miniature digital MEMS microphones, the construction of large count microphone arrays is financially within the reach of the basic researcher. Using various mathematical techniques such as the Helmholtz-Kirchhoff integral and the equivalent source method (ESM), dual surface microphone arrays provide the ability to separate incident and scattered fields instantaneously in time. When these arrays form a closed surface, this separation is extremely accurate. However, one of the concerns with any microphone array is the calibration of the individual elements, especially near the resonance frequency of the diaphragm. In this paper, a self-calibration technique is proposed, based on ESM, that uses external loudspeakers in a simple laboratory facility not required to be anechoic. The approach is tested numerically on a dual surface, 256 element pair spherical microphone array, and uses an algorithm for the minimization of a cost function based on the extinction of the scattered field when the unknown individual calibration coefficients of the array elements are varied. Use of a Burton-Miller approach to model the ESM sources of the scattered field improves dramatically the accuracy at specific frequencies. The problematic interior Dirichlet eigenfrequencies are addressed. [Work supported by the Office of Naval Research.]

Numerical Analysis of CNC Milling Chatter Using Embedded Miniature MEMS Microphone Array System

With the increasingly common use of industrial automation for mass production, there are many computer numerical control (CNC) machine tools that require the collection of data from intelligent sensors in order to analyze their processing quality. In general, for high speed rotating machines, an accelerometer can be attached on the spindle to collect the data from the detected vibration of the CNC. However, due to their cost, accelerometers have not been widely adopted for use with typical CNC machine tools. This study sought to develop an embedded miniature MEMS microphone array system (Radius 5.25 cm, 8 channels) to discover the vibration source of the CNC from spatial phase array processing. The proposed method utilizes voice activity detection (VAD) to distinguish between the presence and absence of abnormal noise in the pre-stage, and utilizes the traditional direction of arrival method (DOA) via multiple signal classification (MUSIC) to isolate the spatial orientation of the noise source in post-processing. In the numerical simulation, the non-interfering noise source location is calibrated in the anechoic chamber, and is tested with real milling processing in the milling machine. As this results in a high background noise level, the vibration sound source is more accurate in the presented energy gradation graphs as compared to the traditional MUSIC method.

A contactless ultrasonic surface wave approach to characterize distributed cracking damage in concrete

We describe an approach that utilizes ultrasonic surface wave backscatter measurements to characterize the volume content of relatively small distributed defects (microcrack networks) in concrete. A simplified weak scattering model is used to demonstrate that the scattered wave field projected in the direction of the surface wave propagation is relatively insensitive to scatterers that are smaller than the propagating wavelength, while the scattered field projected in the opposite direction is more sensitive to sub-wavelength scatterers. Distributed microcracks in the concrete serve as the small scatterers that interact with a propagating surface wave. Data from a finite element simulation were used to demonstrate the viability of the proposed approach, and also to optimize a testing configuration to collect data. Simulations were validated through experimental measurements of ultrasonic backscattered surface waves from test samples of concrete constructed with different concentrations of fiber filler (0.0, 0.3 and 0.6%) to mimic increasing microcrack volume density and then samples with actual cracking induced by controlled thermal cycles. A surface wave was induced in the concrete samples by a 50kHz ultrasonic source operating 10mm above the surface at an angle of incidence of 9°. Silicon-based miniature MEMS acoustic sensors located a few millimeters above the concrete surface both behind and in front of the sender were used to detect leaky ultrasonic surface waves emanating from concrete. A normalized backscattered energy parameter was calculated from the signals. Statistically significant differences in the normalized backscattered energy were observed between concrete samples with varying levels of simulated and actual cracking damage volume.

Miniature multi-contact MEMS switch for broadband terahertz modulation.

A miniature MEMS switch is designed, fabricated, and incorporated in a reconfigurable metallic mesh filter for broadband terahertz modulation. The mechanical, electrical, and geometrical properties of the MEMS switch are set to enable broadband terahertz modulation with relatively low modulation voltage, high modulation speed, and high device reliability. The implemented miniature MEMS switch exhibits an actuation voltage of 30 V, a fundamental mechanical resonance frequency of 272 kHz, and an actuation time of 1.23 μs, enabling terahertz modulation with a record high modulation depth of more than 70% over a terahertz band of 0.1-1.5 THz, with a modulation voltage of 30 V and modulation speeds exceeding 20 kHz.

Fabrication and Characterization of a Novel Nanoscale Thermal Anemometry Probe

The development, fabrication, and characterization of a novel nanoscale thermal anemometry probe (NSTAP) for measuring velocity fluctuations in turbulent flows are described. This miniature MEMS anemometer consists of a freestanding 30 or 60 × 1 × 0.1 μm platinum filament with electrically conductive pads and a silicon structure. A novel deep reactive ion etching lag-based process for fabricating a 3D silicon support structure is described together with other microfabrication steps. The sensors behave similarly to conventional hotwire anemometers, with an order of magnitude better spatial and temporal resolution. Batch fabrication allows a relatively low-cost high-yield process, which together with its superior frequency response and size, make it an attractive thermal anemometry sensor for velocity measurements in turbulent flows.

Built-in Self-Limitation of Masked Aluminum Anodization using Photoresist

This paper reports on a simple method for wafer-scale production of precisely-shaped 2D patterns of porous anodic alumina (PAA) by masked anodization of aluminum films. The produced dielectric PAA structures, featuring cylindrical nano-pores of very high aspect ratio (>100), are useful for several applications including the production of nanowires and miniature MEMS packages. The fabrication process utilizes a photoresist mask; but to overcome the instability of the photoresist during the anodization process, special borderlines are included in the mask design. These borderlines act as self-synchronized switches for the anodization current, preventing the undesired photoresist delamination and lateral extension of the PAA structures. Employing such borderlines resulted in a reduction of the lateral extension of masked Al anodization on 200mm wafers from more than 300μm to approximately 6μm.

Bowling ball dynamics revealed by miniature wireless MEMS inertial measurement unit

This paper presents a novel sensor technology to deduce the dynamics of a bowling ball. The sensor, a miniature wireless inertial measurement unit (IMU), incorporates MEMS accelerometers and angular rate gyros, a microcontroller, a low power RF transceiver, and a rechargeable battery. When embedded in a bowling ball, the IMU transmits the acceleration and angular velocity data that define the dynamics of the ball starting with the bowler’s delivery and its motion in the lane. Example results from professional bowlers illustrate how this technology can be used to assess bowler skill and ball performance. For instance, the IMU accurately measures the spin dynamics of the ball which are crucial to develop the ball “hook.” An analysis of ball dynamics in the lane is distilled to a measurable “hook potential” metric for further assessing bowler skill. Finally, the sensor presented herein is believed to be the world’s smallest, wireless IMU. This highly miniaturized and wireless design will enable parallel training systems for many sports, including basketball, baseball, crew, cricket, golf, fly fishing, soccer, softball, tennis, rowing, among others.

A miniature in-plane piezoresistive MEMS accelerometer for detection of slider off-track motion in hard disk drives

A miniature in-plane pizoresistive MEMS accelerometer was designed, fabricated and characterized for detection of slider off-track motion in hard disk drives. The structure of the accelerometer consists of a central supporting beam and two stress-magnifying sensing beams. Under geometric constraints imposed by the trailing side of a pico slider, the accelerometer design was optimized to achieve approximately pure axial deformation in the sensing beams and a maximum sensitivity with a specified natural frequency of 300 kHz. Fabricated on a silicon-on-insulator (SOI) wafer, the accelerometer with a half Wheatstone bridge was wirebonded to external pads and interfaced with an amplifier circuit on a printed circuit board (PCB). The noise level, sensitivity, nonlinearity were characterized with vibration testing on a shaker. The miniature accelerometer (1 × 0.3 × 0.3 mm3) with a weight of only 0.2 mg offers a much higher resonant frequency with a comparable sensitivity compared with those in previous work.

Performance of Several Viscothermal Acoustic Finite Elements

Viscothermal acoustics can be described by the linearized Navier Stokes equations. Besides inertia and compressibility, these equations take the heat conductivity and the viscosity of the medium (air) into account. These 'viscothermal' effects are significant in, for example, miniature acoustic transducers and MEMS devices. A finite element for viscothermal acoustics, which can be used to model such devices, is presented. The particular set of equations used in the model of viscothermal acoustics leads to a complex symmetric finite element system matrix. Several different FEM discretizations are studied on a 2D thin gap problem. These discretizations are known, in the context of the Stokes equation, as the Taylor Hood quadrilateral and triangle elements, the Crouzeix Raviart element and the MINI element. All elements are implemented in the FEM software COMSOL. The elements with quadratic velocity and temperature shape functions show the best orders of convergence.

Micro IMU Utilizing Folded MEMS Approach

In this paper, we propose a novel wafer-level approach for fabrication and 3-D integration of MEMS devices in miniature multi-axis assemblies of inertial, acoustic, and optical sensors. The approach is based on simultaneous fabrication of all sensors on the same substrate connected by flexible electrical interconnects, mechanical hinges and latches. A multi-axis sensor system is then obtained by folding the fabricated structures into 3-D cubes, pyramids, or other rigid shapes, and subsequently micro-welded. In the current work, we demonstrate feasibility of the folded cube approach for creation of miniature MEMS IMU with <1 cm3 volume. Design of the IMU consists of a folded cube or pyramid backbone structure with micromachined accelerometers and gyroscopes on its sidewalls. Silicon-on-insulator (SOI) wafers are used as a substrate for fabrication of both the inertial sensors and the folded backbone structure. Fabrication of the sensors consists of lithography, deep reactive ion etching (DRIE), and HF acid release of the inertial proof masses. Flexible polymer hinges connecting faces of the folded structures are defined on the same substrate and incorporate electrical interconnects. To provide rigidity to the assembled 3-D structure, interlocking silicon latches are fabricated along the edges of each sidewall, which are silicon-to-silicon welded after assembly. The approach allows for creating miniature multi-axis sensor systems without compromising performance of individual sensors. Gyroscopes integrated in the current folded cube IMU have experimentally demonstrated 3-dB bandwidth of 250 Hz and angle random walk (ARW) below 0.1 (deg/s)/rt-Hz in atmospheric pressure. Measured uncompensated temperature coefficients of gyroscope bias and scale factor were 313 (deg/h)/degC and 351ppm/degC, respectively. Accelerometers on the cube have been characterized, yielding a noise floor of 570μG/√Hz. Sensitivity of the accelerometers is measured at 40mV/G with a bandwidth of 100 Hz.

Miniature diffraction-based optical MEMS microphones with integrated optoelectronics

Diffraction-based optical interferometric sensing has been shown to be a low-noise displacement detection method for MEMS microphones. For this method to be useful in many important applications such as hearing aids, it needs to be packaged in a small volume and should have power consumption levels suitable for operation with a battery. In this talk, we describe miniature, packaged optical microphones that use solid state vertical cavity surface emitting lasers (VCSELs) as light sources and custom designed photodetectors. The package carries silicon chips with two micromachined biomimetic differential microphones as well as an omnidirectional microphone. It is made by 3-D laser stereo lithography process and has dimensions suitable for behind-the-ear hearing aids. With this configuration, the input referred noise floor for the differential microphone is measured as 42.6dBA, limited by the VCSEL intensity noise in this particular case. In addition to miniature packaging, an optoelectronic chip including VCSEL pulser, photodetectors, transimpedance amplifiers and 1 bit Sigma-Delta ADC has been implemented in 1.5V, 0.35u CMOS technology. The second order ADC structure providing 14-bits of theoretical resolution with 64 over sampling ratio and 20 kHz input signal is described and initial characterization results are presented.

Experiments and simulations of MEMS thermal sensors for wall shear-stress measurements in aerodynamic control applications

MEMS thermal shear-stress sensors exploit heat-transfer effects to measure the shear stress exerted by an air flow on its solid boundary, and have promising applications in aerodynamic control. Classical theory for conventional, macroscale thermal shear-stress sensors states that the rate of heat removed by the flow from the sensor is proportional to the 1/3-power of the shear stress. However, we have observed that this theory is inconsistent with experimental data from MEMS sensors. This paper seeks to develop an understanding of MEMS thermal shear-stress sensors through a study including both experimental and theoretical investigations. We first obtain experimental data that confirm the inadequacy of the classical theory by wind-tunnel testing of prototype MEMS shear-stress sensors with different dimensions and materials. A theoretical analysis is performed to identify that this inadequacy is due to the lack of a thin thermal boundary layer in the fluid flow at the sensor surface, and then a two-dimensional MEMS shear-stress sensor theory is presented. This theory incorporates important heat-transfer effects that are ignored by the classical theory, and consistently explains the experimental data obtained from prototype MEMS sensors. Moreover, the prototype MEMS sensors are studied with three-dimensional simulations, yielding results that quantitatively agree with experimental data. This work demonstrates that classical assumptions made for conventional thermal devices should be carefully examined for miniature MEMS devices.

Thin Film Material Parameters Derived from Full Field Nanometric Displacement Measurements in Non-uniform MEMS Geometries

ABSTRACTMEMS-scale polycrystalline silicon 2 μm thin film specimens fabricated via the Multi User MEMS Processes (MUMPs) have been employed to obtain the non-uniform nanometric displacement fields in the vicinity of prefabricated circular and elliptical micron-sized perforations. For the hole diameter-to-specimen width ratios considered in this work, and for all practical purposes, the displacement solution for a hole in an infinite plate is applicable. This method requires the ability to reliably and repeatably acquire nanometer level displacements on freestanding thin films. These tensile tests were conducted by a custom microtensile tester with the aid of Atomic Force Microscopy (AFM) and a special gripper that makes use of electrostatically assisted UV adhesion to handle and load miniature MEMS specimens. This non-conventional procedure for material parameter determination relies on Digital Image Correlation (DIC) to compare two AFM images, one before and one after specimen loading, and thus compute the nanometer level displacement fields (<50 nm global displacements) in 15×15-μm2 (or smaller) areas, with better than 2–3 nm resolution in displacements. By posing and solving a non-linear least squares inverse problem where, for known applied loads and measured displacement fields in an infinite plate with either a circular or an elliptical hole, it is possible to recover the elastic modulus (E). The main advantage of this approach is the full utilization of high-resolution displacement measurements over a specific specimen area, using only measurements acquired at one load level. Further statistical measurements of material properties may be obtained at varying load levels.