Identification of Test Structures for Reduced Order Modeling of the Squeeze Film Damping in Mems

In this study the dynamic behaviour of perforated microplates oscillating under the effect of squeeze film damping is analyzed. A numerical approach is adopted to predict the effects of damping and stiffness transferred from the surrounding ambient air to oscillating structures ; the effect of hole's cross section and plate's extension is observed. Results obtained by F.E.M. models are compared with experimental measurements performed by an optical interferometric microscope.Comment: Submitted on behalf of EDA Publishing Association (http://irevues.inist.fr/EDA-Publishing

Study On The Accuracy Of Squeeze Film Damping Calculation With Finite Element Analysis

Squeeze film damping due to the air trapped between oscillating membrane structure and a fixed substrate is a critical consideration in the design of MEMS devices because it can adversely affect the dynamic behaviour of the system. Therefore, the development of MEMS depends highly on the modelling and numerical simulation in order to optimize and verify their design before the batch production process. In this thesis, a method to model squeeze film damping with finite element approach to attain sufficiently high accuracy while considerably reducing the degrees of freedom is proposed, and its effectiveness is studied by comparing with other methods. The numerical analysis was performed using commercial ANSYS software. The structures were modelled using three-dimensional (3D) element and two-dimensional (2D) element. Results obtained by finite element models are compared with existing experimental measurements and analytical solutions. It was found that for the computation of damping coefficient, two-dimensional model yields slightly similar accuracy with three-dimensional model with respect to experimental data for low number of holes. In contrast, for highly perforated membranes, the proposed two-dimensional model is shown to be more accurate. The results clearly validate the proposed model to achieve good accuracy for damping coefficient solution while consuming considerably less computer time and memory

CMOS-MEMS resonant pressure sensors: optimization and validation through comparative analysis

The final publication is available at Springer via http://dx.doi.org/10.1007/s00542-016-2878-3An optimized CMOS-MEMS resonant pressure sensor with enhanced sensitivity at atmospheric pressure has been reported in this paper. The presented work reports modeling and characterization of a resonant pressure sensor, based on the variation of the quality factor with pressure. The relevant regimes of air flow have been determined by the Knudsen number, which is the ratio of the mean free path of the gas molecule to the characteristic length of the device. The sensitivity has been monitored for the resonator design from low vacuum to atmospheric levels of air pressure. This has been accomplished by reducing the characteristic length and optimization of other parameters for the device. While the existing analytical model has been adapted to simulate the squeeze film damping effectively and it is validated at higher values of air pressure, it fails to compute the structural damping mechanisms dominant in the molecular flow regime, i.e. at lower levels of air pressure. This discrepancy has been solved by finite element modeling that has incorporated both structural and film damping effects. The sensor has been designed with an optimal geometry of 140 × 140 × 8 µm having 6 × 6 perforations along the row and column of the plate, respectively, for maximum Q, with an effective mass of 0.4 µg. An enhanced quality factor of 60 and reduced damping coefficient of 4.34 µNs/m have been obtained for the reported device at atmospheric pressure. The sensitivity of the manufactured device is approximately -0.09 at atmospheric pressure and increases to -0.3 at 40 kPa i.e. in the lower pressures of slip flow regime. The experimental measurements of the manufactured resonant pressure sensor have been compared with that of the analytical and finite element modeling to validate the optimization procedure. The device has been manufactured using standard 250 nm CMOS technology followed by an in-house BEOL metal-layer release through wet etching.Peer ReviewedPostprint (author's final draft

Surface micromachined MEMS variable capacitor with two-cavity for energy harvesting

In this research, a novel MEMS variable capacitor with two capacitive cavities for energy harvesting was developed that use the wasted energy associated with undesirable mechanical vibrations to power microelectronic sensors and actuators widely found in structures and systems surrounding us. The harvested power, though very small, can have a profound effect on the usage of microsensors. First, the self-powered sensors will no longer require regular battery maintenance. Second, the self-powered chip is a liberating technology. On a circuit board, it can simplify the connection. On a commercial jet, the sensors can greatly simplify cabling. The design, fabrication, modeling and complete set of characterization of MEMS variable capacitors with two-cavity are presented in details in this thesis. The MEMS variable capacitors are unique in its two-cavity design and use of electroplated nickel as the main structural material. The device consists of 2x2 mm² movable capacitive proof mass plates with a thickness of 30 [mu]m suspended between two fixed electrodes forming two vertical capacitors. When the capacitance increases for one cavity, it decreases for the other. This allows using both up and down directions to generate energy. The suspended movable plates are supported by four serpentine springs with a thickness of 3-5 [mu]m that are attached to the address lines on a silicon substrate only at the anchors' points which is made of electroplated nickel. The serpentine suspension beams are made with a width, thickness and total length (four serpentine turns) of 15 [mu]m, 5 [mu]m and 1485 [mu]m. Five gold stoppers with height of 2-4 [mu]m were electroplated on the fixed plates to prevent snap-down of the movable plates by overwhelming electrostatic force. SiO2 and Si3N4 thin layers were patterned on the fixed plates to insulate the stoppers and enhance the dielectric property of capacitive cavities. The MEMS variable capacitor with two-cavity has been designed and modeled using MEMS CAD tool and COMSOL Multi-PhysIncludes bibliographical references (pages 108-118)

UV-LIGA micro-fabrication of inertia type electrostatic transducers and their application

This dissertation discusses the design, working principles, static & dynamic analysis and simulation, mechanics of material, applied MEMS technology, micro-fabrication, and experimental testing of two types of micro-transducers: micro-power relay and micro-accelerometer. Several possible design concepts were proposed, and the advantages and disadvantages of electrostatic working principles were also discussed. Transducers presented in this research used electrostatic force as a driving force in the micro-relay and capacitance as a sensing parameter in the micro-accelerometer. There was an analogy between the micro-relay and the micro-accelerometer in their theoretical approach and fabrication processes. The proposed micro-transducers (micro-relay and micro-accelerometer) were fabricated using UV lithograph of SU-8 & SPR and UV-LIGA process. The advantages and disadvantages of these processes were discussed. The micro-relays fabricated by UV-LIGA technology had the following advantages compared with other reported relays: fast switching speed, high power capacity, high off-resistance, lower on-resistance, low power consumption, and low heat generation. The polymer-based micro-accelerometers were designed and fabricated. Instead of applying SU-8 only as a photo resist, cured SU-8 was used as the primary structural material in fabricating the micro-accelerometers. The great flexibility in size and aspect ratio of cured SU-8 made it feasible to produce highly sensitive accelerometers. The prototype micro-relays and micro-accelerometers were tested for the dynamic characteristics and power capacity. The experimental results in micro-relays had confirmed that reasonably large current capacity and fast response speed was able to be achieved using electromagnetic actuation and the multilayer UV-LIGA fabrication process

A MEMS Light Modulator Based on Diffractive Nanohole Gratings

We present the design, fabrication, and testing of a microelectromechanical systems (MEMS) light modulator based on pixels patterned with periodic nanohole arrays. Flexure-suspended silicon pixels are patterned with a two dimensional array of 150 nm diameter nanoholes using nanoimprint lithography. A top glass plate assembled above the pixel array is used to provide a counter electrode for electrostatic actuation. The nanohole pattern is designed so that normally-incident light is coupled into an in-plane grating resonance, resulting in an optical stop-band at a desired wavelength. When the pixel is switched into contact with the top plate, the pixel becomes highly reflective. A 3:1 contrast ratio at the resonant wavelength is demonstrated for gratings patterned on bulk Si substrates. The switching time is 0.08 ms and the switching voltage is less than 15V

IMPROVED SENSITIVITY OF RESONANT MASS SENSOR BASED ON MICRO TILTING PLATE AND MICRO CANTILEVER

Vapor sensors have been used for many years. Their applications range from detection of toxic gases and dangerous chemicals in industrial environments, the monitoring of landmines and other explosives, to the monitoring of atmospheric conditions. Microelectrical mechanical systems (MEMS) fabrication technologies provide a way to fabricate sensitive devices. One type of MEMS vapor sensors is based on mass changing detection and the sensors have a functional chemical coating for absorbing the chemical vapor of interest. The principle of the resonant mass sensor is that the resonant frequency will experience a large change due to a small mass of gas vapor change. This thesis is trying to build analytical micro-cantilever and micro-tilting plate models, which can make optimization more efficient. Several objectives need to be accomplished: (1) Build an analytical model of MEMS resonant mass sensor based on micro-tilting plate with the effects of air damping. (2) Perform design optimization of micro-tilting plate with a hole in the center. (3) Build an analytical model of MEMS resonant mass sensor based on micro-cantilever with the effects of air damping. (4) Perform design optimization of micro-cantilever by COMSOL. Analytical models of micro-tilting plate with a hole in the center are compared with a COMSOL simulation model and show good agreement. The analytical models have been used to do design optimization that maximizes sensitivity. The micro-cantilever analytical model does not show good agreement with a COMSOL simulation model. To further investigate, the air damping pressures at several points on the micro-cantilever have been compared between analytical model and COMSOL model. The analytical model is inadequate for two reasons. First, the model’s boundary condition assumption is not realistic. Second, the deflection shape of the cantilever changes with the hole size, and the model does not account for this. Design optimization of micro-cantilever is done by COMSOL

Design of a hermetically sealed MEMS resonator with electrostatic actuation and capacitive third harmonic sensing

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Page 146 blank. Cataloged from PDF version of thesis.Includes bibliographical references (p. 138-140).A microscale beam resonator has been designed and fabricated for use as a modular pressure sensor for vacuum applications. The device dimensions have been optimized to provide measurable signals with low noise. Electrostatic actuation and sensing are both performed using only one pair of electrodes. The motion of the cantilever changes the capacitance of the actuation electrodes at a frequency three times that of the actuation signal. This method allows the desired motion to be picked out using a lock-in amplifier with minimal interference from other unwanted signals such as parasitic leakage and noise. Unlike previous work, packaging and electrical contacts have been integrated into the fabrication to create a hermetically sealed device that can easily be incorporated into other MEMS designs. Most resonators operate in vacuum because air damping at higher pressures greatly decreases both resonant frequency and quality factor. This loss is directly related to the pressure of the surrounding air, and therefore has been used in this design to measure the pressure. While the relationship is not linear, it is one-to-one. This means that once the device has been characterized, pressure can be determined uniquely over a range from atmospheric pressure down to ~10- Torr. The device was fabricated from two SOI wafers using standard wafer processing techniques. This means that unlike previous work, it can be readily integrated into other designs via wafer bonding. A single access port on the base provides a connection between the otherwise hermetically sealed sensor and other devices. To prevent squeeze film damping from limiting the motion of the beam, the cantilever tip has been perforated with an array of holes and a cavity was etched above where the cantilever will oscillate. Electrical contact can easily be made with the device as fabricated, so no additional packaging is necessary. While the fabricated devices are hermetically sealed, resonance was never detected due to a combination of factors including: poor wafer bonding, parasitic leakage, a Schottky barrier at one terminal and a design error that led to an unexpectedly high frequency and quality factor. Modifications to the current design are proposed that should eliminate these problems in the next iteration.by Eric B. Newton.S.M