Paper Session II-C - High-Resolution Integrated Micro Gyroscope for Space Applications

In this paper, an integrated capacitive gyroscope fabricated by CMOS-MEMS technology is presented. The CMOS-compatibility of the fabrication process enables full integration of the sensor with interface and signal conditioning circuitry on a single chip. The entire microstructure is single-crystal silicon based, resulting in large proof mass and good mechanical behaviors. Thus, high-resolution and high-robustness microgyroscopes can be obtained. With a resolution of about 0.01°/s/Hz112 , the fabricated gyroscope chip is only as small as 1.5mm by 2mm including the sensing elements and integrated electronics. The robustness, light weight and high performance make this type of MEMS gyroscope very suitable for space navigation applications where payload is critical. The on-chip capacitive sensing circuitry employs chopper stabilization technique to minimize the influence of 1/f noise. The on-chip circuits also include a two-stage fully differential amplifier and a DC feedback loop to cancel the DC offset. The CMOS fabrication was performed through MOSIS by using the 4-metal TSMC 0.35 μm CMOS process. The post-CMOS micromachining processing consists of only dry etch steps and uses the interconnect metal layers as etching masks. Single-crystal silicon (SCS) structures are produced by applying a backside etch and forming a 60μm-thick SCS membrane. This work is sponsored by NASA through the UCF/UF Space Research Initiative

DESIGN AND MICROFABRICATION OF A CMOS-MEMS PIEZORESISTIVE ACCELEROMETER AND A NANO-NEWTON FORCE SENSOR

DESIGN AND MICROFABRICATION OF A CMOS-MEMS PIEZORESISTIVE ACCELEROMETER AND A NANO-NEWTON FORCE SENSOR by Mohd Haris Md Khir Adviser: Hongwei Qu, Ph.D. This thesis work consists of three aspects of research efforts: I. Design, fabrication, and characterization of a CMOS-MEMS piezoresistive accelerometer 2. Design, fabrication, and characterization of a CMOS-MEMS nano-Newton force sensor 3. Observer-based controller design of a nano-Newton force sensor actuator system A low-cost, high-sensitivity CMOS-MEMS piezoresistive accelerometer with large proof mass has been fabricated. Inherent CMOS polysilicon thin film was utilized as piezoresistive material and full Wheatstone bridge was constructed through easy wiring allowed by three metal layers in CMOS thin films. The device fabrication process consists of a standard CMOS process for sensor configuration and a deep reactive ion etching (DRIE) based post-CMOS microfabrication for MEMS structure release. Bulk single-crystal silicon (SCS) substrate was included in the proof mass to increase sensor sensitivity. Using a low operating power of 1.67 m W, the sensitivity was measured as 30.7 mV/g after amplification and 0.077 mV/g prior to amplification. With a total noise floor of 1.03 mg!-!Hz, the minimum detectable acceleration is found to be 32.0 mg for a bandwidth of I kHz which is sufficient for many applications. The second device investigated in this thesis work is a CMOS-MEMS capacitive force sensor capable ofnano-Newton out-of-plane force measurement. Sidewall and fringe capacitance formed by the multiple CMOS metal layers were utilized and fully differential sensing was enabled by common-centroid wiring of the sensing capacitors. Single-crystal silicon (SCS) is incorporated in the entire sensing element for robust structures and reliable sensor deployment in force measurement. A sensitivity of 8 m V /g prior to amplification was observed. With a total noise floor of 0.63 mg!-IHz, the minimum detection acceleration is found to be 19.8 mg, which is equivalent to a sensing force of 449 nN. This work also addresses the design and simulation of an observer-based nonlinear controller employed in a CMOS-MEMS nano-Newton force sensor actuator system. Measurement errors occur when there are in-plane movements of the probe tip; these errors can be controlled by the actuators incorporated within the sensor. Observerbased controller is necessitated in real-world control applications where not all the state variables are accessible for on-line measurements. V

A Comparative Study Between a Micromechanical Cantilever Resonator and MEMS-based Passives for Band-pass Filtering Application

Over the past few years, significant growth has been observed in using MEMS based passive components in the RF microelectronics domain, especially in transceiver components. This is due to some excellent properties of the MEMS devices like low loss, excellent isolation etc. in the microwave frequency domain where the on-chip passives normally tend to become leakier and degrades the transceiver performance. This paper presents a comparative analysis between MEMS-resonator based and MEMS-passives based band-pass filter configurations for RF applications, along with their design, simulation, fabrication and characterization. The filters were designed to have a center frequency of 455 kHz, meant for use as the intermediate frequency (IF) filter in superheterodyne receivers. The filter structures have been fabricated in PolyMUMPs process, a three-polysilicon layer surface micromachining process.Comment: 6 pages, 15 figure

High performance 3-folded symmetric decoupled MEMS gyroscopes

This thesis reports, for the first time, on a novel design and architecture for realizing inertial grade gyroscope based on Micro-Electro-Mechanical Systems (MEMS) technology. The proposed device is suitable for high-precision Inertial Navigation Systems (INS). The new design has been investigated analytically and numerically by means of Finite Element Modeling (FEM) of the shapes, resonance frequencies and decoupling of the natural drive and sense modes of the various implementations. Also, famous phenomena known as spring softening and spring hardening are studied. Their effect on the gyroscope operation is modeled numerically in Matlab/Simulink platform. This latter model is used to predict the drive/sense mode matching capability of the proposed designs. Based on the comparison with the best recently reported performance towards inertial grade operation, it is expected that the novel architecture further lowers the dominant Brownian (thermo-mechanical) noise level by more than an order of magnitude (down to 0.08º/hr). Moreover, the gyroscope\u27s figure of merit, such as output sensitivity (150 mV/º/s), is expected to be improved by more than two orders of magnitude. This necessarily results in a signal to noise ratio (SNR) which is up to three orders of magnitude higher (up to 1,900mV/ º/hr). Furthermore, the novel concept introduced in this work for building MEMS gyroscopes allows reducing the sense parasitic capacitance by up to an order of magnitude. This in turn reduces the drive mode coupling or quadrature errors in the sensor\u27s output signal. The new approach employs Silicon-on-Insulator (SOI) substrates that allows the realization of large mass (\u3e1.6mg), large sense capacitance (\u3e2.2pF), high quality factors (\u3e21,000), large drive amplitude (~2-4 µm) and low resonance frequency (~3-4 KHz) as well as the consequently suppressed noise floor and reduced support losses for high-performance vacuum operation. Several challenges were encountered during fabrication that required developing high aspect ratio (up to 1:20) etching process for deep trenches (up to 500 µm). Frequency Response measurement platform was built for devices characterization. The measurements were performed at atmospheric pressures causing huge drop of the devices performance. Therefore, various MEMS gyroscope packaging technologies are studied. Wafer Level Packaging (WLP) is selected to encapsulate the fabricated devices under vacuum by utilizing wafer bonding. Through Silicon Via (TSV) technology was developed (as connections) to transfer the electrical signals (of the fabricated devices) outside the cap wafers

Design, fabrication and characterisation of silicon carbide resonators

Micro-electro-mechanical systems (MEMS) are integrated mechanical and electrical elements realised with micro-fabrication technology and employed as sensors and actuators. The integration of reliable MEMS switches and resonators into transceiver devices is a challenging and attractive solution to increase the efficiency and reduce the power consumption. Silicon carbide (SiC) is an excellent candidate for developing robust and reliable high frequency MEMS for transceivers applications due to its unique mechanical properties.This thesis presents the design, fabrication and characterisation of 3C-SiC micromechanical vertical resonators. New device architectures have been developed for the study of the electro-mechanical behaviour of the devices with the aim of optimising the actuation efficiency, increasing the resonant frequency and obtaining new device functions.A process for the fabrication of single or poly-crystalline 3C-SiC cantilevers, bridges and rings has been developed with the option of integrating top electrodes made of aluminium (Al) or lead zirconium titanate (PZT). The crystal structure and quality of the SiC layers have been evaluated with X-ray diffraction and Raman spectroscopy. A Young's Modulus of ~ 440 GPa has been calculated for the single crystalline SiC from the mechanical resonant frequency of the fabricated single material cantilevers. The fabricated Al/SiC bridges and rings have been actuated and driven into resonance electro-thermally. It has been found that wide Al electrodes applied close to the beams' anchor can maximise the induced displacement and vibration amplitude thus improving the actuation efficiency. Resonant frequencies in the MHz range have been obtained with the ring architectures therefore achieving higher frequencies compared to beam architectures. In addition, electro-thermal mixing of two input frequencies has been demonstrated and performed with the fabricated Al/SiC structures. Furthermore, piezo-electric transduction has been used for actuating the PZT/SiC cantilevers and for sensing the devices' resonance electrically. The design of the PZT piezo-electric active layer has been shown to influence strongly the devices' resonant frequency and has been optimised to enhance the electrical output by decreasing the electrodes length thus decreasing the feedthrough capacitance.The results obtained in this work can be used for the implementation of SiC MEMS mixer-filters with electro-thermal actuation and piezo-electric sensing for transceiver applications

MEMS Technologies for Energy Harvesting

The objective of this chapter is to introduce the technology of Microelectromechanical Systems, MEMS, and their application to emerging energy harvesting devices. The chapter begins with a general introduction to the most common MEMS fabrication processes. This is followed with a survey of design mechanisms implemented in MEMS energy harvesters to provide nonlinear mechanical actuations. Mechanisms to produce bistable potential will be studied, such as introducing fixed magnets, buckling of beams or using slightly slanted clamped-clamped beams. Other nonlinear mechanisms are studied such as impact energy transfer, or the design of nonlinear springs. Finally, due to their importance in the field of MEMS and their application to energy harvesters, an introduction to actuation using piezoelectric materials is given. Examples of energy harvesters found in the literature using this actuation principle are also presented

A Comprehensive Review on Convex and Concave Corners in Silicon Bulk Micromachining based on Anisotropic Wet Chemical Etching

Wet anisotropic etching based silicon micromachining is an important technique to fabricate freestanding (e.g. cantilever) and fixed (e.g. cavity) structures on different orientation silicon wafers for various applications in microelectromechanical systems (MEMS). {111} planes are the slowest etch rate plane in all kinds of anisotropic etchants and therefore, a prolonged etching always leads to the appearance of {111} facets at the sidewalls of the fabricated structures. In wet anisotropic etching, undercutting occurs at the extruded corners and the curved edges of the mask patterns on the wafer surface. The rate of undercutting depends upon the type of etchant and the shape of mask edges and corners. Furthermore, the undercutting takes place at the straight edges if they do not contain {111} planes. {100} and {110} silicon wafers are most widely used in MEMS as well as microelectronics fabrication. This paper reviews the fabrication techniques of convex corner on {100} and {110} silicon wafers using anisotropic wet chemical etching. Fabrication methods are classified mainly into two major categories: corner compensation method and two-steps etching technique . In corner compensation method, extra mask pattern is added at the corner. Due to extra geometry, etching is delayed at the convex corner and hence the technique relies on time delayed etching. The shape and size of the compensating design strongly depends on the type of etchant, etching depth and the orientation of wafer surface. In this paper, various kinds of compensating designs published so far are discussed. Two-step etching method is employed for the fabrication of perfect convex corners. Since the perfectly sharp convex corner is formed by the intersection of {111} planes, each step of etching defines one of the facets of convex corners. In this method, two different ways are employed to perform the etching process and therefore can be subdivided into two parts. In one case, lithography step is performed after the first step of etching, while in the second case, all lithography steps are carried out before the etching process, but local oxidation of silicon (LOCOS) process is done after the first step of etching. The pros and cons of all techniques are discussed