A Low-Cost CMOS-MEMS Piezoresistive Accelerometer with Large Proof Mass

This paper reports a low-cost, high-sensitivity CMOS-MEMS piezoresistive accelerometer with large proof mass. In the device fabricated using ON Semiconductor 0.5 μm CMOS technology, an inherent CMOS polysilicon thin film is utilized as the piezoresistive sensing material. A full Wheatstone bridge was constructed through easy wiring allowed by the three metal layers in the 0.5 μm CMOS technology. The device fabrication process consisted of a standard CMOS process for sensor configuration, and a deep reactive ion etching (DRIE) based post-CMOS microfabrication for MEMS structure release. A bulk single-crystal silicon (SCS) substrate is included in the proof mass to increase sensor sensitivity. In device design and analysis, the self heating of the polysilicon piezoresistors and its effect to the sensor performance is also discussed. With a low operating power of 1.5 mW, the accelerometer demonstrates a sensitivity of 0.077 mV/g prior to any amplification. Dynamic tests have been conducted with a high-end commercial calibrating accelerometer as reference

Development of a three-axis MEMS accelerometer

While originally developed to deploy air bags for the automotive industry, Microelectromechanical Systems (MEMS) based accelerometers have found their way into everything from video game controllers to cells phones. As prices drop and capabilities improve, it is expected that the use of accelerometers will further expand in the coming years. Accelerometers currently have the second highest MEMS sales volume, trailing only pressure sensors [1]. In this work several single and three-axis accelerometers are designed, fabricated, and tested under a variety of conditions. The designed accelerometers are all based off of the piezoresistive effect, where the value of a resistor changes with applied mechanical stress [2]. When accelerated, the inertia of a suspended proof mass causes stress on piezoresistors placed on support arms. The corresponding changes in these resistor values are then converted to an output voltage using a Wheatstone bridge. To sense acceleration independently in all three axes, structures with three distinct modes of vibration and three sets of Wheatstone bridges are used. Devices were fabricated at the Semiconductor and Microsystems Fabrication Laboratory (SMFL), located at RIT. A modified version of the RIT bulk MEMS process was used, consisting of 65 steps, 7 photolithography masks, bulk silicon diaphragm etch, and top hole release etch [3]. Unfortunately the finished chips show poor aluminum step coverage into contact vias and over polysilicon lines. This results in open circuits throughout the chip, prohibiting proper operation. Process corrections have been identified, and with proper fabrication the designs are still expected to yield working devices. Since the finished accelerometers were not functional, several commercial accelerometers have been tested to characterize sensitivity, linearity, cross-axis sensitivity, frequency response, and device lifetime

Capacitive Microaccelerometers And Fabrication Methods

Disclosed are moveable microstructures comprising in-plane capacitive microaccelerometers, with submicro-gravity resolution (17 pF/g). Themicrostructures are fabricated in thick(> 100 µm) siliconon-insulator (SOI) substrates or silicon substrates using a two-mask fully-dry release process that provides large seismic mass (> 10 milli-g), reduced capacitive gaps, and reduced in-plane stiffness. Fabricated devices may be interfaced to a high resolution switched-capacitor CMOS IC that eliminates the need for area-consuming reference capacitors. The measured sensitivity is 83 mV/mg (17 pF/g) and the output noise floor is -91 dBm/Hz at 10 Hz (corresponding to an acceleration resolution of 170 ng/√Hz). The IC consumes 6 mW power and measures 0.65 mm2 core area.Georgia Tech Research Corporatio

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

System design of a low-power three-axis underdamped MEMS accelerometer with simultaneous electrostatic damping control

Recently, consumer electronics industry has known a spectacular growth that would have not been possible without pushing the integration barrier further and further. Micro Electro Mechanical Systems (MEMS) inertial sensors (e.g. accelerometers, gyroscopes) provide high performance, low power, low die cost solutions and are, nowadays, embedded in most consumer applications. In addition, the sensors fusion has become a new trend and combo sensors are gaining growing popularity since the co-integration of a three-axis MEMS accelerometer and a three-axis MEMS gyroscope provides complete navigation information. The resulting device is an Inertial measurement unit (IMU) able to sense multiple Degrees of Freedom (DoF). Nevertheless, the performances of the accelerometers and the gyroscopes are conditioned by the MEMS cavity pressure: the accelerometer is usually a damped system functioning under an atmospheric pressure while the gyroscope is a highly resonant system. Thus, to conceive a combo sensor, aunique low cavity pressure is required. The integration of both transducers within the same low pressure cavity necessitates a method to control and reduce the ringing phenomena by increasing the damping factor of the MEMS accelerometer. Consequently, the aim of the thesis is the design of an analog front-end interface able to sense and control an underdamped three-axis MEMSaccelerometer. This work proposes a novel closed-loop accelerometer interface achieving low power consumption The design challenge consists in finding a trade-off between the sampling frequency, the settling time and the circuit complexity since the sensor excitation plates are multiplexed between the measurement and the damping phases. In this context, a patenteddamping sequence (simultaneous damping) has been conceived to improve the damping efficiency over the state of the art approach performances (successive damping). To investigate the feasibility of the novel electrostatic damping control architecture, several mathematical models have been developed and the settling time method is used to assess the damping efficiency. Moreover, a new method that uses the multirate signal processing theory and allows the system stability study has been developed. This very method is used to conclude on the loop stability for a certain sampling frequency and loop gain value. Next, a 0.18μm CMOS implementation of the entire accelerometer signal chain is designed and validated

MICROMECHANICAL SENSORS USING MERGED EPITAXIAL LATERAL OVERGROWTH OF SILICON

A novel technology for manufacturing thin silicon diaphragm structures is presented. Controllability of thin silicon diaphragm is one of the most important issues in fabricating silicon micromechanical sensors whose sensitivity depends on the diaphragm thickness. This can be accomplished by epitaxial lateral overgrowth (ELO) of single crystal silicon on a patterned layer of masking material, typically Si02, combined with crystallographic etching of which etching rate depends on the crystal plane. With recent improvement of EL0 material, good quality of lOμm thick, 200μm x lOOOμm single crystal silicon was obtained with its thickness being precisely controlled by growth rate (≤ lμ m/min.). The junction leakage of the p-n junction diodes fabricated on merged EL0 silicon indicated the material quality is comparable to the substrate silicon. Using this technology, a bridge-type piezoresistive accelerometer with four beams and one proof mass was fabricated successfully. Its sensitivity and resonant frequency were comparable to the accelerometers made by other methods. They were analyzed by comparing the experimental results to a simple analytical solution as well as ANSYS stress simulator using a finite element: methods. The experimental results showed a potential application of the new technology to silicon sensor fabrication but some further refinement is remaining. Free-standing single crystal cantilever beams were fabricated using, MELO and RIE, of over lOOOμm long and 5μm by 10μm in cross section. These beams were very short, straight, indicating little residual stress. Wide, short beams were fabricated using EL0 which were also free standing. Special treatment of MELO indicated that diodes and bipolar transistors fabricated on top of the oxide stripes showed nearly ideal characteristics, hence the quality of the MELO was improved. With MELO of thicker than 5μm, no voids were observed. Test structures significantly with all surface micromachining, were designed for further development of silicon membranes

Solid State Circuits Technologies

The evolution of solid-state circuit technology has a long history within a relatively short period of time. This technology has lead to the modern information society that connects us and tools, a large market, and many types of products and applications. The solid-state circuit technology continuously evolves via breakthroughs and improvements every year. This book is devoted to review and present novel approaches for some of the main issues involved in this exciting and vigorous technology. The book is composed of 22 chapters, written by authors coming from 30 different institutions located in 12 different countries throughout the Americas, Asia and Europe. Thus, reflecting the wide international contribution to the book. The broad range of subjects presented in the book offers a general overview of the main issues in modern solid-state circuit technology. Furthermore, the book offers an in depth analysis on specific subjects for specialists. We believe the book is of great scientific and educational value for many readers. I am profoundly indebted to the support provided by all of those involved in the work. First and foremost I would like to acknowledge and thank the authors who worked hard and generously agreed to share their results and knowledge. Second I would like to express my gratitude to the Intech team that invited me to edit the book and give me their full support and a fruitful experience while working together to combine this book

Remotely interrogated MEMS pressure sensor

This thesis considers the design and implementation of passive wireless microwave readable pressure sensors on a single chip. Two novel-all passive devices are considered for wireless pressure operation. The first device consists of a tuned circuit operating at 10 GHz fabricated on SiO2 membrane, supported on a silicon wafer. A pressure difference across the membrane causes it to deflect so that a passive resonant circuit detunes. The circuit is remotely interrogated to read off the sensor data. The chip area is 20 mm2 and the membrane area is 2mm2 with thickness of 4 µm. Two on chip passive resonant circuits were investigated: a meandered dipole and a zigzag antenna. Both have a physical length of 4.25 mm. the sensors show a shift in their resonant frequency in response to changing pressure of 10.28-10.27 GHz for the meandered dipole, and 9.61-9.58 GHz for the zigzag antenna. The sensitivities of the meandered dipole and zigzag sensors are 12.5 kHz and 16 kHz mbar, respectively. The second device is a pressure sensor on CMOS chip. The sensing element is capacitor array covering an area of 2 mm2 on a membrane. This sensor is coupled with a dipole antenna operating at 8.77 GHz. The post processing of the CMOS chip is carried out only in three steps, and the sensor on its own shows a sensitivity of 0.47fF/mbar and wireless sensitivity of 27 kHz/mbar. The MIM capacitors on membrane can be used to detune the resonant frequency of an antenna

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