Navid Yazdi

Micromachined inertial sensors

This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

Noise analysis and characterization of a sigma-delta capacitive microaccelerometer

This paper reports a high-sensitivity low-noise capacitive accelerometer system with one micro-g//spl radic/Hz resolution. The accelerometer and interface electronics together operate as a second-order electromechanical sigma-delta modulator. A detailed noise analysis of electromechanical sigma-delta capacitive accelerometers with a final goal of achieving sub-/spl mu/g resolution is also presented. The analysis and test results have shown that amplifier thermal and sensor charging reference voltage noises are dominant in open-loop mode of operation. For closed-loop mode of operation, mass-residual motion is the dominant noise source at low sampling frequencies. By increasing the sampling frequency, both open-loop and closed-loop overall noise can be reduced significantly. The interface circuit has more than 120 dB dynamic range and can resolve better than 10 aF. The complete module operates from a single 5-V supply and has a measured sensitivity of 960 mV/g with a noise floor of 1.08 /spl mu/g//spl radic/Hz in open-loop. This system can resolve better than 10 /spl mu/g//spl radic/Hz in closed-loop.

A generic multielement microsystem for portable wireless applications

An open-architecture microsystem that can be populated with a variety of sensors and actuators is described. The microsystem is designed for low-power wireless applications where small size and high sensor accuracy are important. It consists of an in-module microcontroller connected to multiple front-end transducers through an intramodule sensor bus. An external interface allows internally processed data to be output through either a hard-wired input/output port or a radio-frequency transmitter. The present microsystem is configured for environmental monitoring and measured temperature, barometric pressure, relative humidity, and acceleration/vibration. It occupies less than 10 cc, consumes an average of 530 /spl mu/W from 6 V, and transmits data up to 50 m. System features such as active power management, the intramodule sensor bus, generic bus interface circuitry, and in-module sensor compensation based on bivariate polynomials are discussed.

Precision readout circuits for capacitive microaccelerometers

This paper presents a review of capacitive readout front-end circuits for high-precision accelerometers. The primary design parameters and the trade-offs affecting the resolution are presented. The discussions apply to all capacitive microsensor interfaces. Also, a high-sensitivity capacitive accelerometer interface circuit for hybrid-integration with a surface/bulk micromachined micro-g accelerometer is described. The first generation of the circuit resolves 75 aF of capacitance on /spl sim/120 pF parasitic capacitance with a 200 kHz sampling rate, and the second generation resolves 20 aF with 1 MHz sampling rate. The overall sensor-circuit module has a noise floor of 1.6 /spl mu/g//spl radic/Hz at ambient atmosphere.

A generic interface chip for capacitive sensors in low-power multi-parameter microsystems

Abstract This paper presents a generic low-power sensor interface chip compatible with smart microsystems and a wide range of capacitive transducers. The interface chip is highly programmable, can communicate with an external microcontroller using a nine-line sensor bus standard, contains a switched-capacitor readout circuit, supports sensor self-test, and includes a temperature sensor. The circuit can interface with up to six external sensors and contains three internal programmable reference capacitors in the range of 0.15–8 pF. The chip measures 3.2×3.2 mm in a standard 3-μm single-metal double-poly p-well process, dissipates less than 2.2 mW from a single 5 V supply, and can resolve input capacitance variations of less than 1 fF in 10 Hz bandwidth.

An all-silicon single-wafer fabrication technology for precision microaccelerometers

This paper reports a novel all-silicon single-wafer fabrication technology for high precision capacitive accelerometers. This technology combines both surface and bulk micromachining to attain a large proofmass, controllable/small damping, and a small airgap for large capacitance variation. The microfabrication process provides large proofmass by using the whole wafer thickness, and a large sense capacitance by utilizing a thin sacrificial layer. The sense/feedback electrodes are formed by a deposited 2 /spl mu/m polysilicon film with embedded 20-30 /spl mu/m thick vertical stiffeners. These electrodes, while thin, are made very stiff by the thick embedded stiffeners so that force rebalancing of the proofmass becomes possible. The polysilicon electrodes are patterned to create damping holes. Several prototype microaccelerometers are fabricated successfully. Sensitivity of the devices with 2 mm/spl times/1 mm proofmass and full-bridge support are measured to be 2pF/g.

A Low-power Wireless Microinstrumentation System For Environmental Monitoring

This paper reports a hybrid microinstrumentation system that includes an embedded microcontroller, transducers for monitoring environmental parameters, interface/readout electronics for linking the controller and the transducers, and custom circuitry for system power management. Sensors for measuring temperature, pressure, humidity, and acceleration are included in the initial system, which operates for more than 180 days and dissipates less than 700/spl mu/m from a 6V battery supply. The sensor scan rate is adaptive and can be event triggered. The system communicates internally over a 1MHz, 9-line intramodule sensor bus, and outputs data over a hardwired serial interface or a 315MHz wireless link. The use of folding circuit platforms allows an internal system volume as small as 5cc.

A high-sensitivity silicon accelerometer with a folded-electrode structure

A high-sensitivity capacitive silicon accelerometer with a new device structure is presented in this paper. The structure uses a fixed rigid electrode suspended between a proof mass and a stiff moving electrode to provide differential capacitance measurement and force rebalancing. High sensitivity is achieved by forming a thick silicon proof mass and a narrow uniform air gap over a large area. The mechanical noise floor is reduced by incorporating damping holes in the electrodes. The accelerometer structure is all silicon and is fabricated on a single silicon wafer. The measured sensitivity for a device with 2.6 mm /spl times/ 1 mm proof mass and 1.4 /spl mu/m air gap is /spl ap/11 pF/g per electrode. The calculated mechanical noise floor for the same device is 0.18 /spl mu/g//spl radic/Hz at atmosphere.

Transient liquid phase (TLP) bonding for microsystem packaging applications

This paper explores the use of transient liquid phase bonding for microsystem packaging applications. Two types of bonds are demonstrated: a thin-film evaporated indium-gold bond and an electroplated nickel-tin bond. Both bonds are formed at 300/spl deg/C for about 1.5 hours in a conventional wafer bonder. The wafers were heated to over 400 /spl deg/C for more than an hour after bonding without any signs of bond degradation. The indium-gold bond demonstrated good electrical contact, but poor permeability performance. However, the nickel-tin bond was void free and sealed cavities with bond ring widths as little as 50 /spl mu/m. The cross section of the nickel-tin TLP bond was analyzed with EDAX software to verify the formation of intermetallic compounds.

Low power, wide range threshold acceleration sensing system

A complete threshold acceleration detection system is presented in this paper, possessing (i) an array of threshold accelerometers, and (ii) a low power interface circuit. The sensors were designed and fabricated using a modified bulk silicon dissolved wafer process. This process offers a wide latitude in sensor threshold levels, as demonstrated in the fabrication of devices, with levels of 1.5 g to 1000 g, bandwidth of 45 Hz to 40 kHz, with mass sizes ranging from 15 programs to 0.7 /spl mu/grams, and low-resistance gold-gold contacts for the switch. The interface circuit is fabricated in-house, using a standard 3 /spl mu/m, p-well CMOS process and connected to the sensor chip, in a multi-chip module. Additionally, the system employs redundancy, to improve detection accuracy and fault tolerance, which is crucial in many applications. The system also supports communication with a standard microcontroller bus, in a smart sensor environment.