Howard K. Rockstad

Characterization of a high-sensitivity micromachined tunneling accelerometer with micro-g resolution

A new high-sensitivity bulk-silicon-micromachined tunneling accelerometer with micro-g resolution has been successfully fabricated and tested at Stanford University. This accelerometer is a prototype intended for underwater acoustics applications and is required to feature micro-g resolution at frequencies between 5 Hz and 1 kHz and can be packaged with circuitry in an 8-cm/sup 3/ volume with a total mass of 8 g. This paper briefly describes the mechanical design of this tunneling accelerometer and focuses on the experiments carried out in our laboratory to test the tunneling transducer as well as on the experimental determination of accelerometer resolution. The exponential relationship between tunneling gap and tunneling current is verified and results in an effective tunneling barrier height of about 0.2 eV. The goal of this paper is to outline the measurements which are necessary to verify that the sensor is actually tunneling and to confirm that the accelerometer performance is consistent with what should be expected from a tunneling accelerometer.

Wide-bandwidth electromechanical actuators for tunneling displacement transducers

A series of displacement transducers have been demonstrated which are based on the detection of electrons that quantum-mechanically tunnel across a narrow gap between electrodes. These transducers have important applications due to the sensitivity of the tunneling mechanism to sub-/spl Aring/ variations in the electrode gap. In this paper, we describe the recent development of wide-bandwidth electromechanical actuators and simple feedback circuitry which have been adapted for use in tunneling displacement transducers. With these actuators and circuits, we have built tunneling transducers with control bandwidths well in excess of 10 kHz. The design, fabrication, operation, and applications of these actuators are described. >

A miniature high-sensitivity broad-band accelerometer based on electron tunneling transducers

Abstract New high sensitivity microsensors have been developed using high-resolution position sensors based on electron tunneling. The design of miniature accelerometers having resolutions approaching 10 −9 g /√Hz is discussed. A new dual-element electron tunneling structure, which overcomes bandwidth limitations of single-element structures, allows design of high sensitivity accelerometers operating in a band from a few Hz to several kHz. A miniature accelerometer based on this structure can thus have application as a sensitive acoustic sensor. Thermal vibration of the proof mass is an extremely important constraint in miniature accelerometers, and can be the dominant limitation on the sensitivity. Thermal noise is analyzed for the suspended masses of the dual-element structure, and compared with electronic noise in the tunneling circuit. With a proof mass of 100 mg, noise analysis predicts limiting resolutions better than 10 −8 g /√Hz between 10 and 100 Hz, and 10 −7 g /√Hz at 1 kHz. Prototype accelerometers have been fabricated by silicon micromachining and tested. A noise resolution of 10 −7 g /√Hz between 4 and 70 Hz and 6 × 10 −7 g /√Hz at 400 Hz is observed in a damped device. The low-frequency responsivity of this device is 100 000 V/ g , decreasing to 1300 V/ g at 600 Hz.

A miniature, high-sensitivity, electron tunneling accelerometer

Abstract Prototype low-noise miniature accelerometers have been fabricated with electron-tunneling transducers. The electron-tunneling transducer permits detection of small displacements of the proof mass with high electrical response; such a transducer is essential for a high-performance miniature accelerometer. Prototype accelerometers have shown self-noise of approximately 10 −7 g ( Hz ) − 1 2 or less between 10 and 200 Hz, and close to 10 −8 g ( Hz ) − 1 2 near the resonance frequency of 100 Hz. Directivity measurements give nulls at least 50 dB below the maximum. A dual-axis prototype designed for underwater acoustic applications is packaged in an 8 cm3 volume with a mass of 8 g.

Micromachined electron tunneling infrared sensors

The authors describe the development of an improved Golay cell. This sensor is constructed entirely from micromachined silicon components. In this device, a silicon oxynitride (SiO/sub x/N/sub y/) membrane is deflected by the thermal expansion of a small volume of trapped gas. To detect the motion of the membrane, an electron tunneling displacement transducer is used. An improved infrared sensor in which the cantilever was dispensed with altogether, and the rebalance force from the feedback circuit applied to the membrane directly, is described.<<ETX>>

Characterization of a high-sensitivity micromachined tunneling accelerometer

We have fabricated successfully and tested a new micromachined tunneling accelerometer. This accelerometer is a prototype intended for underwater acoustics applications, and is required to feature micro-g resolution at frequencies between 5 Hz and 1 kHz. This paper will focus on the experiments carried out in our laboratory to test the tunneling transducer as well as on the experimental determination of accelerometer resolution. The goal of this paper is to outline the measurements which are necessary to verify that the sensor is actually tunneling, and to confirm that the accelerometer performance is consistent with what should be expected from a tunneling accelerometer.

A microfabricated electron-tunneling accelerometer as a directional underwater acoustic sensor

Microfabricated accelerometers have been developed for a wide variety of applications; however, the principal commercial focus has been on signal detection in the milli‐g to tens or hundreds of g accelerations. The development of a microfabricated device to detect accelerations in the 10 to 100 nano‐g range is a substantial technological challenge because of the conflict between the required increase in mass (and reduction in suspension stiffness) and the small volume. In an underwater sensor, designed to be nearly neutrally buoyant, there are additional restrictions on the packaging of the sensor with regard to overall density, resistance to hydrostatic pressure, and flexibility of power and signal leads. The design goal of this project is to demonstrate a two‐axis sensor in an 8 cm3 (and 8 gram) package capable of immersion to 600 meters. The sensor must have a self noise below 100 nano‐g per root hertz from 5 to 1000 Hz. Several of these requirements have been demonstrated with an accelerometer structu...

A miniature, high‐sensitivity, electron‐tunneling accelerometer

A prototype low‐noise accelerometer has been fabricated with an electron‐tunneling transducer. By measuring the tunneling current between an electrode on the proof mass and a feedback‐controlled monitor electrode, very small accelerations can be detected with high responsivity. This particular prototype (10×10×1.5 mm) was designed for underwater acoustic measurement from a few hertz to 1 kHz. The measured responsivity below the fundamental device resonance at 100 Hz is roughly 1500 V per m/s2 with a measured noise spectral density of 10−6 m/s2 per root hertz or less between 30 and 300 Hz. The noise floor is controlled primarily by 1/f noise in the tunneling current although the noise floor reaches the theoretical molecular‐agitation limit at 100 Hz. The responsivity and directivity of the device were measured in a standard gradient‐hydrophone calibrator; the noise floor was determined in a vacuum‐isolation chamber assembled from commercial off‐the‐shelf components; and the detailed dynamics of the proof‐m...

Electron tunneling sensors

Many physical sensors operate by coupling the signal of interest to deflection of a sensor component, and utilizing a displacement transducer to produce an electrical signal. The development of miniature high‐performance sensors have been limited by the availability of small, sensitive transducers. A novel accelerometer has been designed that utilizes an electron tunneling transducer to detect the relative motion of a suspended proof mass. The sensitivity of the tunneling transducer allows miniaturization of the device with resolution near 10−8 g/√Hz from 10 Hz to 1 kHz. Thermal noise in the positions of sensor components is important for high‐resolution miniature accelerometers, and is the dominant factor in selection of design parameters. Thermal noise for this dual element system will be discussed and compared with other noise sources. A simple closed‐loop electronic feedback system that is used with these sensors will also be described. Preliminary test results will be described. This system offers im...