Tunneling Transducer Research Articles

Quantum Tunneling in Mesoscopic Electromechanical Transducers

The paper presents the simulation approach to some quantum transport effects, which arise in mesoscopic electromechanical transducers such as two-electrode tunneling transducer. The tunneling transducers are advanced nanoelectromechanical systems, which can be used for motion detection at nanoscale. It is very promising to adopt tunneling transducers as sensitive elements of the inertial microelectromechanical (or nanoelectromechanical) systems. In theory, their extremely high sensitivity is limited only by fundamental quantum relations. We propose theoretical model of the transducer based on the transfer-matrix formalism in this work. We provide the detailed derivation of transfer-matrix. Scattering potential of the transducer is a stepped representation of the electron potential energy. In addition, by using well-known formula proposed by L. Esaki and R. Tsu for finite superlattices we apply the numerical procedure for current density evaluation. The proposed model of the two-electrode mesoscopic tunneling transducer could be a useful tool for quantum transport simulation in high-performance nanoelectromechanical systems and advanced nanomaterials such as metal-polymer composites.

A micromachined wide-bandwidth magnetic field sensor based on all-PMMA electron tunneling transducer

All-PMMA-based tunneling magnetic sensors were fabricated by hot embossing replication with silicon templates. The silicon templates had smooth surfaces, positive profiles, and pyramid-like pits with a high aspect ratio. With this fast (20 min), simple (one-step), and repeatable method, the all-PMMA tunneling sensor platform yielded sharp tunneling tips with 75 /spl mu/m in baseline and 50 /spl mu/m in depth. The sensors were assembled and fixed with measurement circuits, after their electrodes were patterned with modified photolithography and Co film was deposited with e-beam evaporation. A natural frequency response of 1.3 kHz was observed, and a tunneling barrier height of 0.713 eV was tested. Due to the quadratic relation between magnetic force and the field, the sensor field response (7.0/spl times/10/sup 6/ V/T/sup 2/) was also quadratic. The noise voltage at 1 kHz is 0.2 mV, corresponding to a magnet field of 0.46/spl times/10/sup -6/ T. The bandwidth of this sensor is 18 kHz. This new type of sensor platform is promising for the next generation of microsensing applications.

Design, simulation, fabrication, and characterization of a PMMA tunneling sensor based on hot embossing technique

Polymethyl methacrylate (PMMA) was chosen to fabricate highly sensitive tunneling sensors due to its extremely low cost and quick machining by hot embossing. The structure of an all-PMMA-based membrane vertical tunneling sensor platform was synthesized by ANSYS. The system characteristics and feedback system were evaluated with MatLab SimuLink. The first all-PMMA-based membrane tunneling transducers were produced by hot embossing replication with silicon masters. The fabricated tunneling sensors have presented a tunneling barrier height of 0.17 eV, a sensitivity of 26 V/g, and a dynamic range of 1.5 mg, which demonstrated competitive performances with lower cost, lighter weight, and easier fabrication processing.

A combinative technique to fabricate hot embossing master for PMMA tunneling sensors

A combinative approach of anisotropic bulk etching and modified plasma etching has been successfully employed in a single wafer to fabricate silicon masters for the hot embossing process. The masters hold both pyramid pits and positive profile sidewalls with smooth surfaces and steep angles. The SiO2 layer is utilized as a etching mask with the aid of photoresist in three steps of photolithography patterning. The first polymethyl-methacrylate (PMMA)-based tunneling transducer with polymer membrane structures is fabricated by hot embossing replication with the silicon master. Consequently, the exponential relations between tunneling currents and applied deflection voltages are also reported.

A high-precision, wide-bandwidth micromachined tunneling accelerometer

High-precision miniature accelerometers based on electron tunneling transducers have been successfully fabricated using bulk-silicon micromachining process. This process has been modified to reduce bi-metal effects and results in greatly-suppressed 1/f noise. A comprehensive system model and a robust controller based on mixed /spl mu/ synthesis have also been developed to close the feedback loop, extend the bandwidth, and withstand accelerometer variations due to micromachining process variations. This accelerometer is a prototype intended for underwater acoustics applications and is designed to be packaged in an 8 cm/sup 3/ sphere volume with a total mass of 8 grams. In this paper, we describe the accelerometer design, the feedback control design, and several performance enhancements of these micromachined tunneling accelerometers, which include a high resolution of 20 nano-g//spl radic/Hz and a 5 Hz-1.5 kHz bandwidth.

A micromachined magnetic-field sensor based on an electron tunneling displacement transducer

We describe a micromachined magnetic-field sensor that is based on an electron tunneling transducer. This tunnel sensor is small, very sensitive, operates at ambient temperature and requires very little power. The measured resolution of the sensor is 0.3 nT/√Hz at 1 Hz. The limiting resolution, calculated based on fundamental noise sources, is 0.002 nT/√Hz at 1 Hz. The dominant source of the observed noise in the present device is low frequency air pressure fluctuations.

Resonant tunneling displacement transducer

We propose a new type of a tunneling displacement transducer. An electron energy filter quantum mechanically coupled to a conventional tunneling gap results in an improved ruggedness by enabling at least an order of magnitude increase in gap separation. This is achieved simultaneously with an increased sensitivity as compared to a more conventional electron tunneling transducer. We also show how the new transducer can be used in a force rebalance accelerometer.

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.

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

This paper describes the successful fabrication and demonstration of a new dual-element micromachined silicon tunnel accelerometer that extends the operational bandwidth beyond the resonant frequency of the proof mass.

Micromachined tunneling displacement transducers for physical sensors

We have designed and constructed a series of tunneling sensors which take advantage of the extreme position sensitivity of electron tunneling. In these sensors, a tunneling displacement transducer, based on scanning tunneling microscopy principles, is used to detect the signal-induced motion of a sensor element. Through the use of high-resonant frequency mechanical elements for the transducer, sensors may be constructed which offer wide bandwidth, and are robust and easily operated. Silicon micromachining may be used to fabricate the transducer elements, allowing integration of sensor and control electronics. Examples of tunneling accelerometers and infrared detectors will be discussed. In each case, the use of the tunneling transducer allows miniaturization of the sensor as well as enhancement of the sensor performance.

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 improved resolution, compact geometry, low mass, and low cost relative to present miniature accelerometers or acoustic sensors. A wide range of possible applications exist including underwater acoustics. [Work supported by NAWC, and SDIO/IST.]

Uncertainty-principle noise in vacuum-tunneling transducers.

The fundamental sources of noise in a vacuum-tunneling probe used as an electromechanical transducer to monitor the location of a test mass are examined using a first-quantization formalism. We show that a tunneling transducer enforces the Heisenberg uncertainty principle for the position and momentum of a test mass monitored by the transducer through the presence of two sources of noise: the shot noise of the tunneling current and the momentum fluctuations transferred by the tunneling electrons to the test mass. We analyze a number of cases including symmetric and asymmetric rectangular potential barriers and a barrier in which there is a constant electric field. Practical configurations for reaching the quantum limit in measurements of the position of macroscopic bodies with such a class of transducers are studied.

The magnetostriction of CoFeNiMo metallic glasses measured with a tunneling transducer

The longitudinal magnetostriction as a function of the Co/Fe content of the metallic glass series Co 72− x Fe x Ni 3 Mo 1Si 12B 12, for 0 ≤ x ≤ 10, is reported. The measurement is performed using a novel technique, based on electron tunneling, which provides a highly sensitive, noncontact, quasi-dc and direct measurement of magnetostrictive strains. The minimum value of the longitudinal magnetostriction measured is (−1.65±0.24) ppm for 0% Fe, and the maximum (in this composition range) is (2.51±0.15) ppm for 10% Fe, with a zero-magnetostriction composition of 5.25% Fe. These data are compared to those previously reported for a similar metallic glass series, but with higher Mo content. Increasing the Mo content in these glasses is shown to decrease the Fe content at the zero-magnetostriction composition point.

Tunneling transducers: Quantum limited displacement monitors at the nanometer scale

Displacement transducers based upon quantum mechanical tunneling of electrons can potentially achieve a sensitivity far superior to conventional transducers. The reason for this is that the dynamical influence of the tunneling transducer on a test mass is insignificant compared to that of traditional capacitive, inductive or piezoelectric transducers. Thus the fluctuating ‘‘back action’’ force from electrical noise in the tunneling transducer is nearly eliminated. The ‘‘back action’’ force becomes dominant as the test mass is miniaturized, thus the tunneling transducer may be especially useful in micro-fabricated sensors. In this paper we present an equivalent circuit model for a tunneling transducer including noise and outline a method for calculating the minimum detectable signal in any given application. We also discuss some practical limits for tunneling-based sensors.

The scanning tunneling microscope as a high-gain, low-noise displacement sensor

We consider the capabilities of the tunneling probe of the scanning tunneling microscope as a displacement sensor as distinguished from its better established application to surface imaging. Electromechanical transducers that operate on this principle can achieve very large gain and a noise temperature equal to the minimum required by quantum mechanics for any linear amplifier. We present a two-port network representation of the tunneling transducer, including noise, that allows us to discuss the differences between the tunneling transducer and more conventional electromechanical transducers and to draw analogies between a tunneling transducer and a transistor. We present a simple equivalent circuit for the tunneling transducer including two uncorrelated noise generators, the tunneling current shot noise and the fluctuating force that the tunneling probe exerts on a test mass. In practice the fluctuating ‘‘back action’’ force spectral density is exceedingly small. We give an example of a system in which a tunneling transducer is used to monitor the motion of a very small mechanical harmonic oscillator. A transducer gain of approximately 108 should be achieved in this system that makes negligible the noise contribution of conventional following electronics. The contribution to the noise of the tunneling transducer itself should be near the quantum limit and the most significant remaining source of noise is the mechanical oscillator’s Brownian motion. The tunneling transducer represents a new approach for measuring mechanical displacements and may profoundly influence transducer technology in applications from gravity wave detectors down to measurements on single molecules.

Momentum noise in vacuum tunneling transducers.

Momentum noise in vacuum tunneling transducers.