Tunneling Tunneling AccelerometersAccelerometers

Samantha CruzSamantha Cruz

Kevin LeeKevin Lee

Deepak PonnavoluDeepak Ponnavolu

ME 381Final PresentationDecember 6, 2004

IntroductionIntroduction

High sensitivity

Low range

Applications:

Underwater acoustic detection.

Seismology

Micro-g measurements.

ConceptConcept

Sensor BasicsSensor Basics

(a)(a) On acceleration, the proof On acceleration, the proof mass movesmass moves

(b)(b) This changes distance which This changes distance which changes tunneling currentchanges tunneling current

(c)(c) Feedback circuit fights to Feedback circuit fights to maintain the same tunneling maintain the same tunneling current by changing voltage current by changing voltage of electrodeof electrode

(d)(d) The force required to keep it The force required to keep it at the same position is used at the same position is used to figure out accelerationto figure out acceleration

MicrofabricationMicrofabrication

Counter-electrode Counter-electrode cantilever cantilever

(a) e- beam evaporation, (a) e- beam evaporation,

(b) lithography and ion (b) lithography and ion milling milling

(c) ion milling(c) ion milling

(d) sacrificial layer(d) sacrificial layer

(e) masking and metal (e) masking and metal evaporationevaporation

(f) cantilever release(f) cantilever release

MicrofabricationMicrofabricationTunneling electrode cantileverTunneling electrode cantilever

(a)(a) ee-- beam evaporation beam evaporation

(b)(b) SiOSiO22 deposition and deposition and

etchingetching

(c)(c) SOISOI

(d)(d) removal of back Si, tip removal of back Si, tip mold etchedmold etched

(e)(e) ee-- beam evaporation beam evaporation

(f)(f) mask and ion millingmask and ion milling

(g)(g) cantilever releasecantilever release

MicrofabricationMicrofabrication

SensingSensing

IItt = V = VBB*exp(α*exp(αII√Φ*x√Φ*xtgtg))

WhereWhere

VVBB = tunneling bias across electrode gap = tunneling bias across electrode gap

ααII = 1.025(Å-1eV-0.5) = 1.025(Å-1eV-0.5)

ΦΦ = height of tunneling barrier = height of tunneling barrier

xxtgtg = minimum tunneling gap = minimum tunneling gap

IIt t = tunneling current = tunneling current

Feedback ControlFeedback Control

Feedback CircuitFeedback Circuit

(a)(a) Operational Amplifier Operational Amplifier controls the tunneling controls the tunneling CurrentCurrent

(b)(b) High Voltage supply is High Voltage supply is used to correct for change used to correct for change in deflection voltage for in deflection voltage for proper separation of the proper separation of the proof mass and tip drifts proof mass and tip drifts slowly over timeslowly over time

Noise CorrectionNoise Correction

Equivalent acceleration error Equivalent acceleration error √((4*k√((4*kBB*T*ω*T*ωoo)/(m)/(mpp*Q))*Q))

Where,Where,

kkBB = Boltzmann constant = Boltzmann constant

T = TemperatureT = Temperature

ωωoo = Resonant frequency of proof mass = Resonant frequency of proof mass

mmpp = mass of proof mass = mass of proof mass

Q = Mechanical quality factorQ = Mechanical quality factor

ConclusionConclusion

Amazing SensitivityAmazing Sensitivity Great rangeGreat range High BandwidthHigh Bandwidth

ONLY FOR APPLICATIONS THAT ONLY FOR APPLICATIONS THAT REQUIRE HIGH SENSITIVITYREQUIRE HIGH SENSITIVITY

Questions ???Questions ???