Transcript Slide 1

Tunneling
Accelerometers
ME 381
Final Presentation
December 6, 2004
Samantha Cruz
Kevin Lee
Deepak Ponnavolu
Introduction
High sensitivity
Low range
Applications:
Underwater acoustic detection.
Seismology
Micro-g measurements.
Concept
Sensor Basics
(a) On acceleration, the proof
mass moves
(b) This changes distance
which changes tunneling
current
(c) Feedback circuit fights to
maintain the same
tunneling current by
changing voltage of
electrode
(d) The force required to keep
it at the same position is
used to figure out
acceleration
Microfabrication
Counter-electrode
cantilever
(a) e- beam evaporation,
(b) lithography and ion
milling
(c) ion milling
(d) sacrificial layer
(e) masking and metal
evaporation
(f) cantilever release
Microfabrication
Tunneling electrode cantilever
(a) e- beam evaporation
(b) SiO2 deposition and
etching
(c) SOI
(d) removal of back Si, tip
mold etched
(e) e- beam evaporation
(f) mask and ion milling
(g) cantilever release
Microfabrication
Sensing
 It = VB*exp(αI√Φ*xtg)
Where
VB
αI
Φ
xtg
It
= tunneling bias across electrode gap
= 1.025(Å-1eV-0.5)
= height of tunneling barrier
= minimum tunneling gap
= tunneling current
Feedback Control
Feedback Circuit
(a) Operational Amplifier
controls the tunneling
Current
(b) High Voltage supply is
used to correct for change
in deflection voltage for
proper separation of the
proof mass and tip drifts
slowly over time
Noise Correction
 Equivalent acceleration error
√((4*kB*T*ωo)/(mp*Q))
Where,
kB = Boltzmann constant
T = Temperature
ωo = Resonant frequency of proof mass
mp = mass of proof mass
Q = Mechanical quality factor
Conclusion
 Amazing Sensitivity
 Great range
 High Bandwidth
 ONLY FOR APPLICATIONS THAT
REQUIRE HIGH SENSITIVITY
Questions ???