Transcript Tiny Little Gyroscopes Intro to MEMS final presentation

Micromachined Vibrating
Gyroscopes
Intro to MEMS final presentation
December 12, 2002
presented by:
Kimberly S. Elliot
Parag Gupta
Kyle Reed
Raquel C. Rodriguez
Presentation Outline
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State-of-the-art review
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Two case studies
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Introduction / Applications
Operation Principles
HARPSS Vibrating Ring
Draper’s Tuning Fork
Conclusions and Questions
Introduction
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Performance of micromachined gyroscopes
improves by a factor of 10x every two years
since 1991.
Applications: automotive ride stabilization and
rollover detection.
Applications (Cont’d)
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Guidance and
Navigation
Segway Scooter
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Uses five MEMS
gyroscopes for tilt and
rotation detection.
Basic Operating Principles
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Follows Newton’s Laws
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Force required to change velocity
Resistance to change in velocity increases with mass
Gyroscope provides information about angular
orientation.
Three types of Gyros:
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Spinning Mass – tilting produces precession
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Optical – measure time differences in laser paths
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Impractical in MEMS
Very expensive, but also the best performance
Vibrating – based on Coriolis effect
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The most common
Basic Configuration
of Vibrating Gyroscopes
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Four main components:
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Proof mass
Elastic spring
Dashpot
Sensing method
Proof mass is put into
oscillation (x-axis)
Sensitive to angular
rotation in the z-axis
Induced Coriolis
acceleration (y- axis)
Vibrating Gyroscope Basics
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Coriolis Acceleration:
Fc = 2mv  
Fc – Coriolis force
m – vibratory mass
v – linear velocity
 - angular rotation
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Two degrees of freedom
are required: drive and
sense.
Most common sensing
methods:
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Capacitive
Piezoresistive
Performance concepts
Scale factor: amount of change in output
per unit change rotation [V/(/s)]
• Zero-rate output (ZRO): output in the
absence of angular rate, it’s the sum of
white noise and a slowly varying function
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• Noise defines resolution [(/s)/Hz]
• Slowly varying function defines drift [/s]
HARPSS Vibrating Ring Gyroscope
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High Aspect-Ratio Combined Poly and Single-Crystal Silicon.
Developed at The University of Michigan.
Vibrating ring and eight support springs.
Each spring with two electrodes for drive and sensing, and to
compensate asymmetries.
Capacitive sensing.
Vibrating Ring
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Symmetric design:
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Identical drive and sense flexural modes
45 apart
Same resonant frequency
Less temperature sensitive
Q directly amplifies sensitivity
Filters spurious vibrations
Sense mode amplitude:
qsense = 4Ag.Q/0.qdrive.z
Ag  0.37 – structure angular gain;
Q – structure quality factor;
0 – flexural resonance frequency;
qdrive – drive vibration amplitude;
z – rotation rate.
Fabrication of HARPSS 1
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Deposit and pattern a thin layer of LPCVD silicon nitride.
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Dry etch using Deep Reactive Ion Etch (DRIE).
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This serves as an isolation dielectric layer.
Forms deep trenches with smooth and straight sidewalls.
Deposit a sacrificial oxide layer.
Refill the trenches with polysilicon.
Dope the polysilicon layer with boron to speed its etch rate
Fabrication of HARPSS 2
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Deposit and pattern Cr/Au.
Dry directional/isotropic SF6 silicon etch to release silicon sense
electrodes.
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Involves a deep, directional etch followed by an isotropic SF6 silicon
etch.
Using HF:H2O, etch away the sacrificial oxide
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Creates capacitive air gaps between the sense-electrodes and the ring
structure.
HARPSS Gyroscope
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Advantages:
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Small ring-to-electrode gap
Large structural height
Better structural material
Sensitivity  200 [V/(/s)]
For Q  1200, resolution  0.01 [/s] for 1
Hz bandwidth
Min.detectable signal  5x10-3 [/s] for 10
Hz bandwidth
HARPSS Limitations
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Anchor problem:
excessive undercut of
the substrate at the
post causes the oxide
to be exposed and
etched away resulting
in a soft anchor that
dissipates energy.
HARPSS Limitations 2
Voids and keyholes
generated during the
polysilicon refill
process
RIE lag effect: height
of high aspect narrow
trenches is less than
for the medium and
wide ones.
Basic Tuning Fork Gyroscope
Tines resonated to fixed amplitude.
 When rotated, Coriolis force causes
a sinusoidal force on tines.
 This force detected as a bending of
the tines or as a torsional vibration
i
of junction bar.
 Capacitive, piezoresistive, or
piezoelectric detection mechanisms.
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Draper’s Tuning Fork
Masses
Beams
Comb Structure
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Developed by Charles Stark Draper Laboratory.
Two masses suspended by beams.
Electrostatically vibrated using comb drives.
Generates a Coriolis force that pushes masses in and
out of oscillation plane.
Measured by capacitor plates.
Fabrication of Tuning Fork 1
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Dissolved wafer process involving both
silicon and glass processing.
P-type (100) silicon wafer of moderate
doping (>1-cm) is used.
Mask 1:
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Etch recess with KOH to define the height
of the silicon above the glass.
Diffuse Boron at 1175 C to define
thickness.
Mask 2:
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Reactive ion etch (RIE).
Defines the structure’s pattern features.
Fabrication of Tuning Fork 2
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Glass wafer processed separately.
Mask 3:
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Recess glass 1600 Angstroms.
Deposit and lift off a multi-metal
system.
Result: Metal protrudes 500
Angstroms above the glass.
The metal forms the sense and
drive plates of the capacitor and
the output leads from the
transducer.
Fabrication of Tuning Fork 3
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Silicon wafer is turned upside down
and electrostatically bonded to the
glass.
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375C and potential of 1000 V for
strong chemical bonds.
Silicon and glass drawn tightly together
to ensure a low resistance contact to
silicon.
Final step: Selective EDP etch to
dissolve the undoped silicon.
Draper’s Gyroscope
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Robustness, withstands accelerations of
60,000 g’s
Scale factor > 200 ppm
Bias uncertainty < 50 /hr
Drift 0.05 /s
• Low temperature: 0.003 /s
• Resolution 100-200 /hr (60 Hz bandwidth)
• Best: 25 /hr
Conclusions
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Early development phase for
micromachined gyroscopes.
HARPSS yields a gyroscope with excellent
mode matching, high resolution, low ZRO
and long-term stability.
Draper Lab produces a low-cost
gyroscope of small size and considerable
ruggedness.
Any Questions?