Micromachined Vibrating Gyroscopes Intro to MEMS final presentation December 12, 2002

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Micromachined Vibrating GyroscopesIntro to MEMS
final presentationDecember 12, 2002

• presented by
• Kimberly S. Elliot
• Parag Gupta
• Kyle Reed
• Raquel C. Rodriguez

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Presentation Outline

• State-of-the-art review
• Introduction / Applications
• Operation Principles
• Two case studies
• HARPSS Vibrating Ring
• Drapers Tuning Fork
• Conclusions and Questions

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Introduction

• Performance of micromachined gyroscopes improves
  by a factor of 10x every two years since 1991.
• Applications automotive ride stabilization and
  rollover detection.

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Applications (Contd)

• Guidance and Navigation
• Segway Scooter
• Uses five MEMS gyroscopes for tilt and rotation
  detection.

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Basic Operating Principles

• Follows Newtons Laws
• Force required to change velocity
• Resistance to change in velocity increases with
  mass
• Gyroscope provides information about angular
  orientation.
• Three types of Gyros
• Spinning Mass tilting produces precession
• Impractical in MEMS
• Optical measure time differences in laser paths
• Very expensive, but also the best performance
• Vibrating based on Coriolis effect
• The most common

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Basic Configurationof Vibrating Gyroscopes

• Four main components
• 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)

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Vibrating Gyroscope Basics

• Coriolis Acceleration
• Fc 2mv ? ?
• Fc Coriolis force
• m vibratory mass
• v linear velocity
• ? - angular rotation
• Two degrees of freedom are required drive and
  sense.
• Most common sensing methods
• Capacitive
• Piezoresistive

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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, its the sum of white noise and a
  slowly varying function
• Noise defines resolution (?/s)/?Hz
• Slowly varying function defines drift ?/s

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HARPSS Vibrating Ring Gyroscope

• 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.

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Vibrating Ring

• Symmetric design
• 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.

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Fabrication of HARPSS 1

• Deposit and pattern a thin layer of LPCVD silicon
  nitride.
• This serves as an isolation dielectric layer.
• Dry etch using Deep Reactive Ion Etch (DRIE).
• 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

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Fabrication of HARPSS 2

• Deposit and pattern Cr/Au.
• Dry directional/isotropic SF6 silicon etch to
  release silicon sense electrodes.
• Involves a deep, directional etch followed by an
  isotropic SF6 silicon etch.
• Using HFH2O, etch away the sacrificial oxide
• Creates capacitive air gaps between the
  sense-electrodes and the ring structure.

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HARPSS Gyroscope

• Advantages
• 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

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HARPSS Limitations

• 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.

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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.
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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 of junction bar.
• Capacitive, piezoresistive, or piezoelectric
  detection mechanisms.

i
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Drapers Tuning Fork
Masses
Beams
Comb Structure

• 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.

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Fabrication of Tuning Fork 1

• Dissolved wafer process involving both silicon
  and glass processing.
• P-type (100) silicon wafer of moderate doping
  (gt1?-cm) is used.
• Mask 1
• Etch recess with KOH to define the height of the
  silicon above the glass.
• Diffuse Boron at 1175 ?C to define thickness.
• Mask 2
• Reactive ion etch (RIE).
• Defines the structures pattern features.

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Fabrication of Tuning Fork 2

• Glass wafer processed separately.
• Mask 3
• 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.

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Fabrication of Tuning Fork 3

• Silicon wafer is turned upside down and
  electrostatically bonded to the glass.
• 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.

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Drapers Gyroscope

• Robustness, withstands accelerations of 60,000
  gs
• Scale factor gt 200 ppm
• Bias uncertainty lt 50 ?/hr
• Drift 0.05 ?/s
• Low temperature 0.003 ?/s
• Resolution 100-200 ?/hr (60 Hz bandwidth)
• Best 25 ?/hr

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Conclusions

• 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.

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Any Questions?