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Applications: Angular Rate Sensors contd

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Fabrication. photoresist is removed ... Fabrication. crystalline silicon over the SiO2 defines the tines ... Fabrication ... – PowerPoint PPT presentation

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Title: Applications: Angular Rate Sensors contd


1
ApplicationsAngular Rate Sensors (contd)
  • CSE 495/595 Intro to Micro- and Nano- Embedded
    Systems
  • Prof. Darrin Hanna

2
Angular Rate Sensors
3
Angular Rate Sensors
  • The ring (One Ring to rule them all, One Ring
    to find them One Ring to bring them all and in
    the darkness bind them -)
  • 6 mm diameter
  • suspended by flexural beams anchored to
    10-mm-sq. frame.

4
Angular Rate Sensors
  • Vibratory ring shell similar to the sensor from
    Delphi Delco
  • different excitation and sense methods
  • electric current loops in a magnetic field
  • excite primary mode of resonance
  • same physical loops provide the sense signal

5
Angular Rate Sensors
  • Magnetic field perpendicular to beams
  • Current in a loop interacting with the magnetic
    field
  • Lorentz force
  • The radial component oscillates the ring
  • in the plane of the die
  • 14.5 kHzthe mechanical resonant frequency of
    the ring

6
Angular Rate Sensors
  • Sensing mechanism
  • measures the voltage induced around one or more
    loops
  • Faradays law as the ring oscillates, the area
    of the current loop in the magnetic flux changes,
    generating a voltage.
  • Two opposite loops perform a differential
    voltage measurement.

7
Angular Rate Sensors
Fabrication
8
Angular Rate Sensors
Fabrication
  • Silicon dioxide layer is deposited on a silicon
    wafer
  • lithographically patterned and etched
  • serves to electrically isolate the current
    loops
  • A metal layer is sputtered on, patterned and
    etched
  • current loop
  • bond pads
  • A layer of photoresist is spun on and patterned
    in the shape of the ring and support flexural
    beams
  • serves as a mask for DRIE step
  • trenches

9
Angular Rate Sensors
Fabrication
  • photoresist is removed
  • wafer is anodically bonded to a glass wafer
    with a previously defined shallow cavity on its
    surface.
  • permanent magnet is included

10
Angular Rate Sensors
Specifications
  • output scale factor of 20 mV/(º/s)
  • variation of 3 over a temperature range from
    40 to 85ºC.
  • noise is less than 1 mV rms from 3 to 10 Hz
  • nonlinearity in a rate range of 100 º/s is
    less than 0.5 º/s
  • operating current is a relatively large 50 mA
    at 5-V supply

11
Angular Rate Sensors
Daimler Benz
12
Angular Rate Sensors
Daimler Benz
  • Coriolis Effect
  • deflection of a moving object
  • in a rotating frame of reference
  • Coriolis acceleration
  • a 2O x v
  • V is the velocity of the particle in the
    rotating system
  • ? is the angular velocity vector of the
    rotating system
  • magnitude equal to the rotation rate
  • points in the direction of the axis of
    rotation.
  • Multiply by the mass of the object to produce
    the
  • Coriolis force.

13
Angular Rate Sensors
Daimler Benz
  • tines vibrate out of the plane of the die
  • driven by thin-film piezoelectric aluminum
    nitride actuator
  • top of one of the tines

14
Angular Rate Sensors
Shear stress, in general
Daimler Benz
  • Coriolis forces produce a torquing moment
    around the stem
  • shear stresses sensed with piezoresistive
    elements
  • Shear stress is maximal on the center line of
    the stem
  • optimal location for the piezoresistive sense
    elements

15
Angular Rate Sensors
Fabrication
Uses SOI processes
16
Angular Rate Sensors
Fabrication
  • crystalline silicon over the SiO2 defines the
    tines
  • tine thickness control by precise epitaxial
    growth of silicon over the SOI substrate
  • thickness of the silicon layer, and
    consequently of the tine, varies between 20 and
    200 µm, depending on desired performance
  • shallow silicon etch in TMAH
  • 2-µm-deep cavities in two mirror-image SOI
    substrates

17
Angular Rate Sensors
Fabrication
  • fusion bond substrates together with cavities
    facing each other
  • etch step in TMAH removes the silicon on the
    front side and stops on the buried SiO2
  • Buried SiO2 removed in HF

18
Angular Rate Sensors
Fabrication
  • piezoelectric and piezoresistive elements on
    the silicon surface
  • piezoresistors formed using ion implantation
    and diffusion
  • piezoelectric aluminum nitride
  • sputter aluminum in a controlled nitrogen and
    argon
  • shape plate over tine

19
Angular Rate Sensors
Fabrication
  • Aluminum form electrical interconnects and bond
    pads
  • TMAH etch from the back side to remove Si from
    underneath buried SiO2 is etch stop
  • anisotropic plasma etch from the front side
    releases the tines.

20
Angular Rate Sensors
Fabrication
  • frequency of excitation mode 32.2 kHz
  • torsional secondary mode (sense mode) 245 Hz
    lower
  • frequencies exhibited a temperature dependence
  • temperature coefficient of frequency 0.85
    Hz/ºC.

21
Angular Rate Sensors
Robert Bosch

22
Angular Rate Sensors
Robert Bosch
  • two resonant frequencies in phase, and out of
    phase
  • inphase oscillation mode the instantaneous
    displacements of the two masses are in the same
    direction
  • out-of-phase mode the masses are moving, at
    any instant, in opposite directions

23
Angular Rate Sensors
Robert Bosch
  • select coupling spring for good separation
    between resonant freq.
  • electric current loop generates Lorentz forces
    within magnetic field excite only the
    out-of-phase mode
  • oscillation electromagnetically induces a
    voltage in second current loop proportional to
    the velocity of the masses (Faradays Law)

24
Angular Rate Sensors
Robert Bosch
  • Coriolis forces on the two masses are in
    opposite directions
  • orthogonal to oscillation
  • two polysilicon surface-micromachined
    accelerometers with capacitive comb structures
  • measure the Coriolis accelerations for each of
    the masses
  • difference between accelerations is a direct
    measure of the angular yaw rate
  • sum is proportional to the linear acceleration
    along the accelerometers sensitive axis

25
Angular Rate Sensors
Robert Bosch
  • out-of-phase resonant frequency is 2 kHz
  • maximum oscillation amplitude at this frequency
    is 50 µm
  • quality factor of the oscillator at atmospheric
    pressure is 1,200, sufficiently large to excite
    resonance with small Lorentz forces.
  • stimulated oscillation subjects the masses to
    large accelerations reaching approximately 800G.
  • acceleration not perfectly perpendicular to the
    sensitive axis

26
Angular Rate Sensors
Fabrication
  • both bulk and surface micromachining
  • bulk for masses and the surface for
    accelerometers
  • deposit 2.5-µm layer of silicon dioxide
  • epitaxy over the oxide layer grows a
    12-µm-thick layer of heavily doped n-type
    polysilicon.
  • surface-micromachined sensors, polycrystalline

27
Angular Rate Sensors
Fabrication
  • sputter aluminum for electrical interconnects
    and bond pads
  • time etch back side using potassium hydroxide
  • thin central portion of the wafer to 50 µm

28
Angular Rate Sensors
Fabrication
  • two sequential DRIE steps for structural
    elements of the accelerometers and the
    oscillating masses
  • etch sacrificial SiO2 layer using a gas phase
    process to release the polysilicon combs
  • hydrofluoric acid vapor

29
Angular Rate Sensors
Fabrication
  • silicon cap wafer with recess bonded to front
  • glass wafer anodically bonded to back side
  • seals the device
  • final assembly brings together the sensor and
    circuits inside a metal with permanent magnet

30
Angular Rate Sensors
Specifications
  • sensitivity of the device is 18 mV/(º/s)
  • in the range of 100 º/s over 40 to 85ºC
  • temperature dependence causes offset amplitude
    of 0.5 º/s over the specified temperature range

31
Angular Rate Sensors
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