Transcript One Ring to bring them all and in the darkness bind them

Applications:
Angular Rate Sensors (cont’d)
CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Prof. Darrin Hanna
Angular Rate Sensors
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.
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
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 kHz—the mechanical resonant frequency of the ring
Angular Rate Sensors
• Sensing mechanism
• measures the voltage induced around one or more loops
• Faraday’s 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.
Angular Rate Sensors
Fabrication
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
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
Angular Rate Sensors
Specifications
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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
Angular Rate Sensors
Daimler Benz
Angular Rate Sensors
Daimler Benz
• Coriolis Effect
• deflection of a moving object
in a rotating frame of reference
• Coriolis acceleration
• a = 2Ω x v
• V is the velocity of the particle in the rotating system
• ω 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.
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
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
Angular Rate Sensors
Fabrication
Uses SOI processes
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
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
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
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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.
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.
Angular Rate Sensors
Robert Bosch
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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
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 (Faraday’s
Law)
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
accelerometer’s sensitive axis
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
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
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
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
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
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
Angular Rate Sensors