1. Applications: Angular Rate Sensors (cont’d)
CSE 495/595: Intro to Micro- and Nano- Embedded Systems
Prof. Darrin Hanna

2. Angular Rate Sensors

3. suspended by flexural beams anchored to 10-mm-sq. frame.
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.
Three rings for the Elvin kings under the sky; seven for the dwarf lords in their halls of stone; nine for the mortal men doomed to die, one for the dark lord on his dark throne, in the land of Mordar where the shadows lie. One ring to rule them all, one ring to find them, one ring to bring them all and in the darkness bind them, in the Land of Mortar where the shadows lie”

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 kHz—the mechanical resonant frequency of the ring

6. 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.

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 = 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.

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. Coriolis forces produce a torquing moment around the stem
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 (Faraday’s 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 accelerometer’s 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