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

Angular Rate Sensors

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”

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 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

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

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

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

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