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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 • • • • • 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 • 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