ME 141B: The MEMS Class Introduction to MEMS and MEMS Design Sumita Pennathur UCSBMEMS Case Study: A Capacitive Accelerometer Sumita Pennathur UCSBOutline • Accelerometer Fundamentals • Analog Devices Accelerometer  History  Structure  Design and Modeling  Fabrication and packaging  Noise and Accuracy 5/25/09 3/45Why Accelerometer? • Measurement of acceleration  Central element of inertial guidance systems  Crash detection for air-bag deployment  Vibrational analysis  Steady images in video recorder 5/25/09 4/45Measurement Choices 5/25/09 5/45 • Two approaches to measuring acceleration  Open loop: Measure change due to acceleration • In absence of force feedback  Closed loop: A disturbance in a position control system • Disturbance = measurand • Controller output (counters disturbances) = system output • Most accelerometers are open-loop • In any case, a proof-mass is held by elastic support to rigid frame • Acceleration of frame causes mass to move relative to frame, bending or stretching support • Detection of accelerations by direct observation, or by detection of deformation of support (piezo)Accelerometer Types • Open vs. Closed loop sensing  Open loop: measure change due to acceleration  Closed loop: a disturbance in a position control system • Quasi-Static vs. resonant sensing  Quasi-static sensing: motion of mass follows time-evolution of applied inertial force without significant retardation or attenuation • Mechanical resonant frequency > frequency of acceleration • Measure displacement due to acceleration  Optical, capacitive, piezo, tunneling  Resonant sensing • Measure change in resonant frequency  Due to position-dependant nonlinear spring • Today: quasi-static capacitive accelerometer 5/25/09 6/45Accelerometer Fundamentals • Displacement and acceleration are coupled together by a fundamental scaling law  A higher resonant frequency implies less displacement • High frequency and low sensitivity  Measuring small accelerations requires floppier structures • High sensitivity and low frequency 5/25/09 7/45Accelerometer Fundamentals • Displacement and acceleration are coupled together by a fundamental scaling law  Scale factor depends only on resonant frequency and not affected by choice of large mass and stiff spring or small mass and compliant spring  Only ratio is important  If one needs to make an accelerometer that responds quickly (high resonant frequency), the amplitde of the position signal to be sensed will be small • Ie. 50 g Analog Devices accelerometer has resonant frequency of 24.7kHz, maximum displacement is 20 nm, but if f is 1kHz, max displacement is 1.2 um 5/25/09 8/45Accelerometer fundamentals • Noise due to damping = Brownian motion noise • Turns into an equivalent acceleration  Ie. 24.7 kHz resonant frequency, and mass of 2.2 x 10^-10 kg, and Q of 5, rms acceleration noise = 4.83 x 10-3 m/sec^2/sqrt(Hz)  Thus can get huge SNR with microaccelerometers 5/25/09 9/45Accelerometer Specifications Initial application arena was Automotive crash sensor Navigation sensors have Tighter specs 5/25/09 10/45Piezoresistive accelerometers • Use piezoresistors to convert stress in suspension beam  change in resistance  change in voltage • First MEMS accelerometer used piezoresistors  Bulk micromachined  Glass capping wafer to damp and stop motion • Simple electronics • Piezoresistos generally less sensitive than capacitive detection 5/25/09 11/45Capacitors for position measurement • Single capacitors  Capacitance is a function of gap or area  Can be nonlinear • Differential capacitors  One capacitor increases while the other decreases  Have virtue of cancelling many effects to first order, providing signal that is zero at base state 5/25/09 12/45Using a differential capacitor • Differential drive creates sense signal proportional to capacitance difference • Gives zero output for zero change • Output linear with gap 5/25/09 13/45Bulk micromachined capacitive accelerometer • Fabrication not reported, but here is my best guess (using nested-mask process) 5/25/09 14/45Thermal accelerometer • Thermal convection accelerometer • Gas is proof mass • Movement of gas under acceleration changed thermal profile 5/25/09 15/45Transimpedance circuits • The simplest type of circuit measures the displacement current in a capacitor using transimpedance amplifier  Transimpedance converts current to voltage  Nulls out parasitic capacitance • If source is DC, measure velocity of motion  V0 ~ dx/dt • But velocity is not really what we want…we want position 5/25/09 16/45Transimpedance Circuits • If source is AC, we can determine capacitance directly • First, must use frequency high enough such that velocity term is negligible • Second, operate above corner frequency of LP filter 5/25/09 17/45Non-inverting op-am circuits • Requires close matching of input capacitances to ground • Now there is no virtual ground and parasitic capacitance appears in output • Most suitable for applications in which transistors are integrated with the capacitive position sensing element 5/25/09 18/45AC Methods Require Demodulation • For AC methods, output signal is a high frequency sinusoid (carrier) multiplied by a low frequency signal • This is an amplitude-modulated (AM) signal • We want to retrieve the low frequency component  Peak detector  Synchronous demodulator 5/25/09 19/45Synchronous Demodulation • Use a nonlinear circuit to multiply V0 by an in-phase sinusoid • This modulates to baseband • Relative phase is important 5/25/09 20/45Signal-to-noise issues • To get a big signal, use a big voltage -BUT- • Voltage creates a force that can modify the state of the mechanical system (analogous to the self-heating problem in resistance measurement) • Noise floor minimum often set by LPF bandwidth • But amplifier noise will often dominate 5/25/09 21/45Analog Devices Accelerometer • Genesis: an ADI engineer heard about forming mechanical sensors on silicon • Market