Applications: Pressure Sensors, Mass Flow Sensors, and Accelerometers CSE 495/595: Intro to Micro- and Nano- Embedded Systems Prof. Darrin Hanna

From last time… Differential pressure sensor Absolute pressure sensor

From last time… High temp. pressure sensor using Silicon-on- insulator (SOI) processes

Mass flow sensors The flow of gas over the surface of a heated element produces convective heat loss at a rate proportional to mass flow.

Mass flow sensors Deposit a thin layer of silicon nitride approximately 0.5 µm in thickness Deposit & pattern thin-film heaters and sense elements chemical vapor deposition of a heavily doped layer of polysilicon Deposit & pattern an insulating layer to protect heating & sense elements silicon nitride again but keep contacts exposed Etch silicon in KOH anisotropic etch solution to form the deep cavity

Mass flow sensors Two Wheatstone bridges The 2 heating resistors form the two legs of the first bridge The 2 sensing resistors form the two legs of the second bridge

Mass flow sensors Heating Sensing For equilibrium R1/R2 = R4/R3 In either case two of the bridge resistor pairs are fixed and equal such as R2 and R3. R2 = R3 = R B R 1 > R 4  + Pol R 4 > R 1  - Pol Flow direction

Mass flow sensors Sense Heat Sense 12 Heat Sense 12 Heat2 – some heat, H, transferred to gas Heat1 – very little heat transferred from H Sense1 – some heat transferred from H

Mass flow sensors Sense Heat Sense 12 Heat2 – some heat, H, transferred to gas Heat1 – very little heat transferred from H Sense1 – some heat transferred from H Sense Heat Sense 12 Heat Sense 12

Mass flow sensors Sense Heat Sense 12 Heat1 – some heat, H, transferred to gas Heat2 – very little heat transferred from H Sense2 – some heat transferred from H Sense Heat Sense 12 Heat Sense 12

Mass flow sensors 0 – 1000 std cubic cm 75 mV max output time < 3 ms power ~ 30 mW

Acceleration sensors

The primary specifications of an accelerometer are full-scale range (often given in Gs <9.81 m/s 2 ) sensitivity (V/G) resolution (G) bandwidth (Hz) cross-axis sensitivity immunity to shock

Acceleration sensors Airbag crash sensing full range of ±50G bandwidth of about one kilohertz Measuring engine knock or vibration range of about 1G small accelerations (<100 µG) large bandwidth (>10 kHz) Modern cardiac pacemakers multi-axis accelerometers range of ±2G bandwidth of less than 50 Hz require extremely low power consumption Military applications range of > 1,000G

Acceleration sensors F = m∙a

Acceleration sensors Q and Bandwidth The quality factor (Q) is a measure of the rate at which a vibrating system dissipates its energy into heat A higher Q indicates a lower rate of heat dissipation When the system is driven, its resonant behavior depends strongly on Q Q factor is defined as the number of oscillations required for a freely oscillating system's energy to fall off to 1/535 of its original energy, where 535 = e2π, Resonant frequency Bandwidth

Acceleration sensors Q and Bandwidth Bandwidth is defined as the "full width at half maximum". width in frequency where the energy falls to half of its peak value, dB levelRatio −30 dB1/1000 −20 dB1/100 −10 dB1/10 −3 dB0.5 (approx.) 3 dB2 (approx.) 10 dB10 20 dB dB1000 Voltage and Current is 20 Power and Intensity is 10

Acceleration sensors Q and Bandwidth, Example: Q of a radio receiver A radio receiver used in the FM band needs to be tuned in to within about 0.1 MHz for signals at about 100 MHz. What is its Q? Ans: Q=fres/FWHM=1000. This is an extremely high Q compared to most mechanical systems.

Acceleration sensors Q and Bandwidth, Example: Decay of a saxophone tone If a typical saxophone setup has a Q of about 10, how long will it take for a 100-Hz tone played on a baritone saxophone to die down by a factor of 535 in energy, after the player suddenly stops blowing? Ans: A Q of 10 means that it takes 10 cycles for the vibrations to die down in energy by a factor of 535. Ten cycles at a frequency of 100 Hz would correspond to a time of 0.1 seconds, which is not very long. This is why a saxophone note doesn't “ring” like a note played on a piano or an electric guitar.

Acceleration sensors Q and Bandwidth, Resonant frequency Bandwidth The lower the bandwidth, the higher Q and vice versa The higher the bandwidth, the lower Q and vice versa

Acceleration sensors F = m∙a Brownian noise The change in noise with time is random whereas white noise is random noise Brownian noise is the integral of white noise power amplitude Freq. time

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer Inertial mass sits inside a frame suspended by the spring Two thin boron-doped piezoresistive elements Wheatstone bridge configuration Piezoresistors are only 0.6 µm thick and 4.2 µm long very sensitive Inertial mass Output in response to 1G is 25mV for a Wheatstone bridge excitation of 10V.

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer 6,000G for the inertial mass to touch the frame The device can survive shocks in excess of 10,000G Holes in inertial mass reduce weight and provide a high resonant frequency of 28 kHz

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer {110} Silicon for center {111} plane is perpendicular to the surface, therefore an anisotropic wet etchant can be used

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer Boron implantation and diffusion to form highly doped p-type piezoresistors the piezoresistors are aligned along a dir A silicon oxide or silicon nitride layer masks the silicon in the form of the inertial mass and hinge during the subsequent anisotropic etch in EDP

Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer Deposit and pattern aluminum electrical contacts Pattern and etch shallow recesses in base & lid substrates Bond together using adhesive

Acceleration sensors Capacitive Bulk Micromachined Accelerometer

Acceleration sensors Capacitive Bulk Micromachined Accelerometer Measuring range from ±0.5G to ±12G Electronic circuits sense changes in capacitance using voltages Bandwidth is up to 400 Hz for the ±12G accelerometer Cross-axis sensitivity is less than 5% Shock immunity is 20,000G

Acceleration sensors Capacitive Bulk Micromachined Accelerometer Timed etching

Acceleration sensors Capacitive Bulk Micromachined Accelerometer Contacts On side of wafer  Post- processed

Acceleration sensors Capacitive Surface Micromachined Accelerometer

Acceleration sensors Capacitive Surface Micromachined Accelerometer The overall capacitance is small, typically on the order of 100 fF (1 fF = F) ADXL105 (programmable at either ±1G or ±5G) the change in capacitance in response to 1G is 100 aF (1 aF = F). Two-phase oscillator 0 DC offset

Acceleration sensors Capacitive Surface Micromachined Accelerometer Range from ±1G (ADXL 105) up to ±100G (ADXL 190) Bandwidth (typically, 1 to 6 kHz) The small change in capacitance and the relatively small mass combine to give a noise floor that is relatively large ADXL105 - the mass is approximately 0.3 µg and noise floor is dominated by Brownian noise Bulk-micromachined sensor can exceed 100 µg

Acceleration sensors Capacitive Surface Micromachined Accelerometer Open loop measurement Voltage generated at sense contacts Close loop measurement Applying a large-amplitude voltage at low frequency— below the natural frequency of the sensor—between the two plates of a capacitor gives rise to an electrostatic force that tends to pull the two plates together.

Acceleration sensors Capacitive Deep-Etched Micromachined Accelerometer

Acceleration sensors Capacitive Deep-Etched Micromachined Accelerometer Two sets of stationary fingers attached directly to the substrate form the capacitive half bridge. Structures 50 to 100 µm deep sensor gains a larger inertial mass, up to 100 µg, larger capacitance, up to 5 pF. Larger mass reduces Brownian noise and increases resolution.

More accurate sensor model Experimentally determined that the biosensor behaves like a capacitor in parallel with a resistor Improving the Circuit Design an accurate sensing circuit + Wheatstone Bridge + Differential Amplifier = Sensitivity (1nF ~ 3mV) Wheatstone Bridge – Based on sensor model and optimized using PSPICE 1.0Vpp 3.5kHz Wheatstone Bridge + Differential Amplifier 10x Gain 1.0Vpp 3.5kHz Measuring Capacitance

Variable Capacitor (0-2.1 uF) Variable Resistor (0-210 Ohms) Differential Amplifier (10x Gain) Sensor Attach Point Measuring Capacitance

Variable Capacitor (0-2.1 uF) Variable Resistor (0-210 Ohms) Differential Amplifier (10x Gain) Sensor Attach Point Measuring Capacitance