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

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

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

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 For equilibrium R1/R2 = R4/R3 Heating Sensing  (

R

1

R R

1

B



R B

 )(

R R B

4

R

4 

R B

)

V in

 (

R

1 In either case two of the bridge resistor pairs are fixed and equal such as R2 and R3. R2 = R3 = R B

R B

 

R B R

1 )( 

R

4

R

4  

R B

)

V in

R 1 R 4 > R > R 4 1   + Pol - Pol Flow direction

Mass flow sensors 1 2 1 2 Heat2 – some heat, H, transferred to gas Heat1 – very little heat transferred from H Sense1 – some heat transferred from H

Mass flow sensors Heat2 – some heat, H, transferred to gas Heat1 – very little heat transferred from H Sense1 – some heat transferred from H 1 2 1 2 1 2

Mass flow sensors Heat1 – some heat, H, transferred to gas Heat2 – very little heat transferred from H Sense2 – some heat transferred from H 1 2 1 2 1 2

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

Acceleration sensors

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 =

e

2π 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 level Ratio

−30 dB −20 dB −10 dB −3 dB 1/1000 1/100 1/10 0.5 (approx.) Voltage and Current is 20 Power and Intensity is 10 3 dB 2 (approx.) 10 dB 10 20 dB 30 dB 100 1000

Acceleration sensors Q and Bandwidth

Example: Q of a radio receiver

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 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 power amplitude Freq.

time 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 

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 <111> 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

Timed etching Acceleration sensors Capacitive Bulk Micromachined Accelerometer

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

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 = 10-15 F) • ADXL105 (programmable at either ±1G or ±5G) • the change in capacitance in response to 1G is 100 aF • (1 aF = 10-18 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.

• Design an accurate sensing circuit • + Wheatstone Bridge • + Differential Amplifier • = Sensitivity (1nF ~ 3mV)

More accurate sensor model optimized using PSPICE 10x Gain Experimentally determined that the biosensor behaves like a capacitor in parallel with a resistor

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

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