Applications: Pressure Sensors, Mass Flow Sensors, and Accelerometers

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

2. From last time… Differential pressure sensor Absolute pressure sensor

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

4. 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.

5. 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

6. 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

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

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

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

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

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

12. Acceleration sensors

13. Acceleration sensors • The primary specifications of an accelerometer are • full-scale range (often given in Gs <9.81 m/s2) • sensitivity (V/G) • resolution (G) • bandwidth (Hz) • cross-axis sensitivity • immunity to shock

14. 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

15. Acceleration sensors F = m∙a

16. 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

17. 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         , Voltage and Current is 20 Power and Intensity is 10

18. 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.         ,

19. 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.         ,

20. 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

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

22. Acceleration sensors Piezoresistive Bulk Micromachined Accelerometer

23. 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.

24. 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

25. 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

26. 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

27. 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

28. Acceleration sensors Capacitive Bulk Micromachined Accelerometer

29. 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

30. Acceleration sensors Capacitive Bulk Micromachined Accelerometer Timed etching

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

32. Acceleration sensors Capacitive Surface Micromachined Accelerometer

33. 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

34. 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

35. 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.

36. Acceleration sensors Capacitive Deep-Etched Micromachined Accelerometer

37. 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.

38. Improving the Circuit Wheatstone Bridge + Differential Amplifier More accurate sensor model Wheatstone Bridge – Based on sensor model and optimized using PSPICE 10x Gain 1.0Vpp3.5kHz Experimentally determined that the biosensor behaves like a capacitor in parallel with a resistor 1.0Vpp3.5kHz Measuring Capacitance • Design an accurate sensing circuit • + Wheatstone Bridge • + Differential Amplifier • = Sensitivity (1nF ~ 3mV)

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

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