High-overtone Bulk Acoustic Resonator (HBAR) as passive sensor: towards microwave wireless

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Bulk Acoustic Resonator (HBAR) as passive sensor:

towards microwave

wireless interrogation J.M Friedt& al.

RADAR & sensor basics HBAR Increasing operating frequency Conclusion

High-overtone Bulk Acoustic Resonator (HBAR) as passive sensor: towards microwave wireless

interrogation

J.-M Friedt¹, N. Chr´etien¹,

T. Baron², ´E. Lebrasseur², G. Martin², S. Ballandras^1,2

1 SENSeOR, Besan¸con, France

2 FEMTO-ST Time & Frequency, Besan¸con, France Emails: {jmfriedt,ballandr}@femto-st.fr slides available athttp://jmfriedt.free.fr

November 23, 2012

Nov. 21 2012 – eWise

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towards microwave

Introduction

Context: acoustic wave transducers used as passive, wireless sensors Objective: use of commercially available RADAR for probing cooperative target acting as sensor

• Alternative strategy to energy harvesting: no energy at all !

• Passive transducer acts as sensor remotely characterized

• The sensor itself is tiny (<5×5 mm²) but the antenna is huge

• Analog transducer does not provide identification or anticollision capability

⇒increase operating range to the microwave range forreduced antenna sizeandspatial multiplexingthanks to directive beams with modest antenna dimensions on the reader.

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towards microwave

RADAR & sensor basics

HBAR Increasing operating frequency Conclusion

Basics of RADAR

• bistatic (physically separated emitter and receiver) or monostatic configurations:

isolation defines rage

• pulsed (electromagnetic pulse)^FFT⇔ frequency sweep (FMCW)

• echos due to electromagnetic impedance variations (permittivityε_r and conductivityσ)

v= c

s

ε_r 2

q

1 +_ε^σ2ω²²+ 1

• provides both magnitude and phase informations on the returned pulse

• typical frequency range: 50 MHz-50 GHz^a

aH. Stockman,Communication by means of reflected power, Proc. I.R.E36 pp.1196-1204 (1948)

target

T T

2T

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Basics of RADAR

• bistatic (physically separated emitter and receiver) or monostatic configurations:

isolation defines rage

• pulsed (electromagnetic pulse)^FFT⇔ frequency sweep (FMCW)

• echos due to electromagnetic impedance variations (permittivityε_r and conductivityσ)

v= c

s

ε_r 2

q

1 +_ε^σ2ω²²+ 1

• provides both magnitude and phase informations on the returned pulse

• typical frequency range: 50 MHz-50 GHz^a

aH. Stockman,Communication by means of reflected power, Proc. I.R.E36 pp.1196-1204 (1948)

t

f 2T

target

T T

t f

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RADAR example (2)

HF-VHF RADAR is long range (even over the horizon), but requires excessive antenna dimensions for industrial applications

(λ_m= 300/f_MHz ⇒λ/4 = 1 m at 75 MHz).

Objective: electromagnetic scanvengers, here calledcooperative target

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towards microwave

Basics of Surface Acoustic Wave (SAW) delay lines

• acoustic = propagation of a mechanical wave on a substrate

• most efficient way of converting electromagnetic (EM) to mechanical: piezoelectric substrate + interdigitated transducers

• identification + sensor

• physical quantity measurement function of acoustic velocity

• incoming EM pulse generates mechanical pulse which returns as EM with atime delayfunction of physical quantity (temperature, stress, pressure ...)

• high electromechanical coupling coefficient (LNO)

• mirror = patterned electrodes

• time delay between incoming pulse and reflection

= measurement

• typical velocity: 1500-5000 m/s for most materials

• typical delays: 1-5µs (3µs at 3000 m/s⇒ 4.5 mm path)

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towards microwave

SAW delay line as RADAR cooperative target

Acoustic transducer as RADAR cooperative target:

• complement the passive interface monitoring with sensor interrogation

• linear conversion process from EM to mechanical: no threshold voltage (cf diodes in Si based RFID)

distance to surface (m)

time (s), radar speed ∼ 15 k m /h 20

40

60

80

100

120

140

0 50 100 150 200 250 300 350 400

emitter receiver

Rock

Ice raster scan

R=

√ εr(ice)−√

εr(rock)

√εr(ice)+

√ εr(rock)

Reflection coefficient from Fresnel eq.

50 100 150

−20

−15

−10

−5 0

frequency (MHz)

|S11|

0 0.5 1 1.5 2 2.5

−80

−60

−40

−20

time (us)

|S11|

emitted RADAR pulse spectrum

delay line S₁₁

RADAR echo time domain analysis

Delay line dimensions

4000

2400 31003700 780 SAW

GPR emitter

GPR receiver

RF pulse SAW reflections

Challenge: at 5000 m/s, a sensor operating at 5 GHz would require 250 nm lithography (with λresolution)

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Link budget for delay lines

• RADAR illumination of point-like target: decay as 1/d⁴

• Free Space Propagation Loss (FSPL) 10×log₁₀ λ²

4π ×λ²

4π× 1

(4πd²)²

!

= 10 log₁₀ λ⁴

(4π)⁴d⁴

• Considering we know the range at ice-rock interface and reflection coeffient

εice−εrock

εice+εrock

2

'19 dB

• FSPL_ice−rock+IL_ice−rock =FSPL_SAW +IL_SAW

⇒dSAW =d_ice−rock×10^(IL^ice−rock^−IL^SAW^)/40'40 m assumingd_ice−rock = 100 m, consistent with SNR of a 5 m deep-measurement¹

1J.-M Friedt, T. R´etornaz, S. Alzuaga, T. Baron, G. Martin, T. Laroche, S.

Ballandras, M. Griselin & J.-P. Simonnet,Surface Acoustic Wave Devices as Passive Buried SensorsJ. Appl. Phys. 109(3), pp. 034905 (2011)

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towards microwave

High-overtone Bulk Acoustic Resonator (HBAR)

1 The acoustic wave no longer propagates at the air-crystal interface but in the bulk of the crystal

2 Operating frequencies are defined by layer thicknesses rather than lithography of electrodes

3 Poly-crystalline active layer (AlN, ZnO) or single-crystal (lithium niobate): high coupling

4 Low loss propagation substrate exhibiting appropriate

sensitivity to the measured quantity

substrate loss low

LNO...) sapphire,

(Si, qtz piezo

Typical dimensions: 5-10 µm thick piezo, 300-500 µm thick substrate, 2×2 mm² chip

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High-overtone Bulk Acoustic Resonator (HBAR)

-14 -12 -10 -8 -6 -4 -2 0

5e+07 1e+08 1.5e+08 2e+08 2.5e+08 3e+08 3.5e+08 4e+08 4.5e+08 5e+08

|S11| (dB)

f (Hz)

Frequency comb from 50 to 500 MHz

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High-overtone Bulk Acoustic Resonator (HBAR)

-400 -200 0 200 400 600

0 5e-07 1e-06 1.5e-06 2e-06 2.5e-06

returned power (a.u.)

time (s)

Time domain echos, 0.5-2.2µs

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HBAR measurement strategy

• HBAR spectrum is a comb (in time or frequency domain) of modes

• Frequency domain (resonance identification) or time domain (pulse delay) Digital signal post-processing,no modificationof RADAR hardware

• Acoustic velocity change with physical property

• no need to changeRADAR hardware, only signal post-processing step

Frequency domain caracterisation: incompatible with FMCW RADAR (sweep rateQ/π periods) and pulse mode (unable to recover an accurate frequency)

⇒time domain approach, search for time delay between returned echos (magnitude & phase)

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Experimental demonstration (VHF)

Use of cross correlation to accuractely extract the time delay between echos (= acoustic velocity)

-2e+07 -1.5e+07 -1e+07 -5e+06 0 5e+06 1e+07 1.5e+07 2e+07

0 50 100 150 200 250 300 350 400

cross correlation

time (a.u.)

trace 69 trace 45 -6000

-4000 -2000 0 2000 4000 6000

2e-07 4e-07 6e-07 8e-07 1e-06 1.2e-06 1.4e-06 1.6e-06

returned power (a.u.)

time (s)

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Experimental demonstration (VHF)

Temperature measurement

20 30 40 50 60 70 80 90 100

100 200 300 400 500 600 700 800 900

Pt100 probe temperature (degC)

time (s) 167.8

167.9 168 168.1 168.2 168.3 168.4 168.5

0 100 200 300 400 500 600 700 800 900

cross correlation maximum position (pixel) time (s)

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Experimental demonstration (VHF)

Temperature measurement

167.8 167.9 168 168.1 168.2 168.3 168.4 168.5

20 30 40 50 60 70 80 90 100

cross correlation maximum position (pixel)

temperature (degC) 5.738e−3 pixel/degC

Port1 Port2 HBAR electrode layout

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towards microwave

Towards microwaves: mixing

Using a VHF transducer at microwave frequencies: use a diode next to the sensor as AM demodulator

8 GHz antenna

> 2 GHz

8−0.434/2 8+0.434/28 GHz

434+/−1 MHz oscilloscope

434/2 MHz 8 GHz

LO IF

RF

30 kHz

SMA connector 434 MHz dipole antenna dual resonator sensor

power

frequency

Strategy compatible withelectronic beam steeringon the emission (Space-division multiple access) and omnidirectional receiving antenna

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towards microwave

Towards microwaves: mixing

8 GHz source

434/2 MHz source

mixer returned power @ 434 MHz

switch @ 30 kHz

RF switch

434 MHz antenna 8 GHz antenna

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towards microwave

Towards microwaves: mixing

off resonance at resonance

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towards microwave

Towards microwaves: baseband

• However, adding a rectifying diode brings back the drawback of RFID

• HBAR can reach the microwave frequency range ... if appropriately designed

SU8 LNO

substrate

In this example, the SU8 assembling glue acts as a strong acoustic reflector and generates modes up to 4 GHz

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towards microwave

Conclusion

• use of a widely available tool (RADAR) for probing sensors (“cooperative targets”)

• piezoelectric-based (linear) transducers for improved interrogation range

• signal processing for (time-based) delay line: temperature

• ⇒acoustic delay lines for tagging or sensor applications

• ⇒HBAR for multimode (multiple RADAR instrument) &

time-domain interrogation

Apassivesensor solves the issue oflocalenergy harvesting, and moves the energy requirement to the interrogating RADAR ⇒best suited in environments where sensor maintenance is impossible once installed (buried in plastic, concrete, soil ...)