MEMSBased Aerosol Monitor Employing GHz Thin Film Bulk Acoustic Wave Resonator FBAR Mass Sensor

1
MEMS-Based Aerosol Monitor Employing GHz Thin
Film Bulk Acoustic Wave Resonator (FBAR) Mass
Sensor
Justin P. Black, Professor Richard M. White,
BSAC Drs. Mike G. Apte and Lara A. Gundel, LBNL
Environmental Energy Technologies Division Dr.
Rossana Cambie, LBNL Engineering Division George
Stern and Zhuo Zhang, Interns, LBNL
particle laden air flow
heater

• Outline
• Introduction, Problem Definition
• Overview of Principal of Operation
• Fabrication and Characterization of Componens
• Environmental Chamber and Field Tests
• Summary

electronics PCB
FBAR, CMOS
2
Research Objective

• Portable, compact monitor for airborne
  particulate matter (PM)
• based on principles established at LBNL, since
  2001
• thermophoretic precipitation of particles on mass
  monitor
• size-selective inlet based on competition between
  viscous flow and gravity
• mass measurement using thin-film bulk acoustic
  wave resonator (FBAR) / Pierce oscillator
• optical discrimination of PM composition using UV
  / near-IR LEDs and photodetector
• compatible with PM Federal Reference Methods
• commercializable
• Sponsored by California Air Resources Board and
  California Energy Commission

3
Problem Definition
Particulate Matter (PM)

• Airborne particulate matter (PM), or aerosols,
  are a major public health issue
• Estimated 65,000 excess U.S. deaths per year
• Mechanisms not yet established exposure not well
  known
• No affordable population-based exposure
  assessment tools
• Existing equipment bulky and expensive
• Energy link PM largely due to combustion
• Broad biodefense applications (weaponized
  aerosols)
• Nanoparticles

5 µm
Field Test Instruments

• Particulate matter (PM) / aerosols
• mixture of (in)organic solids liquids, whose
  size ranges from a few nm to tens of µm, 1 - 2.5
  µm breakpoint
• smaller particles originate from combustion and
  recondensed organic and metal vapors
• larger particles originate from plants or the
  break up of larger, solid particles

http//www.esemir.it/partic_i.htm
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Vertical Size-Selective Inlet

• Opposing gravitational and vertical flow drag
  forces
• Practical at low sampling rates (lt20 mL per
  minute)
• Requires highly sensitive mass detector

Empirical results from OPC (2.8 L min-1) with 31
cm diameter inlet
slide by Mike Apte
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Thermophoretic Particulate Matter (PM) Deposition
Cold side

• Thermophoretic (TP) force FT ?T
• Particle impact velocity 0.9 m/s
• Sampler air velocity 0.003 m/s
• Adherence Van der Waals forces
• TP collection very efficient for sub-micron and
  nanoparticles
• Can approach 100 collection efficiency

FT
Molecule velocity
?T
Hot side
F. Zheng, Thermophoresis of spherical and
non-spherical particles a review of theories and
experiments, Advances in Colloid and Interface
Science, vol. 97, pp. 255-278, 2002.
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SEM of Quadrant Heater Array
100 µm
100 µm
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Quartz / Polysilicon Thermophoretic Heater
Thermophoretic force into page
500 ?m
100 µm
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Verification of Heater Operation - ETS Deposited
Onto Al Chip
Before ETS
With ETS
ETS particles
Al surface
2 µm
2 µm
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Optics for Source Discrimination

• Measure change in reflected light intensity
• Example discriminate diesel emissions from
  environmental tobacco smoke (ETS)
• Ultraviolet (UV) Near Infrared (NIR) LEDs
  (light-emitting diodes) for real-time sensors
• Photodiode detector

D Absorbance D Concentration
drawing by Mike Apte
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Optical Module
a)
b)
LEDs
apertures
heater wires
c)
d)
before
before
after
after
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Acoustic Wave Devices for Mass Measurement
Ballantine, D.S., et. al., Acoustic Wave
Sensors Theory, Design, and Physico-Chemical
Applications, Academic Press, 1997. P. Kobrin,
C. Seabury, A. Harker, and R. OToole, Thin-film
resonant chemical sensor with resonant acoustic
isolator, US Patent 5,936,150, 1999.
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FBAR Process Flow Top view and Cross Sections
Silicon Nitride Isolation and Pt Bottom
Electrode LPCVD 800 nm low-stress silicon,
evaporate and pattern 200 nm of Pt.
Top view
Cross sections
A
A
B
B
SiN
Pt
AlN
Al
Si
13
FBAR Process Flow Top view and Cross Sections
AlN Deposition and Ground Plane Vias Reactive
Sputter 2 ?m AlN. Etch AlN vias to expose Pt
ground plane.
A
A
B
B
SiN
Pt
AlN
Al
Si
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FBAR Process Flow Top View and Cross Sections
Aluminum Top Electrode, AlN Structural Layer
Sputter and pattern 200 nm of aluminum. Etch AlN
/ SiN to define resonators.
A
A
B
B
SiN
Pt
AlN
Al
Si
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FBAR Process Flow
XeF2 etch Si bulk to release FBAR membrane
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FBAR Device - Electrical Characterization
current design
1st generation
100 µm
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Interferometric Measurements of ZnO FBAR Motion

• BSAC researcher Xuchun (Bert) Liu took phase and
  amplitude measurements with his interferometer
• Two periods of motion at series resonance
• fs 1.077056 GHz
• Pin 0 dBm
• Amplitude 0.05 nm

Au
Active area 100 ?m x 100 ?m
Al
piezo film
undeformed
extension
piezo film
compression
piezo film
silicon nitride
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FBAR Mass Loading Proof of Concept
droplet

• Dispensed 28 pL droplets of water with dissolved
  tobacco smoke extract onto active area with
  print-head
• Fluid evaporates almost instantaneously leaving
  dissolved mass. Monitor shift in fs with network
  analyzer
• Could not extract mass sensitivity due to
  contamination of LBNLs DI water

piezo film
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One-Dimensional FBAR Model
Stress and displacement continuous at the
boundaries between layers Zero stress on free
surfaces
z
air
PM film
Linear Momentum
Al
Gauss' Law in Piezoelectric
AlN
F
incident
reflected
0
Pt
x
Electromechanical Constitutive Eqs.
air
Wave Equation
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FBAR Impedance from Cascaded ABCD Matrices
Stress Voltage Velocity Current
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Equivalent Circuit of FBAR
Between fs and fp the crystal is inductive with
Lx ideally varying from 0 to ?
Unloaded FBAR
fp
fs
Rx
Lx
Co
Cx
ZL
Loading by Al, Pt, and film
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Impedance of Unloaded Square FBAR (df 0)
theoretical
measured
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Impedance of Unloaded Polygon FBAR (df 0)
theoretical
measured
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Pierce Oscillator Analysis
M2
M3
M1
R3
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Characterization of FBAR Pierce Oscillator
3.5 mW at 2.5 V supply
?(f 10 kHz) -102 dBc / Hz
S.G. Burns and R.S. Ketcham, Fundamental-mode
Pierce oscillators utilizing bulk-acoustic-wave
resonators in the 250-300 MHz range, MTT, vol.
MTT-32, Dec. 1984. B.P. Otis and J. M. Rabaey, A
300- uW 1.9-GHz CMOS oscillator utilizing
micromachined resonators, JSSC, vol.38, 2003.
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Current Commutating Mixer - Active and Reference
FBARs
Mix active (mass sensing) and reference FBAR
oscillator outputs to achieve low-frequency
output and reduce noise associated with
temperature fluctuations
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Solidworks Crossection of Component Stack
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Perspective View of Component Stack
Optical module
Heater
Flow channel
100 µm
Air flow
FBAR / CMOS
100 µm
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Solidworks Rendering of Packaged Module
Drawing by Rossana Cambie
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SMA connector
air outlet
heater
connection for size selective inlet
heater power cable
silicone
supply switch
supply connections
PCB
size selective inlet
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Real-time FBAR Response - One Cigarette

• normalized for -25 ppm / ºC (40 kHz / ºC at 1.6
  GHz) temperature coefficient
• 0.0025 kHz / min per µg / min

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Initial FBAR Response to Environmental Tobacco
Smoke (ETS)
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Real-time FBAR Response - ¼, ½, 1 Cigarette
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Summary of Environmental Chamber and Field Study

• LOD of 18 µg / m3, satisfies EPAs Federal
  Reference Method

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Response to Fresh Diesel Particles
Positive frequency shift in the presence of
diesel PM
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Analysis of Oscillator Loop Gain with Diesel
Particles
Decrease in resistance across FBAR terminals
produces positive frequency shift
Rdiesel 100 k?
Rdiesel 5 k?
Rdiesel 3 k?
Buffer
Vout
C2
C1
Resistance function of diesel film thickness
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Field Test in Berkeley Residence
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Field Test in Berkeley Residence (cont)
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Field Test in Berkeley Residence (cont)
QCM Cascade Impactor
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Existing Instruments for Measurement of
Particulate Matter (PM)
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Species Discrimination of by Thermal Spectroscopy

• At 50 ºC, oscillation in center frequency and
  deterioration of oscillator output spectrum
• Passing through glass transition temperature of
  ETS
• (Complex) stiffness change by several orders of
  magnitude

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Film Acoustic Resonance due To Temperature Shift

• Normally film moves synchronously with resonator
  with small strain gradient (microbalance mode)
• At a film thickness of ?/4, film 180 degrees out
  of phase with FBAR-film interface
• Energy storage (frequency oscillator) and
  dissipation (loss)

film resonance
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Summary

• Developed functional FBAR-based aerosol monitor
• GHz FBARs with Q gt 2000 and Rx 2 ?
• quartz / polysilicon thermophoretic precipitator
• picogram sensitivity
• module weighs 114 g, approximate volume 245 cm3,
  power consumption 100 mW

Future Work

• Sensitivity in the femtogram range
• reduce temperature (lt 1 ppm / ºC) coefficient
  with additional Si layer
• use resonator with preloaded ? / 4 film
• Further miniaturization
• active sensor chip area 6 mm x 6 mm x 2 mm
• thermophoretic heater constructed from bondwires
• Airborne pathogens and nanoparticles

10 kHz shift per fg