FIRAT: A new AFM probe for fast
imaging, material characterization, and
single molecular mechanics
F. Levent Degertekin
G.W. Woodruff School of Mechanical
Engineering
Georgia Institute of Technology
Funding sources: NSF CAREER award, NIH
Outline
 Atomic Force microscopy (AFM) background
 Force-sensing Integrated Readout and Active Tip (FIRAT)
probe structure for AFM
 Integration to commercial AFM system
 Fast imaging with FIRAT
 Experimental setup and initial results
 Quantitative surface characterization with FIRAT
 Time resolved interaction force (TRIF) mode operation
 FIRAT structures with improved dynamics and sensitivity
 Application to biomolecular measurements
 Conclusion and future work
Atomic Force Microscope
• Uses microcantilevers as
force sensors
• Sharp tip determines image
resolution
• Optical lever detection used
to measure cantilever
deflection
• Piezo tube moves sample or
cantilever in x-y-z
• Controller keeps the
cantilever deflection or
oscillation constant while
scanning in X-Y plane
2µm
Appl. Nanostructures
• AFM is one of the most widely used tools in nanotechnology
• Topographic and functional imaging of nanoscale structures
• Metrology of IC structures, hard disk drive surface inspection
• Measurement of biomolecular forces, material properties
Some Limitations of AFM
 Imaging Speed
PHOTODETECTOR
 Bulky piezoactuators are slow
 Integrated piezo or magnetic
actuators can be complex
 Material characterization
 Slope detection leads to tip
rotation
 Point force measurements are
somewhat slow for simultaneous
topography imaging
 High Q of cantilever masks tipsample interaction forces during
tapping mode imaging
 Array implementation
 Parallel biomolecular
measurements
 Parallel imaging, nanofabrication
LASER
X, Y
Z
PIEZOTUBE
CANTILEVER
Vibration spectrum of an AFM cantilever
FIRAT Probe Structure
 New AFM probe structure: Sharp tip on micromachined membrane/beam
 Integrated optical interferometer for tip displacement detection
 Phase sensitive grating
 Low-noise, robust interferometer
 Integrated electrostatic actuator for fast tip actuation
 Imaging speed limited by membrane dynamics (fo ~ up to 10MHz)
 Force-sensing Integrated Readout and Active Tip  FIRAT
Integrated Electrostatic
actuator input
Quartz substrate
Micromachined membrane
and diffraction grating
(bottom electrode)
1st diffraction
order
Photodetector
Tip displacement
signal
incident
beam
Reflected
diffraction orders
Diffraction Based Optical Displacement Detection
reflector
d =l/2
dg
d =l/4
substrate
reflection
diffraction
Gaussian aperture w0=9µm, λ=850nm, 2µm grating period
50
d = l/2
0
y (m)
 Non-moving diffraction
grating on transparent
substrate
 Backside illumination:
Reflected diffraction
pattern
 Reflector displacement
changes the intensity of
diffraction orders
 Photodetectors at fixed
locations are used to
detect intensity variations
 Interferometric sensitivity
achieved in a small
volume
- 50
- 200
- 100
0
100
200
50
d = l/4
0
- 50
- 200
- 100
0
100
200
50
d = l/8
0
- 50
- 200
- 100
0
100
x (m)
Normalized intensity
0
0.5
1
200
Displacement Sensitivity
i=R
I 0  I1 
4
x = RIin
x
d 0
l0
1
I0
I1
0.8
Normalized intensity
 Reflection order intensities
I0 α cos2(2πd/λ), I1 α sin2(2πd/λ)
Output for small deflections Δx + d0
For d0=nλ/8 (n odd)
0.6
0.4
0.2
• Shot noise limit  MDD: √ qIR/(4  IR/λ)
• ~3x10-5Å/√Hz with 450µW laser power
on detector is demonstrated
0
0
0.1
0.2
0.3
0.4
0.5
Gap thickness (d) (  m)
 Several diffraction orders can be detected to reduce laser intensity
noise
 Electrostatic actuation is used to optimize sensitivity
 Several methods have been devised to address range limitation
0.6
0.7
Device Fabrication
Surface Micromachining On Quartz Substrate
Devices for use in air:
 Aluminum membrane (0.7-1µm thick)
 Aluminum grating (0.15µm)
 PR sacrificial layer (1.5µm)
Membrane array (100µm diam.)
Quartz
PECVD
oxide
Photoresist
Aluminum
Back side
Originally developed as microphone arrays!!
Device Fabrication - Tips
 Focused Ion Beam assisted deposition on membranes
 Platinum and Tungsten tips with 50-70nm tip radius
 Image resolution, attractive force levels depend on tip geometry
 Batch fabrication of silicon devices seems feasible
 Cantilever processes with few extra steps are needed
 Silicon and diamond tip integration is underway
Platinum Tip On Aluminum Membrane
10µm
Tungsten Tip On Nitride Membrane
Device Fabrication – FIRAT chips
 Quartz chips mechanically cut using dicing
saw
 Fully cut for operation in air
 Halfway cut for fluid cell
Electrodes
 Alignment limits the sample geometry
FIRAT chip with Al m./ Pt tip
Wafer edge
Immersion chip with dielectric m./ W tip
Etch hole to
be sealed
100µm
Adaptation to Commercial AFM
FIRAT vs. Veeco (commercial) Cantilever holder




Laser stereo lithography is used to
fabricate the holder with specific
angle to use the 11º standard optics
FIRAT chip oriented to use first
diffraction order on the
photodetectors
Nearly plug-and-play with the
Veeco Dimension and MultiMode
system with minimal additional
electronics
Readily adaptable to other
commercial systems
Imaging head with FIRAT on AFM
Experimental Setup





Used Dimension AFM with
custom holder and added
electronics
Integrated electrostatic actuator
of FIRAT used as the only Z
actuator for fast tapping mode
imaging (lower loop) with RMS
detector BW ~12kHz
Direct interaction force imaging
(upper loop) for topography and
material properties with
Dimension’s RMS detector and
controller
Digitizing interaction force signal
at PD output (A) while using
upper loop
FIRAT actuator is also used as
equivalent of tapping piezo
A
Fast Tapping Mode Imaging With FIRAT
16 x 512 pixel images
1Hz
X-Y scan by piezo tube
5Hz
control
oscillation
20Hz
Z motion by
Active Tip
FIRAT line scans
Cantilever line scans
120
120
1 Hz
100
80
80
5 Hz
60
20 Hz
40
20
5 Hz
60
20 Hz
40
20
60 Hz
0
60 Hz
0
-20
1 Hz
100
Surface height (nm)
Surface height (nm)
 FIRAT probe used with
commercial AFM system
 FIRAT tapping frequency
600kHz
 Sample: 20nm high
calibration grating
 60Hz scan rate – limited by
X-Y scanner
 Surface topography limited
by interference curve
 A digital controller based
system is being built
60 Hz
-20
-40
0
0.5
1
1.5
2
2.5
Lateral position ( m)
3
3.5
4
0
0.5
1
1.5
2
2.5
Lateral position ( m)
3
3.5
4
Direct Measurement of Interaction Forces




FIRAT Substrate oscillated, tip displacement recorded
No tip-sample interaction  No signal  Zero background
Transient signals recorded with high bandwidth of the membrane
Every tap provides a dynamic force curve
4
3.5
Control loop for imaging
material properties
I
V
Z-input
3
Displacement (a.u.)
2.5
Controller
II
Control + 2kHz
tapping signal
IV
RMS
detector
2
PD
III
1.5
Piezo tube:
x-y scan & Z motion
for material property
imaging
st
+1 order
1
I
FIRAT probe
& holder
V
II III IV
Z- Piezo disp.
(FIRAT substrate)
0.5
0
-0.5
Tip displacement
(Photodetector output)
Jump to contact
-1
0
Laser
diode
50
100
150
200
250
Time ( s)
300
350
400
450
500
Electrostatic
actuation port
Sample
Simultaneous Topography and
Time Resolved Interaction Force (TRIF®) Measurement
2.5nm thick Pt GT logo on silicon
Topography
Control loop for imaging
material properties
Z-input
Laser
diode
Controller
Control + 2kHz
tapping signal
True constant
force tapping
mode imaging
RMS
detector
PD
TRIF signal
Piezo tube:
x-y scan & Z motion
for material property
imaging
+1st order
FIRAT probe
& holder
Electrostatic
actuation port
Individual tap (TRIF) signals
Sample
150
• Slope: Stiffness
• Tap shape inversion: Elasticity
• Attractive peaks: Adhesion, capillary
hysteresis
• Background: Long range forces
• Contact time
Interaction force (nN)
100
50
0
-50
-100
-150
0
0.5
1
Time (ms)
1.5
Quantitative Characterization TRIF™ Signals
 Contact mechanics and adhesion hysteresis models are used to fit the
digitized tap signals
 Tip remains normal to the sample during interaction
 Fast nanoindenter with high resolution imaging capability
Simulated vs. measured signals on PR and Si
Measured signals on polymers samples and Si
100
150
0
-100
Interaction force (nN)
100
Interaction force (nN)
silicon
Si-experiment
Si-simulation
PR-experiment
PR-simulation
50
0
-50
stiff polymer
-200
-300
-400
soft polymer
-500
-100
-150
-600
0.68
0.7
0.72
0.74
0.76
Time (ms)
0.78
0.8
0.82
-700
150
200
250
300
350
400
450
Time (us)
500
• Polymer samples courtesy of Prof. Ken Gall (GaTech MSE)
550
600
650
Extracting material properties (silicon)
Extracting PEGDMA properties
Mapping a feature of TRIF
 Material property imaging
independent of topography
 Adhesion force peak is used
as imaging parameter
Si substrate
Al (140nm) / Si substrate
Al (140nm) / Cr (150nm) / Si
Cr (150nm) / Si substrate
Cr
Topography
Peak attractive force
Al
150
Interaction force (nN)
100
50
0
-50
Al
-100
Al
Cr
Al
-150
Si
0
0.5
1
Time (ms)
Cr
Al
Si
1.5
Mapping properties of CNT over Si
TEM image
1.5 um
• Internal details seen in
the TEM image of a CNT is
observed in stiffness map
Adhesion energy (mJ/m2)
Stiffness (N/m)
Contact time (%)
• CNT samples courtesy of Prof. Sam Graham (GaTech ME)
Modeling of Device Dynamics
1
0.9
0.8
Normalized Amplitude
 The membrane results in
complicated frequency response
 not ideal for AFM applications
(Because it is a microphone!!)
 Finite element method is used to
model squeeze film stiffening and
damping effects for the complex
membrane structure with vent
holes
 Simple linear model also
accurately predicts the device
behavior
 Using validated model,
structures with desired
characteristics can be designed
0.7
0.6
0.5
0.4
0.3
0.2
Experimental
0.1
Linear model fit
ANSYS simulation
0
1
10
2
10
3
10
4
10
Frequency (Hz)
5
10
6
10
7
10
Other FIRAT Structures
 Cantilever, clamped-clamped
beams can be more suitable for
different applications
 The electrodes can be driven to
provide tip motion in 3-D
 Interplay between device spring
constant, stiffness of air in the gap
and damping determines the
frequency response
 Ideal device:
 Flat response until resonance
frequency (f0 ~ 1MHz)
 Reasonable Q (loss) for low
thermal noise limited force
resolution
 Reasonable Q (4-20) for fast
settling time
TRANSPARENT
SUBSTRATE
Electrostatic
actuation port
Cantilever
(Silicon nitride
etc.)
Clamped-beam (Bridge) Devices
 Aluminum bridge devices: 60µm long 20µm or 40µm wide, 0.8µm
thick. Air gap thickness 2.5µm
 Targeted Q values 4-15, spring constants in the 10-40N/m range,
resonance frequency ~ 1MHz to result in 100kHz imaging
bandwidth
 Also fabricated devices with 7µm gap for large actuation range
Top view of a bridge device
Measured Response for Fast Imaging FIRAT
 Simple model predicts device response
 Ideal response for fast tapping mode imaging – flat below resonance
 Thermal noise less than 1nN over 1MHz bandwidth
Optically measured response
Calculated response
20
20
60m x 20m Bridge simulated
60m x 40m Bridge simulated
18
16
Normalized response
Normalized amplitude
16
14
12
10
8
6
14
12
10
8
6
4
4
2
2
0
3
10
60 m x 20 m measured
60 m x 40 m measured
18
10
4
10
5
Frequency (Hz)
10
6
10
7
0
3
10
10
4
10
5
Frequency (Hz)
10
6
10
7
Bridge Device with FIB Tip
 Platinum tip built on 60µmx40µm, 0.8µm thick
aluminum beam
 Gap thickness 4.5µm, k ~ 40N/m, Q~ 4-5
Structures for Enhanced Sensitivity
 The membrane substrate gap
can be converted to a FabryGrating fingers
Perot cavity
(metal)
 Reflective part of membrane is
made of alternating stack of
silicon oxide-silicon nitride
quarter wave layers
 With 5-6 pairs slope increases
by 10x, shot noise remains the
same  displacement
sensitivity in the 10-5Å/√Hz
levels with 60μW laser power
 Low dynamic range, but very
high transient force sensitivity
Membrane
(metal+dielectric mirror)
Dielectric
mirror
number of dielectric
stack increases
(2,4,6,8)
Output voltage (V)
0.4
(a)
0.3
Extending
Experimental Verification of Enhanced Sensitivity
0.1
0.0
Measured Finesse factor of 10-14
0.4
0.4
Outputvoltage
voltage (V)
Output
(V)

Fabricated device with 5.5 pairs of
dielectric layers
Four-arm structure  Non-contact
measurement of 3-D forces in
vacuum and fluids
0.3
0.3
(a)
(b)
Extending Retracting
0.2
0.2
0.1
0.1
0.0
0.0
Membrane displacement x2 (100nm/div.)
Calculated Finesse factor of ~40
0.3
Normalized Intensity

0.2
0.2
0.1
0.0
Membrane displacement x2(0.25lo/div.)
Bio-application: Single Molecule Force
Spectroscopy
 AFM is used to measure binding forces between molecules and
inside a single molecule  linked to effectiveness of drugs
 Many measurements need to be performed to form a statistical model
P. Hinterdorfer et. al., PNAS, 93, (1996)
Daniel Muller, ICNT 2006
Parallel Force Spectroscopy
Not individually actuated  No force control


Most parallel molecular force
spectroscopy measurements require
individually controlled force probes
Individually actuated cantilevers can
be complex to build, can limit the type
of cantilever to be used
Individual actuator on cantilever Complex structures
FIRAT based solutions
 Functionalized, elecrostatically
actuated membranes conform to
cantilever array, force measurement
is performed by moving membranes
 Accurate, parallel detection of
membrane displacement with
integrated detector  accurate
control of force and molecule
extension
“Locally actuated sample surface”
Transparent
substrate
Immersion Device Fabrication
Transparent substrate
Deposit and pattern
the diffraction grating
20/80 nm Ti/Au
Top view
Spin and pattern the
polymer sacrifical layer
~3 μm of film
Deposit and pattern
The bottom insulator: 0.1 µm SiN/SiO2
The top electrode: 5/80 nm Ti/Au
The top insulator: 1.5 µm SiN/SiO2
Bottom
view
Array with separate actuation electrodes:
Decompose polymer layer at
440 °C
Fabricated both nitride and soft parylene membranes:
Spring constant ranging 20-1000N/m
AFM-FIRAT Combination
 An experimental tool for single molecular force measurements
 Allows for comparison of both methods, calibration of membrane
spring constants etc.
Conventional
AFM head
10x optical
camera
PD array
Motorized
vertical position
control for AFM
head
FIRAT ,
regular AFM
cantilever
Laser
Position
adjustment
knobs
FIRAT Based Devices – Initial Results




Integrated electrostatic actuator moves the membrane in vertical direction
Integrated optical interferometer measures displacement with high
resolution
Nitride and parylene membranes have been coated with PEI cushion and
functionalized with desired proteins
Electrical isolation ensures proper operation in buffer solutions
Molecular system used in experiments
V
Diffracted beam
Incident laser beam
Collaborator: Prof. Cheng Zhu (BME/ME)
Experiment with Piezo Actuation

Piezo
Drive: ON
600
400
200
0
-200
-400
Bond rupture
0
0.1
0.2 0.3
time [s]
0.4
0.01 N/m
AFM Cantilever
800
extend
retract
extend
retract
600
force [pN]

Membrane is passive sample
substrate
Commercial AFM piezo is used to
move the cantilever for
conventional molecular pulling
Adhesion-rupture events
recorded
force [pN]

400
200
0
-200
-400
multiple ruptures
0
0.2
0.4
time [s]
0.6
Experiment with Membrane Actuation


Piezo driver turned OFF (very small motion)
The membrane is driven with 5Hz triangular
signal to provide tip contact
Continuous tip contact  tip
follows periodic membrane
motion with some piezo drift
Piezo
Drive: OFF
V
Force measured by the cantilever (+ve tip pushed up, -ve tip pulled down)
No adhesion
Adhesion/ rupture
Adhesion/Rupture
No adhesion
Array of Membrane Sensors




Soft membranes can be used for both force sensing and tip actuation 
Cantilever is eliminated
High force sensitivity along with soft mechanical structure:
10 fm/√Hz * 1N/m  3pN with 1Hz-100kHz BW
Recently demonstrated < 10 fm/√Hz down to 1-2 Hz on FIRAT system
Polymer membranes with actuation capability are being fabricated
Microactuators for Fast Imaging in Liquids
Imaging probe: AFM
cantilever
Sample
Electrostatic
actuation port
Substrate: Transparent or
silicon
X-Y scan by separate
actuator
Frequency Response - in Water
Optically measured frequency response in liquid
2500
2000
PD Out [mVpp]
 Compatible with existing AFMs
 FIRAT membrane is used as
active sample holder
 Devices with > 100kHz BW
have been fabricated and
tested
 Fast Z-motion provided by the
FIRAT membrane
 Built-in displacement detector
for closed loop operation
 Suitable for molecular imaging
in fluids
1500
1000
500
0
10
100
1000
10000
100000
Frequency [Hz]
0th order; Bias=30Vdc, AC=2Vpp
1000000
10000000
Conclusion
 A new type of AFM probe tip has been
developed
A fully integrated FIRAT probe
 Integrates electrostatic actuation with
interferometric detection in a compact
manner
 Suitable for fast topographic imaging
 Provides sensitive broadband frequency
response for direct measurement of time
resolved interaction forces
 Tip motion normal to the sample simplifies
quantitative analysis similar to
nanoindentation
 Operation in fluids and application to force
spectroscopy has been demonstrated
 Structure is suitable for miniaturization
and array implementation
6mm
PD integrated 9x9 membrane array
MembraneSensor array
1cm
Challenges & Future Work
 FIRAT structures with integrated sharp tips
 More complex than cantilevers
 Silicon tips are feasible, cost can be an issue
 Better sample handling with required accurate
alignment
 Current/future work:






Commercialization
Fast imaging: Improved range and robustness
Parallel molecular force spectroscopy
Silicon tip fabrication
Unique probe structures for specific applications
….
Acknowledgments
 Graduate students and postdocs: Guclu Onaran,
Hamdi Torun, Dr. Mujdat Balantekin, Dr. Krishna
Sarangapani, Wook Lee, Byron Van Gorp, Dr. Jemmy
Sutanto, Dr. Will Hughes (MSE), Brent Buchine (MSE),
Rasim Guldiken, Zehra Parlak
 Prof. Calvin F. Quate (Stanford University)
 Prof. Cheng Zhu
 Prof. Z.L. Wang, Dr. Paul Joseph
 FIB facility, MiRC at Georgia Tech
Sample preparation
 Cantilever tips are coated with Anti-L-selectin mAb DREG56 (100
µg/ml)
 Membranes are coated with
 100 ppm polyethylenimine (PEI) solution to reduce non-specific
adhesions
 5 μl lipid vesicle solution
Probe–sample interaction modeling
• Modeling the membrane
with a spring
100
0
~
)
-100
• Finding rest position yB0
from minimum force in
advancing phase
Force F as a function of distance d
Assuming BCP contact mechanics model and a spherical tip of radius R
d0 : intermolecular distance, E* : effective tip-sample elasticity,  : surface energy
Extracting material properties
Elasticity
~
Surface energy (advance)
~
Surface energy (recede)
FIRAT for Material Characterization
 Broadband, damped response for clean tap signals
 Device geometry and gap adjusted for desired response
Measured and simulated response of 150m x 40 m bridge
5
Measured response
Simulated response
Measured tap signals
Normalized response (dB)
150
Interaction force (nN)
100
50
0
-50
0
-5
-10
-15
-100
-150
0
0.5
1
Time (ms)
1.5
-20
3
10
10
4
Frequency (Hz)
10
5
10
6
FIRAT-AFM Comparison
AFM
“FIRAT”
Fast X-Y scan by
piezo
Fast Z motion
by “Active tip”
• Piezo-tube scanner
• Slow moving piezo tube limits Z-scan speed
• Not integrated, bulky
• Fast X-Y scan
•Optical lever detection
• Requires re-alignment for every new
cantilever  time consuming
• Optical interference from sample
• Cantilever with tip
• Large background signal without tip-sample
interaction  Fast imaging of adhesion,
elasticity not possible
• Array implementation is difficult
• Active tip with integrated actuator
• Membrane dynamics limit Z-scan scan speed
• Z-motion  100x fast compared to piezo tube
• Device volume ~ 200µm x 200µm x 5000µm
• Fast X-Y scan
• Integrated Interferometric detection
• 10-100x more sensitivity  smaller impact force
• Fixed laser-detector location  Simple, fast
probe change
•Direct measurement of tip-sample interaction
• Fast imaging of adhesion, elasticity, subsurface
features
• Measurement of 3-D forces
• Integrated structure is suitable for arrays
Research in Degertekin Lab
 Micromachined integrated acousto-optic devices
 Biomimetic microphones for hearing aids (NIH)
 Optical microphone –novel signal processing integration
(Catalyst Foundation)
 Capacitive micromachined ultrasonic transducers
(CMUTs) for intravascular imaging for cardiology
applications
 Forward looking arrays (Boston Scientific Corp.)
 Side looking arrays with CMOS electronics (NIH)
 Atomic Force microscopy
 Quantitative material characterization (NSF)
 Parallel single molecule force spectroscopy (NIH)