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

2µm

• 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

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

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 tip-sample interaction forces during tapping mode imaging

Array implementation Parallel biomolecular

measurements Parallel imaging, nanofabrication

Vibration spectrum of an AFM cantilever

FIRAT Probe Structure

Integrated Electrostaticactuator input

Quartz substrate

Micromachined membraneand diffraction grating

(bottom electrode)

Reflecteddiffraction orders

Photodetector

Tip displacementsignal

1st diffractionorder incident

beam

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

Diffraction Based Optical Displacement Detection

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

reflection diffraction

substrate

=l/2d

dg

=l/4d

reflector

- 200 - 100 0 100 200- 50

0

50

- 200 - 100 0 100 200- 50

0

50

- 200 - 100 0 100 200- 50

0

50

x (m)

y (

m)

d = l/2

d = l/4

d = l/8

0 10.5

Normalized intensity

Gaussian aperture w0=9µm, λ=850nm, 2µm grating period

Displacement Sensitivity

0 0.1 0.2 0.3 0.4 0.5 0.6 0.70

0.2

0.4

0.6

0.8

1

Gap thickness (d) (m)

Nor

mal

ized

inte

nsit

y

I0I1

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

xRx

dRi =

=0

in0

10 4I

II

l

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)

• Shot noise limit MDD: √ qIR/(4 IR/λ)• ~3x10-5Å/√Hz with 450µW laser power on detector is demonstrated

Device FabricationSurface Micromachining On Quartz

SubstrateMembrane array (100µm diam.)

Back side