firat: a new afm probe for fast imaging, material characterization, and single molecular mechanics...
TRANSCRIPT
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