NANO 225 Micro/Nanofabrication

Characterization: Scanning Probe Microscopy

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History

• Scanning Tunneling Microscope invented in 1982 by, Binning, Rohrer, Gerber and Weibel

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http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm

• Binning and Rohrer won Nobel prize in 1986

• AFM developed in 1986 by Binning, Quate and Gerber

Hierarchy of Techniques

Scanning Probe Microscopy

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http://www.chembio.uoguelph.ca/educmat/chm729/afm/firstpag.htm

• Scanning Tunneling Microscopy

• Atomic Force Microscopy • Contact Mode

• Tapping Mode• Non-contact Mode

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STM

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• Feedback loop keeps current constant• therefore d is

constant

• Sample must conduct electricity

• Capable of detecting atomic scale defects

It ~ Ve-

cd

AFM Contact Mode

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• Scan tip along surface

• Tip contacts sample through adsorbed fluid layer

• Maintains constant cantilever deflection (force) using a split photo diode

• Samples can be in liquid state

• Works in ambient conditions

Hook’s law F=-kx

k limits sensitivity(want low k)

AFM Tapping Mode

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• Cantilever oscillates at or below resonance frequency

• Maintains constant RMS of tip movement

• amplitude

• Position of scanner stored to create image

• Tip must breakthrough water layer without getting stuck

• Also works in ambient or liquid

AFM Non-Contact Mode

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• Tip oscillates but does not touch sample

• above resonance frequency

• The resonant frequency is decreased by van der Waals forces

• Position of scanner used to map 3-D plot of surface

Silicon Nitride ProbeSpring Constant

(k) 0.58, 0.32, 0.12,

0.06 N/m (1)

Nominal Tip Radius of Curvature

20 - 60nm

Cantilever Lengths

100 & 200μm

Cantilever Configuration

V-shaped

Reflective Coating

Gold

Sidewall angles 35° on all 4 sides

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(1)Calculated spring constant values are based on the 0.6μm silicon nitride thickness; however, this value can actually vary from 0.4μm to 0.7μm. Thickness is cubed in the spring constant calculation, thus, actual values can vary substantially.

Silicon Probe

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AFM Imaging and Tip Shape

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The radius of curvature of the tip limits the resolution of the image that can be taken

AFM Imaging and Tip Shape

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The probe cannot image a sidewall that is steeper than the angle of the tip

Silicon nitride probe

AFM Imaging and Tip Shape

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Silicon probe

Surface Roughness Measurements

N

1a N

1R

javgj ZZ

N

ZZN

iavgi

1

2

qR

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Rz : average difference in height between the five highest peaks and five lowest valleys relative to the mean plane

Image Ra : average of the absolute values of the surface height deviations measured from the mean plane

Image Rq : Root mean square average of height deviations taken from the mean plane.

NanoScope Software 6.13 User Guide, Section 6.4.3

Four Point Probe

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University of CaliforniaEECS 143 Manual

• Constant current in two outer probes

• Voltage measurement on two inner probes eliminates contact resistance

• Four point technique required for precision resistance measurements

Four Point Probe

I

Vt

2ln

I

V

I

V

tRs 53.4

2ln

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University of CaliforniaEECS 143 Manual

• Thin film case• Current flow is

restricted to a thin film• t<<s t

Four Point Probe

I

Vs 2

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University of CaliforniaEECS 143 Manual

• Bulk resistivity case• Current flows

through bulk material• s<<t

t