E SC 412 Nanotechnology: Materials, Infrastructure, and ...€¦ · topography can sometimes be...

E SC 412

Nanotechnology: Materials, Infrastructure, and Safety

Wook Jun Nam

Lecture 20 Outline

• SPM Overview

• AFM

• STM

• NSOM

• Other Applications

Copyright 2014 by Wook Jun Nam

SPM overview


Applications of Scanning Probe Microscopy

(SPM)


SPM

AFM STM NSOM

Dip-pen

Litho.Nano-

indentation

MFM

SPM (Movie)

http://virtual.itg.uiuc.edu/training/AFM_tutorial/


Atomic Force Microscopy (AFM):

Operation Modes


• Scanning probe techniques enable nano-scale

characterization.

• AFM is suitable for conducting and insulating

samples.

• AFM does not require vacuum

• A variety of AFM scanning techniques are available

(contact, tapping, etc) for different sample types

• Each technique offers its own benefits and needs to

be chosen based upon the properties of the sample

Why Atomic Force Microscopy (AFM) ?

• AFM measures sample topography by monitoring the

deflection of the probe tip in the Z direction

• The sample is rastered (scanned) beneath the probe tip

and detection is performed by a segmented photodiode

Z Direction

Atomic Force Microscopy (AFM)

• Contact Mode

• Non-contact Mode

• Tapping Mode

AFM: Operation Modes

Atomic Distance vs. Force

SPM training notebook, Bruker Corp.

Tapping mode

AFM: Contact Mode


Note: If the tip is not moving, the

forces must be balanced.

Repulsive force between

tip and sample

Attractive force between

tip and sampleRepulsive

Attractive

The cantilever deflects as a result of various forces between the

tip and sample. The AFM keeps the force between the tip and

sample constant by utilizing a piezoelectric tube scanner.

F=-kx

Tip – Sample Interactions

δ

δ

A – B = (+)

A – B = (-)

A

B

A

B

Photodiode Detector: Contact Mode

AFM: Contact Mode (Movie)



• Advantage

– High scan speed (throughput)

– High resolution (only AFM technique offering “atomic

resolution”

– Rough samples with extreme changes in vertical

topography can sometimes be scanned more easily in

contact mode

• Disadvantages

– Can damage soft samples (e.g., biological samples,

polymers).

– Lateral (shear) forces can distort features in the image.

AFM: Contact Mode (continued)


AFM: Tapping Mode


AFM: Tapping Mode (continued)

http://www.intechopen.com/books/physical-and-chemical-properties-of-carbon-nanotubes/study-of-carbon-nanotube-based-devices-

using-scanning-probe-microscope.

http://www.intechopen.com/source/html/43322/media/image1.png

AFM: Tapping mode (Movie)



• Advantage

– Higher lateral resolution on most samples

– Lower forces and less damage to soft samples

– No scraping

• Disadvantages

– Slightly slower scan speed than contact mode

AFM: Tapping Mode (continued)


AFM: Non-contact Mode


• Advantage

– No force exerted on the sample surface

• Disadvantages

– Lower lateral resolution

– Slower scan speed than tapping and contact modes

– Only works on extremely hydrophobic samples to avoid

contacting the adsorbed fluid layer which results in the

tip getting stuck.

AFM: Non-contact Mode (continued)



System


Feedback

(to PC)

Piezo tube

Scanner

PhotodetectorLaser Diode

Tip sample interactions

Sample

Ceramic

Chip

Probe Tip

Cantilever

Typical Configuration of AFM

Optical Microscope

Probe Head

Sample Holder & Scanner


• Mirror 1: aligns the laser onto the end of the

cantilever

• Mirror 2: aligns the laser’s reflection onto the

photodetector

• AFMs are equipped with an optical microscope to

perform this alignment

Mirror 2 Mirror 1

AFM: Mirrors

• The ceramic chip is used for transporting the tiny

cantilever, which is attached on the end of the probe.

The tip is under the end of the cantilever

Ceramic Chip

Probe

Probe=Cantilever + Tip

Tip size: 10’s Å-10’s nmMagnified 70° view

Cantilever lengths: 100’s

mm

Cantilever width: 10-40 mm

Cantilever thickness: 0.3-2

mm

Top down view

(~1 cm x 1 cm)

Ceramic Chip, Cantilever, and Probe

Probe Cartridges Pre-Mounted Tips

Spring Tool

Contact Tapping

AFM: Tip Holder

x

y

z

Tube-shaped scanner allows for movement in all three directions

(x,y, z)

Movement in x & y: used to raster sample beneath stationary tip

Movement in z: used to maintain constant cantilever deflection

(feedback)

Piezoelectric

Tube Scanner

AFM: Piezoelectric Tube Scanner

Metal Disk Adhesive

Circles

Properly Mounted

Sample

AFM: Sample Loading Disk

Putting it all together…



Image Processing


Schematics and image profile of spheres scanned with a

sharp (left) and dull (right) probe

Tip Diameter

Practical guide to SPM, Bruker Corp.

Sidewall angle measurements of trench with vertical side walls.

Tip Diameter


Schematics and image profile of trenches scanned with a

contaminated tip.

Tip Contamination


Scanning Tunneling Microscopy (STM)


Typical Configuration of STM

Nanofabrication: Principles, Capabilities, and Limits, Zheng Cui, Springer (2008)

STM (Movie)



• STM is operated in high vacuum environment.

• Feedback is controlled by tunneling current.

• Tunneling current changes exponentially as the

distance between the sample and the tip is

changed.

• There are two operation modes: constant

current mode and constant gap mode.

Scanning Tunneling Microscopy (STM)

STM Images (Si)

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_4/illustr/g4_2_4.html

• Si surface in ultra high

vacuum conditions

(otherwise the surface

would immediately oxidize

and we would see

amorphous SiO2

STM Images (Pt)

a Pt surface. Vacancies are clearly visible

http://www.tf.uni-kiel.de/matwis/amat/def_en/kap_4/illustr/g4_2_4.html

Atomic Manipulation: the world’s smallest

movie

http://www.youtube.com/watch?v=QOujRp

2xKTY

http://www.youtube.com/watch?v=QOujRp2xKTY

Atomic Manipulation

http://www.youtube.com/watch?v=BUq2bQkL

1zo

http://www.youtube.com/watch?v=BUq2bQkL1zo

Atomic Manipulation Application

http://www.youtube.com/watch?v=f2OKVQm

ODC8

http://www.youtube.com/watch?v=f2OKVQmODC8

Near-field Surface Optical Microscopy (NSOM)


Near-field Surface Optical Microscopy

(NSOM)

• AFM and STM utilizes atomic force and electric current, respectively.

• NSOM uses light.

• Light diffraction limits resolution of conventional optical microscopy: ~250nm

• NSOM offers high resolution optical image, ~50nm. 25nm resolution image was demonstrated using a light source at wavelength of 488nm.


Typical Configuration of NSOM


http://www.nano-science.de/external/research/instruments/snom/snom.htm

NSOM: Tips

• There are two kinds of tips: apertured (a) apertureless (b).

The tips used in the apertureless mode are very sharp

and do not have a metal coating.

http://en.wikipedia.org/wiki/Near-field_scanning_optical_microscope

NSOM: Operation Mode

• Apertured modes of operation: a) illumination, b) collection, c)

illumination collection, d) reflection and e) reflection collection.

• Although there are many issues associated with the apertured tips

(heating, artifacts, contrast, sensitivity, topology and interference

amongst others), aperture mode remains more popular since

apertureless mode is even more complex to set up and operate, and

is not understood as well.


NSOM: Operation Mode

Apertureless modes of operation: a) photon scanning tunneling

microscopy (PSTM) by a sharp transparent tip, b) PSTM by sharp

opaque tip on smooth surface, and c) scanning interferometric

apertureless microscopy with double modulation


High Resolution Optical Imaging

https://nanohub.org/resources/2005/download/2006.11.02-ece695s-l13.pdf.

SEM

AFM NSOM

83 nm Optical resolution

seen with “100” nm tip

NSOM Applications

• High resolution optical imaging

• Spectroscopy

– Near-field Surface Enhanced Raman Spectroscopy

– Local spectroscopy of semiconductor devices

• Modification of Surface

– Subwavelength photolithography

– High density data storage

– Laser ablation

Other Applications


Dip-Pen Lithography


Other Applications

• Field induced deposition

• Electrochemical etching

• Nano-indentation/scratch

Lecture 20 Outline

• SPM Overview

• AFM

• STM

• NSOM

• Other Applications


E SC 412 Nanotechnology: Materials, Infrastructure, and ...€¦ · topography can sometimes be...

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