High Resolution Imaging of Nanoscale Structures by Scanning Probe Microscopy …deb.univpm.it/Farina/LezioneMicroscopia.pdf · 2012-11-06 · Scanning Probe Microscopy (SPM) In all

High Resolution Imaging of Nanoscale Structures by Scanning Probe Microscopy Techniques

Prof. Marco Farina, Senior Member IEEE Dipartimento di Ingegneria dell’Informazione Università Politecnica delle Marche

Our Team

Andrea Di Donato (Assistant Professor) Giuseppe Venanzoni (Research Fellow) Davide Mencarelli (Research Fellow) Tamara Monti (PhD Student) Francesco Bigelli (PhD Student) Antonio Morini (Associate Professor)

Scanning Probe Microscopy (SPM)

A quite recent class (1981) of microscopy techniques that has improved our understanding of surfaces and materials at sub-nanometric scale. In 1986 this work earned Nobel Prize to Gerd Binnig and Heinrich Rohrer (then @IBM in Zurich)

Today IBM labs still holds records in resolution! Imaging the

charge distribution within a single

molecule F. Mohn et al,

Nature Nanotechnology 7, 227–231

(2012)

Imaging of naphthalocyanine (left) and DFT-model (right)


In all cases a probe is scanned in close proximity of the surface of the sample (or vice-versa) and, depending on the type of probe, variations of some physical parameter arising from the interaction between surface and probe are recorded

SPM techniques may achieve “atomic resolution” at room temperature without need for vacuum!

Generally piezoelectric membranes are used to displace the sample (or the probe) at sub-nanometric scale: membranes are driven by some feedback system


In the Atomic Force Microscope (AFM) in “contact mode”, the device recovers the sample topography by a measurement of the deflection of a mechanical sharp tip, when the latter "touches" -in some sense- the sample surface. The tip is a few atoms at its edge

The deflection is detected by means of a laser beam

Images can be obtained by processing either the lateral or the normal deflection

Atomic Force Microscopy

• Credits animation: J.C. Bean, University of Virginia Virtual LAB

Atomic Force Microscopy

In the “semi-contact mode” the tip oscillates nearby its mechanical resonance; the interaction with the sample modifies amplitude and phase of the mechanical oscillation (there is also a frequency shift)

Useful for softer materials, such as polymers and bio-organic samples; phase allows to detect material inhomogeneity

In the “non-contact” mode, still the tip oscillates, but it is farther

Image: courtesy NT-MDT

Conducting Atomic Force Microscopy

Also called “spreading resistance” microscopy: a bias is applied and the current recorded via a conducting tip. The current is proportional to the sample local resistivity


It may be not as easy as it seems: -in air there is always a water meniscus, -there is chance to damage or contaminate the conductive coating during scans -the measurement depends on the contact area, and hence the landing conditions

Techniques derived from AFM: Electric Force Microscopy (EFM)

Using a conductive tip: there are many versions; e.g. in a common semi-contact “two pass” technique the first pass recovers the topography. During the second pass the cantilever is driven at a given distance and following the surface profile, while it oscillates at resonant frequency and cantilever is biased. Capacitive tip-sample electric force (actually its derivative) leads to resonance frequency shift.

Variations of the capacitance (Scanning Capacitance Microscopy SCM), or the surface potential distribution can be imaged by reporting variations in the oscillation amplitude


Techniques derived from AFM: SCM

Alternative (typical) implementation of SCM:

Variations of the capacitance are detected as frequency shift of a microwave resonator

Transmission line resonator

Output Coupled line

Varactor

Electrode (SCM probe)

Input coupled line

Resonator X-y scanner

Microwave source (around 1 GHz)

Detector

In any case the above measurements are qualitative and differential (variations are shown)

Other AFM-related Techniques

Kelvin Probe Microscopy: measurement of contact potential difference between tip and sample; often a two-pass technique and a static potential is applied by a feedback system to keep the system in equilibrium. This potential maps the contact potential

Magnetic Force Microscopy: Investigation of magnetic domains; several possible modes even in this case

…and many more Image: courtesy NT-MDT

Actually a good deal of confusion arises in classifying the large number of possible modifications in the original SPM (e.g., contact, semi-contact, non-contact, tapping mode….)

...AFM manipulation

AFM tip can be used to move object at nano-scale and to perform some lithography (below some example at our Dept. [DIBET] old name: the whole text is 2mm wide; thickness 10nm)

A different approach: the Scanning Tunneling Microscopy (STM)

Historically, the first SPM technique

Exploits the tunneling current between a sharp tip and a conducting sample Sharp tips are obtained by simply cutting a conducting wire or by electrochemical etching

Very sensitive as tunnel current is exponentially dependent from the distance!

Virtually tunnel current occurs just at level of a single tip atom (or just a few); hence no problem related to the curvature radius of the tip (convolution) : atomic resolution!

In AFM in fact the height image is a convolution between tip shape and surface geometry

the Scanning Tunneling Microscopy (STM)

• Credits: animation: J.C. Bean, University of Virginia Virtual LAB

Notes

The convolution arising in AFM imaging is not necessarily a dramatic issue: by scanning a known profile, the tip profile can be estimated, and images can be processed by deconvolution

STM on other hand can be used for other kind of measurements, such as the surface Density of States (DoS) by measuring the tunneling volt-ampère characteristic


STM: imaging at atomic scale

HOPG (graphite) surface as seen in our lab by the NT-MDT P-47 microscope

Carbonium lattice

Microwave Imaging: basic principles

Near-Field Scanning Microwave Microscopy (SMM): a first successful realization dates back to ‘70s[1] while the idea is credited to E.H. Synge [2], 1928

[1]E. A. Ash and G. Nicholls, "Super-resolution Aperture Scanning Microscope", Nature, vol. 237, pp. 510-512, June 1972

[2] E.H. Synge, "A suggested method for extending microscopic resolution into the ultra-microscopic region", Phil. Mag. 6 356–62, 1928

Microwaves are used to resolve sample details well below the Abbe's barrier, namely the wavelength limit; this is done by exploiting “evanescent” (near field) microwave fields interacting with probe and sample

The general line of reasoning: A sharp tip fed with microwave signal, having curvature radius R0<<l, will generate evanescent waves with wave vector k in the order of 1/ R0, hence rapidly decaying from the metal tip and giving resolution power in the order of R0, in spite of the wavelength

..actually the resolution can be much higher than R0 (shared some principles with synthetic radar)

Main issue: results are a convolution of effects due topography and local composition of the sample; difficult interpretation of results

Scanning Microwave Microscopy

Aperture microscopes: a miniature waveguide is used to generate evanescent fields •Pros: near field interaction may dominate (shielding) •Cons: usually resolution limited (micrometric)

Apertureless microscopes: a sharp tip excites quasi-singular fields •Pros: simple, achieves sub-nanometric resolution •Cons: tip radiates: strong coexistence of local and non-local interaction with sample makes somehow harder quantitative measurements and interpretation of data


Aperture Vs Apertureless

waveguide

aperture

sample

coax

sample

Parallel

strip TL

sample sample

cantilever

shield

coax

sample

coax

STM tip

Microwave Imaging: Aperture vs. apertureless


Common question: How can we use microwaves (centimetric wavelengths) to image nanometeric features?

Answer: we use electromagnetic fields to locally couple the probe and sample when they are very close (reactive interaction), not as radiated “rays”. The Abbe’s barrier (wavelength diffraction limit) does not apply

BUT: radiated fields still exist. Usually both local and non local interaction occur, making difficult data interpretation.



Generally SMM is associated to either AFM or STM. AFM or STM are used to control the tip-to-sample distance (for example: Agilent’ implementation uses AFM)

There are important exceptions: in ‘90s group leaded by Weiss used STM tip/sample junction to generate microwave harmonics, using the microwave signal directly in the feedback controlling the tip distance. Important result: possibility to use STM also on insulating specimens

In the most common implementation the SMM tip is part of a microwave resonant structure: they work at a fixed frequency and frequency shift is recorded by a PLL •Pro: enhanced sensitivity, simpler quantitative measurements •Cons: narrow-band; microwave spectroscopy not possible


Microwave Imaging: Our Approach

VNA for Microwave signal source and reflection measurement:

- 0.01 – 70 GHz (PNA

E8361A)

- max Dynamic range 120 dB

Our Software:

- STM-SMM synchronization

- Data processing: new algorithms for broadband processing. We process the complex reflection coefficient, not the resonant frequency shift

STM/AFM feedback control and spatial resolution (Nt-MDT Solver P 47)

Coax. C

able

Ethernet Cable

Vector NetworkAnalyzer

Conductive Pt/Ir tip

Sample

XYZ piezo

Tunnel current

Voltagesource

SPMcontroller/Feedback


Pros and Cons of STM

• Possible Atomic Resolution, quite easily, in ambient conditions • Currently no longer restricted to conducting surfaces: very sensitive current amplifiers (<1pA) available, so that also measurement on biological samples is possible. • possible to reduce parasitic interaction between tip and sample, owing to the typical shape of the tip. Minimum the “piezo cross-talk” effect.

AFM tip

STM tip

• STM is difficult over relatively large areas, or over dishomogeneous regions • The STM information is never purely topographic • Need to be careful in reducing interaction between microwave signal and STM electronics

Our implementation

We have not inserted resonators: “broadband” (0-70GHz)

Of course the tip is in any case a resonator, the mismatched cable is a resonator, the cavity is a resonator… (generally “broadband” implementations involved aperture probe)

Consequence: the sensitivity will be frequency dependent. Our “broadband” statement refers to data collection and manipulation rather than to a specific hardware implementation!

Data acquisition: We use a Vector Network Analyzer in the STM-assisted system (usually VNAs in literature have been used in AFM-assisted systems). VNA allows unpaired dynamics and extremely broadband


The idea is straightforward: a sample imaged at different (eventually close) frequencies shows the same features but with different amount of noise; one can extract common features among images obtained at different frequencies

Data Processing: Frequency

In many frequency regions the sensitivity will not be sufficient, owing to the lack of resonances. How to improve sensitivity?

This can be done by performing cross-correlation between images at different frequencies, or simply by normalizing and averaging images (M. Farina et al. IET Electronic Letters Jan 2010)


SMM before frequency processing (20.35GHz)

SMM after frequency processing (20-20.5GHz)

STM

Example: HOPG Microwave Imaging: Our Approach to HOPG

Screen-shot of our software

A gallery of SMM results:

Integrated circuit

STM SMM

Example: Calibration grating

chalcagenid glass, with gold surface and aluminum

sublayer; the pattern height is 30 nm, period 278 nm

STM SMM @ 22.8GHz

A gallery of SMM results:

Example: Fixed C2C12 muscle cell

STM (1pA, 8V) SMM (X band)

A gallery of SMM results: C2C12 mouse muscle cell

Example: Fixed C2C12 muscle cell (zoom) A gallery of SMM results: C2C12 mouse muscle cell (detail)

Example: Fixed C2C12 muscle cell (further zoom) A gallery of SMM results: C2C12 mouse muscle cell: detail

TIME DOMAIN?

What we measure is at the input of a “box error”, defining all effects not related to the sample (cable, tip body etc)

Quantitative measurement: calibration (IEEE MTT 2011, M. Farina et al.)

Y(e)

Sample local

admittance

Raw

Admittance

By assuming to know three different loads, we can evaluate the error box and remove it

Note: here we assume that just one port connects the error to the sample: not trivial (multimode or multipath interaction is possible). Generally multipath interaction gives poor imaging.

TIME DOMAIN?

The tip edge assumed to be as a sphere, and, if the tip-sample distance is known on a ground plane, the sphere capacitance becomes the known load!

Our Idea: the known loads

h1

h2

h3 > h2 > h1

C1

C2

C3

TIME DOMAIN? Comparison theory/experiment

0 100 200 300 400 500 6001.4

1.6

1.8

2

2.2

2.4

2.6

2.8

Height (nm)

Ca

pa

cita

nce

(1

0-1

6 F

)

Tip capacitance against tip/ground distance (calculated square Vs theory circle)

TIME DOMAIN? Time Domain?

TIME DOMAIN?

Question: How can we use microwaves (tents of picoseconds) to image nanometric features?

We can borrow some concepts of time-domain reflectometry and to Fourier-transform the recorded reflection coefficient Consider an open transmission line 1.5cm long. In vacuum at the speed of light a microwave signal is reflected back to the source in 100 picoseconds

1nm at speed of light in vacuum 0.003 femtoseconds...

1.5cm

A Fourier-transform of a signal with 20GHz as upper frequency would give a pulse of 50picoseconds (actually worse for windowing): perfectly detectable

Time Domain?

TIME DOMAIN?

This transmission line could be the probe. Any added capacitance (tip-to-sample interaction) changes the effective length of the line (as known by Hertz...)

The ability to resolve a small time-shift will depend on the system dynamic range, rather than on the upper frequency of the frequency acquisition

Time Domain?

(unfortunately the time delay depends on the frequency so that the pulse is also distorted)

Advantages?

•Probably easier understanding and interpretation of what is going on

•It is at zero cost (just a matter of post-process!)

•The signal in time is Real: features sometimes hidden either in the real or the imaginary parts (or mag and phase) of the frequency domain reflection coefficient are combined in a real signal and identified more easily

•Idea: reflections from the region of sample under the tip vertex (closer region) can be disentangled from reflections arising from the radiated waves, as the latter should reach the probe at different times. Appropriate selection of time should allow to disentangle local and non-local probe interactions

Time Domain:why?

A Model

COAXP2

ID=CX2

Di=600 um

Do=2620 um

L=1.36e4 um

K=2.12

A=0.0573

F=0.1 GHz

CAP

ID=C1

C=55.8 fF

RES

ID=R1

R=7.5e4 Ohm

CAP

ID=C2

C=0.7 fFTLIN

ID=TL2

Z0=2.754e4 Ohm

EL=3.382 Deg

F0=30 GHz

RES

ID=R3

R=657 Ohm

TLIN

ID=TL1

Z0=792 Ohm

EL=104 Deg

F0=10 GHz

RES

ID=R2

R=20.5 Ohm

CAP

ID=C4

C=13.9 fF

CAP

ID=C3

C=8.8 fF

CAP

ID=C5

C=11.4 fF

RES

ID=R4

R=51.3 Ohm

TLIN

ID=TL3

Z0=2.294e4 Ohm

EL=8.575 Deg

F0=30 GHz

CAP

ID=C6

C=12.4 fF

CAP

ID=C7

C=18.1 fF

CAP

ID=C8

C=18.3 fF

RES

ID=R5

R=46.2 Ohm

RES

ID=R6

R=42.2 Ohm

RES

ID=R7

R=1000 Ohm

TLIN

ID=TL4

Z0=1.994e4 Ohm

EL=3.835 Deg

F0=50 GHz

TLIN

ID=TL5

Z0=2.294e4 Ohm

EL=4.195 Deg

F0=60 GHz

TLIN

ID=TL6

Z0=2.264e4 Ohm

EL=347.5 Deg

F0=70 GHz

PORT

P=1

Z=50 Ohm

Local tip to sample interaction

Non-local tip-to-sample and tip-to-surround interaction

Parameters

selected to fit the

measured

response; number

of lines depends

on the frequency

band

A model to understand:

Comparison with measured data A model to understand: experiment vs model

Local

interaction

dominates

Non-local interaction

dominates

Time domain circuit simulation A proper selection of the time interval allows to disentangle local and non local interactions

SMM time transform (no correction!)

Original SMM in frequency (Mag @ 20.35GHz)

...After all Time Domain transform involves a combination of spectral data...

Some result

Ti Domain Animation Some result: HOPG

Now STM/SMM in liquid (M. Farina et al. IEEE

MWCL In press vol 22, issue 11 2012)

Advantages?

•The part of the tip immersed in water may change with piezo z-displacement: cross-talk

•Solutions: •- shield going up to water •- and/or time disentangling capability! See below…

STM SMM in liquid

Advantages? STM SMM in liquid (HOPG)

•STM

•SMM time 1

•SMM time 2

Advantages?

•We are testing a new AFM assisted SMM: easier to land and to compare with topography

•However piezo cross-talk is in this case relevant, owing to parasitic tip to piezo capacitance

AFM assisted SMM (M. Farina et al. Applied

Physics Letters November 2012 in press)

Time Domain Animation

Some result: Interaction nanotubes-cells (co-work with University of Trieste; University of

Chieti, Dr. Tiziana Pietrangelo)

AFM Microwave

Time Domain Animation Some result: Interaction nanotubes-cells (smaller area around the nanotube)

AFM Microwave

Documents

High Resolution Imaging of Nanoscale Structures by Scanning Probe Microscopy …deb.univpm.it/Farina/LezioneMicroscopia.pdf · 2012-11-06 · Scanning Probe Microscopy (SPM) In all