REU Wednesdays at One:REU Wednesdays at One:

Scanning Probe Scanning Probe MicroscopyMicroscopy

June 30th, 2010

Susan EndersDepartment of Engineering Mechanics

MicroscopMicroscopyy

Scanning ProbeScanning ProbeOptical Optical ElectronElectron

uses visible light and system of lenses to magnify

oldest and simplest design

new digital microscopes use CCD camera

magnification up to 2000 times

uses a particle beam of electrons to illuminate a specimen

create a highly- magnified image

uses electrostatic and electromagnetic lenses

magnification up to 2 million times

forms images of forms images of surfacessurfaces using a physical probe using a physical probe thatthat scans the specimenscans the specimen

surface image produced surface image produced byby mechanically moving mechanically moving probeprobe in a raster scan of thein a raster scan of the specimen and recordingspecimen and recording probe-surface probe-surface interactioninteraction as a function of positionas a function of position

atomic resolutionatomic resolution

was founded in 1981was founded in 1981

AFM, atomic force microscope BEEM, ballistic electron emission microscope EFM, electrostatic foce microscope ESTM, electrochemical scanning tunneling microscope FMM, force modulation microscope KPFM, kelvin probe force microscope MFM, magnetic force microscope MRFM, magnetic resonance force microscope NSOM, Near-Field scanning optical microscope (or SNOM, scanning near-

field optical microscopy) PFM, Piezo Force Microscopy PSTM, photon sanning tunneling microscope PTMS, photothermal microspectroscopy/microscope SAP, scanning atom probe SECM, scanning electrochemical microscope SCM, scanning capacitance microscope SGM, scanning gate microscope SICM, scanning ion-conductance microscope SPSM, spin polarized tunneling microscope SThM, scanning thermal microscope STM, scanning tunneling microscope SVM, scanning voltage microscope SHPM, scanning Hall probe microscope

SPM Types SPM Types

Scanning tunneling Scanning tunneling microscope - STMmicroscope - STM

powerful technique for viewing surfaces at the atomic levelpowerful technique for viewing surfaces at the atomic level invented in 1981 by Gerd Binnig and Heinrich Rohrer (at invented in 1981 by Gerd Binnig and Heinrich Rohrer (at

IBM Zürich)IBM Zürich) Nobel Prize in Physics in 1986Nobel Prize in Physics in 1986 probes the density of states of a material using tunneling probes the density of states of a material using tunneling

currentcurrent good resolution is considered to be 0.1 nm lateral good resolution is considered to be 0.1 nm lateral

resolution and 0.01 nm depth resolutionresolution and 0.01 nm depth resolution can be used not only in ultra high vacuum but also in air can be used not only in ultra high vacuum but also in air

and various other liquid or gas ambients, and at and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few temperatures ranging from near zero kelvin to a few hundred degrees Celsiushundred degrees Celsius

based on the concept of quantum tunnelingbased on the concept of quantum tunneling

Classic “Tunnel Effect”

Quantum Mechanics: Quantum Mechanics: The Tunnel EffectThe Tunnel Effect

Using the Tunnel EffectUsing the Tunnel Effectfor Imagingfor Imaging

)exp(~ dIT Binnig & Rohrer, 1982, Nobel Prize in Physics 1986

Using the Tunnel Effect Using the Tunnel Effect for Imagingfor Imaging

)exp(~ dIT

Binnig & Rohrer, 1982, Nobel Prize in Physics 1986

pointe

échantillon

piezo-céramiquepm = 0.000000001 mm

Using the Tunnel EffectUsing the Tunnel Effect for Imagingfor Imaging

)exp(~ dIT

movie: by courtesy of Dr. Dirk Sander, Max Planck Institute Halle, Germanysander@mpi-halle.de, www.mpi-halle.de

2 inch

The Real Thing

Surface of Au: atoms are visible!Surface of Au: atoms are visible!

N. Knorr, A. Schneider Dept. Prof. Kern, MPI Stuttgart, Germany

Co atoms on a Copper surface

Atoms and Molecules at Atoms and Molecules at SurfacesSurfaces

4.2 Kelvin

Moving Things AroundMoving Things Around

Fe atoms on Cu (111)Fe atoms on Cu (111)

IBM Almaden , D. Eigler

Fe atoms on Cu (111)Fe atoms on Cu (111)

IBM Almaden , D. Eigler

D. Eigler & E. Schweizer, Nature 344, 524 (1990)

Xe / Ni(110)

Atomic Force Microscope Atomic Force Microscope AFMAFM very high-resolution type of SPMvery high-resolution type of SPM resolution of fractions of a nanometer - 1000 times resolution of fractions of a nanometer - 1000 times

better than the optical diffraction limitbetter than the optical diffraction limit STM precursor to the AFMSTM precursor to the AFM Binnig, Ouate and Gerber invented the first AFM in Binnig, Ouate and Gerber invented the first AFM in

19861986 one of the foremost tools for imaging, measuring and one of the foremost tools for imaging, measuring and

manipulating matter at the nanoscalemanipulating matter at the nanoscale information is gathered by "feeling" the surface with information is gathered by "feeling" the surface with

a mechanical probea mechanical probe Piezoelectric elements facilitate tiny but accurate and Piezoelectric elements facilitate tiny but accurate and

precise movements enable the very precise scanningprecise movements enable the very precise scanning

Cantilevers andCantilevers and their propertiestheir properties

Typically made of Typically made of

SiSixxNNyy

Spring constants Spring constants in the range of 1 - 40 N/m in the range of 1 - 40 N/m (Forces from 0,1nN – 20 (Forces from 0,1nN – 20

µN)µN)

Tips range from a pyramid Tips range from a pyramid to very to very

sharp, high aspect ratio sharp, high aspect ratio tips, to flat punches.tips, to flat punches.

J. E. Sader, Review of Scientific Instruments -- April 2003 -- Volume 74, J. E. Sader, Review of Scientific Instruments -- April 2003 -- Volume 74, Issue 4, pp. 2438-2443Issue 4, pp. 2438-2443

Basic principle Basic principle cantilever with a sharp tip (probe) is used to scan the specimen surfacecantilever with a sharp tip (probe) is used to scan the specimen surface

tip is brought into proximity of a sample surface -> forces between tip and tip is brought into proximity of a sample surface -> forces between tip and sample lead to a deflection of the cantilever according to Hooke’s lawsample lead to a deflection of the cantilever according to Hooke’s law

forces measured in AFM include mechanical contact force, Van der Waals forces measured in AFM include mechanical contact force, Van der Waals forces, capillar forces, chemical bonding, electrostatic forces, magnetic forces, capillar forces, chemical bonding, electrostatic forces, magnetic forces, Casimir forces, solvation forces etc…forces, Casimir forces, solvation forces etc…

deflection is measured using a laser spot reflected from the top surface of deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes the cantilever into an array of photodiodes

if tip was scanned at a constant height -> risk that the tip collides with the if tip was scanned at a constant height -> risk that the tip collides with the surface -> damage -> feedback mechanism adjusts the tip-to-sample surface -> damage -> feedback mechanism adjusts the tip-to-sample distance to maintain a constant force between tip and sampledistance to maintain a constant force between tip and sample

sample is mounted on a piezoelectric tube which moves it sample is mounted on a piezoelectric tube which moves it zz direction for direction for maintaining a constant force, and maintaining a constant force, and xx and and yy for scanning for scanning

AFM can be operated in a number of modes, depending on the applicationAFM can be operated in a number of modes, depending on the application

possible imaging modes are divided into static (also called Contact) modes possible imaging modes are divided into static (also called Contact) modes and dynamic and dynamic

(or non-contact) modes where the cantilever vibrates(or non-contact) modes where the cantilever vibrates

AFM can be used to image and manipulate atoms and structures on surfacesAFM can be used to image and manipulate atoms and structures on surfaces

Atomic Force Microscope (AFM)Atomic Force Microscope (AFM)

Laser Beam DeflectionLaser Beam Deflection

Laser Beam DeflectionLaser Beam Deflection

Laser Beam DeflectionLaser Beam Deflection

Imaging modesImaging modesStatic modeStatic mode the static tip deflection is used as a feedback signal measurement of a static signal is prone to noise

and drift -> low stiffness cantilevers used to boost the deflection signal

close to the surface of the sample, attractive forces can be quite strong -> tip 'snaps-in' to the surface

static mode AFM is almost always done in contact where overall force is repulsive

technique called 'contact mode‘ force between the tip and the surface is kept

constant during scanning by maintaining a constant deflection

tip of the cantilever does not contact the sample surface

cantilever is oscillated at a frequency slightly above its resonance frequency (amplitude ~ few nanometers (<10nm))

van der Waals forces (strongest from 1nm to 10nm above surface)

or other long range force which extends above the surface acts to

decrease the resonance frequency of the cantilever

decrease in resonance frequency combined with feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance

Measuring tip-to-sample distance at each (x,y) data point allows software to construct topographic image of sample surface

AFM does not suffer from tip or sample degradation effects

non-contact AFM preferable to contact AFM for measuring soft samples

in case of rigid samples, contact and non-contact images may look the same

Imaging modes -Dynamic Imaging modes -Dynamic modemode

Advantages and Advantages and disadvantagesdisadvantages AFM provides a true 3D surface profileAFM provides a true 3D surface profile

samples viewed by AFM do not require special treatments samples viewed by AFM do not require special treatments

Most AFM modes work perfectly well in ambient air or even a liquid Most AFM modes work perfectly well in ambient air or even a liquid

Study of biological macromolecules and even living organismsStudy of biological macromolecules and even living organisms

gives true atomic resolution in ultra-high vacuum (UHV) and liquid gives true atomic resolution in ultra-high vacuum (UHV) and liquid environmentsenvironments

high resolution AFM is comparable in resolution to STM and TEMhigh resolution AFM is comparable in resolution to STM and TEM

disadvantage of AFM is image size (maximum height in disadvantage of AFM is image size (maximum height in m and maximum m and maximum scanning area around 150 by 150 scanning area around 150 by 150 m)m)

incorrect choice of tip for required resolution can lead to image artifactsincorrect choice of tip for required resolution can lead to image artifacts

relatively slow rate of scanning during AFM imaging often leads to thermal relatively slow rate of scanning during AFM imaging often leads to thermal drift in the image drift in the image

AFM images can be affected by hysteresis of the piezoelectric material and AFM images can be affected by hysteresis of the piezoelectric material and cross-talk between the (x,y,z) axes ->may require software enhancement cross-talk between the (x,y,z) axes ->may require software enhancement and filteringand filtering

filtering could "flatten" out real topographical features filtering could "flatten" out real topographical features

AFM probes cannot measure steep walls or overhangsAFM probes cannot measure steep walls or overhangs

DualScope™ MicroscopeDualScope™ Microscope

Sample size: Ø 50 mmSample size: Ø 50 mm Sample height: 5 mmSample height: 5 mm

Scan size: 40 x 40 µmScan size: 40 x 40 µm Z range: 2.7 Z range: 2.7

µm µm

www.dme-spm.dk

BearingBearing

www.dme-spm.dk

Wooden FibresWooden Fibres

www.dme-spm.dk

Landing Zone of Hard DiskLanding Zone of Hard Disk

www.dme-spm.dk

MotheyeMotheye

www.dme-spm.dk

Near-field scanning optical Near-field scanning optical microscope NSOM/SNOMmicroscope NSOM/SNOM nanostructure investigation that breaks the far field resolution nanostructure investigation that breaks the far field resolution

limit by exploiting the properties of evanescent waveslimit by exploiting the properties of evanescent waves

done by placing the detector very close (<< λ) to the done by placing the detector very close (<< λ) to the specimen surfacespecimen surface

allows for the surface inspection with high spatial, spectral and allows for the surface inspection with high spatial, spectral and temporal resolving powertemporal resolving power

resolution of the image is limited by the size of the detector resolution of the image is limited by the size of the detector aperture and not by the wavelength of the illuminating lightaperture and not by the wavelength of the illuminating light

lateral resolution of 20 nm and vertical resolution of 2-5 nm lateral resolution of 20 nm and vertical resolution of 2-5 nm has been demonstratedhas been demonstrated

contrast mechanism can be adapted to study different contrast mechanism can be adapted to study different properties (refractive index, chemical structure, local stress)properties (refractive index, chemical structure, local stress)

Dynamic properties can also be studied at a sub-wavelength Dynamic properties can also be studied at a sub-wavelength scale scale

Operating PrincipleOperating Principleoptical microscopyoptical microscopy

1870 Ernst Abbe :1870 Ernst Abbe :

d > λ / (2sinθ)d > λ / (2sinθ)

dd = distance between the = distance between the two objectstwo objectsλλ = wavelength of the = wavelength of the incident lightincident light2θ2θ = angle through which = angle through which the light is collected. the light is collected.

best resolution with optical best resolution with optical light is about 200 nmlight is about 200 nm

Operating PrincipleOperating PrincipleSNOMSNOM

if a subwavelength hole in a if a subwavelength hole in a metal sheet is scanned metal sheet is scanned close to an object, a super-close to an object, a super-resolved image can be built resolved image can be built up from the detected light up from the detected light that passes through that passes through the holehole

light passes through a sub-light passes through a sub-wavelength diameter wavelength diameter aperture and illuminates a aperture and illuminates a sample that is placed within sample that is placed within its near field (distance its near field (distance much less than the much less than the wavelength of the light)wavelength of the light)

achieved resolution is far achieved resolution is far better than conventional better than conventional optical microscopy optical microscopy

Modes of OperationModes of Operation1) Transmission mode imaging - sample is illuminated through the probe - light passing through the sample is collected and detected2) Reflection mode imaging - sample is illuminated through the probe - light reflected from the sample surface is collected and detected3) Collection mode imaging - sample is illuminated with a macroscopic light source from the top or

bottom - probe is used to collect the light from the sample surface 4) Illumination/collection mode imaging - probe is used for both the illumination of sample and for collection of

reflected signal

SNOM - LimitSNOM - Limit

Amount of light that can be transmitted by a small aperture poses a limit on how small it can be made before nothing gets thoughTo a degree this can be lived with, as more optical power can be generated, but the cutoff is so severe that it cannot be made smaller.

Next Step Next Step Field Enhancement MicroscopyField Enhancement Microscopy

instead of using a small instead of using a small aperture, a metal tip is aperture, a metal tip is used to provide local used to provide local excitationexcitation

if a sharp metal tip is if a sharp metal tip is placed in the focus of a placed in the focus of a laser beam, an effect called laser beam, an effect called local field enhancement local field enhancement will cause the electric field will cause the electric field to become roughly 1000 to become roughly 1000 times strongertimes stronger

enhancement is localized enhancement is localized to the tip, which has a to the tip, which has a typical diameter of 10 nmtypical diameter of 10 nm

as this tip is scanned over as this tip is scanned over the surface, an image can the surface, an image can be formed with a resolution be formed with a resolution as fine as the tip as fine as the tip

Julien Toquant, University of Basel

Optical scans made with BioLyser SNOM

Shear Force Imaging

Bovine Kidney Cells, non-contact mode: 10 µm scan

3D view of Bovine Kidney cell sample

Aluminum projection pattern on glas

SPM AdvantagesSPM Advantages

resolution not limited by diffraction, but only by the resolution not limited by diffraction, but only by the size of the probe-sample interaction volume ( few size of the probe-sample interaction volume ( few picometers)picometers)

ability to measure small local differences in object ability to measure small local differences in object height (like that of 135 picometre steps on <100> height (like that of 135 picometre steps on <100> silicon)silicon)

probe-sample interaction extends only across the tip probe-sample interaction extends only across the tip atom or atoms involved in the interactionatom or atoms involved in the interaction

interaction can be used to modify the sample to interaction can be used to modify the sample to create small structures (nanolithography)create small structures (nanolithography)

do not require a partial vacuum but can be observed do not require a partial vacuum but can be observed in air at standard temperature and pressure or while in air at standard temperature and pressure or while submerged in a liquid reaction vessel. submerged in a liquid reaction vessel.

SPM DisadvantagesSPM Disadvantages

detailed shape of the scanning tip difficult to detailed shape of the scanning tip difficult to determine (effect particularly noticeable if the determine (effect particularly noticeable if the specimen varies greatly in height over lateral specimen varies greatly in height over lateral distances of 10 nm or less) distances of 10 nm or less)

generally slower in acquiring images due to the generally slower in acquiring images due to the scanning processscanning process

embedding of spatial information into a time embedding of spatial information into a time sequence leads to uncertainties in metrology sequence leads to uncertainties in metrology (lateral spacings and angles) which arise due to (lateral spacings and angles) which arise due to time-domain effects like specimen drift, feedback time-domain effects like specimen drift, feedback loop oscillation, and mechanical vibration loop oscillation, and mechanical vibration

The maximum image size is generally smallerThe maximum image size is generally smaller not useful for examining buried solid-solid or liquid-not useful for examining buried solid-solid or liquid-

liquid interfaces liquid interfaces

The EndThe End